In this paper the effect of selective anodisation under ac bias on ultrathin (1.5-2.75 nm) silicon dioxide grown at two different temperatures, viz. 700 degrees C and 800 degrees C have been studied. It is shown that ac anodisation is much more effective in improving the electrical properties of the ultrathin oxide compared to selective anodisation carried out under dc condition. Unlike dc anodisation, which only repairs the pinholes but does not improve the interfacial properties, ac anodisation is found to reduce the density of interface states. The parameters during ac anodisation, e.g. signal frequency and dc offset are found to have a major role in the degree of the improvement and have been optimised carefully. Best results have been obtained when ac anodisation has been carried out using a 260 mV peak-to-peak signal of frequency 5 kHz and dc offset of 70 mV. (c) 2007 Elsevier Ltd. All rights reserved.