Study of Conditions for Anisotropic Plasma Etching of Tungsten and Tungsten Nitride Using SF[sub 6]/Ar Gas Mixtures

Abstract

Results of the study of reactive ion etching of tungsten, tungsten nitride, and silicon in SF 6 /Ar gas mixtures are presented. For plasma diagnostics, optical emission spectroscopy ͑actinometry͒ was used. Using the actinometry technique, it was possible to show that etching mechanisms were different for Si-F and W-F chemistries. Anisotropic etching of tungsten/tungsten nitride using conventional reactive ion etcher has been obtained, and conditions of achieving anisotropic etching have been analyzed. A correlation is found between anisotropy of tungsten etching and the ratio of Si/W etch rates. Mechanisms of fluorine redistribution between the bottom/sidewall surfaces due to surface diffusion and/or reflection are proposed as possible reasons for the observed correlation.The use of tungsten and tungsten nitride thin films in microfabrication attracts much attention due to their high thermal stability, good adherence ͑for tungsten nitride͒, and low sheet resistance. Due to the continuous shrinking of microdevice dimensions, demands on the dimensional control and anisotropy of etch profiles are constantly increasing. For etching of W and WN x films, fluorinecontaining gases ͑in particular SF 6 or CF 4 ͒ are commonly used which provide high etch rates. 1-4 For conventional reactive ion etching ͑RIE͒ reactors, considerable undercutting or formation of concave sidewall profiles due to spontaneous etching of tungsten by fluorine in fluorine-containing plasmas was usually reported. 1-3 To avoid this, various techniques have been employed including passivation of sidewalls by carbon-containing species using polymerforming gases 1,4 or substrate cooling. 5 Anisotropic etching was obtained in SF 6 /Ar using low-pressure high-density etchers characterized by relatively high ion-to-radical flux ratios. 2 Usually low anisotropy is achieved with conventional low-density ͑i.e., a low ion-to-radical flux ratio͒ RIE etchers. In some cases, anisotropic etching in RIE experiments was obtained. 3 However, conditions for anisotropic tungsten etching still are not well understood. The objective of the present study was to find and to model conditions of anisotropic etching of W and WN x in a SF 6 /Ar plasma at normal temperatures, without applying sidewall passivation techniques. A conventional RIE medium pressure reactor was used in this study.The characteristic of a RIE process is a specific combination of chemical and physical ͑ion-induced͒ etching mechanisms. The relative contribution of chemical/physical components of the process determines the profile and anisotropy of the etched features. For a better understanding of etching mechanisms it is important to characterize quantitatively particle fluxes at the surface. Here, in order to characterize the density of fluorine radicals in the plasma, an optical emission spectroscopy ͑the actinometry technique͒ 6 was employed.In order to provide considerable variation of radical/ion fluxes to the processed surfaces, the SF 6 /Ar flow ratio was varied widely in the experiments. The speci...

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