HBAR oscillator with controlled aging and low power consumption

This PhD focuses on the development of resonators and oscillators for telecommunications, where increasing frequency and maintaining stability are major challenges. The main objective was to design, fabricate, and validate an HBAR resonator operating around 1 GHz, meeting requirements for miniaturization, low power consumption, and mechanical robustness.

After a comparative study of key technologies (SAW, MEMS, DRO, BAW, FBAR, SMR), the HBAR technology was selected for its high-quality factor, ability to operate at high frequencies, and compatibility with microfabrication processes. Several HBAR configurations were investigated. One of the proposed configuration was the HBAR on LiNbO₃/quartz, which provides a good frequency stability and low insertion losses. Fabrication was achieved through wafer bonding and thinning, enabling optimal thicknesses and orientations. Over 2 000 resonators were produced, with performances matching simulations (Q > 6 000, TCF≈ –5 ppm/°C, insertion losses < 12 dB). Several challenges were overcome, including thickness uniformity, insertion losses due to pads, and the management of cracks in piezoelectric material, thanks to trimming techniques, electrode geometry optimization, and reduction of thermomechanical stresses.

A fundamental challenge was the selection of a single harmonic among the many presents in the HBAR spectrum. An innovative method was developed, based on injecting an external frequency or phase and using the moiré principle, which was validated by simulation. Finally, the thesis led to the proposal of a first HBAR resonator design meeting the main requirements, as well as an architecture enabling precise selection of a single harmonic around 1 GHz.

oscillator, HBAR, resonator, laterally coupled resonator, micromanufacturing, telecommunications, high-overtone bulk acoustic resonators.

Full document (EN/FR) : Confidential until 31/12/2028.