Post-CMOS integration of germanium microstructures

Polycrystalline germanium (poly-Ge) microstructures have been fabricated on standard CMOS wafers. Conventional low pressure chemical vapor deposition (LPCVD) and rapid thermal annealing (RTA) processes were used to achieve low-resistivity (2.3 m Omega-cm) tensile poly-Ge structural films, with a thermal budget which is compatible with Al (2% Si) metallization. The CMOS circuitry was passivated with low-temperature oxide and amorphous Si; the latter served as a mask against HF during the microstructure release etch. Comb-drive microresonators with integrated trans-resistance amplifiers were used to demonstrate feasibility of this integration strategy. Preliminary measurements on test structures indicate that poly-Ge has promising material properties. Its fracture strength is 2.2 GPa +/- 0.4 GPa, which is comparable to that of poly-Si. Clamped-clamped lateral resonator test structures have quality factors in vacuum as high as similar to 30,000. For the process conditions used in this work, the residual stress of as-deposited poly-Ge is -79 MPa (compressive); RTA shifts the stress to 203 MPa (tensile). Deposition and annealing conditions have yet to be optimized to minimize stress.