JP4977535B2 - Pattern transfer method - Google Patents

Description

  The present invention relates to a pattern transfer method and a photomask used in the manufacture of semiconductor integrated circuits and the like.

  Photomasks used in a wide range of applications including the manufacture of semiconductor integrated circuits such as ICs, LSIs, and VLSIs are, for example, photomasks in which a light-shielding film containing chromium as a main component is formed on a translucent substrate A blank is used, and a predetermined pattern is formed on the light-shielding film by a photolithography method using ultraviolet rays, electron beams or the like as exposure light. In recent years, along with market demands such as higher integration of semiconductor integrated circuits, pattern miniaturization has rapidly progressed, and the exposure wavelength has been shortened and the numerical aperture of the lens has been increased to increase the resist resolution in the exposure process. Has been addressed.

  However, there is a problem that shortening the exposure wavelength results in an increase in the cost of the apparatus and materials. In addition, while increasing the numerical aperture of the lens has the advantage of improving the resolution, it results in a decrease in the depth of focus. As a result, there is a problem in that the process stability is lowered and the product yield is adversely affected. As one of effective pattern transfer methods for solving such problems, a “phase shift method” using a “phase shift mask” as a photomask is known (Patent Document 1).

  In these photomasks, a resist is applied on a light-shielding film of a photomask blank in which a light-shielding film is formed on a transparent substrate, and the resist is exposed and developed to form a predetermined pattern, and this resist pattern is masked. As described above, the light-shielding film is etched and then the resist is removed.

Conventionally, when a photomask pattern is transferred to a transfer substrate such as a silicon wafer, exposure in the air has been performed. However, as the pattern line width to be formed becomes narrower, the NA increases. In order to realize the exposure in the number (2), a method in which pure water or a liquid having a high refractive index is supplied onto the substrate to be transferred for exposure has been put into practical use. In addition, in order to further improve the resolution, the light source of the exposure apparatus has also been devised, and modified illumination such as dipole illumination, quadrupole illumination, or annular illumination has been used. When such an exposure light source is used, the exposure light incident on the photomask includes a component that is non-perpendicularly incident on the photomask surface. In addition, a pellicle film is provided on the pattern surface of the photomask in order to prevent adhesion of particles to the pattern surface.
JP-A-7-140635

  As shown in the schematic cross-sectional view of FIG. 1, the light shielding portion of the photomask in which the pattern of the light shielding film 2 such as Cr or MoSi is formed on the transparent substrate 1 corresponds to the film thickness of the light shielding film 2. There is a step. Further, as the pattern is miniaturized, the ratio of the pattern step to the pattern width increases. Since the refractive index of the transparent substrate 1 is generally larger than the refractive index (usually 1) of the exposure atmosphere of the photomask, the larger the incident angle (θ) of the light irradiated to the photomask, The phenomenon that the step of the light shielding part of the mask shields (a part of) the light transmitted through the transmissive part becomes prominent, and there arises a problem that the resolution is lowered due to a decrease in contrast ratio.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a pattern capable of suppressing a decrease in pattern resolution caused by an exposure light component incident non-perpendicularly on a photomask surface. It is to provide a transfer method and a photomask.

  In order to solve this problem, the pattern transfer method of the present invention is characterized in that the pattern formation surface of the photomask is exposed while being covered with a medium having a refractive index larger than 1.

  The medium is liquid or solid, for example, pure water, SOG, silicon oxide, silicon nitride, silicon oxynitride or the like.

  The photomask of the present invention is characterized in that a light-shielding film is patterned on the main surface of a transparent substrate, and a medium having a refractive index greater than 1 is filled in a region where the light-shielding film is not formed.

  Examples of the medium include SOG, silicon oxide, silicon nitride, and silicon oxynitride.

  According to the present invention, the medium having a refractive index larger than 1 (high refractive index medium) reduces the refraction angle at the interface between the transparent substrate and the high refractive index medium as compared with the conventional one. The amount of exposure light that is blocked can be reduced. This effect becomes more prominent as the incident angle of the exposure light to the transparent substrate increases.

  As a result, it is possible to suppress a decrease in pattern resolution caused by an exposure light component incident non-perpendicularly on the photomask surface.

  The structure of the photomask of the present invention will be described below with reference to the drawings.

  [Photomask of the Present Invention]: FIG. 2 is a schematic cross-sectional view for explaining the structure of the photomask of the present invention, on the main surface of the transparent substrate 11 such as quartz or calcium fluoride that is transparent to exposure light. A patterned light-shielding film 12 is provided. The entire main surface of the transparent substrate 11 is covered with a high refractive index medium 13 having a refractive index larger than 1 (n> 1) so as to cover the light shielding film 12. In this photomask, the light shielding film 12 forming region is a light shielding portion, and the light shielding film non-forming region is a light transmitting portion. The high refractive index medium 13 needs to be provided so as to fill (fill) the light transmitting portion, but does not necessarily need to cover the light shielding film 12.

  In general, the medium with which the exposure light exit surface is in contact (the medium with which the photomask pattern surface is in contact) is air or nitrogen, and its refractive index is 1 (n = 1). In the conventional photomask having the structure described above, there arises a problem that the stepped portion of the light shielding portion blocks (a part of) the light transmitted through the light transmitting portion and the resolution is lowered. However, when the high refractive index medium 13 having a refractive index larger than 1 is provided as in the present invention, the refraction angle at the interface between the transparent substrate 11 and the high refractive index medium 13 is smaller than in the prior art. As a result, the amount of exposure light blocked by the light-shielding film 12 can be reduced. This effect becomes more prominent as the incident angle (θ) of the exposure light to the transparent substrate 11 is larger.

  When used as a so-called “light-shielding film”, the light-shielding film 12 has an optical density of 3 or more. When a halftone-type light-shielding film is used, a predetermined transmittance (for example, The film thickness and composition are determined so that the transmittance is 1 to 50%. Examples of such a light-shielding film 12 include metal silicides such as chromium, molybdenum silicide, zirconia silicide, and tungsten silicide, and films containing nitrogen and oxygen. Further, the transmittance may be a film having high light shielding properties such as 0.1% or less, and when used as a halftone phase shift film, it does not substantially contribute to exposure (for example, about 1 to 30%). ) May be used. The light shielding film 12 can be formed by a known method such as sputtering, vapor deposition, or CVD.

  When quartz is used as the transparent substrate 11, the use of a film mainly composed of chromium as the light-shielding film 12 has an advantage that etching selectivity can be provided between the transparent substrate 11 and the transparent substrate 11. .

  When the exposure wavelength is 200 nm or less (for example, 193 nm, etc.), the use of a film mainly containing metal silicide (which can also contain oxygen or nitrogen) reduces the reflectance to a predetermined value. There is an advantage that fluorine-based etching that is advantageous for thinning the light-shielding film and miniaturizing the pattern is possible.

  Further, the light-shielding film 12 has an appropriate transmittance (for example, 1 to 30%), and the phase difference between the exposure light transmitted through the light-transmitting portion and the exposure light transmitted through the light-shielding portion is set to a predetermined value (for example, By setting the angle to 180 degrees, a phase shift effect can be provided. In this case, the phase change of the light-shielding part can be made larger than the phase change of the light-transmitting part as in the case of a normal halftone type phase shift mask. It is possible to make it larger than the phase change. In particular, when the refractive index of the high refractive index medium is large, the thickness of the light shielding portion can be reduced. Therefore, the configuration in which the phase change in the light transmitting portion is larger than that of the light shielding portion is preferable.

  As the high refractive index medium 13, SOG, silicon oxide, silicon nitride, silicon oxynitride, or the like is used. If the thickness is, for example, 1 mm or more, there is an advantage that a pellicle film is not necessary. Further, the high refractive index medium 13 is formed so that the thickness of the high refractive index medium provided on the light shielding film 12 is 10 to 40 nm so as to function as an antireflection film, or the light shielding film 12 is protected. You may make it function as a film | membrane.

  [Pattern Transfer Method of the Present Invention]: FIG. 3 is a diagram for explaining the pattern transfer method of the present invention. The photomask used here is a conventional photomask similar to that shown in FIG. A light shielding film 12 is provided on the main surface of the substrate 11.

  In a state where the pattern forming surface of the photomask is covered with the high refractive index medium 13, exposure for pattern printing is performed on the transfer target substrate 15 coated with the resist 14. The high refractive index medium 13 shown in FIG. 3 is a liquid (for example, pure water) having a refractive index higher than that of air or nitrogen (for example, pure water), which is a normal exposure environment. 1 shows a state in which exposure is performed in a state where the transfer substrate 15 is also immersed in the high refractive index medium 13, but the pattern formation surface (light-shielding film 12) of the photomask and the transfer substrate are illustrated. 15 (the upper resist 14) need not be in the high refractive index medium 13 as long as the pattern formation surface of the photomask is in contact with the high refractive index medium 13.

  For example, only the photomask (the pattern formation surface thereof) may be immersed in the high refractive index medium 13 (FIG. 3B), or an optical element 16 such as a lens is provided on the pattern formation surface side of the photomask. The space between them is filled with a high refractive index medium 13 (FIG. 3C), and further, the photomask (pattern forming surface thereof) and the transferred substrate (the upper resist 14) are formed on separate high refractive index media. It is also possible to immerse or to immerse only the exposed part of the substrate to be transferred like a normal immersion exposure machine with only the photomask (pattern forming surface) immersed in a high refractive index medium. Good.

  Further, the high refractive index medium 13 only needs to cover the pattern formation surface of the photomask, and may cover the pattern formation surface thinly. The high refractive index medium 13 does not need to be a liquid, and may be a solid such as SOG, silicon oxide, silicon nitride, or silicon oxynitride. In this case as well, the solid thin film covers the pattern. Just do it.

  In the case of such a pattern transfer method, even when a conventional photomask is used, the refraction angle at the interface between the transparent substrate 11 and the high refractive index medium 13 is compared with the case where the pattern transfer is performed by the conventional method. As a result, the amount of exposure light blocked by the light-shielding film 12 can be reduced. This effect becomes more prominent as the incident angle (θ) of the exposure light to the transparent substrate 11 is larger.

  The refractive index of the high refractive index medium 13 is preferably larger than 1 and larger than that of the light-shielding film 12, and more preferably larger than the refractive index of the transparent substrate 11.

  FIG. 4 is a diagram for explaining the manufacturing process of the photomask of the present invention in the case where the pattern surface of the photomask is covered with a solid having a high refractive index. The photomask obtained in this example is MoSiN as a light-shielding film. This is a binary mask in which a light shielding layer (60 nm) and an antireflection layer (40 nm) are laminated. In this embodiment, the thicknesses of the light shielding layer and the antireflection layer are set to the above values, respectively, but these film thicknesses may be 30 to 60 nm and 10 to 40 nm, respectively.

  First, a photomask blank provided with the light-shielding film 12 on the main surface of the quartz substrate 11 is prepared (FIG. 4A). The light-shielding film 12 of the present embodiment is formed by laminating a light-shielding layer (60 nm) and an antireflection layer (40 nm) of MoSiN. In general, a metal silicide such as Cr or MoSi or oxygen Alternatively, a film containing nitrogen is formed, and the film formation is performed by sputtering a metal target in an atmosphere in which nitrogen gas is added to Ar gas.

When oxygen is contained in the film, it is in an atmosphere in which oxygen gas is added to Ar gas, and when oxygen and nitrogen are contained, oxygen (containing) gas and nitrogen (containing) gas are contained in Ar gas. Sputter film formation is performed in an atmosphere to which is added. Further, when carbon is included in the film, sputtering film formation is performed in an atmosphere to which a carbon-containing gas is added. Examples of such oxygen (containing) gas, nitrogen (containing) gas, or carbon-containing gas include O ₂ , N ₂ , NO, N ₂ O, CO, CO ₂ , and CH ₄ .

  The light-shielding film 12 may have a two-layer structure of a light-shielding layer and an antireflection layer, or may have a structure of three or more layers in which an antireflection layer or an adhesion improving layer is formed on the quartz substrate 11 side. Further, the reflectivity may be reduced by gradually changing the composition in the film to increase the degree of oxidation or nitridation on the surface. Furthermore, an etching mask layer that serves as an auxiliary film for pattern formation or an etching stopper may be formed. These multilayers may be formed from a single element or a layer containing a different element.

  Next, a resist 14 is applied on the light-shielding film 12 (FIG. 4B), and a predetermined pattern is exposed and developed by an EB drawing apparatus to obtain a predetermined resist pattern 14 (FIG. 4C). ). The resist 14 may be an electron beam, a KrF line, or an ArF line, and may be a positive type or a negative type. Although it is not necessary to use a chemically amplified type, in this embodiment, a chemically amplified positive resist for electron beams is used.

  Using the resist pattern 14 as a mask, the light shielding film 12 is etched and patterned (FIG. 4D). Etching in this case can be performed by either wet or dry methods, but dry etching is preferable for forming a fine pattern.

  In this embodiment, since the light-shielding film 12 is a metal silicide film, dry etching is performed using an etching gas containing fluorine such as CF4 and SF6. When a Cr film is used as the light-shielding film 12, a mixture of chlorine gas and oxygen gas can be used as an etching gas. In addition, such an etching gas may contain an inert gas such as He or Ar.

  The light-shielding film 12 may have a laminated structure of three or more layers by forming an adhesion improving layer on the quartz substrate 11 side on the above-described two-layer laminated film. Alternatively, the reflectance may be reduced by gradually changing the composition in the film in the thickness direction to increase the degree of oxidation or nitridation on the film surface. Further, an etching mask layer or an etching stopper that serves as an auxiliary film for pattern formation may be formed.

  Then, when the resist mask 14 is removed after the patterning of the light-shielding film 12, a conventional photomask having a light-shielding film patterned on the main surface of the quartz substrate 11 is obtained (FIG. 4E).

  Subsequently, an SOG film (n = 1.4) which is a high refractive index medium 13 having a refractive index larger than 1 is formed on the entire main surface of the transparent substrate 11 so as to cover the light shielding film 12 (FIG. 4 (F)). The thickness of the SOG film is about 130 nm.

  Then, the SOG film 13 was planarized to obtain a photomask of the present invention having a light transmitting part and a light shielding part (FIG. 4G). As described above, since the thickness of the light shielding film 12 is about 100 nm and the coating thickness of the SOG film 13 is about 130 nm, a high refractive index medium of about 30 nm is provided on the light shielding film 12. It will be. This portion functions as an antireflection film and as a protective film for the light shielding film 12.

  It has a light-shielding part in which a light-shielding film made of CrON and a 30-nm-thick antireflection film are laminated on a quartz substrate, and an isolated transmission part with a width of 0.2 μm (part where the light-shielding film and the antireflection film are not formed) Is set in an ArF excimer laser exposure apparatus using dipole illumination with an incident angle (θ) to the photomask of 45 degrees as a light source. Then, exposure is performed by filling the space between the photomask and the optical system on the object to be exposed (transferred silicon substrate: not shown) with pure water (refractive index: 1.44) (FIG. 5A). For comparison, exposure is also performed in a state where the space between the photomask and the optical system on the object to be exposed side is a nitrogen atmosphere (refractive index 1) (FIG. 5B).

  When light is incident on the photomask at an angle of 45 degrees with the pattern formation surface of the photomask facing toward the silicon substrate that is the object to be exposed, the space between the optical system on the object to be exposed side of the photomask becomes nitrogen. In the case of (refractive index 1) (FIG. 5B), the angle of light emitted from the mask substrate to the space on the exposure object side is 45 degrees. Of the width of the incoming light, the amount of light of 0.1 μm is attenuated by the light shielding film and the antireflection film. In this case, if the width (0.2 μm) of light incident on the light transmitting portion of the mask is 100%, only a portion corresponding to 50% of the width of the light transmitted through the transmission pattern can be transmitted without being attenuated. Thus, the portion corresponding to the width of 50% subject to attenuation causes a decrease in contrast.

  In contrast, when the space between the optical system on the exposed object side of the photomask and the photomask is filled with pure water (refractive index: 1.44) (FIG. 5A), the photomask is exposed to the exposed object side. Since the angle of the light emitted to the space is about 30 degrees, the width of the transmission pattern subjected to the attenuation action is 0.06 μm, and the light corresponding to 0.14 μm (that is, the amount of light corresponding to the width of 70% light) is attenuated. The light can be transmitted without being affected, and the ratio of the width of light that undergoes attenuation that causes a decrease in contrast is reduced to 30%. In other words, in the case of the present invention, the ratio of the width of light that undergoes attenuation that causes a decrease in contrast can be reduced from 50% to 30%.

It is the cross-sectional schematic for demonstrating the structure of the conventional photomask. It is the cross-sectional schematic for demonstrating the structure of the photomask of this invention. It is a figure for demonstrating the pattern transfer method of this invention. It is a figure for demonstrating the manufacturing process of the photomask of this invention. It is a figure for showing the pattern transfer method of the present invention in comparison with the conventional pattern transfer method.

Explanation of symbols

1, 11 Substrate 2, 12 Light-shielding film 13 High refractive index medium 14 Resist 15 Transfer substrate 16 Optical element

Claims (1)

A pattern transfer method using a photomask having a light-shielding film patterned on a substrate , wherein the pattern forming surface of the photomask has a refractive index larger than 1 in any one of the following modes a to d: Exposure light having an exposure light component that is non-perpendicularly incident on the photomask surface is incident from a surface opposite to the pattern formation surface in a state of being covered with pure water as a medium, and is disposed to face the pattern formation surface. A pattern transfer method in which a resist provided on a transferred substrate is exposed by modified illumination.
a: Only the pattern forming surface of the photomask is immersed in the pure water.
b: An optical element is provided on the pattern forming surface side of the photomask, and the pure water is filled only between the pattern forming surface and the optical element.
c: The pattern forming surface of the photomask and the resist on the transferred substrate are separately immersed in the pure water.
d: only the pattern forming surface of the photomask is immersed in the pure water,
Only the exposed portion of the transfer substrate is immersed.

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