Improving the Plasma-activated Bonding Strength between Polydimethylsiloxane and Lithium Niobate in the Fabrication of Microfluidic SAW Devices

The microfluidic surface acoustic wave (SAW) technology is widely used in biomedicine and environmental monitoring, including micro-particle separation, bioparticle concentration sensing, and non-contact fluid actuation. Although polydimethylsiloxane (PDMS) and lithium niobate (LiNbO, LNO) are widely employed in the fabrication of microfluidic SAW devices, the interfacial strength between PDMS and LNO substrates demonstrates insufficient mechanical robustness. In this study, we employed the microwave vacuum plasma processing system with an operating frequency of 2.45 GHz instead of the widely used 13.56 MHz. As a result, the microwave vacuum plasma exhibited a significantly higher density of oxygen plasma with the process gas of pure oxygen. The effects of key process parameters, including power and gas flow rate on the tensile strength of the bonding interface were investigated through tensile tests. The optimized bonding strength raised by 39 % in comparison with the existing plasma-activated bonding methods, reaching 1.95 MPa. Leakage tests confirmed that the maximum flow rate of the SAW separation device increased by over 75 %, exceeding 210 μL/min. Furthermore, the SAW device separated the polystyrene microspheres with diameters of 3 and 5 μm, achieving the separation efficiencies of 83.9 % and 90.2 %, respectively.