Low cost wafer level packaging of MEMS devices [piezoresistive pressure sensor example]

This paper presents a packaging solution at wafer level for MEMS devices using silicon as packaging material and glass frit wafer-to-wafer bonding techniques for sealing. The MEMS devices wafer is bonded between two other silicon wafers. These cover wafers must assure the protection of the sensitive parts of MEMS devices and also the electrical and mechanical connection with PCB. For characterizing our wafer level packaging (WLP) solution we chose, as a test chip, a sensitive piezoresistive pressure sensor (operating range 0 to 0.4 bars). The main advantage of using this sensor is that the offset variation of the tensometric bridge, measured before and after the packaging, gives us the information on the residual stress induced by the packaging technique. Very good results were obtain; the variation of the tensometric bridge offset was less than 5 mV at a supply voltage of 5 V.