Poly(dimethylsiloxane) (PDMS) is one of the most widely used polymers in microfluidic device applications due to its low cost of fabrication and its physical and chemical properties. With soft lithography being the standard technique used in fabrication of PDMS microfluidic devices, this study proposed an alternative plasma etching process to fabricate microfluidic devices in PDMS. PDMS, which is a silicon-based polymer, requires a fluorocarbon plasma to etch the Si-O backbones and an O2 plasma to etch the organic part. Plasma etching was performed in a Lam AutoEtch 590 plasma etcher and SPR 220-7 photoresist was used as a protective mask. The O2/CF4 compositions were varied to obtain an optimized etch rate and etch profile. It was found that 20 sccm of O2 and 40 sccm of CF4, at 2.8 Torr, 450 W and 0.38 gap spacing results in the fastest etch rate and anisotropic etching of microfluidic patterns into PDMS on a Si wafer. The etched surface was found to be smooth with a surface roughness, RRMS less than 1 nm. For a PDMS microfluidic device on a Pyrex 7740 borosilicate glass wafer, the plasma etch rate was much slower and the RRMS of the etched PDMS surface was 18.433 nm.
The fabrication of a PDMS nanofluidic device was also studied. The nanochannel structure in PDMS was fabricated on both Si and Pyrex wafers using the sacrificial layer lithography (SLL) technique. The microfeatures on the PDMS surface were created by microtransfer molding from a master SU-8 2005 wafer. A Ti sacrificial layer was deposited via e-beam evaporation on one side of the PDMS microfeatures using a self-shadowing technique. The second PDMS layer was spin coated on top and patterned with SPR 220-7 to function as a protective mask when etching in O2/CF4 plasma to create reservoirs and expose the Ti layer. The Ti metal was subsequently removed in HCl solution, leaving a nano space in the PDMS structure.
An integrated micro- and nanofluidic device in PDMS was fabricated on a Pyrex wafer with the fabrication process adapted from the PDMS nanofluidic fabrication on Pyrex wafer. Cracks were found on the etched surface, although the RRMS was 9.510 nm, smaller than that of the PDMS microfluidic device fabricated on a Pyrex wafer.