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hydrogen d.c. plasma source
Surface and Coatings Technology 97 (1997) 158–162
Etching characteristics of Si and SiO with a low energy argon/hydrogen 2 d.c. plasma source A. Strass *, W. Hansch, P. Bieringer, A. Neubecker, F. Kaesen, A. Fischer, I. Eisele ¨ ¨ ¨ ¨ ¨ Universitat der Bundeswehr Munchen, Fakultat fur Elektrotechnik, Institut fur Physik, Werner-Heisenberg-Weg 39, D-85579 Neubiberg, Germany
Abstract The effect of a low energy argon/hydrogen arc discharge on the etch rate of silicon and silicon dioxide is investigated with respect to ultrahigh vacuum ( UHV ) wafer cleaning. At room temperature, the reaction speed depends on the partial pressure of the H radicals and the discharge current. Furthermore, the etch rates show Arrhenius dependence versus substrate temperature with negative activation energies of −1.7 kcal mol−1 and −0.7 kcal mol−1 for Si and SiO , respectively. The process window to 2 operate the plasma source is very large, thereby showing a high grade of reliability for standard wafer cleaning under UHV conditions. © 1997 Elsevier Science S.A. Keywords: Plasma etching; Atomic hydrogen; Wafer cleaning; UHV processing
1. Introduction
2. Experimental set-up
Future semiconductor devices will show dimensions below 100 nm. The fabrication of these devices is critical concerning thermal budget and avoidance of nanoparticle contamination to receive good device quality, high yield and reliability. A crucial step is substrate cleaning before and during device fabrication. Low energy hydrogen plasma etching offers important advantages over widely used wet chemical cleaning processes for Si wafers because it is ultrahigh vacuum ( UHV ) compatible, very suitable to patterned substrates and it effectively removes native oxides and organic contaminants at room temperature [1]. A significant increase in device performance of pin and TBD diodes grown by molecular beam epitaxy (MBE ) on UHV plasma cleaned substrates was achieved compared with growth on RCA cleaned wafers [2]. In this paper we compare the etch rates of Si(100) and SiO (on the same wafer) with regard to standard 2 UHV wafer cleaning and microstructural patterning. We used a plasma source recently developed by Balzers Ltd. to create a low energy argon/hydrogen arc discharge in an UHV chamber. The process is used to clean substrates for the MBE growth of nanoelectronic devices such as vertical metal/oxide/semiconductor field effect transistors with channel lengths below 100 nm.
The process chamber described above is part of the Modular UHV Multichamber System (MUM 545) from Balzers AG, Liechtenstein. The arrangement of the system is described elsewhere [1]. We designed the chamber, which is cooled by water, for in situ plasma cleaning of substrates before further processing such as MBE, the growth of thin electrical insulating films by plasma enhanced evaporation [3] or by plasma enhanced chemical vapour deposition with stable operation conditions and a large process window ( Fig. 1). The turbomolecular pump has a high pumping speed of 2200 l s−1 to quickly remove the gas after the process. Both, the composition of the residual gas as well as ions of the plasma were measured by a QMG 420 quadrupole mass spectrometer (Balzers). A pyramid-shaped radiation oven with halogen lamps (radiation temperature: 3000 K ) for wafer heating up to 600 °C was developed for low temperature plasma processes. The oven is not positioned in the UHV to avoid contamination by any damage of lamps or by reactions of plasma-induced radicals with the hot heater material (e.g. graphite). Furthermore, a service is more easily practicable. The radiation of the lamps, that are arranged in concentric rings, passes a glass window and is lead through a metal tube to the wafer. Each lamp contributes nearly the same amount to the substrate
* Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 14 4 - 8
A. Strass et al. / Surface and Coatings Technology 97 (1997) 158–162
159
for this type of plasma source. After the etch process the residual gas spectrum consists of H , and H O, 2 2 predominantly.
3. Measurements and sample preparation
Fig. 1. UHV chamber for plasma and plasma enhanced processes.
temperature because of an optimal design of vertical and horizontal distance from the wafer. The lamps and the window are cooled by compressed air and water, respectively. The maximum substrate temperature of 600 °C can be reached within 30 s. The temperature of a reference substrate (dummy) was measured before the experiments (under the same conditions) with different methods (thermocouple, IR pyrometer, IR diode) under UHV conditions. The temperature dependence obtained by the thermocouple was used as a gauge curve. We varied the temperature either by dimming or by switching out several lamps. The pressure was controlled with a cold cathode filament of Balzers AG. It worked also with the plasma on. Starting at a base pressure of 5×10−7 Pa, argon is fed into the d.c. plasma source as the working gas for the discharge, while hydrogen is let directly into the chamber. The process pressure amounts to about 5×10−1 Pa and is mainly determined by the flow rate of Ar. An electron arc is created from the negative filament to the chamber walls which act as an anode at ground potential. The discharge current typically amounts to 30 A. The kinetic energy of the electrons lies between 5 and 10 eV [1]. Owing to the absence of potentials above 20 V the damage of the substrate is avoided. The position and shape of the electron beam are influenced by external magnetic coils and by a plate of stainless steal in opposite to the plasma source beneath the substrate holder. During the plasma process the following ions are mainly detected by the QMG: H+, H+, H+ Ar+, 2 3 Ar2+ and ArH+. During the measurement the ionisation unit of the analyser is turned off to be sure to measure exclusively plasma induced primary ions. It should be noted that the presence of argon-related species is typical
To determine both the Si and SiO etch rates on the 2 same wafer, several oxide patterns were created on the entire Si wafer surface. For this purpose, the wafers were wet thermally oxidised and then structured by a photolithographic process. The oxide not covered by the resist was removed with HF. Finally, the resist was removed with boiling acetone. The measurements of the thickness and the refractive index of the oxide layers were carried out at fixed wavelength (633 nm) with a PLASMOS SD 2300 ellipsometer. The system was used in N-fix and N-float mode so that both the refractive index as well as the thickness of the layers could be determined with good accuracy at different locations on the wafer. The validity of the thickness measurements was confirmed by comparison with the results obtained from a mechanical profilometer. After oxide removal, the remaining Si steps were measured again with the mechanical profilometer.
4. Results and discussion The etch rates of both materials, Si and SiO , depend 2 on such parameters as partial H pressure, wafer poten2 tial and arc current. At room temperature the etch rate of Si is five to ten times higher than that of SiO . SiO 2 2 shows higher etch rates with increasing partial H pres2 sure as well as of the arc current ( Figs. 2 and 3). The enlarged error bars in the etch rate of Si result from the limited accuracy of the mechanical profilometer. The SiO layers could be measured with higher accuracy by 2 using the ellipsometer. A saturation effect occurs at high H fluxes where the small mean free path of atomic 2 hydrogen atoms limits the etching effectiveness. The total pressure increased with H flow because the flow 2 of Ar was kept constant in all experiments. To understand better the etching process, experiments were done with grounded and floating samples. With the substrate left floating a negative potential of about 20 V was measured to ground owing to the electron discharge. This enhances the soft bombardment with positive ions such as ArH+ causing surface heating due to the high momentum of these heavy species. For higher substrate temperatures—as discussed later—the SiO etch rate decreases. At higher H pressures, more 2 2 ArH+ is formed, resulting in an enhanced surface temperature. For grounded substrates, the attraction of the substrate for ArH+ ions is less than for the floating substrate (−20 V ). Therefore, the grounded substrate
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Fig. 2. Etch rate of Si and SiO vs. H partial pressure on floating and grounded wafers. 2 2
(at lower surface temperature) shows higher etch rates. No influence of the wafer potential on the etch rates of Si was observed under our experimental conditions. We suppose the reason is the high reactivity of the surface dangling bonds. Surface heating does not play an important role at these H partial pressures. 2 After the etching process, the surface homogeneity over the complete 3 in wafer area reached ±10%,
typically. This can be improved by rotation of the wafer and by a sweeping electron beam. The plot of the logarithm of the etch rates versus the inverse temperature ( Fig. 4) indicates Arrhenius dependence. The activation energy amounts to −1.7 kcal mol−1 for Si and to −0.7 kcal mol−1 for SiO . Above 300 °C (1.7×10−3 K−1) the activation 2 energy for Si etching becomes a higher negative
Fig. 3. Etch rate of Si and SiO vs. discharge current. 2
A. Strass et al. / Surface and Coatings Technology 97 (1997) 158–162
161
Fig. 4. Etch rate of Si and SiO vs. substrate temperature. 2
value: −21 kcal mol−1. The effect of ion bombardment can be excluded because of the low energies of the particles. The activation energy is the amount of energy that has to be brought to a molecule or to a compound to reach a transient state before the chemical reaction. This transient product is in an energetically excited state and thereby more reactive. The activation energy is unique for a single step reaction. For etching of Si and SiO it 2 is an indication that there exists a rate determining step within a complex reaction and hence can deliver some useful information about the type of step which is rate determining. The interaction of atomic hydrogen with Si and SiO surfaces is little understood. A detailed discussion 2 is beyond the scope of this paper. We suggest the following model as illustrated in Fig. 5 [4,5]. It is well known that H radicals are adsorbed by the dangling bonds of the Si surface forming a layer of strongly bound SiH(ads) (reaction (1)). The formation of SiH (ads) (reaction (2)) is enhanced by the influence of 2 reactive ions such as H+ and ArH+. The SiH (ads) 3 2 complex reacts further with a weakly bound H atom to SiH (ads). The forward reactions (4) and (5) lead to 3 etching of Si where gaseous SiH is formed as primary 4 product. Three competitive reactions lower the etch rate of Si by decreasing the SiH (ads) concentration: the 3 decomposition of SiH (ads) (4) and (7), the subtraction 3 mechanism (6). The reduction to SiH (ads) is the rate 3 limiting step for silane production [5]. At temperatures above 300 °C, however, the desorption of hydrogen
from the SiH(ads) state seems to decrease the etch rate, predominantly ( Fig. 4). From the literature it is known that the desorption process occurs between 400 and 600 °C [6 ]. We suppose that this difference can be explained by the existence of ArH+ which owing to its momentum transfer elevates the surface temperature. In Ref. [4] Wang found an Arrhenius dependence for considering ion bombardment, however, only for temperatures above 100 °C. In our experiments two regimes of Arrhenius dependence with different activation energies occur. For temperatures above 300 °C the activation energy amounts to −21 kcal mol−1; below 300 °C we found −1.7 kcal mol−1 which compares well with the result of Wang [4]. This gives indication that there is
Fig. 5. Chemical reactions leading to etching of the Si surface.
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A. Strass et al. / Surface and Coatings Technology 97 (1997) 158–162
one rate limiting step within a complex reaction for a certain temperature regime shown in Fig. 5. The etch rate of SiO is far less sensitive to temper2 ature. Our results are in contrast to former results found by Veprek and Webb [6 ] and by Wang [4] where etching of SiO starts above 400 °C increasing with temperature 2 using a pure H direct current discharge plasma. The 2 reason is again the proposed surface heating. Hence, the heat from the radiation oven plays only a minor role for the etch rate of SiO . 2 The concentration of atomic hydrogen increases with discharge current and H flow. For both parameters we 2 found that the etch rates of SiO increase accordingly. 2 However, compared with other studies [4], an increase of the Si etch rate with increasing discharge current could not be detected. One reason might be the formation of ArH+ ions in the plasma which changes the chemical effectiveness of hydrogen differently for Si and SiO . 2
5. Conclusions Low energy hydrogen plasma etching of Si and SiO 2 as a substrate cleaning process has been characterised with respect to temperature, arc current and hydrogen partial pressure. The large process window makes the plasma source a practicable and reliable cleaning device
under UHV conditions. By changing the arc current as well as the substrate temperature the etching selectivity can be easily adjusted, which offers interesting applications for microstructural patterning.
Acknowledgement The authors thank J. Messarosch, G. Fehlauer and H. Geiger for their assistance in sample preparation and fruitful discussions. The expertise of J. Ramm, E. Beck and R. Slepicka of Balzers AG, Liechtenstein, as well as of C. Wang of the University of Texas at Austin is gratefully acknowledged. This work was supported by ¨ the Bundesministerium fur Bildung und Forschung (Nanoelektronik).
References [1] W. Hansch, E. Hammerl, W. Kiunke, I. Eisele, J. Ramm, E. Beck, Jpn. J. Appl. Phys., Part 1 33 (4B) (1994) 2263–2267. ¨ [2] W. Hansch, I. Eisele, H. Kibbel, U. Konig, Mater. Res. Symp. Proc. 386 (1995) 345–350. [3] A. Strass, W. Hansch, P. Bieringer, A. Fischer, I. Eisele, Vacuum. in press. ¨ ¨ [4] C. Wang, Dissertation, Technische Universitat Munchen, Munich, 1993. [5] J. Abrefah, D.R. Olander, Surf. Sci. 209 (1989) 291–313. [6 ] S. Veprek, A.P. Webb, Proc. 4th Int. Symp. on Plasma Chemistry, ¨ Zurich, 1979, p. 79.
Etching characteristics of Si and SiO with a low energy argon/hydrogen 2 d.c. plasma source A. Strass *, W. Hansch, P. Bieringer, A. Neubecker, F. Kaesen, A. Fischer, I. Eisele ¨ ¨ ¨ ¨ ¨ Universitat der Bundeswehr Munchen, Fakultat fur Elektrotechnik, Institut fur Physik, Werner-Heisenberg-Weg 39, D-85579 Neubiberg, Germany
Abstract The effect of a low energy argon/hydrogen arc discharge on the etch rate of silicon and silicon dioxide is investigated with respect to ultrahigh vacuum ( UHV ) wafer cleaning. At room temperature, the reaction speed depends on the partial pressure of the H radicals and the discharge current. Furthermore, the etch rates show Arrhenius dependence versus substrate temperature with negative activation energies of −1.7 kcal mol−1 and −0.7 kcal mol−1 for Si and SiO , respectively. The process window to 2 operate the plasma source is very large, thereby showing a high grade of reliability for standard wafer cleaning under UHV conditions. © 1997 Elsevier Science S.A. Keywords: Plasma etching; Atomic hydrogen; Wafer cleaning; UHV processing
1. Introduction
2. Experimental set-up
Future semiconductor devices will show dimensions below 100 nm. The fabrication of these devices is critical concerning thermal budget and avoidance of nanoparticle contamination to receive good device quality, high yield and reliability. A crucial step is substrate cleaning before and during device fabrication. Low energy hydrogen plasma etching offers important advantages over widely used wet chemical cleaning processes for Si wafers because it is ultrahigh vacuum ( UHV ) compatible, very suitable to patterned substrates and it effectively removes native oxides and organic contaminants at room temperature [1]. A significant increase in device performance of pin and TBD diodes grown by molecular beam epitaxy (MBE ) on UHV plasma cleaned substrates was achieved compared with growth on RCA cleaned wafers [2]. In this paper we compare the etch rates of Si(100) and SiO (on the same wafer) with regard to standard 2 UHV wafer cleaning and microstructural patterning. We used a plasma source recently developed by Balzers Ltd. to create a low energy argon/hydrogen arc discharge in an UHV chamber. The process is used to clean substrates for the MBE growth of nanoelectronic devices such as vertical metal/oxide/semiconductor field effect transistors with channel lengths below 100 nm.
The process chamber described above is part of the Modular UHV Multichamber System (MUM 545) from Balzers AG, Liechtenstein. The arrangement of the system is described elsewhere [1]. We designed the chamber, which is cooled by water, for in situ plasma cleaning of substrates before further processing such as MBE, the growth of thin electrical insulating films by plasma enhanced evaporation [3] or by plasma enhanced chemical vapour deposition with stable operation conditions and a large process window ( Fig. 1). The turbomolecular pump has a high pumping speed of 2200 l s−1 to quickly remove the gas after the process. Both, the composition of the residual gas as well as ions of the plasma were measured by a QMG 420 quadrupole mass spectrometer (Balzers). A pyramid-shaped radiation oven with halogen lamps (radiation temperature: 3000 K ) for wafer heating up to 600 °C was developed for low temperature plasma processes. The oven is not positioned in the UHV to avoid contamination by any damage of lamps or by reactions of plasma-induced radicals with the hot heater material (e.g. graphite). Furthermore, a service is more easily practicable. The radiation of the lamps, that are arranged in concentric rings, passes a glass window and is lead through a metal tube to the wafer. Each lamp contributes nearly the same amount to the substrate
* Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 14 4 - 8
A. Strass et al. / Surface and Coatings Technology 97 (1997) 158–162
159
for this type of plasma source. After the etch process the residual gas spectrum consists of H , and H O, 2 2 predominantly.
3. Measurements and sample preparation
Fig. 1. UHV chamber for plasma and plasma enhanced processes.
temperature because of an optimal design of vertical and horizontal distance from the wafer. The lamps and the window are cooled by compressed air and water, respectively. The maximum substrate temperature of 600 °C can be reached within 30 s. The temperature of a reference substrate (dummy) was measured before the experiments (under the same conditions) with different methods (thermocouple, IR pyrometer, IR diode) under UHV conditions. The temperature dependence obtained by the thermocouple was used as a gauge curve. We varied the temperature either by dimming or by switching out several lamps. The pressure was controlled with a cold cathode filament of Balzers AG. It worked also with the plasma on. Starting at a base pressure of 5×10−7 Pa, argon is fed into the d.c. plasma source as the working gas for the discharge, while hydrogen is let directly into the chamber. The process pressure amounts to about 5×10−1 Pa and is mainly determined by the flow rate of Ar. An electron arc is created from the negative filament to the chamber walls which act as an anode at ground potential. The discharge current typically amounts to 30 A. The kinetic energy of the electrons lies between 5 and 10 eV [1]. Owing to the absence of potentials above 20 V the damage of the substrate is avoided. The position and shape of the electron beam are influenced by external magnetic coils and by a plate of stainless steal in opposite to the plasma source beneath the substrate holder. During the plasma process the following ions are mainly detected by the QMG: H+, H+, H+ Ar+, 2 3 Ar2+ and ArH+. During the measurement the ionisation unit of the analyser is turned off to be sure to measure exclusively plasma induced primary ions. It should be noted that the presence of argon-related species is typical
To determine both the Si and SiO etch rates on the 2 same wafer, several oxide patterns were created on the entire Si wafer surface. For this purpose, the wafers were wet thermally oxidised and then structured by a photolithographic process. The oxide not covered by the resist was removed with HF. Finally, the resist was removed with boiling acetone. The measurements of the thickness and the refractive index of the oxide layers were carried out at fixed wavelength (633 nm) with a PLASMOS SD 2300 ellipsometer. The system was used in N-fix and N-float mode so that both the refractive index as well as the thickness of the layers could be determined with good accuracy at different locations on the wafer. The validity of the thickness measurements was confirmed by comparison with the results obtained from a mechanical profilometer. After oxide removal, the remaining Si steps were measured again with the mechanical profilometer.
4. Results and discussion The etch rates of both materials, Si and SiO , depend 2 on such parameters as partial H pressure, wafer poten2 tial and arc current. At room temperature the etch rate of Si is five to ten times higher than that of SiO . SiO 2 2 shows higher etch rates with increasing partial H pres2 sure as well as of the arc current ( Figs. 2 and 3). The enlarged error bars in the etch rate of Si result from the limited accuracy of the mechanical profilometer. The SiO layers could be measured with higher accuracy by 2 using the ellipsometer. A saturation effect occurs at high H fluxes where the small mean free path of atomic 2 hydrogen atoms limits the etching effectiveness. The total pressure increased with H flow because the flow 2 of Ar was kept constant in all experiments. To understand better the etching process, experiments were done with grounded and floating samples. With the substrate left floating a negative potential of about 20 V was measured to ground owing to the electron discharge. This enhances the soft bombardment with positive ions such as ArH+ causing surface heating due to the high momentum of these heavy species. For higher substrate temperatures—as discussed later—the SiO etch rate decreases. At higher H pressures, more 2 2 ArH+ is formed, resulting in an enhanced surface temperature. For grounded substrates, the attraction of the substrate for ArH+ ions is less than for the floating substrate (−20 V ). Therefore, the grounded substrate
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Fig. 2. Etch rate of Si and SiO vs. H partial pressure on floating and grounded wafers. 2 2
(at lower surface temperature) shows higher etch rates. No influence of the wafer potential on the etch rates of Si was observed under our experimental conditions. We suppose the reason is the high reactivity of the surface dangling bonds. Surface heating does not play an important role at these H partial pressures. 2 After the etching process, the surface homogeneity over the complete 3 in wafer area reached ±10%,
typically. This can be improved by rotation of the wafer and by a sweeping electron beam. The plot of the logarithm of the etch rates versus the inverse temperature ( Fig. 4) indicates Arrhenius dependence. The activation energy amounts to −1.7 kcal mol−1 for Si and to −0.7 kcal mol−1 for SiO . Above 300 °C (1.7×10−3 K−1) the activation 2 energy for Si etching becomes a higher negative
Fig. 3. Etch rate of Si and SiO vs. discharge current. 2
A. Strass et al. / Surface and Coatings Technology 97 (1997) 158–162
161
Fig. 4. Etch rate of Si and SiO vs. substrate temperature. 2
value: −21 kcal mol−1. The effect of ion bombardment can be excluded because of the low energies of the particles. The activation energy is the amount of energy that has to be brought to a molecule or to a compound to reach a transient state before the chemical reaction. This transient product is in an energetically excited state and thereby more reactive. The activation energy is unique for a single step reaction. For etching of Si and SiO it 2 is an indication that there exists a rate determining step within a complex reaction and hence can deliver some useful information about the type of step which is rate determining. The interaction of atomic hydrogen with Si and SiO surfaces is little understood. A detailed discussion 2 is beyond the scope of this paper. We suggest the following model as illustrated in Fig. 5 [4,5]. It is well known that H radicals are adsorbed by the dangling bonds of the Si surface forming a layer of strongly bound SiH(ads) (reaction (1)). The formation of SiH (ads) (reaction (2)) is enhanced by the influence of 2 reactive ions such as H+ and ArH+. The SiH (ads) 3 2 complex reacts further with a weakly bound H atom to SiH (ads). The forward reactions (4) and (5) lead to 3 etching of Si where gaseous SiH is formed as primary 4 product. Three competitive reactions lower the etch rate of Si by decreasing the SiH (ads) concentration: the 3 decomposition of SiH (ads) (4) and (7), the subtraction 3 mechanism (6). The reduction to SiH (ads) is the rate 3 limiting step for silane production [5]. At temperatures above 300 °C, however, the desorption of hydrogen
from the SiH(ads) state seems to decrease the etch rate, predominantly ( Fig. 4). From the literature it is known that the desorption process occurs between 400 and 600 °C [6 ]. We suppose that this difference can be explained by the existence of ArH+ which owing to its momentum transfer elevates the surface temperature. In Ref. [4] Wang found an Arrhenius dependence for considering ion bombardment, however, only for temperatures above 100 °C. In our experiments two regimes of Arrhenius dependence with different activation energies occur. For temperatures above 300 °C the activation energy amounts to −21 kcal mol−1; below 300 °C we found −1.7 kcal mol−1 which compares well with the result of Wang [4]. This gives indication that there is
Fig. 5. Chemical reactions leading to etching of the Si surface.
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A. Strass et al. / Surface and Coatings Technology 97 (1997) 158–162
one rate limiting step within a complex reaction for a certain temperature regime shown in Fig. 5. The etch rate of SiO is far less sensitive to temper2 ature. Our results are in contrast to former results found by Veprek and Webb [6 ] and by Wang [4] where etching of SiO starts above 400 °C increasing with temperature 2 using a pure H direct current discharge plasma. The 2 reason is again the proposed surface heating. Hence, the heat from the radiation oven plays only a minor role for the etch rate of SiO . 2 The concentration of atomic hydrogen increases with discharge current and H flow. For both parameters we 2 found that the etch rates of SiO increase accordingly. 2 However, compared with other studies [4], an increase of the Si etch rate with increasing discharge current could not be detected. One reason might be the formation of ArH+ ions in the plasma which changes the chemical effectiveness of hydrogen differently for Si and SiO . 2
5. Conclusions Low energy hydrogen plasma etching of Si and SiO 2 as a substrate cleaning process has been characterised with respect to temperature, arc current and hydrogen partial pressure. The large process window makes the plasma source a practicable and reliable cleaning device
under UHV conditions. By changing the arc current as well as the substrate temperature the etching selectivity can be easily adjusted, which offers interesting applications for microstructural patterning.
Acknowledgement The authors thank J. Messarosch, G. Fehlauer and H. Geiger for their assistance in sample preparation and fruitful discussions. The expertise of J. Ramm, E. Beck and R. Slepicka of Balzers AG, Liechtenstein, as well as of C. Wang of the University of Texas at Austin is gratefully acknowledged. This work was supported by ¨ the Bundesministerium fur Bildung und Forschung (Nanoelektronik).
References [1] W. Hansch, E. Hammerl, W. Kiunke, I. Eisele, J. Ramm, E. Beck, Jpn. J. Appl. Phys., Part 1 33 (4B) (1994) 2263–2267. ¨ [2] W. Hansch, I. Eisele, H. Kibbel, U. Konig, Mater. Res. Symp. Proc. 386 (1995) 345–350. [3] A. Strass, W. Hansch, P. Bieringer, A. Fischer, I. Eisele, Vacuum. in press. ¨ ¨ [4] C. Wang, Dissertation, Technische Universitat Munchen, Munich, 1993. [5] J. Abrefah, D.R. Olander, Surf. Sci. 209 (1989) 291–313. [6 ] S. Veprek, A.P. Webb, Proc. 4th Int. Symp. on Plasma Chemistry, ¨ Zurich, 1979, p. 79.