As we know oxygen rf plasma is rich in terms of O(-) ion density. I want to understand how / what way these negative ions contribute to surface modification of substrate kept on powered electrode. do these negative ions reach to substrate surface and engage themselves in any phenomena or they remain in bulk plasma only ?
This is a very tricky question as most of the time we misunderstand DC bias. We think DC bias as a negative voltage developed on the powered electrode w.r.t. plasma. This means that a +ve charged ion will be attracted towards the biased electrode, which is the reason you probably asked this question.
In reality, the sheath potential has both +ve and -ve voltage, although the amplitude of -ve voltage is very large compared to +ve voltage, so on average one sees a negative bias. Please have a look at the attached image.
My guess is that some O2 -ve ions might be reaching the substrate, but their energy must be very low. This means that if your surface is highly reactive to the O2, then it will modify the surface properties. Overall the chances are that it must not change the surface properties of your sample.
It is mostly the O2+, O radical etc. which helps in the surface modification
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