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We present a technology for the fabrication of cantilever arrays aimed to develop an integrated biosensor microsystem. The fabrication process is based on spin coating of the photosensitive polymer and near-ultraviolet exposure. Arrays of up to 33 microcantilevers are fabricated in the novel polymer material SU-8. The low Young's modulus of the polymer, 40 times lower than that of silicon, enables to improve the sensitivity of the sensor device for target detection. The mechanical properties of SU-8 cantilevers, such as spring constant, resonant frequency and quality factor are characterized as a function of the dimensions and the medium. The devices have been tested for measurement of the adsorption of single stranded DNA and subsequent interstitial adsorption of lateral spacer molecules. We demonstrate that sensitivity is enhanced by a factor of six compared to that of commercial silicon nitride cantilevers.

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A simple method is presented to reduce the contact angle of the photo-resist SU-8. A low contact angle is valuable in micro total analysis systems for the fabrication of micro channels where the capillary pressure is linearly related to the cosine of the contact angle, θ. If the surface of the channel is hydrophilic, the capillary pressure can be used as the only means to direct the liquid through the channels and the need for external pumps can be avoided. This is very useful, especially in the fabrication of devices for ‘lab-on-a-chip’ where it is important to keep the design as simple as possible. A commonly used technique for releasing structures fabricated on Si wafers is to use a sacrificial Cr layer. It is shown that the contact angle of SU-8 decreases by 40° after etching this layer. A further reduction in contact angle is desirable and can be achieved by treating the sample with ethanolamine at 50 °C for only 10 min. The resulting contact angle is 23° ± 7°. Using a wet chemical treatment, a selective change in contact angle between different areas of a micro channel system can be achieved, without the need to involve different materials in the fabrication process.

Here, we present the activities within our research group over the last five years with cantilevers fabricated in the polymer SU-8. We believe that SU-8 is an interesting polymer for fabrication of cantilevers for bio/chemical sensing due to its simple processing and low Young's modulus. We show examples of different integrated read-out methods and their characterisation. We also show that SU-8 cantilevers have a reduced sensitivity to changes in the environmental temperature and pH of the buffer solution. Moreover, we show that the SU-8 cantilever surface can be functionalised directly with receptor molecules for analyte detection, thereby avoiding gold-thiol chemistry.