U.S. patent application number 09/739193 was filed with the patent office on 2001-11-08 for arc chamber for an ion implantation system.
This patent application is currently assigned to Thermoceramix, L.L.C.. Invention is credited to Abbott, Richard C..
Application Number | 20010037817 09/739193 |
Document ID | / |
Family ID | 27357226 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010037817 |
Kind Code |
A1 |
Abbott, Richard C. |
November 8, 2001 |
Arc chamber for an ion implantation system
Abstract
The present invention relates to the fabrication of materials
and structures having selected mechanical, thermal and electrical
properties. More particularly, the invention relates to the use of
these materials and structures in ion implantation systems.
Structures comprising boron material provide components for use in
implanters including arc chambers with which a beam of ions is
generated for implantation into a target such as a semiconductor
wafer.
Inventors: |
Abbott, Richard C.;
(Gardner, MA) |
Correspondence
Address: |
THOMAS O. HOOVER
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
Two Militia Drive
Lexington
MA
02421-4799
US
|
Assignee: |
Thermoceramix, L.L.C.,
Boston
MA
|
Family ID: |
27357226 |
Appl. No.: |
09/739193 |
Filed: |
December 18, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09739193 |
Dec 18, 2000 |
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09290471 |
Apr 12, 1999 |
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6239440 |
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09290471 |
Apr 12, 1999 |
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09002748 |
Jan 5, 1998 |
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5914494 |
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09002748 |
Jan 5, 1998 |
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PCT/US97/17938 |
Oct 3, 1997 |
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PCT/US97/17938 |
Oct 3, 1997 |
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08725980 |
Oct 4, 1996 |
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5857889 |
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08725980 |
Oct 4, 1996 |
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08622849 |
Mar 27, 1996 |
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Current U.S.
Class: |
134/1.3 |
Current CPC
Class: |
H01J 2237/31701
20130101; H01J 37/08 20130101; H01J 27/04 20130101; H01J 37/3171
20130101; H01J 27/16 20130101; H01J 27/22 20130101 |
Class at
Publication: |
134/1.3 |
International
Class: |
C25F 001/00 |
Claims
What is claimed is:
1. A method of fabricating a boron structure for an ion source
comprising: positioning a substrate in a processing chamber;
forming a boron material on a surface of the substrate to form a
composite structure; removing the boron material from the chamber;
and assembling an ion source housing with the boron material.
2. The method of claim 1 further comprising machining the composite
structure.
3. The method of claim 1 wherein the forming step comprises
depositing boron in a vacuum chamber.
4. The method of claim 1 further comprising depositing a film
having a thickness in the range of 0.5 to 3.0 mm.
5. The method of claim 1 wherein the substrate comprises
graphite.
6. The method of claim 1 further comprising forming an aperture in
the substrate structure to define a beam path for ions exiting the
ion source.
7. The method of claim 1 further comprising: forming a plurality of
composite boron structures; and assembling the plurality of
structures to provide an ion source chamber.
8. The method of claim 1 further comprising depositing a amorphous
boron film having a density of at least 50% of maximum density.
9. The method of claim 1 further comprising forming the boron
material using a chemical vapor deposition process.
10. The method of claim 1 further comprising providing a chemical
vapor deposition reactor, providing a boron containing gas within
the reactor and heating the substrate.
11. The method of claim 1 further comprising forming an amorphous
boron material on the substrate.
12. The method of claim 10 further comprising depositing the boron
material at a temperature of less than about 1500.degree. C. to
form an amorphous material.
13. An ion source comprising: an ion source housing including an
amorphous boron material having a density of at least 50% of the
maximum density of boron; a cathode; an anode; and an aperture
through which ions exit the ion source.
14. The ion source of claim 13 wherein the boron material comprises
a layer having a thickness in a range of 0.5 mm to 3.0 mm.
15. The ion source of claim 13 wherein the boron material is on a
substrate.
16. The ion source of claim 13 wherein the ion source is a Bernas
source.
17. The ion source of claim 13 wherein the housing comprises a
plurality of assembled boron components.
18. A method for chemical vapor deposition of a boron structure for
an ion source comprising: positioning a substrate in a chemical
vapor deposition chamber; depositing a boron material in the
chamber; removing the boron material from the chamber; and
assembling an ion source housing with the boron material.
19. The method of claim 18 further comprising heating a substrate
in the chamber on which the boron material is deposited.
20. The method of claim 19 wherein the heating step comprises
inductively heating the substrate.
Description
RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. Ser.
No. 09/290,471 filed on Apr. 12, 1999 which is a continuation
application of U.S. Ser. No. 09/002,748 (now U.S. Pat. No.
5,914,494) filed on Jan. 5, 1998 which is a continuation
application of International Application No. PCT/US97/17938 filed
on Oct. 3, 1997 which is a continuation-in-part application of U.S.
Ser. No. 08/725,980 filed Oct. 4, 1996 which is a
continuation-in-part of U.S. Ser. No. 08/622,849 filed on Mar. 27,
1996, the teachings of the above applications being incorporated
herein by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to methods for fabricating
materials and structures having mechanical, thermal and electrical
properties suitable for use in a wide variety of applications.
[0003] Materials fabricated from powder have in the past been
fabricated using a sintering process but do not have a sufficient
density to provide a product having sufficient mechanical strength
or thermal stability.
[0004] For example, boron has a wide variety of uses, but it is a
difficult material to form in a desired geometry and is also
difficult to machine. Arsenic, phosphorus, antimony and boron are
all used as dopants in the fabrication of semiconductor devices.
These materials are selectively ionized and implanted using an ion
implantation system. These systems have an ion source that is used
to generate a beam of ionized particles which are directed onto a
target such as a semiconductor wafer. These systems are complex and
expensive to fabricate, operate and maintain. A particular problem
in the use of these ion implanters is the level of impurities
generated during use which increases maintenance, increases defect
density in the materials produced and reduces production yield in
the manufacture of devices.
[0005] The housing for the ion source in an implanter is often
referred to as an arc chamber. Arc chambers have usually been made
of graphite, molybdenum or tungsten. These materials contribute to
the contamination of the beam, and consequently, they contaminate
the final product.
[0006] In one type of arc chamber electrons are emitted by a
cathode, usually by thermionic emission, and accelerated to an
anode. Some of these electrons have collisions with gas atoms or
molecules and ionize them. Secondary electrons from these
collisions can be accelerated toward the anode to energies
depending on the potential distribution and the starting point of
the electron. Ions can be extracted through the anode,
perpendicular to it, or through the cathode area depending upon the
type of source.
[0007] To increase the ionization efficiency of the electrons in
electron bombardment ion sources, several modifications have been
introduced in existing systems. An additional small magnetic field
confines electrons inside the anode and lets them spiral along the
magnetic field lines, multiplying on their way to the anode and
increasing the ionization efficiency of the ion source. By using a
cylindrical anode and a reflector electrode, the electron path is
further enlarged. Many mass separator ion sources are this type,
such as the Nier, Bemas, Nielsen, Freeman, Cusp and other
sources.
[0008] The Bemas ion source, for example, has a rectangular or
cylindrical arc chamber positioned in an external magnetic field.
The source can contain a single-turn helical filament (cathode) at
one side of the arc chamber and a reflector at the other end.
Electrons from the cathode are confined inside the anode cylinder
by the magnetic field and can oscillate between the filament and
the reflector resulting in a high ionization efficiency. Ions are
extracted perpendicular to the anode axis through a slit of about 2
mm width and about 40 mm length. However, the dimensions can vary,
depending on the specific design.
[0009] A continuing need exists for improvements in the field of
materials fabrication to provide structures having desired
mechanical, thermal and electrical properties. In particular, there
is a need for improvements in ion implantation systems used for the
fabrication of semiconductor devices.
SUMMARY OF THE INVENTION
[0010] The present invention relates to devices and methods of
fabricating components for use in ion implantation systems. More
particularly, the invention relates to the fabrication of boron arc
chambers and other boron components for ion implantation systems.
With the use of boron components in ion implantation systems a
number of advantages are realized, including a reduction in
contaminants due to the use of boron instead of other materials
such as graphite, molybdenum or tungsten; the enhancement of beam
current that can be accommodated due to the lower level of
contaminants; the lighter weight of these components and the
ability to retrofit them onto existing systems as well as their use
in new systems, and the ability to use these components with the
electrical system (e.g. as electrodes) and as a source of boron
particles for ionization.
[0011] The refractory metals are problematic for the ion source
because they are heavy, difficult to fabricate, and highly reactive
with boron trifluoride, a gas used in many systems to provide a
source of boron for ionization.
[0012] Tungsten, for example, is the current preferred material for
the Bemas ion source but is far from ideal. It is one of the
heaviest of all engineering materials having a density of 19.3
gm/cc, is difficult and expensive to machine, and reacts with boron
trifluoride to form another gas, tungsten hexafluoride. The
chemical reaction between fluorine and tungsten not only erodes the
interior of the arc chamber but also acts as a material transport
mechanism for depositing tungsten metal at other regions of the
chamber. This effect shortens the chamber lifetime and considerably
alters its interior geometry. Additionally, tungsten hexafluoride
formation acts to pump unwanted tungsten ions into the boron beam
current, some of which invariably ends up in the target, which is
typically a single crystal silicon wafer or silicon-on-insulator
(SOI) structure used for the manufacture of integrated
circuits.
[0013] Boron is very light, having a density of 2.46 gm/cc (about
13% the weight of tungsten) and therefore, is less demanding on
mounting fixtures and is easier to handle. It is also very hard and
strong, even at the elevated operating temperatures of an arc
chamber. It is more durable than graphite and tungsten, which is
prone to creep (permanent displacement under an applied load). A
boron arc chamber enhances the source beam current by reaction with
free fluorine ions in applications involving the use of boron
trifluoride as a source for boron ions.
[0014] Solid boron has not been utilized in semiconductor
processing systems because it is not a conventional engineering
material. There are currently no known manufacturers of dense boron
products, mainly because specialized materials techniques are
required to form this type of boron.
[0015] Structures made from boron for use in the fabrication of
implanter components can be made using several distinct processes.
A preferred embodiment of a method for making such boron structures
includes providing a mold or die having the desired shape for the
part to be fabricated, positioning a boron material such as an
amorphous boron powder into the mold, treating the boron powder
under selected conditions of temperature and pressure to
crystallize the powder into a more crystalline state to form a
solid unitary boron structure, removing the structure from the mold
and machining the structure as necessary. In many applications it
is desirable to produce a structure having a polycrystalline
lattice with an average crystal size in the range of 1 to 10
microns. In some applications it is desirable to form a structure
having a crystal size in excess of ten microns, including single
crystal material. In some applications with lower tolerance
requirements there may remain a large population of crystals with
diameters of less than 1 micron, typically in the range of 0.5-1
micron.
[0016] It is also preferred that the density of the material
produced be at least 50% of its maximum (theoretical) density, and
preferably in the range of 80-100% in order to increase the
mechanical strength and resistance to erosion. A preferred
embodiment employs boron having a high purity level having an
atomic percentage of elemental boron of at least 95%, and
preferably of at least 99.99% or greater.
[0017] The fabrication process can be pressure sintering methods
such as uniaxial hot pressing and hot isostatic pressing, or a
casting method, a single crystal growth method, by deposition from
the vapor or liquid phase, or by spray forming. The specific
technique employed for a given workpart will depend upon the
requirements for purity, density, and geometry as well as the
mechanical, electrical, optical and/or chemical characteristics
desired.
[0018] For certain ion sources, a preferred embodiment of the
invention utilizes boron material as a source of boron to be
ionized. At sufficiently low pressure and high temperature boron
sublimes at a rate sufficient to produce a flow of gaseous boron
suitable to generate an ion beam for implantation under electron
bombardment.
[0019] In another preferred embodiment, boron is used as a filament
to generate electrons by thermionic emission which are then
accelerated by an electric field to bombard the boron within the
arc chamber to generate the ionized beam. A magnetic field is used
to confine ions within the chamber until they are extracted through
the exit aperture of the chamber to form the ion beam. When highly
pure boron is heated to a sufficient temperature, it becomes highly
conductive. Many arc chambers, for example, operate at temperatures
at which boron is conductive and can thus employ boron as
electrically conductive components. Alternatively, doped boron
structures can be used which are conductive at lower temperatures,
including room temperature. Boron is also highly transmissive in
the infrared region (e.g. 1-8 microns wavelength) of the
electromagnetic spectrum and can be used as an optical window or
lens. A boron window or lens can thus be used with an infrared
sensitive camera such as a charge coupled device to monitor thermal
processes or other infrared imaging applications such as infrared
radar.
[0020] Other components within the implanter that are exposed to
the ion beam, or which are likely to contaminate the beam, can
optionally also be made of boron. These can include, without
limitation, the extraction electrode or grid, components of the
beam analyzer such as the beam trap target, beam deflectors, and
components of the implantation chamber including trays for holding
wafers, electrostatic clamps, and robot arms to control movement of
objects within the implantation chamber.
[0021] The processes described herein can also be used in the
manufacture of dense boron coatings, sputtering targets, for the
preparation of boron coatings for diffusion into other substrates
including semiconductors, and as diffusion furnace components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1A is a schematic diagram of an ion implanter embodying
the invention.
[0023] FIG. 1B illustrates an implantation system providing ion
beam trap.
[0024] FIGS. 2A and 2B are cross-sectional top and side views,
respectively, of an arc chamber in accordance with the
invention.
[0025] FIGS. 2C, 2D, 2E are detailed front, bottom and side views
of a boron filament in accordance with the invention.
[0026] FIG. 3A is a cross-sectional view of a multicusp ion source
in accordance with the invention.
[0027] FIGS. 3B and 3C are top and cross-sectional views of an
anode ring for a multicusp ion source embodying the inventions.
[0028] FIG. 4 is a schematic view of a Freeman source in accordance
with the invention.
[0029] FIG. 5A is a process flow diagram illustrating a hot
pressing technique for fabricating a boron structure in accordance
with the invention.
[0030] FIG. 5B graphically illustrates a pre-compacting process of
boron powder.
[0031] FIG. 5C graphically illustrates a preferred apparatus for
conducting hot press under vacuum.
[0032] FIG. 5D is a graphical representation of a process for
making a boron structure in accordance with the invention.
[0033] FIG. 6 is a process flow diagram illustrating steps in an
isostatic pressing method for the fabrication of components in
accordance with the invention.
[0034] FIG. 7 is a process flow sequence illustrating the steps in
a sintering method for fabricating components in accordance with
the invention.
[0035] FIG. 8 is a process flow sequence illustrating the steps in
a casting method for fabricating components in accordance with the
invention.
[0036] FIG. 9A is a process flow sequence illustrating a method of
fabricating single crystal boron components for an ion implanter by
pulling from a melt.
[0037] FIGS. 9B and 9C graphically illustrate the single crystal
growth method.
[0038] FIG. 10 illustrates a method of using an ion implanter in
accordance with a preferred embodiment of the invention.
[0039] FIG. 11 is a schematic illustration of a method of
fabricating boron components of a processing chamber by deposition
from the vapor or gas phase.
[0040] FIGS. 12A and 12B are top and cross sectional views,
respectively, of an ion source housing fabricated in accordance
with a preferred embodiment of the invention.
[0041] FIGS. 13A and 13B illustrate top and cross-sectional views,
respectively, of a portion of an ion source housing fabricated in
accordance with a preferred embodiment of the invention.
[0042] FIG. 14 is a cross-sectional view of a boron deposited
electrode fabricated in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] A preferred embodiment of the invention is illustrated
generally in the ion implantation 10 system of FIG. 1A. The system
10 typically includes an ion source 16, a power supply 14, an
extracting electrode or grid 18, a magnetic beam analyzer 20 and a
controller 22 which are positioned within an inner housing 12. Ions
are generated at the source 16, extracted from the source 16 with
electrode 18 and conveyed along the ion beam path 15 within the
beam housing 24 into the implantation chamber 34. The beam analyzer
20 selectively separates components of the beam exiting from the
source 16 so that only ions of the desired mass are directed
towards the ion implant chamber 34.
[0044] The source 16 includes an arc chamber housing 25 constructed
from a crystalline boron material. There are several types of
sources having different configurations of the components, however,
these components can include an anode, a cathode, a third electrode
or filament, various ports to introduce materials into the arc
chamber 25 to be ionized, and an exit aperture through which ions
are extracted from the chamber 25. The boron material used in the
chamber will vary depending upon the application, but the material
will usually have a uniform density of either a polycrystalline
material or a single crystal material.
[0045] Other components of the implanter can include a beam
controller 26 that can be used to selectively deflect the beam so
that the beam can be scanned across a target 28. The implantation
chamber 34 includes a support 30 for one or more targets or wafers
mounted within the chamber 34 and a drive mechanism to control
movement of the support 30 relative to the implantation chamber
housing 34. The system can be used, for example in the fabrication
of many size wafers from 50 mm up to 300 mm or higher. A robot arm
can be used to move targets within the chamber and/or insert or
remove targets from the chamber. As described in greater detail,
many components of processing systems can be made using boron
material as well as the arc chamber in order to lower impurity
levels and improve system durability and performance.
[0046] FIG. 1B illustrates another preferred embodiment of an
implantation system providing beam trap. Similar to the system in
FIG. 1A, the implantation system 400 includes a ion beam source 402
which generates ion beam 407 extracted with the help of extracting
electrodes 414 and passes the beam through an enclosed path 404.
The beam is filtered by a magnetic beam analyzer which guides a
portion 408 of the source beam having appropriate mass. The portion
406 having heavier mass is unable to make the turn into the target
guide 426 and is directed to a trap 410. The trap includes a beam
target or collector 428 and a valve 412 to allow removal of trap
debris. The magnetic analyzer 418 is controlled by a mass
controller 416. The desired ion beam 408 is guided to strike a
target 422 which can be mounted on a rotatable disk 420 driven by a
motor 424.
[0047] A preferred embodiment of an arc chamber of an ion source is
of Bernas type and an example is illustrated in FIGS. 2A and 2B,
respectively. The housing 50 comprises a boron material made in
accordance with the methods defined in greater detail below. The
boron material is preferably a polycrystalline material with the
average grain size being in the range of 1 to 10 microns. The
density of the material is at least 50% of the maximum theoretical
density (TD), and preferably greater than 60% TD for machining
purposes. The boron housing uses boron having an atomic percentage
of at least 95% elemental boron and preferably at least 99.99%.
[0048] Other preferred embodiments of the housing material can
include single crystal boron, or alternatively, certain components
can be made with boron compounds such as silicon hexaboride,
tungsten boride, boron nitride or titanium diboride which are
compatible for use in combination with the solid boron structures
described herein. The inner surfaces of the chamber can also be
coated with pure dense boron or these compound materials, with the
specific type of material chosen being dependent upon the type of
ion source, the desired insulating characteristics and the gases
used within the chamber.
[0049] The components within the chamber can either be the standard
materials, or more preferably, these can also be formed from the
same boron material as the housing 50. These can include the
shields 52, the cathode which serves as a source of electrons, an
anode 67 and the repeller 54.
[0050] The cathode filament 58 serves as a source of electrons that
bombards a boron material entering through the port 56 in the
bottom wall 55 of the chamber 50 to form boron ions. FIGS. 2C, 2D
and 2E illustrate front, bottom and side views of a boron filament
58. the filament 58 can include three elements, two posts 73, 75
and cross member 71. Member 71 fits within a groove 79 in one end
of both posts, at the opposite end of both posts 73, 75 are
connectors 77 that extend through the wall of the chamber and
connect to pin connectors to attach the filament 58 to a power
source.
[0051] A portion of the chamber serves as the ion source anode 67.
The cathode 58 can be made of Ta or W. The gas flow is small so the
source will operate in a vacuum environment within the arc chamber
of about 5.times.10.sup.-4 torr. Alternatively, as described in
greater detail below, the filament, the ion source and/or the
cathode can also be boron material.
[0052] The top wall 51 of the chamber 50 has an aperture 62 or
rectangular slit through which ions within a containment region 65
within the chamber are drawn.
[0053] The filament 58 generates electrons which oscillate between
the filament and the reflector. The electrons impact particles
containing boron which have been directed into the electron path to
produce boron ions. The ions are extracted perpendicular to the
anode axis through the aperture 62. Additional ports 69 can be used
to introduce gases into the chamber. The boron source 60 can either
be a standard gas or a boron material that is heated above its
sublimation temperature to produce a sufficient flow of boron
particles.
[0054] The top 51, bottom 55, sides 53,57, plates 52, and end
plates can be made separately as described hereinafter and bonded
at the edges, or alternatively, they can be mounted using an
external fixture, or a large number of plates and rings can be made
separately and pressed together. Alternatively, chamber components
can be formed to near net shape without the need for extensive
further machining.
[0055] In the multicusp ion source, the primary ionizing electrons
are normally emitted from tungsten-filament cathodes. The source
chamber walls or a ring form the anode for the discharge. The
surface magnetic field generated by rows of permanent magnets,
typically of samarium-cobalt or neodymium-iron, can confine the
primary ionizing electrons very efficiently. As a result, the arc
and gas efficiencies of these sources are high.
[0056] The multicusp ion source is relatively simple to operate.
There are four main components in the source: the filaments
(cathode), the chamber, an anode ring, and the first, or plasma,
electrode. A schematic diagram of the multicusp source is shown in
FIG. 3A. The source chamber can be rectangular, square or circular
in cross section and is formed with a boron material. The permanent
magnets can be arranged in rows parallel to the beam axis.
Alternatively, they can be arranged in the form of rings
perpendicular to the beam axis. The back plate also contains rows
of the same permanent magnets. Grooves are generally milled on the
external wall so that the magnets are mounted within approximately
3 mm from the vacuum. These magnets generate multicusp magnetic
fields for primary electron as well as for plasma confinement.
[0057] The open end of the source chamber is closed by a set of
extraction electrodes which can be made with a conductive boron
material. The source can be operated with the first electrode
electrically floating or connected to the negative terminal of the
cathode. The background gas is introduced into the source chamber
through a needle or a pulsed valve. The plasma is produced by
primary ionizing electrons emitted from one or more boron or
tungsten filaments, which are normally biased negatively with
respect to the chamber wall or the anode ring. These filaments are
located in the field-free region of the ion source chamber and they
are mounted on molybdenum holders. The plasma density in the
source, and, therefore, the extracted beam current, depends on the
magnet geometries, the discharge voltage and current, the biasing
voltage on the first extraction electrode, and the length of the
source chamber. The source chamber is normally pumped down to about
10.sup.-6 to 10.sup.-7 torr before gases are introduced for beam
generation.
[0058] The multicusp source for ion implantation and for surface
modification purposes, beams of B.sup.+, P.sup..times., As.sup.+,
and N.sup.+ ions have been extracted from multicusp ion sources. By
operating the source at higher discharge voltages, ions with
multiply charged states have been generated. FIG. 3A and 3B
illustrate top and side cross-sectional views, respectively, of a
boron anode ring for a multicusp ion source. As the anode ring
operates at a temperature of about 600.degree. C., the anode is
conductive.
[0059] Freeman developed another type of source by putting the
cathode of a Nier type mass separator ion source inside the hollow
anode, as in a magnetron, but left the magnetic field low. The
performance of the Freeman ion source has made it successful for
ion implantation and industrial application, especially for
semiconductor implantation.
[0060] The Freeman type ion source as shown in FIG. 4 has a similar
design to a magnetron, but it uses just a low external magnetic
field of about 100G. The arc current is 1 to 3A and the arc voltage
is between 40 and 70 V. A rigid cathode rod 80, usually 2 mm in
diameter and made of tantalum or tungsten, is heated with about 130
A and a few volts to the right temperature. The cathode 80, as well
as the chamber 82 can be made of boron. The axial position of the
cathode in the Freeman ion source shows several advantages
including:
[0061] 1. The inherent magnetic field of the cathode forces the
primary electrons to move around the cathode, concentrating the
electron density in this area.
[0062] 2. The high electron density next to the extraction slit
produces a high ion density in this area, and, thus, a high ion
beam current.
[0063] 3. The straight-filament rod fixes the plasma parallel to
the magnetic field lines and the extraction aperture, which is the
reason for the excellent beam quality of Freeman ion sources.
[0064] 4. The low magnetic field does not force instabilities like
the high field in a magnetron.
[0065] The lifetime of the source is dependent upon the type of gas
used or corrosion rate of the boron or other components in the
source. Changing the polarity of the filament after some time of
operation improves cathode lifetime. Heating by AC has the same
effect but increases plasma instabilities and the energy spread of
the extracted ions.
[0066] The Freeman ion source is especially designed to deliver ion
beams from nongaseous materials. There are many versions with ovens
for various temperatures and for the application of chemical
compounds and in situ chemical synthesis of the required material.
With chemical compounds, corrosion problems occur not only at the
cathode, but also at the anode and with the oven.
[0067] Ion currents of several milliamps can be produced for most
elements and more than 20 Ma for a few elements like arsenic and
phosphorus under favorable circumstances and using large extraction
areas. The extraction slit is usually about 2 mm wide and about 40
mm long. Larger slits are possible, such as 90 mm, but there are
some disadvantages because the current density is not uniform along
the long slit due to the bigger voltage drop along the cathode. The
anode and the magnetic field can be adjusted, however, to overcome
this problem.
[0068] The ion current density is controlled by the arc current,
which is controlled by the filament, the gas pressure, and the
magnetic field. The ion source is mounted inside the vacuum
chamber. The feed-throughs can be mounted on a base flange
perpendicular to the source axis. The extraction slit is usually
40.times.2 mm but designs up to 100.times.5 mm have been realized.
This type of source can use a W, Ta or boron filament, 1.5-2.5 mm
.phi.; alumina or boron nitride insulators; and operates in a
vacuum at less than 7.5.times.10.sup.-6 torr.
[0069] FIG. 5A describes the process flow sequence of a preferred
hot pressing method. The process begins at 200 by preparing boron
of 99.9% purity in dry powder form. FIG. 5B illustrates the
apparatus for the pre-compacting sequence. A first die or mold 306
of desired form is fabricated at 202 to contain the boron powder.
The dry boron powder is then pre-compacted at 204 in the first die
by applying ram force with a punch 302 at room temperature. The
compacted boron material 304 is then released upward by a static
ram 308.
[0070] The pre-compacted boron powder from the first die is moved
to a second graphite die at 206 and placed in a vacuum hot press
system 320 (FIG. 5C). The second die 324 is lined with tantalum
foil 310 of about 0.005 to 0.02 inches of thickness and boron
nitride powder 312 is laid between the tantalum foil and the boron
powder 206. The tantalum and boron nitride liners function as
barriers to prevent the formation of B.sub.4C. The boron material
is heated at 208 under vacuum at the rate of about 50 to
300.degree. C./hr to about 1850.degree. C. At this temperature,
pressure is applied as graphically illustrated in FIG. 5D.
[0071] The system 320 provides a cylindrical vacuum compartment 321
in which the boron material 304 in the second graphite die 324 is
pressed by a ram 322 and ram 326. The compartment provides ports
for receiving argon pressure gas 330 and outlet for vacuum pump
334. The compartment further features a pressure release port and a
sealed door 336 with heating elements 337. The heat is applied
slowly to remove any impurities in the boron material and also to
effect complete transformation of boron from the amorphous to the
crystalline state. At 1850.degree. C. the material is held under
vacuum for 1 hour and ram pressure of about 4000 to 6000 psi is
applied at 210 over the boron material and sustained for about 2
hours. When ram pressure is applied, an argon overpressure of
300-1000 Torr is maintained for the duration of the process. At
214, the material is then allowed to cool, still under the argon
atmosphere, at the rate of about 150.degree. C./hr. At 216, the
cooled and hardened block of boron is machined and polished to a
desired configuration.
[0072] FIG. 6 describes a preferred hot isostatic compacting method
for forming dense boron material. Again, pure boron powder,
preferably of 99.9% purity, is prepared at 218. The powder is
typically wrapped in a flexible mold material, such as silicone
rubber, and compacted at 220 at room temperature using isostatic
pressure, defined as fluid pressure which provides uniform pressure
from all direction. The pre-compacted boron is then encapsulated at
222 with a protective foil such as titanium, tantalum or
molybdenum. The boron powder is then heated at 224 in a high
pressure chamber to about 1500.degree. C. to 2000.degree. C. and
sustained for about 1 to 3 hours. At 226, an argon pressure of at
least 20000 psi is applied over the boron material. At 227, the
material is allowed to cool at a controlled rate of about
300.degree. C./hr to prevent any thermal shock to the material. At
228, the boron structure is machined and polished to a desired
configuration.
[0073] Sintering is yet another method of preparing solid boron
structure and is described in FIG. 7. Here again pure boron of
99.9% purity in dry powder form is prepared at 230. The boron
powder is isostatically pre-compacted at 232 using fluid pressure
at room temperature, similar to the pre-compacting process
described in FIG. 6. The pre-compacted boron is placed in a
graphite container at 234 which is lined with tantalum foil and a
layer of boron nitride powder. At 236, the container is placed in a
vacuum furnace and heated to about 1950.degree. under vacuum at the
rate of 150.degree. C./hr to about 400.degree. C./hr. At 238 to
240, while the heat is maintained at this level for about 2 to 3
hours, 1 to 2 psig of argon pressure is applied. At 242, the heat
level is raised slowly to about 2000-2100.degree. C. and maintained
for about 1 to 3 hours, still under pressure. At 244, again boron
is allowed to cool at a controlled rate of about 300.degree. C./hr
to prevent any thermal shock to the material. At 246, the resulting
boron structure is machined and polished to a desired
configuration.
[0074] FIG. 8 describes the method of melting to form solid boron.
Again, at 254 a suitable boron powder is provided. A xylene fluid
is added to the powder and the mixture is ball milled to achieve a
slip of desired consistency. A plaster mold is fabricated at 256 to
hold the boron slip in a desired shape. Prior to placing the boron
powder, the mold is treated with alginate for about 60 seconds and
drained at 257. At 258, the mold is air-dried for about 2 hours
before being treated with xylene for about 15 minutes at 259. The
boron slip is then placed in the mold for casting at 260. The cast
boron material is removed from the mold and at 261, the boron
material is heated in a boron nitride crucible under vacuum at the
rate of about 300.degree. C./hr to about 1950.degree. C. At 262,
argon pressure of 1 to 2 psig is applied over the material. At 263,
the material is further heated to about 2200.degree. C. and held at
such a heat level for about 0.5 hour. At 264, the material is
allowed to cool at the rate of 150.degree. C./hr to room
temperature. The dense boron structure is then machined and
polished to a desired configuration at 265.
[0075] Another preferred method of making boron components for ion
implantation systems utilizes a process of pulling a boule or ingot
of single crystal material from a melt. This process is described
in FIG. 9A and schematically illustrated in FIGS. 9B and 9C. The
process includes melting boron 272 in a boron nitride crucible,
pulling the single crystal material from the melt at 274 at a rate
and temperature sufficient to provide a desired diameter, cutting
at 276 the single crystal material to a desired shape. The parts
are then machined and polished at 278 to form discrete components
of an arc chamber or other components of an implanter, and then
assembled at 280 and 282 as necessary.
[0076] The system for fabricating a single crystal material uses a
crucible 360 in an oven 356 in which heaters 366 melt the boron
powder to form a fluid 362. A rod 350 with a seed mounted on tip
352 contacts the surface at 364 and is drawn up at selected speed
and temperature such that boule 368 is formed.
[0077] Single crystal boron can also be formed using known methods
by deposition from the vapor phase in order to form a layer of
boron on an existing part. This procedure can be used in forming
filaments or in coating the internal surfaces of arc chamber or
other processing chambers as described herein.
[0078] FIG. 10 describes the ion implantation process using the
preferred boron materials of the present invention. A first boron
filament is sufficiently heated at 284 to produce boron particles
inside an ion-source chamber. A sufficient voltage is then applied
at 286 to a second boron filament to generate electrons inside the
chamber to react with the boron particles. A magnetic field is
generated at 288 by a source to contain such electrochemical
reaction inside the chamber. A repeller of opposite charge is
provided at 290 inside the chamber to further contain the ions
generated by the electro-chemical reaction. The boron ions, thus
formed, are extracted from the chamber by providing a sufficiently
small exit aperture on the chamber and extracting electrodes near
the aperture. The ion beam is then mass filtered at 294 and
direction-controlled by a second magnetic field in a sealed beam
path at 296. The ion beam is then directed to a target for
implantation at 298.
[0079] Machining dense boron materials is generally performed using
diamond tooling including core drills, milling machines and
grinding wheels. Typically cutting boron material is computer
numerically controlled (CNC) for precision. Other methods include
lapping, electrical discharge machining, laser machining, grinding,
and ultrasonic machining. Polishing dense boron materials typically
utilizes a diamond grit paste.
[0080] Another preferred embodiment of a method for fabricating
components for an ion implanter, including a boron ion source, is
to use chemical vapor deposition (CVD) of boron on a substrate. In
this technique, a boron containing gas is caused to precipitate
elemental boron, either by reaction with another gas or by thermal
decomposition, onto a suitably prepared substrate. The process is
schematically illustrated in the process sequence of FIG. 11.
[0081] The first step is to prepare a substrate 518 of high density
such as a small grained graphite or other material, preferably with
a coefficient of thermal expansion approximating that of boron. The
substrate can also be boron nitride, aluminum oxide, molybdenum,
tungsten, tin, silicon carbide or other materials that can be
machined so that when a deposit of boron has been made on its
surfaces, it is the desired dimension.
[0082] A standard CVD reactor is configured so that the substrate
can be heated 520 in the presence of the boron containing gas.
Heating methods can be resistive heating, induction heating, or
infrared heating. Following proper preparation, positioning and
heating of the substrate a boron containing gas such as boron
trichloride or diborane is introduced 522 to the reactor and boron
is caused to deposit 524 onto the hot substrate by, for example,
reaction of the boron trichloride with hydrogen, or alternatively,
by thermal decomposition of the diborane.
[0083] The deposition temperature at which the boron is deposited
is preferably held below about 1500.degree. C. so that the material
remains in the amorphous state rather than the crystalline state.
Applications in which crystalline materials are subjected to large
thermal and/or mechanical stresses can also result in fracture of
these materials along weaker planes within the material. This
problem is eliminated by the use of deposited amorphous boron
films. The temperature is also preferably held above the maximum
operating temperature of the component such as an ion source to
provide the desired thermal stability in the resulting film.
Thermal deposition from diborone can be conducted at about
900.degree. C. Note that for applications requiring the use of
conductive boron films, it can remain advantageous to use
polycrystalline boron to improve conductivity. Both amorphous and
polycrystalline films can also be doped to enhance conductivity.
Dopants such as silicon or carbon can be diffused or implanted
during or after boron film deposition.
[0084] In one example, a plate of graphite having a coefficient of
thermal expansion of 8.2 m/m/.degree. K. was fitted with two
electrode clamps so that a current could be run through it for
resistive heating. The plate and electrodes were then placed in a
reactor tube fitted with end flanges so that-it could be evacuated
and then filled with flowing gases at regulated flow rates. An
infrared transducer was aimed at the surface of the plate for
providing feedback to a temperature controller/power supply
circuit. When the graphite reached a temperature of 1240.degree.
C., the boron trichloride and hydrogen gases were introduced in the
ratio of 2 moles boron trichloride to 3 moles of hydrogen. After
one half hours of operation, a uniformly thick layer measuring 0.5
mm had been deposited on the graphite plate. For preferred
embodiments, it is desirable to deposit films having a thickness in
the range of 0.5-3 mm.
[0085] The substrate and boron film is then removed 526 from the
chamber. Depending upon the particular application or component the
film can optionally be flurther processed by one or more methods
including grinding or machining of one or more surfaces or regions
of the film or the substrate to achieve desired tolerances. The
film can also be separated or cleaved from the substrate.
[0086] After any optional processing one or more of the components
of the processing chamber can be assembled 527 and installed in the
ion implanter or other device.
[0087] Illustrated in FIGS. 12A and 12B are top and cross-sectional
views, respectively, of an ion source housing 550 fabricated in
accordance with a preferred embodiment of the invention. The
chamber 550 is similar in dimensions to that illustrated in
connection with FIG. 2A, however, as shown along section A-A in
FIG. 12B, the chamber includes an inner shell or substrate 556 on
which a thin film of boron 554 has been deposited. Deposition can
be used to uniformly coat the entire surface including ports 552,
or alternatively, can cover all or selected portions of internal
surfaces 558.
[0088] Shown in the top and cross-sectional views of FIGS. 13A and
13B, respectively, is the top 570 of the chamber 550. The aperture
572 can have a beveled shape at 578, as seen in the section B-B of
FIG. 13B. A thin film of boron 576 has been deposited on substrate
574 to provide the ion source aperture.
[0089] In another preferred embodiment, the deposition process can
be used in the fabrication of other components described previously
herein. For example, a filament or wire used in the ion source can
be coated with a thin film of boron to provide an electrode or
boron source for use in the ion source housing. Such a film is
shown in the cross-sectional view of FIG. 14 in which a metal
filament 600 has a thin film 602 of boron thereon.
[0090] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention as defined by the appended
claims.
* * * * *