U.S. patent application number 12/683944 was filed with the patent office on 2010-05-06 for apparatus for in-situ substrate processing.
Invention is credited to Guang-Yaw Hwang, Fu-Lun Lui, Yu-Wei Yang.
Application Number | 20100108262 12/683944 |
Document ID | / |
Family ID | 38321001 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100108262 |
Kind Code |
A1 |
Hwang; Guang-Yaw ; et
al. |
May 6, 2010 |
APPARATUS FOR IN-SITU SUBSTRATE PROCESSING
Abstract
A plasma processing system (system) for processing a substrate
is provided. The system includes a gas distribution system and a
gas flow control assembly coupled to the gas distribution system.
The gas flow control assembly is configured to control the input
gases provided by the gas distribution system. The plasma
processing system also includes a first set of nozzles coupled to
the gas flow control assembly and configured to supply a first set
of gases for processing a first portion of the substrate. The
plasma processing system further includes a second set of nozzles
coupled to the gas flow control assembly and configured to supply a
second set of gases for processing a second portion of the
substrate. The flow rate of the first set of gases is different
from a flow rate of the second set of gases.
Inventors: |
Hwang; Guang-Yaw; (Hsin-Shi,
TW) ; Lui; Fu-Lun; (Hsin-Shi, TW) ; Yang;
Yu-Wei; (Hsin-Shi, TW) |
Correspondence
Address: |
IPSG, P.C.
P.O. BOX 700640
SAN JOSE
CA
95170
US
|
Family ID: |
38321001 |
Appl. No.: |
12/683944 |
Filed: |
January 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11608738 |
Dec 8, 2006 |
7662723 |
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12683944 |
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60750852 |
Dec 13, 2005 |
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Current U.S.
Class: |
156/345.3 ;
118/723R; 156/345.33; 156/345.34 |
Current CPC
Class: |
H01L 21/76808 20130101;
H01L 21/31144 20130101; H01L 21/31138 20130101; H01L 21/02063
20130101; H01L 21/67017 20130101; H01J 37/3244 20130101; H01L
21/31116 20130101; H01L 21/6708 20130101; H01L 21/76811 20130101;
H01J 37/32449 20130101 |
Class at
Publication: |
156/345.3 ;
118/723.R; 156/345.33; 156/345.34 |
International
Class: |
H01L 21/306 20060101
H01L021/306; C23C 16/00 20060101 C23C016/00; C23F 1/08 20060101
C23F001/08 |
Claims
1. A plasma processing system for processing a substrate, the
plasma processing system comprising: a gas distribution system; a
gas flow control assembly coupled to the gas distribution system
and configured to control a set of input gases provided by the gas
distribution system, the set of input gases including at least one
gas; a first set of nozzles coupled to the gas flow control
assembly and configured to supply a first set of gases for
processing a first portion of the substrate; and a second set of
nozzles coupled to the gas flow control assembly and configured to
supply a second set of gases for processing a second portion of the
substrate, wherein the first set of gases represents a first
portion of the set of input gases, the second set of gases
represents a second portion of the set of input gases, and a flow
rate of the first set of gases is different from a flow rate of the
second set of gases.
2. The plasma processing system of claim 1 wherein the first
portion of the substrate represents a center zone of the substrate,
the second portion of the substrate represents an edge zone of the
substrate, and the flow rate of the first set of gases is higher
than the flow rate of the second set of gases.
3. The plasma processing system of claim 1 wherein at least one of
the first set of nozzles and the second set of nozzles represents
part of an electrode of the plasma processing system.
4. The plasma processing system of claim 1 wherein the set of input
gases includes oxygen.
5. The plasma processing system of claim 1 wherein the set of input
gases includes an inert gas configured to collide with plasma
generated from at least one of the first set of gases and the
second set of gases.
6. The plasma processing system of claim 1 configured to execute a
waferless auto clean process.
7. The plasma processing system of claim 1 further comprising a
temperature control system coupled to an upper chamber of the
plasma processing system, the temperature control system including
a heating unit configured to heat an electrode disposed inside the
upper chamber of the plasma processing system.
8. The plasma processing system of claim 7 wherein the electrode
comprises at least one of the first set of nozzles and the second
set of nozzles.
9. The plasma processing system of claim 7 wherein the temperature
control system further includes a cooling unit configured to
transport heat away from the upper chamber.
10. The plasma processing system of claim 7 wherein the temperature
control system includes a dissipation band and a thermally
conductive layer, the dissipation band is disposed between the
heating unit and the upper chamber and is configured to perform
radial dissipation of thermal loads, and the thermally conductive
layer is disposed between the dissipation band and the upper
chamber and is configured to conduct the thermal loads in a
direction that is perpendicular to an interface between the upper
chamber and the thermally conductive layer.
11. The plasma processing system of claim 1 further comprising a
heating unit configured to heat at least one of an electrode and a
plasma processing chamber of the plasma processing system, thereby
reducing an amount of contaminants that attach to the at least one
of the electrode and the plasma processing chamber of the plasma
processing system.
12-29. (canceled)
30. A plasma processing system for processing a substrate, the
plasma processing system comprising: a barrier layer; a deposited
photoresist element; a mean for supplying and ionizing a first set
of gases to partially etch the deposited photoresist element at a
first portion of the substrate in a plasma processing chamber to
result in a remaining portion of the deposited photoresist element;
a mean for supplying and ionizing a second set of gases to
partially etch the deposited photoresist element at a second
portion of the substrate in the plasma processing chamber to remove
the remaining portion of the deposited photoresist element; and a
mean for removing the barrier layer in the plasma processing
chamber, wherein the first portion of the substrate represents an
edge zone of the substrate, the first set of gases and the second
set of gases having the same composition, and the first set of
gases and the second set of gases having different flow rates.
31. The plasma processing system of claim 30 further comprising a
mean for controlling the first set of gases and the second set of
gases such that the first set of gases and the second set of gases
have different flow rates.
32. The plasma processing system of claim 30 further comprising a
mean for controlling a set of input gases to generate the first set
of gases and the second set of gases.
33. The plasma processing system of claim 30 further comprising a
mean for executing a waferless auto clean process.
34. The plasma processing system of claim 30 further comprising a
mean for heating at least a portion of the plasma processing
chamber to reduce an amount of contaminants that is attached to the
plasma processing chamber.
35. The plasma processing system of claim 30 further comprising a
mean for heating an electrode of the plasma processing system, the
electrode is configured to supply at least one of the first set of
gases and the second set of gases.
36. The plasma processing system of claim 30 further comprising a
mean for supplying an inert gas to collide with plasma generated
from at least one of the first set of gases and the second set of
gases, thereby reducing a processing rate of the processing.
37. A plasma processing system for processing a substrate, the
plasma processing system comprising: a mean for injecting a first
portion of a first set of gases into the plasma processing chamber
through a first set of nozzles of a showerhead with a first flow
rate, the first set of nozzles being positioned above a center zone
of the substrate; a mean for injecting a second portion of the
first set of gases into the plasma processing chamber through a
second set of nozzles of the showerhead with a second flow rate,
the first portion of the first set of gases and the second portion
of the first set of gases having the same composition, the first
flow rate being different from the second flow rate, the second set
of nozzles being positioned above an edge zone of the substrate; a
mean for injecting a second set of gases into the plasma processing
chamber; a mean for exciting the first portion of the first set of
gases into a first plasma; a mean for performing at least one of
etching and deposition on the substrate utilizing the first plasma;
a mean for exciting the second portion of the first set of gases
into a second plasma; a mean for processing the substrate using the
second plasma; a mean for providing a temperature control system
surrounding an electrode, the electrode representing the showerhead
for delivering at least one of the first set of gases and the
second set of gases into the plasma processing chamber; and a mean
for heating the electrode using the temperature control system to
increase collision between the first plasma and the second set of
gases, thereby reducing a processing rate of the at least one of
etching and deposition.
38. The plasma processing system of claim 37 further comprising a
mean for using the electrode to inject the first portion of the
first set of gases through the first set of nozzles of the
showerhead and the second set of gases through the second set of
nozzles of the showerhead; and a mean for heating at least a
portion of the plasma processing chamber to reduce an amount of
contaminants that attach to the plasma processing chamber.
Description
PRIORITY CLAIM
[0001] This divisional application claims priority under 37 CFR
1.53(b) of and claims the benefit under 35 U.S.C. .sctn.120 to a
commonly assigned patent application entitled "Methods and
Apparatus for in-situ substrate processing" by Hwang et al.,
Attorney Docket Number P1452/LMRX-P093, application Ser. No.
11/608,738 filed on Dec. 8, 2006, all of which is incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] In the processing of a substrate, e.g., a semiconductor
substrate or a glass panel such as one utilized in flat panel
display manufacturing, plasma is often employed. As part of the
processing of a substrate, for example, the substrate is divided
into a plurality of dies, or rectangular areas, each of which will
become an integrated circuit. The substrate is then processed in a
series of steps in which materials are selectively removed
(etching) and/or deposited (deposition) in order to form electrical
components thereon.
[0003] In an example plasma process, a substrate is coated with a
thin film of hardened emulsion (i.e., such as a photoresist mask)
prior to etching. Areas of the hardened emulsion are then
selectively removed, causing components of the underlying layer to
become exposed. The substrate is then placed in a plasma processing
chamber on a substrate support structure comprising a mono-polar or
bi-polar electrode, called a chuck or pedestal. Appropriate etchant
source are then flowed into the chamber and struck to form a plasma
to etch exposed areas of the substrate.
[0004] Referring now to FIG. 1, a simplified diagram of a
capacitively coupled plasma processing system is shown. In a common
configuration, the plasma processing chamber may comprise a bottom
piece 150 located in the lower chamber, and a detachable top piece
152 located in the upper chamber. A first RF generator 134 (source
RF generator 134) generates a plasma 110 as well as controls the
plasma density, while a second RF generator 138 (bias RF generator)
that generates bias RF is commonly utilized to control the DC bias
and the ion bombardment energy.
[0005] Further coupled to source RF generator 134 and bias RF
generator 138 may be a matching network 136 that attempts to match
the impedances of the RF power sources to that of plasma 110.
Furthermore, a pump 111 is commonly utilized to evacuate the
ambient atmosphere from a plasma processing chamber 102 (formed by
the bottom piece 150 and the detachable top piece 152) in order to
achieve the required pressure to sustain plasma 110.
[0006] Generally, an appropriate set of gases (input gases) is
flowed into chamber 102 through showerhead/grounded electrode 109
from gas distribution system 122 to shut off valve 123 located in
the lower chamber. In general, in order to achieve a substantially
uniform enchant gas distribution across the surface of a substrate,
a showerhead/grounded electrode 109, with a perforated or porous
planar surface, is generally utilized. Gas distribution system 122
commonly comprises compressed gas cylinders containing plasma
processing gases (e.g., C.sub.4F.sub.8, C.sub.4F.sub.6, CHF.sub.3,
CH.sub.2F.sub.3, CF.sub.4, HBr, CH.sub.3F, C.sub.2F.sub.4, N.sub.2,
O.sub.2, Ar, Xe, He, H.sub.2, NH.sub.3, SF.sub.6, BCl.sub.3,
Cl.sub.2, WF.sub.6, etc). These plasma gases may be subsequently
ionized to form plasma 110, in order to process (e.g., etch or
deposit) exposed areas of substrate 114, such as a semiconductor
substrate or a glass pane, positioned with edge ring 115 on an
electrostatic chuck 116, which also serves as a powered
electrode.
[0007] In addition, some type of cooling system 140 may be coupled
to the chuck in order to achieve thermal equilibrium once the
plasma is ignited. Cooling system 140 usually comprises a chiller
that pumps a coolant through cavities in within the chuck, and
helium gas pressurizes the small gap between chuck 116 and
substrate 114. In addition to removing the generated heat, the
helium gas also allows cooling system 140 to rapidly control heat
dissipation. That is, increasing helium pressure subsequently also
increases the heat transfer rate. Most plasma processing systems
are also controlled by sophisticated computers comprising software
programs. In a typical operating environment, manufacturing process
parameters (e.g., voltage, gas flow mix, gas flow rate, pressure,
etc.) are generally configured for a particular plasma processing
system and a specific recipe.
[0008] In a common substrate manufacturing method, known as dual
damascene, dielectric layers are electrically connected by a
conductive plug filling a via hole. Generally, an opening is formed
in a dielectric layer, usually lined with a barrier material (e.g.,
SiCN, SiC, SiON, Si.sub.3N.sub.4, etc.), and then subsequently
filled with a conductive material (e.g., aluminum (Al), copper
(Cu), etc.) that allows electrical contact between two sets of
conductive patterns, thereby establishing electrical contact
between two active regions on the substrate, such as a source/drain
region. Excess conductive material on the surface of the dielectric
layer is typically removed by chemical mechanical polishing (CMP).
A blanket layer of silicon nitride is then deposited to cap the
copper.
[0009] There are generally two commonly utilized approaches for
manufacturing dual damascene substrates: via-first and
trench-first. In one example of the via-first methodology, the
substrate is first coated with photoresist, and then the vias are
lithographically patterned. Next, an anisotropic etch cuts through
the surface cap material, etches down through the low-k layer of
the substrate, and stops on the barrier material, just above the
underlying metal layer. Next, the via photoresist layer is
stripped, the trench photoresist applied, and lithographically
patterned. Typically, some of the photoresist may remain in the
bottom of the via, or the via may be covered by an organic ARC
plug, in order to prevent the lower portion via from being
over-etched during the trench etch process. A second anisotropic
etch then cuts through the surface cap material and etches the
low-k material down to a desired depth. The second anisotropic etch
forms the trench. The photoresist is then stripped, and the barrier
material at the bottom of the via is opened with a very soft,
low-energy etch that will not cause the underlying copper to
sputter into the via. As described above, the trench and via are
filled with a conductive material (e.g., aluminum (Al), Copper
(Cu), etc.) and polished by chemical mechanical polishing
(CMP).
[0010] An alternate methodology is trench-first. In one example,
the substrate is coated with photoresist, and a trench lithographic
pattern is applied. An anisotropic dry etch then cuts through a
surface hard mask (e.g., SiCN, SiC, SiON, Si.sub.3N.sub.4, etc.)
followed by stripping the photoresist. Another photoresist is
applied over a trench hard mask, and then the vias are
lithographically patterned. A second anisotropic etch then cuts
through a cap layer and partially etches down into the low-k
material. The second anisotropic etch forms partial vias. The
photoresist is then stripped for trench etch over the vias with the
trench hard mask. The trench etch then cuts through the cap layer
and partially etches the low-k material down to desired depth. The
second anisotropic etch also clears via holes at the same time,
stopping on the final barrier material located at the bottom of the
via. The final barrier material may then be opened with a special
etch.
[0011] For example, a common via--first process generally may
involve alternating between etch and deposition processes,
performed on different plasma processing systems. To facilitate
discussion, FIG. 2 illustrates a partially simplified set of
example prior-art dual-damascene process steps, in which an
idealized cross-sectional view of a layer stack is shown. In the
discussions that follow, terms such as "above" and "below," which
may be employed herein to discuss the spatial relationship among
the layers, may, but need not always, denote a direct contact
between the layers involved. It should be noted that other
additional layers above, below, or between the layers shown may be
present. Further, not all of the shown layers need necessarily be
present and some or all may be substituted by other different
layers.
[0012] A partially etched substrate is shown at step (a). At the
bottom of the layer stack, there is shown a metal layer 210,
commonly aluminum or copper. Above the first metal layer is
commonly metal barrier layer 212 (e.g., SiCN, SiC, SiON,
Si.sub.3N.sub.4, etc.) that is commonly about 500 .ANG. in
thickness. Metal barrier layer 212 generally provides an etch stop
when etching the low-k material immediately above. Above the metal
barrier layer 212, there may be disposed an intermediate dielectric
(IMD) layer 214 formed of a low-k material, e.g., one of SiCN,
SiOC, BLACK DIAMOND.TM. supplied by Applied Materials, Inc.
(www.appliedmaterials.com), CORAL.TM. supplied by Novellus Systems,
inc. (www.novellus.com), TEOS, etc., that is commonly about 3.2
k.ANG. in thickness. Above the IMD layer 214, there may be placed a
cap layer 216 (e.g., SiCN, SiOC, BLACK DIAMOND.TM., CORAL.TM.,
TEOS, etc.) that is commonly about 500 .ANG. in thickness. Above
cap layer 216, there may be disposed a trench mask layer 218 (e.g.,
TiN, SiN, TaN, etc.) that is commonly about 300 .ANG.. Above trench
mask layer 218 may be another barrier layer 220 (e.g. PEOX, etc.)
that is commonly about 300 .ANG.. Above barrier layer 220 may be a
BARC layer 222 that is commonly about 1.1 k.ANG.. Finally above
BARC layer 222 is photoresist layer 224, patterned with via 226,
and is commonly about 2.8 k.ANG. in thickness.
[0013] The substrate is transferred to a plasma processing etch
chamber 202 (e.g., Lam Research 2300 Exelan.TM., etc.). At step
(b), via 226 is etched down to metal barrier layer 212. At step
(c), photoresist layer 224 and BARC layer 222 are removed
(stripped). The substrate is then transferred to a cleaning chamber
(not shown) and then to a photoresist deposition chamber 204. In
general, the substrate must be cleaned when transported transport
between manufacturing stations.
[0014] During the etch process, it is not uncommon for polymer
byproducts to form on the top and bottom of a substrate. In
general, polymers that form on the substrate during the etch
process are organic and may be composed of Carbon (C), Oxygen (O),
Nitrogen (N), and/or Fluorine (F). However, as successive polymer
layers are deposited as the result of several different etch
processes, organic bonds that are normally strong and adhesive will
eventually weaken and peel or flake off, often onto another
substrate during transport. For example, substrates are commonly
moved in sets between plasma processing systems via substantially
clean containers, often called cassettes. As a higher positioned
substrate is repositioned in the container, a portion of a polymer
layer may fall on a lower substrate where dies are present,
potentially affecting device yield. Hence, it is advantageous to
minimize the number of times a substrate must be transported.
[0015] In general, the substrates are chemically cleaned using a
combination of wet and dry processes. Wet processes generally
involve placing the substrate in a tank (e.g., quartz, plastic,
etc.) or on a spinning turntable, with a combination of solvents
and acids (e.g., H.sub.2SO.sub.4, H.sub.2O.sub.2, NH.sub.4OH, HF,
etc.) in order to remove organic and inorganic contaminant
residues, respectively, and is usually followed by deionized (DI)
water rinse and spine dry processes. Frequently, megasonics is
utilized to enhance the cleaning efficiency of wet cleaning
processes. Megasonics refers to a transducer mounted in the bottom
of the tank that generates high power acoustic energy waves. This
additional energy generally aids in removing particles from the
substrate surface. Common examples of substrate spin process
platforms with megasonic functionality include products supplied by
SEZ Holding Ltd. (www.sez.com), Akrion, inc. (www.akrion.com), and
Semitool, inc. (www.semitool.com).
[0016] For example, a common cleaning method called SC-1 (Standard
Clean Solution #1) includes placing the substrate in a tank (e.g.,
quartz, plastic, etc.) or on a spinning turntable with a mixture of
ammonium hydroxide (NH.sub.4OH), hydrogen peroxide
(H.sub.2O.sub.2), and DI water (H.sub.2O). A typical concentration
ratio for the mix is 1:1:5 for NH.sub.4OH:H.sub.2O.sub.2:H.sub.2O,
although recently ratios as low as 0.05:1:5 have been utilized for
better cleaning performance. SC-1 may be effective at removing
organic contaminants from the surface of the substrate by
continually oxidizing and then etching the surface of the
substrate, thereby dissolving the contaminants into solution. It is
typically operated in the temperature range of 50-70.degree. C.
[0017] Another commonly utilized cleaning method called SC-2
(Standard Clean Solution #2) includes placing the substrate in a
quartz or plastic tank with a mixture of hydrochloric acid (HCl),
hydrogen peroxide (H.sub.2O.sub.2), and DI water (H.sub.2O). A
typical concentration ratio for the mix is 1:1:5 for
HCl:H.sub.2O.sub.2:H.sub.2O. In general, SC-2 solution is optimized
for removing metal contaminants from the substrate surface, and
usually follows the SC-1. Like the SC-1, it removes metals by
continually oxidizing and then etching the surface of the wafer,
thereby dissolving the contaminants into solution. It is typically
operated in the temperature range of 50-70.degree. C.
[0018] In addition, dry cleaning methods are also utilized. In
general, dry cleaning is the process of removing contaminants from
the substrate surface in the gas-phase. Removal of contaminants may
be driven by either conversion of contaminant into volatile
compound through chemical reaction, "knocking" the contaminant off
the substrate surface via momentum transfer, or lift-off of the
contaminant during slight etching of contaminated surface.
[0019] After the substrate is cleaned, it is transported to
photoresist deposition chamber 204. In general, the substrate is
placed on a spindle with a vacuum chuck that can hold the substrate
during a high-speed rotation. Liquid photoresist is applied on the
substrate surface, and the centrifugal force from the substrate
rotation spreads the liquid over the whole substrate. The
photoresist thickness is related to both viscosity and spin rate.
That is, the greater the viscosity, the thicker the photoresist
layer, whereas the greater the spin rate, the thinner the
photoresist layer. The photoresist layer is then baked and exposed
to UV radiation in order to convert it into a tough adhesive film
over the surface of the substrate.
[0020] The substrate is then transferred to another plasma
processing etch chamber 206. Generally, the photoresist is
partially etched, leaving only a portion in via 226 to prepare the
substrate for the subsequently metal barrier layer 212 etch. In a
process commonly known as PREB (photoresist etch back), an O.sub.2
chemistry is commonly utilized in order to etch the photoresist
back to a particular level. Here, that level is a point between the
top and bottom of the via. Consequently, in order to prevent all of
the photoresist from being removed, as would commonly be the case
after a typical etch procedure is completed, the photoresist
removal process must generally be attenuated.
[0021] One method of attenuating O.sub.2 chemistry, and hence
controlling etch rate, may be the addition of a substantial amount
of inert gases to the gas mixture. That is, as the volume of inert
gas, such as argon, increases in the plasma processing chamber, a
greater number of oxygen ions may collide and exchange energy with
the inert gas molecules, absorbing thermal energy away from the
substrate surface and hence reducing the photoresist etch rate.
[0022] Another method of attenuating O.sub.2 chemistry may be to
pre-coat the plasma processing chamber with a material that has an
affinity to the etchant in order to reduce the effective amount of
etchant radicals in the plasma, and thus optimize the photoresist
etch rate. For example, pre-coating the chamber with Cl.sub.2 may
reduce the effective amount of oxygen radicals in the plasma that
are available to etch the photoresist. In general, the greater the
amount of pre-coat material, the fewer the number of available
oxygen radicals available to the etching process.
[0023] However, the plasma processing chamber utilized for PREB may
not be optimized for multiple consecutive process steps (e.g.,
PREB, trench etch, photoresist strip, etc.) that may require
different process chemistries and chamber conditions. For example,
in a typical etch chamber, it may be difficult to optimize the
distribution of O.sub.2, in combination with the inert gases,
across the surface of the substrate. However, as the plasma etches
away the photoresist, areas on the substrate with a higher
topography take longer to etch than areas with lower topography,
creating non-uniform photo resist profiles among the trenches or
vias as the case may be. In addition, the outer edge of the
substrate may collect more electrons than the center (hence
increasing the corresponding etch rate) since the edge is closer to
the plasma potential. Hence it may be advantageous to
asymmetrically distribute O.sub.2 across the surface of the
substrate to insure a substantially uniform etch rate.
[0024] In addition, it is important to maintain substantially clean
chamber conditions to minimize chamber memory effects and to
maintain repeatable results between successive substrates. Since
fully removing PREB contaminant deposits may be time consuming in
many plasma processing systems, the plasma processing chamber is
generally cleaned only when particle contamination levels reach
unacceptable levels. However, it is often difficult to determine
exactly when process conditions change beyond established
parameters. There is generally no effective way to determine if a
plasma process has moved outside of established parameters in-situ,
without first initially processing and then subsequently testing
partially manufacturing substrates. That is, after a batch of
substrates has been processed, a sample substrate is removed from
the batch and tested. Hence, in order to maximize the length of
time between required cleanings, a dedicated plasma processing
chamber is often preferred for a PREB process. Often, in order to
maximize capital investment, older machines may be dedicated to
specific tasks or processes, such as PREB.
[0025] After the PREB process is completed, the substrate may again
be cleaned (not shown) as previously described, then transported to
a plasma etch chamber 208 where barrier layer 220 (e.g. PEOX, etc.)
may be removed, and metal barrier layer 212 may be etched.
Initially, at step (f), barrier layer 220 is removed. In general,
the remaining photoresist 228 after the previous PREB step shields
metal barrier layer 212 from the plasma as barrier 220 is removed.
That is, a portion of photoresist 228 is etched instead of metal
barrier layer 212 etched. Next, in step (g), the remaining
photoresist is removed (stripped) as previously described. Finally,
metal barrier layer 212 is etched. In general, a liner removal
process (LRM) may be utilized to etch metal barrier layer 212.
[0026] However, it is often advantageous in the processing of a
substrate to combine as many steps as processing during a single
processing session (i.e., in-situ) in order to minimize the
handling of each substrate, and hence to improve yield, to improve
the overall production throughput, to help minimize the amount of
plasma processing chambers required, and to minimize substrate
defects caused during substrate transport.
SUMMARY
[0027] An embodiment of the present invention relates to a plasma
processing system for processing a substrate. The plasma processing
system may include a gas distribution system. The plasma processing
system may also include a gas flow control assembly coupled to the
gas distribution system and configured to control a set of input
gases provided by the gas distribution system. The plasma
processing system may further include a first set of nozzles
coupled to the gas flow control assembly and configured to supply a
first set of gases for processing a first portion of the substrate.
The plasma processing system may further include a second set of
nozzles coupled to the gas flow control assembly and configured to
supply a second set of gases for processing a second portion of the
substrate. The first set of gases may represent a first portion of
the set of input gases, and the second set of gases may represent a
second portion of the set of input gases. A flow rate of the first
set of gases may be different from a flow rate of the second set of
gases.
[0028] The above summary relates to only one of the many
embodiments of the invention disclosed herein and is not intended
to limit the scope of the invention, which is set forth in the
claims herein. These and other features of the present invention
will be described in more detail below in the detailed description
of the invention and in conjunction with the following figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The present invention is illustrated by way of example, and
not by way of limitation, in the figures of the accompanying
drawings and in which like reference numerals refer to similar
elements and in which:
[0030] FIG. 1 shows a simplified diagram of a capacitively coupled
plasma processing system.
[0031] FIG. 2 shows a partial simplified set of example prior-art
dual-damascene process steps, in which an idealized cross-sectional
view of the layer stack is shown.
[0032] FIG. 3 shows a partial simplified set of dual-damascene
process steps in which a PREB processing step has been combined
with a metal layer barrier etch processing step in an enhanced
plasma processing chamber, according to one or more embodiments of
the invention.
[0033] FIG. 4 shows a simplified diagram of an enhanced
capacitively coupled plasma processing system with a top electrode
temperature control system and differential plasma gas injection,
according to one or more embodiments of the invention.
[0034] FIG. 5 shows a simplified cross-sectional view of the
temperature control system of FIG. 4, according to one or more
embodiments of the invention;
[0035] FIG. 6 shows a partial magnified cross-sectional
representation of the temperature control system of FIG. 5,
according to one or more embodiments of the invention.
[0036] FIG. 7 shows an illustrative top view representation of a
temperature control device, according to one or more embodiments of
the present invention.
[0037] FIG. 8 shows a simplified diagram of a gas flow control
assembly for a multi-zone showerhead/grounded electrode in a plasma
processing system, according to one or more embodiments of the
invention.
DETAILED DESCRIPTION
[0038] The present invention will now be described in detail with
reference to a few preferred embodiments thereof as illustrated in
the accompanying drawings. In the following description, numerous
specific details are set forth in order to provide a thorough
understanding of the present invention. It will be apparent,
however, to one skilled in the art, that the present invention may
be practiced without some or all of these specific details. In
other instances, well known process steps and/or structures have
not been described in detail in order to not unnecessarily obscure
the present invention.
[0039] While not wishing to be bound by theory, it is believed by
the inventors herein that a PREB processing step may be combined
with a metal layer barrier etch processing step in an enhanced a
plasma processing chamber. In an embodiment, the enhanced plasma
processing chamber may be part of the Lam Research 2300 Exelan.TM.
Flex plasma processing system supplied by Lam Research Corporation
(www.lamrc.com).
[0040] As previously described, PREB processing generally uses
attenuated O.sub.2 chemistry in order to etch the photoresist back
to a particular level. For example, a substantial amount of inert
gases may be added to the gas mixture in order to reduce the etch
rate. In addition, the plasma processing chamber may be pre-coated
with a material that has an affinity to the etchant in order to
reduce the effective amount of etchant radicals in the plasma, and
thus optimize the etch rate. For example, pre-coating the chamber
with a material to which oxygen has an affinity, such as Cl.sub.2,
will reduce the effective amount of oxygen radicals in the plasma
that are available to etch the photoresist. In general, the greater
the amount of pre-coat material, the fewer the number of available
oxygen radicals available to the etching process. Conventionally, a
dedicated plasma processing chamber may generally be utilized
because of contamination or process control concerns.
[0041] In an advantageous manner, an enhanced plasma processing
system may be configured with at least one of a top electrode
temperature control system and differential plasma gas injection in
order to both attenuate the O.sub.2 etch process and to minimize
contamination.
[0042] In an embodiment, the enhanced plasma processing system may
execute a waferless auto clean process in order to substantially
remove PREB contaminants, allowing process conditions to remain
within established parameters during successive processing steps of
a substrate. In addition, since the enhanced plasma processing
system substantially removes most of the PREB contaminants, the
required time between chamber cleanings is substantially maintained
or improved.
[0043] One or more embodiments of the present invention involve a
plasma processing system for processing a substrate. The plasma
processing system may include a gas distribution system (i.e. a gas
supply). The plasma processing system may also include a gas flow
control assembly coupled to the gas distribution system and
configured to control a set of input gases supplied by the gas
distribution system. The set of input gases includes at least one
gas. The plasma processing system may further include a first set
of nozzles coupled to the gas flow control assembly and configured
to supply a first set of gases for processing a first portion of
the substrate. The first set of gases may represent a first portion
of the set of input gases. The plasma processing system may further
include a second set of nozzles coupled to the gas flow control
assembly and configured to supply a second set of gases for
processing a second portion of the substrate. The second set of
gases may represent a second portion of the set of input gases. A
flow rate of the first set of gases may be different from a flow
rate of the second set of gases.
[0044] In one or more embodiments, the first portion of the
substrate may represent a center zone of the substrate, the second
portion of the substrate may represent an edge zone of the
substrate, and the flow rate of the first set of gases may be
higher than the flow rate of the second set of gases.
[0045] In one or more embodiments, the plasma processing system may
further include a temperature control system coupled to an upper
chamber of the plasma processing system. The temperature control
system including a heating unit configured to heat an electrode
disposed inside the upper chamber.
[0046] With the electrode being heated during processing the
substrate, the amount of contaminants (e.g., polymer byproducts)
that attach to the substrate, the electrode, and walls of the
plasma processing chamber may be effectively reduced, and therefore
the amount of contaminants that are evacuated by a pump may be
effectively increased. Accordingly, contamination of substrates
positioned under the substrate may be reduced, and therefore time,
efforts, and costs associated with cleaning substrates may be
reduced. Further, contamination of the electrode may be reduced,
and therefore the frequency of replacing or cleaning electrodes and
associated costs may be reduced. The electrode may represent a
showerhead. Further, contamination of the plasma processing chamber
may be reduced, and therefore the substrate may be processed with
more process steps in the same plasma processing chamber without
being moved to a different plasma processing chamber.
Advantageously, productivity and efficiency may be improved, and
costs may be reduced in processing substrates.
[0047] One or more embodiments of the present invention involve a
method for processing a substrate that includes a barrier layer and
deposited photoresist. The method may include supplying and
ionizing a first set of gases to partially etch the deposited
photoresist at a first portion of the substrate in a plasma
processing chamber. The method may also include supplying and
ionizing a second set of gases to partially etch the deposited
photoresist at a second portion of the substrate in the plasma
processing chamber. The method may further include removing the
barrier layer in the same plasma processing chamber.
[0048] In one or more embodiments, the method may further include
heating an electrode of the plasma processing system. The electrode
may represent a showerhead for delivering at least one of the first
set of gases and the second set of gases into the plasma processing
chamber.
[0049] One or more embodiments of the present invention involve a
method for processing a substrate in a plasma processing system.
The method includes heating an electrode of the plasma processing
system to reduce a processing rate of the substrate.
[0050] The features and advantages of the present invention may be
better understood with reference to the figures and discussions
that follow.
[0051] Referring now to FIG. 3, a partial simplified set of
dual-damascene process steps is shown, in which a PREB processing
step has been combined with a metal layer barrier etch processing
step in an enhanced plasma processing chamber, according to one or
more embodiments of the invention.
[0052] A partially etched substrate is shown at step (a). At the
bottom of the layer stack, there is shown a metal layer 310,
commonly aluminum or copper. Above the first metal layer is
commonly metal barrier layer 312 (e.g., SiCN, SiC, SiON,
Si.sub.3N.sub.4, etc.) that is commonly about 500 .ANG. in
thickness. Above the metal barrier layer 312, there may be disposed
an intermediate dielectric (IMD) layer 314 that is formed of a
material comprising a low-k material (e.g., SiCN, SiOC, BLACK
DIAMOND.TM., CORAL.TM., TEOS, etc.) and is commonly about 3.2
k.ANG. in thickness. Above the IMD layer 314, there may be placed a
cap layer 316 (e.g., SiCN, SiOC, BLACK DIAMOND.TM., CORAL.TM.,
TEOS, etc.) that is commonly about 500 .ANG. in thickness. Above
cap layer 316, there may be disposed a trench mask layer 318 (e.g.,
TiN, SiN, TaN, etc.) that is commonly about 300 .ANG.. Above trench
mask layer 318 may be another barrier layer 320 (e.g. PEOX, etc.)
that is commonly about 300 .ANG.. Above barrier layer 320 may be a
BARC layer 322 that is commonly about 1.1 k.ANG.. Finally above
BARC layer 322 may be photoresist layer 324, patterned with via
326, and is commonly about 2.8 k.ANG. in thickness.
[0053] The substrate is transferred to a plasma processing etch
chamber 302 (e.g., Lam Research 2300 EXELAN.TM., etc. supplied by
Lam Research Corporation). At step (b), via 326 is etched down to
metal barrier layer 312. At step (c), photoresist layer 324 and
BARC layer 322 are removed. The substrate is then transferred to a
cleaning chamber (not shown) and then to a photoresist deposition
chamber 304. The substrate may be cleaned when being transported
between manufacturing stations. In general, the substrates may be
chemically cleaned using one or more of wet and dry processes
(e.g., SC-1, SC-2, megasonics, dry cleaning, etc.) or a combination
thereof. After the substrate is cleaned, it is transported to
photoresist deposition chamber 304.
[0054] However, unlike prior-art methods, the substrate may then be
transferred to an enhanced plasma processing chamber 308 in order
to execute both the PREB processing step at step (e) and the metal
layer barrier etch processing step at step (f), without an
additional cleaning step before step (f). At step (e), the PREB
process, photoresist 328 is partially removed, leaving only a
portion in via 326 in order to prepare the substrate for the
subsequently metal barrier layer 312. In one or more embodiments,
at step (e), enhanced plasma processing chamber 308 is heated such
that a substantial amount of contaminants (e.g., polymer
byproducts) produced at step (e) are evacuated by a pump coupled to
plasma processing chamber 308 without contaminating plasma
processing chamber 308. Accordingly, plasma processing chamber 308
may be in a desirable condition for step (f), and the substrate may
not need to be transferred to a different plasma processing
chamber. After the PREB process is completed, barrier layer 320 is
removed at step (f) in the same enhanced plasma processing chamber
308. Next, in step (g), the remaining photoresist is removed as
previously described. Finally, metal barrier layer 312 is etched at
step (h).
[0055] Referring now to FIG. 4, a simplified diagram of an enhanced
capacitively coupled plasma processing system with a top electrode
temperature control system and differential plasma gas injection is
shown, according to one or more embodiments of the invention. The
enhanced plasma processing chamber may comprise a bottom piece 450
located in the lower chamber, and a detachable top piece 452 and a
temperature control system 453 at least partially located in top
piece 452.
[0056] In one or more embodiments, temperature control system 453
may include a heating unit and a cooling unit (or a heating and
cooling unit) and may be coupled to an upper chamber 482 of the
plasma processing system. The heating and cooling unit may serve to
transport heat into or away from upper chamber 282 through the same
thermal interface. In one or more embodiments, temperature control
system 453 may include a heating and cooling unit that is coupled
via a latching mechanism to an outer surface of upper chamber 482.
In one or more embodiments, the latching mechanism may include a
clamp assembly. In one or more embodiments, the clamp assembly is
integrated directly into the heating and cooling unit.
[0057] In one or more embodiments, a first RF generator 434 (source
RF generator 434) generates the plasma as well as controls the
plasma density, while a second RF generator 438 (bias RF generator
438) generates bias RF and is commonly utilized to control the DC
bias and the ion bombardment energy. A matching network 436 may be
coupled to source RF generator 434 and to bias RF generator 438 and
may be configured to match the impedances of the RF power sources
to that of plasma 440. Furthermore, pump 411 may be utilized to
evacuate the ambient atmosphere from plasma processing chamber 402
in order to achieve the required pressure to sustain plasma 440.
Pump 411 also may be configured to evacuate contaminants from
plasma processing chamber 402.
[0058] In one or more embodiments, an appropriate set of gases is
flowed into chamber 402 from gas distribution system 422 to shut
off valve 423 located in the lower chamber. Unlike commonly
configured plasma processing chambers, showerhead/grounded
electrode 409 may comprise different sets or zones of independently
controlled nozzles (e.g., in order to optimize the substrate
uniformity). Showerhead/grounded electrode 409 may be connected to
a gas flow control assembly 425, which may be located near upper
chamber 482 and which may be coupled to shut off valve 423.
[0059] In one or more embodiments, the zones on a multi-zone
showerhead/grounded electrode comprise a center set of nozzles
principally introducing a first set of plasma gases (process gases)
into the center of the plasma for etching a first zone (e.g.,
center zone) of substrate 444, and an edge set of nozzles
principally injecting a second set of plasma gases (process gases)
into the remaining part of the plasma for etching a second zone
(e.g., edge zone) of substrate 444. Advantageously, the flow rates
of the first set of plasma gases and the second set of plasma gases
may be controlled such that substrate 444 may be etched in a
uniform manner. The flow rates may be different. In one or more
embodiments, the flow rate of the first set of plasma gases may be
higher than the flow rate of the second set of plasma gases.
[0060] In one or more embodiments, gas flow control assembly 425,
comprising one or more of stainless steel conduits, valves,
bypasses, and flow restrictions, provides the necessary gas flow
adjustments at showerhead/grounded electrode 409. These plasma
gases may be subsequently ionized to form a plasma 440, in order to
process (e.g., etch or deposition) exposed areas of substrate 444,
such as a semiconductor substrate or a glass pane, positioned with
edge ring 445 on an electrostatic chuck 416, which also serves as a
powered electrode.
[0061] One or more of the first set of plasma gases and the second
set of plasma gases may include one or more of oxygen and inert
gas. In an advantageous manner, heating the top electrode
(showerhead/grounded electrode 409) may excite the plasma gases,
increasing the number of collisions between oxygen ions and inert
gas molecules potentially reducing the photoresist etch rate.
Alternatively or additionally, referring to FIG. 4, heating
showerhead/grounded electrode 409 may reduce the amount of
contaminants that attach to one or more of substrate 444,
showerhead/grounded electrode 409, top piece 452, bottom piece 450,
and electrostatic chuck 416. Accordingly, a substantial amount of
contaminants may be evacuated from plasma processing chamber 402 by
pump 411.
[0062] Referring now to FIG. 5, a simplified cross-sectional view
of temperature control system 453 of FIG. 4 is shown, according to
one or more embodiments of the invention. A cooling channel 502 is
formed in thermally conductive body 506 to house cooling conduit
504. Cooling channel 502 is a single circuitous channel having a
path that doubles back on itself in order to average thermal loads
over thermal body 506. In this manner, warping of thermally
conductive body 506 may be reduced. Cooling conduit 504 may carry a
fluidic medium selected to efficiently conduct thermal energy away
from upper chamber. Any number of cooling fluids may be utilized in
accordance with the present invention. In one embodiment, water is
utilized as the fluidic medium. Cooling conduit 504 may be attached
with thermally conductive body 506 in any of a number of manners
known in the art. In some embodiments, where more efficient cooling
is required, cooling conduit 504 may be secured in the cooling
channel 502 with a thermally conductive material such as a polymer
resin for example.
[0063] Referring now to FIG. 6, a partial magnified cross-sectional
representation of temperature control system 453 of FIG. 5 is
shown, according to one or more embodiments of the invention. In
one or more embodiments of the present invention, a thermally
conductive material 644 comprises one or more thermal interface
layer. Thermally conductive material 644 may be utilized to improve
the metal to metal contact between surfaces--in this example,
between an upper chamber sidewall 612 and a dissipation band 648.
In one or more embodiments, thermally conductive material 644 may
include a thermal pad. In one or more embodiments, thermally
conductive material 644 may include thermal grease. Dissipation
band 648 may comprise one or more thermal interface layers. In
general, dissipation band 648 functions to distribute thermal loads
evenly over the surface of thermally conductive body 506. The
dissipation of thermal loads by dissipation band 648 differs from
conduction of thermal loads by thermally conductive material 644 in
that dissipation is substantially radial with respect to interface
layer surfaces whereas conduction is substantially perpendicular
with respect to interface layer surfaces. Dissipation band 648 may
be formed of material chosen from any of a number of suitable
thermally dissipative materials well-known in the art. In one or
more embodiments, dissipation band 648 is made of series 6000
aluminum.
[0064] A heater layer 650 also may comprise one or more thermal
interface layers. In one embodiment, heater layer 650 may be a
KAPTON.RTM. etched foil heater. The capacity of the heater is
selected based upon the process demand parameters. Thermal barrier
layer 652 also may comprise one or more thermal interface layers. A
final layer (not shown) may be bonded to an outer surface of the
thermally conductive body 506. This final layer, in some
embodiments, is a thermal arrestor. A thermal arrestor may insulate
a thermal control device from ambient temperature changes thus
resulting in better process control of the device. Each of the
layers discussed may be bonded to each other using any thermally
conductive adhesive well known in the art. In some embodiments,
double sided thermal adhesive tape such as THERMATTACH.RTM. T412
from Chomerics (www.chomerics.com, a division of Parker Hannifin
Corporation) may be utilized. Those skilled in the art will
recognize that the layers illustrated may not be scale
representations of actual embodiments. Rather, the layers are for
illustrative purposes only. Material selection and design
constraints will dictate layer sizing.
[0065] Referring to FIG. 7, an illustrative top view representation
of temperature control device 453 is shown, according to one or
more embodiments of the present invention. In one or more
embodiments of the present invention, control device 453 may be
characterized by its circular configuration. Mounting blocks 732
may be attached with each end of thermally conductive body 506.
Mounting blocks 732 may serve multiple functions. First, mounting
blocks 732 may serve as attachment points for a clamping system 728
to secure temperature control device 453 to an upper chamber of a
plasma processing apparatus. Clamping may be accomplished in any
manner commonly known in the art.
[0066] In one or more embodiments, mounting blocks 732 may serve as
attachment points for one or more of an inlet 720 and an outlet 724
of cooling conduit 502 (shown in FIG. 5). At least one inlet 720
and one outlet 724 may be mounted on either of mounting blocks 732.
Any number of suitable fittings well known in the art may be
utilized as attachment points for the cooling conduit. Handles 716
may be provided to assist in the handling of temperature control
device 453. Handles 716 may be thermally isolated from temperature
control device 453 so as to reduce or eliminate thermal noise in
the system. Additional handles may be added as required without
departing from the present invention. A temperature sensing device
is shown at 704. Temperature sensing device 704 may be utilized to
interlock a plasma control system in the event of an
over-temperature condition. In one embodiment, a resistance
temperature detector (RTD) may be utilized to sense temperature. A
heater attachment point 708 may be connected with thermally
conductive body 506. Heater attachment point 708 may provide stress
relief for heater layer and convenient access to the heater
embedded in temperature control device 453. Ideally, heater
attachment point 708 is thermally isolated from temperature control
device 453 so as not to introduce thermal noise into the
system.
[0067] Benefits of heating the top electrode include increasing the
number of collisions between oxygen ions and inert gas molecules in
order to potentially reduce the photoresist etch rate.
[0068] Referring now to FIG. 8, a simplified diagram of a gas flow
control assembly 425 (as shown in FIG. 4) for a multi-zone
showerhead/grounded electrode in a plasma processing system is
shown, according to one or more embodiments of the invention. In an
advantageous manner, the flow of a plasma gas may be individually
adjusted between the center and the edge of the substrate.
Generally, an appropriate set of gases is flowed into a plasma
processing chamber (not shown) from gas distribution system 422
through gas flow control assembly 425 to showerhead/grounded
electrode 409 located in the top piece 452. Showerhead/grounded
electrode 409 may comprise a set of independently controlled
nozzles, a first set in a center zone and a second set in a
perimeter or edge zone. These plasma processing gases may be
subsequently ionized to form a plasma (not shown), in order to
process exposed areas of a substrate (not shown).
[0069] Gas distribution system 422 may be coupled at junction A to
main shut off valve 802 located in the lower chamber, which may in
turn be coupled via junction B through conduit 808a to
lower-to-upper chamber interface 807. Lower-to-upper chamber
interface 807 may allow top piece 450 (located in the upper
chamber) to be safely removed from bottom piece 452 (located in the
lower chamber) for cleaning and maintenance without damaging gas
flow control assembly 425 and gas distribution system 422.
[0070] Lower-to-upper chamber interface 807 may be coupled to
junction C that forks between a conduit 816, a bypass conduit 810
coupled to edge control valve 806 (second control valve 806) at
junction F, and a bypass conduit 812 coupled to center control
valve 804 (first control valve 804) at junction D. Conduit 816 is
further coupled at junction 1 to restricted flow conduit 880 and
restricted flow conduit 822.
[0071] If edge control valve 806 and center control valve 804 are
both closed, the plasma gas flow to both zones of
showerhead/grounded electrode 409 may be substantially restricted.
Opening one of the valves may tend to increase the plasma gas flow
to the corresponding zone, whereas opening both of the valves may
tend to substantially equalize the plasma gas flow between both
zones.
[0072] Edge control valve 806 may be coupled to variable flow
conduit 818 at junction G, which may in turn be coupled to the
previously mentioned restricted flow conduit 880 at junction J.
Likewise, center control valve 804 may be coupled to variable flow
conduit 814 at junction E, which may in turn be coupled to the
previously mentioned restricted flow conduit 822 at junction H.
[0073] Edge conduit 884 may be coupled, at junction K, to
showerhead/grounded electrode 409, while center conduit 886 may be
coupled at, junction L, to showerhead/grounded electrode 409, which
feed gases into the plasma processing chamber (not shown).
[0074] Benefits of the gas flow control assembly include the
ability to asymmetrically distribute O.sub.2 across the surface of
the substrate to insure a substantially uniform etch rate.
[0075] As can be appreciated from the foregoing, embodiments of the
present invention may improve uniformity in processing substrates,
thereby advantageously increasing yield. Further, embodiments of
the present invention may reduce cleaning requirements and
frequencies for substrates and plasma processing chambers, thereby
increasing productivity and reducing costs. Further, in one or more
embodiments, more processing steps, such as PREB and barrier layer
removal, may be performed in the same plasma processing chamber
without transporting the substrates to a different plasma
processing chamber. Accordingly, the frequency of transporting the
substrates is reduced. As a result, the likelihood of substrate
contamination may be substantially reduced, and the processing may
be substantially simplified. Advantageously, the yield may be
further increased, the productivity may be further increased, and
the costs may be further reduced.
[0076] 100751 While this invention has been described in terms of
several preferred embodiments, there are alterations, permutations,
and equivalents which fall within the scope of this invention. For
example, although the present invention has been described in
connection with plasma processing systems from Lam Research Corp.
(e.g., EXELAN.TM., EXELAN.TM. HP, EXELAN.TM. HPT, 2300 VERSYS.TM.
STAR, etc.), other plasma processing systems may be utilized. This
invention may also be utilized with substrates of various diameters
(e.g., 200 mm, 300 mm, etc.). Also, photoresist plasma etchants
comprising gases other than oxygen may be utilized. It should also
be noted that there are many alternative ways of implementing the
methods of the present invention. Furthermore, embodiments of the
present invention may find utility in other applications. For
example, although etching processes are discussed as examples,
embodiments of the present invention may be applied in deposition
processes. The abstract section is provided herein for convenience
and, due to word count limitation, is accordingly written for
reading convenience and should not be employed to limit the scope
of the claims. It is therefore intended that the following appended
claims be interpreted as including all such alterations,
permutations, and equivalents as fall within the true spirit and
scope of the present invention.
* * * * *