Wafer -level packaging and frequency trimming by localized mass deposition.

Abstract

Silicon-glass anodic bonding, commonly used for MEMS packaging, offers many advantages. However, outgassing during the bonding process and vacuum degradation over time can significantly compromise device performance. This dissertation reports a packaging technique in which a cavity is created by anodic bonding and is subsequently vacuum-sealed by localized polysilicon chemical vapor deposition (CVD). The cavities are formed by bonding incomplete polysilicon rings, each with a heater located at the ring opening, on the silicon substrate to a glass wafer. Gas products generated during the bonding process are evacuated via ring openings, offering low and controlled cavity pressures at near ambient temperature. The cavities are sealed at 250mTorr in silane, gettering heaters located in the cavity further reduces the pressure by gettering the Si atoms. Quality factors (Qs) of ∼2300 are obtained from the embedded resonators, suggesting cavity pressures less than 50mTorr have been achieved. This technique is applied to a capacitive barometric pressure sensor with integrated vacuum reference control. The sensor module is fabricated using a double-polysilicon dissolved-wafer process. The wafers are anodically bonded, thinned, and released in EDP. Localized CVD is used to seal the cavity and to react away residual gas trapped inside to create the desired reference pressure. The estimated cavity pressures are ∼30mTorr, and these barometric pressure sensors are capable of resolving 25mTorr over a 300Torr dynamic range. The described packaging technique is also applied to a trimming module, which is designed to investigate the feasibility of post-packaging frequency trimming by localized CVD. The module contains a resonator for pressure sensing, a gettering heater for pressure control, and a cantilever resonator. Trimming heaters located on the boss of the cantilever resonator are activated for frequency trimming in silane-sealed cavities, followed by getter activation to consume any unused gas. Precise frequency trimming of comb resonators (∼0.2%) has been demonstration by mass deposition.