Die and Wafer-Level Hermetic Sealing for MEMS Applications

Two of the primary causes of MEMS failure are stiction of the moving microparts due to moisture from environment and particulate contamination due to the degradation of organic materials. The use of getters such as sputtered Ti and Ba to maintain a moisture free environment within the MEMS package has been proposed and is well documented. Though getters ensure a moisture free environment within the package, they do not ensure hermeticity over long periods of time. Hermetic packaging is sometimes desirable for MEMS and optical MEMS (especially those that require long shelf lives) in order to guarantee operational reliability. This paper presents a hermetic fluxless die and wafer-level sealing process with adequate bond strength, assessed using the MIL-STD-883E standard. The hermetic sealing process is achieved via 80%Au-20%Sn eutectic solder rectangular seal rings deposited around the top die perimeter via evaporation and sputtering. We chose 80%Au-20%Sn eutectic solder for sealing because of its high resistance to surface oxide film formation as a result of its high gold content thereby eliminating the use of organic materials such as flux, and at the same time, providing hermetic seal at the die bond interface. The paper also discusses the use of linear regression analysis in packaging process development for our devices. The focus here is identifying process variables that significantly affect the process using as few packaged MEMS samples as available for reliability testing due to their high cost. The process parameters investigated in this paper apply to the shear strength of the resulting die stack, and include the applied bonding pressure, the maximum reflow temperature and the dwell time at the maximum reflow temperature. Conclusions are drawn based on experimental measurements conducted at the Texas Microfactory™ at UT Arlington, and at the Bennington Microtechnology Center (BMC) in Vermont, USA.