U.S. patent application number 10/776672 was filed with the patent office on 2004-10-07 for method and apparatus for performing hydrogen optical emission endpoint detection for photoresist strip and residue removal.
Invention is credited to Kawaguchi, Mark N., Papanu, James S., Pavel, Elizabeth G..
Application Number | 20040195208 10/776672 |
Document ID | / |
Family ID | 33101166 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040195208 |
Kind Code |
A1 |
Pavel, Elizabeth G. ; et
al. |
October 7, 2004 |
Method and apparatus for performing hydrogen optical emission
endpoint detection for photoresist strip and residue removal
Abstract
A method for monitoring and detecting a hydrogen optical
emission while performing photoresist stripping and removal of
residues from a substrate or a film stack on a substrate.
Inventors: |
Pavel, Elizabeth G.; (San
Jose, CA) ; Kawaguchi, Mark N.; (Mountain View,
CA) ; Papanu, James S.; (San Rafael, CA) |
Correspondence
Address: |
MOSER, PATTERSON & SHERIDAN, LLP
APPLIED MATERIALS INC
595 SHREWSBURY AVE
SUITE 100
SHREWSBURY
NJ
07702
US
|
Family ID: |
33101166 |
Appl. No.: |
10/776672 |
Filed: |
February 11, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60447625 |
Feb 15, 2003 |
|
|
|
Current U.S.
Class: |
216/59 ;
156/345.24 |
Current CPC
Class: |
H01J 37/32963 20130101;
H01J 37/32935 20130101; H01J 2237/3342 20130101 |
Class at
Publication: |
216/059 ;
156/345.24 |
International
Class: |
H01L 021/306 |
Claims
1. A method of removing a photoresist layer comprising: positioning
a substrate comprising a photoresist layer into a processing
chamber; removing the photoresist layer using a plasma; and
monitoring the plasma for a hydrogen optical emission during the
process.
2. The method of claim 1 wherein the photoresist layer comprises a
hardened crust layer.
3. The method of claim 1 wherein the photoresist layer is implanted
with an implant species.
4. The method of claim 1 wherein the photoresist layer has been
exposed to ions.
5. The method of claim 1 wherein the photoresist layer has been
exposed to an electron beam.
6. The method of claim 2 wherein the monitoring step produces a
signal having a first level while etching the crust and produces a
signal having a second level after the crust has been removed.
7. The method of claim 1 wherein the hydrogen optical emission
occurs at a wavelength of about 656 nm.
8. The method of claim 1 further comprising: monitoring the plasma
for an oxygen optical emission while etching.
9. The method of claim 8 wherein the oxygen optical emission occurs
at a wavelength of about 777 nm.
10. The method of claim 1 further comprising: stopping the etching
upon the hydrogen optical emission obtaining a predetermined
level.
11. The method of claim 8 further comprising: stopping the etching
upon either the hydrogen optical emission obtaining a first level
or the oxygen optical emission obtaining a second level, or
both.
12. The method of claim 2 further comprising: monitoring the plasma
for an oxygen optical emission while etching.
13. The method of claim 12 wherein the oxygen optical emission
monitoring step produces an oxygen optical emission signal having a
first level while etching the crust and a second level after the
crust is removed.
14. The method of claim 13 wherein the oxygen optical emission
signal has a third level after the photoresist is removed.
15. The method of claim 8 wherein the hydrogen optical emission is
correlated with the oxygen optical emission.
16. A method of etching a photoresist layer comprising: providing a
substrate comprising a photoresist layer to a process chamber;
etching the photoresist layer using a plasma; and monitoring the
plasma for both a hydrogen optical emission and an oxygen optical
emission while etching.
17. The method of claim 16 wherein the photoresist layer comprises
a crust.
18. The method of claim 16 wherein the photoresist layer is
implanted with an implant species.
19. The method of claim 16, wherein the photoresist layer is
implanted with at least one of As, B, BF.sub.2, BF.sub.4, P, In, Sb
or H.
20. The method of claim 16 wherein the photoresist layer has been
exposed to an ion beam.
21. The method of claim 16 wherein the hydrogen optical emission
occurs at a wavelength of about 656 nm.
22. The method of claim 16 wherein the oxygen optical emission
occurs at a wavelength of about 777 nm.
23. The method of claim 16 further comprising: stopping the etching
upon either the hydrogen optical emission obtaining a first level
or the oxygen optical emission obtaining a second level, or
both.
24. The method of claim 16 wherein the oxygen optical emission
monitoring step produces an oxygen optical emission signal having a
first level while etching the crust and a second level after the
crust is removed, and wherein the hydrogen optical emission
monitoring step produces a hydrogen optical emission signal having
a third level while etching the crust and a fourth level after the
crust is removed.
25. The method of claim 16 wherein the oxygen optical emission
signal has a fifth level after the photoresist is removed.
26. The method of claim 16 wherein the hydrogen optical emission is
correlated with the oxygen optical emission.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. provisional patent
application Serial No. 60/447,625, filed Feb. 15, 2003, which is
herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to a method for
semiconductor substrate processing. More specifically, the
invention relates to a method for monitoring and detecting optical
emission endpoint(s), for photoresist stripping and removal of
residues from a substrate or a film stack on a substrate.
BACKGROUND OF THE INVENTION
[0003] As a part of semiconductor manufacturing, various layers of
dielectric, semiconducting, and conducting films, such as silicon
dioxide, polysilicon, and metal compounds and alloys, are deposited
on a silicon substrate. Features are defined in these layers by a
process including lithography and etching. Such a process comprises
coating a substrate with photoresist, patterning the photoresist,
and then transferring this pattern to the underlying layers during
etching by using the patterned photoresist as an etch mask. Many of
these etch processes leave photoresist and post-etch residues on
the substrate and must be removed before performing the next
process step.
[0004] Patterned photoresist also serves as an ion implant mask for
preferentially doping semiconductor substrates in selected areas.
The doping or implantation process includes exposing the substrate
to ions or an electronic beam of implant species, for example,
arsenic (As), boron (B, BF.sub.2, BF.sub.4), phosphorous (P),
indium (In), antimony (Sb) and hydrogen (H). The ion implantation
process dehydrogenates the photoresist material, resulting in a
hydrogen deficient, carbonized crust layer that is typically one to
several thousand angstroms thick on top of the bulk photoresist.
This makes the characteristics of the photoresist material
vertically non-uniform such that uniform removal (stripping) of the
photoresist can be difficult. As such, the photoresist removal
process may result in non-uniform removal and substantial
post-implant residue remaining on the substrate. Consequently, a
technique for monitoring removal of the photoresist is necessary
such that the photoresist removal process can be controlled as the
characteristics of the material change.
[0005] Optical emission spectroscopy is commonly used to detect the
endpoint of plasma etch processes. Plasma transitions of reactant
or product species emit photons which can be detected in the
ultraviolet, visible and near-infrared ranges. Thus, the endpoint
is usually based on increasing signal for reactants or decreasing
signal for products. The endpoint is identified when either the
reactants or products attain a specific concentration (i.e., the
respective signals cross a threshold level). However, such an
endpoint detection technique does not account for the variations in
the characteristics of a photoresist layer that has been exposed to
an ion beam.
[0006] Therefore, there is a need in the art for a method and
apparatus for performing optical emission endpoint detection for
photoresist strip and residue removal especially when using a
chamber having a remote plasma source.
SUMMARY OF THE INVENTION
[0007] The invention relates to a method for monitoring and
detecting optical emission endpoint(s), more particularly hydrogen
emissions within a plasma, for photoresist stripping and removal of
residues from a substrate or a film stack on a substrate. The
invention determines and uses a hydrogen optical emission peak to
identify the endpoint of a photoresist stripping process. More
specifically, the invention also relates to a method using optical
emission endpoint in general, and hydrogen peak specifically, to
monitor the transition from crust removal to bulk photoresist
removal for post-implant stripping. Since the hydrogen content of
the crust layer is significantly lower than that of the bulk
photoresist, removal of both crust and bulk can be monitored during
stripping within a remote plasma chamber. By this method, the
hydrogen peak also provides a simpler and more direct endpoint
trace for patterned implant substrates (compared to other peaks
such as oxygen). As such, using the hydrogen peak allows for
simpler endpoint algorithms.
[0008] This invention also comprises the combination of a hydrogen
optical emission monitoring with at least one additional emission
peak (e.g., oxygen or other reactants, or other by-product volatile
gases formed from components of the bulk photoresist) for more
robust and/or flexible endpoint control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] So that the manner in which the above recited features of
the present invention are attained and can be understood in detail,
a more particular description of the invention, briefly summarized
above, may be had by reference to the embodiments thereof which are
illustrated in the appended drawings.
[0010] It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are
therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
[0011] FIGS. 1A and 1B is an illustrative graph of a hydrogen
emission peak for a blanket photoresist and an arsenic implanted
photoresist;
[0012] FIG. 2 is an illustrative graph of a hydrogen emission peak
for three arsenic implanted substrates during a substrate test
showing repeatability of an hydrogen emission peak;
[0013] FIG. 3 is a flow diagram of one embodiment of a method of
the present invention;
[0014] FIGS. 4A-B are illustrative graphs of hydrogen and oxygen
emission traces for stripping of unimplanted photoresist (FIG. 4A),
arsenic implanted photoresist (FIG. 4B), phosphorous implanted
photoresist (FIG. 4C) and boron implanted photoresist (FIG. 4D);
and
[0015] FIG. 5 is a schematic diagram of one embodiment of an
illustrative chamber used to perform the method of the present
invention.
DETAILED DESCRIPTION
[0016] The invention relates to a method for monitoring and
detecting optical emission endpoint(s), more particularly hydrogen
emissions, for photoresist stripping and removal of residues from a
substrate or a film stack on a substrate. In one embodiment of the
invention, a method determines and uses a hydrogen optical emission
peak for identifying an endpoint of a photoresist stripping
process, including blanket and patterned photoresist, post-implant
photoresist, and post-plasma etch photoresist. In addition, the
invention comprises a method to use optical emission endpoint in
general, and hydrogen peak specifically, to monitor the transition
from crust removal to bulk photoresist removal for post-implant
stripping. By this method, the hydrogen endpoint trace is a more
direct measure of stripping for patterned implant substrates
(compared to other peaks such as oxygen).
[0017] The present invention uses, in one embodiment, the hydrogen
optical emission peak at 656 nm to monitor endpoint for ion implant
strip, and can be applied to other reducing chemistry based
stripping processes for strip and residue removal after the etching
of low dielectric constant films (low k films), and for other
applications.
[0018] For the crust removal process in post-implant strip, the
hydrogen signal can be especially useful because the crust layer is
hydrogen-depleted relative to the bulk photoresist. Thus, in accord
with an embodiment of the present invention, monitoring for the
rise and leveling-off of the hydrogen peak (656 nm) indicates that
the hydrogen-depleted crust layer is removed and that the
hydrogen-rich bulk photoresist has been reached. The ability to
accurately identify the crust removal clearing time is of use for
identifying changes in substrate conditions or in situations where
a multi-step stripping recipe is beneficial.
[0019] FIGS. 1A-B depict the hydrogen emission trace that occurs
during removal of an unimplanted photoresist layer (FIG. 1A) and
arsenic implanted photoresist layer (FIG. 1B). The graphs 100 and
102 depict emission intensity (axis 104) versus time (axis 106).
During the stripping of photoresist from the unimplanted substrate,
the hydrogen emission trace 108 increases (portion 110) then levels
off (portion 112), and then decreases (portion 114), allowing
endpoint detection as the photoresist clears. For the implanted
substrate, the clearing of the crust layer can be easily identified
in trace 116. The crust layer is hydrogen deficient as described
above, such that the hydrogen emission is low at the beginning of
the stripping process (portion 118). As the crust is removed, the
hydrogen emission increases (portion 120) until a plateau is
reached (122). Finally, the bulk photoresist is removed and the
hydrogen emission decreased (portion 124). The repeatability of the
hydrogen emission peak during a 100 substrate run with implanted
blanket photoresist monitor substrates is evident in the emission
graphs for substrates 2, 49, and 99 shown in FIG. 2.
[0020] One advantage of the present invention is that the hydrogen
signal is created as a process by-product, rather than a process
reagent like oxygen. Thus, the change in optical emission signal is
a more direct measure of the photoresist removal process, as
opposed to a process reagent which is more of an indirect measure
of photoresist removal and may also include additional reactions
not related to the photoresist removal process (such as reactions
with residues on chamber walls or other locations other than the
substrate). A by-product peak should be less sensitive to
non-uniformity issues, which, for the bulk strip step of
post-implant strip, could lead to overly-short process times. While
other by-product signals may also be used to signal the end of the
crust removal (e.g., the OH peak at .about.311 nm), the hydrogen
signal is significantly stronger in intensity and more well-defined
than any of these other peaks and therefore provides a clearer
endpoint trace. In addition, when using of the hydrogen peak over
the OH peak, it may be advantageous if water vapor is used in the
recipe, where the water vapor may mask the OH peak.
[0021] Furthermore, as a process by-product, the hydrogen emission
can be monitored near the substrate surface in a remote plasma
source reactor as described with respect to FIG. 5 below.
[0022] FIG. 3 is a flow diagram of a method 300 of the present
invention. The method begins at step 302 and proceeds to step 304
where a substrate is positioned in a process chamber capable of
performing photoresist stripping. One such chamber is manufactured
under the trademark AXIOM.TM. by Applied Materials, Inc. and
described with respect to FIG. 5 below.
[0023] At step 306, the method performs a plasma process in the
strip chamber. To remove photoresist, an oxygen-based plasma is
used. For example, an oxidizing gas such as O.sub.2, is applied to
a remote plasma source at a flow rate of 100 to 10,000 sccm. The
oxidizing gas is formed into a plasma when 600 to 6000 watts of RF
energy is applied to the source. The gas pressure in the chamber is
maintained at 0.3 to 3 Torr. The temperature of the substrate is
maintained at 15 to 300 degrees Celsius. In one embodiment of the
invention, an RF bias of 100 to 2000 watts is applied to the
substrate. Various oxidizing gases can be used including, but not
limited to, O.sub.2 O.sub.3, N.sub.2O, H.sub.2O, CO, CO.sub.2,
alcohols, and various combinations of these gases. In other
embodiments of the invention, nonoxidizing gases may be used
including, but not limited to, N.sub.2, H.sub.2O, H.sub.2, forming
gas, NH.sub.3, CH.sub.4, C.sub.2H.sub.6, various halogenated gases
(CF.sub.4, NF.sub.3, C.sub.2F.sub.6, C.sub.4F.sub.8, CH.sub.3F,
CH.sub.2F.sub.2, CHF.sub.3), combinations of these gases and the
like.
[0024] At step 308, the method 300 monitors the hydrogen emission
within the plasma in the chamber. At step 310, the method responds
to the emission magnitude. In one embodiment, the chamber
parameters, (e.g., gases, power levels, pressure, temperature and
the like) may be altered upon detecting a change in the hydrogen
emission. As such, the emission can be used to optimize processing
or to cease processing when the photoresist is removed.
Alternatively, one chemistry or recipe can be used for photoresist
crust removal and a second chemistry or recipe can be used for bulk
photoresist removal. Similarly, the bulk photoresist can be removed
until another emission change occurs, then a third chemistry or
recipe can be used to remove residue that remains from the
stripping process. The method 300 ends at step 312.
[0025] In another embodiment of the present invention, a method
uses a combination of a hydrogen optical emission with one (or
more) additional emission peak(s) for more robust and/or flexible
endpoint control. As such, step 308 can be used to monitor other
emissions (shown in phantom).
[0026] The use of the product hydrogen signal in combination with
other optical emission peaks can provide several advantages. For
example, the reactant oxygen signal provides multiple indicators of
stripping though transition layers between the crust and bulk
photoresist. Also, the method of the present invention permits
identification of an early endpoint indicator by monitoring the
reagent oxygen peak and a late/final indicator by monitoring the
by-product hydrogen peak. FIGS. 4A-B depicts graphs hydrogen and
oxygen optical emission traces during the stripping of blanket
unimplanted (graph 400), arsenic implanted photoresist (graph 420),
as well as phosphorous (graph 440) and boron (graph 460) implanted
photoresist. Each graph depicts emission magnitude (axis 404)
versus time (axis 406). In graph 400, the hydrogen emission is
trace 408 and the oxygen emission is trace 410 and, in graph 420,
the hydrogen emission is trace 418 and the oxygen emission is trace
416. In graph 440, the hydrogen emission is trace 436 and the
oxygen emission is trace 438 and, in graph 460, the hydrogen
emission is trace 456 and the oxygen emission is trace 458. These
data show that the implant species and conditions vary the specific
intensity versus time values, but that the general shape of the
emissions traces is the same, allowing for use of the method
described herein. In this example of an embodiment of the present
invention, the hydrogen and oxygen signals mirror each other since
the hydrogen is a by-product peak and oxygen is a reactant peak. By
measuring and monitoring both wavelengths, the method can
incorporate custom endpoint algorithms to minimize risk of
mis-processing and maximize throughput by optimizing process
duration. In addition, utilization of the present invention can
drastically reduce errors by providing a back-up wavelength. In
other words, using both signals, simultaneously allows for more
robust endpoint capability by providing a backup detection
wavelength--if the endpoint is missed at one wavelength, the
endpoint can be triggered by the other wavelength. Dual wavelength
endpoint triggering occurs when either wavelength meets the
endpoint conditions.
[0027] The dual wavelength optical emission can provide advantages
for other processes, such as post-silicon etch photoresist strip
and residue removal, where the process is switched at step 310 of
FIG. 3 from resist stripping chemistry to residue removal and/or
softening chemistry as the photoresist removal is detected. The
combination of the reactant oxygen and product hydrogen signals is
most useful for controlling the plasma-on time for photoresist
removal. Because residues are sometimes more difficult to remove
when exposed to excessive oxygen radicals, inaccurate endpoint
control can result in overly-long plasma-on times to ensure
complete photoresist removal, which in turn reduces the efficacy of
residue removal post-treatments. Accurate endpoint control limits
the oxidizing plasma exposure, thereby increasing the effectiveness
of residue-removal post-treatments.
[0028] The present inventive method may be used on a variety of
systems as the hardware requirements for the implementation of this
invention are not unique. FIG. 5 depicts a schematic diagram of the
AXIOM.TM. reactor (or chamber) 500 that may be used to practice
portions of the method 300. The AXIOM reactor 500 is described in
detail in U.S. patent application Ser. No. 10/264,664, filed Oct.
4, 2002 and incorporated herein by reference. The reactor 500
comprises a process chamber 502, a remote plasma source 506, and a
controller 508.
[0029] The process chamber 502 generally is a vacuum vessel, which
comprises a first portion 510 and a second portion 512. In one
embodiment, the first portion 510 comprises a substrate pedestal
504, a sidewall 516 and a vacuum pump 514. The second portion 512
comprises a lid 518 and a gas distribution plate (showerhead) 520,
which defines a gas mixing volume 522 and a reaction volume 524.
The lid 518 and sidewall 516 are generally formed from a metal
(e.g., aluminum (Al), stainless steel, and the like) and
electrically coupled to a ground reference 560. The sidewall
comprises a window 594 (quartz) that is used to monitor the optical
emissions within the plasma. The window 594 is coupled to a
light-collecting device 592 that carries the optical signals to the
optical emission spectroscopy (OES) system 590.
[0030] The substrate pedestal 504 supports a substrate (wafer) 526
within the reaction volume 524. In one embodiment, the substrate
pedestal 504 may comprise a source of radiant heat, such as
gas-filled lamps 528, as well as an embedded resistive heater 530
and a conduit 532. The conduit 532 provides cooling water from a
source 534 to the backside of the substrate pedestal 504. The
substrate sits on the pedestal by gravity or, alternatively, can be
mechanically clamped, vacuum clamped, or electrostatically clamped
as in an electrostatic chuck. Gas conduction transfers heat from
the pedestal 504 to the substrate 526. The temperature of the
substrate 526 may be controlled between about 20 and 400 degrees
Celsius.
[0031] The vacuum pump 514 is adapted to an exhaust port 536 formed
in the sidewall 516 of the process chamber 502. The vacuum pump 514
is used to maintain a desired gas pressure in the process chamber
502, as well as evacuate the post-processing gases and other
volatile compounds from the chamber. In one embodiment, the vacuum
pump 514 is augmented with a throttle valve 538 to control the gas
pressure in the process chamber 502.
[0032] The process chamber 502 also comprises conventional systems
for retaining and releasing the substrate 526, internal
diagnostics, and the like. Such systems are collectively depicted
in FIG. 5 as support systems 540.
[0033] The remote plasma source 506 comprises a power source 546, a
gas panel 544, and a remote plasma chamber 542. In one embodiment,
the power source 546 comprises a radio-frequency (RF) generator
548, a tuning assembly 550, and an applicator 552. The RF generator
548 is capable of producing of about 200 to 5000 W at a frequency
of about 200 to 600 kHz. The applicator 552 is inductively coupled
to the remote plasma chamber 542 and energizes a process gas (or
gas mixture) 564 to a plasma 562 in the chamber. In this
embodiment, the remote plasma chamber 542 has a toroidal geometry
that confines the plasma and facilitates efficient generation of
radical species, as well as lowers the electron temperature of the
plasma. In other embodiments, the remote plasma source 506 may be a
microwave plasma source, however, the stripping rates are generally
higher using the inductively coupled plasma.
[0034] The gas panel 544 uses a conduit 566 to deliver the process
gas 564 to the remote plasma chamber 542. The gas panel 544 (or
conduit 566) comprises means (not shown), such as mass flow
controllers and shut-off valves, to control gas pressure and flow
rate for each individual gas supplied to the chamber 542. In the
plasma 562, the process gas 564 is ionized and dissociated to form
reactive species.
[0035] The reactive species are directed into the mixing volume 522
through an inlet port 568 in the lid 518. To minimize charge-up
plasma damage to devices on the substrate 526, the ionic species of
the process gas 564 are substantially neutralized within the mixing
volume 522 before the gas reaches the reaction volume 524 through a
plurality of openings 570 in the showerhead 520.
[0036] The controller 508 comprises a central processing unit (CPU)
554, a memory 556, and a support circuit 558. The CPU 554 may be
any form of a general-purpose computer processor used in an
industrial setting. Software routines can be stored in the memory
556, such as random access memory, read only memory, floppy or hard
disk, or other form of digital storage. The support circuit 558 is
conventionally coupled to the CPU 554 and may comprise cache, clock
circuits, input/output sub-systems, power supplies, and the
like.
[0037] The software routines, when executed by the CPU 554,
transform the CPU into a specific purpose computer (controller) 508
that controls the reactor 500 such that the processes (e.g., method
300 of FIG. 3) are performed in accordance with the present
invention. The software routines may also be stored and/or executed
by a second controller (not shown) that is located remotely from
the reactor 500.
[0038] The AXIOM.TM. chamber has a window port 594 for attaching a
light-collecting device 592 (e.g., a fiber optic probe and cable)
to monitor plasma intensity. The window is located slightly above
the substrate plane for collecting emission intensity along a line
parallel to the substrate. Optical emission spectroscopy hardware
590 based on either a monochromator that can be set to monitor the
emission (above the substrate) of a particular wavelength within
the entire spectrum or hardware based on bandwidth filter(s), or
even a spectrometer, can be used. An exemplary embodiment of the
present invention may use a detector unit with two bandpass filters
on the chamber. In such an embodiment, one of the filters includes
the 656 nm emission, or hydrogen optical emission peak,
wavelength.
[0039] In addition to process control and process recipe
endpointing, the use of hydrogen, optical emission or hydrogen
combined with a second wavelength such as that of oxygen can also
be used to monitor chamber health. In such an embodiment of the
present invention, a detector unit may be utilized with one or more
bandpass filters coupled to the chamber. The oxygen emission
peak(s) of 777 nm and/or 845 nm can also be utilized, either singly
or jointly in combination with the hydrogen emission peak. The
relative intensities of these peaks so measured and monitored could
be indicative of the conditions of the plasma sources and chamber
surfaces and be used to provide a proper "fingerprint" of a clean
or "golden" chamber. The magnitude of the emissions can be used to
determine when a cleaning cycle is necessary or whether components
within the chamber are degrading, i.e., certain emissions are
indicative of chamber health.
[0040] While foregoing is directed to embodiments of the present
invention, other and further embodiments of the invention may be
devised without departing from the basic scope thereof.
* * * * *