Principle: An intense, low energy,
neutron beam illuminates the sample uniformly, producing initially
monoenergetic
charged particles (i.e., H+, He2+) from exoergic nuclear reactions. These
isotropically emitted particles lose energy in passing through the sample.
The energy which they retain upon leaving the sample surface is primarily
dependent on the depth at which the reaction took place. The depth profiles
are obtained by the deconvolution of the measured charged particle energy
spectra. The number of charged particles observed at a particular energy
is directly proportional to the concentration of the nuclide at a particular
depth.
Application: The technique provides
quantitative depth profiles in solids for certain elements, such as He,
Li, B, N, Cl, etc. (see table below). It can be used to probe semiconductor
samples, polymers, optoelectronic materials, metal alloys, and most other
solids. The analysis is nondestructive to samples, leaving only trace amounts
of residual radioactivity.
Sample Requirements: The sample
must be solid, or a liquid with very low volatility. It must present a
flat, smooth surface with a minimum area of at least a few mm2.
Maximum sample areas of a few cm2 can be analyzed. Large samples
up to 300 mm diameter can be scanned.
Quantitation: Concentrations
down to a few parts per million can be determined, depending on the element
and the matrix. Profiling to depths of a few tens of micrometers is obtained,
with resolution in depth varying between a few nanometers and a few tenths
of a micrometer.
Time for Analysis: Each run
takes a few minutes to a few hours, depending on the nuclide, sample, and
the degree of quantitation desired. The instrument is available 24
hrs per day when the facility is operating.
Limitations: The technique
is applicable only to the elements listed in the table. Also, as mentioned
before, the sample must present a smooth surface and have a low vapor pressure.
References:
"Neutron Depth Profiling: Overview and Description of NIST Facilities,"
R. G. Downing, G. P. Lamaze, J. K. Langland, and S. T Hwang. NIST Journal
of Research 98 (1993) 109.
Contacts:
Greg Downing, 301/975-3782, gregory.downing@nist.gov
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