Beam Depth Profiling Techniques and Applications

Ion induced nuclear reactions for light element analysis in general and hydrogen analysis in particular have been extensively discussed in the literature. Only a few ion-induced nuclear reactions have cross sections large enough to be useful for materials analysis, and these are listed in published compilations (Amsel et al., 1971 Mayer and Rimini, 1977). 

Among the reactions most often used for analysis of hydrogen isotopes are the 2H(2H, p)3H reaction between deuterium (2H) ions and deuterium atoms, and the 2H(3He,p)4He reaction between 3He ions and deuterium. Both reactions emit high-energy charged particles, whose number and 

Despite its smaller cross section, the other nuclear reaction used for deuterium analysis, the (2H,p) reaction, can be made extremely sensitive, as Myers (1987) has shown. The key is that the proton is emitted with such high energy, 15 MeV, that the detector can be mounted behind the sample wafer (Fig. 1) ensuring a very low background count rate and hence good sensitivity. Myers measured deuterium amounts as low as 1012 atoms/cm2 in his study of deuterium uptake by Si02. However, the depth resolution of this measurement was poor, l jum. 

A widely used technique for depth profiling hydrogen (in this case the H isotope) uses resonant nuclear reactions (Lanford et al., 1976 Ziegler et al., 1978 Clark et al., 1978), i.e., the reaction 

In almost every case, experimenters who use the resonant nuclear reactions have chosen to detect and count the gamma rays, but the alpha particles can also be used (Umezawa et al., 1987, 1988a). There seems to be no published comparison of the results using alpha particle and gamma ray detection. Fig 2 shows a typical experimental setup using a gamma ray detector. 

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SIMS is by far the most sensitive surface technique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. The principle of SIMS is conceptually simple A primary ion beam (Ar+, 0.5-5 keV) is used to sputter atoms, ions and molecular fragments from the surface which are consequently analyzed with a mass spectrometer. It is as if one scratches some material from the surface and puts it in a mass spectrometer to see what elements are present. However, the theory behind SIMS is far from simple. In particular the formation of ions upon sputtering in or near the surface is hardly understood. The interested reader will find a wealth of information on SIMS in the books by Benninghoven et al. [2J and Vickerman el al. [4], while many applications have been described by Briggs et al. [5]. 

SIMS and SNMS are versatile analytical techniques for the compositional characterization of solid surfaces and interfaces in materials research.92-94 As one of the most important applications, both surface analytical techniques allow depth profile analysis (concentration profile as a function of the depth analyzed) to be performed in materials science and the semiconductor industry with excellent depth resolution in the low nm range. For depth profiling in materials science, dynamic SIMS and SNMS using high primary ion beam doses are applied. Both techniques permit the analysis of light elements such as H, , C and N, which are difficult to measure with other analytical techniques. 

Q.) is based on the ejection of the recoiled particles out of the sample in the forward direction by an energetic heavy ion beam. The measured energy spectra of these recoiled atoms can be related to their concentration profiles. The use of range foil in front of the energy detector to permit selective absorption of the various recoils introduces a few limitations in the application of the technique, e.g. deterioration of the energy resolution and hence the depth resolution, the limitation on the accessible depth in the depth profile information, etc. Indeed, the practical utility of the experimental set-up is enormously reduced in the region where overlapping spectra of various atoms are difficult to separate. 

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