Quantification for Depth Profiles: an Example including Curve Fitting

Introduction

For some, depth profiles are the most common use of XPS while for others such experiments are rarely performed. However, whether frequently used or otherwise, the sizes of these data sets require an efficient means for processing the numerous spectra resulting from these experiments. Figure 1 shows a typical data set from an XPS depth profile. Three hundred and eighty seven spectra were recorded in the course of this experiment and, to measure the chemical changes the spectra require analysis using both peak-fitting as well as simple integration regions.

Figure 1: Depth profile acquired on a PHI 5700 using a monochromatic X-ray source and an argon ion-beam to etch the sample.

Analyzing the Profile Using CasaXPS

Eight narrow energy intervals were monitored during the course of the depth profile, some of which merely require a background and an integration region to measure the intensity for that species. Others are more involved. Aluminium metal and oxide are present in this particular material and the presence of copper also complicates the analysis since the Cu 3s peak lies in close proximity to the Al 2s lines and similarly Cu 3p interferes with the Al 2p doublet; hence the need for a synthetic peak model.

The first step in generating a depth profile from the sequence of spectra is to identify the chemical states of interest. Consider the Al 2s region. To extract the information about the metal and oxide components it is necessary to define a synthetic model for the peaks located in the binding energy region [128,112] eV. Possible peaks found in this interval are Al 2s Oxide, Al 2s Metal, and Cu 3s (Figure 2). The spectra displayed in Figure 2 represent a sample of Al 2s regions taken form different depths in the material. Viewing the data in this way provides a means of assessing the variations in the peak proportions as the material is etched and allows a peak model to be built up which is suitable for the entire Al 2s profile.

Figure 2: A sample of Al 2s spectra taken from various depths within the profile.

Display the spectra by first selecting the VAMAS blocks using the right-hand-side of the Experiment Frame and pressing the Overlay toolbar button .

Useful mechanisms for making a selection in the right-hand-side of the Experiment Frame are: Double-click the first VAMAS block with the left-hand mouse button (this will both select the VAMAS block and display the spectrum in the left-hand-side of the Experiment frame), then holding the Ctrl Key down left-click further VAMAS blocks (the control key adds to the current selection). Alternatively, left-click the first VAMAS block in a range, then holding the Shift Key down left-click the last VAMAS block in the range. The result of using the Shift Key is to select all the VAMAS block between the two mouse clicks.

A Shirley background and synthetic peaks are constructed for an Al 2s spectrum using the options on the Quantification Parameters dialog window (Figure 3). The tools for creating peak models and processing the profile all appear as property pages on the Quantification Parameters dialog.

The first property page is labeled Regions and allows a background to be defined for the spectrum displayed in the active tile of the active Experiment Frame (here after referred to as the active spectrum). To create the initial region it is sufficient to press the Create button on the Regions property page (Figure 3) and adjust the Start and End fields so that the background ties to the spectrum at appropriate points. It is often necessary to average the intensities either side of the tie points in order to reduce the influence of noise on the calculated background. The field labeled Av. Width specifies the number of data channels used to determine the point at which the background should tie to the data. Figure 3 shows the Av. Width set to “1” and results in 2 * Av. Width + 1 data channels determining the tie points, i.e. three data channels, the original center plus one point either side of the center.

The start and end points for an integration region may be entered using the edit text-field. Left click on the item for which a modification is required. This will enable the edit text-field, then enter a new value and press Return. If the Return Key is not pressed the edited value is left unaltered.

A background to an integration region is entered in the field labeled BG Type. Possible backgrounds are: Linear (l), Shirley (s), Tougaard (t), W Tougaard (w), Default (d), Adjust (a), Min (mi), Max (ma), Mean (me), Zero (z) and others relating to the three parameter Tougaard cross sections described elsewhere.

Figure 3: Regions Property Page.

Note that the Relative Sensitivity Factor (RSF) for the Al 2s region shown in Figure 3 is set to zero. A zero RSF value will exclude the integrated intensity from any quantification calculation. For this example, the synthetic peaks will determine the intensities for the Al 2s profile and therefore RSF values for these items must be set appropriately.

Synthetic components (peaks) are constructed via the Components property page (Figure 4). Each time the Create button is pressed a new component is added to the active spectrum.

NB Species/transitions are important in CasaXPS. If the species/transition VAMAS fields are entered with the same strings that are also entered in the Element Library, then when a region or component is created the predefined value for the RSF associated with the species/transition string are entered into the region or component. If the species/transition fields do not match then the energy-ordered scrolled list on the Element Library dialog window can be used to force the Element Library entries to be entered into the newly created regions/components. The selected line in the energy-ordered scrolled-list will be used when a region or component is created.

Figure 4: Components Property Page.

Figure 5 shows a set of fitted peaks for one Al 2s region within the depth profile. These peaks have been created using the Components property page, where the initial settings have been adjusted so as to be appropriate for the current problem.

Asymmetric Line Shape

An asymmetric line-shape is used for the metallic Al 2s peak. The string “SGL(40)PHI(1,10)” specifies the type of function used to introduce asymmetry into the line-shape. The first string “SGL(40)” defines the shape of the symmetric portion as a sum approximation to the Voigt function, where the Voigt approximation is constructed from 0.4 * Lorentzian + 0.6 * Gaussian. An asymmetric tail is defined in terms of an exponential function (Briggs and Seah) where the degree of asymmetry is specified using scale and length parameters. The string appended to the “SGL(40)”string , namely, “PHI(scale, length)” is defined by the Physical Electronics MultiPak software and is identical to the Briggs and Seah tail function when the scale parameter is equal to 1.

Figure 5: Peak parameters showing interval constraints.

Interval Constraints

The constraints in Figure 5 are simply interval constraints. These can be easily managed using the “#” constraint shortcut. If the constraint is entered as a “#” sign, then the interval constraint is set to be identical to the current value for the parameter (thus fixing the parameter). If, however, the constraint field is entered using “#0.2”, say, the constraint interval is set to the current value for the parameter plus and minus the value entered after the “#” sign.

Relative Constraints

An alternative to constraint intervals is constraints with respect to other components. Figure 6 shows the same peaks but where the positions of the peaks in columns A and C are constrained to the position of the peak in column B by an offset and the FWHM for the peak in column A is constrained to the FWHM of the peak in column B using a factor constraint. The column characters entered under the heading “Const Id” refer to the labels used to head up the parameter columns shown in Figure 4.

Figure 6: Peak Parameters showing relational constraints.

Viewing Tables in CasaXPS

The component information shown in the tables within Figure 5 and Figure 6 are forms of annotation. These can be added to a spectrum display using the Component property page on the Annotation dialog window. Similarly, regions and tables based on multiple spectra quantification can also be added to the display using the Region and Quantification property pages of the Annotation dialog window.

Once a suitable peak model has been prepared for the active spectrum, the next task is to transfer (or propagate) the quantification items to other Al 2s regions. This is achieved using the Browser Operations dialog window.

Display the spectrum for which the peak model has been constructed.
In the right-hand-side of the Experiment Frame (Figure 1), select the set of spectra for which the same peak model is appropriate. In this case all the Al 2s regions were selected although subsets may be selected if appropriate.
Move the cursor over the spectrum displaying the peak model in the left-hand-side of the Experiment Frame and right-click the mouse. The Browser Operations dialog window appears (Figure 7).
Select the Propagate operations required. That is, tick the Regions and Components check boxes and confirm that the list blow includes only those VAMAS spectra for which a peak model is required.
Press the OK button and wait while the operations are copied to each of the spectra listed on the Browser Operations window. A progress meter offers a Stop button, which when pressed terminates the copy operation on completion of the current action.

Figure 7: Right-click over the displayed spectrum to invoke the Browser Operation dialog.

Intensities for the regions other than the Al 2s region can be estimated using integration of the background subtracted data. Figure 8 shows a representative sample from one layer of the profile, where integrations regions have been defined for all the spectral regions used in the analysis. Note that the Al 2 spectrum is the only one with synthetic peaks fitted. Once regions have been prepared for individual spectra, the propagate method described for the Al 2s synthetic peaks can be repeated for each of the regions in the profile.

Create a region on one of the spectra.
Select the set of spectra for which similar regions are required in the right-hand-side of the Experiment Frame.
Right-click the mouse over the spectrum displaying the region in the left-hand-side of the Experiment Frame.
Select the propagate option on the Browser Operations dialog window (Figure 7).
Check that the appropriate spectra are listed on the Browser Operations dialog window and press OK if correct. NB if the Match Species/Transition check box is ticked, then only those selected regions with the same Species/Transition VAMAS fields, as the source spectrum will receive the propagation operations.

Figure 8: CasaXPS Program Frame showing the selected VAMAS blocks on the right-hand-side of the Experiment Frame and a representative set of spectra showing the regions and components used to quantify the profile. The Quantification Parameters Dialog window is offering the Report Spec property page.

The names associated with the regions and components are important when generating profiles using the Custom Report. The names form the basis for defining formulae in the Custom report. All intensities labeled using the same name are treated as contributing to the same species and are therefore summed together when a formula in a Custom Report uses that name. Figure 9 shows the state of the Custom Report tables when the Area Report button was pressed. The table behind the Quantification Dialog window shows the result of pressing the button. Each column within the report corresponds to the entries in the Name/Formula table shown in the Custom Report section.

Initializing the Custom Report

The Custom Report is initialized using the buttons between the two tables headed Quantification Item Names and Name/Formula. The Quantification Item Names list shows the list of region and component names defined for the spectra selected in the right-hand-side of the Experiment Frame (Figure 8). The top three buttons initialize the Name/Formula table using the items shown in the table above (the bottom three buttons initialize the Name/Formula using ratio formulae). If the Regions button is pressed, the Names/Formula are entered using the items in the above list corresponding to regions. If however, as is the case in this example, the formulae need to be defined using both regions and components, the button labeled All, when pressed, will fill the Name/Formula table using both region and component labels. After press a button there may be more entries in the Name/Formula table than is required for the profile. The current example includes an entry for the Al 2 regions, and since the Al 2s profile is measured using the metal and oxide components the regions item should be removed from the report.

Figure 9: Report Spec property page together with the Custom Report generated from the formulae listed on the Report Spec.

To edit the Name/Formula list:

Move the cursor over the Name field you wish to alter.
Right-click the mouse over the Name field to invoke an edit dialog window (Figure 10). The Name and Formula fields are entered into the corresponding text-fields.
To delete the chosen entry, press the Delete pushbutton. Similarly a new entry in the table is created from this dialog window by pressing the Insert pushbutton. If the OK button is pressed, any changes made to the text-fields will become the current values for the original table entry.

The report shown in Figure 9 is created when the button labeled Area Report is pressed. The result of pressing the Area Report button is to calculate the atomic concentration using intensities measured from area in counts-per-second ´ eV. If Auger data were used to profile a material then peak-to-peak measurement may be more appropriate and in this case the button labeled Height Report should be used instead. The sensitivity factors determine what type of intensity measurement should be used. In this example XPS sensitivity factors suitable for area measurements were used, hence the use of the area report.

Figure 10: Dialog invoked by right-clicking the mouse over the name field in the Custom Report section.

Once a report has been generated (Figure 9), the resulting table may be saved to disk or taken through the clipboard to other graph plotting programs such as Excel or Sigma Plot. Figure 11 displays the profile tabulated in Figure 9 after formatting within Excel. The File menu offers a “Save As …” option. This allows results to be written as a TAB spaced ASCII file or alternatively if the file is saved with a .vms file extension a VAMAS file is written to disk.

Figure 11: Depth Profile generated by CasaXPS and plotted using Excelä

Converting the PHI MultiPak Data to ISO-14976 (VAMAS) Format

CasaXPS requires the data to be in the ISO-14976 file transfer format, however PHI MultiPak binary data files with file extensions .spe, .ang and .pro are directly converted through CasaXPS to the ISO-14976 file format via the Convert File Dialog window. In this example, the original data file was in PHI MultiPak format and simply selecting the file 01011812.pro via the Convert to VAMAS file dialog window converted the binary data to ISO-14976 format as well as opening an Experiment Frame within CasaXPS.

Figure 12: Dialog window used to convert proprietary file formats to the ISO-14976 VAMAS file format.