Quantification of carbon contamination under electron beam irradiation in a scanning transmission electron microscope and its suppression by plasma cleaning

We have measured the build-up of carbon surface contamination as a function of time and irradiated area size for various specimens in a JEOL 2010F (scanning) transmission electron microscope, employing both t/λ mapping with our Gatan imaging filter and recording changes in annular dark-field image intensity. It is shown that the total number of carbon atoms deposited per time for a given beam intensity is roughly constant at room temperature for as-received specimens while it is significantly lower for plasma cleaned specimens. This explains why contamination is generally only an issue at the highest magnifications where the contamination regions become smaller and the carbon layers correspondingly thicker. A Fischione plasma cleaner was then used to remove these carbon layers, and the rate of carbon removal has been determined for contamination spots produced in stationary spot mode as well as for extended regions scanned for a minute so that optimal cleaning times can be chosen.