CHAPTER: IMPURITY ANALYSIS USING SIMS
Impurity analysis is performed using SIMS element surveys via mass scans as
described in ref. 1. The mass spectrometer (secondary ion analyzer) is scanned from mass 1
to any mass up to the limit of the magnet capability, and the secondary ion intensities are
recorded. Molecular species are suppressed by applying an offset voltage to discriminate
against ions that have narrow energy distributions, which molecular ions have. Thus, primarily
atomic or elemental ions are recorded, except for a few strong dimers that are usually of
obvious identification, including hydrides. Many elements can be uniquely identified by their
isotope fingerprints (natural abundance distributions). Each major element (component) of the
matrix is included in these data, so that SIMS relative sensitivity factors (RSFs) can be applied
to quantify the densities (concentrations) of the impurities, if the appropriate RSFs are known.
If the specific RSFs are not known, they can be estimated using the universal RSF pattern
schemes described in ref. 1 and in Wilson and Novak, 2 or in Chapter: Ion Mass Analysis in
this work.
Two kinds of SIMS analyses are performed to enhance the detection of all elements,
namely, bombardment by oxygen primary ions to enhance the emission of the more
electropositive elements (positive secondary ion detection), and bombardment with cesium
primary ions to enhance the emission of the more electronegative elements (negative
secondary ion detection). The transmission of the spectrometer is made large for these
analyses so that the magnitude of the secondary ion signal of the most intense mass in the
spectrum is just less than the saturation value of the Faraday cup detector in the CAMECA
IMS instrument, which is 6x109 cts/s. Both the Faraday cup and electron multiplier detectors
are then combined to provide a total range of signal detection of 6x10 9, or almost ten orders of
magnitude in secondary ion intensity (detectivity). Because of the differences of RSFs among
elements, that is, relative ionization efficiencies or probabilities, this range of signal detection
translates into higher or lower minimum impurity density/concentration determination capability
for various elements. The general range of minimum concentrations that can be determined
varies between about 10-7 (10 ppb) and 10-11 (10 ppt).
CAMECA SIMS instruments have the capability to be operated under conditions of high
mass resolution. In this mode, all but a few molecular species can be separated from the
atomic (elemental) ions at the same nominal mass. High mass hydrides are often difficult. A
penalty in detection sensitivity of about 20 is paid under high mass resolution operation
because of the reduced signal transmission that results from narrowing the mass resolving
slits. Detection sensitivities of elements that comprise the instrument ambient are reduced,
depending on the vacuum achieved in the analysis chamber. These elements usually include
H, C, N, and O. Detection of elements in the primary beam is also reduced. In the cases
described above, these elements are O and Cs. If the O primary ion beam is not mass
separated, the detection of masses (elements) that are in the beam may be reduced; such
elements in a standard CAMECA instrument are H, N, and Ni (from the intermediate electrode
of the duoplasmatron ion source). Other elements that may exhibit backgrounds in mass
spectra are those of samples that have recently been sputtered in the instrument. This
problem can be greatly reduced by cleaning or replacing the immersion lens cover plate and/or
by sputtering the material of present analysis interest to "cover up" atoms of the previously
sputtered material (that can be resputtered into the mass spectrometer). Both of these
solutions require significant times during which analyses cannot be performed.
References
1. R.G. Wilson, F.A. Stevie, and C.W. Magee, Secondary Ion Mass Spectrometry [Wiley,
1989]
2. R.G. Wilson and S.W. Novak, J. Appl. Phys. 69, 466-74 (1991)