U.S. patent application number 11/304257 was filed with the patent office on 2006-05-11 for processing materials inside an atmospheric-pressure radiofrequency nonthermal plasma discharge.
Invention is credited to Ivars Henins, Hans W. Herrmann, Jaeyoung Park, Gary S. Selwyn.
Application Number | 20060096707 11/304257 |
Document ID | / |
Family ID | 25106417 |
Filed Date | 2006-05-11 |
United States Patent
Application |
20060096707 |
Kind Code |
A1 |
Selwyn; Gary S. ; et
al. |
May 11, 2006 |
Processing materials inside an atmospheric-pressure radiofrequency
nonthermal plasma discharge
Abstract
Apparatus for the processing of materials involving placing a
material either placed between an radio-frequency electrode and a
ground electrode, or which is itself one of the electrodes. This is
done in atmospheric pressure conditions. The apparatus effectively
etches or cleans substrates, such as silicon wafers, or provides
cleaning of spools and drums, and uses a gas containing an inert
gas and a chemically reactive gas.
Inventors: |
Selwyn; Gary S.; (Los
Alamos, NM) ; Henins; Ivars; (Los Alamos, NM)
; Park; Jaeyoung; (Los Alamos, NM) ; Herrmann;
Hans W.; (Los Alamos, NM) |
Correspondence
Address: |
UNIVERSITY OF CALIFORNIA;LOS ALAMOS NATIONAL LABORATORY
P.O. BOX 1663, MS A187
LOS ALAMOS
NM
87545
US
|
Family ID: |
25106417 |
Appl. No.: |
11/304257 |
Filed: |
December 14, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09776086 |
Feb 2, 2001 |
|
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11304257 |
Dec 14, 2005 |
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Current U.S.
Class: |
156/345.43 ;
257/E21.24 |
Current CPC
Class: |
H01L 21/31 20130101;
H01L 21/67069 20130101; H01J 37/32733 20130101; H01J 37/32082
20130101; B08B 7/0035 20130101; C23G 5/00 20130101 |
Class at
Publication: |
156/345.43 |
International
Class: |
C23F 1/00 20060101
C23F001/00 |
Goverment Interests
[0001] The present invention generally relates to material
processing and, more specifically, to the processing of an object
or material by insertion between the electrodes of an
atmospheric-pressure radiofrequency nonthermal plasma discharge.
This invention was made with Government support under Contract No.
W-7405-ENG-36 awarded by the U.S. Department of Energy. The
Government has certain rights in the invention.
Claims
1-25. (canceled)
26. Apparatus for processing materials in an atmospheric pressure
radio-frequency non-thermal plasma consisting of: an electrically
conductive enclosure defining and interior space with a surface and
openings for introductions of a gas and for entry and exit of a
material to be processed while said interior space is at or near
atmospheric pressure; an electrode situated inside said interior
space and spaced apart from said surface of said interior space a
distance sufficient to allow placement of said material to be
processed; a mechanical action for placing said material to be
processed inside said interior space on said electrode or between
said electrode and said electrically conductive enclosure; and, a
radio frequency power supply having a phase applied between said
electrode and said electrically conductive enclosure; wherein a gas
containing a majority of inert gas is introduced into said interior
space through said opening for introduction of a gas creating an
atmospheric pressure plasma in said interior space for processing
said material to be processed within said electrically conductive
enclosure.
27. The apparatus as described in claim 26, wherein said phase has
a frequency of 13.56 Megahertz.
28. The apparatus as described in claim 26, wherein said electrode
and said electrically conductive enclosure are cylindrically
shaped.
29. The apparatus as described in claim 26, wherein said electrode
is a rotating roller.
Description
[0002] Surface cleanliness is of vital importance in many
industries, not the least of which is the semiconductor industry.
Clean substrates and devices are imperative if high quality devices
are to be manufactured. Additionally, other applications require
that materials be etched in a predetermined manner to effect the
desired operation from the material.
[0003] Currently, this cleaning or etching is accomplished through
a variety of methods. Among these methods are low-pressure plasma
processing and atmospheric-pressure RF plasma discharge processing.
By utilizing the reaction of the plasma with selected feed gases,
surface processing has been performed on organic films, fabrics,
and semiconductor wafers. In these processes, the target material
is immersed in the plasma, typically by placing the workpiece
directly on an electrode.
[0004] These discharge processes are effective because of the
action of the ions, which typically are more chemically active than
the corresponding neutral gas species, due to their greater
collision cross section and reaction probability. Also, ions are
accelerated across the sheath region in the plasma. This gives rise
to the directed flux of positive ions onto the workpiece. The
kinetic energy contributed by these ions combined with their
chemical reactivity results in the desired chemical reaction.
Unfortunately, however, these prior art processing methods require
expensive vacuum systems in order to be effective, because the
formation of a sheath is favored by low pressure and to obtain a
high kinetic energy of the ions it is necessary to minimize
gas-phase collisions within the sheath region.
[0005] The primary objective of the present invention, like the
prior discipline of low-pressure plasma processing, is to modify
selected surfaces. This modification can include contamination
removal, surface material removal, known as etching, or changes in
the physical state or property of the surface, known as surface
modification.
[0006] Previous demonstrations of atmospheric pressure, RF plasma
discharge that are related to the present invention are U.S. Pat.
No. 5,961,772, issued to Gary S. Selwyn for "Atmospheric-Pressure
Plasma Jet," and U.S. patent application Ser. No. 09/295,942, filed
Apr. 21, 1999, for "Large Area Atmospheric-Pressure Plasma Jet."
The first involves a coaxial, cylindrical electrode configuration,
and the second a parallel plate electrode configuration. In both of
these demonstrations, the plasma by-products are blown out of the
source region, in which the plasma is generated, and is directed
against a surface to be treated. The target surface is typically a
few millimeters from the source.
[0007] It should be noted that radicals, and particularly ions, in
the plasma discharge are extremely short lived, and cannot be
transported for long distances outside the discharge region.
Metastable species produced inside the plasma, on the other hand,
have longer lifetimes at atmospheric pressure, typically on the
order of hundreds of milliseconds. This longer lifetime allows them
to be carried out of the plasma volume along with the gas flow and
impinge against an external material or surface.
[0008] The fast flow of the reactive gas stream exiting the plasma
volume increases the reaction distance, that is, the distance at
which the plasma jet may be positioned from a workpiece and still
provide effective reaction chemistry. High gas flow also increases
the flux of reactive species onto the workpiece. To accomplish all
of this, high gas flows must be maintained to carry reactive
metastable and other plasma species to the workpiece before they
decay and become nonreactive. Of course, high gas flow rates
increase the cost of plasma processing by increasing the cost of
consumables, or require reprocessing of the spent gas.
[0009] Such downstream treatment of materials as described also has
some distinct advantages. It reduces the likelihood of surface
damage to the workpiece, because after exiting the plasma volume,
most of the charged species have recombined and are neutralized.
One common source of damage in microelectronic devices is due to
the build up of charge on dielectric on semiconductor surfaces.
Therefore, surface charging is not an issue in downstream
processing. Because neutral species are not accelerated to high
kinetic energy in the way that ions are in low pressure plasma
processing equipment, the reaction chemistry of neutral species is
more selective, albeit slower, than ion chemistry.
[0010] Clearly, in cases where selectivity is not an important
issue, such as the removal of organic contaminants from silicon or
metals (because of the innate huge difference in chemical
reactivity between the former and the latter two materials), the
process rate could be improved by incorporating ion-driven
chemistry into the reaction scheme. This may be accomplished
through direct immersion of the workpiece into the volumetric
plasma.
[0011] Because the plasma volume contains significant ionic
components, cleaning and surface treatment of materials may be
accelerated by utilizing ion-driven chemistry. Also, by immersing
the workpiece into the plasma, high gas flow rates are not needed
to drive the reactive species several millimeters before they decay
or recombine. This is due to the fact that the reactive species are
present immediately adjacent to the workpiece because they are
formed in the same volume as the workpiece.
[0012] This is the thrust of the present invention: to provide
means for the utilizing ion chemistry even for a small plasma
volume as is present in the plasma jet source described above, but
without the added risk of surface damage caused by high energy
impact of the ions onto the workpiece. In the present invention the
workpiece is introduced directly into the plasma volume and is
exposed to the ion and neutral chemistry of the plasma, and to the
high-pressure environment of the plasma.
[0013] A clear advantage of this approach relative to the prior art
of low-pressure plasma processing equipment is that the
high-pressure environment of the plasma limits the strength and
dimension of the sheath, which in turn limits the kinetic energy of
the ions. Ions are accelerated less by the weaker, thinner sheath,
and those ions that impinge the surface have lower kinetic energy
as a result of the smaller electric field in this sheath, as well
as the frequent, gas-phase collisions the ions undergo with neutral
species. The resultant lower kinetic energy of ions leads to less
surface damage.
[0014] An advantage of the direct immersion process taught in this
invention relative to the prior art of downstream, atmospheric
pressure plasma processing is that the reaction chemistry benefits
from the added presence of ions, which would be recombined and
therefore lost to the downstream chemistry processing approach.
Also, because gas flow is not needed to carry the reactive species
several mm to the workpiece, lower gas consumption is possible.
[0015] The workpiece may either be as rigid as a silicon wafer or
as flexible as manmade or natural textiles. Because the workpiece
is exposed to radio frequency power in this invention, there is no
limitation that it be conducting at dc power: both dielectric
materials and semiconductors, as well as conductors may be
processed.
[0016] It is therefore an object of the present invention to
provide apparatus and method for cleaning and processing materials
inside a high-pressure plasma discharge.
[0017] It is another object of the present invention to provide
apparatus and method for cleaning and processing materials that use
less process gas.
[0018] It is yet another object of the present invention to provide
apparatus and method for treating materials while creating less
surface damage to the materials.
[0019] It is still another object of the present invention to
provide a means of treating materials that may be conductors,
semiconductors or dielectric in nature.
[0020] Additional objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
[0021] To achieve the foregoing and other objects, and in
accordance with the purposes of the present invention, as embodied
and broadly described herein, the apparatus of this invention for
the plasma processing of materials in an atmospheric pressure
radio-frequency non-thermal plasma comprises an electrically
conductive enclosure defining an interior space with a surface and
with openings for introduction of a gas and for entry and exit of a
material to be processed, with an electrode situated inside the
interior space and spaced apart from the surface of the interior
space a distance sufficient to allow placement of the material to
be processed. Means for placing the material to be processed is
located inside the interior space between the electrode and the
electrically conductive enclosure. When a gas is introduced into
the interior space through the opening for introduction of a gas
and a radio-frequency voltage is applied between the electrically
conductive enclosure and the electrode, a plasma is created in the
interior space for processing the material to be processed within
the electrically conductive enclosure.
[0022] In another aspect of the present invention and in accordance
with its principles and purposes apparatus for processing materials
in an atmospheric pressure radio-frequency non-thermal plasma
comprise an electrically conductive enclosure defining an interior
space with a surface and inlets for a gas and for entry and exit of
a material to be processed with an electrode spaced apart from the
electrically conductive enclosure and capable of placing the
material to be processed inside the interior space between the
electrically conductive enclosure and the electrode, the material
to be processed being in contact with the electrode. When a gas is
introduced into the inlet for gas and a radio-frequency voltage is
applied between the electrically conductive enclosure and the
electrode a plasma is created in the interior space for processing
the material to be processed as it passes through the electrically
conductive enclosure In a still further aspect of the present
invention and in accordance with its principles and purposes
apparatus for processing materials in an atmospheric pressure
radio-frequency non-thermal plasma comprise a grounded enclosure
defining a first interior space, gas inlet and outlets and an
opening for radio-frequency voltage connection, with a
radio-frequency connector in said opening. A radio-frequency
electrode is located in the interior space in electrical contact
with the radio-frequency connector, and defines an opening for the
gas inlet and a second interior space. Grounded means for retaining
a spool of material to be processed is in close proximity to the
radio-frequency electrode. When a gas is introduced through the gas
inlet and a radio-frequency voltage is applied between the
radio-frequency connector and ground, plasma is created between the
radio-frequency electrode and the spool of material to be processed
thereby providing cleaning of the spool of material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
[0024] FIG. 1 is a schematic illustration of one embodiment of the
present invention in which the material to be processed is pulled
by roller through the volume between a ground electrode and a radio
frequency electrode.
[0025] FIG. 2 is a schematic illustration of another embodiment of
the present invention in which the radio frequency electrode is
used as a roller to pull the material to be processed into the
volume between the radio frequency electrode and the ground
electrode.
[0026] FIGS. 3A and 3B are plots of voltage versus plasma current
for several gas flow rates.
[0027] FIG. 4 is an illustration of an embodiment of the present
invention employing cylindrical coaxial electrodes.
DETAILED DESCRIPTION
[0028] The present invention provides atmospheric pressure plasma
processing of materials in an effective and efficient manner. The
invention can be understood most easily through reference to the
drawings.
[0029] In FIG. 1 there is a cross-sectional schematic illustration
of one embodiment of the invention where material plasma processor
10 defines electrically conductive enclosure 11 that forms an
enclosed volume 11a having gas inlet 11b, material inlet 11c and
material outlet 11d. Inside volume 11a, roller 12 and electrode 13
are situated. Roller 12 serves to pull film or textile material 14
through enclosed volume 11a between electrically conductive
electrode 11 and electrode 13. Those with skill in this art will
appreciate that any appropriate means other than roller 12 could be
used to transport film material 14 into place inside enclosed
volume 11a. Roller 12 could also be the RF powered electrode in
some cases.
[0030] With an appropriate gas injected through gas inlet 11b and
the appropriate level of RF voltage applied either to electrically
conductive enclosure or to electrode 13 with respect to the other
serving as a ground electrode, a plasma will be created in enclosed
volume 11a for processing said film material 14 as it is pulled
through ground electrode 11 by roller 12. The appropriate gas used
can be any gas that can provide the proper ion-driven chemistry for
the intended processing. In normal operations, an inert gas is the
major gas component, along with the addition of a reactive gas such
as oxygen in an appropriate amount. However, other gases may also
be added, subject to the arcing performance of the plasma source.
In the preferred embodiment, a gas mixture consisting of 99%
helium+1% oxygen at atmospheric pressure is used to remove organic
contaminants from metal or silicon surfaces.
[0031] The outlet for the gas introduced into enclosed volume 11
may simply be small openings between the components that comprise
the electrically conductive enclosure, or they may be tubing used
for gas reprocessing or exhaust. This is true for all of the
embodiments of the present invention described herein.
[0032] It is important to note that inasmuch as the present
invention utilizes RF energy to create a plasma and to process
materials, electrically conductive enclosure 11 does not
necessarily need to be grounded. In some circumstances it may be
desirable to have electrically conductive enclosure 11 floating and
apply RF energy 15 at some predetermined phase, which can differ by
as much as 180.degree., with respect to RF energy 16 applied to
electrode 13, to enhance the effectiveness of the processing. In
this situation, a protective, grounded casing 14, shown by dashed
lines in FIG. 1, would enclose the invention for safety reasons. An
appropriate frequency for the RF energy used in the present
invention is 13.56 Megahertz (MHz), however other RF frequencies
might also prove useful.
[0033] FIG. 2 illustrates another embodiment of the present
invention where material plasma processor 20 defines electrically
conductive enclosure 21 that forms enclosed volume 21a, gas inlet
21b, and material inlet 21c and material outlet 21d. Inside
enclosed volume 21a, electrode 22 also serves as a roller, such as
roller 12 of FIG. 1. However, as with the previous embodiment, it
is not necessary that electrode 22 be in the form of a roller. Any
other appropriate configuration can be used as long as it is
capable of placing a material inside enclosed volume 21a, between
electrically conductive enclosure 21 and electrode 22, and in
contact with electrode 22
[0034] In this embodiment, any material inserted into enclosed
volume 21a through material inlet 21c and is placed in or is pulled
through enclosed volume 21a by, or on top of electrode 22. In this
embodiment, electrode 22 is in direct contact with the material to
be processed, making the material the part of the electrode. In
this embodiment, the material can receive the full effect of all of
the plasma products. It should be noted that even dielectric or
semi-conducting substrates could become part of electrode 22 and
subject to ion impingement, as RF frequency will penetrate such
media.
[0035] The present invention provides direct immersion of the
material into the plasma, providing an important advantage over the
plasma processing techniques of the prior art. As an example, the
short-lived species present in the plasma volume, such as ions and
certain radicals can attack the material's surface because they are
present within the diffusion distance of the material's surface.
Additionally, since the gas does not need to flow at high velocity
in order to carry reactive species beyond the exit of the plasma
source, the gas flow rate can be reduced significantly. This
results in savings in the cost of the process gas and the overall
processing cost.
[0036] The overall low gas flow rate of the present invention, a
few standard liters per minute (slpm), addresses a limitation of
the prior art Atmospheric Pressure Plasma Jet, namely large usage
of He gas for maintaining an arc-free discharge. The present
invention is based on a study of the discharge electrical
properties of the plasma jet as a function of total gas flow rate.
As shown in FIGS. 3A and 3B, the stable region of plasma discharge
does not change appreciably as the total gas flow rate is decreased
from 40 slpm to 2.5 slpm, a factor of 16, with the gas composition
remaining constant. These data indicate the existence of a stable
region of plasma discharge even at very low gas flow rates, so long
as the gas composition remains constant. The gas composition easily
can be maintained in an airtight (not a vacuum) environment.
[0037] This low gas flow airtight environment is the hallmark of
the previously described embodiments of the present invention. Low
gas flow rate decreases the cost of the process and makes treatment
of relatively low-value added processes, such as textile treatment,
economically viable. By insertion of the material to be processed
into the plasma discharge zone as is done in the embodiments of the
present invention, the maximum benefit of the plasma is achieved.
Compared to the prior art Atmospheric Pressure Plasma Jet, which
relies on metastable and other long-lived neutral species for
chemical reactions outside the jet, the present invention provides
in-situ material processing utilizing the full potential of the
atmospheric pressure plasma discharge, including charged species or
ions, atomic and radical species, as well as potentially the UV
radiation emitted by the plasma to aid material processing. Of
course, the contribution of the metastable and other long-lived
species of the effluent-based plasma treatment of the prior art
also is of value to the processing by the present invention, as
these are still present within the discharge region.
[0038] Referring again to FIG. 2, it should be understood that this
embodiment of the present invention, in having the material to be
processed in contact with electrode 22, maximizes chemical
reactivity of the plasma while at the same time allowing
temperature control of the material. This can be accomplished by
simply heating or cooling the electrode in contact with the
material. This temperature control ability can be used to enhance
the rate of chemical reaction or to limit any detrimental side
effects such as thermal damage to the material to be processed.
[0039] To verify the efficacy of this embodiment, a KAPTON.RTM.
film was processed through enclosed volume 21a using He at a flow
rate of 42 slpm and an O.sub.2 flow rate of 0.36 slpm, and an input
power of 345 watts. KAPTON.RTM. is a flexible, dielectric film
comprised of polyimide. The gap spacing between ground and a flat
10 cm by 10 cm stainless steel RF electrode was 0.16 cm. With this
configuration, an etch rate of up to 9 mg per minute was measured
both for the KAPTON.RTM. film between the electrodes and for the
KAPTON.RTM. film in contact with the electrodes.
[0040] Another embodiment of the invention is illustrated in a
cross-sectional view in FIG. 4, in which outer enclosure 41 defines
enclosed volume 42 in which conformal electrode 43 encloses
electrically conductive object 44. Electrically conductive object
44 represents any spool or object that is in need of cleaning, such
as a printing press roll, or a cylinder intended for recycling from
a laser-printing cartridge or a spool used for treatment of thread
material in need of cleaning. Electrically conductive object 44 is
retained inside conformal electrode 43 by physical connector clamp
45 and threaded shaft 46. In this embodiment, electrically
conductive object 44 is grounded and functions as the grounded
electrode. In this case, it is preferable to ground electrically
conductive object 44 and to position an axially symmetric,
RF-powered electrode concentric to electrically conductive object
44 in order to form a plasma. Because electrically conductive
object 44 can be grounded, it may be left attached to other
equipment without damaging the connected equipment by passage of
the RF current. However, in other circumstances, electrically
conductive object 44 may be RF powered and conformal electrode 43
may be grounded or may be RF powered at a different phase than is
electrically conductive object 44
[0041] Outer enclosure 41 provides an opening for gas tube 47 and
for RF connector 48 that provides electrical connection to
conformal electrode 43. Outer enclosure 41 also provides viewing
ports 49, as does conformal electrode 43. Thermocouple clamp 50
retains a thermocouple for controlling heater 51 for maintaining an
appropriate temperature of electrically conductive object 44.
[0042] The foregoing description of the embodiments of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
* * * * *