Investigation on the Uniformity of Surface Energy in Silicon Direct-Bonding Technique

Using an original and dynamic crack-opening method the distribution of surface energy values is analyzed and compared for various surface pretreatment methods in a low-temperature silicon direct-bonding technique. Warm nitric acid and plasma-surface pretreatments were used prior to bonding. In the case of plasma-enhanced wafer bonding, a great number of bubbles is observed at the bonding interface after a long annealing time, although the annealing temperature is as low as 120°C. For the first time, large fluctuations of surface energy values have been presented in the case of plasma-enhanced wafer bonding even if trenches are etched at the bonding interface to avoid the formation of annealing voids. Contrastingly, wet surface treatment such as warm nitric acid prior to bonding results in a relatively low but uniform surface energy value over the whole bonded wafer interface.