Pattern forming method

A substrate is coated with a chemically amplified negative resist, thereby forming a resist film on the substrate. The chemically amplified negative resist includes: a polymer containing an acrylic acid as a polymerization unit; an acid generator for generating an acid when irradiated with extreme ultraviolet radiation; and a cross-linking agent for cross-linking the polymer in the presence of an acid. Next, the resist film is exposed to extreme ultraviolet radiation to generate an acid in exposed portions of the resist film. Thereafter, the resist film is heated to generate cross-linkage in the exposed portions of the resist film. Subsequently, a silylation reagent is supplied onto the surface of the resist film to form a silylated layer on the surface of non-exposed portions of the resist film. And then the resist film is dry-etched using the silylated layer as a mask, thereby forming a resist pattern out of the non-exposed portions of the resist film.

BACKGROUND OF THE INVENTION
 The present invention relates to a method for forming a resist pattern on a
 semiconductor substrate during a process of fabricating a semiconductor
 integrated circuit device.
 In a conventional process of fabricating a semiconductor integrated circuit
 device, a resist pattern is formed by a photolithography technique using
 ultraviolet radiation. How ever, since a greater and greater number of
 semiconductor integrated circuit devices are integrated on a single chip
 with an increasingly higher density in recent years, the significance of
 fine-line patterning technique is growing day after day. In order to cope
 with such demand, the wavelength of exposing radiation used for
 photolithography should be further reduced.
 Nevertheless, the shorter the wavelength of exposing radiation is, the more
 seriously the accuracy is affected by rays reflected from the substrate
 underlying a resist film and a level difference of the resist film. To
 avoid these adverse effects, according to a suggested pattern forming
 method, a surface resolution process is performed by forming a masking
 layer over the surface of a resist film and patterning the resist film
 using the masking layer. In accordance with this method, ArF excimer laser
 light (wavelength: 193 nm) is used as exposing radiation and a chemically
 amplified resist is used as a resist material.
 Hereinafter, a method for forming a positive resist pattern by exposing a
 chemically amplified negative resist to ArF excimer laser light will be
 described with reference to FIGS. 4(a) through 4(c) and FIGS. 5(a) and
 5(b).
 First, as shown in FIG. 4(a), the surface of a semiconductor substrate 1 is
 coated with a chemically amplified negative resist, thereby forming a
 resist film 2 thereon. The resist contains: polymers; an acid generator
 for generating an acid when exposed to radiation; and a cross-linking
 agent for cross-linking the polymers when heated in the presence of an
 acid. Thereafter, as shown in FIG. 4(b), the resist film 2 is exposed to
 excimer laser light 3 through a mask 4 having a desired pattern. As a
 result, an acid is generated from the acid generator in exposed portions
 2a of the resist film 2.
 Then, as shown in FIG. 4(c), when the semiconductor substrate 1 and the
 resist film 2 are heated, the polymers are cross-linked in the exposed
 portions 2a owing to the function of the cross-linking agent, because the
 acid has been generated there. On the other hand, the polymers are not
 cross-linked in non-exposed portions 2b of the resist film 2, because no
 acid has been generated there.
 Next, as shown in FIG. 5(a), a silylation reagent 5, composed of
 dimethylsilyldimethylamine (DMSDMA) in vapor or liquid phase, is supplied
 onto the entire surface of the resist film 2. As a result, a silylated
 layer 6A is formed on the surface of the non-exposed portions 2b of the
 resist film 2. However, no silylated layer 6A is formed on the surface of
 the cross-linked, exposed portions 2a of the resist film 2.
 Subsequently, as shown in FIG. 5(b), the resist film 2 is dry-etched with
 an etching gas 7A, essentially consisting of O.sub.2 gas, using the
 silylated layer 6A as a mask, thereby forming a positive resist pattern 8A
 out of the non-exposed portions 2b of the resist film 2.
 In order to meet the demand for further increasing the degree of
 integration of, and further decreasing the size of, semiconductor
 integrated circuit devices, a method for forming a fine-line resist
 pattern with a line width of about 0.1 .mu.m should be developed.
 Thus, it is imaginable to use extreme ultraviolet radiation having a
 wavelength in the band of 13 nm or 5 nm, instead of ArF excimer laser
 light (wavelength: 193 nm), for the purpose of increasing the resolution
 of exposing radiation for a photolithography process.
 Hereinafter, a pattern forming method, considered by the present inventors
 as a basis of this invention, will be described with reference to FIGS.
 6(a) through 6(c) and FIGS. 7(a) and 7(b). In the method, a positive
 resist pattern is formed by exposing, to extreme ultraviolet radiation, a
 resist film made of a chemically amplified negative resist having the
 following compositions:
 Polymer: poly(vinyl phenol) (1 g)
 Cross-linking agent: melamine (0.25 g)
 Acid generator: triphenylsulfonium triflate (0.04 g)
 Solvent: diglyme (5 g)
 First, as shown in FIG. 6(a), a resist film 2 made of a chemically
 amplified negative resist and having a thickness of 0.5 .mu.m is formed on
 a semiconductor substrate 1. Next, as shown in FIG. 6(b), the resist film
 2 is exposed to extreme ultraviolet radiation 9, having a wavelength on
 the band of 13nm, using a mask 4, thereby generating an acid from the acid
 generator in exposed portions 2a of the resist film 2.
 Then, as shown in FIG. 6(c), the semiconductor substrate 1 is heated by a
 hot plate at 100.degree. C. for 60 seconds. As a result, the polymers
 react with the cross-linking agent owing to the function of the acid
 generated from the acid generator. Specifically, hydroxyl groups, which
 are cross-linking groups contained in the polymers, react with the
 cross-linking agent, whereby cross-linkage is generated in the exposed
 portions 2a of the resist film 2.
 Subsequently, as shown in FIG. 7(a), a silylation reagent 5, composed of
 dimethylsilyldimethylamine (DMSDMA) in vapor or liquid phase, is supplied
 onto the entire surface of the resist film 2. As a result, a silylated
 layer 6B is formed on the surface of the non-exposed portions 2b of the
 resist film 2. However, no silylated layer 6B is formed in the
 cross-linked, exposed portions 2a of the resist film 2.
 Thereafter, as shown in FIG. 7(b), the resist film 2 is dry-etched with an
 etching gas 7B, essentially consisting of O.sub.2 gas, using the silylated
 layer 6B as a mask, thereby forming a positive resist pattern 8B out of
 the non-exposed portions 2b of the resist film 2.
 However, the region of the resist film 2 where the polymers are
 cross-linked (i.e., the hatched regions with dense dots in FIG. 6(c))
 covers not only the exposed portions 2a but also the non-exposed portions
 2b. Accordingly, the size (i.e., width) of the silylated layer 6B is
 smaller than the size (i.e., width) of the non-exposed portions 2b of the
 resist film 2 as shown in FIG. 7(a). As a result, the size (i.e., width)
 of the resist pattern 8B is unintentionally smaller than the size (i.e.,
 width) of the light-blocking portions of the mask 4 as shown in FIG. 7(b).
 Such a decrease in size accuracy of the resist pattern 8B causes various
 failures during subsequent process steps of a fabricating process of a
 semiconductor integrated circuit device.
 SUMMARY OF THE INVENTION
 In view of these problems, a prime object of the present invention is
 improving the size accuracy of a positive resist pattern obtained by
 exposing a resist film made of a chemically amplified negative resist to
 extreme ultraviolet radiation.
 The present inventors analyzed from various angles why the polymer
 cross-linking region covers not only exposed portions but also non-exposed
 portions of a resist film. As a result, we made the following findings.
 We observed that when a resist film was exposed to extreme ultraviolet
 radiation, an acid was also generated in part of non-exposed portions of
 the resist film abutting on the exposed portions thereof, though the
 amount of acid generated was smaller in the non-exposed portions than in
 the exposed portions. This is probably because the energy of extreme
 ultraviolet radiation is higher than that of ArF excimer laser light
 conventionally used.
 That is to say, we found that since an acid had also been generated from
 the acid generator in that part of the non-exposed portions of the resist
 film abutting on the exposed portions thereof, the polymers were naturally
 cross-linked in that part when the resist film was heated. Our inventive
 concept is that the cross-linkage can be suppressed in that part of
 non-exposed portions of a resist film abutting on its exposed portions if
 polymers contained in a chemically amplified negative resist are limited
 to those exhibiting low cross-linking reactivity.
 Another inventive concept of ours is that polymers contained in a
 chemically amplified resist film may be cross-linked only by the
 irradiation of extreme ultraviolet radiation thereto without heating,
 because the energy of extreme ultraviolet radiation is higher than that of
 ArF excimer laser light.
 Specifically, a first pattern forming method according to the present
 invention includes the steps of: coating a substrate with a chemically
 amplified negative resist to form a resist film thereon, the chemically
 amplified negative resist including: a polymer containing an acrylic acid
 as a polymerization unit; an acid generator for generating an acid when
 irradiated with extreme ultraviolet radiation; and a cross-linking agent
 for cross-linking the polymer in the presence of an acid; exposing the
 resist film to extreme ultraviolet radiation to generate an acid in
 exposed portions of the resist film; heating the resist film to generate
 cross-linkage in the exposed portions of the resist film; supplying a
 silylation reagent onto the surface of the resist film to form a silylated
 layer on the surface of non-exposed portions of the resist film; and
 dry-etching the resist film using the silylated layer as a mask, thereby
 forming a resist pattern out of the non-exposed portions of the resist
 film.
 In accordance with the first pattern forming method, the polymer containing
 an acrylic acid as a polymerization unit includes a carboxylic acid group
 as a cross-linking group, and shows low cross-linking reactivity, i.e.,
 reactivity with the cross-linking agent. Accordingly, even when an acid is
 generated in part of the non-exposed portions of the resist film abutting
 on the exposed portions thereof, the polymer cross-linking region is
 limited to the exposed portions of the resist film.
 A second pattern forming method according to the present invention includes
 the steps of: coating a substrate with a chemically amplified negative
 resist to form a resist film thereon, the chemically amplified negative
 resist including: a polymer containing a methacrylic acid as a
 polymerization unit; an acid generator for generating an acid when
 irradiated with extreme ultraviolet radiation; and a cross-linking agent
 for cross-linking the polymer in the presence of an acid; exposing the
 resist film to extreme ultraviolet radiation to generate an acid in
 exposed portions of the resist film; heating the resist film to generate
 cross-linkage in the exposed portions of the resist film; supplying a
 silylation reagent onto the surface of the resist film to form a silylated
 layer on the surface of non-exposed portions of the resist film; and
 dry-etching the resist film using the silylated layer as a mask, thereby
 forming a resist pattern out of the non-exposed portions of the resist
 film.
 In accordance with the second pattern forming method, the polymer
 containing a methacrylic acid as a polymerization unit includes a
 carboxylic acid group as a cross-linking group, and shows low
 cross-linking reactivity, i.e., reactivity with the cross-linking agent.
 Accordingly, even when an acid is generated in part of the non-exposed
 portions of the resist film abutting on the exposed -portions thereof, the
 polymer cross-linking region is limited to the exposed portions of the
 resist film.
 In the first or second pattern forming method, the polymer cross-linking
 region is limited to the exposed portions of the resist film. Accordingly,
 the size of the silylated layer formed on the surface of the resist film
 is substantially equal to that of the non-exposed portions of the resist
 film. As a result, the size accuracy of a resist pattern obtained by
 dry-etching such a resist film using the silylated layer as a mask can be
 considerably improved.
 In an exemplary embodiment of the first pattern forming method, the polymer
 is preferably a copolymer of an acrylic acid and carboxylic acid ester.
 And in an exemplary embodiment of the second pattern forming method, the
 polymer is preferably a copolymer of a methacrylic acid and carboxylic
 acid ester.
 A third pattern forming method according to the present invention includes
 the steps of: coating a substrate with a chemically amplified negative
 resist to form a resist film thereon, the chemically amplified negative
 resist including an acid generator for generating an acid when irradiated
 with extreme ultraviolet radiation and a cross-linking agent for
 cross-linking the polymer in the presence of an acid; exposing the resist
 film to extreme ultraviolet radiation to generate an acid in exposed
 portions of the resist film and to cross-link the polymer owing to a
 function of the acid generated; supplying a silylation reagent onto the
 surface of the resist film to form a silylated layer on the surface of
 non-exposed portions of the resist film; and dry-etching the resist film
 using the silylated layer as a mask, thereby forming a resist pattern out
 of the non-exposed portions of the resist film.
 In accordance with the third pattern forming method, an acid is generated
 in the exposed portions of the resist film and the polymer is cross-linked
 owing to the function of the acid generated by exposing the resist film to
 extreme ultraviolet radiation. Accordingly, compared to heating the resist
 film after the pattern exposure, the cross-linkage of the polymer is
 suppressed. Thus, even when an acid is generated in part of the
 non-exposed portions of the resist film abutting on the exposed portions
 thereof, the polymer cross-linking region is limited to the exposed
 portions of the resist film. As a result, the size of the silylated layer
 formed on the surface of the resist film is substantially equal to that of
 the non-exposed portions of the resist film.
 Consequently, the size accuracy of a resist pattern obtained by dry-etching
 such a resist film using the silylated layer as a mask can be considerably
 improved.
 In an exemplary embodiment of the first to third pattern forming methods,
 the extreme ultraviolet radiation is light having a wavelength on the band
 of 13 nm or 5 nm.
 In such an embodiment, the resolution of exposing radiation for a
 photolithography process can be increased with more certainty.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 EMBODIMENT 1
 Hereinafter, a pattern forming method according to a first embodiment of
 the present invention will be described with reference to FIGS. 1(a)
 through 1(c) and FIGS. 2(a) and 2(b).
 In this first embodiment, a chemically amplified negative resist, including
 a polymer containing an acrylic acid as a polymerization unit and having
 the following compositions, is used:
 Polymer: poly(acrylate(30 mol %)-methyl acrylate (70mol %)) (1 g)
 Cross-linking agent: melamine (0.3 g)
 Acid generator: triphenylsulfonium triflate (0.03 g)
 Solvent: diethylene glycol dimethyl ether (5 g)
 First, as shown in FIG. 1(a), a resist film 12 made of a chemically
 amplified negative resist and having a thickness of 0.5 .mu.m is formed on
 a semiconductor substrate 11. Next, as shown in FIG. 1(b), the resist film
 12 is exposed to extreme ultraviolet radiation 13 having a wavelength on
 the band of 13 nm using a mask 14, thereby generating an acid from the
 acid generator in exposed portions 12a of the resist film 12.
 Then, as shown in FIG. 1(c), the semiconductor substrate 11 is heated by a
 hot plate at 100.degree. C. for 60 seconds. As a result, the polymers
 react with the cross-linking agent owing to the function of the acid
 generated from the acid generator. However, since an acrylic acid has a
 carboxylic acid as a cross-linking group, the polymer containing the
 acrylic acid as a polymerization unit has low reactivity with the
 cross-linking agent. Thus, even when an acid is generated from the acid
 generator in part of the non-exposed portions 12b of the resist film 12
 abutting on the exposed portions 12a thereof, the polymer cross-linking
 region is limited to the exposed portions 12a of the resist film 12.
 Subsequently, as shown in FIG. 2(a), a silylation reagent 15, composed of
 dimethylsilyldimethylamine (DMSDMA) in vapor or liquid phase, is supplied
 onto the entire surface of the resist film 12. As a result, a silylated
 layer 16 is selectively formed only on the surface of the non-exposed
 portions 12b of the resist film 12.
 Thereafter, as shown in FIG. 2(b), the resist film 12 is dry-etched with an
 etching gas 17, essentially consisting of o.sub.2 gas, using the silylated
 layer 16 as a mask, thereby forming a positive resist pattern 18 out of
 the non-exposed portions 12b of the resist film 12.
 In accordance with the first embodiment, the polymer cross-linking region
 is limited to the exposed portions 12a of the resist film 12. Accordingly,
 the size (i.e., width) of the silylated layer 16 formed on the surface of
 the resist film 12 is substantially equal to that of the non-exposed
 portions 12b of the resist film 12. As a result, a resist pattern 17 can
 be formed with excellent size accuracy.
 Examples of the polymer contained in the chemically amplified negative
 resist used in the first embodiment include: polyacrylates containing an
 acrylic acid as a polymerization unit, e.g., a copolymer of an acrylic
 acid and carboxylic acid ester such as
 poly(acrylate (60 mol %)-tertiarybutyl acrylate (40 mol %)) or
 poly(acrylate (40 mol %)-methyl methacrylate (60 mol %)); and
 polymethacrylates containing a methacrylic acid as a polymerization unit,
 e.g., a copolymer of a methacrylic acid and carboxylic acid ester such as
 poly(methacrylate (35 mol %)-tricyclodecyl methacrylate (65 mol %)),
 poly(methacrylate (40 mol %)-methyl methacrylate (20 mol %) hydroxyethyl
 methacrylate (20 mol %)) or
 poly(methacrylate (30 mol %)-adamantyl methacrylate (50 mol %)-methyl
 methacrylate(2 mol %)).
 Each of the polyacrylates containing an acrylic acid as a polymerization
 unit and the polymethacrylates containing a methacrylic acid as a
 polymerization unit includes a carboxylic acid as a cross-linking group,
 and therefore shows low cross-linking reactivity. Accordingly, even when
 an acid is generated in part of the non-exposed portions 12b of the resist
 film 12 abutting on the exposed portions 12a thereof, the polymer
 cross-linking region is limited to the exposed portions 12a of the resist
 film 12. Consequently, the size (i.e., width) of the silylated layer 16
 formed on the surface of the resist film 12 is substantially equal to that
 of the non-exposed portions 12b of the resist film 12.
 It is noted that the extreme ultraviolet radiation may be radiation on the
 band of 5 nm, instead of radiation on the band of 13 nm.
 EMBODIMENT 2
 Hereinafter, a pattern forming method according to a second embodiment of
 the present invention will be described with reference to FIGS. 3(a)
 through 3(d).
 In this second embodiment, a chemically amplified negative resist having
 the following compositions is used:
 Polymer: polyvinyl phenol)(1 g)
 Cross-linking agent: melamine (0.25 g)
 Acid generator: triphenylsulfonium triflate (0.04 g)
 Solvent: diglyme
 First, as shown in FIG. 3(a), a resist film 12 made of a chemically
 amplified negative resist and having a thickness of 0.5 .mu.m is formed on
 a semiconductor substrate 11. Next, as shown in FIG. 3(b), the resist film
 12 is exposed to extreme ultraviolet radiation 13 having a wavelength on
 the band of 13nm through a mask 14, thereby generating an acid from the
 acid generator in exposed portions 12a of the resist film 12 and
 cross-linking the polymers owing to the function of the acid generated. In
 this case, since the extreme ultraviolet radiation 13 has high energy, the
 polymers are cross-linked in the exposed portions 12a of the resist film
 12 only by the irradiation of the extreme ultraviolet radiation without
 any heat treatment. Also, compared to heating the resist film after the
 pattern exposure, the cross-linkage of the polymers is suppressed.
 Accordingly, even when an acid is generated in part of the non-exposed
 portions 12b of the resist film 12 abutting on the exposed portions 12a
 thereof, the polymer cross-linking region is limited to the exposed
 portions 12a of the resist film 12.
 Subsequently, as shown in FIG. 3(c), a silylation reagent 15, composed of
 dimethylsilyldimethylamine (DMSDMA) in vapor or liquid phase, is supplied
 to the entire surface of the resist film 12. As a result, a silylated
 layer 16 is selectively formed only on the surface of the non-exposed
 portions 12b of the resist film 12.
 Thereafter, as shown in FIG. 3(d), the resist film 12 is dry-etched with an
 etching gas 17, essentially consisting of O.sub.2 gas, using the silylated
 layer 16 as a mask, thereby forming a positive resist pattern 18 out of
 the non-exposed portions 12b of the resist film 12.
 In accordance with the second embodiment, the polymer cross-linking region
 is limited to the exposed portions 12a of the resist film 12. Accordingly,
 the size (i.e., width) of the silylated layer 16 formed on the surface of
 the resist film 12 is substantially equal to that of the non-exposed
 portions 12b of the resist film 12. As a result, a resist pattern 17 can
 be formed with excellent size accuracy.
 It is noted that the extreme ultraviolet radiation may be radiation on the
 band of 5 nm, instead of radiation on the band of 13 nm.