Doppler-broadened Ha emission (656.28 nm) detected from a 13.56 MHz, parallel-plate, radio-frequency discharge in hydrogen indicates the presence of fast excited H atoms throughout the discharge volume. Time and spatially resolved measurements of the Doppler-broadened emission indicate that the fast H atoms are formed primarily at the surface of the powered electrode with kinetic energies exceeding 120 eV. Energetic neutrals produced in radio-frequency (rt) discharges used in the production of microelectronic devices can influence etching rates, the quality of diamond deposition, and plasma cleaning mechanisms. While the anticipated energies of these neutrals have been calculated,1 almost no experimental data exist. In this paper we present a new technique that allows the determination of fast atom velocities parallel to the electrode axis in a parallel-plate rf reactor by. measuring the time-resolved Doppler-shifted optical emission perpendicular to the electrode axis. We apply the technique to the detection of fast H atoms in a 13.56-MHz hydrogen discharge because of the interest2-4in the production and transport of fast H. Doppler-broadened Balmer-alpha (Ha) emission from excited fast hydrogen atoms has been previously observed from dc and low frequency rf discharges (... 300 kHz) in pure hydrogen.2.3.5The fast atoms observed in dc and low frequency discharges have kinetic energies of hundreds of electron volts, far in excess of the kinetic energies (up to approximately 8 eV) that have been reported due to electronimpact dissociative ionization of hydrogen.6 Recent work2 suggests that there are two sources of these fast atoms in dc discharges. The first is charge-exchange collisions between fast ions and the background H2 gas, producing fast atoms moving towards the cathode. The second is the formation of fast H atoms at the cathode surface due to bombardment by fast ions and neutrals formed in the discharge. This produces '

Excited neutrals and fast ions produced in a 13.56 MHz radio-frequency discharge in a 90% argon -10% hydrogen gas mixture were investigated, respectively, by spatially and temporally resolved optical emission spectroscopy, and by mass-resolved measurements of ion kinetic energy distributions at the grounded electrode. The electrical characteristics of the discharge were also measured and comparisons are made with results obtained for discharges in pure HZ under comparable conditions. Measurements of Balmer-alpha (H,) emission show Doppler-broadened emission that is due to the excitation of fast atomic hydrogen neutrals formed from ion neutralization processes in the discharge. Temporally and spatially resolved emission profiles of the H, radiation from the Ar-H, mixture are presented for the "slow" component produced predominately by electron-impact dissociative excitation of Hz, and for the "fast" component corresponding to energies much greater than can be derived from dissociative excitation. For the Ar-Hz mixture, the fast component is significantly enhanced relative to the slow component. The measured kinetic-energy distributions and fluxes of predominant ions in the At-H2 mixture, such as Hl, Hi, H", and At-H+, suggest mechanisms for the formation of fast hydrogen atoms. The interpretation of results indicate that Hf and/or Hl , neutralized and backscattered by collision with the powered electrode, are the likely sources of fast hydrogen atoms that produce Doppler-shifted H, emission in the discharge. There is also evidence at the highest pressures and voltages of "runaway" H+ ions formed near the powered electrode, and detected with kinetics energies exceeding 100 eV at the grounded electrode. I. lNTRODUCl-IONRadio-frequency (r-f) discharges produced in argonhydrogen mixtures are useful for surface cleaning applications,' while discharges involving mixtures of argon, hydrogen, and methane have been used for etching of GaAs wafers.2 An understanding of these processes requires a knowledge of the role of collisions of ions and energetic neutrals with surfaces and other particles in the plasma. Particularly, in colhsion dominated discharges, ion and neutraf transport in the sheath region are important in determining the discharge-surface interactions.In addition to industrial applications, ion transport in Ar-Hz gas mixtures has been considered as a prototype system for experimental investigations and rigorous quantumtheoretical studies.3-10 A recent review article on stateselected and state-to-state cross-section measurements for several ion-molecule reaction systems shows that an increasing interest has been paid to the Ar-Hz system."Few investigations in At--H, mixtures have been performed in low pressure dc'2-'4 or rf" discharges. A recent optical emission study*' of an At-H2 mixture in 13.56 MHz rf glow discharges shows an increase in Doppler-broadened Balmer-alpha (H, , X=656.3 nm) emission when argon is "'Present address: Department of Chemical and NucIear Engineering, University of New Mexico, Albuque...

Kinetic-energy distributions are presented for ions sampled from 13.56-MHz discharges in argon in a capacitively-coupled, parallel-plate, Gaseous Electronics Conference (GEC) radio-frequency reference cell. The cell was modified to allow sampling of ions through an orifice in the grounded electrode. Kinetic-energy distributions are presented for Ar+, Ar++, Ar+2, ArH+, and several trace ions for plasma pressures ranging from 1.3 Pa, where ion-atom collisions in the plasma sheath are not important, to 33.3 Pa, where collisions are important. Applied peak-to-peak radio-frequency (rf) voltages of 50, 100, and 200 V were used, and the current and voltage waveforms at the powered electrode were measured. Dependences of the ion fluxes, mean energies, and kinetic-energy distributions on gas pressure and applied rf voltage are interpreted in terms of possible ion-collision processes. The results agree with previously measured kinetic-energy distributions of ions sampled from the side of the plasma through a grounded probe for similar discharge conditions, verifying that ion kinetics are characteristic of the plasma sheath independent of where it is formed [J. K. Olthoff, R. J. Van Brunt, and S. B. Radovanov, J. Appl. Phys. 72, 4566 (1992)].

Vacuum ultraviolet (VUV) photons emitted from excited atomic states are ubiquitous in material processing plasmas. The highly energetic photons can induce surface damage by driving surface reactions, disordering surface regions, and affecting bonds in the bulk material. In argon plasmas, the VUV emissions are due to the decay of the 1s4 and 1s2 principal resonance levels with emission wavelengths of 104.8 and 106.7 nm, respectively. The authors have measured the number densities of atoms in the two resonance levels using both white light optical absorption spectroscopy and radiation-trapping induced changes in the 3p54p→3p54s branching fractions measured via visible/near-infrared optical emission spectroscopy in an argon inductively coupled plasma as a function of both pressure and power. An emission model that takes into account radiation trapping was used to calculate the VUV emission rate. The model results were compared to experimental measurements made with a National Institute of Standards and Technology-calibrated VUV photodiode. The photodiode and model results are in generally good accord and reveal a strong dependence on the neutral gas temperature.

Kinetic-energy distributions have been measured for different mass-selected ions sampled from 13.56 MHz rf glow discharges in argon inside a "GEC rf reference cell." The electrode geometry of this cell produces an asymmetric discharge and the cell is operated in a pressure regime where ion-molecule collisions in the sheath region of the discharge are significant. Ions are sampled from the side of the plasma perpendicular to the interelectrode axis using an electrostatic energy analyzer coupled to a quadrupole mass spectrometer. Kinetic-energy distributions for Ar+, Art, Ar+ +, and ArH+ are presented as functions of applied rf voltage, gas pressure, and distance of the mass spectrometer entrance aperture from the edge of the electrodes. The distributions obtained for the sampling orifice placed close enough to the electrodes to allow formation of a sheath in front of the orifice exhibit features similar to those observed previously when sampling ions through the grounded electrode of a parallel-plate reactor. The Ar+ and Ar+ + distributions exhibit secondary maxima predicted to result from the formation of low-energy (thermal) ions in the sheath region, such as by charge-exchange and high-energy electron collisions. Kinetic-energy distributions for Art and ArH+ exhibit no secondary maxima and are peaked at high energies indicative of the sheath potential, and consistent with a formation mechanism involving relatively low-energy collisions in the bulk plasma (glow region) .