D. S. Pickard

Ion energy spread and current measurements of the rf-driven multicusp ion source

Axial energy spread and useful beam current of positive ion beams have been carried out using a radio frequency (rf)-driven multicusp ion source. Operating the source with a 13.56 MHz induction discharge, the axial energy spread is found to be approximately 3.2 eV. The extractable beam current of the rf-driven source is found to be comparable to that of filament-discharge sources. With a 0.6 mm diameter extraction aperture, a positive hydrogen ion beam current density of 80  mA/cm2 can be obtained at a rf input power of 2.5 kW. The expected source lifetime is much longer than that of filament discharges.

Development of an rf driven multicusp ion source for nuclear science experiments

Abstract A compact 13.56 MHz radio-frequency (rf) driven multicusp ion source is under development at Lawrence Berkeley National Laboratory (LBNL) for radioactive ion beam applications. In this paper we describe the ion source design and the general ion source performance using H 2 , Ar, Xe gas and a 90% Ar/10% CO gas mixture for generating the discharge plasma. The following ion source characteristics have been analyzed: extractable ion current, ion species distributions, ionization efficiency for nobel gases, axial energy spread and ion beam emittance measurements. This ion source can generate ion current densities of approximately 60 mA/cm 2 .

Multicusp sources for ion beam lithography applications

Application of the multicusp source for ion projection lithography is described. It is shown that the longitudinal energy spread of the positive ions at the extraction aperture can be reduced by employing a magnetic filter. The advantages of using volume‐produced H− ions for ion beam lithography are also discussed.

Improvement of the lifetime of radio frequency antenna for plasma generation

At Lawrence Berkeley National Laboratory different antenna protection schemes have been investigated for the radio frequency-driven multicusp ion source. It was found that the antenna lifetime can be greatly enhanced by an additional shielding, which consists of porcelain, quartz or boron nitride. Different antenna configurations and their influence on the plasma generation will be discussed. Antenna life time greater than 500 hours continuous wave operation has been demonstrated in hydrogen plasma using a novel quartz antenna design.

Beam emittance measurements on multicusp ion sources

Multicusp ion sources are used for various applications. Presently, the implementation of this type of ion source planned for the development of an ion beam lithography machine, which will be used for the projection of sub-0.2 {mu}m patterns onto a wafer substrate. Since, for this application, a very good beam quality and a small ion energy spread are required, emittance measurements have been performed on a multicusp ion source for various source conditions. It is shown that the installation of proper capacitors between the extraction electrodes is necessary to avoid rf-pickup, which otherwise leads to a distortion of the beam emittance. The influence of the magnetic filter field on the beam emittance has been investigated, and the beam emittance of a dc filament-discharge plasma has also been compared to that of an rf-generated plasma.

A compact filament-driven multicusp ion source☆

Abstract A compact filament-driven multicusp ion source has been studied using both hydrogen and helium. Three aspects of the source have been investigated: hydrogen ion species, axial energy spread and extractable current. An atomic ion fraction (H+) of approximately 30% could be obtained with a discharge power of 80 V and 3 A. A magnetic analyzer was used to determine the axial energy spread of the extracted (i.e. accelerated) ion beam species, and an electrostatic energy analyzer was used to determine the energy spread of the ions at the source exit. The energy spread of the extracted beam for the individual species of positive hydrogen ions (H+, H2+, H3+) and that for the negative hydrogen ions (H−) was measured as well. Energy spreads as low as 2.3 eV were obtained for H+, 2 eV for H2+, 1.7 eV for H3+, and 1 eV for H−. The axial energy spread in the source exit without extraction for hydrogen and helium was measured to be approximately 1 eV for both cases. The source can generate a hydrogen beam current density of approximately 12 mA/cm2.

Quartz antenna for radio frequency ion source operation

Radio-frequency (rf) driven multicusp ion sources developed at the Lawrence Berkeley National Laboratory use an internal induction coil (antenna) for plasma generation. The copper rf-antenna with a thin layer of porcelain coating, which is presently used, cannot fully satisfy the increasing demands on source cleanliness and antenna lifetime under high power cw or pulsed operation in applications where water cooling is not possible. A quartz antenna has been designed and operated in the multicusp ion source. It has been demonstrated that the overall performance of the new antenna exceeds that of the regular porcelain-coated antenna. It can be operated with a long lifetime in different discharge plasmas. The quartz antenna has also been tested at the Paul Scherrer Institute for cw source operation at rf power higher than 5 kW. Results demonstrated that the antenna can survive under dense plasma discharge operations.

Technique for the removal of electrons from an extracted, pulsed, H− ion beam

A small, permanent‐magnet insert structure for the removal of electrons from pulsed, extracted, negative ion beams has been developed at Lawrence Berkeley National Laboratory. The device was computer modeled and designed for an extraction field strength of 3 kV/mm. The testing was carried out with a rf driven multicusp ion source optimized for the production of H− ions and pulsed at a few Hz with pulse widths of several hundreds of μs. It is demonstrated that the insert structure together with a collar can remove over 98% of electrons from the extracted H− ion beam without any significant deterioration of the H− ion output. Application to other negative ion beams can be expected from this magnetic collar insert.

Development of ion sources for ion projection lithography

Multicusp ion sources are capable of generating ion beams with low axial energy spread as required by the ion projection lithography (IPL). Longitudinal ion energy spread has been studied in two different types of plasma discharge: the filament discharge ion source characterized by its low axial energy spread, and the rf‐driven ion source characterized by its long source lifetime. For He+ ions, longitudinal ion energy spreads of 1–2 eV were measured for a filament discharge multicusp ion source which is within the IPL source requirements. Ion beams with larger axial energy spread (∼7 eV) were observed in the rf‐driven source. A double‐chamber ion source has been designed which combines the advantages of low axial energy spread of the filament discharge ion source with the long lifetime of the rf‐driven source. The energy spread of the double chamber source is approximately 2 eV.

Plasma production via laser‐induced photoemissiona)

The feasibility of laser‐induced photoemission as a driving mechanism for short plasma pulse production is currently being investigated with a pulsed excimer laser at 248 nm. Low work function materials such as LaB6 and barium are used as cathode materials and the resulting plasma characteristics are being examined. Results from early measurements of the barium photocathode show a strong dependence of the photoemitted current on the source pressure and cathode voltage. Additionally, the temporal behavior of the emitted electron pulse is found to consist of two components: a short 50 ns burst corresponding to laser‐induced photoemission, and a larger, longer 100 ns pulse increasing from the tail end of the photoemitted electron pulse.