An AC Bridge for Semiconductor Resistivity Measurements Using a Four-Point Probe

A basic measurement made on a semiconductor material is its resistivity. This is a measure of the impurity content, and determines the suitability of the material for a particular application and the necessary process parameters for subsequent operations. This measurement also determines whether a process step has been performed satisfactorily. Present methods for making this measurement usually are variations of the basic voltmeter-ammeter circuit, using direct-current power supplies and instruments. Such direct-current methods have many causes for error, several of which, while known to exist, are difficult to evaluate. 885 886 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 19G1 An ac measuring circuit has been developed for measurement of resistivity, retaining the four-point probe, but eliminating or minimizing to a negligible amount the errors inherent in the former dc systems. Every component of the new system is ac operated and grounded, except the test specimen and the four-point probe. By using an ac bridge, neither current nor voltage is measured. Rather, only their ratio is read from an accurate resistance standard, which really is what is required. The circuit can also be used for dc, but then many of its advantages are lost. The new electrical ac- system is accurate to 0.5 per cent when a test current that develops at least 1 millivolt between the two voltage probes is used. It includes built-in calibration for a system check at any time. It does not correct errors in point spacing of the four-point probe, but can provide a presetting for any given spacing, so that the resistance ratio dials read directly the bulk resistivity in terms of a semi-infinite body.