Multilayer Masking Technique for Deep Isotropic Silicon Wet Etching

A multilayer masking technique was presented aiming at the requirements of deep isotropic silicon wet etching. Because the processing time of deep etching is relatively long and etching rate is high, it is very hard to achieve satisfying etching result by using conventional photoresist or metal single layer mask. Thus multilayer mask consisting of photoresist and metal layers is fabricated to exert respective advantages and avoid disadvantages. Based on its excellent chemical and thermal stabilities and high viscosity, Su-8 was selected as the material of photoresist layer. The metal layer was fabricated by chromium because it could alleviate the undercut problem in great extent. Results of etching experiment indicated that no obvious defect of pinhole or crack was found on this multilayer mask after etching to the depth of about 300μm. Thus it is undoubted this masking technology is capable for deep silicon wet etching.