Vacuum Web Coating

AIMCAL Converting School 2010 Schedule

February 16 - 18Nip Roller Workshop
Rochester, NY

February 25 - 26Drives in Web Handling and Converting
Schiller Park (Chicago), IL

March 16 - 17Web Handling and Converting
Atlanta, GA

March 18 - 19Winding: Machines, Mechanics and Measurements
Atlanta, GA

April 13 - 14Solution Preparation and Mixing
Philadelphia, PA

April 13 - 14Web Handling and Converting
Brussels, Belgium

April 15 - 16Web Coating and Drying
Philadelphia, PA

April 15 - 16Winding: Machines, Mechanics and Measurements
Brussels, Belgium

April 20 - 22Web Wrinkling and Spreading Workshop
Rochester, NY

May 4 - 5Solution Preparation and Mixing (Europe)
Brussels, Belgium

May 6 - 7Web Coating and Drying (Europe)
Brussels, Belgium

June 8 - 9Optimizing Extrusion-Based Lamination and Coating Technology
Rochester, NY

June 15 - 17Center Winding Workshop
Rochester, NY

August 17 - 19Roller Performance Workshop
Rochester, NY

October 5 - 6Web Handling and Converting
Philadelphia, PA

October 5 - 6Web Coating and Drying (Europe)
Brussels, Belgium

October 7 - 8Winding: Machines, Mechanics and Measurements
Philadelphia, PA

October 7 - 8Drives in Web Handling and Converting (Europe)
Brussels, Belgium

November 9 - 10Solution Preparation and Mixing
Cleveland, OH

November 11 - 12Web Coating and Drying
Cleveland, OH

Visit www.AIMCAL.org or www.ConvertingSchool.com for more information.

Cooling water quality - process effects.

Below is an article posted on the PVD Forum by Eric Kurman.  It primarily deals with the problem of conductivity of water for use in magnetron sputtering sources.

 

However many plasma cleaning units are based on the same magnetically enhanced design as the sputtering sources and so the same factors apply.  

 

It is also worth noting that over time ‘dirty’ water can fur up pipes, fittings and deposition drums which can adversely affect the cooling performance. Often this furring process will take many months or even years and so the changing performance may not be noticed easily. Also it may be assumed that a slowly degrading performance is just the machine aging.  So periodically during major maintenance it is worth checking that pipes are remaining clean and that the flow is not being restricted.  

 

 

 

‘Dirty’ water cooling of magnetron sputtering sources.

 

There is a little more to the story on water.

First of all, "dirty" is really a misleading term, the water does not have dirt in it, rather it has dissolved ions leading to an increase in conductivity. This is common in Process Cooling Water loops or so-called "house" water and can come from two sources.

a) Intentionally, from treatments placed in the water to inhibit corrosion.
b) From corrosion due to wetted surfaces in contact with the water.

Most engineered PCW systems exercise very close control on which surfaces are allowed to be "wetted" in contact with cooling water. However, most thin film systems in my experience have a profusion of wetted materials including various polymers, brass of different types, copper, zinc, tin, chromium, stainless steels of different types, and occasionally target materials from directly cooled targets.

Another problem is that grounding is often sloppy in thin film systems and the machine frame is used as a common for power line ground and return current from a deposition component like a sputter cathode or electron beam hearth.

As others have pointed out, the conductivity of the water needs to be measured. However it might not be as simple as "replacing the dirty water". The DI water can corrode certain materials particularly copper and brasses. Once this happens the water is no longer "DI", and once the conductivity of the water increases, there is the potential for electrochemical corrosion due to dissimilar materials being wetted and voltage applied to components in the loop.

Even in the case of the heat exchanger technique (we used to call them "mini-loops" ) mentioned in previous message, often the water does not stay "DI" for very long. I have looked into the reservoirs of many a mini-loop to see stuff that looks like it belongs in a mud puddle.

The only way to continuously maintain low conductivity is to include a polishing bed in the recirculating loop....kind of expensive but, if you are running production, often worth it.

Knowing the conductivity of the water one can estimate the power drop due to conduction through the water circuit. Electrical power flows from the high voltage component to the nearest ground or return component. You will need to know the internal cross sectional area of the tube or pipe and its length.

Let's assume you are cooling with typical drinking water that has a conductivity of 0.01 S m^-1 and that you have a magnetron cathode running at 500V, connected to ground with two 1/2" (12.7 mm) plastic tubes that are each 1 m in length (two tubes, inlet and outlet, 1 meter long each). Our tube has a wall thickness of 1/16" and an internal diameter of 3/8", thus its cross sectional area is 0.71 cm^2. The series resistance of this water-filled tube is resistivity times length divided by area, namely (10000 Ohm cm)*(100 cm)/(0.71 cm^2) = 1.41 megOhms (1.41E6 Ohm).

The two tubes are in parallel so the total resistance is 700 kOhm.

Note that we had to convert from conductivity, measured in Siemens (facilities and water people always deal in conductivity) to its reciprocal, resistivity, and to also convert from meters to centimeters. Thus 0.01 S m^-1 = 10000 ohm cm.

Your sputter cathode will conduct 500V / 700E3 Ohm = 712 uA to ground. Total power dissipation through the water is about 356 mW. This might be a lot, if you are running a small R&D cathode at a few Watts, or it might be negligible, if you have a bigger system running at 30, 50, 90, etc., kW.

Note that power loss through the water depends only on the voltage drop and will be the same for a small system as a large system for the same water conductivity and tube sizes.

If we repeat the same calculation using seawater (4.8 S m^-1) as the coolant, resistance is 20 kOhm per tube, 10 kOhm total, power drop would be about 24 W for a load at 500V. Water is just not very conductive, even seawater!

So you have two main choices: change the water, or use a longer tube. The latter is a surprisingly effective way to go. It is not that big a deal to coil up 50 or 100 feet of poly-flow tube, tie-wrap it in place, and fix the problem.

If you are running an EB setup at 10 kV or 20 kV or higher, or maybe an X-ray tube at 50 kV to 100 kV, you have a bigger problem. Low conductivity coolant and a long path length is a requirement here. Oil coolants can be used but they have lower heat capacity than water and a big headache as well.

So....do the measurements. ...do the math....analyze and determine a solution.

 

"Eric Kurman" <ekurman@yahoo.com>

AIMCAL FREE WEBINAR SCHEDULE

January 12, 2010, 11:00 AM -12:00 PM Eastern
Coating Solution Preparation Technology
Ken McCarthy, Clean Room Coating Solutions
Edward Cohen, Ed Cohen Consulting

 

January 19, 2010, 1:00 PM - 2:00 PM Eastern
Your Legal Responsibility for Legacy Machinery

Mike DeRosier, AH&T Insurance

 

February 10, 2010, 11:00 AM - 12:00 PM Eastern
Contract Coating For Development or Manufacturing

Larry Gogolin
Edward Cohen, Ed Cohen Consulting

February 16, 2010, 11:00 AM - 12:00 PM Eastern
Resistance Heated Evaporation of Aluminum - The Basics
Dr. Charles Bishop

 

February 18, 2010, 11:00 AM - 12:00 PM Eastern
Reducing the Carbon Footprint: Making Sustainable Flexible Packaging
Giancarlo Caimmi, Nancy Smith, Rick Gloeckler, Rich Castillo and Stewart Richards

 

March 2, 2010, 2:00 PM - 3:00 PM Eastern
On-line Coating Weight Measurement
Hector Marchand

 

March 16, 2010, 11:00 AM - 12:00 PM Eastern
Winding Webs in Vacuum - including Heat Load
Dr. Charles Bishop

 

April 14, 2010, 11:00 AM - 12:00 PM Eastern
Vacuum Deposit Barrier Coatings
Dr. Charles Bishop

 

May 12, 2010, 11:00 AM - 12:00 PM Eastern
Troubleshooting - Process/Adhesion/Vacuum
Dr. Charles Bishop

 

June 16, 2010, 11:00 AM - 12:00 PM Eastern
Reactive Deposition by Magnetron Sputtering - The Basic Process and System Design Considerations

Dr. Charles Bishop

 

To register for any of these webinars go to http://www.convertingschool.com

and go to the box with the AIMCAL logo and pick the webinar you wish to register for and click on the link. 

 

 

 

 

 

Adhesion – the No. 1 problem

Looking back over the questions asked over the years it appears that adhesion is the problem that causes the most difficulty.  This is either directly as poor adhesion when tested by the simple tape test or as evidenced by problems experienced downstream by customers using the coated films.

 

It does not really matter where the problems are experienced the process for trying to sort out the problem saves the same.  Firstly compare the process from when it worked well to now it does not work and look for the differences. This needs a holistic view of the process starting with the incoming film and things like the weather. In some countries the weather can change hugely both in temperature and humidity from season to season. Treatments such as corona can be affected by changes in humidity and so a change in season may require a re-optimisation of the corona treatment process.  If the film supplier has changed the polymer may contain different additives and the surface chemistry may be different and hence any pre-treatments may no longer be optimised.  Within the vacuum deposition process there can be differences between when the system is freshly cleaned and when the system is dirty and is due to be cleaned.  If the season has changed the cooling water may be at a different ambient temperature and so the cooling of various system parts will be different and the process can differ.   If the pump down performance has changed or the base pressure is different (higher) this can be indicative of a dirty system or a system leak.

 

This comparison is very important as it helps to identify the true source of the problem. Without doing this comparison there is a danger of trying to modify the vacuum deposition process to improve the adhesion when the problem is elsewhere and continues to exist. However this much wider range of possible sources of the adhesion problem means that it is essential to me methodical in comparing all aspects of the process from start to finish.

Asking questions -

Hi, I get many questions, some are posted on the blog directly, some are sent via the AIMCAL 'Ask AIMCAL' service and some are sent to me directly.  I answer all questions that I receive, hopefully quickly but sometimes, if I am travelling or if the answer takes some research, the answer may be delayed.  Most of the answers will find their way onto the blog at some stage but with names removed to protect identities.

As you might imagine I get a large amount of spam e-mail which I trawl through the titles because there are occasional items that are not spam but legitimate e-mails.  A number of these legitimate e-mails contain questions to me relating to vacuum coating.  The reason many of these e-mail end in the spam folder is because there is no topic in the subject area.  Often the subject just says 'hello' os something similar which is picked by the spam filter as a potential spam e-mail.  I do wonder how many questions have gone unanswered because there has been no subject identifier.  For those who may think I have deliberately ingnored your e-mail I apologise but if you have not included a topic in the subject area it may just have been filtered out.

So if any of you are considering sending me an e-mail can I suggest you have at least one keyword that relates to vacuum coating as the e-mail subject so that it will arrive in my e-mail in-box rather the spam folder. 

Thanks.

CAB

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      

 

Pinholes, defects and barrier performance.

I have been reviewing the topic of barrier coatings and some of the finer details of why metallized, silica or alumina vacuum deposited coatings do not deliver the barrier performance expected.   Metal foil is used as an example of a perfect barrier and it has always been a complaint of metallized polymer films that they are not a complete replacement for foils because the barrier performance is not as good.  

 

Many years ago now there was a research programme to look at why the metallized films contained pinholes and this was proved to be due to debris on the polymer surface that was moved after metallization leaving behind a small uncoated area of film.  This uncoated film has no barrier improvement at all and it has been shown that these defects are the primary reason why any vacuum deposited coating does not produce a perfect coating and very high barrier performance.

 

What does not appear to be mentioned so frequently is how devastating pinholes can be to the barrier performance.  It only takes 5% of the metallized area to be missing in the form of pinholes and the barrier performance will be almost the same as for uncoated polymer film.

What is even more worrying is just how few and how small the pinholes need to be for them to reduce the barrier performance by at least an order of magnitude.

 

This means that even if we try to clean up the polymer films before vacuum coating it is likely that we will improve the coatings to some extent but not as much as if we could eliminate the pinholes altogether.  It seems unfair that those vacuum coaters who pay particular attention to keeping the vacuum system clean and substrate quality high may still end up with barrier coated films only slightly better than those who pay little attention to the hygiene issues.

 

In order to produce a surface with no contamination that can deliver a coating without any pinholes is hard to achieve.  If the polymer surface is cleaned by whatever means there will still be debris on the surface of a size below 1 micron or possibly as low as below 0.3 microns. Removing debris below this level is very difficult and so usually the surface is cleaned down to this level and then other measures are used to finish the cleaning process such as coating a polymer onto the substrate to cover up all the remaining particles.  This coating needs to be greater than the largest remaining particle in order to deliver a perfect surface for the barrier coating to be deposited onto.  If this polymer coating is deposited outside the vacuum system the coated surface needs to be protected from re-contamination. This includes from the atmosphere as well as from the back surface of the polymer substrate once the film is re-wound into the roll.  If the polymer is deposited in the vacuum system it too needs protecting from contamination as there can be debris contaminating rolls from previous cleaning activities.  Not only does the surface need protecting from re-contamination but it needs to be suitably prepared for deposition by plasma treating as the surface energy needs to be as high as possible to maximise wetting.  If the depositing species does not wet the polymer surface well the growing coating will be rougher and contain more holes and defects than if the depositing coating wets the surface well. 

 

The barrier performance is dominated by the pinholes bit once these are eliminated the defects in the growing coating become the next most critical target.  Thus wetting and producing a continuous coating at the thinnest possible coating thickness becomes the next most critical target. 

 

In the past the density of pinholes has been used as a measure of improvement but this is really a false target. The real target should be zero pinholes. Achieving zero pinholes will deliver the big change in barrier performance.  Anything other than zero pinholes will only achieve a more marginal barrier performance improvement.  

AIMCAL Converting School programme for 2010

AIMCAL Converting School 2010 Schedule
 New courses for 2010 in Red!

February 16 - 18	Nip Roller Workshop Rochester, NY
February 25 - 26	Drives in Web Handling and Converting Schiller Park (Chicago), IL
March 16 - 17	Web Handling and Converting Atlanta, GA
March 18 - 19	Winding: Machines, Mechanics and Measurements Atlanta, GA
April 13 - 14	Solution Preparation and Mixing Philadelphia, PA
April 13 - 14	Web Handling and Converting Brussels, Belgium
April 15 - 16	Web Coating and Drying Philadelphia, PA
April 15 - 16	Winding: Machines, Mechanics and Measurements Brussels, Belgium
April 20 - 22	Web Wrinkling and Spreading Workshop Rochester, NY
May 4 - 5	Solution Preparation and Mixing (Europe) Brussels, Belgium
May 6 - 7	Web Coating and Drying (Europe) Brussels, Belgium
June 8 - 9	Optimizing Extrusion-Based Lamination and Coating Technology Rochester, NY
June 15 - 17	Center Winding Workshop Rochester, NY
August 17 - 19	Roller Performance Workshop Rochester, NY
October 5 - 6	Web Handling and Converting Philadelphia, PA
October 5 - 6	Web Coating and Drying (Europe) Brussels, Belgium
October 7 - 8	Winding: Machines, Mechanics and Measurements Philadelphia, PA
October 7 - 8	Drives in Web Handling and Converting (Europe) Brussels, Belgium
November 9 - 10	Solution Preparation and Mixing Cleveland, OH
November 11 - 12	Web Coating and Drying Cleveland, OH

Visit www.AIMCAL.org or www.ConvertingSchool.com for more information.

The importance of base and operating pressure.

Many people use base pressure as a key process paramater.  When the vacuum system is pumped out it the air is removed and what is left is primarily water vapour that is outgassed from all the different surfaces along with any trapped air and water vapour from the incoming roll.  The base pressure then becomes a measure of how clean the system is being kept.  As the stray deposition builds up it can absorb more water vaour which is slow to desorb and so slows down the pump down time but also limits the base pressure that can be achieved within a limited time.

 

When the deposition sources are heated up the radiant heat will speed up the desorbtion of the water vapour and so the pressure will rise to give the deposition pressure.  So if the system is clean you will not only have a low base pressure but also a low deposition pressure. Conversly if the system is dirty and the stray deposition thickness is allowed to build up the base pressure will be higher and the process pressure will be higher.

 

A higher deposition pressure will mean there will be more collisions between the depositing vapour and the residual gas and this will lead to a more porous coating and this in turn will make the barrier performance worse.  The more porous coating will also mean the thickness will have to be slightly thicker to achieve the same optical density.  Also the coating contamination rate will be worse as the ratio of water vapour to metal arriving at the substrate will be higher as so there will be a slightly greater proportion of aluminium oxide than for a clean system.

 

 

In theory you do not have to achieve a very low base pressure to have a clean process because it is possible to use a different pumping philosophy and achieve the same results.  If, during the roughing of the system, some dry air or other dry gas is introduced into the system the increased gas bombardment can help to desorb the water vapour more quickly than if the system were simply pumped down without any gas introduction.  Introducing the gas slows down the time to the crossover point where the pumping would be switched from roughing to the diffusion pumps and at this point the introduction of the dry gas would be stopped. If the system were allowed to be pumped down to the ultimate base pressure it would be achieved more quickly than if the dry gas were not introduced.  However the aim is to not pump down to the ultimate base pressure as this could increase the risk of backstreaming oil from the pumps.  Usually in this regime the system is pumped down to a low but not ultimate pressure and once this pressure is achieved the process is started.

 

This different pumping operation will still show a pressure rise from the increased outgassing caused by the radiant heating from the sources and so the process pressure will still be affected in exactly the same way. 

 

No matter how the system is pumped there is no substitute for keeping the system as clean as possible in order to produce higher quality coatings.    

Process stability

In reviewing some questions recently it became apparent that a key factor that keeps causing problems is a lack of stability or consistency in the process.  My definition of the process in this case covers everything from the incoming materials onwards.

    If the substrate is variable then it makes sense that the final product will be variable. Similarly throughout the whole process the target must be to try to make everything consistent and reproducible as much as possible.

An example of where improving the consistency before the vacuum deposition process might improve the product might be;  Some substrates may be pre-treated by corona or flame to increase the surface energy and the lifetime of the treatment may be a variable depending on storage time and temperature after treatment.  This can mean that the surface energy of the substrate can be as high as just after the treatment or back to the original level of the substrate before treatment.  Ideally you want consistency with the surface energy the same every time. It is better to have the consistency even if the surface energy is not at the optimum value rather than have some rolls high and others low.

Within the vacuum deposition process too you need to be looking for the same stability.  You want the wire feed to be as free and easy on every coil of wire and guide tube so that the friction is equal on every wire so that the feed will be consistent.  A consistent wire feed helps towards a consistent molten pool which minimises spitting and keeps the coating quality high.

Once the major variables have been taken care of it is then these minor variables that take attention to detail that become the ones that are important to take care of.