Shiro Satoh

Preparation of high purity zinc selenide single crystals and evaluation through photoluminescence spectra

Abstract High purity zinc selenide single crystals have been grown using commercial high purity selenium and zinc purified by vacuum distillation and overlap zone-melting. The photoluminescence spectra measured at 4.2 K on the grown crystals are confirmed to be affected considerably by the purity of the zinc used. In the case of the purest crystal, the emission intensity related to donor bound excitons is remarkably small compared to that due to the recombination of free excitons. Moreover, no emission band is observed in the phonon energy below 2.4 eV.

Low-temperature aluminum thermo-compression wafer bonding with tin antioxidation layer for hermetic sealing of MEMS

Hermetic Al-Al thermo-compression bonding was demonstrated at the lowest temperature ever reported, 370-390°C, using a thin antioxidation capping layer of Sn. A key factor of hermetic sealing is bonding pressure enough to compress the bonding interlayer metal. From the same point of view, a narrower sealing frame for stress concentration, thicker Al and a higher bonding temperature within the allowable range (<;400°C) are favorable for high yield hermetic sealing. Judging from Al-Sn phase diagram, Sn should uniformly and sparsely exist among Al grains as Al-Sn eutectic, which was also supported by the cross-sectional observation of the bonding interlayer. In such a microstructure, Al-Al direct metal bonding, which is stable at Pb-free solder reflow temperature, should be created. Al is the standard metal of CMOS backend, free from the risk of metal contamination and inexpensive, and thus the bonding technology described in this paper is useful for MEMS-CMOS integration.

Comprehensive Die Shear Test of Silicon Packages Bonded by Thermocompression of Al Layers with Thin Sn Capping or Insertions

Thermocompression bonding for wafer-level hermetic packaging was demonstrated at the lowest temperature of 370 to 390 °C ever reported using Al films with thin Sn capping or insertions as bonding layer. For shrinking the chip size of MEMS (micro electro mechanical systems), a smaller size of wafer-level packaging and MEMS–ASIC (application specific integrated circuit) integration are of great importance. Metal-based bonding under the temperature of CMOS (complementary metal-oxide-semiconductor) backend process is a key technology, and Al is one of the best candidates for bonding metal in terms of CMOS compatibility. In this study, after the thermocompression bonding of two substrates, the shear fracture strength of dies was measured by a bonding tester, and the shear-fractured surfaces were observed by SEM (scanning electron microscope), EDX (energy dispersive X-ray spectrometry), and a surface profiler to clarify where the shear fracture took place. We confirmed two kinds of fracture mode. One mode is Si bulk fracture mode, where the die shear strength is 41.6 to 209 MPa, proportionally depending on the area of Si fracture. The other mode is bonding interface fracture mode, where the die shear strength is 32.8 to 97.4 MPa. Regardless of the fracture modes, the minimum die shear strength is practical for wafer-level MEMS packaging.

Low temperature hermetic sealing by aluminum thermocompression bonding using tin intermediate layer

Aluminum with Sn intermediate layer shows very large deformation even below 400°C. Using this new layer structure as sealing metal, high yield hermetic package of MEMS was demonstrated at only 370°C without any treatment of surface oxide removal. During bonding, the bonding metal is significantly pressed (the reduction rate of thickness ∼90%), which guarantees hermeticity at high yield. Based on SEM, EDX and TEM analysis, the role of tin for hermetic sealing and the mechanism of softening of this layer structure were discussed.