Design and fabrication of micromirror supported by electroplated nickel posts

A

ELSEVIER

PHYSICAL

Sensors and Actuators A 54 (1996) 464-467

Design and fabrication of micromirror supported by electroplated nickel posts Seok-Whan Chung ~'*, Jong-Woo Shin a Yong-Kweon Kim a, Bung-SopHartb Department of Electrical Engineering Seoul Na:itmal University San 56-1 Shinrinz-dong, Kwanak-ku Se ~ I ISI-742, South Korea h Sam.vans Electronics Co., Ltd., 416. Maetan-3 Dung, Paldal.gu. Suwon City. Kyungki-Oo, So:~th Korea

Abstract A 100 p-m >" 100/~m aluminium micromirror has been designed and fabricated using a thick i~hotowsist as a sacrificial layer and as a mould for nickel electroplating. The micromirror is composed of an aluminium mirror plate, two ni,. kel support posts, two aluminitlm hinges, two address electrodes and two landing electrodes. The aluminium mirror plate, which is supported by the two nickel support posts, is overhung about 10/~m from the silicon subslrate. We use thick photoresisl to obtain a l0 #m thick sacrificial layer and electroplate nickel to obtain a support post l0 #m high. The aluminium mirror plate is actuated like a seesaw by electrostatic force generated by an electrostatic potential diflerencc applied between the mirror plate and the address electrode. We use RIE (reactive ion etching) in a final step to release the micromirror plate from the silicon substrate. The edge of the mirr~ r plate contacts the substrale (maximum deflection) when the potential difference between the mirror plate and the address electrode is 35 V, and the mirror is released from tile substrate when the potential difference reduces to 22 V. Keywordv: Micromirmrs;Thick photoresist; Sacrificiallayer; Nickel elect;oplatlng;Rea~ive ion etching ( RIE )

1. Introduction Recently, growing interest in the fabrication of an overhanging microstructuic using a sacrificial layer has been brought about by the rapid progress of micromachining technology [ 1 - 7 ] . An overhanging microstructure consists of a stationary part that sticks to the substrate, and a moving part that is released above the substrate. The overhanging microstructure can be classified by the type of moving part as follows: hinge type [ 1 - 5 ] , cantilever [4,5], bridge type 161 and membrane type [ 7 ] , Also the method for removing the sacrificial layer is classified into wet or dry etching. There are two items that should be considered when fabricating an overhanging microstruetare. One is the determination of the removal method o f the sacrificial layer and the other is the determination of an appropriate material for the sacrificial layer suited to the gap between the moving part and the substrate. In this paper, dry etching by reactive ion etching (RIE) is adopted because wet etching often causes a stiction problem. Thick photoresist is used as the sacrificial layer because the gap between the moving part and the underlying electrodes is about 10 p.m. Though there are a few * Correspondingauthor. 0924-424"H961515.00© 1996El~vier Science S.A All rights re.served PIIS0924-4247(96)011fi1-2

examples of the use of polyimide as a sacrificial layer [ 4,5 ], the exact conditions of dry etching are not presented. The subject of this paper is to establish the fabricauun process of a 10/~m high overhanging microstructure using thick photoresist as a sacrificial layer. A 10 Fm high nickel post supporting the moving part is also fabricated by nickel electroplating using a LIGA ( Lithography. Galvanoformung, Abformung)-Iike technique, and in order finally to release the moving part, the conditions of isotrepic dry etching using RIE are established. 2. S t r u c t u r e and design The structure of the micromirror is shown in Fig. I. It consists of an alummium mirror plate that is suspended above

AI Mirro p

l

~

Hi supportpost dlns Elccttod©

Address Electrode-

~[~ -N-type Si substrat© Fig. I. Schematicview ofrl~ overhangingmicromin'or.

s.-w. Chang et al. /Sensors and Actuators A 54 (1996) 464-467

a silicon substrate by two hinges. The address electrodes and the landing electrodes are formed on the silicon substre,te below the mirror pla~. The aluminium mirror plate is torsionally actuated like a seesaw and the two hinges play the role of a torsional spring. When the mirror plate rotates due to the electrical torque generated by the electrostatic potential difference between the mirror plate and the address electrode, the hinge acting as a torsional spring generates mechanical torque counter to the electrical torque, When these two types of torqne arrive at an equilibrium point, the mirror plate stops rotating. Therefore the rotating angle of the mirror plate can be adjusted according to the variation of the electrostatic potential difference between the mirror plate and address electrode. We designed the dimensions of the overhanging microstructure as follows. The mirror plate is 100 /tmX100 /~mX 1.5/xm, the hinge is 2 0 / t i n × 5 p m X0.2 p.m and the support post is 20 p.m × 20 p,m × 10 p.m.

3. Fabrication

The fabrication process in Fig. 2 can be divided into six steps: electrode fabrication, sacrificial layer fabrication, nickel electroplating, hinge and mirror-plate fabrication, and

l

(a) Electrode fabrication step RIE RIE

~(4&l

1

'&&&F-~

(b) Sacrificial layer fabrication stop

(c) Nickel eleclroplating step

I (d) Mirror plate & hinge fabrication step

I

(el Sacrificial layer removnl step

[ ] St(n-type)

[] sin z

L [ Thick

[]

i.ionCr

M^l

I

ElectroplatedNi

Fig.2. Fabricalionstepsof miemmirror.

465

sacrificial layer removal. The fabrication process shown in Fig. 2 is designed to ascertain the feasibility of an overhanging microstructure. All photoresist steps except for the thick photoresist step are performing using standard positive photnresist (AZI512), normally applied to a 1.2 p m thickness. baked and developed. The photcresist is used as a masking layer to etch the metal layer except for the case of lift-off. The masking photoresist that remains after the etch prcees$ is stripped off in acetone rather than by dry etching. The first step is shown in Fig. 2(a). To begfin the prneess, a (100) n-type silicon wafer is thermally oxidized ( 5000 A). The oxide layer is patterned for backside electroplating of the nickel support posts and for electrical isolation between the landing and address electrodes; the address electrodes are deposited on the SiOz layer. Chrome (adhesion layer) and nickel (seed layer) metal layers for the electr~!~s are deposited using thermal evaporation, with a film thickness fur the Cr layer of 300 A. and lt'orthe Ni layer of 1000./~. These melal layers are defined by coating with photoresist, UV exposure, and the lift-offtechnique. In the second step, in order to define the gap between the mirror plate and the electrodes, thick photoresist (PMER-P 900) is used as the sacrificial layer. The thick photoresist is spun on to a thickness of 10 p.m and prebaked at 85°(2 for 30 rain and semieured ( 121Y'Cfor I h then 18ff'C for 30 rain). To form a mould for nickel eloctroplating, the thick pbotoresist is anisotropicallyetched by RIE using oxygen plasma with a 5000 ,/~ thick aluminium hard mask (shown in Fig. 2(b)). The RIE process is performed using 50 seem of O2 at a chamber pressure of 300 retort with an r.f. power of 180 W for about 20 rain. The etch rate is about 0.5 tan rain- t. As a result of the RIE p r o c e s s , a fine residue of an organic sacrificial layer is formed on top of the electrodes. This residue should be eliminated by DI water cleaning in an ultrasonic bath for a few seconds because it can prevent nickel electroplating. At the third step, nickel electroplating is performed as shown in Fig. 2(el using a nickel sulfate electrolyte solution buffered with boric acid [ 8 I- The electroplating bath is maintained ai room [¢lilperattlse using a current density of approximately 10 m A c m -z. yielding an approximate deposition rate of 0.1 g/.Lm rain- '. The top oxidation layer of the nickel seed layer caused by the O2 plasma (used at the second step in order to form post holes) should he etched for about 25 s in 50:1 solution of H20:HF before nickel electroplating. Fig. 2(d) show the hinge and mirror-plate fabrication step. Aluminium is used as a material for the minor plate and hinge metal. A 1.5/.tin aluminium layer is deposited and the hinge and mirror plate ate patterned simultaneously. Then, the aluminium hinge is wet etched until its thickness is 2000 A, because it should be much thinner than the minor plate for electrostatic actuation with a very low electrostatic potential difference. Finally, the thick photoresist used as a sac~ficial layer is removed isotropically by plasma etching using RIE. The released structure is illustrated in Fig. 2(el. The isotropic

466

,S,-W. Chung et aL / Sensor,v and Actuators ,4 54 (1996) 4 6 4 ~ 6 7

Table I Condition,of isotroplcdry etchingby RIEu~nlg0 2 plasma R.f.power Chamberpressure Gasused Etehiilgtime

: 20 W 700 nltorr 02, 50 sccm = 35-40 rain

R1E process to release the aluminium mirror plate is performed using O2 plasma. Because we did not have any dryetching equipment other than for RIE, we had to develop an appropriate method for increasing isotropy. To increase the etch rate in the direction parallel to the substrate, the substrate was positioned about 2 cm above the electrode. We think this increased the etch rate parallel to the substrate because the substrate was positioned in the plasma body rather than in the sheath area. The conditions of the RIE process are shown in Table 1, and the etch rate parallel to the substrate was about 1.5 ~m rain z.

the silicon substrate as shown in Fig. 4. Therefore the deflection angle of the mirror plate caused by the electrostaticpotential difference can be about 10°. Because the final sacrificial-layer etching step has very large selectivity between thick photoresist and the aluminium mirror plate, the etching time is not critical in this step, So the final releasing step shows a gored uniformity over the eight mirror structures. Fig. 5 is an SEM mierograph of the overhanging microstructure released by 02 RIE. The sacrificial layer was successfully etched away by this step. Fig. 6 shows an SEM microgmph of the nickel support post adhered to the substrate and shows the june fort between the post and hinge. The aluminium hinge is well adhered to the nickel post. The height of the post is about 10 p.m. This is the same as the height of the thick photoresist used as the sacrificial layer, The fabricated mieromirror array was actuated by a potential difference applied between the mirror plate and the address electrode. The deflection increased as the applied

4. Results and diseuss~,m The microstructure fabricated in this paper is an eight micromirror array. This fabrication was performed in order to demonstrate the feasibility of the overhanging microstructure using LIGA-like technology with a thick photoresist as a sacrificial layer and nickel electroplating for a 10/.rm high support post. The microstrueture is illustrated in Figs. 3-5. Fig. 3 is an optical micrograph showi.~g the hinge and the mirror plate. The measured dimensions are 93 p.m × 93 p,m × 1.5/.tm for the mirror plate and 20.4 p.mx4.3 p.mX0.4 p.m for the hinge. The measured dimensions are somewhat different from the designed dimensions, The difference is generated because of overetehing of the aluminium used as a material for the hinge and mirror plate. The thick photoresist used as a sacrificial layer is completely removed through the RIE process as mentioned above (Table 1). The aluminiom mirror plate successfully adheres to the electroplated nickel support post and is overhung fi'om

Fig, 3. S B M nlicrographof the fabricatednlic~mirror,

Fig.4. S E M micl~graph of tilernic~min~r array.

Fig. 5. SEM micrographof lhe micromirror showing the released micromirrorstructure.

S.-W. Chung et al. I Sensors at.~ Actuator~ A 54 f 1996} 464-467

467

I2] L.t Hombeck, Deforraable-mirmr spatial ligl~ nmd~!~¢r, i,n Spatial Light I~lodulato~ and Application IlL SPIE Critical Roy., Vc4. | I~0.

1999.pp 86-102. (311¢ Fischer,H Gracf~u~dW.yon Miinch.Electroslafislical[ydefi~t,~[~ poiysi|icon torsional mirrors, -~ensor,v art~ Acl~fllo,'s A, 44 ( l ~ )

Fig.6. SEMmicrographshowingthejunctionbetweenthe postand hin~/ voltage increased (analog operation), and the mirror plate suddenly landed o~ the substrate when the voItage was above 35 V (down-threshold voltage, digital operaGon). When the applied voltage was decreased from 35 V the mirror plate remained in contact with the substrate. When the voltage was reduced to 22 V (up-threshold voltage), the mirror plate suddenly released from the subs|rate.

$. Conclusions An eight-bit micromirror array, which is 100 # m × 100 pro× 1.5/~m with 80 #m pitch between the mirror plates, was fabricated. This fabrication demonstrates the feasibility of an overhanging mierostructum using LIGA-Iike technology with a thick photoresist as the sacrificial layer and nickel electroplating for a 10 p.m high support post. We used an RIE process to release the micromirror plate from the silicon substrate in the final step. The fabricated micromirror was actuated by an electrostatic force. The down-threshold voltage was 35 V, and the up-threshold voltage was 22 V.

Acknowledgemenls The authors gratufully acknowledge Professor He-Sung Kim of Chung-Ang University, and Dr Eun-Ho L<~ of the National Industry Technology Institute in Korea, and specially thank Bum-Kyoo Choi and So-tin Ahn of Samsung Electronics Co., Ltd. This work is supported by Samsung Electronics Co., Ltd. in Korea.

References [ 1] J.M. Younse, Mirrors on a chip, IEEE Spectrum, (Nov.) (1993) 2731.

g3~

89. [41 C,W.$lOnr~ent.D.A.Borkholder,V. W~terlind,J.W. $u~ N.I. Main[ and G.TA. Kovacs. P'le'~ible,dt'y-rel~tsed proces,~ for alum.inmn electrostalicaclua~oc~,J. Micr.~lectromech. Syst.. 3 (1994) 90-96. [$l C.W.$torn~nt, D.A.Borkholdet.V. Wesleflind,J.W, Sub,N.t. M~uf and G.T.A.Kovacs.Dry-rele,a.sed process for aluminume l e c t actuators. Solid-State Sensors am:l Actuator Warl~hf~p. Hilton Head. SC, USA. 13-16June. 1994, pp. 95-98. 16l N Takeshima.K.J. Gabriel.M. Ozaki.J. Tababashi.H. Honffachi~ ' H. Fuiita,Electrostaticparalleloo~amactu~ors. Prec. 6th I~r. Conf. Solid-State Sensors and Aem~ror~ITron-¢ducer.¢ "91). SonFrancisco. CA. USA. 24-28June, 1991, pp, 63-66. [7] L.J. Hombesk. 128×128 deformable mirror devices. IEEE Treas. Electron Devices. 30 (1983) 539-545. 18] A n. Fr:l/~rand M.G.All,n,Mclallicmicrocstmcttu~fabricatedusing photesensin~e polyimideeleciropl~tingmolds, J. Microelecrtomech, Syst. 2 0993) 87-94.

Biographies Seok.WhanChungreceived B.S. (1994) and M.S. (1996) degrees in electrical engineering from Seoul National University. Korea, in 1994. He is currently an M.S. candidate in electrical engineering at Seonl National University. His research is on the design and fabrication of micro actuators, especially on the micromirror device driven by electrostatic force. tong-Woo Shin received B.S. (1992) and M.S. (1994) degrees in electrical engineering from Seoul National University, Korea. He is currently a Ph.D. candidate in electrical engineering at Seoul National University. His research is on the design and fabrication of micro actuators, especially on the micromirror device. Yong-Kweon Kim received B.S. (1983) and M.S. (1985) degrees in electrical engineering from Seonl National University, Korea. He received the Ph.D. degree in elactrical engineering from Tokyo University in 1990. His Ph.D. lhesis was about a linear actuator using superconducting magnets. He is currently an assistant professor in electrical engineering at Seonl National University~His major concern is the design and fabrication mier<~mirror devices, cell fusion de,vices, vibrating gyroscopes and micro EHD pumps. Bong-Soo Han received B.S. (1989) and M.S. (1991) degrees in mechanical engineering from Seoul National University, Korea. He is currently with Samsung Electronics, Co., Ltd. His research is on micromachined electromechanieal systems.