How to remove aluminum's stubborn oxide layer?

[Newbies_Kid]

I want to make a multilayer coating on Si-wafer using RF magnetron sputtering. My targets will be Cu, Al, Ti (in sequence from the top) and my machine capability is only one target per sputtering process. So i assume a problem will arise on aluminum layer because i'm certainly sure that the oxide layer will instantly formed as soon as i open the chamber to change the target for the next layer. I do some study and found that the oxide layer is strongly bonded and have a porous structure. My first idea is to put a negative bias and back-sputter the Al film before proceeding to Cu layer. But i don't know how much energy should the Ions (Ar+) have to clean out the oxide layer and i also afraid if the film will contaminated by Argon ions due to the porous layer of aluminum oxide. My second idea is to do some acid pickling on the film but i'm not sure what solution is effective. If you have any experience or suggestion, please feel free to reply my post. THANKS~~~

[John Cuthber]

Since the oxide layer reforms very quickly you would need to remove it under UHV conditions. That rules out acid pickling.

[Newbies_Kid]

Yeah, i know the oxide would appear in just some mili sec, but how should i remove it under uhv? After deposition, definitely i will open up the chamber's door to change the target. Even before that, i guess the oxide is readily formed as i stop the vacuum pump when finish depositing.

[johnheritage]

In chemistry, the oxide problem is navigated by treating it with a mercury salt, like the water soluble chloride. That forms an amalgam which prevents the oxide forming as the aluminium takes part in the subsequent reaction you want it for. Mercury is a noble metal, like silver, gold and platinum. The mercury is also gradually rotting through the aluminium. And, if the salt somehow finds it's way onto your skin or into your body, rotting through your neurons; as it is a powerful inhibitor of axonal growth cones.

 

For your application, I don't think washing it or messing with mercury is the way to go.

 

Ideally, you want the pure surface generated inside the controlled atmosphere, where it will remain pure. One method would be to apply the bias. I can't really comment on the argon ions as I have no idea how they're affect the sputtering performance, or if they'd even be a problem. You don't need to backsputter aluminium to remove the oxide. If you think of GTAW welding (TIG), the oxide layer is removed simply by switching the polarity of the power supply (the direction of the electrons); which is done automatically by the power supply. TIG welders don't like running the torch in the 'cleaning' mode constantly because it makes the electrode / torch get really hot, rather than the work piece.

 

Another options is to let the oxide form, but blow a reducing gas in to clean it's surface. Possible candidates include hydrogen and carbon monoxide. Both fire hazards and the monoxide is also neuro and metabolically toxic.

 

Situating the aluminium in specific positions may also be of benefit. Argon is heavier than air. If it is low down in the chamber, it will tend to remain covered in a blanket of argon until disturbed.

 

Also consider using higher purity shielding gas. If the chamber has been vacuumed out and backfilled three times with pure argon, and then pulled down to UHV, there should be very little left in there to react with the surface.

Edited April 23, 2011 by johnheritage