Long-Term Stability of AlN Thickness-Shear Mode Resonators
Aluminum nitride (AlN) single crystals are promising candidate materials for bulk acoustic wave resonators in sensor and actuator systems operable at harsh conditions . They exhibit high phase stability up to the melting point and less susceptibility of the bulk electromechanical properties to surficial oxidation comparing to thin film AlN . Moreover, AlN is nominally oxygen-free, and its high-temperature performance should not suffer from oxygen ion transport, as commonly observed for oxide high-temperature piezoelectrics, e.g. of the langasite family .
The high-temperature and long-term stability of key application-relevant properties of bulk single-crystalline AlN under reducing atmospheres (pO2 ≈ 1e-17 bar at 1200 K) is presented. The studied AlN were (10-10)-oriented resonator plates, piezoelectrically excited to thickness-shear mode oscillations. As determined by SIMS, the crystals contained growth-related impurities of carbon (ca. 70 ppm) and oxygen (ca. 30 ppm). High Q-factors are inherent for such AlN, and the rise of high-temperature electromechanical losses (T = 900–1200 K) is governed mainly by the increase of electrical conductivity via the growth-related electrically active defects at high temperatures . The comparison of figures of merit of AlN with TSM resonators of the langasite family revealed that AlN greatly outperformed the langasite at T > 750K, although catangasite showed substantially lower losses in the whole temperature range. After high-temperature treatment of an AlN TSM resonator for about 22 days under reducing atmosphere neither the platinum electrodes nor the AlN crystals showed visible degradation. The resonance frequency remained virtually constant, dropping by only 0.37%, and the electromechanical losses increased only slightly. Contrary, the electrical conductivity of AlN decreased by a factor of 3, which is tentatively interpreted by the passivation of electrically active defects due to the incorporation of hydrogen. The observed FT-IR spectra show strong increase of the absorption peaks, typically attributed to vibrations of CHn bonds. Altogether, the parameters of AlN TSM resonators remained almost constant, indicating high durability and great potential of AlN for use in high-temperature sensor and actuator applications, in particular, at low oxygen partial pressures, e.g. in high-precision resonant nanobalances.