U.S. patent application number 15/231229 was filed with the patent office on 2016-12-01 for atmospheric-pressure plasma processing method.
This patent application is currently assigned to APJeT, Inc.. The applicant listed for this patent is APJeT, Inc.. Invention is credited to Carrie E. Cornelius, Gregory A. Roche, David W. Tyner.
Application Number | 20160348292 15/231229 |
Document ID | / |
Family ID | 50273397 |
Filed Date | 2016-12-01 |
United States Patent
Application |
20160348292 |
Kind Code |
A1 |
Cornelius; Carrie E. ; et
al. |
December 1, 2016 |
ATMOSPHERIC-PRESSURE PLASMA PROCESSING METHOD
Abstract
Methods for atmospheric pressure plasma discharge processing
using powered electrodes having elongated planar surfaces; grounded
electrodes having elongated planar surfaces parallel to and
coextensive with the elongated surfaces of the powered electrodes,
and spaced-apart a chosen distance therefrom, forming plasma
regions, are described. RF power is provided to the at least one
powered electrode, both powered and grounded electrodes may be
cooled, and a plasma gas is flowed through the plasma regions at
atmospheric pressure; whereby a plasma is formed in the plasma
regions. The material to be processed may be moved into close
proximity to the exit of the plasma gas from the plasma regions
perpendicular to the gas flow, and perpendicular to the elongated
electrode dimensions, whereby excited species generated in the
plasma exit the plasma regions and impinge unimpeded onto the
material.
Inventors: |
Cornelius; Carrie E.;
(Durham, NC) ; Roche; Gregory A.; (Durham, NC)
; Tyner; David W.; (Benson, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
APJeT, Inc. |
Morrisville |
NC |
US |
|
|
Assignee: |
APJeT, Inc.
Morrisville
NC
|
Family ID: |
50273397 |
Appl. No.: |
15/231229 |
Filed: |
August 8, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13826089 |
Mar 14, 2013 |
|
|
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15231229 |
|
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61702919 |
Sep 19, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D06B 1/02 20130101; H05H
2240/10 20130101; H05H 2245/123 20130101; D06B 19/007 20130101;
B23K 10/003 20130101; D10B 2401/021 20130101; H05H 1/46 20130101;
H05H 2001/466 20130101 |
International
Class: |
D06B 19/00 20060101
D06B019/00; D06B 1/02 20060101 D06B001/02 |
Claims
1. A method for atmospheric-pressure plasma processing comprising:
flowing a plasma gas between a region defined by at least one first
electrically conducting electrode having a chosen height and having
at least one first elongated planar surface having a chosen length,
and at least one grounded second electrically conducting electrode
having at least one second elongated planar surface parallel to and
coextensive with the first planar surface, and spaced-apart a
chosen distance therefrom, whereby the plasma gas exits the region
through a long dimension of the at least one first planar surface
and a corresponding long dimension of the at least one second
planar surface; applying RF power to the at least one first
electrode from an RF power source, whereby at least one plasma is
formed; and cooling the at least one first electrode and the at
least one second electrode to a chosen temperature.
2. The method of claim 1, wherein each of the at least one first
electrode and the at least one second electrode comprises a hollow
portion, a fluid inlet to the hollow portion and a fluid outlet
therefrom, whereby the coolant is directed into the fluid inlet,
through the hollow portion and through the outlet of each the at
least one first electrode and the at least one second
electrode.
3. The method of claim 1, wherein each of the at least one first
electrode and the at least one second electrode comprises a hollow
square or rectangular metallic conductor.
4. The method of claim 1, wherein the RF power source comprises RF
impedance matching circuitry for providing RF to the at least one
RF electrode.
5. The method of claim 1, wherein plasma gas is flowed into the at
least one plasma region through a long dimension of the at least
one first planar surface and a corresponding long dimension of the
at least one second planar surface opposite to the at least one
plasma region through the long dimension of the at least one first
planar surface and the corresponding long dimension of the at least
one second planar surface through which the plasma gas exits the at
least one plasma region.
6. The method of claim 1, wherein the chosen height is selected
such that power supplied to the plasma by the RF power source is
minimized.
7. The method of claim 6, wherein the chosen height is between
about 3 mm and about 25 mm.
8. The method of claim 1, wherein the chosen distance is between
about 0.2 mm and about 4 mm.
9. The method of claim 1, wherein said step of flowing a plasma gas
is achieved using at least one electrically non-conducting,
elongated gas block having an elongated chamber therein in fluid
communication with a gas manifold, a nozzle in fluid communication
with the chamber having a chosen width and a length of the at least
one first elongated planar surface and the at least one second
elongated planar surface and disposed therebetween.
10. The method of claim 9, wherein the at least one gas block
further comprises a porous tube disposed within the elongated
chamber of said gas block in fluid communication with the gas
manifold for uniformly supplying gas to the nozzle.
11. The method of claim 10, wherein the porous tube comprises a
Teflon tube.
12. The method of claim 1, wherein the RF comprises frequencies
between about 100 kHz and about 100 MHz.
13. The method of claim 1, wherein the chosen temperature is about
20.degree. C.
14. The method of claim 1, wherein gas exiting the plasma has a
temperature <70.degree. C.
15. A method for atmospheric-pressure plasma discharge processing
of a material, comprising: flowing a plasma gas between a region
defined by at least one electrically conducting first electrode
having at least one first elongated planar surface, and at least
one grounded second electrically conducting electrode having at
least one second elongated planar surface parallel to and
coextensive with the first planar surface, and spaced-apart a first
chosen distance therefrom, whereby the plasma gas exits the region
through a long dimension of the at least one first planar surface
and a corresponding long dimension of the at least one second
planar surface; applying RF power to the at least one first
electrode from an RF power source, whereby at least one plasma is
formed; cooling the at least one first electrode and the at least
one second electrode to a chosen temperature; and moving the
material perpendicular to the long dimension of the at least one
first planar surface and the at least one second planar surface at
a second chosen distance therefrom, and perpendicular to the flow
of the plasma gas out of the plasma region.
16. The method of claim 15, wherein each of the at least one first
electrode and the at least one second electrode comprises a hollow
portion, a fluid inlet to the hollow portion and a fluid outlet
therefrom, whereby the coolant is directed into the fluid inlet,
through the hollow portion and through the outlet of each the at
least one first electrode and the at least one second
electrode.
17. The method of claim 15, wherein each of the at least one first
electrode and the at least one second electrode comprises a hollow
square or rectangular metallic conductor.
18. The method of claim 15, wherein the RF power source comprises
RF impedance matching circuitry for providing RF to the at least
one RF electrode.
19. The method of claim 15, wherein plasma gas is flowed into the
at least one plasma region through a long dimension of the at least
one first planar surface and a corresponding long dimension of the
at least one second planar surface opposite to the at least one
plasma region through the long dimension of the at least one first
planar surface and the corresponding long dimension of the at least
one second planar surface through which the plasma gas exits the at
least one plasma region.
20. The method of claim 15, wherein the chosen height is selected
such that power supplied to the plasma by the RF power source is
minimized.
21. The method of claim 20, wherein the chosen height is between
about 3 mm and about 25 mm.
22. The method of claim 15, wherein the first chosen distance is
between about 0.2 mm and about 4 mm.
23. The method of claim 15, wherein said step of flowing a plasma
gas is achieved using at least one electrically non-conducting,
elongated gas block having an elongated chamber therein in fluid
communication with a gas manifold, a nozzle in fluid communication
with the chamber having a chosen width and a length of the at least
one first elongated planar surface and the at least one second
elongated planar surface and disposed therebetween.
24. The method of claim 23, wherein the at least one gas block
further comprises a porous tube disposed within the elongated
chamber of said gas block in fluid communication with the gas
manifold for uniformly supplying gas to the nozzle.
25. The method of claim 24, wherein the porous tube comprises a
Teflon tube.
26. The method of claim 15, wherein the RF comprises frequencies
between about 100 kHz and about 100 MHz.
27. The method of claim 15, wherein the chosen temperature is about
20.degree. C.
28. The method of claim 15, wherein gas exiting the plasma has a
temperature <70.degree. C.
29. The method of claim 15, wherein the second chosen distance is
between about 0 mm and about 5 mm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S.
Provisional Patent Application No. 61/702,919 for
"Atmospheric-Pressure Plasma Processing Apparatus And Method" which
was filed on Sep. 19, 2012, and is a divisional of patent
application Ser. No. 13/826,089 for "Atmospheric-Pressure Plasma
Processing Apparatus And Method which was filed on Mar. 14, 2013,
the entire contents of which applications are hereby specifically
incorporated by reference herein for all that they disclose and
teach.
FIELD OF THE INVENTION
[0002] The present invention relates generally to apparatus and
method for plasma processing of materials and, more particularly,
to an atmospheric-pressure plasma processing apparatus capable of
producing a stable discharge having a neutral gas temperature that
can be controlled using a cooling system, for generation of active
chemical species including gas metastable and radical species
effective for large area plasma processing, whereby active chemical
or active physical components of the plasma exit the discharge
electrode region and impinge unimpeded onto a substrate disposed
externally from the discharge region, and without simultaneous
exposure of the substrate to the electrical influence of the
plasma.
BACKGROUND OF THE INVENTION
[0003] The use of ionized gases (plasma) for treating, modifying
and etching of material surfaces is well established. Both
vacuum-based plasmas and those that operate at or near atmospheric
pressure, have been used for surface modification of materials
ranging from plastic wrap to non-woven materials and textiles, the
plasma being used to provide an abundant source of active chemical
species, which are formed inside the plasma, from the interaction
between resident electrons in the plasma and neutral or other gas
phase components of the plasma. Typically, the active species
responsible for surface treatment processes have such short
lifetimes that the substrate must be placed inside the plasma
("in-situ" processing). Thus, the substrate and at least one stable
"precursor" gas are present together inside a process chamber in
contact with the plasma ranging in excitation frequencies from DC
to microwave frequencies so that the short-lived active chemical
species generated by the plasma are able to react with the
substrate before decay mechanisms, such as recombination,
neutralization or radiative emission can de-activate or inhibit the
intended surface treatment reactions.
[0004] In addition to vacuum-based plasmas, there are a variety of
plasmas that operate at or near atmospheric pressure. Included are
dielectric barrier discharges (DBDs), which have a dielectric film
or cover placed on one or both of the powered and ground electrodes
(which may be planar or annular in design); corona discharges,
which typically involve a wire or sharply-pointed electrode;
micro-hollow discharges, which consist of a series of
closely-packed hollow tubes that form either the rf or ground
electrode and is used with a counter electrode to generate a
plasma; a "flow-through" design, which consists of parallel-placed
screen electrode and in which a plasma is generated by the passage
of gas through the two or more screen electrodes; plasma jets in
which a high gas fraction of helium is used along with electrical
power in the 2 MHz-100 MHz range and a close electrode gap to form
an arc-free, non-thermal plasma; and a plasma "torch", which uses
an arc intentionally formed between two interposed electrodes to
generate extremely high temperatures for applications such as
sintering, ceramic formation and incineration.
[0005] The use of atmospheric pressure gases for generating a
plasma provides a greatly simplified means for treating large or
high volume substrates, such as plastics, textiles, non-wovens,
carpet, and other large flexible or inflexible objects, such as
aircraft wings and fuselage, ships, flooring, commercial
structures. Treatment of these substrates using vacuum-based
plasmas would be complicated and prohibitively expensive. The
present state of the art for plasmas operating at or near
atmospheric pressure also limits the use of plasma for treatment of
these commercially-important substrates.
[0006] Of the various atmospheric pressure plasmas, the Dielectric
Barrier Discharge (DBD) is the most widely used, and is
characterized by the use of a dielectric film or cover on one or
both of the electrodes to prevent formation of a persistent arc
that would otherwise form between the electrodes. Accumulating
charge on the surface of the dielectric as an arc forms, quenches
the arc, which typically reforms elsewhere on the electrode. The
substrate itself may function as the dielectric cover, provided
that it fully covers the exposed electrode. In some situations, a
high gas fraction (>50%) of helium is added to the process gas
to help homogenize the discharge. DBDs have the advantage of having
a large gap between the electrodes, so that a thick substrate can
readily be placed on one of the electrodes. However, since
electrical power must be transmitted through the dielectric cover,
the power density that a DBD discharge can achieve is limited. Low
power density typically produces slow processing, because low-power
density in the plasma also results in a slow generation rate of the
active, chemical species responsible for materials processing. The
dielectric cover on the electrode also inhibits heat removal since
most electrical insulators also function as thermal insulators.
Because of this, the gas temperature inside a DBD can often reach
temperatures as high as 100.degree. C.-200.degree. C. during
prolonged plasma operation.
[0007] The atmospheric-pressure plasma jet (APPJ) uses a process
gas mixture consisting of >95% helium, electrical energy between
1 MHz and 100 MHz and a narrow gap between two conducting
electrodes to achieve a stable, non-arcing plasma. Electrodes may
be planar and parallel, or annular in design, but must have a
uniform gap between the rf and ground electrodes. The use of helium
gas mixtures with an electrode gap in the range of between 0.5 mm
and 2.5 mm has been found to assist in the prevention of arcing
when appropriate high frequencies are used to power the electrodes.
Gas flow may be either along the longitudinal axis of the
electrodes for the annular design, or may be along the planar axis
for the parallel plate design. The advantages of this design over
other atmospheric pressure discharges are the ability to generate a
large-area discharge having high-power density suitable for fast
processing, and the ability to efficiently cool the neutral gas
temperature since dielectric coverings are not required, and since
the use of solid metal electrodes permits internal water cooling to
efficiently remove heat from the gases in the plasma.
[0008] An annular APPJ discharge apparatus where the gas is flowed
between the rf and ground electrodes through a series of
perforations in one of the uncooled electrodes has been used in a
cleaning process.
[0009] A flow-through electrode design using a gas flow consisting
predominantly of helium feed gas that flows through two metal
screens that function as electrodes, one rf-powered and the other
grounded has been reported. The discharge is created in the gap
between the parallel, screen electrodes, which generally have the
same spacing as the inter-electrode gap of the APPJ discharge. High
gas flow rates through a large open area are required since the
active chemical species must transit the distance between the point
of creation in the plasma and the substrate which may be located
several millimeters from the closest perforated electrode. Further,
the metal screens cannot be water-cooled, leading to a high,
neutral gas temperature (>150.degree. C.), especially if high rf
power is used since heat removal is limited to conduction at the
point of contact with the housing, and from the heat capacity of
the gas as it exits the plasma.
SUMMARY OF THE INVENTION
[0010] Accordingly, it is an object of embodiments of the present
invention to provide an atmospheric-pressure plasma processing
apparatus effective for producing a large area,
temperature-controlled, stable plasma discharge, wherein active
species generated in the plasma exit the discharge and impinge
unimpeded on a material to be processed disposed outside of the
discharge, but in close proximity thereto.
[0011] Another object of embodiments of the present invention is to
provide an atmospheric-pressure plasma generating apparatus for
producing active chemical species, wherein the plasma electrodes
are cooled, thereby producing a neutral gas temperature that can be
controlled.
[0012] Yet another object of embodiments of the present invention
is to provide an atmospheric-pressure plasma generating apparatus
effective for producing active chemical species, including gas
metastables, ionic species and active physical components.
[0013] 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.
[0014] To achieve the foregoing and other objects, and in
accordance with the purposes of the present invention, as embodied
and broadly described herein, the atmospheric-pressure plasma
processing apparatus hereof includes: at least one first
electrically conducting electrode having a chosen height and at
least one first elongated planar surface having a chosen length; at
least one grounded second electrically conducting electrode having
at least one second elongated planar surface parallel to and
coextensive with the first planar surface, and spaced-apart a
chosen distance therefrom, forming thereby at least one plasma
region; an RF power supply in electrical connection with the at
least one first electrode; a source of coolant having a chosen
temperature for cooling the at least one first electrode and the at
least one second electrode; a source of plasma gas; and a gas
manifold for flowing plasma gas through the at least one plasma
region and exiting the at least one plasma region through a long
dimension of the at least one first planar surface and a
corresponding long dimension of the at least one second planar
surface; whereby a plasma is formed in the at least one plasma
region.
[0015] In another aspect of the present invention and in accordance
with its objects and purposes, the apparatus for
atmospheric-pressure plasma processing of a material hereof
includes: at least one first electrically conducting electrode
having a chosen height and at least one first elongated planar
surface having a chosen length; at least one grounded second
electrically conducting electrode having at least one second
elongated planar surface parallel to and coextensive with the first
planar surface, and spaced-apart a first chosen distance therefrom,
forming thereby at least one plasma region; an RF power supply in
electrical connection with the at least one first electrode; a
source of coolant having a chosen temperature for cooling the at
least one first electrode and the at least one second electrode; a
source of plasma gas; a gas manifold for flowing plasma gas through
the at least one plasma region and exiting the at least one plasma
region through a long dimension of the at least one first planar
surface and a corresponding long dimension of the at least one
second planar surface; whereby a plasma is formed in the at least
one plasma region; and means for moving the material perpendicular
to the long dimension of the at least one first planar surface and
the at least one second planar surface at a second chosen distance
therefrom, and perpendicular to the flow of the plasma gas out of
the plasma region.
[0016] In still another aspect of the invention and in accordance
with its objects and purposes, the method for atmospheric-pressure
plasma processing hereof includes: flowing a plasma gas between a
region defined by at least one first electrically conducting
electrode having a chosen height and having at least one first
elongated planar surface having a chosen length, and at least one
grounded second electrically conducting electrode having at least
one second elongated planar surface parallel to and coextensive
with the first planar surface, and spaced-apart a chosen distance
therefrom, whereby the plasma gas exits the region through a long
dimension of the at least one first planar surface and a
corresponding long dimension of the at least one second planar
surface; applying RF power to the at least one first electrode from
an RF power source, whereby at least one plasma is formed; and
cooling the at least one first electrode and the at least one
second electrode to a chosen temperature.
[0017] In yet another aspect of the invention and in accordance
with its objects and purposes, the method for atmospheric-pressure
plasma discharge processing of a material hereof includes: flowing
a plasma gas between a region defined by at least one electrically
conducting first electrode having at least one first elongated
planar surface, and at least one grounded second electrically
conducting electrode having at least one second elongated planar
surface parallel to and coextensive with the first planar surface,
and spaced-apart a first chosen distance therefrom, whereby the
plasma gas exits the region through a long dimension of the at
least one first planar surface and a corresponding long dimension
of the at least one second planar surface; applying RF power to the
at least one first electrode from an RF power source, whereby at
least one plasma is formed; cooling the at least one first
electrode and the at least one second electrode to a chosen
temperature; and moving the material perpendicular to the long
dimension of the at least one first planar surface and the at least
one second planar surface at a second chosen distance therefrom,
and perpendicular to the flow of the plasma gas out of the plasma
region.
[0018] Benefits and advantages of the present invention include,
but are not limited to, faster plasma processing speed resulting
from an unobstructed path and a minimum distance for the excited
species formed in the plasma to the material being processed. The
lower neutral gas temperature, and the location of the material to
be processed away from the electrical influence of the plasma,
permit heat-sensitive substrates to be treated and heat-sensitive
processes to be employed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] 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:
[0020] FIG. 1 is a schematic representation of a side view of a
PRIOR ART plasma processing apparatus illustrating an RF electrode
having liquid cooling channels, and at least one gas inlet tube
port having a recessed gas distribution tube, a plurality of
tubular grounded electrodes adapted for liquid cooling around which
the plasma generated between the RF electrode and the tubular
grounded electrodes passes, and the material to be processed
disposed outside of the plasma near the grounded electrodes.
[0021] FIG. 2 is a schematic representation of a side view of a
generalized embodiment of the apparatus of the present invention
illustrating a plurality of rectangular plasma volumes formed
between alternating RF powered and grounded parallel opposing
planar electrode surfaces supplied with plasma gas, the plasma
regions being oriented perpendicular to the material to be
processed such that excited species therefrom impinge on the
material to be processed without obstruction, and at a chosen
distance which may be minimized.
[0022] FIG. 3 is a schematic representation of a more detailed side
view of the embodiment of the present invention shown in FIG. 2
hereof illustrating modular gas injection blocks having nozzle gas
exits for controlling delivery of the plasma gas through the plasma
volume between the electrodes, stanchions for supporting the
water-cooled electrodes, and a source of RF power for the powered
electrodes.
[0023] FIG. 4 is a schematic representation of a perspective view
of an embodiment of the gas block of FIG. 3, hereof, illustrating a
porous tube through which flow is established through the ends
thereof, the gas uniformly exiting the tube along its length before
passing through a nozzle gas exit extending the length of the gas
block to achieve uniform gas flow.
[0024] FIG. 5 is a schematic representation of a side view of the
gas block illustrated in FIG. 4, hereof.
[0025] FIG. 6 is a schematic representation of a perspective view
of either the water-cooled ground electrode or the water-cooled RF
powered electrode shown in FIG. 3, hereof, illustrating the water
cooling tubes and representative supporting stanchions.
[0026] FIG. 7 is a graph of the effectiveness of plasma processing
of polyester poplin fabric as a function of the process gas dose in
liters per square yard of fabric.
[0027] FIG. 8 is a graph of the power required in Watts for
achieving 100 on the AATCC.TM. 22 Spray test as a function of
electrode height, for 0.25 in, 0.5 in, and 1 in.
DETAILED DESCRIPTION OF THE INVENTION
[0028] FIG. 1 is a schematic representation of a side view of a
PRIOR ART (U.S. Patent Application Publication No. 2009/0200948,
published Aug. 13, 2009) plasma processing apparatus illustrating
plasma processing apparatus, 10, showing RF electrode, 12, having
liquid cooling ducts, 14a, 14b, powered by RF power supply and RF
matching network, 16, in electrical connection with electrode 12,
whereby first chosen spacing, 20, between RF electrode 12 and
planar ground electrode, 22, constructed using parallel, grounded,
hollow circular or oval tubes, 24a-24d, having chosen diameter
(major diameter for oval tubes), 25, is maintained. Electrical
energy is supplied in a frequency range between about 1 MHz and
about 100 MHz, the RF matching network being used to adjust for a
load deviation from 50 Ohms in the apparatus. Chiller, 26, supplies
liquid coolant to cooling ducts 14a, 14b and to hollow tubes
24a-24d, adapted for liquid cooling. Material to be processed, 28,
is disposed outside of the plasma in the proximity of ground
electrode 22, and maintained spaced-apart therefrom at second
chosen spacing, 30. Material 28 may be moved during processing
using an appropriate moving apparatus, 32. At least one gas inlet
tube, 34, supplied by gas supply and manifold, 36, provides the
appropriate gas mixture to at least one gas distribution tube, 38,
there being at least one gas inlet tube 34 for each gas
distribution tube 38, to maintain approximately constant gas
pressure across gas distribution tube 38. Gas distribution tube 38
has holes spaced apart along the length thereof and facing grounded
electrode 22, such that gas emerges through tapered channel, 40
opening out of bottom surface, 41, of RF electrode 12. Tapered
channel 40 holds gas distribution tube 38a-38c firmly in place, and
recessed from surface 41. Radiofrequency electrode 12 is shown to
be divided into two portions, 12a and 12b, such that the channels
14a, 14b and 40 may be readily machined and gas distribution tube
38 may be installed, and for cleaning and maintenance as needed
during operation of discharge apparatus 10. The direction of gas
flow is through the opening between grounded tubes 24a-24d. Flowing
gas is employed in the plasma generation process and to carry
active components produced in the plasma discharge between the RF
and ground electrodes in spacing 20 out of the plasma through the
narrow spaces, 44a-44d, between tubes, 24a-24d, of grounded
electrode 22, and onto workpiece 28.
[0029] Modeling and plasma observations of the PRIOR ART invention
by the present inventors has shown that the densest plasma is
formed between surface 41 of rf electrode 12 and surfaces 48a-48d
of grounded tubes 24a-24d of electrode 22, that the gas flow is
impeded and must traverse the diameter of the grounded tubes, and
therefore only a small number of activated species reach substrate
28. The diameters of the circular tubes 24a-24d were reduced, and
the spacing therebetween was increased by the present inventors,
with the result that the flux of activated species increased (from
the observation that the substrate processing speed increased).
Increasing the flux of activated species by increasing the plasma
density to increase the number of active species, improving the
flow of the active species by eliminating ground tube electrodes
24a-24d as physical obstacles, and bringing substrate 28 closer to
the plasma sources, such that a greater number of active species
may reach the substrate unimpeded before they decay and become
inactive, was expected by the present inventors to improve the
apparatus processing speed.
[0030] Briefly an embodiment of the present plasma processing
apparatus includes at least one first electrode having at least one
first elongated planar surface; at least one grounded second
electrode having at least one second elongated planar surface
parallel to and coextensive with the first planar surface, and
spaced-apart a chosen distance therefrom, forming thereby at least
one plasma region; an RF power supply (frequencies between about
100 kHz and 100 MHz are effective); an RF matching circuit for
coupling the RF from the RF power supply to the at least one first
electrode; a source of coolant having a chosen temperature for
cooling the first electrode and the second electrode; a source of
plasma gas; a gas manifold for flowing plasma gas through the at
least one plasma region and exiting the at least one plasma region
perpendicular to an elongated dimension of the at least one first
planar surface and the at least one second planar surface; whereby
an atmospheric-pressure plasma is formed in the at least one plasma
region. The material to be processed may be disposed at a chosen
distance, which can be minimized, from the exit of the plasma gas
from the at least one plasma region and moved perpendicular to the
gas flow and perpendicular to the elongated electrode
dimensions.
[0031] The plasma processing apparatus operates at
atmospheric-pressure and produces a large area, non-thermal, stable
discharge at power densities between about 5 W/cm.sup.3 and
approximately 50 W/cm.sup.3, more specifically, up to about 25
W/cm.sup.3 with a helium/nitrogen mixture, and up to about 50
W/cm.sup.3 with a helium/oxygen mixture, with a neutral gas
temperature that can be controlled using a cooling system for the
electrodes. It should be noted that use of dielectric coatings on
the plasma electrodes would significantly reduce this power
density, and that severe arcing is controlled by the electronics.
Moreover, arcing does not damage the fabric or other substrate
being treated since these materials are outside of the plasma
discharge. Therefore, dielectric coatings are not needed for the
embodiments of the present invention. Typically, the chosen gas
temperature entering the plasma regions may be about 20.degree. C.,
while the neutral gas temperature exiting the plasma regions may be
<70.degree. C. In what follows, the term "atmospheric pressure"
means pressures between about 500 Torr and about 1000 Torr. The
active chemical species or active physical species of the plasma
exit the plasma discharge before impinging on a substrate disposed
outside of the discharge, thereby permitting substrate surface
processing, without simultaneous exposure of the substrate to the
electric fields between the electrodes. The high power densities,
minimum distances between the plasma sources and the substrates,
the lower operating plasma temperatures, and the placement of the
material to be processed exterior to the plasma, permit accelerated
processing rates, and treatment of most substrates.
[0032] The present plasma source may be used for polymerization
(either free radical-induced or through dehydrogenation-based
polymerization), surface cleaning and modification, etching,
adhesion promotion, and sterilization, as examples. As will be
discussed in more detail, hereinbelow, the addition of small
amounts of N.sub.2 or O.sub.2, or other gases, or mixtures thereof
to a noble gas, such as helium, as an example, or a mixture of
noble gases, depending on the substrate and the desired coating
chemistry, assist in the creation of longer lived, active species
in the plasma that may be used for surface activation of materials
or polymerization of monomers located externally to the plasma.
Active chemical or physical species exiting the plasma impact the
substrate before these species, which are generated in the plasma,
are deactivated by collisions, thereby generating chemical and/or
physical changes to the workpiece without exposure of the workpiece
to the electrical field between the electrodes.
[0033] Reference will now be made in detail to the present
embodiments of the invention, examples of which are illustrated in
the accompanying drawings. In the Figures, similar structure will
be identified using identical reference characters. It will be
understood that the FIGURES are presented for the purpose of
describing particular embodiments of the invention and are not
intended to limit the invention thereto. Turning now to FIG. 2, a
schematic representation of a side view of a generalized embodiment
of the apparatus of the present invention is shown, illustrating a
plurality of rectangular plasma volumes or regions, 50a-50d, formed
between alternating RF powered 52a-52d and grounded parallel
opposing planar electrode surfaces, 54a-54d, respectively, supplied
with plasma gas from source and manifold 36, directed into gas
inlet tubes 34a-34d, and then into gas distribution tubes 38a-38d,
as will be discussed in more detail hereinbelow. Plasma regions
50a-50d are oriented perpendicular to material to be processed 28,
and the material may be brought arbitrarily close to the excited
plasma gases exiting the discharge region of the electrodes, 55,
(which may be between 0 mm and about 5 mm). Thus, excited species
impinge unimpeded on the material from arbitrarily close ranges.
Radiofrequency electrodes 12a and 12b, powered by RF source 16,
which may include impedance matching circuitry, and grounded
electrodes 22a-22c have elongated dimensions perpendicular to the
illustrated side view. As will also be described in more detail
hereinbelow, a chosen number of plasma regions 50a-50d may be
included in a module, the plasma regions either being identical or
differing in gas composition, flow rate or applied RF power density
(with appropriate RF power matching, as needed, because of
different discharge impedances) determined by their desired
function.
[0034] FIG. 3 is a schematic representation of a more detailed side
view of the modular five electrode embodiment of the present
invention shown in FIG. 2 hereof illustrating modular gas injection
blocks, 56a-56d, four shown for a five electrode module, having
elongated gas exit nozzles, 58a-58d, fed by gas inlet channels,
60a-60d, (in place of the gas inlet tubes 38a-38d of FIG. 2,
hereof) for delivery of the plasma gas through plasma volumes
50a-50d between RF electrodes 12a and 12b, and ground electrodes
22a-22c, held by stanchions, 62a, 62b, and 62c-62e, respectively,
for supporting the electrodes cooled by fluid cooler 26, and RF
power source 16, which may include RF matching circuitry, for
providing RF energy to the powered electrodes. Other methods for
cooling the electrodes may be anticipated. The electrode and gas
injection block module may be housed and supported in a plastic
block fabricated from thermoplastics such as polyetherimide (Ultem)
or polyetherketone (PEEK), as examples. Gas exits gas nozzles
58a-58d (which may be between about 0.2 mm and approximately 4 mm
in width, with about 1.6 mm being used in the EXAMPLES hereinbelow)
of gas injection blocks 56a-56d into the plasma regions 50a-50d.
Electrode lengths, widths, gap spacings, and the number of
electrodes are chosen depending on the material to be treated. For
example, a typical textile prototype apparatus for testing the
AATCC TM 22 water repellency of samples would be three,
1/4''-square.times.10''-long electrodes with 1.8 mm spacing between
the electrodes, and two plasma regions. An example of an apparatus
for industrial-scale textile fabric treatment might have five,
1/4''-square.times.72''-long electrodes with 2 mm spacing between
electrodes, and four plasma regions.
[0035] Typical electrode spacings formed between alternating RF
powered 52a-52d of FIG. 2 and grounded parallel opposing planar
electrode surfaces, 54a-54d, respectively, may be between about 0.2
mm and approximately 4.0 mm, more typically between about 1.5 and
about 2.5 mm. Electrodes may be fabricated from hollow, square or
rectangular stainless steel, aluminum, copper, or brass tubing, or
other metallic conductors, to permit water cooling. The edges of
all electrodes about 1'' from the ends thereof were chamfered in
order to prevent arcing and edge effects. As will be described in
the EXAMPLES, one-half inch wide electrodes having heights between
about 1/4 in. and about 1 in. were examined at 13.56 MHz. When only
two of the four plasma regions were powered, and the process gas
flow was adjusted to have the same total flow (Liters/min.) as was
used in the four plasma region module, the processing results were
identical (speed to achieve a 100 score on the AATCC.TM. 22 water
repellency spray test) to those for the four plasma region module.
Therefore, with the same total helium flow and one-half the RF
power, two plasma regions were found to generate the same
processing characteristics, thereby providing significant apparatus
cost reduction. Further, process throughput has been found to be
limited by the available helium flow rate, as will be illustrated
in the EXAMPLES hereinbelow. The apparatus of FIG. 3, with two
operating plasma regions was found to provide an approximately
18-fold improvement in processing speed per applied power (YPM/kW)
over that for FIG. 1 (Prior Art) for the same substrate chemistry.
This improvement results from: (1) the reduced RF power requirement
due to the smaller plasma volume as a consequence of the smaller
electrode height; (2) the reduced RF power requirement due to the
smaller number of plasma slots, while keeping the total plasma gas
flow constant; and (3) the fact that the distance from the plasma
discharge to the substrate may be made arbitrarily small.
[0036] FIG. 4 is a schematic representation of a perspective view
of an embodiment of gas injection block 56 of FIG. 3, hereof,
illustrating porous tube, 64, through which flow is established
through gas inlet tubes, 34i, and, 34ii, and end blocks, 66a, and,
66b, from gas source 36, the gas uniformly exiting tube 64 along
its length before passing through gas exit nozzle 58. Gas injection
block 56 is shown split into two parts, 56i, and, 56ii, for ease of
assembly, with appropriate gas sealing and mechanical assembly
components shown in exploded view. Parts 56i and 56ii form nozzle
58, when assembled. Porous tube 64 may include Teflon, ceramic and
metal tubes. Commercially available Teflon tubes having porosity
between about 18% and approximately 73% have been used in
embodiments of the present invention. Selection of tube porosity
over this range may be made to provide a desired gas delivery
uniformity for a given gas flow/pressure. Lower porosity tubes have
higher back pressures, and tend to be more uniform; however, they
allow less plasma gas flow, and consequently limit substrate
processing speed.
[0037] As stated hereinabove, typical plasma gases may include
helium or other noble gases or mixtures thereof, and small amounts
of additives such as nitrogen or oxygen, as examples. The substrate
may be treated with a chosen composition, which may react in the
presence of the species exiting the plasma and, as will be
discussed hereinbelow, a monomeric species may be polymerized and
caused to adhere to the substrate by such species.
[0038] FIG. 5 is a schematic representation of a side view of the
gas block illustrated in FIG. 4, hereof. Gas injection block 56 has
been shown to prevent "parasitic plasma" formation above gas
injection nozzle 58.
[0039] FIG. 6 is a schematic representation of a perspective view
of either the water-cooled ground electrode 22 or the water-cooled
RF powered electrode 12 shown in FIG. 3, hereof, illustrating water
(or other coolant) inlet tube, 68, supplied with coolant from
cooler 26, outlet cooling tube, 70, which may be returned to cooler
26 or discarded, and representative supporting stanchions,
62i-62iii.
[0040] Having generally described the invention, the following
EXAMPLES provides additional details:
EXAMPLE 1
[0041] In the following EXAMPLE 1, use of the embodiment of the
present invention illustrated in FIG. 3, hereof, with two powered
plasma regions (13.56 MHz), for processing fabrics is described.
Clearly, many substrates may be treated by embodiments of the
present apparatus. A monomer, which is polymerized by the action of
free radicals, such as an acrylate, is applied to the fabric by
spraying, as an example. The monomer may have various functional
groups suitable for imparting desired properties to the fabric
including repellency, wicking, antimicrobial activity, flame
retardancy, as examples. After application to the fabric, the
treated portion is moved into the vicinity of plasma regions
50a-50d such that excited species therefrom impinge thereon. The
monomer is cured as the treated fabric is exposed to the plasma
products, forming thereby a polymeric material which adheres to the
fabrics. As an example, the hydrocarbon portions of polymerized
2-(Perfluorohexyl) ethyl acrylate (commonly referred to as C6) bond
to each other and to the fabric, while the fluorinated chains face
away from the fabric and repel water and oil.
[0042] When C6 coated polypoplin (polyester) fabric was treated
using the apparatus of FIG. 1 (Prior Art), the addition of nitrogen
showed no improvement to polymerization. With the apparatus of FIG.
3, however, when small amounts of nitrogen (0.1%-1.5% by volume)
were added to helium plasma gas, an improvement in water, alcohol
and oil repellency was observed. Nitrogen is inexpensive, does not
require special handling and exhaust procedures. In contrast to the
prior art, the plasma stability and process results are also
unaffected by humidity when the present apparatus is employed.
Similarly, when oxygen was added to the helium plasma gas
(0.25%-0.31% by volume), surface changes on the fabric were
observed, whereas no such changes were observed when oxygen was
added to the apparatus of FIG. 1. The oxygen additive rendered the
polypoplin more hydrophilic as determined by wicking and contact
angle testing.
[0043] FIG. 7 is a graph of the effectiveness of plasma processing
of polypoplin fabric as a function of the plasma gas dose (helium
plus a small amount of nitrogen) in liters per square yard of
fabric. Approximately 90 L of plasma process gas per square yard of
fabric yielded a 100 spray test result. The spacing between the
fabric and the electrodes was 2 mm.
EXAMPLE 2
[0044] When the plasma gas is exposed to sufficient electric field
from the electrode, active species generation occurs. Electrode
heights investigated range from 1'' to 1/4''. The thinner
electrodes have smaller plasma volume, and hence require less RF
power to maintain the plasma at a constant power density;
therefore, RF power can be saved and smaller power generators can
be used. Since the process results remain the same, there is an
observed improvement in the YPM/kW metric. FIG. 8 is a graph of the
power required in Watts at 13.56 MHz for achieving a score of 100
using the AATCC TM 22 water repellency spray test for undyed
polyester poplin as a function of electrode height, for 0.25 in,
0.5 in, and 1 in. The fabric speed was 5 Yd/min., the process gas
dose was 90L/Yd.sup.2 of He/N.sub.2 gas blend, the power density
was 16 W/cm.sup.3 in each of 4 plasma regions (5 electrodes), the
tube porosity was 53%, the electrode length was 12'', the electrode
gap was 2 mm, and the electrodes to fabric spacing was 0.5 mm. It
is anticipated by the present inventors that smaller electrode
heights, for example, 1/8'', will provide further reduction in the
RF power requirements for a given processing speed.
[0045] The foregoing description of the invention has been
presented for purposes of illustration and description and 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.
* * * * *