Yuan Lu

This work presents the laterally vibrating Lamb wave resonators (LVRs) based on a 30% aluminum scandium nitride (Al_0.7Sc_0.3N) thin film with three interdigited transducer pairs operating in the S₀ mode. In order to reduce the anchor loss, perfect matched layer-based finite element analysis simulations are utilized to design and optimize the device. Thanks to the high quality AlScN using magnetron sputtering with a single alloy target, vertical etching profile, and designed device structure, 1-μm-thick Al_0.7Sc_0.3N-based LVRs with high performance are fabricated. The resonator equivalent electric parameters are extracted utilizing the modified Butterworth–Van Dyke model. The best Al_0.7Sc_0.3N LVR achieves an electromechanical coupling coefficient (k_t²) of 9.7% and a loaded quality factor (Q_r) of 1141.5 operating at approximately 305 MHz. The same resonator shows a motional quality factor (Q_m) of 1507.2, resulting in a high figure-of-merit (FoM = k_t² · Q_m) of 146.2. A 1.8 MHz tuning range is measured for an Al_0.7Sc_0.3N LVR by applying DC voltage in the range of −40 to 40 V due to the ferroelectric property of high Sc doping in Al_0.7Sc_0.3N. With the high FoM, Q_r, Q_m, and low motional resistance (R_m), the Al_0.7Sc_0.3N-based LVRs show strong potential in applications of radio frequency communications and piezoelectric transducers.

Doped AlN thin films, especially high Sc-ratio AlScN film, have been reported to significantly improve the piezoelectric properties and draw attention for high performance resonators, transducers and integrated ferroelectric applications. However, many demonstrated devices were limited by poor film stress control, abnormal oriented grains and lack of a good etching profile. Compared to costly single alloy target, co-sputtered AlScN films can benefit customized doping concentrations and provide a unique solution for high Sc-ratio AlScN film quality and device studies. In this work, the optimized co-sputtering deposition and ICP etching processes of 500 nm AlScN thin film were developed and released AlScN Lamb-wave resonators were demonstrated. The influence of stress control by changing N ₂ process gas on the crystalline orientation, abnormal orientation grains, film roughness and piezoelectric property of AlScN thin films were discussed in detail. The AlScN film with a high Sc content requires a lower deposition pressure to obtain good crystalline quality. Al _0.85 Sc _0.15 N thin films with FWHM of 1.75°, an average stress of −14.5 MPa and a stress range of 156 MPa were obtained. 130 nm/min etching rate and 77° sidewall profile were achieved by optimized ICP etching of Al _0.78 Sc _0.22 N film. Lamb-wave resonators were fabricated based on both Al _0.78 Sc _0.22 N and Al _0.85 Sc _0.15 N thin films, achieving a quality factor of over 1000 at resonant frequency of approximately 300 MHz. The electromechanical coupling coefficients were improved by 152% and 80% compared to pure AlN devices.[2021-0210]

reports novel aluminum scandium nitride (Al _0.7 Sc _0.3 N) thin film acoustic delay lines (ADLs). Low loss ADLs are implemented on sputtered Al _0.7 Sc _0.3 N thin film using cross-field excited single-phase unidirectional transducers (SPUDT), which generate S0 mode Lamb wave between two ports. ADLs are fabricated with up to 300 $\mu \text{m}$ flat delay gap by optimized low stress Al _0.7 Sc _0.3 N thin film. A large electromechanical coupling factor ( $K^{ {2}}$ ) of 7.8% provided by approximately 1 $\mu \text{m}$ Al _0.7 Sc _0.3 N thin film allows for an overall improvement. Compared to implemented aluminum nitride (AlN) thin film counterparts, Al _0.7 Sc _0.3 N ADLs achieve remarkably lower insertion loss (IL), wide fractional bandwidth (FBW), and better impedance matching, featuring a minimum IL of 2.27 dB and a 3-dB FBW of 7.1% at approximately 780 MHz. The increase in relative permittivity due to Sc doping reduces the imaginary part of the matched network by 40%. For fundamental symmetric (S0) Lamb waves, a high group velocity of 8532 m/s, a propagation loss of 0.07 dB/ $ {\lambda }$ , and a propagation quality factor (Q) of 338 are extracted. Upon further development, scandium (Sc) doped AlScN thin films could enable CMOS-compatible piezoelectric acoustic platform for signal processing, sensing, and computing applications.

Wurtzite Al_1−xSc_xN thin films with scandium Sc concentrations up to x = 0.41 were prepared by reactive pulsed DC magnetron co-sputtering at heater temperatures between 300 °C and 400 °C on Al₂O₃ substrates. Spectroscopic ellipsometry was used to determine the dielectric functions of wurtzite Al_1−xSc_xN by modeling the spectra with a three-layer model involving parametric oscillator functions for the Al_1−xSc_xN layers. By combining ellipsometry with transmission spectroscopy, we determined the composition dependence of the optical band gap E_g. For x < 0.25, the experimentally determined band gap follows the theoretical prediction with an offset, which arises from residual sub-band gap absorption. For higher Sc concentrations, for which the band gap is expected to become indirect, the experimental data deviate from the theoretical values for the direct band gap. No absorption that can be attributed to cubic ScN crystallites was observed up to x = 0.41, indicating a high phase purity in line with X-ray diffraction data.

The piezoelectric and pyroelectric properties of aluminum nitride (AlN) can be further enhanced by alloying AlN with scandium nitride (ScN). However, while the pyroelectric effect can strongly affect the performance of electroacoustic devices such as surface acoustic wave (SAW) resonators, the pyroelectric properties of aluminum scandium nitride (AlScN) and their thermal dependence are not well known. In this work, the crystallographic, microstructural, and pyroelectric properties of 900 nm thick pyroelectric Al_1−xSc_xN thin films (x = 0 to 0.3) are investigated. All films have wurtzite‐type crystal structure and are highly c‐axis oriented. The pyroelectric coefficient in the temperature range of 20–80 °C is determined by measuring the generated current as a function of sinusoidal temperature excitation. The effective pyroelectric coefficient of AlScN increases by approximately 85% in the investigated composition range. Moreover, no temperature dependence of the effective pyroelectric coefficient is observed for all samples. The increase of the effective pyroelectric coefficient is explained qualitatively by considering the contribution of primary and secondary pyroelectric effect as well as the substrate clamping.

Reactive pulsed DC magnetron co‐sputtering is used to grow piezoelectric aluminum nitride (AlN) and aluminum scandium nitride (AlScN) thin films on Si(001) substrates. By using grazing incidence X‐ray diffraction (GIXRD), scanning electron microscopy (SEM), and piezoresponse force microscopy (PFM), we investigate how the microstructure and the presence of misoriented grains affect the piezoelectric properties of the material. N₂concentration and target‐to‐substrate distance are finely tuned to achieve thin films without misoriented grains, resulting in Al_0.87Sc_0.13N thin films with low roughness, high degree ofc‐axis orientation, homogenous polarity, and piezoelectric coefficientd₃₃ = −12.3 pC/N.