Monolithic integration of MEMS devices with driving and controlling electronics is advantageous for improving performance and potentially lowering cost. Polycrystalline silicon-germanium (poly-Si(l-x)Ge(x), where x is between 0 and 1), which has mechanical and electrical properties similar to poly-Si, is a promising candidate for the structural-layer material of post-CMOS integration of MEMS because poly-SiGe can be deposited (and annealed, if necessary) at much lower temperatures than poly-Si. To be successfully integrated with state-of-the-art electronics fabricated by IC foundries, any post-CMOS MEMS process with temperature higher than 450°C must be avoided.
While low-resistivity poly-SiGe can be easily obtained utilizing in situ p-type (i.e., boron) doping during deposition, poly-SiGe films as-deposited at 450°C or lower generally exhibit some level of residual stress and strain gradient. For optical switching/modulation and inertial-sensing applications, residual stress and strain gradient of the structural layers have to be minimized. From previous experimental results , it has been found that the stress and strain gradient of as-deposited poly-SiGe are dependent on deposition conditions, including temperature, pressure, and germanium content.
A full-factorial experiment was conducted to investigate the optimal deposition condition to achieve low stress and low strain gradient. Low residual stress (-9 MPa, compressive) and low strain gradient (2.4E-5µm-1) was achieved in as-deposited 2 µm poly-Si0.4Ge0.6 film deposited at 450°C and 600 mT. The residual stress and strain gradient were each generally found to increase significantly with decreasing deposition temperature. A 2 µm poly-Si0.4Ge0.6 film deposited at 425°C exhibited -45 MPa stress and 3E-4µm-1 strain gradient.
To further bring down the effective strain gradient, a bi-layer approach is currently under investigation. By depositing film with compressive stress on top of film with tensile stress, a bending-down moment can be created to cancel out the bending-up moment in general as-deposited films. Stress cancellation between two layers can also further lower the effective residual stress.