Reactor geometry and plasma uniformity in a planar inductively coupled radio frequency argon discharge

The effects of chamber height on plasma uniformity are examined for a cylindrically symmetric planar inductive radio frequency (rf) (13.56 MHz) argon plasma generated with a single-turn circular loop antenna. Plasma uniformity is characterized by two-dimensional (r, z) spatial profiles of plasma properties measured with a rf-filtered Langmuir probe. The results show a tendency Tor the radial location of the plasma density maximum to shift from the centre outward as the chamber height is decreased and pressure is increased. The location of the density maximum is insensitive to changes in discharge power. These observations are explained as a consequence of changes in axial and radial transport of ionizing energetic electrons as the reactor height L is varied. Because electron heating is localized at the radius of the maximum induced rf electric field R(E). the critical scale lengths are R(E) and L, in addition to the energy relaxation length for ionizing electrons lambda(epsilon). The competition between axial and radial losses can lead, for R(E) similar to L, to a sensitive dependence of plasma uniformity on discharge operating conditions.