ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Realization of Perfect Silicon Corrugated Diaphragm UsingKOH Etching

Norhayati Soin and Burhanuddin Yeop Majlis, SMIEEEInstitute of Microengineering and Nanoelectronics (IMEN)

Universiti Kebangsaan Malaysia43600 UKM, Bangi

Selangor Darul Ehsan, MALAYSIAEmail: norhavatisoin(Rum.edu.my

Abstract This article presents cornercompensation mask design in order torealize perfect silicon corrugateddiaphragm using KOH etching technique.The corner undercutting of silicon (100) inKOH is quite serious and withoutcompensation it is difficult to construct acomplete convex corner structures since thesilicon diaphragm is etched from both topand bottom directions in order to form thecorrugated structure. The introduction ofthe additional mask layout for theprotection of convex corners at all convex-mask geometry of the corrugateddiaphragm during the KOH etching processhas been proved by simulation to producealmost perfect square corners

1. INTRODUCTION

The anisotropic wet etching with aqueousKOH is an important technique formicromachining. The square or rectangulardiaphragm and other complicated three-dimensional structure can be formed by thismethod. In this case the square corrugatedstructure has been realized by using this KOHanisotropic wet etching. The disadvantage isthat the corners undercutting of {100} siliconin KOH is quite serious, and withoutcompensation it is difficult to constructcomplete convex corner structure [1].

For solving this problem, Puers andSansen added the square or rectangular andtriangle mask patterns [1], M. Bao et al., added{l101 strips [2] and Mayer et al. added {100}bar on the convex corners [31 respectively. Byutilizing different shapes and/or size ofcompensation structures (Fig. 1), the problemof fast etching of high-index crystal planes thatis inevitably occurs at convex corners can besolved. The general idea of undercuttingcompensation is to delay the moment when themesa of the corner is etched. Depending on the

compensation structure shape, the etch rate ofdifferent crystal planes have to be taken intoaccount in order to calculate the dimensions ofthe compensation itself.

r

Zhang [41 Bao [21

Enoksson [5] Offereins [61

Fig. 1 Different convex corner compensationprinciples postponing the convex cornerundercutting.

II. DIAPHRAGM STRUCTURE

Two diaphragms used in this study have beensimulated using two different masks andhaving the same characteristic parameters.One is the diaphragm designed with originalmask and the other is designed with cornercompensation mask. The size of the squarediaphragms is 4864 x 4864 gMm2. Thecorrugated diaphragms have 5 corrugationsand its thickness is 43 ,um. The depth and thewavelength of the corrugation are 216 ,um and584 jim respectively. Square corrugations ofthe silicon diaphragm have been introduced inorder to realize the diaphragm using KOH

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

etching technique. The schematic cross-sectional view of the corrugated diaphragm isshown in Fig. 2.

43 gm 887 gm 584 gm4,,< , , ._ ,

216gm4864mm --

Fig. 2 The schematic cross-sectional view of thecorrugated diaphragm.

III. CORNER COMPENSATION MASK DESIGN

The mask-layout for the corrugated diaphragmwithout convex corner compensation mask isshown in Fig. 3. It can be seen that both topand bottom masks of the corrugateddiaphragm consist of convex shaped maskpatterns. Therefore special structures oradditional masks have to be introduced tothese convex shaped mask patterns in order toreduce or to prevent convex cornerundercutting.

.. .............. ........... ..

,

(b)

Fig. 3 Mask layout of corrugated diaphragm (a)top and (b) bottom structure.

The additional mask pattern at eachconvex corner of each corrugation part hasbeen introduced as a corner compensationmethod. The corner compensation maskdesign for a single convex corner of a singlecorrugation part is shown in Fig. 4. Theadditional mask pattern is shown in the dottedline whereas the shaded area is represented asthe required results. This additional maskpattern has been assumed to be standard foreach convex corner for all corrugation parts

since all the five corrugations have beendesigned with the same specifications such asthe depth, d and width, W of the groove to beetched. In this method of corner compensationthe width, L and the length, or indirectly isrepresented by x as shown in Fig. 4 shouldsatisfy certain conditions in order to obtain acomplete convex corner structure.

The width of the additional mask patternis influenced by the depth of the groove to beetched: L = 2 d, where L and d are the widthand the etching depth [4]. Meanwhile, theappropriate length of additional mask patternis determined by trial and error method atconstant value of width as mentioned earlier inorder to achieve the design requirements withperfect convex corner structure. Thedimensioning of the corner compensationmask layout has been carried out as follows:the depth, d of the groove has been primarilycalculated to be 216 [tm, therefore:L= 2*d= 432 [tm

The value of x is determined by trial and errormethod in AnisE process etching simulator tobe 130 plm. The final mask layout with theadditional corner compensation mask isillustrated in Fig. 5.

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Fig.4 Layout of the corner compensation designfor single groove (corrugation)

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

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Fig. 5 Compensated mask layout of corrugateddiaphragm (a) top and (b) bottomstructure.

IV. SIMULATION RESULTS AND ANALYSIS

IntelliSuite CAD simulation software has beenused for the simulation analysis in this study.The KOH etching process has been performedby using etch simulation tool of IntellisuiteAnise in order form the corrugated structureson the silicon wafer. The simulated etchingcondition that has been applied to realize thedesigned corrugated diaphragm is shown in

Table. 1.

Table 1. Summaryconditions

of the simulated etching

Parameters Value

Temperature 80 °C

Concentration 35 wt%

Etching time 3.5 hours

Etching rate:

Si ( 10} 72.55 gm/hrSi{110} II1.71tm/hr

Diaphragm thickness 38 5 gm

The simulated etched structures of the topdiaphragm when the etching process is appliedfrom the top direction of the wafer using thecompensated and compensated mask layoutare illustrated in Fig. 6. It can be seen that thetop corrugated diaphragm etched by using

Fig. 6 Comparison of simulation results of thetop diaphragm with: (a) uncompensatedand (b) compensated mask.

uncompensated mask produced etch frontwhich is composed of areas with differentsurface morphology at each convex corner.One type of areas exhibits a smooth andregular fine structure, while others look quiterough and have irregular shape. This due to thefact that {4 11 } planes are etched faster than{100} planes and resulting in undercutting theconvex corner where these planes are openedto the etching solution [7]. A closer look at thesimulated etch front of one of the convexcorners of a corrugated structure on { 100}silicon is depicted in Fig. 7.

Fig. 6(b) shows the etched structure of thecorrugated diaphragm which comprises almostperfect convex corner. It can be concluded thatthe introduction of the compensated mask atthe convex shaped mask pattern couldeliminate the convex corner undercuttingphenomenon.

The simulated etched structure of thecorrugated diaphragm which is etched fromboth top and bottom direction of the wafer isillustrated in Fig. 8. It can be seen that theetched structure using the uncompensatedmask produced holes at convex corners due tothe convex corner undercutting phenomenon atboth top and bottom side of the diaphragm. Inthe case of etching process using compensatedmask, perfect corrugation structure can beformed as shown in Fig. 8 (b).

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(a)

(b)

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Fig. 7 Close-up view of a simulated etchedconvex corner of top corrugated structure.

(a)

(b)

Fig. 8 Comparison of simulation results of thecross-sectional view of completecorrugated diaphragm structure with:. (a)uncompensated and (b) compensated mask

The profile changes of the corner undercuttingof the corrugated diaphragm etched in KOH isshown in Fig. 9. The diaphragm started themodification of the shape at the convexcorners, changing to a different shape havinghigh index plane throughout the etchingprocess. It can be seen that the penetration ofthe diaphragm has been occurred at everyconvex corner according to the undercutphenomenon. This effect is much moresevered upon the completion of the etchingprocess.

The effect of the additional mask atthe convex shaped mask pattern in delayingthe undercutting at the convex comer through-out the etching process is presented in Fig. 10.The close-up view of this phenomenon isshown in Fig. I1. It has been shown that theperfect corrugated diaphragm has been

produced by using the corner compensationmask. The perfect convex corner producedfrom the simulation result is achieved based onthe etching condition parameters that havebeen set-up in order to meet the designrequirement. Therefore, if the depth of thecorrugated diaphragm to be etched is increasedor the thickness of the diaphragm is reducedthe corner compensation mask has to beredesigned to with stand longer etching time.

V. CONCLUSIONS

The under cut during etching of {100} siliconin KOH has been proved as a serious problemfor getting a perfect corrugated diaphragm inwhich the top and bottom surface area isminimized results in poor diaphragmperformance. The perfect silicon diaphragmwith square corrugation pattern has beenrealized by using KOH etching processsimulator. This is achieved by the introductionof the corner compensation mask at the convexshaped mask pattern. It can be concluded thatthe geometrical design of the cornercompensation mask for corrugated diaphragmis based greatly on the corrugation depth andthe thickness of the diaphragm.

VI. REFERENCES

[I] B. Puers, W. Sansen, "Compensation structuresfor convex corner micromachining in silicon",Sens. Actuators A2 1-A23, pp. 1036-1041 (1990).

[2] M. Bao, C. Burrer, J. Esteve, J., Bausells, &S.Marco,. 1993, "Etching front control of <1 10>strips for corner compensation", Sensors andActuators A. 37 - A38, pp.727-732 (1993).

[3] G. K. Mayer, H.L. Offereins, H. Sandmaier, K.Kuhl, "Fabrication of non-underetched convexcomers in anisotropic etching of (100) silicon inaqueous KOH with respect to novelmicromechanic elements", J Electrochem. 137(12) pp. 3947-395 1 (1990).

[4] Q. Zhang, L. Liu, Z, Li, "A new approach toconvex corner compensation for anisotropicetching of (100) Si in KOH", Sensors andActuators A 56, pp. 251-254 (1996).

[5] P. Enoksson, "New structure for cornercompensation in anisotropic KOH etching", J.Micromech. Microeng. Vol.7, pp. 141-144(1997).

[6] H.L. Offereins, H. Sandmaier, K. Marusczyk, K.Kuhl, and A. Plettner, Compensating comerundercutting of (100) silicon in KOH Sens.Mater. Vol..3, pp. 127- 44 (1992).

[7] K. E. Bean, "Anisotropic etching of silicon",IEEE Trans. Electron Dev. 25 (10) pp. 1185-93(1978).

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

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Fig.9 The formation of convex corner phenomenon (complete diaphragm) according to etching time for(I 10) silicon in 35 %wt KOH at 80°C.

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

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Fig. II Close-up view of AnisE simulation results of etched {100} Sicompensated mask in KOH etchant of 35 wt% at 80° C.

corrugated diaphragm with

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