Precise Mask Alignment Design to Crystal Orientation of (100) Silicon Wafer Using Wet Anisotropic Etching

摘要

A high precision alignment of mask pattern to crystal orientation is essential for fabricating most microk-mechanical devices. With an accurage alignment on a (100) silicon wafer, the time for obtianing a smoothly etched sidewall surface can be reduced and the undercut phenomenon can also be minimized during the anisotropic etching process, In this article, a series of circules are made as an aligning pattern. These cricles are evenly distributed at each 0.1deg along the smae are of radius 43mm. A wafer is etched in the TMAH solvent for a certain period of time, and the final etched pattern is served as an alignment markl THe present method relies upon the fact that when a (100) silicon wafer is etched for a surfficiencly long period of time in a orientation-dependent etchant through a circular window, the etched out portion will form a cavity of pyramidal shape with its surfaces along the <100> directions because the etching process is almost stopped at the (111) planes. However, this pyramidal cavity will continue to grow along the <111> direction when the etching process is continued. This is cuased by the undercut phenomenon occurred at the contact line of the passivation layer and the Si substrate. Despite of the ciruclar window used on the passivation layer, the top view of the yramidal cavity caused by undercut on the Si substrate will be a square opening. Through a visual inspection from the top view, one can find that the corners of some neighboring square openings contact to each other but some are not after a surfficient time of etching. The contact proximity at the corners of two neighboring square opernings is taken as an accurate rule for the wafer's alignment. Such a pre-aligning pattern allows one to determine the <100> orientation within accuracy of 0.05deg.