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. Therefore, the n-layer is not etched during electrochemical etching. As there is chemical etching rate in the propose silicon etchant, two step etching is used. First, almost of the silicon substrate is etched in a solution of HF : $HNO_3$ : $H_2O$, and the rest of silicon substrate is etched in a solution of HF and $H_2O$.
The self-aligning property is related with the resistivity of the silicon substrate and high resistivity substrate shows more positive feedback effect which results in self-aligning property. Experimental and computer simulation results proves the relation between self-aligning property and the resistivity of the silicon substrate.
Proposed electrochemical etching method is applied to an inkjet printhead and a crystalline Active-Matrix LCD. Conventional inkjet printhead has limitations in density and cost because it is fabricated in hybrid type. The proposed self-aligning electrochemical etching method is applied in a monolithic thermal inkjet printhead in forming main ink feedthrough. The inkjet printhead can be made monolithically because the main ink feedthrough can be formed after all the processes are finished. Conventional AMLCD has weak point to integrate the driver circuit with the switching device because the pixel switching device is fabricated on the amorphous or poly-crystalline silicon. Though the property of the single crystal silicon is superior to the that of a-Si or poly-Silicon, there is a little study of the single crystal silicon in AMLCD because the c-Si cannot be grown on the glass or quartz. Crystalline silicon AMLCD is proposed utilizing the proposed electrochemical etching method. Only the transparent area in the pixel is etched accurately using the proposed method. High density and integration of driver circuit are possible in the proposed single crystal silicon AMLCD.
