Spherical tip-plasma resonance probe with an improved upper pressure limit for electron density measurements in low-density plasmas

Le résumé fourni par la source

Abstract Plasma resonance probes (PRPs) are used for electron density measurements based on the observation of the plasma resonance (PR) negative peak frequency at the electron plasma frequency in the transmission microwave spectrum. The cylindrical tip-plasma resonance probe (CT-PRP), also known as the cutoff probe, is widely used due to its simple structure. A recent study indicated that increasing the sheath width around the CT-PRP tip resulted in the PR negative peak frequency deviating from the electron plasma frequency and acted as a source of lowering the upper operating pressure limit. This disturbance becomes significant in low-density plasmas ( < 10 10 c m − 3 ) , where thicker sheaths are unavoidable. This study proposes a spherical tip-plasma resonance probe (ST-PRP) to reduce the sheath width effect via the use of a lower sheath impedance of its spherical tip structure. The improved upper operating pressure limit of the ST-PRP was investigated using a three-dimensional electromagnetic simulation model. The results showed that the ST-PRP extended the upper limit of operating pressure and reduced the sensitivity of the upper pressure limit for changes in sheath width greater than the CT-PRP. An experimental study in low electron density capacitively coupled plasma sources was conducted to compare the upper pressure limit between the ST-PRP and the CT-PRP. The results were consistent with the numerical simulation results. We believe that the proposed ST-PRP with an extended upper pressure limit can be a reliable tool for accurate plasma electron density measurements of low-density plasmas using PRPs.

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