Irreversible bonding of PDMS-LiNbO3 heterostructure for microfluidic application by stepwise plasma modification

Abstract

Owing to the surface acoustic wave produced by lithium niobate (LiNbO3), the combination of LiNbO3 and polydimethylsiloxane (PDMS) was recently applied in microfluidic devices to realize fluid actuation and particle manipulation. However, direct bonding of LiNbO3 substrate to PDMS surface is still a challenge due to the large coefficient of thermal expansion mismatches at the interface. In this work, an efficient method for bonding PDMS-LiNbO3 heterostructure was proposed to improve the bonding strength. Both PDMS and LiNbO3 surfaces were plasma-modified by a stepwise treatment at room temperature. The hydrophilicity, morphology and chemical composition of the modified surface were characterized to analyze the effect of plasma modification. Results showed that plasma modification produced active chemical groups which made the surface extremely hydrophilic, but had no obvious effect on surface morphology. Besides, the bonding strength under different gas composition, flow and time ratio was studied. Compared with oxygen plasma modification, the bonding strength was further increased by oxygen‑nitrogen dual modification, reaching a maximum value of 1396.2 kPa. The bonded devices also showed good seal performance in the leakage test for microfluidic chip. The abundant active chemical groups introduced by dual plasma modification on the surface are the main reason for the improvement of bonding strength. When the modified surfaces contact with each other, these groups at the interface will react with each other, and finally the two interfaces are connected by covalent bonds.