The availability of very sharp, wear-proof, electrically conductive probes is one crucial issue for conductive atomic force microscopy (AFM) techniques such as scanning capacitance microscopy, scanning spreading resistance microscopy, and nanopotentiometry. The purpose of this systematic study is to give an overview of the existing probes and to evaluate their performance for the electrical techniques with emphasis on applications on Si at high contact forces. The suitability of the characterized probes has been demonstrated by applying conductive AFM techniques to test structures and state-of-the-art semiconductor devices. Two classes of probes were examined geometrically and electrically: Si sensors with a conductive coating and integrated pyramidal tips made of metal or diamond. Structural information about the conductive materials was obtained by electron microscopy and other analytical tools. Swift and nondestructive procedures to characterize the geometrical and electrical properties of the probes prior to the actual AFM experiment have been developed. Existing contact models have been used to explain variations in the electrical performance of the conductive probes.
High-frequency impedance biosensors with nanometer gaps have been prepared for the detection of biomolecular interactions such as protein-antibody and protein-aptamer binding. The sensor principle is based on electrical impedance changes measured at 1.2 GHz due to changes of the effective dielectric constant within the 68 nm gaps between two gold electrodes. As a model system, the specific binding of the blood clotting factor human thrombin with different concentrations to its ribonucleic acid (RNA) α-thrombin aptamer, as well as the immobilization process of the RNA-aptamer, have been detected in real time. By using a similar 68 nm-gap sensor blocked with bovine serum albumin and a reference sensor with 10μm electrode spacing, signal changes due to variations of the bulk dielectric constant due to buffer/analyte solutions, and unspecific binding events have been analyzed.
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