Vacuum Web Coating

PVD coloured coatings

I am a graduate student in design at Stanford and I am currently making eyeglasses from stainless steel and am interested in PVD finishing. I understand that watch companies use PVD to finish watches but the colours seem to be limited. I'm hoping for more varied colours like red and was wondering if you know of cutting edge vacuum coating processes that accommodate this (ideally corrosion and scratch resistant). Any pointers to resources, experts of companies would be appreciated. Thanks.

ANSWER
generally all colours are available through the mechanism of mixing materials such as combined deposition of TiN and TiC or similar such as TaN or ZrN, etc.  Alternatively it is possible to grow oxides and get the more traditional interference colours using titania on titanium.  The whole rainbow of colours can be obtained; in fact some years ago there was a trend for titanium jewellery using this titania/titanium material.

In the UK there is a company called Bodycoat who already offer spectacle frames that are coloured using the very PVD technique you are asking about. I believe they have a website that shows examples of these frames.

There is a PVD Group at PVD_coatings@yahoogroups.com where people can ask
questions. I do not know if it is possible to access the archives, as there have been, in the past, some postings as to the change of alloy composition and the expected colour.

What are acceptable partial pressure levels during deposition?

I would like to ask you if you can provide reference values for the acceptable partial pressure levels that one would typically recommend in thin film deposition.

We are working with thin film deposition on silicon wafers with a thermal oxide, we have done RGA analysis of the process chamber (evaporation tool) and I have included the base pressure RGA analysis.

From this we can conclude a corresponding water partial pressure of around 1*10-6 Torr, which corresponds to about a monolayer of water molecule impact pr. sec. this sounds high, but on the other hand it very difficult to relate this to actual oxidation issues for e.g. the Al or Ti layers that we deposit other metals onto or for that matter the impact on the adhesion to the oxide due to this.

Water would provide the following components:                                                                                                                                           

Component:                m/z:                                  Partial pressure in chamber:             

(H2O)+                         18                                        7*10-7 Torr                                    

(HO)+                           17                                        5*10-7 Torr                           

As all originate from water the sum would indicate the partial pressure of water in the chamber:            

(H2O + HO)                Sum:                                      1.3*10-6 Torr

(O)+                              16                                        1*10-7 Torr                           

(H)+                               1                                          NA                                       

At a partial pressure of 1*10-6 Torr one monolayer hit the surface pr. second

If you could provide some reference RGA levels that you have experience with, which do provide good results for thin film adhesion then this would be very helpful.

ANSWER

My experience is primarily with web coaters which generally carry in huge quantities of water and hence I do not have particular levels of water vapour background that are or are not acceptable for deposition onto wafers.  However there are some general comments that might help.

The quality and purity of the coating does not just depend on the background pressure but it also depends on the deposition rate.  The aluminium metallization of polymer webs typically requires the system to be pumped out into the 10^-6 mBar range & when the deposition starts the pressure typically rises into the 10^-4 mBar region.  If we take the pressure to be 1 x 10^-4 mBar the time to form a monolayer is only 0.01s.  Typically the polymer passes through the aluminium deposition zone in 0.1s (or less) and so there would be 10 monolayers of contaminants arriving at the same time as the aluminium arrives.   At the typical metallizing thickness this represents a contamination of approximately 1% -2%.  For slower deposition processes this contamination level will be higher.

So if you know what the acceptable purity of your coating is you can calculate the minimum deposition rate you require to have the appropriate arrival rate of coating to contaminant.  It will depend on the deposition source what the deposition rate is. With evaporation sources being many times faster than sputtering sources.  This gets more complicated if you are also requiring a minimum density to the coatings as evaporated coatings will be less dense because of the lower energy of the deposition process. If additional energy is added the density of the coating will increase but the deposition rate will fall.

The adhesion is a separate matter.  Often surfaces are plasma cleaned to make sure the surface is chemically clean. Aluminium deposition can have improved adhesion if there is oxygen at the interface.  Again sputtered deposition gives higher adhesion because of the higher energy deposition that can remove the poorly adhered    material which evaporation tends not to do. 

If you want to speed up the outgassing of water from surfaces during the pumpdown then using either a plasma or a UV light source can put energy into surfaces that will encourage the outgassing.

Thus a simple measure of the partial pressure of the system may not be only part of the information you need. In aluminium metallizing the partial pressures are largely unknown. The base pressure and deposition pressure is often used to provide information about when the system needs to be cleaned as a dirty system takes longer to pump down and the pressure rise on starting deposition can be higher too for a dirty system.

So although I have not been able to directly answer your question I hope the response is of some use.

Mirror coatings onto Polycarbonates.

Currently we are trying to coat a PC film by PVD process.  We do RF plasma before coating.  But, still have delamination problem.  Currently the coating process is :-

1. RF plasma.

2. coated with pure Chromium. (ratio : 30%)

3. Then coated with pure aluminium.(ratio:70%)

The reason of choosing chromium and aluminium are due to the mirror appearance effect.

I would like to try below combination:-

1. 1st by aluminium then follow by chromium.

2. 1st by tin then follow by aluminium.

From your expert point of view, do you think which way should be the best way?

Among Cr, Al, Sn, which one have higher surface energy?  Does higher surface energy imply to better adhesion to PC film?

Looking forward for your kind answer.

ANSWER

With plasma treatment it is possible to treat the polymer and make the adhesion worse than without treating the surface if the plasma treatment is not done well.  When plasma treating the surface there are two main mechanisms that you are looking to achieve, one is to remove any loose, or low molecular weight material from the surface, secondly is to break some bonds and provide more anchor points for the coating to stick.  If you are using an argon plasma this can break bonds but cannot remove any carbon based contamination from the surface. It may sputter off some carbon but it will most likely end up back on the surface and still be a problem to good adhesion.  It is best to use an oxygen containing plasma. This causes the oxygen to combine with any carbon into a volatile species, which can then be pumped away. 

The time of plasma treatment is important and needs to be optimised. Too little time or too much time will not produce the best adhesion.  To much time will cause too much chain scission and produce smaller and smaller polymer chains and this will tend towards a weak boundary layer which will be seen as a decrease in adhesion over the optimum.

The method of testing the optimum time is to test the surface energy of the polymer. As the time is increased the surface energy increases up to a point where the surface energy plateaus. If one plots the adhesion strength this too will increase as the surface energy does until the surface energy reaches the plateau where the adhesion only reaches a peak and then immediately starts to fall. Hence the aim is to just reach the peak of surface energy but not to add some extra time.

I am most familiar with roll-to-roll coating systems where the time available for plasma treatment is low. These systems use most often use a DC plasma and these may be magnetically enhanced to increase the plasma density so that the plasma treatment can be achieved within a second.

Increasing the substrate surface energy improves the wetting of the depositing metal. If you imagine the nuclei as growing as hemispheres on the untreated surface then as the surface energy increases the nuclei flatten, increasing in diameter and reducing in height. This means the coatings become continuous at a lower thickness and are more likely to be smoother and conformal.

The chromium is generally used as seed layer. Normally it is expected that the seed layer will nucleate more readily and increase the number of nucleation sites as well as being better anchored to the surface.  To do this seed layers are often only ~2nm in thickness. It is not expected that the coating will be produced as a continuous layer and the effectiveness of the seed layer can be less if the thickness is too high. 

The brightness of any mirror depends on two things, the perfection of the surface and the quality of the material that is deposited.  The substrate should be smooth and flat. The coating should be conformal and the crystal structure should be such that it remains equally as smooth on the surface.  The quality of the vacuum and the deposition rate play a part. The faster the deposition rate the smaller the crystal size and, more importantly, the cleaner the coating.  All vacuum systems have some background contamination, usually water vapour from outgassing of the system walls, and this will be bombarding the substrate. Typically at aluminium metallizing rates the contamination will arrive at the substrate at around 1 monolayer pre second and at typical deposition rates there will be 1-2% oxygen contamination from the residual water vapour in the system.  This will still appear as a bright mirror surface.  Where things can go wrong is that if the deposition rate is slowed down the contamination rate will increase. If the vacuum is poor and the rate is low this can result in a change in the coating growth and the inclusion of more oxygen can reduce the mirror performance. In the worst case this can result in the surface of the coating becoming milky, or dull in appearance, or even yellowish. This can be because he coating is more porous, or the crystal size has increased making the surface much more rough, or because the aluminium has been converted to aluminium oxide and there is an interference layer appearing giving the yellow colour.  The crystal size will increase with thickness too and so the aim should always be to produce the best coating at the minimum thickness.

Where the ultimate brightness mirrors are required it is common to deposit silver. As this can be corroded easily a silica layer often protects it. The silica is also used as an antireflection coating to further enhance the reflectance.  It would be possible to improve the aluminium reflectivity using a silica antireflection coating too.

If I were starting I would start from the beginning and try to optimise the plasma treatment and deposit only aluminium without any seed layer. Once the plasma treatment was optimised I would deposit the aluminium at the fastest possible speed to just above the minimum thickness that would achieve the desired reflectivity. In doing this I would also check on the performance of the vacuum system that the deposition conditions were consistent with delivering a high quality coating.

It would be only if I could not achieve the desired adhesion and/or reflectivity that I would look to using a seed layer.

If I needed to use a seed layer I would still aim for the optimised plasma treatment and aim for the minimum thickness of seed layer.

Offhand I do not know the respective energies of the metals Al, Sn, Cr. I would have to search through some books and papers to not only check the respective energies but also to check if they form alloys and look at the electrochemistry to see if they are likely to have an affinity to form a strong bond. Hence I cannot comment which system would be preferable. However I think that there you may not need to use a seed layer.

I hope this help even if it does not quite answer your question directly.

Double-side chrome onto polymer web.

I am looking to see if it is possible to do a double-sided chrome coating on polymer.

The chrome needs to be equal OD on both sides and should be around 2.5 OD.

Any companies that can do this?

Answer

Yes it is possible to double side coat polymer film. Few companies will have a double side single pass coating machine. Most would coat the film on one side and then re-load and coat the second side.   Chromium generally is deposited either by electron beam deposition or magnetron sputtering.

So you are looking for companies other than standard metallizers.
Flex Products Inc (part of JDSU) in Santa Rosa, CA (707-525-7007) have an electron beam deposition machine that could do the job. I know that in the past they have coated Ni for a solar control film application using their system.  They are used to depositing very precisely optical coatings and so they would be worth a call.

Others would include CPFilms Inc in Martinsville VA (276-627-3332) who sell a number of solar control products and so are familiar with the specifications required.

Bekaert Speciality Films LLC  in San Diego CA  (858-571-0200) also have a number of different systems and I am sure could help.

Sheldahl in Northfield MN (507-663-8000) would also be on my contact list. They deposit gold and often this requires a seed layer of chromium and so may well have experience of depositing the chromium layers as well as the right machines.

Optical Density and barrier performance.

Question

'Will coated PET, treated PET, or plain PET metallized to same OD show
different MVTR & OTR?'

Answer
The answer to this is yes there will be differences.
The structure of the metallized coating at a microscopic level will be different for un-treated and treated PET. The wetting of the aluminium will be different and so the adhesion porosity will differ. Thus to get to the same OD the metal thickness are likely to be different.

Thus it is always very hard to replicate someone else's process and produce the same performance film. The source of the film can be different, as too can the debris level leading to differences in pinhole (pin window) size and distribution, as well as humidity levels, etc..

All of these factors can affect the metallization as well as the lamination process.
The samples quoted in the graph were probably produced in either the UK or US and so if nothing else the temperature and humidity of the material from manufacture through to lamination is likely to have been very different to that seen by films produced in more humid climates. Hence I would expect the results to be different.

If you are looking to improve the barrier performance then I would look to the standard problems in the material. Debris levels causing pinholes (pin-windows), surface energy causing poor adhesion which can lead to pick-off and more pinholes as well not delivering the optimum wetting and coating integrity.

The graphs in the AIMCAL Tech.Ref. are meant to be a guide and not a definitive measure.  The trends will be correct but the results would be expected to be different for materials produced from different machines (even in the same factory) let alone sited on different continents.

Polymer deposition in vacuum

I recently read your answer about a polymer layer for optical applications and I'm curious: You mention TPGDA as a polymer layer one can deposit under vacuum. Could you please explain how they deposit and cure this material? It seems to be a perfectly common photopolymerizing product. So if the deposition technique is evaporation, how do you bring in the activator? And otherwise, how do you implement it?

Answer

The original work was patented some 20yrs ago by GE in the US and has since been exploited by two main centres, Battelle NW institute in the US and Sigma International Inc in Tucson AZ USA.

Basically the monomer (or monomer + initiator) is metered and pumped into the vacuum system through an ultrasonic spray head where the monomer mist is directed onto a hot plate where the fine droplets are vaporised.  The vapour is kept within a heated enclosure where there are baffles to assist in equalizing the pressure and there is a linear exit slot that the vapour escapes through. The vapour exiting the nozzle is directed onto a passing web that is wrapped around a cooled deposition drum and hence the vapour condenses onto the web as a uniform thickness as the web moves through the vapour jet.  A short time later a beam of electrons from an electron gun hits the coating.  The fact that the whole process is an a vacuum means that the electron gun is only running at around 25kV rather than the 200kV that is required for atmospheric e-beam curing of polymer coatings.

It is also possible to mix the monomer with photoinitiator and then instead of using an electron beam gun to use a plasma to cure the coating. Plasmas have over 50% of their effective action from the UV content of the plasma and so it does not require any UV lamp to work within the vacuum.

These polymer coatings have been used to provide a smoothing layer prior to depositing inorganics. This is generally to improve the barrier performance of the inorganic material. Sometimes a second organic layer is added on top of the inorganic layer to protect the inorganic material from damage.  Even so it is still very difficult to obtain a perfect coating and so for the ultra barrier layers used for OLEDs or Solar Cells these pairs of inorganic/organic layers can be deposited multiple times to progressively improve the barrier performance.

The choice of monomer needs care, the starting material needs to be of the right viscosity to be pumped into the vacuum system and as it hits the hot plate it must be possible to vaporise the droplets and not simply polymerise the monomer onto the plate. Ideally any mixture will not fractionate and the deposited coating will match the feedstock composition. The viscosity should also be compatible with condensing onto the cooled web on the deposition drum where the coating must be able to level and on polymerisation must not change dimensions so much that the web curls up through too much stress. 

The option to not cure or partial cure the coatings has also been used to advantage. Depositing the monomer but only partially curing the coating can make the coating capable of being dissolved off at a later stage. In this way it can be used to manufacture flake pigments. Deposition materials such as aluminium onto the partially cured polymer allows the polymer to be dissolved in a solvent bath which then floats off the aluminium coating which breaks up into flakes. These flakes are as flat and smooth as the substrate surface and generally this is much smoother than the standard ball-milled and polished flakes. Thus metallic inks and paints are getting brighter and more reflective using this type of manufacturing process.

The work on vacuum deposited curable polymers has been limited and so there is a limited expertise and knowledge base on how many of the standard materials will work in vacuum.

John Affinito did work on an alternative deposition source where he evaporated the monomer from within the vacuum using a resistance/radiant heated enclosed crucible. What he found was that the temperature control was critical to the process and needed to be within a fraction of 1 degree C throughout the whole source otherwise the evaporation rate varied too much and the coating thickness was too variable.

There is work being done elsewhere on evaporation sources that might improve this aspect, until then the pumped, metered, ultrasonic spray, flash evaporation type source will remain the source of choice.

I hope this gives you a more detailed insight into the in-vacuum polymer deposition.

What is the maximum OD to be deposited on BOPP?

Question

We intend metallising 18-micron plain treated BOPP and 18-micron metallising grade plain treated BOPP. Is there a constraint on the maximum OD levels that one can achieve on a BOPP? I am made to understand that normally BOPP cannot be vacuum metallised beyond 2.2 OD.

Answer

The limitations on the OD that can be deposited onto BOPP will depend on a number of things.

The critical part of the process is managing the heat load the film sees during the deposition zone. The film is pulled tight onto the deposition drum. The reason for this is to make a good intimate contact between the polymer web and the cooled surface of the deposition drum. In this way the heat transfer coefficient is increased. The heat transfer coefficient has several components, radiation, convection and conduction. The radiation is small, the conduction depends on the contact surface area and this will depend on the polymer and deposition drum surface roughness as well as the contact pressure. Thus for any particular film/drum combination it can be seen that contact pressure can have an influence.  This is limited because as the film heats up the yield strength of the film decreases and so the tension that it can withstand before it suffers some permanent dimensional change.  This is the same for all metallizers and so the limitations are similar.  Some machines can cool the deposition drum to a lower temperature than others and so will have an advantage allowing a slightly thicker coating than those with a higher deposition drum temperature.  Also a larger diameter deposition drum can have an advantage as the longer deposition zone allows slightly more time to deposit the coating and remove the heat.

The one factor I have left out was that of conduction. I was found that any absorbed moisture in the film could be an advantage in that it increased the heat transfer coefficient. Under the effect of heat the moisture evaporated from the surface and filled the gaps between the film and the deposition drum and then some additional heat was transferred through convection currents. Some modern machines have deliberately made use of this idea to improve the heat transfer coefficient. Where the film comes in contact with the deposition drum gas is injected into the gap and becomes trapped between the web and drum. This trapped gas provides the means to substantially increase the heat transfer coefficient. This enables either a thicker aluminium layer to be deposited or a faster deposition rate to be used.

Incidentally the gas wedge also reduces the coefficient of friction between the web and the drum allowing the web to move laterally more easily than without the gas present. This reduces the propensity of the web to buckle off the surface causing tramlines (railroad lines), which is also a heat related problem.

Thus there can be quite a large difference in performance between a system that has poor deposition drum cooling and no gas insertion mechanism and a small deposition drum and one that has a lower deposition drum temperature, larger drum diameter as well as a gas wedge insertion unit.

Added to this can be the degree of orientation or more precisely the tensile performance of the BOPP as the higher the tensile performance the greater the tension that can be pulled.

Thus if you are wanting to coat to a high OD it is worth reviewing what machines you have available to see if one is more suitable than the others.

Is there a relationship between OD and CoF?

                      Is there a relationship between OD and CoF?

ANSWER

Generally polyester films have some low molecular weight material on the surface.  This is most frequently oligomer that is residual material from the polymerisation process. There may also be other materials added that can migrate to the surface, such as slip agents in OPP, that may also reduce the coefficient of friction (CoF). 

   Often the surface of the polymer is treated to change the surface energy to improve the adhesion. This treatment to increase the surface energy can crosslink the low molecular weight material into the bulk polymer or can volatilise the material and remove it. Either way the low surface energy material is removed and the CoF will get worse (increase).   

    Even if there is no surface treatment the metallization itself will cover up the low molecular weight low surface energy surface and again the CoF will increase.

    Thus there is no fixed CoF that specifically relates to a particular Optical Density (OD) but rather the CoF relates to the history of the polymer web and any surface modification and surface treatment that has been carried out.  Pure PET films will readily block and so they are usually have fillers added to provide a controlled surface roughness. The surface roughness reduces the contact area and hence the CoF. The size, shape, type and distribution of these fillers controls the precise value of the CoF.  This is usually higher than is ideal for and easy to handle film. Coupled with the residuals this can make the ease of handling acceptable.

    Sometimes the back surface of the web is not treated and thus there may be the transfer of some of the low molecular weight low surface energy material from the back surface to the front surface once the metallized web has been re-wound.   This can cause a problem in reduced adhesion of any subsequent coatings to the metallized layer but the transferred material may also aid the ease of handling.

            Where a link may exist between OD and CoF is in the metallization nucleation and growth.  Any low molecular weight, low surface energy contaminant on the surface, that helps reduce the CoF, will stop the aluminium wetting the surface well. This will make the aluminium nucleate and grow as a series of hemispherical islands. This will make the coating quite thick before it completely covers the surface. Whereas it the surface has been treated to raise the surface energy and correspondingly the CoF rises the aluminium will wet the surface well and instead of the hemispherical-like growth the islands will be very flat and spread well over the surface this helps make the coating completely cover the film at a lower thickness. Thus the same OD may be achieved at a slightly lower thickness than for a non-wetting surface, lower CoF film.  However there will not be a direct correlation as the CoF can be varied by the filler or surface roughness as well as by any surface contaminant.

Thus for some materials there may appear to be a link between OD and CoF but this link may disappear depending on the film processing or with a change of film supplier and may never be apparent in other materials. 

Vacuum deposition of flake pigments.

Question.

Is book describe manufacturing process of PVD pigments, so can we manufacture it?

Answer

There are two books

Metallic effect pigments  by Peter Wissling       ISBN 3-87870-171-3  2006

Special effect pigments  by  Ralf Glausch et al  ISBN 3-87870-541-7  1998

both published by Vincentz Verlag, Hanover Germany.

Distributed in some countries by William Andrew Publishing

The books describe the manufacturing process but not necessarily in such detail that you would have enough to start manufacturing immediately.  Within the book they do reference much more information including some of the many patents that cover many of the processes and products that are described. From the patents there is often more information that is useful about the process particularly where they provide illustrations of producing material. Things like the supplier and trade name of release coatings or chemical formulations may be included in some of the patents.

The manufacture of thin film pigments can be dangerous. Some of the thin film pigments oxidise exothermically and there are many accounts of fires and explosions associated with the manufacture of such thin film pigments. So it is essential that the safety information is taken seriously and safety is considered in great detail if you are thinking of manufacturing such pigments.