U.S. patent application number 15/952168 was filed with the patent office on 2018-08-16 for toroidal plasma abatement apparatus and method.
The applicant listed for this patent is MKS Instruments, Inc.. Invention is credited to Xing Chen, David Lam, Ilya Pokidov, Feng Tian, Ken Tran, Kevin W. Wenzel.
Application Number | 20180233333 15/952168 |
Document ID | / |
Family ID | 50732285 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180233333 |
Kind Code |
A1 |
Chen; Xing ; et al. |
August 16, 2018 |
TOROIDAL PLASMA ABATEMENT APPARATUS AND METHOD
Abstract
An apparatus for abatement of gases is provided. The apparatus
includes a toroidal plasma chamber having a plurality of inlets and
an outlet, and at least one chamber wall. One or more magnetic
cores are disposed relative to the toroidal plasma chamber. The
plasma chamber confines a toroidal plasma. A second gas inlet is
positioned on the toroidal plasma chamber between a first gas inlet
and the gas outlet at a distance d from the gas outlet, such that a
toroidal plasma channel volume between the first gas inlet and the
second gas inlet in the is substantially filled by the inert gas,
the distance d based on a desired residence time of the gas to be
abated.
Inventors: |
Chen; Xing; (Lexington,
MA) ; Pokidov; Ilya; (North Reading, MA) ;
Tian; Feng; (Salem, NH) ; Tran; Ken; (North
Chelmsford, MA) ; Lam; David; (Fitchburg, MA)
; Wenzel; Kevin W.; (Belmont, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MKS Instruments, Inc. |
Andover |
MA |
US |
|
|
Family ID: |
50732285 |
Appl. No.: |
15/952168 |
Filed: |
April 12, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15484072 |
Apr 10, 2017 |
9991098 |
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15952168 |
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14212398 |
Mar 14, 2014 |
9630142 |
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15484072 |
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61783360 |
Mar 14, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/32935 20130101;
H05H 2001/4667 20130101; H01J 37/32651 20130101; H01J 37/32458
20130101; H01J 37/32449 20130101; H01J 2237/3321 20130101; H01J
37/32339 20130101; H01J 37/32669 20130101; B01D 53/323 20130101;
Y02C 20/30 20130101; H01J 2237/334 20130101; H05H 1/46 20130101;
H05H 2245/121 20130101; H01J 37/32844 20130101; B01D 53/76
20130101; B01D 2257/2064 20130101; H01J 37/32357 20130101 |
International
Class: |
H01J 37/32 20060101
H01J037/32; B01D 53/76 20060101 B01D053/76; H05H 1/46 20060101
H05H001/46; B01D 53/32 20060101 B01D053/32 |
Claims
1. A method for abating process gas within a plasma source, the
method comprising: flowing via a first gas inlet, a first gas for
ignition into a plasma into a toroidal plasma chamber having a
primary winding coupled to the plasma chamber and a plurality of
magnetic cores oriented such that the plasma chamber passes through
each of the plurality of magnetic cores; generating a toroidal
plasma in the toroidal plasma chamber along a plane extending
through the toroidal plasma chamber; positioning a second gas inlet
between the first gas inlet and a gas outlet at a distance d from
the gas outlet along the plane, the distance d based on a desired
residence time of a gas to be abated; and flowing via the second
gas inlet, the gas to be abated into the toroidal plasma chamber
such that the gas to be abated reacts with the toroidal plasma.
2. The method of claim 1, further comprising adjusting the position
of the second gas inlet to control the desired residence time of
the gas to be abated.
3. The method of claim 1, further comprising adjusting a flow rate
of the first gas flowing into the plasma chamber such that a first
toroidal plasma channel volume between the first gas inlet and the
second gas inlet in the toroidal plasma chamber is substantially
filled by the first gas and a second toroidal plasma channel volume
between the second gas inlet and the gas outlet is substantially
filed with the gas to be abated.
4. The method of claim 1, further comprising providing a reactant
gas to mix with the gas to be abated before flowing into the plasma
chamber via the second gas inlet.
5. The method of claim 1, further comprising providing a water
vapor to mix with the gas to be abated before flowing into the
plasma chamber via the second gas inlet.
6. The method of claim 1, further comprising monitoring an emission
from the plasma chamber via an optical and/or an infrared sensor
and adjusting a flow rate of the reactant gas in response to the
emission from the plasma chamber such that a concentration of the
reactant gas is at a desired level to react with the gas to be
abated.
7. The method of claim 1 further comprising coupling the gas outlet
to a subsequent treatment device.
8. The method of claim 1 wherein generating the toroidal plasma
further comprises: coupling an RF power supply to the primary
winding; and delivering power to the toroidal plasma.
9. The method of claim 3, further comprising coupling an RF power
supply to the primary winding.
10. The method of claim 9 further comprising delivering a constant
plasma current through the first toroidal plasma channel volume and
the second toroidal plasma channel volume.
11. The method of claim 1 wherein positioning the second gas inlet
further comprises orienting the second gas inlet at an acute angle
to the plane.
12. The method of claim 1 wherein positioning the second gas inlet
further comprises positioning the second gas inlet between the
first gas inlet and the gas outlet at a distance approximately 2 to
5 inches from the gas outlet along the plane.
13. The method of claim 1 further comprising selecting the second
gas inlet from a series of selectable gas inlet ports positioned
along the toroidal plasma chamber.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. application Ser. No.
15/484,072, filed Apr. 10, 2017, which is a division of U.S.
application Ser. No. 14/212,398, filed Mar. 14, 2014, now U.S. Pat.
No. 9,630,142, which claims the benefit of and priority under 35
U.S.C. .sctn. 119(e) to U.S. Provisional Patent Application No.
61/783,360, filed Mar. 14, 2013, all of which are incorporated by
reference herein in their entireties.
FIELD OF THE INVENTION
[0002] The invention relates generally to the field of process gas
abatement. More specifically, the invention relates to apparatuses
for and methods of abating process gas using a toroidal plasma.
BACKGROUND OF THE INVENTION
[0003] In semiconductor processes such as etching and chemical
vapor deposition, chemically reactive gases can be used during
processing or produced as a result of processing. These toxic or
otherwise harmful process gases can be treated or abated before
their release to atmosphere. A number of technologies have
traditionally been used for the abatement of such gases. For
example, thermal abatement processes are commonly implemented by
burning halogen-containing gases with a hydrogen-containing fuel to
convert fluorine (F) or chlorine (Cl) to hydrogen fluoride (HF) or
hydrogen chloride (HCl). In those exemplary processes, the
resulting HF or HCl can be subsequently removed in a wet
scrubber.
[0004] Another example of current abatement processes is catalytic
thermal abatement. Catalytic thermal abatement is typically
performed by exposing halogen-containing gases to metal oxides at
high temperatures to convert halogen to salts. Another example is
plasma abatement, which can be performed at sub-atmospheric
pressures, or at atmospheric pressure using microwave plasmas.
[0005] In current thermal abatement processes, energy usage can be
inefficient. Many of the halogen-containing compounds can be
chemically stable, requiring temperatures (e.g., of 1000K to 2000K)
to achieve thermal abatement. For example, this difficulty can
occur in the abatement of highly stable carbon tetrafluoride
(CF4).
[0006] Current thermal abatement processes conducted at atmospheric
pressure can require large amounts of purge gas to be added at
vacuum pumps of the plasma device for protection of the pumps. As a
result, a high level of energy can be wasted just in heating the
purge gas.
[0007] Catalytic thermal abatement processes can achieve higher
abatement efficiency, but still can suffer from high maintenance
cost and high consumable cost. During semi-conductor processing the
flow of halogen-containing gases is often not continuous, toggling
on and off in each wafer cycle. Since the thermal time constant of
a burner to reach operating temperature can be much longer than a
wafer cycle, a thermal abatement unit is often kept on
continuously, dramatically reducing the energy efficiency.
[0008] In plasma abatement, existing technologies have demonstrated
high abatement efficiency (e.g., greater than 95%). Plasmas can be
generated in various ways including DC discharge, radio frequency
(RF) discharge, and microwave discharge. DC discharges are achieved
by applying a potential between two electrodes in a gas. RF
discharges are achieved either by electrostatically or inductively
coupling energy from a power supply into a plasma. Parallel plates
are typically used for electrostatically coupling energy into a
plasma. Induction coils are typically used for inducing current
into a plasma. Microwave discharges are achieved by directly
coupling microwave energy through a microwave-passing window into a
discharge chamber containing a gas.
[0009] The existing technologies, e.g., either inductively coupled
plasma or microwave plasmas, can have a limited operating range. To
achieve high abatement efficiency, gases to be abated are typically
excited and reacted in the plasma. It is desirable for the plasma
to cover as much of the gas flow path as possible such that gas
molecules cannot bypass the plasma region without interacting with
the plasma.
[0010] During abatement, a pressure increase can be generated, for
example, due to restrictions to the gas flow path. Since an
abatement device is often located downstream of a process chamber
in which the gases to be abated are used and/or generated, a
pressure rise in the abatement device can directly impact the
processes in the process chamber. Therefore, it is desirable to
limit pressure rises during abatement.
[0011] Existing plasma abatement devices can suffer from limited
ranges of gas flow rate and operating pressure. They can suffer
from surface erosions due to, for example, reactive plasma
chemistries and high energy ions generated in the plasma
sources.
SUMMARY OF THE INVENTION
[0012] One advantage of the invention is that it provides a high
flow conductance of the plasma source for abatement of process
gases due to, for example, the shape and configuration of the
plasma channel (e.g., greater than 500 L/s). Another advantage is
that the residence time of the process gas to be abated can be
adjusted and optimized. Gas residence time in plasma can be
critical to abatement efficiency. For example, too short of a
residence time can result in insufficient gas-plasma interactions
and lower abatement rate, while too long a residence time can
result in overheating of gases and lower energy efficiency. Another
advantage of the invention is that its topology allows for
structural scalability, such that plasma channel dimensions can be
sized to accommodate a wide range of process requirements and/or
facilitate efficient abatement performance. Gas inlet and outlet
fittings can be scalable to, for example, match user pumping
equipment, allowing for easier integration into existing
semiconductor fabrication systems. A further advantage of the
invention is a low erosion of the plasma chamber surface due to low
electric fields in a toroidal plasma source.
[0013] Another advantage of the invention is a high abatement
efficiency of greater than 95% achieved with a high energy
efficiency in operation (e.g., 3-6 kW for 100-200 sccm of CF4), in
the abatement of the highly stable CF4. Another advantage of the
invention is a broad operating pressure range, from 0.1 Torr to
tens of Torrs, and in some cases, up to atmospheric pressure.
Another advantage of the invention is a low pressure drop of less
than or equal to 0.1 Torr. Parameters such as high energy
efficiency and low pressure drop in an abatement system can lead to
improved product lifetime, fast response to changing gas flow
rates, and lower capital and operation costs of the abatement
apparatus.
[0014] In one aspect, the invention includes an apparatus for
abatement of gases. The apparatus includes a plasma chamber having
a first gas inlet receiving an inert gas for ignition into a
plasma, a second gas inlet receiving a gas to be abated, a gas
outlet, and at least one chamber wall for containing the gas. One
or more magnetic cores are disposed relative to the toroidal plasma
chamber such that the toroidal plasma chamber passes through each
of the one or more magnetic cores. A primary winding is coupled to
the one or more magnetic cores. The plasma chamber confines a
toroidal plasma. The toroidal plasma extends along a plane
extending through the plasma chamber. The second gas inlet is
positioned on the toroidal plasma chamber between the first gas
inlet and the gas outlet at a distance d along the plane from the
gas outlet, such that a toroidal plasma channel volume between the
first gas inlet and the second gas inlet in the is substantially
filled by the inert gas. The distance d is based on a desired
residence time of the gas to be abated.
[0015] In some embodiments, the distance d that is along the plane
is approximately 2 to 5 inches. In some embodiments, the second gas
inlet is a series of selectable gas inlet ports positioned along
the toroidal plasma chamber. In various embodiments, the gas to be
abated is mixed with one or more reactant gases prior to entering
the plasma chamber. In some embodiments, the one or more reactant
gases mixed with the gas to be abated is a water vapor.
[0016] In various embodiments, the apparatus includes a sensor
positioned relative to the gas outlet and configured to monitor an
emission from the plasma chamber. In various embodiments, the
sensor includes an optical and/or an infrared sensor.
[0017] In some embodiments, the gas to be abated is mixed with one
or more reactant gases prior to entering the plasma chamber. In
some embodiments, the one or more reactant gases mixed with the gas
to be abated is a water vapor. In various embodiments, the gas to
be abated received by the second gas inlet is a chlorine compound.
In some embodiments, the gas to be abated received by the second
gas inlet is a perfluorocarbon compound. In various embodiments,
the perfluorocarbon compound received by the second gas inlet
includes carbon tetrafluoride.
[0018] In another aspect, the invention includes a method for
abating process gas within a plasma chamber. The method involves
flowing via a first gas inlet, a first gas for ignition into a
plasma into a toroidal plasma chamber having a primary winding
coupled to the plasma chamber and a plurality of magnetic cores
oriented such that the plasma chamber passes through each of the
plurality of magnetic cores. The method also involves generating a
toroidal plasma in the plasma chamber along a plane extending
through the plasma chamber. The method also involves positioning a
second gas inlet between the first gas inlet and a gas outlet along
the plasma chamber at a distance d from the gas outlet along the
plane, the distance d based on a desired residence time of a gas to
be abated. The method further involves flowing via the second gas
inlet, the gas to be abated into the plasma chamber such that the
gas to be abated reacts with the toroidal plasma.
[0019] In some embodiments, the method further involves adjusting
the position of the second gas inlet to control the desired
residence time of the gas to be abated. In various embodiments, the
method further involves adjusting a flow rate of the first gas
flowing into the plasma chamber such that a toroidal plasma channel
volume between the first gas inlet and the second gas inlet in the
toroidal plasma channel is substantially filled by the first gas.
In some embodiments, the method further involves providing an
reactant gas to mix with the gas to be abated before flowing into
the plasma chamber via the second gas inlet. Further, in various
embodiments, the method involves monitoring an emission from the
plasma chamber via an optical and/or an infrared sensor and
adjusting a flow rate of the reactant gas in response to the
emission from the plasma chamber such that a concentration of the
reactant gas is at a desired level to react with the gas to be
abated. In some embodiments, the method involves coupling the gas
outlet to a subsequent treatment device.
[0020] In another aspect, the invention includes an apparatus for
abatement of gases. The apparatus includes a plasma source having a
gas inlet directing a gas to be abated along a gas flow path, a gas
outlet, and at least one chamber wall for containing the gas. The
apparatus also includes a toroidal plasma channel oriented along a
plane extending through the plasma source, the toroidal plasma
channel having a toroidal plasma channel inlet portion, a main
toroidal plasma channel portion, and a toroidal plasma channel
outlet portion, the toroidal plasma channel inlet portion and the
toroidal plasma channel outlet portion each having a width that is
less than a width W of the main toroidal plasma channel portion.
One or more magnetic cores disposed relative to the toroidal plasma
source such that the toroidal plasma channel passes through each of
the one or more magnetic cores. A primary winding is coupled to the
one or more magnetic cores. The plasma source generates a toroidal
plasma along the plane extending through the plasma source and
confined in the toroidal plasma channel.
[0021] In some embodiments, the gas inlet is oriented such that the
gas flow path is substantially perpendicular to the plane extending
through the plasma source. In some embodiments, the width W of the
main toroidal plasma channel portion is approximately equal to a
cross-sectional diameter of the toroidal plasma.
[0022] In some embodiments, the gas inlet comprises a curved
portion that reduces friction and drag of the gas to be abated
flowing into the toroidal plasma channel. Further, in various
embodiments, the curved portion of the gas inlet is spherically
blunted cone.
[0023] In some embodiments, a major diameter of the toroidal plasma
channel is greater than or equal to the diameter of the gas inlet.
In various embodiments, the diameter of the gas inlet is between
approximately 1 and 10 inches.
[0024] In some embodiments, the gas to be abated is mixed with one
or more reactant gases prior to entering the toroidal plasma
channel. In various embodiments, the toroidal plasma channel
comprises at least one metal layer to shield the toroidal plasma
from electrostatic coupling. In some embodiments, one or more
dielectric gaps are positioned along the toroidal plasma channel to
prevent an induced electric current from flowing in the toroidal
plasma channel.
[0025] In another aspect, the invention includes a method for
abating process gas within a plasma source. The method involves
directing via a gas inlet a gas to be abated along a gas flow path
into a plasma source having a primary winding coupled to the plasma
source and a plurality of magnetic cores positioned such that a
toroidal plasma channel oriented along a plane extending through
the plasma source passes through each of the plurality of magnetic
cores. The method also involves generating a toroidal plasma along
the plane extending through the plasma source and confined in the
toroidal plasma channel. The method also involves positioning the
gas inlet such that the gas flow path is substantially
perpendicular to the plane extending through the plasma source.
Further, the method also involves flowing the gas to be abated into
the toroidal plasma channel such that the gas to be abated
interacts with a main body of the toroidal plasma having a high
electron density.
[0026] In various embodiments, the method also involves providing
one or more reactant gases to mix with the gas to be abated prior
to the gases entering the toroidal plasma channel. In some
embodiments, the method involves positioning one or more dielectric
gaps along the toroidal plasma channel to prevent an induced
electric current from flowing in the toroidal plasma channel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The advantages of the invention described above, together
with further advantages, can be better understood by referring to
the following description taken in conjunction with the
accompanying drawings. The drawings are not necessarily to scale,
emphasis instead generally being placed upon illustrating the
principles of the invention.
[0028] FIG. 1 is a schematic representation of a plasma source for
producing activated gases, according to an illustrative embodiment
of the invention.
[0029] FIG. 2 is a schematic cross-sectional representation of a
plasma chamber, according to an illustrative embodiment of the
invention.
[0030] FIG. 3 is a schematic representation of a plasma abatement
device, according to an illustrative embodiment of the
invention.
[0031] FIG. 4 is a flow chart showing a method for abating process
gas within a plasma chamber, according to an illustrative
embodiment of the invention.
[0032] FIG. 5a is a graph showing CF4 concentration over time at a
gas outlet of a plasma chamber, according to illustrative
embodiments of the invention.
[0033] FIG. 5b is a graph showing CF4 concentration over time at a
gas outlet of a plasma chamber, according to illustrative
embodiments of the invention.
[0034] FIG. 5c is a graph showing CF4 concentration over time at a
gas outlet of a plasma chamber, according to illustrative
embodiments of the invention.
[0035] FIG. 6 is a graph showing CF4 destruction efficiency versus
plasma chamber pressure, according to an illustrative embodiment of
the invention.
[0036] FIG. 7a is schematic cross-sectional representation of a
plasma source, according to an illustrative embodiment of the
invention.
[0037] FIG. 7b is schematic cross-sectional representation of a
plasma source, according to an illustrative embodiment of the
invention.
[0038] FIG. 8 is a flow chart showing a method for abating process
gas within a plasma source, according to an illustrative embodiment
of the invention.
[0039] FIG. 9a is a graph showing abatement rate versus pressure,
according to illustrative embodiments of the invention.
[0040] FIG. 9b is a graph showing abatement rate versus power,
according to illustrative embodiments of the invention.
[0041] FIG. 9c is a graph showing abatement efficiency versus
pressure, according to illustrative embodiments of the
invention.
[0042] FIG. 9d is a graph showing abatement efficiency versus
power, according to illustrative embodiments of the invention.
DETAILED DESCRIPTION
[0043] FIG. 1 is a schematic representation of a plasma source for
producing activated gases, according to an illustrative embodiment
of the invention. The source 10 includes a power transformer 12
that couples electromagnetic energy into a plasma 14. The power
transformer 12 includes a high permeability magnetic core 16, a
primary coil 18, and a plasma chamber 20 that contains the plasma
14, which allows the plasma 14 to form a secondary circuit of the
transformer 12. The power transformer 12 can include additional
magnetic cores and primary coils (not shown) that form additional
secondary circuits.
[0044] One or more sides of the plasma chamber 20 are exposed to a
process chamber 22 to allow charged particles and activated gases
generated by the plasma 14 to be in direct contact with a material
to be processed (not shown). A sample holder 23 can be positioned
in the process chamber 22 to support the material to be processed.
The material to be processed can be biased relative to the
potential of the plasma.
[0045] A voltage supply 24, which can be a line voltage supply or a
bus voltage supply, is directly coupled to a switching circuit 26
containing one or more switching semiconductor devices. The one or
more switching semiconductor devices can be switching transistors.
The circuit can be a solid state switching power supply. An output
28 of the switching circuit 26 can be directly coupled to the
primary winding 18 of the transformer 12.
[0046] The toroidal low field plasma source 10 can include an
apparatus for generating free charges that provides an initial
ionization event that ignites a plasma in the plasma chamber 20 as
described herein. An inert gas, such as argon, can also be inserted
into the plasma chamber 20 to reduce the voltage required to ignite
a plasma. Free charges can be generated in numerous ways as
described herein. For example, free charges can be generated by
applying a short high voltage pulse to an electrode inside of the
plasma chamber 20. Also, free charges can be generated by applying
a short high voltage pulse directly to the primary coil 18. A high
electric voltage signal can be applied to an electrode, located
outside of a dielectric plasma chamber 20 but capacitively coupled
to the plasma volume, to generate free charges to assist ignition
in the plasma chamber 20.
[0047] In another embodiment, an ultraviolet light source 34 is
used to generate free charges that provide an initial ionization
event, which ignites a plasma in the plasma chamber 20. The
ultraviolet (UV) light source 34 is optically coupled to the plasma
chamber 20. The UV light source 34 can be optically coupled to the
plasma channel through an optically transparent window. The UV
light source 34 can either be a continuous wave (CW) light source
or a pulsed light source depending on the duty cycle of the plasma
source.
[0048] The toroidal low field plasma source 10 can also include a
measuring circuit 36 for measuring electrical parameters of the
primary winding 18. Electrical parameters of the primary winding 18
include the current driving the primary winding 18, the voltage
across the primary winding 18, the bus or line voltage that is
generated by the voltage supply 24, the average power in the
primary winding 18, and the peak power in the primary winding 18.
The electric parameters of the primary winding can be continuously
monitored.
[0049] The plasma source 10 can also include an apparatus for
measuring electrical and optical parameters of the plasma 14
itself. For example, the source 10 can include a current probe 38
that is positioned around the plasma chamber 20 to measure the
plasma current flowing in secondary of the transformer 12. Also,
the voltage on the plasma secondary can be measured, for example,
by positioning a secondary winding on the magnetic core parallel to
the plasma 14. Alternatively, the electric power applied to the
plasma can be determined from measurements of the AC line voltage
and current and from known losses in the electric circuit.
[0050] The plasma source 10 can also include an optical detector 40
for measuring the optical emission from the plasma 14. The electric
and optical parameters of the plasma 14 can be continuously
monitored. In addition, the plasma source 10 can include a power
control circuit 42 that accepts data from at least one of the
current probe 38, the power detector 40, and the switching circuit
26 and then adjusts the power in the plasma by adjusting the
current in the primary winding 18.
[0051] In operation, a gas is bled into the plasma chamber 20 until
a pressure that is substantially between 1 mTorr and 100 Torr is
reached. In some embodiments, a gas is bled into the chamber 20
until a pressure that is between about 0.1 mTorr and about 1,000
Torr is reached. The gas can comprise an inert gas, a reactive gas
or a mixture of at least one inert gas and at least one reactive
gas. The switching circuit 26 containing switching semiconductor
devices that supply a current to the primary winding 18 that
induces a potential inside the plasma chamber 20.
[0052] The magnitude of the induced potential can depend on the
magnetic field produced by the magnetic core 16 and the frequency
at which the switching semiconductor devices operate according to
Faraday's law of induction. An ionization event that forms the
plasma can be initiated in the chamber 20. The ionization event can
be the application of a voltage pulse to the primary winding or to
the electrode 30 positioned in the chamber 20 as described herein.
Alternatively, the ionization event can be exposing the inside of
the plasma chamber 20 to ultraviolet radiation.
[0053] Once the gas is ionized, a plasma is formed in the plasma
chamber 20 that completes a secondary circuit of the transformer
12. The circumference length of the toroidal plasma chamber 20 can
be from 10 to 40 inches. The shape of the cross sectional area of
the plasma chamber 20 can vary from circular to non-circular (oval,
etc.). In one embodiment, the diameter of a circular plasma chamber
20 can vary from approximately 0.5 to 2.0 inches depending upon the
operating conditions. Changing the circumference length or
cross-sectional diameter of the plasma chamber 20 can change the
gas flow dynamics and the plasma impedance and allows the plasma
source to be optimized for different operating ranges (i.e.
different power levels, pressures ranges, gases, and gas flow
rates).
[0054] The electric field of the plasma can be substantially
between about 1-100 V/cm. If only inert gases are present in the
plasma chamber 20, the electric fields in the plasma 14 can be as
low as 1 volt/cm. If, however, electronegative gases are present in
the plasma chamber 20, then the electric fields in the plasma 14
are considerably higher. In some embodiments, operating the plasma
source 10 with low electric fields in the plasma 14 is desirable
because a low potential difference between the plasma 14 and the
chamber 20 will substantially reduce erosion of the chamber 20
caused by energetic ions. This will substantially reduce the
resulting contamination to the material being processed. Reducing
erosion of the chamber 20 is not required in some embodiments.
[0055] The power delivered to the plasma can be controlled by a
feedback loop 44 that comprises the power control circuit 42, the
measuring circuit 36 for measuring electrical parameters of the
primary winding 18 and the switching circuit 26 containing one or
more switching semiconductor devices. In addition, the feedback
loop 44 can include the current probe 38 and optical detector
40.
[0056] In one embodiment, the power control circuit 42 measures the
power in the plasma using the measuring circuit 36 for measuring
electrical parameters of the primary winding 18. The power control
circuit 42 compares the resulting measurement to a predetermined
value representing a desired operating condition and then adjusts
one or more parameters of the switching circuit 26 to control the
power delivered to the plasma. The one or more parameters of
switching circuit 26 include, for example, voltage and current
amplitude, frequency, pulse width, and relative phase of the drive
pulses to the one or more switching semiconductor devices.
[0057] In another embodiment, the power control circuit 42 measures
the power in the plasma using the current probe 38 or the optical
detector 40. The power control circuit 42 then compares the
measurement to a predetermined value representing a desired
operating condition and then adjusts one or more parameters of the
switching circuit 26 to control the power delivered to the
plasma.
[0058] In one embodiment, the plasma source 10 can include
protection circuits to ensure that the plasma source 10 is not
damaged either through abnormal environmental conditions or through
abnormal usage. The temperature of the plasma source 10 can be
monitored at numerous locations to ensure that an appropriate
amount of cooling fluid is flowing and that an abnormally high
amount of power is not being dissipated in the source. For example,
the temperature of the mounting blocks for the switching devices,
the plasma chamber 20 itself, and the magnetic core can be
monitored. Also, the current flowing though the FET devices can be
monitored. If the current exceeds predetermined values the plasma
source 10 can be shut down, thereby protecting the switching
devices against possible damage.
[0059] FIG. 2 is a schematic cross-sectional representation of a
plasma chamber, according to an illustrative embodiment of the
invention. Magnetic cores 102a and 102b are disposed relative to a
plasma chamber 101, such that the plasma chamber 101 passes through
each of the magnetic cores 102a and 102b.
[0060] The plasma chamber 101 includes a chamber wall 104, a first
gas inlet 106, a second gas inlet 110, a third gas inlet 112 and a
gas outlet 108. In some embodiments, the third gas inlet 112 is not
included with the plasma source 100. In some embodiments, more than
three gas inlets can be positioned along the plasma chamber 101 for
introducing process gases, including inert gases, reactive gases,
and/or gases to be abated, to the plasma chamber 101. In various
embodiments, the second gas inlet 110 can consist of a series of
selectable gas inlet ports positioned along the toroidal plasma
chamber.
[0061] The plasma chamber 101 includes a first gas flow path 122
from the first gas inlet 106, a second gas flow path 126 from the
second gas inlet 110, and a third gas flow path 128 from the third
gas inlet 112. The plasma chamber 101 includes a gas outlet flow
path 124 from the gas outlet 108.
[0062] During operation, a toroidal plasma 120 that forms within
the plasma chamber 101 flows substantially within a loop (e.g.,
circle or oval) on a plane 150. The plane 150 extends along an x
axis and a y axis as shown in FIG. 2. It is apparent to one of
ordinary skill in the art that the boundaries on plane 150 shown in
FIG. 2 are for illustration purposes only and that the plane 150
can extend beyond those boundaries.
[0063] The first gas inlet 106 is positioned on the plasma chamber
101 such that first gas flow path 122 is oriented substantially
parallel to the plane 150. In some embodiments, the first gas flow
path 122 is oriented substantially perpendicular to the plane 150.
The first gas flow path 122 can also be oriented at an acute angle
relative to the plane 150. The second gas inlet 110 is positioned
on the plasma chamber 101 at a distance d from the gas outlet 108.
In some embodiments, the second gas flow path 126 is oriented at an
acute angle relative to the plane 150. In some embodiments, the
second gas flow path 126 is oriented to form a helical gas flow in
the plasma chamber 101 to increase interactions of gases to be
abated with the plasma 120. The third gas inlet is positioned on
the plasma chamber 101 at a distance d from the gas outlet 108. In
some embodiments, the third gas flow path 128 is oriented
substantially parallel to the plane 150. In some embodiments, the
third gas flow path 128 is oriented at an acute angle relative to
the plane 150. In some embodiments, the third gas flow path 128 is
oriented to form a helical gas flow in the plasma chamber 101 to
increase interactions of gases to be abated with the plasma
120.
[0064] The position of the gas inlets, such as the second gas inlet
110 and the third gas inlet 112, can be based on a desired reactive
plasma volume and/or residence time of the gas to be abated. The
desired reactive plasma volume and/or residence time can allow
optimization of gas abatement efficiency and/or energy
efficiency.
[0065] In operation, an inert gas, such as argon, can be inserted
into plasma chamber 101 along the first gas flow path 122 from the
first gas inlet 106 to ignite and sustain the toroidal plasma. The
toroidal plasma and the toroidal plasma current 130 flow in a loop
around the plasma chamber 101. A process gas to be abated is
inserted into plasma chamber 101 along the second gas flow path 126
from the second gas inlet 110. Chlorine compounds and
perfluoroncarbon compounds, such as carbon tetrafluoride, are
examples of process gases that can be inserted via the second gas
inlet 110 for abatement. Additional reactant gases, such as oxygen,
hydrogen, and water vapor, can be fed into plasma chamber 101
together with the gases to be abated through the second gas inlets
126 and the third gas inlet 128. In some embodiments, the reactant
gases are fed into the plasma chamber 101 through additional gas
inlets positioned along the plasma chamber 101.
[0066] In a toroidal plasma source shown in FIG. 2, the
circumference length of the plasma chamber 101 or the length of the
plasma 120 can be based on the size of the magnetic core 102. The
size of the magnetic core can be dependent on the electric voltages
required to ignite and sustain the plasma as well as the properties
of magnetic materials. For abatement applications, the desired
volume of plasma for interacting with the gas to be abated can be
based on the rates of gas excitation, chemical reactions in the
plasma, and/or the gas resident time in the plasma. Too short a
residence time can result in an insufficient gas-plasma interaction
and/or low abatement rate, while too long a residence time can
result in overheating of gases and low energy efficiency. In some
embodiments, the required plasma volume is smaller than the overall
volume of the toroidal plasma.
[0067] In the exemplary embodiment shown in FIG. 2, a first volume
132 between the first gas inlet 106 and the second gas inlet 110 is
filled primarily with the inert gas. A second volume 134 between
the second gas inlet 126 and the third gas inlet 112 is filled
primarily with the gas to be abated. Filling the first volume 132
with the inert gas can decrease the second volume 134. The inert
gas can have lower plasma impedance compared to the plasma
impedance of the gas to be abated. With the plasma current 130 a
constant along the toroidal plasma 120, lower impedance in the
first volume 132 can allow for the power coupled to the toroidal
plasma to be delivered to the portion of the toroidal plasma that
interacts with the gas to be abated. By focusing the power
delivered to the toroidal plasma to the second volume 134, the gas
abatement can be more efficient. One or more reactant gases can be
mixed with the gas to be abated prior to entering the plasma
chamber 101 via the second gas inlet 110. In some embodiments,
reactant gases are delivered at other ports located near the second
volume 134. The one or more reactant gases can improve abatement
efficiency. In some embodiments, water vapor is mixed with the gas
to be abated.
[0068] FIG. 3 is a schematic representation of a plasma abatement
device 300, according to an illustrative embodiment of the
invention. Plasma abatement device 300 includes a plasma source
302, an RF power supply 304, a control module 306, a process
monitoring sensor 308, and a reactant gas delivery system 310.
[0069] The plasma source 302 can include a plasma chamber (e.g.,
plasma chamber 101 as described above in FIG. 2).
[0070] The RF power supply 304 can be any RF power supply as is
known by those of ordinary skill in the art capable of delivering
sufficient power for plasma abatement processing (e.g., voltage
supply 24 as described above in FIG. 1).
[0071] The process monitoring sensor 308 can be positioned and
configured to monitor gas emissions flowing from the outlet of
plasma source 302. In some embodiments, process monitoring sensor
308 is an optical sensor, an infrared sensor, and/or any sensor
known to those skilled in the art to monitor gas emissions flowing
from the plasma source 302.
[0072] The control module 306 can be coupled to the process
monitoring sensor 308 and the RF power supply 304, and can be any
microprocessor or other controller known to those skilled in the
art for controlling and/or monitoring a power system. In some
embodiments, the control module 306 is also coupled to the reactant
gas delivery system 310. The reactant gas delivery system 310 can
be any reactant gas delivery system or device known to those
skilled in the art to deliver a chemical to a process chamber. In
various embodiments, the reactant gas delivery system 310 provides
a water vapor to the plasma source 302.
[0073] During operation, an abatement process can be monitored by
the process monitoring sensor 308 and controlled by the control
module 306 through measuring the input and output gas parameters of
the abatement device 300, as well as the parameters of the plasma
source 302 and the RF power supply 304. The process monitoring
sensor 308 can sense and/or measure optical emissions from a
toroidal plasma and from gases that are downstream of the toroidal
plasma. The measured emission spectra can be compared with a
reference spectra. The reference spectra can be from a look-up
table and/or a spectra measured previously under standard process
conditions and stored in the abatement device 300. For example, the
look up table and/or measure spectra can be stored in the control
module 306 and used to determine desired gas and plasma conditions
(e.g., intensity) of the plasma source 302.
[0074] The flow of chemical reactants into and out of the plasma
source 302 can be adjusted by the control module 306. The control
module 308 can control the gas flow such that the concentration of
the reactants is substantially stoichiometrically equal to or
greater than the concentration that is desired to react with the
process gas to be abated. The RF power supply 304 can also be
adjusted (e.g., by control module 308) such that the concentration
of unreacted gas exiting the plasma source 302 can be substantially
below a predetermined level. The power can be maintained to achieve
a desired abatement level, e.g., an abatement level that results in
maximum energy efficiency (e.g., 3-6 kW for 100-200 sccm of CF4)
can be achieved. Monitoring and controlling of the flow of chemical
reactants and RF power can be particularly important for an
abatement system with intermittent flows of gases to be abated. In
semiconductor processing, for example, the flow of gases to be
abated can follow each wafer cycle, which can range from a few
seconds to a few minutes. The energy efficiency of the abatement
system can improve when power and/or the reactant flow are
controlled and/or adjusted during a cycle.
[0075] The chemical reactant delivered by the reactant gas delivery
system 310 can be an oxidizing agent such as oxygen, a reducing
agent such as hydrogen, or a combination of both. In some
embodiments, a water vapor is delivered by the reactant gas
delivery system and used for abatement of fluorocarbon compounds
such as CF4. Such a reaction is illustrated as follows, where water
vapor is mixed with CF4 and reaction is facilitated by the
plasma:
CF 4 + H 2 O plasma CO 2 + 4 HF EQN . 1 ##EQU00001##
[0076] The chemical reactants can also include a halogen-containing
gas. A process gas can often include precursors used in chemical
vapor deposition, which can contain compounds of silicon and
carbon, and can be converted to halogen compounds during
abatement.
[0077] FIG. 4 is a flow chart showing a method 400 for abating
process gas within a plasma chamber, according to an illustrative
embodiment of the invention. The method involves flowing via a
first gas inlet (e.g., first gas inlet 106 as shown above in FIG.
2), a first gas for igniting a plasma in a plasma chamber (e.g.,
plasma chamber 101 as shown above in FIG. 2). (Step 410) The plasma
chamber has a primary winding coupled to the plasma chamber and a
plurality of magnetic cores oriented such that the plasma chamber
passes through each of the plurality of magnetic cores. In some
embodiments, the method further involves providing a reactant gas
to mix with the gas to be abated before flowing into the plasma
chamber via the second gas inlet (e.g., second gas inlet 110 as
shown above in FIG. 2).
[0078] The method also involves generating a toroidal plasma in the
plasma chamber along a plane that extends through the plasma
chamber (e.g., plane 150 as shown above in FIG. 2) (Step 420). In
some embodiments, the method further involves adjusting a flow rate
of the first gas flowing into the plasma chamber to control
distribution of power in the toroidal plasma.
[0079] The method also involves positioning a second gas inlet
along the plasma chamber at a distance d from a gas outlet along
the plane (e.g., gas outlet 108 as shown above in FIG. 2), the
distance d based on a desired residence time of a gas to be abated
(Step 430). In various embodiments, the method also involves
adjusting the position of the second gas inlet to control the
desired residence time of the gas to be abated.
[0080] The method also involves flowing via the second gas inlet,
the gas to be abated into the plasma chamber such that the gas to
be abated interacts with the toroidal plasma (Step 440). In some
embodiments, the method further involves monitoring an emission
from the plasma chamber via an optical and/or an infrared sensor
and adjusting a flow rate of the reactant gas in response to the
emission from the plasma chamber such that a concentration of the
reactant gas is at a desired level to react with the gas to be
abated, as well as the RF power such that sufficient abatement
efficiency is achieved. The method can also involve coupling the
gas outlet to a subsequent treatment device.
[0081] FIGS. 5a, 5b, and 5c are graphs 501, 502, and 503,
respectively, showing CF4 concentration over time at the gas outlet
of plasma chamber under various conditions, according to
illustrative embodiments of the invention.
[0082] As shown in FIG. 5a, for a CF4 flow rate of 1,000 sccm and
H.sub.2O flow rate of 2,500 sccm, at 25 Torr, CF4 concentration at
the gas outlet of the plasma chamber is 60,740 ppm with no plasma
(e.g., data point 510); CF4 concentration at the gas outlet is
10,000 ppm with plasma on at 5.1 kW (e.g., data point 520).
[0083] As shown in FIG. 5b, for a CF4 flow rate of 1,000 sccm and
H.sub.2O flow rate of 2,500 sccm, at 2 Torr, CF4 concentration at
the gas outlet is 13,021 ppm with plasma on at 4.1 kW (e.g., data
point 530). For a CF4 flow rate of 1,000 sccm and H.sub.2O flow
rate of 2,500 sccm, at 25 Torr, CF4 concentration at the gas outlet
is 9,791 ppm (e.g., data point 540).
[0084] As shown in FIG. 5c, for a CF4 flow rate of 500 sccm and
H.sub.2O flow rate of 1,200 sccm, at 475 Torr, CF4 concentration at
the gas outlet is 2 ppm with plasma on at 8.3 kW (e.g., data point
550). For a CF4 flow rate of 500 sccm and H.sub.2O flow rate of
1,200 sccm, at 475 Torr, CF4 concentration at the gas outlet is
26,927 ppm with no plasma (e.g., data point 560).
[0085] FIG. 6 is a graph 600 showing CF4 destruction efficiency
versus plasma chamber pressure, according to an illustrative
embodiment of the invention. At a CF4 flow rate of 500 sccm, CF4
destruction efficiency at chamber pressure of 25 Torr is
approximately 73% (e.g., data point 610). CF4 destruction
efficiency at chamber pressure of 85 Torr is approximately 99%
(e.g., data point 620).
[0086] FIGS. 7a and 7b are cross-sectional views of a
high-conductance plasma source 700, according to an illustrative
embodiment of the invention. Plasma source 700 includes a gas inlet
702, a gas outlet 704, a chamber wall 708, and a toroidal plasma
channel 720. The toroidal plasma channel 720 includes a toroidal
plasma channel inlet portion 722, a main toroidal plasma channel
portion 723, and a toroidal plasma channel outlet portion 724.
[0087] In the toroidal plasma channel 720, the main toroidal plasma
channel portion 723 is recessed into the chamber wall 708 such that
a width W.sub.1 of the toroidal plasma channel inlet portion 722 is
less than the width W of the main toroidal plasma channel portion
723. A width W.sub.2 of the toroidal plasma channel outlet portion
724 is less than the width W of the main toroidal plasma channel
portion 723. In some embodiments, the width W.sub.1 of the toroidal
plasma channel inlet portion 722 is substantially equal to the
width W.sub.2 of the toroidal plasma channel outlet portion
724.
[0088] Magnetic cores 710a and 710b, generally 710, are disposed
relative to the toroidal plasma channel 720, such that the toroidal
plasma channel 720 passes through each of the magnetic cores 710. A
primary winding 712 is coupled to at least one of the magnetic
cores 710.
[0089] During operation, a toroidal plasma 730 that flows within
the plasma source 700 flows substantially within a loop (e.g.,
circle or oval) on a plane 750. The plane 750 extends along an x
axis and a z axis as shown in FIG. 7b, where the z axis projects
outward from the page. It is apparent to one of ordinary skill in
the art that the boundaries on plane 750 shown in FIG. 7b are for
illustration purposes only and that the plane 750 can extend beyond
those boundaries.
[0090] The plasma source 700 includes a gas flow path 706 from the
gas inlet 702. The gas inlet 702 is positioned on plasma source 700
such that the gas flow path 706 is oriented substantially
perpendicular to the plane 750 extending through the plasma source
700. In some embodiments, the cross-sectional area of the toroidal
plasma channel 720, relative to the direction of gas flow, is
approximately the product of the circumferential length of the
plasma channel 720 and the width W.sub.1 of the inlet portion 722
of the toroidal plasma channel. The cross-sectional area of the
toroidal plasma channel can be based on a desired gas flow
conductance and/or a desired gas-plasma interaction time for the
abatement process. In some embodiments, the diameter of the
toroidal plasma channel 720, D, is greater than or equal to the
diameter of the gas inlet 702. In some embodiments, the
cross-sectional area of the toroidal plasma channel 720, relative
to the direction of gas flow, is greater than or equal to the
cross-sectional area of the gas inlet 702. In some embodiments, the
radius of gas inlet 702 is between approximately 1 and 4
inches.
[0091] The gas inlet 702 includes a curved portion 760 for guiding
the gas flow 706 into the toroidal plasma channel 720. The curved
portion 760 can be shaped to minimize skin friction and pressure
drag along the gas flow path 706 from the gas inlet 702. The curved
portion 760 can be shaped such that pressure rise within the entire
plasma source 700 can be minimized.
[0092] Various shapes can be used for the curved portion 760. For
example, for subsonic flows, the curved portion 760 can be an
elliptically shaped cone, a tangent ogive cone, and/or a
spherically blunted cone. In another example, for transonic flows,
a Von Karman ogive cone and/or a parabola cone can be used.
[0093] In operation, an inert gas, such as argon, can be inserted
into the plasma source 700 along the gas flow path 706 from the gas
inlet 702 to ignite and sustain the toroidal plasma 730 that flows
in a loop within the toroidal plasma channel 720, with most of
plasma in the main plasma channel portion 723. A gas to be abated
is directed into the plasma source 700 along the gas flow path 706
from the gas inlet 702. The gas to be abated flows along the curved
portion 760 to enter the toroidal plasma channel 720 through the
toroidal plasma channel inlet portion 722. The main toroidal plasma
channel portion 723 is recessed, as described above, such that the
gas to be abated is directed to interact with a region of the
toroidal plasma 730 having a substantially peaking electron density
(e.g., an electron density of 10.sup.12 cm.sup.-3). Less gas is
flowed to the edge of plasma or the plasma-wall boundary region
where electron density decreases due to plasma loss to the walls,
as the boundaries of the plasma remains in the recessed edges of
the main toroidal plasma channel portion 723. The gas exits the
main toroidal plasma channel portion 723 through the toroidal
plasma channel outlet portion 724. The gas can exit the plasma
source 700 through the gas outlet 704.
[0094] The width W of the main toroidal plasma channel portion 723
can be based on a diffusion length of the toroidal plasma 730 at
normal operating conditions. The width W of the main toroidal
plasma channel portion can be based on the width required to fill
the main toroidal plasma channel portion 723 with the toroidal
plasma 730 during described operation.
[0095] The width W of the main toroidal plasma channel portion 723
can also be based on type of gas to be abated, pressure and flow
rate of the gas, and/or diffusion lengths of electrons and/or ions
in the plasma channel.
[0096] The width W of the main toroidal plasma channel portion 723
can be approximately equal to a natural width of the toroidal
plasma 730 (e.g., width of the plasma without the presence of
walls). The diffusion lengths of electrons and ions in the plasma
channel can decrease with increasing pressure and can also decrease
in electronegative gases. When the width W of the main toroidal
plasma channel portion 723 is narrower than the natural width of
the toroidal plasma 730, the plasma source can be inefficient due
to high rate of loss of plasmas near the walls. When the width W of
the main toroidal plasma channel portion 723 is wider than the
natural width of the toroidal plasma 730, the plasma can exist only
in a portion of the main toroidal plasma channel portion 723,
allowing part of gas to be abated to flow through the toroidal
plasma channel 720 without being activated and reacted, thereby
reducing the abatement efficiency.
[0097] In some embodiments, a depth of the recess (e.g., the
difference between the width W.sub.1 of the toroidal plasma channel
inlet portion 722 and the width W of the main toroidal plasma
channel portion 723) is approximately equal to a thickness of a
boundary layer of the toroidal plasma 730. Having a depth of the
recess approximately equal to the thickness of the boundary layer
can allow for most of the gas to be abated to flow through a region
of toroidal plasma 730 where the plasma density is high. For
example, for a gas containing fluorocarbon and oxygen at pressure
of 0.1-0.5 Torr, a width W of the toroidal plasma channel 720 can
be 30 mm, with a depth of recess of 5 mm. For other applications,
the optimal width W of the toroidal plasma channel 720 can range
from 10 mm to 50 mm, with an optimal depth of recess ranging from 1
mm to 20 mm.
[0098] In some embodiments, the toroidal plasma channel 720 is made
of a metal such as aluminum, with its surface covered by a
protective coating. In some embodiments, electrostatic coupling is
shielded by the metallic layer of toroidal plasma channel 720. The
protective coating can be a layer of chemically stable dielectric,
such as Y2O3, an anodized coating on aluminum, or an oxide layer
formed via Plasma Electrolytic Oxidation. The inductively-coupled,
low-field toroidal plasma 730 can be operated at electric fields of
2-10 V/cm to eliminate energetic ions and to reduce ion-induced
erosion. In some embodiments, one or more dielectric gaps (e.g.
dielectric gasps 716a and 716b shown in FIG. 7a) along the toroidal
plasma channel 720 prevent induced electric current from flowing
along the toroidal plasma channel 720. The induced electric voltage
across each dielectric gap can be limited to below 100 V.
[0099] Abatement efficiency can depend on the rates of gas
excitation, chemical reactions, and the time of the gases staying
within the volume of toroidal plasma 730. Toroidal plasma 730
flowing in the toroidal plasma channel 730 can excite, heat, or
dissociate the gases to be abated and raise chemical reactivity of
the gases. The gas residence time in the volume of toroidal plasma
730 is proportional to the plasma volume V, as follows:
t = v s .apprxeq. .pi. Dw 2 s , ##EQU00002##
where
S = d v d t ##EQU00003##
is the pump speed.
[0100] For plasma abatement, the gas residence time t can be of the
order of the gas chemical reaction time. If the residence time is
too short, the abatement efficiency can suffer as there can be
insufficient time for the gases to be activated and reacted in the
toroidal plasma 730. If the residence time is too long, the energy
efficiency can reduce as power can be consumed in re-heating and
re-activating the reacted species.
[0101] The circumference or major diameter D of the toroidal plasma
channel 720 can based on a desired volume of toroidal plasma 730
and/or desired abatement efficiency. While the width W of the main
toroidal plasma channel portion 723 can be determined by the width
of the toroidal plasma 730, there is no such a limitation on the
major diameter D of the plasma channel. For example, for a toroidal
plasma abatement device to match or exceed the flow conductance of
a pipe line of diameter x, the major diameter of the toroidal
plasma channel 720 can be selected by D>x.sup.2/4 W, so that the
cross sectional area of the toroidal plasma channel in the
direction of the gas flow exceeds the cross sectional area of the
pipe line. In addition, the major diameter D of the toroidal plasma
channel 720 can be selected to achieve a desired gas residence time
t in the plasma, as the volume of the plasma in the toroidal plasma
channel 720 is approximately V.apprxeq..pi.Dw.sup.2.
[0102] FIG. 8 is a flow chart showing a method 800 for abating
process gas within a plasma source, according to an illustrative
embodiment of the invention. The method involves directing via a
gas inlet (e.g., gas inlet 702 as shown above in FIGS. 7a and 7b) a
gas to be abated along a gas flow path (e.g., gas flow path 706 as
shown above in FIGS. 7a and 7b) into a plasma source (e.g., plasma
source 700 as shown above in FIGS. 7a and 7b). The plasma source
has a primary winding coupled to the plasma source and a plurality
of magnetic cores positioned such that a toroidal plasma channel
oriented along a plane that extends through the plasma source
passes through each of the plurality of magnetic cores (Step
810).
[0103] The method also involves generating a toroidal plasma along
the plane that extends through the plasma source and confined in
the toroidal plasma channel (e.g., toroidal plasma channel 720 as
shown above in FIG. 7b) (Step 820). In various embodiments, the
method also involves positioning one or more dielectric gaps along
the toroidal plasma channel to prevent an induced electric current
from flowing in the toroidal plasma channel.
[0104] The method also involves positioning the gas inlet such that
the gas flow path is substantially perpendicular to the plane
extending through the plasma source (e.g., plane 750 as shown above
in FIG. 7b) (Step 830).
[0105] The method also involves flowing the gas to be abated into
the toroidal plasma channel such that the gas to be abated
interacts with a main body of the toroidal plasma having a peak
electron density (Step 840). In some embodiments, the method also
involves providing one or more reactant gases to mix with the gas
to be abated prior to the gas to be abated entering the toroidal
plasma channel.
[0106] FIGS. 9a and 9b are graphs showing abatement rate versus
pressure and power, according to illustrative embodiments of the
invention. The magnitude of plasma current, to the first order, can
be proportional to the density of plasma in the toroidal plasma
channel. The plasma current can be controlled by adjusting the RF
power supply that powers the plasma source. Higher plasma density
increases the rate of gas-plasma reactions and the abatement rate
of the gas to be abated. Excess plasma current, however, decreases
the energy efficiency of the abatement process. A preferred
abatement efficiency can range from 80% to 100%, depending upon the
type of gas to be abated.
[0107] As shown in FIG. 9a, at a CF4 flow rate of 100 sccm, CF4
abatement efficiency at source pressure of approximately 0.5 Torr
is approximately 98% for a plasma current of 33 A (e.g., data point
910).
[0108] As shown in FIG. 9b, CF4 abatement efficiency at source
power of approximately 4 kW is approximately 97% for a plasma
current of 33 A (e.g., data point 920).
[0109] FIGS. 9c and 9d are graphs showing abatement rate versus
pressure and power, according to illustrative embodiments of the
invention.
[0110] As shown in FIG. 9c, at a CF4 flow rate of 200 sccm, CF4
abatement efficiency at source pressure of approximately 0.4 Torr
is approximately 97% for a plasma current of 37.5 A (e.g., data
point 930).
[0111] As shown in FIG. 9d, CF4 abatement efficiency at source
power of approximately 6 kW is approximately 97% for a plasma
current of 37.5 A (e.g., data point 940).
[0112] The invention can be used for reducing PFC emissions from
microelectronics manufacturing processes, particularly from
dielectric etch and CVD tools. The invention can be used to abate
gases to remove unwanted species in a gas stream. It can also be
used for decomposing CVD precursors inside or upstream of a process
chamber to assist deposition. It can be used in chemical plants to
abate harmful gases before releasing to atmosphere.
[0113] While the invention has been particularly shown and
described with reference to specific embodiments, it should be
understood by those skilled in the art that various changes in form
and detail can be made therein without departing from the spirit
and scope of the invention as defined by the appended claims.
* * * * *