The high kinetic energy of these Oxygen ions can be more than 200 eV for diode and RF sputtering, is there an effect on the crystallographic orientation of the deposited films? If so, can it be quantified without optical spectrometry and langmuir probes?
Depending on the material the ions might interact the growth by annealing the film or doping. The impact of the ions could influence the crystallinty (X-ray, electrical measurements) and as first indication it might influence the color of the plasma.
Thank you, Gunther. The ions of the plasma are Argon and Oxygen. I have no references where at medium magnetron powers (200-300 W) in a RF magnetron the tail of highly energetic ions can produce big changes (apart from H.Toyoda et al.). The Ar plasma is pink, intense for pure Argon and pale or green pale when Oxygen is added till 100% of Oxygen. I have seen very intense and bright green Ar plasmas but I cannot relate this (without optical spectroscopy) with a surface modification.
The importance of negative ions (O-) and of recoiled working gas atoms (Ar) (their effect on the film microstructure is very hard to discern!) is somewhat discussed in the following two paper in which RF and DC magnetron sputtering discharges applied in the deposition of ZnO and ZnO:Al films are systematically compared. You can perhaps find other references within as well.
R. Cebulla, R. Wendt and K. Ellmer, J. Appl. Phys. 83 (1998) 1087-95
K. Ellmer, J. Phys. D: Appl. Phys. 33 (2000) R17–R32
The authors report that the effect of negative ions on the resulting film properties should be less pronounced in the RF discharges since the cathode sheath potential drop that accelerates them towards the substrates is significantly lower than in the DC discharges operated at the same power. I hope this may help.
Thank you Matej. As I suspected, in RF systems this effect is less important, but maybe I am working in the threshold so I need to be sure. I will look at the references you gave me, thanks again!
During the sputtering process negative ions made of target material can be formed. They are generally repelled from the substrate across the sheath, but if they have enough energy they can reach the growing film. They are generally very energetic only if generated in the target sheath. This kind of negative ions have a strong influence on the growing film (as confirmed by many authors): they can cause etching, they can influence the structure by momentum transfer. It is important to underline that the negative ions which contribute to growing films properties modification are ones coming from target sheath due to their high energy (influenced by other process parameters such as the pressure in the deposition chamber, etc...).
Maria is right, the formation of negative ions in the target, which are accelerated to the substrate are referred as a common effect in the deposition process of aluminum nitride thin films when very high nitrogen percentage is used in the gas discharge. Commonly , a large amount of these ions are neutralized after acceleration and the species traveling in the gas discharge are neutrals, which moves easily because the size is lower and the interaction with other atoms lower. The additional bombardment of the substrate can generate damage and residual stress. However re-sputtering is hard to achieve because he low mass of the atoms of nitrogen.
Thank you for your answers. I have checked the \alpha\beta\gamma > 1 condition established by A.Anders, which gives me a hint with my coating precesses. Fortunately I am working in low energy regime...
We worked in a low energy RF plasma (30 W) of N2O and Ar. We were able to observe film resputtering at high N2O partial pressures (and consequently high O2- formation) . We also diagnose the plasma using mass spectrometry with a Residual Gas Analyzer. About using optical emission spectrosocopy it works fine, just look for the lines of interests that are easy to detect with an spectrometer. Langmuir probe will tell you about negative and positive ions, so placing it next to your substrate could tell you about the amounts of negative ions impinging your surface.
Dear Guillermo. The \alpha\beta\gamma > 1 condition you mention is appliable only on high plasma density discharges into which diode or RF sputtering do not classify. Unless you use very high power densities and high electron confinements, usually in pulsed power mode. You can find more details on the subject of high-density discharge operation within the following paper. Hope this helps.
Article The "recycling trap": A generalized explanation of discharge...
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