The secondary-electron yield of air-exposed metal surfaces

Abstract

The secondary-electron-yield (SEY) variation of atomically clean metal surfaces due to air exposure and during subsequent heat treatments is described. As an example SEY results are presented for the case of a sputter-deposited Nb thin film. Corresponding variations in the surface chemical composition have been monitored using AES and SSIMS. On the basis of these results, and of previously obtained SEY results for metals and metal oxides, the origin of the SEY variations is discussed. The SEY increase, which is generally observed during long-lasting air exposure of clean metals, is mainly caused by the adsorption of an airborne carbonaceous contamination layer. The estimated value of about three for the maximum SEY of this layer is higher than that of all pure metals. Only in some cases can the air-formed oxide contribute to the air-exposure-induced SEY increase, while many oxides have a lower SEY than their parent metals. From the experimental data it can also be excluded that the SEY increase during air exposure is mainly due to an increased secondary-electron escape probability.