A planar four-probe test structure for measuring bulk resistivity

A small planar four-probe test structure for measuring the bulk or collector resistivity of silicon wafers was designed and fabricated with a bipolar transistor process. Analogous to a mechanical square array four-point probe, the planar four-probe structure consists of a large-area base diffusion which is broken at four points through which contact is made to the undiffused collector material. A probe spacing of 2.25 mil (57.2 µm) allows the resistivity of the silicon wafer to be measured with good spatial resolution. A correction factor was derived to obtain the true resistivity from measurements on a wafer with finite thickness and a conducting backside, and it is presented along with the correction factors for other cases. The test device was fabricated in silicon wafers whose resistivities ranged from 0.013 Ω. cm to 12 Ω. cm in n-type material and from 0.7 Ω. cm to 30 Ω. cm in p-type material. Planar four-probe resistivity values are compared with mechanical four-probe values taken on the same wafers before fabrication, and the results are generally in agreement within ±3 percent.