Outstanding quality factors of bulk acoustic wave resonators at cryogenic temperature

International audienceBulk acoustic wave resonators have already been tested at cryogenic temperature and high quality factors (Q-factors) have been observed in such conditions. Main results obtained at this time, and confirmed latter. So, it was tempting to investigate again this domain, with more accurate and easy-to-use means of measurements than in the years 60's - 70's, and even up to the years 90's. As a matter of course, specific means of investigation have been set up in this way. The Q-factor behavior of a quartz crystal resonator versus temperature when the temperature decreases from the ambient to cryogenic temperatures is quite well described at least up to above 10K. The Q-factor first increases, then goes down in a first peak at about 50 K, before increasing again with just a disturbance around 20 K. The increasing is essentially noticeable below this temperature. In the area of 50 K, the increasing of the Q-factor is attributed to a relaxation phenomenon due to the presence of Na+ impurities. These ions are combined with Al3+ ions substituted to some silicon ions Si4+ in the lattice. Interactions of the acoustic wave with acoustic phonons would be responsible of the deterioration of the Q-factor near 20 K. When the temperature decreases below 10 K, the increasing of the Q-factor can sometimes slow down because of the presence to the amorphous layer of silica at the resonator surface. In 1988, J. J. Suter has shown that an electrodeless resonator of the BVA type exhibits higher Q-factor than a conventional electrode-deposited resonator, at low temperature, especially below 5K. The fact that a metallised resonator has a lowest Q-factor has also been mentioned by Fraser