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The analysis of hydrogen is important in many areas of materials science. Hydrogen has a high diffusion coefficient in many materials and is probably the most common impurity in thin film materials. Hydrogen also has important effects on the chemical, physical and electrical properties of many materials.
It depends on the application, whether the incorporation of hydrogen is desired. For example, the mechanical stability of materials such as zirconium, which is used in nuclear industry, is reduced by the incorporation of hydrogen. On the other side, in order to obtain the required electrical properties of amorphous silicon, which is used for solar cells and flat panel display applications, the incorporation of hydrogen is necessary.
The analysis of hydrogen is difficult or impossible for many traditional materials analysis methods. At the Tandetron lab at UWO the scattering of energetic ion beams is used for analysis of hydrogen. The effects of hydrogen are studied in many materials which are important for todays and future technical applications.
Elastic Recoil Detection (ERD) [ecu76, doy79] is a ion beam analysis technique for quantitative analysis of light elements in solids. The sample which has to be analyzed is irradiated with an ion beam (e.g. He, C or O ions) of several MeV energy. Light elements (e.g. H, D) from the sample are scattered in forward directions and can be detected with a Si detector. From the measured energy spectrum of the recoils a concentration depth profile can be calculated.
The detection of scattered ions from the incident ion beam is normally suppressed in order to avoid background. The easiest and most common method is the use of a foil which stops the scattered ions, but allows the passing of the recoils which have a lower stopping power.
For low ion energies (<0.3 MeV/amu) the scattering cross section can be calculated assuming Rutherford scattering. For higher energies experimentally measured cross section have to be used in most cases. The depth profile calculation can be done directly from the spectrum as described by Doyle [doy88] or by using spectrum simulation programs such as RUMP [doo85] or SENRAS [viz90]. The accuracy is about 10%, limited by the uncertainties in the stopping power values of the sample material, experimental cross section values and geometrical uncertainties.
The ERD method provides absolute concentration values and is not affected by matrix effects. Furthermore ERD is non invasive, e.g. the sample is not damaged on a macroscopic scale.
More information about hydrogen analysis with ERD and other methods can be found in a review from W.A. Lanford [lan92] and references therein.
One of the ERD systems at UWO is connected to the 1.7 MV Tandetron accelerator. The detection system is located in the MEIS chamber which has a base pressure of 10e-10 mbar. The chamber is equipped with a 6-axis precision manipulator which allows the adjustment of the sample angle and position. The ERD system is mounted on a turntable which enables recoil detection angles in the angular range from 0 to 40 degrees. The distance of the ERD detector from the sample is adjustable from 55 to 130 mm. Two wheels containing a selection of stopping foils and detector apertures are mounted in front of the detector and allow the selection of solid angle, rectangular or curved slit and energy range. All above mentioned adjustments are possible within minutes and without breaking vacuum, thus allowing quick adjustment of the optimum parameters for different samples.
Most commonly used ion beams in this system are 4He with an energy of 2 to 3 MeV and 12C with an energy of 6 to 8.6 MeV. The beam currents depend very much on the ion beam species and sample. A typical value would be 20 nA for a beam size of 2 mm x 0.5 mm.
The accuracy of the solid angles is checked by the use of an alpha-source with known activity. The calibration is verified using H implanted Si wafers and polyimide foil (Kapton, C22H10N2O5) as standards.
The time requirements for the measurement of one sample depends strongly on the sample itself and the required information. A better depth resolution or a lower concentration of the analyzed element does generally lead to a longer acquisition time. The typical analysis time is on the order of 1 h. Including loading and unloading of the sample the total time per sample is typically 2 h.
The system is used routinely for the analysis of 1H and 2H. Hydrogen measurements have been done on a wide range of materials such as semiconductors, metals and polymers. Considering the maximal available beam energy, the upper limit of detectable elements is probably carbon.
The limits in term of depth range, depth resolution and sensitivity are difficult to quantify because they depend on each other (e.g. a better depth resolution can be obtained by sacrificing depth range and sensitivity). For the described system the maximum depth range is about 1000 nm, the best depth resolution is about 10 nm and the detection limit is on the order of 20 At.ppm.
Sample size min.: 2 x 5 mm^2
Sample size max.: 15 x 15 mm^2
Solid angle: 5 x 10^-4 to 4 x 10^-2 sr
Detection angle: 5 to 40 degree
Depth range: 100 to 1000 nm
Depth resolution: 10 to 100 nm (10% of max. depth range)
Sensitivity: 1000 to 20 At.ppm H
Time requirements: 2 h/sample
[ecu76] J.L.'Ecuyer, C. Brassard, C. Cardinal, J. Chabbal, L. Deschenes and J.P. Labrie, J. Appl. Phys. 47 (1976) 381.
[doo85] L.R. Doolittle, Nucl. Instr. Meth. B9 (1985) 344.
[doy79] B.L. Doyle and P.S. Peercy, Appl. Phys. Lett. 34 (1979) 811.
[doy88] B.L. Doyle and D.K. Brice, Nucl. Instr. Meth. B35 (1988) 301.
[lan92] W.A. Lanford, Nucl. Instr. Meth. B66 (1992) 65.
[viz90] G. Vizkelethy, Nucl. Inste. Meth. B45 (1990) 1.
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