Aluminum nitride on titanium for CMOS compatible piezoelectric transducers

Piezoelectric materials are widely used for microscale sensors and actuators but can pose material compatibility challenges. This paper reports a post-CMOS compatible fabrication process for piezoelectric sensors and actuators on silicon using only standard CMOS metals. The piezoelectric properties of aluminum nitride (AlN) deposited on titanium (Ti) by reactive sputtering are characterized and microcantilever actuators are demonstrated. The film texture of the polycrystalline Ti and AlN films is improved by removing the native oxide from the silicon substrate in situ and sequentially depositing the films under vacuum to provide a uniform growth surface. The piezoelectric properties for several AlN film thicknesses are measured using laser doppler vibrometry on unpatterned wafers and released cantilever beams. The film structure and properties are shown to vary with thickness, with values of d(33f), d(31) and d(33) of up to 2.9, -1.9 and 6.5 pm V-1, respectively. These values are comparable with AlN deposited on a Pt metal electrode, but with the benefit of a fabrication process that uses only standard CMOS metals.