A novel technique for plasma density measurement using surface-wave transmission spectra

Abstract

A technique for the measurement of the absolute electron density in low-pressure plasmas using microwaves is described. It is based on observing the propagation of electromagnetic surface waves (SW) at a plasma-sheath boundary, guided by a dielectric cylinder immersed in the plasma. The transmission spectrum is measured between two antennas situated at either end of the dielectric cylinder and connected to a network analyser. Analytical theory based on the Trivelpiece-Gould work (Trivelpiece and Gould 1959 J. Appl. Phys. 30 1784, Trivelpiece 1967 indicates that the lowest frequency at which the SW can propagate is equal to 1/ √ 2 of the plasma frequency, which is directly related to the electron number density at the plasma-sheath boundary. We call this probe the plasma transmission probe (PTP) in contrast to the plasma absorption probe proposed by Sugai and co-workers (Kokura et al 1999 Japan. J. Appl. Phys. 38 5262). The PTP is promising for the measurement of low densities ( 10 9 cm −3 ) at relatively high gas pressure ( 1 Torr). An axi-symmetric finite element model of the probe is presented and used to calculate transmission spectra. Experimental spectra measured in a radio-frequency capacitively coupled discharge in argon at various plasma densities and pressures (40-750 mTorr) are presented and compared with the calculated ones. Plasma densities derived from the transmission spectra were compared with those obtained with a Langmuir probe. The PTP was also compared with a microwave 1/4-wave resonator ('hairpin probe') at low pressure (5-45 mTorr) in an ICP discharge in argon. The densities determined by the PTP were found to be lower by a factor of 0.5-0.7 compared with those obtained with a Langmuir and a hairpin probe. We believe this can be attributed to the pre-sheath plasma density gradient, as the PTP determines the sheath edge electron density, not the bulk value.

Modeling and simulation of the plasma absorption probe

“…15 ͑This approach-the use of a finite element method ͑FEM͒ code to simulate surface wave-based plasma density measurements-was pioneered by Dine et al 11 for the case of the plasma transmission probe. The probe response ͑ab-sorbed power͒ is calculated from the real part of the displacement current on the surface of the inner electrode, Figure 2 shows the simulated probe response together with an absorption spectrum measured in our experiments.…”

“…This idea, time honored of course, has found re newed interest in recent years as the basis of industry compatible diagnostic methods. [3][4][5][6][7][8][9][10][11][12] In this letter, we concern ourselves with a recently proposed variant, the plasma absorption probe ͑PAP͒ ͑or surface wave probe͒ designed by Kokura et al 13 The PAP is schematically depicted in Fig. 1.…”

Computational characterization of cutoff probe system for the measurement of electron density

“…Another probe method, the plasma absorption probe method, 4 is applicable to any type of processing plasma, yet this method is limited to electron densities greater than 10 10 cm À3 and to discharge pressures of less than 0.5 Torr. 11 A recently proposed method known as the plasma transmission probe method, 12 which uses surface waves (SW) propagating along the plasma-sheath boundary, gives good results when used to measure the local electron density, but with this method, the selection of the launching frequency point of SW is not clear at low pressure. A hairpin probe 13,14 is free from the problems mentioned above; however, it needs high frequency electronic instruments.…”

Parametric Instability of Surface Electron Cyclotron Tm-Modes

“…Second, SECTM-mode's application can be useful for diagnostics of periphery of fusion plasma [20] and for searching possibility to decrease plasma periphery heating. Third, surface waves are prospective for developing electronic devices based on application of new plasma-like meta-materials, which allows one to construct miniaturized electronic devices [21,22].…”