Thin film technologies for hermetic and vacuum packaging of MEMS.

摘要

This thesis describes the development of wafer-level, low temperature thin film packages. Two packages are presented: a hermetic package to seal miniaturized implantable devices, and a hollow cavity vacuum package for microelectromechanical systems (MEMS). The low-profile hermetically sealed package (less than 350°C fabrication temperature) utilizes a 3mum film of gold that is electroplated over a 2mum polyimide layer to enclose a section of an implantable system, which may contain integrated electronics. This package was implemented on an implantable silicon probe substrate to demonstrate a future application of the technology, with a yield of 88%. By virtue of the small bond ring (100mum) of this wafer-level package, costs of this technology should be minimal. The vacuum package (less than 250°C fabrication temperature) is fabricated by electroplating a 40mum thick nickel film over an 8mum sacrificial photoresist that is removed prior to package sealing. Multiple sealing technologies were examined; solder bumping proved to be the most versatile process. It enabled construction of a MEMS package that also forms the basis of an electrical interconnection structure. In order to apply the requisite fluxless, lead-free solder to the MEMS wafer, a mold and transfer technique to pre-form and outgas solder balls in a silicon mold prior to transfer to a MEMS wafer was developed. This package has utility in multiple commercial and military applications, including MEMS oscillators, micro inertial reference, and low cost infrared imagers.; These packages were characterized through the development of integrated sensors, including an integrated leak detecting resistor (LDR) and a micro Pirani gauge (thermal pressure sensor). The LDR predicted the implantable package to have a lifetime of greater than 30 years when soaking in Phosphate Buffered Saline. The wide area (400mum), thin gap (800nm) Pirani gauge can measure absolute pressure from atmospheric pressure to less than 1mTorr. This sensor can be integrated with common vacuum packaging technologies to measure leak rates as low as 6.0 x 10-16sccm. These gauges demonstrated that the vacuum package has a MTTF of 78.5 years in accelerated test conditions (autoclave and thermal cycling tests). Failure analysis and finite element modeling of stress provided methodologies for design improvements.