Coatings for SEM

There are no resolvable variations among different metal coatings sputtered to thicknesses of 20 nm or more, when examined in microscopes 

A study of various coating devices was conducted several years ago to identify the best conductive coating for high resolution SEI taken 

Clay and Peace [275] compared evaporative, sputtering and ion beam sputter coatings and showed that IBS produces a fine grained, uniform film of gold. In a comprehensive review of recent advances in metal coatings Echlin [276] 

In summary, compromises must be made in the choice of conductive coating techniques, depending on resolution. Issues to consider are capital cost, coating deposition time, ease of use and the resolution required for observation. 

It is appropriate to discuss the potential imaging effects , or artifacts, in specimen preparation. For 

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The fracture surface morphology was examined using a scanning electron microscope (Oxford-XL30 PhUips) for specimens coated with gold. The specimens were cleaned with water using a Bran Sonic 52 and dried in an oven at 30 °C for 30 minutes before coating for SEM. 

Walther P, Hermann R, Wehrli E and Muller M 1995 Double layer coating for high resolution low temperature SEM J. Microsc. 179 229-37... 

Fig. 26. SEM micrographs illustratiag the effects of airborne basic chemical contamination, (a) This image was formed when a positive-tone CA resist was processed without any delay after coating, (b) This image was formed when an identical film was stored after coating for 15 minutes in an atmosphere containing 10 parts per billion on A/-methylpyrrohdone, and then processed identically to the first film.

Fig. 4.17 SEM image of HT (high-temperature) M0S2 nanowire arrays on the adhesive surface at two magnifications. The surface was coated with 10 nm of gold for SEM imaging. (Reprinted with permission from [171], Copyright 2009, American Chemical Society)...

This technique can be applied to samples prepared for study by scanning electron microscopy (SEM). When subject to impact by electrons, atoms emit characteristic X-ray line spectra, which are almost completely independent of the physical or chemical state of the specimen (Reed, 1973). To analyse samples, they are prepared as required for SEM, that is they are mounted on an appropriate holder, sputter coated to provide an electrically conductive surface, generally using gold, and then examined under high vacuum. The electron beam is focussed to impinge upon a selected spot on the surface of the specimen and the resulting X-ray spectrum is analysed. 

Because SEM-AIA is often used to explain behavior under specific processing conditions, samples are prepared in the same size in which they are received. Coal samples with their included mineral matter are prepared for image analysis by mixing samples. of the dry coal with polyethylene powder (as a diluent) and molten carnauba wax in a volume ratio of 1 2 2. Pellets are then cut along the cylindrical axis to expose a vertical cross section of coal and mineral matter and polished using standard petrographic procedures. The surfaces are coated with 150 A of carbon to provide a conductive surface for SEM examination. 

Samples suitable for SEM measurements include most solids which are stable under vacuum (metals, ceramics, polymers, minerals). Samples must be less than 2 cm in diameter. Non-conducting samples are usually coated with a thin layer of carbon or gold in order to prevent electrostatic charging. 

Swab and Deldrin samples for SEM/EDX examination were subjected to a concentration/cleanup procedure, the apparatus for which is given in reference 185. All samples were carbon-coated using a Biorad E6430 automatic vacuum controller before examination in the SEM. 

A further test involved repeatedly treating a sample with water prior to carbon coating for manual SEM/EDX examination, with a duplicate, untreated sample acting as a control. When examined, both samples had a high concentration of particles encompassing the complete range of particle types. The sample treated with water did not show any noticeable difference. 

The brominated sticks are embedded in Spurr epoxy resin (Spurr 1969) and sectioned with a diamond knife or glass knife mounted on an ultramicrotome to give cross sections of 0.15//m thickness for TEM-EDXA or 0.5//m thickness for SEM-EDXA. The sections are then placed on a carbon-coated collodion film on the specimen support grid. The specimens are carbon-coated again to avoid charging. 

The loaded specimens were sectioned at two perpendicular directions as shown in Figure 4. The specimens were then moimted in cold mount, polished with 12 grades of sandpaper and gold-coated to be prepared for SEM photography. 

The morphology was determined using a Quanta-200 3D scanning electron microscope (SEM) operated at an accelerating voltage of 1200 V. The sample (0.5 mg) was moimted onto a 5 x 5 mm silicon wafer affixed via graphite tape to an aluminum stub. The powder was then sputter-coated for 105 s at a beam current of 20 mA/dm with a 100-A layer of gold/palladium alloy. 

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