Periodically Poled Lamb Wave Resonators Based on Ferroelectric Aluminum Scandium Nitride

This paper introduces periodically poled Lamb wave resonators (PPLWR) that utilize the ferroelectricity (FE) of aluminum scandium nitride <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{Al}_{1-\mathrm{x}}\text{Sc}_{\mathrm{x}}\mathrm{N})$</tex> to remove the need for interdigitated transducers (IDTs). Instead, laterally periodic poling of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Al}_{0.72}\text{Sc}_{0.28}\mathrm{N}$</tex> allows use of continuous top and bottom electrodes. Through this process, drawbacks introduced by IDTs in traditional LWRs can be largely omitted, while the invaluable characteristic of the LWR – lithographically defined frequencies – can be retained. PPLWR prototypes with domain periodicities from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$14.4\ \mu \mathrm{m}$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.8\ \mu \mathrm{m}$</tex> were fabricated and tested. Acoustic resonances were realized at all target frequencies from 555 MHz to 3.48 GHz, with the highest <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k_{\mathrm{t}}^{2}$</tex> reaching 13.6%, when the device was working in the cross-sectional Lamé mode (CLM). To our knowledge, this is the first demonstration of IDT-free <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Al}_{1-\mathrm{x}}\text{Sc}_{\mathrm{x}}\mathrm{N}$</tex> LWRs at a higher overtone. This innovation provides a workable approach for further scaling LWRs well into the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$X/K_{\mathbf{u}}$</tex>-band region.