Control of Isotropic and Anisotropic Etching and Surface Cleaning of Silicon and Silicon Dioxide in a Hydrogen Plasma
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CONTROL OF ISOTROPIC AND ANISOTROPIC ETCHING AND SURFACE CLEANING OF SILICON AND SILICON DIOXIDE IN A HYDROGEN PLASMA S. Veprek, Ch. Wang and G. Ratz, Institute for Chemistry of Information Recording, Technical University Munich, Lichtenbergstrasse 4, D-8046 GarchingMunich, Federal Republic of Germany ABSTRACT We present data on the temperature dependence of the etch rate of silicon and silicon dioxide in order to elucidate optimum conditions for the selective oxygen removal from the silicon surface. Both, the etching temperature and ion bombardment have a pronounced influence on the surface morphology. The conditions yielding a minimum surface roughness will be presented. A careful control of the oxygen impurities of the hydrogen plasma in the range between about 1-3 ppm and 60 ppm allow us to control the degree of anisotropy of etching of patterned silicon wafers. INTRODUCTION Dry cleaning of silicon by atomic hydrogen attracted the attention of several research groups because of its potential application for in-situ surface treatment prior to the low temperature epitaxy [1-9]. A number of earlier papers provides some insight into the mechanism of the etching of silicon (e.g. [10-14]) and silicon dioxide [15-17] with atomic hydrogen. The data published in the earlier papers [10-12] show that the etching of silicon requires a very low impurity content in the hydrogen plasma because of the different temperature dependence of the etch rate of silicon and silicon dioxide. The etch rate of silicon in a clean hydrogen plasma shows a pronounced maximum around 60 - 800 C (depending on the plasma density [11,12]), it decreases at higher temperatures and becomes almost unmeasurable above about 400 0 C. These results have been recently confirmed in UHV studies of the reaction of silicon with thermally produced H-atoms [13,14]. In contrast, the etching of silicon dioxide in a hydrogen plasma occurs only at elevated temperature of ?400 0 C [15-17]. Therefore, small oxygen impurity can contaminate the silicon surface and prevent the etching at low temperatures. At high temperatures, the oxide can be removed but the etching of silicon stops. A perfectly anisotropic etching of patterned single crystal silicon in a glow discharge under low energy electron bombardment has been reported by Veprek and Sarott some time ago Mat. Res. Soc. Symp. Proc. Vol. 282. 01993 Materials Research Society
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[12]. In view of more recent results to be reported here, the anisotropy arises probably due to side wall passivation by minor oxygen impurities. The effect of the ion bombardment on the etching is a complex process; we refer to some previous papers [18,19] for details. With respect to the surface cleaning it has a pronounced effect on the decrease of the surface roughness as it will be shown here. We shall further demonstrate that, in analogy with the effect of the electron bombardment [12], the ion bombardment in combination with minor oxygen impurities in the range of a few 10 ppm results in highly anisotropic etching of patterned silic
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