U.S. patent application number 13/527684 was filed with the patent office on 2013-12-26 for compositions for extending ion source life and improving ion source performance during carbon implantation.
The applicant listed for this patent is Lloyd A. Brown, Serge Campeau, Ashwini K. Sinha. Invention is credited to Lloyd A. Brown, Serge Campeau, Ashwini K. Sinha.
Application Number | 20130341568 13/527684 |
Document ID | / |
Family ID | 48628302 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130341568 |
Kind Code |
A1 |
Sinha; Ashwini K. ; et
al. |
December 26, 2013 |
COMPOSITIONS FOR EXTENDING ION SOURCE LIFE AND IMPROVING ION SOURCE
PERFORMANCE DURING CARBON IMPLANTATION
Abstract
A novel method and system for extending ion source life and
improving ion source performance during carbon implantation are
provided. Particularly, the carbon ion implant process involves
utilizing a dopant gas mixture comprising carbon monoxide and one
or more fluorine-containing gas with carbon represented by the
formula CxFy wherein x.gtoreq.1 and y.gtoreq.1. At least one
fluorine containing gases with carbon is contained in the mixture
at about 3-12 volume percent (vol %) based on the volume of the
dopant gas mixture. Fluoride ions, radicals or combinations thereof
are released from the ionized dopant gas mixture and reacts with
deposits derived substantially from carbon along at least one of
the surfaces of the repeller electrodes, extraction electrodes and
the chamber to reduce the overall amount of deposits. In this
manner, a single dopant gas mixture provides carbon ions and
removes problematic deposits typically encountered during carbon
implantation.
Inventors: |
Sinha; Ashwini K.; (East
Amherst, NY) ; Brown; Lloyd A.; (East Amherst,
NY) ; Campeau; Serge; (Lancaster, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sinha; Ashwini K.
Brown; Lloyd A.
Campeau; Serge |
East Amherst
East Amherst
Lancaster |
NY
NY
NY |
US
US
US |
|
|
Family ID: |
48628302 |
Appl. No.: |
13/527684 |
Filed: |
June 20, 2012 |
Current U.S.
Class: |
252/372 |
Current CPC
Class: |
C09K 3/00 20130101; C23C
14/564 20130101 |
Class at
Publication: |
252/372 |
International
Class: |
C09K 3/00 20060101
C09K003/00 |
Claims
1. A gas composition for use in a carbon implantation system,
comprising: a dopant gas mixture in an ion source apparatus
comprising carbon monoxide and one or more fluorine-containing
gases with carbon represented by the formula CxFy wherein
x.gtoreq.1 and y.gtoreq.1, the mixture characterized by the absence
of additional oxygen; wherein the fluorine containing gases with
carbon is in an effective amount of about 1-20 volume percent (vol
%) based on the volume of the gas mixture; wherein the dopant gas
mixture releases carbons ions to produce a carbon ion beam under
conditions sufficient to reduce carbon-based deposits and
oxide-based deposits during carbon implantation.
2. The gas composition of claim 1, wherein at least one of the
fluorine containing gases with carbon is in an effective amount of
about 3-15 vol % based on the volume of the gas mixture.
3. The gas composition of claim 1, wherein at least one of the
fluorine containing gases with carbon is in an effective amount of
about 5-10 vol % based on the volume of the gas mixture.
4. The gas composition of claim 1, wherein the one or more fluorine
containing gases with carbon is selected from the group consisting
of C2F6, CF4, C4F8, C2F4 and mixtures thereof.
5. The gas composition of claim 1, wherein the one or more
fluorine-containing gases with carbon is CF4 in an effective amount
of about 3-10 vol % based on the volume of the gas mixture.
6. The gas composition of claim 1, wherein the fluorine-containing
gas with carbon is C2F6 in an effective amount of about 3-10 vol %
based on the volume of the gas mixture.
7. The gas composition of claim 1, wherein the fluorine-containing
gas with carbon is C2F6 in an effective amount of about 5-10 vol %
based on the volume of the gas mixture.
8. The gas composition of claim 1, wherein the fluorine containing
gas with carbon is selected from the group consisting of C2F6, CF4,
C4F8, C2F4 and mixtures thereof, and further wherein fluorine
containing gas is contained in an effective amount of about 3-15
vol % based on the volume of the gas mixture.
9. The gas composition of claim 1, wherein x=1 to 6 and y=1 to
10.
10. A gas composition for use in carbon implantation, comprising: a
dopant gas mixture in an ion source apparatus comprising a first
carbon-based species of carbon monoxide and a second carbon-based
species of fluorine-containing gases with carbon represented by the
formula CxFy wherein x.gtoreq.1 and y.gtoreq.1, the first and the
second carbon-based species each contained in an effective amount
to ionize at least a portion of said first carbon-based species and
said second carbon-based species to produce carbon ions said
mixture characterized by the absence of additional oxygen; wherein
the dopant gas mixture releases carbons ions to produce a carbon
ion beam under conditions sufficient to reduce carbon-based
deposits and oxide-based deposits during carbon implantation.
11. The gas composition of claim 10, wherein the second
carbon-based species with carbon is in an effective amount of about
1-20 volume percent (vol %) based on the volume of the gas
mixture.
12. The gas composition of claim 10, further comprising
hydrogen.
13. The gas composition of claim 10, wherein the second
carbon-based species is selected from the group consisting of C2F6,
CF4, C4F8, C2F4 and mixtures thereof.
14. The gas composition of claim 10, wherein the second
carbon-based species is CF4 in an effective amount of about 3-10
vol % based on the volume of the gas mixture.
15. The gas composition of claim 10, wherein at least a portion of
the fluorine ionizes from the second carbon-based species.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a novel gas composition for
extending ion source life and improving ion source performance
during carbon implantation.
BACKGROUND OF THE INVENTION
[0002] Ion implantation is an important process in
semiconductor/microelectronic manufacturing. The ion implantation
process is used in integrated circuit fabrication to introduce
controlled amounts of dopant ions into semiconductor wafers. An
ion-source is used to generate a well-defined ion beam for a
variety of ion species from a dopant gas. Ionization of the dopant
gas generates the ion species which can be subsequently implanted
into a given workpiece.
[0003] Carbon has emerged as a widely used dopant in the
semiconductor industry for a variety of material modification
applications such as inhibiting diffusion of co-dopants or
enhancing stability of the doped region. In this regard, carbon
dioxide (CO.sub.2) and carbon monoxide (CO) are two commonly used
dopant gas sources for carbon implantation. However, CO2 and CO are
prone to accumulation of deposits along surfaces of the ion
chamber. Additional deposits can form along surfaces of electrodes
of the ion source apparatus. The deposits may form directly from
the dopant gas or from interaction of the dopant gas and/or its
ionization product with the chamber components.
[0004] Such deposit formation is problematic. Deposits along
surfaces of the electrodes of an ion implantation system create
conditions susceptible to energetic high voltage discharge. Voltage
discharge results in momentary drops in the beam current, commonly
referred to as "beam glitching". Deposits on the extraction
aperture degrade the beam uniformity and hence the uniformity of
dopant levels in the doped region. Beam uniformity and the number
of beam glitches (i.e., glitch rate) during the operation of an ion
source can be key metrics for the performance of an ion
implantation system, such as, for example, a ribbon beam ion
implantation system as commonly known in the art.
[0005] Based on the process sensitivity, there may be an upper
threshold to the glitch rate and/or beam non-uniformity beyond
which the implant process cannot operate with acceptable
efficiency. In the event the ion source performance degrades beyond
the upper threshold, the user must stop the implant operation and
perform maintenance or replace the ion source. Such downtime
results in productivity loss of the ion implantation system. Hence,
it is desirable to maintain proper functioning of the ion source
for extended periods of time in order to perform an efficient
implant process.
[0006] As will be discussed, among other advantages of the present
invention, an improved method and system for minimizing deposits
and beam glitching during an ion implantation process is
desired.
SUMMARY OF THE INVENTION
[0007] The invention relates, in part, to a composition for
extending ion source life and improving ion source performance. The
composition of the dopant gas utilized has been found to have a
significant impact on the ability to reduce the accumulation of
deposits within the ion apparatus and improve ion source
performance.
[0008] It has been found that utilizing a dopant gas mixture
comprising a first carbon-based species of carbon monoxide and a
second carbon-based species of fluorine-containing gas having
carbon and represented by the formula CxFy in an effective amount
reduces carbon-based deposits and virtually eliminates tungsten
regrowth and oxide deposits during a carbon implantation process.
As a result, ion source life is extended. Additionally, glitching
of the ion beam is significantly reduced in comparison to
conventional ion implant processes and systems. The generated beam
current is maintained at a sufficiently high level to achieve
implantation of the ionized dopant gas at the desired dosage.
[0009] In a first aspect, a gas composition is provided, comprising
a gas mixture comprising carbon monoxide and one or more
fluorine-containing gases with carbon represented by the formula
CxFy wherein x.gtoreq.1 and y.gtoreq.1, wherein the fluorine
containing gases with carbon is in an effective amount of about
1-20 volume percent (vol %) based on the volume of the gas
mixture.
[0010] In a second aspect, a gas composition is provided,
comprising a first carbon-based species of carbon monoxide and a
second carbon-based species of fluorine-containing gases with
carbon represented by the formula CxFy wherein x.gtoreq.1 and
y.gtoreq.1. The first and the second carbon-based species are each
contained in an effective amount to ionize at least a portion of
said first carbon-based species and said second carbon-based
species to produce carbon ions.
[0011] Advantageously, the system of the present invention can be
constructed utilizing system components that are commercially
available, thus enabling and simplifying the overall assembly of
the system and method of use thereof. Aspects of the ion
implantation process can be carried out using standard techniques
or equipment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The objectives and advantages of the invention will be
better understood from the following detailed description of the
preferred embodiments thereof in connection with the accompanying
figures wherein like numbers denote same features throughout and
wherein:
[0013] FIG. 1 shows an ion implantation apparatus incorporating the
principles of the invention;
[0014] FIG. 2 shows the results of an ionization test utilizing a
CO2-based dopant gas mixture; and
[0015] FIG. 3 shows the results of an ionization test utilizing a
dopant gas of CO;
[0016] FIG. 4 shows the results of an ionization test utilizing a
dopant gas mixture containing CO and 10% CF4;
[0017] FIG. 5 show the results of an ionization test utilizing a
dopant gas CO within the ion apparatus of FIG. 1;
[0018] FIGS. 6a and 6b show the results of an ionization test
utilizing a dopant gas mixture containing CO and 5% CF4 within the
ion apparatus of FIG. 1;
[0019] FIGS. 7a and 7b show the results of an ionization test
utilizing a dopant gas mixture containing CO and 10% CF4 within the
ion apparatus of FIG. 1; and
[0020] FIGS. 8a and 8b show the results of an ionization test
utilizing a dopant gas mixture containing CO and 15% CF4 within the
ion apparatus of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0021] As used herein, all concentrations are expressed as
volumetric percentages. With reference to FIG. 1, an exemplary ion
implantation apparatus 100 in accordance with the principles of the
invention is shown. The ion source apparatus 100 depicted in FIG. 1
has various components, including an indirectly heated cathode
(IHC) 115 which serves as the ionizing source for the dopant gas
mixture. It should be understood that other types of ion sources
known in the art can be used in the present invention, including,
for example, the Freeman sources, Bernas sources and RF plasma
sources. The ion source apparatus 100 of FIG. 1 is preferably used
for producing carbon ions for implantation into semiconductor
substrates. As will be explained, the present invention allows the
ion source life to be extended compared to conventional carbon
dopant implantation systems by significantly reducing the amount of
carbon-based and oxide-based deposition onto surfaces of the
apparatus 100. The term "carbon-based deposits" as used herein
includes elemental carbon, tungsten carbide, and other deposits
containing carbon. The term "oxide-based deposits" as used herein
includes oxidation of various ion chamber components, such as, for
example, tungsten oxide (WOx). Further, the present invention
advantageously eliminates elemental tungsten (W) regrowth onto
surfaces of the apparatus 100. Additionally, the overall
performance of the ion source is improved with respect to
reductions in beam glitch rate and more uniform beam current
compared to conventional ion beam sources generated for carbon ion
implantation.
[0022] Referring to FIG. 1, a dopant gas mixture 102 is introduced
into the ion source chamber 112 through a gas feed line 113
extending through arc chamber wall 111. The term "dopant gas
mixture" as used herein means that each of the species of the
dopant gas mixture contributes to the implantation of the desired
ion species into a given workpiece. In the preferred embodiment,
the dopant gas mixture 102 is a carbon dopant gas mixture that
includes two carbon-based species. The first carbon-based species
is CO. The second carbon-based species is a fluorine-containing gas
with carbon. The carbon dopant gas mixture 102 inside the chamber
112 is subject to ionization by applying a predetermined voltage
from a power supply source (not shown) to resistively heat a
filament 114 positioned in close proximity to the IHC 115. The
filament 114 may be negatively biased relative to the IHC 115.
Insulator 118 electrically and thermally isolates IHC 115 from the
arc chamber wall 111. Electrons emitted from the filament 114
accelerate towards the IHC 115 to heat the IHC 115 to its own
thermionic emission temperature. The electrons emitted by the
cathode 115 accelerate and travel towards the arc chamber 112 to
ionize the carbon dopant gas mixture 102 located therein. The
ionized gas molecules of the carbon dopant gas mixture 102 produce
a plasma environment. Repeller electrodes 116 may be provided
placed diametrically opposed to the IHC 115 to confine the plasma
environment and control the ionization of the dopant gas mixture
102 within the chamber 112. Ionization of the carbon dopant gas
mixture 102 causes the fluorine to be released as either fluorine
anions, cations, radicals or combinations thereof, which can then
react in-situ by etching any carbon deposits which may have
accumulated onto surfaces of the wall 111, filament 114,
suppression electrodes 119, ground electrodes 120 and/or repeller
electrodes 116. Such deposits are converted into volatile carbon
fluorides, thereby reducing the overall accumulation of
carbon-based deposits along the various surfaces. Additionally, the
fluoride ions and radicals can simultaneously recombine with carbon
ions and carbon radicals present in the gas phase and formed from
ionization of the dopant gas mixture. As a result, further
carbon-based deposition within the chamber 112 is prevented. The
net result is a significant reduction of carbon-based deposits
along surfaces of the ion source apparatus 100.
[0023] In addition to the above mentioned scavenging effects of the
fluorine-containing gas with carbon, each of the species of the
dopant gas mixture upon ionization produces carbon ions that form a
portion of the resultant carbon ion beam. In this manner, unlike
the prior art, the present invention utilizes a dual purpose dopant
gas mixture in which each of the species of the dopant gas mixture
provides a source of carbon ions for implantation while
simultaneously capable of in-situ self-cleaning of deposits and
prevention of specific types of deposition (i.e., W regrowth and
oxide-based deposits).
[0024] The carbon ions are extracted from the ion source chamber
112 in the form of the carbon ion beam of desired energy. The
techniques for suitable extraction can be carried out by applying a
high voltage across extraction electrodes, which consists of
suppression electrodes 119 and ground electrodes 120. As shown in
FIG. 1, each of these suppression and ground electrodes 119 and
120, respectively, has an aperture aligned with the extraction
aperture 117 to ensure that the ion beam 121 extracted out of the
arc chamber 112 is well-defined. The resultant ion beam 121 can be
transported through a mass analyzer/filter to select a specific
mass ion to be implanted into a workpiece. The ion beam 121 can
then be transported to the surface of a workpiece, such as a
semiconductor wafer, for implantation of the carbon ions therein.
The carbon ions of the beam 121 penetrate the surface of the
semiconductor wafer to form a doped region of a certain depth with
desired electrical and/or physical properties.
[0025] Applicants have recognized the benefits of CO over CO2. The
CO species of the dopant gas mixture 102 behaves as a reducing gas
in the ion source chamber 112 environment. CO2, on the other hand,
behaves as an oxidizing gas which tends to oxidize tungsten chamber
components to form WOx deposits. These WOx deposits are typically
found within regions of the repeller electrodes 116, cathode
electrodes 115 and/or extraction assembly (i.e., suppression
electrodes 119 and ground electrodes 120) of the apparatus 100. WOx
deposits can be conductive in nature and can cause electrical
shorting as well as ion source glitching. To further compound the
problem, the WOx deposits generate particles in the ion beam line.
Additionally, WOx formation on the W liner or other W components
can deteriorate the electrical properties, thereby requiring higher
voltage to sustain a stable plasma.
[0026] CO avoids such processing challenges. The reducing nature of
the CO species creates an environment within the chamber 112 that
is not conducive to formation of oxide layer deposits so as to
eliminate detection of any oxide layer formation along surfaces of
the ion implantation apparatus 100. Although lack of oxygen content
in CO compared to CO2 reduces the accumulation of WOx deposits, CO
by itself is not a suitable dopant gas source for carbon ion
implantation because of the large amount of carbon and tungsten
carbide deposits formed. Specifically, C deposits can be formed as
a result of plasma decomposition of CO, whereas WC deposits can
form due to interaction of CO and its plasma fragmented products
with W chamber components.
[0027] To mitigate the carbon-based deposits resulting from CO
alone, Applicants identified that a dopant gas mixture of the CO in
combination with a fluorine-containing gas having carbon performed
significantly well to reduce the carbon deposition observed when CO
alone is used. In a preferred embodiment, the fluorine-containing
gas is carbon tetrafluoride (CF4) that does not contain oxygen. In
an alternative embodiment, the fluorine-containing gas may be a
hydrofluorocarbon that does not contain oxygen.
[0028] In yet another embodiment, it is contemplated that CO mixed
with higher order fluorocarbons of formula C.sub.aF.sub.b where
a.gtoreq.2 and b.gtoreq.2 may also be used to form the dopant gas
mixture. Examples of suitable fluorine-containing gases of higher
order fluorocarbons may include but are not limited to
C.sub.2F.sub.6, C.sub.2F.sub.4, C.sub.3F.sub.8 and
C.sub.4F.sub.8.
[0029] The combination of the carbon monoxide with the
fluorine-containing gas consists of the entire dopant gas mixture
and specifically excludes incorporation of additional oxygen, for
the purpose of eliminating oxide layer growth and retaining longer
ion source life, lower beam glitch rate and uniform beam
current.
[0030] Further, Applicants have demonstrated that it is necessary
to maintain the concentration of fluorine containing gas in the
dopant gas mixture of the present invention in a narrow range to
achieve desired performance for carbon implantation. The
concentration of the fluorine-containing gas within the dopant gas
mixture is about 1% to about 20%, more preferably between about 3%
to about 13%, and more preferably between about 5% to about 10%,
based on the volume of the dopant gas mixture. When the
fluorine-containing gas is below the lower limit, the carbon
deposition tends to increase to an unacceptable high level to
severely shorten ion source life. On the other hand, when the
fluorine-containing gas is above the upper limit, the fluorine has
a tendency to interact with chamber components formed from
tungsten, thereby causing volatile tungsten fluorides (e.g., WFx,
where x=1 to 6) to form and thereafter migrate to hotter surfaces
inside the ion source where they can redeposit as elemental
tungsten (W) and potentially cause premature failure of the ion
source. Operating the ion source apparatus 100 with a dopant gas
mixture having the fluorine-containing gas in a specific
composition range unexpectedly provides a sufficient amount of
fluorine to reduce the amount of deposits onto chamber surfaces
derived from carbon yet not exceed an upper limit whereby tungsten
redeposition is prone to occur.
[0031] Accordingly, the combination of CO with the
fluorine-containing gas within a specific concentration range
creates an improved dopant gas mixture capable of minimizing a wide
array of problematic deposits onto surfaces of the chamber 112
without compromising the ability of the CO and the
fluorine-containing gases to each provide carbon ions to produce a
carbon ion beam having sufficient beam current. The dopant gas
mixture within the specific concentration range is a novel hybrid
composition which can mitigate deposition but yet maintain a
required ion beam current to meet productivity requirements.
[0032] Applicants have performed several experiments to compare the
dopant gas mixture of the present invention with other dopant gas
materials, as will now be discussed.
COMPARATIVE EXAMPLE 1
[0033] An ionization test was performed to evaluate the ability of
a dopant gas composed of CO2 to produce a beam of carbon ions. The
ionization process was required to be aborted after a short
duration of operation due to accumulation of WOx deposits within
the ionization chamber. FIG. 2 shows deposits formed on a substrate
disposed inside the ion-source chamber which indicates the extent
of deposits formed inside the ionization chamber. The deposits on
the substrate plate were analyzed utilizing a x-ray spectroscopy
technique as known in the art. The WOx deposits appeared flaky in
nature. Excessive residue formation and short ion source life was
attributed to the oxidizing nature of CO.sub.2 plasma which
resulted in oxidation of tungsten (W) components in the source
chamber to produce the tungsten oxide (WOx) deposits.
COMPARATIVE EXAMPLE 2
[0034] An ionization test was performed to evaluate the ability of
a dopant gas composed of pure CO to produce a beam of carbon ions.
Although WOx deposits were not detected, it was observed that
utilizing pure CO as the dopant gas source resulted in formation of
heavy carbon (C) deposits and tungsten-carbide (WC) deposits during
the operation of the ion source (FIG. 3). Deposits were analyzed by
x-ray spectroscopy. The ionization process was required to be
aborted, as the observed C and WC deposits caused glitching of the
ion beam, which resulted in unstable beam current during the
ionization process. It is believed that the C deposits were the
result of plasma decomposition of CO, and that the WC deposits were
the result of interaction of the CO and its plasma fragmented
products with tungsten chamber components. The effects of C/WC
deposits were similar to WOx and as a result, a CO-based dopant gas
suffered from similar concerns of short ion source life as a
CO.sub.2-based dopant gas mixture.
EXAMPLE 3
[0035] An ionization test was performed to evaluate the ability of
a dopant gas composed of CO and 10% CF4 on a volume basis to
produce a beam of carbon ions utilizing the same ion source chamber
performed for the tests above. The amount of deposits observed
along the surface of the substrate plate was substantially less
than the deposits which formed when utilizing pure CO or CO2-based
dopant gases, as indicated by the less dark ring of deposits around
the substrate plate. It was observed that utilizing CO +10% CF4 did
not cause significant accumulation of any oxide deposits, carbon
deposits, tungsten carbide deposits or tungsten oxide deposits
(FIG. 4). Analysis of the deposits by x-ray spectroscopy indicated
some carbon deposition, but not to the extent that ion beam
glitching occurred.
[0036] The above tests in Examples 1-3 demonstrate that CO+10% CF4
performed better than CO alone or a CO2-based dopant gas mixture.
The next series of tests shown below compared various concentration
levels of CF4 in a dopant gas mixture with the balance CO.
EXAMPLE 4
[0037] For purposes of establishing a baseline and confirming the
results obtained in Comparative Example 2, an ionization test was
performed to evaluate the ability of a dopant gas composed of CO
and not containing CF4 to produce a beam of carbon ions. The ion
source apparatus utilized was similar to that shown in FIG. 1. CO
dopant gas was introduced into an ion source apparatus. Voltage was
applied to the ion source IHC to ionize the CO. During the
ionization process, a large amount of C and WC deposits were
observed along the surfaces of the suppression electrodes as shown
in FIG. 5.
EXAMPLE 5
[0038] Having established a baseline when using CO in the ion
source apparatus 100 of FIG. 1, an ionization test was performed
employing a dopant gas mixture of CO+5% CF4. During ionization of
the dopant gas mixture, a relatively small amount of carbon-based
deposits (C and WC) were observed along the surfaces of the
suppression electrodes, as shown in FIG. 6a. Virtually no W
regrowth was observed along the repeller electrodes, as evident by
the absence of whisker-like structures along surfaces of the
repeller electrodes shown in FIG. 6b.
EXAMPLE 6
[0039] An ionization test was performed to evaluate the ability of
a dopant gas mixture composed of CO and 10% CF4 on a volume basis
to produce a beam of carbon ions utilizing the ion beam apparatus
of FIG. 1. During ionization of the dopant gas mixture, a
relatively small amount of carbon-based deposits (C and WC) were
observed along the surfaces of the suppression electrodes, as shown
in FIG. 7a. The amount of C and WC deposits was comparable to that
of FIG. 6a. Some W whisker-like structures along surfaces of the
repeller electrodes were observed, as shown in FIG. 7b.
EXAMPLE 7
[0040] Another ionization test was performed, utilizing CO+15% CF4
as the dopant gas mixture. The amount of C and WC deposits along
the surfaces of the suppression electrodes is shown in FIG. 8a. The
amount of C and WC deposits observed was comparable to that when
ionizing CO+5% CF4 in Example 5 and ionizing CO+10% CF4 in Example
6. However, the amount of W regrowth observed, as shown in FIG. 8b,
along the surfaces of the repeller electrodes was significantly
higher in comparison to the previous tests.
[0041] The above tests of Examples 4-7 indicate that a dopant gas
mixture containing increased amounts of CF4 will not necessarily
minimize the amount of C, WC and W deposition. The experiments
reveal that there is an upper limit to the concentration of CF4
within the dopant gas mixture. Specifically, the Examples indicate
that a dopant gas mixture at 15% CF.sub.4 addition results in
excessive W regrowth on the repeller, as shown in FIG. 8b.
Excessive W deposition adversely impacts the ion source performance
and results in shorten ion source life. Accordingly, there appears
to be a concentration range of CF4 which minimizes formation of the
deposits.
[0042] Adequate ion beam performance as measured by a sufficiently
high beam current, is another design attribute that the dopant gas
mixture must exhibit. A reduced beam current requires the ion
apparatus to run harder (i.e., utilize greater power consumption
requirements). As a result, the workpieces need to be processed
longer to achieve the required dopant dosage that is implanted into
the workpieces. Longer processing times and higher power
consumption requirements can translate into substantially less
productivity of the beam equipment, as well as a tendency for the
ion source to degrade more quickly. Further, even if a given dopant
gas can generate a higher beam current, the dopant gas may be prone
to a greater accumulation of deposition due to longer processing
times, thereby causing the ion source to be subject to more
frequent beam glitching and/or non-uniformity of beam current. By
way of example, although CO by itself is capable of generating
relatively high beam currents, it has a tendency to deposit a large
amount of carbon and tungsten carbide deposits such that the ion
source life is severely shortened so that the desired workpiece
productivity is never realized. Accordingly, the dopant gas mixture
must be capable of generating and maintaining a sufficiently high
and uniform beam current while minimizing deposition during the ion
implantation process in order to achieve the desired dopant dosage
and maintain acceptable productivity.
[0043] In the present invention, it has been found that introducing
a dopant gas mixture in an ion source chamber that comprises CO and
a second carbon-based species represented by the formula CxFy
wherein x.gtoreq.1 and y.gtoreq.1, allows the requisite balance of
ion source life with ion source performance to be achieved when the
CxFy is present in an optimal concentration range. Generally
speaking, too high a level of fluorine-containing species with
carbon (i.e., CxFy) results in an unacceptably low ion beam current
level, and too low a level of the CxFy species results in a high
amount of carbon and tungsten deposition and therefore poor ion
source life. A specific range of the CxFy at about 3-12 volume
percent (vol %) achieves sufficient ion source life and improved
ion source performance in comparison to conventional carbon ion
implant processes.
[0044] It is also envisioned that CO may be mixed with more than
one fluorine containing C gas to achieve desired dopant gas mix.
The fluorine containing C gas may be chosen from but not limited to
CF4, C2F6, C2F4, C2F2, C4F8.
[0045] It should be understood that the gas composition of the
present invention has other applications. For instance, the gas
composition can be utilized in chemical vapor deposition or atomic
layer deposition processes under suitable processing conditions to
alter the gas mixture chemistry so as to cause deposition of a thin
film carbon layer. Alternatively, the gas composition can also be
utilized to reduce a metal oxide layer to a metallic layer. By way
of example, tungsten oxide can be annealed in a CO+CF4 environment
to reduce the tungsten oxide to a metallic tungsten layer. CO acts
as a reducing gas to extract oxygen from the tungsten oxide layer,
thereby reducing the tungsten oxide to elemental tungsten.
Additionally, the presence of the CF4 may accelerate the reduction
of tungsten oxide to elemental tungsten by potentially fluorinating
the tungsten oxide layer, thereby enhancing its removal rate. The
net result is the ability for tungsten oxide to revert to a pure
tungsten layer more quickly.
[0046] While it has been shown and described what is considered to
be certain embodiments of the invention, it will, of course, be
understood that various modifications and changes in form or detail
can readily be made without departing from the spirit and scope of
the invention. It is, therefore, intended that this invention not
be limited to the exact form and detail herein shown and described,
nor to anything less than the whole of the invention herein
disclosed and hereinafter claimed.
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