Elsevier

Ultramicroscopy

Volume 110, Issue 8, July 2010, Pages 962-970
Ultramicroscopy

The GIF Quantum, a next generation post-column imaging energy filter

https://doi.org/10.1016/j.ultramic.2010.01.009Get rights and content

Abstract

We describe a new post-column imaging energy filter for (scanning) transmission electron microscopy from 60 to 300 kV operating voltage. The completely redesigned GIF quantum has a gradient magnetic prism, dodecapole optics, a 10-times faster 40 Mpixel/sec CCD camera, a 1 μs electrostatic shutter, and new user interface, control, and auto-alignment software. An 8 dodecapole lens system, performs full 2nd and 3rd, and partial 4th and 5th order aberration correction. The improved aberration correction has allowed the size of the entrance aperture to be nearly doubled to 9.0 mm compared to current generation post-column designs. The electrostatic shutter provides exposure control down to 1 μs, extending the exposure time range to over 7 orders of magnitude. Spectroscopy operation has been improved with a larger 2 keV field of view at 200 kV, and a maximum acquisition rate of 1000 spectra per second. A high-speed DualEELS mode simultaneously acquires core- and low-loss spectra up to 2 keV apart. A more intuitive user interface includes new capabilities such as automated exposure control and optimized full spectrum acquisition. The auto-alignment software has been significantly enhanced to use the full flexibility of the dodecapole lens system.

Section snippets

Post-column imaging filters to date

To date, post-column imaging filters have been based on the design of the GIF200 introduced in 1992 [1]. These filters introduce energy dispersion by bending the electrons over 90° with a 100 mm radius magnetic prism. Two post prism quadrupoles magnify and focus the spectrum onto an energy-selecting slit, and 4 post-slit quadrupoles project either an energy-filtered image or an energy-loss spectrum onto a CCD camera. The prism’s tilted and curved entrance and exit faces, in combination with a

Need for a next generation GIF

While the GIF Tridiem series was very successful and productive, a number of considerations led us to design a dramatically improved imaging filter:

  • Field of view and transmissivityA post-column filter’s largest field of view is limited by the magnification from the entrance aperture to the CCD camera. For a given CCD size, a larger entrance aperture will result in an increased field of view. For the Tridiem, the largest field of view depends on the TEM model but is typically around 20 μm. The

GIF Quantum design

An overview of the GIF Quantum and its key components are shown in Fig. 1.

A comparison with earlier GIF generations, shown in Table 1, shows the significant incremental improvement across all specifications.

Electronics

The GIF Quantum uses completely new electronics and enclosures. The dodecapoles use current drivers with 20 bit resolution and better than 1 bit noise and 1 ppm/°C stability. The thermal management (airflow and conduction) of the electronics was carefully simulated and verified experimentally to ensure the cooling necessary for optimal stability and reliability. The cables from the electronics rack to the GIF Quantum are 5 m long, allowing the electronics to be located in a separate equipment room

CCD camera

The GIF Quantum has the same 4 port, 2 k×2 k, full frame CCD as the GIF Tridiem, but uses completely redesigned camera electronics. The new camera has a 1 and a 10 MHz read-out rate (per output) and flexible digital correlated multiple sampling. The 1 MHz mode is used for the highest quality data acquisition; the 10 MHz mode is used for searching, focusing, and high-speed spectrum acquisition. The 10-times faster read-out rate has increased the frame rate to 8 frames per second at 2 k×2 k pixels, 15

BF/DF detector

The STEM imaging can be a very efficient process allowing nearly all the electrons passing through the sample to be acquired simultaneously through multiple acquisition signals. The synchronous collection of EELS and ADF data, for instance, offers accurate spatial correlation between spectral and image features.

To this end, we have designed an optimized annular dark-field (ADF) detector into the GIF Quantum that efficiently collects the electrons just outside the 5.0 mm spectroscopy aperture.

Software

The software supporting the GIF Quantum for both operation and data acquisition has been extensively revised. Automation plays a key part of the new software interface, both during filter alignment and during the optimization of parameters (e.g. CCD exposure) for data acquisition. Many of the key improvements have been made possible by the ultra-fast electrostatic shutter, since this provides highly linear exposure control with ample range to attenuate the full intensity range of the EELS

Further models

We have extended the GIF Quantum design to models optimized for different CCD cameras and for lower and higher voltage operations

  • The GIF Quantum SR Model 964 features a 4 k×4 k CCD camera with 14 μm pixels.

  • The Low Voltage GIF Quantum is designed for operation from 15 to 60 kV, and uses a gradient prism with a 50 mm bending radius. It uses the same camera as the regular GIF Quantum but the scintillator/fiber optic stack is optimized for low voltage imaging.

  • The High Voltage GIF Quantum is designed for

Conclusion

The GIF Quantum represents a significant advance over earlier generations of post-column filters. Completely new optics, camera, and software have resulted in significant improvements in terms of performance and user friendliness in both energy-filtered imaging and electron energy-loss spectroscopy. The flexibility introduced by the electrostatic shutter, combined with the additional information provided by the DualEELS mode, and the efficiency of the automated optimized data acquisition hold

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