Langmuir probe diagnostic studies of pulsed hydrogen plasmas in planar microwave reactors

摘要

Langmuir probe techniques have been used to study time and spatially resolvedelectron densities and electron temperatures in pulse-modulated hydrogendischarges in two different planar microwave reactors (fmicrowave= 2.45 GHz,tpulse= 1 ms). The reactors are (i) a standing-wave radiative slotted waveguidereactor and (ii) a modified travelling-wave radiative slotted waveguidereactor, which generate relatively large plasmas over areas from about 350 cm^2to 500 cm^2. The plasma properties of these reactor types are of particularinterest as they have been used for basic research and for plasma processing,e.g. for surface treatment and layer deposition. In the present study thepressures and microwave powers in the reactors were varied between 33 and 55 Paand 600 and 3600 W, respectively. In regions with high electromagnetic fieldsshielded Langmuir probes were used to avoid disturbances of the probecharacteristic. Close to the microwave windows of the reactors both theelectron density and the electron temperature showed strong inhomogeneities. Inthe standing-wave reactor the inhomogeneity was found to be spatially modulatedby the position of the slots. The maximum value of the electron temperature wasabout 10 eV and the electron density varied between 0.2 and 14*10^11 cm^-3. Thesteady state electron temperature in a discharge pulse was reached within a fewtens of microseconds whereas the electron density needed some hundreds ofmicroseconds to reach a steady state. Depending on the reactor the electrondensity reached a maximum between 80 and 200 microseconds after the beginningof the pulse.