Wafer-level packaging for MEMS devices with nanoporous materials

Abstract

Over the last a few decades, microelectromechanical system (MEMS) technology has established itself as a 21st century key technology thanks to the maturity of bulk and surface micromachining process. Nevertheless, packaging remains as a bottleneck to commercialization because most packaged MEMS devices have been developed individually in-house and their process cost and temperature is relatively high. For commercialization, MEMS require hermetic wafer-level packaging (WLP) with a low process temperature. The main research objective in this thesis is the development on new hermetic packaging methods of MEMS devices at wafer-level scale with nanoporous materials. In chapter 3 and 4, the proposed packaging methods have advantages of simplicity, cost-effectiveness and a low thermal budget (<350 $^\circC$). A Cu sacrificial layer was etched away with a wet-etchant through the vertical nanopores of AAO and columnar Cr membrane to release the MEMS devices. Subsequent sealing is achieved through the deposition of PECVD SiNx over the membrane for vacuum packaging and BCB for $N_2$ atmospheric pressure packaging. The long-term hermeticity was monitored by measuring maximum central deflection change of the sealing membranes over 180 days. In addition, to determine the reliability of the package, the accelerated life time test was performed in a temperature and humidity chamber ($90 ^\circC$ and 92% RH) for 500 hours, and the results show that the lower bound of the 90% confidence interval of MTTF was estimated to 14.4 years under jungle condition ($35 ^\circC$ and 95% RH) and 603 years under office condition ($25 ^\circC$ and 40% RH). In chapter 5, the encapsulated Ni Pirani gauges were successfully fabricated with the use of the nanoporous columnar thin film (Cr). The thickness of the sealing layer was carefully chosen considering the central deflection and stress in the cavity edge. The precise pressure of the packaged cavity (~0.7 Torr) and short-term (30 days) ...