Activation Energy for the Parabolic Rate Constant during Sequential

In this communication we report on an analysis of recent experimental data (i) which clearly shows that transport proper-ties of thermally grown Si02 change as a function of temperature. Our analysis is consistent with other observations where the refractive index (2,3), density (3), and interfacial stress (4) in Si02 vary with temperature. These changes account for the apparent anomalous effect of temperature on the activation energy for the parabolic rate constant, B, reported by several in-vestigators (5-7). Figure i shows this effect in an Arrhenius plot of these data (5-8). The activation energy varies from 0.9 eV at I150 ~ to 2.6 eV at 780~ Recent experiments by Irene (i) shed considerable light on the mechanism for this change in activation energy. He grew oxides in dry 02 at 1000 ~ to l~m thick-nesses, then recorded oxidation rates for subsequent oxidations at lower temperatures. The rates measured at lower temperatures should have therefore exhibited the proper-ties of a "I000 ~ ' oxide, so long as the lower temperature oxidations did not signifi-cantly affect their structure. In a reanalysis of Irene's data, using an improved lag time formulation, we indeed observe this effect (9). The oxidation rates were analyzed using the linear parabolic rate law: dL = B [i] dt 2L+A where L is the thickness, B is the parabolic rate constant B/A is the linear rate constant, and dL/dt is the growth rate. Since dL/dt and L are measured quantities, B can be deter-mined if A is known; since L>>A for these experiments, the error caused by an uncer-tainty in A is not significant