White Papers

White Papers

High kt2 Q Lamb-wave ScAlN-on-Silicon UHF and SHF Resonators

Abstract – This paper reports, for the first time, on waveguide-based resonators implemented in scandium-doped aluminum nitride-on silicon (ScAlN-on-Si) stack to simultaneously benefit from large piezoelectric constants of ScAlN and low acoustic dissipation in single crystal silicon. 1 μm-thick ScAlN film with Sc content of 7% is reactively sputtered on silicon substrates using ac-powered dual target S-gun magnetron with Al targets containing embedded pure Sc pellets. A Cl2/H2 based low-power plasma etching recipe is developed to pattern resonators with smooth vertical sidewalls. In- and out-of-plane waveguide-based resonator prototypes with large electromechanical coupling coefficient (kt2) and high quality-factor (Q) are implemented over 80 MHz – 3.5 GHz demonstrating kt2 of 0.7%-2.9% and Q of 2000-6400. Specifically, a high f0 x Q of 4.3 x 1012 is measured for a resonator at 3.5 GHz, and a high kt2 x Q of 51 is measured at 108 MHz. The large kt2 x Q of ScAlN-on-Si waveguide-based resonators along with lithographical frequency tailorability demonstrate their potential for realization of highly integrated front-end filters for multi-band 5G systems.

AlN thin films grown on epitaxial 3C–SiC (100) for piezoelectric resonant devices

Abstract — Highly c-axis oriented heteroepitaxial aluminum nitride AlN films were grown on epitaxial cubic silicon carbide 3C–SiC layers on Si 100 substrates using alternating current reactive magnetron sputtering at temperatures between approximately 300–450 °C. The AlN films were characterized by x-ray diffraction, scanning electron microscope, and transmission electron microscopy. A two-port surface acoustic wave device was fabricated on the AlN/3C–SiC/Si composite structure, and an expected Rayleigh mode exhibited a high acoustic velocity of 5200 m/s. The results demonstrate the potential of utilizing AlN films on epitaxial 3C–SiC layers to create piezoelectric resonant devices.

Piezoelectric aluminum nitride nanoelectromechanical actuators

Abstract — This letter reports the implementation of ultrathin 100 nm aluminum nitride AlN piezoelectric layers for the fabrication of vertically deflecting nanoactuators. The films exhibit an average piezoelectric coefficient d31−1.9 pC/N, which is comparable to its microscale counterpart. This allows vertical deflections as large as 40 nm from 18 m long and 350 nm thick multilayer cantilever bimorph beams with 2 V actuation. Furthermore, in-plane stress and stress gradients have been simultaneously controlled. The films exhibit leakage currents lower than 2 nA/cm2 at 1 V, and have an average relative dielectric constant of approximately 9.2 as in thicker films. These material characteristics and actuation results make the AlN nanofilms ideal candidates for the realization of nanoelectromechanical switches for low power logic applications.

Surface acoustic wave propagation properties in AlN/3C–SiC/Si composite structure

Abstract — Highly c-axis oriented aluminum nitride (AlN) films were grown on epitaxial cubic silicon carbide (3C–SiC) layers on Si (100) substrates using alternating current (AC) reactive magnetron sputtering at temperatures between 300 °C to 450 °C. The AlN thin films were characterized by X ray diffraction, scanning electron microscope, and transmission electron microscopy. Two-port surface acoustic wave (SAW) devices were fabricated on the AlN/3C–SiC/Si layered structure. The SAW propagation properties in the AlN/3C–SiC/Si layered structure were theoretically and experimentally investigated. The Rayleigh mode exhibited a high acoustic velocity of 5,200 m/s due to the epitaxial 3C–SiC layer. The results demonstrate the potential of AlN thin films grown on epitaxial 3C–SiC layers to create piezoelectric acoustic devices for frequency control and harsh environment applications.

Temperature-compensated aluminum nitride lamb wave resonators

Abstract — In this paper, the temperature compensation of AlN Lamb wave resonators using edge-type reflectors is theoretically studied and experimentally demonstrated. By adding a compensating layer of SiO2 with an appropriate thickness, a Lamb wave resonator based on a stack of AlN and SiO2 layers can achieve a zero first-order temperature coefficient of frequency (TCF). Using a composite membrane consisting of 1 μm AlN and 0.83 μm SiO2, a Lamb wave resonator operating at 711 MHz exhibits a first-order TCF of −0.31 ppm/°C and a second-order TCF of −22.3 ppb/°C2 at room temperature. The temperature-dependent fractional frequency variation is less than 250 ppm over a wide temperature range from −55°C to 125°C. This temperature-compensated AlN Lamb wave resonator is promising for future applications including thermally stable oscillators, filters, and sensors.

Optimization of thin AlN sputtered films for X-band BAW Resonators

Abstract — We investigate the sputter growth of very thin aluminum nitride (AlN) films on iridium electrodes for high frequency filtering applications. The structure and piezoelectric activity of AlN films are assessed through XRD, FTIR, stress and frequency response measurements. A combination of pre-deposition rf plasma treatment of the Ir bottom electrode followed by a two-step ac reactive sputtering of the AlN film allows to optimize the crystal quality and residual stress of AlN films with a thickness as low as 160 nm. BAW resonators tuned around 8 GHz are built on top of polished Bragg reflectors composed of porous SiO2 and Ir layers. Material coupling factors kmat2 of 6.7% and quality factors Q close to 900 are achieved. The films obtained are competitive for X-band filter fabrication.