Snapping microswitches with adjustable acceleration threshold

E LS EVI E R

Sensorsand ActuatarsA 54 ( 1996i 579-583

Snapping microswitches with adjustable acceleration threshold Jeung Sang G o a, Young-Ho Cho ~'*, Byung Man Kwak ~, Kwanhum Park b • Department of Mechanical Engineering, Korea/Idvanced Insgi~ute of Science and Technology. 373- I Kusong-dong, gusong-ku, Taejon 305-701. South Korea Research and DevelopmentDepartment. Hyundai Motor Company. 772-1 Clu~ngduk-ri. Namyang-myun. Kyunggi 445-850. South Korea

Abstract This paper presents the design, fabrication and testing of prestressed bimarph micreheams for applications to tunable acceleration switches. The prestressed bimorph beams are buckled due to the residual stress difference between two dissimilar films, thereby generating initial beam deflections upon fabrication, Necessat'/and sufficient conditions for snapping action of the deflected bimoq~h beam have been derived from snap-through buckling analysis. A set of SiOz/p+-silieon biraorph beams has been designed and fabricated in three different lengths. 800, 900 and 1000 p,m, The electrostatic snap-through voltage for each microheam has been measured as 32. 56.3 and 76.5 V, respectively. Micromechanical properties of beam materials have been measured from on-chip test structures. It is demonstrated that the three different microswitches with a 7/tg proof-mass can be applicable to acceleration switches, where threshold acceleration levels cart he adjustable within the ranges 0-14, 0-35 and 0-47g, respectively, under inter-electrode bias voltages of 0-76 V. Keywards: Accelerationswitches ; Bimofphs; Microswitches

1, Introduction Recently, increasing attention has been shown to microswitching devices utilizing the snap-through behaviour [ 1 ] of a buckled beam, such as non-volatile micromemory cells [2], microvalves [3], microactuators [4] and microtoggle switches [ 5]. In this paper we present the design, fabrication and tesung of a buckled mierobe:,m tot tunable accelerationswitch applications. The threshold acceleration level of the present microswitch can be adjusted by an inter-electrode bias voltage, while that of the previous g-switch [6,7] has been fixed for each device. In a theoretical study, necessary and sufficient conditions for the snap-through switching function have been derived for a clamped-clamped shallow beam of prismatic cross section. For experimental investigations, a set of buckled microswitches has been designed and fabricated using prestressed bimorph microbeams in different sizes. Micromechanical properties of the beam materials have been measured from pressurized on-chip membrane structures. Electrostatic snapping voltages of the fabricated devices are also measured and compared with the estimated values. On this basis, adjustable ranges of the threshold acceleration have been estimated for each microbeam switch, * Correspondingauthor. Phone: + 8 2 42 869 3038. Fax: +82 42 861 1694. E-mail:[email protected]. 0924-42471961515.00© 1996ElsevierScienceS,A. All rightsreserved Pll S0924-4247 t 96 ) 01233-2

Fig. I. ScI'¢ n~'ic diagram of an acccl©ration switch,

2. Design and analysis Fig, I shows a schematic diagram of the microswitch using a prestressed bimorph microbeam, For a bimorph microbeam made of two dissimilar films, initial deflection is generated by the residual stress difference [8] between the two films. The initial deflection at the centre of a multi[ayer microbeam is obtained as: ( w ~ ) ~ = -M°L2 - - ~ j ~ ~ E"j,

(I)

J.S. Go et aL / Sensors and Actuators A 54 ( I F~J 579-583

5SO

where M~ denotes the moment caused by the residual stress difference between two dissimilar films; L, the beam length: E, the Young's modulus; and I, the cross-sectional moment of each layer. From a buckling apalysis [ 1] of a clamped-clamped shallow beam with an initial deflection, the necessary condition for snap-through be~-n buckling has been obtained as: (Wo)~ 4 h a f~

(2)

where ( W o ) ~ is the initial beam deflection at the mid-point and h is the beam thickness. Eq. (2) states that the necessary condition for snap-through is decided by the thickness and initial deflection of the beam, independent of material properties. The sufficient condition for snapping action has been derived from the principle of minimum potential energy [ I ]. The critical load for snap-through is obtained in terms of the size, initial defection and mechanical properties of the microbeam, as follows: ,~. = [(A 2 - lfl~ ''~ _

L~

3

(b)

]

8A]{÷E~{÷q J&,L2,@~ AL2}

(4)

Without inertia force, the threshold voltage for an electro-

static snap-through is obtained as: f 2l 2 I-{A2 - 16~a12

II I

I

m

(3)

where A is ( W o ) ~ / ! / ( I / A ) ; ! and A are the cross-sectional moment and the cross-soctional area of the beam, respectively. The sufficient condition for the snapping action can also be expressed as the sum of the inertia force, F,, caused by applied acceleration, and the electrostatic force, F o adjustable by an inter-electrode voltage: F*=F,+Fc

(d) i

"](rr2EiVlr21~'l ~:2

where ~ is the permirtivity; V, the inter-electrode bias voltage; 5, the area ~f electrode; and l, the gap between electrodes.

3. Microfabrication

Figl 2 illustrates a six-mask micromaehiniag process for the microswitch fabrication: five masks are used for microswitch fabrication and the other mask for counter electrode definition on a gl ass plate. In the silicon bulk-micromachining process described in Fig. 2, EDP solution is used as anisetropic silicon etchant while a p+-silicon layer and thermally grown silicon dioxide are used as the etch-stop layer and the etch mask, respectively. The fabrication starts with an align key definition through a 500/am thick, n-type (100) silicon wafer, in Fig. 2(a), an 8.4/zm deep etch-stop is prepared for the inter-alectrode gap. The processing step of Fig. 2(h) includes a 14 h boron (p+)

YIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIA

Silicon

p+ silicon

SiO2

AI

Olmss

Fig. 2. Fabricationprocessfor the accelerationswitch. diffusion process, followed by thermal silicon oxide growth, Next, a backside silicon etching has been performed to define a square p +-silicon membrane. In the process of Fig. 2(c), n 300 A thick silicon dioxide layer is grown on the square membrane. In Fig. 2(d), the microbeam with an initialdeflection is obtained from the patterning of the SiO2/p+-silicon bimorph membrane. The initial deflection of the bimorph beam is mainly due to the compressive residual stress in the sin2 layer In the step of Fig. 2(e), aluminium electrodes and contact pads are defined on the microbeam and the silicon wafer, respectively. The fabrication process is completed by the processing step of Fig. 2(f), in which a glass plate with upper electrodes is aligned and attached to the microbeam with lower electrodes. Fig. 3 shows an SEM photograph of the microbeams fabricated in three different lengths, 800, 900 and 1000 #m.

4. Material property measurement and mieroswiteh test For experimental investigations, the micromechanical properties and residual stresses of the beam materials are

5St

J.S. Go et ul. /Sensors and hcruators A 54 (1996) 579-583

Table I Mechanical properfi~ measuredfromthin-filmmen',l~afies

Imm

Fig. 3. SEM of microfabricateddevices.

i

Mat©~ials

£(Pa) ± 63%

~o(MPa) ± 13%

v

p*-Si

125

4-77

O.2g

AI

65

- 15

sin2.

73 [13]

-240

0.31 [ 12] 0.17 D3'

[Lil

15 - -

required. On-chip square membranes (Fig, 4) ofp+-silicon, SiO2/p+-silicon and Al/p+-silieon are prepared for micromechanical property measurement. From a pressurized blister test [ 9 ] of the test specimens, Young's modulus and residual stresses can be obtained simultaneously. Load-deflection relations for the pressurized monomorph and bimorph square membranes have been derived and compensated the finite-element method by a similar method to that used in Ref.[ I0]. For a monomorph square membrane, the compensated load-deflection relation is obtained as pa 2

E

- ~ t = 1.37 ( t .496 - 0.431 v ) ~

Pa 2

Eitt

w2

---w--= 1.37( 1.496--0.431/, 0 1 - vt + 3.49o'lh + 1.37( 1.462-0.360 v2) E2t 2

3

W 2

X _--~2-a-T i + 3.59cr2h

(7)

where o',, r,, E, denote the residual stress, thickness and Young's modulus of each film. Micromechanical properties and residual stress measured from the pressurized membrane test are summarized in Table 1. The rilechanical properties and residual stress of the s i n 2 layer are estimated indirectly from the load-deflection rel8tions of SiO2/p+-silicon bimorph membranes; while those of the aluminium and p÷-

II Fig. 4. Square rnerobnUl¢specimenfor micromalenaltest.

0 0

(6 )

where P is the pressure; a, the length of square membrane; w, the maximum deflection of the square membrane; t, the membrane thickness; v, the Poisson's ratio; and o'o, the initial residual stress. For the bimorph square membrane, the compensated load-deflection relation is expressed as

i

9

w2

~-~ + 3.49o'o

L- °'] 900 /.an It00

12

20

40

!

gil

;

!i : ~i!

; ; 60

gO

tO0

Appliedvoltage(V) Fig. 5. Mca.su~cdthw.Sll~]dvoltagefor electro.stoicsrlap-t~'ough. silicon layers are measured directly from aluminium and p + silicon monomGrph membranes, respectively. Threshold voltages for snap-through av~ measured by an electrostatic snapping test of the raicro~,;,ams of three different lengths (Fig. 5). in Fig 5, non-=ere current means the on-~tate of the micros'.',ltch. Table 2 compares the measured and estimated performance of the microswitches. For the microfabticated beams, :Ia'eshold voltages are measured as 32, 56.3 and 76.5 v respectively. Based on the material properties and residual stres.tes of each film (Table 1), adjustable rang~ o f the threshold acceleration ( Fig. 6) have been estima:ed for the microbeams with a 7 p.g proof-mass attached :o the central mieroheam area o f 230/,an × 230/.tin. Fig. 6 dlustrates that through the adjustment of the interelezltode bias voltage, the threshold acceleration level for ~'ach microdevice is tunable within the ranges 0-14, 0-35 and 0--47g, respectively. The microfabricated switches proved to have a switching function well under the inter-electrode bias voltages, as Table 2 Comparisonof the measuredand estimatedperformanceof microswitches Beamlen~h (~m)

Beamdeflectiont~m)

Thmabo~ voltage(V}

Metered

Estimated

Measured Estinla~d

8~

6.5~8

9~0 10GO

8.7~1.0 113±1.4

7.1 8~ [:0

32 56.3 76.5

10.0

27.5 40.0

E S. Go et al./Sensors and Actuators A 54 (1996) 579-593

582

References 800 /an ] .......... 900 ,¢m tO00 /zm

.

40

-

.

N

g

x ,

10

o

",

~

",

6~

'

~o

Applied Voltage (V) Fig. 6 Estimated thresholdaccelerationof the microswfichwith 7/zg proofmass under an inter-aleclrede bias collage. shown in Table 2. However, discrepancies between the estimated and the measured threshold voltages are found in Table 2. The major source of the error comes from the builtin stresses in the deflected beam, which have been neglected in the analytical buckling analysis. Other sources of the error include uncertainty in the measured mechanical properties and residual stresses of SiO2 layers.

$. Conclusions In this paper, we investigated a prestressed bimorph beam for use in tunable acceleration switches. Initial deflection of the microbeam has been generated by the residual stress difference in two dissimilar materials. The necessary condition for the beam snapping turned out to be a pure geometric relation, independent of material properties. The sufficient condition was expressed in terms of inertia force and electrostatic force, thereby enabling the acceleration threshold level to be tuned through the adjustment of electrostatic bias voltages. The fundamental behaviour of the fabricated microswitch showed a good agreement with the designed performance. Experimental measurements of threshold voltage showed some discrepancies, mainly due to the uncertainty in the measured mechanical properties and residual stress of the microbeam materials. The present microswitches can be applicable to acceleration sensors in electronic airbag systems [6,7,14], switching devices in electronics and telecommunication systems, as well as the previously mentioned application areas [ 2 - 5 ] .

Acknowledgements This work was supported by Hyandal Motor Co., Ltd., as a part of the Korea HAN (Highly Advanced National) Project for Next Generation Automotive Technology.

[I] s.J. Similes. An IntrtMuction w the Elastic Stability of Structures. Prentice Hall, Englewood Cliffs, NJ. 1976,pp. 189-215. [2] B. H~lg, On a nonvolatile memory cell b~ed on micro-elecffomechanics, Prec. Micro Electro Mechanical Systems Workshop (MEMS '90), Napa Valley, CA. USA. 1990. pp. 172-176. [3 ] T. Lisec.S. Ho~rschelmann,H.J. Quenzer,B. Wagner and W, Benccke, Thermally driven microvalve with buckling hehevior for pneumatic applications, P-,~c, Micra Electro Mechanical Systems Workshop (MEldS '94), Oiso, Japan, 1994, pp. 13-17. [4] W. Rielhmuller and W. Benecke. Thermally excited silicon microactuators, IEEE Trans. Electron Devices. ED-35 (1988) 758765. 15] H. Matoha. T. lshiknwa, C.-J. Kim and R.S. Muller. A bistable snapping microactuator, Prec. Micro Electro Mechanical Systems Workshop (MEMS '94), Oiso, Japan, 1994, pp. 45-50. 16l C Robinson, D, Overman. R. Warner and T, Biomquis|, Problems encountered on the development of a microscaleg-swilch using three desigu approaches, Tech. Digest, 6th Int. Conf. Solid.State Sensors and AcmaWrs eTronsducers '87), Tokyo, Japan, 2-5 June, 1987, pp. 410-413. [7] W. Frohenius, S. Zeitman. M. White. D. O'Sulgvnn and R. HameL Micromialature ganged threshold accaleromeler$ compatible wilh integrated circuit lechnology, IEEE Trml,r. Electron Devices, ED-19 ¢1972) 37-40. [8] M.W. Judy, Y.-H Cho, RT. Howe and AP. Pisano. Self-adjusting mJcrostrtlcelr~, prec. Micro Electro Meelvanicat Systent~ Workshop (MEMS '91), Nara, Japan, 1991, pp. 51-56. [9l E.I. Bromley, LN. Randall. D.C. Flandels and R.W. Mounlaln, A technique for the delermination of stress in thin film. J. Vac. Sci. TechnoL BI (1983) 1364-1366. II0] S,D. Senturia. Microfabricated slmctures for the measurement of mechanicalpropertiesand adhesion of thin films, Tech. Digest. 6th Int. Cant Solid.State Sensors and Actuators [Transducers '87), Tokyo, Japan. 2-S June. 1987, pp. 11-16. [ I 11 T.H, Ning and C. Hilsum (eds.), Properties of Silicon, INSPEC,New York. 1988,p. 654. [ 12l W.D. Nix, Mechanical ptopenies of thin films, Metallurgical Trans., 20A (Ins9) 2217-2245. [ 13] S.C.H. Lin and I.P. Mur~zkiewicz, Local stress measurement in thin thermal SiO2films on Si substra|es. J. App£ Phys. 43 (1972) 119125. [ 14] Development of Electronic Single Point Sensing System, High-S~fety Chassis Design Technology: Passenger Protection Equipment,

Ministry of Trade, Industry and Resources, Republic of Korea, 1994..

Biographies Jeung Sang Go received the B,S. degree in mechanical engineering from Pusan National University, Pusan, Korea, in 1993, and the M.S. degree in mechanical engineering from the Korea Advanced Institute of Science and Technology ( K A I S T ) , Taejon, Korea, in 1994. He is currently pursuing a Ph.D. degree in mechanical engineering at KAIST. His research interests include the design, fabrication, and development of intelligent sentaation microsystems for automotive, aerospace electronics and telecommunication applications. Young-Ha Cho was born in Taegu, Korea, in 1957. He received the B.S. degree summa cum lande from Youngnam University, Taegu, Korea, in 1980; the M.S. degree from the

J.S. Go et al./Se~ao.~ and Actuat~-~.~A 54 (19ffO1579--583

Korea Advanced Institute of Science and Technology (KAIST), Seoul, Korea, in 1982; and the Ph.D. degree from the University of California at Berkeley for his MEMS work completed in December, 1990. From 1982 to 1986 he was a rasearch scientist at CAD/CAM Research Laboratory, Korea Institute of Science and Technology (KIST), Seoal, Korea. Doring 1987-1990, he worked as a graduate student researcher (1987-1990) and a post-doctoral researcher ( 1991 ) at the Berkeley Sensor and Actuator Center (BSAC) at the University of California at Berkeley. In August 1991, Dr Cho moved to KAIST, where he is currently an assistant professor in the Department of Mechanical Engineering. Dr Cho's research interests are focused on the development of functional microstroctores, microactuators, solid-state sensors, micro optoelectremeehanieal devices and systems, In Korea he has pioneered MEMS research and has been active in contract research on the development of practical miorndevicts and microsystems for applications to printing and display devices, automotive electronics systems, inertial navigation systems, and so on. Dr Cho is a member of 1EEE and ASME. Bynng Man £ w a k earned B.S. and M.S. degrees from Seoul National University in 1967 and 1971, and the Ph.D. in 1974 from the University of Iowa (UI) in the area of optimal

583

design. He worked br~fly for the US Army Armamen~ Command and as an assistant orofessor at UI. Ha spent two batical leaves; one year a~ Mayo Clinic, MN, an~ a n g e r at the Department of Biomedical Engineenng of Ul. Dr Kwak's aeademi:: reeeareh interesls are optimal/engineering design, contact stress analysis and bk~solid mechanics. He is ~tive in contrac~ res,~.aretl on topics in engineering analysis/design of meeh,'mical sys~ms an.d software dev©loorann;. H¢ has been active in the promotion of academia-induslry and intern•tional cooperation and in the activities of professional societies. Kw,~nhum Park was born in 1955. He reo-ived the B.S. degree from Pusan National University, Pa.~an, Korea, in 1979 and the M.S. degree from the Korea Advanced Ias~Jmte of Science and Technology (KAIST), Seoal, K~ea, in 198 I. From 1981 to 1985 he was a researcher at CAD/CAM Research Laboratory, Korea Inslituta of Science and Technology (gIST). Seoul, Korea. During 198,5-1987. he worked as a research assistant in the Depa.~nent of Meehamcal Engineering, UniversityofWashington, Seattle. la 1987 |~emoved to Hyundai Motor Company, Korea, where he is currently an assistant manager in the Research and Developrc~nt Depar,taunt. His research interests are focused on automotive safety devices and systems.