Wafer-Level Hermetic Seal Bonding at Low-Temperature with Sub-Micron Gold Particle Using Stencil Printing

Abstract

Low-temperature hermetic bonding using sub-micron gold (Au) particles has been developed for wafer level packaging of MEMS [1]. The new method of hermetic sealing uses a rim structure covered with sub-micron Au particles (0.3 μm in diameter), which is formed by Au paste (AuRoFUSETM) using a high-precision screen printer with stencil metal mask [2]. As shown in Fig.1 (a), after the alignment between the rim wafer and the metal mask, the printing was completed on the rim structure at a high speed of less than 10 seconds per wafer. The wafer-level printing has been demonstrated with 4 inch wafers in this study. After sintering the paste at 200°C in Fig. 1 (b), the coating thickness of the sub-micron Au particles on the rim top was in a range of 3-5 μm on the entire surface of 4 inch wafer. Then, wafer level thermo-compression bonding between the rim and a diaphragm wafer was performed in a wafer bonding chamber under a pressure of 10-3 Pa at 200°C for 30min with a bonding pressure range of 100-200 MPa. The Au particle pattern was pressed by the rim structure and the height of the pattern was reduced from 3-5 μm to 0.6 μm. The hermetic sealing was successfully realized by dense structure of the sub-micron Au particles as shown in Fig. 1 (c). The deformable property comes from the porous structure of sintered Au, which is advantage in terms of the compliance to surface irregularity as well as the insensitivity to surface roughness. A bonding shear strength measured by a die shear tester is 44 N for 4.3 mm-square rim in 10 μm width. The typical fracture was happened at the inside of the dense Au structure, showing a fractured surface of ductile deformation. After that, the bonded wafer was cut into individual laminates by using a conventional blade dicing.The hermeticity of sealed cavities was confirmed visually by deflection of the diaphragm. The size of diaphragm is 2.4 mm-square, and the concave deflection is -5 μm in atmospheric pressure. Measuring the displacement of the diaphragm precisely, the encapsulated pressure inside the cavity was estimated as 100 Pa. A helium (He) leak rate of sealed cavities was evaluated by an ultra-fine leak test. The detection limit of a usual fine leak test, which is about 1×10-1 2 Pa∙m3/s, is insufficient for the hermeticity measurement of wafer level MEMS packaging. On the other hand, the ultra-fine leak test system using a ‘Capsule-Accumulation Method’ can detect a leak rate down to 4×10-15 Pa∙m3/s (He). In the ‘Capsule-Accumulation Method’, a released He gas from the bonded wafer stack is accumulated during a time of 1,000 seconds in a small-volume capsule, and then introduced into a mass spectrometer. Prior to the measurement of He leak rate, the bonded wafer stacks were immersed in He gas atmosphere at a pressure of 0.617 MPa for 2h or 72h. The samples taken out of the He chamber were kept in air atmosphere before the measurement of He leak rate.Though a small amount of He was detected after the samples were loaded in a measurement chamber, this would indicate that the He initially adsorbed on the Si surface was released to be detected. The measurement showed that the maximum leak rate of the sealed cavity bonded with the sub-micron Au particles was estimated in a range of 10-14 Pa∙m3/s (He), which is sufficient for most of MEMS applications. REFERENCES Hiroyuki Ishida, Toshinori Ogashiwa, Yukio Kanehira, Hiroshi Murai, Takuya Yazaki, Shin Ito, Jun Mizuno, Surface Compliant Bonding Properties of Low-Temperature Wafer Bonding, IEEE ECTC 2013, pp. 1519-1523 T. Ogashiwa, K. Totsu, M. Nishizawa, H. Ishida, Y. Sasaki, M. Miyairi, H. Murai, Y. Kanehira, S. Tanaka, M. Esashi, "Hermetic Seal Bonding at Low-temperature with Sub-micron Gold Particles for Wafer Level Packaging", in Proc. of 48th International Symposium on Microelectronics (IMAPS), Orlando, Florida, USA, October 26-29, 2015, pp. 73-78. Fig.1 Schematic illustration of stencil printing (a), sub-micron Au particle pattern sintered at 200°C (b) and a dense structure after thermo-compression bonding (c). Figure 1