The recently developed High Aspect Ratio Si Etch (HARSE) process is widely used for applications requiring silicon structures with high aspect ratios. This process relies on the alternation of sidewall passivation and silicon etching phases and enables the obtainment of high silicon etch rates and highly anisotropic pro®les. This paper reports an innovative approach to improve the sidewall roughness through a multiple-step HARSE process using an ICP system. Unlike the standard HARSE process, the etching conditions for this new process are gradually altered in order to reinforce the silicon etch ef®ciency as a function of the silicon depth previously etched. Trenches with aspect ratios as high as 40 can be achieved. The sidewall roughness along the entire etching depth is less than 8 nm rms. In comparison with the standard HARSE process in which ripples appear on the trenches sidewall, the sidewall roughness is improved by a factor of 4.
IntroductionThe recently developed High Aspect Ratio Si Etch (HA-RSE) process, e.g. La Èrmer and Schilp (1996), ensures the achievment of features with high depth-to-width aspect ratios, anisotropic pro®les, high Si etch rates and good dimensional control at room temperature. The HARSE process relies on the sidewall passivation technique. The approach is to use a cyclic method composed of deposition and etch phases for each cycle. It requires the use of a high density plasma source such as Inductively Coupled Plasma (ICP). Fluorine-based chemistry is used to etch silicon by producing volatile etch products. Since this etching actually has an intrinsically isotropic behaviour, the deposition step enables a maximum reduction of the lateral silicon etching. The desired silicon etching anisotropy is then obtained. Nevertheless, the formation of wavy sidewalls, e.g. Adams et al. (1997), can be restricting for many applications requiring very smooth sidewall surfaces. Some investigations were done, e.g. Hynes et al. (1999), in order to lessen these undulations. To completely eliminate these ripples for high aspect ratio features, we successfully developed a new multiple-step HARSE process using an ICP system. This paper presents the principle of etching, results of silicon etching and measurements of the sidewall roughness obtained by this multiple-step HARSE process. Comparisons are also made with the standard HARSE process.
Multiple-step HARSE processThe new multiple-step HARSE process to etch silicon anisotropically uses the same basic principle that the standard HARSE process does; alternating deposition phase and etch phase. Whereas the parameters of the HARSE process are constant throughout the entire silicon etching, the different parameters of etching for the multiple-step process are modi®ed progressively during the process. The principle consists of gradually changing the deposition/ etch balance in favour of the etch phase to reinforce the silicon etch ef®ciency.The different physical and chemical effects controlling the silicon etching behaviour can be varied by some externa...
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