Improved direct bonding of Si and SiO2 surfaces by cleaning in H2SO4:H2O2:HF

A method for silicon surface preparation prior to wafer bonding is presented. By cleaning the wafers in a H2SO4:H2O2 mixture in which a small amount of HF is added, and then rinsing in H2O, the bonding behavior of the surfaces is improved, compared to other pretreatments used for bonding. The modified SPM cleaning results in a highly fluorinated chemical oxide on the Si surface. A subsequent water rinse causes substitution of F by OH groups, which increase the initial attraction of the mating surfaces. Higher contact wave velocities and bond strengths than reported for other surface pretreatments have been measured, both for bare and thermally oxidized silicon surfaces.