Optimization of corner compensations in wet etching of silicon for a MEMS Z-axis accelerometer

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Fabrication of planar MEMS accelerometers using micromachining and wet etching of surface structures on monocrystalline silicon wafer substrates poses several problems. One such problem is the preferential undercut of convex corners in the structure during chemical etch of silicon wafers. Resultant structures after etching are no longer the same as the design structure, leading to degradation in the performance of the device. The preferential undercut arises due to ‘wet anisotropic etching’ wherein the etchant has different etch rates along different crystal planes of the substrate. One of the methods to minimise this preferential undercut is to use compensation extension structures in the planar geometry of the device. In this paper, an accelerometer structure using a square extension has been studied in detail and the resulting dimensions are experimentally validated through fabrication and tests. Design steps and selection of optimal dimensions of the square compensation geometry are discussed considering etch depth with minimal undercut using KOH etching solution. Several samples of structures with three different dimensions of square extensions were fabricated and resultant device geometry was studied to establish the optimal extension dimensions with minimal undercut for the desired depth of etch.