I wonder if someone could explain why, in RF sputtering the target material is preferentially sputtered! I understand how sputtering works, but with an AC electric field I cant quite grasp why material is preferentially sputtered from the target and not equally from the chamber walls!
I understand from doing it that a bias voltage "appears" on an insulating target, which I can see would attract the argon ions for the sputtering process, but im not clear how this forms. And what would for example happen if you tried RF sputtering a conducting material? And why does an RF plasma seem to go everywhere in the sputtering system (compared to a DC plasma which is very localised.)
With rf sputtering you can sputter (metals, insulators and semiconductors)
you mention preferential sputtering, which means the target is probably multicomponent/multielement. Naturally the element with a lower binding energy will sputter out much faster in comparison to the element which has a higher binding energy. This is known as sputttering yield. For example lead will sputter out much faster than zirconium or titanium.
Please refer to some good books written by (Chapman, Maissel and Glang, and Bunshah). You will get a lot of information.
Insulating targets (containing inorganic oxides), when sputtered, will definitely loose oxygen very easily, and become oxygen deficient. A white colored target will soon become brownish/yellowish.
DC bias generation. During RF sputtering a DC bias is generated on the target, irrespective of the fact whether it is metal/semiconducltor/insulator. In an rf plasma you have both ions and electrons. The electrons closer to the target are vibrating much faster at the rf frequency, and thereby will be hitting the target surface more number of times in comparison to the more heavier ions. As a result the electrons when hitting an insulator target for example will leave a net negative charge on the surface, and this negative bias will allow the attraction of the positively charged ions, and allow the sputtering of insulators to proceed in a typical rf sputtering process.
I don't think the plasma is going everywhere. The bombardment is occurring only on the target surface, and is focussed onto the negatively charged target in case of DC sputtering, and in case of rf sputtering the imposed DC bias as explained above attracts the positively charged heavier argon ions. All that you are seeing is the glow in the chamber.
K. Sreenivas
As the ions in the plasma (Argon) is of very low energy (
Please note in Rf sputtering we do not apply any bias.
Ions are less mobile than electrons. They are all under the same field, same frequency. Electrons with negative charge, and being more mobile than ions, are hitting the target more often, and creating a self bias. Kindly refer to some good books which explain this in detail.
K. Sreenivas
Many thanks k. Sreenivas RE bias voltage. I think that is the key I have been missing. (Saldyi have not found any good books on RF sputtering yet, that dont simply gloss over the matter!)
One strange thing we have noted however when initially sputtering some oxides, they appear to have negligible bias generated on the target (of the order of 2v) but the deposition rate is actually good. Yet I dont see how there can be sputtering in the absence of a high target bias. (I could understand the lack of bias being due to surface contamination / lack of oxygen.. but the sputtering...)
- Badges
- Science topic
- Similar topics
- Filing