Bonding of Bulk Piezoelectric Material to Silicon Using a Gold-Tin Eutectic Bond
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BONDING OF BULK PIEZOELECTRIC MATERIAL TO SILICON USING A GOLD-TIN EUTECTIC BOND Kevin T. Turner1, Richard Mlcak2, David C. Roberts1 and S. Mark Spearing1 1 Massachusetts Institute of Technology, Cambridge, MA 02139 USA 2 Boston MicroSystems, Inc., Woburn, MA 01801 USA ABSTRACT A class of MEMS devices, which utilizes microfabrication technology and bulk piezoelectric material, is currently being developed to produce high power density transducers. A thin-film gold-tin eutectic solder bond has been developed to bond electrically and mechanically bulk piezoelectric elements to microfabricated silicon structures in these devices. A 4.3 µm thick multilayer film structure, consisting of a titanium adhesion layer, a platinum diffusion barrier, a gold-tin (80 wt.% Au - 20 wt.% Sn) alloy layer, and a pure gold capping layer, was sputter deposited on the piezoelectric components to be bonded. Bonding was accomplished by mating the piezoelectric components with silicon components metallized with a titanium-platinum-gold multilayer film and heating to approximately 300°C in a reducing atmosphere. The bonding technology allows thin, electrically conductive bonds to be formed between dissimilar materials with minimal amounts of applied pressure during bonding. Successful bonding has been achieved between single crystal silicon and polycrystalline lead-zirconate-titanate (PZT-5H) as well as between silicon and single crystal lead zinc niobate-lead titanate (PZN-PT). The process was optimized to produce mechanically robust, void-free bonds. The absence of voids was verified through scanning electron microscope examinations of bond cross-sections. Tensile tests conducted on representative structures indicated that the strength of the bond was limited by the strength of the titanium – PZT-5H interface. INTRODUCTION A class of MEMS devices known as micro-hydraulic transducers (MHT), are currently being developed for actuation and power generation applications [1,2]. To achieve the desired power densities and actuation capabilities, there is a need to integrate bulk piezoelectric (rather than thin-film) material into these devices. These devices are constructed by bonding multiple micromachined glass and silicon wafers. Figure 1 illustrates the basic construction of an MHT device and demonstrates the final bond step, a novel anodic bond between glass and silicon in which the piezoelectric elements (typically cylinders 0.5-2 mm in diameter and 1 mm thick) are simultaneously bonded using a gold-tin eutectic bond within the structure. The bond between the piezoelectric elements and silicon layers must provide both a mechanical and electrical connection, and must be compatible with the preferred anodic bond temperature of 300°C. Goldtin eutectic (80 wt.% Au - 20 wt.% Sn) solder bonding is an attractive option for this application because it has a eutectic temperature slightly less than 300°C, it is a hard solder with a relatively high yield strength, and is electrically conductive. The current work details the gold-tin bonding process dev
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