Cap Fabrication and Transfer Bonding Technology for Hermetic and Quasi Hermetic Wafer Level MEMS Packaging

This article describes a new wafer level capping technology for hermetic or quasi-hermetic 1st level device sealing. The technology is based on fabrication of cap structures with bond frames and optional recesses at temporary carrier wafers and their subsequent transfer bonding to the device wafer. The final release of the caps from the carrier wafer is obtained by laser assisted de-bonding. The cap fabrication relies on deposition and structuring methods from advanced packaging featuring mask aligner lithography for pattern definition. Based on that, bond frames and also the cap outlines can be defined almost freely by layout and thus, fully arbitrary shaped cap structures with irregular forms, sizes and locations and with irregular pitch become possible. The bond frames can be made of adhesive, metal or metal with solder cap to provide just mechanical or air tight device sealing to the target wafer. Since the caps are formed from a wafer, which is bonded to a carrier wafer, even fabrication of ultra-thin caps in the range of 50 μm or less is possible by adding related grinding and polishing steps. For the transfer of the caps to the device wafers, so-called donor wafers are used which can be handled with standard wafer-to-wafer alignment and bonding equipment. Due to the toughness of the temporary adhesive, which holds the caps on the carrier, wafer bonding processes under vacuum with temperatures up to 370 °C and pressures in the MPa range can bet utilized for the cap transfer bonding.The technology was applied for quasi-hermetic sealing of RF-MEMS switches on 200 mm BiCMOS wafers using adhesive bond frames as well as for hermetic sealing operations on 200 mm test wafers and on 200 mm MEMS wafers using soldering of AuSn bond frames. The related cap fabrication and bonding processes as well as associated results are described in this paper.