Anodic bonding for monolithically integrated MEMS

This paper presents a study on the feasibility of packaging monolithically integrated MEMS by anodic bonding. We measured the concentration of mobile ions (N/sub m/), the effective oxide charge (N/sub eff/), and the density of interface traps (D/sub it/) before and after bonding, using MOS capacitors as test structures. The capacitors were situated outside the glass and within glass cavities. In the cavities, the vertical distance d between the gate electrode and the glass ceiling was either d=2 /spl mu/m, d=10 /spl mu/m, or d=200 /spl mu/m. All parameters N/sub m/, N/sub eff/, and D/sub it/ were found to increase due to the anodic bonding. The highest increases were observed on the capacitors situated outside the glass, and the lowest increases were observed on the capacitors with d=10 /spl mu/m and d=200 /spl mu/m. A protective nitride layer reduced the increase in N/sub m/. The results suggest that the increase in N/sub m/ also caused the increase in N/sub eff/ and D/sub it/ in the capacitors outside the glass. Within the glass cavities, the increases in N/sub eff/ and D/sub it/ are thought to be due to Negative Bias-Temperature Instability (NBTI).