The Distribution of A-C Resistance in Oxide Films on Aluminum

F i lms of 7 A1203 produced by anodizing superpurity a luminum at 20v in neutral ammonium tartrate are th inned un i fo rmly when placed in sod ium chromate solution of pH range 7-9. Dur ing film thinning the meta l remains passive and the inherent specific resistance of the film substance is not modified by the dissolution process. Uniform thinning can continue from an initial film thickness of around 240A down to the passive film thickness 20-25A. Measure-ment of capacitanCe and dielectric loss at frequencies of 1000 to 100,000 cps during thinning has permitted determination of the variation of electronic and ionic resistivity throughout the majority of the thickness of the alumina films. Corrections for a-c resistance of the solution have been found to be unnecessary at 1000 cps but mandatory at 100,000 cps in order to obtain ac-curate results. The films possess a region of low electronic resistivity up to 60-80A from the oxide-metal interface. At greater thickness, electronic re-sistivity is constant. Ionic resistivity is only constant across a central region o