Characterization and strain gradient optimization of PECVD poly-SiGe layers for MEMS applications

Poly-SiGe offers an attractive alternative for low temperature MEMS post-processing above CMOS. This paper presents several investigations made to obtain a crystalline material with excellent mechanical (low stress, low stress gradient) and electrical (low resistivity) properties. Two different techniques were used to enhance the crystallization of the plasma enhanced chemical vapor deposition (PECVD) layers at low temperatures. The use of a high hydrogen dilution (H2/(SiH4 + GeH4) ≈ 90) leads to microcrystalline SiGe (μcSiGe:H) at temperatures as low as 350 °C. In this work ∼0.6 and 2 μm thick μcSiGe:H layers were characterized and optimized. Alternatively, a chemical vapor deposition (CVD) crystallization layer can be used below the PECVD layer in order to deposit thick (4–10 μm) polycrystalline films of high quality at temperatures around 450 °C. For both layers, the strain gradient can be optimized by the use of a compressive top layer. In the case of the μcSiGe:H layers an alternative new strain gradient optimization method, which uses a variable hydrogen dilution to effectively fine-tune the mechanical properties of the μcSiGe films, is presented. Also new results on the surface roughness of the layers are presented.