Probes, spreading resistance technique

Figure 1 Illustrates the experimental procedure used In making spreading resistance measurements. Two probes are carefully aligned and then stepped across the bevelled surface of a semiconductor sample at each point, the probes are lowered onto the sample surface and the resistance between the two probes Is measured and plotted. The technique Is referred to as the spreading resistance technique because the dominant resistance of a point contact diode occurs In a very small volume beneath the probe, where the current rapidly spreads out Into the sample. Spreading resistance profiles are usually computer-processed to yield resistivity or dopant concentration profiles.

Therefore, we make resistivity measurements by first generating calibration curves on known-reslstlvlty samples of the same type, orientation, and surface finish as the test specimens to be profiled. Calibration curves are generated for a particular pair of probes at a particular time, using known-reslstlvlty samples of the highest quality available. This calibration procedure Is a particularly noteworthy characteristic of the spreading resistance technique. It means that spreading resistance Is a comparison method, and that Its ultimate accuracy Is therefore limited only by the calibration material available. Fortunately, It s now possible to obtain complete sets of calibration samples from the National Bureau of Standards. 

One technique is referred to as the "spreading resistance" method. In this procedure, a wafer is fractured and the edge containing the film is beveled, as shown in Figure 15. Then, a two-point probe is used to measure resistivity at a sequence of points traversing the interface between the substrate and the epi film. By relating the local resistivity to carrier concentration, one is able to deduce the concentration of dopant atoms over the epi layer. This technique is effective for even highly-doped layers. 

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