U.S. patent number 3,786,359 [Application Number 05/811,361] was granted by the patent office on 1974-01-15 for ion accelerator and ion species selector.
This patent grant is currently assigned to Alpha Industries, Inc.. Invention is credited to William J. King.
United States Patent |
3,786,359 |
King |
January 15, 1974 |
ION ACCELERATOR AND ION SPECIES SELECTOR
Abstract
The apparatus disclosed herein provides high energy positive
ions, suitable for semiconductor doping, by projecting positive
ions through an electron stripping gas at relatively low energy
thereby to obtain positive ions which are multiply ionized or
charged. Those ions which are raised to a preselected ionization
level or state are segregated, and then accelerated by a relatively
high accelerating voltage to achieve an energy suitable for ion
implantation in a semiconductor matrix. Since the ions subjected to
the relatively high accelerating voltage are multiply ionized, the
energy imparted thereto, measured in electron volts, is
substantially equal to an integer multiple of the accelerating
voltage.
Inventors: |
King; William J. (Reading,
MA) |
Assignee: |
Alpha Industries, Inc. (Woburn,
MA)
|
Family
ID: |
25206343 |
Appl.
No.: |
05/811,361 |
Filed: |
March 28, 1969 |
Current U.S.
Class: |
315/500;
250/492.1; 976/DIG.437; 438/514; 250/423R; 250/492.2 |
Current CPC
Class: |
H01J
37/3171 (20130101); G21K 1/14 (20130101); H01J
49/284 (20130101) |
Current International
Class: |
G21K
1/00 (20060101); H01J 49/28 (20060101); G21K
1/14 (20060101); H01J 49/26 (20060101); H01J
37/317 (20060101); H01j 023/10 (); H01j
037/26 () |
Field of
Search: |
;328/233 ;250/49.5T |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Maguire, "Ion Implants Forge Tailor-Made Junctions," Electronics;
April 19, 1963; pages 26-29.
|
Primary Examiner: Segal; Robert
Attorney, Agent or Firm: Kenway, Jenney and Hildreth
Claims
What is claimed is:
1. Apparatus for providing high energy positive ions to a given ion
utilization means, said apparatus comprising:
an ion source for providing positive ions;
means for defining an electron stripping canal;
means for providing to said canal a gas adapted to strip additional
electrons from positive ions projected through said canal;
means for applying a relatively low accelerating voltage to ions
emitted from said source thereby to project ions from said source
through said canal and to thereby increase the extent of ionization
of at least a portion of said ions;
means associated with said canal defining means for selecting a
particular multiply ionized species in the ions emerging from said
canal, species at other ionization levels being blocked; and
means for applying a relatively high accelerating voltage to said
ions of the selected species thereby to further accelerate said
ions of the selected species toward said utilization means, the
energy thusly imparted to said ions of the selected species being
substantially related to the product of said relatively high
accelerating voltage and the multiple charge on each such selected
ion.
2. Apparatus as set forth in claim 1 wherein said utilization means
is substantially at ground potential and said source is at a
relatively high positive potential.
3. Apparatus as set forth in claim 2 including means for scanning a
beam of ions provided by said apparatus.
4. Apparatus as set forth in claim 1 wherein said ion source is a
duoplasmatron.
5. Apparatus as set forth in claim 1 wherein said relatively low
accelerating voltage is generally in the range of from 10 to 150
kilovolts.
6. Apparatus as set forth in claim 1 wherein said relatively high
accelerating voltage is generally in the range of from 100 to 600
kilovolts.
7. Apparatus as set forth in claim 1 wherein said selecting means
comprise means for applying crossed magnetic and electric fields to
ions emerging from said stripping canal.
8. Apparatus as set forth in claim 1 including means for
selectively passing ions of said selected species after
acceleration by said relatively high voltage.
9. Apparatus as set forth in claim 8 wherein said means for
selectively passing ions comprises means for applying an analyzing
magnetic field.
10. Apparatus for providing high energy positive ions to a given
ion utilization means, said apparatus comprising:
an ion source for providing positive ions;
means for defining an electron stripping canal;
means for providing to said canal a gas adapted to strip additional
electrons from positive ions projected through said canal;
means for focusing a beam of said ions on said canal;
means for selecting a particular multiply ionized species of said
ions emerging from said canal, species at other ionization levels
being blocked;
means for focusing a beam of the ions emerging from said canal on
said selecting means; and
means for applying a relatively high accelerating voltage between
said selecting means and said utilization means whereby ions of
said selected species are accelerated to energies which are
substantially proportional to the product of said relatively high
accelerating voltage and the multiply charge on each such selected
ion.
Description
BACKGROUND OF THE INVENTION
It has heretofore been proposed to manufacture various
semiconductor devices by implanting ions in selected portions of a
semiconductor material using an energetic ion beam, thereby to
achieve a desired localized doping of the semiconductor material.
However, to achieve the desired depths of implantation, quite high
energies, e.g., up to a million electron volts, are required in
certain application. Heretofore, it has been contemplated that such
high energies would be provided by employing gas insulated
potential sources such as accelerators of the type typically
employed for scientific investigations. However, for the purpose of
commerically producing semiconductor devices, such machines are
much too inflexible, too difficult to service, and too limited in
ion energy range for given machine size. Further, such machines are
typically quite limited in the maximum available ion current.
Among the several objects of the present invention may be noted the
provision of apparatus for providing energetic ions using a primary
accelerating voltage which is only a fraction of the achieved ion
energy; the provision of such apparatus which may be air insulated;
the provision of such apparatus which will provide a substantial
ion current; the provision of such apparatus which is relatively
flexible and easy to service; the provision of such apparatus which
is readily adaptable to semiconductor manufacture by ion
implantation; the provision of such apparatus which is relatively
simple and inexpensive. Other objects and features will be in part
apparent and in part pointed out hereinafter.
SUMMARY OF THE INVENTION
Briefly, apparatus according to the present invention is adapted to
provide high energy positive ions to a given ion utilization means,
e.g., a means for presenting semiconductor materials to the ion
beam for doping. Positive ions are projected by a relatively low
accelerating voltage into a stripping canal. The stripping canal is
supplied with a gas adapted to strip additional electrons from the
positive ions projected through the canal thereby to increase the
extent of ionization of at least a portion of those ions. Those
ions which are raised to a particular multiple level of ionization
are selected, e.g., by means of a suitable filter, and are then
subjected to a relatively high accelerating voltage. The
preacceleration selection is important if unnecessary beam tube
current loading is to be avoided. Overloading of the beam tube by
undesired ion species might preclude achieving useful currents of
ions of the desired ionization state and species, since the latter
may represent only a small fraction of the total ion current
leaving the stripping canal. The energy thusly imparted to the
selected ions is substantially proportional to the product of the
relatively high accelerating voltage and the charge on the selected
ions. Thus relatively high energies are imparted to the ions
reaching the utilization means.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE is a diagrammatic illustration of an ion
accelerator according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawing, a source of positive ions is
indicated at 11. For facilitating the use of this apparatus in
semiconductor device manufacture, source 11 is preferably of a high
current type such as a duoplasmatron. A stripping canal assembly is
indicated at 13. As illustrated, the stripping canal assembly
comprises a T-shaped tubular structure arranged to permit a beam of
ions to pass through the cross bar of the T and to permit a
stripping gas to be introduced into the beam path through the
transverse tubular section. The ions emitted by source 11 are
focused, e.g., by a so-called Einzel lens as indicated at 15, so
that the ion stream or beam passes precisely through the canal in
the cross bar of the assembly 13. A gas stripping medium is
discussed for illustrative purposes only. Other stripping media
(e.g., solid materials in form of thin diaphragms or fine liquid
jet sprays) could also be used.
Ions emerging from the stripping canal pass through a second Einzel
lens 17 for re-focusing and then through a so-called Wien filter as
indicated at 19. As is understood by those skilled in the art, the
Wien filter applies crossed magnetic and electric fields to the
beam passing therethrough. By proper selection of the field
strengths, the filter operates to deflect all ions passing
therethrough except those which are of a particular selected
species or degree of ionization. Those ions which are deflected are
intercepted by a catcher 21, while ions of the selected species
pass through the catcher 21 into a main accelerating tube 23.
Catcher 21 may, for example, merely comprise an apertured
plate.
A relatively low accelerating voltage, e.g., 10-40 kV, is normally
applied to the ion stream prior to its entry into the main
accelerating tube 23. To achieve higher stripping cross-sections,
however, it may be desirable to raise this voltage to 100-150 kV,
especially on machines where the total available voltage is higher,
e.g., 300-600 kV. As is understood by those skilled in the art, the
distribution of this potential between the ion source 11 and the
catcher 21 will depend upon the configurations of the lens
assemblies 15 and 17. Typically, a substantial portion of the field
gradient will be applied immediately adjacent the source to provide
adequate ion extraction and to impart appreciable energy to the
ions before they reach the stripping canal, while the gradient in
the vicinity of the stripping canal assembly 13 will be minimized
to avoid the deflection of ions there. Since positive ions are to
be accelerated, the source 11 will be positive with respect to the
catcher 21 or the input end of the main accelerating tube 23.
A relatively large accelerating voltage, e.g., 100-600 kV, is
applied across the length of the accelerating tube 23. While such a
voltage is relatively high as compared with the voltage applied
between the source and the input end of the tube 23, it is still
within the range which can be achieved by air insulated voltage
sources.
Assuming that a suitable gas, which normally will have an atomic
weight similar to that of the desired ion species to maximize the
stripping cross-section, is supplied to the stripping canal,
additional electrons will be stripped from at least a portion of
the positive ions projected through the canal so that these ions
will then be multiply ionized, e.g., some of the ions passing
through the stripping canal will become doubly or triply, etc.,
ionized. This is to be contrasted with a reversal in the polarity
of ionization to achieve twice the energy as in the so-called
tandem machine configuration.
The parameters of the filter 19 are adjusted so that only ions of a
particular species pass through the catcher 21 for further
acceleration, e.g., only those ions which are triply ionized.
Accordingly, current drawn from the high voltage source is not
wasted in accelerating unwanted ions. Further, since the ions
passed by catcher 21 are multiply ionized, the energy imparted to
each such ion in transversing the main accelerating tube 23 will be
substantially equal to an integer multiple of the accelerating
voltage, e.g., 600, 900, 1,200, etc, keV in the case of a 300 kV
accelerating voltage. Accordingly, relatively high energies are
obtained even though the source voltage employed is below the range
requiring gas insulation. In order to completely eliminate all
particles other than the desired ion species emerging from the
accelerating tube 23, the ion beam is preferably passed through the
field of an analyzing magnet as indicated at 25. As is understood
by those skilled in the art, such a magnetic field acts as a
mass-energy filter so that stray ions, e.g., such as may be
generated by random collisions within the accelerating tube, are
eliminated. The ion beam emitted from the magnet structure 25 is
thus substantially spectrally pure and may be utilized in the
production of semiconductor devices. A crossed-field filter could
also be used for this purpose.
Means for utilizing the ion beam in this fashion are indicated
generally at 27. Such means may, for example, comprise apparatus
for presenting a substrate 29 of semiconductor material to the beam
for irradiation, thereby to selectively dope the semiconductor
lattice with the material the ions of which are provided by the
source 11. The beam is made to uniformly cover the desired portion
of the semiconductor surface by electrostatically scanning with
vertical and horizontal plates, 30 and 31 respectively. Preferably
the beam utilization means is maintained at ground potential to
facilitate manipulation of semiconductor materials to be treated
while the stripping canal and the ion source are maintained at
relatively high positive potentials with respect to ground. A
vacuum interlock mechanism may be provided to allow insertion and
removal of substrate 29 without the necessity of shutting off the
accelerator.
In view of the foregoing it will be seen that the several objects
of the invention are achieved and other advantageous results
attained.
As various changes could be made in the above construction without
departing from the scope of the invention, it is intended that all
matter contained in the above description or shown in the
accompanying drawing shall be interpreted as illustrative and not
in a limiting sense.
* * * * *