Dual-role plasma absorption probe to study the effects of sheath thickness on the measurement of electron density

Abstract

A sensitive plasma absorption probe (PAP) is reported for measuring electron density in processing plasmas. The sheath formed around the probe tip is important for the resonance of surface waves. For determining the absolute electron density from the absorption frequency of the sensitive PAP, a proper value of sheath thickness relative to the Debye length is required to be assigned in the data processing. In this paper, a dual-role PAP has been proposed to study the effects of sheath thickness on the measurement of electron density. It is used as a Langmuir probe and a sensitive PAP simultaneously. Based on these two functions, the sheath thickness is calibrated before the measurement of electron density. The calibrated value is assigned in the data processing to replace the fitting coefficient used in the previous work. Therefore, the measurement error caused by an inaccurately assigned sheath thickness can be minimized effectively. Because of the bi-functional characteristic, the dual-role PAP is an independent diagnostic tool.

Active plasma resonance spectroscopy: a functional analytic description

“…Although the mathematical structure of (7) to (12) is quite transparent, it is not possible to separate the plasma contributions from the influences of the geometry from the numerical solution.…”

The multipole resonance probe: characterization of a prototype

“…the attempt to exploit for diagnostics purposes the natural ability of plasmas to resonate on or near the electron plasma frequency ω pe = e 2 n e / 0 m e . The idea dates back to the early days of discharge physics [1] and has found many different realizations since then [2][3][4][5][6][7][8][9][10][11][12][13][14]. To structure the field, the following classifications are useful: active plasma resonance spectroscopy [2][3][4][5][6][7][8][9][10][11][12] couples a suitable RF signal into the plasma and records the response, while passive plasma resonance spectroscopy [13,14] merely observes the 'pre-existing' excitation of the plasma by other sources (typically, the RF power input).…”

“…The idea dates back to the early days of discharge physics [1] and has found many different realizations since then [2][3][4][5][6][7][8][9][10][11][12][13][14]. To structure the field, the following classifications are useful: active plasma resonance spectroscopy [2][3][4][5][6][7][8][9][10][11][12] couples a suitable RF signal into the plasma and records the response, while passive plasma resonance spectroscopy [13,14] merely observes the 'pre-existing' excitation of the plasma by other sources (typically, the RF power input). Electromagnetic concepts [2][3][4] are based on the interaction of the plasma with the full electromagnetic field; they operate above the plasma frequency and observe the plasma-induced shift of transmission line or cavity resonances which are already present in vacuum.…”

“…Electromagnetic concepts [2][3][4] are based on the interaction of the plasma with the full electromagnetic field; they operate above the plasma frequency and observe the plasma-induced shift of transmission line or cavity resonances which are already present in vacuum. Electrostatic concepts [5][6][7][8][9][10][11][12], on the other hand, result from an interaction of the plasma with the electrical field alone; they utilize resonances below ω pe which are not present in vacuum (standing surfaces waves) and deduce the plasma parameters from the absolute value of the frequency. Finally, oneport concepts [4,[6][7][8][9][10][11][12][13][14] utilize probes with only one electrical input/output port, while two-port concepts [2][3][4][5] use a probe configuration with two ports (or even two separate probes).…”

Sheath width effect on the determination of plasma frequency in the cutoff probe

“…[7][8][9][10][11] In this letter, the sheath width effect on the determination of the plasma frequency in the cutoff probe and the physics behind it were investigated with two different simulation tools (a simplified circuit simulation and an E/ M simulation). Both simulations showed a consistent result: the cutoff frequency in the TMF spectrum deviates from the plasma frequency while increasing the sheath width on the two probe tips.…”