Wire Bonding of Surface Acoustic Wave (SAW) Sensors for High Temperature Applications

Abstract

SAW sensors are very suitable for high temperature applications up to 1,000 °C or higher. In this work CTGS substrates were used as test sample material. To realize a sensor setup an interdigital transducer (IDT) is necessary and deposited onto the surface of CTGS. In the present work thin film deposited tungsten molybdenum (WMo) and ruthenium aluminum (RuAl) as functional chip metallization will be investigated. For electrical interconnecting of sensor dies wire bonding is the dominant technology in general. Therefore, wire bonding is also the first choice to interconnect SAW sensors. Typical wire bonding materials are Copper, Gold or Aluminum (respectively AlSi1). With regard to high temperature applications up to 1,000 °C these materials have an unsuitable melting point. With regard to reliability the homologous temperature T/Tm should be less than 0.5 i.e. the melting point of the bonding wire has to be at least around 2,275 °C. In [1] Wolfram (TS =3,422 °C) is described as suitable wire material, but it is not common for wire bonding at present. Platinum (TS =1,768 °C) is the best fitting material which is available as adequate bonding wires at the market and already approved by wire bonding. So it is possible to use this material at least up to 748 °C for a homologous temperature of below 0.5. Another critical point besides the wire material is the chip metallization of the IDT and the antenna of the wireless SAW sensors. To reduce the thermoelectric effects at high temperatures and to improve the wire bonding process, platinum finish metallization are being investigated for both, CTGS and antenna on ceramic. For chip metallization thin film technology is used. For antenna either the thin film technology or the thick film technology can be used, while thick film technology is standard for this application. However, a combination of both technologies is applied in this paper to reach best results. Ultrasonic and Thermosonic wire bonding is also compared as main technologies for connecting the SAW chip to the antenna. In according to the DVS guideline 2811 wire pull and ball shear tests were performed on test samples to evaluate the bond quality. In publication [2] an increasing of pull strength after storage at high temperature is described. Therefore, the manufactured test samples were also stored at 800 °C for 2h and 10 h under high vacuum. The pull and shear test results are better as the initial results as expected.