Effect of clamping ring materials and chuck temperature on the formation of silicon nanograss in deep RIE

In this paper, we report the effect of clamping ring materials and the chuck temperature on the formation of silicon grass in deep reactive ion etching (RIE), carried out with an Alcatel A601E etching tool. In the beginning, rectangular trenches of varying widths were etched with conventionally used aluminum clamping rings and later with ceramic rings, keeping other parameters unchanged. Scanning electron microscope images of etched trenches reveal that needle-shaped silicon grass structures were formed on the bottom of the trenches etched with an aluminum ring. Formation of silicon grass was more severe in wider trenches and near the wafer edges; however, the bottom silicon grass was eliminated when trenches were etched with a ceramic ring. Formation of silicon grass is mainly due to "micromasking" caused by sputtered aluminum particles. Due to continuous bombardment of highly reactive radicals (O+, CF3+, and other neutral atoms) on the wafer and surrounding aluminum ring, some aluminum atoms are physically displaced and deposited on the wafer. These micrometer-size aluminum particles deposited on the silicon wafer start acting as a mask layer and prevent the etching of the silicon area beneath them. Due to this aluminum mask, the local silicon etching rate is reduced drastically compared to other silicon areas, hence gradually vertical-grass-type silicon structures start forming. Because more aluminum atoms are deposited near the wafer edges, the amount of silicon grass is nearer the edges than in the center. As the ceramic ring is an insulator, it is not polarized by radio frequency power; hence, there is no ion-bombardment phenomenon and no sputtering of foreign particles. Due to the absence of such micromasking, the silicon grass formation is completely eliminated. (c) 2006 The Electrochemical Society.