Performance of Zr and Ti adhesion layers for bonding of platinum metallization to sapphire substrates

Single crystal sapphire wafers with <1 nm root mean square (RMS) roughness are ideal substrates for chemiresistive sensors that utilize ultra-thin (<50 nm thick) semiconducting metal oxide (SMO) films. Platinum metallization on a highly polished sapphire platform to form electrodes, heater, and a resistive temperature device (RTD) requires the use of a very thin (<20 nm) buffer layer, such as Ti or Zr, to achieve good adhesion at the Pt/sapphire interface. Using AES, secondary ion mass spectroscopy (SIMS), XRD, and wire bond tests before and after annealing treatments, we have found that Zr has superior performance as an adhesion layer compared to Ti. At temperatures of 200–700°C, required for RTD and SMO film stabilization as well as prolonged sensor operation, there is significant migration of Ti through the Pt film, whereas the Zr layer is less mobile. The Pt/Zr/sapphire architecture also minimizes delamination failure of wire bonds to the sensor device.