1-s2.0-S0924424714004622-main

Accepted Manuscript

Title: A novel MEMS electromagnetic actuator with largedisplacement

Author: Xingdong Lv Weiwei Wei Xu Mao Yu Chen JinlingYang Fuhua Yang

PII: S0924-4247(14)00462-2DOI: http://dx.doi.org/doi:10.1016/j.sna.2014.10.028Reference: SNA 8945

To appear in: Sensors and Actuators A

Received date: 22-7-2014Revised date: 8-10-2014Accepted date: 23-10-2014

Please cite this article as: X. Lv, W. Wei, X. Mao, Y. Chen, J. Yang, F. Yang, A novelMEMS electromagnetic actuator with large displacement, Sensors and Actuators: APhysical (2014), http://dx.doi.org/10.1016/j.sna.2014.10.028This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Highlights

x A displacement of more than 55 m was achieved with a magnetic field of 0.14 T and the diving current of 8 mA.

x The actuator utilizing Al as structure material worked under a low driving voltage and can easily be integrated with CMOS circuits.

x The fabrication process is very simple, which ensure the actuator to have a high fabrication yield and good stability.

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A novel MEMS electromagnetic actuator with large

displacement

Xingdong Lv1, 2, *, Weiwei Wei1, 2, *, Xu Mao1, 2, Yu Chen1, 2, Jinling Yang1, 2, **, and Fuhua

Yang1 1Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. CHINA

2 State Key Laboratory of Transducer Technology, Shanghai 200050, P. R. CHINA

*Xingdong Lv and Weiwei Wei equally contribute to this work.

**Correspondent author:

Institute of Semiconductors, Chinese Academy of Sciences,

Qinghua Donglu A 35, Beijing 100083, P. R. China.

Tel.: 86 10 8230 4700

Fax: 86 10 8230 5141

Email: [email protected]

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Abstract

This paper presents a novel MEMS actuator driven by Lorentz force. The actuator has a

structure of folded beams, which is favorable for a large lateral stroke. A displacement of

more than 55 m was achieved with a magnetic field of 0.14 T and the diving current of 8 mA.

The actuator can generate a large displacement under a low driving voltage and can easily be

integrated with CMOS circuits. Lorentz force is proportional to the magnetic field and the

driving current, which results in an easy control of the lateral displacement on the diving

current. The simple structure and fabrication process ensure the actuator to have a high

fabrication yield and good stability.

Keyword

MEMS actuator; Lorentz force; large stroke; high yield.

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1. Introduction

Micro actuators are important building blocks for many MEMS devices, which generate

forces or displacements to realize scanning, tuning, manipulating or delivering function. In

the last decades, a large number of actuators have been developed for various applications

[1-6] and they are usually driven by electrostatic, thermal, piezoelectric, and electromagnetic

methods, each of which has its advantage and restriction. For example, the thermal actuators

can provide a large force and stroke, but suffer from high power consumption and long

response time. The material used in the piezoelectric actuators usually is not compatible with

IC technology, and a magnetic field and magnetic material are needed for the electromagnetic

actuators. Micro electrostatic actuators are more popular than others due to their high

compatibility with mature microfabrication processes, low power consumption, simple

structureand quick responsebut require high operating voltage and show nonlinear behavior.

In many applications, such as optical switches, variable optical attenuators (VOA), and

micro scanners, the MEMS actuators are highly desired to provide large displacements of

several tens of micrometers with a small voltage below 3.3 V or 5 V supplied by CMOS

circuits [2]. A variety of MEMS actuators have been proposed to achieve a large stroke.

Electrostatic [3], electromagnetic [4], thermal actuation methods [5], and a chevron-type

compliant structure [6] have provided large displacements. Lorentz force resulting from the

interaction of an electric field and a magnetic field has been widely employed to excite

MEMS devices [7, 8]. It linearly depends on the current perpendicular to the magnetic field,

requires no magnetic materials, and has no magnetic hysteresis effect. These advantages result

in the actuators of simple structure, linear motion, fast response, reasonable power

consumption, and ideal for large stroke application.

In this work, a novel MEMS actuator driven by Lorentz force was developed and a large

lateral stroke was achieved under a low driving voltage. The actuator was batch fabricated

with high yield (nearly 100%) and can be integrated with CMOS circuits. It has simple folded

beam structure consisting of aluminum and silicon dioxide, the superb stress matching

between the two layers ensures excellent reliability and potential application for large array

systems.

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2. Modeling and analysis

The working principle of the MEMS actuator is schematically shown in Fig. 1. The

actuator consists of folded beams and a moving plate. When applying a current I through the

beams and a static magnetic field B perpendicular to the beam plane, Lorentz force F,

perpendicular to the direction of the current and the magnetic field, will drive the actuator into

displacement in the xy plane.

Fig. 1. Schematic drawing of the MEMS actuator.

The lateral stroke of the actuator linearly depends on the current in terms of Equ. (1),

Lorentz force can be expressed as

, (1)

where B is the magnetic flux density of the magnetic field, I is the current flowing through the

eam, L is the beam length, and is the angle between the current and the magnetic field.

As shown in Fig. 1, the actuator beam is located in xy plane and the magnetic field is

perpendicular to the folded beam. When a current flows through the folded beam, a driving

force F along y direction is generated

. (2)

Fig. 2 is the simplified models of the actuator, several different structures were designed

by changing the size of b. When b = 0, there is no meander and the folded beam turns into a

double-ended beam (Fig. 2 (b)).

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(a)

(b)

Fig. 2. Schematic drawing of the actuator, (a) Folded beam and (b) Double-ended beam.

The folded structure has a low stiffness compared to the double-ended structure and can

achieve much larger displacement at the same current than the double-ended one with the

same length.

The actuator can be considered as a spring with a spring constant k [9], it deforms under an

external force F, meanwhile, an elastic restoring force Fe can be generated

, (3)

where y is the displacement of the beam. At an equilibrium state, the driving force F is equal

to the elastic restoring force Fe. The displacement can be calculated by

. (4)

To achieve a large displacement, B, I, L, and k should be optimized. k is determined by the

mechanical properties of the material and the structure of the actuator.

For the folded beam shown in Fig. 2 (a), the spring constant can be calculated by

, (5)

where E is the Young's modulus, IZ is the bending moment of inertia about the z-axis, n is the

folded times, a and b are the row and line pitch, respectively [10].

x

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For the double-ended beam with b = 0, the spring constant can be computed by

, (6)

where W and H are the width and thickness of the actuator, respectively.

Substituting Equ. (5) into Equ. (3), the displacement can be expressed as

. (7)

As it shown in Equ. (7), the displacement linearly increased with b. The length of b are

designed in different values and the other structural parameters of the actuator are shown in

Table 1.

Table 1. Material and structural parameters of the actuator.

Parameters Value

Youngs modulus of Al (E) 69 GPa

Thickness of Al 1 m

Thickness of SiO2 0.9 m

Beam width (W) 5 m

Row pitch (a) 15 m

Folding times (n) 70

3. Fabrication

The fabrication process flow of the actuator is shown in Fig. 3. It started with a

(100)-oriented Si wafer. Thermal oxidation was done to produce 0.9 m thick silicon dioxide,

then 1 m thick Al layer was deposited by electron beam evaporation and was patterned by

dry etching. As shown in Fig. 3(a), the reactive ion etching (RIE) was employed to remove

the Al and SiO2 layer. Subsequently, the silicon substrate was etched from the backside by

deep reactive ion etching (DRIE) until 50 m thick Si remaining (Fig. 3(b)). Finally, the

silicon substrate was isotropically etched from the front side in Fig. 3(c) with the top Al layer

as the etching mask and the actuator was released.

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(a)

Description: D:\lvxd\Google \VOA\\Al\\tu\Fig. 3(a).jpg

(b)

Description: D:\lvxd\Google \VOA\\Al\\tu\Fig. 3(b).jpg

(c)

Description: D:\lvxd\Google \VOA\\Al\\tu\Fig. 3(c).jpg

Metal Si SiO2

Fig. 3. Cross-sectional view of the fabrication process flow.

Fig. 4 shows the scanning electron microscope (SEM) photograph of a typical actuator. Fig.

5 shows the optical photographs of different actuators. The fabrication yield for the actuator is

very high, nearly 100%. As shown in Fig. 3, the fabrication processes for the actuator is quite

simple and each process step is easy to control. Particularly, the actuator was released by

isotropical dry etching the silicon substrate from the front side (Fig. 3(c)), this is the crucial

for high yield.

(a) (b)

Fig. 4. SEM photographs of the fabricated actuator.

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Fig. 5. Optical photographs of different actuators.

4. Results and discussions

The MEMS actuator was integrated with a printed circuit board (PCB) via wire bonding.

Then the PCB was placed on a permanent magnet, which can provide a static magic field for

the MEMS actuator. The magnetic flux density imposed on the MEMS actuator is about 0.14

T. A digital current source was used to provide the driving current. The testing setup is shown

in Fig. 6. The displacement is measured by an optical microscope and the resistance of the

MEMS actuator was monitored by a multimeter. By changing the applied current, the

displacements of various structures were tested. Fig. 7 shows the optical photographs of the

deformed MEMS actuator in the opposite direction. The displacements and stabilities of

various actuators driven by the current from 0 mA to 8 mA were measured and evaluated. The

resonant frequencies of the actuators have been measured by laser Doppler system.

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Fig. 6. Testing setup. (a) Full view, (b) Enlarged part, and (c) PCB and permanent magnet.

(a) (b)

Fig. 7. The optical photographs of the deformed actuators.

The relationship between the displacement of the actuator and the flowing current from 0

mA to 8 mA with a magnetic flux density of 0.14 T are depicted in Fig. 8. The displacement

increased with the current.

(a)

(b)

(c)

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Fig. 8. Dependence of displacement on applied current from 0 mA to 8 mA with a magnetic flux

density of 0.14 T.

The displacement of the actuator at an applied current of 8 mA and a magnetic flux density

of 0.14 T changed linearly with b, as shown in Fig. 9, which agrees well with the theoretical

results.

Fig. 9 Dependence of displacement on b with an applied current of 8 mA and a magnetic flux density

of 0.14 T.

A lateral stroke of more than 55 m was obtained for the actuator with b = 50 m driven by

a current of 8 mA, which coincided with the calculation results. The displacement is

proportional to the applied current and switch to the opposite direction under a reversed

current, as shown in Fig. 10.

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Fig. 10. Dependence of displacement on applied current from -8 mA to 8 mA with a magnetic flux

density of 0.14 T.

To have a deep understanding on the stability of the actuator, cycling test was carried out.

The cycled displacements versus the driving current are shown in Fig. 11. The actuator has

excellent stability and could precisely be controlled. The slight scattering of the displacements

at one current were caused by the displacement measurement error with the microscope.

Fig. 11. Cycling test for the actuator driven by a current of 0-8 mA under a magnetic flux density of

0.14 T.

As shown in Fig. 12, the resonant frequency are 4.3 kHz and 1.2 kHz for the actuators with

b = 0 m and b = 50 m, respectively.

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Fig. 12. The resonant spectrums of the actuators with b = 0 m (a) and b = 50 m (b) measured in air.

When the current flows through the beam containing the metal layer, heating effect is

distinct. The actuator beam with 1 m thick Al originally has a low resistance, about 85 .

The resistances of metals increase with temperature. When applying a current on the actuator,

Joule heating takes place, changes the resistance and elastic stiffness of the actuator, and

finally influences the current dependence of the displacement.

In order to clarify the effect of Joule heating on the displacement of the actuator, the current

dependence of the resistance, the time dependence of the resistance and displacement were

studied, as shown in Figs. 13-15. The resistances of the actuator driven by a current of 0 8

mA remain unchanged with slight drifts (850.5), as shown in Fig 13. Moreover, while

driven with a current of 8 mA for 2 hrs, the resistances of the actuator still keep unchanged

(850.1), as shown in Fig 14.

When the temperature rises, there will be thermal stress resulting from the different thermal

expansion coefficients of Al and SiO2. The thermal stress may lead to stiffness change of the

actuator and finally the displacement change with the temperature. However, the measured

displacements of the actuator driven by a current of 8 mA for 2 hours keep constant (550.5

m), as shown in Fig. 15.

The generated thermal power by the maximum driving current of 8 mA is 5.44 mW. The

above-mentioned results indicate that thermal dissipation in the suspended bimetallic

structure of Al and SiO2 balances Joule heat, which results in no clear temperature rising.

(a) (b)

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Fig. 13. Dependence of the resistance on the applied current from 0 to 8 mA.

Fig. 14. Dependence of the resistance on the time with the current of 8 mA.


5. Conclusions

R: 85 0.5

R: 85 0.1

D: 55 0.5 m

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A novel MEMS actuator with a large lateral stroke driven by Lorentz force was developed

in this work. By optimizing the structure and geometry of the actuator, a lateral displacement

of more than 55 m was achieved with a magnetic flux density of 0.14 T and a current of 8

mA. The driving voltage is low enough for the MEMS actuator being easily integrated with

IC. The high fabrication yield and good stability ensure potential application of the actuator in

large array systems, such as optical switches, VOAs, and so on.

Acknowledgment

This work is supported by the projects from MOST of China (973 project: 2011CB933102

and 2013YQ16055103) and NSFC projects (61234007 and 61201104).

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References

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[2] Jeong S H, Jong S K and Jan G K 2004 Structural optimization of a large-displacement

electromagnetic Lorentz force microactuator for optical switching applications J. Micromech.

Microeng. 14 15851596

[3] Grade J D, Jerman H and Kenny T W 2003 Design of large deflection electrostatic

actuators J. Microelectromech. Syst. 8 29

[4] Ko J S, Lee M L, Lee D S, Choi C A and Kim Y T 2002 Development and application of

laterally driven electromagnetic microactuator Appl. Phys. Lett. 81 5479

[5] Qui J, Lang J H, Slocum A H and Strumpler R 2003 A high-current electrothermal

bistable MEMS relay Tech. Dig. of 16th IEEE Int. Conf. on Micro Electro Mechanical

Systems (MEMS 2003) (Kyoto, 1923 Jan. 2003) 647

[6] Hwang I H, Shim Y S and Lee J H 2003 Modeling and experimental characterization of

the chevron-type bi-stable microactuator J. Micromech. Microeng. 13 94854

[7] Lin S J, Lee C C, Sung W L, et al. 2013 Displacement enhancement of 1-axis Lorentz

force actuator. In Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS &

EUROSENSORS XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International

Conference on 1591-1594

[8] Sun C M, Wu C L, Wang C, et al. 2012 Implementation of Complementary

MetalOxideSemiconductor Microelectromechanical Systems Lorentz Force Two Axis

Angular Actuator Japanese Journal of Applied Physics 51(6S) 06FL09

[9] Senturia S D 2001 Microsystem design 3 Boston: Kluwer academic publishers

[10] Fedder G K 1994 Simulation of microelectromechanical systems Diss. University of

California

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Biographies

Xingdong Lv received the B.S. degree in electronics science and technology from Jilin

University, P. R. China, in 2011. He is a Ph.D. candidate in electrical engineering at State Key

Laboratory of Transducer Technology, Institute of Semiconductors, Chinese Academy of

Sciences (CAS), P. R. China. His research interests are MEMS devices along with their

associated fabrication, characterization and testing.

Weiwei Wei received the B.S. degree in electronics science and technology from Yanshan

University, P. R. China, in 2009. She is a Ph.D. candidate in electrical engineering at State

Key Laboratory of Transducer Technology, Institute of Semiconductors, Chinese Academy of

Sciences (CAS), P. R. China. She is currently working on MEMS devices.

Xu Mao received the Ph.D. degree in physical electronics from the Chinese Academy of

Electronics and Information Technology, CETC, Beijing, China, in 2006. He was a

Postdoctoral Researcher with Peking University, Beijing, China, where he worked on MEMS

devices and packaging. Since 2009, he has been an Associate Professor with the Institute of

Semiconductors, Chinese Academy of Sciences (CAS), Beijing. He is also with the State Key

Laboratory of Transducer Technology, Shanghai, China. His research interests include

resonator-based MEMS devices and packaging technology for MEMS devices.

Yu Chen received the Ph.D. degree in electronics science and technology from Institute of

Semiconductors, Chinese Academy of Sciences (CAS), P. R. China, in 2007. Since 2009, he

has been an Associate Professor with the Institute of Semiconductors, Chinese Academy of

Sciences (CAS), Beijing. He is currently working on semiconductor pressure sensors.

Jinling Yang received the Ph.D. degree in solid state physics from Institute of Physics, CAS

in 1997. After a two-year stay at Venture Business Laboratory of Tohoku University as a

Post-doc., she joined IMTEK, University of Freiburg, as a Post-doc. and worked on the

reliability of MEMS thin films. In 2002, she joined in Institute of Physics, University of Basel

and IBM Zurich Research Lab, Switzerland, working on scanning force microscopy with

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ultra-small cantilevers. Since 2004, she has worked as a Professor in Institute of

Semiconductors, CAS. Her group focuses on resonator-based MEMS devices, reliability of

MEMS devices.

Fuhua Yang received the Ph.D. degree in solid physics from Paul Sabatier University,

Toulouse in 1998. Currently, he is a professor/director of Engineering Research Center for

Semiconductor Integrated Technology at the Institute of Semiconductors, CAS. His early

research involved 2DEG transport, semiconductor cavity optical properties and light storage

device. Now, His research interests include single electron transistor and their integration,

RTD/HEMT integrated circuit and MEMS.

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Table Captions


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Figure Captions

Fig. 1. Schematic drawing of the MEMS actuator.

Fig. 2. Schematic drawing of the actuator, (a) Folded beam and (b) Double-ended beam.

Fig. 3. Cross-sectional view of the fabrication process flow.

Fig. 4. SEM photographs of the fabricated actuator.

Fig. 5. Optical photographs of different actuators.

Fig. 6. Testing setup. (a) Full view, (b) Enlarged part, and (c) PCB and permanent magnet.

Fig. 7. The optical photographs of the deformed actuators.

Fig. 8. Dependence of displacement on applied current from 0 mA to 8 mA with a magnetic

flux density of 0.14 T.

Fig. 9. Dependence of displacement on b with an applied current of 8 mA and a magnetic flux

density of 0.14 T.

Fig. 10. Dependence of displacement on applied current from -8 mA to 8 mA with a magnetic

flux density of 0.14 T.

Fig. 11. Cycling test for the actuator driven by a current of 0-8 mA under a magnetic flux

density of 0.14 T.

Fig. 12. The resonant spectrums of the actuators with b = 0 m (a) and b = 50 m (b) measured in air.

Fig. 13. Dependence of the resistance on the applied current from 0 to 8 mA.



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Parameters Value

Youngs modulus of Al (E) 69 GPa

Thickness of Al 1 m

Thickness of SiO2 0.9 m

Beam width (W) 5 m

Row pitch (a) 15 m

Folding times (n) 70

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