Self-aligning silicon micromachining using electrochemical etching and its applications

Abstract

In recent years, significant interest has been shown in silicon micromachining and specifically silicon etching. The increasing importance of silicon micromachining, especially bulk micromachining, has generated growing interest in etching through silicon substrate while maintaining precise control of microstructural dimensions and position. To date, conventional orientation-dependent etching methods have been widely used to etch the silicon substrate. Conventional etching methods, however, the final size of the etched hole cannot be defined exactly on the front side of the silicon substrate because the pattern of etching is formed on the back side of the silicon substrate. Furthermore, the pattern requires exact alignment to the specific crystallographic axes of silicon and the final hole size is dependent on the thickness of the silicon substrate. Another disadvantages is that only rectangular geometry of a hole can be realized on the silicon substrate. In this thesis, a new silicon etching method is proposed to overcome the disadvantages of the conventional orientation dependent etching methods. The pattern of etching is defined on the front side of the silicon substrate while the silicon etching takes place on the back side of the substrate. Metal layer is deposited on the patterned silicon substrate so that only the pattern area is electrically contacted to the metal layer. Only the back side of the substrate is in contact with the silicon etchant - 1 : 1 : 1 HF(48-51%) : $HNO_3(69-71%)$ : $H_2O(DI water)$ by volume - and there is no treatment on the back side. Silicon etching proceeds from the back side of the substrate by applying a current and the etched area from the back side converges gradually to the front side hole pattern. This self-aligning property enables the formation of arbitrary shaped holes. Non-conductive n-layer is formed around the pattern area to enhance the resolution because the applied current cannot flows into the n-layer. Therefo...