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 $\Omega$ ) and $f\cdot Q$ product of $2.72 \ast 10^{12}$ at 888 MHz. The temperature coefficient of frequency of this TPoD resonator is measured to be $-9.6 \hbox {p\pm}/^{\circ} \hbox {C}$ , 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 $\sim!!2.9 \mu\hbox {W}/\mu\hbox {m}^{2}$ . $\hfill$ [2012-0099]