Electron optical coupling between TEM projection column and GIF
Although the Gatan imaging filter (GIF) prism is primarily an energy dispersion element, it also acts as a lens that focuses each energy component of the beam (e.g., zero-loss) as it enters the GIF aperture down to a small spot. This design enables the system to form a demagnified image of the final TEM projector lens crossover at the plane of the GIF slit. The quality and stability of the GIF focus (and high order tuning) is thus critically dependent on the size, shape, and stability of the projector lens crossover.
GIF magnification and special GIF modes of the TEM projection column
A key aspect of the GIF optics in image mode is that they are inherently set up to project whatever enters the entrance aperture into a focused, achromatic image that fully covers the CCD sensor at the end of the GIF. It is this basic criterion that fixes the magnification factor the GIF contributes to the overall system magnification (or camera length). In the GIF Tridiem® and GIF Quantum® SE, this magnification factor is 8.75x; for the GIF Quantum ER system the factor is 5x, which provides a larger field of view. In order to a establish a closer correspondence between the TEM viewing screen and the GIF detector field of view, most modern TEM instruments offer optional programs to reduce lens magnification whenever the GIF mode is active on the TEM. Typically, these projection column programs are set up so that whatever is within the central 5 cm disk on the viewing screen in non-GIF mode becomes the full field of view on the GIF when energy-filtered TEM (EFTEM) mode is active.
Effect of projector crossover movements on GIF focus and alignment
The GIF focuses the final crossover of the TEM projection column onto the energy-selecting slit plane. When experimental attributes cause this crossover to move, either laterally or along the optic axis, this may necessitate a complementary fine adjustment of the GIF alignment or focus. Lateral movement will typically result in an apparent energy shift of the zero-loss peak (ZLP); while movement along the optic axis gives rise to an apparent change in optimum GIF focus setting (mainly in Focus X). For a typical TEM, any change to the projection column settings can potentially cause the projector crossover to move; such as projection mode (e.g., imaging, diffraction, scanning), mode magnification or camera length. TEM systems with GIF integration options have special alignment options to minimize crossover movements as the projection column changes. To the extent that these EFTEM alignments are set up and aligned, GIF operation becomes very easy since no GIF adjustments are required as the TEM settings change. In practice, a considerable effort is required to thoroughly complete all these EFTEM alignments between the TEM and GIF. Normally, the TEM service engineer will carry out these alignments at installation. However, due to their complexity and typical time constraints, the EFTEM alignments are often only done for the most commonly used projection column modes and magnifications.
Microscope system control
In order to make GIF operation as simple and convenient as possible, it is necessary to place its various components under the control of a single high level software module. This module has much of the intelligence about the system and its required configurations and adjustments for EFTEM and electron energy loss spectroscopy (EELS) built into the module. In Gatan Microscopy Suite® (GMS) 3, this is controlled via the Microscope System control panel on the left side of the desktop. This is the role of the AutoFilter module in previous version of GMS. The Microscope System control panel coordinates communication with the GIF electron optics and its detector in order to automate mode changes, GIF tuning, and EFTEM and spectroscopy experiments. On the right hand Technique Manager panel, the EFTEM Map technique provides easy access to the features of the GIF and controls data acquisition.
When you use the GIF, one of the first decisions to make is what type of data you desire: images or spectra. This mode switching is done from the Microscope System panel by choosing the EFTEM or EELS buttons in the GIF region of the diagram. For systems without a fast shutter, the software reminds the operator to reduce the beam intensity when switching to Spectroscopy mode to minimize the chance that an intense ZLP will impinge on the detector.
When in EFTEM mode, the Technique Manager should be set to EFTEM Map technique so the Microscope System panel will expose the Tune GIF subpanel. These two features provide fully automated access and optimization of the GIF EFTEM modes.
In the Tune GIF subpanel, use Center ZLP to automatically set the energy zero point for the GIF. The Tune GIF button gives access to the automated EFTEM tuning functions. In general, run the Full Tune mode once per session in a multi-user environment. Run the Quick Tune mode as needed to make fine focus adjustments with the imaging filter. For imaging with slit widths less than 5 eV, enable Spectrum aberrations in the Quick Tune mode, then run at operating magnification at least once.
When in EELS mode, the Technique Manager should be set to the Spectroscopy technique so the Microscope System panel will expose the Tune GIF EELS subpanel. These two features provide fully automated access and optimization of the EELS modes of the GIF or PEELS system.
Click the region below the spectrometer model to control the spectrometer entrance aperture and dispersion. Adjust the energy offset here or from the EELS window in the Spectroscopy technique (during live viewing of a spectrum, the left/right arrows also allow changing of the energy).
In the Tune GIF EELS subpanel, the Align ZLP button will automatically set the energy zero position. The Focus button gives access to the automated EELS tuning functions. Quick Tune mode is recommended for most applications. For high energy resolution EELS, manual adjustment often improves results. The Manual mode gives direct access to the individual lenses. Adjust the lenses to make the ZLP as tall and sharp as possible. To view the aberration figure, run the EELS viewer in a User mode with show 2D enabled.
EELS acquire setup
From the EELS window in the Spectroscopy or related technique, adjust the readout configuration of the spectroscopy camera. Three default camera settings and unlimited user settings are available. In the Spectroscopy technique, the default modes are:
- SNR: Acquire weak signals such as high energy core-loss edges
- View: Survey the sample
- HDR: Use for high intensity signals such as low-loss data
For spectrum imaging applications, the three default modes are set up somewhat differently:
- Fast: Highest acquisition rate possible
- SNR: Map weak signals
- QMap: Map energy loss near edge structure (ELNES) or low-loss signals
In addition to the default modes, you can configure user settings and options from the Configure icon in the EELS panel.
Additional settings and options
- User Setups: In general, high levels of vertical binning improve the sensitivity and speed of the detector at the cost of dynamic range and gain correction purity; the Quality setting reduces the read noise ~3x, but slows down the readout of the detector
- Options: Apply HQ dark correction and Auto-align summed spectra are recommended options and should be enabled by default