Line shapes of atomic hydrogen in hollow-cathode discharges

Optical Diagnostic Studies in Plasmas and Plasma Processing

Optical diagnostic probes have greatly increased our understanding of low pressure discharges used in etching and deposition processes for microelectronics fabrication.Optical emission induced by energetic electron impact excitation provides a qualitative determination of concentrations of a number of important atoms and small radicals.Rare-gas actinometry, combined with high resolution line-shape measurements can convert emission data into quantitative relative number densities, and provide information on mechanism of excited state formation.Emission from products of etching reactions gives insights into mechanisms and serves as an end-point indicator for many commercial processes.Laser induced fluorescence is a sensitive probe which is most useful for small radicals.Quantitative, relative ground-state number densities, internal energy distributions, translational energies and electric fields can be determined by this technique.When optical emission or laser induced fluorescence measurements are performed with spatial resolution and in-phase with respect to the applied field, additional insights are obtained on the dynamics of the discharge processes.Finally, infrared and visible-ultraviolet absorption spectroscopy can be used to measure absolute number densities.This technique also can provide spatial and temporal resolution.

This paper reviews and compares the various optical techniques used in plasma diagnostics with particular emphasis on studies of low pressure radio frequency discharges used in etching and deposition of thin films.

Energetic ion, atom, and molecule reactions and excitation in low-current H2 discharges: spatial distributions of emissions

Spatial distributions of H alpha , H beta , and the near-uv continuum emission from the H2 a ;{3}Sigma g;+ state are measured and compared with a model for low-current electrical discharges in H2 at high E/N and low Nd , where E is the spatially uniform electric field, N is the gas density, and d is the electrode separation. Data are analyzed for 300 Td<E/N<45 kTd , d=0.04 m , and 2 x 10;{21}<N<2.6 x 10;{22} m;{-3} . (1 Td=10;{-21} V m;{2}) The excitation is produced by electrons and by hydrogen atoms and molecules with mean energies from 5 to 1500 eV. Electron-induced emission, dominant at low E/N and low pressures, is distinguished by its buildup toward the anode. Excitation of H alpha by fast H atoms dominates at high E/N and increases toward the cathode. The observed H alpha emission at low E/N is normalized to previous experiments to yield absolute experimental excitation coefficients for all E/N and Nd . Small adjustments of model parameters yield good agreement with H alpha data. Cross sections are derived for excitation of the H2 near-uv continuum by H atoms. Spatial and pressure dependencies of H alpha and H2 near-uv emissions agree well with a model in which reactions of H2+ , H3+ , and H+ ions with H2 lead to fast H atoms and H2 molecules, which then excite H atoms or H2 molecules.

Absolute densities of energetic hydrogen ion species in an abnormal hollow cathode discharge

We develop an optical measurement for densities of fast (units to tens of keV) hydrogen ions in an abnormal hollow cathode discharge in units to tens of mTorr pressure range. This method combines results from previous collisional-radiative models, comparing the intensity of Balmer H_{alpha} due to dissociative excitation of H2 by fast electrons to Doppler-shifted emission arising from charge exchange of energetic ions. The method requires only two inputs: the current density due to fast electrons and a single H_{alpha} spectrum of the characteristic emission channel at the anode. We model in particular the cylindrical interelectrode discharge of an inertial electrostatic confinement device. Experimentally, we find that the density of fast ions emerging from the cathode (bias -5 kV at 20 mTorr ) is in the order 10;{14}m;{-3} , increasing approximately linearly with current in the 10-30 mA range. Calculated densities agree with values obtained in similar apparatus using Langmuir probes and analysis of dust particle motion.