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

Abstract

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 <jats:formula formulatype="inline"><jats:tex Notation="TeX">$c$</jats:tex> </jats:formula>-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 <jats:formula formulatype="inline"><jats:tex Notation="TeX" >$(R_{m})$</jats:tex></jats:formula>. Several resonators were designed with various lateral dimensions and different numbers of support tethers to evaluate the propositions. The lowest <jats:formula formulatype="inline"><jats:tex Notation="TeX" >$R_{m}$</jats:tex></jats:formula> was measured from a multitethered 29th-order thin-film piezoelectric-on-diamond (TPoD) resonator (22 <jats:formula formulatype="inline"><jats:tex Notation="TeX"> $Omega$</jats:tex></jats:formula>) and <jats:formula formulatype="inline"><jats:tex Notation="TeX">$fcdot Q$</jats:tex></jats:formula> product of <jats:formula formulatype="inline"><jats:tex Notation="TeX">$2.72 ast 10^{12}$</jats:tex></jats:formula> at 888 MHz. The temperature coefficient of frequency of this TPoD resonator is measured to be <jats:formula formulatype="inline"><jats:tex Notation="TeX">$-9.6 hbox{ppm}/^{circ} hbox{C}$</jats:tex></jats:formula>, which is much lower than that of the devices fabricated on silicon. Also, this device can withstand input powers up to <jats:formula formulatype="inline"><jats:tex Notation="TeX">$+$</jats:tex></jats:formula>27 dBm, leading to a delivered power density per unit area of <jats:formula formulatype="inline" ><jats:tex Notation="TeX">$sim!!2.9 muhbox{W}/muhbox{m}^{2}$</jats:tex> </jats:formula>.<jats:formula formulatype="inline"><jats:tex Notation="TeX" >$hfill$</jats:tex></jats:formula>[2012-0099]

Low-loss lateral-extensional piezoelectric filters on ultrananocrystalline diamond

“…Thus, to increase the bandwidth of the filter while maintaining the lowest insertion loss possible, it is best to reduce the electrode separation between the input and output electrodes as much as the fabrication technology allows. as shown in [25], increasing mode number generally improves the Il of the Tpos resonator. Hence, monolithic filters were designed in four different order harmonics while keeping the aspect ratio of the device almost constant.…”

“…We have previously shown that multi-tethering of the Tpos resonators improves the q, Il, and power handling, while reducing the spurious modes, some of which could be very close to the target mode [22], [25], [26]. because the operation concept of the monolithic filters is similar to that of Tpos resonators, the same technique is adopted here.…”

“…In general, the frequency separation decreases as the distance between the input and output electrodes increases because of the weaker coupling between the two modes. However, because the electrode separation is increased by decreasing the length of the electrodes, the actuation area decreases, resulting in an inefficient excitation of the two modes and hence larger Il and more spurious modes [25]. considering that the filter structure is identical to the resonator except for the top electrode pattern, same techniques can be employed to improve the Il for both devices.…”

“…considering that the filter structure is identical to the resonator except for the top electrode pattern, same techniques can be employed to improve the Il for both devices. It is beyond the focus of this paper to discuss these methods, as they have been thoroughly explained in [25]. In summary, these techniques comprise multi-tethering and increasing the actuation area by increasing the length and the mode number.…”

“…Three diamond substrates with different young's modulus were prepared for fabrication of the Tpod filters. To improve the quality of the piezoelectric film deposited on the diamond substrate, in this case aluminum nitride (aln), the surface roughness had to be reduced [25]. This was accomplished by depositing ultrananocrystalline diamond (Uncd) with average grain size of less than 10 nm using hot filament chemical vapor deposition (HFcVd) technique, followed by two polishing steps.…”