A new SU-8 process to integrate buried waveguides and sealed microchannels for a Lab-on-a-Chip

This paper shows a novel use of SU-8, a photodefinable epoxy, to monolithically integrate low cost optical sensors and microfluidic structures. The chip consists of optical connectors, multimode waveguides and sealed microfluidic channels patterned in SU8 on a silicon substrate. We describe in detail an SU-8 microfabrication process consisting of (i) four successive photolithographic steps, (ii) a bonding process and (iii) a final releasing step. The core of the waveguide is made of standard SU-8, whereas its cladding, with a lower refractive index, is achieved by diluting the SU-8 in a liquid aliphatic resin. Despite the dilution, the SU-8/aliphatic mixture is still patternable by photolithography allowing us to fabricate microchannels. The input and output optical fibres are horizontally aligned to the waveguides by their insertion in U-Grooves, and vertically aligned by the elevation of the waveguide using a lower cladding layer that allows us to lift it up to meet the centre of a standard optical fibre. Microfluidic channels with smooth vertical walls (125 μm height and 30 μm width) have been achieved. Microchannels are sealed by low temperature adhesive bonding of the SU-8 photopatterned thick-films at a wafer level. Liquid flows in the channels verify good sealing of the microchannels. In order to validate the interaction of these optical-fluidic microcomponents, we carried out an absorption chemical assay. The fabrication procedure described in this article is a fast, reproducible, CMOS compatible and simple way to develop optical Lab-on-a-Chip devices using standard photolithography and bonding equipment.