Characterization of Pt/AlN/Pt-based structures for high temperature, microwave electroacoustic devices applications

Highly c-axis oriented AlN films, 3.15 μm thick, were grown by rf reactive sputtering technique at 200 °C on bare and Pt-covered Si(100) substrates previously oxidized to a thickness of about 2 μm in wet oxygen atmosphere. A Pt film, 2200 Å thick, was then sputtered on the free surface of the AlN/Pt/SiO2/Si multilayer at 200 °C without breaking the vacuum in order to avoid any oxidation effects of the layers. The multilayers were then annealed in air at 900 °C for different time lengths up to 32 h in order to test the materials' resistivity to harsh environment. The influence of this high temperature annealing (HTA) on the thin films' crystallinity, as well as on the c-AlN piezoelectricity and Pt sheet resistivity was investigated at room temperature before and after each annealing. X ray diffraction investigations revealed that the films' crystallinity was improved by the HTA: the full width of half maximum of the AlN(002) and Pt(111) peaks decreases from 0.39° to 0.24°, and from 0.42° to 0.28° after 32-hours-HTA. Scanning electron microscopy, four points probe and piezoelectricity tests revealed that the morphology and the sheet resistivity (in the range from 0.6 to 0.5 Ω/sq) of the outer Pt film, as well as the AlN piezoelectric constants d33 (in the range from 6.2 to 7.4⋅10−12 C/N) was quite unaffected by the HTA even after 32 h of annealing.