Vacuum Web Coating

comment on air-to-air vacuum deposition

From Bill Llewellyn

I read your blog on air to air metallizers and enclosed sources with interest as over the years I have had experience with both types.

 

The air to air machines proved horrifically energy demanding for, as you say, you are continually pumping against a leak. In addition, the seals (then) proved inefficient and the web path required was tortuous to say the least. Additionally the extra costs and investments in such complicated devices could not be justified as there were still breaks in the "continuous" production because of web breaks, the finite lives of evaporation sources, and  the need for cleaning and maintenance of chambers, etc. The frequency of these breaks in otherwise "continuous" production meant that the significant additional investment funding needed for air to air machines and their running cost compared to conventional batch machines could not be justified.

 

The compromise seemed to be the use of machines where the source is maintained at low pressure while reels are changed in atmosphere. In the late 1980s I invested in such a machine - a Galileo 2200 mm machine - that not only included the design feature to maintain sources at low pressure during reel changes, but also included a rotating source device based on a turntable design with a source at either end that included a source in use, with the second source at 180 degrees outside the chamber that would undergo maintenance whilst the first source was in use - the idea was to reduce the time required for source maintenance. The idea of this "source turntable" was later copied by General Vacuum on machines sold in US.

 

There were drawbacks to maintaining a source under vacuum close to operating temperatures. Although it reduced source damage through thermal cycling of the sources there were issues with cooling shields, and once a cycle was broken the benefits were quickly lost.

 

Aerre machines had an interesting layout that was intended to keep unwind and rewind reels at floor level to reduce operator workload, but which also used a series of potential airlocks to introduce the web into a central chamber area that was held under vacuum. This could be modified to air to air use with some design changes to the unwind/rewind chambers. Amsterdam Metallized Products use an Aerre machine that demonstrates the layout, and I have some discs from Aerre that also show the schematic if you have an interest.

 

Dr William Llewellyn
Vice President/Senior Consultant
AWA Alexander Watson Associates BV
Tel: +44 (0) 1443 42 12 57
Fax: +44 (0) 1443 42 12 57
E-Mail wllew@aol.com

Web edge quality

I was discussing winding recently and was surprised to find out that web breaks were more common than I had thought.   Web breaks had largely disappeared from my radar as over the years the tension and winding controls had been improved and the occurrence of web breaks had reduced to being infrequent.

So what has changed?   Where before the web was being simply metallized and sent on now there are a significant number of vacuum processes that require multiple processing steps. This increase in handling has resulted in some rolls being damaged either by other processes not winding the web well or during transit.  As for each process there is a loading and un-loading step where the roll has to have chucks fitted or removed. Between processes the roll is likely to be put into storage. 

Thus if web breaks have become a frequent event it may be worth reviewing the whole manufacturing process including how the roll is handled throughout.  Most web breaks occur through edge damage that starts a tear that spreads across the roll. This may be exacerbated by uneven tension and so poor quality web profile, or curved film, are more likely to be at risk of web breaks if handled poorly.  When handling rolls it may be worth considering using temporary cheek plates to prevent accidental edge damage.  This does not mean that using temporary cheek plates is an excuse to then not review how the rolls are lifted and located into storage racks and for operators not to be better trained in how much care is needed to preserve high quality edges.  

Atmospheric deposition

We are in an age where atmospheric deposition is being used in more and more different ways.   The one that is perhaps getting the most publicity is the printing of the copper indium gallium diselenide by Nanosolar.  This process uses a slurry of powder in a liquid that is dried into a coating which is then heat treated to both recrystallized the coating but also, I think, is used to add the selenium.  This is not really any different to the deposition of some of the Ormocer (Organic Metal Oxide Ceramic) or Sol Gel type coatings but where those coatings have almost been searching for a suitable high volume product to publicise to process Nanosolar have been easily able to get publicity because they produce solar cells. 

            The atmospheric deposition options also include the use of atmospheric plasmas.  These plasmas can take a variety of different forms. One that gained a lot of attention about ten years ago was using flame deposition.  In this case the precursor was injected into the gas flow for the flame and the flame provides the energy to decompose the precursor material.  This process, as might be expected, is a hot process and there was always a problem in getting sufficient coating thickness whilst keeping the substrate temperature low. 

            This nicely leads on to the current atmospheric plasma deposition source that is making progress which is where the corona type plasma treatment process has been modified to stabilise the plasma, reducing the propensity to arcing and hence improving the plasma treatment process as well as providing the possibility of depositing coatings.  Work has been done to deposit coatings from the same precursors as used in vacuum deposition processes. One of the target coatings is silica which is used to make transparent barrier web materials.  These transparent barrier coatings are slower to deposit than aluminium metal and so are more expensive and it is thought that if the coating could be deposited at the same speed but without the expensive vacuum system the coating costs would be reduced.  It has always been the aim to bring the transparent barrier costs down to equal the aluminium metallization costs.  Once the proof of principle had been demonstrated for one material there is the immediate extrapolation that the process can be used for almost any material that can be decomposed from a liquid or gaseous precursor.  Once coatings such as indium tin oxide, aluminium doped zinc oxide, titania or silica can be deposited it opens up a huge opportunity for all kinds of products.  In the area of displays, electronics and solar cells there is a wish to use roll-to-roll coating processes and all atmospheric deposition processes.  There are a number of polymer coatings that can be deposited by printing but currently they are deposited onto vacuum deposited back contacts or coated with transparent front electrodes that are also vacuum processes.  Again it is perceived that the vacuum coating step is an expensive one and so to take cost out of the production process it needs to be replaced by an atmospheric process.  So we have a number of large markets that are interested in atmospheric deposited coatings provided they can come close to matching the performance of the coatings. Currently the costs are not as low as hoped because of the high throughput of helium used in the process to help stabilise the plasma and also the coating deposition rates are low and so the deposition zone is much larger than for the vacuum deposition process. With these limitations this process has not yet become the deposition process of choice, which remains magnetron sputter deposition, but I would think this is only a matter of time.  As developments allow more efficient deposition and ways are found to minimise the use of helium the costs will fall and the use of the process will grow quickly. 

     

Water in roll-to-roll vacuum systems.

            All systems contain water but this will be a variable depending on the system, the operating philosophy as well as the local atmospheric conditions.  When a system is pumped out the volume of gas is removed which primarily leaves whatever is outgassing from the surfaces as the remaining gas load which in most systems will be water vapour.  Thus internal surface area becomes important as the greater the surface area the greater the amount of moisture that can be absorbed onto the surface.  This then leads us on to the operating philosophy or more specifically how frequently and well the system is cleaned.   As any stray deposition material coats the surfaces it massively increases the surface area and so the amount of water that can be absorbed. Thus dirty systems take longer to pump out than clean systems.

            Water is just well enough bonded to the surface that it does not desorb very quickly bit takes time. The speed with which the water is desorbed can be increased if energy is used. This can take many forms. Many high vacuum systems simply use heaters either as heater tape wrapped around pipework or radiant heaters. With vacuum roll coaters this would be too expensive and slow and so energy is usually supplied inside the system.  A quartz lamp will give out ultraviolet light which has enough energy that can be absorbed on the surfaces helping to release the water. Similarly striking up a plasma can achieve the same result both from the energetic bombardment of the surface but also because a significant output of any plasma is in the ultraviolet. 

            An additional water load can be introduced with the roll of material. There are two parts to this water load. The air entrained between the layers of wrapped substrate will contain air of which the water content is usually between 0.6% and 6% depending on the humidity prevalent during winding of the roll.  The second component of the water content of the roll is whatever is absorbed within the bulk of the substrate.  This is a more difficult amount of water to remove as it takes time to diffuse the water from the bulk of the material to the surface which can then be removed.   The substrate material will determine the amount of water that is contained which for some is almost nothing but for others can be more than 1%. As the water at the surface is not removed in vacuum until the surface is exposed there is no driving force for the water to diffuse out of the bulk until the roll is unwound. The time the surface is exposed for is usually too short and so very little water form the bulk will diffuse to the surface and be removed. This means that as the roll is wound back up there is usually still plenty of water left in the bulk to diffuse to the surface whilst the material is in the rewound roll. Thus if it is to be unwound again the pressure rise due to the release of water from the surface will be almost identical to the pressure rise the first time the roll is unwound.  Thus for processes that are thought to be sensitive to water it may be that the substrate needs to be heated under vacuum and wound through very slowly in order to remove as much water as possible before the deposition process.

            Also for any process that is sensitive to water vapour the results of the deposition can vary from run to run as there are multiple variables. The cleanliness of the system and hence the surface area and amount of absorbed moisture, the humidity of the atmosphere will vary from season to season and on humid days it will be easier to absorb more water onto surfaces than on dry days and finally the water content of the roll both in terms of the humidity when the roll was wound as well as the water contained din the bulk of the material. Thus unless all of these are considered and ideally controlled it will be hard to run a reproducible process.    

 

 

Diffusion pump problem.

When pumping a Balzers 500a diffusion pump it gets to all the right settings and pressures but once I go into high vac and bleed in gas for an ion etch the chamber pressure starts to rise and the diffusion pump starts to stall. Could this be a cooling problem with the pump?

 

Answer.

I would start with the usual checks of the diffusion pump oil level and the backing pump oil level.  I would also check the oil quality. There are some cheap oils around that are not very good, some fractionate and lose a proportion to the backing pump.  The backing pump can also lose performance if it becomes contaminated with water and so check the colour of the oil. If it is a milky opaque white/straw colour then it may need to be gas ballasted to remove the water.

 

I would expect that if it was a problem of the cooling water the pump would have over heated during the pumpdown and you would have seen a problem of backstreaming with the internal surfaces feeling oily to the touch. If this looks to be the case look to see the maintenance records and check the time between topping up or changing the diffusion pump oil and if the time is significantly shortened then consider the water cooling as a problem.  The other method of checking the water is to disconnect the water output from the pump and measure the flow and the temperature.  Check the flow against the required flow and that the temperature rise is within the recommended limits.

 

The characteristics you describe would suggest to me a loss of quantity of diffusion pump oil or loss of backing pressure.

Problems with TiN colour consistency

The Ti film varies from the silver-grey metal until enough nitrogen reacts with it to change the colour.  If the TiN film is a lower-density columnar structure, the colour will go from silver-grey to dark bronze to brown.  If it is a denser structure, you will see it go from silver-grey to bright golden as nitrogen consumed.

Where variation in colour can occur is when the process is not constant.  It is common to believe a process is constant but in reality there are often small variations that are present that may go ignored. 

Typically any vacuum system will become coated as the stray coating builds up on surfaces and this coating will be very porous. Thus when the system is brought back up to atmosphere after the coating process and the vessel opened the moisture in the atmosphere will be absorbed onto the surface. This water will be present during the next pumpdown. This occurs at the end of every deposition run. As the coating builds up the amount of water absorbed will be greater. The net result of this is that the pumpdown will take progressively longer to reach the same base pressure or, alternatively, after the same pumpdown time the base pressure would progressively get worse.  Periodically the chamber will get cleaned and this again will change the pumpdown performance. 

The moisture in the atmosphere will vary with season. In the high humidity seasons more water to be absorbed onto the vacuum system surfaces than in the low humidity seasons.

Other things will change with time, such as the erosion track on the sputtering target which will change the current density.  The vacuum system may also develop vacuum leaks. The gases, if being supplied by bottles may also have contaminants. It has been known for bottles to have residual moisture in the bottle so that as the gas content of the bottle falls the moisture content becomes a higher percentage.  This does depend on your bottled gas supplier and the quality they work to.

You gas inputs also need to be in balance with your sputtering requirements with the nitrogen being allowed to vary to match the sputtering rate. If you have an optical monitoring control system this will help.

Thus as you can see from some of the variables above your colour variation may simply be that you do not have as much control over your process as you may require.  If you are able to connect a Residual Gas Analyser you will be able to monitor all the gases in the vessel and check to see if the water levels vary and of there are any leaks.  Note that oxygen either from an air leak, or from water dissociated by the plasma, will oxidise the titanium and affect the colour.

I hope that this help you diagnose your problem.