Study of polyimide as sacrificial layer with O2 plasma releasing for its application in MEMS capacitive FPA fabrication

Polyimide (PI) was a good candidate as the sacrificial layer for its compatibility with CMOS technology. This paper first presented a new patterning method of PI film and then investigated the relationships among undercut rate, the undercut limit length and the releasing hole size in the releasing step, which was helpful and important for its popularity and its application in MEMS capacitive FPA (Focal plane array) fabrication. A new patterning approach of PI film was successfully developed in ICP chamber. The patterning approach selected a PECVD SiO₂ layer as patterning mask. The optimized ICP PI recipe was composed of an O₂ flow of 180 sccm, an electrode power of 400 W and bias plate power 200 W. With the optimized ICP PI recipe, a vertical etching rate about 0.5-0.6 mum/min with a lateral etching rate 0.13 mum/min was realized. With 1 mum PI sacrificial layer, a 0.22 mum/min undercut rate was achieved in a normal barrel etcher. Based on our experimental facts, the optimized releasing hole size was 5 mumtimes5 mum and the distance between lateral releasing holes should be fewer than 17 mum for effectively and completely releasing.