Dual-Mode Scandium-Aluminum Nitride Lamb-Wave Resonators Using Reconfigurable Periodic Poling

Abstract

This paper presents the use of ferroelectric behavior in scandium-aluminum nitride (Sc_xAl_1-xN) to create dual-mode Lamb-wave resonators for the realization of intrinsically configurable radio-frequency front-end systems. An integrated array of intrinsically switchable dual-mode Lamb-wave resonators with frequencies covering the 0.45-3 GHz spectrum. The resonators are created in ferroelectric scandium-aluminum nitride (Sc_0.28Al_0.72N) film and rely on period poling for intrinsic configuration between Lamb modes with highly different wavelengths and frequencies. A comprehensive analytical model is presented, formulating intrinsically switchable dual-mode operation and providing closed-form derivation of electromechanical coupling (kt2) in the two resonance modes as a function of electrode dimensions and scandium content. Fabricated resonator prototypes show kt2s as high as 4.95%, when operating in the first modes over 0.45-1.6 GHz, 2.23% when operating in the second mode of operation over 0.8-3 GHz, and series quality factors (Qs) over 300-800. Benefiting from lithographical frequency tailorability and intrinsic switchability that alleviate the need for external multiplexers, and large kt2 and Q, dual-mode Sc_0.28Al_0.72N Lamb-wave resonators are promising candidates to realize single-chip multi-band reconfigurable spectral processors for radio-frequency front-ends of modern wireless systems.

Keywords: Lamb-wave resonators; complementary switchable; ferroelectric; scandium–aluminum nitride.

Figure 1

(a) Cross−sectional schematic of unit-cell in the Lamb-wave resonators, with strain mode−shapes when operating in ${\mathsf{\Gamma }}_1$ and ${\mathsf{\Gamma }}_2$ modes. (b) Operation State 1 (unified polarization): polarization under all the IDTs is in same direction, enabling the high-$k_t^2$ excitation of ${\mathsf{\Gamma }}_1$ while ${\mathsf{\Gamma }}_2$ mode is turned off. (c) Operation State 2 (alternating polarization): polarization under consecutive IDTs is in the opposite direction, enabling the high-$k_t^2$ excitation of ${\mathsf{\Gamma }}_2$ while ${\mathsf{\Gamma }}_1$ mode is turned off.

Figure 2

(a) Normalized $k_t^2$ of ${\mathsf{\Gamma }}_1$ and ${\mathsf{\Gamma }}_2$ modes across different electrode finger widths and for arbitrary Sc content and electrode thickness. (b) Maximum achievable $k_t^2$ of ${\mathsf{\Gamma }}_1$ mode for different Sc contents and electrode thicknesses.

Figure 3

Fabrication process flow of Sc_0.28Al_0.72N Lamb-wave resonators.

Figure 4

(a) SEM image of Sc_0.28Al_0.72N Lamb−wave resonator. The inset shows the IDT with a 2.4 µm pitch size. (b) Cross−sectional SEM image of the resonator, detailing constituent-layer thicknesses.

Figure 5

(a) Polarization−voltage (P−V) hysteresis loop measured at an IDT port and a $100\mathsf{\mu }\mathrm{m}\times 100\mathsf{\mu }\mathrm{m}$ capacitor. (b) The measured instantaneous current at the IDT port, upon the application of a 45 kHz triangular PUND pulse sequence, highlighting polarization reversal in Sc_0.28Al_0.78N film.

Figure 6

Measured admittance of dual−mode Sc_0.28Al_0.72N Lamb−wave resonators when operating in either of the complementary switchable states defined by periodic poling procedure. The admittances are shown for resonators with (a) 8 mm, (b) 6 mm, (c) 4 mm, (d) 3 mm, and (e) 2.4 mm IDT pitch sizes. (f) The frequency of ${\mathsf{\Gamma }}_1$ and ${\mathsf{\Gamma }}_2$ modes for different IDT pitch sizes, highlighting the coverage of the 0.45−3 GHz spectrum.

Figure 7

Short−span measured admittance of the Lamb−wave resonator with an IDT pitch size of 2.4 mm when operating in (a) State 1 and (b) State 2. The intermediate admittance plots, shown in dashed gray line, highlight the transition between the two operation states.

Figure 8

(a) Measured $k_t^2$ of the two modes for resonators with different IDT pitch sizes, in comparison with values extracted from analytical model. (b) Measured Q_s and Q_p of the two modes, for resonators with different IDT pitch sizes. For each IDT pitch size, the $k_t^2$, Q_s, and Q_p are the average of values measured from ten resonators across the 4-inch substrate.