A novel micromachining technology for multilevel structures of silicon

A novel micromachining technology for multilevel structures of silicon is described in this paper. Complicated multilevel structures can be formed by using a maskless etching following a conventional masked etching in aqueous KOH. The process consists of two successive steps. First, structures with (111) sidewalls are formed for a desired etching depth on the surface of a (100) silicon wafer by a conventional masked anisotropic etching process with a specially designed etching mask. Then, the etching mask is removed except for some areas (such as the frame of the chip) and a maskless etching follows. In addition to the downward etching of the upper and lower (100) planes during maskless etching, fast-etching (311) planes emerge at the upper edges of the (111) sidewalls and cut the (111) sidewalls, while the (111) sidewalls extend downward at the lower ends. As the (311) planes replace the (111) sidewalls with a rate faster than the extension rate of the (111) planes, the (111) sidewalls will finally be replaced by (311) planes. Moreover, by an appropriate mask design and an appropriate etching depth of the previous masked etching, the bottom areas of some etching cavities diminish temporarily so that the downward etching in these areas is stopped for time intervals after the intersection of (111) planes on the bottom and before the (111) planes are replaced completely by the (311) planes. Once the (111) sidewalls are replaced completely by the (311) planes, the bottom areas reappear and the etching downward resumes. Analyses show that levels with different depths can be created in this way for different window apertures and the number, positions as well as the depths of the new levels can be individually decided by the design of the single mask for masked etching and the etch depths of both masked and maskless etching processes. More than 10 interesting multilevel structures with up to five structure levels have been fabricated. The results show that the technology is very versatile and repeatable. Therefore, this technology is a very useful new tool for making silicon micromechanical structures to meet the ever-increasing demands for solid-state sensors and actuators. (C) 1997 Elsevier Science S.A.