A Study of the Precise Alignment of Mask Patterns to the Crystallographic Orientation of Silicon Wafers

In the silicon wet anisotropic etching, which is usually performed in alkaline solutions (e.g. potassium hydroxide (KOH), Tetramethylammonium hydroxide (TMAH), etc.), the etch rate is highly orientation-dependent. {111}Si planes are slowest etch rate planes in all kinds of anisotropic etchants. In order to align the mask patterns with respect to the crystallographic directions on silicon wafer, primary flat is commonly employed as reference. In this case, any degree of misorientation in primary flat leads to the misalignment of mask patterns with respect to crystallographic directions. A small degree of misalignment of the mask edge with crystallographic direction result in oversized microstructure due to the underetching at the misaligned mask edges. Hence, in the fabrication of silicon-based microelectromechanical system (MEMS) structures using wet etching, a high precision alignment of mask pattern to crystal orientation is desirable in order to control the dimensions of fabricated structures. Several studies have been performed for the precise alignment of mask patterns with respect to crystallographic directions on {110} and {100} silicon wafers. All these techniques are based on the development of mask patterns to create the pre-etched pattern for the identification of crystallographic directions, for instance, direction on {100}Si wafers. In the existing methods, mask design for pre-etched patterns varies with wafer orientation.