U.S. patent application number 11/648378 was filed with the patent office on 2007-07-26 for dual mode ion source for ion implantation.
This patent application is currently assigned to SemEquip, Inc.. Invention is credited to Thomas N. Horsky.
Application Number | 20070170372 11/648378 |
Document ID | / |
Family ID | 38023590 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070170372 |
Kind Code |
A1 |
Horsky; Thomas N. |
July 26, 2007 |
Dual mode ion source for ion implantation
Abstract
An ion source is disclosed for providing a range of ion beams
consisting of either ionized clusters, such as
B.sub.2H.sub.x.sup.+, B.sub.5H.sub.x.sup.+, B.sub.10H.sub.x.sup.+,
B.sub.18H.sub.x.sup.+, P.sub.4.sup.+ or As.sub.4.sup.+ or monomer
ions, such as Ge.sup.+, In.sup.+, Sb.sup.+, B.sup.+, As.sup.+, and
P.sup.+, to enable cluster implants and monomer implants into
silicon substrates for the purpose of manufacturing CMOS devices,
and to do so with high productivity. The range of ion beams is
generated by a universal ion source in accordance with the present
invention which is configured to operate in two discrete modes: an
electron impact mode, which efficiently produces ionized clusters,
and an arc discharge mode, which efficiently produces monomer
ions.
Inventors: |
Horsky; Thomas N.;
(Boxborough, MA) |
Correspondence
Address: |
PATENT ADMINISTRATOR;KATTEN MUCHIN ROSENMAN LLP
1025 THOMAS JEFFERSON STREET, N.W.
EAST LOBBY: SUITE 700
WASHINGTON
DC
20007-5201
US
|
Assignee: |
SemEquip, Inc.
|
Family ID: |
38023590 |
Appl. No.: |
11/648378 |
Filed: |
December 29, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11268005 |
Nov 7, 2005 |
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11648378 |
Dec 29, 2006 |
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10170512 |
Jun 12, 2002 |
7107929 |
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11268005 |
Nov 7, 2005 |
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PCT/US00/33786 |
Dec 13, 2000 |
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10170512 |
Jun 12, 2002 |
|
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60170473 |
Dec 13, 1999 |
|
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60250080 |
Nov 30, 2000 |
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Current U.S.
Class: |
250/427 |
Current CPC
Class: |
C23C 14/48 20130101;
H01J 27/08 20130101; H01L 21/26513 20130101; H01L 21/823814
20130101; H01L 21/265 20130101; H01J 37/08 20130101; H01J 37/3171
20130101; H01L 21/26566 20130101; H01J 27/205 20130101; H01J
2237/31701 20130101; H01L 21/2658 20130101; H01J 2237/0827
20130101; H01J 2237/0815 20130101; H01J 27/026 20130101; H01J
2237/082 20130101 |
Class at
Publication: |
250/427 |
International
Class: |
H01J 27/00 20060101
H01J027/00 |
Claims
1. A universal ion source comprising an ionization :volume for
ionizing source gas or vapor; a cathode assembly for generating a
plasma in said ionization volume in a first mode of operation; an
electron gun for generating electrons in a second mode of
operation, said electron gun juxtaposed external to said ionization
volume and configured to direct electrons into said ionization
volume; a source of gas or vapor; and means for switching between
said first mode of operation and said second mode of operation.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of commonly owned
co-pending U.S. application Ser. No. 10/170,512, filed on Jun. 12,
2002, which was nationalized from international patent application
no. PCT/US00/33786, filed on Dec. 13, 2000, under 35 USC .sctn.371,
which, in turn, claims the benefit of U.S. provisional patent
application No. 60/170,473, filed on Dec. 13, 1999 and U.S.
provisional application No. 60/250,080, filed on Nov. 30, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an ion source for the
generation of ion beams for doping wafers in the semiconductor
manufacturing of PMOS and NMOS transistor structures to make
integrated circuits and more particularly to a universal ion source
configured to operate in dual modes, for example, an arc discharge
mode and an electron impact mode.
[0004] 2. Description of the Prior Art
The Ion Implantation Process
[0005] The fabrication of semiconductor devices involves, in part,
the formation of transistor structures within a silicon substrate
by ion implantation. The ion implantation equipment includes an ion
source which creates a stream of ions containing a desired dopant
species, a beam line which accelerates and focuses the ion stream
into an ion beam having a well-defined energy or velocity, an ion
filtration system which selects the ion of interest, since there
may be different species of ions present within the ion beam, and a
process chamber which houses the silicon substrate upon which the
ion beam impinges ; the ion beam penetrating a well-defined
distance into the substrate. Transistor structures are created by
passing the ion beam through a mask formed directly on the
substrate surface, the mask being configured so that only discrete
portions of the substrate are exposed to the ion beam. Where dopant
ions penetrate into the silicon substrate, the substrate's
electrical characteristics are locally modified, creating source,
drain and gate structures by the introduction of electrical
carriers: such as, holes by p-type dopants, such as boron or
indium, and electrons by n-type dopants, such as phosphorus or
arsenic, for example.
Prior Art Ion Sources
[0006] Traditionally, Bernas-type ion sources have been used in ion
implantation equipment. Such ion sources are known to break down
dopant-bearing feed gases,such as BF.sub.3, AsH.sub.3 or PH.sub.3,
for example, into their atomic or monomer constituents, producing
the following ions in copious amounts: B.sup.+, As.sup.+ and
P.sup.+. Such ion sources are known to produce extracted ion
currents of up to 50 mA, enabling up to 20 mA of filtered ion beam
at the silicon substrate. Bernas type ion sources are known as hot
plasma or arc discharge type sources and typically incorporate an
electron emitter, either a naked filament cathode or an
indirectly-heated cathode, and an electron repeller, or
anticathode, mounted opposed to one another in a so-called "reflex"
geometry. This type of source generates a plasma that is confined
by a magnetic field.
[0007] Recently, cluster implantation sources have been introduced
into the equipment market. These ion sources are unlike the
Bernas-style sources in that they have been designed to produce
"clusters", or conglomerates of dopant atoms in molecular form,
e.g., ions of the form AS.sub.n.sup.+, P.sub.n.sup.+, or
B.sub.nH.sub.m.sup.+, where n and m are integers, and
2.ltoreq.n.ltoreq.18. Such ionized clusters can be implanted much
closer to the surface of the silicon substrate and at higher doses
relative to their monomer (n=1) counterparts, and are therefore of
great interest for forming ultra-shallow p-n transistor junctions,
for example in transistor devices with gate lengths of 65 nm, 45
nm, or 32 nm. These cluster sources preserve the parent molecules
of the feed gases and vapors introduced into the ion source. The
most successful of these have used electron-impact ionization, and
do not produce dense plasmas, but rather generate low ion densities
at least 100 times smaller than produced by conventional Bernas
sources.
SUMMARY OF THE INVENTION
[0008] Briefly, the present invention relates to an ion source for
providing a range of ion beams consisting of either ionized
clusters such as B.sub.2H.sub.x.sup.+, B.sub.5H.sub.x.sup.+,
B.sub.10H.sub.x.sup.+, B.sub.18H.sub.x.sup.+, P.sub.4.sup.+ or
As.sub.4.sup.+ or monomer ions, such as Ge.sup.+, In.sup.+,
Sb.sup.+, B.sup.+, As.sup.+, and P.sup.+, to enable cluster
implants and monomer implants into silicon substrates for the
purpose of manufacturing CMOS devices, and to do so with high
productivity. This is accomplished by the novel design of an ion
source which can operate in two discrete modes: electron impact
mode, which efficiently produces ionized clusters, or arc discharge
mode, which efficiently produces monomer ions.
[0009] Borohydride molecular ions are created by introducing
gaseous B.sub.2H.sub.6, B.sub.5H.sub.9, B.sub.10H.sub.14, or
B.sub.18H.sub.22 into the ion source where they are ionized through
a "soft" ionization process, such as electron impact ionization,
which preserves the number of boron atoms in the parent molecule
(the number of hydrogens left attached to the ion may be different
from that of the parent). Likewise, As vapor or P vapor can be
introduced into the ion source (from a vaporizer which sublimates
elemental As or P) to produce an abundance of As.sub.4.sup.+,
As.sub.2.sup.+, and As.sup.+, or P.sub.4.sup.+, P.sub.2.sup.+, and
P.sup.+ ions. The mechanism for producing As and P clusters from
elemental vapor will be described in more detail below. Monomer
ions are produced by creating an arc discharge within the ion
source, producing a dense plasma and breaking down the feed gases
BF.sub.3, AsH.sub.3, PH.sub.3, SbF.sub.5, InCl.sub.3, InF.sub.3 and
GeF.sub.4 into their constituent atoms. This provides high currents
of Ge.sup.+, In.sup.+, Sb.sup.+, B.sup.+, As.sup.+, and P.sup.+
ions as required by many semiconductor processes today. The
invention, as described in detail below, is disclosed by novel
methods of constructing and operating a single or universal ion
source which produces these very different ion species, i.e., both
clusters and monomers, and switching between its two modes of
operation quickly and easily, enabling its efficient use in
semiconductor manufacturing.
Production of Clusters of Arsenic and Phosphorus
[0010] An object of this invention is to provide a method of
manufacturing a semiconductor device, this method being capable of
forming ultra-shallow impurity-doped regions of N-type conductivity
in a semiconductor substrate by implanting ionized clusters of the
form P.sub.4.sup.+ and As.sub.4.sup.+.
[0011] A further object of this invention is to provide for an ion
implantation source and system for manufacturing semiconductor
devices, which has been designed to form ultra shallow
impurity-doped regions of N-conductivity type in a semiconductor
substrate through the use of cluster ions of the form P.sub.4.sup.+
and As.sub.4.sup.+.
[0012] According to one aspect of this invention, there is provided
a method of implanting cluster ions comprising the steps of:
providing a supply of molecules each of which contains a plurality
of either As or P dopant atoms into an ionization volume, ionizing
the molecules into dopant cluster ions, extracting and accelerating
the dopant cluster ions with an electric field, selecting the
desired cluster ions by mass analysis, and implanting the dopant
cluster ions into a semiconductor substrate.
Economic Benefits of As and P Clusters
[0013] While the implantation of P-type clusters of boron hydrides
for semiconductor manufacturing has been demonstrated, no N-type
cluster has been documented which produces large ionized clusters
in copious amounts. If ions of the form P.sub.n.sup.+ and
As.sub.n.sup.+ with n=4 (or greater) could be produced in currents
of at least 1 mA, then ultra-low energy, high dose implants of both
N- and P-type conductivity would be enabled. Since both
conductivity types are required by CMOS processing, such a
discovery would enable clusters to be used for all low energy, high
dose implants, resulting in a dramatic increase in productivity,
with a concomitant reduction in cost. Not only would cost per wafer
decline dramatically, but fewer ion implanters would be required to
process them, saving floor space and capital investment.
Process Benefits of As and P Clusters
[0014] The preferred method of forming drain extensions for sub-65
nm devices is expected to incorporate a wafer tilt .gtoreq.30 deg
from the substrate normal, in order to produce enough "under the
gate" dopant concentration, without relying on excessive dopant
diffusion brought about by aggressive thermal activation
techniques. Excellent beam angular definition and low beam angular
divergence are also desired for these implants; while high current
implanters tend to have large angular acceptances and significant
beam non-uniformities, medium current implanters meet these
high-tilt and precise angle control requirements. Since
medium-current implanters do not deliver high enough currents,
their throughput on high-dose implants is too low for production.
If ion implanters could produce the required low-energy beams at
high dose rates, great economic advantage would be achieved. Since
drain extensions are the shallowest of implants, they are also at
the lowest energies (about 3 keV As at the 65 nm node, for
example); the long, complicated beamlines which typify
medium-current implanters cannot produce enough current at low
energy to be useful in manufacturing such devices. The use of
As.sub.4.sup.+ and P.sub.4.sup.+ cluster implantation in
medium-current beam lines and other scanned, single-wafer
implanters extends the useful process range of these implanters to
low energy and to high dose. By using high currents of these
clusters, up to a factor of 16 in throughput increase can be
realized for low-energy, high dose (.gtoreq.10.sup.14/cm.sup.2)
implants with effective As and P implant energies as low as 1 keV
per atom.
The Chemical Nature of Arsenic and Phosphorus
[0015] As is generally known, elemental, solid As and P are known
to exist in a tetrahedral form(i.e., as white phosphorus, P.sub.4,
and as yellow arsenic, As.sub.). They would therefore seem to be
ideal candidates for producing tetramer ions in an ion source.
However, while these compounds can be synthesized, they are more
reactive, and hence more unstable, than their more common forms,
i.e., red P and grey As metals. These latter forms are easily
manufactured, stable in air, and inexpensive. Importantly, it turns
out that when common red P and grey As are vaporized, they
naturally form primarily P.sub.4 and As.sub.4 clusters in the vapor
phase! [see, for example, M. Shen and H. F. Schaefer III, J. Chem.
Phys. 101 (3) pp. 2261-2266, 1 Aug. 1994.; Chemistry of the
Elements, 2.sup.nd Ed., N. N Greenwood and A. Earnshaw, Eds.,
Butterworth-Heiemann Publishers, Oxford, England, 2001, Chap. 13,
p. 55; R. E. Honig and D. A. Kramer, RCA Review 30, p. 285, June
1969.] Electron diffraction studies have confirmed that in the
vapor phase the tetrahedral As.sub.4 predominates. This tetrahedral
phase is delicate, however, and is readily dissociated, for
example, by exposure to ultraviolet light or x-rays, and
dissociates in plasmas of the type formed by conventional ion
sources. Indeed, it is known that As.sub.4 quite readily
dissociates into 2 As.sub.2 under energetic light bombardment.
[0016] Significant currents of ionized As.sub.4 and P.sub.4
clusters can be produced by vaporizing solid forms of As and P
(either the amorphous or tetrahedral solid phases) and preserving
these clusters through ionization in a novel electron-impact
ionization source, demonstrating that the clusters survive electron
impact.
[0017] Although prior art ion sources have used vaporized solid As
and P to generate ion beams, the tetramers have not been preserved.
The ions produced by these arc discharge sources have consisted of
principally monomers and dimers. Since the tetramer forms As.sub.4
and P.sub.4 are delicate and easily dissociated by the introduction
of energy, to preserve them, the source should be free from
excessive UV (such as emitted by hot filaments, for example) and
most importantly, be ionized by a "soft" ionization technique, such
as electron impact. As will be discussed in more detail below, this
technique is useful in creating As.sub.4.sup.+ ions from vaporized
elemental arsenic and phosphorus.
Advantages of the Novel Ion Source for As.sub.4 and P.sub.4
Production
[0018] The ion source of the present invention introduces gaseous
As.sub.4 and P.sub.4 vapors through a vaporizer which heats solid
feed materials, such as elemental As or P, and conducts the vapor
through a vapor conduit into the ionization chamber of the ion
source. Once introduced into the ionization chamber of the ion
source, the vapor or gas interacts with an electron beam which
passes into the ionization volume from an external electron gun,
forming ions. The vapor is not exposed to a hot, UV-producing
cathode since the electron gun is external to the ionization volume
and has no line-of-sight to the vapors. The ions are then extracted
from a rectangular aperture in the front of the ionization volume
by electrostatic optics, forming an ion beam.
DESCRIPTION OF THE DRAWINGS
[0019] These and other advantages of the present invention will be
readily understood with reference to the following specification
and attached drawing wherein:
[0020] FIG. 1 is a schematic diagram of an exemplary ion beam
generation system in accordance with the present invention.
[0021] FIG. 2 is a schematic diagram of an alternative embodiment
of the exemplary ion beam generation system illustrated in FIG.
1,illustrating a solid vapor source and an in-situ cleaning
system.
[0022] FIG. 3a is a schematic representation of the basic
components of the ion source in accordance with the present
invention which includes an electron gun, an indirectly-heated
cathode, a source liner, a cathode block, a base, an extraction
aperture, a source block, and a mounting flange.
[0023] FIG. 3b is an exploded view of the ion source of the present
invention, illustrating the major subsystems of the ion source
[0024] FIG. 4a is an exploded isometric view of the ion source
illustrated in FIG. 3a, shown with the mounting flange assembly,
electron gun assembly, indirectly heated cathode assembly and the
extraction aperture plate removed.
[0025] FIG. 4b is an exploded isometric view of the ionization
volume liner and the interface or base block showing the plenum and
the plenum ports in the interface block.
[0026] FIG. 4c is an isometric view of the ionization volume
assembly in which the ionization volume is formed from a cathode
block, an interface block, and a magnetic yoke assembly, shown with
the ionization volume liner removed.
[0027] FIG. 5a is an exploded isometric view of a indirectly-heated
cathode (IHC) assembly in accordance with one aspect of the present
invention.
[0028] FIG. 5b is an enlarged exploded view of a portion of the IHC
assembly, illustrating the IHC, a filament, a cathode sleeve, and a
portion of a cathode plate.
[0029] FIG. 5c is an elevational view in cross section of the IHC
assembly illustrated in FIG. 5b.
[0030] FIG. 5d is an isometric view of a water-cooled cathode block
shown assembled to the IHC assembly illustrated in FIG. 5a in
accordance with one aspect of the invention.
[0031] FIG. 5e is an elevational view of the assembly illustrated
in FIG. 5d illustrating the cathode block and the cathode plate of
the IHC assembly in section.
[0032] FIG. 5f is an isometric view of a magnetic yoke assembly
which surrounds the cathode block and ionization volume In
accordance with the present invention.
[0033] FIG. 6a is an isometric view of an emitter assembly which
forms a portion of the external electron gun assembly in accordance
with one aspect of the present invention.
[0034] FIG. 6b is an isometric view of an electron gun assembly in
accordance with the present invention shown with an electrostatic
shield assembly removed.
[0035] FIG. 7 is an isometric view illustrating a magnetic circuit
associated with the electron gun and ionization volume yoke
assembly.
[0036] FIG. 8 is an isometric view of an exemplary dual hot
vaporizer assembly in accordance with one aspect of the present
invention.
[0037] FIG. 9a is an isometric view of a source block in accordance
with the present invention
[0038] FIG. 9b is similar to FIG. 9a but shown with the hot
vaporizer assembly removed.
[0039] FIG. 10 is a diagram which illustrates the typical voltages
applied to each element of the ion source when operating in
electron-impact ionization mode.
[0040] FIG. 11 is similar to FIG. 10 but indicates the typical
voltages applied to each element of the ion source when operating
in arc discharge mode.
[0041] FIGS. 12a and 12b are logic flow diagrams illustrating the
sequence of steps required to establish each operating mode in
succession.
[0042] FIG. 13 is a diagram which shows the thermal interfaces
between source block, interface block, cathode block, and the
ionization volume liner.
[0043] FIG. 14 is a side view in cross section, of the source
assembly, cut in the y-z plane.
[0044] FIG. 15 is similar to FIG. 14 but cut in the x-y plane.
[0045] FIG. 16 is similar to FIG. 14 but cut in the x-z plane
[0046] FIG. 17 is a photograph of the source with the front
aperture plate removed, showing the indirectly heated cathode and
the ionization volume liner.
[0047] FIG. 18 is a photograph showing the mounting flange with
feedthroughs, shown with the vaporizers removed.
[0048] FIG. 19 is a plot of mass-analyzed B.sub.18H.sub.x.sup.+
beam current delivered to an implanter Faraday cup positioned 2
meters from the ion source and downstream from an analyzer magnet
on the left vertical axis, and total ion current extracted from the
same ion source shown on the right vertical axis, as a function of
vapor flow into the ion source.
[0049] FIG. 20 is a B.sub.18H.sub.22 mass spectrum collected from
the ion source of the present invention.
[0050] FIG. 21 is a PH.sub.3 mass spectrum collected from the ion
source of the present invention.
[0051] FIG. 22 is an AsH.sub.3 mass spectrum collected from the ion
source of the present invention.
[0052] FIG. 23 is a P spectrum showing the monomer P.sup.+, the
dimer P.sub.2.sup.+, the trimer P.sub.3.sup.+, and the tetramer
P.sub.4.sup.+.
[0053] FIG. 24 is a As spectrum showing the monomer As.sup.+, the
dimer As.sub.2.sup.+, the trimer As.sub.3.sup.+, and the tetramer
As.sub.4.sup.+.
DETAILED DESCRIPTION
[0054] The present invention relates to ion source for providing a
range of ion beams consisting of either ionized clusters,such as
B.sub.2H.sub.x.sup.+, B.sub.5H.sub.x.sup.+, B.sub.10H.sub.x.sup.+,
B.sub.18H.sub.x.sup.+, P.sub.4.sup.+ or As.sub.4.sup.+ or monomer
ions,such as Ge.sup.+, In.sup.+, Sb.sup.+, B.sup.+, As.sup.+, and
P.sup.+, to enable cluster implants and monomer implants into
silicon substrates for the purpose of manufacturing CMOS devices,
and to do so with high productivity. The range of ion beams is
generated by a universal ion source in accordance with the present
invention which is configured to operate in two discrete modes: an
electron impact mode, which efficiently produces ionized clusters,
and an arc discharge mode, which efficiently produces monomer
ions.
[0055] The universal ion source in accordance with the present
invention is illustrated and described below. FIG. 14 shows, in
cross section, a cut in the y-z plane (i.e., side view) through the
ion source assembly in accordance with the present invention. FIG.
15 is similar to FIG. 14, but shows, in cross section, a cut in the
x-y plane through the source assembly. FIG. 16 shows, in cross
section, a cut in the x-z plane through the source assembly. FIG.
17 is a photograph of the source with the front aperture plate
removed, showing the indirectly heated cathode and the ionization
chamber liner. FIG. 18 is a photograph showing the mounting flange
with feed throughs with the vaporizers removed.
[0056] In order to efficiently produce ionized clusters, the ion
source of the present invention incorporates the following
features: [0057] An electron-impact ionization source is provided,
for example an electron gun which is located external to the
ionization volume and out of line-of-sight of any process gas or
vapors exiting the ionization volume, and the vapors in the
ionization chamber are likewise not exposed to electromagnetic
radiation emitted by the hot cathode in the electron gun; [0058]
When operating in the electron-impact mode, the surfaces exposed to
the vapor introduced into the source are held within a temperature
range which is low enough to prevent dissociation of the
temperature-sensitive parent molecule, and high enough to prevent
or limit unwanted condensation of the vapors onto said surfaces;
[0059] Multiple vaporizers are provided which can produce a stable
flow of vapor into the source (the vaporization temperatures of the
solid borohydride materials B.sub.10H.sub.14 and B.sub.18H.sub.22
range from 20 C to 120 C, while solid elemental materials, such as
As and P, require heating in the range between 400 C and 550 C to
provide the required vapor flows. Thus, one or more "cold"
vaporizers and one or more "hot" vaporizers are incorporated into
the ion source.
[0060] In order to efficiently produce monomer ions, the ion source
of the present invention also incorporates the following features:
[0061] An electron source (cathode), a repeller (anticathode) and a
magnetic field are incorporated into the ion source in a "reflex"
geometry, wherein a strong magnetic field is oriented substantially
parallel to the ion extraction aperture of the ion source, along a
line joining the electron source and repeller; [0062] Electronics
are provided so that an arc discharge can be sustained between the
cathode and the anticathode, such that a plasma column is sustained
along the magnetic field direction, i.e., parallel and in proximity
to the ion extraction aperture; [0063] An ionization volume liner
(an "inner chamber") is provided within the ion source, enclosing
the ionization volume, and is allowed to reach a temperature well
in excess of 200 C during arc discharge operation in order to limit
condensation of As, P and other species onto the walls surrounding
the ionization volume; [0064] A process gas feed is provided to
supply conventional gaseous dopant sources into the ion source.
[0065] Other novel features are provided in the ion source to
enable reliability and performance: It is a feature of the
invention that the ion source incorporates an in-situ chemical
cleaning process, preferably by the controlled introduction of
atomic fluorine gas, and the materials used to construct the
elements of the ion source are selected from materials resistant to
attack by F:
[0066] The ionization chamber liner may be fabricated from titanium
diboride (TiB.sub.2), which is resistant to attack by halogen
gases, and possesses good thermal and electrical conductivity, but
may also be usefully fabricated of aluminum, graphite or other
electrical and thermal conductor which is not readily attacked by
flourine;
[0067] The arc discharge electron source may be an
indirectly-heated cathode, and the portion of which exposed to the
cleaning gas may be formed a thick tungsten, tantalum or molybdenum
disk, and is therefore much more robust against failure in a
halogen environment than a naked filament;
[0068] The indirectly-heated cathode assembly is mechanically
mounted onto a water-cooled aluminum "cathode block" so that the,
limiting its radiative heat load to the ionization chamber and
liner (we note that aluminum passivates in a F environment, and is
therefore resistant to chemical etch); this enables rapid cool down
of the cathode between the time it is de-energized and the onset of
an in-situ cleaning cycle, reducing the degree of chemical attack
of the refractory metal cathode
[0069] The electron gun which is energized during electron-impact
ionization (i.e., during cluster beam formation) is remote from the
ionization volume, mounted externally and has no line-of-sight to
the F gas load during an in-situ clean, and therefore is robust
against damage by F etching.
[0070] Other novel features are incorporated to improve source
performance and reliability: [0071] The aluminum cathode block or
frame is at cathode potential, eliminating the risk of cathode
voltage shorts which are known to occur between indirectly-heated
cathodes and the source chambers of prior art sources. This block
also conveniently forms the repeller structure, being at cathode
potential, thereby obviating the need for a dedicated electron
repeller or anticathode; [0072] The ionization volume liner is
surrounded by a cathode block and a base; the aluminum base and
cathode block are held in thermal contact with a
temperature-controlled source block through thermally conductive,
but electrically insulating elastomeric gaskets. This feature
limits the maximum temperature of the block and base to near the
source block temperature (the source block is typically held below
200 C); [0073] The ionization volume liner is in thermal contact
with the base through a high-temperature, thermally and
electrically conductive gasket, such as aluminum, to limit its
maximum temperature excursion while insuring its temperature is
higher than that of the cathode block and base; Unlike other known
ion sources, no ionization chamber per se is provided. [0074] The
source magnetic field is provided by a magnetic yoke assembly which
surrounds the ionization chamber assembly. It is embedded in the
cathode block. This provides a means for keeping the yoke assembly
at a temperature well below the Curie temperature of its permanent
magnets. [0075] The ion source operates in two discrete modes:
electron impact mode and arc discharge mode. The operating
conditions for each are quite different as described in detail
below.
[0076] When operating in electron impact mode, the following
conditions are met: [0077] The source block is held at a
temperature between about 100.degree. C. and 200.degree. C.
Depending on which specie is run in the ion source; this provides a
reference temperature for the source, preventing condensation of
the source material, such as borohydride or other source materials;
[0078] The indirectly heated cathode is not energized, and cooling
water is not run in the cathode block. The cathode block comes to
thermal equilibrium with the base, with which it is in thermal
contact through a thermally conductive, but electrically
insulating, gasket (the base is in turn in good thermal contact
with the source block, and so rests near the source block
temperature); [0079] The cathode block is held at the same
potential as the base and the ionization volume liner; [0080] The
electron gun is energized by applying a negative potential to the
electron emitter (i.e. the cathode), and applying a positive
potential to the anode and the gun base (i.e. the potential of the
local environment of the electron beam as it propagates through the
gun). The cathode and anode voltages are measured with respect to
the ionization volume. This enables a "deceleration" field to act
on the electron beam as it propagates between the gun base and
ionization volume so that the energy of the electrons which ionize
the gas or vapor can be varied independently of the energy of the
electron beam propagating within the gun, and in particular be
reduced to effect more efficient ionization of the gas molecules;
[0081] A permanent magnetic field provides confinement of the
electron beam as it enters and transits the ionization chamber,
enabling a uniform ion density to be created adjacent to, and along
the ion extraction aperture of the ion source; [0082] The TiB.sub.2
liner (which can also be made of SiC, B.sub.4C, Al, C, or any other
suitable electrically conductive material which is not a
deleterious contaminant in silicon circuits) is in thermal contact
with the base (which is in thermal continuity with the source
block) through an electrically and thermally conductive
high-temperature gasket, and so will settle close to the source
block temperature, since very little power (typically <10 watts)
is dissipated by the electron beam within the ionization volume The
liner is thus always at the same potential as the ionization volume
and the source block.
[0083] When operating in arc discharge mode, the following
conditions are met: [0084] The source block is held at between
100.degree. C. and 200.degree. C. [0085] The indirectly heated
cathode is energized, and cooling water is run in the cathode
block. The cathode block temperature is thus maintained near to the
water temperature, and cooler than the base, which is in thermal
contact with the source block; [0086] The cathode block is held at
the same potential as the cathode, up to 100V negative with respect
to the liner which surrounds the ionization volume. Since the
cathode block also comprises the repeller or anticathode, it is
also at cathode potential. In the presence of the permanent axial
magnetic field, this enables a true "reflex" geometry and hence a
stable plasma column. The arc current is absorbed by the liner,
whose potential establishes the plasma potential. [0087] The
electron gun is not energized, the electron emitter is set to
source block potential, and the gun base is set to cathode block
potential. This prevents any net field from penetrating from the
gun base through the electron entrance aperture in the cathode
block. [0088] With the indirectly-heated cathode energized and an
arc discharge initiated, the liner is exposed to a significant
radiative heat load. This allows the liner to reach an equilibrium
temperature well in excess of the base. The maximum temperature
differential can be "tuned" by reducing or increasing the thermal
contact between liner and base.
[0089] Referring to FIG. 1, a schematic diagram of an exemplary ion
beam generation system which incorporates an ion source in
accordance with the present invention is illustrated. As shown in
this example, the ion source 400 is adapted to produce an ion beam
for transport to an ion implantation chamber for implant into
semiconductor wafers or flat-panel displays. The ion beam
generation system includes an ion source 400, an extraction
electrode 405, a vacuum housing 410, a voltage isolation bushing
415 of electrically insulative material, a vacuum pumping system
420, a vacuum housing isolation valve 425, a reactive gas inlet
430, a feed gas and vapor inlet 441, a vapor source 445, a feed gas
source 450, a reactive gas source 455, an ion source high voltage
power supply 460 and an ion beam transport housing 411. The ion
source 400 produces a resultant ion beam illustrated by the arrow
475.
[0090] The ion source 400 is constructed to provide cluster ions
and molecular ions, for example the borohydride ions
B.sub.10H.sub.x.sup.+, B.sub.10H.sub.x.sup.-,
B.sub.18H.sub.x.sup.+, and B.sub.18H.sub.x.sup.- or, and
alternatively, more conventional ion beams, such as P.sup.+,
As.sup.+, B.sup.+, In.sup.+, Sb.sup.+, Si.sup.+, and Ge. The gas
and vapor inlet 441 for gaseous feed material to be ionized is
connected to a suitable vapor source 445, which may be in close
proximity to gas and vapor inlet 441 or may be located in a more
remote location, such as in a gas distribution box, located
elsewhere within a terminal enclosure.
[0091] A terminal enclosure is a metal box, not shown, which
encloses the ion beam generating system. It contains required
facilities for the ion source, such as pumping systems, power
distribution, gas distribution, and controls. When mass analysis is
employed for selection of an ion species in the beam, the mass
analyzing system may also be located in the terminal enclosure.
[0092] In order to extract ions of a well-defined energy, the ion
source 400 is held at a high positive voltage (in the more common
case where a positively-charged ion beam is generated) with respect
to an extraction electrode assembly 405 and a vacuum housing 410 by
a high voltage power supply 460. The extraction electrode assembly
405 is disposed close to and aligned with an extraction aperture
504 on an extraction aperture plate which forms a portion of the
ionization volume 500. The extraction electrode assembly consists
of at least two aperture-containing electrode plates, a so-called
suppression electrode 406 closest to the ionization volume 500, and
a "ground" electrode 407. The suppression electrode 406 is biased
negative with respect to a ground electrode 407 to reject or
suppress unwanted electrons which are attracted to the
positively-biased ion source 400 when generating positively-charged
ion beams. The ground electrode 407, vacuum housing 410, and
terminal enclosure (not shown) are all at the so-called terminal
potential, which is at earth ground unless it is desirable to float
the entire terminal above ground, as is the case for certain
implantation systems, for example for medium-current ion
implanters. The extraction electrode 405 may be of the novel
temperature-controlled metallic design, described below.
[0093] In accordance with another aspect of the invention, the ion
source 400, illustrated in of FIG. 1, may be configured for in situ
cleaning, i.e. without the ion source being removed from its
operating position in the vacuum housing, and with little
interruption of service. Indeed, for ion sources suitable for use
with ion implantation systems, e.g. for doping semiconductor
wafers, the source chamber or ionization volume 500 is small,
having a volume, for example, less than about 100 ml, and an
internal surface area of, for example, less than about 200
cm.sup.2, and is constructed to receive a flow of the reactive gas,
e.g. atomic fluorine or a reactive fluorine-containing compound at
a flow rate of less than about 200 Standard Liters Per Minute. As
such, a dedicated endpoint detector 470, in communication with the
vacuum housing 410 may be used to monitor the reactive gas products
during chemical cleaning.
[0094] FIG. 2 illustrates an embodiment of an ion source, similar
to FIG. 1, that is configured for conducting in-situ chemical
cleaning of the ion source 400 including the extraction electrode
assembly 405. The in situ cleaning system is described in detail in
International Patent Application No. PCT/US2004/041525, filed on
Dec. 9, 2004, hereby incorporated by reference. Briefly, three
inlet passages are integrated into ion source 400, respectively.
One inlet passage is for reactive gas 430 from a plasma source 455.
Another inlet passage is for feed gas 435 from one of a number of
storage volumes 450 selected. The third inlet is for feed vapor 440
from a vaporizer 445. The plasma-based reactive gas source 455 is
biased at the high voltage of the ion source 400. This enables the
remote plasma source 455 to share control points of the ion source
400 and also enables the cleaning feed gas 465 and argon purge gas
from storage source 466 to be supplied from an ion source gas
distribution box, which is at source potential. Also shown is a
different type of endpoint detector, namely a Fourier Transform
Infrared (FTIR) optical spectrometer. This detector can function
ex-situ (outside of the vacuum housing), through a quartz window.
Instead, as shown in FIG. 2, an extractive type of FTIR
spectrometer may be used, which directly samples the gas in the
vacuum housing 410 during cleaning. Also a temperature sensor TD
may sense the temperature of the de-energized ionization chamber by
sensing a thermally isolated, representative region of the surface
of the chamber. The sensor TD can monitor heat produced by the
exothermic reaction of F with the contaminating deposit, to serve
as end-point detection.
[0095] FIG. 3a is a simplified schematic representation of the
basic components of the ion source, indicating the electron gun
cathode 10, the indirectly-heated cathode (IHC) 20, an ionization
volume liner 30, a cathode block 40, a base or interface block 50,
extraction aperture plate 60, a source block 70, and a mounting
flange 80. The ionization volume liner 30 is preferably made of
TiB.sub.2 or aluminum, but may be usefully constructed of SiC,
B.sub.4C, C, or any other suitable electrically conductive material
which is not a deleterious contaminant in silicon circuits, and can
sustain an operating temperature of between 100 C and 500 C. The
cathode block 40 is preferably of aluminum due to its high thermal
and electrical conductivity, and resistance to attack by halogen
gases. Al also allows for direct water cooling since it is
non-porous and non-hydroscopic. Other materials may be used such as
refractory metals like tungsten and molybdenum which have good
electrical and thermal properties; however they are readily
attacked by halogen gases. Another consideration for the cathode
block is compatibility with ion bombardment of P.sup.+, As.sup.+,
and other species produced under arc discharge operation. Since the
cathode block is unipotential with the IHC cathode 20, it is
subject to erosion by ion bombardment of plasma ions. The sputter
rates of materials under bombardment by ions of interest therefore
must be considered as it will impact useful source life. The base
50, again, is preferably made of aluminum, but can be made of
molybdenum or other electrically and thermally conductive
materials. Since the source block 70, mounting flange 80, and ion
extraction aperture 60 are typically operated at 200 C or below,
they can be usefully constructed of aluminum as well The ionization
volume liner 30 surrounds an ionization volume 35 and is in light
thermal contact with the mounting base 50, which is itself in good
thermal contact with the source block 70. Except for a slot through
the ionization volume liner 30 and the extraction aperture plate 60
through which ions pass, the ionization volume of the ion source is
fully bounded by a cylindrical bore through the ionization volume
liner 30 and the top and bottom plates of the cathode block 40. The
source block 70 is temperature controlled to up to 200 C, for
example. Thus, when the electron gun 10 is active, very little
power is transferred to the ionization volume liner 30, the
temperature of which is close to that of the source block 70. When
the IHC 100 is energized, the ionization volume liner 30 is exposed
to hundreds of watts of power and can attain a much higher
temperature than the source block 70 (up to 400 C or higher), which
is beneficial to limit condensation of gases onto the surface of
the ionization volume liner 30.
[0096] FIG. 3b is an exploded isometric view of the ion source of
the present invention, showing its major subsystems. The ion source
includes an ion extraction aperture plate 60, an ionization volume
or chamber assembly 90, an IHC assembly 100, an electron gun
assembly 110, a source block assembly 120, and a mounting flange
assembly 130. The ion source also includes a low-temperature
vaporizer (not shown) coupled to a port 135. A vapor conduit 137 is
used to transport the vapor into the ionization assembly 90. The
ion source also includes dual hot vapor inlet ports 138, a process
gas inlet port 139, and an optional reactive gas inlet port 140. In
an exemplary, embodiment atomic F may fed to the ionization volume
assembly 90 via the reactive gas inlet port 140. Vaporized As, P,
or SbO.sub.3 into the dual hot vapor inlet ports 138 while
B.sub.18H.sub.22 vapor may be applied to the vapor conduit 137.
[0097] FIG. 4a is an exploded isometric view of the ion source in
accordance with the present invention, shown with the mounting
flange assembly 130, electron gun assembly 110, indirectly heated
cathode assembly 100 and the extraction aperture plate 60 removed.
The ion source includes a source block 120, a cathode block 40,
mounting base or interface block 50, an ionization volume or source
liner 30, a liner gasket 115, a base gasket 125, and a cathode
block gasket 127. As will be discussed in more detail below and as
illustrated in FIG. 4c, when the magnetic yoke assembly 150 is
added, these parts form an ionization volume assembly 90 (FIG. 3b).
The gaskets 125 and 127 are electrically insulating, thermally
conductive gaskets, fabricated from polymer compounds, for example.
Their purpose is to prevent thermal isolation of the parts while
allowing for potential differences between the mating parts. For
example, the cathode block 40 is at several hundred volts below the
base or interface block 50 potential during arc discharge
operation, and so must be electrically isolated. However, during
electron impact operation, the cathode block 40 should be near the
temperature of the base or interface block 50, and so it cannot be
thermally isolated. The gasket 115, however, is a metal gasket
which forms the interface between the ionization volume liner 30
and the base or interface block 50. Metal was chosen because of its
ability to withstand the higher temperatures the ionization volume
liner 30 will reach during arc discharge operation. Since the base
or interface block 50 is effectively heat sunk to the source block
120 (which is a constant temperature reservoir, i.e., it is
actively temperature controlled through embedded ohmic heaters
coupled to a closed-loop controller), it tracks near the source
block 70 temperature. The source block 70 is actively temperature
controlled, and the separate source elements track this temperature
through carefully selected thermal contact paths, as described in
FIG. 13. Closed loop control of the source block 70 temperature may
be implemented using a conventional PID controller, such as the
Omron E5CK digital controller, which can be used to control the
duty cycle of the power delivered to the ohmic heaters embedded in
the source block, as is known in the art.
[0098] FIG. 4b is an exploded isometric view of the ionization
volume liner 30 and the interface or base block 50, showing the
plenum and the plenum ports in the interface block 50. The several
gas and vapor inlet ports, namely vapor port 137, reactive gas port
140, process gas port 139, and dual hot vapor ports 141a and 141b,
feed into a gas plenum 45, formed in the base or interface block
50. The interface block 50 is provided with one or more through
holes 142a and 142b to accommodate mounting conventional fasteners
(not shown) to secure the interface block 50 to the source block
120 and thereby establish electrical conductivity between the
interface block 50 and the source block 120). The gas plenum 45 may
be cavity machined into the interface block 50 and is used to
collect any of the gases fed into the plenum 45 and feed them into
multiple liner ports 32. The multiple liner ports 32 are configured
in a "shower head" design to distribute the gases along different
directions into the ionization volume 35 within the ionization
volume liner 30. By transporting all of the gases or vapors into
the plenum 45, which acts as a ballast volume, which then feeds the
gases through a shower head directly into the ionization volume 35,
produces a uniform distribution of gas or vapor molecules within
the ionization volume 35. Such a configuration results in a more
uniform distribution of ions presented to extraction aperture 60,
and the subsequent formation of a more spatially uniform ion
beam.
[0099] FIG. 4c is an isometric view of the ionization volume
assembly 90, shown with the ionization volume liner removed. The
ionization volume assembly 90 is formed from the cathode block 40,
the interface block 50, and the magnetic yoke assembly 150. The
magnetic yoke assembly 150 is constructed of magnetic steel and
conducts the magnetic flux produced by a pair of permanent magnets
151a and 151b around through ionization volume assembly 90,
producing a uniform magnetic field of about 120 Gauss, for example,
within the ionization volume 35. During electron impact operation,
this permanent field confines the electron beam so that the ions
are produced in a well-defined, narrow column adjacent to the ion
extraction aperture 60. During arc discharge mode, the same field
provides confinement for the plasma column between cathode and the
upper plate of the cathode block 40, which serves as an
anticathode.
[0100] FIG. 5a is an exploded view of the indirectly-heated cathode
(IHC) assembly 100. IHC assemblies are generally known in the art.
Examples of such IHC assemblies are disclosed in U.S. Pat. Nos.
5,497,006; 5,703,372; and 6,777,686, as well as US Patent
Application Publication No. US 2003/0197129 A1, all hereby
incorporated by reference. The principles of the present able
invention are applicable to all such IHC assemblies. An alternate
IHC assembly 100 in for use with the present invention includes an
indirectly-heated cathode 160, a cathode sleeve 161, a filament
162, a cathode plate 163,a pair of filament clamps 164a and 164b, a
pair of filament leads 165a and 165b, and a pair of insulators 167a
and 167b (not shown). The filament 162 emits up to 2 A, for
example, of electron current which heats the indirectly-heated
cathode 160 to incandescence by electron bombardment. Since the
filament 162 is held at a negative potential of up to 1 kV below
the cathode potential, up to 2 kW of electron beam heating capacity
is available for cathode heating, for example. In practice, heating
powers of between 1 kW and 1.5 kW are sufficient, although for very
high arc currents (in excess of 2 A of arc) higher power can be
required. The cathode 160 is unipotential with the cathode mounting
plate 163. The insulators 167a and 167b are required to stand off
the filament voltage of up to 1 kV.
[0101] Referring now to FIGS. 5b and 5c, the IHC 160 is located
onto the cathode plate 163 via a flange 159 and is locked into
position by sleeve 161 through threaded connection 156. The sleeve
161 serves as a radiation shield for the IHC 160, minimizing heat
loss through radiation, except at the emitting surface 157.
[0102] The indirectly heated cathode (IHC) 160 may be machined from
a single tungsten cylinder. An exemplary IHC 160 may be about 0.375
inch thick, and is robust against both F etch and ion bombardment.
As seen in FIG. 5c, the IHC 160 has the appearance of a thick
circular disk joined to a hollow cylinder which has a bottom flange
159 which registers the IHC 160 within its mounting part, cathode
plate 163. Two or more circular grooves 158 or saw cuts are
machined into the cylinder to reduce the conduction of heat from
the cathode emission surface 157 to the cathode plate 163, reducing
electron beam heating requirements. A similar groove 153 is
machined into the sleeve 161 to reduce heat transfer to the cathode
plate 163.The sleeve 161 attaches to the cathode plate 163 via
threads in the plate 163 and the sleeve 161. The sleeve 161 serves
two functions: it "locks down" IHC 158, and acts as a radiation
shield between the IHC 160 and its environment, reducing heating
power requirements. Note that the IHC 160 and its sleeve 161 are
enclosed by the water-cooled cathode block 40 which is designed to
absorb radiation to reduce overall source heating. Filament 162 is
constructed of approximately 1 mm-thick tungsten wire twisted into
a three-bend pattern which provides fairly uniform emission current
coverage onto the bottom of the IHC 160 disk. The filament 162 is
attached to dual clamps 164a and 164b which conduct current through
dual leads 165a and 165b to a vacuum feedthrough and to a 60 A
filament power supply. This power supply, and hence the filament,
is floated to a negative potential relative to the IHC by a high
voltage power supply, so that electron emission current leaving the
filament 162 is accelerated to the IHC 160, providing electron beam
heating. This 2 A, 1 kV power supply provides up to 2 kW of
electron beam heating power to bring the cathode surface 157 to
electron emission. In practice, 1 kW of electron beam heating is
sufficient (1.7 A at 600V, for example), but for IHC arc currents
of over several amperes, higher cathode temperature and hence
higher power is needed.
[0103] The IHC 160, sleeve 161, and filament 162 are preferably
made of tungsten. The filament leads shown in FIG. 5b are crimped
onto the filament 162, and are usefully made of molybdenum or
tantalum, for example. The cathode plate 163 can be made of
graphite, stainless steel, molybdenum, or any high temperature,
electrically conductive material having good mechanical tensile
strength. Since the cathode plate 163 mounts directly to the
cathode block, it is at cathode potential when the IHC 160 is
energized.
[0104] FIGS. 5d and 5e illustrate the indirectly-heated cathode
assembly 100 mounted onto the water-cooled cathode block 40. A pair
of water fittings 41a and 41b are used to transport de-ionized
water through a vacuum interface. The water circulates through the
cathode block 40 and can absorb several kW of power, allowing the
cathode block 40 to remain well below 100.degree. C. at all times.
The IHC 160 is unipotential with the cathode block 40. As such, no
insulation is required between the cathode 160 and cathode block
40, which forms the top and bottom boundary surfaces of the
ionization volume 35. This results in a very reliable system, since
in prior art IHC sources, the IHC is up to 150V different from its
immediate surroundings. This results, in turn, in quite common
failures precipitated by the collection of debris between the IHC
160 and the ionization volume surface through which it penetrates.
Another benefit of the configuration is that it eliminates the
common failure of anticathode erosion since the top plate of
cathode block 40 serves as the anticathode since it is at cathode
potential. The plasma column is bounded by the ionization volume 35
is defined by the bore through the ionization volume liner 30 and
the top and bottom plates of the cathode block 40. This defines a
very stable volume to sustain the plasma column during arc
discharge operation.
[0105] FIG. 5f shows a detail of the magnetic yoke assembly 150
which surrounds the cathode block 40 and the ionization volume 35.
The magnetic yoke assembly 150 is constructed of magnetic steel and
conducts magnetic flux through an ionization volume or chamber
assembly 90, producing a uniform axial magnetic field of about 120
Gauss, for example, within the ionization volume 35. This magnetic
yoke assembly 150 is used to generate a magnetic field to confine
the plasma generated in the ionization volume 35 during an arc
discharge mode of operation. During an electron impact mode of
operation, the electron gun assembly 110 is shielded from the
magnetic field because of a magnetic shield which is inserted
between the yoke assembly 150 and the electron gun, as indicated in
FIG. 7 below.]
[0106] FIGS. 6a and 6b illustrate the external electron gun
assembly 110. In particular, Such electron gun assemblies are
disclosed in detail in U.S. Pat. No. 6,686,595 as well as US Patent
Application Publication No. US 2004/0195973 A1, hereby incorporated
by reference. FIG. 6a is an isometric view of an exemplary emitter
assembly 210 which forms a part of the external electron gun
assembly 110. FIG. 6b is an isometric view of an electron gun
assembly 110, shown with an electrostatic shield assembly 250
removed. The electron gun assembly 110 includes a gun base 240,
which carries an emitter assembly 210, an anode 215, an
electrostatic shield assembly 250 and a magnetic shield 255.
[0107] Electrons emitted from a filament 200 in the emitter
assembly 210 are extracted by the anode 215 and bent through 90
degrees by the magnetic dipole 220, passing through an aperture 230
in the gun base 240. The electron beam is shielded from the
magnetic fields within the ionization volume assembly 90, generated
by the magnetic yoke 150, by a magnetic shield 255. The anode 215,
gun base 240, and the electrostatic shield assembly 250 are all at
anode potential, as high as, for example, 2 kV above the potential
of the ionization volume assembly 90, which is held at the
potential of the source block 120 during electron impact operation.
The filament voltage, for example, is several hundred volts
negative; thus, the electron beam is decelerated between the gun
base 240 and the ionization volume 35, as described in detail, for
example by Horsky in U.S. Pat. No. 6,686,595, hereby incorporated
by reference.
[0108] FIG. 7 is a physical representation of the magnetic circuit
associated with the electron gun assembly 110 and the magnetic yoke
assembly 150. As shown, the magnetic circuit consists of the
magnetic dipole 220, the gun magnetic shield 255, and the magnetic
yoke assembly 150. Magnetic dipole 220 is made of magnetic
stainless steel, and produces a uniform transverse magnetic field
across the poles, bending the electron beam produced by the
electron gun emitter through approximately 90 degrees. Thus
deflected, the electron beam passes through the aperture 230 of
FIG. 6, and into the ionization volume, where it is confined by the
chamber magnetic field.
[0109] FIG. 8 is an isometric view of an exemplary dual hot
vaporizer assembly 301. The dual hot vaporizer assembly 301
includes dual vaporizer ovens 300a and 300b, heater windings 310a
and 310b, and a pair of vapor nozzles 320a and 320b. Solid source
material, such as As, P, Sb.sub.2O.sub.3, or InF.sub.3, resides
within the oven cavities, which are hollow steel cylinders.
Sometimes the material is captured by a graphite crucible which
forms a liner between the material and cylinder, preventing
contamination of the oven walls. The oven heater windings 310a and
310b carry up to 20 A of current at 48V DC, and can dissipate up to
1 kW of heater power. They are brazed onto the ovens for good
thermal contact. The nozzles 320a and 320b are usefully fabricated
of molybdenum for good temperature uniformity, but can be made of
steel or other high temperature, conductive materials. The nozzles
are preferable 1/4 inch tubing and no longer than two inches \long,
to ensure good vapor conductance from oven to ionization volume.
The temperature of the ovens 300a and 300b is monitored by a pair
of thermocouples 330a and 330b. The temperature of the heater
windings 310a and 310b is monitored by a pair of thermocouples 331a
and 331b.
[0110] A mounting plate 340 is used to couple the dual hot
vaporizer assembly 301 to the source block 70. FIG. 9a shows the
source block 70 with the dual hot vaporizer assembly 301 removed
while FIG. 9b illustrates the source block with the hot vaporizer
assembly 301 being inserted.
[0111] FIG. 10 is a diagram which illustrates the typical voltages
applied to each element of the ion source when operating in
electron-impact ionization mode. All voltages are referenced to
source potential Vs, which is positive with respect to ground. The
mounting base or interface block 50, the cathode block 40, and the
source block 70 are held at Vs. The electron gun filament 200 is
held at cathode potential Vc by its related power supply (-1
kV<Vc<-100V), and the potential of the anode 240 Va is
positive (1 kV<Va<2 kV), so that the kinetic energy of the
electrons leaving the filament 200 and forming the electron beam 27
is e(Va-Vc). The ion extraction aperture plate 60 is biased to
either a positive or negative voltage to improve the focusing of
the extracted ion beam (-350V<Vb<350V). The IHC assembly 100
is not energized during an electron-impact ionization mode and is
held at the potential Vs during this mode.
[0112] FIG. 11 is similar to FIG. 10 but indicates the typical
voltages applied to each element of the ion source when operating
in arc discharge mode. All voltages are referenced to source
potential Vs which is positive with respect to ground. The electron
gun assembly 110 is not used, but the cathode supply is connected
to the IHC cathode 160 Vc (-100V<Vc<-0), which is
unipotential with the cathode block 40. Since the electron gun
assembly is not used in this mode, its filament 200 and anode 240
are held at cathode voltage Vc. The IHC filament 162 is at up to 1
kV below the IHC 20 potential (-1 kV<Vf<0), and can provide
up to 2 A, for example, of electron beam heating current. The IHC
160 is up to 100V, for example, different from its immediate
surroundings. FIGS. 12a and 12b are logic flow diagrams of the
sequence of steps required to establish each operating mode in
succession. Since the voltages of the ion source components are
different for the two modes of operation, there is a preferred
sequence for moving between modes:
[0113] When switching from the electron impact mode 600 to the arc
discharge mode 614, as illustrated in FIG. 12a, initially, in step
602, the electron gun assembly 110 is shut off. Next in step 604,
the electron gun anode 215 is decoupled from its power supply. In
step 606, the electron gun anode 215 is set to cathode potential.
This prevents any fields from punching through the cathode block 40
at the upper plate of the cathode block 40, making this an
effective anticathode. In step 608, the bias voltage applied to the
ion extraction aperture plate 60 is interrupted. The extraction
aperture plate 60 bias is only needed in cluster mode, and is not
recommended in discharge mode, especially since the power supply
may draw high currents due to the proximity of a dense plasma. Next
in step 610 water flow into the cathode block 40 is initiated by
automatic sequencing of pneumatically actuated water flow valves.
The water flow valves are interlocked to the ion source control
system through a water flow sensor and relay switch so that the IHC
cannot be energized unless flow has been established The cathode
block 40 must be water cooled during operation of the IHC assembly
100 to prevent undue heating of the source components, and to keep
the magnets 151a, 151b in the magnetic yoke 150 below their Curie
temperature. Finally in step 612, an arc can by initiated by the
introduction of process gas into the ionization volume 35 and
energizing the IHC assembly 100 as is known in the art.
[0114] When switching from the arc discharge 614 to the electron
impact mode 600, as illustrated in FIG. 12b, initially in step, the
IHC assembly 100 is de-energized. Next in step 618, the electron
gun anode 215 is connected to its positive power supply. In step
620, the cathode block 40 and the IHC assembly 100 are connected to
the to the source voltage. In step 622, the bias voltage is set and
connected to the ion extraction aperture plate 60. In step 624,
water cooling of the cathode block 40 is terminated. Finally, in
step 626, the electron gun assembly 110 is energized to establish
an electron beam. Also, vapor is introduced into the ionization
volume 35 to begin ionized cluster formation.
[0115] FIG. 13 shows the thermal interfaces between source block
70, the interface block 50, the cathode block 40, and the
ionization volume liner 30. As further outlined in FIG. 4a, thermal
paths are defined between the cathode block 40, the ion extraction
aperture 60, the interface or mounting block 50, the ionization
volume or source liner 30, and the source block 70 through
thermally conductive gaskets which are in wetted contact to the
surfaces of these components. Thus, the ionization volume liner 30
can attain higher temperatures than the temperature of source block
70, which is actively temperature controlled. In addition, the
water-cooled cathode block 40 has a thermal path to reach the
temperature of the mounting base 50 after water cooling is
disabled.
[0116] FIG. 19 is a plot of mass-analyzed B.sub.18H.sub.x.sup.+
beam current delivered to a Faraday cup positioned 2 meters from
the ion source and downstream from an analyzer magnet, and total
ion current extracted from the ion source. Shown are the extracted
ion current, in mA, on the right vertical axis, and the Faraday
current (similar to on-wafer current) on the left vertical axis.
The currents are measured as a function of B.sub.18H.sub.22 vapor
flow into the ion source, measured as inlet pressure into the ion
source. The vapor was fed into this ion source through a
proprietary closed-loop vapor flow controller which has been
described in detail elsewhere. The transmission through the
extraction optics and beam line of this implanter is about 25%, and
begins to fall off at the highest vapor flows, presumably due to
charge exchange with the residual vapor.
[0117] FIG. 20 is a B.sub.18H.sub.22 mass spectrum collected from
the ion source of the present invention, in electron-impact mode.
The parent peak, B.sub.18H.sub.x.sup.+, represents about 85% of the
beam spectrum. The small peak at half the parent 210 amu mass is
doubly ionized B.sub.18H.sub.x.sup.+, or
B.sub.18H.sub.x.sup.++.
[0118] FIG. 21 is a PH.sub.3 mass spectrum collected from the ion
source of the present invention, in arc discharge mode. Over 10 mA
of .sup.31P.sup.+ current and over 2 mA of doubly ionized
phosphorus was delivered to the Faraday of the implanter at 20 kV
extraction voltage. This performance is comparable to many
commercial Bernas-style ion sources used in ion implantation.
[0119] FIG. 22 is an AsH.sub.3 mass spectrum collected from the ion
source of the present invention, in arc discharge mode. Over 10 mA
of .sup.70As.sup.+ current and about 0.5 mA of doubly ionized
arsenic, as well as 0.5 mA of arsenic dimer was delivered to the
Faraday of the implanter at 20 kV extraction voltage. This
performance is comparable to many commercial Bernas-style ion
sources used in ion implantation.
[0120] FIG. 23 is a phosphorus spectrum showing the monomer
P.sup.+, the dimer P.sub.2.sup.+, the trimer P.sub.3.sup.+, and the
tetramer P.sub.4.sup.+, produced in electron impact mode. The
spectrum is unusual in that the monomer, dimer, and tetramer peaks
are all about the same height (about 0.9 mA), so that the tetramer
yields the highest dose rate, or about 3.6 mA of effective
phosphorus atom current. The spectrum was produced using elemental
P vapors from the hot vaporizer of the dual mode source. The high
cluster yield is due to the fact that the P vapor preferentially
produces P.sub.4, and this fragile cluster is preserved during the
ionization process by electron-impact ionization without exposing
the vapors to intense radiation or heat.
[0121] FIG. 24 is similar to FIG. 23, but collected with elemental
As vapors produced by the hot vaporizer of the dual-mode source.
The As spectrum shows the monomer .sup.70As.sup.+, the dimer
As.sub.2.sup.+, the trimer As.sub.3.sup.+, and the tetramer
As.sub.4.sup.+. At 20 kV extraction, the equivalent of 4 mA of 5
keV As.sup.+ is delivered to the Faraday.
[0122] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than is specifically described
above.
* * * * *