Q: How Clean Is Clean In Vacuum Coating, Or How Long Is A Piece Of String? Part 2 Of 3

By Dr. Charles A. Bishop  
A: It is common for any film-substrate specification to include a phrase about the substrate being clean that can be quite general and without any definition of what is meant by clean. 
Of great importance is the decision as to what technique to use to determine the cleanliness of the surface. The choice of technique is partly determined by what is the definition of cleanliness being used. Is it only particulate contamination, or does it also include chemical contamination? Another key feature that influences the choice of technique is what is the minimum size of contamination that is of interest? The human eye, for someone with good eyesight, can resolve particles down to ~50 microns when the person is not tired and the material is under good illumination. As a proportion of the population will not have good eyesight, or may be tired if particles, or defects below 50 microns are of interest, using some magnification is necessary.
Optical microscopy = a low-cost option
For samples cut from a roll, optical microscopy is a relatively low-cost option that, coupled with image analysis, can speed up the counting and sizing of the particles or defects. This can be enhanced by using a tacky roller that can be rolled for many revolutions over a larger substrate area, and then the collected particles can be transferred to a high-tack pad, and this can be viewed under an optical microscope. This method of sampling a larger substrate area can give a more representative idea of the surface cleanliness. 
Where more visual details are required, or the particle size is below 1 micron, scanning electron microscopy (SEM) may be used but, as the substrates are non-conducting, they typically need to be coated with a conducting metal. It can be argued that if the substrate is to be metallized then instead of sampling the substrate before coating it could be done after metallization. The drawback to this would be that if the cleanliness was not good enough then the cost of metallizing would have been wasted. 
In-line monitoring for defect maps
Modern metallizers can include in-line monitoring of the film surface, usually after metallization, where the whole film width is illuminated, and detectors thus produce a map of the defects present on the whole coated surface. While this is an improvement over not monitoring the film at all, or only monitoring one or two points across the width, it is still limited in benefit as the smallest defects identified are in the range of 50-100 microns, depending on the system used. Applications requiring very high-barrier performance would be difficult to achieve if there were any defects of this size present in the coated film. 
It is possible to improve the resolution of the system to enable smaller particles to be detected, but this usually increases the cost of the measurement system. There are systems that use laser scanning that can measure smaller defects, but these take up more space and, to scan the whole surface at the metallizer, winding speeds need multiple lasers, increasing the cost significantly. Although it is technically possible to improve the resolution by at least two orders of magnitude, it is not done as it is deemed too expensive.