Comparison of electron density measurements in planar inductively coupled plasmas by means of the plasma oscillation method and Langmuir probes

Abstract

The absolute, spatially resolved electron densities in planar inductively coupled plasmas have been measured by two different plasma diagnostic techniques, the plasma oscillation method and Langmuir probes. In the plasma oscillation method a weak electron beam injected into the plasma excites electrostatic electron waves oscillating at the electron plasma frequency, which is proportional to the square root of the electron density. The plasma source is a modified Gaseous Electronics Conference RF Reference Cell. The results for both methods in electropositive, pure rare gas and molecular gas discharges (Ar, Ne, Kr and N 2 ) and in electronegative gases and gas mixtures like O 2 , Cl 2 , CF 4 , BCl 3 , Ar:CF 4 , Ar:O 2 and Ar:Cl 2 are presented and analysed. Over a wide parameter range (gas type, input power and gas pressure), the two techniques yield charge densities which agree within the experimental uncertainty. The electron densities inferred from the plasma oscillation method are generally between the electron and positive-ion density obtained from Langmuir probe measurements. Disagreement between the two techniques is found in molecular gases at higher pressures (p > 3 Pa), where the plasma oscillation method yields electron densities of up to a factor of two higher than the Langmuir probe results.

Effects of charge depletion in dusty plasmas

“…6͑a͒ by a spectrum analyzer. The electron density can then be derived from the maximum intensity of the power spectrum 26 as n e = 1.24 ϫ 10 −2 f max 2 ͑m −3 ͒. Figure 6͑a͒ shows the electron n e,LP and ion n i,LP densities, measured by LP, in comparison with the obtained absolute electron density n e,POM measured by POM for different I D .…”

“…To obtain reliable plasma densities, we have calibrated the LP measurements by the plasma oscillation method ͑POM͒. 25,26 The latter is based on the principle that an electron beam emitted by the emissive probe excites oscillations near the electron plasma frequency, pe ϰ ͱ n e . The cylindrical probe is used as an antenna to detect the plasma oscillations, which are displayed in Fig.…”

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

“…The use of an oscillation probes, 5,6 which can provide the local electron density information, was discovered by Shirakawa and Sugai. It directly gives the absolute electron density without complex modeling and assumptions; however, it still has problems when used to measure the processing plasma, [4][5][6][7][8][9][10] as the filament used as the electron beam source can induce metal contamination in the processing reactor, the filament can be etched by fluorocarbon gases (leading to life time problems with the filament), and the probe measurement is possible only at low pressure. 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.…”

Influence of Negative Ions on Drift Waves in a Low‐Density Ar/O2‐Plasma

“…We have cross-checked our results obtained by the saturation current ratios with the Plasma Oscillation Method (POM) [27,28] and found a good agreement in the charge densities.…”