A Process Control Test for Lateral Charge Spreading Susceptibility

A rapid process control stress test is described, for use in evaluating the susceptibility of MOS circuits-to lateral charge spreading and subsequent channel formation. Measurements of the temperature, time, and stress voltage dependence of the induced channel conductance are described, and these lead to the specification of the standardized stress conditions for the test procedure. The observed temperature dependence is consistent with an activation energy of 1 eV for the charge spreading mechanism. Choice of an appropriate stress voltage is dependent upon field oxide, substrate, and protective overcoat material parameters, in relation to the physical site of the charge spreading. Measurements to separate charge spreading at the overcoat-field oxide interface or in the film bulk are described. The standardized test procedure has been validated for use as a process control by correlating its results with the life test behavior of MOS integrated circuits having various types of overcoat materials. Inversion failure rates of these circuits subjected to conventional life test procedures are found to be proportional to the induced channel conductances observed in the accelerated process control test.