JPS6322576B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Technique) The present invention relates to a dustproof cover for a photomask/reticle, which increases the transmittance of specific light to prevent reflection. (Prior art and its problems) With the recent increase in the degree of integration of semiconductor integrated circuits, the image width of the circuits has become extremely fine, 1 to 3 μm, and the lithography technology for forming this image on a wafer is also There is a transition from the Excision method to the Stepper method. In this lithography process, if dust exists on the photomask or reticle, the dust is also imaged at the same time, causing short circuits and defects in the circuit, reducing the yield of LSI. Particularly in the case of a stepper, several images on the reticle are sequentially reduced and exposed, so one piece of dust on the reticle can cause all LSIs to fail, so it is extremely important to eliminate dust. Become. Therefore, a method has been proposed in which a transparent film is placed on one or both sides of a photomask reticle (hereinafter sometimes abbreviated as "mask") to prevent dust from adhering to the mask ( Special Publication Showa 54-28716). According to this method, dust adheres only to this transparent film, and by focusing on the image on the mask during projection, the dust is out of focus and is not imaged on the wafer. It is possible to prevent defects due to In order to form the image on the mask without distortion, the film used for such a dust-proof cover body needs to be uniform in thickness and free from dust, scratches, etc., and at the same time, it is important that it is transparent to the light to be exposed. be. That is, if the light transmittance is low, it is necessary to lengthen the exposure time, and the throughput deteriorates. Since LSI manufacturing often involves extremely high-volume production, improving throughput is important.In particular, in the case of steppers, each wafer is exposed hundreds of times, so improving light transmission is important. A material with excellent light transmittance is desired since it greatly contributes to the improvement of the light transmittance. Currently, the membranes used for dustproof covers are mainly nitrocellulose thin membranes. The nitrocellulose thin film is exposed to the light normally used for exposure (350~
Although the material does not absorb or scatter light (450nm), when light enters it, reflection occurs on the film surface.
Transmittance will decrease. Therefore, as methods for reducing this reflected light, there are the following two methods. One method is to increase the light transmittance by making the nitrocellulose thin film have a certain thickness so that reflected waves from the front and back surfaces of the film interfere and cancel each other out. In this method, a 0.865 μm film is usually used in the dustproof cover for the stepper in order to transmit light at the exposure wavelength of 436 nm.
0.865±0.03μ for light transmittance of 95% or more
m, ±0.01μm to achieve light transmittance of 98% or more
It is necessary to set the film thickness within the following range. It is quite difficult to produce membranes of such thickness and the yield is low. At the same time, such a thin membrane is easy to tear.
It requires careful handling. In addition, in the case of projection, the exposure light is wide ranging from 350 to 450 nm, and the bright lines of 365, 408, and 436 nm cannot all be transmitted well, so the average light transmittance is
The upper limit is around 92%. In contrast to this method, the nitrocellulose thin film
There is a method of canceling the reflected waves by coating with an inorganic substance such as MgF ₂ , Al ₂ O ₃ , ZrO ₂ or the like by vacuum deposition or the like. This method is an application of the method used for optical lenses, glasses, etc.
By changing the thickness of the anti-reflection coating, it is possible to prevent reflection of light of any wavelength.
Furthermore, even for relatively thick films, a light transmittance of 98% or more can be obtained. As described above, although a dustproof cover using a nitrocellulose film coated with an antireflection coating is excellent, it has several drawbacks. That is, to deposit an inorganic substance onto a nitrocellulose membrane,
It is necessary to use expensive equipment such as a vacuum evaporation equipment.
In addition, when vapor deposition, it is necessary to perform it at a low temperature so as not to damage the nitrocellulose film, but if the vapor deposition is performed at a low temperature, the inorganic substance to be coated will not be deposited densely, and the refractive index will change depending on the temperature conditions. Therefore, it does not show the targeted anti-reflection performance. Furthermore, depending on the deposition material, oxidation may occur in the air after deposition, resulting in a change in the refractive index.
It has the disadvantage that its performance as a dustproof cover is unstable. Therefore, the present inventor completed the present invention as a result of intensive research into an anti-reflection dustproof cover body that does not have such drawbacks. (Structure of the Invention) The present invention has at least a nitrocellulose layer forming the core, and an outermost antireflection layer made of a polymer having a refractive index of 1.42 or less and which does not absorb or scatter light at a wavelength to be prevented from reflecting. A dust-proof cover body for a non-reflective photomask/reticle, which has a multilayer film as the main part, and at least a nitrocellulose layer forms the core, and has a refractive index of 1.42 or less and absorbs and scatters light at wavelengths that should be prevented from reflecting. The outermost antireflective layer is made of a polymer with a refractive index of 1.57 or more and does not absorb or scatter light at the wavelength to be prevented. This is a dustproof cover body for a non-reflective photomask/reticle, the main part of which is a multilayer film disposed between The outermost anti-reflection layer is made of a polymer that does not absorb or scatter light at the desired wavelength, and the intermediate anti-reflection layer is made of a polymer with a refractive index of 1.57 or higher that does not absorb or scatter light at the desired wavelength. When manufacturing a dustproof body for a non-reflective photomask/reticle, which mainly consists of a multilayer film that is sometimes disposed between a cellulose layer and an outermost antireflection layer, the polymers of each of the above layers are dissolved in a solvent. This is a method of manufacturing a dustproof body for a non-reflective photomask/reticle, characterized in that it is formed by a spin coating method. Next, the present invention will be explained with reference to the drawings. In FIG. 1, a nitrocellulose layer 1 forms the core of a multilayer membrane 2. In FIG. The nitrocellulose layer used in the present invention is industrial nitrocellulose specified in JIS K6703, and has a nitrogen content of 10.7% by weight or more and 12.2% by weight or less. Moreover, the degree of polymerization is suitably 30 to 500 as an average degree of polymerization. A higher degree of polymerization is preferable because a tougher film can be obtained, but if the film thickness is thick, a lower degree can be used satisfactorily. The thickness of the membrane is usually 1 to 10 μm. Nitrocellulose membranes can be produced by any method, but since nitrocellulose is not thermoplastic, a method of casting it in a solution is usually used. In addition, for the purpose of this dustproof cover body, the film thickness is as thin as 1 to 10 μm, and uniformity of the film thickness is important, so the manufacturing methods include spin coating method, knife coater method, and dip-pulling method. method, casting method on the fluid surface, etc. are suitable. The outermost antireflection layer 3 is laminated on the surface of the nitrocellulose layer 1 and constitutes a multilayer film 2 as a whole. The outermost anti-reflection layer 3 is made of a polymer having a refractive index of 1.42 or less and which does not absorb or scatter light of the wavelength to be prevented from reflecting. If the refractive index of the antireflection layer is greater than 1.42, the reduction in interference waves will be small, and the minimum light transmittance at which the reflected waves will be maximized due to interference will be 92% or less, resulting in insufficient antireflection effect. refractive index
1.42 or less, for example, if silicone resin (refractive index ≒ 1.42) is used, the minimum light transmittance is
Therefore, when used in a dustproof cover for a projector, a light transmittance of 92% or more can be obtained without the need to set the film thickness to 2.85 μm as in the case of nitrocellulose alone. As a polymer,
For example, silicone polymers and fluorine-based polymers have a refractive index of 1.42 or less, and are good because they have almost no absorption or scattering. In particular, regarding fluorine-based polymers, polymers with various properties can be obtained depending on the type of copolymer monomer and copolymer composition, but among them, the polymers that can be used in the present invention have a refractive index of 1.42 or less and It is necessary that the polymer does not absorb or scatter the exposing light, and furthermore, it is preferable that the polymer does not have a sticky surface. Sticky materials are not preferred because they tend to attract dust and are difficult to remove.
Such fluorine-based polymers include a copolymer of tetrafluoroethylene (hereinafter sometimes abbreviated as TFE) and vinylidene fluoride (hereinafter sometimes abbreviated as VdF), or a copolymer of TFE, VdF, and hexafluoropropylene (hereinafter sometimes abbreviated as VdF). Among the terpolymers (hereinafter sometimes abbreviated as HFP), HFP content is 0wt% or more and 10wt
% or less, the TFE content is 25-51wt%,
In the range where HFP exceeds 10wt%, TFE content is 35~
51wt%, and the content ratio of HFP to VdF is
Mention may be made of polymers with HFP content in the range less than 2.3:1. Compositions other than those mentioned above are not suitable for use in the present invention because scattering occurs due to crystallinity, or the surface becomes sticky due to the formation of an elastomer. In FIG. 2, the structure of the parts other than the multilayer film 2 is basically the same as the conventional one. The dustproof cover body has a structure in which the multilayer film 2 is fixed to the support frame 4 with an adhesive layer 5 or the like without wrinkles or sagging, and the other surface of the support frame 4 has an adhesive layer 6. The size of the support frame 4 made of metal such as aluminum, plastic, etc. is approximately 1 to 6 inches in diameter, depending on the size of the photomask or reticle used.
Or 1 to 6 inches square. The height of the support frame 4 is approximately 2 to 10 mm. The material of the support frame 4 is aluminum alloy, hard plastic, or the like, as it has low dust adhesion, is lightweight, and has strength. The dustproof cover body is stored in a clean case until it is ready for use to prevent dust from adhering to it. In use, a photomask or reticle 8 is used to cover the image 7 of the lithographic apparatus as shown in FIG.
Attach the dustproof cover to the By storing the mask in the manner shown in FIG. 3, it is possible to prevent dust from adhering to the mask, etc., and there is no need to clean the mask, etc. when using it again. In FIG. 4, an intermediate antireflection layer 9 is arranged between the nitrocellulose layer 1 and the outermost antireflection layer 3. In FIG. The intermediate anti-reflection layer 9 is made of a polymer having a refractive index of 1.57 or more and which does not absorb or scatter light of the wavelength to be prevented from reflecting. Such polymers include polystyrene, polysulfone, polyether sulfone, polyphenylene ether, polycarbonate, aromatic polyester. The thickness of the intermediate antireflection layer 9 is suitably about 0.05 μm to 0.5 μm. Outermost antireflection layer 3 and intermediate antireflection layer 9
Due to the cooperative action of these, the minimum light transmittance is 98% or more. In the configuration shown in FIG. 4, the ratio of the refractive index of _each polymer in the intermediate antireflection layer 9 and the outermost antireflection layer 3 is important; Let n ₂ represent the refractive index of n ₁ /
If n ₂ =√1.51, complete anti-reflection can be achieved. However, in the present invention, it is sufficient to have a transmittance of 98% or more, so it is not necessarily necessary to satisfy the above-mentioned ratio condition, and it is sufficient to use a polymer having a refractive index relationship close to this for each layer. The refractive index of polymers is 1.34 to 1.34 in the wavelength range of 350 to 450 nm, which is the exposure wavelength, if there is no absorption or scattering.
Since the refractive index is limited to about 1.63, if a polymer with a refractive index of 1.63 is used for the intermediate antireflection layer 9,
The refractive index of the polymer of the outermost antireflection layer 3
If it is 1.42 or less, its light transmittance is 98%.
That's all. In addition, conversely, the outermost antireflection layer 3
When a polymer with a low refractive index of 1.34 is used, and if the polymer of the intermediate antireflection layer 9 has a refractive index of 1.57 or more, a light transmittance of 98% or more can be obtained as a whole. Especially as the intermediate antireflection layer 9
When a polymer with a refractive index of 1.63 and 1.34 is arranged as the outermost antireflection layer 3, the light transmittance is 99.5% or more, and the antireflection effect is particularly excellent. The thickness of the anti-reflection layer must be equivalent to 1/4 (2n+1) [n=0, 1, 2] of the wavelength of the light to be prevented from reflecting, and if this thickness deviates, the anti-reflection effect will be large. Since the film thickness deteriorates over time, it is necessary to set this film thickness accurately. In addition, the value of the refractive index n is usually 0, that is, the thickness is equivalent to 1/4 of the wavelength of the light to be prevented from reflecting, since the wavelength range in which the anti-reflection effect can be achieved can be widened. It is suitable for preventing reflection of light over a relatively wide range of 350 to 450 nm. Still another embodiment is a multilayer film in which an intermediate layer or the like having antistatic performance is arranged between the outermost antireflection layer 3 and the intermediate antireflection layer 9 or between the nitrocellulose layer 1 and the intermediate antireflection layer 9. However, it is important that this intermediate layer is also made of a polymer that does not absorb or scatter light of the wavelength to be prevented from reflecting. Although various solution casting methods, plasma polymerization methods, and the like can be used to manufacture each aspect of the multilayer film described above, the spin coating method is the most suitable as the easiest and most accurate manufacturing method. Solution casting methods include knife coater method, rod coater method,
There are various methods such as the blade coater method and the impregnation coater method, but these methods have a film thickness of 1/4λ (2n + 1), which is the objective of the present invention, [where λ is the wavelength and n is the refractive index], usually 0.06. It is quite difficult to form a thin film of about 0.5 .mu.m with extremely high precision, and this is even more difficult due to the fact that the nitrocellulose film is as thin as 1 to 10 .mu.m. It has been found that the intermediate layer, the intermediate antireflection layer, and the outermost antireflection layer can be formed on the nitrocellulose layer with sufficiently satisfactory accuracy by the spin coating method. The method of the present invention includes a method in which a nitrocellulose film is formed on a wafer and an antireflection layer is applied thereon using a spin coater, and a method in which a nitrocellulose film prepared in advance is coated on a support frame. This method involves fixing the material, setting it in a spin coater, and applying an antireflection layer, etc. This makes it possible to form an antireflection layer or the like with extremely high uniformity. For example, if the thickness of the antireflection layer is 1/4λ [λ: wavelength to be prevented] and λ = 400nm, adjust the polymer concentration of the solution forming the antireflection layer to 0.5 to 1.5wt%, and apply it to the spin coater. The desired film thickness can be obtained by adjusting the rotation speed to about 500 to 2000 rpm and rotating for 10 seconds to 5 minutes. The actual thickness of the film is extremely thin at approximately 0.1 μm, and it is composite, so it is difficult to measure it directly. Since the wavelength at which the waves are smallest corresponds to 1/4λ, the thickness of the antireflection layer can be estimated. (Effects) The present invention, which has a basic structure of a specific outermost antireflection layer and a nitrocellulose layer, can efficiently transmit light at wavelengths that should be transmitted without being absorbed or scattered, and its performance is stable. can be maintained. The above effects are further enhanced if there is a specific intermediate antireflection layer. Furthermore, according to the method of the present invention, the multilayer film described above can be produced reliably, efficiently, and stably. (Example) Hereinafter, the present invention will be explained in more detail in two examples. Example 1, Comparative Example 1 Nitrocellulose (manufactured by Asahi Kasei Corporation, H-20) was dissolved in n-butyl acetate to form a 7% by weight solution.
In addition, TFE/HFP/VdF terpolymer (TFE=
50.1% by weight, HFP = 29.2% by weight, VdF = 20.7% by weight, n ²⁰ _D = 1.35), perfluoro-2-methyl-
1-oxy-3-thiacyclohexane-3,3-
Dioxide

[Formula] was dissolved to make a 0.6% by weight solution. A silicon wafer (125 mmφ) was set in a spin coater, and a nitrocellulose solution was dropped to form a uniform nitrocellulose coating on the wafer. After drying this with an infrared lamp, TFE/ The HFP/VdF terpolymer solution was added dropwise, rotated at 700 rpm for 20 seconds, and then dried with an infrared lamp. Next, an epoxy adhesive was applied to an aluminum ring of 100 mmφ (outer diameter), overlapped with the film surface formed as described above, and cured. Immerse this material in water, leave it for 30 minutes, separate the wafer and film, collect the thin film stuck to the aluminum ring from the water, air dry it, and place it again in the spin coater with the film side facing up. Set, TFE/
After applying HFP/VdF copolymer solution and rotating film formation,
By drying, the intended dustproof cover body was obtained. As shown in Figure 5, the light transmittance of this anti-reflection film in the vicinity of 400 nm was determined by measuring the spectral characteristics with a spectrophotometer (UV-240, manufactured by Shimadzu Corporation).
The average transmittance was 98% between 96.5 and 99.5%. on the other hand,
The spectral characteristics of a dustproof cover made of nitrocellulose alone without the TFE/HFP/VdF ternary copolymer coating show a light transmittance of 84-99%, large interference waves observed, and an average transmittance of 92%. It was hot. Example 2 Silicone polymer (manufactured by Shin-Etsu Silicon Co., Ltd.) was used instead of the TFE/HFP/VdF terpolymer of Example 1.
KE−108, n _D = 1.41) was dissolved in hexane and 1.0
A weight percent solution was prepared, silicon layers were formed on both sides of the nitrocellulose thin film, and the intended dustproof cover body was obtained. As a result of measuring the spectral characteristics of this dustproof cover, the light transmittance between 350 and 450 nm was 93.5~
The lowest transmittance was 99.8%, which was higher than the average transmittance of the nitrocellulose single membrane, which was 92%. Example 3 In place of the TFE/HFP/VdF terpolymer of Example 1, TFE/VdF copolymer (TFE content = 49.7
A dustproof cover body was obtained by forming a film using a hexamethylphosphoric acid triamide solution (wt%, n _D = 1.40). From the measurement of the spectral properties of this material,
The light transmittance between 350 and 450 nm was 92.5 to 99.5%, and the average transmittance was 96%, indicating an antireflection effect. Examples 4, 5, 6, 7, 8 Nitrocellulose (manufactured by Asahi Kasei Corporation, H-20) was dissolved in ethyl lactate to form a 9% by weight solution.
Also, polystyrene (manufactured by Asahi Kasei Corporation, GP-683, n _D =
1.59), polysulfone (manufactured by Nissan Chemical Co., Ltd., P-3500,
n _D = 1.63), polyether sulfone (manufactured by ICI, P-
2000, n _D = 1.63), polycarbonate (manufactured by Teijin Ltd.,
1250, n _D = 1.59) and polyphenylene ether (Asahi Kasei Corporation, Zylon T-1045, n _D = 1.59) were each dissolved in 1,1,2,2-tetrochloroethane to form a 0.75% by weight solution. And so. Also, TFE/
HFP/VdF terpolymer (TFE=37.6% by weight,
HFP = 18.9% by weight, VdF = 44.5% by weight, n _D =
1.37) as perfluoro-2-methyl-1-oxy-
It was dissolved in 3-thiacyclohexane-3,3 dioxide to give a 0.6% by weight solution. Using a spin coater, these solutions were deposited on a silicon wafer in the order of nitrocellulose, intermediate layer polymer such as polystyrene, and TFE/HFP/VdF polymer, and after adhering to an aluminum ring in the same manner as in Example 1, Separate the silicon wafer in water, and then use a spin coater to coat the intermediate layer with TFE/HFP/
A dustproof cover body was obtained by forming a VdF polymer layer and having a nitrocellulose layer as the center layer. When these spectral characteristics were measured, they were as shown in Table 1, and the minimum light transmittance was 98% or more, indicating sufficient performance as a dustproof cover for a stepper.

[Table] Examples 9 and 10, Comparative Example 2 Dust-proof covers were produced in the same manner as in Example 5, except that the concentration of the nitrocellulose solution was changed to 14% by weight. The spectral characteristic data of the obtained dustproof cover body is shown in FIG. On the other hand, a dustproof cover body consisting only of a nitrocellulose film having a concentration of 14% by weight was prepared, and its spectral characteristics were measured. The results are shown in FIG. The film thickness was calculated from the interference wave shown in FIG. 7 using the following formula. d=kλ ₁ λ _K /2n (λ ₁ - λ _K ) d: Film thickness (nm) λ ₁ : Peak wavelength of interference wave near D line (587.6 nm) (nm) λ _K : kth wavelength from λ ₁ Peak wavelength (nm) n: n _D of nitrocellulose (=1.51) As a result, the film thickness was approximately 9.5 μm. Moreover, from FIG. 7, the light transmittance of the nitrocellulose single film in the vicinity of 400 nm was 87 to 96%, with an average of 91.5%. (Comparative Example 2) On the other hand, in this Example 9, the nitrocellulose film was formed under the same conditions as Comparative Example 2, so it was approximately
Although it is estimated to be 9.5μm, the light transmittance is 400nm.
Interference waves almost disappeared at 99.5% in the vicinity, demonstrating high light transmission performance.

[Brief explanation of the drawing]

FIG. 1 is an enlarged cross-sectional view showing an example of a multilayer film that is a main part of the present invention, FIG. 2 is a cross-sectional view of the entire dustproof cover body, and FIG.
FIG. 4 is an enlarged sectional view showing another example of the multilayer film, FIG. 5 is a diagram showing the wavelength and light transmittance of Example 1 and Comparative Example 1, FIG. 6 is a diagram similarly showing Example 10, 7
The figure similarly shows Comparative Example 2. 1...Nitrocellulose layer, 2...Multilayer film, 3
... Outermost antireflection layer, 4 ... Support frame, 5 ... Adhesive layer, 6 ... Adhesive layer, 7 ... Image, 8 ... Photomask or reticle, 9 ... Intermediate antireflection layer.

Claims (1)

[Claims] 1. At least a nitrocellulose layer forms the core, and an outermost antireflection layer made of a polymer having a refractive index of 1.42 or less and which does not absorb or scatter light at a wavelength to be prevented from reflecting is disposed at the outermost side. A dustproof cover body for non-reflective photomasks and reticles that has a multilayer film as the main part. 2 At least, a nitrocellulose layer forms the core, and an outermost anti-reflection layer made of a polymer with a refractive index of 1.42 or less and which does not absorb or scatter light at the wavelength to be prevented from reflection is disposed on the outermost side, and has a refractive index of 1.57 or more. A non-reflective photomask/reticle whose main part is a multilayer film in which an intermediate anti-reflective layer made of a polymer that does not absorb or scatter light at wavelengths to be prevented from reflecting is disposed between a nitrocellulose layer and an outermost anti-reflective layer. dustproof cover body. 3 At least, a nitrocellulose layer forms the core, and an outermost anti-reflection layer made of a polymer that has a refractive index of 1.42 or less and does not absorb or scatter light at the wavelength to be prevented is disposed on the outermost side, and has a refractive index of 1.57 or more. A non-reflective photomask whose main part is a multilayer film in which an intermediate anti-reflective layer made of a polymer that does not absorb or scatter light at wavelengths to be prevented from reflecting is disposed between the nitrocellulose layer and the outermost anti-reflective layer. - A method for producing a dustproof body for a non-reflective photomask/reticle, characterized in that, in manufacturing the dustproof body for a reticle, each of the above layers is formed by dissolving the polymer in a solvent and using a spin coating method.