This paper describes hermetic seal wafer bonding using Al covered with thin Sn as an antioxidation layer. The bonding temperature is below 400 °C, which is the maximum temperature of CMOS-LSI backend process. Gas tightness over 3000 h at room temperature and sealing stability through heat treatment under a typical reflow condition of 260 °C for 10 min were confirmed for samples bonded at 370 °C and 380 °C. A key for successful hermetic seal bonding is relatively high bonding pressure and stress concentration on sealing frames as narrow as several micrometers. The results of SEM and EDX analysis suggested that the bonding was due to direct Al-Al bonding, while Sn was diffused sparsely among Al grain boundaries. The developed bonding technology is usable for wafer-level integration of LSI and MEMS in conjunction with hermetic sealing.