Abstract: Surface analytical studies using ion scattering spectrometry, Auger electron spectroscopy and secondary ion mass spectrometry

Abstract

Ion scattering spectrometry (ISS), LEED and Auger electron spectroscopy (AES) are combined in one apparatus. By this means the ISS signal can be calibrated as a function of surface composition. This is done for the case of sorption of O and S on Ni. 1,2 By assuming appropriate cross sections an estimate of relative neutralization yields is possible. For the scattering of Ne+ from sulfur adsorbed on nickel the neutralization seems to be a factor of 7 lower than in the case of Ni so that the surface sensitivity is very good (Fig.1). From this figure a detection limit of 10−4 of a monolayer of S on Ni is estimated. In the case of oxygen adsorption of Ag (110) no simple relations between oxygen signals detected by AES and ISS and the observed LEED pattern have been established. Both signals increase in proportion to the change of the work function with increasing exposure, whereas the Ag signal from ISS decreases (Fig.2). But in the exposure range where (3×1) to (2×1) LEED patterns are observed the oxygen concentration according to AES and ISS does not change from 0.3 to 0.5 of a monolayer. A naive interpretation of the Ag−ISS signal by shadowing may not be justified in that case, even though the detection of oxygen by ISS is a strong indication that at least part of the oxygen is adsorbed above the silver atoms. 4 For many problems ISS can be improved by using an electrostatic anlyzer and a quadrupole mass filter in series, 5 especially when a high yield of true secondary ions buries the peaks of backscattered ions (Figs.3 and 4). In the case of the Nb−Cu sample the high ion yield is due to oxygen contamination of the Nb as shown by the He+ backscattering spectrum, whereas in the Cu−Be case it is obviously due to BeOH contamination as shown by the secondary mass spectrum. This is recorded by using the electrostatic analyzer as an energy window, the secondary ions being accelerated to the appropriate energy. Figure Captions Fig. 1. Auger electron spectroscopy and ion scattering from a S−covered Ni surface. The S concentration amounts to about 0.02 of monolayer. From the spectrum with low impact angle (Ψ = 10,0°) and low scattering angle the sensitivity is estimated to 10−4 of a monolayer. Fig. 2. Auger electron spectroscopy and ion scattering from a O−covered Ag surface. The work function change Δ φ is reported by Engelhard et al.. 3 Fig. 3. Ion scattering from a Nb−Cu surface contaminated by O. The sputtered ions are suppressed by using a mass filter in series with an electrostatic analyzer. Fig. 4. Same technique as used for Fig. 3 in case of a Cu−Be sample.