Sub-zeptofarad sensitivity scanning capacitance microscopy

The scanning capacitance microscope technique described is based on a dC/dV measurement at the metal-semiconductor junction between a metallic probe and a sample. The probe forms part of a RF resonator and is scanned in a raster across the sample. Changing dopant concentrations in the sample result in small variations in the junction capacitance, changing the load on the resonator. The sensitivity of a capacitance sensor depends on the operating frequency, the quality factor (Q) of the resonator and sense voltage applied to the resonator. Increasing any of these parameters will increase the sensitivity of the instrument. The instrument described in this paper operates at 2.5 GHz and the resonators have Q values in the range 50-100. Importantly, these resonator designs can operate with low sense voltages (0.1 V-1.5 V), minimizing artefacts that result from larger sense voltages. Capacitance noise response and DC stability of the sensor have been used to demonstrate idealized (unloaded) sensitivities as low as 0.71×10^-21 F/√Hz.