Carbon films are important in both SEM and TEM preparation.
For SEM applications carbon coating is used to make specimen surfaces more conductive for EDS, WDS and EBSD analyses.
For TEM the primary application is for making structure-free carbon support films on grids, as well as replicas of finely structured specimen surfaces. For this application reproducible thickness combined with high quality films are more critical.
Whatever the application, carbon films need to be of consistent and of reproducible quality and thickness.
Control of carbon thickness
Carbon evaporation can be controlled in several ways:
- Timing the duration of evaporation at a set current has been a popular method for carbon coating. However, precise control is difficult especially as sources of carbon vary between batches, often without notice from suppliers. This variability ultimately leads to differences in coating thickness and consistency.
- Film thickness monitors give an indication of the thickness that has been achieved. However, they have proved difficult to use with carbon, which is a light element, to actively and accurately control film thickness, especially with a fixed evaporation current.
Safematic CCU-010 – control algorithm and controlled pulse evaporation
The new carbon coating algorithm on the CCU-010 EM coater systems provides much greater control of carbon deposition. The new enhanced process uses the on-board quartz film thickness measurement system in combination with pulsed evaporation to track the increase in carbon film thickness and then adjust the evaporation current as it nears the user-defined thickness, typically required to be in the region of 3-15nm for EM applications. The new control algorithm and film thickness measurement system are fitted as standard to the CT-010 carbon head which can be used on both the high vacuum and standard vacuum versions of the CCU-010.
Pulsed evaporation of carbon fibre thread also overcomes many of the issues with carbon rods associated with heating. Far less heat is generated by pulsed evaporation from carbon fibre threads compared to rod evaporation,eliminating heat induced artefacts. Pulsed evaporation in combination with film thickness measurement in between pulses not only reduces heat generation but allows the film thickness monitor to update the controller and adjust current output accordingly for subsequent pulses, thus improving the accuracy of the final film layer thickness.
The new algorithm is also available to existing users as a firmware upgrade.
For high quality TEM support films we recommend the Safematic CCU-010HV carbon coater
For routine SEM carbon coating we recommend the Safematic CCU-010LV carbon coater