XPS Quantification and
Related Subjects
The
subjects covered in this section are:
- The basic concepts of
quantification regions and synthetic components.
- Relative Sensitivity Factors
and instrument transmission.
- The Element Library mechanism
in CasaXPS.
- Report generation.
The CasaXPS
interface for preparing a quantification report is the Quantification
Parameters Dialog Window (Figure 1), available under the Quantify menu item on
the Options Menu.
Quantification
Regions
The
intensity of a peak in an XPS spectrum is measured by integrating the recorded
counts per second minus a computed background over an energy interval
delimiting the peak. For the purposes of quantifying a sample, the integrated
intensity must be adjusted for both relative sensitivity of the various
photoelectric lines and energy dependent instrumental/measurement artefacts before
computing the elemental composition for a surface. The primary purpose of
quantification regions as defined in CasaXPS is to provide the information
required for this calculation.
Synthetic
Components
When
photoelectric peaks overlap, the tool most often used to partition the
contributions from the individual transitions to the recorded envelope is optimisation
using predefined line-shapes for modelling the underlying transitions. The
intensity for a peak is determined by fitting a set of synthetic components to
the data by minimising the sum of squares residual, where the intensity for a
peak is extracted as one of the optimisation parameters. A synthetic component
is therefore analogous to a quantification region; however quantities such as
position, area and full-width-half-maximum (FWHM) are determined from an
optimization procedure using a given line-shape rather than, in the case of
quantification regions, being computed directly from the data.
Quantification Items and Reports
Since
quantification regions and a synthetic components both result in characteristic
quantities for XPS peaks, in CasaXPS both are referred to as quantification
items. Quantification reports are constructed from the characteristic values
determined from these quantification items.
A
quantification report may be either a Standard Report or a Custom Report. The
Standard Report option allows a tabulation of quantification items, grouped by
experimental variable, where values taken from each quantification item appear
as a line in the final report. Various options are available for Standard
Reports to permit the user to define the format for each row of information,
however each quantification item, if included, appears as a row in the table of
results. Custom Reports, on the other hand are column orientated and one or
more quantification items can contribute to the values reported in a column.
Essentially, a column in a Custom Report is defined in terms of arithmetic
expressions involving the name fields given to the quantification items and are
aimed at extracting trends within a data set defined by the variation of quantification
items with respect to the experimental variable.
Quantification
of Spectra
The
following is a set of examples illustrating the various methods for preparing a
quantification report. First, however, an overview to the possible analysis
scenarios is presented before the mechanics of performing the tasks is detailed.
Quantification of a Survey
Spectrum
The most
common and basic analysis of a sample is probably the elemental quantification
using quantification regions on a low resolution survey spectrum. Figure 2
shows a survey spectrum acquired from a PMMA sample using a Kratos Axis 165
operating in Hybrid mode at a pass-energy of 160 eV, where the elemental
quantification in the form of an atomic concentration table appears as
annotation on the tile displaying the data.
In CasaXPS,
the method for creating the quantification table may follow several paths:
- Automatic peak identification
leading directly to annotating the display with a region quantification
table.
- Manual peak identification
followed by manual labelling of peaks leading to region creation based on
the annotation labels.
- Region creation using the
Element Library entries.
- Direct region creation using
the Regions Property page.
Quantification
tables based on regions can be displayed over the data, as seen in Figure 2, or
alternatively separate quantification table text files are created from the
information derived from the quantification regions. The route taken leading to
the quantification table depends on the nature of the spectrum under analysis
and also personal preference.
The easiest
method, when it works, is the automatic peak identification and region creation
route. The regions shown in Figure 2 were created using this procedure. The
spectrum in Figure 2 is a very simple PMMA spectrum and therefore lends itself
to automatic analysis because of the good signal to noise and the peaks are
within the expected energy position for the various photoelectric lines; automatic
peak identification generally works provided the peaks can be distinguished
from the noise and the spectrum is calibrated with respect to energy. The steps
to perform an automatic quantification are as follows:
- Display the survey spectrum in
the Active Tile.
- Calibrate the energy scale
using the Processing dialog window.
- Invoke the Element Library
dialog window.
- Press the
- Use the Periodic Table buttons
to add or remove elements from those identified in the previous step until
the set of element markers match the peaks in the data.
- While still displaying the
element markers, press the Create Regions pushbutton
located on the Element Library dialog window. A set of quantification
regions are created and the corresponding quantification table is displayed
over the spectrum.
- At this point it is worth
checking the background start and end positions for each region and, if
required, creating any additional regions. The need to manually create
regions is sometimes required because the automatic creation procedure
will only create a region when a peak can be distinguished from the
background and occasionally minor peaks may be ignored by this process.
Visually reviewing regions: A quick means of checking the energy limits for a
set of regions is to press the Reset toolbar button
. The
Reset toolbar button loads a set of energy intervals on to the zoom list
determined from the current set of quantification regions. Once the zoom list
is loaded in this way, pressing the Zoom Out toolbar button
causes the display to step through the quantification regions.
The regions
created via the Element Library dialog window are chosen from those elements
identified using the
The second
means of creating quantification regions is based on annotating the peaks prior
to creating the regions. A typical spectrum will require the labelling of peaks
for presentation purposes and so as part of the labelling process quantification
regions can be created en route to labelling the spectrum. Peak labels
themselves are created in conjunction with the Element Library dialog window
and since one problem associated with the quantification process is the correct
extraction of the relative sensitivity factors for the peak in question, this
link between the peak label and the element library is helpful when creating
regions. The methodology is as follows:
- First annotate the peaks used to
quantify the sample. The Peak
Labels property page on the Annotation dialog window offers those
lines for which the Element Library has been used to display element
markers and when created via the Peak Label property page, the labels are
assigned the text used in the Element Library.
- Once the peaks intended for use
in the quantification are labelled using the text from the Element
Library, the Regions property page is used to create the quantification
regions. That is to say, the Create
from Label pushbutton causes the creation of a quantification region
for each peak label.
- The remaining peak labels for
those peaks not used in the quantification can now be created using the
same mechanism as that used in step 1.
A survey
spectrum VAMAS block does not initially contain information identifying peaks
in the data, hence the importance of linking the Element Library text strings
with quantification region creation via the annotation peak labels. A further
means of specifying the transition associated with a peak, but without the need
to create peak label annotation, is via the Element Table scrolled-list (Figure
3). The creation of quantification regions is performed by selecting the
Regions property page on the Quantification Parameters dialog window and, on
the Element Library dialog window, ticking the two tick-boxes below the
scrolled list in Figure 3 labelled, Zoom
When Line Selected and Create When
Line Selected followed by selecting a line via the name field in the
scrolled list. These tick boxes modify the behaviour of the scrolled list on
the Element Table property page. The Zoom When Line Selected tick-box causes
the action of picking a transition in the Element Table to zoom the display so
that the energy scale visible in the Active Tile matches a small interval
around the line indicated via the name field of the scrolled-list. The Create When Line Selected tick box
similarly modifies the same action by creating a quantification region using
the energy interval currently used to display the spectrum. The act of creating
a quantification region by selecting a line in the Element Table ensures the
correct relative sensitivity factor is used in the newly created region.
The fourth
method for creating a quantification region is to simply press the Create button on the Region property
page of the Quantification Parameters dialog window. The current zoom state
determines the start and end energies for the new created region, however all
other parameters are merely a repeat of the parameters previously used to
create a quantification region. If the species/transition fields are consistent
with an entry in the Element Library, then the relative sensitivity factor is
retrieved from the library and used in the quantification region. For survey
spectra, owing to the lack of species/transition information, the basic
creation mechanism is of little use, however if narrow scan spectra are
involved, the species/transition fields should be correctly assigned, in which
case the Create pushbutton will
indeed result in the correct assignment for the relative sensitivity of the
line.
Although
the Create pushbutton, when applied
to a survey spectrum, creates a region without retrieving the RSF from the
library, the RSF from the library can be entered using the name field for the
region. The default name field in the created region will not match any element
library entry, however if the region name is re-entered using a name from the
library preceded by the # character, then on pressing the return key the name
field will be entered without the # character and the element library field
matching the string is used to update the RSF in the region.
In the case
of a survey spectrum, an atomic concentration table may be displayed over the spectrum
as seen in Figure 2 or alternatively a quantification report is generated using
the Report Spec property page on the Quantification Parameters dialog window. When
using the Report Spec property page, the VAMAS block containing the survey
spectrum must be selected in the right-hand-side of the Experiment Frame prior
to pressing the button used to generate the report table.
Quantification of High
Resolution Spectra
The
mechanisms described for the creation of quantification regions on a survey
spectrum are also applicable to high resolution data from a sample typically
recorded as a sequence of narrow scan spectra. The key difference is that each
narrow scan spectrum is assigned one or more quantification regions and since
more than one spectrum is included when quantifying a sample, the quantification
table generated from these separate narrow scan spectra requires the use of the
browser in the right-hand-side of the Experiment Frame to perform the task. Those
spectra for which a quantification report is desired must be selected in the
right-hand-side of the Experiment Frame. Further more, quantification using
multiple acquisition regions for a given sample requires all corresponding VAMAS
blocks used in the quantification to appear in the same row of the browser pane
in the right-hand-side of the Experiment Frame. If the VAMAS file does not
result in the correct alignment of the VAMAS blocks in the right-hand-side,
then the values for the species/transition and the experimental variable must
be adjusted until those VAMAS blocks containing the spectra and quantification
regions for each sample all appear in a single row. A quantification report is
generated from only those quantification regions for which the VAMAS blocks are
selected in the right-hand-side and a separate row will appear in the
quantification table for each quantification item in the row of selected VAMAS
blocks in the right-hand-side.
Quantification with Tags
A problem
when quantifying samples, particularly for older instruments, is the variation
of an analyser’s transmission characteristic with pass energy, lens modes and
any other acquisition variables. A consequence of these operating mode
dependent variations in the measured peak areas is that, without transmission correction, quantification must
be performed using the same acquisition settings. Transmission correction
usually takes the form of, so called, transmission functions and in modern
instruments these are typically saved along with the spectra for use during
quantification. Older instruments rarely include transmission correction so, when
more than one operating mode is used, an alternative strategy is offered in
CasaXPS for quantification.
Quantification
using TAGS is designed to allow for the situations where elemental
concentrations determined from the survey operating mode are combined with high
resolution data to reveal chemical state information unavailable in the lower
resolution survey spectrum. Peak models determined from the high resolution
spectra are associated with quantification regions defined on the survey
spectrum and the relative intensity of the synthetic peaks from the high
resolution spectrum are used to proportion the elemental concentration from the
survey data. Thus, the chemical state information present in the high
resolution spectra is referenced back to the survey spectrum. The result is a
combined quantification report in which the transmission variations are
eliminated from the quantification report.
Both
quantification regions and synthetic components include as part of their
definition a TAG field. These TAG fields are text strings, which when the same
string is used for both a region and a corresponding synthetic component, the
region intensity is further tabulated using the relative intensities of
synthetic peaks with the same string in the TAG fields as the string entered
for TAG field in the region. Quantification using TAG fields is one of the
options on the Report Spec property page of the Quantification Parameters
dialog window.
Quantification of Depth
Profiles
The general
philosophy in CasaXPS is to handle larger data sets by identical mechanisms to
those used for smaller set of data. The same quantification items used to
quantify survey and narrow scan regions are used to reduce XPS depth profiles
to traces for each chemical state monitored as a function of depth.
The
structure of a profile experiment is essentially a sequence of survey or narrow
scan spectra recorded following an etch cycle or stage tilt; thus the VAMAS
blocks appear in columns corresponding to the assignments for the
species/transition where each row is aligned with respect to a common etch time
or angle. The route to tabulating a set of intensities as a function of
experimental variable is via defining regions and/or synthetic variables for a
spectrum and then propagating these quantification items to spectra with
similar requirements. There is no requirement to propagate the quantification
items through an entire set of spectra; rather the propagation is performed
only on those VAMAS blocks selected in the right-hand-side of an Experiment
Frame. Thus, a peak model, for example, can be adjusted at intervals during a
depth profile in order to account for new or absent peaks resulting from a
change of matrix material or interfacial anomalies. It should be emphasized,
however, that each spectrum used to profile a sample must have quantification
items assigned and so regions or synthetic components must be created directly
or propagated to each spectrum in the profile.
Survey
Spectra: Creating Quantification Regions, Relative Sensitivity Factors and
Transmission Correction
As
discussed above, the easiest method for defining quantification regions on a
survey spectrum is using the automatic peak identification options on the
Element Library dialog window followed by pressing the Create Regions button on
the same dialog window. For well behaved spectra, such as the data shown in
Figure 2, the results are generally achieved with minimal user intervention,
however not all samples are as accommodating and some form of adjustments to
the automatically created regions becomes necessary. The primary aim of the
following is to describe the options available for the manual creation of
quantification regions. The example chosen to illustrate the manual creation of
quantification regions is also useful in understanding some of the issues
associated with the quantification of a sample. The sample is pure gold, so the
only significant peaks derive from the same material at different kinetic
energies and from a range of symmetries (4s, 4p1/2, 4p 3/2, 4d, 4f and 5p3/2),
which makes it possible to investigate the factors influencing quantification
in a known regime.
Figure 4 is
a gold spectrum taken from a Kratos Axis Ultra at UMIST in
Manual Creation of
Quantification Regions
The basic
means of creating a quantification region is the Create button on the
Quantification Parameters dialog window shown on Figure 1. On pressing the
Create button, a new quantification region is created on the first spectra
displayed in the Active Tile, where the start and end energy values for the
region are determined from the interval displayed in the Active Tile.
Additional information from the element library is entered into the region,
such as the RSF, provided the name field in the newly created region matches
the element/transition fields for an entry in the current element library
loaded in CasaXPS. Since on creation via this route, the name field for the
regions is constructed from the species/transition VAMAS fields for the data in
the Active Tile, the RSF retrieval will only work for spectra in which the
species/transition fields have been correctly assigned in the VAMAS data file.
In the case of a survey spectrum, where multiple species/transitions are
present, the RSF will not be retrieved and therefore must be entered manually.
When creating regions on a survey spectrum such as the gold spectrum in Figure
4, the steps to follow are:
- Display the spectrum in the
Active Tile.
- Zoom into the peak for which
the region must be defined.
- Press the Create button on the
Regions property page on the Quantification Parameters dialog window.
- Set the name for the region
just created by entering the element/transition strings for the name
preceded by the # character and press the enter key on the keyboard.
To display a spectrum in the Active Tile: Using the mouse left-hand button,
double click the VAMAS block on the right-hand-side of the Experiment Frame.
Alternatively, select the VAMAS block using the left-hand mouse button and
press the 
toolbar button.
To zoom into a section of a spectrum: Use the left-hand mouse button to
drag out a box over the display area of interest and once a box is displayed,
click inside the box. The display will rescale to display the rectangular
region previously bounded by the box.
Once the
enter key on the keyboard is pressed, the # character is removed from the
string entered for the name and the RSF is retrieved from the matching entry in
the element library. The region name field can subsequently be altered to any
string not starting with the # character without further updates to the RSF.
Figure 5
illustrates the use of the # character to force the region to retrieve the RSF
from the element library. The element library name field for the doublet peaks
displayed in the Active Tile in Figure 5 is Au 4d, therefore to retrieve the
RSF for the region create for the displayed energy interval, the name field for
the region is entered using the string #Au
4d. When the Enter Key is pressed, the region name is modified to Au 4d and this new string is used to
match an entry in the current element library. If a match is found, both the
RSF and the TAG string are updated within the region parameters, shown in
Figure 6.
Relative Sensitivity
Factors and Other Corrections
If
information is retrieved from the element library, the name entered for the region
must be an exact match for an entry in the element library. In the case of
doublet states, spectral features derive from two peaks; these two peaks are labelled
with the same principal and orbital quantum numbers n and l, but
differentiated by the total angular momentum quantum number j. For example, in a gold spectrum, the
doublet pair at about binding energy 83 eV are assigned to the two transitions
Au 4f5/2 and Au 4f7/2 (Figure 7). The RSF for these
individual peaks are different and so if only one peak from a doublet pair is
used to measure the intensity for an element, the name used for the region
should be identical to the name field in the element library for the chosen
transition. In the case of the gold spectrum shown in Figure 4, the Au 4f
doublet pair overlap to the point where it is necessary to use the total counts
associated with the two peaks, in which case the RSF corresponding to the name
Au 4f is used to scale the intensity prior to comparison with other
transitions. This is in contrast to the 4p1/2 and 4p3/2
transitions, where the peaks are easily resolved and so the individual RSF
entries for the two peaks are included in the quantification table in Figure 4.
Ideally, the use of the combined intensity of the doublet pair is preferred
from a statistical perspective; however in practice the influences of time,
background and interfering peaks often force the use on one peak when
quantifying a material. The default CasaXPS library therefore includes three
entries per doublet pair, one for each individual peak in the doublet and one
for the total intensity for the two peaks. The rule for using doublet peaks is
to use the entry without a subscript for situations where both peaks are included
in the measurement, that is both peaks should have the same RSF, and to only
use the subscripted entries for cases when one peak from a doublet pair is
included in the quantification table.
The default
CasaXPS library contains Scofield cross-sections for the relative sensitivity
factors. These Scofield cross-sections were computed using Hartree-Fock theory
to determine the relative intensity of the transitions throughout the periodic
table for a given photon energy. The cross-sections are relative to the C 1s
transition and the default CasaXPS library includes entries for both Mg and Al
x-ray anodes. The theory used to calculate the cross-sections does not,
however, include corrections for instrumental and sample dependent intensity
variations. Without these corrections, XPS yields good relative measurements
but poor absolute quantities. That is to say, a run of identical samples, when
quantified using Scofield cross-sections, will produce good agreement between
the resulting tables, however the numerical values will not compare favourable
to the true proportions of the materials comprising the samples. It is
therefore important to account for these additional intensity variations as
part of the quantification procedure. The principal corrections are:
instrumental transmission correction, instrument dependent angular distribution
corrections to the measured transition and escape depth variations as a
function of kinetic energy. The quantification table shown in Figure 4 has been
computed using Scofield cross-sections corrected for all three intensity
adjustments. Once these corrections are made to the intensity of the peaks used
in quantification, the only remaining issues are that of background definition
and matrix dependent effects. The former is at the user’s discretion, while the
latter is beyond the scope of a one-size-fits-all
library approach; however multiple CasaXPS libraries are easily maintained via
the Input property page on the Element Library dialog window and so matrix
dependent RSF tables can be accommodated within CasaXPS.
Most modern
instruments offer transmission correction to account for the variation in
performance of a spectrometer as a function of kinetic energy for the ejected
electrons when measured using the different operating modes of the instrument.
Various mathematical models are employed to describe these transmission
functions; however the one that best fits
the Scofield library is the transmission function correction procedure
developed by the National Physical Laboratory (NPL) in the
Once the
instrumental transmission is accounted for, there is still an energy depend
correction required originating from the sample. The depth below the sample
surface from which electrons are collected by the spectrometer varies as a
function of kinetic energy of the ejected electrons; sometimes referred to as
the escape-depth correction, this adjustment to the peak intensities takes the
form of dividing the area of a peak by the kinetic energy of the peak maximum
raised to a constant power n, where n is typically a value
between 0.5 and 1.0. In CasaXPS, the intensity is adjusted by multiplying by
the energy raise to an exponent, and therefore the value used for the exponent
should be a negative number.
Both
transmission correction and escape-depth correction are applied by default,
assuming the information is made available to CasaXPS. The transmission curve
must be present in the VAMAS block as a second corresponding variable; however
the value for the escape-depth correction is either present in the VAMAS block
containing the spectrum or entered on the Regions property page in the text-fields
within the Intensity Calibration section (Figure 1). A configuration file entry
in the ParameterFile.txt file located in the CasaXPS.DEF directory allows the
default value for a new VAMAS block to be specified different from zero. The
values used to correct the peak intensities for transmission and escape depth
are computed at the energy of the peak maximum. There is, however, a subtle
difference between the meanings of “the energy at the peak maximum” for these
two quantities. The transmission curve relates to the peak maximum with respect
to the instrument and therefore relates to the kinetic energy of the electrons
in flight through the analyser. In contrast, the escape-depth is considered by
CasaXPS to relate to the kinetic energy of the ejected electron within the
sample. For these reasons, the transmission correction is computed using the
energy at the peak maximum before any energy calibration is performed, while
the escape-depth correction is performed using the kinetic energy for the
calibrated peak position.
Transmission
and escape-depth corrections are energy dependent adjustments applied to the
measured intensities; however there is a further correction related to the
instrumental geometry and the symmetry of the electronic states used in
quantification. The so called angular distribution correction can be viewed as
a correction, which must be applied to the Scofield cross-sections, to account
for intensity variation resulting from the angle of the x-ray source to the angle
of the ejected electrons (analyser axis). If the angle in question is different
from the, so called, magic angle of 54o 44′ minutes for which
correction is unnecessary, the values for the Scofield cross-sections must be
adjusted to account for the angular distribution of photoelectrons ionized by
unpolarized light impinging on an nl sub-shell.
The
instrument used to acquire the gold spectrum in Figure 4 (a Kratos Axis Ultra)
had an angle of 60o between the x-ray source and the analyser axis,
therefore a direct application of uncorrected Scofield cross-sections would not
yield the quantification shown in Figure 4. In fact, the RSFs used to
characterize the set of gold transitions where determined by correcting the
Scofield cross-sections for both the angular distribution correction factors
and the transmission function determined using the NPL instrument calibration
procedure. The relative percentage concentration values were also computed
using an escape-depth correction where the value for the exponent was -0.69.
The table in Figure 4 illustrates the possibility of measuring the gold signal
from any one of the six transitions resolvable in the gold spectrum. Given the
range of kinetic energies for the peak positions and the variation in the
electronic states, the consistent nature of the relative intensities is very
good. The problem with offering this table as a paradigm for all quantification
lies in the choice of background type and the quantification region limits. In
the case of the gold spectrum in Figure 4, all the quantification regions are
defined with linear background types and the energy intervals over which
integration is performed are chosen to be reasonable
subject to the constraint that a good result is achieved. There is greater
uncertainty associated with real samples and the choice of background type and
integration limits are without doubt the greatest source of error with respect
to the precision of an XPS measurement. Nevertheless, without these corrections
to the intensity calculation, it was not possible to reproduce the
quantification in Figure 4 with the same consistency.
Angular
distribution corrected Scofield cross-sections are calculated in CasaXPS for
properly defined VAMAS files. The required information for the use of
automatically corrected RSFs is a comment line placed in the VAMAS file comment
section of the VAMAS file and an appropriate entry in the ParametersFile.txt
configuration file. At present only VAMAS files converted from PHI MULTIPAK
data files contain the required information, but as an alternative, CasaXPS
libraries of corrected Scofield cross-sections are possible to construct. The
library used to quantify the gold spectrum was created for a specific
instrument, but the corrected Scofield cross-sections are such that spectrometers
characterised relative to the gold spectrum in Figure 4, could write relative
transition functions to the data enabling the same modified Scofield library to
be used in general. Since the gold spectrum was acquired on a Kratos Axis
Ultra, the use of this gold spectrum and associated library is most appropriate
for other Kratos Axis instruments.
Atomic Concentration
Report
The
quantification report displayed over the gold survey spectrum in Figure 4 is
typical of the types of information used to characterise a sample. Various
quantities are computed from the data, such as peak positions, FWHM and peak
area, however the column reported under the heading At%, is in general, the atomic concentration calculated from the
intensities following the application of all the corrections discussed above.
Clearly, in the case of a pure gold sample the atomic concentration is 100%
gold and the quantification table in Figure 4 is merely establishing the
relative accuracy of measuring the gold intensity using any one of the six
regions on display. When quantifying a real
sample, the quantification table derives from a selection of regions, where one
transition is chosen for each element identified as contributing to the peaks
in the data. Given that m elements
are so defined and therefore m
regions defined on the spectrum, the calculation for the relative proportions
of the sample surface or percentage atomic concentration is given by the
formula:
.
The percentage atomic concentration for the ith element Xi is defined by the adjusted intensity Ai as follows:
.
The terms contributing to the adjusted intensity are: the
measured intensity for a peak Ii
(either integrated peak area or peak height), the transmission function evaluated
at the peak position T(Ei), the relative
sensitivity factor Ri,,
kinetic energy Ei and escape-depth
exponent n.
Data
Orientated Discussion
The general
overview presented above is now expanded using specific examples to illustrate
the practical aspects of using CasaXPS to quantify spectra.
Quantification
using High Resolution Spectra
Although
elemental quantification is commonly performed on low resolution survey
spectra, circumstances occasionally require the use of high resolution spectra
for this purpose. A common reason for using narrow scan spectra is poor
signal-to-noise in the data. The greatest source of error when computing the area
of a peak results from noise-levels within the data channels used to tie the
calculated background to the data. For weak peaks it may be necessary to increase
the dwell-time compared to stronger peaks; the aim of most analysts is to
acquire the data in the minimum time subject to the constraint of sufficient
precision. The experiment shown in Figure 8 is an example of the case in point.
A small amount of chromium is barely evident on the survey spectrum in Figure
8; however, using an increased dwell-time for a narrow energy interval
containing the Cr 2p doublet allows efficient use of time when measuring the Cr
signal to the require precision.
Data
acquired using different dwell-times, number of scans and energy step sizes are
accounted for correctly by integrating the peak areas using the units of counts
per sec electron Volts (CPSeV), thus eliminating these parameters from the
quantification calculation. Switching lens modes or pass energy when acquiring
the different narrow scan spectra will require the proper use of transmission
functions to allow quantification to proceed.
The
analysis of the experimental data shown in Figure 8 is as follows.
Charge correction: The first step is usually to correct the data
for energy shifts due to charging of the sample under the influence of the
x-rays. The Calibration property page of the Processing dialog window 
offers a range of procedures for charge
correcting spectra (Figure 9). When the experiment consists of a set of narrow
scan spectra, most likely the set of spectra all require an identical energy
shift, where the value for the energy shift is determined by locating the peak
on one spectrum. There are several ways of performing this task, however in the
example shown in Figure 8 the calibration of the file was performed using the
following approach:
- Define a quantification region
on the Si 2p spectrum (described below).
- Select the VAMAS block
containing the Si 2p data.
- In the True text-field on the Calibration property page, enter the
require energy for the peak position defined by the peak maximum in the
quantification region previously defined on the Si 2p spectrum.
- Un-tick the box labelled Use
Reference Intensity and press the button labelled Apply by Row (1st Region).
The result
of these steps is to calibrate the energy for each spectrum in the same row as
the Si 2p by shifting the energy scale by the amount required to place the
maximum of the Si 2p peak at the energy entered in the True field on the Calibration property page. The position of the Si
2p peak is determined from the first region defined on the Si 2p spectrum. In
this case there is only one quantification region defined on the Si 2p
spectrum, but if more than one were defined, then the region in column A of the
Regions property page of the Quantification Parameters dialog window is the one
used in the shift operation.
It is also
worth noting that each row of spectra
in the right-hand-side, when calibrated using the Apply by Row (1st Region), is independently adjusted.
That is to say, each row in which the appropriate Si 2p VAMAS block is selected
will be treated as a separate calculation to determine the energy shifts. Thus
multiple samples loaded into the same VAMAS file, which appear in separate
rows, can be treated in this manner. The restriction, however, is that every
spectrum in a row is shifted without exception. For situations where for whatever
reasons only a subset of a row should be calibrated identically, other means of
calibrating selected spectra are available on the Calibration property page of
the Processing dialog window.
Quantification issues: Quantification using narrow scan
spectra requires the creation of quantification regions for each significant
peak in the data. The VAMAS block for each narrow scan spectrum used in the
quantification maintains the region information for the data held within the
VAMAS block. It is therefore necessary to individually display the narrow scan
spectra and create appropriate quantification regions. Once the regions are
created, quantification proceeds via the Report Spec property page on the
Quantification dialog window, where quantification is performed on only those
data within the VAMAS blocks selected in the right-hand-side of the Experiment
Frame.
Region creation: For the data in Figure 8, the VAMAS blocks are
assigned the correct species/transition fields, so creating quantification
regions is as simple as displaying the data in the left-hand-side of the
Experiment Frame, invoking the Quantification Parameters dialog window and
pressing the Create button on the Regions property page. The RSF for each
region is retrieved from the current element library; this is because the name
field within the element library matches the strings assigned for the
species/transition in the VAMAS block. Once regions are created for each narrow
scan spectrum, switching to the Report Spec property page on the Quantification
Parameters dialog window and selecting the VAMAS blocks as seen in Figure 10
allows quantification to proceed based on the users choice.
Quantification via the Standard Report: The data shown highlighted in Figure
10 represents four narrow scan spectra in which a single quantification region
has been created on each spectrum, namely, the Si 2p, O 1s, C 1s and Cr 2p
peaks. When a region is initially created using the Create pushbutton on the Regions
property page, the name field for the region is automatically entered using a
concatenation of the element/transition strings for which a match occurred with
an element library. Although the regions are assigned to individual spectra, there
is a feedback mechanism for checking which regions are active with respect to
the Report Spec option as a consequence of the current browser selection: all
quantification items defined on the selected VAMAS block will have their names
entered into the scrolled-list entitled Quantification Items Names in the
Custom Report section of the Report Spec property page. It is therefore worth
glancing through the list following making the selection of those VAMAS block
for inclusion in the quantification table. Note the Quantification Item Names
scrolled-list only includes distinct strings in the list. That is to say, if
two regions are assigned the same name, then only one entry will appear in the
Quantification Item Names scrolled-list, however, a Standard Report will create
an entry in the final table, one for each region or component defined on the selected data, subject to
which Standard Report button in pressed. This is in contrast to the Custom
Report, in which any combination of regions and components defined on the
selected data are always included in the calculation of the quantification
table. Further, if two or more regions and/or components are assigned the same
name fields, then only one entry will appear in the Custom Report table for
identically named quantification items and the intensity from all items with the
same name are summed to produce the intensity used in the resulting table.
The steps
to generate a Standard Report are as follows:
- Select the VAMAS blocks
containing the required data in the right-hand-side of the Experiment
Frame.
- Press the Regions pushbutton on the Standard Report Section of the
Report Spec property page.
- If required, press the copy
button
to transfer the quantification tables
onto the clipboard for use in other applications such as Excel.
The
quantification report shown in Figure 11 was generated using the Regions pushbutton, where the Use Config File tick-box was ticked. A
much wider range of quantification table fields are possible, however for many
only a subset of the full table is desirable and using a configuration file
named RegionQuantTable.txt in the CasaXPS.DEF directory allows the
specification of which fields are displayed when the table is generated. If the
tick box for the use of the configuration file is not ticked, a much more
verbose from of the table is created. It is also worth noting, the
configuration file definition file also allows the user to display additional
fields not visible in the default quantification table.
Quantification
of a Nylon Sample using Tags
The data
file shown in Figure 12 represents an example where a combination of a low
resolution survey spectrum and a high resolution C 1s narrow scan spectrum
results in a more complete quantification table than if the two spectra are
treated separately. Figure 12 shows data taken from a Kratos Axis 165 and,
although the Kratos system does include a method for transmission correction,
the data shown was exported without the original transmission curves, so a
direct comparison of the intensities measured from the survey data and the peak
model shown for the C 1s narrow scan region is not possible. Further, the VAMAS
file containing these spectra include neither the N 1s nor O 1s data acquired
using the same operating modes as the C 1s region, therefore the only way to
exploit the information from the C 1s data envelope is to use the Tag mechanism
in CasaXPS; that is, to proportion the elemental concentration for C 1s
measured using the survey spectrum in accordance with the relative intensities
from the high resolution peak model.
The Tag
mechanism in CasaXPS relies on the proper assignment of strings for each region
or component in the calculation. That is to say, the Tag strings are identical
for only those components and quantification region for which the concentration,
as measured by the region, is to be broken down in the proportions calculated
from the relative intensities of synthetic peaks. In the example of nylon seen
in Figure 12, there are three peaks used in the elemental concentration. When
the regions defining the O 1s, C 1s and N 1s intensities are created, the Tag
field must be entered with a value other than NoTag and must also be
distinct from one another. The default Tag field, when a region is created
using information from the element library, is the same string found in the
Name field for the region. Figure 13 shows the table from the Regions property
page where the Tag fields are entered using the element/transitions for the
regions in question, although any other strings would have been equally as good
provided the three strings are all different. However, whatever the Tag string
used for the C 1s region must also be used for the Tag fields entered in the C
1s components seen in the insert tile in Figure 12. Since the peak model for
the C 1s region also requires the creation of a region, it is as equally
important to ensure the Tag field for the C 1s region defined on the narrow
scan spectrum is assigned the string NoTag. The significance of the
string NoTag is that only quantification items for which the Tag field
is different from NoTag are included in a Tag based quantification report. Since
the area of the peak determined from the region on the narrow scan C 1s data is
of no use when compared to the O 1s and N 1s areas measured from the survey
spectrum (owing to the different instrumental operating modes used to acquire
the data), it is important the region from the C 1s narrow scan spectrum is
excluded from the quantification calculation by this means. A simple way to set
a Tag field to NoTag is to delete all characters from the text-field and press
return. The Tag field will automatically load the NoTag string.
The
quantification report displayed over the survey spectrum in Figure 12 is a form
of annotation
. A
table of information added to the display using the Quantification property page on the Annotation dialog window
(Figure 14) is both very useful, but also very dangerous if misused. The
information in the table does not come directly from the VAMAS block
maintaining the data display in the Active Tile, but rather is dependent on the
VAMAS blocks selected in the right-hand-side of the Experiment Frame. The
danger is clearly that the table shown on the display may derive from a
completely different spectrum in the same file; however the advantage of these
quantification tables is that related spectra can be combined to produce
information as useful as seen in Figure 12. Namely, the survey spectrum
provides the elemental quantification, while the C 1s narrow scan VAMAS block
provides the relative intensities of the four carbon peaks. The mechanism by
which the quantification table is created for the annotation is in fact
identical to the procedure used to generate general quantification tables using
the Standard Report on the Report Spec property page from the Quantification Parameters
dialog window. If the desire is to annotate a spectrum using information only
derived from the spectrum displayed in a tile, then either the Regions or
Component property pages on the Annotation dialog window are appropriate.
One
consequence of using the quantification table annotation is also seen in Figure
12, where the peak model for the C 1s envelope must also conform to the
chemistry of the sample, namely the intensity of the O 1s peak must relate to
the highest binding energy C 1s synthetic peak. It is therefore possible to
adjust the C 1s peak model whilst monitoring the affect of the relative
intensities on the O 1s and C 1s peaks. Further, Figure 12 illustrates a use
for the insert tile, where the parent tile displays the survey spectrum
together with the quantification table, whilst the insert tile displays the C
1s narrow scan region, thus the components for the C 1s spectrum can be changed
whilst still viewing the quantification table. A display such as the one shown
in Figure 12 is created as follows:
- Select the survey VAMAS block
in the right-hand-side of the Experiment Frame.
- Press the Display toolbar
button
. - Draw a zoom box in a vacant
area of the tile sufficiently large to display the C 1s spectrum and press
the Insert key-board key.
- Select the VAMAS block
containing the C 1s spectrum.
- Select the newly created insert
tile and press the Overlay toolbar button
. - Select both the VAMAS blocks
containing the survey spectrum and the C 1s narrow scan region in the
right-hand-side of the Experiment Frame. Given that a quantification table
annotation is defined on the survey spectrum, the act of selecting both
these VAMAS blocks will result in the display of the table shown in Figure
12.
Note:
Quantification annotation behaves in the same way as the Standard Report tables
created from the Report Spec property page. That is, VAMAS blocks can only be
quantified against each other provided the VAMAS blocks appear in the same row
of VAMAS blocks as seen in the right-hand-side of the Experiment Frame.
A table of
quantification results identical to the quantification annotation table in
Figure 12 is generated using the Standard Report 
button
(see Figure 10) on the Report Spec property page of the Quantification
Parameters dialog window. Again, the C 1s and survey VAMAS blocks must be
selected in the right-hand-side of the Experiment Frame prior to pressing the button.
Once the quantification table window is displayed in CasaXPS, the values from
the table are saved to file via the File menu or alternatively placed on the
clipboard via the Copy toolbar button
.
A
Multi-layer Depth Profile Example
The data
set shown in Figure 15 is a profile measured using a PHI Quantum 2000 where the
sample was a multi-layer material. Not all depth profiles have so many interfaces,
however even the presence of one boundary within the profile may require
specific adjustments to both quantification regions and synthetic components to
permit an accurate description of the chemistry as the environment alters
across an interface. The ability to create quantification items on a
spectrum-by-spectrum basis allows variations due to matrix effects to be easily
accommodated within CasaXPS and results in profiles representative of the
material rather than the limitations imposed by requiring one model to fit all.
The key to analysing depth profiles is to realise that:
- Each spectrum requires the
definition of quantification regions and components.
- The browser on the
right-hand-side of the Experiment Frame permits the propagation of a
specific model to only those VAMAS blocks currently selected.
If a
sequence of spectra can all be modelled with the same set of region and
components, the task is therefore to define the required region and components
on one representative spectrum, then using the browser, select the remaining
spectra for which the model is appropriate and propagate the model from the one
spectrum to all those so indicated.
The
importance of maintaining the quantification items as individual parameters for
each spectrum is illustrated in Figure 16, where the niobium spectra are
displayed together with the quantification regions corresponding to the initial
surface before ion-gun action and following the first ion-gun etch cycle. The
action of the ion-gun transforms a well defined doublet pair into an envelope
derived from a number of such doublet pairs. It is clearly preferable to select
spectrum specific start and end energies for the integration regions; the
consequence of only using one set of region parameters applied to all the Nb 3d
spectra is a more noisy profile trace in the final result.
By way of
example, consider the Si 2p spectra in the data set shown in Figure 15. The
silicon signal is effectively reduced to noise when the etch cycle is within
the niobium layer, however at the interface between the silicon and niobium
oxides a small Si 2p peak appears to the right of the silicon oxide peak. In
order to minimise the influence of noise and accommodate the intermittent
appearance of the minor peak in the silicon profile, it is necessary to adjust
the region limits on a spectrum by spectrum basis. One method of creating the
required quantification regions throughout the profile is as follows:
- Select all the Si 2p spectra in
the right-hand-side of the Experiment Frame. If the heading button for the
column of Si 2p VAMAS block in the right-side browser is pressed once,
every VAMAS block below the header button will become selected.
- Press the Overlay
toolbar
button. The entire set of Si 2p spectra will appear overlaid in the Active
Tile. - Using the Regions property page
on the Quantification Parameters dialog window
,
create a quantification region by pressing the Create button. The
element/transition fields for these data are correctly assigned and agree
with the desired entries in the current element library; therefore the RSF
is retrieved from the library when the region is created. Note that the
region is only created on the first spectrum in the Active Tile, namely,
the VAMAS block selected first when the browser selection was made prior
to pressing the Overlay toolbar button. - Adjust the quantification
region parameters, for example the Av
Width field might be set to “1”
and the start and end parameters adjusted under mouse control such that
the region looks plausible for all the spectra display in the Active Tile.
- Propagate the region to all the
Si 2p spectra: ensure the Si 2p column of VAMAS blocks are all selected in
the right-hand-side Browser, right-click the mouse over the tile
displaying the Si 2p spectrum for which the region was defined, ensure the
Region tick-box in the
propagate section of the Browser
Actions dialog window is ticked
and none others, confirm the list of destination VAMAS blocks agrees with
the intended Si 2p list and press the OK
button. Since the Si 2p spectra are overlaid in the Active Tile, the newly
created quantification regions together with the resulting backgrounds
will appear in the display showing all the silicon spectra. Note, however,
the fact that the spectra are displayed in the Active Tile is a convenient
way of visualising the result of the propagation and it is the Browser
selection which dictates the destination for the propagation action, not
the set of spectra in the Active Tile.
- Use the arrow keyboard keys to
step through the set of Si 2p spectra making adjustments to the region
parameters as required. If the Quantification Parameters dialog window is
open and the Region property page is the front-most page, then the mouse
can be used to adjust region limits.
The depth
profile in Figure 15 is a relatively small profile and the interfaces are close
together, however for larger sequences of etch-cycles where sub-ranges of
spectra are accommodated by the same set of region parameters, the propagation
mechanism can be used to selectively transfer a region to a subset of spectra.
That is, the propagation operation is performed on only those VAMAS blocks
selected in the right-hand-side of the Experiment Frame and there is no
requirement to select the entire column of Si 2p spectra as suggested above.
Similarly, regions and synthetic components can be propagated using the
identical mechanism when the use of subsets within a profile sequence becomes
more important; optimised peaks require more attention than regions typically
do. Small adjustments to peak constraints can often make a big difference to
the quality of the final depth profile, particular when these small alterations
to the peak model are applied to the appropriate part of the data set.
Now,
continuing with the remaining element/transitions in the profile, regions
and/or components must be defined for each of the four columns of VAMAS block
seen in Figure 15. Only the O 1s spectra are quantified using synthetic
components, where two O 1s Gaussian-Lorentzian line-shapes are used to fit the
data. These two peaks are labelled O 1s 1 and O 1s 2, principally to emphasize
the importance, when using the Custom Report to generate the quantification
table, of distinct names for quantification items. If two quantification items
(regions or components) are assigned the same name, then the
Custom Report (Figure 17) will add the intensities from
these identically named quantification items together before calculating the
concentration table. If, for example, the region for one of these O 1s spectra
had been named O 1s and the same name O 1s given to both the synthetic peaks,
the result from the Custom Report would be in error by a factor of two. That
is, the intensity from the region and the two synthetic peaks would be summed
together and used to compute the oxygen concentration. It is therefore
important to assign distinct names to quantification items unless it is desired
to sum the intensities involved. There are many situations where two or more
regions might be appropriately assigned the same name with the view to
improving the statistic in a measurement. For example, Figure 4 shows the Au 4p
doublet pair widely separated such that a single region spanning the energy
interval containing both peaks would struggle to define a plausible background.
On the other hand, two regions as seen in Figure 4, easily define the
background for the individual peaks and by assigning the same name to these
regions, the intensity from both would be summed by the Custom Report.
A key point
to note is that the Custom Report always works with all the quantification
items defined on the selected VAMAS blocks, regardless of whether the Region or
Component button is pressed to initialise the Name/Formula table seen in Figure
17. It is the Name/Formula table that defines the report and the buttons
between the Quantification Items table and the Name/Formula table are only a
quick way of initialising the Name/Formula table. Unwanted entries in the
Name/Formula table can be removed by right-click over the name field of the Name/Formula
list; a dialog window appears offering the option to modify, insert or delete a
field in the Name/Formula table.
The
Name/Formula table shown in Figure 17 was prepared by first selecting the
entire data set, then pressing the button labelled All between the two scrolled
lists on the Custom Report section. Since the O 1s spectra have both a region
named O 1s and two components named O 1s 1 and O 1s 2, it is necessary to
exclude the region from the calculation. A consequence of calling these quantification
items by different names is that each has a distinct entry in the Name/Formula
table and therefore the O1s entry corresponding to the region can be deleted.
The strings used in the Name column of the table is at the user’s discretion,
however the Formula entries must be constructed using exactly the strings used
to name the quantification items. Thus the formula to add the intensity from
the two components should be entered as follows: O 1s 1 + O 1s 2, where both the case and the spaces in the names O
1s 1 and O 1s 2 are important. Since formulae in the Name/Formula table can be
any arithmetic expression using constants such as 1 and 2, and the
quantification item names also include these digits, allowing such terms as 2 * O 1s 2 + 1 within a formula forces the use of exact strings for
quantification item names.
The goal of
processing a depth profile is to monitor the trend of each element as a
function of depth. The data set seen in Figure 17 lists the VAMAS blocks in
rows corresponding to etch-time; however the quantification table displayed in
Figure 18 lists the intensity and concentration determined from the VAMAS
blocks as a function of depth. The conversion from etch-time to depth, in this
example, is achieved by specifying the depth of the crater corresponding to the
total etch-time. The toolbar button
,
when pressed, offers a dialog window in which the interval for the crater depth
dimension can be entered corresponding to the sequence of etch-times, and
selecting the Depth radio button prior to pressing the OK button results in the
new abscissa values and labels. The change of experimental variable from
etch-time to depth is made under the assumption of a constant sputter-rate and
a uniform etch-time during the course of the experiment. If the etch-time is
none uniform, the Depth (Interpolate)
radio button should be used rather than the Depth option. If more complex relationships exist between the
etch-time and depth, then the quantification table must be exported to a spreadsheet for
further processing.
The
quantification report shown in Figure 18 is generated by preparing the
Name/Formula table as seen in Figure 17 and then pressing the Area Report button within the Custom
Report section. The resulting quantification report only includes values
determined from the intensity parameters computed using either the integrated
quantification region area or the area parameter from synthetic components. The
raw intensities are adjusted for relative sensitivity of the transitions using
the RSF entered into the quantification items, transmission correction, if
available and selected via the Regions property page, and escape-depth
correction using the value also entered on the Regions property page. The first
column in Figure 18 is the experimental variable and is followed by the
adjusted integrated intensities for each entry in the Name/Formula list, which
in turn is followed by the percentage atomic concentration computed from the
intensity entries.
The Custom
Report is designed for depth-profiles or any other trend experiments. The File
Menu, also displayed in Figure 18, offers a choice of saving the table to an
ASCII file or creating a VAMAS file containing one VAMAS block for each column
in the quantification table. Once selected, the Create Profile menu option results in a new Experiment Frame
appearing within the CasaXPS main window. The trends within the profile can be
overlaid and
annotated with a key as seen in Figure 19 using the Tile Display dialog window
.
Apart from offering many configurable display options, in particular the
Display property page on the Tile Display dialog window allows a key to be
added to the display. A further option on the File Menu available for spectrum
and profile display allows a prepared tile format to be saved to a separate
file from the VAMAS file. These ASCII tile format files default to a file
extension of .tff and can be retrieved at a later time to return the display of
a VAMAS file to an earlier state. Saving and reloading a tile format file is
performed using two options on the File Menu
.
Figure 1: Quantification Parameters
Dialog Window
Figure 2: Quantification of a Survey Spectrum
Figure 3: Element Library Property
Page
Figure 4: Gold Spectrum taken from a Kratos Axis Ultra
Figure 5: Zoomed energy interval showing a newly created region prior to the use of the region name field to trigger the update of the RSF and TAG fields.
Figure 6: Region parameters following the input of the string #Au 4d.
Figure 7: Element Library dialog window showing the three possible choices for the Au 4f transition
Figure 8:
Figure 9
Figure 10: Report Spec Property Page on the Quantification Parameters Dialog Window.
Figure 11: Standard Report Table.
Figure 12: Quantification using Tag
fields.
Figure 13: Tables from the Regions and Components Property Page on the Quantification Parameters dialog window.
Figure 14: Annotation dialog window displaying the Quantification property page.
Figure 15: Multi-layer Depth Profile Data Set (PHI Quantum 2000)
Figure 16: Niobium spectra illustrating the variation in the data envelope before and after the first ion-gun etch cycle.
Figure 17: Custom Report section prepared for generating a profile from the data selected in the right-hand-side of the Experiment Frame.
Figure 18: Custom Report Table.
Figure 19: Depth Profile created from
the table shown in Figure 18.