United States Patent
`
`[19]
`
`Shinagawa
`
`[11] Patent Number:
`
`5,628,871
`
`[45] Date of Patent:
`
`May 13, 1997
`
`USOO5628871A
`
`[54] METHOD OF REMOVING RESIST MASK
`AND A METHOD OF MANUFACTURING
`SEMICONDUCTOR DEVICE
`
`[75]
`
`Inventor: Keisuke Shinagawa, Kawasaki, Japan
`
`[73] Assignee: Fujitsu Limited, Kanagawa, Japan
`
`[21] Appl. No.: 264,915
`
`[22] Filed:
`
`Jun. 24, 1994
`
`[30]
`
`Foreign Application Priority Data
`
`Sep. 17, 1993
`
`[JP]
`
`Japan .................................. .. 5.232013
`
`Int. Cl.° ................................................... H0lL 21/312
`[51]
`[52] U.S. Cl. .......................... 438/514; 438/704; 438/725;
`438/714
`
`[58] Field of Search ................................... .. 156/643, 646,
`156/651, 659.1; 437/229, 20, 22, 931; 148/DIG. 83
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`8/1977 Saiki et al. .............................. 437/236
`4,040,083
`12/1988 Fujimura et a1.
`..
`I56/643
`4,789,427
`8/1989 Fujimura et al.
`..
`156/643
`4,861,424
`8/1989 Fujimura et al.
`..
`437/229
`4,861,732
`7/1990 Fujimura et al.
`..
`156/643
`4,938,839
`4,980,022 12/1990 Fujimura et al.
`....................... 156/643
`
`
`
`OTHER PUBLICATIONS
`
`S. Wolf “Silicon Processing for the VLSI Era; Process
`Technology”, vol. 1., 534, 1986.
`
`Primary Examiner——John Niebling
`Assistant Examiner—Thomas G. Bilodeau
`
`Attomey, Agent, or Firm—Nikaido Marmelstein Murray &
`Oram LLP
`
`[57]
`
`ABSTRACT
`
`The present invention relates to a method of manufacturing
`a semiconductor device including a process of removing a
`photoresist mask or a photosensitive polyimide mask
`remaining after implanting impurity ions into a semicon-
`ductor layer or the like, and has an object to prevent
`generation of oxides of impurities and photoresist explosion
`and arranging it so that no residue remains. The present
`invention comprises the steps of forming a mask composed
`of photosensitive organic matter on a layer,
`implanting
`impurity ions into the layer through the mask, and removing
`the mask through processing including three steps of: expos-
`ing the mask to a plasma activated gas containing hydrogen,
`exposing to the mask to a plasma activated gas containing
`oxygen, and exposing the mask to a solution containing
`nitric acid under conditions suflicient to dissolve alumina
`
`which had formed on the mask during exposure of the mask
`to oxygen.
`
`18 Claims, 8 Drawing Sheets
`
`IMPURITY IONS
`
`1111 114
`
`
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`H: 1 H20
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`PLASMA
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`ii
`
`Intel Corp. et al. Exhibit 1 01 8
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 1 of 8
`
`5,628,871
`
`IMPURITY IONS
`
`»l
`
`J
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`¢
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`FIG. 1A
`
`(PRIOR ART)
`
`FIG . 1B
`
`(PRIOR ART)
`
`FIG . 1C
`
`(PRIOR ART)
`
`
`
`Intel Corp. et al. Exhibit 1018
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 2 of 3
`
`5,628,871
`
`FIG. 1D
`
`(PRIOR ART)
`
`FIG. 1E
`
`(PRIOR ART)
`
`Intel Corp. et al. Exhibit 1018
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 3 of 8
`
`5,628,871
`
`FIG. 2
`
`Intel Corp. et al. Exhibit 1018
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 4 of 8
`
`‘
`
`5,628,871
`
`
`
`J
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`FIG.3B
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`FIG.3C
`
`Intel Corp. et al. Exhibit 1018
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 5 of 8
`
`5,628,871
`
`02 + H20
`
`PK?TINU¥EED C§¥S
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`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 6 of 8
`
`5,628,871
`
`FIG.4A
`
`+++
` ++
`+++
`
`32
`
`IMPURITY IONS
`
`1114111
`
`FIG.4B
`
`NITRIC ACID
`
`
`
`Intel Corp. et al. Exhibit 1018
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 7 of 8
`
`5,628,871
`
`
`
`I
`
`‘\."\—32a
`
`,34
`
`Intel Corp. et al. Exhibit 1018
`
`02+ H20 ACTIVATEDGAS
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`FIG.4E
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`Intel Corp. et al. Exhibit 1018
`
`

`
`U.S. Patent
`
`May 13, 1997
`
`Sheet 8 of 8
`
`5,628,871
`
`
`
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`Intel Corp. et al. Exhibit 1018
`
`Intel Corp. et al. Exhibit 1018
`
`

`
`1
`METHOD OF REMOVING RESIST MASK
`AND A METHOD OF MANUFACTURING
`SEMICONDUCTOR DEVICE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to a method of removing a
`resist mask and a method of manufacturing a semiconductor
`device, and more particularly to a method of removing a
`photoresist mask or a photosensitive polyimide mask which
`remains after implanting impurity ions selectively into a
`semiconductor substrate to form a conductive layer or a steel
`or the like to improve hardness and persistence and a method
`of manufacturing a semiconductor device including a pro-
`cess of removing a resist mask.
`In a method of manufacturing a semiconductor device, it
`is required to introduce impurities such as boron and phos-
`phorus into a semiconductor substrate or the like selectively
`by ion implantation in order to form a pn junction such as a
`source/drain region and the other conductive layers in a
`semiconductor layer, a semiconductor substrate or the like.
`In such a case, such a process is to form a mask using a
`photoresist or a photosensitive polyimide and to remove the
`mask after ion implantation is performed.
`2. Description of the Prior Art
`FIG. 1A to FIG. 1E are sectional views showing how to
`remove a photoresist mask after conducting ion implantation
`selectively into a semiconductor substrate for instance using
`a photoresist mask
`First, a photoresist mask is formed on a semiconductor
`substrate 1 by spin coating. Then, after exposing the pho-
`toresist film, it is soaked in a developer and a photoresist
`mask 2 is formed as shown in FIG. 1A.
`
`Next, as shown in FIG. 1B, impurity ions are implanted
`selectively into the semiconductor substrate 1 through the
`photoresist mask 2.
`At the same time, impurities, such as P, B and As, are
`bonded chemically with the remaining photoresist polymer,
`and the surface layer of the photoresist mask 2 changes in
`quality and becomes a very hard layer. This hard layer is
`referred to as a carbonized layer (transmuted layer) 2b.
`Besides, the photoresist polymer inside the photoresist mask
`beyond reach of impurities remains in a state as it is. This
`interior layer is referred to as an un-changed layer (non-
`transmuted layer) 2a.
`Then, ashing, using an oxygen plasma, is applied to the
`photoresist mask 2 so as to remove it.
`Now, the ashing, using oxygen plasma, demolishes the
`carbonized layer 2b in the surface layer of the photoresist
`mask 2 by utilizing bombardment by ions in oxygen plasma,
`and also the photoresist mask is removed by utilizing an
`oxidation reaction between oxygen ions and the photoresist
`polymer.
`As shown in FIG. 1C, however, in ashing with oxygen
`plasma, impurities (such as P, B and As) implanted into the
`photoresist mask 2 and oxygen formed into plasma react
`with each other, and oxides of these impurities are produced.
`In particular, there is such a problem that those oxides of
`impurities formed in the ashing of the photoresist mask after
`ions are implanted at high dose, are diflicult to volatilize.
`Therefore they remain on the substrate as it is.
`Further, when the wafer temperature rises, because of
`plasma irradiation, before the carbonized layer 2b is
`removed completely, volatile components inside the
`
`5,628,871
`
`2
`
`un-changed layer 2a are gasified and expanded as shown in
`FIG. 1D. In such a case, since the carbonized layer 2b which
`covers the un-changed layer 2a has a structure which is
`dense and does not pass gas 2c, what is called a “photoresist
`explosion”,
`in which the carbonized layer 2b explodes
`becasue of the pressure of the expanded gas 2c, is generated
`as shown in FIG. IE. Further, there is such a problem that
`the pieces of the carbonized layer 2b that have scattered by
`the photoresist explosion form particles, which cause low-
`ering of production yield of semiconductor devices.
`Accordingly, a method of performing hydrogen plasma
`processing, such that the wafer temperature is maintained at
`a low temperature, has been developed as a process in which
`the photoresist explosion is not brought about, and oxides of
`impurities are not produced. This processing method is
`called a 2-step ashing process, and has been announced
`already by the present inventor under a title of “High Ashing
`Rate of Ion Implanted Resist Layer” in DRY PROCESS
`SYMPOSIUM OF 1992.
`
`It has been well known that a compound of an impurity (P,
`B or As) and hydrogen is liable to volatilize, and it was
`found that the carbonized layer had been removed without
`leaving residues when hydrogen plasma processing was
`performed practically. Further, since the wafer is cooled to
`a low temperature of approximately 5° C., the photoresist
`explosion has not occurred. Reactive ion etching (RIE) for
`removing the carbonized layer physically and chemically by
`hydrogen ions is suitable for the hydrogen plasma process-
`mg.
`Further, after the carbonized layer is removed by the
`hydrogen plasma processing, the interior un-changed layer
`appears. In order to remove this un-changed layer, a down-
`stream ashing process using 02 as main reaction gas is very
`often applied because its process has less bombardment of
`the substrate by ions.
`By applying the 2-step ashing processing in which the
`plasma processing by hydrogen gas and the downstream
`processing by oxygen gas described above are performed in
`succession, generation of oxides of impurities is prevented,
`and photoresist explosion is also prevented, thus greatly
`reducing the quantity of produced particles. With this, it is
`possible to aim at improvement of production yield of
`semiconductor devices.
`
`When an ashing process is performed using this 2-step
`ashing process, however, new residues 4 remain along the
`side wall of the removed photoresist mask 2 sometimes as
`shown in FIG. 2, which becomes a problem.
`SUMMARY OF THE INVENTION
`
`It is an object of the present invention to provide a method
`of removing a resist mask and a method of manufacturing a
`semiconductor device, including a process of removing a
`resist mask, capable of preventing generation of oxides of
`impurities and what is called photoresist explosion so that no
`residues remain.
`
`According to the investigation by the present inventor, it
`has been found from u-Auger Electron Spectroscopy (AES)
`analysis that the residue remaining after the 2-step ashing
`process is performed contains alumina as a main component.
`Moreover,
`the residue remains on the side wall of the
`photoresist mask. This residue is of a type which is different
`from the residue composed of oxides of impurities which
`have been generated in oxygen plasma ashing, and such
`residues could not be found after the oxygen plasma ashing
`process giving rise to photoresist explosion. It is believed
`that this is because even the residues existing have been, for
`all practical purposes blown away by the photoresist explo-
`sron.
`
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`Intel Corp. et al. Exhibit 1 01 8
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`Intel Corp. et al. Exhibit 1018
`
`

`
`5,628,871
`
`3
`The origin of the alumina residues is conceivable to be
`aluminum from the component, of the ion implantation
`apparatus. This aluminum is partially vaporized, oxidized
`naturally into alumina residue, and then deposits on the side
`walls of the resist mask.
`
`In order to remove the alumina residue described above.
`which has been ascertained through the investigation men-
`tioned above, without exerting influence upon others, it is
`suflicient to expose the residue to chemicals which dissolve
`alumina. for example at least either nitric acid or phosphoric
`acid. before or after the 2—step ashing process. This method
`is eifective not only with a mask composed of photoresist
`but also a mask composed of photosensitive polyirnide.
`Accordingly. in order to remove a mask remaining after
`impurity ions are implanted selectively into a substrate or
`the like. it is required to remove a carbonized layer in a
`surface layer containing impurities and an un-changed layer
`inside thereof by the 2-step ashing process, and to remove
`alumina residue remaining on the surface of the mask by
`chemical processing using nitric acid or phosphoric acid.
`The 2-step ashing processing consists of plasma process-
`ing first by a gas containing hydrogen for removing the
`carbonized layer. and second, a downstream processing by a
`gas containing oxygen for removing the un-changed layer.
`Reactive ion etching (RIE) of the cathode-coupled parallel
`plate type is suitable for the plasma processing by a gas
`containing hydrogen. This is because, according to the RIE
`method, the carbonized layer is removed by chemical reac-
`tion while hydrogen ions having high energy bombard the
`wafer perpendicularly thereby to demolish the carbonized
`layer physically. The RIE of this type applies high frequency
`electric power between opposed electrodes, forms hydrogen
`gas, introduced in between the opposed electrodes, into
`plasma, and imposes the plasma onto a wafer placed on one
`of the opposed electrodes.
`On the other hand, a method which forms gas containing
`oxygen into plasma, and exposes a mask on a substrate to
`activated gas remaining after removing ions from the gas
`formed into plasma so as to etch the mask, is suitable for the
`second. or downstream, ashing process by a gas containing
`oxygen. The reason for the above is that ion bombardment
`of the substrate is decreased so as to reduce the damage of
`the substrate to a minimum during the ashing process.
`Besides, in order to increase the ashing rate, both in the
`plasma process and the downstream process, it is suflicient
`to add a small amount of water vapor to hydrogen or oxygen.
`This is because the generation efliciency of hydrogen atoms
`or oxygen atoms, which are reaction species for ashing, is
`increased by addition of a small amount of water vapor.
`As described above, it is possible to remove the photo-
`resist mask and the alumina residues completely by joint use
`of the 2—step ashing process and the chemical process with
`nitric acid or phosphoric acid. Further, it is possible to
`prevent generation of oxides of impurities and photoresist
`explosion so as to suppress the generation of particles.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1A to FIG. 1E are sectional views showing a
`conventional method of manufacturing a semiconductor
`device including a method of removing a photoresist mask;
`FIG. 2 is a sectional view showing problems in a con-
`ventional method of removing a photoresist mask;
`FIG. 3A to FIG. 3E are sectional views showing a method
`of manufacturing a semiconductor device including a
`method of removing a photoresist mask according to a first
`embodiment of the present invention;
`
`4
`FIG. 4A to FIG. 4E are sectional views showing a method
`of manufacturing a semiconductor device including a
`method of removing a photoresist mask according to a
`second embodiment of the present invention; and
`FIG. 5 is a side-view showing a structure of a 2-step
`ashing apparatus used in a method of removing a photoresist
`mask according to an embodiment of the present invention.
`
`DESCRIPTION OF THE PREFERRED
`ENLBODIMENTS
`
`Embodiments of the present invention will be described
`hereinafter with reference to the drawings. (1) Description of
`a method of manufactming a semiconductor device includ-
`ing a method of removing a photoresist mask according to
`a first embodiment of the present invention.
`In the method of removing a photoresist mask according
`to the first embodiment of the present invention. chemical
`processing with at least nitric acid and phosphoric acid is
`performed after the 2-step ashing processing.
`(i) Description of an apparatus for 2—step ashing process-
`ing.
`FIG. 5 is a side-view for explaining a structure of the
`2-step ashing processing apparatus used in a method of
`removing a photoresist mask according to an embodiment of
`the present invention.
`In FIG. 5, a reference numeral 11 represents an apparatus
`for 2-step ashing processing, in which a cathode-coupled
`parallel plate type RIE chamber 12 for performing hydrogen
`plasma processing and a downstream chamber 13 for per-
`forming oxygen plasma processing are combined with each
`other. Further, a wafer 15 to be processed is transported
`freely between the RIE chamber 12 and the downstream
`chamber 13 without touching the atmosphere by means of an
`arm 14 provided at a connected portion between the RIE
`chamber 12 and the downstream chamber 13.
`The RIE chamber 12 has a structure as described herein-
`
`the RIE chamber 12 is equipped with a
`after. Namely,
`cooling stage (placing table) 16 including means for placing
`the wafer 15 and cooling the placed wafer 15 and also
`serving as a cathode electrode among opposed electrodes, an
`RF power source 17 connected to the cooling stage 16 and
`for supplying electric power for forming the gas containing
`hydrogen (reaction gas) into plasma, and a plate—shaped gas
`shower 18 for supplying reaction gas onto the cooling stage
`16 and serving also as an anode electrode among opposed
`electrodes.
`
`Further, the downstream chamber 13 has a structure as
`described hereunder. Namely, the downstream chamber 13 is
`equipped with a heating stage (placing table) 19 for placing
`the wafer 15 (a substrate) and including a heater inside, a
`plasma chamber 21 partitioned from the downstream cham-
`ber 13 by a shower head 20 and into which the gas
`containing oxygen (reaction gas) is introduced, a gas inlet
`port 22 for introducing the gas containing oxygen into the
`plasma chamber 21 and a waveguide 24 partitioned from the
`plasma chamber 21 by a microwave transmitting window 23
`and for introducing a microwave into the plasma chamber
`21.
`
`The processing is performed as follows using the appa-
`ratus 11 for the 2-step ashing processing described above.
`First, the plasma processing in the RIE chamber 12 will
`be described. After a wafer is introduced into the RIE
`chamber 12, a gas containing hydrogen is introduced
`therein. The gas containing hydrogen is formed into a
`plasma by RF electric power applied between opposed
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`Intel Corp. et al. Exhibit 1 01 8
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`Intel Corp. et al. Exhibit 1018
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`

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`5,628,871
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`electrodes 16 and 18 and pours onto the wafer 15. Since RIE
`is used at this time, hydrogen ions having high energy react
`on conductivity type impurities (P, B or As) while bombard-
`ing the wafer 15 perpendicularly thereby to physically
`demolish the carbonized layer. Further, the compounds of
`impurities and hydrogen volatilize easily, thus removing the
`carbonized layer an the surface of the photoresist mask.
`Besides, since the cooling stage 16 has a means for
`cooling the wafer 15, it is possible to prevent photoresist
`explosion by cooling the wafer 15 to maintain it at approxi-
`mately 5° C. for instance until the hydrogen and the impu-
`rities finish reacting with each other.
`Next,
`the downstream processing in the downstream
`chamber 13 will be described hereinafter.
`
`The gas containing oxygen is formed into a plasma in the
`plasma chamber 21 by the imposition of microwaves. Then,
`ions are removed out of the plasma due to a fact that the
`plasma flows into the downstream chamber 13 through the
`shower head 20. Furthermore, only activated gas containing
`no ions remaining thereafter contacts the wafer 15. The
`activated gas reacts on the un—changed layer of the photo-
`resist mask, thereby to etch and remove the un—changed
`layer of the photoresist mask.
`The reason why the downstream process by the gas
`containing oxygen is adopted is to reduce ion bombardment
`of the wafer 15 during the ashing process so as to keep the
`damage of the wafer 15 to the minimum.
`Besides,
`in order to increase the ashing rate in the
`downstream processing, it is suflicient to add a small amount
`of water vapor to the hydrogen or oxygen. This is because
`the generation efficiency of hydrogen atoms, or oxygen
`atoms which are reaction species for ashing, is increased by
`the addition of a small amount of water vapor.
`(ii) Description of a method of manufacturing a semicon-
`ductor device including a method of removing a photoresist
`mask.
`
`FIG. 3A to FIG. 3B are sectional views showing a method
`of removing a photoresist mask after implanting impurity
`ions selectively into a semiconductor substrate for instance
`using a photoresist mask.
`First, a photoresist film is formed on a semiconductor
`substrate (a substrate wafer) 15, such as a silicon substrate,
`by a conventional application method. Then, after the pho-
`toresist film is exposed, it is soaked in a developer so as to
`form a photoresist mask 32 as shown in FIG. 3A.
`Next, as shown in FIG. 3B, impurity ions such as boron
`(B) are implanted selectively into the semiconductor sub-
`strate 15 which is exposed through the photoresist mask 32.
`By this process, an implanting layer 34 is formed.
`At this time, aluminum is inadvertently sputtered from the
`apparatus for ion implantation and sticks to the side walls of
`the photoresist mask 32, and reacts further with residual
`oxygen, thus generating an alumina film 33. Further, impu-
`rities are bonded chemically with the photoresist polymer,
`thereby to form a very hard carbonized layer 32b‘ on the
`surface layer of the photoresist mask 32. The thickness of the
`carbonized layer 32b depends on implantation energy of
`impurities. In the case of the energy needed for forming an
`ion implantation layer 34 of normal depth, it is estimated
`that the carbonized layer is approximately several hundred A
`thick. The photoresist polymer inside the photoresist mask
`32, beyond the reach of these impurities, remains as it is.
`This layer is referred to as an un—changed layer 32a.
`Then, the wafer 15 is brought into the RIE chamber 12 of
`the apparatus for the 2-step ashing processing as shown in
`
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`FIG. 5 and placed on the cooling stage 16. Then, the wafer
`15 is cooled by the cooling means and maintained at 5° C.
`Next, the pressure in the RIE chamber 12 is reduced, and,
`when it reaches a predetermined pressure, mixed gas of H2
`at a flow rate of 400 sccm and H20 at a flow rate of 100 sccm
`is introduced into the RIE chamber 12 and maintained at a
`pressure of 1 Torr.
`Then, RF power of 500W is applied between opposed
`electrodes 16 and 18. With this, a mixed gas of H._,+H2O is
`formed into a plasma. Further, hydrogen ions having high
`energy bombard the wafer perpendicularly and remove the
`carbonized layer 32b by chemical reaction while demolish-
`ing the carbonized layer 32b physically. After the carbonized
`layer 32b is removed, the interior un—changed layer 32a
`appears as shown in FIG. 3C. Besides, since a small amount
`of water vapor is added to hydrogen, the generation efli-
`ciency of hydrogen atoms, which are reactive species for
`ashing, is increased, and thus,
`the ashing rate becomes
`considerably high.
`Next,
`the interior un—changed layer 32a which has
`become exposed is removed. For this purpose, the wafer 15,
`to be completed by plasma processing using the mixed gas
`of O2+H2O, is transported into the downstream chamber 13,
`and the wafer 15 is placed on the heating stage 19. Then, the
`wafer 15 is heated by the heating means and maintained at
`a temperature of 200° C.
`Then, the pressure in the downstream chamber 13 in the
`plasma chamber 21 is reduced. When the pressure reaches a
`predetermined pressure, a mixed gas of 02 at a flow rate of
`1,350 sccm and H20 at a flow rate of 150 sccm is introduced
`into the plasma chamber 21 and maintained at a pressure of
`1 Torr.
`
`Next, microwave power of 1.5 KW with a frequency of
`2.45 GHz is applied to a waveguide 24. With this, oxygen is
`formed into plasma and only neutral activated oxygen in the
`plasma flows downstream, and the un—changed layer 32a is
`removed as shown in FIG. 3D. However,
`the alumina
`residue 33a still remains without having been removed.
`Then, in order to remove the alumina residue 33a, the
`wafer 15 is taken out of the processing apparatus. After a
`nitric acid solution is prepared, heated and maintained at 80°
`C., the wafer 15 is soaked for a period of approximately one
`minute as shown in FIG. 3E. With this, the alumina residue
`33a is removed, thus completing the removal of the photo-
`resist mask 32. Besides, in the processing by the nitric acid
`solution described above, the component ratio of the nitric
`acid solution, processing temperature and soaking period of
`time are not limited, but may be set optionally within a range
`that the alumina residue 33a can be removed
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`50
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`As described above, according to the first embodiment of
`the present invention, chemical processing by nitric acid is
`7 performed after the 2-step ashing processing is performed.
`Thus, the carbonized layer 32b in the surface layer of the
`photoresist mask 32 and the interior un—changed layer 32a
`are removed while preventing generation of impurity oxides
`and photoresist explosion by the 2-step ashing process.
`Furthermore, the alumina residue 33a which has been gen-
`erated along the surface of the side wall of the photoresist
`mask 32 is removed by chemical processing using nitric
`acid.
`
`With this, it is possible to remove the photoresist mask 32
`and the alumina residue 33a completely, and also to prevent
`generation of impurity oxides and photoresist explosion
`whereby of the quantity of generated particles is reduced to
`eliminate.
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`65
`
`Next, as showing some examples which the above method
`of removing a resist mask is applied to, it can be used in case
`
`Intel Corp. et al. Exhibit 1 01 8
`
`Intel Corp. et al. Exhibit 1018
`
`

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`5,628,871
`
`7
`
`of removing a used photoresist mask after selectively
`implanting impurity ions to form a source/drain of an
`insulated gate field etfect transistor or to form wells as
`isolation regions or element forming regions in a semicon-
`ductor substrate or to give charges to an insulating layer.
`Further,
`it can be used in case of removing a used
`photoresist mask after selectively implanting impurities into
`a steel to improve hardness and persistence or a silicon
`substrate for micro machines for similar purpose.
`The case of selectively implanting impurities to form a
`source/drain region and thereafter removing a used photo-
`resist mask is explained in the following. (2) Description of
`a method of manufacturing a semiconductor device includ-
`ing a method of removing a photoresist mask according to
`a second embodiment of the present invention.
`In a method of removing a photoresist mask according to
`a second embodiment of the present invention, chemical
`processing using nitric acid is performed before the 2-step
`ashing process.
`FIG. 4A to FIG. 4E are sectional views showing a method
`of removing a photoresist mask after selective ion implan-
`tation into a semiconductor substrate for instance using the
`photoresist mask.
`First, as shown in FIG. 4A, a photoresist film is formed on
`a semiconductor substrate (wafer) 15 by an application
`method. Then, after the photoresist film is exposed, it is
`soaked in a developer. and the photoresist film is removed in
`a region where ion implantation is to be made is thus
`forming a photoresist mask 32.
`Next. impurities such as boron (B) are ion-implanted
`selectively into the semiconductor substrate 15 in the
`exposed areas through this photoresist mask 32.
`At this time, aluminum is inadvertently sputtered from the
`chamber of the apparatus for ion implantation and sticks to
`the side wall of the photoresist mask 32, and it reacts further
`with residual oxygen and an alumina film 33 is generated
`Rlrther, impurities are bonded chemically with the photo-
`resist polymer and the surface layer of the photoresist mask
`32 becomes a very hard carbonized layer 32b. Besides, the
`photoresist polymer inside the photoresist mask 32 remains
`as an un-changed layer 32a.
`Then, in order to remove the alumina film 33, a nitric acid
`solution is prepared, heated and maintained at 80° C., and
`the wafer is soaked therein for approximately one minute
`thereafter. With this, the alumina film 33 stuck to the surface
`of the side wall of the photoresist mask 32 is removed.
`Next, processing similar to the 2-step ashing processing
`described in the first embodiment is performed by the
`apparatus for the 2-step ashing processing. The carbonized
`layer 32b is removed first by plasma processing using mixed
`gas of H2+H2O. Then, the interior un-changed layer 32a,
`which has appeared after hydrogen processing is removed
`by downstream processing using mixed gas of O2+H2O, thus
`completing removal of the photoresist mask 32.
`As described above, according to the method of removing
`the photoresist mask of the second embodiment of the
`present invention, the 2-step ashing processing is performed
`after performing chemical processing with nitric acid.
`Thus, the alumina film 33 on the side wall of the photo-
`resist mask 32 is removed by chemical processing with nitric
`acid. Furthermore, by the 2-step ashing processing,
`the
`carbonized layer 32b in the surface layer of the photoresist
`mask 32 and the interior un-changed layer 32a are removed
`while preventing generation of impurity oxides and photo-
`resist explosion.
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`10
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`20
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`35
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`S5
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`65
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`8
`With this, it is possible to remove the photoresist mask 32
`and the alumina film 33 completely, and also to prevent
`generation of impurity oxides and photoresist explosion,
`thereby suppressing generation of particles.
`Besides, in the first and the second embodiments. a nitric
`acid solution heated to 80° C. is used in order to remove the
`alumina residue 33a and the alumina film 33, but a phos-
`phoric acid solution or a mixed solution of nitric acid plus
`phosphoric acid may be used. Further, the component ratio
`of the solution, processing temperature and soaking period
`of time can be set optionally within a range that the alumina
`residue 33a and the alumina film 33 can be removed.
`
`Further, the present invention is applied to a case that the
`impurity is boron, but the present invention is also appli-
`cable to a case of other impurities such as P and As.
`Furthermore, a semiconductor substrate such as a silicon
`substrate is used, but a polysilicon film and a polycide film
`may also be used as well as a semiconductor substrate
`composed of other materials.
`Further, a photoresist containing novolak resin as base
`polymer is applicable as an exemplification of the photore-
`sist. Moreover a resist of PMMA (polymethylmethacrylate)
`or alicyclic compound system is applicable. For example, a
`resist of a norbomene system or an adamanthyl system can
`be used as a resist of a typical allcyclic compound system.
`Furthermore,
`it is also possible to use a photosensitive
`polyimide as a resist.
`The above-mentioned embodiment is applicable to a case
`that impurities such as boron and phosphorus are introduced
`into a semiconductor substrate or the like by ion implanta-
`tion in order to form a pn junction such as a source/drain
`region and other conductive layers in a semiconductor layer
`on an insulating layer, a semiconductor substrate or the like
`in the manufacture of a field effect transistor, a semicon-
`ductor device of integrated circuit or the like.
`What is claimed is:
`1. A method of removing a resist mask on a substrate
`comprising the steps of:
`(a) forming a mask comprising organic material on a
`surface layer of a substrate;
`(b) implanting impurity ions into the surface layer through
`openings in the mask as well as into a surface layer of
`said mask thereby reacting said impurity ions with a
`surface portion of said mask and thereby forming a
`hard layer on a surface of said mask while leaving a
`substantially unreacted layer of said mask under said
`hard layer; and
`(c) removing the mask by sequentially:
`( 1) exposing the mask to a plasma activated gas con-
`taining hydrogen under conditions suflicient
`to
`vaporize said hard layer and thereby initiate removal
`of said mask,
`(2) after removal of said hard layer, exposing the mask
`to a plasma activated gas containing oxygen under
`conditions suflicient to remove said unreacted layer;
`and
`
`wherein, after said unreacted layer has been removed
`by said plasma activated gas containing oxygen to
`thereby completely remove said mask, aluminum
`oxide residues are left,
`(3) after removal of said mask by said oxygen, expos-
`ing said remaining aluminum oxide residues to a wet
`solution comprising an etchant consisting essentially
`of nitric acid under conditions sufficient to cause said
`nitric acid to remove said aluminum oxide residues.
`
`2. A method of removing a resist mask according to claim
`1, wherein the organic material is at least one of a photoresist
`and a photosensitive polyimide.
`
`Intel Corp. et al. Exhibit 1 01 8
`
`Intel Corp. et al. Exhibit 1018
`
`

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`9
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`5,628,871
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`3. A method of removing a resist