The contamination of surfaces during high-energy electron irradiation

Abstract

Specimen contamination during electron-optical examination has been studied using “holey’ carbon-film specimens in a modified electron microscope. The composition of the residual gas atmosphere was determined by mass-spectrometric analysis. The gas pressure in the vicinity of the specimen was reduced to between 10⁻⁵ and 10⁻⁸ torr by differential pumping. The functional dependence of contamination rate on specimen temperature was also investigated between −85 and +200°c. Both the source of carbon-bearing gas molecules and the growth rate of carbon film caused by the interaction of electrons with adsorbed carbonaceous species were investigated. The results indicate that molecules contributing to the observed rates of film growth originate not only from the specimen surrounds, but also from the region along the electron-beam path, possibly from as far away as the electron gun. However, with both these sources included, calculations showed that the supply of contaminating gas molecules was barely sufficient to explain the observed contamination rates. An additional contribution caused by surface migration of adsorbed gas to the area irradiated by the electron beam was also considered. This surface-diffusion contribution has been included in an expression that describes the growth rate of the surface contamination layer. A numerical method was employed to best fit the theory to the experimental data. From electron-transmission measurements on growing contaminant film, a value of (9·.89 ± 1·27) × 10⁴ cm⁻¹ was obtained for the scattering cross section of carbon for 100 kv electrons.