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[摘要]：The dependence of the field emission effect on distance is applied for displacement sensing and high-resolution positioning. Silicon atomic force microscopy probes were used as a field emission source by applying voltages up to 400 V between this probe and a counter-electrode sample consisting of TiW sputtered on a silicon wafer. From current-voltage characteristics measured for distances varying from 50 to 950 nm, values for the field enhancement factor were determined which show a dependence on the electrode separation. This dependence can be correctly described by a model the authors developed using finite-element calculations and is determined by the emitter geometry and tip radius. Feedback to the probe position was used to maintain a constant current to apply this distance dependence for positioning. When increasing the applied voltage from 5 to 40 V for a constant current of 3 nA, the probe position is raised similar to 90 nm. The nonlinear sensitivity of this positioning method is determined by the varying field enhancement and can be fitted by the same calculated model. Using feedback, the field emitter can be positioned with high lateral resolution and scanned over a conducting surface. Increasing the bias voltage from 3 to 50 V results in an increase in the emitter-sample distance and a decrease in lateral resolution. Damage to the scanned surface has to be prevented by using a current-limiting resistor and by annealing the probe and sample under ultra high vacuum conditions before use. (C) 2008 American Vacuum Society. |
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