Novel low temperature hermetic sealing of micropackages

Abstract

Traditional methods of hermetic packaging are not easily scaled to the task of sealing micropackages, i.e. packages with volumes on the order of one cubic millimeter. Micropackages are fabricated from thin metal shells that are sealed with ceramic or metal covers. They are used to protect MEMS devices, chemical sensors, batteries, and microfluidic components, which all have limited thermal processing tolerance, e.g. less than 150°C. Glass seals are difficult to pattern at this scale and require sealing temperatures in excess of 300°C. Mechanical tolerance issues, fixturing, and electrode design constraints preclude seam sealing. The small size of micropackages makes it difficult to control and remove the heat generated during laser welding. Low temperature soldering is an option for sealing micropackages, but there is a risk of contamination with flux residues and solder wettable metallizations must be provided on both the package and cover. We have successfully sealed ceramic covers to anodized aluminum packages, with internal volumes of one cubic millimeter. The hermetic seal is made by compression of a ring of indium foil between the cover and a flange on the package. The compressive stress on the indium is maintained by an epoxy bond around the perimeter of the package. The seal is made by loading the package into an alignment fixture, placing preforms of indium and epoxy on the package flange, aligning a cover on the package, placing a weight on the cover, and curing the epoxy at 140°C on a hot plate. The process is readily scalable to sealing arrays of packages and covers. It also does not require metalized seal rings on either seal surface as required with soldering. For our application, sealing is conducted within an argon atmosphere, but we believe a dry nitrogen atmosphere would also be adequate.