I am attempting to make structural bonds using silver thin films. I have achieved excellent adhesion of refractory metals to the substrates, which are glass. The glass is first cleaned in a sonic bath with acetone, ethanol, and DI H2O and then spun dry.
Using thermal evaporation under high vacuum (10-6 torr working pressure) Cr, Ti, and Ni have been independently used as adhesion layers and all have excellent adhesion to the substrate and pass the standard "scotch tape" peel test and hold up to mechanical abrasion. However, the silver layer that immediately follows the intermediate layer without breaking vacuum does not adhere well to the intermediate layer and fails the scotch tape test. The silver layer is a thickness 200-500nm and likewise deposited using thermal evaporation at a high vacuum working pressure.
I have tried both high and low deposition rates for the Ag layer and varied the amount of the intermediate layer from only a few nm to 100s of nms. I have also tried heating of the substrate to 200C for deposition of both the intermediate layer and the Ag layer.
Can anyone offer some insight into how the adhesion of the Ag film to the intermediate layer might be improved? Has anyone had success with the formation of an intermediate alloy, maybe Ag-Sn, at the interface with the interlayer? Perhaps growing the adhesion layer with a texture, ie. at a glancing angle of deposition, may increase the interfacial area between the intermediate and the Ag and improve adhesion?
Best,
Steve
Looking at the binary phase diagrams of Silver-Nickel suggests that below about 700 ºC there is almost no solubility for silver in nickel. This means that the two materials would be immiscible and likely to form a sharp interface. This in itself may not necessarily be a showstopper, but at the vacuum you are using, you may still be getting oxidation of the substrate (Cr in particular) which could reduce the adhesion.
If you look at the phase diagram for Al-Ti, which shows a good solubility for silver at temperatures of 600 ºC+ (suggesting a reasonable amount at NTP), which ought to mean that the silver has some chance of sticking. Again, however, at these pressures, the titanium is very likely to oxidise, potentially affecting adhesion.
I couldn't find a phase diagram for Al-Cr. I don't know much about oxidation of Nickel at 10-6 torr.
If you could improve your vacuum, it may help. Heating the substrate may allow some alloying, but also may conversely increase oxidation.
Hi Stephen Peter,
one technique to improve your interlayer adhesion is to deliberately roughen the surface of your Ti, Cr or Ni. The idea is to increase the effective surface that binds your main deposit to the adhesion layer.
One of the best ways to do this is by applying a gentle plasma mill. I used to roughen the surface of my substrates (of diverse nature) by means of an Ar plasma in a sputtering chamber. I did this to improve the adhesion of Ti to the substrate, but if your Ag to Ti/Cr/Ni is poor you can use this technique after the deposition of your intermediate layer and prior to Ag deposition.
Hope this helps,
Giuseppe
Surface roughening is best acquired treating the surface with low energy inert ion beam bombardment that can be done using ion accelerator or even Plasma ion implantation, as suggested above by Gluseppe.
If the Ti and Ag are deposited from separate sources, would a brief co-deposition of Ti:Ag provide better adhesion than a strictly sequential deposition?
We are planning to try a deposition of TiW alloy using sputtering and then follow up with Ar ion bombardment to roughen the surface. We may use an e-beam with arc suppression simultaneously with Ar substrate bombardment during the initial few nm of Ag deposition to try to muttle the interface between the Ag and the TiW.
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