Interface Material Modification to Enhance the Performance of a Thin-Film Piezoelectric-on-Silicon (TPoS) MEMS Resonator by Localized Annealing Through Joule Heating

Abstract

This paper presents a novel approach employing localized annealing through Joule heating to enhance the performance of Thin-Film Piezoelectric-on-Silicon (TPoS) MEMS resonators that are crucial for applications in sensing, energy harvesting, frequency filtering, and timing control. Despite recent advancements, piezoelectric MEMS resonators still suffer from anchor-related energy losses and limited quality factors (Qs), posing significant challenges for high-performance applications. This study investigates interface modification to boost the quality factor (Q) and reduce the motional resistance, thus improving the electromechanical coupling coefficient and reducing insertion loss. To balance the trade-off between device miniaturization and performance, this work uniquely applies DC current-induced localized annealing to TPoS MEMS resonators, facilitating metal diffusion at the interface. This process results in the formation of platinum silicide, modifying the resonator's stiffness and density, consequently enhancing the acoustic velocity and mitigating the side-supporting anchor-related energy dissipations. Experimental results demonstrate a Q-factor enhancement of over 300% (from 916 to 3632) and a reduction in insertion loss by more than 14 dB, underscoring the efficacy of this method for reducing anchor-related dissipations due to the highest annealing temperature at the anchors. The findings not only confirm the feasibility of Joule heating for interface modifications in MEMS resonators but also set a foundation for advancements of this post-fabrication thermal treatment technology.

Keywords: Joule heating; Q-factor; anchor-related losses; localized annealing; piezoelectric; thin-film piezoelectric-on-silicon (TPoS) resonator.

Figure 1

(a–h) Illustration of the fabrication process flow by depicting key steps of the fabrication in perspective, cross-section, and top-view diagrams.

Figure 2

SEM image of a microfabricated, rectangular-shaped ZnO-on-Si resonator.

Figure 3

Modal displacement of a rectangular-plate resonator at its 5th-order mode.

Figure 4

Conceptual illustration of the localized annealing experimental setup, where the strategically chosen amplitude and duration of a DC current is applied to heat the resonator body through its side-supporting anchors and electrodes.

Figure 5

COMSOL Multiphysics thermal simulation of an array of identical rectangular-plate ZnO-on-Si resonator devices under localized Joule heating. The simulation shows that the temperature elevation—indicated by the color bar (in °C)—is confined to the annealed resonator, while adjacent devices remain at significantly lower temperatures. This demonstrates that localized annealing by resistive Joule heating effectively limits the thermal impact on neighboring structures, a result not achievable by conventional annealing methods such as furnace, oven, or hot-plate treatments.

Figure 6

COMSOL Multiphysics thermal simulation of a disk-shaped and a rectangular-plate resonator, both with side-supporting anchors, through localized annealing under conditions identical to those applied during the experimental study conducted in this work.

Figure 7

Top-view diagram of a rectangular-plate resonator to illustrate its key dimensions, as well as the layout of its input/output IDT electrodes (shown in red).

Figure 8

Top-view schematic and SEM photo of a 150 μm radius disk resonator to show its key dimensions and the layout of its two-port input and output electrode configurations.

Figure 9

Simplified diagram to showcase the mechanism and effect of localized annealing [29].

Figure 10

Conceptual illustration of silicide formation through metal diffusion.

Figure 11

TEM cross-sectional images after localized annealing indicate silicide formation.

Figure 12

A comparison of measured frequency responses of a rectangular-plate ZnO-on-Si piezoelectric resonator before and after localized annealing.

Figure 13

A comparison of measured frequency responses of a disk-shaped ZnO-on-Si piezoelectric resonator before and after localized annealing.

Figure 14

TEM images for a FIB-prepared cross-sectional specimen from a rectangular-plate resonator by showing resonator layer interfaces: (a) before annealing and (b) after annealing.

Figure 15

TEM image of snowplow-like regions where PtSi formations were observed. The presence of PtSi was confirmed through Energy Dispersive Spectroscopy (EDS) analysis using the X’Pert HighScore software (version 1.0f). The corresponding reference pattern is presented in Table 1.

Figure 16

TEM image of snowplow-like regions where PtSi formations were observed. The presence of PtSi was confirmed by Energy Dispersive Spectroscopy (EDS) analysis using the X’Pert HighScore software. The corresponding PtSi reference pattern is presented in Table 1.

Figure 17

A comparison of Energy Dispersive Spectroscopy (EDS) analysis results from annealed and unannealed TEM samples, which clearly show that the localized annealed sample has a substantial amount of Pt diffused into the SOI’s Si device layer away from the original Pt/Si interface.