The accelerometer (see figure) is made by combining a resonant beam and a proof mass.

The devices are fabricated on an AlGaN/GaN heterostructure epitaxially grown on a silicon substrate. The device structure was initially optimized for HEMTs. GaN epitaxy on silicon can only be achieved at the cost of a residual stress in the GaN layer. This phenomenon, due to the mismatch in lattice parameters between GaN and Si and to the difference in the coefficient of expansion between the two materials, needs to be carefully controlled in order to release beams that are not deformed by buckling and whose resonant frequency can be reproducibly controlled. In addition, we need to be able to control the total thickness of the GaN layers, since they determine the thickness of the beams and their resonant frequency. Material growth research was carried out by the CRHEA under the supervision of Dr Yvon Cordier.

The resonator beam is released by chemical and plasma etching. The silicon substrate is also etched. The residual stress in the resonator allows detection by increasing or reducing the stress.

The test mass is released and held by four guiding arms. It is also connected to one end of the resonator (GaN beam). Here, the silicon substrate is retained to provide a sufficient mass.

To characterize the accelerometer, it is mounted on a “vibrating pot”, which is a support oscillating at a low frequency (20 Hz). The accelerometer response is obtained by measuring the phase of the resonator (GaN Beam) output signal as a function of time. The signal shows that the accelerometer is operational and that its response is proportional to the applied acceleration (figure). Further details can be found in the publications.