A study on characteristics and processing parameters of photo-patternable adhesives for MEMS Motion Sensor Wafer Bonding

Abstract

In this study, we investigated various photo-patternable materials for the selective adhesive wafer bonding for the encapsulation of MEMS motion sensors. Commercially available photo-patternable materials which have different base resins and photolithography mechanisms were selected. Curing behavior for each photo-patternable material was analyzed using Fourier transform infrared (FT-IR) spectroscopy to obtain the low bonding temperature conditions with a sufficient degree of cure and wafer bonding conditions were optimized. Selective adhesive wafer bonding using the photo-patternable materials was successfully demonstrated without any critical void and defects except the phenol-based resin. Also, mechanical properties, such as bonding strength and residual stress, and reliability were evaluated in order to eventually select the promising 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 result with the bonding strength up to 42.9MPa. Residual stress in the adhesive layer was evaluated using a wafer curvature measurement. The silicone-based resin showed the lowest residual stress of 3MPa with low young`s modulus. Five different reliability tests were performed to evaluate the reliability of the photo-patternable materials. The silicone-based resin showed excellent high reliability. As a result of this study, optimized processing conditions and promising materials were presented for the MEMS motion sensor wafer bonding.