Thin film encapsulation of acceleration sensors using polysilicon sacrificial layers

This work presents a novel wafer-level encapsulation technique for MEMS devices featuring functional, sacrificial, and cap layers all made from polycrystalline silicon. This technique offers several advantages such as a low area consumption, a reduced thickness of the sealed device and a CMOS compatible process flow for a MEMS-first CMOS intergrated sensor. The process flow for the encapsulation of a MEMS accelerometer is presented. The main features of the approach are the use of a second sacrificial layer on top of the device made from silicon, a thick cap layer also made from silicon and the use of gaseous CIF/sub 3/ for the etching of the sacrificial silicon layer. The protection of the cap and functional layer of the device during sacrificial etching is achieved by thin oxide layers confining the sacrificial etching to a predefined volume. The mechanical stability of the cap layer during handling or plastic mold packaging is ensured by a barrel vault design of the MEMS cavity.