How does Scanning Frequency Comb Microscopy (SFCM) measure the sample-resistivity for carrier profiling?
We have previously mentioned this new method for carrier profiling, but would like to clarify its mechanism. For further information, including a patent applications published by the USPTO in December, please see the new LinkedIn group "Scanning Frequency Comb Spectroscopy and Microscopy or contact me through SemiWiki.com.
1. A mode-locked laser generates a comb of microwave harmonics in a scanning tunneling microscope.
2. With a metal sample the microwave power increases with the bias voltage and the tunneling current. With a resistive sample at each bias there is a current for maximum power. For a fixed sample resistivity, each bias-current pair has the same tip-sample distance.
3. The harmonics are generated by optical rectification so the DC bias and current are unnecessary. Feedback control of the tip-sample distance may be based on maximizing the microwave power. The power is maximum when the sample spreading resistance equals the tunneling resistance to match the load to the source.
4. The resistivity of a sample may be determined with consecutive measurements including standards with known resistivities because the maximum microwave power is inversely proportional to the square of the resistivity.
5. SFCM can have finer resolution than Scanning Capacitance Microscopy and Scanning Spreading Resistance Microscopy because the tip is sharper and does not contact the sample. Furthermore, the spot-size is smaller than the tip radius because the current is maximum directly under the apex of the tip.