I sputtered Ni-5.5Al (at%) targets onto a glass substrate, which was preheated to 250C before sputtering. Sputtering time is 60 minutes.
The thickness of the thin film is about 510 nm. The composition of the thin film, determined with EDX is Ni-7.1(at%) while the grain orientation is different from the bulk.
I am trying to understand how this happened. I came across the term "preferential sputtering", however I did not manage to get enough information regarding this phenomenon.
Does this mean that, although it is an alloy, during bombardment, Ni and Al will be split, which causes different atoms to reach the substrates earlier, hence the difference in composition and grain orientation? I did read that pure Ni atoms have very low mfp, but not sure about Al. Could this difference lead to this phenomena?
Also, I am wondering if the heat from the substrates provided energy to the atoms to change the orientation. The orientation of the thin films did not match with any Ni and NiAl alloys. I wonder how would I determine the orientation of the thin films?
Regarding your comment on preferential sputtering you are correct, and it is because of every atom has a different scattering cross section. Also, the temperature of your substrate matters to allow kinetic stabilization of your film. Thermodynamical equilibrium may also be necessary for create the composition you want thus making critical at which temperature your substrate needs to be. The following paper will get you started. Cheers.
Thank you, Mr. Morales. I really appreciate it.
I just went through the paper. Just wondering, it showed the sputtering yield of compound (GaAs) and Ta-oxides. Will the same physics apply on a metal alloy (NiAl) ?
Hello. In practice you need to do at least two tests. 1) slow down your deposition rate maintaining your current substrate temp or 2) increase substrate temperature and maintain your current deposition rate. For 2) I advice you to check on a NiAl Phase Diagram to confirm at what temperature your preferred phase forms. From the phase diagram you should be able to find a better temperature starting point for your substrate. Cheers.
Preferential sputtering relates to differences in atomic masses, surface energy, bonding strength and it will cause an offset in composition of the film and the sputter target used. It can be mitigated by process conditions (sputter gas, energy) but usually needs to be accounted for when choosing the target composition.
Orientation of the grains in a film is affected by the substrate if the process is close enough to thermodynamic equilibrium (250C may be be insufficient) but also sputtering angle, orientation any magnetic field (even if the final composition appears not very magnetic).
Orientation of grains in the bulk can be also different depending on process conditions.
Useful reference: M. Ohring "Materials Science of Thin Films" (Academic Press, 2002) ISBN 0125249756
Mr. Morales, Mr. Lapicki, Mr. Linss,
Thank you for the feedback.
1. When you refer to the effect of "angular distribution", does it refer to the angular between the target (gun ) and the substrate? Would a target-substrate angle at 90 degrees (substrates directly below the target have the highest yield?
The DC sputtering equipment that I used was designed so that the gun is situated around 7 cm from the rotating table, slanted about 30 degrees of the substrate. However, during my sample preparation, the target to substrate distance was set to be 3 cm. Under this condition, although still slanted, the target is on top of the substrate, hence would the angular distribution still take effect?
Also, there is no substrate bias. The substrate is also not rotated.
2. The composition of the bulk was initially analyzed using ICPMS, then re-checked using EDX when I analyzed the sputtered surface condition. The comp data is similar.The comp of the thin film was analyzed using using EDX only.
By the way, is there a database where I can obtain the sputtering yield of different elements? I have some papers on Co, but no data on Ni or Al, or NiAl alloys.
Mr. Linss, Thank you for the link.
When you say matrix, I assume that you are referring to the grain orientation? As in FCC, the sputtering yield (S) is :
(111) > (001) > (011)
Attached is a sketch of the condition of the sputtering gun (Target) and table (substrate). The glass substrates are put directly below the gun. I really appreciate your feedback.
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