Pulsed laser thermal annealing on stress eliminating of poly-SiGe film used in MEMS

Abstract

Poly-SiGe is widely used in MEMS applications as structural films for it's similar to poly-Si and compatible with standard CMOS (Al interconnects) backend processing. However, with its large as-deposited stress, it becomes more and more challenging and critical to control stress relaxation for sensitivity improvement of MEMS. In this paper, we demonstrate eliminating the stress in poly-SiGe films used in pressure sensor as membrane by excimer laser thermal annealing (LTA). First, the threshold melting laser energy density (ED th ) of poly-SiGe, the correlation between melting depth to laser ED and the thermal profile are study by simulation. Then the bare and real device wafers are processed by LTA (308 nm, 150 ns). It is demonstrated that stress can be released to the target exactly by setting a proper ED for the stress are increasing linearly with laser ED. And there's no contact open concern for surface temperature of AL is lower than 400C. Furthermore, the laser annealing seems to reduce surface roughness. Significantly, LTA is promising technique for surface properties modification in MEMS applications.