Design, fabrication and testing of a lateral self-cleaning MEMS switch

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Design, Fabrication and Testing of a Lateral Self-cleaning MEMS Switch By Yong SHI

B.Eng., Materials Sciences and Applied Chemistry

The National University of Defense Technology, P. R. China, 1985 S.M., Aeronautics and Astronautics,

Massachusetts Institute of Technology, 2001

Submitted to the Department of Aeronautics and Astronautics in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy in Aeronautics and Astronautics At the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY August 2004

Signature of Author... ... ... ... ... ... ... ... ... ... ... ... ... ...

Departmen( of Aeronautics and Astronautics Aug. 20, 2004 Certified by ... ... ... ... ... ... ... ... ... .

~ - Sang-Gook Kim Esther & Harold Edgerton Professor of Mech. Eng.

Thesis Supervisor Certified by Certified by ... . Certified by ... ... . ... ... ... ... ... ... .. L/ S. Mark-Sp g Professor of Arfonauticsnd Astronautics

Charles Stark "ra fe2s & Astro. G

e

Barbastathis Esther & Harold Edgerton Professor of Mech. Eng. Certified by ...

Nannaji Saka P ri~~pal Research Scientist A ccepted by ... ... ... ... ... ... ... ... ... ... . ... .... ... . . ... ... ... ... ... ... ... . . ...

Jaime Peraire Professor of Aeronautics and Astronautics

OF T jo Chair, Departmental Committee on Graduate Students

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Design, Fabrication and Testing of a Lateral Self-cleaning MEMS Switch by

YONG SHI

Submitted to the Department of Aeronautics and Astronautics On August 20, 2004 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in

Aeronautics and Astronautics

ABSTRACT

A lateral contact MEMS switch has been developed to address the need for a long life cycle, low contact resistance RF switch. At the present time, there is no commercial MEMS switch that meets all the requirements. The objectives of this research are to understand the functional requirements and the failure modes of such MEMS switches, and to develop a cost effective, compact and highly reliable direct contact MEMS switch.

Major switch performance parameters were investigated to determine the real functional requirements of an RF switch, which leads to a novel switch design. This switch design is characterized by the self-alignment of the contact surfaces, self-cleaning of the particles generated from asperity fracture and deformation, and the anchoring method of the metal contacts in the micro switch structures and the large stroke piezo-actuation by the strain amplifying MEMS mechanism. The analytical model for the contact force - contact resistance relation is established to predict the required contact force, while modeling of the switch isolation provides the required displacement of the actuator.

The 5-mask fabrication process for the device consists of several steps including bottom electrode lift-off, plating mold formation, electroplating, mold removal, switch structure formation and device release. The major issue is the fabrication of the vertical sidewall of gold for electrical contact. A fine control of electroplating current and temperature makes deep and clean vertical metal walls. The device is released with XeF2.

It has been demonstrated that a contact resistance lower than 0.1 is achieved for up to 10 billion operating cycles. The grooved surface exhibited the self-cleaning effect and the parallel-beam design of the switch structure guaranteed the perfect contact during the switch operation. In addition, no failure has been observed in the anchoring of the gold metal to the switch structure. Finally, molded electroplating proved to be an effective way to create vertical metal sidewall for electric contact. The electroplated gold surface is more uniform and the microstructure is denser than that deposited by e-beam evaporation.

Thesis Supervisor: Sang-Gook Kim

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Acknowledgements

I would like to thank my advisor Prof. Sang-Gook Kim sincerely for his supervising and supporting on this project, especially his encouraging and emphasizing on the way of creative thinking, and the ability to balance looking into the "big pictures" as well as the details in research.

My sincere thanks also go to all the memebres of the MIT Mcro/Nano Systems Laboratory and my office mates Dr. Yongbae Jeon, Dr.

J.

H. Jeong, Dr. Cheewei. Wong, Dr. Y. A. Song, Nick Conway, Raj Sood, Tarek A. El Aguizy, Clemens Mueller-Falcke, Sunil Doddabasanagonda, Ray Hardin, the MIT Microsystems Technology Laboratory staff Dave Terry, Kurt Broderick, Vicky Diadiuk, Dennis Ward and my friends Dr. Hanqin Li and Dr. Hongwei Sun for their help and advice.

I would also like to thank Prof. Mark Spearing for the partial RA support and Prof. Carol Livermore and Prof. Joel Voldman for the TA support during the course of this work.

Finally I want to thank my wife Zhihong Wang, my son Caleb and my daughter Isabel for their love and making all this meaningful.

This project was originally funded by the Manufacturing Institute of MIT and the Korea Institute of Machinery and Materials (KIMM).

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83

5.1 INTRODUCTION .... ... 83

5.2 PROCESS EVALUATION ... _.83

5.3 FABRICATION PROCESS FLOW... 86

5.4 ISSUES AND PROBLEMS OF THE DEVICE FABRICATION... 91

5.4.1 Electroplating in general ... 91

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5.4.3 E lectroplating... 95

5.4.4 U nderplating ... 97

5.4.5 O ther issues ... 100

5.5 FABRICATION RESULTS AND SUMMARY ... 101

6. DEVICE TESTING RESULTS AND DISCUSSION ... 106

6 .1 T E ST SET -U P ... 106

6.2 CONTACT SURFACE CHARACTERIZATION... 108

6.3 CONTACT RESISTANCE MEASUREMENT... 110

6.3.1 Dummy design resistance measurement... 110

6.3.2 Contact resistance measurement... 111

6.4 HOT AND COLD SWITCH TEST... 114

6.4.1 H ot test... 114

6.4.2 C old T est ... 116

6.5 TEST RESUL TS DISCUSSION... 118

7. RESEARCH SUMMARY, CONCLUSIONS AND CONTRIBUTIONS ... ... . 123

7.1 RESEARCH SUMMARY... 123

7.2 C ON CLU SION S ... 124

7.3 CONTRIBUTIONS... 125

7.3.1 MEMS switch design... 125

7.3.2 M odeling... ... ... ... 125

7.3.3 MEMS switch fabrication ... 126

7.3.4 MEMS switch test and Analysis... 126

7.4 RECOMMENDATIONS FOR FUTURE WORK ... 126

7.4.1 D esign ... .... ... .. 126

7.4.2 Fabncation... ... ... 127

7.4.3 Device integration, packaging... 127

7.4.4 Testing... 128

REFERENCES ... ... ... 129

APPENDIX A ... 134

P RO CESS D ETA ILS ... ... ... 1 34 APPENDIX B... 137

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List of Figures

Figure 2-1 Sw itch concept... 32

Figure 2-2 Mechanical anchoring of the contacts to the switch ... 33

Figure 2-3 MEMS switch simplified configuration ... 35

Figure 2-4 Switch equivalent m odel... 35

Figure 2-5 Insertion loss vs. contact resistance... 37

Figure 2-6 Micro-strip transmission line ... 38

Figure 2-7 Micro-strip impedance vs. the ratio of w/h ... 38

Figure 3-1 Sidewall surface of e-beam evaporated Gold... 41

Figure 3-2 A sim ple contact surface... 42

Figure 3-3 Single asperity elastic contact ... 43

Figure 3-4 Volume conservation after plastic deformation... 45

Figure 3-5 Constriction resistance between surfaces Al and Ac ... 46

Figure 3-6 Contact resistance vs. contact force for single asperity ... 52

Figure 3-7 Influence of asperity size on the contact force-contact resistance relation...53

Figure 3-8 Contact force-contact resistance for varying plastic index... 59

Figure 3-9 Comparison of the contact resistance-force relations from the single asperity model and the distributed asperity model ... 60

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Figure 4-1 Curved electrode electrostatic actuator... 61

Figure 4-2 PZT m icro gripper... 62

Figure 4-3 Comb drive actuator ... 63

Figure 4-4 Scratch drive actuator...64

Figure 4-5 Actuator work density vs. cycling frequency ... 66

Figure 4-6 Principle of the transverse mode of piezoelectric actuator... 69

Figure 4-7 Displacement from a simple PZT actuator...70

Figure 4-8 Bow actuator model...72

Figure 4-9 Free body diagram of the switch beam ... ... 74

Figure 4-10 The coupled switch -actuator system...76

Figure 4-11 Switch schematics ... 82

Figure 5-1 Su-8 structure with e-beam evaporated Gold...84

Figure 5-2 Close-up view of the Gold film on the sidewall ... 85

Figure 5-3 Surface quality of the sidewall...85

Figure 5-4 Step 1: Growth of thermal oxide on the Si substrate and the 5 masks ... 86

Figure 5-5 Step 2: Photolithography and bottom electrode lift-off...87

Figure 5-6 Step 3&4: Thin Film PZT deposition, patterning and top electrode lift-off ... 87

Figure 5-7 Step 5: Preparation of photo resist mold for electroplating...88

Figure 5-8 Step 6:Electroplating of the contact metal...88

Figure 5-9 Step 7: Electroplated contact metal after electroplating mold is removed ... 89

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Figure 5-12 Positive photo resist mold cross-section... 93

Figure 5-13 Su-8 mold cross-section on flat surface ... 94

Figure 5-14 Su-8 mold after parameters trade-off...95

Figure 5-15 Plating results with un-cleaned mold ... 96

Figure 5-16 Typical de-bonding between the Su-8 mold and substrate ... 99

Figure 5-17 Underplating at the edges of electrode... 99

Figure 5-18 Plated Gold contacts after mold removal... 102

Figure 5-19 SEM picture of device with two rows of actuators after it's released ... 103

Figure 5-20 SEM picture of device with single row of three actuators after it's released ... 103

Figure 5-21 SEM picture of the switch part of the released device ... 104

Figure 5-22 Picture of the released device showing the undercut of the release (darker area) ... 104

Figure 5-23 SEM picture of the contact area of the released device... 105

Figure 6-1 Test set-up schem atic... 106

Figure 6-2 The probe station and the measuring system... 107

Figure 6-3 The actuator driving system ... 107

Figure 6-4 SEM picture of the sidewall surface of gold by molded electroplating... 108

Figure 6-5 SEM picture comparison of the contact surfaces ... 109

Figure 6-6 AFM image of the mold surface... 109

Figure 6-7 Circuit for contact resistance measurement... 111

Figure 6-8 Relationships between contact force and contact resistance... 114

Figure 6-9 Contact resistance vs. number of operation cycles for hot test... 116

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Figure 6-11 Picture of the device under testing using four-probe method... 119 Figure 6-12 SEM picture of the contact area after the cycling test ... 121 Figure 6-13 Zoom-in SEM picture of the contact area after cycling test ... 122

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List of Tables

Table 1-1 Comparison of MEMS switch to traditional switches ... 19

Table 1-2 Performance review of MEMS switches developed by industry ... 20

Table 1-3 Performances review of MEMS Switches developed by academia...21

Table 1-4 Comparison of metal contacting and capacitive coupling...22

Table 2-1 Switch isolation for given geometry... ₃₆

Table 3-1 Au Material properties and asperity size... 50

Table 3-2 Plastic index and surface topography... ₅₈

Table 4-1 Actuator performance comparison... ₆₅

Table 4-2 Actuation efficiency of micro actuators ... 67

Table 4-3 Driving voltage or current comparison of different actuators for MEMS switch. 67 Table 4-4 Bow actuator size and performances ... 72

Table 4-5 Modal analysis of the bow actuator ... ₇₃

Table 4-6 Actuators performances summary...73

Table 4-7 Sw itch design m atrix ... 80

Table 4-8 Switch Beam thickness ... 81

Table 6-1 Resistance measurements on dummy design B15D ... 111

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Table 6-3 D riving voltage vs. contact resistance ... 112 Table 6-4 Contact force Vs. Contact Resistance... 113 Table 6-5 Test matrix for long cycle contact resistance measurement... 115 Table 6-6 Hot contact resistance measurement (sample 1-15-8# at 12 V driving voltage) 115 Table 6-7 Hot contact resistance measurement (Sample 2-36-3 # at 9 V driving voltage).. 116 Table 6-8 Contact resistance measurement for cold test ... 117

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Contact radius

Area of the contact surface

Contact radius at plastic deformation Switch beam width

Stiffness under constant electric field Off- state capacity of the switch Capacity of PZT actuator

Gap between two switch contacts/distance between to contacts Piezoelectric coefficient in 1-3 direction

Electric displacement, electric displacement in 3 direction Young's modulus of the switch beam material

Young's modulus of contact materails 1 and 2 Electric field and electric field in the 3 direction Piezoelctric coefficient in 1-3 direction

Nomenclature

a A, b C11E C d D, D3 E EL, E3

C, t

e13

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Force on the contact surface from the actuator

FnT

Force normal to the contact surface Total contact force

Force paralell to the contact surface X and y components of Fn

Force on the actuator from the switch Contact conductance

Switch structure thickness Brinell hardness

Current

Stiffness matrix of the switch beam and actuator Stiffness of the PZT actuator

Length of the switch beam

Deformation of the switch beam in x and y direction due to sliding Moment on the switch beam

Mass matrix of the structure and actuator Normal direction

Total number of asperties Contact presure FS Ge h H I KpzT I Al,, Al n N P F

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Pc Yield stress

PP₀₁

Q

q

Poling of the PZT Charge on the surface

Change generated by PZT actuator Radius of the asperity

Contact resistance/constriction resistance

S parameter of transmissiom coefficient from port 1 to port 2 Strain, strain in 1 direction

Compliance under constant electric field Compliance at open circuit

Stress , stress in 1 direction Voltage

Width of the microstrip (electrode width on the switch beam) Sliding motion between two contact surfaces

Characteristic impedance indentation

Critical indentation when plastic deformation occurs Plastic indentation

Dielectric constanct at constant strain

S21 SS, SD T, T, V w x zo 8s

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Dielectric constanct at constant stress

(z)

(ppi, <p,

Asaperity height distribution fuction

Potential and potential on surface A, and surface A, Friction cofficient

Angle of the contact surface Electrio-mechanical coupling term Resistivity

Standard deviation of the asperity heights Signal frequency

Plastic index

Electric mode shape Mechanical mode shape 0

p0

(-to,

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1. Introduction

1.1 Background and Motivation

Radio frequency (RF) MEMS switches are devices that provide a short circuit or open circuit in the RF transmission line by micro-mechanical movement. RF switches usually operate at radio frequency to millimeter wavelength (frequencies of 0.1 to 100 GHz) [1]. They have wide applications from satellite communication to wireless sensors. In the past, semiconductor switches such as GaAs or InP p-i-n diodes as well as FET (Field Effect Transistors) have been used to perform the switching function. In the last 15 years, the performance of GaAs HEMT (high-electron mobility transistors) devices and silicon CMOS (complementary metal-oxide-semiconductors) have had tremendous advances, but the performance of the semiconductor switches has no significant improvements until the emergence of the MEMS (Microelectromechanical system) technology. The cut-off frequency, which is an indication of the low-loss performances of the switch, is 1-2 THz for GaAs p-i-n diodes and 0.2-0.5 TEIz for FET switches respectively, while this frequencies for MEMS switches is 30 to80 THz, which are much higher that of semiconductor switches. The isolation of MEMS switches could be as low as -40 dB at 40 GHz, while that of the semiconductor switches is only about -5 dB [1].

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Because of their broad range of applications, hybrid technology and the huge market potential, RF MEMS switches have attracted a great deal of research interests. Different kinds of MEMS switches have been developed by a number of companies and universities [2]-[8]. Most of the current RF MEMS switches are designed with electrostatic actuators [3], although there are some designs using thermal [4] or magnetic [5] actuators. One of the major problems of MEMS switch is the low reliability or short lifecycle. The typical life cycle requirement for a RF switch for radar systems and other instrumentation systems is over 40 to 100 billion cycles [1]. The best practice of the RF MEMS switch reported can achieve about 10 billion life cycles. There is a big gap between the current technology and the market requirement. To address these issues, the functional requirements of switches, which reflect customers' demands, have to be investigated. The failure mechanisms of MEMS switches and the contact mechanics and physics of the switching members have to be taken into account. The primary goal of this research is to fully investigate the contact mechanics and failure modes of MEMS switches, design and fabricate a low contact resistance and long lifecycle RF MEMS switch.

1.2 Objectives

The objectives of this research are three-fold:

1) To investigate the functional requirements of MEMS switches, determine the key factors that influence the major switch performance parameters, in order to design and build a MEMS switch emphasizing the uniqueness of micro fabrication rather than just miniaturizing a bulk conventional product.

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between the two contact surfaces are also studied to further investigate the failure modes of MEMS switches.

3) To verify experimentally the model and analysis through the design, fabrication and testing of a new MEMS switch. The major performance target is to achieve a low contact resistance of less than 0.1 Q (about 0.5 0 is the current best practice) over its entire life cycle. The test and analysis results should provide guidelines for further switch performance improvements in order to build a cost effective, robust and highly reliable MEMS switch with a lifetime of more than 100 billion cycles.

1.3 Literature review 1.3.1 General switch performances

Petersen reported the first MEMS switch in 1979 [9]. Since then, a large number of RF MEMS switches have been developed. Gabriel M. Rebeiz summarized the performance of MEMS RF switches and those of traditional RF switches, which are compared in Table 1-1 [1].

Table 1-1 Comparison of MEMS switch to traditional switches

RFMEMS PIN FET

Voltage (V) 20-80 ± 3-5 3-5 Power consluription 0.05-0.1 mW 5-100 mW 0.05-0. 1mW Switching time 1-300 ps 1-100 ns 1-100 ns Rs Q 0.5-2 2-4 4-6 Isolation(1-10 GHz) -40~ -60 dB -40--20 dB -30--10 dB Isolation (10-40 -30-40 dB -20 ~-5 dB 0 -- 5 dB GHz) Loss (1-100 GHZ) 0.05-0.2 dB 0.3-1.2 dB 0.4-2.5 dB

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From Table 1-1 we can see that the advantages of MEMS RF switches are the low power consumption, high isolation and low insertion loss. Besides, MEMS switches are very linear devices and require very low intermodulation. The disadvantages are the longer switching time and higher driving voltages. The latter will be addressed further since it is related to the actuation mechanism.

In Table 1-2 and Table 1-3, the performance of some of the recently developed MEMS switches from both the industry and academia is compared [10].

Table 1-2 Performance review of MEMS switches developed by

industry

cenhza A voage CWmq, es Suik& fte Rentuie Idagen Less IeMuIfesn

4 G~r,

V mW ps sN A a a ydes

meteru h zakedenA 40-5e 0* 2-4 100 1-2 -44 > 6x1010

Anabgmke Baesb.ck 70-30 0 3-6 100 >6x1013

OaMn Ueduk& 17-20 0 300 1000 1-2 -40 -0.15 >1x10'

2000-CreMa nhual 5 200 10,000 3000 -30 -0.25 >x10'

n mdrsms ed i 60 0 8-10 50-150 0.0-2 -50 -0.1 >1x10'

SumwUg xerU e& 5-s 0 100 50-100 0.5-1 -35 -0.15 >5x10

nI. zedresds 2e-3 0 30-40 50-100 1-2 -44 -0.15 v1x10'

DUIaR " edkedk& 73 0 el 50-150 1-2 -40 -0.15 >lx10

ST-jmaddia nmr 5 0 300 100-200 -44 -0.5 >1x108

dedredus

cretoa maadeswK 5 0 500 50-150 -50 -0.2 >1x108

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Table 1-3 Performances review of MEMS Switches developed by academia

Power Switch Contact Resistanc Isolatio Proven life University Actuation voltage Consumptian time force e n lass time

4 GHz, V mW pS pN 0 dB dB cycles Northeastr Eletrostatic 60-80 0* 2-3 1-1.5 -40 -0.15 Michigan Eletrostatic 30-40 0 26-30 0.5-1 -35 -0.15 Berkeley Electrostatic 30-110 0 0.2-0.4 -37 -0.2 1000-UCDavis Thermal 6-8 30-40 3000 -36 -0.5 Fraunhofer Germany Electrostatic 20-260 0 5-30 1-3 Beijing U. Electrostatic 43-135 0 50 1 x107 U. Illinois Electrostatic 0 1-1.5 -25 -0.1

It can be seen that the majority of the switches utilize electrostatic actuators that require high driving voltages from 20 volt up to 260 volt. The typical ones need driving voltages of 60-80 volts. The high voltage requires expensive dc-dc converter, which prevents low cost RF MEMS switch eventually. The contact resistances of these switches range from 0.5 Q to 2 Q, while the life cycles can reach as high as 1010, although in several cases no lifetime data is reported. In addition, most of MEMS switches cannot handle more than 20-50 mW powers. Furthermore, they also need to be packaged in inert gas environment and in very low humidity, which results in very high cost.

1.3.2 MEMS Switch classification

MEMS switches can be classified according to the actuation methods. These included electrostatic [11], thermal-electric [12], electromagnetic [13-16], piezoelectric [17,18]. Electrostatic actuators need higher driving voltages, while thermal-electric and electromagnetic actuators require higher power consumption. This will be discussed in detail

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According to the switch configurations, MEMS switches can be classified as vertical contact [8] and lateral contact [19, 20, 21]. The dynamic behavior of lateral contact switches is superior to many vertical contact switches. For example, the switch contact can be made through a linear controllable motion provided by the thermal actuator as reported in [19], which avoid pull-in (unstable condition) of the switch member. Pull-in is a phenomenon typical for electrostatic actuators. However, the contact resistances of the lateral contact switches are much higher due to the higher roughness on etched sidewall surfaces and the contact materials for the existing switches. Lateral contact switch fabricated by bulk micro-machining method [20,21] may also need wafer bonding which makes the process more complex. To take advantage of the lateral contact switch, a new fabrication method has to be developed to make smoother sidewalls for contact.

According to the switch contact methods, there are two types of switches: direct metal contacting [8] and capacitive coupling [22]. Performances of switches with the two methods are compared in Table 1-4 [10].

Table 1-4 Comparison of metal contacting and capacitive coupling

It can be seen that metal-metal contact switches provide large application frequency Power Handlig mW Frequency range

GHz Metal-metal 0.5 - 5 available DC-60 GHz

contact _{10-100 low reliability}

Capacitive 30-300 > 10 GHz

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contact switches. The major reasons are that metal-metal contact switches have low power handling capability and their reliability is also relatively low. Contact resistance of a switch controls its power-handling capacity, while the lack of an effective way to maintain a low contact resistance reduces its reliability.

1.3.3 Contact mechanics and switch failure modes

Several types of contact mechanics models have been proposed. The Greenwood-Williamson contact model is one of the basic "asperity-based-model" [23]. The model is valid for an elastic contact. Chang et al [24] expanded this model by introducing plastic deformation, so that the model can operate at both low and high contact force situation.

In addition, Negus-Yovanovich [25] has proposed a thermal contact model, while Leung and Hyman [26] used numerical method called " thermal network analysis".

J.

Tringe, et al. [27] studies the contact problem of electroplated gold thoroughly. They measured gold contact resistance of 100 mQ with a contact force of 100 tN. They found that as a contact metal, gold is relatively inert, forming only modest contamination layers, and there is no insulating oxide that must be broken with a large force in order to obtain the required contact resistance. They observed that arcing could occur, causing sufficient energy transfer to the contact surfaces to destroy the switch.

D. Hyman et al. [26] studied the contact physics of electroplated gold probe tip on sputtered pure gold substrate. They found that heat dissipation is the critical design parameter for maintaining a low contact resistance, a high power handling capability, and a minimum of surface adhesion in a metallic contact switch. This limiting factor can be addressed through the design of a proper heat sink and thermal modeling throughout switch development. D. Hyman suggested that the switch contact electrodes should be fabricated of

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films, which also have high thermal conductivities, with a minimum thermal path to a sink substrate.

Y. Wang, et al. [19] developed low-voltage lateral-contact micro-relays which is surface micro-machined in 2002. The thermal actuator has a length of 200 tm, width and thickness of 2 gm, center offset of 10 gm. The maximum displacement calculated is 5.4 yxm. The sidewall of the switch is sputtered gold with a thickness of 0.5 ym. It has a skin depth of 0.71 ytm at 12 GHz and 0.45 ym at 30 GHz. They believe sputtered gold has a higher hardness that gives less surface damage for metallic micro-contacts. The structure of the micro-relay is polycrystalline silicon. The actuator part and the contact head is connected through a 0.6 tm low stress silicon nitride serving also as electric and thermal isolation. They found that contact heads with round and square shapes showed better reliability than the angled-shaped contact head. The failure modes they observed were due to metal contact welding. Besides, surface roughness on the sidewall results in bad contact and a high adherence force of gold also plays a role in contact degradation. They suggested that a gold and nickel alloy be considered as the contact metal due to its small adherence force and relatively

low

resistance.

1.3.4 Summary

Most MEMS RF switch designs utilize electrostatic actuators that require high operation voltage and packaging in inert gas environment resulting in high cost. The switch functional requirements are not fully investigated. Lateral metal-metal contact switches have shown to have promising characteristics compared to other configurations, especially their better dynamic behavior and their large application frequency range. However, the reliability

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because their contact surfaces (sidewall) are usually etched, so that they have high surface roughness.

1.4 Approach

Beginning with an extensive literature review to summarize the current status of MEMS switch, the present research focuses on the conceptualization of novel switch configuration and contact method to satisfy fully the requirements for an RF switch. Failure analysis of currently available MEMS switches will be carried out to better understand the switch function and also provide feedback for satisfying the functional requirements of RF switches. There are several parameters which have been utilized to define the performance of an RF switch, such as cut-off frequency, isolation, insertion loss, power carrying capacity, switching speed, contact resistance and reliability etc. However, not all of these parameters are independent. Contact resistance is one of the dominant factors. A low contact resistance has positive influence on most of these operating parameters. These parameters will be investigated to determine the major functional requirements for MEMS switches in order to generate a decoupled and optimized design.

Low contact resistance is the primary requirement for high performance RF MEMS switches. To achieve a low contact resistance, a higher contact force is required to generate a sufficiently large real contact area through elastic-plastic deformation of the asperities between the surfaces. Contact mechanics and contact resistance will be modeled to provide guidelines for switch and actuator design.

The ability to maintain the low contact resistance directly influences the life cycles of the switches. Switch design will focus on how to maintain the low contact resistance.

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Self-cleaning switch configurations will be developed to satisfy this requirement. In the mean time, the configurations should not dramatically increase the complexity

for fabrication

to reduce the cost [28].

Different actuators will be compared with each other according to their performance. An actuator for the switch will be selected and designed based on its performance (displacement, force, driving voltage, controllability). Optimization of the

actuator will be conducted to improve the switch performance.

Finally the RF switch will be fabricated and tested. The contact resistance and long term performance of the switch will be measured, analyzed and compared with the theoretical prediction. Further suggestion and recommendation will be given for the

commercialization of the switch.

1.5 Organization of the document

This document is organized in the same way as the problem is approached.

Chapter 2 presents the analysis of the failure modes of currently available RF MEMS switches, and also investigates the functional requirements of these types of switches. The main switch performances will be examined and the major functional requirements of a MEMS switch will be determined: contact resistance and isolation. These major functional

requirements are further investigated and decoupled into a set of independent functional requirements. Each of the functional requirements is satisfied by an independent design parameter. This leads to the novel design of the switch, which is a lateral contact series switch and is capable of self-cleaning, self-alignment and maintaining a low contact resistance over a long life cycle.

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Chapter 3 presents the modeling of contact mechanics and contact resistance. A higher contact force is required to generate large enough real contact area through elastic-plastic deformation of the asperities on the contact surfaces. However, if the contact force is too large, the frictional force will also be very large resulting in a high wear rate of the contact materials. This obviously will reduce the life cycles of the switches. The modeling of the contact resistance consists of more steps: surface characterization, real contact area determination using Hertz 's law for elastic contact, and the contact resistance determination using Holm's equation. For larger force, the deformation of the asperity is probably in plastic region and is treated accordingly. The modeling provides the force and displacement requirements for the low contact resistance requirements.

Chapter 4 presents the selection of the actuation methods. Based on the literature review, a PZT actuator has been selected because of its high force and low driving voltage. However, the displacement from piezoelectric actuators is relatively small. An amplification mechanism is required to obtain the required displacement. Finite element analysis is applied to verify and optimize the actuator design. After that, an actuator-switch coupled analysis is conducted to determine the appropriate switch geometry. Chapter 5 discusses the key fabrication issues of this MEMS switch and the corresponding fabrication results. Process verification tests have been done to determine the method to deposit Au on the sidewall of the structure for metal-metal contacts. Electroplating of Au is chosen to deposit the metal film on the contact areas over E-beam evaporated Au. Pt by e-beam evaporation is used as a seed layer. To obtain high quality contact metal by electroplating, the mold for electroplating is critical. The mold should have near vertical sidewall and should be removed easily after electroplating. Spin-on Su-8 has been chosen for the mold and also the switch structure materials for its unique vertical sidewall and compatibility with the actuator. PZT

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actuator is deposited using the sol-gel method. The final fabrication process utilizes five masks. Special considerations have been given to ensure the process compatibility with surface micro machining techniques, such as photolithography, lift-off, and XeF2 dry release

etc.

Chapter 6 presents the principle of the measurements, the design of the test set-up, and the experiments conducted. The surface quality of the electroplated contact areas are studied under SEM and compared with that of e-beam evaporated surface. The former is far smoother than the latter. Contact-force and contact-resistance relationships are determined and the results correlated with the theoretical prediction. Both dynamic tests (power on when cycling) and static tests (power off when cycling) are conducted. The results have demonstrated the self-cleaning effect of the modulated contact surface design.

Chapter 7 _{summarizes and concludes the research. Compared with the existing} MEMS switches, the uniqueness of the device lies in the self-alignment of the contact surfaces, self-cleaning of the particles generated from asperity fracture and plastic deformation, and the anchoring method of the metal contact to the micro switch structure. By introducing a modulated surface to modify the tribological behavior of the contact surfaces, low contact resistance of 0.1 Q can be maintained for billions of operating cycles without sacrificing the benefits of MEMS switches such as low insertion loss, near zero power consumption, and very high isolation.

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2. _{RF MEMS Switch Design}

2.1 Switch failure mode analysis

Failure analysis of currently available MEMS switches provides a deep understanding of the switch functional requirements and also feedback for satisfying the functional requirements. The major failure modes of MEMS switches are damage, pitting and surface hardening of the contact area [26]. These are caused by the asperity fracture, plastic deformation, and repeated impact from the opposite switching members, which gradually reduces the real contact area and increases the contact resistance.

Another failure mode is micro welding between switching members. Micro welding, which causes the switch fail to open, is due to Joule heating. The increase in contact resistance results in the increase of Joule heating, which increases the local temperature, then further increases contact resistance and causes more plastic deformation and micro welding. In order to minimize the two failure modes, a mechanism to maintain low contact resistance has to be developed.

Other failure modes are mostly decoupled from the system and can be easily avoided. For example, adding a protecting circuit can minimize arcing, while packaging of the device in inert gas environment can almost eliminate the deposition of organics and

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2.2 _{Functional design of the switch system}

The switch performance is investigated to determine the real functional requirements of an RF switch. There are several parameters which have been used to define the performance of an RF switch, such as cut-off frequency, isolation, insertion loss, power carrying capacity, switching speed, contact resistance and reliability etc. However, not all of these parameters are independent. Contact resistance is one of the dominant factors. A low contact resistance has a positive influence on most of these parameters. Another independent parameter is the isolation that can be determined by the separation of the two switching members. The functional requirements of a new RF MEMS switch can be summarized as how to provide and maintain the low contact resistance and high isolation over a high number of operating cycles.

2.2.1 Functional requirements:

" FR1: Provide low resistivity at contact

e FR2: Remove particles periodically between contact surfaces " FR3: Provide low off-state capacitance

Each of the functional requirements is to be satisfied by an appropriate design parameter or solution. The new design parameters are generated to meet individual functional requirement it's associated with, but not to couple to other functional requirements.

2.2.2 Design parameters:

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9 DP3: Piezoelectric actuation with amplified strokes

2.3 Switch concept and design

2.3.1 The general concept

The switch design is shown in Figure 2-1. It is a lateral contact series switch that consists of fixed switching members, movable switching members and the position stopper to prevent excessive contact forces. Each switching member consists of two parallel beams with angled contact surfaces at the tips that are floating and induce small scale sliding between fixed and movable contacts. Gold or other noble metals are to be deposited on the sidewall or the angled contact surfaces as well as the transmission line along the beams and pads. When the movable members meet the fixed members under a linear controllable motion, the physical contacts between the two pairs of angled surfaces will create a short circuit in the transmission line from one of the fixed members to the other. When a certain amount of separation (or gap) is maintained between the two pairs of angled surfaces, there will be an open circuit between the two fixed members.

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Contac

u rfac e

. .

.F

_{ix e d}

Undulated

surface

n

sto

-- 25 pm

MovablI6

Figure 2-1 Switch concept

2.3.2 _{The self-alignment of the contact surfaces}

Several novel ideas are devised for the new design. First of all, geometric or position mismatch of the contact surfaces from device operation or fabrication will reduce the normal contact area and even prevent a real contact. The two pairs of identical beams shown in Figure 2-1 will deform equally in magnitude but opposite in directions. This will ensure a good contact between the two pairs of surfaces during switch operation. A slight torsional movement of the two pairs of beams can also compensate for any the contact surface sidewall slope resulting from the fabrication process.

2.3.3 The self-cleaning of the damaged surface

The debris or loose particles generated on the contact area during operations are to be cleaned through micro sliding motion between the two surfaces, and then trapped in the micro grooves fabricated on one of the surfaces. The concept of undulation of low friction

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from it. Low contact resistance can thus be maintained throughout the long life cycles of the switch.

2.3.4 Attaching of the gold contacts to the structure

Noble metals or alloys have weak adhesion to the sidewall of the switch structure, such as Si or SU-8 (an epoxy). The connecting parts of the switch have been designed as a series of dovetails. The contact metal is filled into these dovetail trenches, and thus is embedded and anchored in the switch structures. Mechanically anchoring the contact metal

into the structure ensures the two have secure physical and mechanical connection as shown in Figure 2-2.

Sftructue

Contact mnetals

4 pmrr

Figure 2-2 Mechanical anchoring of the contacts to the switch

2.3.5 Contact force adjustment

To achieve a low contact resistance, a higher contact force is required to generate a sufficiently large real contact area through elastic-plastic deformation of the asperities between the surfaces. However, if the contact force is too large, the frictional force will also be very large resulting in a high wear rate of the contact materials. This obviously will reduce

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the life cycle of the switches. The angle between the contact surfaces and the switch beams provides the adjustment of the contact forces, which ensures the capability to optimize the switch performance.

2.4 Switch materials selection

There are several materials that could be used as the structural materials both for the switches and the actuators. The most commonly used material is Si since it is available as the substrate materials and also its fabrication techniques are mature. However, we have chosen SU-8 as the switch and actuator structural material. There are few reasons. First of all, SU-8 is a negative, epoxy-type, near-UV photoresist (365 nm) [32]. It can be patterned directly by photolithography and the thickness of the structures can be as high as 2 mm with an aspect

ratio of up to 25. This will dramatically simplify the fabrication process avoiding deep reaction ion etch and chemical-mechanical polishing (CMP) for Si structures. Besides, it has been demonstrated that the sidewall of the SU-8 structure can be nearly vertical, which is crucial for forming the switch contact surfaces. In addition, SU-8 is an epoxy resin with a Young's modulus of about 4.4 GPa and a Poisson's ratio of about 0.22. Its low stiffness allows the switch beams to deform or bend easier because this bending is required by the self-cleaning mechanism. This is also advantageous for the strain amplification PZT actuator as can be seen later.

2.5 Switch modeling 2.5.1 Equivalent model

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metal

Actuator

d

Substrae

Figure 2-3 MEMS switch simplified configuration

This lateral switch is equivalent to a capacitor at off state and a resistor at on state [1] as is shown in Figure 2-4. zo

Off State:

On State:

zo

Cs _zo

zo

Figure 2-4 Switch equivalent model 2.5.2 Switch isolation

Isolation is defined as the ratio of the power delivered to the load for an ideal switch in the "ON" state to the actual power delivered to the load when the switch is in the "OFF" state [33].

Isolation can be found from the transmission coefficient parameter, S21, as following

=

4w 2CSz2 _(2-1)

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1 OLog(S )2 ₌

20Log,12>CZ0

|

(

For a given set of switch geometry and signal frequency, the isolation of the switch can be computed as listed in Table 2-1, where the impedance is assumed to be 50 0.

Table 2-1 Switch Thickness h

pm

10 10 10 10

isolation for given geometry

AVid th b pm 10 10 10 10 Gap d pm 3 5 3 Fr.

qe

.ncy Frequency GHz 4 4 40 40 Isolation dB -67 -62 -47 -43

It is shown from the table that the isolation is comparable to the existing MEMS switches even with a small gap of 3 pm.

2.5.3 Switch insertion loss

The insertion loss is defined as the ratio of the power delivered to the load in the

"ON" state of the ideal switch to the actual power delivered by the practical switch, in the ON state [33]. An idea switch is assumed to have no power loss.

Insertion loss can be determined from the S parameters:

InsertionLoss = -20Log(l - c 2ZO

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Assuming the impedance is again 50 Q, the insertion loss vs. contact resistance is shown in Figure 2-5. 0.8 -0.7 0.6 0 . -03 S0.2-0 0.1-0 1 2 3 4 5 6 7 8

Contact resistance ohm

Figure 2-5 Insertion loss vs. contact resistance

For a contact resistance of 0.1 , the insertion loss is only 0.01 dB.

2.5.4 Micro strip transmission line design

A micro strip transmission line [33] as shown in Figure 2-6 below has been used in most RF MEMS design due to its simplicity of fabrication. The critical parameters of a micro strip are the ratio of metal layer width to dielectric layer thickness (w/h). The candidate material for the switch structure is SU-8, which has been explained in 2.4. The dielectric constant of SU-8 is around 4.8 and the impedance of the transmission line can be chosen

from 50 to 100 Q depending on the application. From Figure 2-7 [33], it can be determined

that w/h should be around 1-2.5. In this research, a range of switch beam widths (w) and

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Figure 2-6 Micro-strip transmission line

ItI I I 1

-7 C4J Ci Id Ui

-6l 66660 N M lt L 0 I N 0 M I-t L 000 00

w/h

Figure 2-7 Micro-strip impedance vs. the ratio of w/h

1000 500 400 300 200 100 50 40 30 20 10 4) Q C cc E .2 5 4 3 2

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2.6 Summary

A novel MEMS switch was conceptualized based on switch performance analysis. Several novel ideas are devised for the new switch design. First of all, compliant supports of the contact surfaces deform accordingly to compensate for the geometric or position mismatch of the two contact surfaces, resulting from either device operation or fabrication. Second, the debris from damages generated on the contact area during operation are to be cleaned through micro sliding motion between the two surfaces, and then trapped in the micro grooves fabricated on one of the surfaces. Low contact resistance is thus maintained throughout the long life cycles of the switch. Thirdly, noble contact metals and alloys, which are to be used as contact materials for their low resistivity, have weak adhesion to the sidewall. Anchoring the contact metal into the switch structure mechanically solves this problem, so that the two have secure mechanical connection. The equivalent model of the switch is established and the insertion loss and signal isolation loss has been predicted to provide guideline for the design of the switch.

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3. _{Contact Mechanics and Contact}

Resistance

In this research, the primary goal is not to develop a better contact model, but rather use the existing models to provide a guideline for the selection and design of the actuator. There are several models for the mechanical contact behaviour of rough surfaces. Greenwood and Williamson proposed the basic elastic contact model in 1958 [23]. The modeling of the contact resistance consists of a few more steps. First the contact surface is characterized to determine the distribution of asperity diameter and height, then, a single asperity contact analysis is made to find the relation between contact force and the radius of the contact area for either elastic or plastic contact. With the radius of contact, we can find the constriction resistance for the single asperity. Finally, the total contact resistance is found by integrating all the contact asperities over the whole area using Greenwood and Williamson's model.

3.1 Contact surface characterization

The contact resistance of two surfaces is closely related to the mechanical behaviour of the two contact surfaces.

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Ii -- hi

If we zoom in any surface of a real material, we find that the surface consists of asperities. An example is the sidewall surface of e-beam evaporated gold film by SEM as shown in Figure 3-1.

/84nm

10nm

7G 3.7 nm

Acc V Spot Magn Det WD 200 nmn

10 0 kV 3_0 80000x GSE 8.0 4.1 Tourr

Figure 3-1 Sidewall surface of e-beam evaporated Gold

In general, we can assume that contact between a plane and a normally flat surface covered with a large number of asperities; the asperities are all spherical and the heights vary randomly. An example surface is presented schematically in Figure 3-2.

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Z

\)

d

Reference

Figure 3-2 A simple contact surface

If the two surfaces come together until their reference planes are separated by a distance d. Then the probability of making contact at any asperity of height z is

prob(z > d)=

#

(z)dz (3-1)

d where * (z) is a probability function.

3.2 Contact mechanics

3.2.1 Hertz contact

To study the contact behaviour of the two surfaces, we can start with a simple case, the contact of two spherical bodies as shown in Figure 3-3. Hertz first solved this problem in the elastic regime [34].

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m~ m.-. U 2W -~ - - T~ W~~EL~

ZI

E

₁

.,o

D

R

₁

L2

Figure 3-3 Single asperity elastic contact

From the theory of elasticity, the vertical deformation can be expressed as

a = (ki + k)go 7 2a

2 (3-2)

The contact radius, a, is given by

a= (k +k2) K

4p9

From force balance, we have

go 2 3F~ 2a3 _{= F --> go =} _" a 3 "2ra 2 where (3-3) - v2l 1 1 v 7E₂ R₁+R2 2RR ₂ (3-4)

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Substituting equation (3-4) into equation (3-3) and simplifying, we have the normal contact force

431 1 1

F, =-a3(-+-- (3-5)

3 R R2 ;r(k+ k₂)

For R2 -> 0X, the contact radius, indentation and contact force can be related by the

following equations I I 4 3 F '=-ERI a 2 (3-6) 3 1 1-v 2 ₁ E El E2

Through these equations, contact radius a and the contact force Fn is related to each other by the indentation a if the deformation is in elastic regime. They can be use to determine the contact radius from the force applied.

3.2.2 Plastic contact

Under higher contact forces, plastic deformation will occur and the Hertz contact solution is no longer valid. From the work of Tabor (1951), it can be shown that yield occurs when the contact pressure

PC = 0.6 H ( 3-7)

where H is the Brinell hardness of the contact material.

Since the contact area is of prime interest while the actual shape of the deformed asperity outside the contact is less important, W. R. Chang (1987) assumed that the volume

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c

ap

2a

Figure 3-4 Volume conservation after plastic deformation

From equation (3-6) and (3-7), it can be found that the critical indentation when plastic deformation occurs is,

ac 0.3rH 2R

E (3-8)

Based on the assumption, the control volume before and after plastic deformation is constant. The diameter of the contact area after plastic deformation occurs can be expressed as

a 2=RaC ₍₃₉₎

Where C is related to the plastic indentation and can be expressed as

-- a-,

a _(3-10)

a=aC +a,

By substituting equation (3-10) into (3-9), the contact radius after plastic deformation can be determined as

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a Ria (2 -- a a

(3-11)

3.3 Constriction resistance

R. Holm (1967) [29] described the constriction resistance due to the geometry change in a conductor. Assuming two arbitrary surfaces A1 and Ac with potential <p and <p

as shown in Figure 3-5.

A

1

(Pi

n

Ac

(PC

Figure 3-5 Constriction resistance between surfaces Al and Ac

From Ohm's law, the constriction resistance is simply given by

-

|P -(|

_

Q

I IC (3-12)

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1=::- dA

I = a 8P d A

Substituting in (3-12), we have

RC =

4xrC

Assuming the arbitrary Ac is circular with radius a, then

RC = (3-13)

2a

This equation relates the contact radius with the constriction resistance. With all these relations, we are able to determine the overall contact resistance.

3.4 Overall contact resistance

3.4.1 Elastic Contact

If the contact pressure P on an asperity satisfies P < Pc = 0.6H,

The deformation of the contact material is in the elastic region.

If we assume the total asperities number is N, then the expected number of contacts can be estimated as

n = N $(z)dz (3-14)

Since a=z-d, the contact area

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The mean contact area is

fra(z - d)#(z)dz

The total expected area of contact is given by

A = irNR, J(z - d)#(z)dz

d

(3-16)

(3-17)

The total expected load is then

Fr =

3NER2

f(z - d)2

#(z)dz

(3-18)

And the total conductance is

2NR12

-GC = -f(z -d) 2 0 (Z)dZ

P d

(3-19)

This is the Greenwood and William model [23]. 3.4.2 Plastic contact

If the contact pressure P on an asperity satisfies P > Pc = 0.6H,

The deformation of the asperity subjected to such a high force will experience plastic deformation, while the deformation of the other asperities might still be in elastic regime. The critical indentation ac associated with this critical pressure is expressed in Equation (3-8). The total expected load is

I d+ac 3 00

F =3 NER1I

f

(z -d)2

#(z)dz+0.6;rRNH

J[2(z -d)-a,]#(z)dz

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(3-20)

The total conductance is

GC = 2NR2 d f , V-2 NR-(z -d) 2

#b(z)dz±

+ 1Jd [2(z - d) -a ]2

#(z)dz

d

From this conductance equation, we can find the total contact resistance.

3.5 Computing Examples

To provide the actuator design guideline, we assume the contact requirement for the proposed MEMS switch is 0.1 Q. The surface profile of the contact surface is similar to that described in section 3.1. We will look at the contact behavior of two cases: 1). Single asperity. 2). Distributed asperities.

3.5.1 Contact resistance and force with a single asperity Assuming the contact resistance requirement is

RC = 0.1 Q

From (3-13),

2RC (3-22)

From (3-6),

a = RI2a 2

Equalizing the two equations, we have,

p

2RC

1 1

R12ca 2

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a = P 2R R (3-23) Substituting (2-23) into (2-3), 1 3 F =-ERR 12a 2 3 _E Cp 6 RI R (3-24)

To evaluate the force requirement, we choose Au to the contact material and its material properties and asperity size are listed in Table 3-1.

Table 3-1 Au Material properties and asperity size

Parameters

Asperity radius m

1.1xl10-Young's modules GPa 77.2

Resistivity Q.m 2.2 x 10-8

Poisson's ratio 0.42

Brinell Hardness GPa 2

Substituting the parameters in Table 3-1 into (3-23) and (3-24), we can find the indentation

and contact force,

a = 1.1 x10~7 M

And F, =7.6 x104 N

Design, fabrication and testing of a lateral self-cleaning MEMS switch

e

Acknowledgements

J.

Contents

83

List of Figures

List of Tables

Nomenclature

C, t

Q

1. Introduction

1.1

Background and Motivation

industry

J.

low

for fabrication

1.5

Organization of the document

2.

RF MEMS Switch Design

2.1

Switch failure mode analysis

2.2

Functional design of the switch system

2.3

Switch concept and design

Contac

u rfac e

. .

.F

ix e d

Undulated

surface

n

sto

MovablI6

metal

Actuator

d

Substrae

Off State:

On State:

zo

zo

=

|

pm

qe

2.6

Summary

3.

Contact Mechanics and Contact

Resistance

3.1

Contact surface characterization

Z

\)

d

Reference

#

3.2

Contact mechanics

E

1

.,o

D

R

c

3.3

Constriction resistance

A

(Pi

n

Ac

(PC

|P -(|

_

Q

_{RF MEMS Switch Design}

_{Functional design of the switch system}

_{ix e d}

_{Contact Mechanics and Contact}

₁