Frequently Asked Questions
In the case of compositional analysis for elemental distribution or quantification, the 5 mm aperture is better suited because it allows a bigger collection angle and more signal into the spectrometer. For chemical analysis, the 2.5 mm is better suited since it delivers a slightly higher energy resolution.
This depends on the type of experiment. For chemical mapping fine structure analysis, a higher energy resolution is required. Therefore higher dispersions (smaller eV/channel numbers) are required.
For compositional analysis, only lower dispersions (big numbers) are suited. In general for compositional analysis and fast mapping, a dispersion of 1 eV/ch seems to be optimal because it gives the biggest field of view (2000 eV in the GIF Quantum® system) and the highest intensity on the CCD and EELS.
For general mapping, choose the shortest camera length available in the microscope. However, it really depends on the converge angle required. You always want to have your collection angle much bigger than the convergence angle; ideally two or three times.
The electron energy loss spectrum is probably saturated, since the low energy portion shows the highest number of counts. Reduce the exposure time or the binning.
In my STEM EELS acquisition, the interfaces that were straight in the surveyimage are tilted or wavy.
This is typically caused by specimen drift during the acquisition. Wait to get the microscope more stable or use the spatial drift correction function.
By focusing the spectrum using the spectrum focus function via the automatic routine or manually adjusting each single lens FX, FY, SX, and SY; mostly the former two.
Ideally you want to make the zero-loss peak (ZLP) as sharp as possible. The energy resolution is measured at the full width half maximum (FWHM) of the ZLP.
This depends on the amount of drift. The more severe the drift, the more frequent the correction needs to be set up.Note: The drift correction procedure increases the total exposure time for the entire dataset.
A common cause is excessive chromatic aberration in the image. Follow the below steps to solve this issue:
- Use a smaller objective aperture (typical aperture sizes are 8 – 24 mrad at 200 kV)
- Limit chromatic aberration by using a smaller energy selecting slit
Note: Both of these methods will reduce your image intensity. Increase the beam current or exposure time to compensate.
Poor focus and image stigmation are common contributors. Follow the below steps to solve this issue:
- Stigmate the image in normal phase contrast mode using the Diffractogram method if possible. Be sure to have the same objective aperture inserted while stigmating as it will be used for the EFTEM data acquisition.
- Acquire a View mode camera image at an energy offset of 400 eV (the inelastic is more sensitive than the elastic image to poor alignment)
- Slit width should be about 50 eV
- The energy and slit width can be configured via the Filter State control of the GIF and EELS Control UI (aka AutoFilter)
- Focus the image at this energy loss. Look to optimize the sharpness of the features. You can condense the beam smaller than the field of view to get more current if needed. If you do, be sure to optimize the beam to just larger to image field of view before data acquisition.
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.
The aperture size can be optimized based on the energy loss, slit width and required spatial resolution, but this is generally not necessary. You must use an aperture to limit chromatic blurring or your maps will be poor. Generally an aperture on the order of 8 – 16 mrads at 200 kV is adequate.
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
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.
This is typically caused by afterglow of the phosphor. To minimize this effect:
- Acquire the data in reverse direction using the “Acquire high to low” option in the spectrum imaging software
- Avoid exposing the camera to the unfiltered or elastic (ZLP filtered) image prior to acquiring a core-loss EFTEM SI
- Do not use the option “Align ZLP before acquire” option in the spectrum imaging software
This is caused by tuning of the Filter or an incorrectly centered zero-loss peak (ZLP). It is good practice to tune the filter (using the Tune for spectrum focus option in AutoFilter) at the magnification you will be using to acquire your maps. Do this at the early stage of the setup to avoid scintillator afterglow.
Why does a semicircular shadow appear in a corner of my EFTEM SI (the shadow may shrink into a circle or drift across the image field of view)?
This is typically caused by a poorly calibrated Condenser Adjustment. As the HT is changed to acquire the EFTEM SI, an automated adjustment must be made to the condenser focus and deflectors. To run this calibration, make sure the system is in Power User mode and run the menu item “EFTEM | Calibrate | Calibrate Condenser Adjustment …”. This is best done with a sample in the TEM and moderately thick area in the field of view (1 – 2 mean free paths). Simply follow the online prompts.
At the start of an energy-filtered transmission electron microscope (EFTEM) mapping session, or if strange shadows exist in acquired images or maps during a session, perform Tune GIF in the AutoFilter® palette and repeat. For more details refers to the tuning document.