Spectroscopic measurement of the degree of ionization in a helium electron cyclotron resonance discharge in a simple cusp field

Abstract

For an electron cyclotron resonance (ECR) discharge, a simple cusp field can improve electron confinement and enhance the degree of ionization (DOI) without sacrificing accessibility to the plasma. In this study, the spatial distribution of the DOI is experimentally revealed in a helium plasma produced with widely used 2.45 GHz and 800 W microwaves. The DOI is evaluated from the electron density and ground state atom density measured using HeI emission line intensities and by collisional-radiative model analysis. It is found that the DOI increases to more than 15% within a reasonably large volume surrounded by the ECR surface and locally reaches as high as 25%.

Helium atom line-intensity ratios as an integrated diagnostic tool for low-pressure and low-density plasmas

“…The method can measure not only T e and n e but also the metastable and ground-state atomic densities. The former is essential in atom lithography, 8,9 stimulated desorption, 10 and the removal of hydrocarbon contaminants, 11 and the latter is necessary to evaluate the ionization degree 12 and the depletion 13 and dynamics 14 of neutrals. The method is thus usable as an integrated diagnostic for discharges.…”

“…The procedure is the basis of our previously reported application. 12 Note that we use the term "low-pressure" in the sense that the effect of neutral collisions is negligible in the excitation and deexcitation of atoms.…”

“…A standard procedure is to measure as many excited-state densities as possible and simultaneously determine all the unknown parameters by least-squares fitting of the calculated densities to the measured values. 3,4,7,12 We performed experiments using a helium ECR discharge plasma. 12 Briefly, the plasma was produced using 2.45 GHz and 0.8 kW microwaves in a simple cusp field.…”

“…3,4,7,12 We performed experiments using a helium ECR discharge plasma. 12 Briefly, the plasma was produced using 2.45 GHz and 0.8 kW microwaves in a simple cusp field. The shape of the resonance layer was a spheroid with major and minor axes of 168 and 92 mm, respectively.…”

“…Since all the observed transitions are optically thin, their intensities were converted to local emissivities e pq ðrÞ using the Abel inversion while taking into account the numerical error. 12 The upper-state density n p ðrÞ was then calculated from the relation e pq ðrÞ ¼ ðh pq =4pÞn p ðrÞA pq , where h is Planck's constant and pq is the transition frequency. The results are shown in Fig.…”