What exactly changes when we use 40 KHz, 13.56 MHz or 2.4 GHz plasma source for plasma surface activation of polymers at low pressure in CCP-RF type plasma ?
Why 40 KHz gives better results compared to 13.56 MHz plasma ?
When you go to GHz (microwave discharges), the period of the electric field is too short for any particles to gain some momentum - even the electrons just "shake" and don't move much across the volume. These discharges are then closer to equilibrium having the temperatures (electron, excitation, vibrational, rotational and kinetic) rather close compared to the lower frequency.
When exciting with MHz (RF), the electrons now follow the electric field, usually managing to move across a substantial part of the reactor, forming sheaths at the edges. The ions, on the other hand, are still not able to follow and usually are considered static. The plasma is usually "on" the whole time.
In kHz, the period is already so long, that the plasma often quenches for some part of the period. The ions are now able to follow the field, but often the recombination is faster than the drift (at least for high pressure).
As for your last question - "Why 40 KHz gives better results compared to 13.56 MHz plasma ?" - to answer this for a particular case, serious effort is required. Often you need to combine extensive plasma diagnostics combined with modelling and look for correlations with the activated sample properties - making enough results for several papers. Here, the answer is almost never obvious.
If you have good results at such low frequency, I would check for thermal damage of the samples at higher frequencies, or try to arrange the treatment not be the "glowing" plasma but the afterglow.
When you go to GHz (microwave discharges), the period of the electric field is too short for any particles to gain some momentum - even the electrons just "shake" and don't move much across the volume. These discharges are then closer to equilibrium having the temperatures (electron, excitation, vibrational, rotational and kinetic) rather close compared to the lower frequency.
When exciting with MHz (RF), the electrons now follow the electric field, usually managing to move across a substantial part of the reactor, forming sheaths at the edges. The ions, on the other hand, are still not able to follow and usually are considered static. The plasma is usually "on" the whole time.
In kHz, the period is already so long, that the plasma often quenches for some part of the period. The ions are now able to follow the field, but often the recombination is faster than the drift (at least for high pressure).
As for your last question - "Why 40 KHz gives better results compared to 13.56 MHz plasma ?" - to answer this for a particular case, serious effort is required. Often you need to combine extensive plasma diagnostics combined with modelling and look for correlations with the activated sample properties - making enough results for several papers. Here, the answer is almost never obvious.
If you have good results at such low frequency, I would check for thermal damage of the samples at higher frequencies, or try to arrange the treatment not be the "glowing" plasma but the afterglow.
In the MHz range, generally the plasma density increase with frequency. For Capacatively Coupled Plasmas, CCP, the density goes goes as f squared. So for 40 MHz you have a lot more plasma density (maybe similar to the results for 13.56 MHz for 9 times the plasma time).
When You go to Microwave frequencies the plasma is wave driven and quite possibly of lower density than at 40 MHz.
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