High-Frequency Thin-Film AlN-on-Diamond Lateral–Extensional Resonators

In this paper, low-impedance lateral-extensional microresonators are fabricated on a stack of aluminum nitride (AlN) directly deposited on a polished ultrananocrystalline diamond (UNCD) film. The large acoustic velocity of UNCD is utilized to extend the frequency of such resonators beyond 1 GHz while the frequency-defining features are not reduced excessively. In order to promote the growth of a c-plane piezoelectric AlN film, the surface of the UNCD film is polished after deposition. Three different UNCD films with different Young's modulus values were prepared, and frequencies up to two times that of similar devices fabricated on silicon have been achieved. The finite-element analysis is employed to evaluate the effect of various physical parameters on the performance of the thin-film piezoelectric-on-substrate resonators in order to achieve very low motional resistance (R
_m
). Several resonators were designed with various lateral dimensions and different numbers of support tethers to evaluate the propositions. The lowest R
_m
was measured from a multitethered 29th-order thin-film piezoelectric-on-diamond (TPoD) resonator (22 Ω) and f · Q product of 2.72 * 10
¹²
at 888 MHz. The temperature coefficient of frequency of this TPoD resonator is measured to be -9.6 ppm/°C, which is much lower than that of the devices fabricated on silicon. Also, this device can withstand input powers up to +27 dBm, leading to a delivered power density per unit area of ~2.9 μW/μm
²
.