Question about retrofitting a glow discharge

Hi, We have a very old Dynavac Evaporative Metalliser and would like to obtain better adhesion in our substrates by building a Glow discharge. While the principle is simple (DC transformer connected to an Aluminum rod and the chamber, then keep the vacuum level low while energizing). Looking at your diagram on page 229 of your book the ideal setup would be to have Vacuum at 10 -1 with voltage at 900 DC, to achieve this vacuum we would install a bleed value at the opposite end of the chamber to the pumps. My questions are does this setup sound ok and can we use filtered air instead of an inert gas, or do you suggest some further reading before we jump into this?

Answer

The glow discharge sounds as if it would work however if I were to be modifying a system I would aim for a magnetically enhanced plasma to treat the surface. The power requirement can be the same but the plasma density with the magnetic enhancement will be greater as it can run at the lower voltage but will carry a greater current.  The design is very similar to a magnetron source with the magnets making a magnetic circuit for the electrons to race around.  The higher density plasma will give a faster treatment time than the glow discharge and so is more useful at the faster winding speeds.  It will also operate down to lower pressures and so can be more flexible in where it is sited in the system.

As to gases, any gas will allow a plasma to be struck but the surface treatment will be dependent upon the gases present.  Argon is a heavy atom and when it strikes a polymer surface it can break bonds but argon is inert and so cannot react with anything on the surface. Thus it can hit contaminants on the surface and may crosslink them to the substrate or may further fragment any short chain molecules or oligomers. To remove any hydrocarbons the plasma needs to contain a reactive gas, oxygen for preference which can react with the hydrocarbons to form volatile species that will desorb from the surface and can be pumped away.  If you have a roll of polymer in the system there will always be some oxygen around from the air trapped in the roll and also from any moisture in the air and absorbed into the polymer. The water will be cracked in the plasma and more oxygen released.  Thus you introducing dry, filtered air will provide the surface with two gases both of which are oxidising gases with oxygen being more active than the nitrogen.

Practically if the pumps are sited to the side of the winding system there will be a small pressure gradient from one side of the chamber to the other. When you introduce the gas there is a danger that you will exaggerate the pressure gradient. Sometimes I have seen a tube used with holes drilled in the tube to allow the gas out. By suitable positioning of the tube and varying the size of the holes the pressure can be balanced. There may still be some variation across the web from plasma generated by-products which will be present at a higher proportion towards the pump. 

If you use a magnetically enhanced plasma the system can be run at a lower pressure and this will reduce this effect somewhat.