Roughning and smoothing dynamics during KOH silicon etching

Sensors and Actuators 74 Ž1999. 18–23

Roughning and smoothing dynamics during KOH silicon etching Ralu Divan a

a,)

, N. Moldovan a , H. Camon

b

National Institute of Microtechnology Bucharest, PO Box 38-160, Bucharest, Romania b LAASr CNRS, 7 AÕenue du Colonel-Roche, 31077 Toulouse Cedex, France

Abstract We studied the influence of surface preparation prior to KOH etching and of surfactants added to the etchant over the etching rates and roughness of the Si Ž111. and Si Ž100. surfaces. The investigated etchants were 25% KOH at 708C, and 25% KOH with small amounts of anionic, cationic and non-ionic surfactants. The surface preparation refers to the use of the following solutions for the native oxide removal: HF:H 2 O 1:10 Žfollowed by DI water rinsing and drying., HF:C 2 H 5 OH 1:10 Ždried without any further rinsing., and 10% HCl in HF:H 2 O 1:1 Žalso dried without rinsing.. The evaluations were made by mechanical profilometry and AFM. No difference between the samples dipped in HF–water and HF–alcohol could be observed. The etching rate of the samples dipped in HCl-containing solutions were greater, while their roughness was diminished. We analyzed the influence of surfactants on the roughness and the anisotropy. The etch rates increased when using cationic and anionic surfactants and decreased with non-ionic ones. The anisotropy is modified by surfactants. Tentative explanations for the roughning mechanisms are proposed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Anisotropic etching; Roughness; Anisotropy; Surfactants; Contaminants

1. Introduction Bulk micromachining of Si has found a wide application range in microelectronics, microoptics and micromechanical systems, due to the well-known anisotropic behaviour of some high-pH etchants ŽEDP, KOH, H 2 N–NH 2 , NH 4 OH, TMAH.. Etchants based on KOH are preferred, but show an inconvenient roughness development. There are two levels at which roughness manifests w1x: a microscopic level, due to the random removal of atoms from the boundary of the crystal, and a macroscopic level Ždue the gases and metallic contaminants, hydrogen bubbles, lattice defects. usually exceeding the microscopic roughness with orders of magnitude Ždomain 10–100 nm.. Our goal was to characterize the macroscopic roughness developed by KOH based etchants, to put into evidence the effect of the contaminants and to find solutions for the decreasing of roughness. 2. Experimental The etching experiments were carried out with p-type Ž12 V cm. Ž100. and Ž111. Si wafers which were masked )

Corresponding author

˚ . films. Two kinds of samples with LPCVD Si 3 N4 Ž800 A Ž . were used: 111 Si wafers with squares of 5 = 5 mm2 openings in the masking layer on the polished front and on the unpolished backsides of the wafer for roughness measurement and Ž100. Si wafers with radial gratings Ž18 steps, 18-mm diameter. for measuring the etching anisotropy. The roughness was measured with a Tencor P-1 Instrument before and after the immersion of the samples in the etchant. The etching rates were established by depth-offocus measurements on optical microscopes Žfor deeply etched samples above 5 mm. and using the Tencor P-1 Instrument Žfor shallow etched samples.. Atomic force microscopy was used to evaluate the root-mean-square surface roughness on low-roughness surfaces and a Tencor a-step 200 Instrument was used for evaluating higher roughness values. The standard etchant used was 25% KOH at 708C. The etching experiments were carried out in Teflon vessels. The solutions were stirred with a magnetic stirrer and the temperature controlled within "0.58C Žthe sensibility of the controller.. Prior to all experiments, the wafers were cleaned using a standard RCA treatment. The removal of oxide films exposes the silicon surface to metallic impurities present in the HF-dip solution. The silicon is known to be susceptible to metallic impurities, which have higher oxidation poten-

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 3 2 7 - 6

R. DiÕan et al.r Sensors and Actuators 74 (1999) 18–23

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Table 1 Standard electron potential for different metalrion couples No.

Metalrion couple

Standard electron potential E 0 ŽV. 258C

1 2 3 4 5

Siq6Fy ™SiF62y q4ey Co 2q q2ey ™Co Cu2q q2ey ™Cu Agq qey ™ Ag Auq qey ™ Au

y1.2 y0.277 q0.337 q0.799 ;1.68

tials than Si in the electrochemical potential series ŽTable 1.. Some of these metal ions are reduced to the elemental metal and are deposited as such on the Si surface, remaining there up to the KOH etching step. Adding hydrochloric acid ŽHCl. to the HF oxide-removing solution was reported to suppress the pitting and surface roughening, by removing the metallic impurities in non-reactive compounds w2x. The cleaning methods play a major role not only in the prevention of particulate contamination, but leaves the solid surface saturated by hydrogen or fluorine atoms. We experimented three types of solutions for removing the native oxide and preparing the surface: HF:H 2 O 1:10 Žfollowed by DI water rinsing and drying., HF:C 2 H 5 OH 1:10 Ždried without any further rinsing. and HF:H 2 O 1:1 with 10% Žby volume. HCl Žalso dried without rinsing.. In separate experiments, we estimated the influence of different types of surfactants added to the KOH etchant, on the etching anisotropy of SiŽ100. and on the saturation value of the roughness. Surfactants were reported to reduce roughness w3x. These surfactants typically belong to the category of ethers of polyethylene oxide alcohol or alkylphenol poly glycidol. Van der Waals interactions and H-bonds dominate the binding of organic molecules with a

Fig. 1. The evolution of the etching depth Žmm. on the unpolished backside of the wafer SiŽ111..

Fig. 2. The evolution of the etching depth Žmm. on the polished frontside of the wafer SiŽ111..

hydrophobic surface such as H-terminated silicon. Until the critical micelle concentrations of the surfactants in KOH solution, water molecule adsorption on the silicon surface and the dissociation of Si–H bonds will be strongly hindered. These phenomena could change the anisotropy of etching and the roughness of the etched surfaces, by intervening directly in the atomic scale mechanisms of etching. In our experiments, the addition of surfactants to the standard KOH solution was done in a way to avoid the critical micelle concentrations Žcmc.. We used 5 ppm and 10 ppm anionic— Žsodium dodecyl-sulfate C 12 H 25 –O– q . SOy 3 Na , SDS, cmc SDS s 12 ppm ,70 ppm cationic— Žcetyltrimethylammoniumbromide C 19 H 42 Nq Bry, CTA– Br, cmc CTA – Br s 80 ppm. and 20 ppm and 15 ppm nonionic-surfactants Žnonylphenol ethylene oxide C 9 H 19 – C 6 H 4 –O– ŽC 2 H 4 –O.16 –OH, cmc non-ionic s 30 ppm. in 25% KOH solution. All surfactant solutions were prepared

Fig. 3. The evolution of the roughness Žrms. on the unpolished backside of the wafer SiŽ111..

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R. DiÕan et al.r Sensors and Actuators 74 (1999) 18–23

4. The influence of the initial surface preparation on etching

Fig. 4. The evolution of the roughness Žrms. of the polished frontside of the wafer SiŽ111..

in DI water. The cmc in basic solution was calculated with the formula: lg Ž cmc . s y3.7671 y 0.2133 lg Ž cm q w OH x s w KBr x . Ž 1. where cm is cmc in pure water w4x.

3. Results and discussions Basically, all the measurements regarding roughness and anisotropy have to be compared with the standard process, i.e., HF:H 2 O-dip and KOH 25%r708C etching. In this process, for Ž100. Si and Ž111. Si the roughness increases with time on the front Žpolished. side and decreases on the back Žlapped. side, until a steady-state roughness is reached. The saturation value of roughness is installed after about 4 h of etching on Ž111. Si, but a much longer time is needed on Ž100. Si wafers w5x. That is why we measured the etching depths, rates and the roughness on the front- and backsides of Ž111. Si wafers.

No differences between the samples dipped in HF–water and HF–alcohol could be observed by etching them in the standard solution. The etching rate of the samples dipped in HCl-containing solutions was greater ŽFigs. 1 and 2. but the roughness on the unpolished backside was diminished ŽFig. 3.. On the polished frontside, due to the limit of the instrumental resolution, the results are arbitrarily ŽFig. 4.. ŽWe tried to show what is the general tendency of the evolution of the roughness.. The saturation value of roughness is sooner installed for the wafers prepared in this solution. The etching rate on the back Žrough. side is greater than on the front side, due to the initial pits of the unpolished surface. The first solution HF:H 2 O leaves the silicon surface saturated by hydrogen atoms and the second HF:C 2 H 5 OH mainly by fluorine atoms w6x. The lower dielectric constant of ethanol and the lower dissociation constant expected for HF in ethanol and some other reactions including polymerisation of HF2y ions by hydrogen bonding were expected to influence the interface chemical properties of these cleaning solutions. The saturation by fluorine atoms of the silicon surface also prevents the further growth of the native oxide. However, the manifestation of these initial conditions of the surface during the alkaline etching was minimal. The surface of the samples etched after the dip in these solutions, show a higher roughness. The HCl-containing solution leaves the surface also saturated by hydrogen atoms, but this time, the metallic contaminants are removed. The absence of metallic contaminants provides a decrease of the roughness ŽFig. 3.. This certifies the fact that metallic impurities, when they initially contaminate the surface, their effect will persist on the interface during the whole etching period. The decrease of the etching rate with roughness and the persistence of the roughening mechanism during the whole period of etching, sustain the proposed model for the macroscopic contamination-borne roughness.

Table 2 Results obtained on the radial structures Etchant

Etching depth on the small featurea Žmm.

Etching depth on large featuresb Žmm.

Etching time Žmin.

KOH 25%, 708C 70 ppm CTA–Br in KOH 25% 6 ppm SDS in KOH 25% 9 ppm SDS in KOH 25% 20 ppm non-ionic in KOH 25% 14 ppm non-ionic in KOH 25%

25 18 27 ŽV-limited. 5 23 24

25.5 29.5 63 28 23 21

20 20 40 20 40 40

a b

Measurements at the edge of radial gratings of 37 mm. Measurements at the edge of radial maxim gratings of 147 mm.

R. DiÕan et al.r Sensors and Actuators 74 (1999) 18–23

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5. The influence of surfactants The chemical reaction responsible for the etching of silicon in KOH solutions was described to proceed mainly by the breaking of the Si–Si back-bonds due to the reaction with neutral water molecules w7x. Surfactants, which are supposed to occupy the solid-etchant interface, should be able to influence the reaction mechanism and change the anisotropy. At least, the adherence of bubbles and solid particles to the silicon surface should be severely diminished. This way, the macroscopic roughness of Si etched in KOH solution should be decreased. We tried to verify these hypotheses experimentally. The surfactants chosen were of three different types: anionic, cationic and non-ionic. The etching experiments revealed an increase of the etch rate when using cationic and anionic surfactants, and a small decrease with non-ionic ones, as compared with the solution without surfactants ŽTable 2.. To detect the eventual changes in anisotropy, we examined the figures of underetching on the 18-step radial gratings, on Ž100. Si wafers and compared them with similar samples etched in the standard solution. The results are presented in Fig. 5. The only surfactant preserving a clear underetching figure was the cationic one: the secondary minima oriented towards the w100x directions are accentuated, and the lobes of the figure are narrowed. The etching figure of the non-ionic surfactant is dominated by the effect of roughening, which covers the usual lobes. The roughening in this case is entirely produced by the accu-

Fig. 6. Change of the dynamics of etching at the samples etched with anionic surfactants Žscale 500:1..

mulation of pyramidal spikes. An interesting effect is the concentration of these spikes on the centre of radial grating, where the structures are finer. A diminishing of the etching rate in this zone is also noticeable. The anionic surfactant produces a similar figure of etching, but some proximity effects additionally alter the image: the total underetching in the directions of maxima does not start at the centre of the grating, but somewhere at the middle of the rays, where the trenches have an initial spacing of 52

Fig. 5. The modification of the anisotropy during the etching of SiŽ100. in KOH solution with surfactants Žscale 5:1..

R. DiÕan et al.r Sensors and Actuators 74 (1999) 18–23

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Fig. 7. The evolution of the roughness during the etching of SiŽ100. in KOH solution with surfactants Žscale 1000:1..

mm ŽFig. 6.. The accumulation of spikes on the narrow places is intensely manifested. The explanation of the above-mentioned effect is that the spiky surface in the centre of the grating reduces the etching rate, preserving the initial structure of the rays, while large fields, starting at the middle of the grating rays, will continue their etching with the initial rate. The accumulation of spikes in the narrow part of the rays could be an effect of changing the reactant concentration. The wide field roughness for the samples etched with the three surfactants is presented in Fig. 7. The cationic surfactant generates a texture of shallow valleys with no spikes, the anionic one—a very smooth surface with rare local spikes up to 0.3 mm height, while the non-ionic one —a highly spiked surface, typical to illustrate the spikedriven roughening. The density and height of the spikes

differ with the concentration of the surfactants. Table 3 presents the numerical values sustaining the above-mentioned considerations.

6. Conclusions An experimental study of the roughness evolution during the etching of silicon in KOH after different cleaning treatments was realised. The metallic contaminants affect the roughness. We could demonstrate that for the evolution of the roughness during the anisotropic etching the initial state of surface has a great importance. The results suggest that is possible to obtain a more acceptable performance with respect to the requirements of etch rate, roughness and initial state of surface.

Table 3 Results obtained by etching large areas Etchant

Etching depth Žmm.

Etching time Žmin.

Average etching rate Žmmrmin.

Peak-to-peak roughness Žmm.

Maximal height of spikes Žmm.

Density of spikes Žmmy2 .

70 ppm CTA–Br in KOH 25% 6 ppm SDS in KOH 25% 9 ppm SDS in KOH 25% 20 ppm non-ionic in KOH 25% 14 ppm non-ionic KOH 25%

29.1 62.0 32.8 30.8 30.9

20 40 20 40 40

1.455 1.55 1.64 0.77 0.77

0.609 0.262 0.099 5.86 2.347

– 1.3 0.3

0 442 572 3385 703

3.3

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The addition of surfactants modifies the etch rate: this increases when using cationic and anionic surfactants, and a small decrease is observed for non-ionic ones. The only surfactant preserving a clear underetching figure for the anisotropy was the cationic one. The etching figure of anionic and the non-ionic surfactant are dominated by the effect of roughening which covers the usual lobes. The roughening in this case is entirely produced by the accumulation of pyramidal spikes.

w2x

w3x

w4x

Acknowledgements The European Commission is gratefully acknowledged for supporting this study by the PECOrHCM project No. ERBCIPDTC940008. References w1x K. Sato, M. Shikida, T. Yamashiro, M. Tsunekawa, S. Ito, Characterization of Anisotropic Etching properties of Single-Crystal Silicon:

w5x

w6x

w7x

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