RBS (Rutherford Backscattering)

RBS is used to get relative concentrations of atoms within the sample and to get the depth distribution of these atoms.

Standard

Accelerated ions, usually helium, are sent into a user’s test sample. The beam size on the sample surface is usually about 1 mm in diameter, but varies from about 0.5 mm to 1 cm. Size is mostly determined by how much beam the sample can stand and on the mass of the elements in the sample. A small fraction of the ions rebound from atomic nuclei in the sample and are caught in an energy-sensitive detector. When the energy spectrum of these ions is plotted, the shape of the curve can be used to analyze the elemental composition of the near-surface region (usually about the first micron of depth). Sensitivity is very high for heavy elements and very low for the lightest elements.

Users commonly analyze RBS spectra to get relative concentrations of atoms within the sample and to get the depth distribution of these atoms. Thin films can therefore be analyzed for composition, thickness and uniformity with depth. The values obtained for element concentration are absolute values needing no further calibration. The depth and thickness numbers are not absolute unless the density of the material is correctly known. Many thin films are not as dense as the "handbook" values. Users must take the depth data with some caution.

Separate identification for light elements is generally easy, but very difficult for very heavy atoms. Since the heavier element crowd together at the upper end of the spectrum, their separation mostly depends on the energy resolution of the particle detector. The laboratory normally uses silicon particle detectors of about 12 keV resolution. They are small, easy to use, and give rapid results. There is no current indication that they can be improved by a factor of 2 or more. For highest resolution, one must use the HIRRBS facility described below.

The laboratory has two beamlines equipped for standard RBS and one beamline for high-resolution RBS (the HIRRBS facility). In addition, the Microbeam system uses RBS analysis on micron-sized portions of a sample.

HIRRBS

Silicon particle detectors are normally used for RBS measurements, as described above for standard RBS. Their resolution limit is currently about 10 to 12 keV. By switching to use of a magnetic mass spectrometer, the HIRRBS facility improves resolution by more than a factor of 4.

The mass spectrometer particle detector is a HIMAG 5000 system, built by AMACC. It was designed with a theoretical resolution of 1 part in 4000. Conventional mass spectrometer designs use a single particle detector, with data taken by stepping slowly through a range of magnetic field, pausing at each step to collect particles for a fixed amount of time. The result is very slow operation. In the HIRRBS facility, the magnetic field remains fixed while data are taken simultaneously from detectors sensing from many different locations.

INVITATION TO USE THE HIRRBS FACILITY