Question.
Good morning,we have a running metalliser in which we are facing a problem with a long time, in our system vacuum is coming fast in 8.0 X 10-4 torr with in 10 mintues . we can not to start the film directly to for first heating.if we start the metallising in first heating the is fall. so precautionary we start metallising after second or 3rd heating.Is threr is any system in new plants so metallising start in first heating. please suggest for the problem how we can remove this?
Answer
If I understand your problem correctly when you pump the system down and then heat up the boats the first time you do this the pressure rises to high for you to start metallizing and so you cool the boats and pump back down again and repeat this so that on the third time you are able to start metallizing.
If this is what you are seeing then what is happening is that there is a lot of water vapour present in the chamber walls and shield surfaces. When the heat hits these surfaces that added energy speeds up desorption of the water and this raises the pressure. When you switch off the heat the speed of desorption decreases and the pressure decreases again but when the heat is reapplied the speed of desorption again increases and the pressure rises. This process will continue until you reach a point where the desorption rate is matched, or exceeded, by the pumping speed.
The total water load is not just from the chamber walls and the shields but is also from the CPP as it unwinds. The aim is to try to minimise the water load from both sources and if it cannot be reduced then to add more pumping to match the high load.
Keeping the chamber and shields as clean as possible and removing as much of the stray deposition as possible on a regular basis is important. This is not just cleaning the close deposition shields but also making sure all other surface are not accumulating a porous coating which will absorb a large amount of moisture. Remember a single drop of water will expand to 1 million times the volume when under vacuum at the pressures you want to work at and it is easy for a porous coated surface to absorb a drop of water.
If the humidity of the area where the rolls are stored is high it is worth having an area where the rolls can be stored at a controlled lower humidity for a few days before use. This will reduce the amount of moisture trapped within the rolls.
If you are already doing these things to reduce the moisture content of the rolls and chamber surfaces then the next thing to look at is how you can pump water faster within the system. The use of cryopanels is extensively used to do this. The most common system is made by Polycold and this name is used as a generic term for these cryopanels even where they are manufactured by their competitors. The cryopanel is usually in the vacuum system as a serpentine copper pipe that has a cold gas pumped through that cools the surface of the pipe down to appoint where it has a large capacity for condensing and freezing water onto the surface. This process of condensing and freezing the water takes the water out of the atmosphere inside the vacuum and hence is a type of capture pump. Where the diffusion pumps may be 30,000 lit/sec type of pumping speed a small Cryopump may well be 200,000 lit/sec for water. The cryopanels do not pump anything other than water but are very quick for water. Where the system suffers from a high water load more than one of these Polycolds may be required in the system. In large metallizers that coat paper the paper can contain up to 20% by weight of moisture and so several Polycolds may be used to pump this large amount of water. Often these serpentine pumping coils will be placed around the side wall of a chamber. If you consider adding one to your system then it is worth considering where best to put the coils. If your winding zone is separated from your deposition zone by a low conductance gap and the pressure in each zone is different it is worth having some of the cryopanel pumping in each zone this gives a better performance than if all the pumping is only in one zone or the other.
Another possible thing to try is to try to speed up the outgassing of the water from surfaces during the initial pumpdown time. This can be done by using ultraviolet (UV) light. Installing a UV light inside the vacuum system and switching it on during initial pumpdown will speed up the water desorption as the UV light is absorbed by the water and the additional energy is converted and results in faster desorption. An Alternative might be to strike up a plasma within the vacuum system as any glow discharge contains more than 50% of UV light and so will do the same as using a UV light.
I hope I have understood the problem correctly and that my thoughts have given you some ideas on how to improve the problem.
Best regards
CAB


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