U.S. patent application number 09/748060 was filed with the patent office on 2002-01-31 for downstream sapphire elbow joint for remote plasma generator.
Invention is credited to Cox, Gerald M., Kamarehi, Mohammad.
Application Number | 20020011310 09/748060 |
Document ID | / |
Family ID | 22437347 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011310 |
Kind Code |
A1 |
Kamarehi, Mohammad ; et
al. |
January 31, 2002 |
Downstream sapphire elbow joint for remote plasma generator
Abstract
A remote plasma generator, coupling microwave frequency energy
to a gas and delivering radicals to a downstream process chamber,
includes several features which, in conjunction, enable highly
efficient radical generation. In the illustrated embodiments, more
efficient delivery of oxygen and fluorine radicals translates to
more rapid photoresist etch or ash rates. A single-crystal,
one-piece sapphire applicator and transport tube minimizes
recombination of radicals in route to the process chamber and
includes a bend to avoid direct line of sight from the glow
discharge to the downstream process chamber. Microwave transparent
cooling fluid within a cooling jacket around the applicator enables
high power, high temperature plasma production. Additionally,
dynamic impedance matching via a sliding short at the terminus of
the microwave cavity reduces power loss through reflected energy.
At the same time, a low profile microwave trap produces a more
dense plasma to increase radical production. In one embodiment,
fluorine and oxygen radicals are separately generated and mixed
just upstream of the process chamber, enabling individually
optimized radical generation of the two species.
Inventors: |
Kamarehi, Mohammad;
(Pleasant Hill, CA) ; Cox, Gerald M.; (Lafayette,
CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
22437347 |
Appl. No.: |
09/748060 |
Filed: |
December 22, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09748060 |
Dec 22, 2000 |
|
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|
09546750 |
Apr 11, 2000 |
|
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6263830 |
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60128859 |
Apr 12, 1999 |
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Current U.S.
Class: |
156/345.12 ;
118/723ME |
Current CPC
Class: |
H01J 37/32256 20130101;
H05H 1/46 20130101; H01J 37/32192 20130101; H01J 37/32357 20130101;
H05H 1/4622 20210501 |
Class at
Publication: |
156/345 ;
118/723.0ME |
International
Class: |
H01L 021/3065 |
Claims
We claim:
1. A plasma generator in a semiconductor processing reactor,
comprising a microwave choke including quarter-wavelength shorted
coaxial conductors, the shorted coaxial conductors defining a choke
enclosure, the enclosure filled with a solid material having a
dielectric constant greater than about 3.
2. The plasma generator of claim 1, wherein the solid material
comprises a ceramic.
3. The plasma generator of claim 1, wherein the solid material has
a dielectric constant greater than about 5.
4. The plasma generator of claim 1, wherein the solid material has
a dielectric constant of about 9.
5. A microwave plasma generator in a semiconductor processing
reactor, comprising: a microwave power source; a microwave energy
waveguide extending from the energy source at a first end to a
second end; an microwave cavity in communication with the second
end of the waveguide; a gas carrier tube extending from an upstream
gas source through the cavity; and a cooling jacket surrounding the
carrier tube within the cavity, the cooling jacket filled with a
perfluorinated cooling fluid transparent to microwave energy.
6. The plasma generator of claim 5, wherein the cooling fluid
comprises Galden.TM..
7. The plasma generator of claim 5, wherein the gas carrier tube
comprises a sapphire section within the cavity.
8. The plasma generator of claim 5, wherein the upstream gas source
comprises fluorine.
9. The plasma generator of claim 7, wherein the microwave power
source can couple at least about 3,000 W of power to the gas within
the microwave cavity.
10. A plasma generator comprising a hollow sapphire tube extending
from a gas source through a microwave cavity to a process chamber,
the tube including an elbow joint defining an angle of greater than
about 35.degree. between the microwave cavity and the process
chamber.
11. The plasma generator of claim 10, further comprising a
microwave power source coupled to the microwave cavity.
12. The plasma generator of claim 11, wherein the microwave power
source can couple at least about 3,000 W of microwave energy at
about 2,450 MHz to gas within the sapphire tube.
13. The plasma generator of claim 10, wherein the gas source
comprises fluorine.
14. The plasma generator of claim 10, wherein the elbow joint
defines an angle of about 90.degree..
15. A remote plasma generator for generating a plasma within a gas
carrier tube upstream of a process chamber, the generator
comprising: a microwave energy generator; a microwave energy
pathway from the generator, including: an isolator module in
communication with the generator, the isolator module configured to
protect the energy generator from reflected power; a waveguide
communicating at a proximal end with the isolator module; and a
microwave cavity communicating at a proximal end with a distal end
of the waveguide, the cavity including a gas influent port and a
radical effluent port, a sliding short defining a variable distal
end of the microwave cavity, the sliding short dynamically
controlled to match impedance of the microwave cavity with the
waveguide.
16. The remote plasma generator of claim 15, wherein the microwave
energy pathway includes a directional coupler measuring reflected
energy directed toward the microwave energy generator, the
directional coupler generating signals controlling movement of the
sliding short.
17. The remote plasma generator of claim 15, wherein preset tuning
is conducted via a fixed tuning knob within the waveguide and fine
tuning is conducted dynamically by the sliding short.
18. A dual plasma source downstream reactor, comprising: a first
plasma source cavity; a first plasma energy source coupled to the
plasma source cavity; a first gas carrier tube extending through
the first plasma source cavity; a second plasma source cavity; a
second plasma energy source coupled to the plasma source cavity; a
second gas carrier tube extending through the first plasma source
cavity; a plasma mixer chamber in fluid communication with each of
the first gas carrier tube and the second gas carrier tube
downstream of first and second plasma source cavities; and a
process chamber downstream of and in fluid communication with the
mixer chamber.
19. The reactor of claim 18, further comprising a first perforated
baffle plate positioned between the process chamber and the mixer
chamber.
20. The reactor of claim 19, further comprising a second perforated
baffle plate positioned between the process chamber and the mixer
chamber, wherein the first and second baffle plates have
non-aligned perforations.
21. The reactor of claim 18, wherein the first gas carrier tube
comprises sapphire and the second gas carrier tube comprises
quartz.
22. The reactor of claim 21, wherein the first gas carrier tube
communicates with a source of fluorine and the second gas carrier
tube communicates with a source of oxygen.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit under 35 U.S.C.
.sctn. 119(e) from provisional Application No. 60/128,859 of
Kamarehi et al., filed Apr. 12, 1999.
FIELD OF THE INVENTION
[0002] The present invention relates generally to remote plasma
generators, for semiconductor processing equipment, and more
particularly to microwave power plasma generators for ashing or
stripping photoresist and removing polymeric residue from
semiconductor substrates.
BACKGROUND OF THE INVENTION
[0003] In fabricating integrated circuits, photoresist is coated
over semiconductor substrates and patterned through selective
exposure to developing light and removal of either the developed or
undeveloped portions. The patterned resist forms a mask used to
extend the pattern into underlying layers, such as oxides or metal
layers, by etching through the holes in the mask. Masks are also
commonly used to selectively dope regions of the substrate by ion
implantation. Once the mask has been employed, it is typically
removed by an oxidation process. The oxidizing process is referred
to in the industry as resist "stripping" or "ashing."
[0004] Increased throughput is a primary objective in the
semiconductor industry, particularly in the current era of
single-wafer processing. Any reduction in the time required for
processing each substrate serially in single-wafer processing
systems can lead to significant cost savings in a highly
competitive industry. In the case of resist stripping, the rate of
processing can be increased by supplying reactive oxygen free
radicals to the substrate. For example, dissociation of
oxygen-containing gases, such as diatomic oxygen gas (O.sub.2),
results in atomic oxygen (O), also known as oxygen free
radicals.
[0005] The addition of fluorine, in the form of NF.sub.3, CF.sub.4,
SF.sub.6 or fluorine free radicals (F), often aids the stripping
process where resist chemistry has been complicated by prior
processing. For example, it is difficult to remove photoresist that
has been subjected to ion implantation, such as that employed in
electrically doping the semiconductor substrate through the mask.
Similarly, reactive ion etch (RIE) through resist masks,
particularly where metal is exposed during the etch, tends to form
polymeric residue, which is also difficult to remove by oxidation
alone. In each of these situations, application of fluorinated
chemistries aids cleaning the resist and residue from the
substrate. Fluorine is also commonly used in other cleaning or
etching steps.
[0006] Maximizing the generation of oxygen (and/or fluorine) free
radicals positively influences the rate at which resist can be
stripped, thus increasing substrate throughput. Such free radicals
are commonly produced by coupling energy from a microwave power
source to oxygen-containing gas. Remote microwave plasma generators
guide microwave energy produced in a magnetron through a waveguide
to a resonant cavity or "applicator," where the energy is coupled
to a gas flowing through the cavity. The gas is excited, thereby
forming oxygen free radicals (O). Fluorine free radicals (F) are
similarly formed when fluorine source gas is added to the flow.
Common source gases include O.sub.2 for providing O, and NF.sub.3,
CF.sub.4, SF.sub.6 or C.sub.2F.sub.6 for providing F. Nitrogen
(N.sub.2) or forming gas (N.sub.2/H.sub.2) is often added to the
flow to increase particle kinetics and thereby improve the
efficiency of radical generation.
[0007] While microwave radical generators can lead to significant
improvements in ash rates, conventional technology remains somewhat
limiting. The plasma ignited by the microwave power, for example,
includes highly energetic ions, electrons and free radicals (e.g.,
O, F, N). While O and F free radicals are desirable for stripping
and cleaning resist from the substrate, direct contact with other
constituents of the plasma can damage the substrate and the process
chamber. Additionally, the plasma emits ultraviolet (UV) radiation,
which is also harmful to structures on the substrate.
[0008] Direct contact between the plasma and the process chamber
can be avoided by providing a transport tube between the microwave
cavity or applicator and the process chamber. The length of the
tube is selected to encourage recombination of the more energetic
particles along the length of the tube, forming stable, less
damaging atoms and compounds. Less reactive F and O radicals reach
the process chamber downstream of the microwave plasma source in
greater proportions than the ions. Because the process chamber is
located downstream of the plasma source, this arrangement is known
as a chemical downstream etch (CDE) reactor. By creating a bend in
the tube, the process chamber is kept out of direct line of sight
with the plasma, such that harmful UV radiation from the glow
discharge does not reach the substrate.
[0009] The tube itself, however, places several limitations on the
CDE reactor. Conventionally, both the applicator and the transport
tube are formed of quartz. Quartz exhibits advantageously low rates
of O and F recombination, permitting these desired radicals to
reach the process chamber while ions generated in the plasma source
recombine. Unfortunately, quartz is highly susceptible to fluorine
attack. Thus, the quartz transport tube and particularly the quartz
applicator, which is subject to direct contact with the plasma,
deteriorates rapidly and must be frequently replaced. Each
replacement of the quartz tubing not only incurs the cost of the
tubing itself, but more importantly leads to reactor downtime
during tube replacement, and consequent reduction in substrate
throughput.
[0010] An alternative material for applicators and/or transport
tubes is sapphire (Al.sub.2O.sub.3). While highly resistant to
fluorine attack, sapphire tubes have their own shortcomings. For
example, sapphire transport tubes exhibit much higher rates of O
and F recombination as compared to quartz, resulting in lower ash
rates. Additionally, sapphire is susceptible to cracking due to
thermal stresses created by the energetic plasma, limiting the
power which can be safely employed. Lower plasma power means less
generation of free radicals, which in turn also reduces the ash
rate. While employing single-crystal sapphire somewhat improves the
strength of the tube relative to polycrystalline sapphire, safe
power levels for single-crystal sapphire are still low compared to
those which can be employed for quartz tubes. Moreover, bonding
material at the joint between sapphire sections that create the
bend in the transport tube, which prevents UV radiation from
reaching the process chamber, is typically as susceptible to
fluorine ion attack as is quartz.
[0011] Other limitations on the production of radicals in a
conventional microwave plasma generator relate to the efficiency of
the energy coupling mechanism. Much of the microwave power supplied
by the magnetron is lost in power reflected back up the waveguide,
where it is absorbed by an isolator module designed to protect the
magnetron.
[0012] Energy also escapes where the applicator carries source gas
in and free radicals out of the resonant cavity. The plasma-filled
tube acts as a conductor along which microwave energy travels out
of the cavity, thus effectively extending the plasma and reducing
plasma density. In addition to reducing plasma density, and
therefore reducing generation of radicals, the extension of the
plasma also increases the risk of ions surviving to reach the
process chamber and substrate housed therein. Microwave traps can
confine such microwave leakage. For example, U.S. Pat. No.
5,498,308 to Kamarehi et al., entitled "Plasma Asher with Microwave
Trap," discloses a resonant circuit trap. Even employing such
traps, however, the plasma expands outside the plasma source cavity
along the tube to the outer edges of the traps.
[0013] Accordingly, a need exists for more efficient microwave
generators to improve resist ash rates.
SUMMARY OF THE INVENTION
[0014] In satisfaction of this need, a remote plasma generator is
provided for coupling microwave frequency energy to a gas and
delivering radicals to a downstream process chamber. The plasma
generator includes several features which, in conjunction, enable
highly efficient radical generation and consequently high
photoresist ash rate. Such high ash rates can be achieved for both
standard photoresist and chemically more problematic residues, such
as those created by ion implantation and reactive ion etching.
[0015] In accordance with one aspect of the invention, high power
can be coupled to gas flow that includes a fluorinated chemistry. A
single-crystal, one-piece sapphire applicator and transport tube
resists fluorine attack. The tube can be lengthened and provided
with an elbow joint, aiding recombination of ions and protecting
the process chamber from UV radiation produced in the plasma
discharge.
[0016] In accordance with another aspect of the invention,
microwave transparent cooling fluid enables high power, high
temperature plasma production. While useful for increasing
practicable power for applicators of any material, liquid cooling
is of particular utility in conjunction with sapphire applicators,
which are susceptible to stress cracking.
[0017] In accordance with still another aspect of the invention,
dynamic impedance matching via a sliding short at the terminus of a
microwave cavity reduces power loss through reflected energy. At
the same time, a low profile microwave trap produces a more dense
plasma to increase radical production.
[0018] In accordance with yet another aspect of the invention,
different radicals are separately generated and mixed just upstream
of the process chamber, enabling individually optimized radical
generation of the two species. In the illustrated embodiment,
fluorine radicals are generated in a sapphire applicator, while
oxygen radicals are generated in a quartz applicator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] These and other aspects of the invention will be apparent
from the detailed description below, and from the appended
drawings, which are intended to illustrate and not to limit the
invention, and wherein:
[0020] FIG. 1 is a schematic view of a semiconductor reactor
incorporating a remote microwave plasma generator, constructed in
accordance with a preferred embodiment of the present
invention;
[0021] FIG. 2 is a right, front and top perspective view of a
plasma generator, constructed in accordance with the preferred
embodiment;
[0022] FIG. 3 is a left, front and top perspective view of the
plasma generator of FIG. 2;
[0023] FIG. 4 shows a subsystem of the plasma generator of FIG.
3;
[0024] FIG. 5 is a right, front and top perspective view of the
subsystem of FIG. 4, shown with a protective sheath removed from a
carrier tube;
[0025] FIG. 6 is an exploded view of the subsystem of FIG. 4, taken
from the angle of FIG. 5;
[0026] FIG. 7 is a side sectional view of the subsystem, taken
along lines 7-7 of FIG. 4;
[0027] FIG. 8 is a top down sectional view of the subsystem, taken
along lines 8-8 of FIG. 4;
[0028] FIG. 9 is a graph illustrating reflected power against
microwave generator power, for both fixed tuning and dynamically or
in-situ tuned impedance matching;
[0029] FIG. 10 is a graph illustrating reflected power against
total gas flow, for both fixed tuning and dynamically or in-situ
tuned impedance matching;
[0030] FIG. 11 is a graph illustrating reflected power against gas
pressure, for both fixed tuning and dynamically or in-situ tuned
impedance matching;
[0031] FIG. 12 is a cross-sectional view of a component of the
subsystem of FIG. 4, including a low profile, co-axial microwave
choke;
[0032] FIG. 13 is a rear, right perspective view of a semiconductor
reactor incorporating dual plasma generators, constructed in
accordance with a second embodiment of the present invention;
[0033] FIG. 14 is a rear elevational view of the reactor of FIG.
13;
[0034] FIG. 15 is a cross-sectional view taken along lines 15-15 of
FIG. 13; and
[0035] FIG. 16 is a pair of graphs illustrating ash rates and
uniformity using the plasma generators of the preferred
embodiments.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] While the illustrated embodiments are described in the
context of a resist stripping or ashing system, the skilled artisan
will readily find application for the devices and methods disclosed
herein for other systems. Within the semiconductor industry, for
example, plasma or free radical generation is desirable for
assisting or enhancing many chemical etch and chemical vapor
deposition processes. For many of these processes, remote
production of plasmas advantageously avoids damage to the
substrate.
[0037] With reference initially to FIG. 1, a chemical downstream
etch (CDE) reactor 10, according to a preferred embodiment, is
schematically illustrated. The reactor 10 includes a microwave
plasma generator 12 upstream of a process chamber 14. A substrate
16, typically comprising a monolithic semiconductor wafer, is
supported upon a chuck 18 over a pedestal 20 within the chamber
14.
[0038] With reference to FIGS. 1-4, the plasma generator 12
includes a microwave power source 22, which can be a conventional
magnetron. For example, suitable microwave power sources are
commercially available under the trade names NL10230 and NL 10250
from Richardson Electronics of LaFox, Ill. The NL10230 magnetron
generator is capable of producing about 3,000 W of microwave power
at 2,450 MHz (nominal). The skilled artisan will readily appreciate
that, in other arrangements, the power source can be of any
construction suitable for coupling power to a gas. Other plasma
generators, for example, can employ radio frequency power, and
energy can be coupled inductively or capacitively to the gas being
ionized.
[0039] The illustrated plasma generator 12 further includes,
adjacent the power source 22, an isolator module 24, which can also
be of conventional construction. As is known in the field, the
isolator 24 protects the magnetron from reflected power by
diverting such reflected power to a dummy load. Desirably, the
isolator 24 includes an integrated directional coupler, which also
measures reflected power in order to match impedance along the
microwave energy pathway, as will be discussed in more detail with
respect to the section entitled "Impedance Matching," below.
[0040] Microwave power is directed through the isolator 24 to a
waveguide 26, which extends into a microwave cavity or plasma
source 28. As best seen from FIG. 2, the illustrated waveguide 26
is S-shaped, enabling a stacked configuration and reducing reactor
footprint on the fabrication floor. The waveguide includes a fixed
tuning knob 30 (FIG. 1), which operates in conjunction with an
autotuner module 32 to match impedance of the microwave energy path
(including the isolator 24, waveguide 26 and cavity 28) to that of
the power source 22. The autotuner module 32 is described in more
detail in the section entitled "Impedance Matching," below.
[0041] A gas carrier tube 34 extends from a gas source 36 through
the microwave cavity 28. The tube 34 axis extends transversely to
the waveguide axis. In the illustrated embodiment, the gas source
includes an oxygen source gas (preferably O.sub.2), a fluorine
source gas (preferably CF.sub.4 or NF.sub.3), and a carrier gas
(preferably N.sub.2). As discussed in more detail in the section
entitled "Single-Crystal Transport Tube," below, the carrier tube
34 includes an upstream section 38, an applicator section 40, and a
transport tube section 41. The transport tube 41 includes a bend or
elbow joint 42 between the cavity 28 and the process chamber
14.
[0042] A pair of microwave emission barriers 44, 46, surrounding
the applicator 40 immediately upstream and downstream of the cavity
28, respectively, serve to prevent microwave energy escaping the
cavity 28. The construction of the emission barriers 44, 46 is
discussed in more detail in the section entitled "Microwave Choke,"
below.
[0043] Single-crystal Transport Tube
[0044] Referring to FIG. 1, as briefly noted above, the gas carrier
tube 34 includes three sections: the upstream section 38, carrying
gas from the gas source 36; the applicator section 40, extending
through the microwave cavity 28; and the transport tube section 41,
extending downstream of the cavity 28 to the process chamber
14.
[0045] In operation, microwave power conducted from the waveguide
26 is coupled to gas flowing through the applicator portion 40 of
the tube 34 (within the cavity 28), exciting the gas and igniting a
plasma. The applicator 40, including portions of the tube 34 within
the cavity 28 as well as sections immediately adjacent the cavity
28, is directly subjected to energetic particles of the plasma
discharge and are consequently subject to faster deterioration than
upstream portions, and slightly faster than downstream portions.
Since the upstream section 38 of the tube 34 is not subject to the
plasma discharge and therefore does not deteriorate rapidly, the
upstream section 38 is preferably a conventional stainless steel
gas line and is provided separately from applicator 40, such that
it need not be replaced when the applicator 40 is due for
replacement.
[0046] In the illustrated embodiment, both fluorine and oxygen
source gases are provided to the applicator 40, generating 0 and F
free radicals as well as a variety of ionic species and electrons.
As noted in the Background section above, fluorine is particularly
corrosive to quartz tubing. Accordingly, the applicator 40
preferably comprises sapphire for resistance to fluorine attack.
Most preferably, the applicator 40 is formed of single-crystal
sapphire, providing superior physical strength to withstand the
stresses generated by exposure to the plasma.
[0047] As discussed in the Background section above, the length of
the transport section 41 of the carrier tube 34 is selected to
allow recombination of ions prior to introduction of the energized
gas to the process chamber 14. Preferably, the transport section
41, from the end of the microwave cavity 28 to the process chamber
14, is at least about 5 inches long, more preferably at least about
10 inches, and is about 14.5 inches in the illustrated embodiment.
The total length of the applicator 40 and transport section 41 is
about 21.5 inches in the illustrated embodiment. The skilled
artisan will understand, however, that shorter lengths of transport
tubing can be used where ion content is reduced in alternative
manners.
[0048] The transport section 41 preferably includes the bend or
elbow joint 42, best seen in FIG. 7, thus avoiding direct line of
sight between the glow discharge within the cavity 28 and the
process chamber 14 (FIG. 1). Preferably, the bend 42 defines at
least a 35.degree. angle, and more preferably greater than about a
45.degree. angle. In the illustrated embodiment, the bend 42
defines a 90.degree. or right angle. The substrate 16 is thus
shielded from harmful UV photons released by the glow
discharge.
[0049] As the transport tube 41 is also subjected to energetic
plasma products, including excited fluorine species, this section
is also preferably formed of sapphire, and more preferably
single-crystal sapphire. Additionally, the transport tube 41 is
preferably formed integrally with the applicator tube 40.
[0050] Unfortunately, the crystalline quality of sapphire tends to
degrade for tube lengths over 12 inches. As previously noted, the
resultant polycrystalline structure of longer lengths of sapphire
tubing is susceptible to stress cracking when subjected to high
power, high temperature plasmas. The desirability of forming an
elbow joint within the transport tube section 41 also necessitates
joining at least two sections of single-crystal sapphire. Typical
bonding materials, however, are incompatible with fluorine, such
that employing such bonds would negate the very advantage of
sapphire tubing.
[0051] Accordingly, the illustrated transport tube 41 and
applicator 40 are provided as sections of single-crystal sapphire,
bonded at the elbow joint 42 without bonding materials susceptible
to fluorine attack. In particular, the sections 40 and 41 are
bonded by eutectic bonding, as disclosed in PCT publication number
WO 09 856 575, entitled Eutectic Bonding of Single Crystal
Components, published Dec. 17, 1998 (the "PCT application"). The
disclosure of the PCT '575 application publication is incorporated
herein by reference. Single-crystal sections pre-bonded in the
manner of the PCT '575 application are available from Saphikon,
Inc. of Milford, N.H.
[0052] Applicator Cooling System
[0053] The employment of single-crystal sapphire provides
resistance to fluorine attack and greater strength than
polycrystalline sapphire. Accordingly, the single-crystal sapphire
tube, serving as an integral applicator 40 and transport tube 41,
enables coupling relatively high power to the gas, while
withstanding fluorine attack for applications such as
post-implantation ashing.
[0054] Coupling high power to the gas desirably increases the rate
of O and/or F radical production, thereby increasing ash rates.
However, the kinetic energy created in a high power plasma
introduces negative effects as well. Within the cavity 28, rapid
and frequent collisions between energized particles, and between
such particles and the applicator walls, raises the temperature of
the applicator 40, creating thermal stresses on the tube. While
single-crystal sapphire is more resistant to such stresses than
polycrystalline sapphire, sapphire remains subject to stress
cracking under high power, high temperature operation, as compared
to quartz. Moreover, the high temperature of the applicator 40
encourages recombination of dissociated particles. While
recombination of ions and electrons is desirable, recombination of
free radicals (F, O) is counterproductive.
[0055] Accordingly, the preferred embodiments employ a cooling
mechanism within the applicator 40, thereby reducing
kinetically-induced recombination within the applicator 40.
Lengthening the transport tube 41 compensates for reduced ion
recombination within the applicator 40. Energetic ions recombine
over the length of the transport tube 41 in greater proportions
than radicals, due to additional electrostatic reactions
encouraging such recombination. At the same time, cooling the
applicator 40 enables use of higher power for a given tolerance for
thermal stress. For sapphire applicators, in particular, the
cooling mechanism reduces the occurrence of stress cracking while
boosting the efficiency of free radical generation.
[0056] With reference to FIGS. 7 and 8, a cooling jacket 50
surrounds the applicator section 50 of the carrier tube 34. The
space between the jacket 50 and the applicator 40 is filled with a
coolant fluid. Advantageously, the fluid is circulated through the
jacket 50, entering at a fluid inlet 52 (FIG. 7) and exiting at a
fluid outlet 54 (FIG. 8). The inlet 52 and outlet 54 are arranged
at 90.degree. to one another, to encourage circumferential
circulation of the fluid. The cooling jacket 50 preferably extends
upstream and downstream of the microwave cavity 28, as shown.
[0057] Desirably, both the jacket 50 and the cooling fluid comprise
microwave transparent materials, thus maximizing microwave energy
coupling to the gas within the applicator 40, rather than direct
absorption by the jacket 50 and coolant. The cooling jacket 50
preferably comprises quartz.
[0058] The coolant fluid is selected to have minimal hydrogen (H)
content, which readily absorb microwave energy. Preferably, the
coolant contains no hydrogen, and in the illustrated embodiment
comprises a perfluorinated, inert heat transfer fluid. Such fluids
are available from the Kurt J. Lesker Company of Clairton, Pa.
under the trade name Galden.TM.. Advantageously, this liquid
coolant is available in multiple formulations having different
boiling points. Thus, the most appropriate formulation can be
selected for cooling the microwave applicator, depending upon the
desired parameters for operating the microwave plasma
generator.
[0059] Accordingly, recombination of desirable radicals within the
applicator 40 is reduced by provision of liquid cooling of the
applicator 40. Moreover, greater power can be coupled to the gas
without damage to the applicator 40. In the illustrated embodiment,
the power source 22 can be operated at full power (about 3,000 W)
under normal operating conditions (i.e., continuous or intermittent
operation while sequentially ashing photoresist from multiple
wafers), without inducing stress cracks in the sapphire applicator
40. It will be understood that improved power tolerance, and hence
more efficient radical production, are also applicable to quartz
applicators, which are generally more desirable for non-fluorinated
chemistries. It will be understood that operable power levels may
be considerably higher for quartz applicators with liquid
cooling.
[0060] To a lesser extent, the downstream transport tube 41 is also
heated by exposure to plasma by-products. The downstream tube,
however, is not directly contacted by the glow discharge. Rather
than liquid cooling the transport tube 41, therefore, the preferred
embodiments provide an insulated shroud 56 around the tube, as
shown in FIGS. 2-4 and 6, reducing risk of burns to technicians.
Preferably, fans direct air through the insulated shroud 56,
cooling the transport tube 41 by convection.
[0061] Impedance Matching
[0062] With reference to FIGS. 1-3, the microwave energy generated
by the power source 22 is propagated through an energy path
including the isolator 24, waveguide 26 and microwave cavity 28.
The impedance of the various sections of the energy path should be
closely matched to avoid energy loss through reflected power. By
careful impedance matching, a standing wave or resonant condition
is created in the microwave waveguide system 26, 28, where the
power is coupled to gas flowing through the applicator 40. While
the power source is protected from reflected energy by the isolator
24, reflected energy absorbed by the dummy load in the isolator 24
represents wasted power that would otherwise be available for
radical generation.
[0063] Impedance matching is complicated by the fact that the
medium within which the microwaves propagate is of variable
composition. The density and conductivity of gas flowing through
the microwave cavity 28 varies with different process recipes.
Since a reactor is typically utilized repetitively for the same
recipe by a semiconductor manufacturer, impedance matching is
typically performed for a given process recipe. Tuning the
impedance of the waveguide 26 (including the cavity 28) is thus
necessary when the reactor is first shipped to the manufacturer, as
well as each time the process recipe is changed.
[0064] A common method of tuning impedance of the waveguide 26 is
by employing three tuning knobs within the waveguide 26. By
adjusting the amount of protrusion of these conductors tranversely
across the waveguide 26 at three different locations along the
energy propagation axis, impedance of the waveguide 26 can be
matched to that of the isolator 24 and power source 22, thus
minimizing reflected power for a given process recipe. This manner
of impedance matching is known as a triple stub tuner. While
effective in minimizing reflected power, triple stub tuners are
expensive.
[0065] In the preferred embodiments, however, impedance matching is
controlled by the combination of a fixed tuning knob 30 (FIG. 1)
within the waveguide 26 and a sliding short 60, shown in FIG. 8.
The tuning knob 30 is preferably factory preset for gross tuning,
while the sliding short 60 dynamically fine tunes the impedance
matching.
[0066] The sliding short 60 is driven by a motor actuator 62 within
the autotuner module 32. The sliding short 60 comprises a conductor
which extends across the walls of the microwave cavity 28, thus
providing a movable end wall for the cavity 28. The position of the
sliding short 60 can be changed until impedance of the waveguide 26
(including the microwave cavity 28) closely matches that of the
isolator 24, at which point a resonant condition is achieved within
the cavity 28. Moreover, adjustment of the sliding short 60 along
the energy propagation axis, in place of transverse movement of a
triple stub tuner in the waveguide 26, facilitates arranging the
standing wave pattern to optimize coupling of energy to the gases
flowing through the applicator 40.
[0067] Moreover, as the name implies, the autotuner 32 matches
impedance dynamically via closed loop control. Reflected power is
continually measured at the isolator module 24 (FIGS. 1-3) and
sends signals to an electronic controller (not shown). The
controller, in turn, sends signals to the motor actuator 62, which
drives the sliding short 60. After the sliding short 60 moves, the
change in reflected power is recognized by the controller, which
then further adjusts the position of the sliding short 60, and so
on until reflected power is minimized.
[0068] In the illustrated embodiment, the sliding short 60 is
initially positioned one half of the microwave energy wavelength
(as measured in the waveguide system) from the center of the
applicator 40 running through the cavity 28. In this position, the
magnetic field is maximum, and a generally low electric field
strength reduces damage from ion acceleration downstream. If
necessary, the short 60 can be moved to one quarter of the
microwave energy wavelength for initial ignition and then moved to
one half wavelength after the plasma is essentially
self-supporting. The motor actuator 62 preferably enables deviation
forward or back from this default position by up to about 0.25
inch, more preferably up to about 0.5 inch, and in the illustrated
embodiment, the sliding short 60 is movable by about 0.75 inch to
either side of the default (1/2 wavelength) position. In another
embodiment, the actuator 62 enables deviation from the default
position of up to a half wavelength in either direction (.+-.1.7
inches), for fully adjustable impedance matching. The autotuner
module 32 further includes optical sensors to prevent over-movement
of the sliding short 60. It has been found that, not only does the
above-described arrangement (short 60 at {fraction (.lambda./2)}
from applicator center) minimize reflected power, but it also
maximizes microwave magnetic field intensity within the
applicator.
[0069] Dynamic closed loop control of impedance matching thus
accommodates fluctuations in parameters in operation of a single
process recipe. Additionally, autotuning can accommodate various
process recipes. FIGS. 9-11, for example, illustrate the effect of
reflected power against various process parameters, including
differences in power source output, total gas flow and gas
pressure. It will be understood that other process parameters, such
as constituent gas makeup, will also affect reflected power. As
illustrated, dynamic or in-situ impedance matching reduces losses
from reflected power, relative to fixed tuning, without the
downtime required for manually tuning for each different process
recipe. Such autotuning is particularly advantageous for research
and development of new processes, where it is desirable to test
many different processes for optimization.
[0070] Microwave Choke
[0071] With reference to FIG. 1, while the autotuner module 32
minimizes the power reflected back toward the power source 22,
impedance matching does not address the possibility of microwave
leakage through the openings in the cavity 28 through which the
applicator 40 extends upstream and downstream. Such leakage is
disadvantageous for a variety of reasons discussed in the
Background section above, and in U.S. Pat. No. 5,498,308 to
Kamarehi et al., entitled "Plasma Asher with Microwave Trap"
(hereinafter "the '308 patent"). The disclosure of the '308 patent
is incorporated herein by reference.
[0072] In order to minimize this leakage, therefore, the preferred
embodiments are provided with the upstream microwave choke or
emission barrier 44 and the downstream microwave choke or emission
barrier 46. FIGS. 4-8 show these emission barriers 44, 46 in
relation to the microwave cavity 28 and the carrier tube 34,
specifically in relation to the applicator section 40 of the
carrier tube 34.
[0073] FIG. 12 shows the emission barrier 46 in isolation. As
illustrated, the emission barrier 46 comprises an inner conductor
70, an outer conductor 72, and a dielectric medium 74. Each of
these components is rectangular (see the exploded view of FIG. 6)
and surrounds the applicator 40 just outside the microwave cavity
28. When assembled, the inner and outer conductors 70, 72 define a
choke cavity filled with the dielectric medium 74, having a gap or
opening 75 between the inner and outer conductors 70, 72 at a
distal end of the choke cavity.
[0074] The inner conductor 70 and the outer conductor 72 define
co-axial conductors selected to have an electrical length of a
quarter of a wavelength of the microwave energy of interest. As
disclosed, for example, in "Fields and Waves in Communication
Electronics," Ramo, Whinnery and Van Duzer, p. 46, Table 1.23
(hereinafter, "Ramo et al.") the impedance of an ideal quarter
wavelength line is given by the following formula, 1 Z = Z 0 2 Z
L
[0075] where Z is the impedance of the coaxial line, Z.sub.0
represents the characteristic impedance of the medium through which
the electromagnetic waves travel and Z.sub.L represents the load
impedance. In the illustrated embodiment, the coaxial line is
shorted across the inner conductor 70 and outer conductor 72 at the
proximal end 76. As the load impedance Z.sub.L of a shorted line is
ideally zero, the microwave energy propagating from the microwave
cavity 28 toward the distal end of the choke 46 meets with an
impedance approaching infinity at the opening 75 of the choke
cavity, regardless of the characteristic impedance Z.sub.0.
[0076] The high impedance of an open ended, shorted, quarter wave
coaxial line can alternatively be shown by using the formula for
the impedance of a shorted line, as also disclosed in Ramo et
al.:
Z=jZ.sub.0 tan(.beta.l)
[0077] The phase constant .beta. equals 2{fraction
(.pi./.lambda.)}, while the length of the line l has been selected
to be a quarter wave, or {fraction (.lambda./4)}. Inserting these
values into the formula above, the tangent term (tan .beta.l)
becomes the tangent of {fraction (.pi./2)}, which approaches
infinity.
[0078] Referring again to FIG. 1, while the high impedance
microwave chokes 44, 46 limit leakage of microwaves past the
opening 75 of the choke cavity, the energy still propagates along
the inner conductor 70 to the opening 75 of the choke. Energy thus
continues to couple to the gas within the applicator 40 to this
point, expanding the plasma beyond the confines of the microwave
cavity 28, both upstream and downstream of the cavity 28. Such
plasma expansion is disadvantageous for a number of reasons. As
noted above, the expansion of plasma downstream of the cavity 28
increases the likelihood that energetic ions and/or UV radiation
from plasma glow discharge will reach the process chamber 14.
Moreover, the expansion of the plasma disadvantageously reduces
plasma density. As will be recognized by the skilled artisan,
increasing the plasma density facilitates more efficient generation
of free radicals for a given power input.
[0079] Accordingly, the dielectric medium 74 is selected to have a
high dielectric constant. In contrast to air (dielectric
constant=1), the illustrated dielectric medium 74 preferably
comprises a solid material having a dielectric constant of at least
about 3.0, more preferably greater than about 5, and comprises
about 9 in the illustrated embodiment. The exemplary material of
the illustrated embodiment comprises a ceramic, more particularly
Stycast.TM. Hi K, available from Emerson & Cuming.
[0080] Microwaves travel on the surface of conductors within the
high dielectric medium 74, i.e., along the interior of the choke
cavity. Thus, the absolute distance of a quarter wave in the
exemplary ceramic is much shorter than the absolute distance of a
quarter wave in air (about 1.2 inches for 2,450 MHz microwave
energy). In the illustrated embodiment, a quarter wave through the
ceramic translates to an absolute distance of about 0.4 inch, since
the quarter wave length is proportional to the square root of the
medium's dielectric constant.
[0081] The effective volume of the plasma generated within the
cavity 28 and the leakage out to the opening of the choke cavity is
thus reduced with increasing dielectric constant of the dielectric
medium 74. Consequently, the density of the plasma is improved for
a given power input, and the efficiency of radical generation
improves. Improved radical generation, in turn, results in
increased ash rates in the illustrated plasma ash reactor.
[0082] Segregated Plasma Sources
[0083] FIGS. 13-15 illustrate a plasma ash reactor 100 constructed
in accordance with a second embodiment of the invention. It will be
understand that the reactor 100 preferably includes one or more,
and more preferably all of the above-noted features of the
invention. As the reactor 100 includes many features which can be
similar or identical to those of the previously discussed
embodiments, like features will be referred to by like reference
numerals, with the addition of the number 100.
[0084] The illustrated reactor includes a first process chamber 114
and a second process chamber 115. As the two chamber 114 and 115
can have identical construction, the present description will focus
on the first chamber 114 and the plasma generators associated
therewith.
[0085] The chamber 114 has two plasma generators 112 and 112', each
of which can have a similar construction as that disclosed above
(with particular distinctions noted below). Each of the two plasma
generators 112, 112' leads generated radicals, via transport tubes
141, 141' to the first process chamber 114, as shown. The free
radicals from each generator are mixed, prior to introduction to
the process chamber 114, in a mixer chamber 145.
[0086] With reference to FIGS. 13 and 14, it can be seen that the
microwave cavities 128 and 128' of the two generators 112 and 112'
are transverse to one another. This arrangement enables closer
packing of the modules within the reactor frame, saving footprint
on the clean room floor.
[0087] With reference to FIGS. 14 and 15, the transport tubes 141,
141' each communicate with the mixer chamber 145 via injectors 147,
147'. Advantageously, the injectors 147, 147' are configured to
inject radicals tangentially near the circumference of the mixer
chamber 145, thus facilitating mixing of the radicals from the two
different sources 128, 128'. Most preferably, the injectors 147,
147' inject with opposite orientation, such as clockwise and
counter-clockwise, creating turbulence and aiding the mixture of
reactive species from each of the plasma sources 128, 128'.
[0088] The interior walls of the mixer chamber 145 and the
injectors 147, 147' preferably comprise anodized aluminum, but can
also be fabricated of polished sapphire for improved surface
smoothness. In either case, the chamber 145 preferably has the same
chemical makeup as sapphire (Al.sub.2O.sub.3), which is
advantageously resistant to fluorine attack. The mixer chamber 145
has a low profile, preferably less than about 1.0 inch in height,
more preferably less than about 0.5 inch, and is about 0.22 inch
high in the illustrated embodiment. The low profile presents less
wall surface to the radicals, and thus reduced recombination of
free radicals.
[0089] As shown in FIG. 15, mixer chamber 145 includes a relatively
small central window 149 in the floor, opening into a first plenum
chamber 153 defined above a first perforated baffle plate 151. A
second baffle plate 155 below the first baffle plate 151 defines a
second plenum chamber 157 between the baffle plates 151, 155. The
second baffle plate 153 includes perforations (not shown) which are
misaligned relative to the perforations of the first baffle plate
151. Together, the baffle plates 151, 155 ensure uniform delivery
of free radicals to the process chamber 114 below.
[0090] The separate plasma generators 112, 112' advantageously
enable individual optimization for different reactants. For
example, the material used for the applicator and transport tubes
have individual advantages and disadvantages, which tend to favor
one material for a certain process recipe and another material for
another process recipe. As noted above, sapphire advantageously
exhibits resistance to fluorine attack. The disadvantage of
sapphire, however, is that it exhibits undesirable recombination of
free radicals. The table below illustrates, by way of example, the
recombination coefficient (.gamma.) of various materials.
1 Material Recombination efficiency (.gamma.) silver 1.000 copper
0.708 iron 0.150 nickel 0.117 aluminum oxide (sapphire) 0.009 glass
(quartz) 0.001 Teflon .TM. 0.0001
[0091] The above table illustrates that sapphire exhibits about
nine times more recombination of desirable radicals than quartz.
Thus, while desirable for resisting fluorine attack, sapphire
significantly reduces the efficiency of radical delivery to the
process chamber.
[0092] Thus, in the illustrated dual plasma source reactor 100, the
first plasma generator 112 includes a single-crystal sapphire,
integral applicator 140 and transport tube 141,
* * * * *