Christine Charles (a), Helen Smith (a), Rod Boswell (a) Kenneth Trantham (b) and Stephen Buckman (b)

(a)Plasma Research Laboratory,
(b)Atomic and Molecular Physics Laboratories
Research School of Physical Sciences & Engineering, Australian National University, ACT 0200, AUSTRALIA cxc112@rsphysse.anu.edu.au

This paper looks at the breakdown behaviour in rf (13.56 MHz) discharges for a range of pressures and electrode separations. Breakdown is defined to commence when the electrons are sufficiently heated by the applied voltage so that the ionisation rate outweighs losses to the walls, and finishes when the density becomes large enough for the Debye length to enter the system and sheaths to form. The time required to break the plasma down depends strongly on the gas pressure and the electrode separation. At a typical operating pressure of 50 mTorr and an electrode separation of 5 cm, the breakdown time is of the order of a microsecond (i.e., ~ 10 rf cycles). At pressures below 1 mTorr, the ionization probability is greatly decreased and the time necessary for breakdown can increase by orders of magnitude to hundreds of microseconds. Experimental measurements of the rf breakdown characteristics for a large range of pressures and interelectrode distances and their relationship to the Paschen curve are presented. At pressures below 1 mTorr metastables play a major role in the breakdown of the discharge.