Radio-frequency discharges in oxygen: II. Spatio-temporally resolved optical emission pattern

Abstract

Axially and temporally resolved optical emission structures were investigated in the rf sheath region of a parallel plate capacitively coupled rf discharge (13.56 MHz) in pure oxygen and tetrafluoromethane. The rf discharge was driven at total pressures of between 10 and 100 Pa, gas flow rate of 3 sccm and rf power in the range 5–100 W. In particular, the emission of the atomic oxygen at 844.6 nm (3p3P → 3s3S0) and the atomic carbon at 193 nm (3s1P0 → 2p1D) were imaged with a lens onto the entrance slit of a spectrometer and detected by a fast ICCD-camera. The spatio-temporally resolved analysis of the emission intensity during the rf cycle (73.75 ns) provides two significant excitation processes inside the rf sheath: the electron impact excitation at the sheath edge, and heavy particle impact excitation in front of the powered electrode. In oxygen plasma the emission of atomic oxygen was found in both regions whereas in tetrafluoromethane the emission of atomic carbon was observed only in front of the powered electrode. The experimental results reveal characteristic dependence of the emission pattern in front of the powered electrode on plasma process parameters (self-bias voltage, pressure) and allow an estimation of the excitation threshold energy and effective cross section of energetic heavy particle loss.

Radio-frequency discharges in oxygen: I. Particle-based modelling

“…Some of the cross sections significantly deviate from the ones previously used [36]. Our simulations indicate, however, that the modifications are essential for obtaining bulk densities in accordance with experiments [27,30].…”

“…Both have to be adjusted to the particular experimental conditions. For the experiments [27,30], n res ≈ 10 8 -10 9 cm −3 and k B T res ≈ 10 eV , resulting in approximately 10 5 simulated particles.…”

“…[27,30]. Ignoring the electric asymmetry between the powered and grounded electrode, this part of the discharge can be simulated by the planar, one-dimensional model shown in figure 1.…”

“…It can be, for instance, accompanied by a double layer (internal sheath [20]). Electro-negative gas discharges are thus rather complex and the investigation of the spatio-temporal structure of the discharge as a function of external control parameters (current, voltage, frequency, pressure, and geometry) is a great theoretical [21,22,23,24] and experimental [25,26,27,28,29,30] challenge. In addition, electro-negative processing gases are reactive molecular gases, with internal degrees of freedom, which lead to a quite involved plasma chemistry.…”

“…1: Schematic geometry of the rf discharges used in Refs. [27,30]. We use a planar, one-dimensional model to simulate the central axial part of the discharge (thick solid line).…”