GB2235086A

GB2235086A - Ion beam source

Info

Publication number: GB2235086A
Application number: GB8912569A
Authority: GB
Inventors: Anthony Cyril Evans
Original assignee: Ion Tech Ltd
Current assignee: Ion Tech Ltd
Priority date: 1989-06-01
Filing date: 1989-06-01
Publication date: 1991-02-20
Also published as: GB8912569D0

Abstract

An ion beam source comprises an RF excited plasma with means to extract the ions from the envelope of the plasma, the RF energy being applied capacitatively to the plasma. The extraction can be by means of extraction electrodes or by magnetic fields or both and the capacitor for applying the RF energy may have plates spaced along or transverse to the direction of the beam. As shown in Fig. 1, two semi-cylindrical capacitor plates 1, 2 are mounted externally either side of a silica tube plasma chamber, ions being extracted by virtue of the field between electrodes 3, 4. Alternatively, as in Fig. 3, the RF is applied to alternate spaced bands 2 and the plasma may be enhanced by a magnetic field formed by sets of magnets, neighbouring sets having the opposite polarity. <IMAGE>

Description

ION BEAM SOURCE Ion beam sources have been described in which electrons, provided by a heated filament, are accelerated to ionise a gas. The resulting plasma is confined in a chamber in the presence of a magnetic field to increase the length of electron trajectories, so increasing the probability of ionisation. Ions are extracted from the plasma through a grid at which a negative potential is applied with respect to the plasma (1).

Such sources are generally operated with inert gases, but when a reactive gas such as oxygen is introduced the life of the filament is drastically curtailed. To overcome this problem, RF excitation may be used to excite the plasma. In one version the RF is applied to electron emitting electrodes in the form of multiple hollow cathodes. The cathodes are shaped so as to minimise erosion by ion bombardment. The output from this source is less than that obtainable from thermal electron sources.

Another method is to apply RF field inductively direct to the ionisable gas, by surrounding the plasma chamber with a coil.

Care must be taken to minimise induction currents in adjacent metal components.

An improved method to induce an RF field is by capacitative coupling. A possible configuration consists of two semi cylindrical capacitor plates mounted either side of the plasma chamber (fig.1.), which may be a silica tube.

A preferred way in order to achieve high output, is to mount the capacitor electrodes as a set of bands round the chamber the RF potential being applied to alternate bands with the neighbouring bands earthed (fig.2.).

The plasma may be enhanced by applying a magnetic field. The most effective configuration consists of magnets with a set of poles of the same polarity mounted between each capacitor band, neighbouring sets having the opposite polarity.

The magnetic field has the further effect of aiding the extraction of the beam from the plasma chamber. Without any potential being applied to electrodes 3 and 4, a beam is extracted on application of an RF field in the presence of the magnetic field.

(1) MR Kaufman et al. J.Vac Sci Technol. 21, 725 (1982). Broadly defined, the invention comprises an ion beam source comprising an envelope for an ionisable gas, an extraction electrode for extracting ions from the envelope, a bias electrode for applying a bias in one direction relative to the extraction electrode and a capacitor for applying an RF field to the interior of the envelope. The plates of the capacitor are preferably outside the envelope and their spacing may be at right angles to said direction.

Alternatively their spacing may be parallel to said direction and the plates may be interdigitated. Magnetic poles may be applied to provide magnetic field line looping into the envelope around each finger of the interdigitated plates, adjacent poles in said direction being of opposite polarity.

The invention in another aspect provides a method of generating ions in an envelope comprising applying an RF field to an ionisable gas in the envelope by means of a capacitor, preferably in the form of interdigitated plates. Magnetic fields may loop into the envelope around each finger of the interdigitated plates, adjacent fingers having loops in opposite directions.

Claims

CLAPIS :

1. An ion beam source comprising an envelope for an ionziable gas, a bias electrode for applying a bias in one direction within the envelope and a capacitor for applying an RF field to the interior of the envelope and extraction means for extracting a beam of ions from the envelope.

2. A source as claimed in Claim 1 wherein the extraction means comprises an extraction electrode arranged so that the applied RF field is located between the electrodes.

3. A source as claimed in Claim 1 wherein the extraction means comprises means for generating a magnetic field in the region of the capacitor plates within the envelope.

4. Apparatus as claimed in Claim 2 comprising means for generating a magnetic field within the envelope in the region of the capacitor plates.

5. Apparatus as claimed in ary one of the preceding Claims wherein the capacitor plates are located outside the envelope.

6. A source as claimed in any one of Claims 1 to 5 wherein the capacitor plates are spaced at right angles to the direction in which the ion beam is extracted.

7. A source as claimed in any one of Claims 1 to 5 wherein the capacitor plates are spaced along said direction.

8. A source as claimed in Claim 7 wherein said plates are interdigitated.

9. A source as claimed in Claim 7 or Claim 8 together with Claim 3 or Claim 4 wherein the magnetic field generating means comprises poles arranged so as to generate looped magnetic fields around the capacitor plates.

10. An ion beam source substantially as hereinbefore described with reference to the accompanying drawings.