| Due to the unique structure and small scale of Micro-Electro-Mechanical Systems (MEMS), the inherent residual stresses during the deposition processes can have a profound affect on the functionality and reliability of the fabricated MEMS devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not controlled during deposition. The residual stress, for example, tends to vary significantly for different deposition methods. Currently, few techniques are available to measure the residual stress in MEMS devices. In this dissertation, μRaman spectroscopy is used to measure the residual and induced stresses in MUMPs polysilicon and GaAs MEMS devices. μRaman spectroscopy was selected since it is nondestructive, fast, and provides the potential for in situ stress monitoring. Raman spectroscopy line and mapping scans were performed to obtain Raman residual stress profiles on unreleased and released MEMS fixed-fixed beams, cantilevers, and micromirror flexures. These profiles were compared to analytical models to assess the accuracy of the Raman stress profiles. Finite element residual and induced stress profiles are obtained from MEMCAD modeling software and used to assess the viability of μRaman spectroscopy as an in situ stress measurement technique. Post-processing techniques to include thermal annealing, phosphorous diffusion, and phosphorous ion implantation were performed to investigate methods to alter or control the residual stress within MEMS devices. μRaman spectroscopy is used to characterize and monitor the residual stress levels in the unreleased MEMS structures following each post-processing experiment to determine the magnitude of stress relaxation. Significant residual stress relaxation is observed in the Raman stress profiles and verified with on-chip test structures following the thermal anneals and doping. The MUMPs foundry fabricated residual stress levels can be significantly reduced by over 90% to stress levels less than 1 MPa following post-processing. The reduced residual stress levels can significantly improve device performance, reliability, and yield as MEMS devices become smaller. In addition to the polysilicon stress profiles, the first μRaman stress measurements in III-V MEMS are presented. |
