Multi-cusp ion source

Details Multi-cusp ion source Multi-cusp ion source

1998-05-19

2001-09-25

Day, Michael H. (Department: 2879)

Electric lamp and discharge devices

With positive or negative ion acceleration

Extraction or target electrode

C313S231310, C313S231410

Reexamination Certificate

active

06294862

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to an ion source for ion implantation equipment and more specifically to an ion source having a magnetic field that enhances performance of the ion source.
BACKGROUND OF THE INVENTION
Ion implantation has become a standard accepted technology used in doping workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of the contamination of the workpiece by airborne particulates.
Conventional ion sources consist of a plasma confinement chamber, which may be formed from graphite, having an inlet aperture for introducing a gas to be ionized into a plasma and an exit aperture through which the plasma is extracted to form the ion beam. The plasma comprises ions desirable for implantation into a workpiece, as well as ions which are not desirable for implantation and which are a by-product of the ionization process. The plasma also includes electrons of varying energies.
One example of an ionizing gas is phosphine (PH
3
). When phosphine is exposed to a high energy source, such as high energy electrons or radio frequency (RF) energy, the phosphine can disassociate to form positively charged phosphorous (P
+
) ions for doping the workpiece and hydrogen ions. Typically, phosphine is introduced into the plasma confinement chamber and then exposed to the high energy source to produce both phosphorous ions and hydrogen ions. The phosphorous ions and the hydrogen ions are then extracted through the exit aperture into the ion beam. If hydrogen ions in the beam or high energy electrons find their way to the surface of the workpiece, they may be implanted along with the desired ions. If sufficient current densities of hydrogen ions or high energy electrons are present, these ions and electrons may cause an unwanted increase in the temperature of the workpiece that may damage structures such as resists on the surface of the substrate, which are employed to mask regions of the workpiece.
In order to reduce the number of unwanted ions and high energy electrons contained within the ion beam, it is known to provide magnets within the source chamber to separate the ionized plasma The magnets confine undesirable ions and high energy electrons to a region of the source chamber away from the exit aperture and confines the desirable ions and low energy electrons to a region of the source chamber near the exit aperture. Such a magnet arrangement is shown in the applicant's commonly-owned, co-pending U.S. patent application Ser. No. 09/014,472, filed Jan. 28, 1998, entitled Magnetic Filter For Ion Source, now U.S. Pat. No. 6,016,036, issued Jan. 18, 2000, which is incorporated by reference herein as if fully set forth. Other related examples of magnet configurations within an ion source chamber are shown in U.S. Pat. Nos. 4,447,732 and 4,486,665 to Leung et al. The Leung references show a magnetic filter comprised of a plurality of longitudinally extending magnets oriented parallel to each other. The Leung '665 patent also shows a negative ion source having a plasma grid assembly. The plasma grid assembly has a plurality of spaced-apart conductive grid members positioned adjacent the ion extraction zone.
An object of the present invention is to improve upon known ion sources having magnetic filters by forming an ion source having an enhanced magnetic field.
SUMMARY OF THE INVENTION
The ion source of the present invention achieves the objects of the invention by providing a plasma electrode which can form a generally planar wall section of an ion source confinement chamber and having at least one primary magnet and an opposing magnet oriented relative to an opening in the plasma electrode, such that the magnets form a magnetic field extending across the opening. This magnetic field improves the confinement of the plasma within the confinement chamber and filters high energy electrons from the ion beam.
One aspect of the invention provides for an ion source having a plasma electrode with at least one opening for allowing an ion beam to exit the confinement chamber and having at least one primary magnet and an opposing magnet. The primary magnet is coupled to the plasma electrode and is oriented to present one pole along an edge of the opening in the plasma electrode. The opposing magnet is coupled to the plasma electrode and is oriented to present an opposite pole along an opposing edge of the opening in the plasma electrode. The primary magnet and the opposing magnet generate a magnetic field that extends across the opening in the plasma electrode through which the ion beam passes.
According to another aspect of the invention, improved ion beam performance is achieved through a removable and replaceable plasma electrode. The plasma electrode includes at least one opening for allowing an ion beam to exit the confinement chamber and includes at least one primary magnet and an opposing magnet. The primary magnet and the opposing magnet are oriented relative to edges of the opening in the plasma electrode such that they generate a magnetic field that extends across the opening.
Other features of the invention include a power supply for negatively biasing the plasma electrode relative to the plasma confinement chamber and an insulator for electrically insulating the plasma electrode. The openings in the plasma electrode can be fashioned as elongated slots or circular opening aligned along an axis. In the case of an array of circular openings, the primary magnet and the opposing magnet are positioned relative to the openings such that the magnetic field is generally oriented at an angle &THgr; relative to the axis, where the angle &THgr; is greater than 0 degrees and less than 90 degrees. The invention can further include cooling tubes for transferring heat away from the magnets coupled with the plasma electrode. The cooling tubes can be mounted adjacent to the magnets or the tubes can enclose the magnets.


REFERENCES:
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patent: 0 054 621 (1982-06-01), None
Forrester, A. Theodore,Large Ion Beams, Fundamentals of Generation and Propagation, (A Wiley-Interscience publication, 1988) pp. 204-227. (No month).
Takagi, K., et al., “A High Current Sheet Plasma Ion Source”,Nuclear Instruments and Methods in Physics ResearchB37/38 (1989) pp. 169-172 (North-Holland, Amsterdam) (No month).

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