Quantifying resistivity using scanning impedance imaging

Noncontact scanning impedance imaging (SII) has the ability to produce impedance based images of a thin sample. This paper describes how SII can be used to produce quantitative measurements related to the specific electrical impedance (impedivity) of a sample. This is made possible through hardware improvements to previously reported systems and the use of numerical simulations to quantify the degree of current confinement occurring in the coaxial impedance probe. Experiments conducted in homogeneous saline solutions show that measured impedance increases linearly with probe height, confirming simulation results and the concept that current through the sample is being confined into a cylinder of constant area. Microfabricated structures of the photoresist SU-8 are used as a test case to demonstrate that SII probes can accurately determine the quantity ρh (resistivity × sample height). For the experiments described, the dielectric contribution to the measured impedance is negligible; in these cases the sample resistivity is reported instead of the impedivity. This results in a resistivity measurement for SU-8 of ρ = 3 × 106 Ω-cm. Two-dimensional resistivity scans of various samples are also shown.