Mark Bowden

MEASUREMENTS OF ELECTRON TEMPERATURE, ELECTRON DENSITY, AND NEUTRAL DENSITY IN A RADIO-FREQUENCY INDUCTIVELY COUPLED PLASMA

Electron temperature, electron density, and neutral atom density were measured in a radio‐frequency (rf) inductively coupled plasma using Thomson and Rayleigh scattering of laser radiation. Measurements were made in an argon discharge for pressures from 1 to 20 mTorr and input rf powers from 100 to 500 W. Spatial distribution profiles were measured for discharges with different aspect ratios. Electron temperature was found to depend on pressure but only weakly on power. Electron density depended strongly on both pressure and power. The neutral density was found to be significantly depleted in the plasma center and this depletion was attributed to heating of the neutrals by charged particle collisions. These results were compared to a simple model of inductively coupled plasmas.

Diagnostics of low-density glow discharge plasmas using Thomson scattering

In recent years, the method of incoherent Thomson scattering has been applied to make measurements of electron properties of glow discharges used for industrial applications. These plasmas have electron density of the order of and electron temperature of a few eV. This paper reviews the recent progress in this research area. Details of the experimental systems are given and examples of measurements made in several different types of glow discharges are presented. A method in which Thomson scattering is combined with Rayleigh scattering to provide direct measurements of the neutral density is also described and an example of measurements is given. The electron velocity and energy distribution functions are also an important property of these plasmas, and the extent to which Thomson scattering can measure the distribution function is discussed. Future trends for Thomson scattering in glow discharges are also discussed.

Thomson scattering measurements of electron temperature and density in an electron cyclotron resonance plasma

Electron temperature Te and density ne in the source region of an electron cyclotron resonance discharge have been measured by incoherent Thomson scattering of the beam from a 0.5 J yttrium aluminum garnet laser. This is the first experiment in which this technique, routinely used on fusion plasmas, has been applied to a processing plasma. Measurements were made in an argon discharge at pressures from 0.3 to 2 mTorr and microwave powers from 250 to 1000 W. Velocity distributions were measured both parallel and perpendicular to the magnetic field and a slight anisotropy of electron temperature was observed for low‐pressure discharges. Temperatures in the range of 1–5 eV and densities in the range of 2–10×1017 m−3 were measured. Te and ne were found to strongly depend on pressure but only weakly on the input power and discharge magnetic field. No deviations from a Maxwellian velocity distribution were observed.

A study of electron energy distributions in an inductively coupled plasma by laser Thomson scattering

The electron energy distribution function (eedf) in an inductively coupled plasma was studied using the method of laser Thomson scattering. eedfs were measured for various plasma conditions, at different gas pressures, and at different input rf powers. In high electron density plasmas, the eedf was observed to be Maxwellian, while in low electron density plasmas, a non-Maxwellian eedf was observed. The transition between Maxwellian and non-Maxwellian eedfs was attributed to the thermalization of the electron population in higher density plasmas. In order to completely characterize the eedf, spatial and temporal dependencies were measured and measurements were also made in the afterglow period of a pulsed discharge. Discussions are made of the electron behavior and the shape of the eedf.

A Thomson scattering diagnostic system for measurement of electron properties of processing plasmas

In recent years, the method of incoherent Thomson scattering has been used in the study of glow discharge plasmas to measure electron properties such as electron density and electron temperature. In this paper, we report the development of a Thomson scattering diagnostic system that is appropriate for measurements in glow discharges containing reactive chemical species. The new scattering system is based on a low-pulse-energy, high-repetition-rate YAG laser, a multipass cell for the laser beam, and an ICCD camera for simultaneous detection of the entire scattered spectrum. Measurements made in an electron cyclotron resonance plasma using the new system are presented. The lower detection limit of electron density is estimated to be .

Comparison of electron property measurements in an inductively coupled plasma made by Langmuir probe and laser Thomson scattering techniques

Langmuir probes and Thomson scattering are two independent methods of measuring electron properties in glow discharges. In this article, measurements of electron properties of an inductively coupled plasma were made using both methods, and the results obtained with the probe were compared with those obtained using the more reliable laser method. The values of electron temperature and density obtained by both methods were similar over a range of plasma conditions but significant differences in the electron energy distribution function were observed. In addition, clear evidence was obtained that the presence of the probe perturbed the discharge. The advantages and disadvantages of each method are discussed with respect to the suitability of each technique for measuring electron properties in inductively coupled glow discharges.

MEASUREMENT OF NON-MAXWELLIAN ELECTRON ENERGY DISTRIBUTIONS IN AN INDUCTIVELY COUPLED PLASMA

The electron energy distribution function (EEDF) in an argon planar inductively coupled plasma was measured by the method of laser Thomson scattering. In a low‐pressure discharge, a clear departure from a Maxwellian distribution function was observed; an unambiguous measurement of a non‐Maxwellian distribution in a low‐density glow discharge. The non‐Maxwellian EEDF became Maxwellian in a higher pressure discharge. These observations were attributed to the lack of randomizing collisions in the low‐pressure discharge and the increase in the number of these collisions at higher pressure.

Electric field measurements in an argon glow discharge using laser spectroscopy

Direct measurements of the electric field in the sheath of a dc glow discharge were made using laser spectroscopic measurements of argon atoms. The effect of the electric field on the 4s⇒7f transitions was used to determine the electric field. Both laser opto‐galvanic spectra and laser‐induced fluorescence spectra were obtained. The magnitude of the electric field was determined using an experimentally obtained calibration of the change in wavelength produced by the electric field.

Measurements of sheath electric fields in a high pressure helium radio frequency discharge

The importance of capacitively coupled radio frequency (rf) discharges has resulted in many attempts, by experiment and by simulation, to understand the dynamics of the discharge. Because of the time varying nature of the sheath potential, the sheath region is of special interest in these plasmas. Direct measurements are reported of the sheath electric fields in a helium plasma obtained using a laser induced fluorescence method. In the interpretation of these measurements, the time dependence of the fluorescence spectrum had to be considered. The measured electric field distributions can be used to deduce sheath widths.

Observations of bi-Maxwellian and single Maxwellian electron energy distribution functions in a capacitively coupled radio-frequency plasmas by laser Thomson scattering

Electron energy distribution functions in low-pressure capacitively coupled radio-frequency argon discharges were measured using the technique of laser Thomson scattering. It was found that the distribution functions changed from a bi-Maxwellian at lower pressures to a single Maxwellian at higher pressures. These measurements provide independent confirmation of probe measurements made in similar discharges. The electron temperature and density of the cold group of electrons were measured with an accuracy of better than 10%.