Anodic bonding for monolithically integrated MEMS

Abstract

This paper presents a study on the feasibility of packaging monolithically integrated micro electro-mechanical systems (MEMS) by anodic bonding. We measured the concentration of mobile ions ( N m), the effective oxide charge ( N eff), and the density of interface traps ( D it) before and after bonding. Metal-oxide-semiconductor (MOS) capacitors were used 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 μm, d=10 μm, or d=200 μm. During anodic bonding, N eff increased with ∼10 12 cm −2 in the capacitors situated outside the glass. Within glass cavities, the capacitors with d=2 μm had Δ N eff of about 4×10 11 cm −2. For the capacitors with d=10 μm and d=200 μm, the increase was in the 10 10 cm −2 range. D it increased by quantities similar to the increases in N eff, except that lower increases were observed in the capacitors outside the glass. N m increased with ∼10 13 cm −2 in the capacitors situated outside the glass. Within the glass, Δ N m was in the low 10 11 cm −2 regime. A protective nitride layer deposited on top of the capacitors retarded the increase in N m during bonding. The results suggest that the increase in N m also caused the increases in N eff and D it in the capacitors situated outside the glass. Within the glass cavities, the increase in N eff and D it are thought to be due to negative bias-temperature instability (NBTI).