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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 braided carbon thread determine thickness, not really variable
  • Flash evaporation without control
  • Carbon fibers 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 or braids. Often limited by the power supply and leads to breaking vacuum and reloading source for multiple layers.
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.

go to Carbon Rods for Evaporation
go to Carbon Fiber for Evaporation

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

PELCO easiShaper Carbon Rod Shaper - SharpenerPELCO easiShaper™
Carbon Rod Shaper for stepped carbon rods