1. Field of the Invention
The present invention relates to a diffraction grating, a laser apparatus, and a manufacturing method for the diffraction grating.
2. Description of the Related Art
Conventionally, there is a diffraction grating used in an ArF or Krf excimer laser apparatus as a reflection-type diffraction grating, which is used, for example, for dispersing a wavelength of far ultraviolet rays. Such a diffraction grating is used as a band-narrowing element and acts as a kind of a resonator by combining an output mirror in a discharge chamber. Here, the high proportion of light quantity that returns at a predetermined order upon the reflection, that is, high diffraction efficiency, is important for utilizing incident light without being wasted it as far as possible, and for obtaining a preferable operation of the laser apparatus. The reflection-type diffraction grating is typically an asymmetrical triangle-shaped grating, includes a reflective layer made of a metal and a dielectric layer formed on the reflective layer for protecting the reflective layer from influences such as oxidation. However, the dielectric layer is contributes significantly to the diffraction efficiency. Japanese Patent No. 4549019 discloses a diffraction grating having a dielectric layer including Al2O3, for enhancing the antioxidant effect more than a conventional one and for enhancing the diffraction efficiency by specifying a thickness of the dielectric layer. U.S. Patent Publication No. 2009/0027776 additionally discloses a method for enhancing the diffraction efficiency in which no dielectric layer or a dielectric layer thinner than a blaze surface is formed on a counter surface on which no light or small amount of light is incident, and which is generally adjacent to the blaze surface at the light incident side.
The condition of the dielectric layer on the counter surface is important when forming the dielectric layer to enhance the diffraction efficiency. Depending on the condition, absorption of light on the counter surface may occur and the diffraction efficiency may more significantly lower than the case where the reflective layer is disposed as an outermost layer. However, Japanese Patent No. 4549019 does not disclose the absorption of light on the counter surface. In contrast, U.S. Patent Publication No. 2009/0027776 discloses the absorption of light on the counter surface under the limited condition described above. However, in fact, the diffraction efficiency may be significantly lower than the case where the reflective layer is disposed as the outermost layer even when the dielectric layer thinner than the blaze surface is formed on the counter surface. For example, the diffraction efficiency lowers to 43.2% when forming an MgF2 single layer film that serves as the dielectric layer on the reflective layer in spite of the diffraction efficiency being 62.6% when forming an aluminum film that serves as the reflective layer. Noted that, the thickness of the blaze surface is about 56 nm in this case, which is approximately optimum when considering the diffraction efficiency, and the thickness of the counter surface is 33 nm. Specifically, it does not mean that all the diffraction grating acquires high efficiency even when satisfying the condition disclosed in the specification of United States Patent Application No. 2009/0027776. In contrast, with reference to the condition in which no dielectric layer is formed on the counter surface, an extremely strict control of the film-forming conditions, such as an incident angle on the dielectric layer, is needed in the film-forming process, and manufacturing thereof is technically difficult and is achieved with difficulty.
The present invention provides, for example, a diffraction grating having high diffraction efficiency and is easily manufactured.
The present invention is a reflection-type diffraction grating that has a grating, the cross section of which is an asymmetrical triangular shape, and comprises a first layer formed on the cross section having an asymmetrical triangular shape and of which a material is a metal, for reflecting light; and a second layer formed on the first layer, on a surface of a long side of the triangular shape and of which a material is a dielectric, wherein given that a refractive index of the dielectric of the second layer is “n”, the thickness of the second layer is 15.38×(n−1)−065 (nm) or below.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
First, a description will be given of a diffraction grating according to a first embodiment of the present invention. The diffraction grating according to the present embodiment is a reflection-type diffraction grating that has a reflective layer, and the cross section having an asymmetrical triangular shape that is continuously disposed. The diffraction grating is used, for example, for an ArF excimer laser apparatus (used for dispersing light upon generating ArF excimer laser light), and also acts as a kind of a resonator by combining an output mirror in a discharge chamber, in parallel with narrowing the band.
Next, a description will be given of a thickness condition of the dielectric layer 4 for enhancing the diffraction efficiency of the diffraction grating 1 (hereinafter referred to as “thickness condition with high efficiency”). Typically, a condition about how the dielectric layer 4 is formed (added) is significantly important for realizing a diffraction grating having high efficiency. This is because the diffraction efficiency may drastically be lowered due to the absorption of light occurring at the dielectric layer 4 on the counter surface 9 (on the surface) as described above. A fact also disclosed is that the diffraction efficiency may be lowered even when the thickness of the dielectric layer 4 on the counter surface 9 is thinner than that of the dielectric layer 4 on the blaze surface 6, as described in the description of the related art. In contrast, also with reference to a condition in which the dielectric layer 4 is not added to the counter surface 9, manufacturing thereof is also technically very difficult and such a diffraction grating is realized with difficulty due to a requirement of strict control of the film-forming conditions in the film-forming process, as described above.
Based on the matters described above, in the present embodiment, the thickness condition with high efficiency is set as below. The thickness condition with high efficiency is determined by calculating the diffraction efficiency (hereinafter, refer to as only “efficiency”) that is executed by an information processing unit (computer) by using a rigorous coupled-wave analysis (RCWA), which is a type of the electromagnetic field analysis methods. First, a description will be given of a value that is set in advance when calculating the efficiency. The diffraction grating 1 is used under a condition in which an emitted light is reflected in the same direction as the incident light, a so-called Littrow layout, in the ArF excimer laser apparatus. Additionally, an incident angle (incident angle of light rays a) that is an angle at which the light rays 5 are incident to the diffraction grating 1 (angle to a normal of the grating plane 7), is equal to an emission angle, 79.60 degrees. A wavelength λ of the light that is excited and discharged by argon-fluorine and irradiated to the diffraction grating 1 is 193.00 nm. The grating interval d is 10.7081 μm (93.3636 grating grooves per 1 mm), the order m is 109, and vertex angle φ is 85.50 degrees. The material of the metal layer 3 is aluminum, and its film thickness is 220 nm. The refractive index of aluminum is 0.14+2.35i. Moreover, the dielectric layer 4 is a single film of which the material is MgF2, and the refractive index of MgF2 is 1.45 in the calculation below. Based on these setting values, the information processing unit executes calculating the efficiency by using each thickness (film thickness) of the MgF2 layer on the blaze surface 6 and the MgF2 layer on the counter surface 9, which serve as the dielectric layer 4, as parameters.
Based on the change of the efficiency described above, the thickness condition of the dielectric layer 4 having high efficiency is that the thickness of the dielectric layer 4 (here, the MgF2 layer) on the counter surface 9 is thinner than a given thickness (smaller than a given value). Additionally, while the efficiency drastically lowers when the thickness of the dielectric layer 4 on the counter surface 9 is thicker than the given thickness, the efficiency increases and is finally saturated when the thickness further increases. For example, a case is considered of forming the MgF2 film such that the thickness of the MgF2 layer on the blaze surface 6 is 56 nm and the thickness of the MgF2 layer on the counter surface 9 is 20 nm, with respect to the diffraction grating having the efficiency of 61.9% when forming the metal layer 3 (when forming aluminum film). In this case, the efficiency after forming the MgF2 film is consequently 61.7%, which is almost same value as the case where the metal layer 3 is disposed as the outermost layer. That is, the efficiency that is almost same as the case where the metal layer 3 is disposed as the outermost layer may be acquired by allowing the dielectric layer 4 on the counter surface 9 to have appropriate thickness.
Additionally, a high efficiency needs to be defined when regulating the thickness of the dielectric layer 4 on the counter surface 9 to achieve the high efficiency. In the present embodiment, “high efficiency” is defined as an efficiency that is always higher than the efficiency when the thickness of the dielectric layer 4 on the counter surface 9 is a given thickness or above. Specifically, a high efficiency is the efficiency of 97.0% or above with respect to the efficiency when the metal layer 3 is disposed as the outermost layer, and correspondingly, a thickness condition with high efficiency is a case where the thickness of the dielectric layer 4 on the counter surface 9 is between 0.1 nm and 25.8 nm. Note that for acquiring high efficiency, it is important to satisfy the thickness condition with high efficiency, and the thickness of the dielectric layer 4 on the counter surface 9 does not have to be thinner than that of the dielectric layer 4 on the blaze surface 6. Additionally, a configuration in which the dielectric layer 4 is not formed only on the counter surface 9 (that is, the thickness of the dielectric layer 4 is zero), which involves great difficulty in manufacture, is not needed.
Here, the thickness condition with high efficiency described above changes little due to the variation of the material of the metal surface of the metal layer 3 and the grating shape (triangular shape), and therefore it seems to be less dependent on them. In contrast, the thickness condition with high efficiency largely depends only on the refractive index of the material (dielectric) configuring the dielectric layer 4 on the counter surface 9. A maximum value “a” of the thickness of the dielectric layer 4 on the counter surface 9 that satisfies the thickness condition with high efficiency is calculated by changing the refractive index “n” of this dielectric as shown below.
[Formula 1]
a=15.38×(n−1)−0.65 (1)
That is, the diffraction grating 1 can realize a high efficiency if the maximum value “a” is in the range denoted by formula (2) as the condition formula below, with respect to the refractive index “n” of the dielectric.
[Formula 2]
0<a≦15.38×(n−1)−0.65 (2)
Thus, the thickness of the dielectric layer 4 on the counter surface 9 may be designed so as to satisfy the condition with high efficiency as described above (15.38×(n−1)−0.65 (nm) or below) (designing step), and the metal layer 3 may be formed on the resin layer 2 and the dielectric layer 4 may be formed on the metal layer 3 (forming step). Accordingly, the diffraction grating 1 can acquire a high efficiency by using a relatively easy manufacturing method.
As described above, the diffraction grating having a high diffraction efficiency and that is easily manufactured, and a manufacturing method for the diffraction grating can be provided according to the present embodiment. Additionally, by using the diffraction grating 1 according to the present embodiment, the laser apparatus allows utilizing the incident light without being wasted it as far as possible and efficiently generating laser.
Next, a description will be given of a diffraction grating according to a second embodiment. The diffraction grating in the case where the dielectric layer 4 is configured of the MgF2 single layer is exemplified in the first embodiment. In contrast, as a case of configuring the dielectric layers 4 having plural layers including a plurality of dielectrics, the present embodiment exemplifies a case where a LaF3 layer is further configured on the MgF2 layer. Note that the same reference numerals are provided to each of the elements and the parts of the diffraction grating of the present embodiment corresponding as each of the elements and the parts of the diffraction grating 1 according to the first embodiment.
Here, serving as values previously set before calculating the efficiency in the present embodiment, the refractive index of the LaF3 layer is 1.61, the thickness of the MgF2 layer on the blaze surface 6 is 25 nm, and the thickness of the LaF3 layer on the blaze surface 6 is 30 nm. Note that other calculation conditions are identical to those in the first embodiment. The thickness of the MgF2 layer on the counter surface 9 and the thickness of the LaF3 layer on the counter surface 9 are then changed as parameters to derive the thickness condition with high efficiency in a manner similar to the defined one in the first embodiment.
Next, a description will be given of a diffraction grating according to a third embodiment of the present invention. In the first embodiment, a diffraction grating that is assumed to apply to the ArF excimer laser and in which the wavelength λ of light is 193.00 nm is exemplified. In contrast, in the present embodiment, the diffraction grating that is assumed to apply to the KrF excimer laser and in which the wavelength λ of light is 248.30 nm is exemplified. Based on this assumption, the thickness condition with high efficiency obtained by executing the calculation of the efficiency in a manner similar to the first embodiment is as described below.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2014-045909 filed Mar. 10, 2014, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
---|---|---|---|
2014-045909 | Mar 2014 | JP | national |