Highly textured growth of AlN films on sapphire by magnetron sputtering for high temperature surface acoustic wave applications

Abstract

Piezoelectric aluminum nitride films were deposited onto 3 in. ͓0001͔ sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly ͑002͒-textured AlN films to be used in surface acoustic wave ͑SAW͒ devices for high temperature applications. The synthesized films, typically 1 m thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction ͑XRD͒, transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly ͑002͒-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3°can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250°C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800°C for 90 min. The frequency response, recorded after the annealing process, confirmed that the thin films were still strong in a high temperature environment and that they had retained their piezoelectric properties.

Correlation between structural properties of AlN/Sapphire and performances of SAW devices in wide temperature range

“…This strain is the sum of the extrinsic stress due to the thermal expansion mismatch between the crystal and substrate, and the intrinsic stresses developed during the growth of the film [9]. In order to establish a correlation between the structural properties and SAW device performances, the variation of the net peak area of the (002) plan and the relative insertion loss (IL curve was reported in previous works [4]) with the variation of the annealing temperature is shown in figure 4. One can observe that the evolution of the crystalline quality of the AlN film (net area of (002) peak) with temperature follows similar trends as the evolution of IL.…”

“…EXPERIMENTAL DETAILS 1.3 μm-thick (002) AlN films were epitaxialy grown with the c-plane of the sapphire substrate by magnetron sputtering technique in Ar/N2 gas mixture. The experimental details are fully described in the reference [4]. In situ X-ray diffraction (XRD) annealing experiments were performed with a Brucker D8 diffractometer equipped with a lynxeye PSD detector and using the CoK α radiation (λ = 0.179026 nm).…”

“…At each temperature, the scan range of 2θ was 35° to 47° (XRD patterns presented in figure 1 are between 2θ = 41.5° and 43°). XRD measurements were made in the closest possible conditions to those used for measuring S parameters of SAW device [4]. Indeed, similar temperature profile was used and the chamber was pumped to reach a pressure of 2·10 −2 Torr.…”

Effects of sputtering atmosphere on the properties of c-plane ScAlN thin films prepared on sapphire substrate

“…The lattice mismatch between c-oriented AlN and [0001] sapphire is reasonable (13 %) and SAW velocities on sapphire are close to that of AlN, which is possible to make AlN/sapphire-based devices to work at frequencies as high as 5 GHz [7].…”

Surface Acoustic Wave Devices for Harsh Environment

“…Then the as grown films, 0.26 to 4.7 µm thick, were thermally annealed at 900°C in air for 1 to 18 hours and the AlN structural characteristics were evaluated after each thermal cycle. References are available in the literature concerning the high thermal annealing (HTA) of AlN on Si performed at 700 -1200 °C for 2 to 12 hours in controlled atmosphere (in oxygen or nitrogen flux) or in high vacuum; AlN films on Al 2 O 3 were heated at 950 °C in air for 30 minutes [20] to 1 hour [21], at 900 to 1200 °C for 10 s in flowing N 2 [22], and at 800 °C for 90 minutes in air [23]; bulk AlN was annealed in oxygen at 900 -1150 °C for 6 hours [24]. In the present work, the HTA of the AlN films on Si and on Al 2 O 3 substrates have been performed up to 32 and 18 hours, respectively.…”