Diagnostics of DC-pulse SiH4 plasmas using laser-induced fluorescence and optical emission spectroscopy

Plasmas produced with short-pulse power show promise for deposition of hydrogenated amorphous silicon films (a-Si:H). For high electron temperature and density compared to those in continuously excited plasmas, the reactants for a-Si:H deposition can be produced efficiently in the pulse plasmas. In the afterglows, the average energy of ions impinging on the substrate is expected to be low, allowing one to grow less-damaged films through reactions of long-lifetime reactants such as SiH₃. To optimise the pulse plasmas for the a-Si:H deposition process, it is necessary to understand the production and transport of the reactants in connection with the plasma structure. DC-pulse plasmas through pure SiH₄ and SiH₄/H₂ mixtures are diagnosed by laser-induced fluorescence. The spatial profiles of SiH radicals are measured to investigate the plasma structure. Optical emission spectroscopy and the time-resolved Langmuir probe measurement are also conducted to characterise the plasma in terms of electron density and temperature. In a DC-pulse plasma through hydrogen, the electron density reaches approximately 10¹¹ cm^-3 one to two orders of magnitude higher than the electron densities in conventional radio-frequence (RF) plasmas. For a SiH₄/CF₄ mixture, CF₄ molecules are dissociated in the DC-pulse plasmas for the high energy electrons produced in the initial stage of plasma production. By contrast, they are scarcely decomposed in continuous RF plasmas. The spatial profiles of SiH density for the DC-pulse plasmas in pure SiH₄ suggest that the discharge has no positive column and that SiH radicals are produced mostly in the negative glow