Evidence of local power deposition and electron heating by a standing electromagnetic wave in electron-cyclotron-resonance plasma

Microwave plasmas excited at electron-cyclotron resonance were studied in the 0.5–15 mTorr pressure range. In contrast with low-limit pressure conditions where the plasma emission highlights a fairly homogeneous spatial structure, a periodic spatial modulation (period ∼6.2 cm) appeared as pressure increased. This feature is ascribed to a local power deposition (related to the electron density) due to the presence of a standing electromagnetic wave created by the feed electromagnetic field (2.45 GHz) in the cavity formed by the reactor walls. Analysis of the electron energy probability function by Langmuir probe and optical emission spectroscopy further revealed the presence of a high-energy tail that showed strong periodic spatial modulation at higher pressure. The spatial evolution of the electron density and of the characteristic temperature of these high-energy electrons coincides with the nodes (maximum) and antinodes (minimum) of the standing wave. These spatially-modulated power deposition and electron heating mechanisms are then discussed.

Figure 1

(a) Schematic of the ECR plasma reactor showing the magnets (in the bottom) and the microwave antenna located near the side of the reactor (black line). (b) Optical images of the plasma were taken for different working pressures between 0.5 and 15 mTorr from the top windows schematized in (a). Langmuir probe measurements were performed in the region in the middle of the reactor and the optical emission measurements reported were performed in the region between the two dotted lines.

Figure 2

Typical emission spectrum recorded for plasma in ${\mathrm{O}}_2$ and rare gases mixture at 1.5 mTorr.

Figure 3

Ne (585.2 nm) and ${\mathrm{O}}_2^{+}$ (602.6 nm) band emission intensities as a function of position along the antenna for 0.5, 1.5, and 5 mTorr in plasmas in a mixture of ${\mathrm{O}}_2$ and rare gases.

Figure 4

Comparison of the electron current measured by Langmuir probe (symbols) and the segmented electron current constructed using the electron populations determined by OES (lines). The results are shown for plasmas in ${\mathrm{O}}_2$ and rare gases mixture at 0.5, 1.5, and 5 mTorr.

Figure 5

Electron temperatures determined by OES as a function of position along the antenna (see text for description of $T_e^{\mathrm{high}}$ and $T_e^{\mathrm{tail}}$).

Figure 6

Ratio of the measured Ne emission intensity at 585.2 nm by the rate coefficient calculated for an electron temperature equal to $T_e^{\mathrm{tail}}$ and the operating pressure. Such ratio is linked (in arbitrary units) to the electron density (see text for more details).