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Evaporation with Carbon Rods versus Carbon Fibers

Carbon coating is widely used in electron microscopy and microprobe analysis for:

support films for TEM (high vacuum (HV) evaporation)
conductive films for SEM/EDS samples (low vacuum evaporation)
conductive surface films for non-conductive microprobe samples (HV evaporation)
conductive surface films for non-conductive EBSD samples (HV evaporation)
conductive surface films for non-conductive FESEM samples (HV evaporation)
conductive surface films for non-conductive FIB samples (HV evaporation)

The most commonly used methods are carbon evaporating using a pre-shaped carbon rod or from a woven thread of carbon fiber in a vacuum system.

Features of carbon rod evaporation using a Bradley type pre-shaped rod fed source

High purity carbon rods with low surface area remain pure
Thickness can be easily controlled between 1 to 50nm from a single rod
Control over power and evaporation time determines thickness
Clean carbon layer
More powerful source (up to 185A) is needed
Multiple evaporation possible with one set of rods
Pulse mode without breaking vacuum can be used to built up a thicker carbon layer to reduce thermal load

Features of carbon evaporation using flash evaporation of woven carbon fiber thread

High surface area of the carbon fibers has more adsorbed contaminants
Diameter of carbon thread determine thickness, not really variable
Flash evaporation without control
Carbon fiber tend to fall on surface causing imaging artifacts
Smaller size power supply, approx. 50A
Single evaporation process, source needs reloading after each evaporation
Thicker layers can be built up by using multiple threads. Often limited by the power supply and leads to breaking vacuum and reloading source

Considering the above features, it is clear that using pre-shaped rods with the correct controls system (pulse, continuous and auto-cycle modes) delivers not only the better quality carbon layer but also give much better control over the thickness of the carbon layer.

Note: The quality of the carbon coating depends on the vacuum system used when evaporating the carbon. In a turbo pumped high vacuum system (typical 10^-5 mbar range or better) the carbon is deposited a virtually amorphous structure not resolvable by SEM or FESEM imaging. This type of coater needs to be used for FESEM, Microprobe, EBSD and TEM applications. For standard SEM/EDS applications a much simpler system can used, utilizing a rotary vane pumping system delivering a vacuum level of 10^-3 mbar. At these vacuum levels the carbon layer has a structure, which can be revealed at magnifications of approx 25,000x.

Possible carbon pointed rod shapes and their setups. The top illustration, with neck rods, is usually preferred.

Carbon rod shaper for stepped carbon rods.