Atomic hydrogen temperature in silane plasmas used for the deposition of a-Si:H films

Abstract

Atomic hydrogen temperature in a silane plasma used for the deposition of a-Si:H films was measured using a two-photon laser-induced fluorescence technique. The temperature was found to be 2200±600 K and higher than both the parent-gas temperature and the atomic hydrogen temperature in a hydrogen plasma. We considered the mechanism of atomic hydrogen temperature determination in the plasma as follows. Hydrogen atoms generated by the dissociation of gas molecules due to electron impact in the plasma have a kinetic energy of several eV, which they then lose through collisions with gas molecules. At the same time, they disappear from the plasma due to chemical reactions and diffusion before thermal equilibrium with gas molecules has been established. Thus, an atomic hydrogen temperature higher than that of gas molecules becomes the equilibrium value in the plasma. We show, in the new experiment, that this model provides a good estimation of atomic hydrogen temperature in the plasma under various conditions of silane-hydrogen mixtures.

Langmuir probe plasma parameters and kinetic rates in a Ar–SiH4–H2plasma during nc-Si films deposition for photovoltaic applications

“…In pure silane RF discharges at similar pressures, the mean temperature of H atoms determined by LIF was 2000±500 K [35] (kinetic energies of ∼0.26 eV) with decreasing values as H 2 is added. In the LEPECVD discharge the H atom energy distributions observed by mass spectrometry (energy-resolved residual gas analysis and ion energy spectra de-convoluted from plasma potential) show a double peak distribution with mean energies at about 0.7 and 1.7 eV and calculated average H atom energies E = 0.15-0.08 eV (table 4).…”

Energy distributions of H and H2in a Ar–SiH4–H2plasma during transition from a-Si to nc-Si film deposition

“…As first steps in the validation of the method, two obvious results can be mentioned: the evolution of H density obtained by ERRGA matches the one estimated from kinetic rates, while the average kinetic energies and temperatures of the atomic H evaluated from ERRGA energy distributions are close to those obtained by LIF in SiH 4 -H 2 plasmas at similar pressures [9]. This paper presents the basis of a new method where accuracy can be increased by improving the instrument or the accuracy in cross-sections values.…”

“…While laser induced fluorescence (LIF) and optical molecular spectroscopy methods provide average temperatures for H or H 2 species [9,10], the analysis by mass spectrometry (MS) should allow an observation of their energy distribution. Due to the complex energy distributions of H and H 2 , the well-known MS methods such as residual gas analysis (RGA) and threshold ionization (TIMS), which are performed at a fixed value of the observed ion energy, are leading to density underestimations.…”

Kinetic modelling of atom production and thermalization in CCRF discharges in H2

“…These fast or hot H atoms can relax their excess kinetic energy through elastic collisions with the background gas molecules [13]. The thermalization of H atoms in RF discharges was studied by Miyazaki et al [14] using a twophoton laser-induced fluorescence (LIF) technique. They studied H atoms in SiH 4 , SiH 4 : H 2 , and H 2 plasmas: the temperature of H atoms was found to be as high as 2800 K in pure SiH 4 but H atoms were found to have essentially room temperature in the case of pure H 2 .…”

“…In view of their importance, fast atoms have been investigated numerically in several past works. Miyazaki et al [14] proposed a theoretical approach based on energy loss assuming a Maxwellian velocity distribution. They were able to account for the experimental value of H atom temperature in the three mixtures considered.…”