A Kelvin probe (KP) determines the contact potential difference (CPD), or the relative work function, of a sample by measuring the external voltage required to eliminate surface charging when the sample is electrically connected to a reference electrode. Simply put, the CPD represents the difference between two materials' work functions as seen in the following equation. Since we know the reference material, and therefore its work function, we can calculate the work function of the sample.
CPD = Vext = WF sample - WF reference
Kelvin probe measurements are surface sensitive. Water adsorption, other adsorbed contaminants, and oxide layers can greatly affect the CPD by changing the physical, chemical, and electrical properties of the sample being investigated. Thus, Kelvin probe microscopy is widely used as a "single material" quality control tool in the semiconductor (contaminants) and steel (corrosion) industries. Though the Kelvin probe produces precise results, its accuracy has yet to be determined when comparing samples made of different materials.
We plan to determine the accuracy of the Kelvin probe by controlling variables, including surface roughness, humidity, and contaminating layers. We monitor the roughness of our samples (and polish them if necessary). We place our samples under UHV, reaching 6.0 x 10-8 Torr, and we clean them using a GDC procedure. By controlling and reducing the known variables, we plan to examine how accurate (and useful!) Kelvin probe microscopy can be.