EFTEM

As you prepare for an experiment, it is useful to understand EFTEM and the fundamental acquisition modes that support this technique.

Follow these EFTEM mapping best practices to ensure you can isolate electrons from distinct energy ranges to form your image or diffraction pattern of interest.

Basic Preparation

Below are common steps to prepare your sample and system for an EFTEM experiment.

  1. Always start with a well-prepared sample.

    1. For elemental mapping, sample thickness should be between 0.5 – 15 mfp (0.5 – 2.0 for edges >1 kV)

  2. Align the TEM; gun tilt, condenser, and optical axis alignment are critical.

  3. For mapping, a high beam current is desirable.

    1. Use the largest aperture and spot size if your sample is not beam-sensitive.

  4. Set the TEM objective lens current at its optimal value.

  5. Adjust the sample height to achieve a coarse focus.

  6. Center an appropriate objective aperture to limit chromatic blurring.

  7. Operate the TEM in GIF or EFTEM mode to ensure a stable projector lens crossover.

  8. Focus and carefully stigmate the image while you observe the image on the Gatan imaging filter (GIF) camera.

  9. You are now ready for EFTEM imaging or spectrum imaging (EFTEM SI) acquisition.

If these symptoms occur when you switch to the GIF mode on your TEM, the mode is not correctly set up.

  • The region of interest center on the GIF camera in GIF mode is not the same as in conventional TEM mode.

  • The GIF mode image is significantly out of focus when compared to the conventional TEM mode.

  • The image is cut off or not visible on the TEM view screen when you enter GIF mode.

  • Diffraction patterns are not centered in GIF mode.

To correct these abnormalities, please consult the TEM documentation or service organization for the appropriate settings.

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Mapping Workflow

Similar to spectroscopy, EFTEM is a simple workflow for enhancing contrast, mapping, and analyzing chemicals or elements.

  1. Find the region of interest you want to map.

  2. Insert and center an appropriate objective aperture.

  3. Align the zero-loss peak.

  4. Focus and stigmate the image.

  5. Take a thickness map to decide if the sample has the correct thickness to proceed.

  6. Move to an energy offset appropriate for the type of map you want to acquire.

  7. Focus and stigmate at the energy loss.

  8. Acquire maps.

  9. Adjust manual settings if needed.

Follow these step-by-step instructions for the above workflow when you use DigitalMicrograph® software.

Zero-Loss Image

Configure zero-loss image A zero-loss image is made using only electrons in the ZLP; that is, they have not undergone any inelastic scattering. When you exclude the inelastically scattered electrons, you can improve an image's contrast and resolution.

  1. Access to this capability depends on the mode you use.

    1. SingleMap mode – Press the Select ZLP button to open the Configure Zero-Loss Image dialog

    2. MultiMap mode – Specify in the MultiMap Configuration dialog

  2. EFTEM zero-loss peakIndicate parameters within the Acquire Zero-Loss Image dialog.

    1. Window Settings – Specify the energy loss (0 eV) and slit width for the filter (ideally, it should be wide enough just to cover the ZLP)

    2. Detector – Specify the detector settings you want to use

  3. Click Capture.

Zero-loss peak
As shown here, you can insert the slit to isolate electrons from the ZLP to create the ZLP image.

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Unfiltered Image

Unfiltered Image DialogAn unfiltered image contains both elastic and inelastically scattered electrons and is acquired with no energy-selecting slit inserted. When you obtain an unfiltered image, it is useful as an accompaniment with zero-loss imaging.

  1. To acquire an unfiltered image, only specify the camera parameters.

  2. Access to the Configure Unfiltered Image dialog will vary based on your chosen mode.

    1. SingleMap mode – Press the Select Unfiltered button

    2. MultiMap mode – Specify in the MultiMap Configuration dialog

  3. Unfiltered Image DataIndicate parameters within the Acquire Unfiltered Image dialog.

    1. Window Settings – Specify the energy loss (0 eV) and slit width (None) for the filter

    2. Detector – Specify the detector settings you want to use

  4. Click Capture.

Unfiltered image
In contrast to the zero-loss imaging, you will fully retract the slit to create an unfiltered image.

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Thickness Map

You can relatively easily produce a relative thickness map using the EFTEM technique by acquiring an unfiltered and zero-loss image from the same region under identical conditions. Once acquired, you can compute the relative thickness map when you utilize the Poisson statistics of inelastic scattering:

\(t/\lambda = -ln (\frac{I_{o}}{I_{t}})\)

where

  • \(I_{t}\) = Total intensity (unfiltered)

  • \(I_o\) = Zero-loss intensity (elastic or ZLP filtered)

  1. Thickness mapTo acquire a thickness map, specify the slit width for the zero-loss acquisition and the camera parameters to be used.

    1. The same camera parameters will be applied for both the zero-loss and unfiltered image acquisition.

  2. Access the Configure Thickness Map dialog will vary based on the mode you choose.

    1. SingleMap mode – Press the Select Thickness Map button

    2. MultiMap mode – Specify via the MultiMap Configuration dialog

      1. Window Settings – Set the energy loss (0 eV) and slit width for the filter

      2. Detector – Specify the detector settings you want to use

  3. Click Capture.

Thickness map data

The routine will

  • Acquire an unfiltered image followed by a zero-loss (elastic) image

  • Correct  any spatial drift between the images

  • Compute and display the thickness map

References

Malis, T.; Cheng, S. C.; Egerton, R. F. EELS log ratio technique for specimen-thickness measurement in the TEM. J. Electron Microscope Technique. 8:193; 1988.

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Electron Spectroscopic Imaging

Electron spectroscopic imaging dialogThe electron spectroscopic imaging (ESI) acquisition routine allows you to acquire a single energy-filtered image from an arbitrary energy range of the electron energy loss spectrum. This is useful for imaging features that show an intensity increase at particular energy losses (e.g., plasmon imaging or contrast tuning).

  1. To acquire an ESI, specify the EFTEM energy loss offset, the energy slit size, and camera parameters.

  2. Access to the Configure ESI Image dialog will vary based on the mode you choose.

    1. SingleMap mode – Press the Select ESI button

    2. MultiMap mode – Specify via the MultiMap Configuration dialog

      1. Electron spectroscopic imaging dataWindow Settings – Specify the energy loss and slit width for the filter; the Use Current button will copy the current spectrometer settings

      2. Detector – Specify the detector settings you want to use

  3. Click Capture.

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Jump-Ratio Map

Two techniques are commonly used in EFTEM to map elemental distribution: 2-window ratio mapping and 3-window elemental mapping. The jump-ratio approach requires two energy-filtered images, one you position just before the ionization edge (pre-edge) and one just after the edge (post-edge). In the jump-ratio approach, the post-edge image is divided by the corresponding pre-edge image to produce a map that indicates the distribution of the element you select.

  1. To acquire a jump-ratio map, specify:

    1. Element of interest

    2. Camera parameters

    3. Slit width and filter energies to be used for the pre- and post-edge acquisitions

  2. Access the Configure Jump Ratio dialog will vary based on your chosen mode.Configure Jump Ratio

    1. SingleMap mode – Press the Select Jump-Ratio Map button

    2. MultiMap mode – Specify via the MultiMap Configuration dialog

    3. Indicate parameters within the Configure Jump Ratio dialog

      1. Window settings (eV)

        1. Setup – Allows you to recall pre-saved or default settings for a particular element

        2. Slit width – Specify the energy-selecting slit width you want to use for both the pre- and post-edge acquisitions

        3. Post-edge – Designate the energy loss setting that the filter will use to acquire the post-edge image

        4. Pre-edge – Indicate the energy loss setting that the filter will use to acquire the pre-edge image

        5. Edge – Specify the edge you want to use for ratio mapping

      2. Detector – Specify the detector settings you want to use

      3. Spectrum display – Generates and displays a simulated spectrum at the top of the dialog to give you visual feedback regarding the feature shape and the selected acquisition window positions

  3. Click Capture.

    Jump-ratio map data

The routine will

  • Acquire the post-edge image followed by the post-edge image

  • Correct for any spatial drift between the images

  • Compute and display the ratio map

Jump ratio image
When you create a jump-ratio map, first insert the slit and focus the image at an energy loss (typically 400 eV and 50 eV slit) before acquiring a pre-edge image. Acquire the pre-edge image, then shift the energy to after the edge to acquire the post-edge image. To calculate the ratio, divide the post- by the pre-edge image value.

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Elemental Map

The 3-window technique for elemental mapping requires three energy-filtered images; two positioned before the ionization edge (pre-edge images), and one positioned just after the edge.

  1. To acquire an elemental map, specify:

    1. Element of interest

    2. Camera parameters

    3. Slit width and filter energies to be used for the pre- and post-edge acquisitions

  2. Access the Configure Elemental Map dialog will vary based on the mode you choose.Configure Elemental Map dialog

    1. SingleMap mode – Press the Select Elemental Map button

    2. MultiMap mode – Specify via the MultiMap Configuration dialog

  3. Indicate parameters within the Configure Elemental Map dialog.

    1. Window Settings (eV)

      1. Edge – Specify the edge you want to use for ratio mapping

      2. Pre-edge 1 and 2 – Indicate the energy loss setting that the filter will use to acquire the pre-edge image

      3. Post-edge – Designate the energy loss setting that the filter will use to acquire the post-edge image

      4. Slit width – Specify the energy-selecting slit width you want to use for both the pre- and post-edge acquisitions

      5. Setup – Allows you to recall pre-saved or default settings for a particular element

    2. Detector – Specify the detector settings you want to use

    3. Spectrum display – Generates and displays a simulated spectrum at the top of the dialog to give you visual feedback regarding the feature shape and the selected acquisition window positions

  4. Click Capture.

Elemental Map data

The routine will

  • Acquire a post-edge image followed by the two pre-edge images

  • Correct for any spatial drift between the images, if specified

  • Compute and display the elemental map

Elemental Map data

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EFTEM MultiMap

The EFTEM MultiMap Acquisition software allows you to perform multiple EFTEM acquisitions (e.g., zero-loss, elemental maps) from the same region during a single experiment. This will enable you to collect any combination of maps and images, but researchers frequently use this software to obtain a series of elemental maps. One key advantage is that you can acquire the pre- and post-acquisition thickness map to help evaluate the effect of the beam on your sample.EFTEM palette

  1. Select MultiMap in the EFTEM palette under the EFTEM Map technique.

    1. Click on the Add button to open the Setup EFTEM Multi-Mapping dialog

  2. Use these group boxes within the Setup EFTEM Multi-Mapping dialog to define your experimental parameters.

    EFTEM MultiMap dialog

    1. Map List – Allows you to specify which features to acquire and in what order

    2. Detector – Defines the individual detector parameters for each acquisition in the MultiMap list

    3. Window settings (eV) – Configures the individual window settings for each acquisition in the MultiMap list

    4. Spectrum display – Generates and displays a simulated spectrum at the top of the dialog to give you visual feedback about the feature shape and the selected acquisition window positions (Display all edges will show potential overlap problems in the list of elements you have selected)

    5. Load/Save Setup List – Saves the EFTEM MultiMap list and all associated parameters as an analytical list profile for future use

      EFTEM MultiMap data
      Multi-mapping of a semiconductor device during a single acquisition.

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Optimize Maps

EFTEM mapping is generally limited by three effects:

  • Shot noise

    • Symptom – Noisy maps, especially in areas that should have a high, uniform composition of the element; results from the inherent statistical variation in the arrival rate of electrons on the detector

    • Solution – Acquire more electrons per pixel; typically, you optimize a combination of beam current, exposure time, and binning

  • Chromatic aberration

    • Symptom – Fringes along boundaries that separate dark and bright parts of the image; typically occurs when you do not use an objective aperture

    • Solution – Reduce the slit width and objective aperture to minimize the effects of chromatic aberration; however, this will reduce the number of electrons collected and increase the shot noise

  • Image drift

    • Symptom – Unidirectional fringes along interfaces that separate dark and bright parts of the image and in areas with a lot of diffraction contrast

    • Solution – Repeat the drift correction function with different parameters or in manual mode; ensure the sample is not damaging or shrinking; reduce exposure time and/or increase beam intensity

  • Image focus

    • Symptom – Blurred EFTEM maps at an energy loss

    • Solution – Focus the image 

You can choose several general parameters to optimize your EFTEM maps further when you use DigitalMicrograph® software. These parameters affect all EFTEM acquisition modalities (e.g., SingleMap, MultiMap), and you can locate them in the EFTEM Mapping Preferences dialog.

  1. Go to the EFTEM palette, then click the Setup button.

  2. The EFTEM Mapping Preferences dialog will then open.

Drift measurement

The sample may drift during EFTEM data acquisition involving multiple images (e.g., summed image acquisition or mapping using multiple windows), resulting in a spatial mismatch between images acquired successively. This spatial mismatch will introduce artifacts into the final map or image if not accounted for. The EFTEM mapping routines have drift correction procedures you can configure via the Drift Correction tab in the EFTEM Mapping Preferences dialog.

  • Plane-to-plane alignment – Post-processing steps that correct drift between successive energy integration ranges in an EFTEM acquisition

  • Cumulative alignment – Fully automates drift correction during a cumulative (or summed) acquisition between successive planes at the same energy integration range

Map computation

The parameters set here globally affect how you perform map computations.

  • Automatically remove x-rays – Replaces erroneously high or low values in the source images with their local neighborhood average prior map computation

  • Hole-count threshold – Treats values below the set threshold as zero and omits them for the computation of maps; these typically are holes in the sample (e.g., regions with no sample)

  • Background model – Specifies the elemental mapping model you will use to compute the post-edge background contribution from the pre-edge images during the creation of a 3-window elemental map

Options

When MultiMap is present, the Options tab is available. It contains the options that will affect all MultiMap acquisitions globally.

  • Colorize thickness map – When this option is selected, the thickness map will be colorized
  • Automatically save and organize maps – Mapping routines will automatically save all acquired images at the end of acquisition using the Group Save options

Auto Exposure

The Auto Exposure dialog contains a fast and robust auto-exposure and -binning routine to help simplify the acquisition of elemental maps with optimal image intensity. The behavior of the auto-exposure and -binning routines depends on the preferences you specify and the particular acquisition type.

  • Exposure – Sets the minimum and maximum exposure times the auto-exposure routine uses for EFTEM single and multi-mapping

  • Intensity – Determines the target intensity the auto-exposure routine will aim for, along with the minimum and maximum acceptable limits

  • Binning – Allows charges from adjacent pixels to be combined to increase readout speeds

Note: You can use Binning to trade signal-to-noise ratio (SNR) for a number of pixels. Fewer pixels (higher binning) give better SNR but lower resolution. Typically, 2x or 4x is used for edges up to 1 keV, and 4x or 8x is used above 1 keV.

Energy offset method

The EFTEM modes adjust the TEM high voltage to offset the energy during acquisition. This helps ensure the lower column and the GIF remain in focus and aligned throughout the experiment. The other methods of offsetting the energy are used for spectroscopy and are described here.

Note: EFTEM at high energy losses is an incoherent imaging process. Conventional bright-field TEM imaging is a coherent imaging process, and lens defocus is used to add contrast to the image (for example, Scherzer focus is typically used for HREM imaging). Incoherent images are only sharp for one value of focus (Gaussian focus). In addition, the large angular distribution of the energy loss electrons results in a very narrow depth of focus. This makes focusing at a reasonably large (~400 eV) energy loss prior to the start of acquisition critical for EFTEM mapping. Since the high voltage offset is used in EFTEM acquisition and the energy of the detected electrons stays fixed, once the Gaussian focus is found, the focus does not need to be changed when the energy offset is changed.

References

Berger, A.; Kohl, H. Optimum imaging parameters for elemental mapping in an energy-filtering TEM. Optik. 92:175 – 193.

Kothleitner, G.; Hofer F. Optimization of the signal to noise ratio in EFTEM elemental maps with regard to different ionization edge types. Micron. 29349 – 357; 1998.

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Related FAQ's

  • Why do I see bright and/or dark bands at interfaces?

    When you see bright and/or dark bands at the interfaces of computed maps, this means your sample may be drifting:

    • Decrease exposure time (should not exceed 30 s for each frame)
    • If more than one frame is chosen select align images to remove the spatial drift
    • Ensure that spatial drift has been removed by aligning the images correctly before extracting elemental maps

    Another cause is the application of excessive smoothing of the power law background exponent. This option is set in the EFTEM Mapping Preferences dialog. Consider reducing the smoothing half width or changing to a straight pixel-by-pixel power law model. The maps can be recalculated using the menu item “EFTEM | Compute elemental maps” without reacquiring the data.

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  • My map and/or images are noisy. What can I do?

    Make sure the sample is thin by acquiring a relative thickness map. If it is thick (e.g., relative thickness >1), try moving to another sample area. Increase the number of collected electrons by one (or all) of the following:

    • Increase the beam intensity
    • Increase the camera exposure
    • Increase the CCD binning
    • Increase the slit width
    • Increase the number of frames summed
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  • I see very sharp, periodic contrast that is not from the sample in bright areas, why?

    If you see very sharp contrast that is not from the sample in bright areas, you may have saturated the detector in ones of the images. Decrease the detector exposure time and/or the sample illumination, then repeat.

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