Vacuum Web Coating

Pumping Problem

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      

 

Pumping oxygen

Question

How can I dilute oxygen when it is passed in vacuum chamber? If lose contact of boats is there a possibility of explosion when oxygen is passed. We are using silicon oil in diffusion pump and roughing pump which oil is used for oxygen gas passed.
Please guide me.

 

 

Answer

Let’s start with the composition of air which is approximately 80% Nitrogen and 20% oxygen and it is perfectly safe to pump air through diffusion and backing pumps using hydrocarbon oils.

 

Once the vacuum system is pumped down the predominant gas load is that of water vapour but because the system is under vacuum the quantity is very small.

 

It is common to feed in a mixture of oxygen and argon into the plasma treatment zone bit this too is typically no more than 10% oxygen.

 

Where things can become interesting is when the process requires the use of oxygen at a higher concentration than 20% and may also require processing at higher than normal metallizing deposition pressures. If you have the combination of very high concentration of oxygen and high operating pressure then you may have to protect the system from the diffusion pump oil being degraded or catching fire and this is typically achieved by changing the oil from a hydrocarbon oil to a Fomblin oil. This Fomblin oil is much higher cost oil than the hydrocarbon oils and so it is only used where there is no alternative.  A simple alternative can be to add another gas to dilute the oxygen back to being less than 20% of the total gas load. This additional gas could be dry nitrogen and could be added at the throat of the diffusion pump to limit the effect of the additional gas on the process.

 

The more exciting problem is the backing pump where the gas is further compressed and the oxygen passes over hot oily surfaces which can result in the ideal conditions for triggering an explosion.  There are three ways to deal with this problem. The first is to dilute the oxygen as mentioned above.  If the volume of gas is low and the diffusion pump does not need protecting the diluent gas could be injected directly into the backing line.  This is generally done as a fail-safe feature and hard wired into the controls so that to get the oxygen feed to work the nitrogen must already be flowing into the backing line.  The quantity of nitrogen is usually take to be sufficient such that if the oxygen line were switched to the maximum flow the proportion of oxygen would still be less than 20%.  Thus for lower flows of oxygen the proportion would be very much less than 20%. 

A second option is to change the oil for a Fomblin one and third option is to change the pump from an oil lubricated pump to a dry pump.  These dry pumps were developed for the semiconductor industry and are based on the use of wear resistant ceramics with fine tolerances allowing the pump to run without lubrication and so there is no hot oil to trigger any explosion.

 

So for your process check the base pressure as well as the operating pressure and the oxygen flow.  Use this along with your pump information to know what compression you have and hence what pressure the oxygen will reach through each of the pumps.  From this you can calculate the volume of nitrogen you need to introduce to bring the oxygen content back to that of air at 20%.   If the pressure of the process is too high that with the nitrogen included the flow of gas is too much for the pumps you may need to consider changing the pump system to a dry pump to eliminate the need for using nitrogen.

 

 

I hope this helps.

  

 

 

 

 

 

 

Problem with Rootes Blower

What could be the cause of a roots blower pressure to plunge intermittently?

Answer

There are some questions and checks that you can do.

Is the same pressure change seen by more than one pressure gauge?

If the pressure variation is seen only on one gauge then does the noise the Rootes pump makes change with the change in pressure indicating that the extra gas load is real or if you measure the current to the pump does this change with the change in pressure also indicating a real change in gas load.  If the sound or current does not change then it might mean that the pressure gauge is faulty rather than the pump.

Assuming it is not a faulty gauge the ask what has changed between the pump not having a problem and the problem appearing?

This could range from nothing at all to things such as the system being overhauled, or maintained where water or air joints have been undone and re-made, or the system has been greased/oiled as part of a routine maintenance.  If something has been done to the system then this could be the source of the problem. Whatever has been done may have disturbed something such as a seal is not longer sitting properly and so is leaking, etc

Check the simple things such as the oil level in the pump, has the correct oil been used, and check the pressure gauges are reading correctly. 

Having done all this then there are two basic possible problems.

The first is that there is a problem with the vacuum. This could be a water leak where the water is leaking at a low rate into the vacuum such that as it evaporates into the vacuum the heat required to evaporate is taken from the water and pipe and this is cooled to the point where the water freezes. The ice then has a lower vapour pressure and so the vacuum pressure falls. As the evaporation rate has fallen this takes less heat and so the thermal mass begins to heat the ice and it returns to water and so the higher rate evaporation returns and the pressure rises. The cycle then repeats and the water freezes and the pressure falls again.  This generally only occurs for relatively small leaks and the two pressures are characteristic of the vapour pressure of water and ice.

There could also be a small air leak and a pipe that is deforming so that sometimes the leak is closed and other times open.

The second possible type of problem is that there is a problem with the Rootes pump.  This could relate to gas loading. If the gas load is too high the rotors could be bending and touching the housing causing an increase in friction and so slowing down the rotation speed. Other problems could be a problem with the seals that is causing an intermittent leak.

I hope this gives you a few things to think about and try.

Pumping problem

I have 2 metallisers in my company.
We have bought the 2nd one recently from a company in USA. It was never used before, so it was practically brand new. We have been operating the first system for a while now. I am facing a typical problem. I am loosing a lot of diffusion pump oil from the second machine. On an average this can be as much as 1 gallon a week. We utilize one charge in a year in the first system. When I asked the company about it they say there are 2 reasons.         1. Poor foreline vacuum or        2. Leakage.

I have checked both and am not able to figure out where the problem is, because the readings are as good as normal. Could you please help me out with this?

Answer.

The poor foreline pressure would lead to backstreaming, particularly at low pressures.

The backing pump is not matched to the diffusion pump requirements and so the diffusion pump pressure rises and this causes some of the oil to backscatter and enter the main vacuum chamber.

What you should be looking for is evidence of oil in the main chamber.

Generally most vacuum systems will have a small amount of oil that can leave a thin smear of oil on the surface but if you are losing 1 gallon per week then I would expect that there would be much more pronounced evidence of oil further into the chamber and possibly even signs of oil drips on the bottom edges of some of the metal surface nearby.

If you have any baffle across the diffusion pump orifice I would expect it to be well covered in oil. If this baffle is also your cryo coil then I would inspect the water that you collect when the system is brought back to atmosphere and see if it has oil included in the water.

The problem of leaking would work the other way round. The gas load is so great on a permanent basis that the diffusion pump oil is being swept out and through the backing pumps.  This might be noticed if the backing pump oil levels have increased and possibly they are throwing a lot more oil out in the backing pump exhaust.  If you have a backing line with a low spot in the tubing where oil could collect, it may be worth checking to see if oil is accumulating there. However several gallons I would have expected would have become evident by now and so I would put this as a lower priority than other checks.

Both of these problems would suggest that the system base pressure is not as good as it could be. A poor foreline pressure would mean the diffusion pump would never reach its ultimate base pressure. A leak also would not allow it to reach its ultimate pressure. A quick check for a leak is to pump the system down and then seal it off and measure the pressure rise with time. If the pressure rise continues with a reasonably constant rate of rise in pressure with time then the system has a leak, with the higher the slope the greater the size of leak. If the pressure rise is initially quick but then becomes slower and levels off then the system does not have a major leak and you need to be looking to a problem with the backing pressure.

If you purchased the system that was aimed for a different location it is worth a check that it is not a different voltage problem that is making the backing pumps not perform as well as expected.  Sometimes the change in voltage can mean they are operating below full capacity and many system designers minimise the cost of the machine by using the minimum pumping they can get away with and so a small drop in performance can radically affect the pumping performance.

Other checks on the backing pumps are the standard ones, check the oil level is correct and that the pumps have not collected too much water and need gas ballasting. If the oil level and condition is OK then the same pressure check can be done with the backing line. Pump out the backing lines and then seal them off and check the pressure rise. It will be the same as for the main chamber, if the pressure continues to rise you have a leak but if there is an initial pressure rise and then the rise slows and stops then there is not leak.

Also check the backing pumps to see if the oil is one of the recommended ones for the pump. Sometimes oils can be sold cheaply because they do not have the same operating performance. 

I think that this should give you a few things to try.

Will this improve the pumpdown speed?

Question

We have a metalliser that has two deposition zones. The volume is very large (15.53m 3). We are not use currently the second deposition source so we think to put up an empty airtight box in the empty space. Is there any benefit for the vacuum or is there any disadvantage in the sense of degassing of the metal surface?

Answer.

Essentially what you propose is possible and by reducing the volume you will reduce the pumpdown time.

There are a couple of points to bear in mind when making this type of change.  The box MUST be leak tight otherwise you will have a virtual leak permanently within the vacuum system and this will both reduce the pumpdown speed and the base pressure.  Depending where the volume is positioned will depend on if it collects any stray deposition. If it does it needs to be cleaned like any other surface to keep the surface area down to a minimum, otherwise you will be absorbing more water than before and this too could reduce the pumpdown speed, reducing the benefit. The increase in surface area of just the box surface is small compared to the surface area that is produced by any stray coating and so generally would not significantly change the amount of outgassing that would occur during the pumpdown. It is only if it collects stray deposition and it is difficult to clean and so gets left with the coated material on that it would affect the pumping.  Finally check the position with respect to any conductance gaps and the positioning of the pumps. You do not want the box to block any pumping orifice. If the box reduces the size of the pump throat then this would act as a constriction reducing the pumping speed of the orifice and slowing down the pumping of the vessel.

Assuming the box is leaktight, kept clean and suitably positioned then you can expect a pumping speed improvement.

Rootes pumps and Booster pumps

What is the difference between the roots pump and the booster pump?

ANSWER

Booster pumps are usually a similar type of pump to a diffusion pump in that some oil is vaporised and directed down a reducing volume to help knock the gas down the tapered tube to compress the gas and so increasing the pressure to make it easier to pump away by the backing pumps.

The rootes blower to some extent does the same job but it is usually classed as a backing pump and is used to improve the crossover as the backing pumps (often rotary vane pumps) run out of pumping speed and before the diffusion pump becomes effective. Often the rootes pumps are place in-line with the roughing line. The rootes pump is often left to freewheel without being driven when the chamber is initially roughed down. Once the pressure falls sufficiently the rootes pump starts to be driven where the rotating lobes can cause a reducing volume forcing the pressure up and making the rotary pumps more effective.

Hence using either can make the operation of the total system more efficient.  Modern vacuum systems appear to favour the rootes plus backing pump combination over the combined diffusion pump plus booster pump combination.  I must confess I have never checked on why, it could be that the cost benefits or speed is better with the rootes pump than with the booster. 

Question re. condensing gases onto cryocold surfaces

Question.

At the moment we are using a PolyCold and cryogenic panel to remove retained solvents on 12μ polyester in our metallizer.

This works ok on a web of 700mm, but will not work on a web of 1400. Would it be better to use liquid nitrogen as a cooling medium instead. (not sure as to what temp. solvent condenses at).

*************************************************************

Answer.

When you talk about polyester film and using a cryogenic panel to remove retained solvents I am curious about your substrate material.  Usually polyester does not have much if anything by way of retained solvents. However it does contain moisture and depending upon the manufacturing process and then the subsequent winding and storage conditions this can be as much as 2% by weight of moisture.  Most Polycold cryopanels are used specifically to pump water vapour and nothing else.

The way any cryopanel works is to lower the temperature of the panel to the point where any atoms hitting the surface have a high sticking coefficient. The temperature governs what atoms will stick and which will not. The Polycold type cryopanels have the temperature reduced enough such that they have a very high sticking coefficient for water molecules but little else.  Gases such as oxygen or hydrogen are generally only captured if the water vapour traps the gas as it condenses and solidifies into ice.

The performance of the cryopanel depends upon the capacity of the system. As the gases condense and solidify an ice layer builds up. Ice is not as good a thermal conductor as the copper tube or panel surface. The ice may not form as a dense layer as the density depends on the rate of arrival of the gas and if the rate is very high the layer is likely to be porous. A porous ice layer is an even worse thermal conductor. The arriving gas is at a higher temperature than the cryopanel and so the cryo panel will warm up. The amount it warms is dependent upon the gas load and the capacity of the cryopump to keep up with the gas load.  The capacity of the cryopump has to be sized to pump the last amount of water vapour from a roll every bit as well as the first few meters otherwise there will be a progressive pressure rise through the process as the pumping performance declines and the ice layer builds up and the porosity gets progressively worse.

Thus on your system if the cryopanel has been sized to cope with the gas load from a 700mm wide roll of polyester and it then has in effect double the load applied then it is likely that the cryopanel temperature will rise to a higher level during the process and the ice layer formed is also likely to be less dense because of more gas arriving which means the pumping performance of the cryopanel is likely to decline.  If the cryopumping system has the capacity it may be that simply doubling the surface area of the cryopanel will cure the problem.  If the cryopanel surface is already at a maximum it may be that you need a second cryopump.

Worried by backstreaming?

Backstreaming is a common concern and yet many operators still insist that vacuum systems need to be pumped to a very low base pressure before starting the deposition process.  These two facts seem to be contrary, as we all know that backstreaming becomes increases with reducing pressure & time spent at the low pressure.

Therefore it seems only sensible not to pump down to a low base pressure.

However if I suggest this method the immediate response is what about the water vapour absorbed on the walls?  The belief is that pumping down to a low base pressure helps remove the water from the walls. 

            This is not true; the ability to pump to a low pressure is an indication that the water has been removed. The lower pressure does not actually remove the water.

There are several ways of speeding up the removal of the water. One is to use heat and this is commonly used on ultra-high vacuum systems with trace heating elements wrapped around all the external pipe-work. The second method is to apply energy by some other means such as ultra-violet lamps that excite the water & assist in breaking the bond to the surfaces.

            The final method, that has been little used, but works is to only pump to a set pressure which can either be where the roughing pumps are still working or to a point just below the stalling point of the diffusion pump.  At the elevated pressure there is increased bombardment of all surfaces because of the higher gas pressure.  The greater number of gas molecules means that there are more molecules bouncing off the walls & the kinetic energy is sufficient to break the bonds holding the water to the surfaces & hence it speeds up the release of the water.

            Thus feeding the system with a small leak of dry nitrogen for a set time is better at reducing the water than trying to pump to a lower pressure. If there is dry nitrogen in the system it may be possible to strike a plasma which will further increase the energy & so will further increase the water evolution rate.

            The nitrogen, or other inert gas, needs to be dry otherwise the process is counterproductive.

            By this method it is possible to reduce the pumpdown time slightly as well as never having to pump to the ultimate base pressure and risk backstreaming.  In turn this will them improve the system productivity & the film quality.

  If in doubt try it out   -    & let me know how you get on.

Common Pumping Problems.

Water has to be the biggest cause of pumping problems.  The source of the water can vary from, a wetter than normal substrate, more ambient moisture absorbed when the machine is at atmospheric pressure or even a water leak directly into the system.

Other problems can relate to a lack of routine maintenance or too infrequent maintenance.  This can be everything from not cleaning the deposition shields frequently enough to not checking the oil levels in the pumps or ballasting the roughing pumps to remove any excess water vapour.

Below I will highlight each of these problems and some of the methods for identifying the possible source of the problem.