Darrin Leonhardt

Contamination-free sounding rocket Langmuir probe

A technique for removing surface contaminants from a sounding rocket spherical Langmuir probe is presented. Contamination layers present on probe surfaces can skew the collected data, resulting in the incorrect determination of plasma parameters. Despite following the usual probe cleaning techniques that are used prior to a launch, the probe surface can become coated with layers of adsorbed neutral gas in less than a second when exposed to atmosphere. The laboratory tests reported here show that by heating the probe from the interior using a small halogen lamp, adsorbed neutral particles can be removed from the probe surface, allowing accurate plasma parameter measurements to be made.

Beam-generated plasmas for processing applications

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis ...

Plasma diagnostics in large area plasma processing system

A series of plasma diagnostic have been carried out in our large area plasma processing system which is based on a modulated electron-beam produced plasma. These discharges were created in both electrically conducting and insulated vacuum chambers operated in 30–120 mTorr of pure gases (argon, oxygen, and nitrogen). Langmuir probes, microwave transmission, mass spectrometry, and external current sensors show robust discharges were made over fairly wide parameter ranges resulting in plasma densities of 1–4×1011 cm−3 and temperature ranging from 0.2 eV for the molecular gases and 1.4 eV for argon. The effects of various experimental techniques, parameters, and contamination issues are discussed in detail.

On the extraction of positive and negative ions from electron-beam-generated plasmas

The results of investigations using pulsed, electron-beam-produced plasmas in Ar/SF6 mixtures are reported. Time-resolved, positive, and negative ion fluxes were measured at a biased electrode located adjacent to the plasma. The measurements indicate that plasmas form with large negative ion densities due to electron attachment to SF6 at the low electron temperatures associated with electron-beam-generated plasmas.

Time-resolved diagnostics in a pulsed, electron beam-generated plasma

Time-resolved ion flux and energy distributions were measured at an electrode located adjacent to a pulsed, electron beam-generated plasma produced in argon. A Langmuir probe was also used to determine time-dependent electron temperature and plasma density. Temporal variations in the incident Ar/sup +/ and Ar/sup +2/ ion energy and flux were correlated to changes in the electron temperature and plasma density. The evolution of the plasma density and ion flux is understood by considering the loss mechanisms of each ion species.

Ion energy distributions in a pulsed, electron beam-generated plasma

In this work, we investigate the ion flux at a grounded electrode located adjacent to a pulsed, argon plasma generated by a high-energy electron beam. The plasmas, produced in 100 mTorr, are characterized by high plasma densities (>1011 cm−3) and low electron temperatures (<1.5 eV). An energy selective mass spectrometer was used to measure temporally resolved ion kinetic-energy distributions at the electrode surface. In addition, ion energy distributions are presented for various electrode locations. The ion energy distributions correlate well with Langmuir probe measurements of the plasma potential.

Study of photoresist etching and roughness formation in electron-beam generated plasmas

A modulated, electron-beam generated plasma processing system was used to study plasma-polymer interactions for 193 and 248nm photoresists (PRs) that differed significantly in polymer structure. Because of the low plasma potential of the electron-beam generated plasma, the authors were able to study plasma etching and surface roughening of the photoresists at very low ion energies ( 1014cm−2). Typical conditions in the experiments were 2kV∕4ms electron-beam pulses with a 20ms period. The effects of ion bombardment energy, chemically assisted etching using fluorine, and the presence of a thin fluorocarbon overlayer on surface roughness formation during PR etching were examined. Gas mixtures containing SF6 resulted in much higher etch rates and an increased surface roughness relative to values measured in pure Ar plasmas. However, the rms roughness per nanometer of photoresist removed was greater for pure Ar plasmas. Overall the 248nm PR showed less surface roughness...

Fundamentals and applications of a plasma-processing system based on electron-beam ionization

Plasmas generated from moderate energy (2–5keV) electron beams (e-beam) have unique, attractive characteristics that are ideal for materials processing applications. These plasmas possess low electron temperatures (<0.5eV), variable plasma densities (109–1012cm−3) with an improved control of plasma species generation, and perhaps most importantly, a direct scalability to processing areas exceeding one square meter. These characteristics are due to the plasma ionization being driven by the e-beam instead of an external electromagnetic field as used in conventional processing plasma sources. Theoretical and experimental system details are discussed in terms of plasma operating conditions applied to three different surface modification approaches: metal nitriding, negative ion etching, and polymer surface energy tailoring.

Effect of plasma flux composition on the nitriding rate of stainless steel

The total ion flux and nitriding rate for stainless steel specimens exposed to a modulated electron beam generated argon-nitrogen plasma were measured as a function of distance from the electron beam axis. The total ion flux decreased linearly with distance, but the nitriding rate increased under certain conditions, contrary to other ion flux/nitriding rate comparisons published in the literature. Variation in ion flux composition with distance was explored with a mass spectrometer and energy analyzer as a possible explanation for the anomalous nitriding rate response to ion flux magnitude. A transition in ion flux composition from mostly N2+ to predominantly N+ ions with increasing distance was observed. Significant differences in molecular and atomic nitrogen ion energy distributions at a negatively biased electrode were also measured. An explanation for nitriding rate dependence based on flux composition and magnitude is proposed.

Etching with electron beam generated plasmas

A modulated electron beam generated plasma has been used to dry etch standard photoresist materials and silicon. Oxygen–argon mixtures were used to etch organic resist material and sulfur hexafluoride mixed with argon or oxygen was used for the silicon etching. Etch rates and anisotropy were determined with respect to gas compositions, incident ion energy (from an applied rf bias) and plasma duty factor. For 1818 negative resist and i-line resists the removal rate increased nearly linearly with ion energy (up to 220nm∕min at 100eV), with reasonable anisotropic pattern transfer above 50eV. Little change in etch rate was seen as gas composition went from pure oxygen to 70% argon, implying the resist removal mechanism in this system required the additional energy supplied by the ions. With silicon substrates at room temperature, mixtures of argon and sulfur hexafluoride etched approximately seven times faster (1375nm∕min) than mixtures of oxygen and sulfur hexafluoride (∼200nm∕min) with 200eV ions, the diffe...