JP3606630B2 - Optical multilayer filter - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an optical multilayer filter having an optical multilayer film formed by alternately laminating a plurality of material films having different refractive indexes on a substrate.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, there has been known an optical multilayer filter that obtains predetermined filter characteristics by alternately forming a plurality of high refractive index material films and low refractive index material films on a glass substrate. The optical multilayer film is usually formed by sputtering, vacuum deposition, or the like, but the glass substrate is polished as thinly as possible in order to suppress the increase in the optical path length due to the insertion of the filter and the propagation light scattering caused thereby.
[0003]
In this type of optical multilayer filter, an internal stress is usually generated in the optical multilayer film due to a difference in coefficient of thermal expansion from the substrate material, a structural change, or the like. In general, the internal stress of a thin film caused by sputtering or the like includes a tensile stress in a direction in which the film tends to shrink and a compressive stress in a direction in which the film tends to spread. Therefore, when the substrate thickness is reduced, the internal filter warps due to the internal stress. In the case of tensile stress, the surface on which the optical multilayer film is formed becomes concave, whereas in the case of compressive stress, it becomes convex.
[0004]
In order to prevent such warpage of the optical multilayer filter, for example, it is desired to select a combination of materials and film thicknesses of a high refractive index material film and a low refractive index material film to reduce the internal stress as much as possible. . However, the selection of these materials and film thickness is limited due to the required optical filter characteristics.
Conventionally, the following method has been proposed as a method for preventing such warpage of the optical multilayer filter.
[0005]
(1) A separable film is formed on a substrate in advance, an optical multilayer film is formed thereon, and then only the optical multilayer film is separated.
(2) A polyimide film is applied to a substrate and an optical multilayer film is formed thereon, and then the substrate is removed to make polyimide the final substrate.
[0006]
[Problems to be solved by the invention]
In the method {circle around (1)}, the substrate is finally removed, so that the mechanical strength of the optical multilayer filter is weak, which makes it difficult to mount, for example.
In the method (2), since polyimide, which is an organic material, serves as the substrate, problems remain in terms of mechanical strength and reliability.
[0007]
The present invention has been made in view of the above points, and an object thereof is to provide an optical multilayer filter that prevents warping due to internal stress without impairing mechanical strength.
[0009]
[Means for Solving the Problems]
The present invention, in the optical multilayer filter having an optical multilayer film obtained by forming a plurality of layers stacked films of different materials having a refractive index on the base plate alternately, the substrate, the heat resistance of the starting substrate to be finally removed It is characterized in that it is a glass layer formed by a flame deposition method (FHD method) and sintered and having an internal stress whose direction and size are equal to the internal stress of the optical multilayer film.
[0011]
[Action]
According to the present invention, by forming the starting substrate a glass layer internal stress similar to the optical multilayer film is produced by FHD method, by removing the starting substrate, you are possible to prevent the Ri anti filter of the optical multilayer film it can. Although the starting substrate is removed, since a sintered glass layer is used as the final substrate, mechanical strength is sufficient, there is no problem in mounting, and reliability is high unlike an organic material.
[0012]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows an optical multilayer filter according to an embodiment of the present invention. In this embodiment, first, as shown in FIG. 1A, a buffer layer 12 is formed on a BK-7 substrate 11 by sputtering, and then an optical multilayer film 13 is formed thereon by sputtering. When the internal stress of the optical multilayer film 13 is a compressive stress, the buffer layer 12 is assumed to generate a tensile stress that counteracts this, and when the internal stress of the optical multilayer film 13 is a tensile stress, the buffer layer 12 It is assumed that a compressive stress that cancels this occurs.
Then, as shown in FIG. 1B, the substrate 11 is finally polished to have a thickness of about 30 μm as a whole.
[0013]
Specifically, in this embodiment, the optical multilayer film 13 includes a low refractive index SiO ₂ film 13a (refractive index = 1.46) and a high refractive index TiO, as shown in an enlarged view in FIG. _It is assumed that a plurality of _two films 13b (refractive index = 2.25) are alternately formed with a necessary optical film thickness (for example, a film thickness corresponding to λ / 4). When each film thickness is equivalent to λ / 4, the film thickness of the SiO ₂ film is about 1.5 times that of the TiO ₂ film. The optical multilayer film 13 obtained at this time becomes compressive stress. Therefore, as the buffer layer 12, a TiO ₂ film is used in this embodiment so that a tensile stress that cancels the internal stress of the optical multilayer film 13 is generated.
[0014]
In this embodiment, in order to effectively cancel the stress of the multilayer film 13 by the buffer layer 12, the selection of the film thickness is also important. That is, when the film thicknesses of the high refractive index layer and the low refractive index layer constituting the optical multilayer film 13 are dH and dL, and the stresses per unit film thickness are σH and σL, respectively, the optical multilayer film 13 as a whole is obtained. The stress σT is
σT = σH ΣdH + σL ΣdL
It becomes.
[0015]
On the other hand, the buffer layer 12 has a stress σB per unit film thickness and a film thickness dB, and the overall stress is σB · dB. Therefore, in order to cancel the stress of the optical multilayer film as a whole,
σB · dB = −σT
It is necessary to select a material and a film thickness so as to satisfy the above.
[0016]
FIG. 3 shows the results of measuring the internal stress of sputtered films of various dielectric materials formed on a quartz substrate with respect to ion assist current density, which is one of the sputtering conditions. From this data, the optical multilayer film 13 of SiO ₂ / TiO ₂ of the example has a compressive stress as a whole because the TiO ₂ film is thick. Therefore, it can be seen that if the TiO ₂ film formed with the ion current density under the condition of tensile stress is used as the buffer layer 12, the stress cancellation is possible.
Instead of TiO ₂ , an MgF ₂ film or the like can be used as the buffer layer.
[0017]
FIG. 2 shows a manufacturing process of an optical multilayer filter according to another embodiment of the present invention. In this embodiment, as shown in FIG. 2A, a heat-resistant quartz substrate 21 is used as a starting substrate, on which SiO ₂ soot is deposited by the FHD method, and this is sintered at a high temperature to obtain SiO 2. _Two layers 22 are formed. Next, as shown in FIG. 2B, an optical multilayer film 23 is formed by sputtering. As shown in an enlarged view in the figure, the optical multilayer film 23 includes a SiO ₂ film 23a that is a low refractive index material and a Ta ₂ O ₅ film 23b that is a high refractive index material (refractive index = 2.1 to 2.2). Are alternately laminated by λ / 4. Finally, as shown in FIG. 2C, the quartz substrate 21 as the starting substrate is polished and removed.
[0018]
As shown in FIG. 3, the SiO ₂ film exhibits a substantially constant compressive stress regardless of the film formation conditions, and this does not change even when the FHD method is used. Therefore, as the film forming conditions for the other Ta ₂ O ₅ film constituting the optical multilayer film 23, the conditions for the same compressive stress as the SiO ₂ film in FIG. 3 are selected. As a result, when the starting substrate 21 is removed, there is no obstacle to the expansion or compression of the film due to internal stress, and the occurrence of warpage is prevented.
The SiO ₂ layer 22 by the FHD method has sufficient mechanical strength and chemical stability compared to the case where an organic film such as polyimide is used.
[0019]
【The invention's effect】
According to the present invention as mentioned above, as the final substrate, Ri by the the use of sintered glass layer due to internal stress and the size and direction are equal FHD the optical multilayer film, the effect to the optical multilayer film The warping of the filter can be prevented.
[Brief description of the drawings]
FIG. 1 shows an optical multilayer filter according to an embodiment of the present invention.
FIG. 2 shows an optical multilayer filter according to another embodiment of the present invention.
FIG. 3 shows internal stress data of a dielectric material film formed by sputtering.
[Explanation of symbols]
11 ... BK-7 substrate, 12 ... SiO ₂ film (buffer layer), 13 ... Optical multilayer film, 13a ... SiO ₂ film, 13b ... TiO ₂ film, 21 ... Quartz substrate, 22 ... FHDSiO ₂ layer (final substrate), 23: Optical multilayer film, 23a: SiO ₂ film, 23b: Ta ₂ O ₅ film.

Claims (1)

In an optical multilayer filter having an optical multilayer film formed by alternately laminating a plurality of material films having different refractive indexes on a substrate, the substrate is subjected to a flame deposition method on a heat-resistant starting substrate to be finally removed An optical multilayer filter, characterized by being a glass layer formed by sintering and having an internal stress equal in direction and size to the internal stress of the optical multilayer film.

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