I have Ti-Al alloy that is sputtered onto a copper foil. To control the Ti-Al ratio, I will need to use an etchant to etch out some of the Ti. However, most etchants that attack Ti will also react with Cu and Al.
Kroll's reagent is the standard for metallographic preparation of titanium surfaces, perhaps somethings similar will work for your application. Indeed, transene titanium etchant from https://transene.com/ti/ looks to be somewhat similar to Kroll's (they both contain HF) and looks to be promising for your purposes.
Alternatively, if you are sputtering with Ti and Al targets simultaneously then you could simply just lower the power going to the Ti target to reduce the fraction of Ti in your sputtered film.
Andrew J Gross Thank you for the information. Actually, I use an alloy target with 50-50 Al-Ti ratio, which means they are well mixed instead of a layer by layer structure. Further, Kroll's etchant will probably attack Al, and Transene will attack Cu. It seems difficult to do this selective etching.
For this purpose, it is probably worth trying the etchant manufactured by Transene; the supplier states that it has better than 20:1 selectivity in etching Ti over Cu.
Also, when sputtering with two targets, if the sputtering tool has multiple chimneys/power-supplies/etc. then deposition with multiple materials can be performed simultaneously to achieve the desired mixture of the target materials, rather than depositing layer by layer.
I agree with Andrew's approach, but, if etching is the only route then you might need to consider the concentration of your etchant. In the very distant past I was fabricating Dolan shadow masks from Al supported on Cu and had to etch Cu from under a layer of Al. I used Nitric acid for this but I also had to increase the concentration (and reduce the etching time) as freshly exposed Al during undercutting would also dissolve in water, thus, etching the feature I was trying to create.
Having said that, I am not sure that the structure of your sputtered alloy will a good pltform for selective etching to alter the material ratio.
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