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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The application of cantilevered structures as check valves or flow sensors can provide new possibilities towards the integration of accurate sample preparation systems within a lab-on-a-chip. The cantilevers presented in this paper act as flaps enclosed within a channel in a direction perpendicular to the flow. This orientation allows simpler designs and easier integration of the valve or flow sensor within the microfluidic network. The cantilevers have been embedded in a microfluidic channel by low temperature full wafer adhesive bonding. In this way, electrodes, microchannels, microchambers and cantilevers can be fabricated and sealed at the same time at a wafer level. To the author's knowledge, this is the first example of flap cantilevers embedded in a polymeric microfluidic channel. The mobility of the structure and the leakage are dependent on the size of the sealing gaps between the cantilever and the enclosing channel. In this paper, we present three different fabrication methods for a range of bottom sealing gaps from the micro to the nanometer size. The top sealing gap is determined by the adhesive bonding and is 11 µm wide. Furthermore, various geometrical features have been introduced in order to optimize a valve or flow sensor. The characterization of the structures comprises measurements of the sensitivity of each cantilever design by obtaining their relative spring constant, measurements of their elastic and plastic working regimes and Young's modulus of the SU-8.

The fabrication, characterization and packaging of novel microfluidic-optical integrated biosensors for label-free biochemical detection is presented in this paper. The integrated device consists of a three-dimensional embedded microchannel network fabricated using enhanced CMOS compatible SU-8 multilevel polymer technology on top of a wafer containing Mach-Zehnder Interferometer (MZI) nanophotonic biosensor devices. PMMA housing provides connection to the macro-world and ensures robust leakage-free flow operation of the devices. This macro-microfluidic module can operate at pressure drops up to 1000 kPa. Fluid flow experiments have been performed in order to demonstrate the robustness of our microfluidic devices. The devices have been designed to operate under continuous flow. Steady-state flow rates ranging from 1 to 100 µl min−1 at pressure drops ranging from 10 to 500 kPa were measured in the laminar flow regime. Experimental results are in good agreement with laminar flow theory. The first interferometric sensing measurements are presented in order to demonstrate the functionality of these novel integrated devices for lab-on-a-chip and label-free biosensing applications. A bulk refractive index detection limit of 3.8 × 10−6 was obtained, close to the minimum detected up to now by label-free biosensor devices without microfluidic integration. As far as we know, this is the first time that a label-free biosensor device is integrated within a microfluidic network using a wafer-level CMOS compatible process technology.