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.