EE C245 - ME C218. Fall 2003

Transcription

1 EE C45 - ME C8 Introduction to MEMS Dein all 003 Roer Howe and Thara Srinivaan Lecture Electrotatic Actuator II EE C45 ME C8 all 003 Lecture Today Lecture Linear (v. diplacement) electrotatic actuation: vary overlap area: electrotatic comb drive Electrotatic prin: poitive (comb levitation) Second-order effect in electrotatic actuator: chared dielectric, work function, depletion, and Caimir Readin:. W. C. Tan, M. G. Lim, and R. T. How Electrotatic comb drive levitation and control method, Journal of Microelectromechanical Sytem,, (99).. B. D. Jenen, S. Mutlu, S. Miller, K. Kurabayahi, and J. J. Allen, Shaped comb finer for tailored electromechanical retorin forc Journal of Microelectromechanical Sytem,, (003). 3. Kudrl T. D., et al, Pull-in uppreion and torque manification in parallel plate electrotatic actuator with ide electrod th Int. Conf. on Solid-State Senor, Actuator, and Microytem (Tranducer 03), Boton, Ma., June 8-, 003, pp EE C45 ME C8 all 003 Lecture

2 Interdiitated Comb Drive Common bia: DC offet P connected to huttle throuh poly0 round plane EE C45 ME C8 all 003 Lecture William Tan, Ph.D. EECS Dept., 990 (thi device by Clark Nuyen, Ph.D. 994) 3 Electrotatic orce: a irt Pa* t tator (fied electrode) rotor (not but movin) L ap, thickne t L finer lenth overlap lenth EE C45 ME C8 all 003 Lecture W. C. Tan, Ph.D. EECS Dept., 990 4

3 irt-pa Electrotatic orce (Cont.) Nelect frinin field Parallel-plate capacitance between tator and rotor r r ε ot ε ot (, r) q(, ) d N d N r 0 0 W e W r r r d ε ot N independent of! Have we forotten anythin? The ubtrate! EE C45 ME C8 all 003 Lecture 5 Comb Drive orce: a Second Pa Enery mut include capacitance between the tator and rotor and the underlyin round plan which i typically biaed at the tator voltae why? t L z o - - r EE C45 ME C8 all 003 Lecture 6

4 Comb-Drive orce with Ground Plane Correction iner diplacement chane capacitance from tator and rotor to the round plane modifie the electrotatic enery W d p d rp r d r ( ) r EE C45 ME C8 all 003 Lecture Gary edder, Ph.D., pp. 9-, Capacitance Epreion Conider cae where r p 0 C p depend on whether or not finer are enaed C p N Cr NC, e C p u ( L )] [ Cp, r EE C45 ME C8 all 003 Lecture Gary edder, Ph.D., pp. 9-, 994 8

5 Simulation (D inite Element) N ( Cr Cp, e Cp, u ) 0-40% reduction of e EE C45 ME C8 all 003 Lecture Gary edder, Ph.D., p. 3, ertical orce (Levitation) z W z dz EE C45 ME C8 all 003 Lecture p Conider r 0 a hown: dz rp r z dz r ( ) r ( C C ) d p, e N dz W. C. Tan, JMEMS, 99 (reader) r 0

6 Levitation orce electrical prin cont. e, z ke( Δze Δz) contant Levitation force add to the mechanical prin contant in the z direction increae the reonant frequency EE C45 ME C8 all 003 Lecture Gary edder, Ph.D., p., 994 ertical Reonant requency Mut account for electrical prin in findin MEMS reonant frequencie comb (-ai) k e 0 comb (z-ai) k e > 0 parallel plate k e < 0 EE C45 ME C8 all 003 Lecture W. C. Tan, JMEMS, 99 (reader)

7 Relative orce for Surface Microtructure L ap μm, thickne t μm finer lenth L 00 μm overlap lenth 75 μm y r 0 ε t EE C45 ME C8 all 003 Lecture Comb drive (-direction) ( ) o Differential plate (y-direction) ( 0, ) ε ot y y ε ot ε ot plate win bi for urface MEMS 3 Levitation Suppreion Pattern Poly 0 into differentially biaed electrode to minimize field line terminatin on top of comb Penalty: -ai force i reduced EE C45 ME C8 all 003 Lecture W. C. Tan, JMEMS, 99 (reader) 4

8 Eperimental Meaurement Shuttle i pulled down (toward the ubtrate) with zero applied voltae Why? EE C45 ME C8 all 003 Lecture W. C. Tan, JMEMS, 99 (reader) 5 Chared Dielectric: No Applied oltae Needed! Minimize epoed dielectric! Nitride chare inferred from deflection and imulated field ditribution i conitent with typical value EE C45 ME C8 all 003 Lecture W. C. Tan, JMEMS, 99 (reader) 6

9 Work unction Difference Eample: p tructure over n poly 0 electrode z p poly-si n poly-si Equilibrium band diaram n poly-si EE C45 ME C8 all 003 Lecture p poly-si E z How i chare echaned to reach equilibrium? Anwer: mobile chare on dielectric urface 7 Depletion Effect in Silicon n type ilicon (SOI tructure) depletion reion EE C45 ME C8 all 003 Lecture ρ() E air (ε Si / ε o )E Si Q f() Q qn d X d qn d -X d E() -X d -Q Nonlinear chare-torae affect electrotatic force 8