Vacuum Web Coating

Brief report on AIMCAL Fall Technical Conference – part 2

Continuing with papers that caught my interest at this years AIMCAL Fall Conference held earlier this month.  Don McClure who is a consultant to 3M since he retired from 3m presented a paper on the development of a roll-to-roll process for making thin film transistors.  This process requires a mixture of processes including patterning, fine detail registration, encapsulation and scribing. This paper also included details of the high quality substrate and the effects on yield of contamination and film damage and benefits of cleaning.

There were a group of papers that talked about different aspects of in-vacuum patterning.  There were two papers from Leybold Optics, one detailing the use of a new modification of the existing oil masking process and the second explaining their measurement system that can look at free margins as well as small metallized areas.  The modified oil masking process uses oil pumped into an evaporation chamber where it is flash evaporated through nozzles onto the web. By this method they keep the oil level down to a minimum and improve the edge quality.  The paper that captured most interest was given by Nick Copeland from Bobst GVE who presented their in-register in-vacuum pattern metallization process. This is where they can keep the in-vacuum pattern precisely located with a pattern printed or embossed onto the incoming film. This requires keeping the pattern in-register both in the machine direction and the transverse direction. The paper included details of the measurement system that was used as the control signal to enable adjustments to be made. This first system also highlighted some of the difficulties and where GVE will make future improvements.

Of the numerous papers on barrier coatings the most interesting was from Senthil Ramadas from Singapore who described a novel approach of using nano-sized fillers to fill in any of the cracks or holes in the vacuum deposited coatings. These fillers really are of a nano size and within the holes, pores or cracks the filler either blocks the holes or provides a tortuous path improving barrier by slowing down the permeation. This is the same process as is used by the polymer layers that are used between the inorganic layers in the ultra barrier layers but improves on this by incorporating inorganic filler that has a higher barrier performance.  

DuPont Teijin Films gave details of the improvements made to their PET and PEN films to meet the needs of the electronics industry.  This included the smoothing of the surface by planarisation, minimisation of oligomers, cleaning of the surface as well as the thermal stabilisation of the film to aid dimensional stability during multilayer processing where in-register precision is important.

I hope these couple of summaries prove to be of interest. For those who wish to have full details of the Conference contact AIMCAL at www.aimcal.org where the proceedings can be purchased.

Brief report on the AIMCAL Fall Technical Conference – Part 1

I have just returned from my travels which included attending the AIMCAL Fall Technical Conference in the USA.  There was as always a good selection of papers on a whole range of topics some of which I will share some brief comments below.

Barrier coatings. There were several papers on barrier coatings of which Nicolas Schiller of FEP was the runner up in the John Matteucci prize for best paper for his contributions which described the FEP process for adding a plasma to evaporated aluminium to produce a good barrier performance aluminium oxide thin coating.  Although the paper was interesting, detailed and proved the benefits of the process it did become apparent that the cost may still be a limiting factor.  The process uses a series of cold cathode plasma sources across the web width. What was slightly surprising was the power used by each of these cold cathodes which to get the best coating density was 400A per source.  Thus the increased capital cost, of adding these sources, along with the increased running costs, do not make this necessarily a cheap process. I would estimate the increase in costs to be something around one third, or more, higher. This would make it uncompetitive against the material produced by the more simply modified metallizers that are currently producing alumina barrier coatings. 

Cleaning films. There were a couple of different papers that took my interest. One was a good paper that highlighted that over-treatment of webs was easily achieved for both corona and atmospheric plasma treatment although the better distributed atmospheric plasma showed a more uniform treatment and less variability.  It also demonstrated that using dyne pens could never tell the whole story of how much surface treatment had been done.

The second cleaning paper was one from Teknek that described their latest developments in trying to improve their tacky roll performance.  Tacky rolls will remove debris in general but there at the small end of the debris it can be more difficult to remove the debris and, of course, this is the end that is of most interest to the electronics industry.  What they now do is to texture the surface of the tacky roll so that the small debris more closely fits the surface depressions and so has a larger area surface contact and so can more effectively lifted off the surface. They also explained that the interim solution to cleaning webs in vacuum has been to use one tacky roll on the process before the roll enters the vacuum system. In this way there is less debris to be removed by the tacky roll inside the vacuum and so it less likely that the accumulation roll becomes clogged up.

There was a theme of sustainability or improved energy use throughout the conference. This included the vacuum deposition of aluminium and transparent silica coatings onto PLA biodegradable films. Other papers included reduced energy use by more effective cooling processes, improved maintenance of ovens to minimise heat losses, as well as some recommending down gauging, minimising the number of layers as well as looking towards the future and the increasing need to make packaging solutions capable of recycling, down-cycling or benefiting of some other energy recovery process.

   

A problem of poor adhesion

Sir we are having a PET film product that is having one side copolymer extruded layer & other side corona treated surface, we are metallizing this film on corona treated side but we are facing a problem of metal transfer after metallizing, after handling & after slitting ,in all three stages in first stage the metal transfer pattern is because of TD buckles because of loose winding & in second stage metal transfer because of pressing roll on foam sheet or finger marks & after slitting very fine unmetallized spots after & before winding. So over all metal transfer is there. In metallizer we have taken so many precautions like reduction of heat load by decreasing chill roll temp. to minimum ,increasing heat transfer rate by increasing gas inject flow, evaporators heating reduced to min., a special type of evaporators used to reduce heat load, Rollers cleaning rollers alignment to maintain constant traction.

Could you suggest where we are wrong & what else we can do to reduce metal transfer, Is there is some thing that we can do in base film to improve situation, however we have also conducted a trial in which this PET film is one side copolymer extruded layer & other side chemically coated & metallization done on chemical coated side to improve metal bond strength that can help to reduce metal transfer but results are same.

ANSWER

As with all problems of poor adhesion the first action is always to try to find out the precise plane of failure.  It is always assumed that the failure on adhesion is at the interface but it may not be it may be that the failure is cohesive and in the top layers of the polymer film. It will be difficult to do this as the polymer is transparent and if the failure is in the polymer the small amount left on the metal coating will be difficult to see.  Using a microscope to see if there is residual polymer or testing to see if the surface energy of the back of the metal is the same as the polymer surface may help give clues to where the failure is. Freshly deposited metal has a higher surface energy than the polymer and so if there are differences in energy it is likely that the failure is at the interface.

If the failure is at the interface this then is a problem of adhesion and this is dependent on the cleanliness and quality of the polymer surface and the type and amount of pre-treatment that is done to the surface before metallization.  Polymers can have proprietary treatments which may include coatings as well as include additives that help in the polymerisation process or modify the handling of the film. These treatments or additives can be present on the surface and may not help the adhesion.  Pre-treatments are often used to counteract or improve on any of these earlier surface modifications. Any residual low molecular weight material will be a cause of low adhesion and so either needs to be removed or needs to be crosslinked into the rest of the polymer surface to provide a better anchor to the deposited coating.  The pre-treatments such as flame, corona, atmospheric plasma or in vacuum plasma are all used not only to help remove or stabilise the low molecular weight material but also to change the chemistry of the polymer surface to help increase the amount of direct bonding of the depositing aluminium to the polymer.  The plasma bombardment will break some polymer chains and the oxygen in the plasma will replace some of the existing atoms and the aluminium will bond well to the oxygen.  If the surface energy of the polymer is measured before the plasma treatment and after the plasma treatment the surface energy will hopefully have been increased by something like 10 dynes above the untreated surface.

If the pre-treatments done outside the vacuum system such as a corona treatment it will be effective for some time after treatment but with time the surface energy will degrade back to the original level and the surface will need to be treated again. If the pre-treatment is corona there may also be some variability to the treatment depending on the humidity. If the corona treater is set at a fixed level it will be better at some humidity levels than others. The conductivity of air is dependent on the moisture level in the atmosphere and so the corona will reflect this and there will be a different effect if the power is fixed. Ideally the power will be adjusted to suit the conditions but this is not always the case.  Humidity also affects the surface of the polymer as on high humidity days the polymer will have more moisture on the surface than on dry days and this moisture has to be removed before any plasma can get to the surface to start the chemistry.  Thus if the surface treatment has been optimised on a dry day and then you have a high humidity day you may find the treatment less effective.

Pre-treatments will increase the surface energy but need to be optimised as it is possible to over treat the surface and although the surface energy is high the adhesion can have fallen off and become poor sometimes worse than if no treatment were used at all.  The plasma can be used to cause chain scission that is breaking the polymer chains to allow for direct bonding of the coating to the polymer. This is good in small amounts but if too much is done the amount of broken bonds means that the polymer chains are reduced to very small bits and this then becomes a weak boundary layer and will reduce the adhesion. The amount of oxygen at the surface will still be increased which is why the surface energy will still measure high but the strength of the bond will have fallen. This is why the process needs to be optimised and not simply assume that the highest surface energy will automatically give high adhesion.

As the plasma treatment starts the surface energy will rise and so too the adhesion. With an increase in power or time the surface energy will continue to rise and so too will the adhesion. At some level the surface energy will stop rising and will start to plateau out and at this point the adhesion will have reached a maximum and any additional treatment will result in a fall in the adhesion whilst the surface energy still is high and on the plateau.

So for your particular problem I would look at the failure and if it is an adhesive failure, which it does sound to be, I would then look back at the history of the film and any surface treatment that has been done and make sure that if there has been surface treatment that it has been optimised particularly making sure there is not over treatment.  If there is no pre-treatment then it would be worth considering using some pre-treatment.

I hope this helps.

Aluminium Oxide transparent barrier coatings

Aluminium has long been the main vacuum coated barrier material but it does, of course, have the property of being opaque. So for those wanting a transparent barrier coating they have always looked to other materials of which the most popular has been some version of silica.  Most time this is a sub-stoichiometric version of the silica and in the case fo the plasma enhanced chemical vapour deposition (PECVD) process it also contains an amount of carbon, anywhere up to 20%, or more, depending on process conditions.

The cost of these alternative materials has been high with the lowest still being of the order 2x to 3x the cost of aluminium metallizing.  This is because of two significant costs, the capital cost of either a PECVD or an electron beam deposition system is a much higher cost than a resistance heated boat type metallizer and the deposition speed is often less than a half that of aluminium metallizing.

The one company that appeared to have solved this problem was Camvac who managed to produce an aluminium oxide coating from their modified standard metallizer.  It would appear that by them carefully introducing the oxygen at the right point and in the right quantity they could control the oxidation fo the coating but also not damage their resistance heated boats. 

Needless to say this technology has been patented and so for many years this has been the only low cost process that I have known to be operating.  However in recent times I have heard that there have been two others who have managed to develop a similar process. One I know has been checked out to make sure it was not infringing any patents and that they were clear to run production the other I do not know if it has been checked out but they are prepared to talk about the process and will be giving a paper on the process at the 2009 SVC conference in Santa Clara.  This process was developed by the Fraunhofer Institute in Dresden (FEP) in conjunction with a polypropylene converter and the Applied Materials built a machine using the technology for use in production.  The FEP specialise in using additional plasmas during deposition to activate any reactive gases and to also promote surface reactions and densification of the growing coating.  Thus I expect that this process uses that expertise to evaporate the aluminium quickly but then use the plasma to activate the oxygen to speed up the conversion of the metal to the oxide.

As aluminium oxide is less dense than the metal this conversion from metal to oxide can also help improve the barrier as it will add a compressive component to the coating, as it swells, that will close pores down and make diffusion through grain boundaries slightly harder.  If this technology is now available through one of the system builders then this could mean that the cost of obtaining transparent barriers could be due for a fall in the next couple of years as these coatings start to become available.

Air-to-air vacuum coating systems.

With the surge of interest in vacuum coating machines for the deposition of photovoltaic materials there has been the development of evaporation sources.  This has included those evaporation sources that have a very high material efficiency.  This type of source can offer opportunities to change the design of the vacuum system.  The source no longer needs to be brought to atmosphere after the deposition of each roll.  It is only the unwind and rewind polymer rolls that need to be changed.  This leads to the option of using load-lock chambers for the unwind and rewind rolls which would allow the main chamber to be held under vacuum during the roll change over.

There is a trade-off between the system cost of a more complex winding system and the load-lock design and the possible reduction in some of the pumping capacity as the main volume does not need to be pumped out as frequently and so it is usually allowed slightly longer pumping time thus requiring a slightly smaller pump set. 

A second design option is to go to an air-to-air winding system. This still uses a load lock for roll changes but there is no need to vent and pump any vessel around the unwind and rewind rolls.  However the pumping requirement is much higher as to get the film into and out of the vacuum system there is, in effect, a continuous air leak that has to be pumped.  The pressure is reduced as the film passes through a series of chambers. Each chamber is up to two orders of magnitude different to the adjacent chambers.  Thus not only the pumping system is more substantial but also the winding system has considerably more rolls included. The air-to-air systems, although it has attractions, may also have problems such as increased contamination as the air that enters the vacuum system continuously will contain airborne particles that can become pressed into the film as the film passes through the nip roll that restricts the quantity of the air passes into the system.  A second detraction that has been an irritation to a number of operators in the past is that of noise.  The velocity of the air is high and this can cause a continuous loud noise requiring ear defenders .

Both of these system options also allow the deposition sources to be kept under vacuum and hot during roll changes.  This can reduce the down time between deposition runs as there is no need to cool the source and then re-heat of which the cooling generally is a rate limiting part of the process.  There is usually a small reduction in temperature in order to slow down the evaporation rate whilst maintaining the bulk of the heat. This smaller temperature variation also helps in reducing any source temperature variations and can thus be of benefit in improving deposition uniformity.

As more of these different systems are built more experience will be gained and the designs refined with the best features retained.  The economics of the different capital cost and running costs can be evaluated and this may lead to changes in the design of metallizers. So do not be surprised if these is a new batch of air-to-air metallizers produced in the near future.

Lifetime of metallized film.

Hi sir, follow up question on the lifetime of metallized but this time on the adhesion of the metallized coating on the cpp or pet film. Some customer complains that after a year their un-used laminated opp/vmcpp film delaminate. We observed that the metallized coating completely transfers to the opp. Did the bond between the cpp and the metallized was overcome by the bond of adhesive between opp and metallized coating. Did surface contamination plays part on this after a long period of storage.

Answer

As with all adhesion failures the recommendation is to first confirm the plane of the failure. The reason for this is that you do not want to be trying to solve the wrong problem.  Assuming the failure really is at the interface it is possible that this relates to the initial adhesion and then is time and temperature dependent.  Ideally the metal is bonded to the polymer at all possible points uniformly across the whole surface.  If the surface is not suitably prepared the metal will only be intermittently bonded to the polymer. In the spaces between bonds it is possible for material to migrate into the space and it is this material that can degrade the adhesion.  This migrating material could be moisture or low molecular weight unpolymerised fragments or any additives that might have been added to the polymer.

The rule of thumb is generally that the higher the original adhesion the longer the lifetime of the metal adhesion. The lower the adhesion the easier it is for material to migrate into the space between the unbonded metal and polymer swelling the space and leading to premature delamination.  Heating the material can accelerate any degradation process as it increases the rate of migration or diffusion through the polymer of any potential contaminant.

If you have a known problem of failures after a year of storage it should be possible to plot the progressive decrease in adhesion over the year.  The force used for delamination should reduce progressively.

Thus examining the surfaces may give some indication about contamination. If you have some material that is known to fail then sending this to a surface analytical laboratory and allowing them to delaminate some material in their controlled environment and then examine each of the metal and polymer surfaces by a technique such as X-ray Photoelectron Spectroscopy should allow you to identify the chemical composition of both surfaces.  If there has been some contamination it is likely that you will have the same material present on both the metal and polymer surface and it will not be the same chemistry as the original polymer surface.

There is also another factor that may be coming occurring and that is the adhesive between the OPP and MCPP may be also aging but in this case may be slightly improving in adhesion. It depends on the adhesive type but some do not fully cure immediately but take some time and so with time can increase in adhesion.  Thus if the adhesion on one side of the metal is reducing and on the other side it is increasing the failure may switch from one side to the other over time.

I hope this gives you something to work on.

Question about retrofitting a glow discharge

Hi, We have a very old Dynavac Evaporative Metalliser and would like to obtain better adhesion in our substrates by building a Glow discharge. While the principle is simple (DC transformer connected to an Aluminum rod and the chamber, then keep the vacuum level low while energizing). Looking at your diagram on page 229 of your book the ideal setup would be to have Vacuum at 10 -1 with voltage at 900 DC, to achieve this vacuum we would install a bleed value at the opposite end of the chamber to the pumps. My questions are does this setup sound ok and can we use filtered air instead of an inert gas, or do you suggest some further reading before we jump into this?

Answer

The glow discharge sounds as if it would work however if I were to be modifying a system I would aim for a magnetically enhanced plasma to treat the surface. The power requirement can be the same but the plasma density with the magnetic enhancement will be greater as it can run at the lower voltage but will carry a greater current.  The design is very similar to a magnetron source with the magnets making a magnetic circuit for the electrons to race around.  The higher density plasma will give a faster treatment time than the glow discharge and so is more useful at the faster winding speeds.  It will also operate down to lower pressures and so can be more flexible in where it is sited in the system.

As to gases, any gas will allow a plasma to be struck but the surface treatment will be dependent upon the gases present.  Argon is a heavy atom and when it strikes a polymer surface it can break bonds but argon is inert and so cannot react with anything on the surface. Thus it can hit contaminants on the surface and may crosslink them to the substrate or may further fragment any short chain molecules or oligomers. To remove any hydrocarbons the plasma needs to contain a reactive gas, oxygen for preference which can react with the hydrocarbons to form volatile species that will desorb from the surface and can be pumped away.  If you have a roll of polymer in the system there will always be some oxygen around from the air trapped in the roll and also from any moisture in the air and absorbed into the polymer. The water will be cracked in the plasma and more oxygen released.  Thus you introducing dry, filtered air will provide the surface with two gases both of which are oxidising gases with oxygen being more active than the nitrogen.

Practically if the pumps are sited to the side of the winding system there will be a small pressure gradient from one side of the chamber to the other. When you introduce the gas there is a danger that you will exaggerate the pressure gradient. Sometimes I have seen a tube used with holes drilled in the tube to allow the gas out. By suitable positioning of the tube and varying the size of the holes the pressure can be balanced. There may still be some variation across the web from plasma generated by-products which will be present at a higher proportion towards the pump. 

If you use a magnetically enhanced plasma the system can be run at a lower pressure and this will reduce this effect somewhat.