Environmental Scanning Electron Microscope

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The Environmental Scanning Electron Microscope (ESEM) is a special variant of the scanning electron microscope . The main difference to a conventional scanning electron microscope (SEM or SEM) is the lower vacuum (higher pressure) in the sample chamber and the specially adapted detector.

Image examples

history

In continuation of the previous works by Lane, Robinson, Spivak and Shah, the principle of ESEM was developed by the Greek Gerasimos Danilatos (approx. 1988-1990), this was adopted in 1989 by the ElectroScan Corporation in the USA in a commercial scanning electron microscope. Later, with the acquisition of ElectroScan, the patents were transferred to Philips (now FEI Company ), which offers this principle as an additional option in their conventional scanning electron microscopes. Other manufacturers also have devices with the option of a weaker vacuum in their product range, but due to patent protection they usually name them VPSEM ( variable pressure scanning electron microscope ), i.e. a SEM with the ability to vary the pressure in the sample chamber. The pressure range and the detector used by these companies differ from the following description due to the patent.

Working principle

Schematic structure

As with a conventional scanning electron microscope, the specimen is scanned by a focused electron beam and the signal generated when it interacts with the specimen is used to generate the image . However, the sample chamber is not under a high vacuum, but a gas with a gas pressure of typically 130 to 1300 Pa is located around the sample  . Suitable gases include water vapor, nitrogen or air.

When the electron beam hits the sample, there are various interactions in the sample surface. The formation of low-energy secondary electrons (0 to 50 eV), which leave the sample surface as relatively slow electrons, is important for imaging in ESEM mode.

The gas in the sample chamber itself is used to amplify the signal in the ESEM. The secondary electrons are accelerated towards the detector by an applied voltage of a few hundred volts between the sample and the detector. On the way to the detector there are collisions between the electrons and the gas atoms. The atoms are ionized and new electrons are created (amplification cascade). The image resulting from this signal corresponds mainly to a topographic contrast.

Due to their positive charge, the ionized gas atoms are accelerated in the opposite direction in the direction of the sample, where they neutralize the charges that could arise in samples with non-conductive surfaces. The “detector” is neither light nor temperature sensitive.

The pressure difference between the high vacuum area with cathode (electron beam generation) and the sample chamber with poor vacuum is realized by a series of fine apertures in the beam path and by a differential pump system.

Advantages and disadvantages compared to conventional SEM

The advantages of the technology over conventional scanning electron microscopy include:

  • Samples that are not vacuum-stable or outgassing can be examined with increased residual gas pressure in the sample chamber. Changes due to evacuation are reduced. Application: e.g. B. Examination of biological samples without prior fixation or exchange series, some mites even survive such conditions and move under the electron beam.
  • If water vapor is used specifically as the gas, the relative humidity in the vicinity of the sample can be regulated between 0 and 100% by varying the pressure and temperature in the sample chamber. This makes it possible to study drying or wetting processes. Application: e.g. B. Characterization of hardening processes in the cement industry, drying of paints, swelling of superabsorbents, analysis of the lotus blossom effect, ...
  • The charge compensation (neutralization by the residual gas in the sample chamber) enables non-conductive samples to be examined directly. A previous vapor deposition or metallization of the sample is not necessary. Application: e.g. B. Analyzes in criminology, where evidence may not be changed, investigation of dynamic experiments in the electron microscope (in situ), which leads to a change in the sample geometry.
  • Since the detector is neither light nor heat sensitive, the ESEM is also suitable for analyzing the temperature behavior of a sample in heating table experiments with a temperature range of up to 1000 ° C and beyond. Application: Observation of melting processes or chemical reactions at high temperatures.

On the other hand, there are also some disadvantages compared to working in a high vacuum:

  • In the ESEM mode, very small magnifications (<50x) can only be achieved with difficulty or not at all.
  • Liquids are opaque and can obscure the actual surface.
  • Due to the lower screen speed, the image build-up takes longer than in the conventional operating mode.
  • An X-ray microanalysis ( EDX ) is considerably more complex and requires subsequent corrections.

literature

  • WC Lane: The environmental control stage. Scanning Electron Microsc. 1970, p. 43-48 .
  • VNE Robinson: A wet stage modification to a scanning electron microscope . In: 8th Int. Congr. El. Microsc., Australian Academy of Science . tape II , 1974, p. 50-51 .
  • J. Shah, A. Beckett: A preliminary evaluation of moist environment, ambient temperature scanning electron microscopy . In: Micron (1969) . tape 10 , 1979, p. 13 , doi : 10.1016 / 0047-7206 (79) 90015-3 .
  • GV Spivak, EI Rau, NM Karelin, IE Mishustina: Scanning electron microscopy of moist, live, and frozen objects. In: Izv. Akad. Nauk SSSR, Ser. Fiz. 41, 11, 1977, pp. 2238-2251 (Russian)
  • DG Danilatos: Foundations of environmental scanning electron microscopy . In: Peter W. Hawkes (Ed.): Advances in Electronics and Electron Physics . tape 71 . Academic Press, 1988, ISBN 0-12-014671-1 , pp. 109-250 .
  • GD Danilatos: Theory of the gaseous detector device in the environmental scanning electron microscopy . In: Peter W. Hawkes (Ed.): Advances in Electronics and Electron Physics . tape 78 . Academic Press, 1990, ISBN 0-12-014678-9 , pp. 1-102 .
  • DG Danilatos: Bibliography of environmental scanning electron microscopy . In: Microscopy Research and Technique . tape 25 , 1993, pp. 529 , doi : 10.1002 / jemt.1070250526 ( danilatos.com ).

See also