62.6 GHz ScAlN Solidly Mounted Acoustic Resonators

Abstract

We demonstrate a record-high 62.6 GHz solidly mounted acoustic resonator (SMR) incorporating a 67.6 nm scandium aluminum nitride (Sc0.3Al0.7N) piezoelectric layer on a 40 nm buried platinum (Pt) bottom electrode, positioned above an acoustic Bragg reflector composed of alternating SiO2 (28.2 nm) and Ta2O5 (24.3 nm) layers in 8.5 pairs. The Bragg reflector and piezoelectric stack above are designed to confine a third-order thickness-extensional (TE) bulk acoustic wave (BAW) mode, while efficiently transducing with thickness-field excitation. The fabricated SMR exhibits an extracted piezoelectric coupling coefficient (k2) of 0.8% and a maximum Bode quality factor (Q) of 51 at 63 GHz, representing the highest operating frequency reported for an SMR to date. These results establish a pathway toward mmWave SMR devices for filters and resonators in next-generation RF front ends.

Figures (7)

• Figure 1: SMR structure, device layout, and key dimensions. (a) Cross-sectional schematic of the Pt/ScAlN/Pt stack on a SiO2/Ta2O5 Bragg reflector on a high-resistivity Si substrate. In the Bragg reflector, SiO2 is shown in light gray and Ta2O5 in dark gray; only a subset of the 8.5 pairs is drawn for clarity. (b) Top-view schematic of the device layout. Dimensions are listed in Table II.
• Figure 2: FEA mode shape at third-order TE resonance of 49.2 GHz. (a) Displacement confined to the Pt/ScAlN/Pt cavity with exponential decay into the SiO2/Ta2O5 reflector. (b) Axial stress $T_z$ with alternating sign and stress antinodes at the Pt/ScAlN interfaces.
• Figure 3: Simulated admittance of the SMR. (a) Simulated wideband admittance magnitude and key extracted parameters. (b-c) Zoomed-in admittance (b) magnitude and (c) phase around 50GHz.
• Figure 4: Structural verification of the SMR stack. (a) EDS line-scan confirming the expected compositional periodicity of the SiO2/Ta2O5 Bragg reflector and the Pt/ScAlN/Pt structure. The top protective metal is used only for TEM sample preparation. (b) Cross-sectional TEM of the layer stack. (c) XRD showing strong c-axis orientation of the ScAlN film. Note that the protective metal layer in Fig. 4(b) is deposited only during TEM sample preparation and is not the patterned top Pt electrode in the measured devices.
• Figure 5: Fabrication flow and device images. (a) As-deposited layer stack of Sc_0.3Al_0.7N on buried Pt on SiO2/Ta2O5 Bragg reflector on HR-Si. Only a subset of the Bragg reflector pairs is illustrated (schematic not to scale). (b) Mesa definition and ion-mill etching of ScAlN/Pt. (c) Low-temperature PECVD SiO2 backfill and lift-off. The PECVD SiO2 backfill/isolation oxide is shown in green to distinguish it from the Bragg-stack SiO2 layers. (d) Top Pt electrode patterning and deposition. (e) Thick electrode metal (300nm Al) deposition. (f) Optical microscope image of a completed SMR device. (g) Zoomed-in optical image of the resonator region.