United States Patent [191
`Hwang et a1.
`
`lllllllllllllllllllllllllllllllllllllllllllllllllllllllllIlllllllllllllllll
`USO05174856A
`Patent Number:
`5,174,856
`Dec. 29, 1992
`Date of Patent:
`
`[11]
`[45]
`
`[54] METHOD FOR REMOVAL or
`PHOTORESIST ovER METAL WHICH
`ALSO REMovEs OR INAcrIvATEs
`CORROSION-FORMING MATERIALS
`REMAINING FROM PREVIOUS METAL
`ETCH
`[75] Inventors: Jeng H. Hwang, Cupertino; Steve Y.
`Mak, Pleasanton, both of Calif.
`Applied Materials, Inc., Santa Clara,
`Calif.
`Appl. No.: 749,733
`Filed:
`Aug. 26, 1991
`
`[73] Assignee:
`
`[21]
`[22]
`[51]
`[52]
`
`[58]
`
`Int. Cl.5 ....................... .. B44C 1/22; B29C 37/00
`US. Cl. ...................................... .. 156/643; 134/1;
`156/651; 156/655; 156/668
`Field of Search ............. .. 156/643, 646, 651, 655,
`156/657, 659.]. 662. 668, 345; 204/l92.36,
`298.33; 134/1. 19,31
`References Cited
`U.S. PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`Toy, David A., “Choose the Right Process to Strip
`Your Photoresist", Semiconductor International, Feb.,
`1990, pp. 82-87.
`Primary Examiner-William A. Powell
`Attorney, Agent, or Firm—.lohn P. Taylor
`[57]
`ABSTRACT
`A process is described for removing from an integrated
`circuit structure photoresist remaining after a metal
`etch which also removes or inactivates a sufficient
`amount of any remaining chlorine residues remaining
`from the previous metal etch to inhibit corrosion of the
`remaining metal for at least 24 hours. The process in
`cludes a first stripping step using either 03 gas or a
`combination of 03 gas and N3 gas and/ or a ?uorocarbon
`gas associated with a plasma followed by a subsequent
`step using a combination of O3 and N113 gases associated
`with a plasma. When ?uorocarbon gas is used in the
`?rst stripping step, a ?ushing step may be used prior to
`introduction of the NH; gas to flush out any remaining
`?uorocarbon gas. Preferably, the plasma is generated in
`a microwave plasma generator located upstream of the
`stripping chamber and the stripping gases pass through
`this generator so that reactive species produced from
`the gases in the plasma enter the stripping chamber.
`
`20 Claims, 1 Drawing Sheet
`
`4.980.022 12/1990 Fujimura et al. ................. I. 134/1 X
`5.057.187 10/1991 Shinagawa et al. .............. ., 156/643
`
`FOREIGN PATENT DOCUMENTS
`
`63-216346 9/1988 Japan ................................. .. 156/643
`2-140923 5/1990 Japan ................................. .. 156/643
`
`FLOWING A MIXTURE OF OXYGEN GAS,
`NITROGEN GAS, AND A FLUOROCARBON
`GAS THROUGH A MICROWAVE PLASMA
`GENERATOR INTO A STRIPPING CHAMBER
`CONTAINING AN INTEGRATED
`CIRCUIT STRUCTURE COMPRISING A
`PREVIOUSLY ETCHED PATTERNED METAL
`LAYER AND A PHOTORESIST MASK
`OVER THE UNETCHED METAL
`
`0
`
`IGNITING A PLASMA IN THE GENERATOR
`WHILE THE GASES ARE FLOWING THROUGH
`THE GENERATOR INTO THE CHAMBER
`
`EXTINGUISHING THE PLASMA AND
`SHUTTING OFF THE FLOW OF NITROGEN GAS
`AND FLUOROCARBON GAS WHILE FLUSHING
`THE CHAMBER WITH OXYGEN GAS TO
`REMOVE ANY REMAINING FLUOROCARBON
`GAS AND HEATING THE STRUCTURE TO A
`TEMPERATURE OF ABOUT 245'C
`
`FLOWING AMMONIA GAS AND OXYGEN GAS
`THROUGH THE PLASMA GENERATOR INTO THE
`CHAMBER AND IGNITING A PLASMA IN
`THE GENERATOR AFTER ABOUT 10 SECONDS
`OF GAS FLOW TO COMPLETE THE REMOVAL
`OF PHOTORESIST AND REMOVAL 0R
`INACTIVATION OF CHLORINE RESIDUES
`
`LAM Exh 1006-pg 1
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`US. Patent
`
`Dec. 29, 1992
`
`5,174,856
`
`FLOWING A MIXTURE OF OXYGEN GAS,
`NITROGEN GAS, AND A FLUOROCARBON
`GAS THROUGH A MICROWAVE PLASMA
`GENERATOR INTO A STRIPPING CHAMBER
`CONTAINING AN INTEGRATED
`CIRCUIT STRUCTURE COMPRISING _A
`PREVIOUSLY ETCHED PATTERNED METAL
`LAYER AND A PHOTORESIST MASK
`OVER THE UNETCHED METAL
`
`IGNITING A PLASMA IN THE GENERATOR
`WHILE THE GASES ARE FLOWING THROUGH
`THE GENERATOR INTO THE CHAMBER
`
`EXTINGUISHING THE PLASMA AND
`SHUTTING OFF THE FLOW OF NITROGEN GAS
`AND FLUOROCARBON GAS WHILE FLUSHING
`THE CHAMBER WITH OXYGEN GAS TO
`REMOVE ANY REMAINING FLUOROCARBON
`GAS AND HEATING THE STRUCTURE TO A
`TEMPERATURE OF ABOUT 245'C
`
`FLOWING AMMONIA GAS AND OXYGEN GAS
`THROUGH THE PLASMA GENERATOR INTO THE
`CHAMBER AND IGNITING A PLASMA IN
`THE GENERATOR AFTER ABOUT 10 SECONDS
`OF GAS FLOW TO COMPLETE THE REMOVAL
`OF PHOTORESIST AND REMOVAL OR
`INACTIVATION OF CHLORINE RESIDUES
`
`LAM Exh 1006-pg 2
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`1
`
`METHOD FOR REMOVAL OF PHOTORESIST
`OVER METAL WHICH ALSO REMOVES OR
`INACTIVATES CORROSION-FORMING
`MATERIALS REMAINING FROM PREVIOUS
`METAL ETCH
`
`5
`
`5,174,856
`2
`rosion of the metal by such chlorine-containing resi
`dues.
`It would, therefore, be desirable to provide an im
`proved process for the removal of photoresist remain
`ing'after a metal etch step which would not only re
`move the photoresist mask, but also remove or inacti
`vate a sufficient amount of any remaining chlorine-con
`taining residues from the metal etching step so that the
`remaining metal or metals will be passivated or free
`from corrosion for at least 24 hours after such process
`ing.
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to a process for removing pho
`toresist remaining over a metal layer after etching of the
`metal layer. More particularly, this invention relates to
`a dry process for removing photoresist and also remov
`ing or inactivating corrosion-forming etch residues re
`maining over a metal layer after etching of the metal
`layer.
`2. Description of the Related Art
`A photoresist mask, which remains over portions of
`one or more metal layers on an integrated circuit struc
`ture after patterning such metal layers through the pho
`toresist mask. has been conventionally removed by dry
`etch techniques using plasmas of oxygen and ?uorocar
`bon gases. Such dry etch techniques are preferred over
`wet etch techniques since the underlying metals are not
`attacked. and because the dry etching is sometimes
`more effective in removal of photoresist residues, par
`ticularly when the photoresist has been altered by reac
`tive ion etching. high temperature post bakes, or the
`like.
`However, such dry etch techniques have been found
`to be less than satisfactory in removing or inactivating
`certain sidewall etch residues remaining from the previ
`ous metal etch (patterning) step. Such metal etch pro
`cesses conventionally use chlorine-based chemistry.
`e.g., Cl; and BCI3, which may leave chlorine-containing
`residues on/in sidewalls of the photoresist mask and
`underlying metal layer portions after the metal etch.
`Ifsuch chlorine-containing residues (regardless of
`their source) in the sidewall residues, remaining after
`the metal etch step, are not removed or inactivated
`during the subsequent removal of the photoresist mask,
`such chlorine-containing residues may cause corrosion
`ofthe underlying metal prior to subsequent downstream
`processing steps which may include washing (solvent
`rinse) steps resulting in removal of such chlorine-con
`taining residues.
`'
`Since it is know that such subsequent processing steps
`can result in removal of any chlorine-containing resi
`dues remaining in sidewalls from the metal etch step, it
`has become conventional to judge the effectiveness of
`the photoresist removal step in also removing or inacti
`vating such chlorine-containing residues based on how
`much corrosion occurs during a 24 hour period follow
`ing the photoresist removal step. If no corrosion of the
`underlying metal occurs within 24 hours after the pho
`toresist removal step, the photoresist removal step is
`judged to have successfully removed or inactivated a
`sufficient amount of such corrosion-causing residues,
`since it is assumed that within 24 hours the integrated
`circuit structure will have been subjected to subsequent
`processing which will include at least one subsequent
`washing step which will remove any remaining chlo
`rine-containing residues.
`However, the currently practiced photoresist re
`moval process using 03 and CF4, has not succeeded in
`providing this desired 24 hours of protection from cor
`
`SUMMARY OF THE INVENTION
`The invention comprises a process for removing pho
`toresist remaining after a metal etch which also removes
`or inactivates a sufficient amount of any remaining chlo
`rine-containing residues, in sidewall residues from the
`metal etch step, to inhibit corrosion of the remaining
`metal or metals for at least 24 hours. The process in
`cludes a ?rst stripping step using either 03 or a combina
`tion of and a ?uorocarbon gas and/or nitrogen gas,
`associated with a plasma; and followed by a subsequent
`step using a combination of O3 and NH; gases also asso
`ciated with a plasma. When a fluorocarbon gas is used
`in the ?rst stripping step, a ?ushing step may be used
`prior to introduction of the NH; gas to flush out any
`remaining fluorocarbon gas.
`
`BRIEF DESCRIPTION OF THE DRAWING
`The sole FIGURE is a flow sheet illustrating the
`process of the invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The process of the invention is utilized on an inte
`grated circuit structure whereon a metal layer, or a
`composite of metal layers, previously deposited on an
`integrated circuit structure, and which, for example,
`may comprise a titanium nitride barrier layer and an
`overlying aluminum layer, hasjust been etched through
`a photoresist mask, leaving titanium nitride barrier por
`tions and aluminum portions under the photoresist
`mask. After the metal etch step, sidewall etch residues
`remain on the sidewall surfaces of the photoresist mask,
`and on the sidewall surfaces of the remaining underly
`ing metal layer portions. These sidewall etch residues,
`in turn, contain chlorine etch materials or residues
`therein which can cause corrosion of such metal por
`tions.
`In accordance with the invention, the chlorine-con
`taining residues in such sidewalls are removed or inacti
`vated and remaining metal portions of the metal layers
`are passivated during the process for the removal of the
`photoresist mask portions.
`The metal layer or layers, partially removed during
`the preceding metal etch step to leave the patterned
`metal portions, may comprise any metal conventionally
`used in the formation of integrated circuit structures,
`such as, for example, aluminum, titanium, tungsten, etc.
`When multiple metal layers are etched to form metal
`portions, the upper metal portions will usually comprise
`aluminum and the underlying metal portions will usu
`ally comprise a metal compound such as titanium ni
`tride which serves as a barrier layer to prevent spiking
`of aluminum to underlying silicon contacts. Such under
`lying electrically conductive metal-containing barrier
`materials will be referred to herein as metal layers,
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`5,174,856
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`regardless of whether they comprise pure metals. metal
`During this ?ushing step, the temperature of the
`alloys, or metal compounds.
`wafer may be slowly ramped up. at a rate of about 10°
`The process of the invention ?nds greatest utility
`C./second. to a temperature of from about 150° C. to
`when the upper metal portions over the barrier layer
`about 400° C.. typically about 245° C., at which temper
`comprise aluminum or any other metal which may be
`ature the wafer is maintained during the remainder of
`subject to corrosion by chlorine-containing residues. or
`the process.
`'
`other corrosion-forming metal etch step residues, if
`While the wafer is heating up to the desired operating
`such residues are not removed during the photoresist
`temperature, e.g., 245° C., ammonia gas is ?owed
`through the plasma generator into the stripping cham
`removal step. By aluminum is meant either pure alumi
`num or an aluminum alloy such as, for example, an
`ber at a rate equivalent to from about 10 to about 300
`aluminum alloy containing 1-2 wt. % silicon and 0-4
`sccm, and typically about 65 sccm, into a 5 liter cham
`ber, while the O2 is ?owed through the plasma genera
`wt. % copper.
`-
`The integrated circuit structure (wafer) is removed
`tor into the stripping chamber at an equivalent rate
`from the metal etch chamber and placed in a vacuum
`within the range of from about 2000 to about 5000 sccm,
`stripping chamber. unless the same chamber is to be
`typically about 3000 sccm. The ratio of NH3 gas to 03
`used for both processes. The stripping chamber is main
`gas ?owing into the stripping chamber should range
`tained at a pressure ranging from about 0.5 to about 3
`from about 0.5 volume % NH3 to about 10 volume %
`Torr, typically about 2 Torr. The temperature of the
`NH3, with the balance 03 gas.
`wafer at this stage is within a range of from about 40° C.
`The O3 and NH 3 gases are ?owed through the plasma
`generator into the stripping chamber for a period of at
`to about 100° C.
`'
`During the ?rst stripping stage, 03 gas may be used
`least about 10 seconds, after which a plasma is ignited in
`alone orin combination with a ?uorocarbon and/or N3
`the plasma generator and maintained for a period of at
`gas. When a ?uorocarbon gas is used in this step, the
`least about 40 seconds. preferably at least about 60 sec
`?uorocarbon gas may comprise, for example, CF4.
`onds, during which the ?ow of the O3 and NH; gases
`through the plasma generator into the stripping cham
`CHI-'3. CgF<,. C3F<,. C3H4F3, or the like. However, the
`use of CF4 gas is preferred.
`ber is maintained. Longer periods of time can be used,
`both for the gas ?ow prior to the plasma ignition, as
`The rate of ?ow of such gas or gases in this ?rst step
`well as the period during which the plasma remains on,
`will be at a rate equivalent to a rate of from about 1000
`to about 2000 standard cubic centimeters per minute
`but such longer periods are deemed to be unnecessary.
`After the plasma is extinguished and the ?ow of gases
`(sccm). typically about 1400 sccm. of 03: from about 0 .
`shut off. the wafer may be removed from the stripping
`to about 150 sccm. typically about 100 sccm. of N3; and
`chamber and subject to further processing, e.g., topside
`from about 0 sccm to about 80 sccm. typically about 45
`processing, as desired. The process of the invention
`sccm, of the ?uorocarbon gas into a 5 liter stripping '
`chamber.
`removes all of the photoresist and also removes or inac
`A plasma having a power level of about 500 watts to .
`tivates a sufficient amount of any chlorine-containing
`residues remaining from prior metal etching to provide
`about 2500 watts, typically about 1400 watts, is then
`ignited in the gas ?ow, preferably upstream ofthe strip
`freedom from any corrosion for at least 24 hours follow
`ping chamber, and maintained for about 10 seconds
`ing the resist removal process of the invention.
`after which the plasma is extinguished and the ?ow of
`To further illustrate the invention, a silicon wafer was
`the ?uorocarbon gas such as CF4 is shut off. The ?ow of
`provided with a layer of oxide thereon, a layer of alumi
`N3 into the chamber may also optionally be shut off at
`num containing 1 wt. % silicon and 0.5 wt. % copper
`this time.
`deposited over the oxide layer, and a photoresist mask
`While any type of conventionally generated plasma
`formed over the metal layer. The metal layer was ?rst
`conventionally etched through the photoresist mask
`may, in general. be used in the practice of the invention.
`preferably the plasma used in the process of the inven
`using a mixture of BCl; and C13 etchant gases in a 11.5
`tion is generated by a microwave plasma generator such
`liter vacuum etch chamber.
`as, for example. a Model AURA plasma generator com
`The wafer was then placed in a 5 liter stripping cham
`mercially available from the GaSonic division of Ato
`ber where the photoresist mask was then stripped and
`mel Corporation of Sunnyvale, Calif, which is located
`the underlying metal passivated by removal or inactiva
`upstream of the stripping chamber.
`tion of any etch residues remaining from the metal etch
`In this type of apparatus, the gas ?owing toward the
`step by ?rst ?owing a mixture of O2, CF4, and N2
`stripping chamber ?rst passes through the microwave
`through a microwave plasma generator located up
`plasma generator located upstream of the stripping
`stream of the stripping chamber, and then into the
`chamber and the plasma generated therein produces
`chamber at a rate of 1400 sccm O2, 45 sccm CF4, and
`reactive species from the gases ?owing through the
`100 sccm N2, for a period of about 10 seconds during
`plasma generator to the stripping chamber, and such
`which a plasma was ignited in the plasma generator and
`reactive species then ?ow into the stripping chamber.
`maintained at a power level of about 1400 watts.
`After the plasma is extinguished. and the flow of
`The plasma was then extinguished and the ?ow of
`?uorocarbon gas into the chamber is shut off, at the end
`CF4 and N2 shut off, while the ?ow of 02 was increased
`of the ?rst step, the ?ow of 03 may be increased from
`to 3000 sccm to ?ush out any remaining CF4 gases.
`1400 sccm (or its equivalence in a larger or smaller
`During this ?ushing period the wafer was ramped up to
`chamber) up to from about 2000 sccm to about 4000
`a temperature of about 245° C. at a rate of about 10° C.
`sccm, typically about 3000 sccm, to ?ush out any re
`per second, e.g., over about a 25 second period.
`maining ?uorocarbon gas prior to the next step of the
`After the wafer reached 245° C., as measured by a
`process. This ?ushing step, which need only be carried
`thermocouple contacting the back of the wafer, NH3
`out if ?uorocarbons are used in the ?rst step, is carried
`was ?owed through the plasma generator into the
`out for a minimum time period of at least about 10 sec
`chamber at a rate of about 65 sccm. After about 10
`ends.
`seconds, the plasma was reignited in the plasma genera
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`tor and maintained for about 60 seconds following
`which the ?ow of NH; was shut oft" and the plasma
`extinguished.
`The wafer was then removed from the stripping
`chamber and allowed to stand in the open atmosphere
`for 24 hours. The wafer surface was then examined,
`using 500 X light ?eld and dark ?eld optical micro
`scopes, and a 50,000 X scanning electron microscope
`(SEM). All of the photoresist was removed and no
`evidence of corrosion was found on the exposed metal
`surfaces.
`‘
`Thus, the process of the invention results in complete
`removal of photoresist from the surface of an integrated
`circuit structure after patterning of a metal layer be
`neath the resist mask and the remaining metal is passiv
`ated by removal or inactivation of any corrosion-pro
`ducing residues remaining from the prior metal etch
`step.
`Having thus described the invention what is claimed
`15'
`1. A process for removing from an integrated circuit '
`structure, in a stripping chamber. photoresist remaining
`after a metal etch which comprises:
`a) exposing said structure to 03 gas for a period of at
`least about 10 seconds; and
`b) then exposing said structure to 03 gas and NH; gas
`for a period of at least about 40 seconds.
`2. The process of claim 1 which further comprises
`igniting a plasma associated with said stripping chamber
`during said step b).
`3. The process of claim 2 wherein said plasma ignited
`during said step b) is maintained at a power level of
`from about 500 to about 2500 watts.
`4. The process of claim 3 wherein said plasma is gen
`erated in a microwave plasma generator upstream of
`said stripping chamber.
`5. The process of claim 4 wherein said NH; gas is
`?owed through said plasma generator into said strip
`ping chamber at a rate equivalent to a ?ow of from
`about 10 seem to about 300 sccm into a 5 liter chamber.
`6. The process of claim 4 wherein said 03 gas is
`?owed through said plasma generator into said strip
`ping chamber during said step b) at a rate equivalent to
`a ?ow of from about 2000 sccm to about 5000 sccm into
`a 5 liter chamber.
`7. The process of claim 4 wherein the ratio of NH;
`gas to 03 gas ?owing through said plasma generator
`into said stripping chamber during said step b) should
`range from about 0.5 volume % NH; to about 10 vol
`ume % NH; .
`8. The process of claim 7 which further comprises
`igniting a plasma in said plasma generator during said
`step a).
`9. The process of claim 8 wherein said step a) further
`comprises exposing said wafer to a combination of 03
`gas and N3 gas or a ?uorocarbon gas.
`10. The process of claim 8 wherein O3, and optionally
`N1 gas and/or a ?uorocarbon gas, is ?owed through‘
`said plasma generator into said stripping chamber dur
`ing said step a) at a rate equivalent to ?owing from
`about 1000 sccm to about 2000 sccm of said 03 gas, from
`0‘ sccm to about 150 sccm of said N3 gas, and from 0
`seem to about 80 sccm of said and from 0 seem to about
`80 sccm of said fluorocarbon gas into a 5 liter chamber.
`11. The process of claim 8 wherein said plasma during
`said step a) is maintained at a power level of from about
`500 to about 2500 watts.
`
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`12. The process of claim 10 wherein a ?uorocarbon
`gas flows through said plasma generator into said strip
`ping chamber during said step a) and which includes the
`additional step of continuing to ?ow 03 through said
`plasma generator into said stripping chamber after ex
`tinguishing said plasma and shutting off the ?ow of said
`?uorocarbon gas into said chamber during said step a)
`to thereby purge said stripping chamber of any ?uoroe
`carbon gas in said stripping chamber prior to com
`mencement of step b).
`13. The process of claim 1 wherein said wafer is main
`tained at a temperature of from about 100° C. to about
`400° C. during said step b).
`14. The process of claim 13 wherein said wafer is
`maintained at from about 40° C. to about 100° C. during
`said step a) and then heated to said step b) temperature
`range at a rate of about 10° C. per second.
`15. The process of claim 1 wherein said stripping
`chamber is maintained at a pressure of from about 0.5
`Torr to about 3 Torr during said process.
`16. A process for removing, from an integrated cir
`cuit structure, photoresist remaining after a metal etch
`which comprises:
`a) ?owing through a microwave plasma generator
`into a stripping chamber containing said integrated
`circuit structure a mixture of 03, N3, and ?uorocar
`bon gases for a period of at least about 10 seconds
`at a rate equivalent to ?owing from about 1000
`seem to about 2000 sccm of said 03 gas, from about
`50 sccm to about 150 sccm of said N3 gas. and from
`about 20 sccm to about 80 sccm of said ?uorocar
`bon gas into a 5 liter chamber;
`b) igniting a plasma in said plasma generator while
`said 03. N1, and ?uorocarbon gases are ?owing
`through said plasma generator into said stripping
`chamber;
`c) then extinguishing said plasma and shutting off said
`?ow of said N3 gas and said ?uorocarbon gas into
`said stripping chamber while maintaining said ?ow
`of 03 into said chamber to purge said chamber of
`?uorocarbon gas;
`d) then ?owing NH; and O3 gases through said
`plasma generator into said stripping chamber at a
`rate equivalent to a ?ow of from about 10 sccm to
`about 300 sccm of NH; and from about 2000 sccm
`to about 5000 sccm of 03 into a 5 liter chamber; and
`e) igniting a plasma in said plasma generator at least
`about 10 seconds after said NI-l3 and O3 gases begin
`?owing through said plasma generator into said
`stripping chamber.
`17. The process of claim 16 wherein said hydrocar
`bon gas comprises CF4.
`18. The process of claim 16 wherein said plasma is
`maintained at a power level of from about 500 watts to
`about 2500 watts while said 02, N2, and CF4 gases are
`?owing through said plasma generator into said strip
`ping chamber.
`19. The process of claim 16 .wherein said plasma is
`maintained at a power level of from about 500 watts to
`about 2500 watts for a period of at least about seconds
`while said 03 and NH3 gases are ?owing through said
`plasma generator into said stripping chamber.
`20. A process for removing. from an integrated cir
`cuit structure, photoresist remaining after a metal etch,
`which process also removes or inactivates chlorine
`residues remaining from said previous metal etch, com
`prising:
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`5,174,856
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`a) ?owing through a microwave plasma generator
`into a stripping chamber containing said integrated
`circuit structure a mixture of 03. N3. and CF4 gases
`for a period of at least about 10 seconds at a rate
`equivalent to ?owing from about 1000 sccm to
`about 2000 sccm of said 03 gas. from about 50 sccm
`to about 150 sccm of said N3 gas, and from about 20
`sccm to about 80 sccm of said CF4 gas into a 5 liter
`chamber;
`b) igniting a plasma in said microwave plasma genera
`tor and maintaining said plasma in said generator at
`a power level of from about 500 watts to about
`2500 watts while said 03, N3, and CF; gases are
`?owing through said plasma generator into said
`stripping chamber;
`0) then extinguishing said plasma and shutting off said
`?ow of said N3 gas and said Ct} gas into said strip
`
`8
`ping chamber while maintaining said ?ow of 03
`into said chamber to purge said stripping chamber
`of said CF4 gas:
`d) then ?owing NH; and O3 gases into said through
`said plasma generator into said stripping chamber
`at a rate equivalent to a ?ow of from about 10 sccm
`to about 300 sccm of NH; and from about 2000
`sccm to about 5000 sccm of 03 into a 5 liter cham
`ber; and
`e) igniting a plasma in said microwave plasma genera
`tor at least about 10 seconds after said NH3 and O3
`gases begin ?owing through said generator into
`said stripping chamber and maintaining said plasma
`at a power level of from about 500 watts to about
`2500 watts for a period of at least about 40 seconds.
`*
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