Philip G. Steen

Atomic oxygen surface loss coefficient measurements in a capacitive/inductive radio-frequency plasma

Spatially resolved measurements of the atomic oxygen densities close to a sample surface in a dual mode (capacitive/inductive) rf plasma are used to measure the atomic oxygen surface loss coefficient β on stainless steel and aluminum substrates, silicon and silicon dioxide wafers, and on polypropylene samples. β is found to be particularly sensitive to the gas pressure for both operating modes. It is concluded that this is due to the effect of changing atom and ion flux to the surface.

Structure observed in measured electron energy distribution functions in capacitively coupled radio frequency hydrogen plasmas

Electron energy probability functions measured with a passively compensated Langmuir probe in asymmetric capacitively coupled hydrogen and deuterium plasmas exhibit structure. The otherwise relatively continuous distribution appears to have an abrupt peak in electron density near 5 eV. This structure occurs at a higher energy in deuterium than hydrogen and there is a correlation between floating potential and the voltage at which the structure is observed in the second derivative of the I(V) characteristic. While the cause of the structure has yet to be clarified, spectroscopic observations and computer-based hydrogen models indicate that the high energy tail of the distribution is strongly modulated during the radio frequency cycle. The effect of this modulation on plasma properties and probe measurements has yet to be explored.

Role of low-energy electrons in Ar emission from low-pressure radio frequency discharge plasma

Optical emission spectra from a low-pressure Ar plasma were studied with high spatial resolution. It has been shown that the intensity ratios of Ar lines excited through metastable levels to those excited directly from the ground state are sensitive to the shape of electron energy distribution function. From these measurements, important information on the spatial variation of plasma parameters can be obtained.

Characterization of a high-density, direct-current reflex discharge plasma source operating in Ar and N2

The characterization of a direct current, low-pressure, and high-density reflex discharge plasma source operating in argon and in nitrogen, over a range of pressures 1.0–10−2 mbar, discharge currents 20–200 mA, and magnetic fields 0–120 G, and its parametric characterization is presented. Both external parameters, such as the breakdown potential and the discharge voltage–current characteristic, and internal parameters, like the charge carrier’s temperature and density, plasma potential, floating potential, and electron energy distribution function, were measured. The electron energy distribution functions are bi-Maxwellian, but some structure is observed in these functions in nitrogen plasmas. There is experimental evidence for the existence of three groups of electrons within this reflex discharge plasma. Due to the enhanced hollow cathode effect by the magnetic trapping of electrons, the density of the cold group of electrons is as high as 1018 m−3, and the temperature is as low as a few tenths of an el...

Electrical and optical characterisation of capacitively and inductively coupled GEC reference cells

Abstract The Gaseous Electronics Conference (GEC) reference cell concept is designed to provide a standard discharge for comparative studies. It exists in both a capacitively and inductively coupled form. The electrical characterisation, by derivative probes, provides an accurate, reproducible measurement of the power dissipated in the plasma. Langmuir probe measurements of the electron energy distribution function show transitions between different heating modes. Time-resolved optical emission, when correlated with time- resolved potential measurements, can indicate the relative contributions of different plasma heating mechanisms.

Temporal phenomena in inductively coupled chlorine and argon–chlorine discharges

Reproducible modulations in low-pressure, inductively coupled discharges operating in chlorine and argon–chlorine mixtures have been observed and studied. Changes in the light output, floating potential, negative ion fraction, and charged particle densities were observed. Here we report two types of unstable operational modes in an inductively coupled discharge. On the one hand, when the discharge was matched, to minimize reflected power, instabilities were observed in argon–chlorine plasmas over limited operating conditions of input power and gas pressure. The instability window decreased with increasing chlorine content and was observed for chlorine concentrations between 30% and 60% only. However, when operating at pressures below 5mTorr and the discharge circuit detuned to increase the reflected power, modulations were observed in a pure chlorine discharge. These modulations varied in nature from a series of sharp bursts to a very periodic behavior and can be controlled, by variation of the matching c...

Negative ion enhancement in caesium-seeded hydrogen discharges - a volume or surface effect?

Measurements of plasma parameters, including H− ion densities, made in conjunction with wall temperature, visible and vacuum ultraviolet emission spectroscopy verify that there is little caesium in the plasma volume of the H− ion source. Surface work function measurements indicate that there is significant caesium coverage of the inner walls of the ion source. It is found that, as the work function of a test surface decreases due to caesium seeding, the H− ion fraction in the discharge volume increases. These observations combine to indicate that, in the present source, the H− ion enhancement mechanism is a surface dominated effect.

High-density and low electron temperature direct current reflex plasma source

A new type of direct current, high-density, and low electron temperature reflex plasma source, obtained as a hybrid between a modified hollow-cathode discharge and a Penning ionization gauge discharge is presented. The plasma source was tested in argon, nitrogen, and oxygen over a range pressure of 1.0–10−3 mbar, discharge currents 20–200 mA, and magnetic field 0–120 Gauss. Both external parameters, such as breakdown potential and the discharge voltage–current characteristic, and its internal parameters, like the electron energy distribution function, electron and ion densities, and electron temperature, were measured. Due to the enhanced hollow-cathode effect by the magnetic trapping of electrons, the density of the bulk plasma is as high as 1018 m−3, and the electron temperature is as low as a few tenths of electron volts. The plasma density scales with the dissipated power. Another important feature of this reflex plasma source is its high degree of uniformity, while the discharge bulk region is free o...

Analysis procedure for calculation of electron energy distribution functions from incoherent Thomson scattering spectra

Incoherent Thomson scattering (ITS) provides a nonintrusive diagnostic for the determination of one-dimensional (1D) electron velocity distribution in plasmas. When the ITS spectrum is Gaussian its interpretation as a three-dimensional (3D) Maxwellian velocity distribution is straightforward. For more complex ITS line shapes derivation of the corresponding 3D velocity distribution and electron energy probability distribution function is more difficult. This article reviews current techniques and proposes an approach to making the transformation between a 1D velocity distribution and the corresponding 3D energy distribution. Previous approaches have either transformed the ITS spectra directly from a 1D distribution to a 3D or fitted two Gaussians assuming a Maxwellian or bi-Maxwellian distribution. Here, the measured ITS spectrum transformed into a 1D velocity distribution and the probability of finding a particle with speed within 0 and given value v is calculated. The differentiation of this probability ...