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Ion Microbeam Source

IP.com Disclosure Number: IPCOM000050560D
Original Publication Date: 1982-Nov-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 5 page(s) / 59K

Publishing Venue

IBM

Related People

Cuomo, JJ: AUTHOR [+2]

Abstract

A typical electron-bombardment ion source is shown in Fig. 1. The gas to be ionized is introduced into the outer shell 2 of the ion source. Energetic primary electrons are emitted by the cathode 3. At the low pressures normally of interest, the primary electrons would rapidly escape to the anode 4, except for the magnetic field lines 5 between the anode and cathode. The primary electrons process around the axis of the discharge chamber (the volume enclosed by the anode 4 and the ends of the outer shell 2) until collisions permit them to cross the magnetic field lines. Some of the collisions generate ions, which, if they reach the aperture 6, can be extracted by the negative potential on electrode 7. The ions of interest pass through aperture 8 and become the departing ion beam 9.

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Ion Microbeam Source

A typical electron-bombardment ion source is shown in Fig. 1. The gas to be ionized is introduced into the outer shell 2 of the ion source. Energetic primary electrons are emitted by the cathode 3. At the low pressures normally of interest, the primary electrons would rapidly escape to the anode 4, except for the magnetic field lines 5 between the anode and cathode. The primary electrons process around the axis of the discharge chamber (the volume enclosed by the anode 4 and the ends of the outer shell 2) until collisions permit them to cross the magnetic field lines. Some of the collisions generate ions, which, if they reach the aperture 6, can be extracted by the negative potential on electrode 7. The ions of interest pass through aperture 8 and become the departing ion beam 9.

Both the primary electrons and the electrons produced in ionizing collisions must cross the magnetic field lines by a series of collisions. This general process of crossing magnetic field lines is indicated in the cross section of Fig. 1B. An electron path is followed starting at point 10. This point can be the point of origin for a secondary electron generated by the ionization of a gas atom, or it can be a collision point for an electron previously emitted by the cathode. Under the influence of some small radial electric field between the cathode 3 and the anode 4, the electron will follow a cycloidal path 11 until it has another collision 12. (The electric field between the anode and cathode is usually greatly reduced by the presence of the discharge-chamber plasma.) After this new collision, the electron will, on the average, follow another cycloidal path 13 that is closer to the anode 4. After following this new path, it will have another collision 14 which, on the average, will result in a new cycloidal path 15 that is again closer to the anode 4. In the case of path 15, it is shown to intersect the anode.

The outer shell 2 is normally at the potential of the cathode so that, with most collisions being oblique, it is energetically difficult for electrons to escape to the outer shell. The total electron current to the anode therefore consists of the electrons from the cathode together with the secondary electrons from the ionizing collisions. The total collision cross section for electrons increases sharply at a few eV (the normal Maxwellian energy range) due to greatly increased electron electron cross sections (the cross section varies as (eV) /-2/). (The large cross section at low energy also is the cause of the randomization of low energy electrons into a Maxwellian distribution.) As the result of the preferential low energy collisions, the Maxwellian electrons preferentially diffuse across the magnetic field lines -- relative to the higher energy primary electrons. This preferential mobility is desirable in that it tends to conserve electron energy within the discharge-chamber plasma, rather than let it escape to the anode....