Stable thin film encapsulation of acceleration sensors using polycrystalline silicon as sacrificial and encapsulation layer

We show a wafer-level encapsulation technique for microelectromechanical system (MEMS). The process flow for fabrication and 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 ClF3 for the etching of the sacrificial silicon layer. The protection of the cap and functional layer of the device during sacrificial etching was achieved by thin oxide layers confining the sacrificial etching to a predefined volume. The acceleration sensors were electrically tested before dicing and after plastic mould packaging. The presented technique offers several advantages such as CMOS compatibility, a low area consumption, a reduced total thickness of the sealed device and an integrated cap with high mechanical stability for damage-free handling and second level plastic mould packaging.