CMOS Compatible Hermetic Wafer Level Packaging For Inertial MEMS
Recently, there has been an increased demand for MEMS devices where the MEMS structure is integrated with CMOS circuitry at wafer level. Such devices could be fabricated in a conven- tional foundry using CMOS compatible materials in the manufacturing process. Currently, wafer level packaging of inertial MEMS devices either involves traditional glass frit or metal systems involving gold eutectic which is expensive and not compatible with CMOS front-end processing. The use of Al as a bonding metal is compatible with CMOS integration since Al is already used as a bond pad metal and has no contamination concerns in a standard CMOS clean room environment. Al-Ge based eutectic wafer bonding has been shown to provide a practical solution for MEMS-CMOS integration and hermetic wafer level packaging due to both Al and Ge being CMOS fab friendly, elec- trically conductive and orders of magnitude less permeable than glass. This applications note describes a practical Al-Ge bonding process where Al is used as a seal ring metal on the MEMS device wafers while Ge is used as a cap wafer material. The Aluminum- Germanium system [2, 3] is a simple eutectic system with three phases (a) liquid (b) fcc (Al) solid solution and (c) diamond cubic (Ge) solid solution as shown in Figure 1. The eutectic point of this system has not been reliably reported but most published data points at a eutectic point of 420 °C ± 4 °C placing the atomic per centage of Ge at 28.4 % to 30 %.