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Velocity boundary conditions for positive ions entering radio-frequency sheaths in electronegative plasmas
- Source :
- Journal of Applied Physics. 122:053302
- Publication Year :
- 2017
- Publisher :
- AIP Publishing, 2017.
-
Abstract
- Under certain conditions in radio-frequency (rf) plasmas, the amplitude of the low-energy peak in ion energy distributions (IEDs) measured at an electrode depends sensitively on the velocity at which ions approach the sheath. By measuring IEDs, incident ion velocities can be determined. Here, IEDs were measured in inductively coupled plasmas in 1.3 Pa of CF4, at rf sheath voltages up to 100 V at 1 MHz, obtained by biasing a counterelectrode. From measured IEDs and sheath voltages, we determined the incident velocities of all significant positive ions: CF3+, CF2+, CF+, and F+. At higher bias voltages, we detected essentially the same velocity for all four ions, suggesting that some collisional process keeps different ions at the same velocity as they emerge from the presheath. For all four ions, measured velocities were significantly lower than the Bohm velocity uB and the electropositive ion sound speed cs, because of negative ion effects. From the measured velocities, an upper bound for negative ion temperature is obtained. The velocities determined here do not agree with boundary conditions that have been previously proposed, because the latter neglect either the reduction in ion sound speed due to negative ions or the acceleration that occurs as ions pass from the point where quasineutrality is violated to the point where electron density becomes negligible. Both of these effects are treated to fair approximation, for collisionless sheaths, by setting the initial velocity to twice the ion sound speed modified by negative ions.
Details
- ISSN :
- 10897550 and 00218979
- Volume :
- 122
- Database :
- OpenAIRE
- Journal :
- Journal of Applied Physics
- Accession number :
- edsair.doi...........e9f3dca21407df38321a840a37a6617b
- Full Text :
- https://doi.org/10.1063/1.4997168