Hi,
I am using a pulsed DC plasma (~10 kHz pulsing frequency) with CH4 and H2 in a simple plate condenser set up. I use pressures of a few mbar and temperatures of 700°C-900°C. If I use non conducting substrates (e.g. glass, quarz) I see a dark spot above the substrate in the plasma (see attached pictures). What could be the reason for this dark spot? Does the substrate charge up and thus no plasma can be sustained? Or is it because there is no plasma sheath and thus no potential drop at this spot? Does anyone know something about such a plasma?
Look up Debye Sheath. There should be quite a bit of literature available. Basically it is caused by the accumulation of positive ions at the surface as a result of their relatively slower speed in the plasma than electrons. This charges the surface negatively and creates an ionic double layer boundary.
Hi @Ron Reiserer,
thank you for your answer. Isn't it an accumulation of electrons on the surface of he substrate? Then a negative potential is built up and thus free electrons are getting repelled by the negative potential of the substrates surface? That could be the case.
So lets pretend that my bottom electrode has a potential of -500V. Thus there will be a relatively big sheath, because electrones are getting repelled by the bottom electrode. If I don't see that sheath above my substrates, this means that either the sheath is extremely big or way smaller than the sheath around the bottom electrode. Case 2 seems more probable. So this would mean that the potential of my substrates surface is way smaller than the -500V of the bottom electrode right?
Electrons do accumulate on the substrate surface causing some repulsion of electrons, but also, attraction of more dense positive ions which are moving slower and easily absorb the energy of the incoming electrons, quenching the plasma formation in this area.
May I know what voltage anode and cathode are biased? when I am experimenting with the semiconducting materials inside PECVD chamber, the plasma sheath interestingly only forms above the substrate. I have not come across the situation like yours . However, my situation is exactly opposite to yours. I really want to form the plasma uniformly above the susceptor but seems like plasma sheath gets accumulated only above the Si substrate.
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