Properties of aluminum nitride thin films for piezoelectric transducers and microwave filter applications

Aluminum nitride thin films have been grown by reactive magnetron sputter technique using a pulsed power supply. The highly (002)-textured columnar films deposited on platinized silicon substrates exhibited quasi-single-crystal piezoelectric properties. The effective $d_{33}$ was measured as 3.4 pm/V, the effective $e_{31}$ as 1.0 C/m². The pyroelectric coefficient turned out to be positive (4.8 μC m⁻² K⁻¹) due to a dominating piezoelectric contribution. Thin-film bulk acoustic resonators (TFBAR) with fundamental resonance at 3.6 GHz have been fabricated to assess resonator properties. The material parameters derived from the thickness resonance were a coupling factor $\text{k=0.23}$ and a sound velocity $v_s\text{=11\hspace{0.05em}400\hspace{0.05em}}\mathrm{m/s}.$ With a quality factor Q of 300, the TFBARs proved to be apt for filter applications. The temperature coefficient of the frequency could be tuned to practically 0 ppm/K.