Transfer of metal MEMS packages using a wafer-level solder transfer technique

This paper presents a modular, low profile, wafer-level encapsulation technology for microelectromechanical systems (MEMS) packaging. Electroplated caps are formed on top of a solder transfer layer previously deposited on a carrier wafer, then simultaneously transferred and bonded to a device wafer by a novel solder transfer method and transient liquid phase (TLP) bonding technology. The solder transfer method is enabled by the dewetting of the solder transfer layer from the carrier wafer and TLP bonding of the cap to the device wafer during bonding. The bond and transfer cycle has a maximum temperature of 300°C and lasts about 2.5 h. This approach has been demonstrated with nickel (Ni) caps as thin as 5 microns, with thicker caps certainly possible, ranging in size from 200 μm to 1 mm. They were transferred with a lead-tin (Pb-Sn) solder layer and bonded with nickel-tin (Ni-Sn) TLP bonding with greater than 99% transfer yield across the wafer.