RBS with high depth resolution using a small magnetic spectrometer


RBS with high depth resolution using a small magnetic spectrometer

Grötzschel, R.; Klein, C.; Mäder, M.

Rutherford Backscattering Spectrometry ( RBS) and Elastic Recoil Detection Analysis (ERDA) are well established techniques for quantitative thin film and surface analysis. The advantage of these methods consists in the simple physics they are basing on, namely the stopping of energetic ions in matter and the binary scattering of at the Coulomb potential of atomic nuclei.
In particular RBS with He ions is widely used at many small accelerators utilizing standard surface barrier or implanted silicon detectors with typical depth resolution values of 12 keV, which provides a depth resolution in the order of 10 nm.
The increasing importance of ultra-thin layers for novel technologies demands quantitative analysis techniques with a depth resolution of atomic monolayers, which can be obtained for RBS and ERDA by magnetic spectrometers only.
We operate at the 3 MV Tandetron accelerator a magnetic spectrometer consisting of an UHV scattering chamber and a simple magnet with circular field boundaries (Browne-Buechner spectrometer). Since in many cases of high resolution IBA the samples must be prepared in situ in UHV, the chamber with a base vacuum of 4x10-10 mbar is equipped with an Ar ion sputter gun and two OMICRON EFM3 low rate e-beam evaporators for in situ layer deposition.
A RHEED system is used to check the surface reconstruction and monitor the layer growth. Samples are transferred, together with a BN heater, to the precision 5-axes channelling goniometer . The magnet with a mean radius of 0.65 m is mounted vertically and can be positioned either at 35.5° or 144.5°. The backward position offers the advantage of a high mass resolution, but the Rutherford cross sections are a factor of about 100 lower than at the forward angle, which is the preferred position if kinematically possible.
Using a two-dimensional position sensitive focal plane detector the transversal position provides the possibility for kinematical corrections since the transversal focusing of the magnet is small. A multiple-pad PSD with a pitch of 4 mm is adequate.
This spectrometer is described, recent applications are discussed and as well as he influence of charge state transitions of ions on the obtainable depth resolution will be demonstrated.

  • Invited lecture (Conferences)
    14th Hellenic Symposium on Nuclear Physics, 21.-23.05.2004, Athens, Greece

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Publ.-Id: 9897