A New Method for Analyzing Thin Sidewall Inhibitor Layers
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A New Method for Analyzing Thin Sidewall Inhibitor Layers J.P. McVittie, T.A. Un and A.J. Bariya Stanford Integrated Circuits Laboratory and Dept. of Chemical Engineering Stanford University, Stanford, CA 94305
Abstract Etch inhibitorlayers are the key to anisotropy for a number of dry etch processes,yet little is known about these layers because of the difficulty in analyzing them on the side walls where they form. In this paper we will show that an Al grid suspended above an etching surface can be used to suppress ion bombardment and allow the inhibitorto form on large horizontalsurfaces which can be easily analyzed. The effect of ion bombardment on the nature of the inhibitor layer can also be elucidated using this technique. In conjunction with X-ray Photoelectron Spectroscopy, this method was used to look at the polymeric inhibitor formed during Si etching in SF6'C2 ClF5 with and without the presence of photoresist.
Introduction The discharge that is struck during plasma etching is a complex environment containing a myriad of chemical species. Some of these species can undergo polymerization reactions leading to the formation of polymeric residues on surfaces exposed to the discharge. These residues play an important role in the etch process. They may affect subsequent processing steps and may require additional steps for their removal. Often, they are responsible for obtaining good selectivities. Finally, they may act as inhibitor layers, reducing the etch rate. These inhibitor layers are removed, or are modified, by ions that bombard any surface exposed to the discharge. The ion flux is directed normally onto the surface so that residues that form on horizontal surfaces are removed, whereas those that form on sidewalls are little affected. This interaction between ion bombardment and the inhibitor layers leads to anisotropic etch profiles, as illustrated in Fig 1. Despite the widespread use of sidewall layers for anisotropic etching of poly-Si[1], silicides and aluminum[2], little is known about their properties. In most cases these layers are believed to be composed of halocarbon polymers because their effect on lateral etching is only seen when there is a high carbon to halogen ratio in the source gas, and increases in the carbon to halogen ratio lead to polymer formation on all surfaces. Cobum[3] has suggested that for the case of silicon etching in a mixed halocarbon gas, a thin film of fluoropolymer ( a few monolayers thick) could retard the etch rate by lowering the reaction probability of the etching species. Inorganic sidewall layers also appear to be important in that Danner et al[4] formed SiO 2-like sidewall layers while etching silicon ina mixture of CCl2F2/0 2. The importance of residues in the etch process calls for their better understanding. Further, the effect of ion bombardment on the nature of these residues needs elucidation. This paper describes a technique that may be used to study the effect of ion bombardment on the residues. One of the problems in analyzing the residues is that the
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