Highly sensitive plasma absorption probe for measuring low-density high-pressure plasmas

Abstract

This article reports a new type of sensitive plasma absorption probe (PAP), which is characterized with a thin wire antenna directly exposed to plasma. In the sensitive PAP, the power reflection coefficient resonantly decreases at a certain frequency due to absorption of a surface wave, which is excited along a sheath formed around the antenna. The electron density is derived from the measured absorption frequency in comparison to a wave dispersion relation: the dispersion is calculated under assumptions that the sheath width is twice the Debye length and that wavelength is twice the antenna length. This sensitive PAP also enables measurements of very low electron densities (∼108 cm−3) and very high pressures (∼10 Torr), in comparison to a conventional standard PAP. In addition, both electron temperature and electron density can be measured using a pair of sensitive PAPs of different antenna radii.

Modeling and simulation of the plasma absorption probe

“…The employed surface wave dispersion relations rest on an idealization of the geometry which may be too drastic, particularly in the limit d → 0. ͑It may be valid, however, in the longer and narrower probe geometry proposed by Nakamura et al 14 …”

Computational study on reliability of sheath width measurement by the cutoff probe in low pressure plasmas

“…The circuit model reproduces the zero crossing resonance frequencies of PSR and PR (Plasma Resonance) in accord with the FDTD simulation, even though, the phase offset below the PSR frequency and phase jump by cavity resonances observed in the FDTD simulation are not reproduced by the circuit model. The origin of this discrepancy below the PSR frequency is not well analyzed yet, but it is believed to be the effect of the propagation only a function of surface waves ( f surface < f p / √ 2) [18]. The PSR frequency is the function of the sheath width if all other variables (n e , T e , etc.)…”

Plasma frequency measurements for absolute plasma density by means of wave cutoff method

“…This corresponds to a surface wave resonance absorption monitored by using a sensitive plasma absorption probe. 8 The reflected power difference between that in the plasma and in a vacuum ͑without plasma͒ at the plasma frequency ͑2.45 GHz for P rf ϭ200 W͒ was so small that we could not find a clear peak which corresponded to the plasma frequency. The reason for such a small reflected power difference is thought to be as follows.…”

“…The plasma absorption method is applicable to any type of plasma but has a limitation of electron density Ͼ10 10 cm Ϫ3 and discharge pressureϽ0.5 Torr. 8 To expand the applicability of plasma absorption method, Sugai's 1 group has developed a sensitive plasma absorption method. 8 The sensitive plasma absorption method is applicable at higher pressure and lower density of plasma but the sheath effect and electron temperature should be considered to minimize the measurement error.…”