A COMPARISON OF 2 XPS METHODS FOR QUANTIFICATION OF CONCENTRATION PROFILES

Binary Al-Li alloys are known for well pronounced segregation at ambient temperatures. Literature provides a good knowledge of segregation behaviour of these alloys. An exponential concentration profile with depth can be assumed. We compare the concentration profiles obtained from SIMS measurements with results from XPS. SIMS measurements [M. Vonbank and P. Varga, Vak.-Tech., 37 (1988) 220.] performed on a specimen with 9.1 at.% Li after storage at room temperature for more than 48 h gave a surface composition of 50 at.% Li and a gradient of -20 at.% Li nm-1. We investigated the same specimen after heat treatment for 2 h 20 min at 150-degrees-C. For comparison, XPS with variable take-off angle gives 75 at.% Li and -17.8 at.% Li nm-1 assuming an exponential depth profile, 70 at.% Li and -9.9 at.% Li nm-1 with a linear response and our new imaging XPS method with a linear response gives 92 at.% Li and -15.4 at.% Li nm-1. The standard deviations of surface concentration and gradient are approximately 9 at.% Li and 8 at.% Li nm-1. The differences are due to: the statistical significance of measured data (8% relative error in case of variable take-off angle experiments and 25% for imaging experiments) especially due to weak Li1s signals; the necessity of a specimen transfer in ambient atmosphere; and uncertainties in sputter depths of about 10%. In principle the imaging method is similar to methods in which the surface is etched with a constant flux Ar ion beam and changes in the composition of the surface with time are monitored using XPS.