Imaging mechanism and effects of adsorbed water in contact-type scanning capacitance microscopy

In order to demonstrate its unique capabilities, a combined microscope comprised of a scanning capacitance microscope and a contact mode atomic force microscope was used to image a buried structure. The atomic force microscopy images surface topography, while the scanning capacitance microscope simultaneously images buried structure. By applying a modulation technique, the voltage derivative of the capacitance (dC/dV) was used as a scanning capacitance microscope signal. The sample used for the demonstration was a silicon wafer with metal lines 4 nm in thickness covered by a 40-nm-thick layer of silicon oxide. The scanning capacitance microscope image was dependent on the bias voltage applied to the probe and the sample. Since the scanning capacitance microscope signal above the metal lines is zero, the bias voltage dependency of the contrast in scanning capacitance microscope images corresponds directly to the scanning capacitance microscope signal versus V at locations where the metal line is not buried. However, the bias dependency of the contrast did not agree with the scanning capacitance microscope signal observed between metal lines. A possible mechanism for the discrepancy between these bias dependencies is discussed based upon charge deposition on the sample surface caused by tip contact and lateral movement of the charges in the adsorbed water on the sample surface. (C) 1996 American Vacuum Society.