Dynamic SIMS: Working with Cameca Data

 

Depth profile data from Cameca IMS-6f SIMS instruments can be exported via ASCII formatted files. These files, when written to disk, are assigned a file extension of .dp_ascii and, if exported from the raw data, include experimental parameters plus two columns of X/Y pairs representing the etch-time per cycle and the secondary ion intensity in counts per second (Figure 1). CasaXPS will convert these .dp_ascii files only when the raw data is exported. Conversion is performed using the Convert option on the File menu or via the toolbar button. A File dialog window is invoked by these options, in which the .dp_ascii file containing the raw data is selected and the Open button pressed. A new VAMAS file will be written to the same directory containing the original ASCII data after which the profile will appear in a new Experiment Frame in CasaXPS. When more than one profile is of interest, the data can be combined into a single Experiment Frame via the Convert and Merge option on the File menu. Again a File dialog window appears, in which each .dp_ascii file within the current directory can be selected using the mouse and the Ctrl key; on pressing the Open button on the dialog window the entire set of selected dp_ascii files are converted to VAMAS files and the resulting files loaded into a single Experiment Frame. The directory containing the original dp_ascii files will also contain corresponding VAMAS files. Since the conversion process involves writing new VAMAS files, it is important that the directory containing the data must have write permission and there is sufficient space on the disk to receive the new VAMAS file.

 

Figure 1: Cameca IMS-6f ASCII formatted data file.

 

 

After conversion to VAMAS format, both the timing information and the intensity units are adjusted to those used internally by CasaXPS. Most notably, the intensity unit in the .dp_ascii file is counts per second, however the intensity in the VAMAS file will be measured in counts per cycle and the abscissa becomes cycle index. All the associated timing information is also save with these abscissa and ordinates so that the calibration to depth and atomic density can be computed later.

 

Mass channels used to measure the secondary ion intensity are labelled within the .dp_ascii file using the nominal mass and the element name abbreviation. When entered into the VAMAS file, the block identifier is assigned the original string used to label the mass channel, however CasaXPS attempts to extract the nominal mass and the element abbreviation for use in the element and transition fields used by the VAMAS format. The concatenation of these element and transition fields provides the information used to align the VAMAS blocks within the Experiment Frame and is also used to determine the isotopic abundance ratio, which in turn is used to determine elemental from isotopic relative sensitivity factors. If the element/transition fields for a profile are not correctly assigned, should the user request elemental sensitivity factors an error message will result. In the event that the element and transition fields do not contain the element and isotopic information required to determine the relative abundance for an isotope, then these fields must be edited to reflect the true isotope used to measure the profile. To edit these fields:

  1. Select the VAMAS block(s) in the right-hand-side of the Experiment Frame.
  2. Press the  toolbar button and enter, on the resulting dialog window, the correct element abbreviation and nominal mass for the isotope of those VAMAS blocks in the selection.

When the information on the dialog window is accepted, the VAMAS blocks in the right-hand-side of the Experiment Frame will be re-organised to reflect the new assignment.

 

One way to check the possible values for the element/transition entries is to use the Exact Mass Calculator on the Element Library dialog window. Enter a string such as “Si28” into the text-edit field on the Exact Mass property page and press the Add Formula button. If a valid isotope string has been entered, then the string will be entered into the scrolled list above the text-field. Otherwise, an error dialog will indicated an error occurred. If elemental RSF values are desired, it is important the string above the VAMAS block in the Experiment Frame is accepted by the Exact Mass calculator, because this means the database includes the information necessary for the conversion of the RSF.

 

Computing the RSF for Si Data in Figure 1 and Figure 2

 

Figure 2: Initial state of the data following conversion to VAMAS format and displayed in CasaXPS.

The following steps lead to calibrated depth and intensity scales for the data in the original .dp_ascii file.

 

  1. Convert the .dp_ascii file using the Convert option on the File menu.
  2. Display the 69Ga data in the Active Display Tile and left-click on the Active Tile to enable the toolbar buttons.
  3. Invoke the Dynamic SIMS Calibration dialog window by pressing the Dynamic SIMS toolbar button. The top left-most text-field displays the Block Id for the data which is the current focus of the Dynamic SIMS dialog window.
  4. Press the Define Matrix button on Dynamic SIMS dialog window (Figure 3). Provided no cycles are selected in the scrolled list below the Define Matrix button, a dialog window will inform the user that no selection is active and ask whether all cycles should be updated with the profile in the Active Tile. Press the Yes button and observe that the Matrix Index column of the scrolled list is updated with the string 69Ga. If any cycle selection has been made within the scrolled list, then all cycles must be selected before pressing the Define Matrix button is pressed.

Figure 3

 

The second column in the scrolled list is labelled Interface. For this particular profile, the same matrix is present throughout the etch cycles and therefore the use of the Interface column is unnecessary. If the material were a multilayer sample, where RSF and sputter-rates varied between layers, then the Interface column would need to be populated with strings corresponding to the different materials characterising the layer structure. Once the layers are established, the Interface definition allows the assignment of RSF and sputter-rates to the individual layers.

  1. Double-click on the 28Si VAMAS block so that the silicon profile is displayed in the Active Tile.
  2. Press the Define Surf/BG button. Two regions are created on the 28Si profile and these mark the surface limit and the background to the secondary ion signal. Adjustments to these regions are performed using the Quantification Parameters dialog window.
  3. On the Dynamic SIMS dialog window, enter into the Dose and the depth fields the appropriate values for the standard (Dose = 2e14 atoms per cm2, Depth = 1776.4 nm)
  4. Press the buttons labelled Compute RSF and Compute Sputter Rate. The text-fields below these buttons will be updated with the computed values (Figure 4).
  5. Select one cycle in the scrolled list on the Dynamic SIMS dialog window and press the Apply RSF/SR to Matrix button. The values from the two text-fields for the RSF and the sputter-rate will be entered into the corresponding fields in the scrolled-list for each cycle for which the matrix index is identical to the one previously selected.
  6. Press the button labelled Calibrate Depth Profile. The RSF and sputter-rate entered into the 28Si table are used to compute the atomic density and the depth; the computed profiles are entered into a new Experiment Frame (Figure 5).

 

Figure 4: RSF and Sputter Rates are computed from the Dose and Depth, and then entered into the scrolled list.

 

The new Experiment Frame contains a VAMAS block for each VAMAS block in the original file. Within these new VAMAS block, several corresponding variables are defined which are assigned values for the intensity in counts per second plus the RSF and sputter-rate used to calibrate the profile. These values can be viewed using the Crtl PageUp/Crtl PageDown mechanism for stepping through the corresponding variables in a VAMAS block.

 

NB: To compute the elemental RSF from an isotope profile, the tick-box labelled Elemental RSF must be ticked and the element/transition field for the VAMAS block in use must be set appropriately.

 

 

Making Adjustments to the Sputter Rate

 

A depth profile prepared for calibration must have an RSF and sputter rate defined for each matrix within the analysis volume.  If at a later time it is desired to alter the RSF or sputter rate for a given matrix, the following sequence of steps should be used:

  1. Select a cycle from the scrolled-list to identify the matrix for which the RSF or sputter-rate requires adjusting.
  2. Press the pushbutton labelled Restore RSF/SR. The values for the matrix identified by the chosen cycle are entered into the corresponding text-fields for the RSF and Sputter Rate on the dialog window shown in Figure 3.
  3. Either the RSF, the sputter rate or both the RSF and the sputter rate can be recalculated before again pressing the Apply RSF/SR to Matrix button. The matrix defined by the selected cycle will be updated with the modified values.

The same procedure is used to transfer RSF and/or sputter rates from standard materials to unknown samples. The RSF and sputter rate from a standard sample are moved from the profile data to the Dynamic SIMS Calibration property page by first, displaying the data for an element from the standard in the Active Tile, selecting a cycle in the scrolled-list and then pressing the Restore RSF/SR button. The RSF and sputter-rate are copied from the selected cycle and entered in to the RSF and Sputter Rate text-fields. Provided the corresponding trace in the unknown sample is prepared with matrix information and a cycle is selected, displaying the unknown trace in the Active Tile then pressing the Apply RSF/SR to Matrix button completes the transfer.

 

Display of Calibrated Profiles

 

The labels for identifying the X and Y axes in a display tile are determined from the first VAMAS block with respect to the order of selection in the right-hand-side of the Experiment Frame at the time the VAMAS blocks are overlaid in the display tile. If a row of VAMAS blocks are selected by clicking on the experimental-variable value for the row, then the left-most VAMAS block will determine the axes labels. When a profile is calibrated, in the event that the left-most VAMAS block in row is an un-calibrated profile, for example a matrix trace, the data in the un-calibrated profile will be scaled to the range of the data for all the calibrated profiles in the row and will therefore be assigned a Y axis label of “Arbitrary Units”. To display an overlay of the profiles where the Y axis is labelled with respect to a calibrated profile, namely “Atomic Density”, it is necessary that a calibrated VAMAS block is selected first. To select VAMAS block in an arbitrary order, left-click the first VAMAS block, then hold the Ctrl keyboard key down and left click the other blocks required for the display. The traces are displayed in the Active Tile by pressing the overlay toolbar button and are ordered with respect to the selection sequence.

 

 

Figure 5: Calibrate Depth Profile