Thin-Film Piezoelectric-on-Silicon Resonant Transformers

Abstract

This paper reports the performance of leadzirconate-titanate (PZT)-on-silicon electromechanical resonators as thin-film piezoelectric transformers. The PZT-on-silicon resonators rely on the odd harmonics of the contour lengthextensional modes to provide a convenient means for voltage and current transformations with a single layer of piezoelectric PZT. The resistive-load-dependent voltage gains and efficiencies are derived along with the peak efficiencies and open circuit voltage gains. The models of efficiency and voltage gain are compared to the experimental measurements of fabricated PZTon-silicon piezoelectric transformers with 2, 4, and 10 μm of device silicon. The resonant frequencies of the devices range between 14 and 19 MHz. Peak efficiencies as high as 62% are measured and open circuit voltage gains as high as 5.7 are extracted from the measurements. The measured efficiencies with 50-loads compare within 14% of the models on average (25% peak error). The extracted open-circuit voltage gains and their models compare within 22% on average (67% peak error). The trade between the load-dependent efficiencies and voltage gains are also presented for the n =3, 5, 9, and 13 harmonics. [2012-0339]

Thin-film piezoelectric transformers operating in harmonics of out-of-plane flexure modes

“…When comparing the two design types, similar efficiency peaks of ~55% result, however, the corresponding resistive load at peak efficiency, R L,peak differ by a factor of 2X (with C in > C out ), which is not the expected 3X. This is largely due to the differences in routing resistances at the output port which has a direct impact on the electrical quality factor of the effective load and the transformer performance; this impact has been previously documented [1]. A summary of expected performances for the four different device types is shown in Table 2.…”

“…Comparison with Extensional-mode Transformers In Fig. 7 the TxFOMs for the current devices are compared with length-extensional PZT-on-Si transformers previously reported [1]. The prior work explored harmonics of length-extensional modes with 0.5 μm PZT on silicon devices and resonant frequencies between 14 and 19 MHz; the device silicon thicknesses assessed were 2, 4, and Disc-2-parallel 10 μm.…”

“…In contrast, thin-film electromechanical resonators and transformers offer extremely high equivalent electromechanical inductance densities (e.g. >100,000 nH/mm 2 at 14-20 MHz frequency [8]), but there are still challenges in maintaining the product of quality factor (unloaded) and electromechanical coupling factor, k eff 2 Q u , which has a direct impact on device efficiency and voltage gain [1].…”

“…Although there have been significant prior art detailing thin-film piezoelectric resonators for RF filters, there has been limited work on thin-film MEMS piezoelectric transformers (piezo-transformers) [1], [11][12][13][14][15]. While this work builds upon prior electrode shaping methods used for thin-film PZT resonators [16], it does contrast prior thinfilm PZT piezoelectric transformer work relying on the use of the typical extensional modes presented in the MEMS literature [1] and utilized with bulk-scale piezo-transformers [17].…”

“…While this work builds upon prior electrode shaping methods used for thin-film PZT resonators [16], it does contrast prior thinfilm PZT piezoelectric transformer work relying on the use of the typical extensional modes presented in the MEMS literature [1] and utilized with bulk-scale piezo-transformers [17]. Herein, flexure-mode devices are explored and feature asymmetric electrodes needed for impedance, voltage, or current transformation.…”