DIELECTRIC MULTILAYER FILM MIRROR

Abstract

Provided is a dielectric multilayer film mirror including: a substrate; a first multilayer film structure formed on the substrate and including alternately stacked layers of a first low refractive index material having a refractive index equal to or lower than a refractive index of a second low refractive index material and a first high refractive index material having a refractive index higher than a refractive index of a second high refractive index material; and a second multilayer film structure formed on the first multilayer film structure and including alternately stacked layers of the second low refractive index material and the second high refractive index material, the second high refractive index material having a refractive index higher than a refractive index of the second low refractive index material and having an extinction coefficient lower than an extinction coefficient of the first high refractive index material.

Claims

1. A dielectric multilayer film mirror comprising: a substrate; a first multilayer film structure formed on the substrate including alternately stacked layers of a first low refractive index material and a first high refractive index material, the first low refractive index material having a refractive index equal to or lower than a refractive index of a second low refractive index material, and the first high refractive index material having a refractive index higher than refractive indices of the first low refractive index material and a second high refractive index material; and a second multilayer film structure formed on the first multilayer film structure including alternately stacked layers of the second low refractive index material and the second high refractive index material, the second high refractive index material having a refractive index higher than a refractive index of the second low refractive index material and having an extinction coefficient lower than an extinction coefficient of the first high refractive index material.

2. The dielectric multilayer film mirror according to claim 1, wherein the first low refractive index material and the second low refractive index material are silicon oxide.

3. The dielectric multilayer film mirror according to claim 1, wherein the first high refractive index material is hafnium oxide.

4. The dielectric multilayer film mirror according to claim 1, wherein the second high refractive index material is aluminum oxide.

5. The dielectric multilayer film mirror according to claim 1, further comprising: a protective layer formed on the second multilayer film structure and formed of a material which is one of the second low refractive index material and the second high refractive index material that is different from a material disposed on an outermost surface of the second multilayer film structure.

6. The dielectric multilayer film mirror according to claim 5, wherein an optical thickness of the protective layer is an integral multiple of /2 of a desired wavelength .

7. The dielectric multilayer film mirror according to claim 1, wherein a number of stacked layers in the second multilayer film structure is equal to or more than 8 and equal to or less than 48.

8. The dielectric multilayer film mirror according to claim 1, wherein a total of a number of stacked layers in the second multilayer film structure and a number of stacked layers in the first multilayer film structure is equal to or more than 26 and equal to or less than 70.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0018] FIG. 1 is a view showing a configuration of a conventional dielectric multilayer film mirror.

[0019] FIGS. 2A-2B are graphs showing a relationship between a reflectance and the number of stacked layers in a conventional dielectric multilayer film mirror including alternately stacked layers of silicon oxide and hafnium oxide.

[0020] FIGS. 3A-3B are graphs showing a relationship between a reflectance and the number of stacked layers in a conventional dielectric multilayer film mirror including alternately stacked layers of silicon oxide and aluminum oxide.

[0021] FIG. 4 is a view describing a reflectance in the vicinity of a surface of a dielectric multilayer film mirror including alternately stacked layers of silicon oxide and aluminum oxide.

[0022] FIG. 5 is a view showing a structure of a dielectric multilayer film mirror of one embodiment according to the present invention.

[0023] FIGS. 6A-6B are graphs showing reflectance characteristics of the dielectric multilayer film mirror of the present embodiment.

DESCRIPTION OF EMBODIMENTS

[0024] As described above, since, in a dielectric multilayer film mirror including alternately stacked layers of silicon oxide SiO.sub.2 and hafnium oxide HfO.sub.2, the reflectance cannot exceed the upper limit (99.67%) even when the number of stacked layers is increased, the present inventors have considered that it is necessary to develop a dielectric multilayer film mirror having a new structure in order to increase the reflectance equal to or higher than the upper limit of the reflectance, and thus the present inventors have examined various materials and configurations. Before describing a specific embodiment of the dielectric multilayer film mirror according to the present invention, the examined contents will be described.

[0025] The present inventors have considered that aluminum oxide Al.sub.2O.sub.3 that is a material having an extinction coefficient lower than that of hafnium oxide HfO.sub.2 is used as a high refractive index material in order to obtain a reflectance higher than that of a conventional dielectric multilayer film mirror. Then, the present inventors have investigated a relationship between the number of stacked layers and a reflectance, in a dielectric multilayer film mirror including alternately stacked layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2. The results are shown in FIG. 3A. In addition, FIG. 3B is a partially enlarged view of the vicinity of a central wavelength (250 nm) of ultraviolet light to be reflected by the dielectric multilayer film mirror.

[0026] Even in a case where aluminum oxide Al.sub.2O.sub.3 is used as the high refractive index material, similarly to a case where hafnium oxide HfO.sub.2 is used as the high refractive index material, the reflectance is increased as the number of stacked layers is increased. As shown in FIGS. 2A-2B, in the dielectric multilayer film mirror in which hafnium oxide HfO.sub.2 is used, when the number of stacked layers is increased up to 30 (15 pairs), the reflectance reaches the upper limit (99.67%). However, in the dielectric multilayer film mirror in which aluminum oxide Al.sub.2O.sub.3 is used, when the number of stacked layers is increased up to 70 (35 pairs), the reflectance is increased and reaches the upper limit (99.80%).

[0027] Since the aluminum oxide Al.sub.2O.sub.3 has an extinction coefficient of 250 nm smaller than that of the hafnium oxide HfO.sub.2, the reflectance continues to increase until the number of stacked layers is larger than that in the case where the hafnium oxide HfO.sub.2 is used. However, the refractive index of the aluminum oxide Al.sub.2O.sub.3 is 1.68 (@250 nm), which is smaller than the refractive index of the hafnium oxide HfO.sub.2 of 2.18 (@250 nm). Accordingly, a difference in refractive index between the aluminum oxide Al.sub.2O.sub.3 and the silicon oxide SiO.sub.2 is small as compared to the dielectric multilayer film mirror in which the hafnium oxide HfO.sub.2 is used as the high refractive index material. Therefore, in the dielectric multilayer film mirror in which the aluminum oxide Al.sub.2O.sub.3 is used, 70 layers (35 pairs) are required to be stacked in order to increase the reflectance up to the upper limit. In the dielectric multilayer film mirror in which the aluminum oxide Al.sub.2O.sub.3 is used, the maximum reflectance is higher than the maximum reflectance (99.67%) of the dielectric multilayer film mirror in which the hafnium oxide HfO.sub.2 is used, but it takes a lot of time to produce the dielectric multilayer film due to an increase of the number of stacked layers, and the cost also increases.

[0028] After the above examination, the present inventors have found that the loss of the amount of light due to light absorption is suppressed by disposing aluminum oxide Al.sub.2O.sub.3 (that is, used as the second high refractive index material) having a small extinction coefficient at 250 nm in a region where the amount of incident light is large, and a reflection efficiency is increased at the interface between hafnium oxide HfO.sub.2 and silicon oxide SiO.sub.2 by disposing the hafnium oxide HfO.sub.2 (that is, used as the first high refractive index material) having a high refractive index in a region where the amount of incident light is relatively small.

[0029] As illustrated in FIG. 4, according to the simulation (wavelength of 250 nm) performed by the present inventors, in the dielectric multilayer film mirror including alternately stacked layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2, 14% of the incident light is reflected at 2 layers (1 pair) positioned at the outermost surface, 23% of the incident light is reflected at 4 layers (2 pairs) (that is, 9% of the incident light is reflected by adding a third layer and a fourth layer), and 32% of the incident light is reflected at 6 layers (3 pairs) (that is, 9% of the incident light is reflected by adding a fifth layer and a sixth layer). That is, since the incident light is mostly reflected at a small number of layers positioned at the outermost surface, the amount of light that reaches a layer located deeper than the layers positioned at the outermost surface and is absorbed in the deep layer is not large. Therefore, the present inventors have reached the conclusion that it is possible to produce a dielectric multilayer film mirror having a high reflectance with low costs by adopting a configuration in which a multilayer film structure including alternately stacked layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2 is disposed near a surface of the dielectric multilayer film mirror, and a multilayer film structure including alternately stacked layers of hafnium oxide HfO.sub.2 and silicon oxide SiO.sub.2 is disposed near the substrate (deep layer side).

[0030] FIG. 5 is a view showing a configuration of a dielectric multilayer film mirror of one embodiment according to the present invention. The dielectric multilayer film mirror of the present embodiment generally includes a substrate 10, a first multilayer film structure 20 formed on the substrate 10, and a second multilayer film structure 30 formed on the first multilayer film structure 20. The first multilayer film structure 20 is a structure including alternately stacked first low refractive index material layers 22 and first high refractive index material layers 21. The second multilayer film structure 30 is a structure including alternately stacked second low refractive index material layers 32 and second high refractive index material layers 31.

[0031] Since the second multilayer film structure 30 is disposed near a surface of the mirror where the amount of incident light is large, based on the above consideration, aluminum oxide Al.sub.2O.sub.3 having an extinction coefficient lower than that of hafnium oxide HfO.sub.2 is used for the second high refractive index material layer 31. The second low refractive index material layer 32 is formed of silicon oxide SiO.sub.2 similarly to the related art. Therefore, an amount of absorbed light is suppressed and an amount of incident light is mostly reflected. The outermost layer of the second multilayer film structure 30 also serves as a protective layer 33 for preventing damage of the surface of the mirror. In the present embodiment, the protective layer 33 is formed of silicon oxide SiO.sub.2 similarly to the second low refractive index material layer 32 at a thickness twice that of each of the first low refractive index material layer 22 and the second low refractive index material layer 32 (silicon oxide) in the first multilayer film structure 20 and the second multilayer film structure 30. The second high refractive index material layer 31 (aluminum oxide Al.sub.2O.sub.3) that is positioned adjacent to the protective layer 33 and is used in the second multilayer film structure 30 may also be used for the protective layer 33. However, in the present embodiment, silicon oxide SiO.sub.2 having a further excellent environment resistance is used. In addition, in the present embodiment, a thickness of the protective layer 33 is set to be twice that of each of other layers (that is, an optical thickness is 212). The optical thickness of the protective layer 33 may be an integral multiple of 212, and is not necessarily limited to 212.

[0032] On the other hand, since the first multilayer film structure 20 is disposed at a deep layer portion where the amount of incident light is small, based on the above consideration, hafnium oxide HfO.sub.2 having a refractive index higher than that of aluminum oxide Al.sub.2O.sub.3 is used for the first high refractive index material layer 21. The first low refractive index material layer 22 is formed of silicon oxide SiO.sub.2 similarly to the second low refractive index material layer 32 used in the second multilayer film structure 30. Therefore, in the first multilayer film structure 20, a difference in refractive index between the high refractive index material and the low refractive index material is large as compared to that in the second multilayer film structure 30, and thus the light passed through the second multilayer film structure 30 is efficiently reflected. In the present embodiment, although both the first low refractive index material layer 22 and the second low refractive index material layer 32 are formed of silicon oxide SiO.sub.2, a material having a refractive index lower than that of the second low refractive index material layer 32 is used for the first low refractive index material layer 22, so that the difference in refractive index can be further increased.

[0033] In the configuration shown in FIG. 5, when the number of stacked layers in the first multilayer film structure 20 is 30 (15 pairs), and the number of stacked layers in the second multilayer film structure 30 is 10 (5 pairs), as shown in FIGS. 6A-6B, the high reflectance of 99.82% of ultraviolet light of 250 nm can be obtained. This reflectance is higher than both the maximum reflectance (99.67%) of the dielectric multilayer film mirror obtained by stacking 40 layers (20 pairs) formed of hafnium oxide HfO.sub.2 and silicon oxide SiO.sub.2, and the maximum reflectance (99.80%) of the dielectric multilayer film mirror obtained by stacking 70 layers (35 pairs) formed of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2. Materials and physical thicknesses of the respective layers constituting the dielectric multilayer film mirror of the present embodiment are shown in the following table.

TABLE-US-00001 TABLE 1 Refractive index Physical thickness Layer No. Material (@250 nm) (nm) Incident medium Air 1.00 40 SiO.sub.2 1.49 83.98 39 Al.sub.2O.sub.3 1.68 37.11 38 SiO.sub.2 1.49 41.99 37 Al.sub.2O.sub.3 1.68 37.11 36 SiO.sub.2 1.49 41.99 35 Al.sub.2O.sub.3 1.68 37.11 34 SiO.sub.2 1.49 41.99 33 Al.sub.2O.sub.3 1.68 37.11 32 SiO.sub.2 1.49 41.99 31 Al.sub.2O.sub.3 1.68 37.11 30 SiO.sub.2 1.49 41.99 29 HfO.sub.2 2.18 28.64 28 SiO.sub.2 1.49 41.99 27 HfO.sub.2 2.18 28.64 26 SiO.sub.2 1.49 41.99 25 HfO.sub.2 2.18 28.64 24 SiO.sub.2 1.49 41.99 23 HfO.sub.2 2.18 28.64 22 SiO.sub.2 1.49 41.99 21 HfO.sub.2 2.18 28.64 20 SiO.sub.2 1.49 41.99 19 HfO.sub.2 2.18 28.64 18 SiO.sub.2 1.49 41.99 17 HfO.sub.2 2.18 28.64 16 SiO.sub.2 1.49 41.99 15 HfO.sub.2 2.18 28.64 14 SiO.sub.2 1.49 41.99 13 HfO.sub.2 2.18 28.64 12 SiO.sub.2 1.49 41.99 11 HfO.sub.2 2.18 28.64 10 SiO.sub.2 1.49 41.99 9 HfO.sub.2 2.18 28.64 8 SiO.sub.2 1.49 41.99 7 HfO.sub.2 2.18 28.64 6 SiO.sub.2 1.49 41.99 5 HfO.sub.2 2.18 28.64 4 SiO.sub.2 1.49 41.99 3 HfO.sub.2 2.18 28.64 2 SiO.sub.2 1.49 41.99 1 HfO.sub.2 2.18 28.64 Substrate Synthetic 1.51 quartz Total thickness: 1496.94

[0034] A physical thickness of each of the layers in the first multilayer film structure 20 and the second multilayer film structure 30 is set such that the product of the physical thickness and the refractive index becomes a quarter of a desired wavelength (in the present embodiment, 250 nm). That is, in the first multilayer film structure 20, a physical thickness of the first low refractive index material layer 22 (silicon oxide) is 41.99 nm, and a physical thickness of the first high refractive index material layer 21 (hafnium oxide) is 28.64 nm. In the second multilayer film structure 30, a physical thickness of the second low refractive index material layer 32 (silicon oxide) is 41.99 nm, and a physical thickness of the second high refractive index material layer 31 (aluminum oxide) is 37.11 nm. A physical thickness of the protective layer 33 positioned at the outermost surface is 83.98 nm.

[0035] An optical path difference of a half wavelength (/4+/4) is generated in the light reflected at each interface between layers stacked at an optical thickness of a quarter wavelength (/4) of the incident light. In addition, a phase of light that is incident from a low refractive index layer and is reflected at the interface between the low refractive index layer and a high refractive index layer is inverted at the time of reflection (the same effect as in the generation of the optical path difference of 212). On the other hand, a phase of light that is incident from the high refractive index layer and is reflected at the interface between the low refractive index layer and the high refractive index layer is not inverted. As a result, the phases of the light reflected at each interface between the high refractive index layers and the low refractive index layers are aligned (optical path difference /2+effect 212 obtained by phase inversion effect=).

[0036] As described above, in the dielectric multilayer film mirror of the present embodiment, a higher reflectance (99.82%) is obtained as compared to both the upper limit (99.67%) of the reflectance of the conventional multilayer film mirror including alternately stacked layers of hafnium oxide HfO.sub.2 and silicon oxide SiO.sub.2 and the upper limit (99.80%) of the reflectance of the conventional dielectric multilayer film mirror including alternately stacked layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2. In addition, in the dielectric multilayer film mirror including alternately stacked layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2, 70 layers (35 pairs) are required to be stacked in order to obtain an upper limit value of the reflectance. On the other hand, in the dielectric multilayer film mirror of the present embodiment, the total number of layers in the first multilayer film structure 20 and the second multilayer film structure 30 is reduced to 40 (20 pairs), and thus the dielectric multilayer film mirror can be easily produced with low costs. The reflectance obtained when the number of layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2 is the same as in the present embodiment, that is, a total of 40 layers (20 pairs) are stacked is 98.17%. In the dielectric multilayer film mirror of the present embodiment, a sufficiently higher reflectance is obtained.

[0037] The embodiment is merely an example and can be appropriately changed within the spirit of the present invention. In the embodiment, although the number of stacked layers in the first multilayer film structure 20 is 30 (15 pairs) and the number of stacked layers in the second multilayer film structure 30 is 10 (5 pairs), the number of stacked layers can be appropriately changed in consideration of a balance between a level of the reflectance to be obtained and the cost. For example, in a case where the cost is reduced (that is, the number of stacked layers is reduced) with the same reflectance as in the related art, the number of stacked layers in the first multilayer film structure 20 may be 18 (9 pairs), and the number of stacked layers in the second multilayer film structure 30 may be 8 (4 pairs) (reflectance: 99.68%). Alternatively, in a case where the same number of stacked layers (70 layers) as in the conventional dielectric multilayer film mirror including alternately stacked layers of aluminum oxide Al.sub.2O.sub.3 and silicon oxide SiO.sub.2 is allowable, when the number of stacked layers in the first multilayer film structure 20 is 22 (11 pairs), and the number of stacked layers in the second multilayer film structure 30 is 48 (24 pairs), a high reflectance of 99.84% is obtained.

[0038] In addition, in the embodiment, silicon oxide is used as the first low refractive index material and the second low refractive index material, hafnium oxide is used as the first high refractive index material, and aluminum oxide is used as the second high refractive index material, but the present invention is not necessarily limited to only this combination. As described above, an appropriate material having a refractive index equal to or lower than that of the second low refractive index material can be used as the first low refractive index material, an appropriate material having a refractive index higher than that of the first low refractive index material can be used as the first high refractive index material, and an appropriate material having a refractive index higher than that of silicon oxide and having an extinction coefficient lower than that of the first high refractive index material can be used as the second high refractive index material.

REFERENCE SIGNS LIST

[0039] 10 . . . Substrate [0040] 20 . . . First Multilayer Film Structure [0041] 21 . . . First High Refractive Index Material Layer [0042] 22 . . . First Low Refractive Index Material Layer [0043] 30 . . . Second Multilayer Film Structure [0044] 31 . . . Second High Refractive Index Material Layer [0045] 32 . . . Second Low Refractive Index Material Layer [0046] 33 . . . Protective Layer