Space charge neutralization of an ion beam

Details Space charge neutralization of an ion beam Space charge neutralization of an ion beam

1998-05-22

2001-12-11

Anderson, Bruce (Department: 2881)

Radiant energy

Electrically neutral molecular or atomic beam devices and...

C250S492210

Reexamination Certificate

active

06329650

ABSTRACT:

BACKGROUND
This invention relates to devices which use ion beams, specifically devices which use ion beams to treat a workpiece such as a substrate, for example, by implanting materials in the substrate or sputtering or depositing material on the substrate.
Ion beams are used in a variety of technologies to treat substrates. For example, ion implantation is a process of generating an ion beam, focusing the beam, analyzing the beam to separate a species of ions for implantation and directing it towards a substrate to implant the ions in the substrate.
A positive ion beam does not have an equal density of ions and electrons and has an inherent potential that is typically distributed nonuniformly across a cross-section of the beam. (This inherent potential of a beam is also known as the space charge of the beam.) Because the ions in the beam repel each other, they result in the beam diverging and losing density and focus as the beam travels along the beam path. This tendency depends on the perveance of the beam. A beam that has a high current and a low energy (e.g., less than 15 KeV preferably less than 5 KeV), and therefore a low speed, typically has a high perveance.
In the case of devices that use ions beams to treat substrates, fast moving particles collide with residual gases in the device and the walls of the device and generate low energy ions and secondary electrons. A positively charged ion beam traps these electrons and simultaneously rejects the positive ions. However, these electrons are typically not present in sufficient density to completely compensate for the space charge of the beam (typically referred to as “neutralizing” the space charge of the beam). This is especially the case for low energy beams, which have a low cross-section for producing secondary electrons.
Various methods have been used to further neutralize the space charge of ion beams. The goal of these methods is to generally increase the current and the stability of an ion beam by reducing the space charge of that beam. Examples of these methods include generating free electrons for introduction into the path of the beam for neutralizing the beam potential. The electron may be generated by electron sources or by plasma sources which generate a plasma formed mainly of positive ions and free electrons.
SUMMARY
In one general aspect, the invention features a device for treating a workpiece with positively charged ions. The device includes an apparatus including an ion source for producing a positive ion beam and directing the positive ion beam toward a surface of a work piece. The device further includes a source for introducing negative ions into the beam path in at least one selected region downstream of the ion source.
In another general aspect, the invention features a device for implanting positively charged ions in a workpiece. The device includes apparatus including an ion source for producing an ion beam having a perveance in the order of or greater than 0.02 (mA) (amu)
½
(KeV)
−{fraction (3/2)}
, a plurality of magnets constructed and arranged to direct the ion beam toward the surface of the work piece, a workpiece holder to hold the workpiece, and a source for injecting negative ions into the beam path in at least one selected region downstream of the ion source.
Preferred embodiments may include or more of the following features.
The source for injecting negative ions into the beam path may be a second ion source for generating the negative ions, and a passage for introducing the negative ion into the beam. The source may also be a gas injector positioned to add an electronegative gas to the ion beam in at least one selected region downstream of the ion source. A gas injector may be arranged to add the electronegative gas at a region between the plurality of magnets for directing the ion beam toward the surface of the work piece and the ion source.
The electronegative gas is a gas having an electron affinity in the order of greater than or equal to 1 eV. The gas is selected from a group of gases comprising: CF
3
, CClF
3
, BF
3
, NF
3
, SF
6
, SiF
4
, GeF
4
, O
2
, and NO
2
. The ions in the beam is selected from a group of material comprising argon, nitrogen, boron, arsine, phosphine, phosphorus, arsenic, and antimony.
The device can include any one of analyzer, scanner, or collimator magnets. A gas injector can then be arranged to add the electronegative gas at a said selected region between the ion source and the analyzer magnet, between the analyzer and the scanner magnets, in or adjacent to a gap in the scanner magnet, between the scanner magnet and the workpiece, or between the scanner and the collimator magnets.
The ion beam is a low energy ion beam, e.g. has an energy less than 5 KeV.
Embodiments of the invention may include one or more of the following advantages.
Embodiments of the invention can be retro-fitted to existing devices which use ions beams treat substrates.
A beam neutralized in accordance with the teachings of the invention can have high current density and be stable. Embodiments of the invention can also result in such a beam being analyzed effectively. Moreover, embodiments of the invention prevent beam blow up after the ion source.


REFERENCES:
patent: 3846636 (1974-11-01), Zehr et al.
patent: 4419203 (1983-12-01), Harper et al.
patent: 4585945 (1986-04-01), Bruel et al.
patent: 5055696 (1991-10-01), Haraichi et al.
patent: 5466929 (1995-11-01), Sakai et al.

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