A Packaging Solution for Pressure Sensor MEMS

In this paper, a wafer-level packaging solution for pressure sensor microelectromechanical system (MEMS) is reported. Sensor and glass cap wafers are anodically bonded at a bonding temperature less than 400°C. Bubble free interfaces are obtained and the bond strength is higher than 20 MPa. Sensor and bottom silicon cap wafers are bonded at a temperature of 400–450°C with the assistance of a gold intermediate layer. The bond strenght is higher than 5 MPa. The via holes, used for feedthroughs leading out the circuit, on bottom silicon cap wafer are anisotropically formed in KOH etching solution. Aluminum layer is sputtered on the bottom silicon wafer for electrical connection, re-routing circuit and the seed layer of under bump metallization (UBM). During sputtering process, the sidewalls of via holes are also sputtered with aluminum film. At the same time, the metal pads on sensor wafer are also built up to connect with metallized via holes. It is found that the cavities are vacuum sealed. Sputtered Cr/Ni/Au layers are used for UBM layers. Finally, solder bumps can printed or plated on the UBM. The whole process leads to promising performance of the devices.