Carbon Coating & Evaporation - Physical Considerations (2)

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2. Physical Considerations (Part 2)

Table of Contents:

1. Introduction
2. Physical Considerations
3. Physical Considerations (Part 2)
4. Turbo Pumped, High Vacuum Systems
5. Surface Treatment

The apparent point source of the filament also suggests another consideration in coating a flat substrate, which is related to the angle for differing heights. If the angle is large the variation of coating of a central area to that on an outer edge will be somewhat different. This is given in the table showing the percentage coating for an angle of c which can also be related to the height h to radius r, although again due to collision with the short path length it is not as sensitive as may be anticipated from the table. However it serves as a good basis for considering the height range requirement (usually variable) and specimen table diameter (usually fixed). It might be reasonable to consider a figure of 85% as the acceptable limit, with a ration of 0.4. If we use a specimen table with a diameter of 50mm (radius=25mm), then h would need to be some 65mm. In practice the system has a 50mm diameter specimen table with variable height facility centring of 65mm with a +/- 20mm. range, giving 45mm to 85mm. As has already been mentioned however, in practice the coatings tend to be much more evenly deposited over this range. Figure 3 gives examples of slides that have "blanked off" areas for visual effect, indicating varying grey tones from uncoated to a reasonably heavy coated slide. This is covering a range of thickness, and indicates the versatility of the technique. The accompanying table giving the range of parameters used. NOTE: Reproduction may not repeat readily the changes in grey tones apparent on the original slides.

Figure 3:

Although the thicknesses are estimated, what is usually significant is the high repeatability. The total heat Input of the system and the final Heat Input to the specimen are also low compared to conventional carbon rod evaporation.

This is usually demonstrated with freshly broken polystyrene, however a typical specimen would, of course, only be subject to a relatively small percentage of the total energy input. It can be considered that if we satisfy the Physical considerations discussed then we should be able to produce Carbon Films which are Fine Grain, very repeatable, with a flexible range in Carbon "Flash" Evaporation systems.

Micrograph of polystyrene coated with carbon

Figure 4: Scanning Electron Micrograph of expanded polystyrene coated with carbon. Magnification x 1500 this is a very sensitive sample that is free of heat damage.

Micrograph of polystyrene coated with gold

Figure 5: Similar specimen to 4, cool sputtered with Gold included as an undamaged standard.

Figure 6: Emitech K450X Carbon Coater, which is suitable for use with a range of Carbon Fibre Filaments now available. The K450X is a fully automatic unit outgassing the fibre and subsequently flash coating. Note: This unit is also available as the semi-automatic K400X.

Figure 7: Emitech K250 Carbon Coating Attachment, which is manually operated

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