CORRELATION OF SIO2 LIFETIMES FROM CONSTANT AND RAMPED VOLTAGE MEASUREMENTS

This paper investigates constant voltage stress (CVS) and ramped volta ge stress (RVS) for thermally grown oxides on single crystal and on po lycrystalline silicon. CVS is a standard stress for the measurement an d prediction of oxide lifetimes. However, RVS has the advantage over C VS of recording the breakdown properties in a very fast time and is, t herefore, widely used in industry. The aim of this work is the assessm ent of a correlation between times to breakdown of RVS and CVS. Times to breakdown of RVS and CVS are compared directly and it has been foun d that they do not correlate with a simple classical model. An anomaly is reported for thick oxides grown on polycrystalline silicon: greate r breakdown voltages are recorded for slow ramps than for fast ramps. It is shown that the times to breakdown which are estimated from RVS a re longer than times to breakdown of CVS. The increase in times is dep endent on the oxide thickness. This is verified with CVS measurements on pre-stressed oxides. In contradiction to the literature, results fr om pre-stressed oxides were found to have greater time to breakdown an d charge to breakdown values than data of virgin oxides. The results o f this work give evidence that the time to breakdown and the injected charge to breakdown of the voltage stress measurements are strongly in fluenced by charge trapping effects in the oxide layer. Longer times t o breakdown resulting from a RVS have to be taken into account when li fetimes at use conditions are predicted from RVS results. If this is n ot considered oxide lifetimes at operating voltage will be overestimat ed. Copyright (C) 1996 Elsevier Science Ltd.