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Low temperature direct Cu bonding for MEMS packaging

The sealing and contact bonding of IMU (inertial measurement unit) are important procedures in the MEMS device fabrication. Conventional process is Al-Ge eutectic bonding developed by InvenSense, for which the process temperature is above 450°C and the bonding time is longer than 1 hr. This study proposes an innovative and efficient process combined with an air plasma bombardment and a following catalyzed formic acid vapor treatment, which can achieve robust Cu to Cu bonding high than 30MPa when bonded at 250°C for 5 min under a pressure of 10 MPa in N2. Instead of surface activation, the low vacuum air plasma bombardment is to induce defects and thus harden the faying surface. It is found that bonding strength was closely related to subsurface hardness. An acceleration in self diffusion of copper atoms by plasma bombardment accounts for the enhancement in bonding strength.