Micro/Nanosystems Technology

Micro/Nanosystems Technology Wagner / Meyners 1

Micro/Nanosystems Technology

Prof. Dr. Bernhard Wagner

Dr. Dirk Meyners

Surface micromachining


Outline

Introduction to surface micromachining (SMM)

SMM process sequence

Structural and sacrificial layers

Sticking issue

Anti-stiction release technologies

Summary


Surface micromachining (SMM)

Suspended microstructured device layer

pinned to wafer surface by one or several anchor points

released by sacrificial layer etching

Only front side waferprocessing (as in IC technology)

- wafer remains rigid

- no need for double side aligning

anchor

suspended

structure


Surface micromachining process

Deposit sacrificial (sac) spacer layer

Pattern sac layer (mask 1)

Deposit device (structural) layer

anchored on substrate

Pattern device layer (mask 2)

Release etch:

complete removal of sac layer

selective etching

to structural layer and substrate

device layer

sac layer

anchor


Fedder


SMM process with anchor hole filling

Deposit and pattern sac layer (mask 1)

Via filling: use of CMP rigid anchoring

Deposit and pattern device layer (mask 2)

Release etch

Process leads to rigid anchoring

avoids problems of anchor step coverage

in case of thin device layers or thick sac layers


Single mask SMM process

Deposit sac layer

deposit device layer

(or use SOI wafer)

pattern device layer (mask 1)

Release etch

time-controlled underetching of device

sac layer is not removed completely

sac layer anchor is remaining

Top view:

complete underetching of beam

partial underetching of anchor point anchor


SMM without sacrificial layer

SCREAM: Single-Crystal Reactive Etching And Metallization

Industrialized for high volume manufacturing of acceleration sensors (Kionix)

Isotropic structure underetch

(Si substrate etch) using RIE with SF6

Single mask process

no sacrificial layer

device layer thickness not well

controlled

Fedder


Device layer (structural layer)

mechanical material properties determine device performance

stress control is of paramount importance to produce flat device structure

thickness: 0.2 µm - 10 µm

poly-Si is most common structural material: thermal match to substrate

but also metallic and dielectric layers

device layer is perforated

formation of multiple etch holes

allows underetching of wide structures

Poly-Si structural layer

ISIT


Sacrificial layer (spacer layer)

uniform deposition well-defined gap between device and substrate

high selectivity of etch process

layer thickness: 0.5 – 5 µm

underetching etch rates << bulk etch rates

(diffusion limited process)

Ristic 4.13

Mechanism of

phosphosilicate glass

sacrificial layer etching

through etch hole


Combinations of device layers and sac layers

Device layer Sacrificial layer

Si, Poly-Si SiO2 (PSG, TEOS)

Al resist, amorphous Si

Ni Cu

SiO2/ Si/ SiO2 Al

SiO2, Si3N4, Si3N4 / Si / Si3N4

Poly-Si

Polyimide Cu


SMM process issue: Stiction during wet release process

Sticking of microstructure during drying step:

capillary force of rinsing liquid is greater than restoring spring force

microstructure is pulled down to substrate

microstruture remains stuck due to adhesion forces:

solid bridging, van der Waals force, hydrogen bonding, electrostatic forces

Note:

stiction can also happen in operation: in-use stiction


Drying process of etched cantilever

Abe JMM 1996

Sticked cantilever tip


Stiction test structure

Cantilever beam array: long beams are sticking

Mastrangelo 1999


Surface tension forces

g

AF Cla cos2

A wetted area

la surface tension at liquid–air interface

C contact angle between liquid and solid

g gap (liquid layer thickness)

Attractive force between plates for C < 90°

Tas JMM 1996

Capillary force by thin liquid layer between two plates


Anti-sticking release (1): physical countermeasures

Reduce contact area:

- introduce rough surfaces

- introduce dimples: local spikes by design

(additional mask layer)


Anti-sticking release (2): chemical countermeasures

Low surface tension rinsing liquids:

Replace DI water by methanol, IPA, penthane, …

Chemical surface modification:

deposition of hydrophobic layers with low intersolid adhesion energy

wetting angle C > 90° water repellend

e.g. coating with self-assembled monolayers (SAM) during rinse

hydrocarbon or fluorcarbon chlorosilane based SAMs


Anti-sticking release (3): avoid direct liquid-gas transition

Avoid drying by evaporation step: a) Sublimation drying

b) Supercritical drying (critical point drying, CPD)

C-J Kim, S&A 1998, 17

CO2 critical point drying:

rinse with liquid CO2

drying via supercritical phase

at

T > 31 °C and p > 73 bar !

CPD Equipment: e.g. Tousimis

supercritical

phase region


Anti-sticking release (4): dry release etch

HF gas phase etch: SiO2 sac layer Equipment: e.g. Primaxx

XeF2 gas etch (no plasma): Si sac layer Equipment: e.g. Memsstar

SF6 RIE isotropic etch: Si sac layer or underetch into Si substrate

O2 plasma etch: organic sac layer, e.g. polyimide


Surface micromachining with multiple device and sac layers

Poly-MUMPS process: www.memsrus.com

two poly-Si device layers

two SiO2 sac layers


Summary

Surface micromachining is most important for MEMS structure fabrication

SMM uses sacrifical layer technology with front-side wafer processing only

MEMS standard uses Si device layer and SiO2 sacrifical layer

Specic equipment needed for release etching in order to avoid sticking,

e.g HF gas etch, XeF2 gas etch, critical point drying


Literature

Madou Ch. 5

Documents

Micro/Nanosystems Technology