Catalyzed Gaseous Etching of Silicon
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CATALYZED GASEOUS ETCHING OF SILICON
N. SELAMOGLU, J. A. MUCHA, D. L. FLAMM AND D. E. IBBOTSON AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT Etching of silicon by molecular fluorine is accelerated when trace quantities of copper are present on the surface. Copper is a residue formed when sputter-deposited aluminum (containing 0.5 % copper) is selectively removed by HF from the silicon surface. The temperature dependence of the etching rate was studied in the range 60-290"C. At temperatures higher than 80"C copper causes a -100-fold increase in the rate of etching of the underlying silicon (100), compared to unmetallized samples. Above 180'C, F2 exhibits a higher absolute etch rate than equivalent concentrations of fluorine atoms. Preliminary results for other metal contaminants and etchant gases indicate that silver also accelerates F2 etching, and copper enhances etching by NF 3 . The results are interpreted in terms of a catalytic mechanism.
INTRODUCTION Surface condition and surface contamination effects on gaseous etching have not been extensively studied. Generally, only inhibiting effects have been reported; however, there are a few examples of acceleration of silicon etching. The formation of "wormholes" in silicon below 300"C in the presence of tungsten and WF 6 has been interpreted [1] by a mechanism involving a vapor-solid-solid transport process with metal silicide as a likely intermediate. This is reminiscent of the SLV (solid-liquid-vapor) process described by Wagner [21 to explain the high temperature (750'C) etching of silicon and germanium by HCI in the presence of Au, Ni, and Pt. More directly related to plasma etching, alkali metal residues were shown to enhance the etching of silicon in CF 4/0 2 plasmas [3,41. This was attributed [41 to the formation of Na 2SiF 6 and its decomposition to NaF and SiF 4 . Finally, the well-known enhancement of silicon etching in Cl-containing plasmas owing to the "doping effect" of n-type impurities [5] might loosely be considered a surface effect. Here, n-type doping raises the Fermi level and reduces the barrier to electron transfer from silicon to chlorine. As a result, incident atoms are more readily chemisorbed and the etch rate increases [61. We report a new effect on the etching of silicon by reactive inorganic fluorine-bearing gases. When traces of copper are present on a silicon surface, the etch rate is greatly enhanced. For instance, we measured the etch rate of clean undoped silicon in F2 at 1 Torr and 200"C to be about 2000 A/min.; but when traces of copper were present it increased about 100-fold, to 20Am/min., The enhancement is interpreted in terms of a catalytic cycling of copper oxidation states that increases the efficiency of fluorine transfer to silicon.
Mat. Res. Soc. Syrup. Proc. Vol. 75. '1987 Materials Research Society
460
EXPERIMENTAL METHODS Typically, 4000X of thermal Si0 2 was grown on 4-inch diameter (100) silicon wafers and a resist pattern was formed by photolithography. This pattern was then transferred to the SiO 2
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