Studies on the Polymer Adhesive Wafer Bonding Method Using Photo-Patternable Materials for MEMS Motion Sensors Applications

In this paper, various photo-patternable polymer adhesive materials were investigated for the selective wafer bonding of microelectromechanical system (MEMS) motion sensors. Commercially available photo-patternable materials which have different base polymer resins and photolithography mechanisms were selected. Curing behavior for each photo-patternable material was analyzed using the Fourier transform infrared spectroscopy to optimize the lower bonding temperature conditions with a sufficient degree of cure and wafer bonding process capability. Selective polymer adhesive wafer bonding method using photo-patternable materials was successfully demonstrated without any critical voids and defects except the phenol-based polymer resin. Also, mechanical properties, such as bonding strength and residual stress, and reliability were completely evaluated in order to select the best materials which meet the requirements for the piezoresistive MEMS motion sensors. The bonded wafers were strong enough to endure the dicing process. Bonding strength was measured quantitatively by a tensile test. The epoxy-based resin showed the best bonding strength with up to 42.9 MPa. Residual stress in the adhesive layer was evaluated using a wafer curvature measurement, because the residual stress may cause the changes of MEMS sensor performance. The silicone-based resin showed the lowest residual stress of 3 MPa because of lower Young's modulus. Five different reliability tests were performed to completely evaluate the reliability of the photo-patternable materials. The silicone-based resin showed the best reliability performance. As a result, photo-patternable polymer adhesive materials and wafer processing conditions were optimized for the MEMS motion sensors wafer bonding.