U.S. patent application number 11/616324 was filed with the patent office on 2008-07-03 for plasma generator apparatus.
This patent application is currently assigned to Novellus Systems, Inc.. Invention is credited to David Cheung, Vincent Decaux, James A. Fair, Anirban Guha, Peter Jagusch, John Keller.
Application Number | 20080156264 11/616324 |
Document ID | / |
Family ID | 39582147 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080156264 |
Kind Code |
A1 |
Fair; James A. ; et
al. |
July 3, 2008 |
Plasma Generator Apparatus
Abstract
Embodiments of a plasma generator apparatus for ashing a work
piece are provided. The apparatus includes a container adapted for
continuous gas flow there through from an inlet end to an outlet
end thereof. The container is fabricated of a dielectric material
and adapted for ionization therein of a portion of at least one
component of gas flowing therethrough. A gas flow distributor is
configured to direct gas flow to a region within the container and
a coil surrounds at least a portion of side walls of the container
adjacent the region of the container to which the gas flow
distributor directs gas flow. A radio frequency generator is
coupled to the coil.
Inventors: |
Fair; James A.; (Mountain
View, CA) ; Decaux; Vincent; (San Francisco, CA)
; Guha; Anirban; (Milpitas, CA) ; Cheung;
David; (Foster City, CA) ; Keller; John;
(Newburgh, NY) ; Jagusch; Peter; (Los Gatos,
CA) |
Correspondence
Address: |
INGRASSIA FISHER & LORENZ, P.C.
7010 E. COCHISE ROAD
SCOTTSDALE
AZ
85253
US
|
Assignee: |
Novellus Systems, Inc.
San Jose
CA
|
Family ID: |
39582147 |
Appl. No.: |
11/616324 |
Filed: |
December 27, 2006 |
Current U.S.
Class: |
118/723E |
Current CPC
Class: |
H01J 37/321 20130101;
H01J 37/32651 20130101; H01J 37/32467 20130101; H01J 2237/335
20130101; H01J 37/32458 20130101; H01J 37/32449 20130101; H01J
37/32633 20130101; H01J 37/3244 20130101 |
Class at
Publication: |
118/723.E |
International
Class: |
C23C 16/00 20060101
C23C016/00 |
Claims
1. A plasma generator apparatus for ashing a work piece, the
apparatus comprising: (a) a container adapted for continuous gas
flow there through from an inlet end to an outlet end thereof, the
container comprising a dielectric material and further adapted for
ionization of a portion of at least one component of gas flowing
therethrough; (b) a gas flow distributor configured to direct gas
flow to a region within the container; (c) a coil surrounding at
least a portion of side walls of the container, the portion of the
side walls adjacent the region of the container to which the gas
flow distributor directs gas flow; and (d) a radio frequency
generator coupled to the coil.
2. The apparatus of claim 1, further comprising a gas diffuser at
an outlet of the container.
3. The apparatus of claim 1, wherein the container is cylindrical
and the gas flow distributor comprises a circular baffle plate
axially centered in a cylindrical container to direct gas flow
towards side walls of the container.
4. The apparatus of claim 1, wherein the container comprises a
cylindrical lower portion capped with a domed or conical upper
portion.
5. The apparatus of claim 4, wherein the gas flow distributor
comprises a nozzle located proximate an apex of the domed or
conical upper portion, the nozzle directing gas flow along inner
walls of the domed or conical upper portion.
6. The apparatus of claim 5, further comprising a Faraday shield
interposed between the coil and the at least a portion of the side
walls of the container.
7. The apparatus of claim 5, wherein the nozzle comprises a
hemisphere with a plurality of through holes, the through holes
directing gas flowing therethrough to at least a portion of the
side walls surrounded by the coil.
8. The apparatus of claim 5, wherein the nozzle comprises a tube
extending axially into the container, the tube comprising a
plurality of through holes in a side wall thereof, the through
holes directing gas flow to the portion of the side walls of the
container surrounded by the coil.
9. The apparatus of claim 1, wherein the coil comprises a
symmetrical coil.
10. The apparatus of claim 9, wherein the symmetrical coil
comprises at least about 2 segments.
11. The apparatus of claim 1, wherein a largest dimension of the
container, transverse to gas flow in the container, approximates a
dimension of a surface of a work piece, the surface to be treated
by the apparatus.
12. An apparatus for ashing a work piece, the apparatus comprising:
(a) a container configured for flow there through from an inlet end
to an outlet end, the container comprising side walls adapted for
ionization of a portion of at least one component of the gas
flowing through the container; (b) a gas flow distributor adapted
to direct gas flow to a region within the container; (c) a
segmented coil surrounding at least a portion of side walls the
container, the portion adjacent the region of the container to
which the gas flow distributor directs gas flow, the segmented coil
free of any Faraday shield interposed between the segmented coil
and the portion of the side walls; (d) a radio frequency generator
coupled to the segmented coil; and (e) a diffusion plate at an exit
end of the container to direct gas flow onto a surface of a work
piece to be ashed.
13. The apparatus of claim 12, wherein the container is cylindrical
and the gas flow distributor comprises a circular baffle plate
axially centered in a cylindrical container to direct gas flow
towards side walls of the container.
14. The apparatus of claim 12, wherein the container comprises a
cylindrical lower portion capped with a domed or conical upper
portion.
15. The apparatus of claim 12, wherein the gas flow distributor
comprises a nozzle located proximate an apex of the domed or
conical upper portion, the gas flow distributor directing gas flow
along inner walls of the domed or conical upper portion to the
portion of the side walls of the container surrounded by the
segmented coil.
16. The apparatus of claim 14, wherein the container and the
dome-shaped or cone-shaped upper portion are comprised of
quartz.
17. The apparatus of claim 15, wherein the gas flow distributor
comprises a hemispherical nozzle with a plurality of through
holes.
18. The apparatus of claim 15 wherein the gas flow distributor
comprises a tube extending axially into the container, the tube
comprising a plurality of through holes in a side wall thereof, the
through holes directing gas flow along inner walls of the domed or
conical upper portion to the portion of the side walls of the
container surrounded by the segmented coil.
19. The apparatus of claim 12, wherein the segmented coil comprises
at least 2 segments.
20. A plasma generator apparatus for ashing a work piece, the
apparatus comprising: (a) a container of larger diameter configured
for continuous gas flow there through from an inlet end to an
outlet end, the container comprising side walls of a dielectric
material adapted for containing charged particles produced by radio
frequency discharge into a gas, the container having a domed-shaped
or cone-shaped upper portion; (b) a gas flow distributor proximate
the dome-shaped or cone-shaped upper portion of the container, the
gas flow distributor adapted to direct gas flow to a region within
the container; (c) a diffusion plate proximate an outlet end of the
container to direct gas flow from inside the container onto a
surface of a work piece to be treated; (d) a symmetrical coil
comprising at least 2 segments, the symmetrical coil surrounding at
least a portion of the side walls of the container, the portion of
the side walls adjacent the region within the container to which
the gas flow distributor directs gas flow; and (e) a radio
frequency generator coupled to the symmetrical coil.
Description
TECHNICAL FIELD
[0001] The present technology relates generally to apparatus used
in the fabrication of semiconductor devices, and more particularly,
the present technology relates to plasma generator apparatus for
generating plasma used in ashing and surface treatment
procedures.
BACKGROUND
[0002] In semiconductor manufacturing, plasma ashing is the process
of removing the photoresist from an etched wafer. Plasma in this
context is an ionized form of a gas. A gas ionizing apparatus, also
referred to as a plasma generator, produces a monatomic reactive
species of oxygen or another gas required for the ashing process.
Oxygen in its monatomic or single atom form, as O rather than
O.sub.2, is the most common reactive species. The reactive species
combines with the photoresist to form ash which is removed from the
work piece with a vacuum pump.
[0003] Typically, monatomic oxygen plasma is created by exposing
oxygen gas (O.sub.2) to a source of energy, such as a RF discharge.
At the same time, many charged species, i.e. ions and electrons,
are formed which could potentially damage the wafer. Newer, smaller
circuitry is increasingly susceptible to damage by charged
particles. Originally, plasma was generated in the process chamber,
but as the need to avoid charged particles has increased, some
machines now use a downstream plasma configuration, where plasma is
formed remotely and channeled to the wafer. This reduces damage to
the wafer surface.
[0004] Monatomic oxygen is electrically neutral and although it
does recombine during the channeling, it does so at a slower rate
than the positively or negatively charged particles, which attract
one another. Effectively, this means that when substantially all of
the charged particles have been neutralized, the reactive neutral
species remains and is available for the ashing process.
[0005] Current plasma generating apparatus present a variety of
challenges during ashing procedures. Generally, plasma is generated
using a coil, often copper, wrapped around a dielectric tube, such
as quartz or aluminum/sapphire tube. The coil is energized with a
radio frequency (RF) voltage from an appropriate RF generator.
Plasma formation is initiated by capacitively coupling the electric
field through the quartz to the rarefied gas inside the quartz
tube. As the power level and current through the coil are
increased, the plasma switches from a capacitively coupled mode to
an inductively coupled mode. Significant voltages exist on the
coil. Difficulties arise in trying to isolate the high voltage
components to prevent these components from breaking down and
arcing to cause damage to other components. In addition, the high
voltages generate a high electric field across the quartz and can
cause significant ion bombardment and sputtering on the inside of
the quartz tube thus reducing its lifespan and increasing its
maintenance needs. A reduction in the ion bombardment energy may be
helpful.
[0006] In addition, as illustrated in schematic cross section in
FIG. 1, prior art plasma sources 10 for ashing have smaller
diameter plasma generation regions 12, in quartz cylindrical
containers 15, than the work pieces 20 that are to be treated.
Accordingly, plasma flows from a smaller diameter plasma generation
region 12 of the quartz cylinder 15 of about 76 mm diameter that is
surrounded by a RF induction coil 14, to a larger diameter
distribution region 16 of a diameter approximating the work piece
diameter, often about 300 mm diameter. In the distribution region
16, the oxygen atoms (O), which are the desired product in the
plasma generator effluent, are spread out or dispersed over a
larger cross sectional area than that of the generation region 12
in an attempt to control the flux of O atoms to the surface of a
work piece 20. In addition, the distribution region 16 includes a
diffuser 18 of some kind to further facilitate a desired plasma
distribution over the surface of the work piece 20. Significant
numbers of O atoms are lost in this process.
[0007] Ion bombardment of the quartz cylinder 15 poses another
significant challenge. When a small diameter plasma source 10 is
used, the plasma density should be very high in order to generate
enough O atoms to perform ashing at an acceptable rate. This high
plasma density coupled with the high energy fields (E-fields)
present in the coil 14 cause significant ion bombardment of the
quartz container 15 and a reduced container lifespan. One method to
ameliorate this effect is to place a Faraday shield 22 between the
quartz container 15 and the coil 14, as illustrated in the
schematic cross section of FIG. 2. This effectively prevents the
E-fields from penetrating the quartz container 15 and consequently
reduces the sputtering of the quartz container 15. The addition of
the Faraday shield 22 reduces one problem at the expense of
creating additional problems. The Faraday shield 22 is complex,
increases cost, requires water cooling and consumes power that
would otherwise be delivered to the plasma.
[0008] In addition, present day plasma generator apparatus suffer
from non-uniform plasma production. Generally, when an
oxygen-containing gas flows through the container, plasma
generation is initiated in the tube adjacent the coil. But since
the E-field has limited penetration into the container, the peak
area for energy dissipation is near the inner wall of the
container. Due to this limited penetration of the E-field, the
plasma forms a ring 25 inside the quartz container 15, as seen from
above, and as schematically shown in FIG. 3, with the area of peak
power dissipation being near the inner wall of quartz container 15.
There is a hole 26 corresponding to a nearly field-free region
where there is little or no energy dissipation from the excitation
fields. For example, in the 76.2 mm diameter tube on the Gamma
2130.TM. of Novellus Systems, Inc. [San Jose Calif.], the size of
the central hole 26 in the ring 25 is small, although quite visible
under certain conditions. While gas flows through the entire cross
section of the quartz container 15, oxygen in the gas flow is
mainly dissociated in the ring 25 to produce O atoms. Very little
of the oxygen in the remainder of the gas flow is dissociated to O
atoms. Accordingly, a large portion of the incoming gas flow,
namely gas in the vicinity of the center of the cylindrical gas
flow in container 15, is not subjected to sufficient energy for
ionization.
[0009] In addition, present day plasma generators are difficult to
adapt to ashing larger wafers. If the quartz container 15 is
increased in diameter, the peak plasma region remains approximately
the same size and is still located near the wall. The hole 26 in
the ring 25 increases in size dramatically as the diameter of the
quartz container 15 is increased. The majority of the gas flows
down the center of the quartz container 15 and is never directly
ionized. Thus, few O atoms are produced in the central region of
the quartz container 15. The efficiency of producing O atoms in
larger diameter quartz containers is therefore expected to be
low.
[0010] Accordingly, it is desirable to provide an improved plasma
generation apparatus that is suitable for use in ashing procedures
in semiconductor fabrication. It is also desirable to provide an
apparatus that is able to provide a more uniform distribution of O
atoms over a large diameter work piece, such as a 300 mm or larger
wafer. It is further desirable to provide a plasma generator that
does not require Faraday shields, but that also provides an
acceptable quartz container lifespan. In addition, it is desirable
to provide a plasma generation apparatus and/or process that
converts oxygen more efficiently to O atoms. Other desirable
features and characteristics of the present technology will become
apparent from the subsequent detailed description and the appended
claims, taken in conjunction with the accompanying drawings and the
foregoing technical field and background.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A more complete understanding of the present technology may
be derived by referring to the detailed description and claims when
considered in conjunction with the following figures, wherein like
reference numbers denote like elements throughout the figures and
wherein:
[0012] FIG. 1 is a simplified cross sectional view of a prior art
plasma generator for use in ashing in semiconductor
fabrication;
[0013] FIG. 2 is a simplified cross sectional view of a prior art
plasma generator with a Faraday shield for use in ashing in
semiconductor fabrication;
[0014] FIG. 3 is a cross sectional view along line 3-3 of FIG.
2;
[0015] FIG. 4 is a simplified schematic of an induction coil
typical of prior art plasma generators;
[0016] FIG. 5A is a simplified side view in cross section of a
large diameter plasma generator in accordance with an exemplary
embodiment that utilizes an embodiment of a gas dispersion nozzle,
and a quartz container with a conical upper portion;
[0017] FIG. 5 B is a simplified side view in cross section of a
large diameter plasma generator in accordance with an exemplary
embodiment that utilizes an embodiment of a gas dispersion nozzle,
and a domed quartz container;
[0018] FIG. 6 is a simplified side view in cross section of a large
diameter plasma generator in accordance with an exemplary
embodiment that utilizes an embodiment of a gas dispersion nozzle,
and a domed quartz container, with a Faraday shield interposed
between the RF coil and the quartz container;
[0019] FIG. 7A is a simplified top view of a surface of an
exemplary nozzle used in connection with plasma generators in
accordance with exemplary embodiments;
[0020] FIG. 7B is a simplified side view in cross section of a
nozzle with hemispherical outlet end used in connection with plasma
generators in accordance with exemplary embodiments;
[0021] FIG. 8 is a simplified side view in cross section of a large
diameter plasma generator in accordance with an exemplary
embodiment that utilizes an alternative embodiment of a gas
dispersion nozzle, and a quartz container with a conical upper
portion;
[0022] FIG. 9 is a simplified side view in cross section of a
plasma generator in accordance with another exemplary embodiment
utilizing a flow directing baffle; and
[0023] FIGS. 10 A-B illustrate simplified schematic representations
of embodiments of symmetrical multi-segmented induction coils for
use with plasma generators in accordance with exemplary
embodiments.
DETAILED DESCRIPTION
[0024] The following detailed description is merely exemplary in
nature and is not intended to limit the described embodiments or
the application and uses of the described embodiments. Furthermore,
there is no intention to be bound by any expressed or implied
theory presented in the preceding technical field, background,
brief summary or the following detailed description.
[0025] In accordance with an exemplary embodiment, a plasma
generator apparatus includes means for diverting a portion of an
incoming gas flow into a region of higher plasma density than
another region of the apparatus. The region of higher gas density
is located in a container of suitable dielectric material, such as
a quartz container, and specifically within or proximate the
strongest region of a plasma-generating energy field to which the
container is subjected. Accordingly, a higher proportion of the
incoming ionizable components in the gas flow is ionized (or
"converted to plasma") when sufficient appropriate excitation
energy is applied.
[0026] Another embodiment provides symmetrical segmented coils for
generation of an electrical field of a frequency that will generate
ionization of a gaseous component in a plasma generator. The use of
such coils, as explained below, reduces or eliminates the necessity
for a Faraday shield. This results in significant cost savings. In
addition, by reducing the creation of free radicals that are
produced at high voltages there is reduced etching of the quartz
tubes often used in the plasma generator. Thus, the symmetrical
segmented coils increase the useful life of the quartz tubes.
[0027] An example embodiment of a plasma generator apparatus 100
with a conical upper portion is illustrated in FIG. 5A, and another
embodiment with a domed upper portion is illustrated in FIG. 5B.
The following description applies to both figures, except with
respect to the differences relating to the shape of the upper
portion of the container. The plasma generator is not limited to
use in semiconductor fabrication to ash work pieces, but may also
be used in other applications. The apparatus described herein can
also be used for general surface treatment, such as cleaning
organic material from any surface and not just in the semiconductor
industry. Coupled with different chemistry that contains nitrogen,
oxygen, hydrogen and compounds that might contain carbon and
fluorine, this apparatus may be used for cleaning and surface
treatment of a variety of work pieces, for example: cleaning
organic material from parts, removing biological contamination,
enhancing adhesion prior to deposition of another layer, reduction
of metal oxides, or for etching a range of materials.
[0028] The plasma generator 100 includes an upper portion 110 that
is conical (FIG. 5A) or domed (FIG. 5B) and that caps a gas flow
tube 125. The apex 112 of the cone 110, or highest point 112 of the
dome 110, is uppermost for receiving gas entering in gas stream 300
(depicted by arrows in the drawings) at a plasma generator inlet
101. The cone or dome 110 and tube 125 of the plasma generator 100
may be fabricated of quartz, as is conventional, or another
suitable material. The ionizable gas component in the gas stream
300 may be, for example oxygen, argon, helium, hydrogen, nitrogen,
and fluorine-containing compounds. The diameter 120 of the tube 125
may be from about 200 mm up to 500 mm and/or typically about 300 mm
for a work piece that approximates that size. The gas flow tube 125
has a larger diameter 120 with features to permit dispersion of
plasma generated across the entire cross section of the larger
diameter container 100 in which the plasma is generated. The term
"larger" diameter in the specification and claims with reference to
a container, within which plasma is generated, means a container
diameter that approximates, but may not precisely equal, the
diameter of a work piece to be subjected to ashing. In certain
embodiments the larger diameter therefore may encompass a cylinder
125 having a diameter 120 in excess of about 200 mm, or in the
range from about 300 mm to about 500 mm or more. A larger diameter
may be typically at least about 300 mm for a work piece of that
size, or more for larger work pieces. In the embodiment shown, the
apex 112 of the cone 110, or highest point 112 of the dome 110, has
a gas distribution nozzle 114 with a plurality of through holes 116
disposed in the nozzle 114, as shown more clearly in the top view
of FIG. 7A. The through holes 116 direct incoming gas along the
sloping inner sidewalls 118 of the cone or dome 110 toward the
region of higher plasma density 130.
[0029] The nozzle 114 shown in top view in FIG. 7A may be
spherical, hemispherical or pyramidal, or of any other suitable
shape. Another exemplary embodiment of a nozzle 170 with a
hemispherical-tip 172 is illustrated in FIG. 7B. The nozzle tip 172
has a plurality of through holes 116 therein to direct gas along
inner surfaces of walls of the cone 110 (or dome, if it is a domed
container) to a region of higher gas density 130 for plasma
generation in that region.
[0030] Referring to FIGS. 5A-B and FIG. 7A, the nozzle 114 directs
gas flow via through holes 116 (shown in FIG. 7A). The gas exiting
the through holes 116 (gas shown by downstream arrows 300 in the
Figures) is directed to preferentially flow along the inner
sidewalls 118 of cone or dome 110. The gas flow then impacts the
inner walls 126 of the gas flow tube 125. The impact area is in the
higher plasma region 130 which encompasses the intersection of
inner sidewalls 118 and inner tube walls 126. The directed gas flow
creates a region 130 of high gas pressure (high gas density) as the
gas flow changes direction from flowing parallel to the inner
sidewalls 118 of the dome 110 to flowing downward in tube 125
parallel to its inner walls 126. An energizing coil 140 surrounds
the outer surface 113 of the cone or dome 110 and the outer surface
124 of the tube 125 to supply energy at the appropriate frequency
into this region 130 to ionize gas components.
[0031] Because a large proportion, or even a major portion, of the
gas flow is directed by the nozzle 114 and the container inner
walls 118 into region 130, region 130 is a zone of highest plasma
density 130. Excitation energy is applied from the outside of the
tubular container 125 directly into this region 130. This permits
more efficient gas component ionization because it ameliorates the
effect of the energy level diminishing (and ionization decreasing)
as the energy penetrates farther into the container. Of course, the
flowing of more gas through the region of highest power
dissipation, region 130, increases the production of radicals and
atoms as well, in this case O atoms.
[0032] A gas distributor plate 150 is disposed at the exit end 102
of the generator 100. This gas distributor plate 150 has a
plurality of through holes, or is of a porous construction. It
provides means to control the O atom flux that impinges upon the
work piece being treated. As the gas impinges upon and travels
through the gas distributor plate 150, some charged species are
neutralized thereby reducing the potential for charged particle
damage to the work piece 200.
[0033] In accordance with another exemplary embodiment, a diameter
120 of the tube 125 and a diameter 210 of work piece 200 are
approximately the same. While equality of diameter is not
necessary, embodiments may have equal diameters of tube 125 and
work piece 200, or diameters that approximate equal size. This
feature significantly or completely reduces the need to expand the
tube 125 near its exit end 102 to approximate the work piece
diameter to facilitate distribution of the gas flow. In general, it
is preferable that a characteristic dimension of the apparatus,
such as tube diameter in the example of a quartz cylinder,
approximates a characteristic dimension of a work piece, such as
the diameter of a circular work piece surface that is presented
transverse to the direction of gas flow. In this regard, the plasma
generation region is increased in size thereby allowing a reduction
in overall plasma density while still increasing the O atom
production generated in the flowing gas. Increasing the volume of
the plasma reduces the plasma density in the region near the
container wall. This in turn results in less ion bombardment and
less container wall heating.
[0034] The plasma generator 100 may be used in conjunction with a
Faraday shield 144, shown in FIG. 6, or may be used in conjunction
with an induction coil circuit 160 that has a symmetrical coil 140
that has reduced peak voltage, as discussed below. Preferably, but
not necessarily, to reduce damage to the quartz components (dome
110 and tube 125) the peak voltage V.sub.p should be reduced by a
factor of about 2 or even by a factor of about 4, if necessary to
protect the container from premature aging.
[0035] As a preliminary matter, the prior art of driving the
induction circuit 160 is shown in FIG. 4. One end of the coil 140
is grounded and the other end is powered by a high frequency
alternating current generator 162 through a matching network and
capacitor. The peak voltage V.sub.p is seen at one end of the coil,
and the other end is grounded. According to an exemplary embodiment
of the present technology, the induction circuit 160 is configured,
as shown in FIG. 10A, so that there are two capacitors 164, one
outside each end of the coil 140. This configuration, and
configurations like this example, will be referred to as a
"symmetrical coil" configuration. The capacitors 164 are chosen
such that the total impedance of each capacitor is one half the
impedance of the original capacitor shown in FIG. 4. This maintains
the total impedance of the capacitor-coil induction circuit 160
unchanged. Accordingly, the voltage drop V.sub.p across the entire
coil 140 will be identical for the same current flowing through the
coil 140 and, therefore, the resulting plasma generation capability
will be the same. Thus, instead of a real ground (zero voltage, for
example) located at only the end of the coil 140 as in FIG. 4,
there is a now a pseudo ground (also zero voltage like the real
ground, in this example) located at the center 166 of the coil 140
as well. This means that the highest voltage seen on the coil
relative to ground at points 168 is V.sub.p/2. This reduces the
peak voltage by a factor of 2, and thereby reduces all electric
fields by a factor of 2. Furthermore, this also reduces the
electrical field across the quartz walls of the plasma generator
(which reduces ion bombardment energy) by a factor of 2. It also
reduces all of the other electrical standoff voltage requirements
by a factor of 2.
[0036] The effect may be further enhanced by dividing the coil into
a plurality of symmetrical segments, as shown in FIG. 10B. As shown
in FIG. 10B, subdividing the coil 140 into two symmetrical segments
reduces peak voltage V.sub.p to one-quarter of the peak for an
asymmetrical coil, V.sub.p/4. Accordingly, dividing the coil into N
segments, reduces peak voltage to 1/(2N) of the peak voltage of an
asymmetrical coil, which is shown in FIG. 4 for comparison. When
peak voltage VP has been reduced so that any voltage-induced
effects to the quartz components of the plasma generator 100 are at
an acceptable level, there is no longer any need for a Faraday
shield.
[0037] In accordance with an exemplary embodiment of the present
invention, illustrated schematically in FIG. 8, the gas inlet 101
is of a different design than the nozzle 114 of FIG. 5. The inlet
101 includes a tube 180 that has a closed end 181, and a series of
outlet holes 182 in the vicinity of the closed end 181 that direct
gas to the region 130, proximate an intersection between the dome
or cone 110 and the tube 125. Region 130 is adjacent induction coil
140 that is wrapped around the outer surface 113 of the dome 110
and the outer surface 124 of the tube 125 to provide excitation
energy to gas in region 130. In region 130, the gas 300 is ionized
and flows downward in the cylindrical container 125 to outlet 102.
At the outlet 102, the gas encounters a gas distributor plate 150
which neutralizes some charged gas species that were formed during
gas ionization. The gas stream 300 passes through holes or pores
184 in gas distributor 150, and exits as gas stream 302 to impinge
on an upper surface of the work piece 200 to perform a desired
function, such as surface ashing.
[0038] FIG. 9 illustrates another exemplary embodiment of a plasma
generator apparatus 100. This apparatus lacks a dome or cone upper
section. Rather, incoming gas flows into an apparatus 100 that
includes a tube 125 and is diverted within the tube 125 to flow
around an axially-centered round baffle plate 190. This diversion
causes gas stream 300 to flow towards sides of the inner wall 126.
An induction coil 140 surrounds the outer surface 124 of container
125 in an area adjacent region 130 where the diverted gas flow 300
impacts the inner wall 126 of the container 125. The induction coil
140 applies energy in that region 130 to ionize gas components. By
diverting gas out of the central region 135 of the tube and forcing
the gas towards the inner walls 126 of the tube, gas flow is forced
into region 130 and gas is concentrated in that region 130. The
application of energy to the concentrated gas results in a greater
ionization of the gas. The diameter of the baffle plate 190, or
other characteristic baffle dimension if not a circular baffle, may
be selected taking into account the inside diameter 120 of tube 125
and the gas flow rate, to determine an optimum gas flow rate and
pressure in the region 130 in which the gas is closest to the coil
140. After ionization, the gas 300 then flows downward in the
cylindrical container 125 to outlet 102. At the outlet 102, the gas
encounters a gas distributor plate 150 which neutralizes some of
the charged gas species that were formed during gas ionization. The
gas stream 300 passes through holes or pores 184 in gas distributor
150, and exits as gas stream 302 to impinge on an upper surface of
the work piece 200 to perform a desired function, such as surface
ashing.
[0039] While at least one example embodiment has been presented in
the foregoing detailed description, it should be appreciated that a
vast number of variations exist. It should also be appreciated that
the example embodiment or embodiments described herein are not
intended to limit the scope, applicability, or configuration of the
invention in any way. Rather, the foregoing detailed description
will provide those skilled in the art with a convenient road map
for implementing the described embodiment or embodiments. It should
be understood that various changes can be made in the function and
arrangement of elements without departing from the scope of the
invention as set forth in the appended claims and the legal
equivalents thereof.
* * * * *