An evaluation of critical issues for microhydraulic transducers : silicon wafer bonding, strength of silicon on insulator membranes and gold-tin solder bonding

Abstract

Microhydraulics transducers (MHT) are a class of microelectromechanical systems (MEMS) currently being developed to produce bi-directional transducers with high power densities (500-1000 W/kg). The development of these devices, which combine microfabrication technology and piezoelectric materials, requires the use of variety of materials and fabrication technologies that are not fully developed. Three materials and structures issues, which are essential to the development of MHT devices, are silicon wafer bonding, strength of silicon-on- insulator (SOI) membranes, and gold-tin bonding. Each of these topics was addressed independently. The mechanical integrity of silicon fusion bonds as a function of processing parameters was examined using a four-point bend delamination specimen. The study showed that the specimen was effective for characterizing low toughness bonds and that certain processing conditions can have a profound impact on bond toughness. Bond toughness increased with anneal time and temperature, but, initial contacting conditions, such as time and clamping pressure, proved to have little effect on final bond toughness. The fracture strength of membranes fabricated from SOI wafers using deep reactive ion etching was experimentally measured. Results showed that the strengths of these membranes was less than that of structures etched from bulk silicon and that the strength was dependent on SOI manufacturer. Finally, a thin film gold-tin solder bond was developed to bond bulk piezoelectric material to silicon structures. The process, which uses a sputtered gold-tin eutectic alloy (80wt%Au-20wt%Sn), was refined to produce void-free bonds. Preliminary tensile tests indicated failure was likely to occur in the piezoelectric material itself or along the solder-piezoelectric material interface. The results of these three studies provide information that is essential to the development of MHT devices as well as a wide range of MEMS devices.