Abstract
The purpose of this project was to optimize the bonding parameters to develop an adhesive wafer bonding process that produces a defect free and relatively strong bond using SU-8 as an intermediate adhesive layer at the lowest possible processing temperature. There are a variety of bonding parameters that have a significant impact on void formation at the bond interface, defect density and the bond strength. The influence of several bonding parameters on the resulting bond quality was studied to identify the Key Process Input Variables of soft bake temperature, UV exposure dose and bonding temperature. Three different values were chosen for each process variable and the bonding process was repeated in random order for all possible combinations of process variables. The quality of a wafer bond is determined by the fraction of interfacial area in intimate contact and the strength of the bond interaction. Therefore, the void area and fracture strength of each bonded sample was considered as Key Process Output Variables. Bond characterization, which involves bond interface imaging to detect defects and measure void area, and tensile load testing to calculate the fracture strength, was done on all the bonded wafer pairs. All the bonded wafers were scanned using a Sonoscan Scanning Acoustic Microscope (SAM) and C-Scan images were used to determine void area. The six inch bonded wafers were diced into 5mm X 5mm size test specimens and each was scanned using SAM to calculate the interface bond area. Tensile load testing was done on the diced test specimens of each bonded pair to calculate the fracture strength. A general linear statistical model was developed to perform analysis of variance to determine the impact of the process input variables soft bake temperature, exposure dose and bonding temperature on the process output variables void area and fracture strength. The bonding parameters soft bake temperature, exposure dose and bonding temperature were successfully optimized to develop an adhesive wafer bonding process that can produce a defect free and strong bond using SU-8 as an intermediate adhesive layer at the optimum conditions of 90 oC soft bake temperature, 100 mJ/cm2 exposure doses and 115 oC bonding temperature.