This paper describes a novel fabrication process based on successive wafer-level bonding and releasing steps for stacking several patterned layers of the negative photoresist EPON SU-8. This work uses a polyimide film to enhance previous low temperature bonding technology. The film acts as a temporary substrate where the SU-8 is photopatterned. The poor adhesion between the polyimide film and SU-8 allows the film to be released after the bonding process, even though the film is still strong enough to carry out photolithography. Using this technique, successive adhesive bonding steps can be carried out to obtain complex 3-D multilayer structures. Interconnected channels with smooth vertical sidewalls and freestanding structures are fabricated. Unlike previous works, all the layers are photopatterned before the bonding process yielding sealed cavities and complex three-dimensional structures without using a sacrificial layer. Adding new SU-8 layers reduces the bonding quality because each additional layer decreases the thickness uniformity and increases the polymer crosslinking level. The effect of these parameters is quantified in this paper. This process guarantees compatibility with CMOS electronics and MEMS. Furthermore, the releasing step leaves the input and the output of the microchannels in contact with the outside world, avoiding the usual slow drilling process of a cover. Hence, in addition to the straightforward integration of electrodes on a chip, this fabrication method facilitates the packaging of these microfluidic devices.

This paper describes a novel fabrication method for the manufacture of three-dimensional (3D) interconnected microchannels. The fabrication is based on a full wafer polymer bonding process, using SU-8 polymer epoxy photoresist as a structural material. The technology development includes an improvement of the SU-8 photolithography process in order to produce high uniformity films with good adhesive properties. Hence, 3D embedded microchannels are fabricated by a low temperature adhesive bonding of the SU-8 photopatterned thick films. The bonding occurs at temperatures (100-120 • C) lower than those usually applied in bonding technology. The bonding process parameters have been chosen in order to achieve a strong and void-free bond. High bond strengths, up to 8 MPa, have been obtained. Several examples using this new technology are shown, including bonding between different combinations of silicon and Pyrex wafers. This method also allows us to bond wafers with previously surface micromachined structures. Interconnected microchannels with vertical smooth walls and aspect ratios up to five have been obtained. Channels from 40 to 60 µm depth and from 10 to 250 µm width have been achieved. Liquid has been introduced at different levels into the microchannels, verifying good sealing of the 3D interconnected microchannels. The fabrication procedure described in this paper is fast, reproducible, CMOS compatible and easily implementable using standard photolithography and bonding equipment.

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This paper shows the potential of microfluidic devices as advanced artificial systems capable of modeling in vivo nutrient and oxygen gradients during tumor evolution.