Small footprint wafer-level vacuum packaging using compressible gold sealing rings

Abstract

A novel low-temperature wafer-level vacuum packaging process is presented. The process uses plastically deformed gold rings as sealing structures in combination with flux-free soldering to provide the bond force for a sealing wafer. This process enables the separation of the sealing and the bonding functions both spatially on the wafer and temporally in different process steps, which results in reduced areas for the sealing rings and prevents outgassing from the solder process in the cavity. This enables space savings and yields improvements. We show the experimental result of the hermetic sealing. The leak rate into the packages is determined, by measuring the package lid deformation over 10 months, to be lower than 3.5 × 10−13 mbar l s−1, which is suitable for most MEMS packages. The pressure inside the produced packages is measured to be lower than 10 mbar.

Wafer-Level Vacuum Packaging Enabled by Plastic Deformation and Low-Temperature Welding of Copper Sealing Rings With a Small Footprint

“…This conservatively estimated leak rate is smaller than the reported data from cavities based on Au sealing rings [37] and Au bumps [38] by three and two orders of magnitude, respectively. Although the absolute pressure inside the cavity is not as good as the work reported in [38], it should be noted that the significantly smaller cavity volume in the present work yields a higher surface/volume ratio, which makes it more difficult to maintain the vacuum level inside the cavity.…”

“…This thickness variation was caused by the varying etch rates from the center to the edge of the wafer. The deflection of a diaphragm depends on the thickness of the diaphragm and the pressure difference of atmospheres inside and outside of the enclosed cavity [37]. This approach potentially offers a much lower detection limit for leak rate measurements compared with bubble leak test and helium fine leak test, although at the cost of a longer measurement period [47].…”

“…In order to evaluate the leak rate of the sealed cavities, the deflection changes of the 4015 µm × 4015 µm cavity diaphragms were monitored over a period of 97 days by whitelight interferometry (Veeco Wyko NT9300) [37]. Fig.…”

“…In addition, the Cu ring in the cap substrate maintained an intact line shape after bonding, verifying that no reflow had occurred. The excellent bonding strength in combination with the small sealing ring footprint achieved here enables further miniaturization of vacuum packages compared to alternative metal-based vacuum packaging techniques, which use sealing rings that are typically at least 100 µm wide [13], [14], [16], [18], [22], [23], [25], [33], [36], [37], [39].…”

“…The high ductility of Au enables vacuum sealing based on cold welding of the Au sealing structures [36]- [38]. However, the proposed sealing techniques either rely on reinforcement of the bond strength by additional materials such as epoxy underfill [36], or solder patches [37], or the process is based on wafer bonding in combination with sealing of vent-holes by plastic deformation of Au plugs [38].…”