a two - head probe apparatus measures sheet resistivities of the thin layer on a semiconductor wafer . on one side of the wafer , a first probe head makes contact for injecting current into the wafer and measuring the voltage difference between two surface points . on the other side of the wafer , a second probe head makes contact for monitoring the substrate potential at two probed points on the wafer substrate . the second probe is disposed below a platen supporting the wafer , and the probe electrodes are moved into spring biased contact with the wafer through apertures in the platen by a vacuum actuated piston . circuitry coupled to the two probe heads computes leakage current and displays sheet resistivity .