The hillocks created by the application of ramped voltage stress on thin oxide films have been imaged using different modes of the atomic force microscope (AFM) and using conductive or insulating tips, leading to the conclusion that these anomalous hillocks correspond to real (physical) modification of the oxide’s surface. Electric force microscopy has also been used, which shows that negative charges are trapped in the oxide layer after the ramps and contribute to the contrast of AFM images although their role may not be predominant. Comparisons between ramps operated in air and in dry atmosphere or vacuum emphasize the role of the water layer covering the sample in the apparition of the hillock. The authors’ results tend to accredit that the formation of the hillock is a complex phenomenon involving a chemical (oxidation), electrical (trapped charges), and physical (electrothermal effect) mechanisms.

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