U.S. patent application number 15/592857 was filed with the patent office on 2017-11-16 for lanthanated tungsten ion source and beamline components.
The applicant listed for this patent is AXCELIS TECHNOLOGIES, INC.. Invention is credited to NEIL K. COLVIN, TSEH-JEN HSIEH, PAUL B. SILVERSTEIN.
Application Number | 20170330725 15/592857 |
Document ID | / |
Family ID | 58745508 |
Filed Date | 2017-11-16 |
United States Patent
Application |
20170330725 |
Kind Code |
A1 |
COLVIN; NEIL K. ; et
al. |
November 16, 2017 |
LANTHANATED TUNGSTEN ION SOURCE AND BEAMLINE COMPONENTS
Abstract
An ion implantation system is provided having one or more
conductive components comprised of one or more of lanthanated
tungsten and a refractory metal alloyed with a predetermined
percentage of a rare earth metal. The conductive component may be a
component of an ion source, such as one or more of a cathode,
cathode shield, a repeller, a liner, an aperture plate, an arc
chamber body, and a strike plate. The aperture plate may be
associated with one or more of an extraction aperture, a
suppression aperture and a ground aperture.
Inventors: |
COLVIN; NEIL K.; (MERRIMACK,
NH) ; HSIEH; TSEH-JEN; (ROWLEY, MA) ;
SILVERSTEIN; PAUL B.; (SOMERVILLE, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AXCELIS TECHNOLOGIES, INC. |
BEVERLY |
MA |
US |
|
|
Family ID: |
58745508 |
Appl. No.: |
15/592857 |
Filed: |
May 11, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62336246 |
May 13, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 2237/061 20130101;
H01J 37/08 20130101; H01J 37/3171 20130101; H01J 37/3002
20130101 |
International
Class: |
H01J 37/30 20060101
H01J037/30; H01J 37/317 20060101 H01J037/317 |
Claims
1. A conductive component for an ion implantation system, wherein
the conductive component is comprised of one or more of lanthanated
tungsten and a refractory metal alloyed with a predetermined
percentage of a rare earth metal, and wherein the conductive
component is generally passivated during operation of the ion
implantation system.
2. The conductive component of claim 1, wherein the conductive
component is a component of an ion source.
3. The conductive component of claim 2, wherein the conductive
component comprises one or more of a cathode, cathode shield, a
repeller, a liner, an aperture plate, an arc chamber body, and a
strike plate.
4. The conductive component of claim 3, wherein the liner comprises
an ion source liner.
5. The conductive component of claim 3, wherein the aperture plate
is associated with one or more of an extraction aperture, a
suppression aperture and a ground aperture.
6. The conductive component of claim 3, wherein the conductive
component comprises between 1% and 3% lanthanum.
7. An ion implantation system comprising a conductive component,
wherein the conductive component is comprised of one or more of
lanthanated tungsten and a refractory metal alloyed with a
predetermined percentage of a rare earth metal.
8. The ion implantation system of claim 7, wherein the conductive
component comprises one or more of a cathode, cathode shield, a
repeller, a liner, an aperture plate, an arc chamber body, and a
strike plate.
9. The ion implantation system of claim 8, wherein the liner
comprises an ion source liner.
10. The ion implantation system of claim 8, wherein the aperture
plate is associated with one or more of an extraction aperture, a
suppression aperture and a ground aperture.
11. The ion implantation system of claim 7, wherein the conductive
component is positioned upstream of a mass analyzer.
12. The ion implantation system of claim 7, wherein the conductive
component is generally passivated during operation of the ion
implantation system.
13. The ion implantation system of claim 7, wherein the conductive
component comprises between 1% and 3% lanthanum.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/336,246 filed May 13, 2016, entitled
"LANTHANATED TUNGSTEN ION SOURCE AND BEAMLINE COMPONENTS", the
contents of which are herein incorporated by reference in their
entirety.
TECHNICAL FIELD
[0002] The present invention relates generally to ion implantation
systems, and more specifically to an improved ion source and
beamline components that improve a lifetime, stability, and
operation of various aspects of an ion implantation system.
BACKGROUND
[0003] In the manufacture of semiconductor devices, ion
implantation is used to dope semiconductors with impurities. Ion
implantation systems are often utilized to dope a workpiece, such
as a semiconductor wafer, with ions from an ion beam, in order to
either produce n- or p-type material doping, or to form passivation
layers during fabrication of an integrated circuit. Such beam
treatment is often used to selectively implant the wafers with
impurities of a specified dopant material, at a predetermined
energy level, and in controlled concentration, to produce a
semiconductor material during fabrication of an integrated circuit.
When used for doping semiconductor wafers, the ion implantation
system injects a selected ion species into the workpiece to produce
the desired extrinsic material. Implanting ions generated from
source materials such as antimony, arsenic, or phosphorus, for
example, results in an "n-type" extrinsic material wafer, whereas a
"p-type" extrinsic material wafer often results from ions generated
with source materials such as boron, gallium, or indium.
[0004] A typical ion implanter includes an ion source, an ion
extraction device, a mass analysis device, a beam transport device
and a wafer processing device. The ion source generates ions of
desired atomic or molecular dopant species. These ions are
extracted from the source by an extraction system, typically a set
of electrodes, which energize and direct the flow of ions from the
source, forming an ion beam. Desired ions are separated from the
ion beam in a mass analysis device, typically a magnetic dipole
performing mass dispersion or separation of the extracted ion beam.
The beam transport device, typically a vacuum system containing a
series of focusing devices, transports the ion beam to the wafer
processing device while maintaining desired properties of the ion
beam. Finally, semiconductor wafers are transferred in to and out
of the wafer processing device via a wafer handling system, which
may include one or more robotic arms, for placing a wafer to be
treated in front of the ion beam and removing treated wafers from
the ion implanter.
[0005] Ion sources (commonly referred to as arc ion sources)
generate ion beams used in implanters and can include heated
filament cathodes for creating ions that are shaped into an
appropriate ion beam for wafer treatment. U.S. Pat. No. 5,497,006
to Sferlazzo et al., for example, discloses an ion source having a
cathode supported by a base and positioned with respect to a gas
confinement chamber for ejecting ionizing electrons into the gas
confinement chamber. The cathode of the Sferlazzo et al. is a
tubular conductive body having an endcap that partially extends
into the gas confinement chamber. A filament is supported within
the tubular body and emits electrons that heat the endcap through
electron bombardment, thereby thermionically emitting ionizing
electrons into the gas confinement chamber.
[0006] Conventional ion source gases such as fluorine or other
volatile corrosive species can etch the inner diameter of cathode
and repeller seals over time, thereby allowing the volatile gases
to escape and damage nearby insulators, such as a repeller assembly
insulator. This leakage will shorten the useful lifetime of the ion
source, thus resulting in shutting down of the ion implanter in
order to replace parts therein.
SUMMARY
[0007] The present disclosure thus provides a system and apparatus
for increasing the lifetime of an ion source. Accordingly, the
following presents a simplified summary of the disclosure in order
to provide a basic understanding of some aspects of the invention.
This summary is not an extensive overview of the invention. It is
intended to neither identify key or critical elements of the
invention nor delineate the scope of the invention. Its purpose is
to present some concepts of the invention in a simplified form as a
prelude to the more detailed description that is presented
later.
[0008] In accordance with one aspect of the disclosure, an ion
implantation system is provided having one or more conductive
components comprised of lanthanated tungsten and one or more of a
refractory metal alloyed with a predetermined percentage of a rare
earth metal. In one example, an ion source is provided, comprising
an arc chamber having a body defining and interior region of the
arc chamber. A liner is operably coupled to the body of the arc
chamber. In accordance with another exemplary aspect, an electrode
having a shaft and a head is further provided, wherein the shaft
passes through the body and the hole in the liner. The shaft is
further electrically isolated from the liner. The electrode, for
example, may comprise one or more of a cathode, repeller,
anti-cathode, and cathode shield.
[0009] The conductive component that is comprised of lanthanated
tungsten and one or more of the refractory metal alloyed with the
predetermined percentage of a rare earth metal may be a component
of the ion source, such as one or more of the cathode, cathode
shield, repeller, liner, an aperture plate, an arc chamber body,
and a strike plate. The aperture plate may be associated with one
or more of an extraction aperture, a suppression aperture and a
ground aperture.
[0010] In accordance with another exemplary aspect of the
disclosure, an ion source, such as an ion source for an ion
implantation system, is provided. The ion source, for example,
comprises the arc chamber and a gas source, wherein the gas source
is further configured to introduce a gas to the interior region of
the arc chamber body.
[0011] In another example, the ion source comprises a repeller
disposed opposite the cathode. An arc slit may be further provided
in the arc chamber for extraction of ions from the arc chamber.
[0012] To the accomplishment of the foregoing and related ends, the
disclosure comprises the features hereinafter fully described and
particularly pointed out in the claims. The following description
and the annexed drawings set forth in detail certain illustrative
embodiments of the invention. These embodiments are indicative,
however, of a few of the various ways in which the principles of
the invention may be employed. Other objects, advantages and novel
features of the invention will become apparent from the following
detailed description of the invention when considered in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a block diagram of an exemplary vacuum system
utilizing an ion source cathode shield in accordance with several
aspects of the present disclosure.
[0014] FIG. 2 is a perspective view of an exemplary ion source in
accordance with several aspects of the present disclosure.
[0015] FIG. 3 illustrates an exemplary arc chamber in accordance
with several aspects of the present disclosure.
[0016] FIG. 4 illustrates a conventional tungsten cathode and
shield after 20 hours of operation.
[0017] FIG. 5 illustrates a lanthanated tungsten cathode and shield
after 20 hours of operation in accordance with several aspects of
the present disclosure.
[0018] FIG. 6 illustrates a conventional arc chamber after running
30 hours with no co-gas.
[0019] FIG. 7 illustrates a lanthanated tungsten arc chamber after
running 30 hours with various source materials with no co-gas in
accordance with several aspects of the present disclosure.
[0020] FIG. 8 illustrates a graph of emission characteristics of
pure tungsten and lanthanated tungsten in accordance with several
aspects of the present disclosure.
[0021] FIG. 9 is a chart illustrating various characteristics of
various compounds in accordance with several aspects of the present
disclosure.
DETAILED DESCRIPTION
[0022] The present disclosure is directed generally toward an ion
implantation system and an ion source associated therewith. More
particularly, the present disclosure is directed toward components
for said ion implantation system that are comprised of lanthanated
tungsten for improved lifetime, stability, and operation of the ion
implantation system.
[0023] Accordingly, the present invention will now be described
with reference to the drawings, wherein like reference numerals may
be used to refer to like elements throughout. It is to be
understood that the description of these aspects are merely
illustrative and that they should not be interpreted in a limiting
sense. In the following description, for purposes of explanation,
numerous specific details are set forth in order to provide a
thorough understanding of the present invention. It will be evident
to one skilled in the art, however, that the present invention may
be practiced without these specific details. Further, the scope of
the invention is not intended to be limited by the embodiments or
examples described hereinafter with reference to the accompanying
drawings, but is intended to be only limited by the appended claims
and equivalents thereof.
[0024] It is also noted that the drawings are provided to give an
illustration of some aspects of embodiments of the present
disclosure and therefore are to be regarded as schematic only. In
particular, the elements shown in the drawings are not necessarily
to scale with each other, and the placement of various elements in
the drawings is chosen to provide a clear understanding of the
respective embodiment and is not to be construed as necessarily
being a representation of the actual relative locations of the
various components in implementations according to an embodiment of
the invention. Furthermore, the features of the various embodiments
and examples described herein may be combined with each other
unless specifically noted otherwise.
[0025] It is also to be understood that in the following
description, any direct connection or coupling between functional
blocks, devices, components, circuit elements or other physical or
functional units shown in the drawings or described herein could
also be implemented by an indirect connection or coupling.
Furthermore, it is to be appreciated that functional blocks or
units shown in the drawings may be implemented as separate features
or circuits in one embodiment, and may also or alternatively be
fully or partially implemented in a common feature or circuit in
another embodiment. For example, several functional blocks may be
implemented as software running on a common processor, such as a
signal processor. It is further to be understood that any
connection which is described as being wire-based in the following
specification may also be implemented as a wireless communication,
unless noted to the contrary.
[0026] Referring now to the Figures, in accordance with one aspect
of the present disclosure, FIG. 1 illustrates an exemplary vacuum
system 100. The vacuum system 100 in the present example comprises
an ion implantation system 101, however various other types of
vacuum systems are also contemplated, such as plasma processing
systems, or other semiconductor processing systems. The ion
implantation system 101, for example, comprises a terminal 102, a
beamline assembly 104, and an end station 106.
[0027] Generally speaking, an ion source 108 in the terminal 102 is
coupled to a power supply 110, whereby a source gas 112 (also
called a dopant gas) supplied thereto is ionized into a plurality
of ions to form an ion beam 114 that is extracted via extraction
electrodes 115. The ion beam 114 in the present example is directed
through an entrance 116 of a mass analyzer 117 (e.g., a
beam-steering apparatus), and out an aperture 118 towards the end
station 106. In the end station 106, the ion beam 114 bombards a
workpiece 120 (e.g., a semiconductor such as a silicon wafer, a
display panel, etc.), which is selectively clamped or mounted to a
chuck 122 (e.g., an electrostatic chuck or ESC). Once embedded into
the lattice of the workpiece 120, the implanted ions change the
physical and/or chemical properties of the workpiece. Because of
this, ion implantation is used in semiconductor device fabrication
and in metal finishing, as well as various applications in
materials science research.
[0028] The ion beam 114 of the present disclosure can take any
form, such as a pencil or spot beam, a ribbon beam, a scanned beam,
or any other form in which ions are directed toward end station
106, and all such forms are contemplated as falling within the
scope of the disclosure.
[0029] According to one exemplary aspect, the end station 106
comprises a process chamber 124, such as a vacuum chamber 126,
wherein a process environment 128 is associated with the process
chamber. The process environment 128 generally exists within the
process chamber 124, and in one example, comprises a vacuum
produced by a vacuum source 130 (e.g., a vacuum pump) coupled to
the process chamber and configured to substantially evacuate the
process chamber. Further, a controller 132 is provided for overall
control of the vacuum system 100.
[0030] The present disclosure provides a system and apparatus
configured to increase utilization and decrease downtime of the ion
source 108 in the ion implantation system 101 discussed above. It
shall be understood, however, that the apparatus of the present
disclosure may be also implemented in other semiconductor
processing equipment such as CVD, PVD, MOCVD, etching equipment,
and various other semiconductor processing equipment, and all such
implementations are contemplated as falling within the scope of the
present disclosure. The apparatus of the present disclosure
advantageously increases the length of usage of the ion source 108
between preventive maintenance cycles, and thus increases overall
productivity and lifetime of the system vacuum 100.
[0031] The ion source 108 (also called an ion source chamber), for
example, can be constructed using refractory metals (W, Mo, Ta,
etc.) and graphite in order to provide suitable high temperature
performance, whereby such materials are generally accepted by
semiconductor chip manufacturers. The source gas 112 is used within
the ion source 108, wherein source gas may or may not be conductive
in nature. However, once the source gas 112 is cracked or
fragmented, the ionized gas by-product can be very corrosive.
[0032] One example of a source gas 112 is boron tri-fluoride
(BF.sub.3), which can be used as a source gas to generate Boron-11
or BF.sub.2 ion beams in the ion implantation system 101. During
ionization of the BF.sub.3 molecule, three free fluorine radicals
are generated. Refractory metals, such as molybdenum and tungsten,
can be used to construct or line the ion source chamber 108 in
order to sustain its structural integrity at an operating
temperature of around approximately 700.degree. C. However,
refractory fluoride compounds are volatile and have very high vapor
pressures even at room temperature. The fluorine radicals formed
within the ion source chamber 108 attack the tungsten metal
(molybdenum or graphite) and form tungsten hexafluoride (WF.sub.6)
(molybdenum or carbon fluoride):
WF.sub.6.fwdarw.W.sup.++6F.sup.- (1)
or
(MoF.sub.6.fwdarw.Mo.sup.++6F.sup.-) (2)
[0033] Tungsten hexafluoride will typically decompose on hot
surfaces. For example, in an ion source 200 illustrated in FIG. 2,
the tungsten hexafluoride or other resultant material may decompose
on surfaces 202 of various internal components 203 of the ion
source, such as on surfaces of a cathode 204, a repeller 206 and
arc slit optics (not shown) associated an arc chamber 208 of the
ion source. This is called a halogen cycle as shown in equation
(1), but the resultant material can also precipitate and/or
condense back onto walls 210 or liners 212 or other components of
the arc chamber 208, as well as the arc slit in the form of a
contaminant material 214 (e.g., solid-state particulate
contaminants). The liners 212, for example, comprise replaceable
members 215 operably coupled to a body 216 of the arc chamber 208,
wherein the liners are comprised of graphite or various other
materials. The replaceable members 215, for example, provide wear
surfaces that can be easily replaced after a period of operation of
the arc chamber 208.
[0034] Another source of contaminant material 214 deposited onto
the internal components 203 arises from the cathode 204 when the
cathode is indirectly heated (e.g., a cathode composed of tungsten
or tantalum), whereby the indirectly heated cathode is used to
start and sustain the ion source plasma (e.g., a thermionic
electron emission). The indirectly heated cathode 204 and the
repeller 206 (e.g., an anti-cathode), for example, are at a
negative potential in relation to the body 216 of the arc chamber
208, and both the cathode and repeller can be sputtered by the
ionized gases. The repeller 206, for example, can be constructed
from tungsten, molybdenum, or graphite. Yet another source of
contaminant material 214 deposited on the internal components 203
of the arc chamber 208 is the dopant material (not shown), itself.
Over time, these deposited films of contaminant material 214 can
become stressed and subsequently delaminate, thereby shortening the
life of the ion source 200.
[0035] Surface condition plays a significant role between a
substrate and films deposited thereon. London dispersion force, for
example, describes the weak interaction between transient dipoles
or multi-poles associated with different parts of matter,
accounting for a major part of the attractive van der Waals force.
These results have significant implications in developing a better
understanding of atomic and molecular adsorption on different metal
substrates. Multi-scale modeling integrating first-principles
calculations with kinetic rate equation analysis shows a drastic
reduction in the growth temperature from 1000.degree. C. to
250-300.degree. C.
[0036] As the formation of a strong atomic bond within the
interfacial region is unlikely to happen, the thermal expansion
coefficient differences between the substrate (e.g., the cathode
204, liners 212, and/or repeller 206) and the deposited contaminant
material 214, the thermal cycling when transitioning between high
power and low power ion beams, and the dissociation of implant
materials residing within the uneven plasma boundary can cause
premature failure. The residual stresses in these deposits are of
two types: one arises from imperfections during film growth; the
other is due to mismatch in the coefficients of thermal expansion
between substrate and the deposited film.
[0037] The demand from device manufacturers for longer source life,
increased ion beam currents, ion beam stability and non-dedicated
species operation has pushed conventional ion source designs to
their limits. Each of these demands are not mutually exclusive,
however, whereby one or more performance characteristics are
typically sacrificed to provide an ion source that does not fail
prematurely.
[0038] The highly corrosive nature of fluorides and oxides
generated from cracking GeF.sub.4, BF.sub.3, SiF.sub.4, CO, and
CO.sub.2 challenges the conventional refractory metals used to
construct the ion source 200 and components associated therewith.
The formation of tungsten fluorides (e.g., WF.sub.x) which
subsequently decompose (e.g., via a halogen cycle) and deposit
tungsten onto heated surfaces such as the cathode, repeller (e.g.,
anode) and arc slit optics (not shown) typically degrades
performance of the ion source 200. The tungsten fluorides
(WF.sub.x), for example, can also react with insulators in the ion
source 200, thereby forming a conductive coating that can cause ion
beam instabilities and shortened lifetimes of the ion source and
associated components. The formation of WO.sub.2 and WO.sub.3 on
the internal source components, for example, can negatively impact
transitions to other species, such as .sup.11B and .sup.49BF.sub.2,
until the residual oxygen released from the tungsten oxides is
below some threshold level.
[0039] The present disclosure utilizes lanthanated tungsten (WL) or
other refractory metals alloyed with a predetermined percentage of
a rare earth metal for components (e.g., internal arc chamber
components) associated with the ion source 200. For example,
providing such lanthanated tungsten components may advantageously
eliminate the need and/or use of a co-gas such as hydrogen to tie
up residual fluorine and/or oxygen to prevent the aforementioned
damage to the ion source. The reaction of F.sup.- and O.sup.- with
lanthanum, for example, results in a protective surface layer which
is very stable at temperatures greater than 2000.degree. C.,
whereas tungsten fluorides and oxides are very volatile (e.g.,
halogen cycle) and lead to shorter lifetimes of the ion source, as
well as increased ion beam instabilities. Further, the ion source
200 of the present disclosure provides improved cathode electron
emission due to its lower work function and decreased formation of
tungsten carbide or oxides on the cathode tip, thus reducing
cathode electron emission for carbon implants.
[0040] In addition to using lanthanated tungsten or other
refractory metals alloyed with a predetermined percentage of a rare
earth metal to construct the arc internal components, the arc
chamber body and other components of the ion implantation system
that are downstream of the arc chamber can also be constructed
utilizing such a material. For example, extraction electrode optics
(e.g., suppression and ground apertures) and any other downstream
ion beam defining apertures, liners, and ion beam strike plates can
be formed of such a lanthanated tungsten material. Any components
that are susceptible to etching or sputtering by extracted fluorine
or oxygen ions are considered as being candidates for being formed
of such a material, where volatile corrosive conductive gases
formed in conventional systems would typically coat critical
insulators.
[0041] The present disclosure advantageously provides for the use
of lanthanated tungsten in ion implant systems. The lanthanated
tungsten material is resistant to fluorine and therefore mitigates
etching and contamination issues. The lanthanated tungsten material
further obviates the need for the use of certain co-gases.
[0042] In accordance with other exemplary aspects of the present
disclosure, FIG. 3 illustrates another exemplary an arc chamber 300
in which the present disclosure may be utilized. The arc chamber
300 of FIG. 3 is similar in many ways to the arc chamber 208 of
FIG. 2. As illustrated in FIG. 3, the arc chamber 300 has a body
302 defining and interior region 304 of the arc chamber. The arc
chamber 300, for example, comprises one or more electrodes 304. The
one or more electrodes 305, for example, comprise a cathode 306,
and a repeller 308. The arc chamber 300, for example, further
comprises an arc slit 310 for extraction of ions from the arc
chamber. One or more liners 312 are operably coupled to the body
302 of the arc chamber 300. The body 302, for example, may further
comprise one or more walls 314 operably coupled to, or integrated
with, the body. In one example, a cathode shield 316 generally
surrounds a periphery of the cathode 306.
[0043] In accordance with the present disclosure, one or more of
the electrodes 305 (e.g., one or more of the cathode 306 and
repeller 308), the cathode shield 316 comprise or are comprised of
lanthanated tungsten. Further, one or more of the liners 312, walls
314, and/or extraction aperture 310 of the arc chamber 300 can
comprise or are comprised of lanthanated tungsten. The present
disclosure presently appreciates that lanthanated tungsten is more
resistant to chemical attack as compared to pure tungsten used in
convention ion sources. The presently considered theory is that
lanthanated tungsten forms a lanthanum oxide layer on the exposed
surface during the ionization process taking place in the arc
chamber 300. Since this lanthanum oxide layer is chemically more
stable than conventional chemistries, it generally inhibits further
corrosion.
[0044] FIG. 4 illustrates a conventional cathode 400 and its
corresponding cathode shield 402 (e.g., a tubular member that
covers the cathode) after running carbon dioxide (CO.sub.2) in an
arc chamber for 20 hours, wherein the cathode and cathode shield
are comprised of conventional tungsten. As illustrated in FIG. 4,
severe oxidation 404 of the cathode shield 402 and its subsequent
deposition through thermal decomposition onto a sidewall 406 of the
cathode 400 are present. As illustrated, the cathode shield 402 has
been oxidized such that the cathode shield has been deleteriously
separated into two pieces 408A, 408B.
[0045] FIG. 5 illustrates cathode 500 and corresponding cathode
shield 502 of the present disclosure after similarly running carbon
dioxide (CO.sub.2) in an arc chamber for 20 hours, wherein the
cathode and cathode shield are comprised of lanthanum tungsten. As
illustrated in FIG. 5 the reduction in the oxidation 504 of the
cathode shield 502 and reduced tungsten deposition onto a sidewall
506 of the cathode is readily apparent when compared to the
conventional cathode 400 and cathode shield 402 of FIG. 4.
[0046] FIG. 6 illustrates the conventional cathode 400 of FIG. 4 in
a conventional arc chamber 410 after running 30 hours of GeF.sub.4
with no co-gas. Excessive deposition 412 of tungsten onto the
cathode 400 and the repeller 414 and the etching 416 of arc chamber
liners 418 is clearly present.
[0047] FIG. 7 illustrates an arc chamber 508 of the present
disclosure having one or more components, such as one or more of
the cathode 500, cathode shield 502, repeller 510, chamber walls
512, liners 514, and extraction aperture (not shown) comprised of
lanthanum tungsten after running GeF4, SiF4 and BF3 with no co-gas
for 10 hours each. While all exposed surfaces 516 within the arc
chamber 508 illustrated in FIG. 7 are comprised of lanthanated
tungsten, such an example is not to be considered limiting, whereby
some components may not be comprised of lanthanated tungsten, or
may comprise a coating of lanthanated tungsten. As shown in FIG. 7
there is no significant deposition of tungsten onto the cathode 500
and the repeller 510 (e.g., no halogen cycle is present), and there
are minimal signs of etching of the arc chamber liner(s) 514. It is
further noted that the present disclosure can advantageously
eliminate running a co-gas when running CO and CO.sub.2.
[0048] FIG. 8 is graph 600 illustrating emission characteristics of
pure tungsten and lanthanated tungsten, where the maximum stable
emission of 4 A/cm.sup.2 is at 1900 K (e.g., reference to thoriated
tungsten of 3 A/cm.sup.2 at 2100 K). Thermionic emission for pure
tungsten, for example, is one hundred times less at
.about.2300K.
[0049] FIG. 9 is a table 700 illustrating characteristics for
various materials after reacting with fluorine and oxygen.
Lanthanum oxide, for example, has a melting point approximately
1000 C higher than standard tungsten dioxide, which indicating that
lanthanum oxide is much more stable. The arc chamber liners
described above typically operate at approximately 700-800 C,
whereby the cathode operates at approximately 2500 C, and the
cathode shield operates at approximately 2000 C. Accordingly,
components of the arc chamber being comprised of lanthanated
tungsten provide a stable compound which doesn't break down easily
at high temperatures after reacting with fluorine.
[0050] The present disclosure thus provides lanthanated tungsten as
an alloy or mixed with other refractory metals (e.g., molybdenum,
tantalum, etc.) for construction of the arc chamber (e.g., ion
source chamber) and/or other electrode components. The present
disclosure provides a lanthanated alloy having a predetermined
amount of lanthanum (e.g., 1-3% lanthanum powder). For example, the
predetermined amount of lanthanum is mixed with the desired metal
(e.g., tungsten) and isostatically pressed to form the component.
The lanthanated tungsten, for example, may define the whole
component, or the component may be coated or otherwise deposited
over the component.
[0051] When lanthanated tungsten components of the present
disclosure are exposed to process gases, for example, the resulting
compounds are generally stable at operation temperatures of the ion
source. For example, La.sub.2F.sub.3 is formed in the presence of
fluorine, and is stable up to greater than 2000 C. Thus, surfaces
associated with the ion source can be considered to be passivated.
Likewise, when lanthanated tungsten is exposed to oxygen in process
conditions, an oxide compound is formed, which is also quite
stable. Thus, the lanthanated tungsten "ties up" the fluorine or
oxygen and generally prevents the fluorine or oxygen from etching
tungsten off the components. In one example, any component that is
exposed to the plasma in the arc chamber comprises or is comprised
of lanthanated tungsten.
[0052] A halogen cycle associated with fluorine (e.g., WF.sub.6),
for example, when exposed to a hot surface, provides a gas that is
volatile and subsequently decomposes, leaving residual tungsten
behind, whereby the fluorine goes back into the plasma to scavenge
more tungsten. By providing the components exposed to the plasma as
being comprised of lanthanated tungsten, the halogen cycle is
basically eliminated or broken. As such, less WF.sub.6 is formed,
and less material is present to decompose on the hot surfaces to
make them increase in mass, etc.
[0053] Conventionally, halogen cycles have been problematic when
fluorine-based dopant gases are utilized. Germanium, for example,
is a worst case, followed by BF.sub.3 and SiF.sub.4, whereby the
liners and other components of the ion source are conventionally
etched, pitted, etc. When used with oxygen, the cathode may be
oxidized. When used with tungsten, WO.sub.2 is formed. Deposits of
carbon on the cathode can further create tungsten carbide. Because
the cathode is indirectly heated, it will emit electrons at
approximately 2500 C. When this happens, the source does not emit
electrons, whereby plasma arc current drops, plasma density drops,
and ion beam current drops. By providing the cathode as being
comprised of lanthanated tungsten, for example, there is also no
need to run the co-gas with carbon. The ion source may be operated
with no co-gas because the emissivity of the cathode is reduced
(e.g., 100 times better than standard tungsten) due to the
lanthanum. As such, the ion source generally protects itself, its
work function is lower, and it can emit electrons more easily at a
lower power.
[0054] When running carbons with oxygens (or any gas with oxygen),
a significant amount of tungsten dioxide and tungsten trioxide can
be formed in the arc chamber. When a subsequent transition to boron
(BO.sub.3) is desired, the ion source is unstable until the oxygen
disposed therein is removed. Thus, until the oxygen is removed, the
previous tuning solution associated with the ion source will not
work well. Thus, in accordance with the present disclosure, since
there is no tungsten dioxide formed, the lanthanated tungsten
provides for a passivating of the chamber, thus protecting it, and
not forming a significant amount of WO.sub.2 or WO.sub.3.
[0055] In accordance with one example, extraction electrodes
utilized in extract the ions from the ion source (e.g., optics
plates) can comprise or otherwise be made of lanthanated tungsten.
When fluorine is utilized in a conventional tungsten extraction
electrode, for example, the fluorine will sputter the apertures and
combine to form tungsten fluoride (WF) gas, which is corrosive.
Further, insulators are often provided between the extraction
plates, whereby the fluorinated tungsten will attack the insulators
(Al.sub.2O.sub.3), which further creates a deleterious conductive
coating on the insulator. Thus, in accordance with the present
disclosure, the aperture plates are comprised of lanthanated
tungsten, thus mitigating such deleterious conduction.
[0056] The present disclosure further contemplates components
upstream of the mass analyzer 116 (e.g., AMU magnet) of FIG. 1 to
be comprised of lanthanated tungsten, such as the ion source 108,
extraction electrodes 115, and components of the ion source
chamber. Arc chamber internal components may be comprised of
lanthanated tungsten, such as any liners, arc slit, cathode,
repeller, and cathode shield associated with the ion source
chamber, as described above. Further, the entrance 116 to the mass
analyzer 117 can also be comprised of lanthanated tungsten.
Additionally, components further downstream of mass analyzer 117
(e.g., anywhere along the beamline) may be comprised of lanthanated
tungsten in a similar manner.
[0057] Although the invention has been shown and described with
respect to a certain embodiment or embodiments, it should be noted
that the above-described embodiments serve only as examples for
implementations of some embodiments of the present invention, and
the application of the present invention is not restricted to these
embodiments. In particular regard to the various functions
performed by the above described components (assemblies, devices,
circuits, etc.), the terms (including a reference to a "means")
used to describe such components are intended to correspond, unless
otherwise indicated, to any component which performs the specified
function of the described component (i.e., that is functionally
equivalent), even though not structurally equivalent to the
disclosed structure which performs the function in the herein
illustrated exemplary embodiments of the invention. In addition,
while a particular feature of the invention may have been disclosed
with respect to only one of several embodiments, such feature may
be combined with one or more other features of the other
embodiments as may be desired and advantageous for any given or
particular application. Accordingly, the present invention is not
to be limited to the above-described embodiments, but is intended
to be limited only by the appended claims and equivalents
thereof.
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