Direct wafer bonding was performed under medium vacuum condition. The effect of the sequence of wafers contact and vacuum application, plasma treatment, and annealing temperature on the bonding quality was investigated. From the comparison of the bonding efficiency, contacting two wafers in air rather than in vacuum produces much less bubbles. Besides, it was also found that ex-situ plasma treatment degrades the bonding quality. It is believed that the degradation is caused by contamination and damage induced by the plasma treatment before vacuum wafer bonding. For vacuum bonding without plasma treatment, good bonding can be achieved at temperature as low as 300 °C.

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