Neutron depth profiling

NDP is a prompt nuclear analysis technique, which employs a nuclear reaction that results in emission of charged particles with a specific kinetic energy. It is one of the most powerful non-destructive techniques for depth profiling of some light elements especially for and Li, which have very high thermal neutron capture cross-sections of 3837 and 940 barn respectively. 

The samples are kept in a vacuum chamber and are irradiated with thermal neutrons produced by the nuclear reactor. 

TOF-NDP (time of flight NDP) is a variation of the traditional NDP. It uses time-of-flight techniques with superior resolution compared to conventional energy measurements conducted with surface barrier detectors. 

The solid state sample is preferred. Depending on the nuclear reaction used and the type of the analysed material, the sample surface region down to a depth of several micrometers can be analysed. 

If the thermal neutron capture takes place beneath the sample surface, the energy loss of charged particle stopping in the sample can be used to obtain information about the Li or B concentration profile (Fig. 13.3). The amount of energy loss is related to the distance that the charged particle has travelled within the specimen. 

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NAA cannot be used for some important elements, such as aluminum (in a Si or Si02 matrix) and boron. The radioactivity produced from silicon directly interferes with that ftom aluminum, while boron does not produce any radioisotope following neutron irradiation. (However, an in-beam neutron method known as neutron depth profiling C3J be used to obtain boron depth profiles in thin films. ) Another limitation of NAA is the long turn-around time necessary to complete the experiment. A typical survey measurement of all impurities in a sample may take 2-4 weeks. 

Neutron depth profiling technique (NDP) [13]. NDP is a speeial method for depth profiling of few light elements, namely He, Li, B and N in any solid material. The method makes use of speeifie nuelear reaetions of these elements with thermal neutrons. The samples are plaeed in the neutron beam from nuclear reactor and the charged products of the neutron indueed reactions (protons or alpha particles) are registered using a standard multiehannel spectrometer. From the measured energy spectra the depth profiles of above mentioned elements can be deduced by a simple computational procedure. 

Figure 9. Spreading resistance, SIMS, and NDP (Neutron Depth Profiling) profiles of a 8 lon-lmplant Into an N-type substrate. Reproduced with permission from Ref. Copyright 1984 American...

The development of the neutron depth profiling technique has been motivated by the importance of boron in both optical and microelectronic materials. Boron is widely used as a p-type dopant in semiconductor device fabrication and in the insulating oxide barriers applied as an organometallic or in vapor phase deposition glasses. 

Interfacial Profiling. Neutron depth profiling is well suited for measurements across interfacial boundaries. Kvitek et al. ( ) and others (16,17,21,30) have studied profiles of boron implanted and diffused across the interfacial region of Si/Si02. Other NDP experiments (33,34) have been described for interfaces of silicon, silicon dioxide or metal on metal, where diffusion distributions and segregation coefficients were studied. 

Neutron depth profiling has been applied in many areas of electronic materials research, as discussed here and in the references. The simplicity of the method and the interpretation of data are described. Major points to be made for NDP as an analytical technique include i) it is nondestructive il) isotopic concentrations are determined quantitatively iii) profiling measurements can be performed in essentially all solid materials, however depth resolution and depth of analysis are material dependent iv) NDP is capable of profiling across interfacial boundaries and v) there are few interferences. 

Finally, the development of two- and three-dimensional neutron depth profiling should be possible through the use of position sensitive detectors and ion optics, providing an even more advanced tool for the further understanding of microelectronic materials. 

TOE ERDA Time-of-Ehght Elastic Recoil Detection Analysis, 9 TOE-NDP Time-of Ehght Neutron Depth Profiling, 25 TPD Thermal Desorption Spectroscopy ... 

A necessary condition for cBN phase formation in thin films is that boron and nitrogen are incorporated in a nearly 1 1-ratio. Based on highly accurate composition measurement using neutron depth profiling, Hackenberger et al. [41] investigated the relationship between film stoichiometry and the phases present in the film. From these measurements, together with an analysis of data from the literature, they found evidence that film stoichiometry is one of the factors that stabilize the cubic phase. 

Whitney S., Biegalski S. R., Huang Y. H., Goodenough J. B. Neutron Depth Profiling Applications to Lithium-Ion Cell Research, J. Electrochem. Soc. 2009, 156, A886-A890. 

Nagpure S. C., Downing R. G., Bhushan B., Babu S. S., Cao L. Neutron depth profiling technique for studying aging in Li-ion batteries, Electrochim. Acta 2011, 56, 4735-4743. 

Oudenhoven J. F. M., Labohm F., Mulder M., Niessen R. A. H., Mulder F. M., Notten R H. L. In Situ Neutron Depth Profiling A Powerful Method to Probe Lithium Transport in Micro-Batteries, Adv. Mater. 2011, 23, 4103-4106. 

It should be noted that even at a low energy of implanted species (100 keV) the size of nanopores that are formed in the implanted layer turns out to be enough to make the insertion of large molecules possible (for instance, the dicarbollyl complex of cobalt readily diffuses into polyethylene implanted with 150-keV ions [75]). In the case of energetic ions (with energies of several hundreds of MeV), the pore size increases and the implanted polymer can be doped with fullerenes [61]. Thus, the concentration of C o molecules that difhise into polyimide implanted with 500-MeV ions from toluene solution amounts to as much as 1.8 x 10 fullerene molecules per track (the fullerene concentration was evaluated by a neutron depth profiling technique using Li ions, known to form the insoluble adduct with Cfio as the tracer [61]). 

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