FORMATION AND BEHAVIOR OF SURFACE-LAYERS ON ELECTRON-EMISSION GLASSES

The thermochemical reduction of thin surface layers on multicomponent lead-silicate glasses is fundamental to their use in electron multiplier and microchannel plate devices. These surface layers can exhibit a specific conductivity as high as 10-2 (Ω cm)-1 and secondary electron yields up to 3.5. However, due to the complex processing used in the fabrication of the devices, a basic understanding of the chemical and structural surface characteristics responsible for these properties has not been established. Moreover, the effects of prolonged electron bombardment upon the chemical characteristics of the surface have not been extensively investigated, nor related to any associated degradation of the electron emission properties. In this study, the clean fracture surfaces of these glasses were investigated. The effects of hydrogen reduction, chemical etching, and prolonged electron bombardment were determined. Ion-scattering spectroscopy (ISS) was used for its monolayer sensitivity, especially to alkali species, while secondary ion mass spectroscopy (SIMS) provided depth profiles. The hydrogen profile created by the reduction could also be obtained with SIMS. X-ray photoelectron spectroscopy (XPS) was employed selectively to examine changes in the oxidation state of the surface species. It was found that the hydrogen reduction of these glasses creates a thin 20-50 nm silica-rich surface layer. The layer of reduced lead atoms is beneath this zone, and is visible to depths of the order 5 μm, but the hydrogen profiles which are found in these surfaces extend only 0.5 μm in depth. The electron bombardment of these surfaces leads to a decrease in concentration of alkali and lead in the surface monolayer, and to a change in the hydrogen profile. The cross-section for this bombardment-induced change in the surface composition correlates with the reported gain degradation in microchannel plate devices. © 1990.