Electrical Properties and Noise of Poly SiGe Deposited at Temperatures Compatible With MEMS Integration on Top of Standard CMOS

Abstract

The rapid development of MicroElectroMechanical Systems (MEMS) implied monolithic integration of these devices with the driving, controlling and signal processing electronics on the same CMOS substrate as this can improve performance, yield and reliability as well as lower the manufacturing, packaging and instrumentation costs. The post-processing route (fabricating MEMS on top of CMOS) is preferred as it avoids introducing any changes in standard foundry CMOS processes. Depending on the specific metallization type and the design requirements, post-processing limits the maximum fabrication temperature of MEMS to 450°C or 520°C [1], as this avoids introducing any damage or degradation to interconnects. This temperature constraint is quite strict for post processing surface micromachined MEMS, as it might affect relevant physical properties of the active element of MEMS such as crystallization, growth rate, mechanical properties, dopant activation, electrical resistivity, etc.. Polycrystalline silicon germanium (poly SiGe) is an attractive material for such applications due to its low amorphous to polycrystalline transition temperature, which can be as low as 400°C (with the appropriate germanium content). Furthermore, the mechanical properties of poly SiGe proved to be suitable for surface micromachining application [2]. In addition it is compatible with standard IC fabrication process, and hence it enables monolithic integration of MEMS with the driving electronics.