A supersonically expanding arc plasma in argon is analyzed both experimentally and theoretically. The plasma is created in a cascaded arc and extracted through a hole in the anode. It emanates in a large vacuum system, where it expands supersonically. This expansion is limited by a shock wave. After the shock wave a subsonic plasma beam is created. A quasi one‐dimensional model, based on the conservation of mass, momentum and energy is presented. The shock wave is treated as a discontinuity. The electron density, the gas velocity and the gas temperature are measured as a function of the position in the expansion by means of Stark broadening and Doppler spectroscopy. The model calculations agree well with the measurements, especially in the first part of the supersonic flow.
Results from emission spectroscopy measurements on an Ar/SiH4 plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I = 60 A, V = 80 V, Ar flow = 60 see/s and pressure 4 X lo4 Pa) to expand to a low pressure ( 100 Pa) background. In the resulting plasma SiH4 is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities ( -lOi* m-3), electron temperatures ( -5000 K) as well as concentrations of H+, Si+, and Art particles. The emission spectrum of the SiH radical, the A 'A-X 211 electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting ,in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si + .
Experiments to study negative ion densities in a radio-frequency CF4 plasma have been carried out using a photodetachment technique. Electrons are photodetached from the negative ions using the pulse of a Nd-YAG laser at the tripled (355 nm) or the quadrupled (266 nm) frequency. The photodetached electrons are detected by a microwave method as a sudden increase of the electron density in the plasma. The negative ion density, which consists mainly of F- is found to be typically four times higher than the stationary electron density at a pressure of 13 Pa, an RF power of 15 W and a CF4 flow of 15 SCCM. The measured decay of the detached electrons after the laser pulse has been interpreted in terms of electron attachment and ambipolar diffusion. The results demonstrate the possibilities for use of this technique to evaluate attachment coefficients in active plasmas. The attachment rate constant for CF4 is found to be (7+or-1)*10-17 m3 S-1 at RF powers of 15 W. The electron diffusion coefficient is 0.13+or-0.12 m2 s-1 at standard conditions of 1 Torr and 300 K.
Experiments to study negative ion densities have been carried out using the photodetachment effect in a rfplasma in CF.~. Electrons are detached from the negative ions under the influence of the pulse of a Nd:YAG laser. The induced increase of the electron density is measured as a function of time using the shift of the resonance frequency of a microwave cavity containing the plasma. The negative ion density IF-] is found to he about (4 ± 1) X 10 15 m--3 , a factor
Abstract. A high-density expanding recombining plasma is investigated for deposition of a-Si:H thin films. The deposition method allows nigh growth rates and it relies on separation of plasma production in a high-pressure thermal arc, and transport of lragments of injected SiH, monomer lo the substrate. Some characteristics of the plasma are discussed together with an explanation of the dominant chemical kinetics. which proceed mainly through heavy-panicle interactions. The deposition results indeed show very high growth rates from 2-30 nm s-' on areas of 30 cm2. The properties of the layers are characterized by measuring their refractive index (in the range 3.1-3.8) and bandgap (1.2-1.5 eV). Analysis of the oxygen content in the deposited films shows oxidation of the samples in air, which is probably associated with the microstructure of the layers
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