ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Convex-Corner Undercutting on Corrugated DiaphragmNorhayati Soin and Burhanuddin Yeop Majlis, SMIEEE

Institute of Microengineering and Nanoelectronics (IMEN)Universiti Kebangsaan Malaysia

43600 UKM, BangiSelangor Darul Ehsan, MALAYSIAEmail: norhayatisoin@um.edu.my

Abstract KOH process simulation etching ofcorrugated diaphragm with bossedstructure on silicon (100) is presented withemphasis on convex corner behavior. Theinfluence of the KOH etching temperatureand concentration on the convex cornerundercutting of corrugated diaphragm areobserved. The convex corner behavior isanalyzed based on the geometricalparameters and the new emergent highindex silicon planes. It was found that theconvex corner undercutting phenomena issignificantly reduced at low etchingtemperature and high KOH concentrationrespectively.

1. INTRODUCTION

KOH etching is a key technology infabricating micro-electromechanical systems(MEMS) including diaphragm structures thatto be used for example in pressure sensors.The etching of square convex corners in KOHsolution leads to a deformation of the edgesdue to corner undercutting phenomena. Mask-corner undercut during etching on a (100)silicon wafer is a widely recognizedphenomenon when a convex-shaped maskpattern is used in etching.

At the undercut are, a typical facetedprofile appears. The facet depends on theetching conditions [1-6]. Offereins et al.developed a new type of measuring system toidentify the facet planes, and concluded thatthey were {41 11} planes when the solution wasKOH [1,2]. Mayer et al. [2], Puers and Sansen[4], Wu and Ko [5] investigated the change ofthe facet structures with the change of theetching solutions. Kampen and Wolfffenbuttelsystematically studied the undercutphenomena, and they found that {41 1} planeis the responsible for the undercutting [7].

However, these previous studies [ 1-7]have been applied to planar structures thathave been etched from either top or bottom

directions. The aim of this simulation study isto analyze the effect of KOH etchingtemperature and concentration on the convexcorner undercutting of corrugated diaphragmwhich is etched from both top and bottomdirections of silicon wafer.

II. INTELLISUITE ANISOTROPIC ETCHSIMULATION SOFTWARE

IntelliSuite CAD simulation software has beenused for the simulation analysis in this study.IntelliSuite CAD for MEMS provides theability to simulate with high accuracy differentclasses of MEMS devices inducedmechanically, electrostatically andelectromagnetically and then to obtain thegraphical representation of the appearance ofeach simulated device. It is an integratedsoftware complex which assists designers inoptimizing MEMS devices by providing themaccess to manufacturing databases and byallowing them to model the entire devicemanufacturing sequence, to simulate behaviorand to see the obtained results visually withouthaving to enter a manufacturing facility [8].

Etch simulation tool of Intellisuite Anisepermits the generation of 3D model foranisotropic etching of silicon. With AnisE usercan layout microstructure, view 3Drepresentation, access information about theetch rates of different etchants and simulateautomatically the etching under different time,temperature, and concentration parameters [9].

Il1. DIAPHRAGM STRUCTURE

The diaphragm structure to be used in thesimulation analysis has been modeled asshown in Fig. 1. The values of the diaphragmthickness, h and the corrugation depth, dshown in Fig. 1 are based on the originaldesign specifications. In the simulationanalysis both values are varied according tothe etching condition applied.

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

h 43 gm , 887 gm 584gm

d=216 ,um4864 gtm

Fig. Schematic cross-section view of corrugateddiaphragm structure.

IV. SIMULATION RESULTS AND ANALYSIS

Fig. 2 displays a simulated etch front of a

convex corner of a corrugated structure on

{100} silicon etched in pure KOH with 20wt0/o at 80 'C. In this case the silicon wafer isetched from the top direction. At each convex

corner, the etch front is composed of areas

with different surface morphology. One typeof areas exhibits a smooth and regular fmestructure, while others look quite rough andhave irregular shape. This due to the fact that(411 } planes are etched faster than {100}planes and resulting in undercutting theconvex corner where these planes are openedto the etching solution [2]. The close-up view

of this convex corner undercuttingphenomenon is illustrated in Fig. 3.

Top view of an etched convex cornerfrom the simulated results can be presentedgraphically as shown in Fig. 4, with thedefinition of the measured angle a anddistances d, and d2. This result shows that theAnisE etching simulation tool is able to predictthe arising etching shape at convex corners. Acloser look to the comer undercuttingphenomena occurred on the completecorrugated diaphragm structure is shown inFig. 5. It can be seen that holes have beencreated at the convex corners due to theetching process which took place from bothtop and bottom wafer.

Fig. 2 Simulation result of the diaphragmstructure etched from top of the wafer.

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Fig. 3 Close-up view of a simulated etchedconvex corner of top corrugated structure.

Fig.4 Top view of an etched convex corner, withthe definition of the measured angle a anddistances djand d,

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Table. I Variation of convex corner parameters asa function of etchant temperature.

Fig. 5 Simulation result of the cross-sectionalview of complete corrugated diaphragmstructure.

(i) Dependence of undercutting of convexcorners on the KOH temperature.

Etching d, d, AngleTemp.(°C) (urm) (Erm) (deg)

60 68.2 119.2 151.165 114.0 173.1 149.970 182.4 224.0 148.275 265.2 306.3 146.980 374.3 416.0 144.482 421.5 476.3 143.7

The variation of convex corner parameters as afunction of etchant temperature is shown inTable. 1 The definitions used in order tocharacterize the convex corner features in thistable can be referred to Fig. 4. As can be seenin Fig. 6 the undercutting of convex corners ismuch more pronounced with increasing of theKOH etchant temperature. Both the distancesd, and d? increased with increasing KOHtemperature.

The angle of etched convex corner (a)decreased as the temperature increased. Thedotted lines in Fig. 6 indicate values of anglesresulting from {41 1 } and {31 11} bevelingplanes. At the beginning the angle between theintersection lines had a value above the valuecorresponding to {41 11} planes. Withincreasing the KOH temperature the value ofthe angle decreased continuously andapproached the value corresponding to {3 1 1}planes. Therefore the beveling planes thatoccurred at the convex corners cannot beidentified as simple crystal planes.

The variation of the convex cornerundercutting phenomena as a function oftemperature for the complete corrugateddiaphragm structure is shown in Fig. 8. Thediaphragm started the modification of theshape at the convex comers, changing to adifferent shape having high index plane. It canbe seen that the holes created due to theetching process from the top and bottomdirection at every convex corner of thecorrugation part became larger as thetemperature increased. The penetration of thediaphragm at the convex corner during theetching process has been occurred at the 750 Cwith 35wt% KOH.

500

400

300

200

100

060 65 70 75

Ternp. (deg C)

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80 85

Fig.6 Dependence of distances di and d2 on theKOH temperature. For definition of d, andd2see Fig. 4.

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150

60 65 705 80

75 80 85!

Temp. (deg C)

FIG. 7 Dependence of the angle of etched convexcorner (a) on the KOH temperature. Fordefinition of a see Fig.4.

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143.13- = f 31 1 )

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

(a) at 60°C

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(c) at 70°C

..rr.-

(e) at 80°C

(b) at 65UC

(d) at 75°C

(f) at820C

Fig. 8 AnisE simulation results at 100% completion of corrugated silicon diaphragm at different etchingtemperatures at constant KOH concentration of 35wt%.

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(ii) Dependence of undercutting of convexcorners on the KOH concentrations.

The variation of convex comer parameters as afunction of etchant concentration is shown inTable. 2. The definitions used in order tocharacterize the convex corner features in thistable can be referred to Fig. 9. The dependenceof the distances d, and d2 on the KOHconcentrations is presented in Fig. 4. Bothvalues of di and d2 have been increased up tothe point at which the KOH concentration is20 wt %. Beyond this point, the distances d,and d2 have been declined as the KOHconcentration increased.

The angle of the convex corners, a hasbeen measured from the simulation results andthe dependence of the angle a on the KOHconcentration is shown in Fig. 10. At lowerKOH concentration the angle of convex cornerhas a value near the one corresponding to{311} planes (143.130). The value of thisangle is decreased when the concentration isincreased to 20wt%. The value of the anglescan be seen continuously increasedapproaching the value corresponding to {41 1 }planes when the KOH solution became moreconcentrated.

The description of the convex cornerundercutting phenomena changed as the KOHconcentration increased is illustrated in Fig.11. It can be seen that the holes created due tothe convex corner undercutting during top andbottom etching process are less pronounced asthe KOH concentration increased. Finally atthe KOH concentration of 50 wt%, the holes atthe convex corners are not existed at all due tothe low etching rate. Therefore it can beconcluded that the depth of the structure to beetched must be taken into account whendesigning the comer compensation to solve theproblem of convex corner undercutting.

Table.2 Variation of convex corner parameters asa function of etchant concentration

KOH d, d, AngleConc.(%wt) (pm) (pm) (deg)

15 368.0 368.3 143.020 388.0 388.5 142.525 204.0 205.5 143.730 357.0 358.2 145.135 317.0 318.9 145.840 143.0 148.5 146.445 245.0 249.1 147.050 194.0 200.3 148.3

EI 0

-1 5 20 25 30 35 40 45 50 5.

KOH concentration (%wt)

Fig.9 Dependence of distances d, and d, on theKOH concentration

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152

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148

146

144

142

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15 25 35 45 551

Conc. (wt%)

Fig. 10 Dependence of the angle of etched convexcorner, a on the KOH concentration

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

(a) 15 wt% KOH

(c) 35 wt% KOH

(b) 25 wt% KOH

(d) 40 wt% KOH

..4. "I %.

19

...:.

11

t,I RI

(e) 45 wt% KOH (f) 50 wt% KOH

Fig. II AnisE simulation results at 100% completion of corrugated silicon diaphragm at different KOHconcentrations at constant temperature of 80°C.

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V. CONCLUSIONS

Characterization of convex comer propertiesof corrugated diaphragm by KOH anisotropicetching has been performed. It is shown thatno etching conditions influence the sharpconcave corner shape of the corrugateddiaphragm. However, it is observed that theetching parameters such as temperature andconcentration of the KOH solution have beenfound to be the factors that affect the cornerundercutting phenomena of corrugateddiaphragm structure. The significanceinfluence of the factors has been determined interms of the geometrical size and the newemergent crystal plane of silicon. It can beconcluded that the prominent facetscontributing to the undercutting of the convexcorners of the corrugated diaphragm for thegiven etching conditions coincide with the{311} and {411} planes.

Vi. REFERENCES

[1] H.L. Offereins, H. Sandmaier, K. Marusczyk, K.Kuhl, A. Plettner, "Compensating cornerundercutting of (100) silicon in KOH", Sens Mater.3 (3) (1992) 127-144 (1992).

[2] H.L. Offereins, K. Kuhl, H. Sandmaier, "Methodsfor the fabrication of convex corners in anisotropicetching of (100) silicon in aqueous KOH", Sens.Actuators A 25-27 pp. 9-13 (1991).

[3] G. K. Mayer, H.L. Offereins, H. Sandmaier, K.Kuhl, "Fabrication of non-underetched convexcorners in anisotropic etching of (100) silicon inaqueous KOH with respect to novel micromechanicelements", J. Electrochem. 137 (12) pp. 3947-3951(1990).

[4] B. Puers, W. Sansen, "Compensation structures forconvex corner micromachining in silicon", Sens.Actuators A2 1-A23 pp. 1036-1041. (1991).

[5] X. P. Wu, W. H. Ko, "Compensating comerundercutting in anisotropic etching of (100)silicon", Sens. Actuators 18 pp. 207-2 15 (1989).

[6] R Dizon, H. Han, A. G. Russell, M. L. Reed, "Anion milling pattern transfer technique for fabricationof three-dimensional micromechanical structures",J Microelectromech. Systems 2 (4) pp. 151-159.(1993).

[71 R.P. Van Kampen, R. F. Wolfffenbuttel, "Effects of<100> oriented corner compensation structures onmembrane quality and convex corner integrity in(100)-silicon using aqueous KOH", J. Micromech.Microeng. 5 pp. 91-94. (1995).

[8] A.V. Bogdanov, E.N.Stankova and E.V.Zudilova, "Visualization Environment for 3DModeling of Numerical Simulation Results",Proc. Of the IS" SGI Users Conference, pp.487-494

[9] N. Finch , Y. He, J. Marchetti , General MEMSProcess Physics Simulation an its Application,Intellisense Corporation, 36 Jonspin Road,Wilmington, MA USA.

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

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