| Micro-electro-mechanical systems (MEMS) with identical properties cannot be readily produced. There are sub-micron differences in size and other variations, which especially in resonators, cause frequency scatter, and reduce yield. A post-fabrication process was developed to correct variations in micromachining processes. It consisted of electrically-heating suspended polysilicon structures in a silane environment to locally decompose the gas and deposit polysilicon. The volume of polysilicon deposited followed the temperature distribution of the structures. The temperature during selective deposition was found using an ABAQUS finite element electro-thermal analysis. The electrical current necessary to obtain deposition was found experimentally for some samples and predicted for others. The development of the electro-thermal simulations involved estimating material properties for polysilicon. A study of the sensitivity of temperature to the values of the thermal conductivity and the temperature coefficient of resistance was done. Significant dependence on these parameters was found.; In order to design microbeam resonators, a versatile numerical method of solving the pull-in problem was developed. Solutions for various boundary conditions (compliant supports, fixed and cantilever beams) are presented.; A new, special comb-drive resonator which had the standard four-beam suspension on one side replaced with a single-beam was designed. The latter feature enables one to predict the thermal hotspots. A finite-element thermal-stress simulation was developed to design against buckling at deposition temperatures, while keeping frequency at levels measurable by optical observation. Depending on the initial resistance of the sample, the power dissipation during heating and process time used, some variability in the deposited polysilicon (volume and texture) was found. Continuously and uniformly deposited layers, judged to be the best, were found for longer process times. Increases in frequency from 0.7% to 2.6% were obtained from the selective deposition of polysilicon on the special comb-drive resonators. For certain continuous and uniform depositions, the percent change in frequency due to the location and amount of deposition was determined and validated by using simulation. Large changes in frequency were predicted for situations in which deposition occurred at the root of a beam and its anchor. |
