Investigations of Anodic Bonding for MEMS Wafer Level Package Applications

Wafer bonding technology is widely used in MEMS devices. Among them, anodic bonding is the most popular and can also provide hermetic sealing for wafer level package applications between silicon and glass wafer. However, the thermal mismatch between silicon and glass causes the residual stress, and consequently induces glass or silicon chipping and wafer warpage. Moreover, for package applications, the reliability issues of anodic bonding should be taken into account. Since the glass was usually used as cap wafer, a cavity is created to hermetically accommodate the MEMS devices. As a result, the effective bonding areas could affect the reliability in the long run.

This study was focused on above issues and relevant anodic wafer bonding phenomena. The anodic bonding limit was first investigated. Multiple cavities in Pyrex 7740 substrate were first etched by HF solutions for later bonding with p-type silicon. The etched cavities could provide the housed space for the various MEMS devices. Anodic bonding areas were defined by different cavity dimensions. The anodic bonding pairs were then diced for the bonding ability tests and the hermetic tests, including shear tests and helium-leaking tests (JESD22-A109). Shear strength up to 30MPa and helium-leaking rate below to 5.0*10e-8atm*cc/sec of the anodic bonding pairs were obtained in the study.

It was found that the shear force is proportional to the boning area, and the hermeticity could be achieved by the sufficient bonding area (>20%). The elements distributions near the interface of anodic bonding pairs were probed by the energy dispersive spectrometer (EDS) elemental mapping. It was found that the oxygen anion diffusion layer thickness is proportional to the shear strength. Further investigation of the oxygen anion diffusion layer in silicon before/ after temperature cycling tests (TCT, JESD22-A104) by the EDS elemental mapping were done and will be described.