Gatan Imaging Filter (GIF)

This section will discuss the Gatan implementation of energy-filtering and spectroscopy instrumentation. Other systems have similar controls and considerations.

Before you begin an experiment, it is important to familiarize yourself with the various controls, parameters, and adjustments of the Gatan imaging filter (GIF) system. This will help you understand the basic functionality you will use to collect energy-filtered TEM (EFTEM) images, elemental maps, and electron energy loss spectra from your transmission electron microscope (TEM) sample.

Optical column

Entrance apertures

The GIF entrance aperture determines what portion of the TEM beam will contribute to EFTEM images and electron energy loss spectra. Each GIF has four entrance apertures that are selectable under pneumatic control. You can use the largest entrance aperture (5 mm –  GIF Quantum^® SE, 9 mm – GIF Quantum ER system, and GIF Continuum^®) for all GIF imaging applications. This aperture and the TEM magnification fully determine the specimen field of view the GIF will capture. Imaging apertures for the GIF Quantum and Continuum are square and are not useable for spectroscopy. The smaller apertures (2.5 and 5 mm, round) are useful for spectroscopy applications. There is also the option for a 1 mm aperture for high-resolution Continuum systems. The smallest of these apertures cut out the portion of the electron beam that contributes to small residual aberrations of the GIF prism focus (see below) and thereby helps achieve the best possible energy resolution in the electron energy loss spectra. Apertures also control the collection angle and signal intensity in electron energy loss spectroscopy (EELS) experiments. Finally, there is a special mask aperture with a precision fabricated square array of holes. Use this solely for automated tuning and alignment of your GIF.

Multipole optics

Sophisticated multipole optical elements (e.g., deflectors, quadrupoles, sextupoles) pre-­shape the beam to compensate for the residual focus errors of the GIF prism. Although complex, the fine adjustments of this multipole system are fully automated via the Tune GIF function of the DigitalMicrograph^® software.

Energy-dispersing magnetic prism

The heart of the GIF is its magnetic prism. This element generates a magnetic field that bends the electron beam when it enters the GIF through 90°. This disperses the electrons along a vertical line in accordance with their energies. Zero-loss electrons have the most energy, are bent the least, and appear at the bottom of the vertical distribution. Core-loss electrons, which have lost the most energy and are the least energetic, are bent the most and appear at the top of the distribution. The prism also acts like a lens with focusing properties. In this respect, its main function is to form a sharp (demagnified) image of the final TEM projector system crossover at the plane of the energy selection slit.

Energy selection slit

The energy selection slit focuses on the energy-dispersed electron beam. During unfiltered imaging and spectroscopy applications, the slit is fully retracted. For EFTEM applications, the slit is inserted then the width is adjusted to select the electron energy loss range you desire in each EFTEM image. Typically, at 200 kV, the width of the energy selection varies from 2 – 100 eV, which represents a physical slit opening of 5 – 250 µm on the GIF Quantum system.

Energy offset controls

While you can adjust the slit to select a variable energy range, the centroid of that energy range varies when you electron optically move the dispersed electron beam up and down relative to the slit edges. You can accomplish this in any of three ways.

Spectrum offset (TEM high tension) – The spectrum offset function increases the TEM high tension by an amount equal to the average energy loss you wish to have in an EFTEM image. Primary electrons that now have higher energy will lose energy in the sample and pass through the slit at the original ZLP energy. This is the recommended offset method for all EFTEM imaging applications.

Energy shift (prism current) – The energy shift function varies the strength of the prism magnetic field when you change the current in the drive coil. This method is recommended when a large energy offset is required (up to 10 keV) and for monochromated systems operating at low (<80 kV) voltage. However, a disadvantage is that you may observe a small hysteresis in energy offset when the current varies over a large range. Therefore, this method should not be used for precision edge onset measurements.

The prism current is also used when you adjust the energy zero of the spectrum or center the ZLP in the energy-selecting slit.

Drift tube voltage – This function applies a precise voltage to the drift tube that temporarily accelerates the beam as it passes through the prism, resulting in a net downward deflection of the dispersed beam. This method is recommended for spectroscopy (EELS) since it is more important to keep the illumination conditions (e.g., probe focus) constant in this case, especially when you work in scanning transmission electron microscope (STEM) mode. This method is suggested when you measure chemical shifts in the spectrum because the offset does not suffer from lens hysteresis. However, the drift tube voltage has a limited range (1 or 2 kV, depending on the hardware options) and introduces a small defocus of the EELS spectrum at low operating voltages.

Multipole projection optics

After the electron beam passes through a crossover at the energy­ selection slit plane, it is projected back into either an image or a spectrum at the plane of the GIF detector. Multipole lenses between the slit and detector enable this capability. The multipole lenses are ideal due to the inherent asymmetry of the prism, which introduces and permits optimum coupling of image and spectral data to the pixels and natural readout directions of the CCD sensor. The multipole elements of the GIF projection system allow automated correction of image magnification and higher-order distortions to sub-1% levels depending on the GIF model. Also, the multipole optics allow automated correction of prism-induced chromatic aberration (achromaticity).

Low-level filter control

Fundamentally, you can mediate all GIF adjustments via the low-level FilterControl application. This is the only program that talks directly to the GIB electronics. All other higher-level software (see below) controls the GIF by sending messages to FilterControl. FilterControl manages all the lens programs and installation settings of the GIF optics and is normally only operated by Gatan-authorized service engineers. However, some basic controls and diagnostics remain accessible to operators and may be useful in certain circumstances and operating modes.

CCD detector for GIF Quantum

US1000FT and US1000FT-XP with 4-port readout electronics

The GIF Quantum detector is based on the UltraScan^® (US) 1000 CCD camera technology from Gatan. At the heart of this detector is a 2048 x 2048 pixel CCD with 14 µm square pixels and four active readout ports, one near each corner of the CDD. These outputs are coupled to the camera controller for rapid readout of image or spectral data captured during each exposure cycle. The camera controller program moves the charge in each quadrant of the CCD to a different readout register and can digitize all four outputs simultaneously, achieving high data readout rates.

Phosphor scintillator with HCR optical stack

Electrons projected by the GIF optics onto the detector impinge on a phosphor that emits a pulse of light for each beam electron that strikes it. A novel optical stack captures each light pulse and channels it to the immediately adjacent CCD pixels. This proprietary HCR technology from Gatan ensures that light pulses are captured with a very narrow point spread function, thereby preserving optimum resolution in the images and electron energy loss spectra.