Long-Term and Accelerated Life Testing of a Novel Single-Wafer Vacuum Encapsulation for MEMS Resonators

TLDR

The authors developed a single‑wafer, 20‑µm polysilicon vacuum encapsulation for MEMS resonators, achieving <1 Pa pressure and validating it with accelerated hydrogen‑leakage tests at elevated temperatures. The encapsulation remained pressure‑stable over 10 000 h at ambient, survived >600 thermal cycles from –50 °C to 80 °C, and withstood repeated hydrogen‑leakage tests up to 400 °C, demonstrating high robustness attributed to the clean, high‑temperature sealing process.

Abstract

We have developed a single-wafer vacuum encapsulation for microelectromechanical systems (MEMS), using a thick (20-mum) polysilicon encapsulation to package micromechanical resonators in a pressure <1 Pa. The encapsulation is robust enough to withstand standard back-end processing steps, such as wafer dicing, die handling, and injection molding of plastic. We have continuously monitored the pressure of encapsulated resonators at ambient temperature for more than 10 000 h and have seen no measurable change of pressure inside the encapsulation. We have subjected packaged resonators to >600 cycles of -50 to 80degC, and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated temperature. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400degC have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400degC without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high-temperature environment during the sealing process