Sputter-deposited AlN Research Articles

High-Q AlN/SiO2 symmetric composite thin film bulk acoustic wave resonators

High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO(2)-AlN-SiO(2) sandwiched between electrodes have been developed. Acoustic isolation was achieved by means of deep silicon etching to obtain membrane type thin film bulk acoustic wave resonators (TFBARs). Three different device versions were investigated. The SiO(2) film thicknesses were varied (0 nm, 70 nm, 310 nm, and 770 nm) while the piezoelectric AlN film had a constant thickness of 1.2 microm. The sputter-deposited AlN film grown on the amorphous, sputter-deposited SiO(2) layer exhibited a d(33,f) of 4.0 pm/V. Experimental results of quality factors (Q) and coupling coefficients (k(t)(2)) are in agreement with finite element calculations. A Q of 2000 is observed for the first harmonic of the 310 nm oxide devices. The most intense resonance of the 770 nm oxide device is the third harmonic reaching Q factors of 1450. The temperature drift reveals the impact of the SiO(2) layers, which is more pronounced on the first harmonic, reducing the TCF to 4 ppm/K for the 3rd harmonic of the 310 nm oxide devices.

Optical second-harmonic generation in sputter-deposited AlN films

Results of a study of optical second-harmonic (SH) generation in AlN films deposited on borosilicate glass by reactive direct current magnetron sputtering are reported. As determined by x-ray diffraction (XRD), the films were highly c-axis oriented. The transmitted SH signal for film thickness ranging from 340 to 7800 Å was measured. The signal was consistent with the existence of a dead layer (in which no SH was generated) on the film surface with an effective thickness of ∼300 Å. A set of 7800-Å-thick films were deposited at substrate temperatures from ambient to Ts=400 °C. For films deposited at 100 °C or higher, minimal variation in the SH signal with Ts was found. Values of |d33|=4 pm/V and |d31|=0.04 pm/V, with d33 and d31 having the same sign, were obtained for Ts=400 °C. The approximate tilt angle of the optic axis of the films with respect to the substrate surface normal was determined. XRD results showed that the tilt was due to a tilt of the crystallographic texture axis.

Ultraviolet second harmonic generation in radio-frequency sputter-deposited aluminum nitride thin films

Optical second harmonic generation in radio-frequency sputter-deposited AlN thin films has been studied for harmonic wavelengths from ultraviolet to near infrared. The effective second-order nonlinearity χ(2)(ω) was determined to have a nonresonant background value of ∼5×10−9 esu for second harmonic wavelengths longer than 400 nm, and it increases dramatically as the second harmonic frequency approaches the bandgap of 6.2 eV. This is likely due to resonance of the second harmonic frequency with the critical point transition associated with the direct bandgap of AlN.

Second order optical nonlinearities of radio frequency sputter-deposited AlN thin films

Polarized second harmonic generation measurements were performed on AlN films deposited on (100) sapphire substrates by the reactive rf sputtering technique. The bulk effective second order nonlinearity observed in these films is typically about 6×10−9 esu at 1.06 μm, several times larger than that of quartz or KTP. The tensorial properties of the nonlinearity are consistent with the crystal symmetry of AlN and the microcrystallinity of these films.