We purchased a 70 atomic percent silver: 30 atomic percent gold sputtering target, however when I sputter I consistently get the ratio of 55% silver: 45% gold by atomic percent.
I have tried sputtering between an Argon pressure of 5mT to 20mT, and using a power density between 5 and 15 watts/cm^2.
Do you have any advice? I was told sputtering tends to produce the same ratio as the target, however after many trials I consistently get a different ratio than the target itself.
Thank you for any advice
Unfortunately it is often the case : the sputtering yield of Au and Ag are not the same and it is not straightforward to get on the substrate the same atomic ratio than that of the target.
You can try to change also the target to substrate distance, or you can (if possible in your chamber) use an off-axis configuration : that is when the target (cathode) is mounted perpendiculary to the substrate (anode). Such configuration was successfully used in case of complex oxides like YBaCuO. If you have a multitarget system, the simplest way is to use one target for each element, or to add an Ag target to your present Ag/Au target in order to get the 70% Ag that you expect.
Despite the different sputter yields of Ag and Au, the flux of sputtered atoms leaving the target will - after a short initial period of preferential sputtering - correspond to the target composition, provided that target cooling is sufficient to suppress diffusion. However, scattering of sputtered Ag and Au atoms due to collisions with Ar atoms will be different. It is likely that the lighter Ag is more effectively scattered than the heavy Au. Thus, you will have less Ag in the film, which corresponds to your experimental finding.
You may consider to reduce the Ar pressure (which will increase the mean free path and thus reduce the number of scattering events) and reduce the target-substrate distance. Both should contribute to an increase of the Ag content in the film.
I agree with Christian Mitterer on the preferential sputtering at the target.
But what's about the substrate? You might have a selfbias at the substrate which could occur in RF sputtering. Maybe you can check the composition with grounded and "floated" substrate.
As has already been commented, the deposit tends to follow the preferential sputter yield of the material rather than the target, and gold is by far the easiest material to sputter.
I do wonder however why you are using RF to sputter a metal as it would not be the most obvious choice, and you may find things improve slightly in your favour if you used DC.
Indeed Phil Denby, DC sputtering might be better in this case.
Concerning preferential sputtering at the target: As Christian Mitterer explained, after a while the outgoing flux ratio at the target should be the the same as the composition of the target.
However this is not the case at the substrate during growth and with a bias.
Dear Mr. Keller. You mentioned the composition will not be the asme as the composition of target with a substrate bias in this case, if I understood coreectly. I know that the bias plays a role in the crystalline of film. But could you explain how the substrate bias(DC or rf?) affect the film composition? Many thanks !
Dear Mr. Liu, the preferential sputter process at the target will after a short time stop otherwise you will end up with a Au target (see comment of Mr. Mitterer.
On the substrate side however you always start fresh layers with the initial target composition so preferential sputtering will continue to occur during film growth.
I recommend that you try with dc instead of rf, you might get slight improvement
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