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.