General
In the single-beam FIB, such as our Hitachi FB-2000A FIB, some milling will occur during observation of the specimen. Surface features, such as thin films, can be milled away during this process. In these cases, some form of protective coating must be applied prior to FIB processing. The following is a guide to selecting the best form of protection for particular types of specimens.
Surface Features
If the specimen feature of interest is relatively close to or on the surface (~< 1 µm) of the bulk specimen, steps to protect the target need to be taken due to the slight milling that occurs prior to W-deposition. A layer of 0.5–1 µm thickness coating of evaporated carbon is commonly used as a protective layer. The thick carbon layer allows the user to clearly separate the surface layer from the carbon layer in the TEM. As a rule, the steps to take are, in sequence: carbon evaporation, then FIB W-deposition.
Features Below the Surface
If the specimen feature of interest is ~1 µm or more below the surface of the bulk sample, it may not be necessary for the user to perform carbon coating prior to FIB milling. Either way, it is still advisable to deposit a W layer.
Multi-Layered Specimens
A ~0.5–1 µm thick layer of carbon, followed by W deposition, may be necessary to protect multi-layered specimens without a sacrificial layer on the specimen surface.
Particles in a Matrix
Locating particles of interest can be troublesome using the single-beam FIB. If the particles are visible in the optical microscope, the area can be marked with a "Sharpie" pen. The pen marks can be seen in the FIB image. Otherwise the particles will have to be located in the FIB using the M0–50 beam. Some form of a protective layer may be required as described in the preceding paragraphs.
Powders
An easy procedure for preparing large powders (> 10 µm) for TEM is to paint a conductive epoxy onto the edge of the special TEM grid designed for FIB work. Make the conductive adhesive by mixing Duco cement with carbon particles from rod sharpening. Dip the grid into the powder while the adhesive is still soft. After drying, a particle protruding from the epoxy is usually found that can be cross-sectioned. Lift-out is not required.
Semiconductors
When preparing site-specific cross-sections, such as a contact region of a memory chip, the user may mill a box on both the right and left sides of the area of interest to assist in navigating, extracting, and/or aligning the beams to a specific feature. Some form of protective layer may be required as described in preceding paragraphs.
Biological Specimens
For biological specimens, the FIB performs best when specimens are embedded in plastic. Normal preparation includes aledehyde-fixation to preserve the material, staining with osmium tetroxide to increase conductivity, and plastic embedding to protect the specimen. Subsequently, the specimen is ready for sectioning by the FIB system. Target areas in the specimen near the surface may need protective coatings. After sectioning, the user is able to view the interior of microstructures in the TEM.