Research Techniques

In RBS measurements, the accelerator produces a beam of specific ions of known energy. The beam then impinges on the sample and scatters in different directions with different intensity. A sensitive detector records the energies of the ions bouncing off the sample at a specific angle.

Since the initial energy of the ion is known and the collision between atoms can be considered completely elastic, the principles of the conservation of momentum and energy can be used to calculate the mass of the atom struck by the ion. This technique allows for a very detailed analysis of a material’s composition within the first micron depth of the material. 

The elemental concentration determined with RBS are absolute and generally do not need further calibration or corrections. However, values of the depth and thickness are not absolute, unless the density of the material is accurately known. Many thin films are not as dense as reported in the literature. Therefore, users must use the depth data with some caution. 

Identification of different light elements using RBS is generally easy but can be difficult for very heavy atoms. Heavier elements crowd together at the upper end of the energy spectrum and their separation is limited mostly by the energy resolution of the particle detector. 

The IBL normally uses silicon particle detectors with an energy resolution of about 12 keV. They are small and easy to use, and they yield rapid results. For measurements requiring the highest resolution, researchers must use the High-resolution RBS (HIRRBS) facility described below.  

To determine concentration and distinguish heavy elements, a magnetic mass spectrometer is used in conjunction with traditional RBS, improving the resolution by a factor of four.

The mass spectrometer particle detector at the IBL is a HIMAG 5000 system, built by AMACC. It was designed with a theoretical resolution of one part in 4,000. 

Conventional mass spectrometers use a single particle detector. Data is collected in a slow and laborious process that involves stepping slowly through a range of magnetic fields while pausing at each step to collect particles for a fixed amount of time. 

In IBL’s HIRRBS facility, the magnetic field remains fixed while data are taken simultaneously from detectors positioned at many different locations — producing a high-resolution energy spectrum in a very short time period. 

Normal analysis method of materials in the IBL uses a beam of particles of about 1 or 2 mm in diameter, with results averaged over a similar sized area of the sample.

However, in the Microbeam method, the particle beam is squeezed down to about 1 to 4 microns in diameter and the sample is then probed using conventional RBS or PIXE techniques. 

Analysis with a very narrow beam is not useful if the researcher cannot tell where the beam is hitting the sample. Therefore, the Microbeam method has several features to help researchers locate the beam. 

The first feature is a closed-circuit television system that uses a moderate power microscope lens and provides an approximate 3 mm diameter image of the target.  

The second feature is the use of a raster-scanning mode like that of a scanning electron microscope. The beam is scanned over about 1.2 mm of the sample. The electrons knocked loose by the beam are captured and the data is displayed on a computer screen — producing an image akin to a microscope view.  

When the researcher locates the designated site on the sample, they use the visual information to position the beam precisely on the area of interest and collect data for as long as necessary to obtain good signal-to-noise ratio.  

The Microbeam method can also be used to take data while scanning the beam over a sample, resulting in a 2D map of the location of chemical elements within the scanned area. The RBS or PIXE spectrum acquired continuously at one location can produce quality results in five to ten minutes. Good 2D maps produced with the Microbeam method often take one to four hours. 

In PIXE experiments, accelerated ions — usually of Helium or Hydrogen — strike the test sample. Electrons within the sample are displaced to higher energy levels creating unoccupied lower energy levels. The electrons at higher energy jump to the unoccupied levels and the excess energy is emitted as X-rays and the observed X-ray spectrum is a characteristic of that specific atom.  

By analyzing the X-ray spectrum, researchers can identify most of the elements present in the sample. While it is possible to obtain the absolute concentrations of such elements, most researchers use the technique for generalized identification of elements. 

PIXE measurements can be made simultaneously with RBS on the IBL’s 45 beamline and on the Microbeam station. Researchers frequently use PIXE and RBS with the scanned Microbeam to map the location and concentrations of elements on surfaces.

Single crystals of semiconductor materials used in manufacturing have orderly periodic arrays of atoms. If a particle beam arrives in parallel to their atomic planes, most of the beam will pass through the space between the planes and penetrate deep within the crystal. 

Channeling studies take advantage of this phenomenon to determine the structure of the crystal and to locate the placement of atoms within the array. 

Channeling measurements can use ion scattering (RBS) and X-rays produced by the ion beam (PIXE). Both are available simultaneously on the normal RBS-PIXE beamline. 

The light elements — from hydrogen to fluorine — are not easily analyzed by conventional RBS or PIXE techniques. In many cases, these elements can be detected by using a resonance effect.

Under the right conditions, accelerated particles interact with the target atom nucleus, give off gamma rays or secondary particles. Such interactions happen only if the incoming particle has exactly the right velocity (such as energy). NRA takes advantage of this resonance effect. 

In a typical case, the mass 15 isotope of nitrogen will interact with hydrogen in the target if the nitrogen has an energy equal to 6.385 MeV.  

If the nitrogen ion beam has precisely this energy and hits atoms of hydrogen on the sample’s surface, gamma ray emission will occur. Gamma ray measurements are then used to determine the absolute concentration of hydrogen on the sample’s surface.  

If the hydrogen is buried below the sample surface, then the nitrogen will not generate gamma rays — unless the beam energy is increased to compensate for the sample slowing down the nitrogen, thus increasing ion penetration. The depth of the hydrogen can be determined by measuring the difference between 6.385 and the beam’s energy before it starts to penetrate the sample. 

Researchers often use standard NRA techniques in the IBL to measure hydrogen, lithium, fluorine, sodium and aluminum distributions. Less frequently, they use NRA to measure deuterium, boron, carbon, nitrogen-14, oxygen and phosphorus.