Langmuir 1998, 14, 2925-2928
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Notes Contact Angle, Gas Bubble Detachment, and Surface Roughness in the Anisotropic Dissolution of Si(100) in Aqueous KOH Theo Baum, John Satherley, and David J. Schiffrin* Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, U.K. Received November 3, 1997. In Final Form: February 17, 1998
Introduction Micromachining for the production of three-dimensional structures with micrometer dimensions has become a new area of industrial activity, with an increasing number of microdevices being produced which make use of the anisotropic etching of single-crystal silicon in alkaline solutions.1 The production of microstructures with nanometer precision requires a fundamental understanding of the surface chemistry of the etch process. Although there is a consensus of opinion that the slowest step in the dissolution of silicon in KOH is the hydrolysis of surface Si-H groups,2-5 the origin of surface roughness is unclear at present. Roughness is associated with the formation of pyramids, which has been attributed, among other reasons, to the attachment of hydrogen bubbles to the surface,6 the purity of the chemicals used,6 hydrogen diffusion into the lattice,7 and incomplete dissolution of hydrated oxides.8 It must be recognized though, that Si(100) surfaces are intrinsically unstable, and indeed, Chabal et al.9 has reported microfaceting of these surfaces when in contact with HF. Improvements of surface finish are achieved by the addition of isopropanol (IPA)10 and/or oxidation agents (O2, H2O2, [Fe(CN)6]3-)7,11,12 to the etch bath, by imposing convection conditions,13 and also by applying potentials positive to the free etching potential.12 IPA is the bath (1) (a) Mandle, J.; Lefort, O.; Migeon, A. Sens. Actuators 1995, 46, 129. (b) Mizuno, J.; Nottmeyer, K.; Cabuz, C.; Minami, K.; Kobayashi, T.; Esashi, M. Sens. Actuators 1996, 54, 646. (c) Noell, W.; Abraham, M.; Mayr, K.; Ruf, A.; Barenz, J.; Hollricher O.; Marti O.; Guthner, P. Appl. Phys. Lett. 1997, 70, 1236. (d) Smith, L.; Soderbarg, A.; Bjorkengren, U. Sens. Actuators 1994, 43, 311. (e) Strandman, C.; Backlund, Y. J. Microelectromech. Syst. 1997, 6, 35. (2) Baum, T.; Schiffrin, D. J. J. Electroanal. Chem., in press. (3) Allongue, P.; Costa-Kieling, V.; Gerischer, H. J. Electroanal. Soc. 1993, 140, 1009. (4) Jacob, P.; Chabal, Y. J.; Raghavachari, K.; Becker, R. S.; Becker, A. J. Surf. Sci. 1992, 274, 407. (5) Bressers, P. M. M. C.; Kelly, J. J.; Gardeniers, H. J. M.; Elwenspoek, M. J. Electrochem. Soc. 1996, 143, 1744. (6) Campbell, S. A.; Cooper, K.; Dixon, L.; Earwaker, R.; Port, S. N.; Schiffrin, D. J. J. Micromech. Microeng. 1995, 5, 209. (7) Abbott, A. P.; Campbell, S. A.; Satherley, J.; Schiffrin, D. J. J. Electroanal. Chem. 1993, 344, 211. (8) Tan, S.; Han, H.; Boudreau, R. J. Microelectromech. Syst. 1994, 7, 147. (9) Dumas, P.; Chabal, Y. J.; Jakob, P. Surf. Sci. 1992, 269, 867. (10) Lee, D. E. J. Appl. Phys. 1969, 40, 4569. (11) Schnakenberg, U.; Benecke, W.; Lochel, B.; Ullerich, S.; Lange, P. Sens. Actuators 1991, 25, 1. (12) Bressers, P. M. M. C.; Pagano, S. A. S. P.; Kelly, J. J. J. Electroanal. Chem. 1995, 391, 159. (13) Adachi, S.; Ikegami, T.; Utani, K. Jpn. J. Appl. Phys. 1993, 32, 4398.
additive commonly employed to achieve an improved surface finish. The purpose of the present work was to investigate the influence of the attachment of hydrogen bubbles produced during the etching of Si(100) on surface roughness, with a view to understanding the mechanism by which the presence of alcohols and/or oxidizing agents improves the surface finish. Since the surface chemistry determines etching characteristics, contact angle measurements have been carried out under different conditions. Experimental Section All experiments were carried out in 2 M KOH, and when appropriate, 5% isopropanol (BDH, Aristar) was added. Potassium hydroxide (Fluka, puriss. p.a.) was used as supplied. Argon (Pureshield), nitrogen, and oxygen were obtained from BOC, U.K. Water was purified with a Milli-Q 185 system (F g 18.2 MΩ cm). p-type Si〈100〉 wafers (F ) 7-13 Ω cm) were cut in squares of 1 cm2, sonicated for 5 min in water, and blown dry in a stream of argon. The native oxide layer was removed by a 30 s immersion in HF (Fluka, puriss p.a.) buffered with NH4F (Aldrich, A.C.S.), as previously described. This results in a hydrophobic hydrogenterminated surface (Si-H).5,14,15 Contact angle measurements were carried out at room temperature in argon or oxygen atmospheres. These determinations were made by measuring the profile of sessile droplets and bubbles attached to the dissolving surface using the automated video imaging system, digital data acquisition, and profile analysis previously described.16 The video camera was controlled by a computer that stored the profile coordinates of the drops or bubbles detected line by line by the video raster;16 only the edge and a sharp change in refractive index were recorded. The coordinate data produced by the digitizer employed did not require further processing. Typical acquisition and transfer rates for a profile were 100 ms, which ensured good time resolution and low uncertainty of the digitized coordinates due to unwanted vibrations. The coordinate data points close to the silicon surface were fitted to a third-order polynomial from which the contact angle was calculated by differentiation. The glass cell used had two parallel planar optical windows and openings for the sample holder and gas inlet and outlet. Etching under ultrasound was studied in order to impose highshear stresses at the surface and, hence, greatly enhance the rate of dislodgment of the evolving gas bubbles. The experiments were carried out with a laboratory sonicating bath (Ultrawave Limited, model U400) at 60 °C. The temperature was controlled by a glass coil immersed in the sonicating bath through which water from a thermostat (LTD 6, Grant Ltd, Cambridge, U.K.), providing a temperature regulation better than (0.05 °C, was circulated. The glass cell in which the etching experiments were carried out was secured in the sonicating bath, and transmission of ultrasound occurred through the cell walls. The etchant was allowed to reach the required temperature and was then saturated with either oxygen or argon for 30 min prior to the experiment. The appropriate gas was bubbled continuously through the solution during etching to maintain saturation conditions. Surface images of the etched samples were obtained by atomic force microscopy (AFM) (NanoScope III) using contact mode. (14) Trucks, G. W.; Raghavachari, K.; Higashi, G. S.; Chabal, Y. J. Phys. Rev. Lett. 1990, 65, 504. (15) Rappich, J.; Lewerenz, H. J.; Gerischer, H. J. Electrochem. Soc. 1993, 140, L187. (16) Girault, H. H.; Schiffrin, D. J.; Smith, B. D. V. J. Electroanal. Chem. 1982, 137, 207.
S0743-7463(97)01195-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
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Notes
Figure 2. Schematic representation of the contact angles for hydrogen bubbles and sessile droplets on Si(100).
like etch masks, as can be seen from the hillock geometry forming pyramids corresponding to the slow etching planes.5 The significant improvement observed when high surface stress hydrodynamic conditions are imposed, that is forcing the detachment of hydrogen, clearly demonstrates that surface finish is strongly influenced by wetting of the dissolving surface by the gas bubbles. This is a mechanical effect; although high intensity ultrasound is known to generate OH• radicals, the concentration that can be generated of these species is too low to affect the etching characteristics,17 in particular, for the very low power ultrasonic cleaning bath employed in the present work. Hydrogen Bubble Detachment. Bubble size, contact area, and attachment time are closely related to the contact angle θ at the solid-gas-liquid interface. This is described by the Young equation18 (Figure 2):
cos θ )
Figure 1. AFM images of a Si(100) surface etched in 2 M KOH + Ar for 1 h at 60 °C (a, top) in the absence and (b, bottom) in the presence of ultrasonic radiation.
Results and Discussion Etching under Ultrasound. The etching reaction of silicon in 2 M KOH is accompanied with vigorous formation of hydrogen bubbles. By simple visual observation, these can be seen to remain attached to the silicon surface for different times, locally blocking the surface. Their effect on surface roughness was investigated by mechanical removal with soft ultrasonic radiation. Figure 1a shows an AFM image of a Si(100) surface etched in 2 M KOH for 1 h at 60 °C. This exhibits a rough appearance with hillocks of height greater than 3 µm. For the same experimental conditions, etching under ultrasound, where bubble attachment is greatly reduced, results in a significant decrease in hillock size to approximately 100 nm (Figure 1b). This clearly shows that attached bubbles are a major contributory factor to surface roughness, as they induce temporary local etch stops. These behave
γSG - γSL γLG
(1)
where γLG, γSL, and γSG are the interfacial tensions of the liquid-gas, solid-liquid, and solid-gas, interfaces. A decrease in θ leads to surfaces with a higher degree of wettability and, therefore, to better bubble detachment. For this reason, the influence of the saturation of the etchant with oxygen or addition of IPA on the contact angles of droplets of KOH solution and of hydrogen bubbles formed at the silicon-KOH interface has been studied. Contact Angle Measurements of Sessile Droplets. Parts a and b of Figure 3 compare typical digitized profiles of droplets of deaerated or oxygen-saturated 2 M KOH placed on a p-Si(100) surface in an argon or oxygen atmosphere. It can be seen that the size of the droplet decreases with time due to water evaporation. However, neither the contact angle nor the contact base dimension changes with time (Figure 3a), showing that the surface hydrophobicity remains unchanged. Since this is governed by the surface termination of silicon, it can be concluded that, in these experiments, the hydrophobic Si-H termination is not replaced by a hydrophilic surface species such as Si-OH.5,19 In contrast, for oxygen-saturated solutions, a large increase of the droplet base and a decrease in contact angle (Figure 3b) are observed, indicating an increase in adhesion between the etching solution and the silicon surface. This results from an increase in hydrophilicity due to a partial replacement of Si-H by Si-OH. The change in surface termination is most likely due to the oxidation reaction of Si-H to yield Si-OH. This can occur via hole injection in the valence (17) Kondo,T.; Kirschenbaum, L. J.; Kim, H.; Riesz, P. J. Phys. Chem. 1993, 97, 522. (18) Adamson, A. W. Physical Chemistry of Surfaces; Interscience: New York, 1964; p 103. (19) Ba¨cklund, Y.; Hermansson, K.; Smith, L. J. Electrochem. Soc. 1992, 139, 2299.
Notes
Langmuir, Vol. 14, No. 10, 1998 2927
Figure 4. (a) Chemiluminescence in 2 M KOH + N2 + 10 µm lucigenin when saturating the solution with O2. (b) Chemiluminescence in 2 M KOH + O2 + 10 µm lucigenin with N2 saturation.
observed for etching with an externally applied potential at potentials positive to the free etching potential, thus revealing the similarity of the effects on surface termination of an increase in the rate of hole attachment to surface Si-H as the potential is made more positive.2,12 If the effect of oxygen is to chemically modify the surface by reaction with Si-H according to7
Si-H + O2 + H2O f Si-OH + H2O2
Figure 3. Sessile drop coordinates of 2 M KOH solutions placed on an initially hydrogen-terminated p-Si(100) wafer at 25 °C. The cell was filled with the same gas used in saturating the solution, and the times are indicated (a) argon-saturated solution; (b) oxygen-saturated solution; (c) argon-saturated solution with the addition of 5% IPA.
band from an oxidizing agent such as oxygen or ferricyanide12 or by a direct chemical reaction of oxygen with Si-H.7 Saturating the solution with oxygen shifts the open circuit potentials from -1.320 to -1.210 V. It is not possible at present to distinguish between an electrochemical or a chemical route for the resulting shift in open circuit potential, but nevertheless, either pathway will lead to the shift in surface termination mentioned above. Bressers et al. showed the close relationship between the concentration of an oxidation agent (ferricyanide) and the hydrophilicity and surface termination of p-Si.12 A similar change in surface Si-H for Si-OH groups and improvement in the surface finish were
(2)
hydrogen peroxide should be formed as a product. To investigate this point, peroxide was determined by its specific chemiluminescence reaction with lucigenin.20 Figure 4a shows the steady-state lucigenin chemiluminescence produced during etching in the absence and presence of oxygen. A significant increase in light emission was observed when saturating the solution with oxygen; purging with nitrogen results in a large decrease of the chemiluminescence reaction (Figure 4b), confirming that oxygen oxidizes surface Si-H with the formation of HO2-. Alternatively, peroxide can also be produced by hole injection from oxygen and the oxidation of Si-H following an electrochemical pathway.2 Figure 3c shows the time evolution of a sessile drop of argon-saturated 2 M KOH with the addition of 5% IPA. The contact angle is very small (θ ) 23°) in comparison with those of the solutions without IPA. Recent work has shown that IPA can become chemically attached to a silicon surface by the formation of the corresponding surface (20) Campbell, A. K. Chemiluminescence; VCH, Ellis Horwood Ltd.: 1988.
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alkoxy.21 Also, STM imaging indicates IPA chemisorption.22 Thus, a change in surface termination from Si-H to Si-OCH(CH3)2 will not significantly decrease the surface hydrophobicity. Therefore, from eq 1, the decrease in contact angle predominantly occurs by the decrease in surface tension of 2 M KOH on addition of IPA; at 25 °C the surface tension decreases from 77.7 mN m-1 to a calculated value of 55.7 mN m-1.23,24 If it is assumed that (γSG - γSL) is constant, a contact angle of 33° can be calculated (eq 1). The deviation from the experimentally observed contact angle of 23° is most likely due to a decrease in surface hydrophobicity and consequently in γSL, as it is well-known to occur for surfactants.18 Contact Angle Measurements of Sessile Bubbles. Contact angle measurements of hydrogen bubbles formed during silicon dissolution have confirmed the previous observations. The contact angle θ decreases significantly for 2 M KOH + Ar (θ ) 62°) when oxygen (θ ) 42°) or IPA (θ ) 34°) is added to the KOH solution, resulting in the corresponding decrease in bubble size. However, these values are larger than those observed with the sessile drops. Since in these measurements the bubbles are growing continuously, a dynamic contact angle is being measured in agreement with the observations by Lin et (21) Bitzer, T.; Richardson, N. V.; Schiffrin, D. J. Surf. Sci. Lett. 1997, 382, L686. (22) Allongue, P. Phys. Rev. Lett. 1996, 77, 1986. (23) Vazquez, G.; Alvarez, E.; Navaza, J. E. J. Chem. Eng. Data 1995, 40, 611. (24) CRC Handbook of Chemistry and Physics; 1980, Weast, R. C., Ed.; CRC Press: Boca Raton, FL, p F-43. (25) Lin, J. N.; Banerji, S. K.; Yasuda, K. Langmuir 1994, 10, 936.
Notes
al.25 on the detachment of air bubbles from a hole on a horizontal plane. The dynamic contact angle is larger than the static value, as observed in the present case. Contact angle measurements could not be carried out at higher temperatures due to the large increase in the rate of hydrogen evolution and, importantly, the decrease in bubble size in the presence of oxygen and IPA, indicative of very low values of θ. Conclusions It is proposed, therefore, that the improvements in surface finish observed result from the enhancement of hydrogen bubble detachment when decreasing the contact angle either by decreasing the surface tension of the etchant or by increasing the hydrophilicity of the dissolving surface. The principles of this phenomenon are the same as those of classical detergency for the removal of oil from surfaces.18 Acknowledgment. This work was carried out with the support of the European Union Human Capital and Mobility Programme, Contract No. ERB-CHBI-CT941118. Useful advice and technical support by Dr. J. Kremesko¨tter, Dr. Y. Cheng, and Dr. R. Wilson, all from Liverpool University, are gratefully acknowledged. The authors also acknowledge the University of Liverpool for support for the purchase of a NanoScope III Surface Probe Microscope. LA9711950
