W. B. Kunkel

Optimization of H−production from a small multicusp ion source

A small multicusp source has been developed to generate volume‐produced H− ion beams in pulsed operation. To obtain high H− current densities (J−>250 mA/cm2), this source requires relatively high gas pressure and high discharge power. Experiments have been conducted to improve the arc and gas efficiencies, the beam pulse shape, and the H− to electron ratio in the extracted beam by optimizing the filter magnetic field, the thickness and axial position of the extraction aperture in the plasma electrode, and by mixing xenon or other elements with hydrogen in the discharge. The biggest improvement is achieved by adding cesium to the source, resulting in a fivefold increase in the extractable H− current and a substantial drop in the e/H− ratio. In order to improve the lifetime of the cathode, both filament and coaxial type LaB6 cathodes have been developed and have been operated successfully in this H− source.

Production of low energy spread ion beams with multicusp sources

Abstract The use of multicusp sources to generate ion beams with narrow energy spread has been investigated. It is found that the presence of a magnetic filter can reduce the longitudinal energy spread significantly. This is achieved by creating a uniform plasma potential distribution in the discharge chamber region, eliminating ion production in the extraction chamber and in the sheath of the exit aperture and by minimizing the probability of charge exchange processes in the extraction chamber. An energy spread as low as 1 eV has been measured.

Electron‐suppression experiments in a small multicusp H− source

Several techniques for suppressing the electrons before they form part of the extracted beam have been studied in a small multicusp H− source. It is found that some schemes reduce both the electron and the H− output currents. Other approaches, such as the installation of a collar at the extraction aperture, the addition of xenon or cesium to the hydrogen discharge, or the reduction of the source plasma potential, not only can reduce the electron current substantially, but bring about an enhancement in the extracted H− current.

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 u2002mA/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.

Production of atomic nitrogen ion beams

A small multicusp ion source has been used to generate beams of nitrogen ions. To achieve a high percentage of N+ ions, the source was equipped with a magnetic filter and operated with a high N2 gas density. A nearly pure atomic nitrogen ion (N+) beam (98.5% N+, 1.5% N2+), suitable for implantation purposes, was produced. The operating characteristics of this source, including species mix, are presented.

Axial energy spread measurements of an accelerated positive ion beam

Abstract A multicusp ion source has been designed for use in ion projection lithography. Longitudinal energy spreads of the extracted positive hydrogen ion beam have been studied using a retarding field energy analyzer. It has been found that the filament-discharge multicusp ion source can deliver a beam with an energy spread less than 3 eV which is required for the ALG-1000 machine. The multicusp ion source can also deliver the current required for the application.

Applications of the constant-current variable-voltage dc accelerator

Abstract Variable beam energy at constant current is useful for industrial applications such as surface hardening and passivation or semiconductor processing. We discuss a dc accelerator, under development for the DOE magnetic fusion energy program, which uses electrostatic quadrupole (ESQ) focusing to maintain constant current while the voltage is variable over a wide range. The use of ESQs also allows a conservative design that alleviates known causes of dc voltage breakdown. The beam is accelerated by a series of 100 kV ESQ modules, stackable to a megavolt or more. The energy is tunable to as low as 20 keV without reduction in current. Single-beam and multiple-beam systems have been designed, capable of accelerating typically 50 mA of N + or O + per beam.

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.

Multicusp sources for ion beam projection lithography

Multicusp ion sources are capable of producing positive and negative ions with good beam quality and low energy spread. The ion energy spread of multicusp sources has been measured by three different techniques. The axial ion energy spread has been reduced by introducing a magnetic filter inside the multicusp source chamber which adjusts the plasma potential distribution. The axial energy spread is further reduced by optimizing the source configuration. Values as low as 0.8 eV have been achieved.

The virtual cathode: Key to the numerical simulation of negative ion extraction

The simulation of volume produced negative ions from a plasma is by far more complicated than the extraction of positive ions, while in experiments the only difficulty seemes to be connected with the power of the electrons, which are extracted at the same time. The reason for this complication in simple minded simulations is the infinite space charge, which builds up in the turning point of the positive ions in the extraction aperture for the negative ions. Smearing out the energy of the positive ions seems to help, however, this is mostly not justified by experiments, showing a low ion energy, especially in the region between the magnetic filter and the extraction hole. This difficulty may be overcome by using experience from virtual cathode formation in magnetically focused, decelerated electron beams. The decelerated electrons behave similarly to the reflected positive ions and are forming a virtual cathode in the reflection zone. From the analysis of the electron deceleration experiment, a simple powe...