Stress control in masking layers for deep wet micromachining of Pyrex glass

In this paper, we report three etching masking technologies of Cr/Au, Cr/Cu and PECVD amorphous silicon developed for Pyrex glass micromachining in hydrofluoric acid solution. Our study reveals that the residual stress, especially the tensile stress, in the mask layers is responsible for the pinholes on the glass surface and notch defects on the edge formed at the etch edges of glass due to the breakage of highly stressed mask layers during the etching process. The Cr/Au metal mask can achieve a glass etch depth up to 100 mum, along with a number of pinholes and notch defects on the edge due to the high tensile residual stress in the Cr/Au layer. The Cr/Cu metal masking layer improves the glass etch quality by the reduced residual stress. Detailed studies have been done using the amorphous silicon film as a glass etch mask. The PECVD process and the subsequent annealing process have been optimized to reduce the compressive residual stress in the amorphous silicon layer. The maximum etch depth in the glass can be as high as 200 mum and almost without pinholes and notch defects on the edge. To our knowledge, this is the best result reported in the literature so far. (C) 2004 Elsevier B.V. All rights reserved.