Oxygen Plasma and Warm Nitric Acid Surface Activation for Low-Temperature Wafer Bonding

In this paper, the void formation and the surface energy for low-temperature Si-Si wafer bonding technique as a function of the annealing time and temperature as well as warm nitric acid and O2-plasma-assisted surface pretreatments are considered and compared. The analysis of the surface energy vs annealing time exhibits two main bonding mechanisms: (i) rapid reaction between silanol groups which leads to a quick enhancement of the bonding strength, and (ii) slow further increase of bonding strength and improvement of the bonding uniformity thanks to the out-diffusion of interface voids.