Metal-bonded, hermetic 0-level package for MEMS

This paper presents a zero-level packaging technology for hermetic encapsulation of MEMS. The technology relies on the “chip capping” of the MEMS using a metallic bond made by means of diffusion soldering of a Cu-Sn system at a temperature of around 250°C. For this, on a “capping wafer” a sealing ring (or bond frame), composed of a double layer of Cu/Sn, is grown, and on the MEMS wafer a matching ring of a single Cu layer is made. Next, the “capping chip” is assembled onto the “MEMS die”, either in a die-to-wafer (D2W) or a wafer-to-wafer (W2W) fashion. The thicknesses of the layers (Cu/Sn and Cu) and the bonding process parameters (temperature and force profile) have been optimized so as to achieve a strong, hermetic package, that remains stable up to temperatures as high as ∼415°C. Leak testing, based on the “membrane deflection method”, revealed that the packages are air tight and He leak tight. No noticeable change of the deflection of the cap (thinned down to 20–50 µm) was observed as a result of pressurizing the packages for 11 days under He at 30 MPa.