Cantilever based sensors for surface stress measurements

The work presented in this Ph.D. thesis deals with the development of cantilever surface stress sensors. The aim is to use this sensor as a biochemical sensor; small micrometer sized cantilevers have been shown to be sensitive to the surface stress that is created when molecules adsorb on the surface of the cantilever. The readout technique for measuring the cantilever bending investigated in this work is the deformation of an integrated resistor. This deformation creates a resistance change of the resistor that is a measure of the bending of the cantilever. This readout technique is believed to be useful for making compact devices with many cantilevers for the purpose of measuring several biological species simultaneously. Such a measurement is usually made by inserting the cantilever into the liquid sample. In order to measure specific chemical species, the cantilever needs to be coated with a layer that binds these with a high degree of selectivity to other chemical components in the sample. A device like this can be thought of as a candidate for point-of-care analysis, which is diagnostic testing taking place at, for example, a physician’s office, directly where the sample is taken. A cantilever sensor, with polysilicon as the strain gauge material and titanium silicide as the on-chip wiring, was developed. Both resistors and wiring were encapsulated in LPCVD silicon nitride to make efficient protection of the electrical on-chip circuit against the buffer liquid that contains the analytes in biochemical measurements. To gain in resolution a new sensor was designed and partially realised, which features single crystalline silicon strain gauges. Single crystalline silicon makes more sensitive strain gauges than polysilicon and, at the same time, the electrical noise from a single crystalline silicon resistor is much lower than that of a polysilicon resistor. The principle resolution limit for a device made by the design described here should be close to the resolution limit set by thermal mechanical noise. A device including an array of cantilevers made entirely in the polymer SU-8 was presented. The integrated strain gauge is made of gold. This presents a potentially cheap device with a processing time much lower than that of comparable silicon devices.

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