Vacuum Web Coating

Optical density follow-up question

I have a similar issue of measuring the density of Ni layer on Tin. Can anyone tell the equipment to be used and give out more the experimental details?

Answer.

There are a number of options that can and have been used for depositing multilayers.

Both materials are conducting and so you have the option of using other measurement techniques of which the most widely used is to measure the resistivity.

The first step in this process is to deposit a series of coatings of differing thickness to span the range from too thin to too thick for your application.  On each of these coatings measure both the optical density and resistivity to give calibration curve you can then use the resistivity measurement to control the optical density.  This can be repeated for the second coating.  There are some options here, one is to take the first metal layer at the thickness/OD that you want and then repeat the series of different thickness coatings of the second layer on top of this background conducting first layer coating.  You then will have a calibration curve for the conductivity and OD of both coatings combined with the first layer optimised.

This process can be done for the optical density by measuring the transmittance for the first layer and after optimising the first layer for the combination of layers.

A third method depends on the sources used.  If you are using a wire fed resistance heated boat source or an electron beam deposition source this will not be appropriate. However if you are using a sputtering source for either of the coatings it is possible having produced a similar series of thickness coatings to not only have a calibration curve of resistivity vs OD but also to have the voltage/current/pressure/winding speed information for each thickness of coating.  As magnetron sputtering sources are very stable many use the voltage/current/pressure/winding speed information directly to deposit a particular thickness coating and do not necessarily measure the coating performance but trust the settings to deliver the correct coating.  If your system is not so precise then it would make sense to confirm the deposition by in-vacuum monitoring.

If you are looking to a company that can install a system to monitor OD then I would suggest Mr. Tim Emmerich of NAGY Messsysteme GmbH who would be able to provide a suitable system.  Tel: +49-7032-76670 * Fax: +49-7032-72189
emmerich@nagy-instruments.de      http://www.NAGY-Instruments.de

I hope this helps.  If others have other experiences that provide a different view of the problem please feel free to add them.

Question re. Aluminium oxidation.

The question.

I am student of packaging institute presently working in multinational company.please tell me how can we prevent oxidation of aluminium in metallized films. Is there any lacquer or coating available to prevent oxidation.Thank u very much in advance for ur help.

Answer as follows.

Aluminium is highly reactive and oxidises whilst it is within the vacuum system. Even at metallization pressures there is oxygen present that will bombard each surface with a monolayer of oxygen per second. Thus by the time the aluminised material is removed from the vacuum system the oxide on the aluminium will be of the order 1nm thick. 

The rate of oxidation slows as the oxide layer builds up.  The oxide is less dense than the metal and so the oxide as it forms swells on the metal surface & so the oxide is in compression.  This is what makes the oxide a good oxygen barrier that slows down the continued growth of oxide.  Thus the rate of oxide growth is approximately as follows;

Monolayer             =            immediately

1nm                  =            1 hour

2nm                  =              1 week

3nm                  =             1 month

5nm                  =            1 year

Essentially there is nothing you can do to prevent the oxidation of the aluminium.  Overcoating with lacquers may slow the process down but it will be less effective than the native oxide.  Where overcoatings may have a part to play and that is where metallized coatings are bent around very sharp radii or even used with a dead-fold. If the aluminium coating breaks or cracks the fresh surface will immediately oxidise if exposed to the atmosphere. An overcoating can change the neutral axis making cracking less likely and may also reduce the speed of oxidation if cracking does occur.

Thus the short answer is you cannot stop aluminium coatings from oxidising. 

If you believe there is a problem of oxide growth the problem needs to be specified more accurately. 

For example, if the post oxidation is causing the optical density to change it may be that the aluminium thickness needs to be increased to compensate for this. This may impact the vacuum metallizer throughput and the material costs. 

Question re. on-line monitoring.

Is there a system of checking for number of pinholes\weak spots in a metallised film online ie.during metallising\slitting?

The answer is yes & no.

Yes it is possible to monitor the defect levels on-line however there are limitations.

The systems often use scanning lasers with strategically placed detectors.

On wide metallizing machines, that in some cases can metallize at up to 1250m/min, the methods used may be able to operate at the same speed but at the loss of some resolution. Hence many smaller pinholes would not be measured/counted. 

Part of the problem comes with the combination of web width, web speed and laser spot size. To raster a single laser across the whole surface quickly, and still detect the signal, can only be done by broadening the scanning beam which loses resolution. The alternative would be to restrict the scanning to a smaller area & either use this lane as being representative of the whole roll or to have multiple laser systems side by side to give each a shorter rastered scan length and hence retain the fine spot size and resolution.

If you want to talk to someone who is an expert in this field try Tim Potts of Dark Field Technologies, Inc   tapotts@darkfield.com