Vacuum Web Coating

Deposition sources

There are changes coming in deposition sources largely driven by the new enthusiasm for high rate deposition for photovoltaic devices.

The photovoltaic materials generally have to be deposited to a thickness of greater than 1 micron and so the deposition process has to be fast.  Although some of the companies are using sputtering the more successful companies depositing the copper indium gallium diselenide (CIGS) materials are using evaporation.  The CIGS compound is evaporated from a series of sources of the individual elements enabling the compound stoichiometry to be graded through the thickness.  Although the compound has to be graded through the thickness the requirement is for the thickness and stoichiometry uniformity across the web to be precise.  Thus there has been work done to improve the stability of the deposition process and control the thickness uniformity. 

There have been two different approaches to the sources. One has been to develop the jet vapour source where the evaporation is done into an enclosed volume where all the internal surfaces are kept hot to prevent condensation and the exit slot is the full width of the substrate.  The internal vapour pressure evens out any evaporation variations so that the exiting vapour uniformity is very good.

The second approach has been to take existing high stability vapour sources that have well known characteristics and to use arrays of these sources to provide the uniformity.  The Knudsen source is the basis of this type of source.  The semiconductor industry has used this type of source for many years and it has been modelled extensively so that the interaction of multiple sources is well understood. The design of these sources has been developed and optimised so that the temperature is controlled to a fraction of a degree and so the evaporation rate is more precisely controlled than most evaporation sources. 

What becomes clear is that these sources could be adapted to the deposition of aluminium. The benefits of doing so would be not only that the uniformity of the deposition would be improved but also that the material efficiency would be significantly improved.  Currently evaporation from resistance heated boats can have an efficiency of anywhere from 35% up to more than 50% depending on the system design including source to substrate distance and deposition drum size.  The vapour jet source, in particular, can have a material deposition efficiency of greater than 95%.

Where these new sources are unproven is in two aspects. One is the replenishment of the sources. Neither of these sources has a replenishment facility. The whole inventory of material has to be loaded and heated at the start. This is because any feed process has to be done via a hole where vapour can escape which is not only a material loss, a cooling point and a possible problem through condensation of the vapour that could close up the hole causing feeding problems.  The Knudsen sources can usually hold sufficient material for several deposition runs to be completed.  This leads to the second area of process uncertainty which is that of source cooling.  At the end of the first deposition run the source needs to be cooled to a safe temperature for the system to be vented.  As these sources are designed for temperature uniformity they usually include radiation shielding which both limits the energy losses but also slows down the cooling because of this minimisation of heat losses. In the past gas quenching has been used to accelerate the cooling process.  In some systems the need for cooling the sources has been followed by using load locks for the un-wind and re-wind rolls with the main vessel kept under vacuum for the multiple depositions the inventory of the source can allow. As the material efficiency is so high the need for shield cleaning is reduced and so keeping the main vessel under vacuum is possible.

Another advantage of using this different evaporation source is that the range of materials that can be evaporated is increased.  It has always been one of the limitations of resistance heated evaporation sources that the range of materials is very limited.  Sidrabe have developed an alternative source where they extended the range of materials by using tungsten rods as the core of the evaporator and refractory materials for the enclosure.

I hope this gives a glimpse of what is coming. It may take a few years for these developments to filter through to the aluminium evaporators but it will eventually be adopted.  As the material efficiency can be improved so much the energy requirement can similarly be reduced and this will become increasingly important as energy process continue to rise.

Wrinkling - additional monitoring

Recently I was visiting a well known metallizer manufacturer and this was an observation that came from some of our conversations.

I had been looking at metallizers and thinking that in so many ways they had not really changed significantly in decades.  The basic wire fed evaporation source was easily recognised as working in the same way.  There have been changes behind the panels in that the power supplies are more sophisticated and power is applied better and more uniformly and similarly the wire feed is better than it was. 

I was looking round at things that make the life of the machine operator easier. One change stood out as being both simple and helpful and that was the view into the system.  I have lost count on the number of occasions where it would have helped in diagnosing a problem if I had only been able to see better into the system to look at different parts of the winding to see where winding problems were beginning.  Generally we had one or possible two windows to look through and possibly a strategically placed mirror added inside the system to give some view of an obscured part of the system.  This has all changed with the miniaturisation and price reduction of camera systems.  I had previously worked on a vacuum process for roll coating explosives where we controlled the system from behind a blast wall and so our only view of the system was via cameras. This made me familiar with split screen multiple view displays but even the 10 – 15 years ago the camera technology was still quite bulky and so these cameras were sited outside the system pointing in at various angles to give us the necessary vision.  This was never as much or as good as we wanted.   Now the cameras have become small enough that they can be fitted with easy within the system and cheap enough that it is possible to consider using many cameras within the system to give all the necessary views to help with diagnostics.  What is more it is almost as cheap to use colour cameras as it is to use black and white.  The same rule applies as ever it did and that is that it is critical the lens of the camera is protected from any stray deposition.

Thus it is now conceivable to have a view of the film as it comes off any high wrap, spreader or bowed roll as well as out of the metallizing zone. This means that identifying where wrinkles start should no longer be a guessing game. Our first thought is always that the deposition has caused the problem but there are occasions where that is not true and it is always expensive in time and money to be trying to solve the wrong problem.

Retrofitting is generally more problematic than installing during the system build but as most of this is just cabling this should not be too difficult.

So if this has persuaded any of you to retrofit some cameras or for those of you ahead of the game that might have already done so I would be delighted to hear how you have got on with them and if you have experienced any problems.

Web Processing for Barrier

AIMCAL Converting School          Web Processing for Barrier   October 9 - 10, Cleveland, Ohio

Room Rate Guarantee Deadline is September 24!

The Web Processing for Barrier course provides an overview of the technologies that can be applied to deliver barrier materials for markets such as food packaging, display, and photovoltaic (solar) applications. Initial discussion focuses on the fundamentals of diffusion and permeation to provide an understanding of the contributions and limitations of materials and processes. Advanced topics include newer technologies such as nanotechnology, scavengers, and indicators that result in "smart" materials.

Course Outline
Day 1: 8:30AM - 5PM
Day 2: 8:30AM - 4PM

Introduction
The Basics of Barrier
Markets
Terminology
Techniques and standards for barrier measurement
Materials
Performance
Blends and laminates
Nanomaterials
Selection
Smart/active packaging
Indicators
Scavengers
Technologies
Film making and extrusion
Coating
Lamination
Vacuum deposition
Substrates, surfaces and quality
Relating materials and surfaces to barrier
Pre-treatment (wetting, adhesion)
Cleaning and barrier
Variation in barrier during processing
Who will benefit from this course?
Anyone working at facilities that convert or use barrier materials including engineers, designers, quality control, stability and production personnel.

Date for this course:
| October 9 - 10, Cleveland, Ohio |

Comparison between CPP and BOPP

As we know CPP and BOPP exhibits different properties. Especially sealing properties are quite different. CPP exhibit better seal strength than BOPP. I request you to enlighten me on technical grounds why such difference is exhibited?

What will be the performance if we use same tool to seal CPP and BOPP?

Answer

The acronyms are as follows CPP = Cast PolyPropylene  and BOPP = Biaxial Oriented PolyPropylene.

Cast polypropylene is produced by extruding polypropylene and chilling it and so the polymer chains within the film are randomly distributed throughout the film in all three dimensions.

BOPP starts out in the same way, the polypropylene is extruded and chilled but then the polymer is re-heated and stretched in two directions hence the 'bi-axial orientation' in the description.  This stretching is done when the polymer is softened and so the polymer chains are able to rearrange themselves to some extent and so they become oriented into the two stretching directions.  This changes the mechanical performance of the polymer. The tensile performance is higher along the polymer chain length than it is across the polymer chains thus in either of the two stretched directions the tensile performance is improved but in the third axis the tensile performance is reduced.   In the ordering of the polymer chains many of them become ordered enough that they become crystalline. This too effects a change in the performance, as crystalline material is a better barrier than amorphous material.  Crystalline polymer is denser than amorphous polymer and the amorphous polymer will soften at a lower temperature than the crystalline.

Thus using the same heat sealing tool may well work OK but may need a higher temperature for the BOPP than for the CPP.

I hope this answer helps.

Rotatable vs. planar magnetron sputtering burn-in

I have been wondering just how sensitive rotatable aluminium targets are to developing a oxide layer requiring burn in to break through to the metal as compared to planar targets.

In my case planars burn in almost instantly while there is a lot of technique applied by operators to get rotatable targets to finally burn through.

Answer

Removing the oxide is not only dependent upon the cathode design, magnetic arrangement and strength, power applied and sputtering pressure but is also dependent upon the history of the target.  The targets for the rotatables will have a different manufacturing history and so are likely to have a different oxide thickness to the planar targets.  It is probably more difficult to make the tubular shape and so more heat will have been used in the processing which will have increased the oxide thickness.  As the oxide sputters at a slower rate than the metal, the target surface area several times larger on the rotatable than the planar and the rotation of the target allowing time between sputtering for the surface to re-oxidize it can be expected for the rotatable to take more time to clean up compared to the planar.

If you want to try to minimize the burn-in time it may be worth mechanically cleaning up the target surface just before installing then into the vacuum system.  I know people have used abrasives to clean the surface to get to bright metal. This needs care as the dust can be a fire or an explosion hazard and as it generates dust it is important that the target is cleaned well afterwards to make sure the dust does not cause sealing problems or arcing on the target surface. The target will immediately oxidize but the oxide thickness will have been minimized.  I would expect a target precleaned in this way to clean up in a similar time (per unit area) to a planar for the same sputtering conditions.

An expensive way of investigating the target differences would to use one of the surface analysis techniques to determine oxide thickness of the different types of target.

Supplementary question

On the burn in issue for “new” targets I can concur on the variables you mentioned, what I can’t understand is that after the machine has produced a batch for several hours (and now targets are supposedly really clean) then the machine vented, the need for substantial burn in between batches is always present.  The targets just do not get to a high enough voltage to sputter until they finally “break thru.”  Again I do not see this with the planar aluminum…metallize for hours…vent…pump down and I can immediately hit them with full power and the voltage is high and they sputter…a couple minutes of burn and they are stable and ready again.

Answer

The burn in of the rotatable is no surprise even on a well established target.  Once the sputtering has stopped the oxide will build up. Nominally it will be the same thickness as on the planar but on the planar the target does not move and so the negative aspect of the planar which is the erosion profile that is often a 'V' shape becomes a benefit to burning in the aluminium. When the aluminium switches between oxide and metal it does not switch instantaneously across the whole racetrack but it breaks through where the magnetic field is strongest and parallel to the surface then as the metal suddenly sputters very quickly some will be backscattered to the target surface as well as the metal track widening and so it gives the appearance of breaking through across at least most of the racetrack.  The rotatable may have a similar magnetic design and strength but before the breakthrough takes place the target has rotated slightly and moved into a slower sputtering rate position and so the oxide is either removed at a slower rate or it may even begin to build up again.  The front surface of the target will be hot and so will rapidly oxidise. Thus for your rotatable it will take many more passes across the sputtering zone before the target breaks through.

I suspect if you were able to stop the rotation that you would be able to get breakthrough at a similar speed to the planar but obviously not around the whole cylindrical target.

Question about pressure variation when using plasma treater

We have metallizer. When operating plasma treater there is a variation or rise in evaporation zone pressure throughout the metallizing cycle. Theoretically the pressure of winding should rise as plasma treater is located in winding section, but the winding zone pressure is not varying, what is the reason behind this?

Answer

This sounds to me to probably be related to a problem of cooling.

I would monitor any cooling systems that you are using. If you have different cooling water to the plasma treater, the deposition drum, protective shields and the evaporation boats I would monitor each separately. I would check both the input temperature and output temperature.

Typically once you have pumped out your system the residual gas is almost all water that is out gassing from all surfaces including from the unwinding roll from trapped air and from water contained within the polymer.  Once the roll is unwinding there will be an increase in the gas load because of the release of the trapped air as the roll keeps presenting fresh surfaces. 

As you suggest this is a fairly constant process and so you would expect everything to remain constant throughout the deposition run.

If you have a problem with a cooling system what happens is that initially everything is cool as the process has only just started and there may be a large thermal mass that has to be warmed up. If you are recirculating the water (or coolant, which may be a water /glycol mix or something similar) there is also the thermal mass of the water.  What happens is that the water passes through something that is heating up and so the water is heated up, but usually to a lower temperature than whatever they are passing through. The coolant then returns to the chiller that returns the coolant to a constant starting temperature.  If the heating load is higher than the chiller capacity you will progressively see that the coolant is no longer returned to the same starting input temperature but this temperature gradually creeps up.  As the coolant temperature creeps up it no longer cools as well as it ought to and so the internal surfaces become hotter.  If these surfaces have any stray deposition on them they will have a very high surface area and so can contain a lot of water. As the temperature increases water can be released from the surface thus raising the pressure.

Most systems have cryopanels that will pump away the water very well but it does depend on the water having access to the cryosurfaces.  It is common to see systems with only cryopanels in the unwind zone whereas it has been shown by Telemark Cryogenics  Ltd as presented at AIMCAL that it is better if the cryopanels are distributed across different zones.  The deposition zone will always heat up the web and internal surfaces more that anything occurring in the unwind zone. The deposition zone also has the greatest potential for having coated surfaces that can mop up very large quantities of water.

If the problem does not appear what the plasma treater is not used it could be that some of the cooling systems are linked and that it is only when the plasma treater is used that the chiller cannot keep the temperature low enough.  Even if the coolant systems are not linked the plasma treater does add another heat load within the system. It is possible that the hot plasma treater is radiating heat to another surface that is the releasing the water.

Another option is that the extra heat is reaching the cryopanel surface and that over time the surface of the ice no longer remains frozen but stays as a liquid with a correspondingly higher vapour pressure.  The cryopanels are always a balancing act as the core remains cold but the growth of the ice can affect the total capacity of the panel. If the water is released within the system very quickly the ice may be very porous and this is then has a worse thermal conductivity than if the ice was built up slowly and was very dense.  The dense ice will allow more water to be collected than the porous ice. If there is too much water within the system over time the ice will build up to a thickness where no more water can be condensed and so the system pressure will rise.

Finally it is possible you have a small water leak that is normally closed but under heating opens up slightly and leaks water out.

I hope this helps you sort out the problem.

Question re. Aluminium degradation due to heat & light

If a vacuum metallized film has an adhesive applied to it and it is adhered to a car windshield (inside facing out) will the aluminium still oxidize? If so, how long will it take? Also, I have heard that sunlight will degrade aluminium, is this correct?

Answer

There is no straightforward answer.  For years aluminium metallized film has been available with an adhesive coating for laminating to both windows for housing as well as car rear windows.

The stability of the aluminium depends on many things including the thickness of the aluminium.  The regulations are that the front windshield has to have a high visible transmittance and so the aluminium has to be very thin and so other factors need to be well controlled for the aluminium to have a long life.  For rear side and rear windows the visible transmittance can be much lower and so the aluminium thickness can be much greater and the stability is generally greater too.

The aluminium when it is evaporated onto the polymer film needs to stick well and wet the polymer surface. If the aluminium does not do this it can have many microscopic holes through the aluminium metallic structure which make it easier for the aluminium to oxidise. Aluminium oxide is transparent and so the aluminium disappears with time.  High heat and humidity will accelerate any corrosion process.

As well as the aluminium deposition conditions and the surface quality and state of the polymer the quality and type of adhesive can also play a part.  Many of the adhesives are water based so that the film can be cut to shape dry and the fitting checked before the adhesive is activated by water and the film stuck and squeegeed to the window.   This water can be absorbed by the polymer as well as by the adhesive and if there are any defects in the aluminium it can work on these defects making them grow and become easily visible.

Sunlight can be a source of energy to accelerate various processes.  The polymer film will normally degrade with exposure to UV light but as the aluminium is a good UV filter the polymer will last longer because the aluminium is facing the sunlight and protects the polymer.

Aluminium is normally quite stable; it is when it comes in contact with moisture and possibly chemicals that corrosion can be accelerated.

Thus if you have a high quality aluminium metallized film suitable for rear windows then the film is likely to be stable for years.  Lower quality film could be expected to be reduced.  If you are considering a film for the front window then it will need to be a much thinner aluminium coating and this becomes much more difficult to get at very high quality and so is correspondingly more vulnerable to corrosion which because of the very thin coating thickness becomes much more visible.

Both sunlight and heat can speed up any degradation process.  Aluminium vacuum deposited onto polymer film is not pure aluminium. As part of the process the aluminium will contain at least 1% (and often a higher percentage) of oxide or in some cases hydroxide. The polymer is not a perfect barrier to oxygen or moisture diffusion and so more water and oxygen can reach the aluminium film from the air inside the vehicle.  Thus any existing defects are likely to grow in size with time.  Chemical reactions are speeded up with heat or other energy source.  Light contains UV energy and this can accelerate some chemical reactions.  So both light and heat can accelerate aluminium degradation.

Aluminium oxidises naturally. All aluminium has an aluminium oxide surface and it is the aluminium oxide that helps slow down further oxidation of the surface. The oxide is less dense than the metal and so wants to take up more space and so the oxide is under compression and provides a better oxygen barrier helping to slow down oxygen reaching the metallic aluminium.

With semi-transparent aluminium the thickness of the aluminium is only a few tens of nanometers thick and so small amounts of oxidation can become visible very quickly.  Thus any defects in the aluminium deposition can cause holes or cracks in the aluminium often too small to be visible to the naked eye but which can allow oxygen to pass through the aluminium.  The more defects there are the faster the oxidation of the aluminium.

Thus degradation depends on the quality of the aluminium deposition as well as the temperature the film will reach and the time it spends at an elevated temperature.

The UV part of the light can be absorbed to an extent providing energy which also can accelerate corrosion depending on the chemicals present.

Humidity also plays a critical role and high temperature and high humidity together are the worst combination.

Neither heat nor light will degrade aluminium by themselves, both require other components such as oxygen or moisture and can be further accelerated by other chemicals that maybe present within the polymer, adhesive or even migrating from the glass (such as sodium).

If comparing two films in the same situation of being laminated to the same type of glass and in the same environment the comparison can be done of the quality of the aluminium deposition.  If the same film is to be used in differing conditions then the aluminium structure and quality can be ignored and the differences in heating, and other materials (glass, adhesive, atmospheric contaminant content) need to be considered in comparing potential lifetimes.

Scratches during Slitting

We are having 920 mm slitters. We are facing intermittent problems of formation of scratches in the machine direction mainly near the Heavy Edge metallization. Sometimes it continues through and through till the end of the reel and sometimes it appears and then again it vanishes. Most of the time it is appearing for higher thickness ( 7 to 10 um ) films. We have tried our best with all possible trials but could not able to find the route cause and hence not able to remove the problem permanently.

Answer

The direction of the scratches is important. If they are only in the machine direction the cause becomes limited.  Scratches are produced by a roll stopping or running slowly in comparison to the web.  If the tensions are set with too much of a difference or the winding speed of the rewind poorly set it is possible to produce intermittent scratching as the tensions hunt to some point of stability.  As you are able to wind some film well this is unlikely.  If some of the rolls are only tendency driven and the bearings wear or become dry they may become more difficult to turn and so may not precisely match the web speed.

If you look at the scratches under an optical microscope it may be possible to tell if the scratches are continuous, which would suggest that a roll has stopped completely or if the scratches are made up of a large quantity of very tiny short length micro-scratches which is more characteristic of the roll not quite matching the web speed.

Scratches can also be worse if the polymer web is dirty.  If there is debris on the surface of the web and the surface is metallized the debris is metallized too. The debris can be several microns in size so that when it comes in contact with a roller it has to lift the web up if it is to pass around the roller without moving. If the tension is high it may become difficult to do this and so the debris may start to slide along the surface of the web causing scratching.  If, when the scratches become visible, you reduce the tension and the scratches reduce or disappear this may be an indication of this being the cause of the scratches.  If this is the case then improved cleaning of the web before metallizing could also help as could cleaning all the rolls in the winding system to make sure there is no residual debris on any of the rolls.

This could also explain why it is the heavily metallized edge that is the one that shows more scratching than elsewhere.  Any part of the web that is thicker then the rest will have more tension applied to it and so as the web passes around a roller it will have a greater pressure on the thicker area of the coated web i.e. the more heavily metallized region. Thus it will be this area where it will be harder for the debris to lift the web up to allow the space for the debris to pass around the roll.   

Question - Seasonal variations – are they a problem?

I have a query regarding bond strength in metallized film. We have long rainy season and unfortunately we don’t have any humidity control system in our metallization hall, we are also producing PET chips & PET film at same premise but since past few days of rainy season we are continuously getting low bond strength in all different types of PET metallized film even in copolymer coated metallized for which we never get bond strength less than 600 grm force/inch currently getting 150~200 grm force, we have tried to increase solid content in copolymer coating we got some improvement and value raises to 300 grm force/inch & in other type of films like plain PET & corona treated PET we are constantly getting ~ 50 grm force/inch earlier we are getting around 110 grm force/inch. Can high humidity like 80% RH can play this big role or we need to look for some other causes?

Answer

Humidity has all sorts of effects throughout the process and so if the ONLY difference between the good bond strength and the low bond strength is the season and the increase in humidity then your answer is yes the humidity is the cause of the problem. You do need to be sure that this is the only difference.

If we look at some of the variations that are related to humidity we can see how it can affect the process.

PET chips will absorb moisture from the surrounding atmosphere and so in high humidity will contain more water than in dry conditions.  PET chip is usually dried before extrusion but it depends on what controls there are on the drying as to whether the PET chip enters the extruder with the same moisture content or is slightly wetter in the high humidity season.  If the chip is only dried for a set time there will be variations. If the drying is done using dry air and the output moisture content monitored and the chip always used once the moisture is below a set level the content ought to be similar irrespective of the season.  The moisture level in the PET chip will be reflected in the moisture content of the PET film produced as well as giving small variations in the polymer chemistry that can affect things like the oligomer level too. Thus the more consistent the input material the more consistent the film output can be expected to be.

As the film is wound on the production line it will trap the surrounding atmosphere between the layers and this will have the high or low humidity depending on the season. The PET will absorb the moisture and so in the high humidity season the moisture content will be higher.  If the film is corona treated there can be variations in the treatment level depending on the corona treatment settings and ambient humidity.  High humidity air is more conducting than low humidity air and so the corona treater, if it is left with the same settings, will produce different treatment levels at high humidity than for low humidity.  Again if this is known about and the settings are changed, to compensate for this conductivity difference, the treatments may be similar.

Between the film manufacture and corona treatment (if any) and the vacuum deposition process the film will be stored and during this time the film will also equilibrate with the surroundings and will either absorb or desorb moisture depending on the humidity levels.

Once the film reaches the vacuum system and it is pumped down there may be observable changes such as the base pressure is not as low as for film pumped down in the dry season. This may be difficult to check as there is also the factor of the stray deposition that absorbs moisture and so the base pressure can also be related to the system cleaning and quality of the cleaning. Without any film in the system if the system is pumped down it will reach a different base pressure if the system is clean from when the system has stray deposition coating shields and other surfaces.  The stray deposition is porous and so has a very high surface area and so will absorb much more moisture than the cleaned surfaces.  The difference in surface area can easily be a factor of ten or more.  Thus this may hide the fact that there is also a higher moisture load from the roll of film made in the high humidity season however both will be related to some extent as the both the absorbed moisture in the stray deposition coating and the roll of film will be significantly higher in the high humidity season than the dry season.

It is likely that to allow for variations in cleanliness and seasons you have a base pressure set that once the system is below this level it is OK to begin metallizing.  What you may see more easily is the time to reach this base pressure is longer in the high humidity season than in the dry season.

Once the process starts there will be a higher level of moisture given up as the trapped air layer is revealed as the web unwinds. This moist air will have the moisture captured by any cryosurfaces within the system but the background moisture level will be higher than in the dry season. This higher moisture level will affect any plasma cleaning process in as much that the moisture will be a proportion of the gases involved in the plasma thus if you think you have a particular ratio of argon to oxygen it will really be a ratio of argon to oxygen to water where the water content is dependent upon the season.  The water will be split to oxygen and hydrogen and so will change the argon to oxygen ratio.  Depending on how much of a change this is will depend on how much of a change it will make to the plasma treatment but if the plasma treatment has been optimised in one season it could well mean that it is no longer optimised in a different season.

If the moisture level of the polymer is not controlled during film making and so there is a variation in oligomer level in the PET film then this will be seen as a different level of oligomer on the surface which may also mean the plasma treatment needs to be modified to take account of the different level of oligomer.  The worst case scenario is if some of these changes are cumulative such as if in the high humidity season there is more oligomer, and the corona treatment has been optimised in the dry conditions and so is worse in the high humidity season and also the moisture level is higher in the vacuum system which also has had the plasma treatment optimised in the dry season and so this too is not optimised.  The cumulative effect of all this could lead to a surface that is very different to that produced in the dry season and so the wetting and adhesion is poor by comparison.

Thus there are a number of things that you may be able to check if you have been monitoring and recording them over time such as system base pressure and pump down time.  The pump down time is likely, on average, to be longer, even after cleaning, in the high humidity season than in the dry season.  If you carry out a corona treatment check the settings to see if they are fixed or are modified with changes in humidity.  If they are fixed it would be worth planning to check the surface energy as a routine measurement over a year to relate the performance to seasonal changes such as humidity and temperature. Plotting the temperature and humidity against the system pumpdown time and noting the cleaning points too is also worth doing.  It would be difficult to plot the surface energy after the vacuum plasma treatment and so it may be that measuring the bond strength of the coating against daily humidity would be easier.

Measuring oligomer levels may be more difficult as this would involve extraction and weighing the oligomer to a high precision and this may be very time consuming.

I know this does not solve your immediate problem but it will give you the information in the future to know which areas to concentrate on in improving monitoring and control.

In the short term I would check that the corona treatment and plasma treatments are both optimised for the current conditions.  Both may have been optimised in the past but as explained above this optimisation may no longer be correct.

A question relating to plasma / sputter cleaning

Please go through the following abstract of a sputter engineer:

It is possible to get a plasma in your cathode with gas flow of 80-100 sccm of Argon and Oxygen. But if you use so little quantity of Oxygen gas, the target will go up with the sputter etch. That means you start with a sputter process. But this is not the goal of the pre-treatment system!!!! You will have a plasma for cleaning of the film and to get a better adhesion for the layer That means with minimum gas flow 1200 sccm Oxygen, the target do not go up with the sputter etch.The life time of the target will be longer than one or two years, depend of your production. I don’t now why you use this small quantity of oxygen gas but I know this quantity is too small, gas flow set points: Argon 400 - 500  sccm Oxygen 1200 - 1500 sccm.

Sir, based on above comments I would like to know:

How low gas flow of oxygen can increase sputtering process because I had read the sputtering will take place because of bombardment of heavier ions of argon gas to target surface

The goal of Plasma pre-treatment in metallizer is only the cleaning? I think we need both cleaning as well as sputtering

Answer

What you have is a magnetically enhanced sputtering source.  The way you use it will determine quite what happens to your substrate.  If the substrate passes through the plasma some distance away from the source it will be immersed in the plasma and bombarded by it. If the substrate is passed very closely over the surface of the source the polymer itself becomes the target material and the polymer surface is sputtered off.  If the polymer is passed through the plasma it may not only be bombarded by the plasma but also the plasma will be sputtering whatever target is used on the source.  If the pressure is low the sputtered material will undergo few collisions and will arrive at the substrate and may even deposit on the surface. Thus the polymer surface may contain a very low percentage of the sputter cleaning target material.  Often this target material is carbon and so if a chemical analysis is done of the surface it may well be that no contaminant is found because of the carbon present in the polymer masks the carbon deposited on the surface. To limit this deposition, the pressure of the plasma treatment zone is raised to a high pressure which causes the sputtered material to undergo many collisions causing the sputtered material to be scattered, some back to the cathode and the rest to all surfaces rather than just onto the web substrate.

Thus your engineer is suggesting that increasing the plasma cleaning zone pressure will limit the deposition of any target material.

Argon is a heavier atom and so will have more impact energy than the oxygen and this aids chain scission and crosslinking. The argon will not convert hydrocarbons into any volatile species that can then be pumped away and so it is usual to have a combination of argon and oxygen to be able to do this process.  The ratio of argon to oxygen is usually in the Argon 80% to 90% range and the oxygen 20% to 10% range.  This is safer than having very high levels of oxygen passing through the diffusion and rotary pumps. The rotary pumps have a thin film of hot oil over the moving metal surfaces and excessively high levels of oxygen can result in explosions. Thus if high levels of oxygen are to be used the roughing line needs to have nitrogen injected into the line to dilute the oxygen level back to atmospheric levels i.e. approximately 20%.

Cleaning of the surface includes all parts of the plasma process, sputtering, chain scission, crosslinking as well as chemical reactions.  The total pressure, as well as power, can affect the balance and speed of surface treatment.  This is part of the reason why, for any given substrate, the process needs to be optimised to provide the highest adhesion.  The variables you have available to change are the argon : oxygen ratio, the total flow and hence process pressure and the power to the magnetron cathode.  Most of then the gas ratio is fixed and the total flow is fixed and only the power is varied to optimise the adhesion.