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

The plasma of a 13.56 MHz capacitively coupled oxygen discharge of the Gase ous Electronics Conference Reference Cell type exhibits fluctuations of the plasma potential and of light emission in the kilohertz range. This behavi or can be explained in terms of an attachment-induced ionization instabilit y in connection with a sufficiently high fraction of negative ions. A neces sary condition for the appearance of this type of instability is a strong p ositive dependence on electron temperature of the electron attachment coeff icient describing the formation of negative oxygen ions. A strong volume lo ss of negative ions by neutral particles-metastable molecules in the presen t case-is an additional condition. Calculations of the temperature dependen t coefficients for ionization and attachment by solving the time-independen t Boltzmann equation in the two-term approximation in combination with the fluid equations of the charge carriers show that the criteria for the devel opment of an attachment-induced ionization instability are fulfilled in the pressure domain around 30 Pa and for medium radio frequency voltages aroun d 300 V. (C) 1999 American Institute of Physics. [S0003-6951(99)01540-5].