Characterization of low-temperature wafer bonding using thin-film Parylene

This paper presents detailed experimental data on wafer bonding using a thin Parylene layer, and reports results on: 1) bond strength and its dependence on bonding temperature, bonding force, ambient pressure (vacuum), and time, 2) bond strength variation and stability up to two years post bond, and 3) bond strength variation after exposure to process chemicals. Wafer bonding using thin (< 381 nm) Parylene intermediate layers on each wafer in a standard commercial bonder and aligner has been successfully developed. The Parylene bond strength is optimized at 230 degrees C, although Parylene bonding is possible at as low as 1130 degrees C. The optimized bonding conditions are a low-temperature of similar to 230 degrees C, a vacuum of similar to 0.153 mbar, and 800 N force on a 100 mm wafer. The resultant Parylene bond strength is 3.60 MPa, and the strength for wafers bonded at or above 210 degrees C is maintained within 93% of its original value after two years. The bond strength is also measured after exposure to several process chemicals. The bond strength was reduced most in undiluted AZ400K (base) by 69% after one week, then in BHF (acid), MF319 (base), Acetone (solvent), and IPA (solvent) by 56%, 33%, 20%, and 8%, respectively, although less than one hour exposure to these chemicals did not cause a significant bond strength change (less than 11%).