Ion source

Details Ion source Ion source

1998-05-22

2001-02-06

Anderson, Bruce C. (Department: 2881)

Radiant energy

Ion generation

Field ionization type

C250S492210

Reexamination Certificate

active

06184532

ABSTRACT:

BACKGROUND
This invention relates to an ion source, specifically an ion source for use in an ion implanter for implanting ions in a substrate.
In manufacturing semiconductors through ion implantation several types of ion sources are typically used. Ion implantation requires ion sources with long operational life and high ion source efficiency. One ion source used in ion implantation is the Bernas type ion source which has been widely accepted in ion implantation.
FIG. 1
shows a top view of a single filament Bernas type ion source
1
with its top plate removed. Ion source
1
has a cathode
12
connects to a power source that drives cathode
12
to therminiocally emit electrons. Walls
14
of ion source
14
are biased relative cathode
12
so as to act as an anode. A repeller plate
18
is positioned behind cathode
12
and another repeller plate
16
is positioned across from cathode
12
. The ion source is placed in a uni-directional magnetic field, as shown in FIG.
1
.
During operation, a gas to be ionized is discharged into the chamber and is ionized by electrons emitted from cathode
12
. Repeller plates
16
and
18
reflect primary fast electrons emitted from cathode
12
and generate an oscillatory electron movement along the axis of the magnetic field. In this manner, a plasma is generated in the ion source between cathode
12
and walls
14
for extraction by an extraction electrode outside ion source
1
.
When the ion source operates, material such as vaporized metal from cathode
12
are deposited and sputtered on walls
14
and create a film on walls
14
. Because this material is usually adhered weakly to walls
12
, it can generate particles and file-flakes which in turn can short out the cathode and anode, for example, by resting acrose insulations
18
.
SUMMARY
In one general aspect, the invention features an ion source constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction and a source of ionizable material for creating ion. The ion source includes a chamber, defined by walls, and a relatively narrow outlet aperture for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is positioned with respect to the aperture, the cathode and the predetermined direction of the magnetic flux to cause ions produced in the chamber to concentrate near the aperture.
In another general aspect, the invention features an ion source constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction. The ion source includes a chamber defined by walls, and a relatively narrow, elongated outlet slit for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is elongated and positioned adjacent to and generally parallel to the slit. The ion source and magnet being relatively positioned such that the magnetic flux lines are generally parallel to the anode and at an angle to an electrical field produced between the anode and the cathode.
In yet another aspect, the invention features an ion implanter for implanting ions in a work piece. The ion implanter includes an ion source, a plurality of magnets to focus and scan the ion beam in a first direction, and a workpiece holder to hold the workpiece and to move perpendicular to the first direction. The ion source is constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction. The ion source includes a chamber defined by walls, and a relatively narrow, elongated outlet slit for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is elongated and positioned adjacent to and generally parallel to the slit. The ion source and magnet being relatively positioned such that the magnetic flux lines are generally parallel to the anode and at an angle to an electrical field produced between the anode and the cathode.
Preferred embodiments of the invention may include one or more of the following features.
The aperture is a relatively narrow, elongated slit. The anode is elongated and positioned adjacent to and parallel to the aperture and may extend substantially the full length of the slit-form aperture. The anode is of generally rod form. The elongated anode is arranged to be substantially parallel with the predetermined direction of the magnetic flux.
The chamber is elongated in the direction of the elongated slit, two cathodes are located at each of the two ends. The cathodes are positioned symmetrically at either end of the chamber relative to the elongated slit and the anode. A negatively biased electrode can be used for sputtering material into the chamber for ionization.
The walls of the chamber can have a potential selected to deflect electrons. The walls of the chamber can have substantially the same potential as the cathode. The cathode can be a hot, indirectly heated, or cold cathode. The cathode can be a coil of tungsten wire, the coil having a generally circular form.
A magnet produces a magnetic field having flux lines in the above predetermined direction. The anode and chamber lie within the magnetic field. The magnet is arranged relative to the aperture and electrical field condition produced within the chamber to apply a force to the ions in the direction of the aperture. The lines of the magnetic field cross lines of an electrical field generated between the cathode and the anode. The anode is positioned with respect to the cathode to cause an electrical field between the anode and the cathode to concentrate the ions near the anode. The anode is positioned with respect to the aperture, the cathode, and the magnetic flux lines to cause ions near the anode to drift towards the aperture.
Embodiments of the invention may include one or more of these advantages.
Embodiments of ion source have efficient ion production because the anode being separated from the walls of the source allows the walls of the ion source to float relative to the anode and reach a potential close to that of the cathode potential. This results in the walls acting as an electron reflector rather than an anode. Therefore, the electrons can only be absorbed by an anode that is smaller than the walls. Therefore, the electrons trace an extended path in the source and increase the efficiency of the ion source.
In some embodiments, the material deposited on the walls strongly adhere to the walls, reducing the flaking of deposited material. This in turn reduces the possibility of the flakes short circuiting the source.
In other embodiments, because the cathode and the walls have the same potential, arcing in the source is reduced.
In some embodiments, the location of the anode relative to the magnetic field in which the ions source operates causes the plasma to drift towards the ion source emission slit and to concentrate near the emission slit. This increases the efficiency of extracting ions from the ion source and the current of the extracted beam.


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