Thermal Control Design and Packaging for Surface Acoustic Wave Devices in Acoustofluidics

This article presents a thermal control design method for a surface acoustic wave (SAW) device. We designed a heat-dissipation structure and packaging scheme to solve three key issues observed in SAW devices using anisotropic crystals as piezoelectric substrates in acoustofluidics (e.g., lithium niobate): SAW chip cracking caused by thermal stress, SAW chip cracking caused by mismatched thermal expansion coefficients of the packaging materials, and enhancement of the structural strength and stability of the SAW chip. This study establishes the physical model of the designed structure and the relationship between the steady-state working temperature and the physical properties of the material. By comparing these physical properties and numerical calculations, we identified nanosilver adhesive as the most effective bonding material between the SAW chip and the heat sink. In addition to designing and fabricating, we also evaluated our SAW devices experimentally. The results not only confirmed that the abovementioned three key problems were solved but also demonstrated the significant enhancement of the stability of the SAW device.

I. Introduction

Surface acoustic waves (SAWs) are widely employed in microfluidic operations such as pumping, mixing, atomization, particle manipulation, and jetting, which involve reduced fluid volumes that are too small to control. Furthermore, SAWs have be successfully used in digital microfluidics for liquid handling in both closed microchannels and free droplets [1]–[3]. Their relatively cost-effective manufacturing technology involves the use of at least one set of metallic interdigital transducers (IDTs) fabricated on a piezoelectric substrate and a microchannel bonded with polydimethylsiloxane (PDMS). Moreover, owing to their small size, these could be added to a microscale system to successfully realize a lab-on-a-chip system. Hence, owing to the rapid development of electronic systems and microfabrication techniques, including mechanical structures, SAW technology has increasingly attracted attention for building multifunctional, multifrequency, miniaturized, and integrated acoustofluidic platforms [4].