Attachment-induced ionization instability of a radio frequency excited discharge in oxygen

Abstract

The plasma of a 13.56 MHz capacitively coupled oxygen discharge of the Gaseous Electronics Conference Reference Cell type exhibits fluctuations of the plasma potential and of light emission in the kilohertz range. This behavior can be explained in terms of an attachment-induced ionization instability in connection with a sufficiently high fraction of negative ions. A necessary condition for the appearance of this type of instability is a strong positive dependence on electron temperature of the electron attachment coefficient describing the formation of negative oxygen ions. A strong volume loss of negative ions by neutral particles—metastable molecules in the present case—is an additional condition. Calculations of the temperature dependent coefficients for ionization and attachment by solving the time-independent Boltzmann equation in the two-term approximation in combination with the fluid equations of the charge carriers show that the criteria for the development of an attachment-induced ionization instability are fulfilled in the pressure domain around 30 Pa and for medium radio frequency voltages around 300 V.

Alpha–gamma transition in RF capacitive discharge in low-pressure oxygen

“…As was shown in paper [13], attachment-induced ionisation instability can be excited in the RFcapacitive discharge in oxygen. In the RF discharge in oxygen there are a large number of negative ions O À , whose concentration can be comparable to or even can exceed the concentration of electrons [7,8].…”

“…The authors of this paper derived the criterion for the onset of this instability in plasma containing negative ions. This criterion was used by the authors of papers [13,24] for elucidating the nature of the instability they had observed in RF-capacitive discharge in oxygen.…”

“…A large number of experimental and theoretical papers studying the characteristics of this type of the discharge were published (see, e.g. [4][5][6][7][8][9][10][11][12][13][14][15]). But up to now the data are actually absent on the regimes of RF-capacitive discharge in oxygen and on the transition of the discharge from a weak-current (a-) to a strong-current (g-) regime (a-g transition).…”

Double layer onset inside the near-electrode sheath of a RF capacitive discharge in oxygen

“…They also presented [16] numerical evidence regarding a new type of self-sustaining mechanism for RF oxygen plasma when new electrons are generated in the bulk plasma by the detachment reaction from negative ions with high density under the condition of a relatively high bulk field. Katch et al [17] found that the plasma of a 13.56 MHz capacitively coupled oxygen discharge of the GEC reference cell exhibits fluctuations of the plasma potential and light emission in the kilohertz range. They explained this behaviour in terms of an attachment-induced ionisation instability in connection with a sufficiently high fraction of negative ions.…”

“…A large number of experimental and theoretical papers studying the characteristics of this type of the discharge were published (see, e.g., [6][7][8][9][10][11][12][13][14][15][16][17]). Let us present a brief review of main results obtained with the RF discharge in oxygen.…”