Environmental Scanning Electron Microscope

The Environmental scanning electron microscope ( ESEM ) provides a special variant of the scanning electron microscope dar. The main difference from a conventional scanning electron microscope (SEM, or engl. SEM ) is the lower vacuum (higher pressure) in the sample chamber and the specially adapted detector.

History

In continuation of previous works of Lane, Robinson, Spivak and Shah, the principle of ESEM by the Greek Gerasimos Danilatos (ca. 1988-1990) was developed, this was taken over in 1989 by the company Electro Scan Corporation in the United States in a commercial scanning electron microscope. Later, the patents were transferred to the corporate takeover of Electro Scan to the company Philips (now FEI Company), which offers this principle as an additional option in their conventional scanning electron microscopes. Other manufacturers have also devices with a weaker vacuum in the program option, rename it, however, because of patent protection usually with VPSEM ( engl. variable pressure scanning electron microscope ), that is, to vary a SEM with the possibility of the pressure in the sample chamber. The pressure range, and the detector used by such companies differ because of the patent, but by the following description.

Principle of operation

Just as in a conventional scanning electron microscope, the sample is scanned by a focused electron beam and the signal produced during the interaction with the sample used for image formation. However, the sample chamber is not here under high vacuum, but around the sample there is a gas with a gas pressure of typically 130-1300 Pa. Suitable gases are, among others, water vapor, nitrogen or air.

Does the electron beam on the sample, so there is in the sample surface various interactions. Important for imaging in ESEM mode is the emergence of low-energy secondary electrons ( 0-50 eV), which leave the sample surface as a relatively slow electrons again.

For signal amplification in the ESEM, the gas is used in the sample chamber itself. By an applied voltage of several hundred volts between the sample detector and the secondary electrons are accelerated towards the detector. On the way to the detector it comes to collisions between electrons and gas atoms. The atoms are ionized and in this case it creates new electrons (amplification cascade). The resulting image from this signal corresponds mainly to a topographical contrast.

The ionized gas atoms are due to their positive charge, opposite in direction sample accelerated and ensure there for a neutralization of charges, which could result in samples with non-conductive surfaces. The " detector " is neither light nor temperature sensitive.

The pressure difference between the high- vacuum region with the cathode ( the electron gun ) and the sample chamber with a poor vacuum is realized by a number of fine apertures in the beam path and by means of a differential pumping system.

Advantages and disadvantages over conventional SEM

Which may be mentioned, among others, as advantages of technology over conventional scanning electron microscopy:

• Stable or non-vacuum outgassing the sample can be analyzed in the sample chamber at increased residual gas pressure. Changes caused by the evacuation be reduced. Application: eg investigation of biological samples without prior fixation or exchange rows, some mites survive even such conditions and move under the electron beam.
• If one uses specially water vapor as a gas, it can be controlled by varying the pressure and temperature in the sample chamber, the relative humidity in the vicinity of the sample between 0 and 100%. This makes it possible to study drying or wetting processes. Application: eg characterization of curing processes in the cement industry, drying of paint, swelling of superabsorbent polymers, analysis of the lotus blossom effect, ...
• Due to the charge compensation ( neutralization by the residual gas in the sample chamber ) non-conductive samples can be examined directly. A previous evaporation or metallization of the sample is eliminated. Application: eg analysis in forensics, where evidence may not be changed, Investigation of Dynamic experiments in the electron microscope ( in situ ), where there is a change in the specimen geometry.
• Since the detector is neither light nor heat sensitive, the ESEM is also suitable for the analysis of the temperature behavior of a sample in Heiztischexperimenten with a temperature range up to 1000 ° C and beyond. Application: observation of melting processes or chemical reactions at high temperatures.

Contrast, however, can be seen also some disadvantages compared with the work in a high vacuum:

• In the ESEM mode, very low magnifications (< 50-fold ) can only poorly or not implemented.
• Liquids are opaque and may obscure the actual surface.
• The frame lasts through the lower scanning speed longer than in the conventional mode.
• An X-ray microanalysis ( EDX) is considerably more complex and requires retroactive corrections.
• ...