Fabrication of an autonomous surface stress sensor with the polymer SU-8

The aim of this thesis was the demonstration of a new sensor concept for the measurement of surface stress changes due to biomolecular interactions. The novelty of the sensor is that the readout is autonomous, which means that no external energy source is required. The core part of this device is a container filled with a coloured marker solution and closed by a thin polymer flapper. The surface of the device is functionalized with receptor molecules. Specific binding of analyte to the receptors results in a change of surface stress. As a consequence, the flapper bends mechanically and the marker solution is released. This colour change is detected by the naked eye or with an optical microscope. The focus of this thesis was on the design and fabrication of the autonomous sensor. A high actuation of the flapper is required to release the coloured marker. This is challenging because biomolecular interactions result in very small changes of surface stress. The fabrication was done with the negative epoxy photoresist SU-8. The advantage of this polymer compared to traditional silicon-based materials is that its Young’s modulus is about 50 times lower. This allows for high actuation due to changes in surface stress if the thickness of the flapper is minimized. In a first part of the thesis, the fabrication of thin SU-8 cantilevers was redesigned. These devices were used as a model system for process optimization. A new release method for thin SU-8 cantilevers was introduced using a fluorocarbon coating as release layer. The processing of thin SU8 films was redesigned to obtain high structural stability and low residual stress in the polymer. Further process optimization was required to minimize the initial out-of-plane bending of the cantilevers and to achieve high fabrication yield. With the optimized process, the fabrication of arrays of 2-μm-thick SU-8 cantilevers with a length of 500 μm an initial bending of less than 20 μm was demonstrated. Stability of the devices upon storage for several months was investigated and minimal influence was observed for devices fabricated with the optimized process. First, the optimized processes were used for the fabrication of SU-8 cantilever chips for the measurement of surface stress with optical read-out. There, new approaches for process-integrated metal coating, surface functionalization and passivation of cantilever-based sensors were introduced. The fabricated sensors were used for the measurement of surface stress that is generated by DNA-hybridization. The design and characerization of these chips should allow for conclusions on the expected actuation of the flapper due to biomolecular interactions. Finally, the flapperchip was designed and fabricated. The fabrication and release of flappers with an area of 400x400 μm and a thickness of only 2 μm was demonstrated. The thin SU-8 flapper had to be integrated in a microfluidic system that allows testing of the sensor. For this purpose, a Pyrex cover was thermally bonded to the SU-8 structures. The wafer-scale release of the flapperchips bonded to the Pyrex was possible. Finally, methods for coating of the flappers with a biopolymer were evaluated. Degradation of the biopolymer coating through hydrolysis or enzymes is expected to result in a high actuation of the flappers. In conclusion, the main achievement of this thesis was the optimization of existing and new processes for the fabrication of thin cantilever-based surface stress sensors with the polymer SU-8. The efforts were directed towards the fabrication of sensor for autonomous read-out of a change in surface stress but the discussed processes might be used for the fabrication of other sensors.