Sensors and Actuators A 137 (2007) 338–344 Quantifying resistivity using scanning impedance imaging Brian G. Buss, Daniel N. Evans, Hongze Liu, Tao Shang, Travis E. Oliphant, Stephen M. Schultz, Aaron R. Hawkins ∗ Electrical and Computer Engineering Department, 459 Clyde Building, Brigham Young University, Provo, UT 84602, United States Received 15 September 2006; received in revised form 1 March 2007; accepted 8 March 2007 Available online 14 March 2007 Abstract 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 × 10 6 -cm. Two-dimensional resistivity scans of various samples are also shown. © 2007 Elsevier B.V. All rights reserved. Keywords: Impedance imaging; Resistivity; Conductivity; Noncontact measurement 1. Introduction Specific electrical impedance (impedivity) is one of the most distinguishing characteristics for individual solids and liquids. Resistivity, the real component of impedivity, exhibits differ- ences between good insulators and good conductors of 20 orders of magnitude. The large change in resistivity also exists in biological materials. For example, cancerous tissue exhibits resistivity differences compared to healthy tissue [1,2]. With this in mind, if the impedance of different portions of a heteroge- neous media could be measured, natural impedance differences would provide large measurement contrasts. A two- or three- dimensional impedance map of a sample would allow for the visualization of material boundaries and compositions, and even the identification of materials based on impedance properties. Impedance is measured by applying a voltage across a sample and then measuring the current flow. The current flow tends to spread out from a measurement electrode resulting in a dramatic reduction in the spatial resolution. To complicate matters, the ∗ Corresponding author. Tel.: +1 801 422 8693; fax: +1 801 422 0201. E-mail address: hawkins@ee.byu.edu (A.R. Hawkins). current spreading is dependent on material impedance. There- fore, even though impedance based imaging and measurement provides a very promising technique in terms of image contrast, obtaining images of any significant resolution is a challenge. Despite the issue of current spreading, the technique of electri- cal impedance tomography (EIT) has been shown to produce impedance images on the macroscopic scale simply from mea- surements between two or more electrodes that surround a sample [3,4]. Recent research has also extended conventional atomic force microscopy (AFM), combining nanoscale imaging and impedance spectroscopy. A modified conducting AFM tip is used as a conducting electrode to measure frequency-dependent impedance properties [5–8]. This technique is often referred to as scanning impedance or resistance microscopy and is capa- ble of producing high-resolution impedance based images for materials very close to the surface of a sample. The current spreading and contact resistance problems asso- ciated with the EIT and resistance microscopy have been addressed by the development of a technique called scanning impedance imaging (SII) [9], which involves immersing a sam- ple in a conducting solution and measuring the current flow through the sample under an applied electric field. The SII system uses a coaxial probe geometry to dramatically reduce 0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.03.005