While learning for an exam, I stumbled over the following Question:
According to Material Science of thin Films by Milton Ohring,
"RF sputtering essentially works because the target self-biases to a negative potential. Once this happens, it behaves like a DC target where positive ion-bombardment sputters away atoms for subsequent deposition."
So far so good. What I don't understand is
- why exactly a self-bias voltage is appearing?
- why this bias voltage does not lead sputter bombarding of the substrate, that should be coated?
I understand that it relates to the high mobility of electrons compared to the ions, but I don't see why this implicates the self-bias yet. The above mentioned source tries to explain it as follows:
"Negative target bias is a consequence of the fact that electrons are considerably more mobile than ions and have little difficulty in following the periodic change in the electric field. The disparity in electron and ion mobilities means that isolated positively charged electrodes draw more electron current than comparably isolated negatively charged electrodes draw positive ion current. For this reason the discharge current-voltage characteristics are asymmetric and resemble those of a leaky rectifier or diode [...]"
This is not clear to me, as I would think that this explanation would hold for the sputter substrate as well. I would be very glad if someone could make that clear by a somehow intuitive explanation. Thank you for all suggestions. :)