Effects of Wafer Cleaning and Annealing on Glass/Silicon Wafer Direct Bonding

We studied cleaning and annealing effects in glass/Si direct bonding using 4 inch Pyrex glass and silicon wafers. SPM cleaning (sulfuric-peroxide mixture, $H2SO4:H2O2=4:1,$$120°C),$ RCA cleaning $(NH4OH:H2O2:H2O=1:1:5,$$80°C),$ and combinations of these two methods were examined to investigate the wafer cleaning effect. When wafers were cleaned by RCA after SPM cleaning, maximal bonding quality at room temperature was gained. Surface roughness, as measured by AFM (atomic force microscope), was found to correspond with bonding quality at room temperature. Bonding strength increased as the annealing temperatures increased to $400°C,$ but debonding occurred at $450°C.$ The difference in thermal expansion coefficients of the glass and the Si wafer used led to this debonding. When wafers bonded at room temperature were annealed at 300 or $400°C,$ the bonding strength increased for 28 hours, and then it decreased with further annealing. The decrease in bonding strength caused by further annealing was due to sodium ion drift through the glass/Si interface.