JPH07107563B2 - Anti-reflection film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is formed so as to minimize the reflectance at the wavelength of an excimer laser used as an exposure light source in an optical system of a semiconductor exposure apparatus (hereinafter referred to as a mask aligner). The present invention relates to an antireflection film.

[Conventional technology]

In recent years, in semiconductor devices such as VLSIs, the miniaturization of patterns has advanced due to the demand for higher integration, and submicron line widths have begun to be put to practical use. In such a pattern transfer process of a semiconductor element, a reduction projection exposure apparatus (hereinafter referred to as a stepper) is mainly used in consideration of resolution and yield, but further improvement in resolution is required.

Practical resolution R of photolithography by stepper
It is given by R = kλ / N _A. Where λ is the exposure wavelength,
N _A is the numerical aperture of the lens, and k is a constant that is determined by the development contrast of the resist and is usually about 0.8. Therefore, in order to improve the resolution, the wavelength of the exposure light is shortened or the numerical aperture is increased. A stepper using an excimer laser, which is a high-power short-wavelength light source, has been attracting attention as a target for shortening the wavelength. The optical system of this stepper, especially the projection lens, is high at the excimer laser wavelengths of KrF (λ = 248 nm) and XeCl (λ = 308 nm) in order to prevent the occurrence of light loss, ghost, flare, etc. due to the surface reflection of the lens. It is necessary to form an antireflection film having an antireflection effect and high light resistance.

[Object of the Invention]

One of the objects of the present invention is to use the excimer laser wavelength of the stepper exposure light, specifically KrF (λ = 248 nm), XeC.
An object of the present invention is to provide an antireflection film having a good antireflection effect for l (λ = 308 nm).

Another object of the present invention is to provide an antireflection film which has a good antireflection effect against excimer laser light and is physically and chemically stable, and particularly excellent in light resistance.

[Means for Solving the Problems] The above object of the present invention is to provide a first layer having an intermediate refractive index substance layer and a second layer having a low refractive index on a substrate that transmits light having a wavelength of 160 nm or more. An antireflection film for use in an optical system using an excimer laser, which is formed by laminating a material layer, an intermediate refractive index material layer that is a third layer, and a low refractive index material layer that is a fourth layer in the same order, In the wavelength region of 160 nm or more, the refractive index of the intermediate refractive index material is 1.6 to 1.8, the refractive index of the low refractive index material is 1.5 or less, and when the design reference wavelength is λ ₀ , optical thickness is lambda _0/2, and when the optical film thickness of the second layer and _{n 2 d 2 1.4λ 0/4} <n 2 d 2 <1.6λ 0/4,
And an optical film thickness of the third layer and the fourth layer is achieved by an anti-reflection film, which is a lambda _0/4.

In the antireflection film according to the present invention, the target wavelength region is the ultraviolet region, particularly the deep ultraviolet region, so that the film material is an important problem in designing and manufacturing. That is,
The film material of the antireflection film must be a stable substance that does not absorb in the deep ultraviolet region and has high transmittance. Materials that can be used under such conditions include MgF ₂ , CaF ₂ , LiF, and Na.
_{3 AlF 6, LaF 3, NdF} 3, and the dielectric of fluoride ThF ₄ such, SiO _2, A
Examples thereof include oxide dielectrics such as l ₂ O ₃ , Y ₂ O ₃ , Sc ₂ O ₃ , HfO ₂ and ZrO ₂ . On the other hand, as shown in Table 1, the antireflection conditions of the antireflection film are known to be obtained by using a high refractive index substance in any structure of single layer, two layers, three layers and four layers.

The following Table 1 is a table showing antireflection conditions (theoretical solution and film configuration example) in the low refractive index substrate.

Among the above film materials, those having a high refractive index, Y ₂ O ₃ , Sc ₂
O ₃ , HfO ₂ and ZrO ₂ can be mentioned. Since these high-refractive-index substances have a large absorption in the far-ultraviolet region as compared with other materials, they become a factor that lowers the laser damage threshold of the film.
It cannot be said that it is an appropriate material when high light resistance is required. Therefore, the antireflection film of the present invention is an antireflection film formed by using a dielectric medium refractive index material and a low refractive index material having small absorption in the far ultraviolet region.

Of these, the low refractive index substance used in the present invention is Mg.
A substance selected from F ₂ , CaF ₂ , LiF and Na ₃ AlF ₆ , SiO ₂ and a substance selected from LaF ₃ , NdF ₃ , ThF ₄ and Al ₂ O ₃ are suitable as the intermediate refractive index substance.

The antireflection film of the present invention is an antireflection film having a four-layer structure as shown in FIG. In FIG. 1, reference numeral 1 denotes a substrate made of a substance that transmits light in a region of 160 nm or more including a deep ultraviolet region, specifically, an optical element such as a lens made of synthetic quartz, artificial quartz, CaF ₂ or the like. Layers 2 and 4 laminated on the substrate 1 are layers of an intermediate refractive index substance, and 3 and 5 are layers of a low refractive index substance. In order to form each of these layers, a vacuum deposition method (ion plating) is usually used. , Sputtering, etc.) are used.

In the antireflection film of the present invention, in order to reduce the reflectance for the excimer laser beam, the optical film thickness of the first layer from the substrate side with respect to the design wavelength lambda ₀ is lambda _0/2, and the second
When the optical film thickness of the layer and _{n 2 d 2 1.4λ 0/4} <n 2 d 2 <1.6λ
_0/4, the optical thickness of the third layer is lambda _0/4, the optical film thickness of the fourth layer is a lambda _{0 /.}

It should be noted that the optical film thickness of each layer is optimized by calculating, for example, by an electronic computer by a combination of the substrate and the film material so that the reflectance has a minimum value in a desired wavelength range.

As a method of improving the laser damage threshold value of the antireflection film, it is known to improve it by providing a low refractive index layer having an optical film thickness of λ / 2 on the substrate side. In the above embodiment, MgF ₂ or Si having an optical thickness of λ / 2 with respect to the laser wavelength λ is used.
By adding the O ₂ layer as the first layer on the substrate, it is possible to obtain an antireflection film having higher light resistance (high laser resistance) without changing the reflectance at the laser wavelength.

Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1 In the antireflection film of FIG. 1, the substrate 1 was a lens made of synthetic quartz, and LaF ₃ was used as the intermediate refractive index substance of 2,4 and MgF ₂ was used as the low refractive index substance of 3,5. The material was hard coated by vacuum deposition. Table 2 shows the design reference wavelength λ ₀
Shows the refractive index and the optical film thickness of the antireflection film having this film configuration, where is 238 nm. FIG. 2 shows the spectral characteristics of the antireflection film manufactured with this film structure. According to this, the reflectance at the excimer laser wavelength λ = 248 nm could be suppressed to 0.1% or less.

Next, regarding the durability, the adhesion, abrasion resistance, solvent resistance, and humidity resistance tests according to MIL-C-675 (C) were satisfied.
Also, continuous irradiation test average of KrF laser is 200mJ, 25Hz,
Even after irradiation for 50 hours with a pulse width of 15 ns, laser damage and deterioration of transmittance were not observed, and laser resistance was also confirmed.

In Example 1 of the present invention, the actual wavelength used (λ = 248 nm)
However, the design reference wavelength (λ ₀ = 238 nm) is made different. This point will be described with reference to Comparative Example 1 and FIG.

Comparative Example 1 Table 3 shows the refractive index and optical film thickness of the antireflection film having this film configuration when the design wavelength λ is ₀ 248 nm. In Comparative Example 1, the same substrate and film material as in Example 1 were combined, and the optical film thicknesses were 0.5λ, 0.375λ, 0.
It has the same configuration as 25λ and 0.25λ.

The spectral characteristics of the antireflection coatings of Example 1 and Comparative Example 1 are shown in FIG. According to this, in Example 1, the reflectance at the excimer laser wavelength λ = 248 nm can be suppressed to 0.1% or less. However, in Comparative Example 1, the reflectance at λ = 260 nm can be suppressed to a small value, but the desired wavelength range (λ
= 248 nm), the reflectance does not have the minimum value.

Thus, in the first embodiment, the desired wavelength range (λ = 248n
In m), the design reference wavelength λ ₀ is optimized by making it different from the incident light wavelength λ so that the reflectance has a minimum value.
In this respect, the same applies to Examples 2, 3, 4, and 5 described later.

In Example 1 of the present invention, the optical film thickness of the second layer is deviated from 1 / 4λ. This point will be described with reference to Comparative Example 2 and FIG.

Comparative Example 2 Table 4 shows the refractive index and the optical film thickness of the antireflection film having this film structure when the optical film thickness of the second layer is 1 / 4λ. In Comparative Example 2, the same substrate and film material as in Example 1 were combined, and the optical film thicknesses were 0.5λ and 0.2 in order from the first layer.
5λ, 0.25λ, 0.25λ.

The spectral characteristics of the antireflection films of Example 1 and Comparative Example 2 are shown in FIG. According to this, in Example 1, the reflectance at the excimer laser wavelength λ = 248 nm can be suppressed to 0.1% or less. However, in Comparative Example 2, the desired wavelength range (λ = 24
The reflectance does not reach the minimum value at 8 nm).

Thus, in the first embodiment, the desired wavelength range (λ = 248n
In m), the optical film thickness of the second layer is deviated from 1 / 4λ so that the reflectance has a minimum value. In this respect, the same applies to Examples 2, 3, 4, and 5 described later.

Example 2 Al ₂ O ₃ as an intermediate refractive index substance and MgF ₂ as a low refractive index substance
Was used to form an antireflection film having the structure of the present invention. The film structure at this time is as shown in Table 5. The spectral characteristics obtained with this configuration are shown in FIG. Also in this example, an antireflection film having optical characteristics and chemical and physical stability equivalent to those of Example 1 was obtained.

Example 3 Fluorite (CaF ₂ ) as a substrate, LaF ₃ as an intermediate refractive index material,
Using MgF ₂ as the low refractive index material, the antireflection film having the constitution of the present invention was formed. The film structure at this time is shown in Table 6, and the obtained spectral characteristics are shown in FIG. Since fluorite, which is a substrate, is an ionic crystal, when an electron gun is used as an evaporation source, a color center may be generated by the electrons generated from the electron gun. Therefore, a resistance heating method is suitable as an evaporation source, A combination of LaF ₃ and MgF ₂ fluorides according to this embodiment is preferable. In addition, since the substrate itself is a fluoride, a combination of fluorides is physically used as a film material.
It is also suitable because of its chemical stability. Also in this example, an antireflection film having optical characteristics and chemical and physical stability equivalent to those of Example 1 was obtained.

Example 4 Synthetic quartz was used as a substrate, and XeCl was formed according to the configuration of Example 1.
An antireflection film for (λ = 308 nm) was formed. The film structure at this time is shown in Table 7, and the obtained spectral characteristics when the design reference wavelength λ ₀ is 295.6 nm are shown in FIG. Also in this example, an antireflection film having optical characteristics and chemical and physical stability equivalent to those of the other examples was obtained.

Example 5 Synthetic quartz is used as a substrate, and the structure of Example 1 is used for ArF.
An antireflection film for (λ = 193 nm) was formed. The film structure at this time is shown in Table 8. When an experiment was carried out with the design reference wavelength λ _{0 set} to 185.2 nm, an antireflection film having optical characteristics and chemical and physical stability equivalent to those of the other examples was obtained also in this example.

[Effects of the Invention] As described above, the antireflection film of the present invention has excellent optical characteristics of suppressing reflection on the surface of a substrate such as a lens at a desired excimer laser wavelength and solving problems such as flare and ghost. have. Further, it has excellent chemical stability such as solvent resistance and moisture resistance, and at the same time has excellent physical stability such as adhesion, abrasion resistance and laser resistance, and is extremely useful in practice.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining the structure of a four-layer antireflection film of the present invention, FIG. 2 is a graph showing the spectral characteristics of the antireflection film produced in Example 1, and FIG. FIG. 4 is a graph showing the spectral characteristics of the antireflection film of Example 1 and Comparative Example 1, FIG. 4 is a graph showing the spectral characteristics of the antireflection film of Example 1 and Comparative Example 2, FIG. 5, FIG. FIG. 7 is a graph showing the spectral characteristics of the antireflection films produced in Examples 2-4. 1 ... Substrate, 2, 4 ... Medium refractive index material layer, 3, 5 ... Low refractive index material layer.

Claims (3)

[Claims] 1. A substrate which transmits light having a wavelength of 160 nm or more,
The intermediate refractive index material layer that is the first layer, the low refractive index material layer that is the second layer, the intermediate refractive index material layer that is the third layer, and the low refractive index material layer that is the fourth layer are in the same order. An antireflection film used for an optical system using an excimer laser that is laminated, wherein the refractive index of the intermediate refractive index substance is 1.6 to 1.8 in the wavelength region of 160 nm or more, and the refractive index of the low refractive index substance. Is 1.
5 or less, when the design reference wavelength is lambda _0, the optical thickness of the first optical film thickness of lambda _0/2 layer and a second layer
n ₂ d ₂ and for the _{1.4λ 0/4 <n 2 d} 2 <1.6λ 0/4, and the optical thickness of the third layer and the fourth layer is characterized by a lambda _0/4 Antireflection film. 2. The excimer laser has a K wavelength of 248 nm.
The antireflection film according to claim 1, which is an excimer laser of rF or XeCl having a wavelength of 308 nm. 3. The low refractive index material layer comprises MgF ₂ , CaF ₂ , LiF and Na ₃ A.
Of lF ₆ and SiO ₂ , the intermediate refractive index material is LaF ₃ , NdF ₃ ,
The antireflection film according to claim 1 or 2, which is selected from ThF ₄ and Al ₂ O ₃ , respectively.