S. Maeno

Plasma Sputter‐type Ion Source with Wire Electrodes for Low‐energy Gallium Ion Extraction

Low‐energy ions of gallium (Ga) and argon (Ar) were extracted from a plasma sputter‐type ion source system that utilized a tungsten (W) wire extractor geometry. The 90% transparent W wire extractor configuration had shown that the system was capable of producing an ion beam with the energy as low as 10 eV in a dc filament discharge and 50 eV in a radio frequency (rf) excited system. In the present investigation, Ar plasma was sustained in an ion source chamber through an inductively coupled 13.56 MHz rf power source. Negatively biased liquid Ga target suspended on a W reservoir was sputtered and postionized prior to extraction. Mass spectral analyses revealed a strong dependence of the Ga+ current on the induced target bias.

Gallium ion extraction from a plasma sputter-type ion source

A broad mixed ion beam containing positive ions of gallium (Ga) was produced with a plasma sputter-type ion source. Liquid Ga was suspended on a tungsten reservoir to be sputtered and postionized in argon (Ar) plasma excited by a radio frequency (rf) power at 13.56 MHz. Optical emission spectra from the plasma near the Ga sputtering target had indicated that the release of Ga into plasma increased with increasing negative bias to the sputtering target. The ratio of Ga(+) current to Ar(+) current was measured to be about 1% with a quadrupole mass analyzer at 100 V extraction voltage for incident rf power as low as 30 W. Ions in the plasma were extracted through a pair of multiaperture electrodes. The homogeneity of Ga flux was examined by making a Ga deposition pattern on a glass substrate located behind the extractor electrodes.

Characteristics of low-energy ion beams extracted from a wire electrode geometry

Beams of argon ions with energies less than 50 eV were extracted from an ion source through a wire electrode extractor geometry. A retarding potential energy analyzer (RPEA) was constructed in order to characterize the extracted ion beams. The single aperture RPEA was used to determine the ion energy distribution function, the mean ion energy and the ion beam energy spread. The multi-cusp hot cathode ion source was capable of producing a low electron temperature gas discharge to form quiescent plasmas from which ion beam energy as low as 5 eV was realized. At 50 V extraction potential and 0.1 A discharge current, the ion beam current density was around 0.37 mA/cm(2) with an energy spread of 3.6 V or 6.5% of the mean ion energy. The maximum ion beam current density extracted from the source was 0.57 mA/cm(2) for a 50 eV ion beam and 1.78 mA/cm(2) for a 100 eV ion beam.

Hydrogen negative ion production in a 14 GHz electron cyclotron resonance compact ion source with a cone-shaped magnetic filter

The plasma electrode structure of a 14 GHz ECR ion source was modified to enlarge the plasma volume of low electron temperature region. The result shows that the extracted beam current reached about 0.6 mA/cm(2) with about 40 W microwave power. To investigate the correlation between the volume of the low electron temperature region and the H(-) current, a vacuum ultraviolet (VUV) spectrometer had been installed to observe light emission in the VUV wavelength range from the plasma. From the results of the negative ion beam current and that from VUV spectrometry, production rate of vibrationally excited hydrogen molecule seems to be enhanced by increasing the volume of low electron temperature region.

Ion energy distribution functions of low energy beams formed by wire extraction electrodes.

The two-electrode extractor system made of 0.1 mm diameter tungsten wires separated by 0.7 mm has formed an argon ion beam with 50 V extraction potential. Energy spreads of the extracted beams were typically less than 2 eV when the beam current density was low. The beam intensity rapidly decreased as the distance between the extractor and the beam detector increased, indicating space charge limited transport of the beam. Problems associated with the emittance measurements are also discussed.

Extraction of negative hydrogen ions from a compact 14 GHz microwave ion source

A pair of permanent magnets has formed enough intensity to realize electron cyclotron resonance condition for a 14 GHz microwave in a 2 cm diameter 9 cm long alumina discharge chamber. A three-electrode extraction system assembled in a magnetic shielding has formed a stable beam of negative hydrogen ions (H(-)) in a direction perpendicular to the magnetic field. The measured H(-) current density was about 1 mA∕cm(2) with only 50 W of discharge power, but the beam intensity had shown saturation against further increase in microwave power. The beam current decreased monotonically against increasing pressure.

Extraction of low-energy negative oxygen ions for thin film formation

Coextraction of low‐energy positive and negative ions were performed using a plasma sputter‐type ion source system driven by a 13.56 MHz radio frequency (rf) power. Titanium (Ti) atoms were sputtered out from a target and the sputtered neutrals were postionized in oxygen/argon (O2/Ar) plasma prior to extraction. The negative O ions were surface‐produced and self‐extracted. Mass spectral analyses of the extracted ion beams revealed the dependence of the ion current on the incident rf power, induced target bias and O2/Ar partial pressure ratio. Ti+ current was found to be dependent on Ar+ current and reached a saturation value with increasing O2 partial pressure while the O− current showed a peak current at around 1:9 O2/Ar partial pressure ratio. Ti+ current was several orders of magnitude higher than that of the O− current.

Sputtering of liquid metal target by a capacitively coupled radio frequency self-bias

Performance of a liquid metal sputtering system induced by a radio frequency (RF) power source has been investigated. The system was equipped with a spherical glass reservoir for liquid metal in order to minimize contamination due to metallic impuritiesl. The system was fitted with a planar magnetron electrode in order to confine the gas discharge near the target surface and away from the substrate location. The system was divided into two chambers separated by a 2.5-mm diameter orifice; an analysis chamber was used for flux measurements of sputtered materials while the discharge chamber was used for plasma irradiation of a target material. Capacitive coupling of the RF power to the gas discharge was accomplished by employing two configurations. In the first configuration, a 50-mm wide copper strip was wound around the side of the glass chamber to act as a counter electrode of the magnetron electrode. In the second configuration, a 40-mm × 60-mm copper counter electrode was placed inside the chamber. Due to the insulating nature of the glass chamber, the system configuration largely affected the power coupling to the discharge. Liquid gallium (Ga) suspended upon the bottom part of the glass chamber was irradiated with argon and nitrogen plasmas separately. Ga was confirmed to reach the analysis chamber through the orifice by means of a quadrupole mass analyzer. Optical emission spectral analyses revealed the presence of Ga species in the discharge chamber directly above the Ga target surface. Properties of gallium nitride films deposited on glass and silicon substrates will be presented.

Mass separation of ion beams using transverse radio frequency electric field (abstract)

A condition to achieve a mass separation of an ion beam at a given energy by means of applying a transverse radio frequency (rf) electric field was derived. Effectiveness of the scheme was examined with a He ion beam of energy below 2 kV. An array of four-stage electrodes produced distributed 13.56 MHz rf field to realize a function of a velocity filter. When the extraction voltage of the beam was adjusted to yield a velocity predicted from the theory, the beam current showed transmission as large as 80% through the array of electrodes.