Improved Crystallinity of Annealed 0002 AlN Films on Sapphire Substrate.

AlN is a piezoelectric material used in telecommunication applications due to its high surface acoustic wave (SAW) velocity, stability, and mechanical strength. Its performance is linked to film quality, and one method to achieve high-quality films goes through the process of annealing. Consequently, c-orientated AlN film with a thickness of 1.1 µm deposited on sapphire was annealed at temperatures of 1100 °C and 1150 °C in a N2 controlled atmosphere. This was compared to annealing at 1100 °C, 1450 °C, and 1700 °C with N2 flow in an open atmosphere environment. Sample rotation studies revealed a significant impact on the ⍵-2θ X-ray rocking curve. A slight variation in the film crystallinity across the wafer was observed. After the annealing, it was found that the lattice parameter c was increased by up to 2%, whereas the screw dislocation density dropped from 3.31 × 1010 to 0.478 × 1010 cm-2, and the full width at half maximum (FWHM) of reflection (0002) was reduced from 1.16° to 0.41° at 1450 °C. It was shown that annealing in a N2-controlled atmosphere plays a major role in reducing the oxidation of the AlN film, which is important for acoustic wave devices where the electrodes are placed directly on the piezoelectric substrate. The face-to-face arrangement of the samples could further reduce this oxidation effect.

The impact of area on BAW resonator performance and an approach to device miniaturization.

The dependence of the performance of thin film bulk acoustic resonator (FBAR) and solidly mounted resonator (SMR), on their areas is studied with the aid of finite element method (FEM) software. Dual step frame method is applied for both types of the resonators in order to improve their quality factors at resonance and at antiresonance frequency when they are miniaturized. The important role of the material quality in promoting the benefit of this method is also emphasized in this study.

A Design Approach for High-Q FBARs With a Dual-Step Frame.

This paper presents a new approach for the design of film bulk acoustic resonators (FBARs) with high-quality factor ${Q}_{a}$ at antiresonant frequency ${f}_{a}$ . Lamb waves are among the main contributors to the loss of energy at ${f}_{a}$ and the deterioration of ${Q}_{a}$ . The targeted approach is an optimization process of a dual-step frame structure to alleviate the problem with Lamb waves at ${f}_{a}$ . Such a frame is a combination of two neighboring steps that have different acoustic impedances reflecting the two Lamb modes that carry the largest amount of energy. The dimensions of these steps are computed using the diffraction grating method. To do so, the dispersion curves of Lamb waves and their associated power flows are calculated using a global matrix method and the acoustic Poynting's Theorem. This results in an enhancement in ${Q}_{a}$ of nearly 82% for an FBAR with a dual-step frame in comparison with an FBAR with no frame, as well as a smoother electrical response, at frequencies above ${f}_{a}$ but below the resonant frequency ${f}_{r}$ spurious resonances are moderately increased.