This work investigates the Thin Film Bulk Acoustic Resonator operating at low frequencies. This study aims to substitute quartz resonators in the 4-27 MHz band and to fabricate selective filter for frequencies lower than 1GHz with quality factor higher than 10000. In this paper, we present the design, fabrication and testing of two different types of resonators. It consists of aluminum nitride film (0.8 μm) sandwiched between two aluminum electrodes (0.2 μm each). The first resonator is made by clamped edge beam and the second one is a free-free beam construction anchored in the middle of the cantilever. A demonstrator was achieved and the resonators are manufactured on a silicon substrate; AlN and Al layers were deposited on silicon using standard cathode sputtering technique. The resonators operate in extensional mode and the thicknesses of each of the materials are lower than 1μm. ANSYS, a Finite Element Analysis, has been performed to simulate the static, modal and harmonic behaviour. The simulation has been used, on the one hand, to determine the thickness of each material so as to reach the desired frequency range, on the other hand, to compare theoretical and experimental frequency values. First resonant frequencies between 2 and 10MHz were measured for resonators with dimensions of 20-40μm wide and 200-1000μm long and were found close to theory. Quality factor under 10000 operating in air has been achieved. These results confirm that such an integrated solution will replace Quartz oscillators and/or Surface Acoustic Wave filters in very compact applications.
Piezoelectric aluminum nitride (AlN) thin films have been developed to realize ultrasonic transducers. AlN up to 1.5m is deposited at low temperature (140 degree(s)C) by reactive DC magnetron sputtering of an Al target in argon and nitrogen on Si, Si/SiO2/Al, and Si/Al substrates, and is wet etched (rates from 0.1 micrometers /min to 0.2 micrometers /min and selectivity of 1:10 with Al, and no etching with Si). SiO2/Al/AlN/Al, Al/AlN/Al and Si/AlN/Al square and circular membranes, from 10 micrometers to 1.5 mm size are fabricated using silicon deep reactive ion etching (DRIE), which gives etch profiles about 90, which allows larger integration density than wet anisotropic etching for ultrasonic transducers arrays. By varying size and thickness of membranes, resonance frequencies from 10 kHz to 20 MHz are expected, acoustic and electrical measurements are in progress. Ultrasonic transducers using this technology will be used to measure flows velocity by Doppler method. Other potential applications for ultrasonic transducers include medical ultrasounds and sonar. Other structures are also in progress such as Thin Film Bulk Acoustic Resonator (TFBAR), and Lamb wave devices using this technology.
AlN films were deposited on Si(100), and Al/SiO2/Si substrates by reactive Direct Current (DC) magnetron sputtering of an Al target, under different conditions of substrate temperature, pressure, N2/N2 + Ar ratio. The film properties were investigated by X-ray Diffraction, scanning electron microscopy and atomic force microscopy. Deposition rates in the range of 1.2 to 1.8 micrometers /h were obtained, the film grain size was around 40nm. To fabricate test structures, wet chemical etching was developed to etch AlN with a good selectivity respect to Al and Si. Visual aspect and surface roughness show that the maximum temperature must be less than 300 degrees C. X-ray diffraction together with dielectric constant measurement show that films are better oriented on Si(100) than on Al/Si(100).
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