Ion implantation apparatus capable of increasing beam current

Details Ion implantation apparatus capable of increasing beam current Ion implantation apparatus capable of increasing beam current

2002-04-09

2004-09-21

Lee, John R. (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S3960ML, C250S42300F

Reexamination Certificate

active

06794661

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ion implantation apparatus.
2. Description of the Related Art
In the recent technology of ion implantation, along with the requirement to provide fine structures in semiconductor devices, the energy applied to ions, which are used for implantation, has been decreased more and more in order to decrease the depth of implantation. At low energies, ions traveling along a beam line from an ion source to a substrate increase the beam size in radius, because the ions in an ion beam repel each other by means of their own charge. This is referred to as the space charge effect, and this effect occasionally causes a problem in that a sufficiently large ion beam current cannot be obtained due to the decreased efficiency in the transportation of the ions.
The ion beam diverges since the positive charges therein are forced to be separated from each other due to the repulsive force therebetween. This space charge effect becomes more prominent with a decrease in the ion energy as well as with an increase in the ion beam current. Therefore, it is very important to suppress the space charge effect and thereby to decrease the divergence of the ion beam due to the repulsion of charged particles in order to obtain a high ion beam current, particularly in an ion implantation apparatus in which low-energy ions are used.
The space charge effect resulting from the fact that the ions each have a positive charge can be reduced by the charge cancellation or charge neutralization, if electrons and/or ions having a negative charge exist among the positive charges. Referring now to
FIG. 1
, the neutralization mechanism will be explained.
In
FIG. 1
, ions generated in an ion source
11
can be extracted as an ion beam with the aid of a set of extraction electrodes
12
. The ions thus extracted pass through both a mass analysis magnet (not shown) and a mass analysis slit
13
, and then are implanted into a substrate
14
, which is disposed in a process chamber
15
. The pathway of the ion beam from the ion source
11
to the process chamber
15
is hermetically sealed by means of a housing, i.e., a vacuum chamber
16
. The constitution extending from the ion source
11
to the process chamber
15
is referred to as an ion beam line.
In the ion beam line, there are a number of electrons, secondary electrons, and negative ions. The electrons are generated by the ionization of residual gas molecules in the vacuum chamber
16
, when the ion beam collides with the residual gas molecules. The secondary electrons are generated from structural elements arranged along the ion beam line when the ion beam collides with these elements. Irradiating molecules or atoms with the electrons generates the negative ions.
In
FIG. 1
, P
1
represents the collision of ions with the extraction electrode
12
. P
2
represents the collision of ions having a greater mass than the primary ions with the inner wall of the vacuum chamber
16
. P
3
represents the collision of ion beams with the residual gas in the vacuum chamber
16
. P
4
represents the collision of ions having a lighter mass than the primary ions with the Inner wall of the vacuum chamber
18
. P
5
represents the collision of the ion beam with the mass analysis slit
13
. P
6
represents the collision of the ion beam with the substrate
14
which is a target.
In order to increase the beam current at the final stage by reducing the space charge effect, using the above-mentioned effect, the following method has been proposed. A gas is intentionally introduced into the vacuum chamber
16
constituting the ion beam line, and then the gas is ionized by the ion beam. As a result, the space charge effect is reduced by the electrons generated due to the ionization.
As described, for instance, in Japanese patent publication No. 2765111, the introduction of the gas into the ion beam transportation line causes the frequency at which the ions in the ion beam collide with gaseous molecules in the ion beam trajectory to increase. As a result, the actual number of electrons in the ion beam increases so that the space charge effect in the ion beam can be reduced.
As described in Japanese Unexamined Patent Publication No. 11-96961, another method has been proposed, wherein molecules, for instance, water (H
2
O), which can easily be polarized and thereby provide negative ions, are used to neutralize the space charges. The above-mentioned two methods are proposed for attaining the same object.
The above method of introducing the gas into the vacuum chamber provides a greater amount of secondary electrons. However, the ion beam current loss increases due to the collision of the ions with the gas. As a result, the beam current reaches a maximum at a certain gas pressure and thereafter decreases, even if the amount of introduced gas is further increased.
As described above, the generation and disappearance of electrons are repeated in the vacuum chamber
16
, and thereby the amount of electrons at each moment is determined by the balance of the generation and disappearance. From this viewpoint, it is necessary to increase the number of electrons at a certain balance in order to completely eliminate the space charge effect.
In other words, it is effective to lengthen the lifetime of the electrons and negative ions existing during the interval between the generation and the disappearance in order to substantially increase the amount of electrons necessary for neutralizing the charges in the ion beam without loss of the ion beam current itself.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an ion implantation apparatus capable of easily increasing the beam current, particularly in the low energy range.
An ion implantation apparatus according to the present invention comprises an ion beam line extending from an ion source for generating ions to a substrate. The ion beam line includes extraction electrodes for extracting ions from the ion source as an ion beam, a mass analysis magnet apparatus for carrying out mass analyzing of the extracted ions, and a mass analysis slit through which the analyzed ions pass. The ions passed through the mass analysis slit are implanted into the substrate directly or after being accelerated or decelerated.
According to aspect of the present invention, the ion beam line is provided with a magnet assembly for forming cusp magnetic fields.
For instance, the magnet assembly is disposed inside the mass analysis magnet apparatus and in sections of the ion beam line that continue to front and rear parts of the mass analysis magnet apparatus.
The magnet assembly may be disposed in a section of the ion beam line that extends from the vicinity of the extraction electrodes to the vicinity of the exit of the mass analysis magnet apparatus.
The magnet assembly may be disposed in a section of the ion beam line that extends from the vicinity of the extraction electrodes to the entrance of the mass analysis magnet apparatus.
The magnet assembly may be disposed in a section of the ion beam line that extends from to the exit of the mass analysis magnet apparatus to the vicinity of the mass analysis slit.
The magnet assembly may be disposed in a section of the ion beam line that extends from the vicinity of the mass analysis slit to the substrate.
The magnet assembly may be disposed in a section of the ion beam line that extends from the vicinity of the entrance of the mass analysis magnet apparatus to the vicinity of the mass analysis slit.
The magnet assembly may be disposed in a section of the ion beam line that extends from the vicinity of the extraction electrodes to the vicinity of the mass analysis slit via the mass analysis magnet apparatus.
The magnet assembly may be disposed in a section of the ion beam line that extends from the vicinity of the extraction electrodes to the vicinity of the substrate via both the mass analysis magnet apparatus and the mass analysis slit.
According to another aspect of the present invention, the distance be

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