Microwave bonding of polymer-based substrates for potential encapsulated micro/nanofluidic device fabrication

Microwave-based bonding of polymer substrates is presented in this paper to illustrate a promising technique for achieving precise, localized, low temperature bonding. Microwave power can absorbed by a very thin film metal layer deposited on a polymer (PMMA) substrate surface. The intense thin-film volumetric heating promotes localized melting of refractory metals such as gold. One of the advantages of the process is that PMMA is relatively transparent to microwave energy in the 2.4GHz regime. This makes it an excellent substrate material for microwave bonding. Selective heating and melting of the thin layers of metal also causes localized melting of the PMMA substrates and improves adhesion at the interface. We have shown that ∼1μm of interfacial layer can be generated that is composed of the melted gold and PMMA, and which can hold two substrates together under applied tension greater than 100lb/in. ² (7kg/cm²). We also used lithographically patterned metal lines on a PMMA substrate to demonstrate that the PMMA remains optically transparent after microwave processing. A numerical simulation was also performed and validated with experimental results to show that globally the PMMA substrates indeed remained below its melting point during the microwave bonding process. The novel bonding process will open up possibilities for precise and total encapsulation of polymer-based micro/nanofluid devices - which are impossible to built using existing polymer bonding techniques.