Selective and deep etching of SiO/sub 2/ in high density low pressure fluorocarbon plasmas

Abstract

Summary form only given. Plasma processes using high-density sources have been extensively developed to meet more and more stringent constraints required by integrated circuits fabrication. Among the various steps, dielectric etching is the more challenging as the process relies on polymerizing hydrofluorocarbon gases that produce simultaneously deposition and etching. It is thus difficult to achieve adequate SiO/sub 2//mask etch selectivity and to continue etching in high aspect ratio features at the same time. More recently, the development of integrated optical components leads to new technological challenges in particular concerning deep etching (= 10 gm). Hence, fabrication of MEMS (micro electro mechanical systems)and O-MEMS (Optical MEMS) requires several conditions: (i) a higher etch rate to reduce the process time (400 nm/min), (ii) an extreme selectivity (> 15), (iii) a much longer etching process. In order to improve SiO/sub 2//Si selectivity, there have been several studies of high density plasma etching processes. Usually, selective etching of silicon oxide with a silicon mask is obtained in RIE plasmas using fluorocarbon gases. The formation of a fluorocarbon film by CF/sub n/ radical deposition on the silicon surface reduces the silicon etch rate whereas oxide etching is weakly affected leading to a considerable improvement of the selectivity. However, the low ion flux generated by the RIE system leads to very low oxide etch rates less than 50 nm/min. In contrast, in high density plasma sources, the high degree of dissociation and the high ionic density enable to reach higher oxide etch rates (> 200 nm/min). Nevertheless, the SiO/sub 2//Si selectivity is strongly affected. The present work is based, in a first step, on the comparison between three fluorocarbon gases (CF/sub 4/, C/sub 2/H/sub 6/, CHF/sub 3/) and their mixture with hydrogen or methane for a better understanding of the selective etching of SiO/sub 2/ using a silicon mask. We show that adding methane instead of hydrogen to any fluorocarbon gas allows to increase considerably the SiO/sub 2//Si selectivity without decreasing too much the oxide etch rate. However, we demonstrate that is not possible to increase both the selectivity and the etch rate by changing the hydrofluorocarbon mixture, and that the only way to obtain this combined increase, is to reduce the gas residence time. Finally, pattern transfer into silicon dioxide with high SiO/sub 2//Si selectivity was performed with success.