Development of cantilevers for biomolecular measurements

Abstract

The main objective of this thesis has been the research in the design and fabrication of micro-cantilevers that are one of the most used mechanical transducers because of their versatility. The use of polysilicon piezoresistive cantilevers has been explored in order to detect binding forces between biomolecules. Force resolution under 100 pN was required. A detailed analytical study has been performed in order to calculate sensitivity and resolution when applying a force at their free end. The results obtained with this analysis have been confirmed by the use of FEM simulations and hence used to determine the optimum design of the piezoresistive sensor. U-shaped polysilicon cantilevers have been fabricated at CNM clean room facilities using a novel and dedicated technology. Designs were made following the criteria imposed by the previously obtained analytical results. The high force resolution required implied the fabrication of some cantilevers among the softest piezoresistive cantilevers reported up to date (elastic constants down to 0.5 mN/m). With the final optimized fabrication process, a yield of 95% has been achieved. Using a commercial CMOS technology (0.8 m from AustriaMicroSystems), polysilicon piezoresistive cantilevers have been designed and fabricated following again the criteria imposed by the theoretical analysis and, in this case, also design rules from the CMOS technology. Cantilevers were integrated with a filtering and amplifying circuitry to reduce noise. The softest piezoresistive CMOS integrated cantilevers have been obtained with a high yield and with an undamaged circuitry. In order to determine the actual sensitivity of such soft sensors and their gauge factor, a characterization method (consisting in AFM actuation) has been developed. Gauge factor for polysilicon deposited at CNM and at AustriaMicroSystems was -12 and -9 respectively. The maximum force sensitivity and force resolution obtained for CNM fabricated sensors have been 11 V/nN and 28 nN respectively. The maximum force sensitivity and force resolution obtained for CMOS fabricated sensors have been 11 V/pN and 27 pN respectively. In both cases, resolution is limited by the noise in the circuit, whose main contributions are Hooge noise (or 1/f) and Johnson noise (or thermoelectric). Conductive, but isolated, nitride cantilevers (with a wrapped gold layer) with a sharp tip (that has an opened contact) have been designed and fabricated to be used in conductive measurements in liquid environments. Polysilicon tips definition has been optimized to improve the whole probes fabrication process, achieving apex radii smaller than 20 nm using a dry etching by means of a DRIE equipment followed by sharpening oxidation. A complete and novel technological process has been developed for the fabrication of AFM cantilevers. Different tip materials and machining processes have been analyzed, obtaining the best results for crystalline silicon tips defined using a DRIE equipment to machine rocket tips. Isotropic processes with low cross-wafer dispersion and anisotropic processes with low cross-wafer dispersion and low scalloping have been achieved. After a sharpening oxidation, apex radii smaller than 5 nm have been achieved. Complete AFM probes have been fabricated. In order to test the developed technology, probes with similar characteristics to commercial ones were fabricated and used to raster scan some samples (in contact and non-contact mode) yielding results similar to those obtained with commercial probes. In addition, some special probes have been fabricated for nanoindentation over polymers and also to improve Kelvin Probe Force Microscopy (KPFM) performance. Thus, the availability of a technology that allows the fabrication of customized cantilevers is very useful for the development of new SPM applications.

Date of Award	14 Dec 2006
Original language	Undefined/Unknown
Supervisor	Joan Bausells Roige (Director)