Thick single-layer positive photoresist mold and poly(dimethylsiloxane) (PDMS) dry etching for the fabrication of a glass–PDMS–glass microfluidic device

Abstract

Poly(dimethylsiloxane) (PDMS) is an elastomer widely used in microfluidic devices due to desirable characteristics such as its low cost and moldability. Photoresist patterned to form microfluidic features is often used as a mold, and SU-8 is commonly used. However, negative photoresists like SU-8 are difficult to remove, as compared to positive photoresists. Here, a method of forming microfluidic structures in the PDMS with an AZ9260 thick positive (Novolak-based) photoresist using standard microfabrication technique is reported. The chip consisted of a sandwich structure with glass on the top and bottom and PDMS with microfluidic channels in the middle; these structures have PDMS sidewalls, but are free of PDMS on the bottom and top. A single layer of the positive AZ9260 photoresist was spun to controllable and uniform thicknesses of up to 49 µm and used as a sacrificial mold to create PDMS microfluidic features. Reactive ion etching (RIE) with CF4 and O2 gases and a liftoff technique was used to create these features, and subsequent irreversible bonding to glass was achieved by activation in oxygen plasma by RIE. The final depths of channels were about 20–30 µm. The effect of the gas flow rate ratios on the bonding ability was investigated, and the surfaces of etched PDMS were studied under a scanning electron microscope (SEM), and the bonding strength was measured to be 0.85 MPa. The fabricated microfluidic device was tested and shown to be leak free.