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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low pressure plasma generation systems
and, more particularly, to coil configurations for improving plasma
uniformity in a plasma generation system.
2. Description of the Related Technology
Ionized gas or "plasma" may be used during processing and fabrication of
semiconductor devices. Plasma is used to etch or remove material from
semiconductor integrated circuit wafers. Plasma may also be used to
deposit or sputter material onto integrated circuit wafers. Use of plasma
gases in the fabrication of integrated circuits is widespread in the
semiconductor manufacturing industry.
During fabrication, a semiconductor integrated circuit wafer may require
materials to be added or removed, or selectively etched through a mask. To
use plasma in the integrated circuit fabrication process, typically, a low
pressure process gas is introduced into a process vessel chamber
surrounding an integrated circuit wafer. The process vessel is used to
maintain the low pressures required for the plasma and to serve as a
structure for attachment of the energy source. The process gas is ionized
into a plasma by the energy source, either before or after entering the
chamber, and the plasma flows over the semiconductor integrated circuit
wafer.
Ideally, uniformly ionized plasma would flow over the entire surface of the
wafer. Any difference in the plasma ionization strength will cause uneven
reaction characteristics along the surface of the wafer. Uneven reaction
characteristics may cause problems when etching thin films associated with
semiconductor manufacturing. Some of the problems created are etch rate
non-uniformity across a substrate from edge to center, profile or line
width variation across the substrate, and semiconductor device damage.
Plasma may be created from a low pressure process gas by inducing an
electron flow which ionizes individual gas molecules through the transfer
of kinetic energy through individual electron-gas molecule collisions.
Various methods of inducing an electron flow in the process gas are well
known to those skilled in the art. Typically, electrons are accelerated in
an electric field such as one produced by radio frequency energy. Low
frequencies (below 550 KHz), high frequencies (13.56 MHz), or microwaves
(2.45 GHz).
Using microwave radio frequency energy to generate plasma has the advantage
of more readily transferring energy to the process gas rather than to
surrounding objects such as the walls of a process chamber or the
semiconductor wafer. Another way of generating a plasma is with an
electron cyclotron resonance (ECR) system. The ECR system requires a
controlled magnetic field to induce circular electron energy into the
process gas and not into the process chamber walls.
Other methods for improving the efficiency of plasma generation are
magnetically enhanced plasma generation systems and inductively coupled
electron acceleration, more commonly called inductively coupled plasma.
Magnetically enhanced plasma systems use a constant magnetic field
parallel to the integrated circuit wafer surface and a high frequency
electric field perpendicular to the wafer surface. The combined magnetic
and electric forces cause the electrons to follow a cycloidal path, thus,
increasing the distance the electrons travel relative to the more direct
straight path induced by the electric field alone. A major drawback in
using a magnetic field to increase the electron travel distances is the
requirement of a strong magnetic field which is both costly and difficult
to maintain.
In the inductively coupled plasma process, the electrons also follow an
extended circular path. Two techniques may be used to generate plasma by
inductive coupling, both of which use alternating current to transfer
energy to the gas by transformer coupling. The first technique utilizes a
ferrite magnetic core to enhance transformer coupling between primary and
secondary windings, and uses low frequencies, for example, below 550 KHz.
The second technique uses a solenoid coil surrounding the gas to be
ionized. This technique may use either low frequencies or high frequencies
in the range of 13.56 MHz. Neither of these techniques provides a uniform
plasma proximate and substantially parallel with the surface of an
integrated circuit wafer.
U.S. Pat. No. 4,948,458 describes a method and apparatus for obtaining a
more uniform and substantially parallel (planar) plasma for use during
processing of integrated circuit wafers. The invention disclosed in this
patent comprises an enclosure having an interior bounded at least in part
by a radio frequency transparent window. A planar coil is disposed
proximate to the window, and a radio frequency energy source is coupled
through an impedance matching circuit to the coil. The planar coil
radiates the radio frequency energy such that a planar magnetic field is
induced in the interior of the enclosure. This planar magnetic field
causes a circulating flow of electrons to be induced into the process gas.
The circulating flow allows the electrons to travel a path a much greater
distance before striking the enclosure. The circulating electrons flow is
substantially planar and has minimal kinetic energy in the non-planar
direction. The planar coil is substantially parallel with a support
surface. The support surface, therefore, is oriented substantially
parallel to the circulating electron flow and is adapted to hold a
semiconductor integrated circuit wafer during process fabrication. Thus,
the support surface holds the semiconductor wafer substantially parallel
to the electron flow.
The purpose of the invention disclosed in the above mentioned patent is to
generally limit the wafer treatment to only the chemical interaction of
the plasma species with the integrated circuit wafer. This is accomplished
by minimizing the kinetic velocity of the plasma flux in the non-planar
direction, thus reducing the kinetic impact on the wafer.
Referring to FIGS. 1 and 2, isometric and cross-sectional views of the
prior art, respectively, are illustrated schematically. A plasma treatment
system 10, for etching individual semiconductor wafers W, includes an
enclosure 12 having an access port 14 formed in an upper wall 16. A radio
frequency transparent window 18 is disposed below the upper wall 16 and
extends across the access port 14. The window 18 is sealed to the wall 16
to define a vacuum-tight interior 19 of the enclosure 12.
A planar coil 20 is disposed within the access port 14 adjacent to the
window 18. Coil 20 is formed as a spiral having a center tap 22 and an
outer tap 24. The plane of the coil 20 is oriented parallel to both the
window 18 and a support surface 13 upon which the wafer W is mounted. In
this way, the coil 20 is able to produce a planar plasma within the
interior 19 of the enclosure 12 which is parallel to the wafer W.
Referring now to FIGS. 1-3, the planar coil 20 is driven by a radio
frequency (RF) generator 30. The output of the generator 30 is fed by a
coaxial cable 32 to a matching circuit 34. The matching circuit 34
includes a primary coil 36 and a secondary loop 38 which may be mutually
positioned to adjust the effective coupling of the circuit and allow for
loading of the circuit at the frequency of operation. Conveniently, the
primary coil 36 is mounted on a disk 40 which may be rotated about a
vertical axis 42 in order to adjust the coupling therebetween.
A variable capacitor 44 is also provided in series with the secondary loop
38 in order to adjust the circuit resonant frequency with the frequency
output of the RF generator 30. Impedance matching maximizes the efficiency
of power transfer to the planar coil 20. An additional capacitor 46 is
provided in the primary circuit in order to cancel part of the inductive
reactance of coil 36 in the circuit.
Referring now to FIGS. 2 and 4, process gas is introduced into the interior
19 of the enclosure 12 through a port 50 formed through the side of the
enclosure 12. The gas is introduced at a point which provides for
distribution throughout the interior 19.
The flat spiral coil 20 may consist of equally spaced turns. Referring to
FIG. 8, a graph representing test measurements of the current density
versus position relative to the center of an equally spaced planar coil is
illustrated. The graph of FIG. 8 illustrates maximum plasma density at or
near the center of the equally spaced planar coil. This is also described
in U.S. Pat. No. 4,948,458, column 6, lines 35 to 41.
Further tests, however, indicate that the equally spaced turns of the coil
create a non-uniformity in the plasma generated. This is so because the
side walls of the enclosure 12 cause more losses to the periphery of the
plasma than toward the center of the plasma. Referring to FIG. 9, the
current density versus position of an unmodified equally spaced spiral
planar coil and a modified planar coil having unequal spacing of the
turns, is illustrated. The current density 90 of the unmodified coil has a
lower current density at the outer periphery 92 and 94 than does the
modified coil current density 96. Thus, in contrast to the invention
claimed in U.S. Pat. No. 4,948,458, a more uniform plasma over the entire
surface of the semiconductor wafer requires not more but less RF power
near the center of a planar spiral wound coil.
SUMMARY OF THE INVENTION
The present invention utilizes new, novel and non-obvious coil
configurations for the purpose of enhancing the RF power delivered toward
the periphery of the circulating plasma and reducing the RF power
delivered toward the center of the plasma. By delivering inversely
gradiated RF power to the plasma stream, a more uniform plasma density
results in practice. The present invention more effectively compensates
for loss of plasma at the walls of the enclosure without creating plasma
"hot spots" toward the center of the plasma that lead to problems in
etching of thin films. These problems may include etch rate non-uniformity
across the wafer from edge to center, profile or line width variation
across the wafer from edge to center, and possible wafer surface damage.
The present invention utilizes a non-uniformly wound planar coil or coils
to create a current density in the plasma such that the plasma has
consistently even characteristics from the periphery to the center of the
surface of a semiconductor wafer. A number of embodiments may be utilized
to produce the required current density throughout the plasma.
Unevenly Spaced Spiral Coil
A non-uniformly spaced spiral coil having wider spacing between the turns
toward the center and closer spacing of the turns toward the outside
radius of the coil is illustrated in FIG. 10. The more closely spaced
turns toward the outside of the coil create a higher density radio
frequency field than the field produced by the wider spaced turns toward
the center of the coil. By careful selection of the number of turns of the
coil and the various spacings between the different turns, a current
density may be configured that results in a uniform plasma density across
the entire working surface of the enclosure. This is especially important
when processing the newer eight inch diameter semiconductor wafers, flat
panel displays and future larger diameter semiconductor wafers.
Radio frequency power from an RF source is coupled to a matching network by
means of coaxial cable. The matching network is used to insure maximum
transfer of RF power from the source into the coil. The transferred RF
power is radiated from the matched coil into the process gas flowing into
the work chamber where the gas becomes plasma.
An object of the present invention is to compensate for plasma energy loss
near the side walls of the enclosure by having more RF energy available
toward the circumference of the plasma than toward the center. This
results in the plasma having a uniform energy density throughout.
Doughnut Coil Configuration
A partially spiral coil in the form of a doughnut having coil turns
predominately toward the outside radius of the coil is illustrated in FIG.
11. The turns in doughnut coil configuration create a higher density radio
frequency field around the periphery of the coil. By careful selection of
the number of turns of the coil and the turn spacing, a current density
may be configured that results in a uniform plasma density across the
entire working surface of the enclosure.
In similar fashion to the non-uniform spiral coil described above, radio
frequency power from an RF source is coupled to a matching network by
means of coaxial cable. The matching network is used to insure maximum
transfer of RF power from the source into the coil. The transferred RF
power is radiated from the matched coil into the process gas flowing into
the work chamber where the gas becomes plasma.
Doughnut Coil Plus Separate Independently Powered Center Coil
A partially spiral doughnut shaped exterior coil and an interior coil
concentric to the exterior coil, both having separate RF sources, are
illustrated in FIG. 12. The exterior and interior coils allow a current
density pattern to be created that generates a uniform plasma field by
adjusting each of the respective power sources. The power of each power
source may be independently adjusted for best current density pattern.
Each RF source may be phase locked together so as to maintain the same
frequency. Phasing of the two RF sources may be adjusted over a 0-180
degree range for fine tuning of the resulting plasma density. Individual
matching networks are used to insure maximum transfer of power from the
respective RF power sources to respective coils.
A single RF power source may be utilized with the two coil embodiment of
the present invention. FIG. 12a illustrates a simplified schematic block
diagram of the single RF power source embodiment. The power source
connects to a RF power divider which supplies a portion of the RF power
source to each matching network. The power divider may be utilized to
balance the power distribution between the interior and exterior coils.
Phasing between coils may be varied over a 0-180 degree range by varying
the length of one of the coaxial cables between the RF power divider and
the respective matching networks.
Spiral Coil With Moveable Tap
A spiral coil having an adjustable tap connected to an RF power source is
illustrated in FIG. 13. The adjustment of the coil tap results in a radio
frequency field that results in a uniform plasma density across the entire
working surface of the enclosure. The RF power flows mainly between the
tap connection and ground. The ungrounded portion of the coil does not
radiate a significant amount of RF power but may produce a phase inversion
feedback that beneficially modifies the RF current density toward the
center of the tapped coil.
Radio frequency power from an RF source is coupled to a matching network by
means of coaxial cable. The matching network is used to insure maximum
transfer of RF power from the source into the coil. The transferred RF
power is radiated from the matched coil into the process gas flowing into
the work chamber where the gas becomes plasma.
An object of the present invention is to easily adjust a planar coil so as
to compensate for plasma energy loss near the side walls of the enclosure
by having more RF energy available toward the circumference of the plasma
than toward the center.
Additional Independently Powered coil Around Exterior Wall of Plasma
Chamber
As illustrated in FIG. 14, a spiral coil may be placed on top of the
chamber housing and a side coil may be placed around the side wall of the
chamber biased toward the top coil. Independent RF power sources may be
utilized for adjusting the amount of RF introduced into the chamber
interior for creation of the plasma. The two power sources may be phase
locked together to maintain the same frequency. Phase adjustment of 0-180
degrees may be made between the two RF sources by phase adjustment means
well known in the art of signal generators and transmission lines. One RF
power source, a power divider and coaxial phasing lines may also be
utilized as illustrated in FIG. 12a.
The side coil adds RF energy to the outer circumference of the plasma field
where there is the most plasma energy loss due to side wall absorption.
Matching networks are utilized for maximum power transfer from the RF
power sources which may be, for example, 50 ohms impedance. The matching
networks adjust the impedance of the respective coils to match the typical
50 ohm impedance of the coaxial cable used to connect the RF power source
to the matching network.
A third RF power source may be connected to the wafer support surface to
impart plasma energy in the tangential direction to the wafer surface. RF
frequencies in the high frequency region of 13.56 MHz, the microwave
region of 2.45 GHz, or the low frequency region below 540 KHz may be
utilized separately, or in combination, to produce a desired result during
processing of the integrated circuit wafer or other objects such as, for
example, flat screen display panels.
S-Shaped Coil
An S shaped coil is illustrated in FIG. 15. This S shaped coil more evenly
distributes the RF energy into the process gas than does a circular spiral
coil. The S shaped coil may be utilized at microwave (2.45 Ghz) or high
frequency (13.56 MHz). Adjustment of the spacing between the coils may be
used to adjust the RF radiation pattern into the process gas to create the
uniformly energized plasma. A matching network and RF source which
function as mentioned above are also illustrated.
Other and further objects, features and advantages will be apparent from
the following description of presently preferred embodiments of the
invention, given for the purpose of disclosure and taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of a prior art apparatus for producing a planar
plasma;
FIG. 2 is a cross-sectional view of the apparatus of FIG. 1;
FIG. 3 is a schematic view of the circuitry of the apparatus of FIG. 1;
FIG. 4 is a detailed view of a process gas introducing ring employed in the
apparatus of FIG. 1;
FIG. 5 is a partial elevational view illustrating the magnetic field
strength produced by the apparatus of FIGS. 1 and 2;
FIGS. 6 and 7 are schematic views of prior art apparatus for producing a
planar plasma;
FIG. 8 is a graphical representation of test data taken with the apparatus
of FIG. 1;
FIG. 9 is a graphical representation of test data taken with the apparatus
of FIG. 1 and a modified planar coil in accordance with the present
invention;
FIG. 10 is a schematic block diagram of a non-uniformly spaced spiral coil
embodiment of the present invention;
FIG. 11 is a schematic block diagram of a doughnut spiral coil embodiment
of the present invention;
FIG. 12 is a schematic block diagram of a doughnut spiral coil and
concentric spiral coil embodiment of the present invention having
independent RF power sources;
FIG. 12a is a schematic block diagram of a doughnut spiral coil and
concentric spiral coil embodiment of the present invention having a common
RF power source connected through an adjustable power divider and phasing
coaxial cables;
FIG. 13 is a schematic block diagram of a spiral coil embodiment of the
present invention having an adjustable tap and connected to an RF power
source;
FIG. 14 is a schematic elevational view of an embodiment of the present
invention illustrating top and side coils both having independent RF power
sources; and
FIG. 15 is a schematic block diagram illustrating an S shaped coil
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The development and characterization of uniform, large area, high density
plasma sources capable of clean and rapid processing of integrated circuit
substrates is crucial to the semiconductor industry. The present invention
is used in a radio frequency induction (RFI) plasma processing system. The
RFI system is used to economically produce a uniform planar plasma during
the process fabrication of modern integrated circuit semiconductor wafers.
Referring now to the drawings, the details of the preferred embodiments are
schematically illustrated. In the drawings, the letter C designates
generally an apparatus for enhancing the RF power delivered toward the
periphery of the circulating plasma and reducing the RF power delivered
toward the center of the plasma. Like elements are numbered the same, and
similar elements are represented by the same number and a different lower
case letter thereafter.
Non-Uniformly Spaced Spiral Coil
Referring now to FIG. 10, a non-uniformly spaced spiral coil embodiment of
the present invention is illustrated schematically. An RF generation
system having even distribution of RF energy for creating a planar gas
plasma is generally referenced by the letter C. The RF generation system C
comprises a non-uniformly spaced spiral coil 1002, an impedance matching
network 1004, coaxial cable feed line 1006 and an RF power source 1008.
Power source 1008 typically has an output power of from 100 to 2000 watts
into a 50 ohm load. Coaxial cable 1006 may have 50 ohm impedance and have
a power handling capacity at the frequency of use sufficient for the RF
source 1008. The cable 1006 feeds RF power from the source 1008 to the
matching network 1004. Network 1004 is used to obtain a proper match to
the coaxial cable 1006 and source 1008.
In addition, network 1004 may also tune coil 1002 to resonance at the
frequency of source 1008. A matched and tuned condition results in maximum
transfer of RF power from the source 1008 to the coil 1002. RF power is
radiated by the coil 1002 into the chamber 19 (FIG. 2) where the RF energy
causes the process gas to become a plasma.
The coil 1002 has a winding pitch which varies according to the distance
from the center such that the windings become more tightly wound further
away from the center. A center winding spacing distance 1010 is greater
than an outer circumference winding spacing distance 1012. Thus, the
winding spacing distances between the spiral coil 1002 turns starts widest
at the center 1014 and decreases toward the outer circumference 1016.
Ground return connections are illustrated by grounds 1018 and 1020.
The coil 1002 produces an RF energy field that is greater toward the outer
circumference 1016 than toward the center 1014. Having the RF energy field
biased toward the outer circumference of the coil 1002 compensates for the
greater plasma energy loses nearer the walls of the housing 12 (FIG. 1).
Referring now to FIGS. 8 and 9, current densities of the prior art coil 20
and the coil 1002 of the present invention are graphically illustrated.
The current density of the prior art coil 20 is greatest toward the center
region 802. Current densities 804 are illustrated on the vertical axis of
the graph for different RF power levels 806 to the coil 20. The variation
of the current density is plotted on the horizontal axis of the graph for
various distances 808 from one edge of the chamber to the other. Distance
802 is representative of the current density at the center of the coil 20.
The graph of FIG. 9 illustrates the difference in current densities between
the unmodified prior art coil 20 and the coil 1002 of the present
invention. The current density curve 906 of coil 1002 is greater and more
evenly distributed than the current density curve 900 of the prior art
coil 20. At end points 902 and 904, the curve 906 has a greater current
density than does curve 900. This increase in current density at the outer
periphery greatly helps in overcoming the plasma energy loses from the
proximate enclosure 12 walls.
Doughnut Coil Configuration
Referring now to FIG. 11, a partially spiral coil in the form of a doughnut
having coil turns predominately toward the outside radius of the coil is
schematically illustrated. The turns in doughnut coil 1102 create a higher
density radio frequency field around the periphery of the coil 1102. By
careful selection of the number of turns of the coil and the turn spacing
1104, a current density may be configured that results in a uniform plasma
density across the entire working surface of the enclosure 12 (FIG. 1).
In similar fashion to the non-uniform spiral coil 1002 described above,
radio frequency power from the RF source 1108 is coupled to a matching
network 1104 by means of coaxial cable 1106. The matching network 1104 is
used to insure maximum transfer of RF power from the source 1108 into the
coil 1102. The transferred RF power is radiated from the matched coil 1102
into the process gas flowing into the work chamber 19 (FIG. 2) where the
gas becomes plasma.
Doughnut Coil Plus Separate Independently Powered Center Coil
Referring to FIG. 12, a partially spiral doughnut shaped exterior coil and
an interior coil concentric to the exterior coil, both having separate RF
sources, are schematically illustrated. The exterior coil 1202 and
interior coil 1204 allow a current density pattern to be created that
generates a uniform plasma field by adjusting each of the respective RF
power sources 1206 and 1208. The power of each of the power sources 1206
and 1208 may be independently adjusted for best current density pattern.
RF sources 1206 and 1208 may be phase locked together so as to maintain
the same frequency. Phasing of the two RF sources 1206 and 1208 may be
adjusted over a 0-180 degree range for fine tuning of the resulting plasma
density. Individual matching networks 1210 and 1212 are used to insure
maximum transfer of power between the respective RF power sources 1206 and
1208, and the respective coils 1202 and 1204.
A single RF power source may be utilized with the two coil embodiment of
the present invention. Referring now to FIG. 12a, a simplified schematic
block diagram of the single RF power source embodiment is schematically
illustrated. The power source 1220 connects to an RF power divider 1222
which supplies a portion of the RF power to each matching network 1224 and
1226. The power divider 1222 may be utilized to balance the power
distribution between the interior and exterior coils 1204 and 1202,
respectively. Phasing between coils may be varied over a 0-180 degree
range by varying the length of one of the coaxial cables 1228 or 1230
between the RF power divider 1222 and the respective matching networks
1224 and 1226.
Spiral Coil With Moveable Tap
Referring now to FIG. 13, a spiral coil having an adjustable tap connected
to an RF power source is schematically illustrated. A spiral coil 1302 has
an adjustable tap 1304 biased toward the center 1306 of the coil 1302. The
adjustment of the coil tap 1304 creates a radio frequency field that
results in a uniform plasma density across the entire working surface of
the enclosure 12 (FIG. 1). RF power from the RF source 1308 flows mainly
between the tap 1304 connection and ground 1310. The ungrounded portion
1312 of the coil 1302 does not radiate a significant amount of RF power
but may produce a phase inversion feedback that beneficially modifies the
RF current density toward the center 1306 of the tapped coil 1302.
Radio frequency power from the RF source 1308 is connected to a matching
network 1314 by means of a coaxial cable 1316. The matching network 1314
is used to insure maximum transfer of RF power from the source 1308 into
the coil 1302. The transferred RF power is radiated from the matched coil
1302 into the process gas flowing into the work chamber 19 (FIG. 2) where
the gas becomes plasma.
Additional Independently powered Coil Around Exterior Wall of Plasma
Chamber
Referring to FIG. 14, a preferred embodiment of the present invention is
illustrated in schematic elevational view. A spiral coil 1402 may be
placed on top of the chamber housing 16, and a side coil 1404 may be
placed around the side wall of the chamber 12 biased toward the top coil
1402. Independent RF power sources 1406, 1408 and 1410 may be utilized for
adjusting the amount of RF introduced into the chamber 12 interior 19 for
creation of the plasma. The two power sources 1406 and 1408 may be phase
locked together to maintain the same frequency. Phase adjustment of 0-180
degrees may be made between the two RF sources 1406 and 1408 by phase
adjustment means well known in the art of signal generators and
transmission lines. One RF power source, a power divider and coaxial
phasing lines may also be utilized as illustrated in FIG. 12a.
The side coil 1404 adds RF energy to the outer circumference of the plasma
field where there is the most plasma energy loss due to the enclosure 12
side wall absorption. Matching networks 1416, 1418 and 1420 are utilized
for maximum power transfer from the RF power sources 1406, 1408 and 1410,
respectively, which may be, for example, 50 ohms impedance. The matching
networks 1416, 1418 and 1420 adjust the impedance of the coils 1402, 1404
and work surface 1422, respectively, to match the typical 50 ohm impedance
of the coaxial cables 1426, 1428 and 1430, respectively, which are used to
connect the RF power sources 1406, 1408 and 1410, respectively, to the
matching networks 1416, 1418 and 1420, respectively.
The RF power source 1410 may be connected to the wafer support 1422 surface
to impart plasma energy in a tangential direction to the surface of the
wafer W. RF frequencies in the high frequency region of 13.56 MHz, the
microwave region of 2.45 GHz, or the low frequency region below 540 KHz,
may be utilized separately or in combination to produce a desired result
during processing of the integrated circuit wafer W or other objects such
as, for example, flat screen display panels (not illustrated).
S-Shaped Coil
Referring now to FIG. 15, an S shaped coil is schematically illustrated.
The S shaped coil 1502 more evenly distributes the RF energy into the
process gas than does the circular spiral coil 20. The S shaped coil 1502
may be utilized at microwave (2.45 Ghz) or high frequency (13.56 MHz).
Adjustment of the spacing 1504 between the turns of the coil 1502 may be
used to adjust the RF radiation pattern into the process gas to create a
uniformly energized plasma. A matching network 1510 and RF source 1506 are
also illustrated and function as mentioned above.
The present invention, therefore, is well adapted to carry out the objects
and attain the ends and advantages mentioned, as well as others inherent
therein. While a presently preferred embodiment of the invention has been
given for purposes of disclosure, numerous changes in the details of
construction, interconnection and arrangement of parts will readily
suggest themselves to those skilled in the art and which are encompassed
within the spirit of the invention and the scope of the appended claims.
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