SAW sensors on Aln/diamond/Si structures

In this work we present preliminary results on surface acoustic waves (SAW) chemical sensors based on a new AlN/diamond/Si multilayered structure. The high SAW velocity in diamond allows it to operate at higher frequencies at moderate interdigital transducer (IDT) line-width resolution in order to increase the sensor output signals, with the aim to increase the sensor sensitivity. Aluminium nitride has been chosen as piezoelectric layer because of its high SAW velocity together with excellent electrical, mechanical and chemical properties. The SAW phase velocity in the experimented structure is 10716 m/s for the Sezawa mode, more than three times that in ST-cut quartz. Both SAW delay line and 1-port resonator have been implemented and tested, under the following propagation conditions: acoustic wavelength λ=8 μm, normalized AlN film thickness h/λ=0.225, operation frequency f≅1.35 GHz. The thickness of the diamond layer (22 μm) is such that it can be considered as a semi-infinite substrate. The two test structures have been coated by thermal evaporation with a sensible thin (10 nm) layer of Co-tetra-phenyl-porphyrin which allowed us to detect small concentrations of ethanol and CO.