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Claims  |
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Having thus described my invention, what I claim as new and desire to
secure by Letters Patent is as follows:
1. An ion beam deposition apparatus for producing low temperature epitaxial
growth of a semiconductor material including at least a first ion source,
said ion source providing a particular pattern, said ion beam deposition
apparatus including
a magnet means having means for mass-analyzing an ion beam from at least
said first ion source including means for focussing said particular
pattern in at least one plane into a pattern of high aspect ratio at a
focal or crossover point located prior to a target.
2. An apparatus as recited in claim 1, wherein said magnet means includes
an arcuate ion optical axis, two pole pieces joining poles of a first
magnet positioned near a center of said arcuate ion optical axis and
corresponding poles of a second magnet positioned opposite said ion beam
for said first magnet, thereby defining a cavity within said magnet means.
3. An apparatus as recited in claim 2 wherein said first and second magnets
are electromagnets.
4. An apparatus as recited in claim 1, further including at least one
second ion source.
5. An apparatus as recited in claim 4, further including means for
converging an ion beam from said at least one second ion source with said
beam from said first ion source.
6. An apparatus as recited in claim 5, wherein said means for converging an
ion beam from said at least one second ion source with said beam from said
first ion source includes
means for selectively deflecting said beam from said at least one second
ion source to selected locations within said cavity of said magnet means.
7. An apparatus as recited in claim 6, wherein said means for selectively
deflecting said beam comprises a split deflector, respective portions of
which are electrically isolated from each other and adjacent to opposite
sides of said ion beam.
8. An apparatus as recited in claim 1, wherein said ion source is a
multi-aperture ion source and
said magnet means includes means for adjusting the focal length thereof to
converge beams from said multi-aperture source in said at least one plane.
9. An apparatus as recited in claim 8, further including at least one
second ion source.
10. An apparatus as recited in claim 9, further including means for
converging an ion beam from said at least one second ion source with said
beam from said first ion source.
11. An apparatus as recited in claim 10, wherein said means for converging
an ion beam from said at least one second ion source with said beam from
said first ion source includes
means for selectively deflecting said beam from said at least one second
ion source to selected locations within said cavity of said magnet means.
12. An apparatus as recited in claim 11, wherein said means for selectively
deflecting said beam comprises a split deflector, respective portions of
which are electrically isolated from each other and adjacent to opposite
sides of said ion beam.
13. An apparatus as recited in claim 9, further including means for
confining a plasma occurring within said magnet means.
14. An apparatus as recited in claim 13, wherein said means for confirming
a plasma comprises electrostatic plasma confinement means.
15. An apparatus as recited in claim 13, wherein said means for confining a
plasma comprises magnetic mirror plasma confinement means.
16. An apparatus as recited in claim 13, wherein said means for confining a
plasma comprises multipole magnetic plasma confinement means.
17. An apparatus as recited in claim 9, further including means,
independent of said ion beam, for producing a plasma from background gas
within said magnet means.
18. An apparatus as recited in claim 17, wherein said means for producing a
plasma includes RF magnetic field producing means.
19. An apparatus as recited in claim 18, wherein said RF magnetic field
producing means includes a pair of coils within said cavity of said magnet
means and adjacent said first and second magnets and oriented to produce
an RF magnetic field generally perpendicular to a magnetic field produced
by said magnet means.
20. An apparatus as recited in claim 17, wherein said means for producing a
plasma includes a microwave wave guide means for conveying microwave
energy to background gas in said cavity of said magnet means.
21. An apparatus as recited in claim 17, wherein said means for producing a
plasma includes a capacitive coupling means for coupling RF energy to
background gas in said cavity of said magnet means.
22. An ion beam deposition apparatus for producing low temperature
epitaxial growth of a semiconductor material including at least a first
ion source including
a magnet means having means for mass-analyzing an ion beam from at least
said first ion source including means for focussing said ion beam into a
pattern of high aspect ratio, and
means for confining a plasma occurring within said magnet means.
23. An apparatus as recited in claim 22, wherein said means for confining a
plasma comprises electrostatic plasma confinement means.
24. An apparatus as recited in claim 22, wherein said means for confining a
plasma comprises magnetic mirror plasma confinement means.
25. An apparatus as recited in claim 22, wherein said means for confining a
plasma comprises multipole magnetic plasma confinement means.
26. An ion beam deposition apparatus for producing low temperature
epitaxial growth of a semiconductor material including at least a first
ion source including
a magnet means having means for mass-analyzing an ion beam from at least
said first ion source including means for focussing said ion beam into a
pattern of high aspect ratio, and
means, independent of said ion beam, for producing a plasma from background
gas within said magnet means.
27. An apparatus as recited in claim 26, wherein said means for producing a
plasma includes RF magnetic field producing means.
28. An apparatus as recited in claim 27, wherein said RF magnetic field
producing means includes a pair of coils within said cavity of said magnet
means and adjacent said first and second magnets and oriented to produce
an RF magnetic field generally perpendicular to a magnetic field produced
by said magnet means.
29. An apparatus as recited in claim 26, wherein said means for producing a
plasma includes a microwave wave guide means for conveying microwave
energy to background gas in said cavity of said magnet means.
30. An apparatus as recited in claim 26, wherein said means for producing a
plasma includes a capacitive coupling means for coupling RF energy to
background gas in said cavity of said magnet means.
31. A method of depositing a material on a surface by ion deposition of a
material from an ion beam from at least one ion source providing a
particular pattern including the step of
simultaneously mass analyzing and focussing said particular pattern in at
least one plane at a focal or crossover point located prior to a target
plane with a single magnet means having an arcuate ion optical axis.
32. A method as recited in claim 31, including the further step of
subjecting at least a portion of said ion beam to a selectable path length
different from a path length of another portion of said beam in dependence
on the location along a width of said magnet means where said portion of
said ion beam enters said magnet means.
33. A method as recited in claim 32, including the further step of
producing said another portion of said ion beam from said at least one ion
source and producing said portion of said ion beam with a further ion
source.
34. A method as recited in claim 33 including the further step of steering
said portion of said ion beam to a selected location along said width of
said magnet means.
35. A method as recited in claim 31, including the further step of
confining plasma produced within said magnet means to locations separated
from poles of said magnetic means.
36. A method as recited in claim 31, including the further step of
producing a plasma within said magnet means independently of said ion
beam.
37. A method as recited in claim 35, including the further step of
producing a plasma within said magnet means independently of said ion
beam.
38. An apparatus for producing an ion beam within a volume of ionizable gas
at high vacuum, a predetermined fraction of said ionizable gas being
ionized by said ion beam including
means for increasing the concentration of electrons available for space
charge neutralizing said ion beam from said ionizable gas.
39. An apparatus as recited in claim 38, wherein said means for increasing
said concentration of electrons comprises means for confining said
electrons within said volume of said ionizable gas.
40. An apparatus as recited in claim 38, wherein said means for increasing
said concentration of electrons comprises means for ionizing an additional
fraction of said ionizable gas. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to charged particle beam systems
and, more particularly, to ion beam systems for depositing material on a
semiconductor structure during manufacture of an electronic device,
especially in providing epitaxial growth of layers thereof.
2. Description of the Prior Art
The manipulation of beams of charged particles with electrical and magnetic
fields has long been known and many devices exploiting this effect have
been developed. For example, cathode ray tubes in televisions and
oscilloscopes manipulate an electron beam to produce visually perceptible
images. Electron beam lithography is also used in the production of highly
accurate patterned areas in the manufacture of very large scale integrated
circuits (VLSI). It is also known to produce and manipulate beams of other
kinds of charged particles, such as in ion beam devices. Such ion beam
devices have been used to advantage in certain aspects semiconductor
device manufacture, such as in impurity implantation.
Impurity implantation by means of an ion beam is desirable for a number of
reasons. The ion beam current and implantation energy can also be very
accurately controlled to provide extremely accurate concentrations and
distributions of impurities and implantation depths. Such ion implantation
processes can also be carried out at low temperatures, allowing the use of
low temperature masking materials.
Moreover, the mass of the ion in relation to the charge thereon affects the
degree to which it is accelerated both axially and transversely by an
electrostatic or magnetic field. Therefore, the beam which reaches a
desired area of a chip can be made very pure since ions of differing
molecular weight will be deflected to positions away from the beam and
implantation of other than desired materials can be avoided. This feature
of the ion beam optics of ion beam devices is known as mass analysis and
is typically carried out by deflecting the beam through an arc and using
an exit aperture of a size which will effectively separate ions of
different molecular weight.
Such implantation processes use kinetic effects and are done at high energy
to implant the ions within the body of a semiconductor material. More
recently, efforts have been made to use an ion beam Process for purposes
which require deposition on the surface of a target material, such as for
welding. As can be readily understood, a deposition process produced from
an ion beam would require the energy of the ion particles to be very much
lower than the energies at which implantation is performed. Such reduced
energies of the ions cause some difficulties to be encountered in
maintaining convergence of the ion beam due to the mutual repulsion of
ions bearing a like charge. However, in such an application, the need for
high beam current is not necessary because the amount of material is
typically small.
The formation of monocrystalline epitaxial layers of a semiconductor
material, particularly with conductivity determining impurities, is often
necessary in the manufacture of various types of semiconductor devices.
This process is often carried out through vapor phase deposition at very
high temperatures of approximately 1100.degree.-1200.degree. C. With a few
exceptions, such as P-doped and intrinsic silicon, good quality
monocrystalline deposition is difficult below about 1000.degree. C. This
high temperature requirement for forming a monocrystalline epitaxial layer
therefore has the drawback that, particularly if other doped structures
have previously been formed, out-gassing effects and/or out-diffusion
between regions may occur. In device design, compensation for such effects
is often difficult or impossible and can also limit the minimum dimension
of conductivity region in the device for a particular manufacturing yield
since impurity out-diffusion distances can easily dominate an epitaxial
layer which in thinner than about 2 microns or a region of similar lateral
dimension. Such out-diffusion due to the high temperature process also
results in dopant distribution being less than fully controllable, even
when ion implantation is subsequently used to add impurities to the
monocrystalline epitaxial layer.
It should also be noted that ion implantation, by itself, does not
completely eliminate the need for a high temperature process even though
ion implantation can be carried out at low temperatures since the ion
implantation process causes damage to the crystal lattice structure and
annealing is often necessary to repair the damage before further
processing can be carried out.
The use of an ion beam to provide a low temperature process for producing a
monocrystalline epitaxial layer has been achieved and is disclosed in
detail in Keller et al U.S. Pat. Nos. 4,151,420 and 4,179,312, assigned to
the assignee of the present invention and hereby fully incorporated by
reference. These techniques are characterized by the use of multi-aperture
sources to obtain high ion beam current. Such multi-aperture sources
produce a broad beam and it can be readily understood that a significant
amount of ion beam current is lost at the mass analysis aperture if good
separation of ion masses is to be obtained, even though condensing lenses
are used for each of the superimposed beams.
These techniques achieved a relatively high beam current at the target at
reasonably low energies of about 500 eV. However, these currents were
spread over a relatively large area of the target (e.g. a beam diameter of
about 15 cm). Thus, a beam current of about 1 ma/cm.sup.2 resulted in a
rate of material deposition which limited the throughput of the process.
Also, by using such a large beam diameter, the epitaxial growth process
was limited to performance of the process over the entire wafer and
selective epitaxial growth could not even be limited to the actual chip
areas, wasting beam current directed to areas of the wafer between chips.
It has also been found, by the inventors herein, that even lower ion
energies are desirable for epitaxial growth during the manufacture of a
semiconductor device or other object, such as a mask or calibration grid.
For instance, implantation may be performed at a typical energy of
approximately 20 Kev, whereas, it has been found, by the inventors herein,
that energies of 2 Kev or less are required for epitaxial growth and even
lower values are desirable. While the arrangements of the
above-incorporated Patents achieved energies of about 0.5 KeV at the
target, even faster epitaxial growth can be achieved at energies of 50-300
eV. It has also been found that, for several reasons discussed in more
detail below, energies of about 5 KeV are desirable for good performance
of mass analysis where epitaxial growth consists of a material which may
contain a plurality of elements (e.g. silicon and an impurity element such
as boron and arsenic, depending on the conductivity type desired) and
which must be deposited simultaneously at coincident locations to assure
homogeneity in the epitaxial growth. Such homogeneity also requires that
the ions of the different materials reach the target at the same
impingement angle, preferably perpendicular to the target, in order to
avoid a differential distribution of the elements in the direction of
epitaxial growth, particularly if the beam is to be scanned over the
region where epitaxial growth is desired. It is also necessary to maintain
good beam convergence to assure homogeneity of the epitaxial growth.
Although the arrangements of the above-incorporated patents utilize
deceleration lenses, such a differential between mass analysis energies
and deposition energies is difficult to achieve consistent with high beam
current at the target. Other conflicting requirements also exist in
processes for achieving epitaxial crystal growth with ion beam devices.
Specifically, it is desirable to perform the process at high vacuum to
minimize the possibility of contamination of the deposition and to
maintain beam current which would otherwise be reduced due to charge
exchange between ions and molecules of gas which may be present. If the
charge is removed from an accelerated ion, no further mass analysis can be
performed to guide it and maintain it within the beam, thus reducing beam
current. To avoid contamination, an atmosphere of a noble gas such as neon
and silane (SiH.sub.4) is typically used. A high vacuum is also used to
reduce reduction of beam current by charge exchange.
Since ions carry the same positive charge, they will mutually repel each
other unless oppositely charged particles are available to neutralize the
space charge of the beam. At particle energies of above 10 Kev, even at
high vacuum, the beam energy imparted to the extremely low pressure
atmosphere within an ion beam device will produce a plasma which will
provide substantially full space charge neutralization at high vacuum.
However, when the ion beam energy is reduced to 5 KeV or less, it has been
found by the inventors that a lower vacuum atmosphere of about
1.times.10.sup.-4 Torr is required to neutralize the space charge of the
beam. Even this low pressure severely reduces beam current due to charge
exchange. Alternatively, if the space charge is less fully neutralized,
beam current is lost during mass analysis due to the interfering effect of
the mutual repulsion between ions, particularly if the beam is focussed or
concentrated or current density otherwise increased, as the inventors
herein have found to be desirable to enhance mass analysis.
It is also important to note that electrostatic deceleration lens
arrangements, even when operated at fairly low voltages to reduce the
particle energies from about 1-10 KeV to about 0.5 KeV, causes the beam to
diverge. The divergence of the beam will also be increased since the space
charge neutralizing particles must be removed from the ion beam prior to
deceleration. Therefore, the mutual repulsion between ions will be great
since the particle beam must be focussed at virtually the same point along
the beam path that deceleration is desired. It is therefore particularly
desirable to keep the beam energy low in order to reduce the operating
voltage necessary for the electrostatic deceleration lens for minimization
of beam divergence, especially at high current densities. However, this
severely conflicts with the performance of mass analysis at high current
densities and low energies as pointed out above.
It should also be noted that the deceleration lens is electrostatic and
typically takes the form of an aperture of some type. Additionally, as
disclosed in the above-incorporated patents, the beam is substantially
collimated by a mass separator plate. While the mass separator plate
provides the function of increasing the purity of the deposited material,
it makes the focussing of the beams of ions of different elements
especially critical if it is not to severely reduce beam current or alter
the relative amounts of each type of ion deposited, particularly where
broad multi-aperture ion sources are used to obtain high currents. This is
also true of the deceleration lens and the beam must be accurately
collimated to avoid introducing distribution differentials across the beam
pattern at the target.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and
apparatus for achieving epitaxial growth of a crystalline material with a
mass analyzed ion beam at increased ion beam currents and current
densities.
It is another object of the invention to provide an apparatus and method
for simultaneously depositing a plurality of materials at high current,
high vacuum and at sufficiently low energy for homogeneous epitaxial
crystal formation to occur.
It is another object of the invention to provide an apparatus and method
for producing a wide, focussed, mass analyzed, plural element ion beam
with high resolution, high current density and homogeneity.
To achieve the foregoing and other objects of the invention, an ion beam
deposition apparatus for producing low temperature epitaxial growth of a
semiconductor material including at least a first ion source is provided
including a magnet for mass-analyzing an ion beam from at least said first
ion source including and focussing said ion beam int o a pattern of high
aspect ratio.
In accordance with another aspect of the invention, a method of depositing
a material on a surface by ion deposition of a material from an ion beam
from at least one ion source having a two-dimensional array of extraction
apertures including the step of simultaneously mass analyzing and
focussing said beam in at least one plane with a single magnet means
having an arcuate ion optical axis.
In accordance with a further aspect of the invention, an apparatus for
producing an ion beam within a volume of ionizable gas at high vacuum is
provided including means for increasing the concentration of electrons
available for space charge neutralizing said ion beam from said ionizable
gas.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, aspects and advantages will be better
understood from the following detailed description of a preferred
embodiment of the invention with reference to the drawing, in which:
FIG. 1 is an overall view of the ion beam epitaxial growth apparatus
according to the invention,
FIG. 2 is a cross-sectional view of the focussing magnet along its axis,
taken at Section A--A of FIG. 1,
FIG. 3 is a simplified schematic view of an ion source showing the use of a
divided acceleration plate for beam steering according to the invention,
FIG. 4 shows the apparatus of FIG. 1 with some elements removed for clarity
in illustration of the operation of the divided acceleration plate of FIG.
3.
FIG. 5 is a cross-sectional view of the focussing coil of FIGS. 1 or 4,
taken along section C--C of FIG. 4 in accordance with the invention,
FIG. 6a is a cross-sectional view of the focussing coil similar to that of
FIG. 5, illustrating magnetic mirror plasma confinement,
FIGS. 6b and 6c are cross-sectional views of the focussing coil along
section lines C--C and B--B of FIG. 4, illustrating multi-pole magnetic
plasma confinement,
FIG. 6d is a cross-sectional view of the focussing coil similar to that of
FIG. 5, illustrating electrostatic plasma confinement,
FIG. 7a is a cross-sectional view of the focussing coil similar to that of
FIG. 5, illustrating helicon RF plasma generation,
FIGS. 7b and 7c are cross-sectional views of the focussing coil along
section lines C--C and B--B of FIG. 4, illustrating microwave plasma
generation, and
FIG. 7d is a cross-sectional view of the focussing coil similar to that of
FIG. 5, illustrating capacitively coupled RF plasma generation.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawings, and more particularly to FIG. 1, there is
shown an overall view of the ion beam epitaxial growth apparatus 10 in
accordance with the invention. The principal elements of the apparatus are
the high current silicon ion source 11, boron impurity ion source 12,
arsenic impurity source 13, focussing magnet 14, vacuum chamber 15,
deceleration lens 16 and target wafer 17. It should be recognized that if
other impurity elements are to be deposited, the positions of impurity
sources 12, 13 must be changed to compensate for differences in the ion
mass and to ensure that the ions enter the focussing magnet at
approximately the correct position to be superimposed on the silicon ion
beam. The focal length of focussing magnet 14 can also be adjusted by
altering the angular position of the focussing magnet provided by arcuate
slots 18 for achieving superposition of beams from the different sources.
It should also be appreciated that the silicon component of the ion beam
current is typically far greater than the impurity ion component.
Therefore, the silicon ion source 11 is illustrated as a multi-aperture
source and the impurity ion sources 12, 13 are illustrated as single
aperture sources. It is to be understood that multi-aperture sources could
also be used for developing the impurity component of the ion beam
current, if necessary. For instance, in arrangements for depositing GaAs
or GaAlAs, two, three or more multi-aperture sources would preferably be
used. It is also to be understood that in the so-called single aperture
ion source, the ions are drawn from a plasma in the source through a
grid-like structure which may, in fact, be formed as a plurality of
apertures, as in the preferred embodiment of the invention where a linear
array of such apertures is provided by a screen-like grid.
While the basic configuration and elements of the present invention may
appear superficially similar in structure and operation to the
arrangements disclosed in the above-incorporated patents, the
mass-analysis magnet also functions, most importantly, as a focussing
magnet and the ion sources are modified to provide both electrostatic
focussing and steering in accordance with various aspects of the present
invention to function in combination in a manner which results in greatly
improved performance. Further apparatus for enhancing beam current
delivered to the target, specifically plasma containment and enhancement
structure, not shown in FIG. 1, is also provided in combination with these
modifications in accordance with other aspects of the invention. These
aspects of the invention will now be discussed in turn.
Focussing Magnet 14
It is important to understand that focussing magnet 14 performs both the
mass analysis and focussing function. For this reason, the focussing
magnet is formed in an arcuate shape with a relatively wide gap as shown
at 21 of FIG. 2. The design of this magnet provides a substantially
uniform magnetic field across the entire width of gap 21. The arcuate
shape provides a substantially linear variation in path length through the
magnet with position of entry into the focussing magnet gap.
Considering ions of a given mass from any one of ion sources 11-13, the
magnet provides focussing by virtue of the position at which the beam
component enters the focussing magnet. For example, as is well understood
in the art, an ion entering the focussing magnet along axis 19 is caused
to follow an arcuate path, as shown. Since the focussing magnet has an
arcuate form and maintains a substantially uniform field across the wide
gap, an ion entering along path 19a would follow a longer path through the
magnet and, hence, would be deflected to a greater degree than an ion
following path 19. Conversely, an ion following path 19b would be
deflected less, due to its shorter path through the magnet. Thus, it is
seen that ions entering the arcuate focussing magnet from different
positions, as from a relatively wide multi-aperture source 11 (e.g. having
a two dimensional array of extraction apertures, such as a plurality of
parallel slit apertures), will be brought to a common focus in the radial
plane, resulting in a beam having a potentially high aspect ratio.
Ions of different mass, such as boron, which is lighter than silicon, and
arsenic, which is heavier than silicon, will be similarly affected by the
arcuate focussing magnet but deflected by respectively greater of lesser
amounts due to their difference in mass as shown by paths 19c and 19d.
Therefore it is seen that the arcuate focussing magnet has mass-analyzing
properties which can be used to converge beams of ions of differing masses
while at the same tim | | |
