The increasing development of wireless communication bands has motivated the development of compact, low-loss, and frequency adjustable RF filtering technologies. Acoustic resonators are the ideal solution to these requirements, and tunable implementations offer a path toward reconfigurable front ends. In this work, we investigate epitaxial barium titanate (BTO) grown on silicon as a platform for tunable acoustic resonators operating in the sub-GHz regime. We demonstrate lateral excitation of symmetric lamb (S0) modes in X-cut BTO membranes, in contrast to prior thickness-defined ferroelectric resonators. Devices are designed using finite-element simulations and fabricated with laterally patterned electrodes that enable overtone coupling to multiple resonant modes. Under applied DC bias, ferroelectric domains align, allowing electrical excitation, frequency tuning, and quality-factor enhancement of acoustic modes. Resonances near 300 MHz and 700 MHz exhibit electromechanical coupling up to 8% and bias-dependent frequency tuning, with a distinct transition in behavior near 20 V. These results highlight monolithic BTO on silicon as a promising material system for laterally excited, tunable acoustic resonators for reconfigurable RF applications.
Modern wireless communications and technology have progressively shifted toward smaller, more clustered frequency bands with higher data rates [1]. With each additional communication band comes the addition of more Radio Frequency (RF) components to accommodate these bands and selectively select one signal from hundreds. Acoustic filters, with small size and low insertion loss, are ideal candidates for these tasks, as one can fit far more acoustic filters into cellular devices than electromagnetic (EM) versions [2], [3].
Current acoustic technology assigns one filter per frequency band to select the appropriate information. Thin film piezoelectric materials commonly used include aluminum nitride (AlN), scandium aluminum nitride (ScAlN), Lithium Niobate (LN), and Lithium Tantalate (LT) [4]- [6]. However, an alternative to using one filter per band is to use a single tunable filter across multiple frequency bands. Technologies for tunable integrated resonators/filters include phase change materials [7], ferromagnetics [8], or MEMS varactors and switches [9], [10]. Ferroelectrics offer an alternative route to integration, requiring only a DC bias in addition to the AC signal for tuning. These materials use a tuning of electrome- chanical coupling (k 2 ), or change in effective stiffness to change resonance frequency, and thus change filter frequency or turn off the filter altogether [11]. The most commonly used ferroelectric material in the acoustic domain is ScAlN, but is limited by only changing frequency with changes in effective stiffness, and the device cannot be turned on and off [12], [13]. Barium Titanate (BTO), alongside its lower Curie temperature counterpart Barium Strontium Titanate (BST), is an excellent candidate for tunable filters. Compared to ScAlN, the tunability is far greater through the increase of coupling with DC bias. BTO also offers several other advantages, including epitaxial growth on silicon (Si), control of orientation via growth conditions, and integration with other device types, such as electro-optic modulators [14]- [16]. However, prior BTO/BST resonators typically rely on thickness-defined frequencies and often require bottom electrodes, which limits monolithic integration and makes multifrequency-on-chip difficult without changing film thickness or adding additional process complexity.
The following work focuses on Epitaxial BTO on Si as a material for tunable acoustic resonators. Previous demonstrations of BTO have focused on film bulk acoustic resonators (FBARs) for acoustic devices, utilizing thickness electricalfield profiles to excite acoustic modes [17]- [19]. Here, we demonstrate lateral excitation of symmetric Lamb modes in Fig. 2. COMSOL admittance simulation shows S0 overtones and their stress profiles.
X-Cut BTO in the sub-GHz range.
Simulations were performed in COMSOL Multiphysics to determine optimal electrode configurations for coupling to the following mode profiles. Simulations are performed with 125 nm of BTO and 75 nm of gold for electrodes. Due to high film stress, release conditions were limited to isotropic etching of approximately 10 µm of silicon laterally to limit the out-of-plane deflection of said devices, as can be seen by the gradient of color in the blue released region. A total lateral size of 7.75 µm was chosen, with an electrode size of 1.25 µm and an aperture of 50 µm. Devices utilize the e 11 coefficient to excite fundamental symmetric lamb modes (S0). Due to the lateral spacing between the electrodes and the etch windows, the device functions as an overtone resonator and couples to multiple modes rather than a single mode [20]. The admittance plot of the device is shown in Fig. 2, which depicts the different modes we are coupling into, with progressively higher-order stress nodes.
For this study, intrinsic silicon wafers (R≈ 10000 Ωcm) with 2" diameter were used as substrates. Before BTO deposition, a 5 nm-thick SrTiO 3 (STO) buffer layer was deposited on the clean Si surface by molecular beam epitaxy and subsequently transferred under vacuum to a sputtering system. The BTO layer was deposited by off-axis RF magnetron sputtering at a substrate temperature of 700°C and was grown to a thickness of 120 nm. Epitaxial growth was confirmed using reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) in Fig. 3. The 120-nm BTO films on bulk Si showed an out-of-place lattice constant of 4.036 Å.
Devices were measured using a Vector Network Analyzer (VNA) with an applied DC bias between ports 1 and 2 [21]. When the stack is grown, unit cells have spontaneous polarizations that can point in one of four directions for aaxis BTO, termed ferroelectric domains [22]. For this reason, generally, electromechanical coupling cancels out, and no modes can be seen for unbiased measurements. However, when a DC bias is overlaid with our AC signal, these unit cells align, a net piezoelectric coefficient is realized, and acoustic mo
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