Improved Near Surface Heavy Impurity Detection By a Novel Charged Particle Energy Filter Technique
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Fluorescence (TXRF) etc., which are sensitive to low level surface contamination. However, there
is a need for a non-destructive, quantitative and simple technique. Rutherford Backscattering Spectrometry (RBS), based on elastic scattering of MeV He or H ions, is commonly used for nondestructive, quantitative depth profiling. RBS is especially suited for detection of heavy impurities in light substrates. However, the RBS detection limit for heavy impurities is adversely affected by the underlying background signal, mostly caused by pulse pile-up (chance summing of pulse amplitudes, unresolved in time, from backscattered atoms in the sample matrix). Gunzler et. al.2 have reviewed different experimental techniques to reduce the pileup background. Thin film absorbers, to cut out the lower energy signal and thereby reduce pile up, have been successfully used, however the depth resolution deteriorates because of the inevitable energy broadening caused by straggling in the film. Another ion beam technique being used is backscattering with heavier ions like C or N. Researchers from Sandia National Laboratory 3 have demonstrated a detection limit of 5x10 9 at/cm 2 for Ni and Fe in Si, using a 150 keV nitrogen beam in conjunction with a time of flight technique. However, radiation damage produced by heavy ions in the sample and in the surface barrier detector limits its use. Ion channeling is another way to reduce the signal from the substrate and consequently the pulse pileup. In this study, we present the results of an alternate, inexpensive and simple way to reduce the pulse pileup in a conventional RBS setup, and thereby improve the detection limit of near surface, heavy impurities. An electric field is applied across a pair of plates between the sample and the detector, with the detector staggered with respect to the scattered beam so as to receive the deflected ions in a given energy range. The voltage applied to the plates can be adjusted such that only singly ionized He ions with energies in the range of that corresponding to backscattering from impurities heavier than the substrate are detected. The ions of lower energy corresponding to 399 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
scattering from the lighter substrate atoms, as well as the doubly ionized fraction of the scattered beam are deflected through a greater angle and miss the detector. The undeflected neutral fraction of the scattered beam also does not enter the detector because it is staggered several degrees with respect to the entrance slits defining the scattered beam. Thus, the setup for application of electric field to the scattered beam acts as a "Charged Particle Energy Filter (CPEF)". Due to the suppression of signals from scattering by the abundant substrate atoms, the pulse pileup at higher energies can be drastically reduced. The reduction of the background enables resolution of higher energy signals corresponding to scattering from heavier impurities of low concentration. Ross et. al 4 .have applied a similar con
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