Poly-SiGe is a promising material for modular integration of MEMS with electronics due to its low process thermal budget. Poly-SiGe can be deposited on top of standard CMOS electronics in a LPCVD reactor as a replacement of poly-Si as MEMS structure material. Deposition temperature at 450 °C along with 550 °C rapid thermal annealing can result good electrical and mechanical properties of the SiGe film. SiGe films deposited at lower temperature have degradations in mechanical properties, such as high compressive stress and large stress gradient. To further decrease the thermal budget of SiGe depositions but maintain their good mechanical properties, excimer laser annealing is investigated in this work.
Two laser annealing processes have been developed for stress tuning of the SiGe films. Laser annealing was applied to boron doped bi-layer SiGe films. For the ELA-middle process, laser annealing is the middle step of the bi-layer deposition; for the ELA-last process, laser annealing is the last step after the bi-layer deposition. As-deposited SiGe films have compressive stress in most cases. The average stress becomes more tensile after laser annealing. Most of the single layer as-deposited films have positive strain gradient, resulting in upward curvature of released cantilevers. An appropriate combination of bi-layer depositions together with laser annealing can reduce the average stress and eliminate the strain gradient of the film. With the ELA-last process, the lowest average stress achieved is 7 MPa tensile and the lowest strain gradient achieved is 1×10-5μ m-1. Comparing the two laser annealing processes, ELA-last process is simpler and also has advantages in process control.
Besides the experimental results, this report also discusses the operation and limitation of the equipment. SiGe LPCVD process can be relatively stable with careful monitoring of the injector condition. However, there is always small variation due the drift of the injector over time. In addition, the excimer laser has pulse-to-pulse energy variation and its homogenizer array has non-uniform output.
June 30, 2004
Low, C. W. (2004). Excimer Laser Annealing of Silicon Germanium for MEMS Applications: Research Report. United States: University of California, Berkeley.