Fabrication technique for microelectromechanical systems ... papers/2005/JVB0000… · sets of comb ﬁngers. One set is ﬁxed and stands on the wafer substrate. The other is movable

Fabrication technique for microelectromechanical systems verticalcomb-drive actuators on a monolithic silicon substrate

Q. X. ZhangInstitute of Microelectronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685

A. Q. Liua)

School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue,Singapore 639798

J. LiInstitute of Microelectronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685

A. B. YuSchool of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue,Singapore 639798

(Received 1 June 2004; accepted 25 October 2004; published 28 December 2004)

This article presents a technique to fabricate a microelectromechanical systems vertical comb-driveactuator on a monolithic silicon substrate. This technique employs only two photomasks. The firstphotomask defines all the critical patterns, including a set of movable upper hollow comb fingers,a set of fixed lower comb fingers, and a suspension spring to avoid the alignment problem andmaintain a small finger gap. The second photomask selectively covers the upper fingers to obtain thereleased upper hollow fingers. The vertical comb-drive actuator is fabricated by deep reactive ionetching process on a monolithic silicon wafer using these uniquely designed photomasks to avoidthe residual stress and stiction problems. Different lateral gaps between the adjacent lower solid andupper hollow fingers are obtained with various finger widths. The height of the comb fingers is 10.0µm. The vertical offset between the two sets of comb fingers can be adjusted by controlling theprocess conditions. Both symmetric and asymmetric staggered comb-drives are achieved throughprocess modifications. The mechanism of notching effect is discussed and addressed by multiplespacer oxide deposition. Silicon residue effect, which occurs during the lowering down etchingprocess, is investigated from the fabrication aspect and resolved by combining isotropic andanisotropic etching processes. This kind of vertical comb-drive actuator can be widely applied inoptical switches, scanning micromirrors, and sensors.© 2005 American Vacuum Society.[DOI: 10.1116/1.1835291]

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I. INTRODUCTION

Comb-drive structures are widely applied in microelecmechanical systems(MEMS) devices for electrostatic actution and capacitive sensing. In general, the comb-drivetuators can be divided into two broad categories accordithe direction of the motion:(1) in-plane interdigitated combdrive and (2) vertical comb-drive actuator. The verticcomb-drive actuator is developed to generate out-of-platorsional motions, and is widely used in high-speedhigh-resolution optical scanning and switching applicatiThe vertical comb-drive actuator consists of two staggsets of comb fingers. One set is fixed and stands on thesubstrate. The other is movable and supported by a susion spring. However, the fabrication of such a vertcomb-drive actuator on a single crystalline silicon waferchallenging task as the conventional microfabrication mods basically define only the planar geometry.

The vertical comb-drive actuator fabrication can be csified into three different research methods. The first me

a)Author to whom correspondence should be addressed; electronic
[email protected]
32 J. Vac. Sci. Technol. B 23 (1), Jan/Feb 2005 0734-211X/2005

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is by combining surface micromachining and bulk microchining technologies, in which the vertical comb arraycroactuator is developed by trench etching in bulk siland trench filling of polysilicon.1 In addition, an electrostattorsional actuator is fabricated on silicon-on-insulator(SOI)wafer by using polysilicon surface micromachining and dreactive ion etching(DRIE).2 The second method is basedsingle-crystal silicon developed either on SOI substrate oemploying wafer bonding, such as employing double-sprocess on SOI wafer3 and developing comb structurestwo wafers and assembling them by wafer bonding.4 As thealignment between upper and lower comb is difficult, saligned method is studied to minimize the planar gap angenerate high electrostatic actuation in low operavoltages.5–10 In further efforts, Jeonget al.11 and Pattersonetal.12 developed a self-aligned vertical comb-drive actuatoa single layer of SOI substrate where vertical offsetsrealized by using bimorph cantilevers bending or thick ptoresist hinge reflowing. In the latest research, Kimet al.proposed the surface/bulk micromachining technology(111) single-crystal silicon substrate13 and Chuet al. usedl:
boron etch-stop-assisted lateral silicon etch on a(111)
32/23 (1)/32/10/$19.00 ©2005 American Vacuum Society

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33 Zhang et al. : Fabrication technique for MEMS 33

wafer.14 These two approaches employ multiple masksdeep trench etching steps to form silicon beams with oand utilize alkaline solution to release the vertical costructure in which the spacer oxide or boron diffusion pthe role of sidewall protection.

These techniques have their individual drawbacksshortcomings. Generally, they suffer from some of thproblems:(1) alignment and bonding difficulties when wabonding and multiple-layer techniques are employed,(2) in-herent residual stress problem when polysilicon is usestructure material,(3) photoresist residue problem in hiaspect ratio features, and(4) stiction problem when the wrelease process is carried out. After all, the fabricationsingle-crystal silicon vertical comb-drive actuator onmonolithic silicon wafer with a simple process seems tothe most attractive.

This article proposes a simple fabrication techniqurealize the vertical comb-drive actuator on a monolisingle crystalline silicon substrate by employing tuniquely designed photomasks and multiple DRIE proceavoid the stiction, stress, bonding, or alignment probleThe design of the vertical comb-drive actuator and thetomasks are described in Sec. II. The fabrication procesresults are discussed in Sec. III. The analysis of the notceffect and the silicon residue effect is presented in SecThe last section presents some brief conclusions.

II. THE UNIQUE DESIGN OF THE PHOTOMASKS

The proposed vertical comb-drive actuator consistsset of fixed lower solid comb fingers, a set of movable uphollow comb fingers and a suspension spring that provsupporting and counteracting forces. Figure 1(a) shows theschematic overview of the actuator and(b) is the crosssectional view of the comb fingers. The actuation is opeing when a voltage is applied between the comb fingDuring this process, the upper beams are pulled towarsubstrate with the respective fixed solid fingers. This mois the result of fringing electric fields, which depend onelectrostatic force and the torsional stiffness. The driforce is determined mainly by the relative position ofmovable and fixed comb fingers. The relation betweenactuating force and the relative position is simulated by CventorWare, as illustrated in Fig. 2, where the height offingers is 10.0mm The result shows that the force aproaches maximum when the initial offset is zero and itno significant change until the engagement between thesets of comb increases to half of the beam height. Aftercritical point, the force drops rapidly. Obviously, the initoffset is an important parameter for the vertical comb-dactuator. In addition, the height of the comb fingers isother critical parameter for consideration to achieve loperating range. To suppress the motions along other dtions, the precise alignment between the two sets of finand uniform gap between them are important.

To realize such kind of actuator, two pieces of phomasks are involved in the fabrication. The first photom
comprises the frame of the hollow fingers and solid fingers
JVST B - Microelectronics and Nanometer Structures

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as illustrated in Fig. 3(a), which is used to define the two sof comb on the hard mask layer. This one mask patterwill help to avoid the alignment problem and maintain smfinger gap. The second mask[see Fig. 3(b)] is to accomplishthe hollow fingers by covering the overall hollow fingeThe top and the cross sectional views of the relative posbetween the two photomasks are shown in Figs. 3(c) and3(d), respectively. By employing these uniquely desigphotomasks, the deep trench etching process is dividedtwo steps and two trenches with different depthsachieved. The detail of the fabrication will be describethe following section.

The relative position between the two masks is veryportant in the photomask design. The patterned width osecond mask must not be narrower than the width o

FIG. 1. Schematic of the vertical comb-drive actuator.(a) Overview of thecomb drive.(b) cross sectional view at A-A8 of the comb fingers.

FIG. 2. Relative force simulated as a function of vertical overlap betw
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hollow finger defined by the first mask. Otherwise, thelow fingers cannot be fully covered in the etching down pcess, which may lead to the damage of the upper finHowever, the misalignment tolerance between the two minduced by the process can be as wide as 0.2µm, whichdepends on the lithography machine and undercutting dthe DRIE etching.

The width of the comb fingers and the lateral gap betwthe fingers depend on the photomask design and arestrained by the process capabilities as well. Expandinggap and finger width makes the fabrication easy at the coenlarging the operating region. On the contrary, the sizthe device can be reduced, but the dimension is limitethe fabrication facilities. Additionally, the depth of the figers and the motion range will be sacrificed. As a rethere is a trade-off between the overall size of the devicethe fabrication process. From the design aspect, narrothe hollow finger width is the most effective method toduce the overall size because it is wider than the solid finHowever, the width of the inner trench as shown in Fig.(a)is limited by the fabrication, which makes it difficult to otain deep fingers and hard to realize the spacer oxide dsition. The width of the hollow finger frame also restrictsfinger depth due to the unavoidable undercutting and/osloped beam profiles.

After considering for the simulation results and the facation process, the width of the hollow finger is allowedvary from 3.0 to 7.5µm, whereas the width of the inntrench is allowed to vary from 1.0 to 2.5µm, and the widthof the frame beam is from 1.0 to 2.5µm. The solid combfinger is designed to have a width of 2.0µm with differentplanar gaps of 2.0, 2.5, and 3.0µm. The vertical offset between the two sets of comb fingers is adjusted throughcess control, which may be positive, zero, and negative

III. FABRICATION PROCESS

The process flow is outlined in Fig. 4 that is started w

FIG. 3. Design of the photomasks.(a) First mask consists of hollow ansolid comb fingers.(b) Second mask for selective coverage of hollow cofingers.(c) Relationship between the two photomasks.(d) Cross sectionaview at A-A8.

a 6 in.(100) single-crystal silicon wafer. The fabrication pro-

J. Vac. Sci. Technol. B, Vol. 23, No. 1, Jan/Feb 2005

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cesses of the asymmetric and symmetric vertical codrives using the uniquely designed masks are describelow. The fabrication results are discussed with mhighlight on the hollow comb fingers.

A. Asymmetric vertical comb drive

First, a 2.0µm plasma-enhanced chemical vapor deption (PECVD) tetraethoxysilane(TEOS) oxide was deposited onto a cleaned wafer as hard mask layer. This thickshould be as thin as possible to reduce the stress effectsilicon structure, but should also be sufficiently thick to rethe multiple times of DRIEs. Then the first photomaskemployed to define both the hollow and solid fingers. Thpatterns were then transferred to the hard mask laye2.0 mm oxide RIE etch in the Applied Materials P50etcher using CH4, CHF3, and argon as etching gases[see Fig4(a)]. After photoresist strip and wet cleaning the wafer,second photomask was used to selectively cover the h

FIG. 4. Process flow of the vertical comb-drive on a monolithic silsubstrate.(a) Patterning hard mask.(b) Photoresist coverage and first treetch. (c) Photoresist strip, second trench etch and thinning the oxide(d)Spacer oxide deposition and bottom oxide etch back.(e) Prerelease etch.(f)Release etch using XeF2. (g) Spacer oxide strip.(h) Spacer oxide redepotion and exposing of the solid comb fingers.(i) Lowering down the lowebeams.(j) Spacer oxide strip.

fingers by a 2.0µm thick photoresist as a soft mask while

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exposing the other microstructures. This prepatterningnology avoids resist spinning on high aspect ratio structNext, a time-controlled oxide trimming was carried ouetch away 8000 Å oxide by employing the Applied MaterP5000 etcher with the same etchants. This trimming proreduced the thickness of the hard mask layer over thefingers with a consequence of different oxide thicknessethe hollow and solid fingers. This difference is a key pareter in the lowering down process later.

The first DRIE etching was conducted with an anisotroSurface Technology Systems(STS) inductively coupledplasma(ICP) system. This is an anisotropic silicon etchprocess realized by alternating polymer passivation anding from the bottom to deepen the trenches vertically.balancing the etching and deposition in the cyclic wayaccurate control of the anisotropy was obtained. The proing gas was the mixture of C4F8, SF6, and O2, where C4F8

served as the passivation precursor, while SF6 and O2 servedas the etching gases. The silicon etch rate was a1.0–1.3mm/min for 2.0–3.0mm wide silicon trenches. Thetching selectivity between silicon and oxide was more80, which implies that the oxide hard mask layer on tothe solid comb fingers was consumed little in this treetching process step. Thus, the difference between thelow comb fingers and solid comb fingers on the thicknesthe hard mask layers was increased further. The targdepth of the first trench isd1 [see Fig. 4(b)] and the detaileprocess conditions are listed in Table I.

It should be noted that for the in-plane comb structuthe single DRIE etching step is the only requirement to fthe fingers. However in this process, the DRIE etchingdivided into two steps to obtain trenches with two differdepths. As shown in Fig. 4(c), the photoresist was strippusing oxygen plasma followed by the second deep tretching with the same DRIE system for the depth ofd2.

TABLE I. Silicon DRIE etching conditions.

Processes Process cycles

Gas fl

C4F8 SF6

Etch 30 100First and second trench etch Passivation 160

Etch 0 140Prerelease etch Passivation 90

Isotropic etchsXeF2d Etch ¯ ¯

TABLE II. Lowering down process conditions.

ProcessesProcesscycles

Gas

C4F8

Condition I Anisotropic etch Etch 30Passivation 160

Condition II Anisotropic etch Etch 30Passivation 160

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Spacer oxide was then deposited on the trench sideserving as a protection layer for later release etches. PETEOS oxide of 6000 Å was deposited globally on the wincluding the wafer top surface, sidewall, and bottom oftrenches. The RIE oxide etch followed to clear the olaterally and to expose the bottom silicon for further etchdown the trenches, while the sidewall oxide remained,[seeFig. 4(d)] as a result of anisotropic oxide etching providedthe Applied Materials P5000 etcher. The next DRIE scalled prerelease etching, was then carried out with thesystem for depthd3 [see Fig. 4(e)]. The etching gases uswere same as those for the first trench etching, while thflow rate, supplied power, and the process cyclic time wadjusted to fasten the silicon etching. The etch rate astep could be 2.0–3.0mm/min for 2.0–3.0mm widetrenches. The detailed process conditions are listed in Taas well for comparison with the first and the second treetch. By using isotropic silicon etching with reaction gaXeF2 in the same STS system with the process condilisted in Table I, all the exposed silicon was etched. Bothupper and lower comb fingers were fully released fromsubstrate with various depths, as shown in Fig. 4(f). Anasymmetric comb-drive was obtained when the spacerand residual hard mask were removed. The upper shorlow fingers and the lower high solid fingers were achiewith the same top surface. The oxide trimming in Fig. 4(b) isnot required for asymmetric comb-drive fabrication becait is not necessary to lower down the solid fingers. Howethe double photomask patterning is necessary to realiztwo sets of comb fingers with different depths.

B. Symmetric vertical comb-drive

To realize the symmetric and staggered vertical cstructures, the following processes were developed to l

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down the solid comb fingers. As illustrated by Fig. 4(g), thespacer oxide on the sidewall of the released beamsstripped by isotropic oxide etch in Applied Materials SAtmospheric CVD(SACVD) using C2F6 and O2 as etchinggases at the flow rates of 600 and 700 sccm, respectunder the process power of 500 W. A fresh protection spoxide layer was redeposited using similar processes, astrated in Fig. 4(d). However, the bottoms of the silicon comfingers were covered by oxide[see Fig. 4(h)]. This redeposition is necessary for the lowering down etching procbecause the spacer oxide on the sidewall was attackedwhere during the previous etching processes and therespacer oxide layer hanging beneath the released sbeams, as shown in Fig. 4(f), which interfered with the subsequent processes by scattering the incident ions. Mwhile, the height of the upper fingers can be guaranteethe subsequent processes as the beam bottom was covea new oxide layer. The blank oxide etching in Applied M

FIG. 5. SEM micrograph of the vertical comb structure.

FIG. 7. SEM micrographs of cross sectional of the vertical comb with v


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terials P5000 etcher was employed again to clear themask layer on the top of the solid comb fingers while keing the hollow comb fingers still being covered by oxlayer [see Fig. 4(h)]. This is due to the difference in tthickness of the hard mask, which was achieved at theshown in Fig. 4(b) and in the following etching steps.combined anisotropic and isotropic silicon etching wasadapted to lower down the solid comb fingers, as showFig. 4(i). The anisotropic etching used the same conditionthe first trench etching and the isotropic etching used6and O2 as etching gas that are listed in Table II. Finaisotropic oxide etching was performed to strip spacer oon the sidewall and bottom of comb fingers[see Fig. 4(j)].

The process is different from the other fabrication teniques of high aspect ratio silicon structure in severapects. First, the deep trench etching was divided into

FIG. 6. SEM micrograph of cross sectional of the vertical comb struwith 22.6 µm offset.

arious offsets.(a) Offset of 13.2 µm. (b) 0.0 µm offset with spacer oxide.

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steps to allow the variation in the height of the two setthe comb fingers. Second, the hard mask layer was paretched before the second silicon DRIE to obtain hard mwith different thicknesses for hollow and solid comb fingFinally, the solid comb fingers were lowered down by trming away the top hard mask layer and further etching dthe silicon beams by employing the combination of antropic and isotropic etchings.

C. Vertical comb-drive

The fabricated vertical comb-drive is shown in Fig.where the upper hollow comb fingers and the lower scomb fingers can be observed. The size of the actuaapproximately 1.5 mm30.6 mm. The cross-sectional vieof the symmetric vertical comb-drive is shown in Fig.where the widths of hollow fingers and solid fingers areµm (width of the frame is 1.5mm and width of the trencinside is 1.0mm) and 2.0µm, respectively. The height of thfingers is 10.061.0 µm when the etching depths of the fiDRIE d1 and second DRIEd2 are 15.0µm. The lateral gabetween the two sets of fingers is 2.5µm without any misalignment, and the vertical offset between the upperlower fingers is22.6 µm which can be tailored by adjustithe process conditions as shown in Fig. 7. A positive vergap between upper and lower comb fingers of +3.2mm is

TABLE III. Dimensions of the vertical comb structures.

Parameters/wafers #01 smmd #02 smmd #03 smmd

Width of solid finger beam 2.0 2.0 2.0Width of hollow finger beam 4.0 5.5 6.5Planar gap 2.5 2.0 2.5Vertical overlap 12.6 23.2 0Height of solid finger beam 10.3 11.7 16.1Height of hollow finger beam 10.5 11.1 11.6

Aspect ratio .4 .5 .4

FIG. 8. SEM micrographs of various vertical comb structure:(a) 3.0 mm w4.0 mm wide hollow beam with two 1.5mm frame beams and 1.0mm inne
1.5 mm inner trench.

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shown in Fig. 7(a) and a 0.0mm offset is indicated by Fig7(b), where the sidewall spacer oxide in white color rembefore the oxide stripping.

The designed and fabricated dimensions of the threetical comb-drives are listed and compared in Table III.three different designs have solid comb fingers with a wof 2.0 µm The width of the hollow finger is 4.0mm for thefirst design, 5.5mm for the second design, and 6.5µm for thethird design. Planar gaps for the three designs are 2.5and 2.5 µm, respectively. The vertical offset varies frpositive to zero and to negative. The height of the cfingers is greater than 10.0µm and the aspect ratio of tcomb fingers is greater than 4.0, and can be further increto 15.0 by increasing the trench depth. However, theresome difficulties with the sidewall PECVD oxide coveraunless a better conformal coverage process, such as thoxidation or low-pressure CVD oxide deposition, is utiliz

The unique hollow comb finger structure is investigaparticularly. The cross-sectional scanning electron mscope (SEM) micrographs of the vertical comb structuwith hollow fingers in various widths are shown in Fig.where the width of the solid comb fingers is 2.0µm and thelateral gap is 2.5µm. The widths of the hollow fingers adesigned to be 3.0, 4.0, and 4.5µm, respectively. These thrdifferent designs were fabricated using the same processditions, and the targeted heights of the hollow fingerssolid finger are 10.0µm. The fabricated hollow fingers anarrowed to a certain extent with a negative profile, limithe depth and aspect ratio. The lower solid beams arerowed to 1.9mm, resulting in a wider lateral gap. The fabcated widths of the 3.0µm wide hollow fingers(1.0 mmwide frame and 1.0mm wide trench) are 2.80µm on top, 2.6µm at middle, and 2.39µm at bottom, as shown in Fig. 8(a).This beam narrowing is due mainly to the undercuttinging the deep trench etching. These phenomena are alsserved in Figs. 8(b) and 8(c), where the widths of the hollofingers at the middle part decrease from 4.0 to 3.7µm and

hollow beam with two 1.0mm frame beams and 1.0mm inner trench,(b)nch, and(c) 4.5 mm wide hollow beam with two 1.5mm frame beams an

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from 4.5 to 4.1µm, respectively. The heights of the hollofingers and solid fingers are within the range of 10.061.0µm.

IV. NOTCHING EFFECT AND SILICON RESIDUEEFFECT

Two significant problems arise during the vertical comdrive fabrication process. First, a notching effect occurthe etching steps after the prerelease etching. Seconsilicon residue effect happens during the lowering detching process. The mechanisms for the occurrenceapproaches to address these effects will be discussedsection.

A. Notching effect

A notching effect is the unexpected lateral silicon etchwhich damages the microstructures and degrades the pmance of the comb-drive actuator. The mechanism onotching effect varies in different processes. For exampoccurs at the interface of silicon and buried oxide ducharging on the insulator layer15 when the process is carriout on a SOI wafer. In this study, the lower part of the ssilicon fingers was seriously damaged by the lateral notc

FIG. 10. Notching as a function of planar gap:(a) 2.0


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as observed from Fig. 9, due to the reaction betweensilicon beam and the etching gas. This particular notcoccurs after the prerelease etching[Fig. 4(e)] and the undecut release etching[Fig. 4(f)], where the position is just blow the bottom of the upper fingers at the depth ofd1 fromthe top surface. The worst case, as shown in Fig. 9(b), is thatthe fingers are completely broken after the lowering detching process[Fig. 4(i)]. Hence, the height of the lowfingers is constrained to a very small value by this notceffect, and it is impossible to achieve symmetric staggcomb-drive. The mechanisms for the notching effect inprocess are analyzed in order to solve this problem.

The notching effect has a strong relation with the asratio of the silicon structure as shown in Fig. 10. Whenetched trench was fixed at 30.0µm in depth, with exactly thsame process conditions, the notching effect was most plent on silicon beams with 2.0µm lateral gap[see Fig10(a)]. The beams were better with 2.5µm trench[see Fig10(b)] and showed little damage with 3.0mm trench [seeFig. 10(c)].

The process condition, in particular the prereleaseing, also plays an important role in influencing the notcheffect. A vertical comb-drive with exactly the same des

FIG. 9. Notching effects on the vertical comb structwith 2.0 mm planar gaps.(a) Seriously damaged solfingers.(b) Broken solid fingers due to notching.

mm gap,(b) 2.5 mm gap, and(c) 3.0 mm gap.

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was fabricated with two different prerelease etching cotions. These two conditions result in different oxide csumption rates during the prerelease step, which were300 and 100 Å/min, respectively. The notching effectthese two different prerelease etching conditions is showFig. 11. Obviously, notching[see Fig. 11(a)] with a highoxide consumption rate is more apparent than notching[seeFig. 11(b)] with a relative low oxide consumption raClearly, the process with high oxide consumption rate teto have a more serious notching effect.

The principle of the DRIE based on the Bosch proce16

using the RIE STS ICP system also accounts for the notceffect. This technique consists of a series of alternativeand passivation cycles with each cycle lasting for a fewonds. The passivation step deposits a polymer layer ontfeature to prevent lateral etching, and the etching stemoves the polymer from the bottom of the feature followby silicon etching. Since the DRIE process starts frometch cycle on bare silicon surface, the silicon etch rate duthe first etch cycle is faster. In addition, the silicon etchrate drops along the etching depth due to the loading eTherefore, scallops formed along the trench sidewall areuniform and the highest peaks always appear near theing point of the deep etching. However, as described inprocess flow, the two sets of comb fingers are obtainedtwo times of deep etching. The second etching startsthe depth ofd1. Hence, the scallop peak appears at thesection, which negatively impacts the spacer oxide coveat this critical section compared to the other vertical portiThe next prerelease etching,(especially with a high oxidconsumption rate) attacks the weak spacer oxide ontrench sidewall and reacts with the silicon, which in tresults in the notching effect. The isotropic releasingsXeF2d reacts with silicon wherever it makes contact. Tsilicon fingers are etched laterally when the oxide sp

FIG. 11. Notching as a function of the prerelease condit

coverage is poor, and the fingers will break[see Fig. 9(b)] if


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the isotropic etch is too long. The lowering down etchprocess is also a contributing factor to the notching efwhich damages the fingers.

This explanation was verified by the damaged positiothe lower fingers. The broken point occurs exactly atdepth ofd1 from the top. This is the depth of the first deetching or the starting point of the second deep etching.implies that the damage occurs at the weak joint.

This notching problem was resolved by multiple spaoxide depositions to obtain better spacer coverage.method to improve the spacer coverage is to increasPECVD oxide deposition time, but the top opening willblocked by the deposited oxide and the weak joint willnot be well covered. On the contrary, multiple depositionanisotropic oxide etching are more efficient. This is beclonger deposition periods assure more conformal coveon the sidewall, and the top trench opening is assured banisotropic oxide etching. For trenches with higher asratios, a slow prerelease condition is recommended tovent the notching effect. As a result, fingers with adjustheights can be achieved without the notching problemsfinal cross-sectional views of the fingers using the improprocess are depicted in Figs. 12(a) and 12(b).

B. Silicon residue effect

A silicon residue effect arises during the lowering doetching process[Fig. 4(h)]. This phenomenon can be oserved in Fig. 13, where the silicon residues remain on tothe solid comb fingers after the lowering down etchingcess. This is the result of the spacer oxide protectionand the pure anisotropic process, which is observedmore serious at the corners. As is widely known, silianisotropic deep etching is performed using a mixturSF6, C4F8, and O2, which has a very high etching selectiv

igher oxide consumption and(b) lower oxide consumption.

between silicon and oxide. Consequently, the protective ox-

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ide layer on the solid comb fingers is not removed duringsilicon DRIE, and it acts as shadow masks in the subseqanisotropic silicon etching.

To resolve this problem, a combination of anisotropicisotropic silicon etching processes was developed, wherformer serves to lower down the beam, and the latteremove the silicon residues clearly. The process condifor these two kinds of lowering down etching are listedTable II. The final lowered solid fingers of the vertical codrive without any silicon residue are shown in Fig. 8.

V. CONCLUSIONS

This article demonstrates the feasibility of a vertcomb-drive actuator on a monolithic silicon substrate wicombination of two uniquely designed photomasks. The

FIG. 12. SEM micrographs of the vertical comb structure with modifiedand (b) 5.5 mm hollow fingers with 2.0mm planar gap and +3.2mm offse

FIG. 13. SEM micrograph of silicon residues on th


t

e

s

t

mask defines all the microstructures while the secondselectively covers only the upper hollow fingers. MultiDRIE etchings with the association of spacer oxide protion are developed to obtain the asymmetric comb-dstructure. By employing the lowering down etching procsymmetric vertical comb-drive actuator is fabricated. Inprocess, the notching effect and the silicon residue effecanalyzed. The notching effect results from the two-steptrench etching, and poor sidewall spacer oxide coveraaddressed by multiple spacer oxide depositions. The sresidue effect is overcome by employing isotropic and antropic etching to lower down the beam. This uniquelysigned photomask and dry process have successfully avthe misalignment, residual stresses, and stiction probleachieve asymmetric and symmetric staggered comb-driv

ess:a) 4.0 mm wide hollow fingers with 2.5mm planar gap and −2.0mm offset
proc(
e solid comb after lowering down etching process.

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tuators. Three advantages are observed through this s(1) simple fabrication process without alignment proble(2) adjustable offset between the upper hollow and losolid comb fingers, and(3) compatible with the complemetary metal-oxide-semiconductor process on a monolsingle crystal silicon substrate.

Various designs with hollow finger width varied from 3to 7.5µm and lateral gaps from 2.0 to 3.0µm are obtained bthis fabrication technique. The narrowest hollow comb finachieved is less than 3.0µm and it can be reduced furthPositive, zero, and negative vertical offsets are obtaineadjusting the process conditions. The height of the cfingers is 10.0µm with an aspect ratio of 4.0, which canincreased further. The vertical comb-drive actuator onmonolithic single-crystal silicon substrate has potentiamany applications, such as an optical switch, a scannincromirror, and accelerometers that require vertical and/osional motion.

ACKNOWLEDGMENTS

This research work is financially supported by A*ST(Agency for Science, Technology and Research). The authorwould like to express their sincerest gratitude to all tec
cians from the Alpha Lab at Institute of Microelectronics

y:

y

--

(IME) and Photonic Lab of Nanyang Technological Univsity (NTU) for their help and technical support.

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Documents

Fabrication technique for microelectromechanical systems ... papers/2005/JVB0000… · sets of comb ﬁngers. One set is ﬁxed and stands on the wafer substrate. The other is movable