Sample Prep Equipment
Before a sample can be observed with the SEM it is often necessary to subject the sample to additional processing steps to make the sample conductive, stable in a vacuum, stable under the electron beam, or to optimize the geometry between the sample surface, the electron beam and one (or more) of the detectors attached to the SEM.
An excellent resource on SEM sample preparation is Handbook of Sample Preparation for Scanning Electron Microscopy and X-Ray Microanalysis by Patrick Echlin (2009). The text is available for the UMD community through the UMD Library website.
Standard sizes of available sample holders are 12, 32, 57 and 76 mm in diameter. Samples should be sized to fit any of these holders and should be less than 50 mm tall. A vice holder is available for large irregularly shaped samples. A holder for standard petrographic thin-section samples is also available. Samples to be observed by CL, EBSD, have much more restrictive sample size requirements and users should consult with the laboratory manager prior to sample preparation for these techniques.
Denton Vacuum DeskIV Conductive Coating
Applying a conductive coating to samples is the most common type of sample preparation step for SEM observation. By applying a thin conductive layer (typically carbon or gold) the sample is made electrically conductive. If electrons accumulate on the sample surface during observation with the SEM, significant artifacts would obscure its surface. Therefore, it is commonly necessary to provide a pathway by way of an ultrathin coating for electrons to travel away from the location of the incident electron beam.
Buehler Vibromet Vibratory Polisher
The Buehler Vibromet2 is used exclusively for the final polishing steps required for EBSD analysis. EBSD is very sensitive to near-surface lattice defects and likewise any crystallographic damage induced by sample preparation. To obtain high quality EBSD data the sample needs to be polished in such a way that this near-surface damage is minimized. This is accomplished by using extremely fine polishing compounds (0.05 Ám colloidal silica) on a vibratory polisher. Prior to using the vibratory polisher, samples should be polished to at least 1 µm.
Samples containing significant amounts of water (e.g., biological materials) are unstable in even moderate vacuum environments and are even less stable under the electron beam. This requires that samples either be dehydrated or frozen. Dehydration is done using a method called critical point drying. The water is replaced by a solvent which is in turn replaced under high pressure by liquid carbon dioxide that is quickly evaporated using the Tousimus Autosamdri-810 critical point dryer. This has been the traditional method for preparation of biological materials to be observed with scanning electron microscopy, and remains a viable technique. However, with the advent of variable pressure SEM, flash freezing samples in liquid nitrogen and then observing the samples directly is an acceptable preparation alternative in most cases.