SURFACE-POTENTIAL CONTROL ON THIN OXIDE-FILMS WITH RESPECT TO ELECTRON-STIMULATED DESORPTION STUDIES

These experiments deal with the study of desorption of negative ions s timulated by low energy electron collisions on insulating surfaces cov ered with various adsorbates. For such investigation a careful control of the sample surface potential is required to set the incident elect ron energy accurately as well as to identify the desorbed species by m ass spectrometry. Most of the reported experiments concern this surfac e potential control. We show that, for very thin SiO2 films thermally grown on silicon substrates, a tunnel conduction might set the surface potential accurately. This assertion mainly relies on recording the i ntensity of electrons transmitted through thin oxides as well as the i ntensity of backscattered and secondary electrons re-emitted from the surface. A direct comparison of the O- ion energy distributions confir ms the correct control of the surface potential for a large range of i ncident electron energies. In such condition we noticed that the O- de sorption yields just swiftly varied with incident electron energy. In particular no modification could be detected as the incident electron energy passed the various Auger excitation levels. The discrepancy bet ween this last result and the published data is discussed in the last part of this paper.