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We have demonstrated that room temperature bonding of Si/Si and Si/Cu using surface activated bonding (SAB) method could be successfully applied to vacuum sealing of microcavities. This method will make it possible to seal various MEMS devices on silicon or non-silicon substrates in vacuum by bonding of silicon cap wafers, because the method can be applied to diverse combinations of materials including silicon, metal films and compound semiconductors. In addition, the SAB vacuum sealing can overcome the problem that the sealed cavity pressure increases during heating due to the accumulation of gaseous products originating at the bonding interface. In this study, the quality of vacuum sealing by SAB is examined by estimating the pressure of the sealed cavity; for that, resonant quality factors of microcanti-levers placed in the vacuum-sealed cavity are measured. As a result, the cavities sealed with Si/Si bonding have a good sealing quality with the pressure increase rate of around 2/spl times/10/sup -15/ Pa m/sup 3//sec and their pressure is being maintained at less than 3 Pa. Also, it has been found that the pressure increase rate of Si/Cu sealing is larger than that of Si/Si sealing and is estimated as about 2/spl times/10/sup -14/ Pa m/sup 3//sec.