All systems contain water but this will be a variable depending on the system, the operating philosophy as well as the local atmospheric conditions. When a system is pumped out the volume of gas is removed which primarily leaves whatever is outgassing from the surfaces as the remaining gas load which in most systems will be water vapour. Thus internal surface area becomes important as the greater the surface area the greater the amount of moisture that can be absorbed onto the surface. This then leads us on to the operating philosophy or more specifically how frequently and well the system is cleaned. As any stray deposition material coats the surfaces it massively increases the surface area and so the amount of water that can be absorbed. Thus dirty systems take longer to pump out than clean systems.
Water is just well enough bonded to the surface that it does not desorb very quickly bit takes time. The speed with which the water is desorbed can be increased if energy is used. This can take many forms. Many high vacuum systems simply use heaters either as heater tape wrapped around pipework or radiant heaters. With vacuum roll coaters this would be too expensive and slow and so energy is usually supplied inside the system. A quartz lamp will give out ultraviolet light which has enough energy that can be absorbed on the surfaces helping to release the water. Similarly striking up a plasma can achieve the same result both from the energetic bombardment of the surface but also because a significant output of any plasma is in the ultraviolet.
An additional water load can be introduced with the roll of material. There are two parts to this water load. The air entrained between the layers of wrapped substrate will contain air of which the water content is usually between 0.6% and 6% depending on the humidity prevalent during winding of the roll. The second component of the water content of the roll is whatever is absorbed within the bulk of the substrate. This is a more difficult amount of water to remove as it takes time to diffuse the water from the bulk of the material to the surface which can then be removed. The substrate material will determine the amount of water that is contained which for some is almost nothing but for others can be more than 1%. As the water at the surface is not removed in vacuum until the surface is exposed there is no driving force for the water to diffuse out of the bulk until the roll is unwound. The time the surface is exposed for is usually too short and so very little water form the bulk will diffuse to the surface and be removed. This means that as the roll is wound back up there is usually still plenty of water left in the bulk to diffuse to the surface whilst the material is in the rewound roll. Thus if it is to be unwound again the pressure rise due to the release of water from the surface will be almost identical to the pressure rise the first time the roll is unwound. Thus for processes that are thought to be sensitive to water it may be that the substrate needs to be heated under vacuum and wound through very slowly in order to remove as much water as possible before the deposition process.
Also for any process that is sensitive to water vapour the results of the deposition can vary from run to run as there are multiple variables. The cleanliness of the system and hence the surface area and amount of absorbed moisture, the humidity of the atmosphere will vary from season to season and on humid days it will be easier to absorb more water onto surfaces than on dry days and finally the water content of the roll both in terms of the humidity when the roll was wound as well as the water contained din the bulk of the material. Thus unless all of these are considered and ideally controlled it will be hard to run a reproducible process.
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