Characterization of selective polysilicon deposition for MEMS resonator tuning

Variations in micromachining processes cause submicron differences in the size of MEMS devices, which leads to frequency scatter in resonators. A new method of compensating for fabrication process variations is to add material to MEMS structures by the selective deposition of polysilicon. It is performed by electrically heating the MEMS in a 25 degreesC silane environment to activate the local decomposition of the gas. On a (1.0 x 1.5 x 100) mum(3), clamped-clamped, polysilicon beam, at a power dissipation of 2.38 mW (peak temperature of 699 degreesC), a new layer of polysilicon (up to 1 mum thick) was deposited in 10 min. The deposition rate was three times faster than conventional LPCVD rates for polysilicon. When selective polysilicon deposition (SPD) was applied to the frequency tuning of specially-designed, comb-drive resonators, a correlation was found between the change in resonant frequency and the length of the newly deposited material (the hotspot) on the resonator's suspension beams. A second correlation linked the length of the hotspot to the magnitude of the power fluctuation during the deposition trial. In one sample, at a power dissipation of 10.7 mW (peak temperature of 800 degreesC), the cross section of a suspension beam increased from 6 to 9.4 mum(2) in 15 min. The resonant frequency increased 1.96% from its initial value of 86.6 kHz. This change was in good agreement with the value of 2.2% predicted in a simulation. The mechanisms for changing resonant frequency by the SPD process include increasing mass and stiffness and altering residual stress. The effects of localized heating are presented. The experiments and simulations in this work yield guidelines for tuning resonators to a target frequency.