A MEMS Valve for the MIT Microengine

A MEMS Valve for the MIT Microengine

by

Xue'en Yang

B.S. Mechanical EngineeringUniversity of California, Berkeley (1999)

Submitted to the Department of Mechanical Engineeringin partial fulfillment of the requirements for the degree of

Master of Science in Mechanical Engineering

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

May2001 V"© 2001 Massachusetts Institute of Technology

All Rights Reserved

Author ..........................Depirth t of Mechanical Engineering

May 22, 2001

C ertified by ................................................. . .. . ........ .................... ..... ....Martin A. Schmidt

Professor of Electrical Engineering and Computer ScienceThesis Supervisor

A ccepted by ................................ ....... . .........................................Ain Sonin

Professor, Department of Mechanical EngineeringChairman, Department Committee on Graduate Studies BARKER

MASSAHUSETS TTUTEOF'TEGHN OLOGY

JUL 1 6 001

LIBRARIES

2

A MEMS Valve for the MIT Microengine

by

Xue'en Yang

Submitted to the Department of Mechanical Engineeringon May 22, 2000, in partial fulfillment of the requirements forthe Degree of Master of Science in Mechanical Engineering

Abstract

A microfabricated, electro-statically actuated, on/off gas valve made of silicon materialhas been designed, fabricated and tested. The valve will be a fuel control component in amicro-scale gas turbine engine. Room-temperature testing results using nitrogen havedemonstrated repeatable valve functions and choked flow characteristics.

MIT has initiated a project to build a micro-scale gas turbine generator for high powerdensity output in applications such as portable power source or micro air vehicles. Forclosed-loop operation, a valve is required to be able to withstand 10 atm upstream pressureunder high-temperature operating environment (700K), and result in a maximum flow rateof 600 sccm while has very low gas leakage rate. These system requirements can not bemet by previously reported MEMS valve, many of which are designed for low tempera-ture or low pressure applications.

The microengine prototype valve comprises of three fusion-bonded SOI wafers. Electro-static-actuation is used to lift the silicon boss actuator supported on four L-shaped tethersand open against high pressure. Polysilicon is chosen as the seat material for high-temper-ature operating environment. The flow path of the valve is designed to be choked andbecause of the micro-scale nature, both viscous and compressible effects are taken intoconsideration in flow analysis with axis-symmetric geometric.

It is demonstrated that at operating pressure of 10 atmosphere, the valve can be opened atless than 150 V with power consumption that is less than 0.04 mW. The gas leakage at thesame pressure is estimated to be less than 0.03 sccm Helium, while the open flow rate is43 sccm (3 g/hr) nitrogen. Commercial fluid analysis package CFD FLUET is used tomodel the flow and very good agreement with experimental data is obtained.

In the future, an array of 20 on/off valves (to obtain 5% accuracy in flow rate) will be usedto accomplish the fuel control scheme of the microengine.

Thesis Supervisor: Martin SchmidtTitle: Professor of Electrical Engineering and Computer Science

4

Table of Contents

Table of Contents .............................................................................................................................. 5List of Figures ................................................................................................................................... 7List of Tables ................................................................................................................................... I IAcknowledgments ........................................................................................................................... 13Nomenclature .................................................................................................................................. 151. Introduction ................................................................................................................................ 19

1. 1 Background .................................................................................................................... 191.2 The Valve Team and Facility ......................................................................................... 211.3 Thesis Organization ........................................................................................................ 22

2. Design Process ........................................................................................................................... 232.1 System Requirem ents ..................................................................................................... 232.2 Design of Prototype Valve ............................................................................................. 26

2.2.1 Design Schematic .............................................................................................. 262.2.2 Design History ................................................................................................... 292.2.3 Design Parameters ............................................................................................. 29

2.3 Summ ary ........................................................................................................................ 413. M icrofabrication ......................................................................................................................... 43

3.1 Fabrication Process ........................................................................................................ 443.2 Fabrication Considerations ............................................................................................. 543.3 W afer Bonding and Diesawing ...................................................................................... 553.4 Summ ary ........................................................................................................................ 58

4. Test Package and Testing Setup ................................................................................................. 594.1 Packaging ....................................................................................................................... 594.2 Testing Setup .................................................................................................................. 61

4.2.1 Electrode Characterization ................................................................................ 614.2.2 System Characterization .................................................................................... 624.2.3 Flow Characterization ....................................................................................... 64

4.3 Summ ary ........................................................................................................................ 665. M odeling and Testing ................................................................................................................ 67

5.1 Electrode Characterization ............................................................................................. 685.2 System Characterization ................................................................................................. 71

5.2.1 Quasi-Static M ode ............................................................................................. 725.2.2 Dynam ic m ode ................................................................................................... 75

5.3 Flow Characterization .................................................................................................... 785.3.1 Valve Function .................................................................................................. 855.3.2 Gas Leakage ...................................................................................................... 88

5.4 Summ ary ........................................................................................................................ 896. Conclusions and Future W ork .................................................................................................... 91

6.1 Conclusions .................................................................................................................... 916.2 Future W ork ................................................................................................................... 92

Appendix A M ask Drawings .......................................................................................................... 97Appendix B Valve Process Flow ................................................................................................. 117

B.1 Top W afer .................................................................................................................... 117B.2 Boss W afer .................................................................................................................. 119B.3 Seal W afer ................................................................................................................... 123

Appendix C M ask Drawings ........................................................................................................ 127

5

6

List of Figures

Figure 1.1. Schematic of the control system in the microengine. (Drawing by Diana Park.) ........ 20

Figure 1.2. Cross section of the microengine. (Drawing by Diana Park.).................................21

Figure 2.1. 3D schematic of the three structural layers that comprise the design the first generation

of prototype valve.........................................................................................................27

Figure 2.2. Cross-sectional view of the three structural layers..................................................28

Figure 2.3. Top view of boss supported by tethers. A) Straight L-shaped. B) Revised L-shaped teth-

ers w ith rounded corners............................................................................................ 30

Figure 2.4. Force balance when a voltage is applied to open the valve in its closed position........34

Figure 2.5. Breakdown voltage of nitrogen as function of pd from the generalized Townsend theory

(original data points from Meek & Craggs [13]).....................................................36

Figure 2.6. Cross-sectional view of the valve seat geometry.....................................................37

Figure 3.1. Fabrication flow for top wafer................................................................................. 46

Figure 3.2. SEM im age of top w afer...............................................................................................47

Figure 3.3. Fabrication flow of boss wafer ................................................................................. 48

Figure 3.4. SEM im age of boss w afer.............................................................................................49

Figure 3.5. Fabrication flow of bottom wafer.............................................................................51

Figure 3.6. SEM images of the seal wafer showing two different magnifications............52

Figure 3.7. AFM photos of polysilicon and silicon surfaces scanning on a 5 by 5 area. The grains

and stripes shown on the silicon photo are the scan line artifacts. .......................... 53

Figure 3.8. Profiles of oxide undercut A) without using step-oxide etch and B) after using step-oxide

etch ................................................................................................................................ 54

Figure 3.9. Results of wafer bonding of the three-wafer stack after annealing. The size of the fringe

is a measure of the local gap between the surfaces caused by particles..................56

Figure 3.10. Valve schem atic as bonded..................................................................................... 57

Figure 3.11. Pictures of the valve die showing the top view and contact pads for the various elec-

trodes in the bottom view . ........................................................................................ 57

Figure 4.1. Assembly of the valve chip package that attains both flow and electrical connections for

testing purpose. Drawing by Alexander Hoelke.......................................................60

Figure 4.2. Test package on an air-floating table........................................................................61

Figure 4.3. The circuit used to actuate the valve using a voltage source and obtain voltage and cur-

rent m easurem ents. ....................................................................................................... 62

7

Figure 4.4. 2D and 3D images of the tethers taken by Wyko as they are deflected to the upmost po-

sition. The tether deflection can be read from the 2D profile...................................63

Figure 4.5. A chart representation of the flow test system showing the nitrogen flow path...........65

Figure 5.1. Cross-sectional schematic of the second generation valve to show the probes and the

four contact pads....................................................................................................... 68

Figure 5.2. I-V curves of the four contact pads measured using HP semiconductor analyzer by

sweeping -100V to 100 V across the same contact pad............................................69

Figure 5.3. I-V characteristics between the two parallel plate electrodes...................................70

Figure 5.4. Lumped model of the electrostatic actuator.............................................................71

Figure 5.5. Plot of equilibrium position of boss as function of voltage using measurement data from

T able 5.1. ............................................................................................................ 73

Figure 5.6. Plots of boss deflection measured using Wyko vs. voltage applied between the top ac-

tuator and the boss for two different dies. Also in the plot is the theoretical curve using

m easured dim ensions................................................................................................ 74

Figure 5.7. Tether deflection as voltage is applied across boss and bottom electrode. .............. 75

Figure 5.8. Step response of the boss with a step voltage of 41 V............................................ 76

Figure 5.9. Undamped natural frequency of the system as a function of the voltage input............77

Figure 5.10. Valve open flow rate measured at different absolute pressures of gas inlet for two dies

w ith different seat geom etry. ................................................................................... 78

Figure 5.11. Simplified flow geometry showing the flow direction......................................... 80

Figure 5.12. Open flow rate as function of absolute pressure at low pressure range for Die I. The

model matches the experimental data well at pressure lower than about 1.5 atm........81

Figure 5.13. Open flow rate as function of pressure in high pressure region. The model neglects vi-

sous effect. .................................................................................................................... 82

Figure 5.14. Velocity contours in flow region for 1.2 atm upstream pressure. .......................... 83

Figure 5.15. Flow profile in A) the throat and B) the channel showing subsonic flow that is fully

developed......................................................................................................................83

Figure 5.16. Mach number and pressure contours for 10 atm upstream pressure. .................... 84

Figure 5.17. At 10 atm upstream pressure, A) shows choked flow in the throat and B) shows pres-

sure drops on the boss along the valve seat............................................................ 84

Figure 5.18. Voltage required to open the valve against applied upstream differential pressure...85

Figure 5.19. Leakage current between the boss and top electrode as voltage is applied for Die I. 86

Figure 5.20. Flow rate at certain pressure as voltage is gradually increased to open the valve......87

8

Figure 5.21. Helium leakage rate of two dies with different seat areas....................88

Figure 6.1. Valves distributed on microengine chip. .................................................................. 93

Figure A.1. Mask: ALIGN, wafer level, with streets..................................................................98

Figure A.2. Mask: TOPELEC, die level, with streets...............................................................99

Figure A.3. Mask: Top_.ELEC, device level.................................................................................100

Figure A.4. Mask: TOPOX_2, die level, with streets ................................................................. 101

Figure A.5. Mask: TOPTHROUGH, die level, with streets ....................................................... 102

Figure A.6. Mask: TOPTHROUGH, device level ...................................................................... 103

Figure A.7. Mask: BOSSOX1, die level, with streets.................................................................104

Figure A.8: Mask: BOSSFEET, device level..............................................................................105

Figure A.9. Mask: BOSSOX-2, die level...................................................................................106

Figure A. 10. Mask: BOSSTETHER, die level, with streets ....................................................... 107

Figure A. 11. Mask: BOSSTETHER, device level......................................................................108

Figure A.12. Mask: BOSSDEEP, die level, with streets ............................................................ 109

Figure A.13. Mask: STREETS, wafer level..................................................................................110

Figure A. 14. Mask: SEALOXIDE, die level, with streets .......................................................... 111

Figure A.15. Mask: SEALOX, device level ............................................................................... 112

Figure A.16. Mask: SEALBACK, die level, with streets ........................................................... 113

Figure A. 17. Mask: SEALSEAT, die level, with streets ............................................................ 114

Figure A.18. Mask: SEALCHANNEL, die level, with streets ................................................... 115

Figure C. 1. AutoCAD layout of the valve package: window plate .............................................. 128

Figure C.2. AutoCAD layout of the valve package: top plate ...................................................... 129

Figure C.3. AutoCAD layout of the valve package: spacer plate ................................................. 130

Figure C.4. AutoCAD layout of the valve package: bottom plate ................................................ 131

Figure C.5. AutoCAD layout of the valve package: pin holder....................................................132

Figure C.6. . AutoCAD layout of the valve package: pin holder..................................................133

9

10

List of Tables

Table 2.1. Microengine system requirements for fuel valve and test valve................................24

Table 2.2. Design parameters of valve geometry........................................................................ 40

Table 3.1. Wafer layout and the status of dies as fabricated.......................................................45

Table 3.2. Types of SOI wafers used for the three structural layers..........................................45

Table 5.1. Planar dimensions of fabricated valve as well as constants calculated from these dimen-

sion m easurem ents. ......................................................................................................... 67

Table 5.2. Pull-in Voltages of different dies for both the top electrode and the bottom electrode.

*Die VI has a tether that is buckled and is not considered in statistics.......................74

Table 5.3. Valve performance for four dies at 10 atmosphere upstream pressure......................87

11

12

Acknowledgments

I would like to thank foremost my advisor Professor Martin Schmidt, who has givenme the opportunity for this invaluable research, who has mentored me throughout theproject, and who has always supported his students in many ways. This project could nothave been accomplished without Dr. Alexander Hoelke, who initiated the design, andtaught me through everything about valves, MEMS, graduate life, etc. Vielen Dank, Alex!

I also owe much gratitude to Professor Alan Epstein, who has always guided methrough the project, and Professor Jeffrey Lang, Dr. Auturo Ayon, Dr. Stuart Jacobsenand Dr. Stephen Umans for their advice on the different aspects of the valve.

It has been a great experience to work with the Schmidt group, namely, Samara, Joel,Ole, Becky, Christine, Sam and Zony. I would like to thank them sincerely for their friend-ship and the help they never hesitate to provide. There is another group that I owe manythanks; they are the people I have spent the most time with for a long while. Tom, Ravi,Dennis, Yoav et al., thanks for sharing the many experiences and providing many helps inthe cleanroom! I owe many thanks to the people in the microengine group, whom I learneverything about microengine from. I need to thank Dr. Xin Zhang specifically for herexceptional help on wafer bonding and her bountiful advice.

I am also grateful to Dr. Vicky Diadiuk and the MTL staff, especially Kurt Broderik,Paul Tierney and Bernard Alamariu for their training and caring. I would like to thank Dr.Carol Livermor for her patience with my questions and what I did to the Microvision sys-tem, Paulo Lozano for his prompt help with helium leak detector, Yifang Guo for his gen-erous assistance in using CFD FLUENT and many others who have helped and shared theexperiences in various ways.

The most special thank goes to Simon, who has added different colors to my life andhas tried to change my perspectives on many things for the past year. Le printemps est ici,Simon!

I owe the most to my parents. I thank them deeply from my heart for giving me theguidance and strength: wuyan ganji.

13

14

Nomenclature

Greek

interfacial tension from air to liquid interface

S permittivity of air

dynamic viscosity

V specific heat ratio

0 water contact angle

Or water contact angle on roughened surface

T time constant

COC cutoff frequency

(On natural frequency

Roman

A area where choked flow occurs in the flow path (the throat)

Aactuator area of actuator

b damping constant

C1 constant used in Townsend's theory

C2 constant used in Townsend's theory

d gap used in Paschen's law

D boss diameter

ds gap distance when spark breakdown occurs

E young's modulus

Es electrical field strength

F capillary force

Ftank pressure force acting on the boss from the fuel tank

Ftether tether force

15

g gap between the two parallel plates in the capacitor

go the original gap between the two parallel plates

go gap between the two parallel plates at an operational point

gmnin minimum gap between the two parallel plate electrodes

h distance of gap between boss and valve seat when valve is fully open

I flow region in the seat area

I-XII die numbers

II flow region in the channel area

k spring constant

Kbend minor loss coefficient in the bend

Kinlet inlet minor loss coefficient

1 tether length

m mass of boss

m mass flow rate

p pressure used in Paschen's law

P fluid pressure

PO stagnation pressure

Po fluid pressure at an operational point

P1 pressure at seat inlet

P2 pressure at seat outlet

P3 pressure at channel inlet

Ptank pressure of tank

Q charge on capacitor

Q1 volume flow rate in flow region I

Q2 volume flow rate in flow region II

R resistance

R universal gas constant

r; radius of water droplet

r, radial dimension in flow region I

r2 radial dimension in flow region II

r; radius of flow channel / inner radius of valve seat

16

ro outer radius of valve seat

t tether thickness

TO stagnation temperature

V voltage

VB voltage applied to boss

VBT voltage applied to boss, with top electrode grounded

VBS voltage applied to boss, with bottom electrode grounded

VTB voltage applied to top electrode, with boss grounded

VSB voltage applied to bottom electrode, with boss grounded

VS voltage applied to bottom electrode

VL voltage applied to landing pad

VT voltage applied to top electrode

VP; pull-in voltage

Vr velocity in radial direction

VS spark voltage used in Paschen's law

w tether width

x1 state of charge on capacitor

x10 charge on capacitor at an operational point

X2 state of boss displacement

x2o boss displacement at an operational point

X3 state of boss velocity

z displacement of boss from the original position

zI vertical axis from seat used in flow region I

Z2 vertical axis from seat used in flow region II

Acronyms

AFM atomic force microscope

BOE buffered oxide etch (HF)

17

BOX buried oxide

CFD computational fluid dynamics

DRIE deep reactive ion etch

DSP double side polished

FEM finite element analysis

FIB focused ion beam

LPCVD low pressure chemical vapor deposition

MEMS micro-electro-mechanical system

SOI silicon on insulator

STS deep reactive ion etcher from Silicon Technology Limit

VLSI very large scale integration

18

Chapter

1Introduction

1.1 Background

This thesis examines the design, analysis, fabrication, packaging and testing of a MEMS

(Micro-Electro-Mechanical Systems) fuel valve for the application of a micro gas turbine

engine.

MIT has initiated a research project on micro power systems, with an aim to build

a micro-scale gas turbine generator to produce high density power. This heat engine is

designed to produce tens of watts of electrical power per cubic centimeter, which is about

ten times the energy density of batteries. The microengine technology can be used to

power micro-air vehicles, micro-fluidic control, miniature cooling systems and micro-

rocket engines. The microengine will be built using semiconductor fabrication techniques

(microfabrication) developed in the microelectronic industry. The structural material is sil-

icon and silicon carbide, which possess good mechanical properties such as high strength

and toughness.

For self-contained applications, the design of a microengine includes a built-in control

system for fuel metering as shown in Figure 1. This control scheme includes a start valve

for initial engine stabilization and a fuel valve for fuel level control in response to the sig-

19

20 Chapter 1: Introduction

nals of pressure, temperature and engine spinning speed. Therefore, the valve must be able

to modulate the flow according to the input control signal.

PressurizedFuel Tank

Fuel Valve Fuel LineStart

User ValveInterface

Start Control ControlSignal

Pressure

Tomarature,

RPM

E Igniter

Micro Gas Turbine

Figure 1.1. Schematic of the control system in the microengine. (Drawing by DianaPark.)

Figure 2 is the cross-sectional view of the microengine, showing the compressor, com-

bustor, turbine supported by air bearings and integrated electric generator. The fuel system

consists of the fuel manifold and the fuel injectors, which supply fuel to the combustor

from the fuel tank (not shown in the picture). A valve is placed between the fuel tank and

the plenum to modulate the fuel flow requested by the controller. Microfabrication tech-

nology constrains the design of the microengine to be a 2D-extruded structure. Therefore,

the valve will be designed to stack on top of the engine.

21

Starter/Generator

Flame Fuel Fuel CompressorHolders Manifold Injectors Diffuser Rotor

\Vanes Bades I

CombustionChamber 7I~i

TurbineNozzleVanes

Figure 1.2. Cross

Turbine ExhaustRotor NozzleBlades Centerline Rotor

of Rotation

section of the microengine. (Drawing by Diana Park.)

1.2 The Valve Team and Facility

The valve team consists of post doctoral associate, Alexander Hoelke, me and an advisory

committee including Professors Alan Epstein, Martin Schmidt, Jeffrey Lang, Dr. Arturo

Ayon, Dr. Stuart Jacobsen, and Dr. Stephen Umans.

Dr. Hoelke initiated the preliminary design of the fuel valve in the summer of 1999.

After I joined him in the Fall, we fabricated the first generation of the prototype valve and

tested its functions. Based on the testing results, I revised the design, fabricated, and test-

ing the second generation.

The valve was microfabricated in Microsystems Technology Laboratories (MTL) at

MIT. MTL possesses sufficient facilities for the valve fabrication, including photolithog-

raphy, DRIE (Deep Reactive Ion Etch), plasma etch, thermal oxidation, nitride deposition

and fusion wafer bonding.

IEMM

Section 1.2: The Valve Team and Facility

Gasnlet Path

22 Chanter 1: Introduction

1.3 Thesis Organization

This thesis will emphasize the design, fabrication, packaging, testing and modeling of the

second generation of the prototype valve.

Chapter 2 presents the design of the second valve based on the preliminary design and

the testing results of the first generation.

Chapter 3 explains the detailed fabrication process and presents the results of fabrica-

tion.

In Chapter 4, the packaging design and testing apparatus needed for different testing

purposes are described.

In Chapter 5, the testing results of the second valve are presented, together with model

analysis for comparison.

The final chapter concludes the work and lays out the future work for the valve.

22 Chapter 1: Introduction

Chapter

2Design Process

This chapter introduces the design of a prototype valve based on the system requirements

of the microengine. Two generations of the valve were fabricated and tested. This chapter

will emphasize the design of the second generation, which has improved function com-

pared to the first one.

2.1 System Requirements

The goal of the microengine is to produce 20 Watts mechanical power, while consuming

about 45 grams per hour of propane fuel. To achieve this goal, the fuel valve must operate

under high temperature and high pressure. Such requirements rule out many currently

available designs of microvalves developed in both industry and academe.

A valve can be categorized mainly by its actuation type and sealing material. Conven-

tional valves used in engines typically employ solenoids for magnetic actuation. On the

micro scale, however, induced magnetic forces are usually too weak to act against high

pressure flows [2]. Successfully commercialized microvalves that use bimetallic and ther-

mopneumatic driving techniques have been reported [3]. These valves often operate under

relatively low temperatures (between 0*C to 60*C in the case of thermally actuated

valves) because of the materials used. Piezoelectric and electrostatic actuation have been

23

widely used in design because of their low power consumption [4][5]. Such drivers

require high voltage input and small deflections in order to produce large actuation forces.

Other actuation techniques include electrolysis-bubble and shape memory alloys [6][7].

Conventional valve sealing materials can be either hard or soft. To achieve extremely low

leakage rate, many researchers have employed soft materials as contact surfaces such as

polyimide or silicone for their high flexibility and fatigue resistance [8]. However, such

materials are not apt for high temperature applications. Other valves conveniently use sili-

con as hard contact surfaces, typically in the form of cantilever and diaphragm [9]. Vari-

ous studies have shown that sealing properties are not only dependent on materials used,

but also on the fabrication process.

Table 2.1. Microengine system requirements for fuel valve and test valve.

Application Microengine Prototype Valve

Throttle

Fluid Propane N2, Propane

Mount Engine Test Package

Temperature 700K 300K

Flow Rate (g/h) 45 2.25 (5% of 45)

A Pvalve, max (atm) 6 9

Ptank (atm) 10 10

Modulation 0.5-1 On/Off

Precision 5% 5%

Time Response ms ms

Shock Resistance 1Og log

Size (cm 2) 2.1 0.25

Mass (mg) 1200 80

24 Chapter 2: Design Process

Table 2.1 lists the system requirements of the microengine fuel valve as well as a pro-

totype valve. The goal of the valve project is to build a prototype valve that satisfies the

design criteria of the microengine and to evaluate the function of the valve in a test pack-

age before integrating it with the engine. In other words, the prototype valve is used to val-

idate the design concept. For simplicity, testing conditions are set at room temperature

while nitrogen is used as the testing fluid. The items in Table 2.1 will be explained in the

following paragraphs.

For a portable engine design, it is desirable to integrate the valve with the engine by

wafer bonding. Doing so also reduces packaging complexity and improves power density.

Integration with the engine, however, will cause rapid heat transfer from the combustion

walls to the valve and as a result, the valve will be heated. For a combustion temperature

at 1600 K, it is estimated that the valve will be operating at about 700 K. The operating

temperature requirement forbids the use of polyimide or elastomer as a sealing material.

Furthermore, as will be explained in the fabrication chapter, the 1100*C annealling tem-

perature of the valve precludes the use of metal for actuation.

In order for the valve to control fuel level, two possible design schemes have been pro-

posed. One is a proportional valve that adjusts flow according to an input actuation signal,

another is an array of on/off valves that modulate the flow by turning on an appropriate

number of valves in response to the input signal. The second scheme is often easier to

design and implement. However, an array of valves requires complex wiring path. In addi-

tion, flow accuracy is limited by the number of elements in the array. For a maximum

engine flow rate of 45 g/h, in order to obtain an accuracy of 5%, 20 on/off valves are

needed, each of which will supply a fuel flow of 2.25 g/h when turned on. The number of

valves in the array will also be limited by the planar dimension of the microengine, which

is 2.1 cm 2 .

Section21l: System Reqient 25

26 Chanter 2: Design Process

The pressure of the fuel tank will initially be at 10 atmospheres. The tank walls form

the external shell of the microengine package and they enclose about 800 cm 3 of fuel.

This package is designed to supply tens of hours of electrical power between refuelling.

The maximum pressure drop across the valve should be about 6 atm. The prototype valve,

however, will be tested at an outlet pressure of one atmasphere, for design simplicity.

It is desired that the valve have a response time in milliseconds and a shock resistance

of 100 g. The size of the final valve is limited by that of the microengine. For the engine to

be efficient, the valve should weight no more than 1.2 g.

2.2 Design of Prototype Valve

Based on the requirements stated above, a first design of the prototype valve has been cre-

ated. The valve uses electrostatic force as the actuation method. This is chosen because the

microengine is integrated with an electric generator and is designed to provide 300 V of

electrical signal. The actuation mechanism can be described as a parallel plate capacitor

acting on a mass-spring-damper mechanical model. We have chosen silicon material for

the valve seat because soft materials are eliminated in our design by high temperature con-

straints.

2.2.1 Design Schematic

The valve is comprised of three 4" wafers fusion bonded together at room temperature and

subsequently annealed at 1100*C. Figure 2.1 shows the 3D cross-sectional view of the

three layers.

The top wafer contains the valve inlet, view-port, main electrode and landing pads.

Gas fuel enters the valve from the tank above via the inlet. A view-port is opened for test-

ing purposes so that a fiber optic sensor can be inserted to detect the motion of the boss.

26

The main electrode is primarily a thin layer of single crystal silicon on top of a 1 gm thick

silicon dioxide layer that acts as insulation. The substrate is a 500 pm thick silicon wafer.

Such a three-layer structure is commonly called an SOI (silicon on insulator) wafer. The

landing pads are holes etched into the silicon dioxide insulation layer. They are used as the

mechanical stop and prevent the boss from crashing onto the main electrode upon pull-in.

The substrate is grounded so as to avoid being electrically floating.

Valve Inlet

Viewport

Main Electrode

Landing Pad

Landing Feet

Boss

Tethers

Valve Seat

Secondary Electrode

Valve Outlet

Figure 2.1. 3D schematic of the three structural layers that comprise the design the first gen-eration of prototype valve.

The middle wafer features a movable boss that is supported by four L-shaped tethers,

which offer the boss sufficient flexibility in the vertical direction. The tethers are made

also from an SOI layer to ensure uniform thickness. We are only interested in the vertical

Section 2.2: Design of Prototype Valve 27


motion of the boss and will not consider its in-plane rotation. The boss initially closes

down under 10 atm pressure from the fuel tank. When a voltage is applied between the top

electrode and the boss, electrostatic force will attract the boss to displace upwards and

hence open up the valve. During pull-in, the landing feet on top of boss will touch down

on the landing pads and make electrical contact with the top substrate, which is also

grounded. By doing so, a minimum gap is formed between the two electrodes and a short

circuit is avoided.

The bottom wafer includes three main objects: valve outlet, valve seat, and secondary

electrode. The outlet is a through-hole in the silicon wafer and it leads the gas to the com-

bustor through the engine manifold and the injectors. When the boss moves down, it lands

on the valve seat and hence closes the flow path. The secondary electrode is added for test-

ing purpose. It forms another parallel plate capacitor with the boss and is used to attract

the boss in the downward position. This setup helps to characterize the total boss displace-

ment.

To illustrate the design further, Figure 2.2 shows a schematic cross-sectional view of

the valve with the three wafers bonded together.

Valve Inlet Landing Foot

Tether View Port Main Electrode

Top

Seat

Backup Electrode Valve Outlet Pin Ports

Figure 2.2. Cross-sectional view of the three structural layers.

29

2.2.2 Design History

Two generations of the valve have been successfully fabricated. The first generation used

smooth silicon as the valve seat. This valve allowed us to validate the design and develop

the fabrication techniques needed for the process. Testing results showed that this valve

had fully functioning actuation mechanism and predicted flow characteristics. However,

there were a few drawbacks. First, the actuation force was too weak to open against full

pressure flow. Second, the current leakage between the electrodes worsened after high

voltage usage. Finally, over time, the boss adhered to the valve seat because of stiction.

Based on these problems, a second generation was designed and fabricated. The new

design has improved geometry and fabrication process, and uses polysilicon as the sealing

material. This section will introduce the design concept and the design variables.

2.2.3 Design Parameters

The goal of the preliminary valve is to be able to open against 10 atm pressure with a volt-

age no more than 300 V, and a response time of milliseconds. Therefore, the design

dimensions must be able to satisfy the following conditions:

1. The tethers must have appropriate stiffness to support the boss. Furthermore, the

resonant frequency of the tether-boss structure should be much less than that of the

rotor, which has achieved a speed of 1.2 million RPM.

2. The silicon based capacitor should exert enough attraction force over the boss to

counteract the pressure force when the valve is in the closed position.

3. The dimensions of the gap between the boss and valve seat, as well as the diameter

of the gas outlet, should be chosen such that the gas flow rate is 2.25 g/h.

Section 2.2: Design of Prototype Valve


Based on these design considerations, an analysis of the tether structure, parallel

capacitor electrodes and fluid dynamics is carried out to estimate the valve dimensions.

This analysis will then be compared with testing result to check its validity.

Tether Design

A top view of the boss supported by L-shaped tethers are shown in Figure 2.3.A. The

tethers are L-shaped rather than straight because such a design has many advantages. It

allows more linear downward deflection and offers better attenuation over packaging

stresses and thermal stresses [10]. During fabrication, however, we have found that this

boss structure is very fragile and that the fabrication yield is low. Tethers tend to break

along the straight corners due to stress concentration. To resolve this problem, we have

revised the straight corners with round fillets. Also, the width of the tether is increased to

make it stiffer. Such a design is illustrated in Figure 2.3.B).

A) B)

1W

Figure 2.3. Top view of boss supported by tethers. A) Straight L-shaped. B) Revised L-shaped tethers with rounded corners.


The tether thickness is determined by that of the SOI layer, which is used in our design

to ensure uniformity throughout the wafer during the etching process. Given the tether

length 1, width w and thickness t, the total force FTether exerted on boss for tether end deflec-

tion z can be expressed as

Ftether Ewt3 Z (2.2)

where E is the Young's modulus of silicon. In our design, we use (100) wafers and the

tethers are oriented at 450 with the primary flat, i.e., in the [001] direction. The Young's

modulus corresponding to this direction is 130 GPa [10].

The tether-boss structure can be modeled as spring-damper-mass mechanical system,

which can be represented by the following system equation,

mz + bz + kz = a (2.3)

where b is the damping coefficient, m is the mass of boss, k is the spring constant of the

tethers, and a is the input signal. The natural frequency of this system is then

O) = -z (2.4)

Time constant t for a step function can be expressed as,

m (2.5)

Squeezed-Film Damping

The damping constant can be evaluated using squeezed-film damping theory, which

applies as the boss moves up and down relative to the stationary, parallel actuator. Consid-

ering the top electrode, we assume the following:

1. The gap between boss and top electrode is much smaller than the boss diameter.

Section 2.2: Design of Prototyp av 31

2. The gas obeys the ideal gas law, is fully developed and isothermal.

3. The boss moves in slow motion such that the gas attains a small Reynold's number,

and viscous effects dominate.

4. There is no pressure gradient in the vertical direction.

5. The no-slip boundary condition can be applied because the ratio of the mean free

path of the gas molecules to the gap is small.

Applying the Navier-Stokes equations and combining with the ideal gas law, the

squeezed-film damping phenomenon can be described by the Reynolds equation [11]:

3

(Pg) = &V2p 2 (2.6)6g.

where P is the pressure of the film that is a function of radius and time, g is the gap

between the two plates, and R is the dynamic viscosity of the fluid film. This partial dif-

ferential equation is nonlinear, and in order to obtain an analytical solution, linearization is

performed near an operating point Po and go. We can then find the pressure response to a

velocity impulse. The total force acting on the plate can be calculated by integrating the

pressure over the plate. A first order approximation of this force in Laplace transform is

[11]

F(s) = b sz(s) (2.7)+ _

where b is the damping constant

b 96gr4 (2.8)g go

oC is the cutoff frequency defined by

32 Chapter 2: Desigzn Process

33

2^2'it g0 P0O =C 2O (2.9)

C 2R 21211r

and r is the radius of the actuator.

This solution demonstrates that b is a function of geometry only. The existence of oC

suggests that the gas behaves also like a spring due to the compressibility effect. At low

frequency, the compressibility effect can be ignored, and b can be viewed as a constant.

However, at high frequency, we have to take into consideration the spring effect. If o is

much larger than the frequency that we are interested in, we can use b as a constant.

There is also a damping effect from the bottom electrode. As the radius of the seat is

much smaller than the radius of the boss, we will ignore this damping effect and consider

only the top electrode.

Force Analysis

Actuation force for the boss comes from a parallel plate capacitor formed by the top wafer

and the boss. When a voltage V is applied between these two electrodes, the electrostatic

force exerted on the boss for a gap distance g is

F uactuator ctuatorV2 (2.10)2g2

where,

e = permittivity of air, which is 8.85x10- 2 F/m, and

Aactuator = Actuation area defined by the enclosed area of top wafer electrode and boss

Under the 10 atm pressure of the fuel tank, the valve is normally closed. The boss

experiences a net pressure force due to the pressure difference in the valve seat area. For

design purposes, the worst case of this pressure force is evaluated in order to estimate the

largest actuation force needed. This case corresponds to the largest pressure drop AP


34Chpe2:DsgPrcs

across the valve channel (9 atm) and the vacuum condition on the valve seat area. This

external pressure force can be expressed as

Ftank = A~r+P tankC(r -r?) (2.11)

where

ri = inner diameter of the valve orifice

r0 =outer diameter of the valve seat

To open the valve from its close position, force balance requires that

Factuator + Ftether > Fank (2.12)

The force diagram of the boss is demonstrated in Figure 2.4.

+jriro

V

Figure 2.4. Force balance when a voltage is applied to open the valve in its closed position.

Minimum GapThe landing feet create a minimum air gap between the top electrode and the boss when

the boss is in its upmost position. Without the landing feet, the boss would crash onto the

top electrode upon pull-in, causing the two surfaces to stick. A proper choice of the mini-

mum gap prevents breakdown between the two electrodes.

The dielectric strength of air is usually reported as 3x10 6 V/m at atmospheric pres-

sure. This value, however, does not hold when the gap between the two plane electrodes is

Chapter 2: Design Process34

reduced to micron dimensions. It has been observed from experiments that the breakdown

voltage depends on the product of the gas pressure and the gap separation, as stated in Pas-

chen's law [13]

V, = f(pd) (2.13)

where p is the gas pressure and d is the gap separation. This law can be interpreted as that

the breakdown voltage at small gaps can be predicted by using data taken at small pres-

sures. Experiments carried out in vacuum, i.e., very low (pd) values, have shown that

many gases exhibit a minimum breakdown voltage. At lower (pd) values, the breakdown

strength of gases will increase. This behavior is captured in Townsend's breakdown the-

ory, which explains the breakdown phenomenon as a number of collision processes that

ionize the gas [13]. At very low pressure or gas separation distances, particle collisions are

less likely to occur, therefore making sparking breakdown more difficult. The criterion for

breakdown is given as

c,%e -( 1 = 1 (2.14)

where c, and c2 are constants that can be obtained by measurements of pre-breakdown

ionization current, c1 is the coefficient representing ionization by electrons, c2 represent-

ing ionization of gas by positive ions, and d, is the gap distance when spark breakdown

occurs. The breakdown voltage then relates to the gap distance by Vs = Esd,, where Es is

the strength of the uniform electrical field. Using this criterion, the breakdown characteris-

tics of nitrogen can be described by the curve shown in Figure 2.5.

Section 2.2: Design of Prototyve Valve 35


550-

500-

450-

400-

C: 350-

0

( 300-

250-

200

0 10 20 30 40 50 60

pd (atm-um)

Figure 2.5. Breakdown voltage of nitrogen as function of pd from the generalized Townsendtheory (original data points from Meek & Craggs [13]).

The curve suggests that the minimum breakdown voltage for nitrogen is about 300 V.

At 10 atmospheres, the gap separation at this breakdown voltage is about 1 gm. Experi-

ments at very low gap separation, however, have shown that this breakdown voltage is not

always achievable. Surface contamination or surface roughness could cause the actual

breakdown voltage to be much smaller. In such cases, the electric field can be locally con-

centrated, enhancing gas ionization and resulting in a lower average breakdown field.

Therefore, a larger gap separation will be advantageous. In the design of the valve, a min-

imum gap of 2 gm is chosen.

Flow Analysis

In order to control the mass flow rate, choked flow is designed in the gas flow path. A

magnified cross-sectional view of the valve seat geometry is shown in Figure 2.6.


37

Po, To h

Patm

Figure 2.6. Cross-sectional view of the valve seat geometry.

The geometry is axisymmetric so that only the 2D case needs to be considered. For

invisid flow in a duct, the choked condition determines the maximum mass flow rate to

area ratio to be

S1/2(l V -(V + 1)

0a + Y-2 ((1 - 1) (2.15)

where

m = mass flow rate

A = 2nrih, is the area of the flow path where flow is choked

h= distance of gap between boss and valve seat when valve is fully open

PO = stagnation pressure of flowing fluid, same as the pressure of the fuel tank

To = stagnation temperature of flowing fluid

v = specific heat ratio of flowing fluid; 1.13 for propane; 1.14 for nitrogen

R = gas constant; 189 J/kg-K for propane; 287 J/kg-K for nitrogen



This equation might be used to estimate the flow rate at high pressure, where the Rey-

nold's number is relatively high. But in the real situation, there will be pressure drops

across the seat and within the channel because of viscous effects. Also, various minor

losses in the flow inlet and the bend have to be considered. For design simplicity, this

equation will be used to estimate the size of the flow path.

StictionIn Figure 2.5, we have shown three forces acting on the boss. However, a stiction force is

omitted from the picture. This section is devoted to describe this force, which is hard to

quantify as it depends on surface roughness, humidity and other factors. Nonetheless, dur-

ing testing of the first generation of the valve, we often observed that the boss easily

adhered to the top or the bottom surface, and could not be released. The second generation

aims to prevent these problems by using rougher material as the valve seat and by reduc-

ing the valve contact area.

Stiction is more well known in the microscopic world and often proves to be detrimen-

tal to MEMS devices. The causes of stiction include capillary forces, Van Der Waals

attraction and electrostatic forces. For a water droplet between two parallel plates, the

attracting capillary force is given as [15]

F = 2ycos0 r2 (2.16)h 1

where y is the interfacial tension from air to liquid interface, 0 is the water contact angle,

h is the gap between two surfaces and r, is radius of the water droplet. This equation says

that the capillary force is proportional to the plate area and the cosine of the water contact

angle. At a contact angle of 90*, the force is zero. To reduce the capillary force, possible

solutions are then to decrease the area of the plate and to increase the water contact angle.


A completely hydrogen terminated silicon surface is hydrophobic with a contact angle

around 90'. When the surface is exposed to air or water, a native oxide forms on the sili-

con surface, causing it to be hydrophilic with a contact angle less than 60'. This contact

angle can be increased by roughening the silicon surface. It is derived from quasithermo-

dynamic model that the water contact angle on roughened surface can be expressed as [16]

cosor = rcos0 (2.17)

where Or is the contact angle of the roughening surface and r is the ratio of the actual area

of roughened surface to the projected area. Therefore, increasing the roughness reduces

the value of cos0 and hence the capillary force. In experiments, however, such an appar-

ent relationship is not always achievable. The advantage of roughening the surface, fur-

thermore, is that it reduces the actual contact area and as a result, the Van der Waals force

and the electrostatic force will also be reduced [17].

Conventional ways to modify silicon surfaces include using focused ion beam (FIB) to

create dimples [18], chemical etching by NH4F [19], or silicon anodization [20]. For a

valve, however, there is a trade-off between the roughness of the valve seat surface and

the leakage rate. Rougher surface produce less flow resistance in closed position and result

in larger leak. Polysilicon is chosen as the seat material because of its rough surface and

the fabrication advantage. Deposition of polysilicon is compatible with VLSI, and the thin

film structure is uniform and stable. Furthermore, roughness of polysilicon can be con-

trolled through deposition conditions.

Chosen Dimensions

Summarizing the structural and flow analysis from above, we chose the dimensions of

various geometry and some resulting constants as listed in Table 2.2. Comparison of the

39

4040Chte2:DinPrcs

dimensions used in the 1st generation and the 2nd generation are also made in the table.

Table 2.2. Design parameters of valve geometry.

Items Units 1st Generation 2nd Generation

Tether:

Tether Thickness t jim 17 (SOI) 17 (SOI)

Tether Width w pim 20 60

Tether Length 1 jim 680 800

Total Spring Constant K N/m 161 300

Boss:

Boss Diameter D gm 670 1080

Boss Mass m Kg 4.1x10 7 10.5x10~

Natural Frequency oo KHz 2.76 2.68

Original Gap g, pm 6.2 4.9

Actuation Gap g gm 7.2 5.4

Minimum Gap gmin Im 2.86 2

Valve Seat:

Inner Radius r; jm 15 18

Outer Radius ro jm 100/200 34/42

Flow Gap h jm 4 3.2

Oscilliary Feature:

Landing Feet Diameter jm 50 30

Landing Pad Diameter jm 100 60

View Port Diameter pm 100 290

The second generation aims not only to increase fabrication yield but also to allow the

valve to open against full pressure. In order to do so, the tether stiffness is increased by tri-

pling the tether width. Furthermore, the actuation area is enlarged while the actuation gap

was decreased, thus resulting in an increased actuation force. The valve seat radius is also

decreased by three times hence reducing the pressure force acting on the boss from the

Chapter 2: Design Process

Section 2.3: Summary 41

fuel tank by about 17 times. Given these dimensions, we would predict that for the second

generation, the valve would be able to open against 10 atm (1.013x106 Pa) at an actuation

voltage of 148 V. Because the flow gap is decreased as a result of the smaller actuation

gap, the flow channel radius is increased from 15 ptm to 18 gm in order to obtain a simi-

lar flow rate.

2.3 Summary

A prototype valve was designed to meet the system requirements of the MIT microengine.

Some design issues are discussed. Dimensions of the valve are assigned based on the pre-

liminary structural, electrical and fluidic analysis.

An on/off valve is proposed as it is more practical to build by microfabrication. Fuel

control can be accomplished by using an array of such valves. Electrostatic actuation is

chosen as the actuation method as voltage can be supplied from the engine generator. A

boss supported by tethers forms the actuation mechanism, which can be modeled as a par-

allel plate capacitor with mass, damper and spring. The flow is designed to be choked at

the valve seat at high pressure. For high temperature application, silicon is used for the

valve seat.

Design of the valve involves several interesting phenomena. The first one is the

squeezed-film damping, which occurs when two parallel plates have relative motion.

Damping turns out to be caused both by viscous (dashpot) and compressibility (spring)

effects. At frequencies much lower than the cutoff frequency, spring effects can be

ignored. The second one is the electrical breakdown between two parallel plates separated

by submicron distance. A minimum voltage appears as the distance decreases further, in

which case, the breakdown voltage is limited by the surface condition. The third one is

41Section 2.3: Summary


stiction, which is a function of contact area and water contact angle. Effective ways to

reduce stiction include reducing the area and roughening the contact surface.

This chapter introduces the design concept of the prototype valve. The second genera-

tion of the design differs to the first one by: 1) a revised geometry to increase the net valve

opening force; 2) using polysilicon as the valve sealing surface instead of smooth silicon

to reduce stiction; and 3) improving the fabrication process to reduce current leakage

between the electrodes. The fabrication process will be introduced in the next chapter.

Chapter

3Microfabrication

Fabrication of the prototype valve was carried out in the MIT Microsystem Technology

Laboratories (MTL). The techniques of microfabrication were similar to VLSI, i.e., using

photolithography for mask patterning, and various wet and dry etching methods for cut-

ting exposed geometries. In particular to MEMS fabrication, surface micromachining

refers to the process of making free-standing thin-film structures by use of sacrificial lay-

ers and bulk micromaching refers to the process of etching deep into the substrate [11].

Deep etches are usually achieved by using thick photoresist as a masking material or by

using a high selectivity material such as an oxide thin-film. MEMS microfabrication often

involves bonding two or more wafers together to achieve various geometries. In this case,

the surface of the wafer must be flat and smooth in order for wafer bonding to be success-

ful.

It is worth noting that there are a couple of current technologies which have made the

current valve design feasible:

1. SOI (silicon on insulator) wafers are made by thermally growing oxide on a silicon

substrate and subsequently bonding to another silicon substrate, which would then

be thinned down and chemical-mechanically polished. We used these wafers as the

starting material for two purposes: 1) They possess excellent uniformity as well as

43

surface smoothness for wafer bonding. For this purpose, the SOI layer was used for

tether structures; and 2) the buried oxide could be used as an electrical insulation

from the substrate to the electrode.

2. The high aspect ratio silicon structures in the design would not have been accom-

plished without access to deep reactive ion etching (DRIE). In particular, we used

the time-multiplexed deep etching technique developed by Robert Bosch. This tech-

nique cycles an etching phase (using SF6) and a sidewall passivating phase (using

C4F8to prevent etching of the sidewalls) [12]. With this technique, etching of silicon

as deep as 300 gm with thin walls is possible.

The following section will explain the valve fabrication process.

3.1 Fabrication Process

The valve requires a total of fifteen masks to be patterned on three wafers, four sides, and

approximately twelve shallow plasma etches, four deep plasma etches, three thermal oxi-

dations and two thin film depositions. Fabrication of the second generation differs from

the first one mainly because of the use of polysilicon in the bottom wafer. In this section,

we will discuss only the process flow for the fabrication of the second generation. The

wafer layout and all the masks used in fabrication are shown in Appendix A. Fabrication

details of each wafer are described in Appendix B.

Each wafer layout has twelve evenly spaced devices containing four different designs

of valve seat geometry (the numbering of dies on the wafer is shown in Figure A.1 in

Appendix A). The performance of each die after fabrication is listed in Table 3.1. Note

that because of fabrication constraints, all dies with grooves were not successfully fabri-

cated and as a result, only 6 dies contained functioning valves.

Chapter 3: Microfabrication44

Section 3.1: Fabrication Process 45

Table 3.1. Wafer layout and the status of dies as fabricated.

Die Number Seat Outer Diameter Seat Pattern Fabrication Status

I, VIII, X 34 flat good

II, VII, IX 42 flat good

III, V, XII 34 grooved, 2 rings bad

IV, VI, XI 42 grooved, 2 rings bad

Table 3.2 lists the three wafers used for the three structural layers. The fabrication pro-

cess for each wafer will be explained in detail in the following sections.

Table 3.2. Types of SOI wafers used for the three structural layers.

Structural Layer SOI Thickness Buried Oxide Thickness Silicon Substrate Thickness

gm gm

Top Wafer (SOI) 0.34 1 500

Boss Wafer (SOI) 17 0.4 380

Seat Wafer (DSP) N/A N/A 450

Top Wafer

The top wafer contains the top electrode, landing pads and the view port. The S01 layer is

used as an electrode and the buried oxide as an insulator. Figure 3.1 shows the fabrication

flow.

The major fabrication steps are:

Section 3. 1: Fabrication Process 45

46 Chapter 3: Microfabrication

Silicon Oxide Polysilicon Nitride

Substrate

1. Etch alignment marks Buried Oxide

Sol

2. Plasma shallow etchtop electrode

3. BOE etch oxide step *

Stepped oxide etch--

4. DRIE through etch forview port and gas inlet a1

Landing 7View PortLandingContact Pad to

Pad Top Electrode Landing Pad

Figure 3.1. Fabrication flow for top wafer.

1. Prepare the SOI wafer and etch alignment marks on both sides (mask: ALIGN). The

top side alignment is to be used for aligning to the other two wafers during wafer-

bonding.

2. Shallow plasma etch the SOI layer to define the top electrode area as well as the

landing pad areas (mask: TOPELEC). The top electrode has a diameter of 1100

gm and the diameter of the landing pad is 80 gm. The valve is enclosed in a square

of 1.6 mm2

3. Etch the buried oxide layer in buffered HF (BOE) (mask: TOPOX_2). Instead of

etching through the oxide using the same mask as in step 2, this oxide etch creates a

projected oxide layer (section 3.2 provides further explanation).

4. DRIE to etch through the wafer to open the flow inlet as well as a through hole for


Section 3. 1: Fabrication Process 4

viewing purposes under microscopes (TOPTHROUGH). Diameter of the hole is

290 pm.

Figure 3.2 shows an image of the fabricated top wafer using a Scanning Electron

Microscope (SEM). The white band shown in the picture is the "electrical wire" from the

electric contact located near the edge of the die. The wire is insulated from the rest of the

wafer surface by the oxide thin film below it. We could also see the step-oxide etch by not-

ing the color contrast at the edge of the cylinder.

Figure 3.2. SEM image of top wafer.

Boss Wafer

The boss wafer forms another half of the parallel plate capacitor. It contains the mov-

able boss that is supported by four tethers. The fabrication process is shown in Figure 3.3.

and the process flow can be summarized into the following steps.

47

48 ChaDter 3: Microfabrication


1. Thermal oxidation, 1 pm;Nitride deposition; Etchalignment marks

2. Wet etch oxide anddefine device area

3. Plasma shallow etchlanding feet, 3.4 pm

4. BOE etch oxide fromback side

5. DRIE etch tether fromback side, using BOXas etch stop

6. DRIE etch boss fromfront side using BOXas etch stop

7. Wet etch oxide to releaseboss; Remove nitride inphosphoric acid hot bath

NitrideThermal OxideSubstrateBuried Oxide

Landing Feet

Bos

TetherContact Pad

to BossContact Through Hole

to Top Electrode

Figure 3.3. Fabrication flow of boss wafer

1. Thermal oxidation of SOI wafer under wet conditions at 11000 C for 1.5 gm.

LPCVD nitride deposition of 0.1 gm. This nitride layer is used as the etch mask for

releasing the tether structure in BOE in the last step. Etch alignment marks on both

48 Chap~ter 3: Microfabrication

Figure 3.4. SEM image of boss wafer.

6. DRIE to etch the boss structure (mask: BOSSDEEP). The buried oxide again is

used as the etch stop.

7. Etch the buried oxide in BOE and release the boss structure using the nitride film as

49

sides of wafer (mask: ALIGN).

2. Plasma etch nitride and then use BOE to etch top oxide layer to define the device

area for landing feet etching (mask: BOSSOX_1).

3. Plasma shallow etch of silicon to create the four landing feet, each with diameter of

30 gm. Etch depth is 3.4 pm (mask: BOSS_FEET_2).

4. Plasma etch nitride and use BOE to etch backside oxide layer (mask:

BOSSOX_2).

5. DRIE to etch the tether from the back side for 17 gm (mask: BOSSTETHER).

The buried oxide is used as the etch stop.

50 Chanter 3: Microfabrication

the etch mask. Remove nitride layers in hot phosphoric acid.

An SEM photo of the as fabricated boss wafer is shown in Figure 3.4. The rough walls

seen in this picture are the result of DRIE process, which etches the side walls slightly.

Bottom WaferThe bottom wafer has the valve seat and flow orifice. The process flow is shown in

Figure 3.5.

1. Thermal oxidation of double sided polished wafer under wet conditions at 1 100 0C

for 0.7 gm after etching of alignment marks on both sides (mask: ALIGN in the

front and mask: STREETS at the back). This oxide thin film is used for two pur-

poses. First, it is the insulation from the bottom electrode and the sealing material,

which is polysilicon. Second, it is used as an etch mask (in replacement of photore-

sist) to etch the bottom electrode.

2. LPCVD polysilicon deposition at 625 C, for 0.95 gm. This polysilicon layer is

used as the seat material.

3. Thermal oxidation on top of polysilicon under wet conditions at 1 100*C for 1 jm.

This process consumes about 0.5 gm of polysilicon and hence the final thickness of

polysilicon is 0.45 gm.

4. Three etches in a row to define the valve seat area and the bottom electrodes (mask:

SEALOX). First use BOE to etch the top oxide, then plasma-etch the polysilicon

thin film, and finally use BOE again to etch the bottom oxide thin film.

50

Section 3.1: Fabrication Process 5


1. Thermal oxidation,0.7 gm

2. Undoped polysilicodeposition, 0.95 pt

3. Thermal oxidation,1 jm

4. DRIE etch flow outfrom back side, 22

5. Etch top three layeand define seat anbottom electrode

6. Etch top two layersexcept the seat are

7. Etch flow channel,100 jm

8. Etch bottom electrcusing oxide as mas100 sim

9. BOE remove oxide

4 Thermal Oxide4 Substrate

4 Polysilicon

4- Thermal Oxide

nn

let0 gm

rsd

aOR

de

Bottom Electrode A- Valve Seat

Through Hole for Flow Outlet Through Hole forBoss Contact Landing Pad Contact

Figure 3.5. Fabrication flow of bottom wafer.

5. DRIE to etch the flow outlet from the bottom side (mask: SEALBACK). The etch

depth is 240 gm. The reason for this etch is to reduce the etch depth of the small

channel from the front. It is difficult to use DRIE to produce straight walls for wafer-

51

deep features, and especially in this case, where the cylindrical wall of the valve seat

has a thickness of only 16 jm. Furthermore, because of thinning of photoresist at

the corners during etching, the hole diameter tends to expand. Therefore, longer etch

times will result in less accurate geometric dimensions.

6. Etch the top two thin films for nested mask (mask: SEALSEAT). The oxide left on

the top of the valve seat and the bottom electrode will be used as the etch mask for

the DRIE etch of these features later.

7. DRIE to etch the flow channel from the top side for 100 jm (mask:

SEALCHANNEL). After this flow channel is etched, it would be very difficult to

spin on photoresist and expose the next mask evenly. This is the reason why the

oxide is used as the etch mask.

8. DRIE to etch down the bottom electrode as well as to etch through the flow channel

using only oxide as mask. The etch depth here for the bottom electrode is 100 jm.

9. Remove oxide layer using BOE.

Figure 3.6. SEM images of the seal wafer showing two different magnifications.


Two SEM photos of the fabricated seal wafer are shown in Figure 3.7. The one on the

right is an enlarged view of the valve seat area with the bottom electrode. With an etch

depth of 100 pim, straight walls of the channel are obtained using DRIE.

We will now examine the roughness of polysilicon deposited in this process. Figure

3.7 shows the topography of polysilicon surface using tapping mode Atomic Force Micro-

scope (AFM). The average roughness is measured to be 21 nm, and the difference between

the actual surface area and the projected area is 5% as provided by AFM analysis. A com-

parison with smooth silicon is also made in the picture (note that the z scales in the two

images are different). The roughness of silicon shown here is 0.4 nm, and the area differ-

ence is 0.08%. As have been described in Equation (2.17), the increase of actual surface

area in polysilicon results in an increase of water contact angle. Quantitative comparison

in the actual stiction force exerted on silicon and polysilicon surfaces, however, is not able

to be made in this experiment.

NanOsanpe Tapping AFN

Scs e 5.000 ON

scan rate 0.4984 LaNumbser of Sam

X1 000 pa/div2 150.000 nw/Aiv X~-" 1.000 Pm/dIV

Z 20.000 nA4/div

Polysilicon Silicon

Figure 3.7. AFM photos of polysilicon and silicon surfaces scanning on a 5 by 5 Rm2 area.The grains and stripes shown on the silicon photo are the scan line artifacts.

53

Chapter 3: Microfabrication

3.2 Fabrication Considerations

This section will discuss a number of techniques employed in fabrication that are essential

to the valve function, and why some dies failed to be fabricated as expected.

Firstly, we have used the step-oxide etch method to eliminate possible leakage current

between two electrodes. Because etch of oxide in HF is isotropic, undercut of oxide after

etch is unavoidable (Figure 3.8.A). The hydrophilic oxide attracts water molecules and

creates a "shelter" for moisture and debris, which are the two possible sources for shorting

the SOI electrode with the silicon substrate. By using a different mask for the oxide etch,

we can etch the oxide far from the SOI edge and hence reduce the chance of current leak-

age path (Figure 3.8.B). Testing results have proved this technique to be effective.

Oxide Oxidie

A) Oxide undercut B) step-oxide etch

Figure 3.8. Profiles of oxide undercut A) without using step-oxide etch and B) after usingstep-oxide etch.

Secondly, it is very important to protect the surfaces of the wafer for bonding during

fabrication processes. For DRIE etch, the wafer is mounted on a quartz or silicon handle

wafer for through etch. Thick photoresist (10 gm) or oxide thin films are used as etch

masks to protect the front side of the wafer. The back side of the wafer, however, can be

damaged during etching by footing or pitting, which occurs when the ions reflected off the

handle wafer attack the back side. This problem can be avoided by coating the back side

with thin resist. To protect the wafer further, oxide or nitride thin films prove to be useful.

54

55

Finally, the grooved seal geometries were not fabricated successfully. This problem is

caused by the nested-mask process, which requires two oxide thin films to be grown.

Etching of each oxide film results in different degrees of undercut depending on the time

etched. In order for the polysilicon layer to survive the undercut, it can be found that at

least 2.2 Jtm of margin must be used in mask design. Because of over-etch during the pro-

cess, the grooves which had widths of 5 gm and 6 gm failed to survive the etches and

hence resulted in seat-less dies.

3.3 Wafer Bonding and Diesawing

After the three wafers were fabricated, they were bonded together using direct fusion

bonding. When two flat and smooth silicon or thermally oxidized silicon surfaces are

pressed to each other to atomic distances, the interfacial forces such as the Van der Waals

forces and electrostatic forces will attract the two surfaces and form good adhesion. Upon

annealing at high temperature, atoms at the two wafer surfaces migrate and reorient them-

selves to reach a state of minimum free energy, and in doing so, filling up the macroscopic

voids and result in bonding strength at the interface as strong as silicon. This mass trans-

port model is analogous to the sintering mechanism in metal surface [21].

To prepare for bonding, firstly, the wafers must be flat. By our experience, wafer bows

greater than 20 pm usually cause the wafers to fail to adhere to each other. Secondly, the

surface of the wafer must to be cleaned using NH 4 C1 and HCl solutions to remove metals

or any organic contaminants. Direct bonding is most vulnerable to surface contaminants.

Any particles on the surface will create gaps and depending on the height of the particles,

the bond may fail locally or globally on the whole wafer. During cleaning, the wafer sur-

face is also hydrated. This step is important as hydrophilic surface will result in greater

bond energy. The presence of hydroxyl group will attract the water molecules in the envi-

Section 3.3: Wafer Bonding and Diesawing

Chapter 3: Microfabrication

ronment and enhance the hydrogen bonds. Thirdly, the wafers will be contacted by press-

ing to each other. After the alignment contact, the wafer stack is continued to be

compressed under 4 atmospheric pressure for as long as more than 10 hours. This is to

ensure good adhesion. Finally, the wafer stack is annealed at 1 100'C for 1 hour. Bonding

is then complete.

Figure 3.9 shows an optical interference photo of the bonding stack after annealing.

Bonding is shown to be successful in most areas except a few local gaps seen as fringes on

the picture. Since these gaps are located outside of the valve area, they are "harmless" to

the structures.

Figure 3.9. Results of wafer bonding of the three-wafer stack after annealing. The size of the

fringe is a measure of the local gap between the surfaces caused by particles.

Figure 3.10 shows a schematic view of the cross-section of the bonded three-wafer

stack. This schematic shows two contact holes for making electric contact from outside

electronics to the boss and the landing pads. The contact hole for the top electrode is omit-

ted in this picture. It is worth noting that the contact made to the boss is through the tether

layer, and that there is a thin oxide film between the tether and the boss. This means that

56

57

when a voltage is applied to the contact pad, the actual voltage of the boss is floating. This

is the weakness of the current design that is difficult to overcome.

Landing

LandingFeet

Contact Pto Boss

Pad View Port Flow Path

Top Electrode

S V f Bottom Electrode

Tether alve Seat

Flow Outlet Conto L

tact Padanding Pads

Figure 3.10. Valve schematic as bonded.

We diced the wafer stack using a stainless steel blade impregnated with diamond. It is

important to place tapes on both sides of the wafer stack to ensure no water and slurry get

into the valve structure during the operation. Photos of the valve die taken from the top

and bottom sides are shown in Figure 3.11. The bottom view shows the contact pads that

make electrical contacts by using Pogo pins. How this is done will be explained in detail in

Chapter 4.

Figure 3.11. Pictures of the valve die showing the top view and contact pads for the variouselectrodes in the bottom view.

Section 3.3: Wafer Bonding and Diesawing

ad

3.4 Summary

This chapter has introduced necessary details of how the valve is microfabricated. The

fabrication process for each wafer is illustrated and explained. Because of a mishap

between design and fabrication, only six dies out of the twelve are rendered useful valves.

Three fabricated wafers were bonded together successfully, resulting in 100% bonding

yield.

It is also mentioned that the use of silicon-on-insulator (SOI) wafers and deep reactive

ion etch (DRIE) methods have enabled the current design of the valve. Several other tech-

niques employed in fabrication are also essential to the function of the valve. They are

namely: 1) creating an oxide step between SOI and substrate to avoid current leakage

paths; 2) protecting the back side of wafer during DRIE etch with a thin resist, oxide, or

nitride; and 3) using a nested-mask process to etch through the flow channel as well as 100

gm deep bottom electrode. In conclusion, it is found from the valve fabrication experi-

ence that microfabrication in an important sense constraints the design, and therefore a

successful MEMS design must take into consideration the details of fabrication issues.

58 Chapter 3: Microfabrication

Chapter

4Test Package and Testing Setup

For the prototype valve, a package was designed specifically for testing purposes. This

package need to contain both electrical and flow connections, be able to withstand 10

atmosphere pressure and deliver a voltage of 300 V. Different testing setups are needed in

order to characterize the electrodes, system dynamics, gas flow rate, and leakage rate.

4.1 Packaging

In MEMS devices, packaging is known to be costly and as important as the device itself.

Hence, design of MEMS is a process inseparable from the consideration of packaging

issues. For testing purposes, the valve chip has incorporated contact pads for making elec-

trical contacts as explained in Chapter 3.

The housing of the package is made of Plexiglas, as it is simple to machine. The pack-

age assembly is shown in Figure 4.1, and the AutoCAD layout of each plate is included in

Appendix C. The valve chip fits in the middle of a spacer plate, which is made of alumi-

num and lies between the top and bottom plates that have through holes of 2 mm outer

diameter on the side for flow connections. Stainless steel tubes made by Scanivalve will be

fixed into the side holes and sealed by epoxy and make flow connections with the test sys-

tem through Teflon tubing. The window plate of 5 mm thick closes the flow compartment.

59

60Chpe4:TsPakganTetnSeu

O-rings are used for sealing between the chip and the plates. The bottom plate has pin-

holes with a diameter of 1 mm. Pogo pins made of nickel and silver alloy with gold-lined

interior will fit into these holes using instant glue1 . These special probes have a spring

mechanism to ensure ohmic contact with the valve chip. They are installed in receptacles

that are attached to wires and thus enable connection to the outside electronics. The design

of the package provides all electrical access to the valve from the bottom side of the chip.

This setup has been taken into consideration in the design of the fabrication process.

Window Plate

Top Plate

O-ring Groove

Valve

Spacer

Bottom Plate

Pin Holder

Figure 4.1. Assembly of the valve chip package that attains both flow and electrical connec-tions for testing purpose. Drawing by Alexander Hoelke.

Figure 4.2 shows a picture of the assembled testing package with flow and electri-

cal connections. This package was used in all the flow tests. For electrode characterization

where no flow is involved, the top plate was replaced by a thin window plate that had a

hole opened in the middle to allow microscope access to the valve.

1. These Pogo pins are products of IDI, Interconnect Devices, INC.

Chapter 4: Test Package and Testing Setup60

Section 4.2: Testing SetuD 6

Figure 4.2. Test package on an air-floating table.

4.2 Testing Setup

This section will explain the methodology and instrumentation used in each testing setup.

4.2.1 Electrode Characterization

To ensure the electrodes have proper functions, we first characterize the contact resistance

as well as the current leakage between two electrodes.

To obtain I-V curves, an automated Hewlett-Packard semiconductor analyzer is used,

which can be programed to provide a ramp voltage signal from -100 V to 100 V. The data

is graphically displayed and can also be converted to text format by using a special soft-

ware provided by the vendor. This test can be done using a probe station without the use of

the testing package.

61

4.2.2 System Characterization

The purpose of this test is to examine the actuator and how it responds to a voltage input.

We are interested in both the quasi-static response of the system to input voltage as well as

the dynamic response. This can be used to verify the spring constant of the tether, the time

response and the natural frequency of the system.

In tests that involves using the voltage source to actuate the valve, the circuit shown in

Figure 4.3 is used. When the circuit is switched on, a voltmeter is used to measure the

actual voltage across the valve, and a Keithley Picoammeter is used to measure the current

leakage through the valve while the voltmeter is disconnected (to eliminate the current

drawn by the voltmeter). When the circuit is switched off, an RC circuit is used to dis-

charge the voltage across the valve. The voltage and current data are used to estimate the

power consumption of the valve.

O On

Off

Sourge - Valve V Voltmeter

Picoammeter

Figure 4.3. The circuit used to actuate the valve using a voltage source and obtain voltageand current measurements.

To test the quasi-static response, voltage is applied between the top/bottom electrode

and the boss. Boss displacement is measured by using a Wyko surface profilometer, which

is an optical profilometer that renders 3 dimensional images of surfaces. Deflection of

62 Chapter 4: Test Package and Testin2 Setup

Section 4 2: Testing Setun 6

boss can be obtained by measuring the tether deflection. A Wyko measurement displayed

in 2D and 3D image form is shown in Figure 4.4. This image is taken with the boss in its

upmost position.

1.9 mmI

/000-1.2 00

-200-

0.4 2 00-0'100 200 300 400 _ 000 0 '0 700 800

'm 46 22 um 069 um0.0 TI IIR_ 7M.21_um -2.07 um

0.0 0.5 1.0 1.5 2.0 2.5 D: 50 =m -276 m

2D Front View Tether Profile

-2 9 2.5

0.0 .mm

3D Perspective View

Figure 4.4. 2D and 3D images of the tethers taken by Wyko as they are deflected to theupmost position. The tether deflection can be read from the 2D profile.

To test the dynamic response, the Computer Microvision system developed by Profes-

sor Dennis Freeman at MIT has been suggested [22]. This system is designed to visualize

the in-situ motion of MEMS structures to an accuracy of one nanometer by combining the

techniques of light microscopy, video imaging and machine vision. It does so by using a

light microscope to magnify the image and then project it to a camera. To detect the Z

motion, stroscopic illumination is used to take sequence of images at multiple planes of

focus. And at each plane of focus, images are taken at multiple stimulus phases that are in

synchronization with the source signal generated by the computer. This process can be

63

repeated at different frequencies and the images will be analyzed by Computer Microvi-

sion algorithm, which output the motion in three axes as a function of frequency. Using

this algorithm, bode plots can be generated and the dynamics of the system will then be

revealed. The Computer Microvision system has been used in many applications, includ-

ing measuring 3D motion of fatigue structures, mirror alignment in optical system, linear

and nonlinear behavior of a gyroscope and in-plane motion of an MIT tethered-motor.

The source signal of the Computer Microvision system, however, is limited to 10 V

DC. For the valve, this results in displacement that is comparable to the noise. Therefore,

in order to use this system, an amplifying circuit will be needed.

The Computer Microvision system will be a very useful tool to provide information

about the dynamics of the system. Currently, experiments using this system have not been

adequate to make any conclusions. This method, however, will continue to be of interests

for future valve testing.

4.2.3 Flow Characterization

We are interested mainly in three types of flow tests. 1. Flow rate of the valve when it is

fully open at different tank pressures. 2. Voltage needed to open the valve at different tank

pressures. 3. Flow leakage rate when valve is completely closed. A flow chart of the test-

ing setup for the flow tests is shown in Figure 4.5. The setup is part of the fluid control/

measurement system C. C. Lin has used for his microturbine air bearing rig [23].

A Mass-Flo Controller by MKS Instruments is used to measure the upstream flow rate

of the valve. A Honeywell pressure transducer is used to measure the upstream pressure

that is set by the pressure regulator. Data acquisition is done via a National Instruments

GPIB board and the LabVIEW program is used for the PC interface to output the values of

pressure and flow rate. The program is explained in detail in C. C. Lin's Doctoral thesis.

Chapter 4: Test Package and TestinR Setup64

The Mass-Flo Controller has a full scale range of 200 sccm and an accuracy of 1%.

This accuracy, however, will not be enough to detect the leakage rate of the valve. Hence a

Mass-Flo Meter is used to measure the flow rate downstream with a full scale of 50 sccm.

The measurement can be improved by using a more accurate flow meter; however, MKS

mass flow meters have a range limit of 10 sccm and therefore the best accuracy is 0.1

sccm. For very low leakage rate, a helium leak detector will be better.

PCItrae TasueLabview Pressure Power Supply

Pressure Regulator In Flow AtmosphereMKS Mass-Flo MKS Mass-Flo

FlwPah Controller Meter

Nitrogen Valve Package

Figure 4.5. A chart representation of the flow test system showing the nitrogen flow path.

Helium leak detectors are commonly used in vacuum system. Using this tool, helium

leak rates as low as 1 x10- 10 atm-cc/sec can be measured. However, some modification is

needed in order to adapt the hose of the leak detector to the Teflon tubes from the valve

package; this can be done by using Teflon plates. The flow setup is similar to the flow

measurement setup, but instead of nitrogen, a helium source is used to supply the flow,

and the upstream pressure is directly read from the dial. The leak rate is displayed in the

detector, which has an upper limit of l0x 10- atm-cc/sec1 .

1. The helium detector used in this test is 959 Portable Leak Detector manufactured by Varian Vac-uum Inc.

65

66 Chapter 4: Test Package and Testing Setup

4.3 Summary

This chapter introduces the package design for both electrode and flow testing of the

valve. This package is designed to withstand 9 atm pressure drop and high voltage input.

Testing methodology and instrumentation are also described. We are mainly interested in

three types of testing: 1) Obtaining I-V curves and access current leakage in the actuator;

2) Verifying function of the electrode in both static and dynamic modes, and 3) Evaluating

flow characteristics, including flow rate, opening voltage and gas leakage of the valve at

different pressure drops.

Chapter

5

Modeling and Testing

This chapter presents the results for the testing of the actuation mechanism and flow char-

acteristics. These results are then compared with those obtained from the lumped element

and flow models.

Before the tests are carried out, the valve dimensions are measured using Wyko and

Electronic Vision tools. Table 5.1 lists the average measurement values across the wafer

Table 5.1. Planar dimensions of fabricated valve as well as constants calculatedfrom these dimension measurements.

Items Units Designed Measured

Tether:

Tether Height t Rm 17 (SOI) 17.5±0.1

Tether Width w gm 60 55.6±0.4

Tether Length 1 gm 800 800± 1

Total Spring Constant K N/m 300 303±6

Boss:

Landing Feet Height pm 3.4 3.6±0.1

Boss Diameter D pim 1080 1080±2

Boss Height jm 378 378±0.1

Boss Mass m Kg 10.5x10-7 7.97 ± 0.08x10 7

Natural Frequency oo KHz 2.68 3.1 ± 0.1

Valve Seat:

Inner Radius r; jim 20 22.0±0.6

Outer Radius ro jm 34/42 31.1/39.1±0.6

67

68 Chapter 5: Modeling and Testing

as well as the uncertainties, which are the standard deviations of the measurements made.

5.1 Electrode Characterization

The purpose of these tests is to characterize the contact resistance of the metal-semicon-

ductor contacts and the leakage current of the parallel capacitors. I-V curves within each

contact pad are obtained. It is found that for n-doped wafers, the wafer-metal contacts

exhibit diode behavior. The leakage current between two electrodes is also measured.

While the leakage current is very low between the boss and the top electrode, this is not

the case between the boss and the bottom electrode. Ideally, the leakage current should be

extremely low and hence the resistance in the circuit does not matter. However, if the leak-

age current is high, the actuation behavior will be affected.

A schematic of the valve cross-sectional view is shown in Figure 5.1. The four contact

pads are illustrated as those for the boss (which is actually the tether), the landing pad, the

top electrode and the bottom electrode.

Landing Pad: VL= VB

Boss: VB BottomElectrode: Vs

Top Electrode: VT

Figure 5.1. Cross-sectional schematic of the second generation valve to show the probes and

the four contact pads.

Section 5.1: Electrode Characterization 69

Note that the boss and the landing pad are shorted so that the landing pad is not electri-

cally floating. The voltages applied to the boss, the landing pad, the top electrode and the

bottom electrode are VB, VL, VT and VS respectively. In the following sections, VBT

denotes a voltage applied between the boss and top electrode, with the top electrode

grounded, and VTB means the same except that the boss is grounded. (That is, the second

subscript denotes the grounded electrode.) Other cases such as VBS, VSB are similar. When

voltage is applied between two electrodes, the third electrode is usually electrically float-

ing.

To evaluate the contact resistance, I-V curves are obtained by putting two probes to the

same contact pad. Figure 5.2 shows the measurement curves. All wafer substrates used are

lightly doped, and the n-type boss wafer and bottom wafer exhibit the typical Schottky

diode behavior. This diode behavior may affect the actuation voltage of the electrodes.

x1 0-3

e~ -1C.)

Top Electrode- .- Landing Pad

- Boss0 9 Bottom Electrode

Voltage (V)

Figure 5.2. I-V curves of the four contact pads measured using HP semiconductor analyzerby sweeping -100V to 100 V across the same contact pad.

4-

3-

2-

50 -100 50 100

-2-

-3-

-4-


When voltage is applied across the top electrode and the boss, it is found that the elec-

trodes behave differently depending on the voltage polarity. The boss behaves normally

when VBT is applied. If the boss is grounded, i.e., when VTB is applied, it oscillates when it

touches the landing pad. This phenomenon can be described by the I-V characteristics

shown in Figure 5.3.A. In this test, the boss is grounded. When voltage is negative, there is

little current measured. When voltage is positive, there is a high current leakage upon

breakdown, causing the actual voltage between the two capacitors to drop significantly.

The boss therefore releases, and when the voltage drops, the capacitor becomes charged

again, forcing the boss to move up. These experiments suggest that the top electrode

should have a lower potential than the boss in order for the actuator to work.

X 10- x 10-3

1- - I

50 -100 -5 0 50 100 1

- - - - 0.5 ------------ -- - - - 7I-1 . -

TAI

Votlage (V) Votlage (V)

A) I-V curve between boss and top electrode B) I-V curve between boss and bottom electrode

Figure 5.3. I-V characteristics between the two parallel plate electrodes.

The current leakage between the boss and bottom electrode is much more substantial

in both polarities, as shown in Figure 5.3.B. The step-etch approach is not applied to the

bottom electrode, and there might be residuals from fabrication process that act as a cur-

-8 ----------------

----- - ----- 6--------- - -----

--------- - - -

o 76- 1

U)

0

-1

71

rent path between these two electrodes. Because of the diode behavior as illustrated in Fig-

ure 5.2, current leakage is not obvious until voltage breakdown takes place. This could

explain the sudden rise of current at about 60 V in the plot.

5.2 System Characterization

We are ultimately interested in determining the pull-in voltage, the resonant frequency and

the time constant for the actuation system. To do so, we model the system by lumped ele-

ment method using a parallel plate capacitor, and a mass-spring-damper mechanism. In

this section, we will compare the model and the experimental results in quasi-static mode,

and then predict the time response and resonant frequency using the dynamic model.

RFixed Electrode

Z m\ MechanicalVin Stops

T- k b

FixedSurrounding

Electrical Domain Mechanical Domain

Figure 5.4. Lumped model of the electrostatic actuator.

The lumped model of the electrostatic actuator is shown in Figure 5.4. Here, m is the

mass of the boss, k is the spring constant of the tethers, and b is the squeezed-film damp-

ing constant. The two mechanical stops are the landing pads in the top wafer and the valve

seat in the bottom wafer; they restrict the motion of the boss to a certain stroke. R is the

Section 5.2: System Characterization

lumped resistance including the resistance of the power source, the contact pads and the

wafers.

5.2.1 Quasi-Static Mode

In quasi-static mode, voltage is applied across the electrode and the equilibrium positions

of the boss are measured. In the case where there is no current leakage between the elec-

trodes, V = Vn, and hence the value of R is not important. From this measurement, the

pull-in voltage of the electrostatic actuator and the spring constant are determined.

At each equilibrium position, we can equate the electrostatic force with the spring

force, such that

E AV22 = kz (5.1)

2(g 0 - z)

where go is the original gap space between the two electrodes as the tethers are unde-

flected. Using this formula, we can express the voltage as a function of boss displacement

z and by using the design dimensions listed in Table 2.2, a plot of the displacement versus

voltage across top electrode and boss is determined (Figure 5.5). As voltage increases,

there are two solutions for the displacement, but one of the rootshis not stable, and hence is

not the real solution. There exists a maximum voltage at which the displacement con-

verges to a single root in the plot; this point is where pull-in occurs. By setting the deriva-

tive of V with respect to z to be zero, we can find that this maximum voltage happens at

1Zg = 3go. The pull-in voltage is therefore

8kgo (5.2)27eAactuator

This is the minimum voltage required to actuate the valve.

72 Chapter 5: Modelin2 and Testin2

73

6

5

4

0 3

2

0.00 10.00 20.00 30.00 40.00 50.00 60.00

Voltage (V)

Figure 5.5. Plot of equilibrium position of boss as function of voltage using measurementdata from Table 5.1.

Figure 5.6 plots the measured displacements as function of voltage VBT. Also shown in

the plot is the theoretical curve using the measured dimensions listed in Table 5.1. An ini-

tial upward displacement from 0.3 to 0.5 ptm is observed on all dies, which can be caused

by the initial residual stress in the SOI film. In the plot, however, this initial displacement

is trimmed and the zero stress state is set at zero displacement. Two measurements are

made for each die, and it can be seen that the experimental curves are repeatable. The pull-

in characteristics of 5 different dies are listed in Table 5.2. The table shows the pull-in

voltages from both the top electrode and the bottom electrode, the maximum boss dis-

placements of each stroke, as well as the spring constant. The spring constant is calculated

by using Equation (5.2) and uses g as the sum of the average maximum upward displace-

ment and the landing feet height. From the table, we can estimate that the spring constant

of tethers as fabricated is 316 ± 32 N/m, which is within the theoretical value of 303 N/m.

-- -- --- - ---- -- - - - - - --- - -NN

Unstable-- - - --- - - - --- --- - -- - - - - --- - - - - - - -

------------- -- --- -- ---Pull-In

Stable


Table 5.2. Pull-in Voltages of different dies for both the top electrode and the bottom electrode.

*Die VI has a tether that is buckled and is not considered in statistics.

Die Initial Tether Vpi of Top Maximum Spring Vpi of Bottom Maximum

Number Deflection Electrode Upward Constant Electrode Downward

Displacement Displacement

(V) (N/m) (V)

I 0.21 36.2 2.65 280 162 0.34

IV 0.21 39.1 2.79 300 141 0.36

VI* 0.36 45.5 2.89 385 180 0.41

V 0.35 40.5 2.70 340 108 0.39

XI 0.31 36.2 2.31 345 162 0.29

Average 0.29 38 2.7 316 140 0.36

STD 0.07 2 0.2 27 22 0.04

Notice from the table that the pull-in voltage of the bottom electrode is significantly

larger although the gap is much smaller. Figure 5.7 shows a typical tether response using

the bottom electrode. Although the theoretical pull-in voltage is 17 V, the actual applied

3.

S 2 --- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -De#

r 2.5---- L -4---------------- ------- D

00

2 1. ----------------- -------------- ---------rtI I I De A r

O i Ix Die #1

II * Die#40

W 0.5 -------- --- - -- - - - - - - - -

0 I I

0 10 20 30 40 50

Voltage Applied to the Actuator (V)

Figure 5.6. Plots of boss deflection measured using Wyko vs. voltage applied between the

top actuator and the boss for two different dies. Also in the plot is the theoretical

curve using measured dimensions.

Chapter 5: Modefing and Testingz74

75

voltage VBS has to be much larger because of current leakage (as shown in Figure 5.3) that

causes large voltage drop. The pull-in voltage differs significantly among dies, due to the

unknown resistance that puts the two electrodes in electrical contact.

) 2 4P 69 89 QO 1

-V o -- - - - --F------

Voltage (V)

-- Theoretical

x Die XI

Figure 5.7. Tether deflection as voltage is applied across boss and bottom electrode.

5.2.2 Dynamic mode

Taking into consideration of dynamics, we could represent the lumped model of the elec-

trostatic actuator in Figure 5.4 by a set of state-space equations.

The governing differential equation for the electrostatic actuator is

mz+bz + kz = -V2 (5.3)2(go - z )2

V(go - Z)The charge of the parallel capacitor can be expressed as Q= M . If we take the

charge Q, the boss displacement z and velocity v as the three state variables x1, x2 and x3 ,

and take Vin as the input, then we can represent the system in state-space form,

C

0

-0.1-

-0.2

-0.3

-0.4

-0.5

-0.6 -

-0.7


- Vi X1 (go - X2) 54x1 = - -(5.4)X R EA

X2 = X3 (5.5)

2

X3 = - kx2 - bx 3) (5.6)

We are interested in obtaining the time response to a step voltage input of the system.

If we assume that the electrical domain has much faster time response than the mechanical

domain, which is usually the case, then V=Vin. Matlab Simulink can be used to obtain the

step response of the system. Figure 5.8 shows displacement of the boss when a step volt-

age slightly greater than the pull-in voltage is applied. In this plot, R is estimated to be 13

KQ, and b is evaluated using Equation (2.8) at the position where the gap between boss

and top electrode is minimum. This overestimates b and gives the time constant of the step

response to be 4 ins. It can also be shown that the cutoff frequency in the squeezed-film

damping model is two orders of magnitude greater than the natural frequency of the

spring-mass system. Therefore, we can neglect the spring effect in our damping model.

x 10-631

2.5

2

1.5

1

0.5

00

Figure 5.8. Step

0.005 0.01 0.015 0.02

Time (sec)

response of the boss with a step voltage of 41 V.

Chapter 5: Modeling and Testing76

We can also linearize the above system about an operating point. By using the Jaco-

bian matrix, the linearized system can be written as

(go -x 2 o) x 10 0 - --

8xI E A LA 6x 1

8X2j 0 0 1 8x2 + 1 6V, (5.7)

x6 _k bo 6 X

mEA m m

where at the operating point, x 2o is found by Equation (5.1) and choosing the stable

Vino(g -x 2 o)root, x10 = A , and bo can be calculated by Equation (2.8). Using this model,

we can obtain a plot of the undamped natural frequency of the system versus the input

voltage, as shown in Figure 5.9. It is observed that the resonant frequency shifts as the

input voltage increases. As the voltage is approaching pull-in, the frequency drops rapidly.

Such phenomenon is often called "spring softening." At zero input voltage, the natural fre-

quency is 3.1 KHz, the same as the spring-mass system.

3.5-

1. -- ----- - ---- ------ - ----- -------

U.

Z - - - - - - - - - - - - - - - -

00 10 20 30 40 50

Voltage Input (V)

Figure 5.9. Undamped natural frequency of the system as a function of the voltage input.

Section 5.2: Systemn Characterization 77

5.3 Flow Characterization

At the fully opened position, the flow rate of the valve is measured at different upstream

pressures and compared with analytical and finite element results. At low pressures, the

flow can be considered as incompressible as the Reynold's number is low. At high pres-

sures, however, viscous effect can not be neglected. As analytical result is difficult to

obtain, flow analysis is done by commercial FEM package CFD FLUENT A simplified

geometry is able to predict the flow with reasonable accuracy. Where choked flow occurs,

a series of normal shock waves are observed.

When the valve is fully opened, its flow rate is observed to be rather linear with the

differential pressure. Figure 5.10 plots the testing results of two dies with different outer

seat geometry. Die IV has a seat radius 8 gm bigger than Die I. This results in more flow

resistance and hence slightly less flow (by about 2 sccm from the plot). However, such a

difference is usually hard to distinguish from that caused by uncertainty in dimensions.

Also plotted in the figure is the flow rate obtained using CFD FLUENT, which shows very

60

50 ----- -L - - j ----- ---- IL- --- IL ----

I40-- --- --- -- ------ - ------I II

1 1 Die 1, Test 1

50 I A

30 ---- --- - -- - --- -- A- --- -- - ---- - 9 Die 1, Test 2

cc I

Die IV

0 20 --- --- ---- - - ---- ------ 7- -- -- CFD Model

00 2 4 6 8 10 12

Pressure (x 105 Pa

Figure 5. 10. Valve open flow rate measured at different absolute pressures of gas inlet fortwo dies with different seat geometry.


79

good agreement with the experimental data. At 9 atm differential pressure, the volume

flow rate for the smaller seat geometry is about 43 sccm, corresponding to 3 g/hr. This

flow rate is larger than desired. The difference is caused by the isentropic model used to

choose the design dimensions, as well as the fabrication error. The testing and modeling

results will be further discussed. All modeling results are obtained by using the measure-

ments values listed in Table 5.1, and for the seat, the smaller diameter is used.

Low Pressure Region

Flow at very low pressure is examined first as solutions can be obtained analytically.

When the Mach number is less than 0.3, the compressibility effect can be usually

neglected. A simplified flow geometry can be used for this analysis as drawn in Figure

5.11. Assuming steady state, fully developed flow, and neglecting gravity effects, we can

write the Navier-Stoke's equation in region I as

2

0 = + r (5.8)r az 2

with boundary conditions:

Vr(zj = 0) = 0

Vr(zi = h) = 0

Similarly, in region II,

P = [ r Vz (5.9)aZ2 r2 r2 2

with boundary conditions:

JVz2(r2 =) = 0 (5.10)3r2

Vz2(z2 = 0) = 0 (5.11)

Section 5.3: Flow Characterization


Po,To h z I 2P3

*z2

II

Seat Profile

atm

Figure 5.11. Simplified flow geometry showing the flow direction.

Solving the above equations, we can obtain the volume flow rates in the two regions

as:

region I:

(P 1 -P2)th'Q, = (5.12)

6gln -or.

region II:

(P3 - P at,,,) iQ2 = (5.13)8gl

We also need to take into consideration of the minor loss at the entrance and the bend.

For sharp-edged entrance, we choose Kent = 0.5, and for a sharp bend, Kbend = 1.1 [24]. A

Matlab program is set up to do the calculations. It is found that at pressures lower than

about 1.5 atm, the flow rate calculated complies quite well with the experimental data, as

shown in Figure 5.12. This value corresponds to a Mach number of about 0.5.

Chapter 5: Modelingy and Testingy80

Section 5.3: Flow Characterization 81

16-

14 --- ------------ ----- --- -

12 - --- -- ------ ------ - - ----

0 10------------------------ --- ------------ Experimental

2 8-------------------- ------ ------------ Model

6 -- - - -- ------- ------- - - -- --- -

0U: 4 ----------- ------------- IL---------

2 ---------------------- L

0 I0 1 2 3 4

Pressure (x 10 5 Pa)

Figure 5.12. Open flow rate as function of absolute pressure at low pressure range for Die I.

The model matches the experimental data well at pressure lower than about 1.5

atm.

High Pressure Region

At 10 atmospheric pressure, the Reynold's number reaches 1500. Although viscous effect

may not be negligible, it would be interested to see how the isentropic model compares to

experimental results.

Assuming isentropic, choked flow, Equation (2.15) can be used to calculate the flow

rate as a function of stagnation pressure. Figure 5.13 compares the calculated values with

the experimental data. It is worth noting that using the isentropic model, we could predict

the flow rate with a simple formula within 8 sccm. Therefore, it is a quick way to access

the design dimensions.


70-

60 ------- -------- - -------

E 40 -------50-- ------

,u- 4 0 -- - - - -- - - - -- - - - --- Experimental

30- -------------------------------------- Model.220 -----------LL

10 -- ------------------- -- ------

06 8 10 12 14

Pressure (x 105 Pa)

Figure 5.13. Open flow rate as function of pressure in high pressure region. The modelneglects visous effect.

CFD FLUENT Model

It is been shown in Figure 5.10 that the CFD FLUENT results are in good agreement with

the experimental data. Furthermore, at low pressure, laminar and fully developed flow is

observed as predicted, and at high pressure, normal shock waves are present.

At 1.2 atm upstream pressure, the velocity distribution in the flow region near the seat

area is shown in Figure 5.14. Note that the geometry drawn is rotated 900, and therefore

the flow inlet is from top and outlet to the right. The contours show rather smooth transi-

tion of velocity. At the throat of the flow path, the Mach number only reaches 0.26, as

revealed in Figure 5.15.A. Downstream in the channel, the flow is seen to be steady and

fully developed. Figure 5.15.B shows half of the parabolic velocity profile of a typical

Poiseuille flow.

83Section 5.3: Flow Characterization

9.31e+01

8,38e+01

745e+01

4.66e+O1

3.72e+01 c o i wro r.2.tmustea.resue

2,79e+01

1t860+01

9.31e+00

O.0O+00

Figure 5.14. Velocity contours in flow region for 1.2 atm upstream pressure.

EzJU

0.3

0.25

0.2

0.15

0.1

0.05

n

0

50-

40-

30-

20 -

10-

n

0 1 2 3 4Position from Seat (0) to Boss (3.2 gm)

A) Mach number profile in throat

Figure 5.15. Flow profile in A) the throat and B)fully developed.

0 5 10 15 20

Distance from Center Line (gm)

B) Velocity profile downstream in channel

the channel showing subsonic flow that is

For a 10 atm upstream pressure, the flow patterns become more interesting. Figure

5.16 shows both the Mach number and the static pressure contours of the flow region. The

flow is choked at the outlet of the throat as expected. The Mach number profile plotted in

- - - - - - - - -

----------------

--- -- -------- - -- -- - -- -- - --

-- -- - -- ----

* I I

I v


Figure 5.17.A further demonstrates this. As flow suddenly expands in the channel, flow

separation is observed near the wall region. Not far from the throat, a normal shock clearly

dominates the flow pattern, and result in a large pressure gradient. Downstream, the

boundary layer thickens and the shocks following are not as obvious. The flow gradually

diverges to subsonic before reaching the outlet of the channel. Such flow pattern resem-

bles the case of a supersonic nozzle.

3060+00 1010+06

2.76e+00 .130+05

2AS.e+O0 8.13e+05

2 1 4e-+O 713e+05

1.84+00 &13e+05

115*+W 5.13e+05

1.2f*004.13o.-0S

a, 13".01 3.13e+05

6,13e-1 m o m2.13e+05

3,06"01 1.13e+05

1,760-6 1.32e+04

A) Mach number contour B) Pressure (Pa) contour

Figure 5.16. Mach number and pressure contours for 10 atm upstream pressure.

1.2 12

0

E

0. -- - - - - -- - - - - -4

040.0 1.0 2.0 3.0 4.0 0 40 80 120 160 200 240

Position from Seat (0) to Boss (3.2 gm) Position from Center Line (gm)

A) Mach number profile in throat B) Pressure distribution on boss

Figure 5.17. At 10 atm upstream pressure, A) shows choked flow in the throat and B) showspressure drops on the boss along the valve seat.

85

Figure 5.17.B shows the pressure distribution on the boss. It is seen that pressure drop

along the seat area can be approximated by linear profile, and the pressure drop in the

channel area has a rather small variation around 6 atm.

5.3.1 Valve Function

In this section, we will examine the voltage and power required to open the valve against

an applied pressure. This ultimately demonstrates how the valve functions.

The current valve closes at a very small pressure drops. In this test, a step voltage is

applied, and the lowest voltage that fully opens the valve is recorded. Figure 5.18 plots the

testing results of Die I. The opening voltage that is predicted using the worst case scenario

as depicted by Equation (2.11) is also plotted. As expected, the actual opening voltage is

less than the designed value. At 10 atmospheres (132 psig), the opening voltage is 136 V.

180160 ---- - -------- --- --- |140 -------------- ------ -- ------

120-------------------------- ----- L

4) 100 --------+------ ---- ----- - - -- ------

0 2 4 6 8 10 12

Pressure (x 1O5 Pa)

Figure 5.18. Voltage required to open the valve against applied upstream differential pres-sure.

Section 5.3: Flow Characterization

86Chpe5:MdlnanTetn

To access the power consumption, the leakage current between the boss and the top

electrode is measured during flow operation and displayed in Figure 5.19. The curve

shows the characteristic shape of ionized current between two parallel plate electrodes. As

mentioned before, current leakage has been a major problem in the 1st generation of the

valve design. We have solved this problem by creating the step-oxide etch as discussed in

Chapter 3.

x 10-90.6

0.3 ----- --------- - - - - -- -- -- ---

I I

0.5 ----------- I--- -

0.1 ------

00 20 40 60 80 100 120 140 160

Voltage Across Boss and Top Electrode(V)

Figure 5.19. Leakage current between the boss and top electrode as voltage is applied forDie I.

It is shown that at 136V, the leakage current is 0.5 nA, resulting power consumption of

68 nW. The opening function has been very repeatable at different times over more than

one hundred cycles. Table 5.3 lists the performance of other tested dies. All dies have been

tested under voltage as high as 300 V and no voltage breakdown is observed. Die VI is

discarded because its current leakage is too high (0.8 mA at 300 V) and hence not able to

open the valve. The table reveals that opening voltage differs more than 10 V for different

dies and current leakage in particular varies significantly. Nonetheless, we could conclude


that the valve can be opened with voltage less than 150 V, and it consumes power less than

0.04 mW.

Table 5.3. Valve performance for four dies at 10 atmosphere upstream pressure.

Die Number Seat outer Opening Flow Rate Current PowerDiameter Voltage Consumption

gm (V) (sccm)/(g/h) (nA) (nW)

I 31.1 136 45.1/3.08 0.50 68

X 31.1 126 41.6/2.84 13.3 1,683

Average 131 43.4/2.96

IV 39.1 143 43.6/2.97 0.61 87

XI 39.1 142 43.1/2.94 270 38,232

Average 142.5 43.4/2.96

It will be interesting to observe the pull-in phenomenon at an upstream pressure. Since

the boss deflection can not be obtained directly during flow tests, the flow rate is instead

measured. Figure 5.20 shows the experimental curves for Die I at four different pressures.

At higher pressures, the valve pulls in much more suddenly and hence a gradual increase

in flow is difficult to record.

0LL

0

E

12

10

8

6

4

2

00 20 40

Voltage (V)

60

-- 4.9 gpsi

_e10.9 gpsi

-*-20 gpsi

--- 29.8 gpsi

80

Figure 5.20. Flow rate at certain pressure as voltage is gradually increased to open the valve.

---- ----- T ---- -------- ----- - ---------

-- -------- - ------- -- - ------ ---------

-- - - - -I - - - - I - - - -

- - - - - - - -

Section 5.3: Flow Characterization 87

88

5.3.2 Gas Leakage

Gas leakage when the valve is closed is of fundamental interest. It can be used to deter-

mines whether the valve is suitable for the desired applications.

The leakage rate measured using a 50 sccm flow meter reads 0.01 sccm, which is the

minimum measurement the meter could detect. In order to measure the small flow more

accurately, a helium leak detector is used. Because of the upper detection limit of 104

cm 3 /s of the particular detector used, the upstream pressure is limited to be less than 2.4

atm (20 psig). Two dies with different seat geometry are tested. The results are plotted in

Figure 5.21.

It can be determined from the trend of the two curves that the larger seat area results in

smaller gas leakage. A leakage model, however, is difficult to obtain, because the surface

roughness of the polysilicon is on the same order of magnitude with the gas mean free

path, and hence the fluid can not be treated as a continuum. If we assume that the flow rate

x 10-3

(Die I

7

----------------- -. 4 - - - - - --- - - - - - - - - Small Seat

cc (Diel1)3 A-- - - - -- - - -L- - - - -- .- -- --

Large Seat2 ------- ------ '------- ------- (Die VI)

00 5 10 15 20

Pressure (psig)

Figure 5.21. Helium leakage rate of two dies with different seat areas.

Chapter 5: Modeling and Testing

is linear at higher pressure, we could estimate that at 10 atmospheres, the valve has leak-

age rates of 0.03 sccm for the small seat and 0.02 sccm for the large seat, which gives per-

centage leak of 0.07% and 0.04%, respectively.

5.4 Summary

Various testing results and corresponding analysis have been presented in this chapter.

They can be summarized as follow.

1. Electrodes

" I-V curves of the contact pads show Schottky diode behavior for n type wafers. This

has caused the actuator to behave abnomally in one polarity. In the future, this can be

avoided by using heavily doped p type wafers.

- Attributing to the step-oxide technique, very little current leakage is observed

between boss and top electrode. Since this technique is not applied, the bottom elec-

trode shows substantial leakage.

2. System

- Equilibrium position of boss at applied voltage is measured using Wyko, and the

experimental value corresponds well with the quasi-static model. Average pull-in

voltage is found to be 38 V and the spring constant is 316 N/m.

* Using lumped element method, the dynamic model of the electro-static actuator is

obtained. The model predicts that the time constant for a step response for this sys-

tem should be less than 4 ms. The system also exhibits "spring softening" effect, i.e.,

its resonant frequency drops as the input voltage approach pull-in voltage.

3. Flow

- In the low pressure region, the low can be considered incompressible and hence an

analytical model is obtained. The model complies well with experimental data.

Section 5.4: Summary 89

- In the high pressure region, viscous effect may not be neglected. Compared to the

isentropic model, the experimental data displays about 15% less flow rate. However,

the simple formula of isentropic chocked model is a quick access in the design of the

dimensions.

- CFD FLUENT is used to analyze flow characteristics. Laminar, fully developed flow

pattern is observed at 1.2 atm upstream pressure as predicted. As pressure is

increased higher, chocked flow occurs, and a set of normal shock waves appear in

the flow path, resembling the case of a supersonic nozzle.

4. Valve Function

- At 10 atm, the valve can be opened at voltage less than 150 V in average, while con-

suming less than 0.04 mW of power.

5. Gas Leakage

" Gas leakage using helium detector is estimated to be less than 0.07% of the full flow.

Larger seat diameter results in less gas leakage.

" Depending on the requirement of the application, gas leakage might have to be

improved. This can be done by using smoother seal surface, or increasing the seat

area. But the trade-off between leakage and stiction forever exists.

90 Chapter 5: Modelin2 and Testin2

Chapter

6Conclusions and Future Work

6.1 Conclusions

The MIT microengine prototype valve has been fabricated, tested, and found to be fully

functional. Two generations of the prototype valve were built and the second generation

was improved based on the testing results of the first one. This thesis emphasizes the fab-

rication and testing of the second generation.

The microengine valve employs electrostatic actuation and uses silicon material as the

valve seat. The actuation mechanism can be described using a parallel plate capacitor and

a lumped spring-mass-damper model. The flow is designed to be choked at the seat. Fabri-

cation of the valve is made possible using SOI wafers and the DRIE process.

The first generation of valve has demonstrated functional electrodes and choked flow

characteristics. However, it fails to function as a valve in three aspects: 1) the actuation

force is too weak to open against pressure force; 2) current leakage is high; and 3) the

valve adheres to a surface, rendering any actuation force useless. Design of the second

generation aims to resolve such problems. First, actuation area is increased and the valve

seat diameter is reduced in order to increase the net opening force. Second, a step-oxide

etch is employed where two electrodes are separated by a thin layer of oxide, preventing

91

undercutting between the two conducting layers. This method has eliminated current leak-

age along the edges. Third, instead of silicon, which has a very smooth surface, polysili-

con is chosen as the sealing material, becuase it has a much rougher surface. The

advantage of using polysilicon, however, is not solely justified. Reduction of the seat area

not only has greatly diminished the pressure force acting on the boss, but also decreased

stiction forces. Nonetheless, use of polysilicon is an alternative for fabrication; deposition

of polysilicon gives more flexibility in choice of thickness compared to using an SOI

wafer.

The new valve is shown to have met most of its specifications. At 10 atmospheres, the

valve can be opened with less than 150 V and consumes less than 0.05 mW of power. The

flow rate at this pressure is 3 g/h. The opening function is very repeatable. Stiction has not

been observed even at 100% ambient humidity. The time response of the valve to a step

voltage is estimated to be in milliseconds. The gas leakage when the valve is fully closed

at 10 atmospheres is estimated to be less than 0.03 sccm.

CFD FLUENT is used to model the flow and has very good agreement with the exper-

imental data. At low pressures, the flow is laminar, fully developed and compressibility

effect can be neglected. At high pressure, flow is choked at the seat and series of shock

waves are predicted downstream as would be observed in supersonic flow in nozzles.

6.2 Future Work

The prototype valve has demonstrated the feasibility of the design of an MIT microengine

fuel valve. It will be of great importance to perform cyclic tests and have a better appreci-

ation of the repeatability of the opening function. To incorporate this valve design into the

microengine will required more experimental work. The prototype valve could also be

used in other application, such as in the microrocket that MIT is developing.

92 Chapter : Conclusions and Future Work

For the cyclic testing, an electrical switch can be used together with the LabVIEW pro-

gram to access the cyclic performance of the valve. The failure mode can be either

mechanical fatigue, or electrical breakdown. Therefore, this test could also be of interests

to study fatigue in silicon or electrical breakdown phenomenon.

The microengine will require an array of valves (designed to be 20) in order to modu-

late the flow and accomplish the control scheme. Distribution of valves on the

microengine valve chip can be planned as shown in Figure 6.1, where the array of valves

is drawn to show the relative position in the engine plenum. An extra valve is set to be the

start valve, which will require a much larger initial flow rate to start the engine. This valve

can be made by using the same design but varying the design dimensions to fit the flow

requirements. Wiring also needs to be carefully designed on the chip in order to effectively

switch on the correct numbers of valves.

L1, 0 N

Figure 6.1. Valves distributed on microengine chip.

Section 6.2: Future Work 93

94 Chapter: Conclusions and Future Work

To expand the applicability of the valve, it will be interesting to carry out testing in liq-

uid fuel. The microrocket engine uses liquid fuels such as ethanol, kerosene or JP-7. How-

ever, liquid is usually has higher conductivity and therefore, current leakage will be large

between the electrodes, resulting in much greater power consumption even if the valve

functions. Nonetheless, there are fluids that possess very good electrical properties in high

strength fields that can be used as the first testing fluids, such as silicon oil. If the valve

can accomodate liquid fuels, it can be expected that its future applications will be much

wider.

References

[1] A. Epstein, et al. Micro Gas Turbine Generators, Third Semi-Annual InterimTechnical Profess Report, January 1997.

[2] D. J. Sadler, K. W. oh, et al. A New Magnetically Actuated Microvalve For LiquidAnd Gas Control Applications, Transducers '99, June 7-10, 1999, Sendai, Japan, pp1812-1815.

[3] R. Zengerle, H. Sandmaier. Microfluidics in Europe, 28th AIAA Fluid DynamicsConference & 4th AIAA Shear Flow Control Conference, June 29 - July 2, 1997,Snowmass Village, CO.

[4] I. Chakraborty, W. C. Tang, D. P. Bame, T. K. Tang. MEMS Micro-Valve For SpaceApplications, Transducers '99, June 7-10, 1999, Sendai, Japan, pp 1820-1823.

[5] Mike L. Philpott, David J. Beebe, et al. Switchable Electrostatic Micro-Valves WithHigh Hold-Off Pressure, Solid-State Sensor and Actuator Workshop, Hilton HeadIsland, South Carolina, June 4-8, 2000, pp 226-229.

[6] A. P. Papavasiliou, D. Liepmann, Al. P. Pisano. Electrolysis-Bubble Actuated GateValve, Solid-State Sensor and Actuator Workshop, Hilton Head Island, SouthCarolia, June 4-8, 2000, pp 48-51.

[7] G. Hahm, H. Kahn, etc. Fully Microfabricated, Silicon Spring Biased, ShapeMemory Actuated Microvalve, Solid-State Sensor and Actuator Workshop, HiltonHead Island, South Carolia, June 4-8, 2000, pp 230-233.

[8] C. Vieider, 0. Ohman, H. Elderstig. A Pneumatically Actuated Micro Valve with ASilicone Rubbber Membrane for Integration with Fluid-Handling Systems,Transducers' 95, Eurosensors IX, Stockholm, Sweden, June 25-29, 1995, pp 284-286.

[9] J. Ulrich, H. Fuller, R. Znegerle. Static And Dynamic Flow Simulation of A KOH-Etched Micro Valve, Transducers'95, Eurosensors IX, Stockholm, Sweden, June 25-29, 1995, pp 17-20.

[10] Mitchell J. Novack. Design and Fabrication of a Thin Film MicromachinedAccelerometer, Master Thesis in the Department of Mechanical Engineering atMassachusetts Institute of Technology, September 1992.

[11] Stephen D. Senturia. Microsystem Design. Kluwer Academic Publishers. Boston2000, pp 332-338.

95

96 References

[12] A. A. Ayon, R. Braff et al. Characterization of a Time Multiplexed InductivelyCoupled Plasma Etcher, Journal of The Electrochemical Society, 146 (1), 1999, pp339-349.

[13] J. M. Meek, J. D. Craggs. Electrical Breakdown of Gases. John Wiley & Sons, Ltd:New York, 1978, pp2 10 -3 18 .

[14] Jo-Ey Wong. Analysis, Design, Fabrication, and Testing of a MEMS Switch forPower Applications. Doctoral Thesis at the Massachusetts Institute of Technology,June 2000, pp2 9 -32 .

[15] C. H. Mastrangelo, C. H. Hsu. Mechanical Stability and Adhesion of Micro Structureunder Capillary Forces, Journal of Microelectromechanical Systems, March 1993,pp33-43.

[16] Arthur W. Adamson. Physical Chemistry of Surfaces. Interscience Publishers, Inc.,New York. 1960, pp2 6 1-2 7 5 .

[17] K. Komvopoulos. Surface Texturing and Chemical Treatment Methods for ReducingHigh Adhesion Forces at Micromachine Interfaces. Part of the SPIE Conference onMaterials and Device Characterization in Micromachining, Santa Clara, California,September 1998. SPIE Vol. 3512, pp 106-122.

[18] Y. Ando, J. Ino et al. Friction and Pull-off Force on Silicon Surface Modified by FIB,Micro Electro Mechanical Systems, 1996, MEMS '96, Proceedings. An Investigationof Micro Structures, Sensors, Actuators, Machines and Systems. IEEE, The NinthAnnual International Workshop, 1996, pp 349 -353.

[19] M. Houston, R. Maboudian, etc. Ammonium Fluoride Anti-Stiction Treatments forPoysilicon Microstructures, The 8th International Conference on Solid-State Sensorsand Actuators, and Eurosensors IX, Stockholm, Sweden, June 25-29, 1995, pp 210-213.

[20] Y. Matsumoto, T. Shimada, etc. Novel Prevention Method of Stiction Using SiliconAnodization for SOI Structure, Sensors and Actuators A 72 (1999), pp 153-159.

[21] M. Horiuchi, S. Aoki. A Mechanism of Silicon Wafer Bonding. Proceedings of theFirst International Symposium of Semiconductor Wafer Bonding: Science,Technology and Applications. January 1992, pp 46-52.

[22] http://umech.mit.edu/MEMS.html

[23] Chuang-Chia Lin. Development of a Microfabricated Turbine-Driven Air BearingRig. Doctoral Thesis at the Massachusetts Institute of Technology, June 1999.

[24] Munson, Yound & Okiishi. Fundamentals of Fluid Mechanics. Third Edition. JognWiley & Sons, Inc. New York: 1998.

96 References

Appendix

AMask Drawings

97

02 pedi:Ms Daig

-i*

4 4#

Figure A. 1. Mask: ALIGN, wafer level, with streets

- - - a Q -1d - --- - _- -1 -; 11 2=

Appendix : Mask Drawins98

Appendix: Mask Drawings

Figure A.2. Mask: TOPELEC, die level, with streets

99

100 Appendix: Mask Drawings

Figure A.3. Mask: TopELEC, device level

0

0 0

0

100 Appendix : Mask Drawingzs

Appendix: Mask Drawings 101

Figure A.4. Mask: TOPOX_2, die level, with streets

.01

101Appendix : Mask Drawin~s


eK2

Figure A.5. Mask: TOPTHROUGH, die level, with streets


( )

Figure A.6. Mask: TOPTHROUGH, device level

103Appendix : Mask Drawings


Figure A.7. Mask: BOSSOX1, die level, with streets

104 Appendix : Mask Drawings


00

Figure A.8: Mask: BOSS_FEET, device level


0

Figure A.9. Mask: BOSSOX_2, die level

106 Appendix : Mask Drawings

ADuendix: Mask Drawines10

0

Figure A.10. Mask: BOSSTETHER, die level, with streets

107


Figure A. 11. Mask: BOSSTETHER, device level

Amcendix : Mask Drawings108


0

Figure A.12. Mask: BOSSDEEP, die level, with streets


__________________________________ 1±:


_____________%

Figure A.13. Mask: STREETS, wafer level


0

Figure A.14. Mask: SEAL-OXIDE, die level, with streets

112 A Dendix : Mask Drawings

Figure A.15. Mask: SEAL_OX, device level

112

Appendix : Mask Drawinas13

00

Figure A.16. Mask: SEALBACK, die level, with streets

113

1 ilAnedx Ms1raig

Figure A.17. Mask: SEALSEAT, die level, with streets

Appendix : Mask Drawings

0

114


0 0

0

Figure A.18. Mask: SEALCHANNEL, die level, with streets



Appendix

BValve Process Flow

B.1 Top Wafer

In this process, nitrite is used as protection layer.

1. Nitrite Deposition: all wafersICL, Tube A5Recipe: G460Target: 0.1 umDeposition Time: 36 min

Wafer Number Film Thickness(um) STD

Monitori 0.1012 0.0006Monitor2 0.1001 0.0006

2. Mask 1: ALIGN, both sidesPhotolithography

Coating: Standard thin resist at 3000 rpm

Coat on one sidePrebake 10 min at 90 degCCoat the other sidePrebake 25 min at 90 degC

Exposure: EVI for 2 secBack side alignment

Develop: 10 secExposure: Front side alignmentDevelop: 55 secPostbake: 30 min at 120 degC

117

118

AME Etch: both sidesRecipe: Nitride STD SF6 for nitrideTime: 60 secRecipe: Undoped Poly for SOITime: 60 sec

Step Height Measurement:==> SOI thickness: 0.43 um

BOE 12 min

Double Piranha Strip

3. Mask 2: TOPELECPhotolithography

Coating: Standard thin resist @ 3000 rpmPrebake 30 min at 90 degC

Exposure: EVI for 2 secFront side alignment

Develop: 60 secPostbake: 30 min at 120 degC

AME 5000 EtchRecipe: Nitrite STD SF6Time: 60 secRecipe: Undoped PolyTime: 80 sec (intended for 50 sec)Target: 0.34 um

SOI Thickness Measurement: 0.44 um

4. Mask 3: TOPOX_2Photolithography

Coating: Standard thin resist at 3000 rpmPrebake 30 min at 90 degC

Exposure: EVI for 2 secFront side alignment

Develop: 60 secPostbake: 30 min at 120 degC

AME for NitriteRecipe: Nitrite STD SF6Time: 60 sec

BOE for BOX, 1 umTime: 13 min

Appendix B: Valve Process Flow

Appendix B: Valve Process Flow 119

5. Mask 4: TOPTHROUGHPhotolithography

Coating: NT1-3: Thick resist at 1000 rpm: 10 umNT4,5: Thick resist at 2000 rpm: 8 umCoat on front sidePrebake 60 min at 90 degC

Exposure: EVI for 21 secTop side alignment

Develop: 180 sec

Coating: Thin resist at 3000 rpm back sidePostbake: 30 min at 90 degC

Mounting6" quarze waferThick PR; 2.2 krpmCenter Dot; Middle Ring; Outer RingSoft Bake: 15 min at 90 degC

Through Etch: STS2Recipe: MIT_37Time total: 4 hr 20 min

B.2 Boss Wafer

1. Thermal oxidation, 1.5 umICL, Tube A3Recipe: G22410 min dry oxidation280 min wet oxidation


VB1 1.539 0.005VB2 1.547 0.002VB3 1.549 0.002VB4 1.549 0.002VB5 1.548 0.002VBM1 1.543 0.003VBM2 1.532 0.004


120

2. Mask 1: ALIGNPhotolithography

Coating: Standard thin resist at 3000 rpmCoat on one sidePrebake 10 min @ 90 degCCoat the other sidePrebake 20 min @ 90 degC

Exposure: EV1 for 2 secBack side alignment

Develop: 10 secExposure: Front side alignmentDevelop: 55 secPostbake: 30 min @ 120 degC

BOE: 1.54 um oxideHF:H20 7:1 Buffer20 min

Nitrite EtchAME 5000, Chamber ARecipe: Nitrite CF4Time: 60 sec

Alignment Mark Etch: AME 5000All wafers; Both sidesRecipe: POLYSTDTime: 135 secEtch Depth: 1 um

3. Mask 2: BOSSOX1Photolithography

Coating: Standard thin resist @ 3000 rpmCoat on front sidePrebake 30 min @ 90 degCExposure: EV1 for 2 secTop side alignment

Develop: 60 secBack Side Coating: Standard thin resist @ 3000 rpmPostbake: 30 min @ 120 degC

Xueen 12/12/00

AME Nitrite Etch: VB5Recipe: NITRITE STD SF6Time: 60 sec


BOE: 1.54 um oxide


19 min, ICL

Double Piranha Strip Photoresist

4. Mask 3: BOSSFEET_2Photolithography

Coating: Standard thin resist @ 3000 rpmCoat on front side

Prebake 10 min @ 90 degCRe-coat PR front sidePrebake 25 min @ 90 degC


Develop: 90 secPostbake: 30 min @ 120 degCResist Height: 2.06 um

AME EtchBOE 15 secRecipe: Undoped PolyTime: 470 secTarget: 3.4 um

Feet Etch Step height Measurement:

Wafer Number Feet Height (um)

VB2 3.39 umVB3 3.68 umVB5 3.78 um

5. Mask 4: BOSSOX_2Photolithography

Coating: Standard thin resist @ 3000 rpmCoat on back sidePrebake 60 min @ 90 degC

Exposure: EV1 for 2 secTop side alignment

Develop: 60 secPostbake: 30 min @ 120 degC

AME for Nitrite: VB5Recipe: Nitirte STD SF6Time: 60 sec


122 Appendix B: Valve Process Flow

BOE for Oxide, 1.54 umTime: 20 min

6. Mask 5: BOSSTETHERPhotolithography

Coating: Thick resist @ 3000 rpm: 6 umCoat on front side

Prebake 60 min @ 90 degCExposure: EVI for 17 sec

Top side alignment

Develop: 120 sec

Coating: Thin resist @ 3000 rpm

Back side

Postbake: 30 min @ 120 degC

Tether Etch: STS1Recipe: MIT_59Etch Rate: 1.5 um/min ?

BOX Thickness: 0.35 umEtch Time: 10 minTether Height Measurement Using Wyko:

Wafer # Step Height

VB2 17.6VB3 17.6VB5 17.7

Double Piranha Strip

7. Mask 6: BOSSDEEPPhotolithography

Coating: Thick resist @ 2000 rpm: 8 umCoat on front side


Top side alignment

Develop: 150 secCoating: Thin resist @ 3000 rpm

Back side

Postbake: 30 min @ 90 degC

Mounting:

4" quarze wafer

122 Avivendix B: Valve Process Flow


Thick PR; 2.2 krpmCenter Dot; Middle Ring; Outer Ring

Softbake: 15 min @ 90 degC

Boss Etch: STS1Recipe: MIT_69Etch Rate: 2.04 um/min

Etch Time: 3 hr

B.3 Seal Wafer

1. Double Alignment

Mask 1 & 2: ALIGN & STREETPhotolithography

Coating: Standard thin resist @ 3000 rpm

Coat on front side

Prebake 10 min @ 90 degCCoat on back side

Prebake 25 min @ 90 degCExposure: Top side alignment (ALIGN)

EV1 for 2 seeDevelop: 8 sec

Exposure: Back side alignment (STREET)EV1 for 2 see

Develop: 55 seePostbake: 30 min @ 120 degC

AME, ICLRecipe: UPDOPEDPOLYTime: 68 see

Target: 0.5 um

2. Thermal oxidation, 0.7 um

Piranha strip photo: double piranha, ICLPiranha clean, ICLRCA clean, ICL

ICL, Tube A3Recipe: G1485 min dry oxidation67 min wet oxidation5 min dry oxidationFilm Thickness: 0.713 um

3. Polysilicon deposition, 0.95 um

124

ICL, Tube A6Recipe: G461Deposition rate: 58 A/minDeposition time: 2 hr 30 min


Dummy 1.007* 0.002

* Estimated *

4. Thermal oxidation, 1 umICL, Tube A3Recipe: G224

5 min dry oxidation135 min wet oxidation5 min dry oxidation135 min wet oxidation5 min dry oxidation

5. Mask 2: SEALBACKPhotolithography

Coating: Thick resist @ 2000 rpm: 8 umCoat on back sidePrebake 60 min @ 90 degC


Top side alignment

Develop: 150 secCoating: Thin resist @ 3000 rpm

front sidePostbake: 30 min @ 90 degC

BOE: 9 min

Mounting6" silicon waferThick PR; 2.2 krpmCenter Dot; Middle Ring; Outer RingSoft Bake: 15 min @ 90 degC

STS2 EtchRecipe: MIT_37AEtch Time: 80 min



Etch Depth: 240 um

Piranha Dismount30 min

Piranha Clean10 min

6. Mask 3: SEALOXPhotolithography

Coating: Standard thin resist @ 3000 rpmCoat on front sidePrebake 60 min @ 90 degC



BOE Etch Top OxideTime: 13 min (intended for 10 min)Target: 1 um

AME Etch PolyRecipe: Updoped PolyTime: 70 secTarget: 0.42 um

BOE Etch Bottom OxideTime: 9 minTarget: 0.7 urn

Piranha clean10 min

7. Mask 4: SEALSEATPhotolithography

Coating: Standard thin resist @ 3000 rpmCoat on front sidePrebake 60 min @ 90 degC

Exposure: EVI for 2 secTop side alignment*Use Arturo's rig for alignment exposure**Make sure use clear area for blokage*



BOE for Top Oxide, TRL15 min

AME for PolysiliconRecipe: Undoped PolyTime: 80 sec

8. Mask 6: SEALCHANNELPhotolithography

Coating: Thick resist @ 2000 rpm: 8 umCoat on front sidePrebake 60 min @ 90 degC

Exposure: EVI for 17 sec

Top side alignmentDevelop: 180 secCoating: Thin resist @ 3000 rpm

Back sidePostbake: 30 min @ 90 degC

BOE UltrasonicTime: 10min*Tencor measurement shows about half micron oxide ontop of silicon. *

Mounting:4" quarze waferThick PR; 2.2 krpmCenter Dot; Middle Ring; Outer RingSoftbake: 15 min @ 90 degC

STS1 EtchMIT 372 hr

Piranha Dismount

9. Wafer BondingRCA Clean

EvAlign-BonderCompressed overnightAnnealing: Tube A2Time: 1 hr

10. Diesaw

126

Appendix

CMask Drawings

127

1.6

0.8

0.65

-- 0.5 --

1.0000±0.0005

Ctearonc

........... ................ ......... . . . .......... 1 1/16__

Clearance Hole for 4-40

Clearance Hole for 4-40 (4 Holes)

RO.03130 0 00 0

Reamed Holes for Dowel Pins-0.0004(2 Holes)

0.50.65

0.8

e Hole for 4-40 (2 Holes)

Material: Polished Plexiglass 1/16 thickTolerances: ± 0.003 unless noted otherwiseAlexander H61ke indow Plate Sheet 109/21/99 of 6

Figure C. 1. AutoCAD layout of the valve package: window plate

00

1.6

0.8

-e - 0.65

--- 0.5

R0.2362"0.0015

[ R6.00mm+0.04m] Clearance Hole for 4-40


R0.03130 00 04 Reamed Holes for Dowel Pins

- - A(2 Holes), tight tolerancesA A on location i 0.001

---

0.50.039[i.00mm - RO.1969[R5.00mm.l6

0.8

4-40 UNC threaded through (2 Holes)0.0394(1.00mm

Section A-A3/64 0.028 ±0.001 [0.7mm]

- ------ - 3/32

1/06 I3/16 0.3 0.024 ±0.001 [0.6mm]

Material: Polished Plexiglass 3/16 thickTolerances: ± 0.003 unless noted otherwiseAlexander Hdlke Top Plate Sheet 209/21/99 of 6

Figure C.2. AutoCAD layout of the valve package: top plate

1.6

0.8

0.65

0.5

0.302 0.001 - - -

4-40 UNC threaded through

Clearance holes for 4-40 (4 holes)

00.06250.0000 Reamed Holes for Dowel Pins<2 HoLes),

0.302±0.001 tight toterances on location t0.001

0.50.65

0.8

Clearance Holes for 4-40 (2 Holes)

NotePerpendicularity of center squareand alignment to the dowel pinsis critical (±0.001)

Material: AluminumTolerances: ± 0.003 unless noted otherwiseAlexander Holke Spacer Plate Sheet 309/21/99 of 6

Figure C.3. AutoCAD layout of the valve package: spacer plate

1.6

1.~)0

- -----

01/8(4 Holes Clearance)

0.0590.001 (1.5mm)

I

1.6

0.8

0.65

0.5 -

Af

0.213

0.039[1.00mm

R0.236+0.0015E R6.00mm+0.04mm0.0000 L0.00MM

Cl

4-th

RO.0 310

0 for- tolerar

--E±iZ + - --- - -00

0 o 0.50 00 )0 000 ~ 0.65++ R.197[R5,00m 1 .

Clearance Holes for0.039[1.00mm

0 0.039 [lm] Through holesfor Pogo Pins (19 Holes)

Tolerances are ±0.001

earance Hole for 4-40

40 UNC threaded 0.030 [0.75mm]

rough 04 Hotes) -- 0.089 [2.25mm]

0.148 [3.75mm30.000 Reamed Holes 0.207 [5.25mm)-0.0004 026[.5mDowet Pins (2 Hotes) - 0.266 [6.75cc)ce for location ±0.001

0.030 10075cc)

0.8 0.089 (225mm)

0.148 [3.75-3 Detail of Center Section0.207 [5.25m ] -

0.266 [6.75mm]

4-40 (2 Holes)

I I I l I I I I I

1/4

1/8

Section A-A

1/016 3/64 1

0.3

Figure C.4. AutoCAD layout of the valve package: bottom plate

0.024 ±0,001 [0.6mm]

Material: Polished Plexiglass 1/4 thickTolerances: ± 0.003 unless noted otherwise

Alexander Hblke Bottom Plate Sheet 409/21/99 of 6

1.60.8

0.65

0.5 --

Clearance Hole for 4-40(4 Holes)

Clearance Hole for 4-40

3 Clearance Hole for 4-40 (4 Holes)

R0.03000$ 4 Reamed Holes for Dowel Pins(2 Holes), toleranceson location are 0.001

I 80.016 #78 Through Holes (19 Holes)for Pogo Pins, same Centers as shown

-- - --- - -for Bottom Plate, Tolerances ±0.001

0.50.65

1.05 .


I III III liii

I ~ [I I II~I ~I I I111111111 II

Material: Polished Plexiglass 1/2 thickTolerances: ± 0.003 unless noted otherwiseAlexander H61ke Pin Holder Sheet 509/22/99 of 6

Figure C.5. AutoCAD layout of the valve package: pin holder

L'.3

.A ALI

a-1 CL00

0 CDI 3 0 u-

CD CCD

Alexander Hlke09/22/99

Assembly Sheet 6of 6

Figure C.6.. AutoCAD layout of the valve package: pin holder

7. -

134

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A MEMS Valve for the MIT Microengine