Ion beam analysis of thin films with heavy MeV ions

Among the wide choice of physical analysis methods to characterise thin and ultra thin films both Rutherford backscattering spectrometry (RBS) and the elastic recoil atom spectrometry (ERDA) have a unique feature. The binary elastic scattering at the Coulomb potential of the involved nuclei can be described analytically by the Ruther-ford cross section formula. This allows an easy and accurate determination of quan-tities like area densities of elements or composition of thin films.
For the detection of all light elements ERDA with heavy incidence ions of energies
> 1 MeV/u is the method of choice. As recoil atom spectrometers heavy ion detec-tors are used which provide not only the energy signal but also particle identification and position information for kinematical corrections, which is particularly necessary at large acceptance angles. We use at the Rossendorf 5 MV tandem Bragg peak ioni-sation chambers, dE-E-ionisation chamber telescopes, time-of-flight (TOF) spectrome-ters or combinations of those. The specific merits and drawbacks of these detectors are discussed. A common feature is the moderate energy resolution dE/E of roughly 0.01. This is the limiting parameter for the obtainable depth resolution. To overcome this limit magnetic spectrometers have to be used which enable depth resolutions in the order of monolayers. We have installed at the tandem a QQDS spectrometer for ERDA with variable dispersion, which is shortly described.
An analogue situation we have in the case of RBS. Despite of many physical advanta-ges of heavy incidence ions [1] mostly He ions are used due to the fact that the energy resolution of solid state detectors decreases with increasing Z of the detected particles. When using magnetic spectrometers for RBS it is possible to profit fully from the high stopping powers of heavy ions. Thus depth resolutions of one monolayer can be achieved. We have installed a magnetic spectrometer of the simple Browne-Buechner type at the 3 MV Tandetron for RBS. The first results with this instrument are discussed.

[1] D.J.O´Connor, T.Chunyu, Nucl. Instr. And Meth. B 36 (1989) 178