This paper reports on the deposition and characterization of piezoelectric Al_XSc_1-XN (further: AlScN) films on Si substrates using AlSc alloy targets with 30 at.% Sc. Films were deposited on a Ø200 mm area with deposition rates of 200 nm/min using a reactive magnetron sputtering process with a unipolar–bipolar hybrid pulse mode of FEP. The homogeneity of film composition, structural properties and piezoelectric properties were investigated depending on process parameters, especially the pulse mode of powering in unipolar–bipolar hybrid pulse mode operation. Characterization methods include energy-dispersive spectrometry of X-ray (EDS), X-ray diffraction (XRD), piezoresponse force microscopy (PFM) and double-beam laser interferometry (DBLI). The film composition was Al_0.695Sc_0.295N. The films showed good homogeneity of film structure with full width at half maximum (FWHM) of AlScN(002) rocking curves at 2.2 ± 0.1° over the whole coating area when deposited with higher share of unipolar pulse mode during film growth. For a higher share of bipolar pulse mode, the films showed a much larger c-lattice parameter in the center of the coating area, indicating high in-plane compressive stress in the films. Rocking curve FWHM also showed similar values of 1.5° at the center to 3° at outer edge. The piezoelectric characterization method revealed homogenous d_33,f of 11–12 pm/V for films deposited at a high share of unipolar pulse mode and distribution of 7–10 pm/V for a lower share of unipolar pulse mode. The films exhibited ferroelectric switching behavior with coercive fields of around 3–3.5 MV/cm and polarization of 80–120 µC/cm².

This work demonstrates an Overmoded Bulk Acoustic Resonator (OBAR) design that incorporates 35% Scandium doped Aluminum Nitride (Sc_0.35Al_0.65N) as the piezoelectric layer. The ScAlN OBAR presented here is a Bulk Acoustic Wave (BAW) resonator that excites a second overtone within a stack formed by a ScAlN layer and a set of alternating metallic layers. The metal electrodes act simultaneously as the acoustic cavity and as acoustic Bragg mirrors. Individual resonators are connected to each other by thick floating electrodes and top interconnects to form the devices demonstrated herein. The fabricated ScAlN OBAR with best performance exhibits a series resonant frequency of 51.3 GHz, electromechanical coupling ( $k_{t}^{2}$ ) of 6.1% and a Quality factor (Q) at series resonance of 108. The measurements of various ScAlN OBAR devices with different geometries show that Q is increasing as the perimeter and area of the individual resonator increases and $k_{t}^{2}$ is increasing as the number of resonators in series increases. Material losses and surface roughness with associated acoustic energy leakage are discussed as possible sources of damping in these mmWave resonators. The investigations trace a path for further technological improvement and show that the ScAlN OBAR is a promising device for mmWave acoustics and filtering applications. [2025-0071]