JP7560264B2 - Optical layer, optical member and optical device - Google Patents

Description

The present invention relates to optical layers that utilize the interference conditions of light to prevent reflection or cut off specific wavelength ranges, optical filters that form optical layers, optical members such as lenses, and optical devices such as cameras that capture images of light that has passed through optical members.

Conventionally, optical components such as optical filters and lenses have been known to have optical films that utilize the optical interference of thin films to provide functions such as anti-reflection and infrared blocking. Optical devices such as cameras that include image sensors incorporate optical components with these optical films formed on substrates such as lenses, and the optical components suppress ghosts and flares caused by reflections that occur within the optical device, as well as image degradation caused by unnecessary light entering the image sensor.

When forming an optical film on a lens substrate or a synthetic resin substrate with a curvature, it is known that the use of atomic layer deposition (ALD) can form a film with good shape conformability and high gas barrier properties. Patent Document 1 discloses a technology in which molecular layers with different physical property values related to optical properties are laminated by ALD to produce a thin film with complex optical properties, and this is further laminated to form an optical multilayer film.

JP 2004-176081 A

However, in the optical multilayer film described in Patent Document 1, the stress in the thin film increases, which can lead to cracks and peeling.

In order to solve the above-mentioned problems, the optical layer of the present invention is an optical layer whose overall optical thickness satisfies an interference condition for a predetermined wavelength, the optical layer comprising: a first thin film layer having a physical thickness of 4 nm or less that mainly contributes to the refractive index of the optical layer; and a second thin film layer disposed adjacent to the first thin film layer and having an internal stress opposite to that of the first thin film layer, the optical thickness of the second thin film layer being an optical thickness at which no optical interference occurs for the predetermined wavelength, a refractive index difference between the optical layer and the first thin film layer being ±3% or less, and the optical layer being configured by a repetition of a set of the first thin film layer and the second thin film layer according to a predetermined thickness ratio.

According to the present invention, it is possible to reduce the stress of the first thin film layer, which mainly contributes to the refractive index of the optical layer, and reduce the possibility of cracks or peeling without significantly affecting the optical film design or optical properties.

1 is a cross-sectional view of an optical layer and an optical member according to the present invention. Configuration examples of optical layers according to the present invention Configuration examples of optical layers according to the present invention 1 is a cross-sectional view of an optical layer and an optical member according to Example 1. 1 is a cross-sectional view of an optical layer and an optical member according to Example 2. 11 is a cross-sectional view of an optical film and an optical member according to a third embodiment. 1 is a cross-sectional view of an optical film and an optical member according to a fourth embodiment of the present invention; Optical device according to the present invention

The embodiment of the present invention will be explained with reference to the figure.

Figure 1 shows a cross-sectional view of the optical layer and optical member of the present invention cut along a plane passing through the optical axis. Figure 1(b) shows a partially enlarged view of the optical layer 2 of Figure 1(a). The optical layer 2 of the present invention is configured by laminating a main material thin film (first thin film layer) 3 that mainly contributes to the refractive index of the optical layer 2 and a molecular layer (second thin film layer) 4 that has a stress opposite to that of the main material thin film, and the optical layer has a thickness that causes optical interference. Here, the molecular layer 4 is formed to a thickness that does not cause optical interference, and optical interference does not occur between the main material thin film 3 and the molecular layer 4. The optical member of the present invention is configured by forming one or more optical layers 2 on a substrate.

In FIG. 1, the optical member 6 is one optical layer, but the optical film may be formed by laminating another optical layer having a different refractive index from the optical layer 2. Here, the other optical layer may be a layer made of a single material. That is, in the optical film of the present invention, at least one optical layer is configured by laminating a main material thin film (first thin film layer) 3 and a molecular layer (second thin film layer) 4 having a stress opposite to that of the main material thin film. In addition, the optical film may have multiple optical layers made of the same combination of materials, or may have optical layers made of different combinations of materials.

(Optical Layer)
The optical layer 2 will be described. The optical layer 2 of the present invention is composed of a main material thin film 3 and a molecular layer 4 having a stress opposite to that of the main material thin film, and has an optically significant film thickness. The optically significant film thickness refers to a film thickness at which optical interference occurs at the target wavelength. The molecular layer 4 forming the optical layer 2 is controlled to a film thickness at which optical interference does not occur, and is formed by ALD. In order to provide the optical member 6 with functions such as anti-reflection and infrared cut, it is preferable that the optical layer 2 has a lower or higher refractive index. For example, when providing an AR function with a single layer, it is preferable that the optical layer 2 has a refractive index as low as possible.

In addition, in an optical film including an optical layer described below, it is preferable to stack layers with a large difference in refractive index, that is, a stack of a low refractive index layer and a high refractive index layer. In other words, when an optical layer is used as a low refractive index layer or a high refractive index layer, it is preferable to provide the molecular layer 4 with a thickness that does not significantly increase or decrease the refractive index of the main material thin film 2. For this reason, it is preferable that the refractive indexes of the main material thin film 3 and the molecular layer 4 that form the optical layer 2 are as close as possible, for example, 0.5 or less, and more preferably 0.2 or less.

Here, the refractive index n of the optical layer 2 can be considered to be a value shown by the following formula (1) when the thickness of the optical layer 2 is D, the refractive index and total thickness of the main material thin film 3 are nM and ΣdM, respectively, and the refractive index and total thickness of the molecular layer 4 are nA and ΣdA, respectively. At this time, it is preferable that the difference in refractive index between the main material thin film 3 and the optical layer 2 is within the range expressed by formula (2). If the difference in refractive index between the main material thin film 3 and the optical layer 2 is at a level that satisfies formula (2), the stress of the optical layer 2 can be alleviated without significantly affecting the optical design. In addition, in the present invention, the refractive index is a value at a light wavelength of 550 nm unless otherwise specified.

-3% ≦ 100×(n - nM)/nM ≦ 3% (2)

The thickness of the molecular layer 4 is the thickness of one molecule, and although it depends on the material, it is preferable that it is, for example, 0.05 nm or more. Furthermore, in order not to exhibit optical interference and so that the optical properties of the optical layer 2 do not deviate significantly from the optical properties of the main material thin film 3, it is more preferable that the film thickness occupied by the molecular layer 4 in the optical layer 2 is 1/3 or less, and even more preferably 1/5 or less.

The optical layer 2 is composed of a plurality of main material thin films 3 and molecular layers 4, and the thicknesses of the main material thin films 3 arranged at different positions in the thickness direction may be the same or different, and the thicknesses of the molecular layers 4 arranged at different positions may be the same or different. Considering the stress during the formation of the optical layer, it is preferable that the main material thin film 3 does not have an extremely thick layer, and it is more preferable that at least the main material thin film 3 is composed of a substantially uniform thickness. In other words, when a combination of the thickness dM per layer of the main material thin film 3 forming the optical layer 2 and the thickness dA of the molecular layer 4 is considered as one set, it is preferable to repeatedly mold this set multiple times so that the optical layer has a desired thickness. At this time, the material of the optical layer 2 that is initially formed on the substrate 1 may be formed from the main material thin film 3 or the molecular layer 4.

On the other hand, as described below, when forming an optical film by combining with other optical layers or other optical layers, the film thickness balance between the main material thin film 3 and the molecular layer 4 of the optical layer 2 may be adjusted in consideration of the stress balance until the entire optical film or optical layers are formed.

For example, if another optical layer having an opposite internal stress to the film stress of the main material thin film 3 is formed before the optical layer is formed, the main material thin film 3 (compressive stress) of the optical layer 2 to be formed may have a region in which the thickness of the main material thin film 3 becomes thinner continuously or stepwise with increasing distance from the optical layer directly below or from the optical layer, as shown in FIG. 2.

Figure 2(a) shows a partially enlarged view of the optical layer 2, and Figure 2(b) shows the film thickness corresponding to each layer. The vertical axis of Figure 2(b) shows the film thickness of each layer stacked up to that point, and when the main material thin film 3 with a thickness of dMn is formed on the outermost part of the optical layer 2, the film thickness becomes D. In other words, in the initial stage of film formation of the optical layer 2, the film may be formed so as to relieve the stress of the optical layer or optical layer 2 directly below as much as possible. At this time, the thickness of the molecular layer 4 forming the optical layer 2 may be uniform, or may become thinner as it moves away from the optical layer or optical layer directly below, like the main material thin film 3, or may become thicker as long as optical interference does not occur.

Also, there may be a region in which the film thickness increases continuously or stepwise as it moves away from the optical layer directly below or from the optical layer. FIG. 3(a) shows an enlarged view of a portion of the optical layer 2, and FIG. 3(b) shows the film thickness corresponding to each layer. The vertical axis of FIG. 3(b) shows the film thickness of the layers stacked up to each layer, and when a thin film of main material 3 with a thickness of dMn is formed on the outermost part of the optical layer 2, the film thickness of the optical layer 2 becomes D.

As shown in FIG. 3, by combining regions in which the thickness of the main material thin film 3 decreases continuously or stepwise with increasing distance from the optical layer directly below or from the optical layer, the stress of the optical layer directly below can be alleviated in the early stages of the optical layer, while the stress of the main material thin film 3 in the optical layer 2 does not suddenly increase in the later stages of molding.

In this embodiment, the main material thin film 3 forming the optical layer 2 can be made of various known materials such as _MgF2 , _SiO2 , _Al2O3 , MgO, _HfO2 , _ZrO2 , _La2Ti2O7 ( _{LaTiO3 )} , Si3N4, _Ta2O5 , _Nb2O5 , _TiO2 , etc. _{The molecular} layer 4 can be made of known materials that can be formed by ALD, _{such as MgF2} , _SiO2 , _{Al2O3 , MgO} , _HfO2 , _ZrO2 , _Si3N4 , _Ta2O5 , _{Nb2O5 , TiO2 , etc.} , _that are appropriately selected according to the main material thin film 3. If some light absorption is acceptable as an optical film, metal atoms such as Al, Mg, Si, and Ti may be used for the molecular layer 4. Metal atomic layers are preferable in terms of ductility and stress relaxation compared to their oxide/nitride layers. A metal compound layer and an atomic layer of a metal element having a thickness similar to that of the molecular layer may be mixed to form the molecular layer.

In the present invention, the main material thin film 3 forming the optical layer 2 can be formed using known film formation techniques, such as physical vapor deposition techniques such as vacuum deposition, ion-assisted deposition, ion plating, and sputtering, and chemical vapor deposition techniques such as CVD (Chemical Vapor Deposition), PECVD (Plasma Enhanced Chemical Vapor Deposition), thermal ALD, and PEALD (Plasma Enhanced Atomic Layer Deposition). However, as mentioned above, considering that the molecular layer 4 is formed by the ALD method, it is preferable to select the chemical vapor deposition method in terms of the manufacturing process.

(Optical film)
In the present invention, the optical film is a film having an optical function, which is composed of a plurality of layers including at least the optical layer 2. The film having an optical function refers to, for example, a cut film such as a multi-AR film or an infrared cut film, a bandpass film, a mirror film, etc. These films obtain the desired spectral characteristics by the optical interference of optical layers having different refractive indices.

For example, the optical film will be described in the case of a multi-AR film. In the multi-AR film, low-refractive index layers and high-refractive index layers are alternately stacked to realize low reflection in a desired wavelength region. When stacking optical layers like a multi-AR film, MgF ₂ , SiO ₂ , Al ₂ O ₃ , MgO, HfO ₂ , ZrO ₂ , La ₂ Ti ₂ O ₇ (LaTiO ₃ ), Si ₃ N ₄ , Ta ₂ O ₅ , Nb ₂ O ₅ , TiO ₂ and the like can be used, but it is preferable to select materials that have stress in different directions for the low-refractive index layer and the high-refractive index layer so that the stress of the entire optical film is as small as possible. For example, it is preferable to select SiO ₂ with compressive stress as a low-refractive index material and TiO ₂ with tensile stress as a high-refractive index material because they have opposing stresses and have a large refractive index difference. If the refractive index difference is large, the number of layers (film thickness) required to obtain the desired spectral characteristics can be reduced, which contributes to suppressing cracks.

For example, when forming a multi-AR film using a synthetic resin lens as a substrate, it is preferable that the film has a structure of 4 to 12 layers, and more preferably a structure of 6 to 10 layers. Multi-AR films and other laminates tend to have higher gas barrier performance as the number of layers increases.

This is because, even if there is a defect in the layer forming the laminate, if there is no defect in the same region between the laminated layers, gases such as oxygen and water vapor will not easily reach the synthetic resin substrate. In other words, the more the number of layers, the easier it is to suppress deterioration such as yellowing of the synthetic resin even if a minute film defect occurs in the layer forming the multi-AR film. On the other hand, if the number of layers is increased too much, the multi-AR film is more likely to crack due to film stress, especially when a synthetic resin lens substrate is used. The number of layers here refers to the optical layer or the number of layers of optical layers that exhibit optical interference. The film thickness of the multi-AR film is preferably about 200 nm or more and 600 nm or less, and more preferably 250 nm or more and 500 nm or less. If the film thickness is too thin, it becomes difficult to sufficiently reduce reflection due to the interference effect of the thin film, and the gas barrier effect of the multi-AR film becomes small. On the other hand, if the film thickness is too thick, cracks are more likely to occur. Furthermore, it is preferable that the optical layer or optical layer is 80 nm or less, and more preferably 50 nm or less, except for the outermost layer. If each layer is too thick, the stress in that layer will be too great, and there is a risk of cracking or peeling, especially when the layer is formed on a synthetic resin lens substrate. On the other hand, the outermost layer is formed so that its optical thickness is approximately λ/4 at the target wavelength λ in order to prevent reflection.

Here, we have described the preferred conditions for forming a multi-AR film on a synthetic resin lens substrate, but suitable conditions may be selected as appropriate for the optical film. For example, when forming an optical film that requires multiple layers and film thickness to obtain optical properties, such as an infrared cut film, on a glass substrate (including lenses) or a synthetic resin substrate that does not have a curved surface, the number of layers and film thickness may be appropriately determined so that cracks do not occur.

(substrate)
The optical layer and optical film according to the present invention can be formed on a substrate 1 to form an optical member 6. As the substrate 1, for example, a substrate made of an inorganic material such as glass or quartz, or various synthetic resin substrates such as polyester, olefin, polyether, acrylic, styrene, PET (polyethylene terephthalate), PES (polyethersulfone), polysulfone, PEN (polyethylene naphthalate), PC (polycarbonate), and polyimide can be used. Furthermore, a substrate made of an organic-inorganic hybrid material, for example, a substrate having a silsesquioxane skeleton, may be used. Furthermore, a lens can be used as the substrate 1 as necessary. Various materials such as glass and synthetic resin can be used for the lens. As a synthetic resin lens, olefin, PC, acrylic, styrene, polyester, polyurethane, etc. can be suitably used in view of moldability and refractive index characteristics. In addition, a thermo- or photo-curable resin such as a multifunctional reactive polymer having an aromatic dendrimer-like multi-branched structure may be used. A synthetic resin substrate is flexible, light, and has good processability compared to glass, but is prone to deformation and deterioration due to heat. For this reason, the synthetic resin substrate should have high heat resistance (high glass transition temperature Tg), for example, the glass transition temperature Tg is preferably 120° C. or higher, more preferably 140° C. or higher. Furthermore, in consideration of deformation of the synthetic resin substrate due to water absorption, olefin-based, especially cycloolefin-based materials are suitable. In addition, materials with high refractive index and small Abbe number are sometimes required for lenses, etc. In this case, for example, PC-based or fluorene-containing polyester resins can be preferably used.

When the optical member 6 of the present invention is used as a lens in applications with a wide operating temperature range, such as in-vehicle cameras and surveillance cameras, it is preferable that the synthetic resin substrate has a small change in refractive index due to temperature. For example, the difference in refractive index at e-line between 0°C and 80°C is preferably 0.8% or less, more preferably 0.6% or less. In addition, it is preferable that the change in optical properties due to high temperatures, i.e., yellowing (ΔYI), is small, and for example, the value of ΔYI, which is the difference between the yellowing index (YI: yellow index) at 120°C for 1500 hours and the initial YI, is preferably 10 or less, and more preferably 8 or less. In addition, the transmittance of the synthetic resin substrate is 80% or more at visible light wavelengths (400-700 nm), and it is preferable that the transmittance at visible light wavelengths at 120°C for 1500 hours is 60% or more.

When multiple substrates are used in the same optical device, such as a lens unit, multiple substrates 1 with different refractive indices and Abbe numbers can be used to adjust optical properties such as aberration.

When the substrate 1 is a resin lens, it can be molded, for example, by injection molding. Specifically, a cavity formed by butting the fixed mirror piece and the movable mirror piece together, which includes the lens section, outer periphery, gate section, etc., is filled with resin material through the gate section, and once the resin has solidified, the mold is opened and the substrate is removed. Finally, the gate section is cut off. With this method, aspheric lenses and high curvature lenses, which are difficult to process using glass, can be made relatively easily. Note that the molding of the substrate is not limited to injection molding, and various other known methods such as injection compression molding and cast polymerization can be used.

When using a synthetic resin substrate, it is preferable to perform an annealing treatment before forming the optical layer and optical film in order to release internal stress caused by molding and to remove moisture absorbed by the substrate. The annealing treatment is preferably performed at a temperature of 80°C to the Tg of the substrate, preferably at a temperature of about 15°C below the Tg of the substrate, for about 0.5 to 24 hours, more preferably 1 to 12 hours.

(Production method)
The optical layers and optical films of the present embodiment can be fabricated, for example, by an ALD process.

Now let us explain ALD. ALD is a vapor phase thin film formation method similar to CVD. In CVD, two types of precursors are simultaneously introduced into the reaction chamber, and the reaction products are deposited on the substrate, whereas in ALD, only one type of precursor is simultaneously introduced into the reaction chamber, and the precursors other than the one adsorbed on the substrate 1 do not chemically react with other precursors, and reaction products are formed only on the surface of the substrate 1. For this reason, ALD can form conformal layers even on substrates 1 with complex shapes.

The ALD film formation process will be described. First, a site, for example, an OH group, to which a first precursor described later can be adsorbed is provided on a substrate or a surface on which an ALD film is to be formed. To provide the OH group, for example, plasma or UV/ _O3 may be used, or water molecules may be used as a precursor to adsorb the OH group on the substrate. Next, the first precursor, which is a source gas of the molecular layer 3 to be formed, is chemically adsorbed on the substrate. At this time, since the chemical adsorption can be performed on the portion of the substrate where the OH group is exposed, the adsorbed first precursor forms a monolayer on the film formation surface regardless of the shape of the substrate. Next, the excess first precursor that could not be adsorbed on the film formation surface and the gas generated when the first precursor is adsorbed on the film formation surface are purged (removed).

Then, a molecular layer of metal oxide, metal, metal nitride, etc. is obtained by using a second precursor or radical ozone plasma that oxidizes, reduces, and nitrides the first precursor adsorbed on the film formation surface. This process can be repeated to form a layer with a desired thickness. N2, Ar, etc. can be used as a carrier gas when introducing each precursor. Since ALD advances film formation by adsorption of a monomolecular layer to the film formation surface, it is possible to form a film with a uniform thickness without being affected by the shape of the substrate, and is suitable for film formation on, for example, high curvature or aspheric lenses. Furthermore, since the layer formed is a monomolecular layer, precise film thickness control is possible. Furthermore, unlike physical vapor deposition, ALD forms a layer even with a monomolecular level thickness, so a layer with few film defects and high gas barrier properties is formed. Therefore, when a synthetic resin is used for the substrate, it is expected to suppress yellowing due to oxidation of the resin, and expansion and hydrolysis due to moisture.

The molecular layer formation process by ALD using an Al ₂ O ₃ layer will be described in detail. First, the deposition surface is irradiated with plasma to provide OH groups. Next, TMA (trimethylaluminum) is supplied into the deposition chamber as a first precursor. At this time, the reaction shown in formula (1) or (2) occurs. Next, excess TMA and generated CH ₄ are removed by purging. Inert gases such as N 2 and Ar are preferably used for purging. Next, water molecules are supplied into the deposition chamber as a second precursor. At this time, the reaction shown in formula (3) or (4) occurs. Furthermore, excess water molecules and generated CH ₄ are removed by purging. These steps constitute one cycle, and the number of cycles is repeated to obtain a desired film thickness. In the case of Al ₂ O ₃ , the film thickness is approximately 1 Å in one cycle.

Al(CH ₃ ) ₃ + :OH → :O-Al(CH ₃ ) ₂ + CH ₄ (3)
Al(CH ₃ ) ₃ + 2:OH → :O-Al(CH ₃ ) + 2CH ₄ (4)

:O-Al(CH ₃ ) ₂ + 2H ₂ O → :Al-O-Al(OH) ₂ + 2CH ₄ (5)
:O-Al(CH ₃ ) + H ₂ O → :Al-O-Al(OH) + CH ₄ (6)

The following describes the preparation of the optical layer 2, which is made of the main material SiO ₂ and the molecular layer Al ₂ O ₃ having a stress in the opposite direction to that of the main material, as described in the above embodiment. First, the substrate is set on the deposition holder and introduced into the reaction chamber. The reaction chamber is evacuated by a vacuum pump, and the substrate 1 is heated by a heater. When the pressure in the reaction chamber and the substrate temperature reach the desired values, deposition begins. First, the main material SiO ₂ is deposited. The SiO ₂ layer is formed by a deposition process that is substantially the same as that described for the Al ₂ O ₃ molecular layer, using TDMAS (trisdimethylaminosilane) as the first precursor and water molecules as the second precursor. At this time, the film thickness per cycle is approximately 0.8 Å. The SiO ₂ molecular layer forming cycle is repeated for the number of cycles required to form the desired film thickness.

Next, a monolayer of Al ₂ O ₃ is formed by the above-mentioned process using TMA as the first precursor and water molecules as the second precursor. At this time, the number of cycles is limited so that the film thickness of Al ₂ O ₃ falls within a film thickness that does not cause optical interference. After the desired number of cycles of the molecular layer of Al ₂ O ₃ are repeated, a SiO ₂ layer is further formed. This process is repeated to form a composite film of SiO ₂ and Al ₂ O ₃ to a desired film thickness. Here, TDMAS is used as the first precursor of SiO ₂ , but it is not limited to this and other aminosilanes may be used, or TEOS (tetraethoxysilane) or silicon tetrachloride may be used. In particular, in the formation of the SiO ₂ molecular layer, the reaction can be promoted by introducing a catalytic gas such as pyridine into the reaction chamber after introducing the first precursor and the second precursor.

For example, when obtaining an optical film in which multiple optical layers with different refractive indexes are formed, such as a multi-AR film, a _TiO2 layer (high refractive index layer) with a different refractive index from the optical layer ₂ made of _SiO2 and _Al2O3 described above is provided. When forming _{a TiO2} film by ALD, for example, TDMAT (tetrakisdimethylaminotitanium) can be used as the first precursor and water molecules can be used as the second precursor. In this case, the film thickness is approximately 0.6 Å per cycle described above. This is repeated the number of cycles required to obtain the desired film thickness. As the first precursor of _TiO2 , other aminotitaniums, titanium tetrachloride, etc. can be used without being limited to TDMAT, and a catalyst gas for promoting the reaction may be introduced as necessary.

After the formation of the _TiO2 layer, a low refractive index layer made of _SiO2 or _SiO2 and _Al2O3 is formed. The necessary number of layers are laminated to obtain the desired _optical characteristics. Of course, the high refractive index layer may also be an optical layer that combines a main material and a molecular layer having a stress opposite to that of the main material. In this case, for example, the main material can be _TiO2 , and the molecular layer having a stress opposite to that of the main _material can be _Si3N4 . In addition, the same first precursor _{as SiO2} can be used to form the molecular layer of _Si3N4 , and ammonia gas or nitrogen gas can be used as the second precursor.

Although the method of using ALD to form the optical layer 2 has been described so far, a film formation process combining CVD and ALD can also be used. For example, when the main material is SiO ₂ and the molecular layer is Al ₂ O ₃ , the main material SiO ₂ can be formed by CVD, and the molecular layer Al ₂ O ₃ can be formed by ALD. The molecular layer Al ₂ O ₃ needs to be controlled to a film thickness that does not cause optical interference, while the main material SiO ₂ does not necessarily need to be controlled at the molecular layer level. Therefore, by forming the main material SiO ₂ by CVD, the film formation rate can be improved compared to forming the optical layer only by the ALD process, and productivity can be improved. Note that, when forming an optical film, which is a laminate of optical layers including an optical layer, CVD and ALD may be combined within the optical layer in a specific optical layer, or the layers may be switched for each optical layer, for example, a part of the low refractive index layer is formed by ALD and the other layers are formed by CVD.

Although the ALD method has been explained using thermal ALD, PEALD, which uses plasma to promote chemical reactions, can also be used. In particular, when a synthetic resin substrate is used as the substrate 1, PEALD is more suitable because it allows for a lower film formation temperature. In this case, in order to avoid damage to the substrate 1 by the plasma, it is preferable to use a process in which the plasma is generated outside the reaction chamber, for example, and only the excited OH radicals reach the reaction chamber. Similarly, the use of PECVD for CVD film formation is particularly suitable when forming a film on a synthetic resin substrate.

Example 1
FIG. 4 shows a cross-sectional view of the optical layer and the optical member according to the first embodiment cut by a plane passing through the optical axis. FIG. 4(b) shows a partially enlarged view of the optical layer 2 in FIG. 4(a). The optical member 6 of the first embodiment uses a cycloolefin resin lens as a substrate 1, and one optical layer 2 is formed on the resin lens. The optical layer 2 is composed of a main material thin film 3, which is a SiO ₂ thin film having compressive stress, and a molecular layer 4 made of Al ₂ O ₃ having tensile stress. The main material thin film 3 with a physical thickness d _M and the molecular layer 4 with a physical thickness d _A are alternately laminated, and the physical thickness D of the optical layer 2 is determined so that the optical thickness is about λ/4 at the wavelength λ. That is, it has an AR function at the wavelength λ. In the first embodiment, the main material thin film 3 (SiO ₂ ) and the molecular layer 4 (Al ₂ O ₃ ) are both formed by PEALD, and an optical layer with the same thickness as the main surface, that is, approximately the physical thickness D, is formed on the end of the resin lens. Here, the main surface refers to the effective surface of the optical member 6, and in the case of a lens, for example, refers to a surface having a curvature.

In Example 1, 24 sets were formed, each set consisting of a 3 nm main material thin film ( _SiO2 ) and a 0.8 nm molecular layer ( _{Al2O3 )} . The refractive indices of _SiO2 and _Al2O3 are 1.46 and _1.62 , respectively, and the refractive index of the optical layer 2 is 1.494 according to formula (1). At this time, the physical film thickness is 91.2 nm, and the optical film thickness of the optical layer 2 at a light wavelength of 550 nm approximately satisfies λ/4.

In the first embodiment, the optical layer 2 is formed in the order of the main material thin film 3 and the molecular layer ₄ from the substrate 1 side, but this is not limiting and the molecular layer 4 may be formed first. In particular, when _Al2O3 is used as the molecular layer 4, it is more preferable that the layer in contact with the substrate 1 is the molecular layer 4, taking into consideration the adhesion with the substrate 1.

Example 2
FIG. 5 shows a cross-sectional view of the optical layer and the optical member according to the second embodiment cut by a plane passing through the optical axis. FIG. 5(b) shows a partially enlarged view of the optical layer 2 in FIG. 5(a). The optical member 6 of the second embodiment uses a cycloolefin lens as the substrate 1 as in the first embodiment, and one optical layer 2 is formed on the substrate 1. The optical layer 2 is composed of a SiO ₂ thin film having compressive stress as the main material thin film 3 and an Al ₂ O ₃ molecular layer 4 having tensile stress, and the physical thickness D of the optical layer 2 is determined so that the optical thickness is about λ/4 at the wavelength λ as in the first embodiment. Here, in the second embodiment, the main material thin film 3 is formed by PECVD, and the molecular layer 4 is formed by PEALD, and the physical thickness is different between the main surface and the end of the resin lens. Specifically, the main material thin film 3 and the molecular layer 4 are formed on the main surface of the resin lens with a physical thickness D, while the main material thin film is partially wrapped around, and only the material of the molecular layer 4 is substantially formed on the end, and the thickness of the thickest part is ΣdA. The physical thickness of the main surface referred to here is the thickness of at least the optically effective range of the optical member 6, and in the case of a lens, for example, it refers to at least the region having a curvature.

In Example 2, 16 sets were formed, each set consisting of a 4 nm main material thin film and a 0.8 nm molecular layer. The refractive indices of _SiO2 and _Al2O3 are 1.46 and _1.62 , respectively, and the refractive index of the optical layer is 1.487 according to formula (1). At this time, the physical film thickness is 91.2 nm, and the optical film thickness of the optical layer 2 at a light wavelength of 550 nm approximately satisfies λ/4. The film thickness at the substrate end is approximately 15.2 nm.
In the second embodiment, as in the first embodiment, the main material thin film 3 and the molecular layer 4 may be formed first from the main material thin film 3, or first from the molecular layer 4, and the optimum conditions may be selected according to compatibility with the substrate 1.

Example 3
Fig. 6 shows a cross-sectional view of the optical film and the optical member according to Example 3, cut along a plane passing through the optical axis. Fig. 6(b) shows a partially enlarged view of the optical film 5 in Fig. 6(a), Fig. 6(c) shows a partially enlarged view of the optical layer 2', and Fig. 6(d) shows a partially enlarged view of the optical layer 2. The optical film 5 is an optical multilayer film formed by alternately laminating the optical layers 2 and 2' so as to satisfy the interference condition for a predetermined wavelength region.

Specifically, the optical member 6 of Example 3 uses a resin lens made of a fluorene structure polyester resin as the base material 1, and a multi-AR film is formed as an optical film 5 in which low refractive index layers and high refractive index layers are alternately laminated on the resin lens. Here, in Example 3, the low refractive index layer and the high refractive index layer are both optical layers 2, 2' consisting of main material thin films 3, 3' and molecular layers 4, 4', and the low refractive index layer uses SiO ₂ as the main material thin film 3, Al ₂ O ₃ as the molecular layer 4, and the high refractive index material uses TiO ₂ as the main material thin film 3' and Si ₃ N ₄ as the molecular layer 4'. In this example, the main material thin films and molecular films of the low refractive index layer and the high refractive index layer are formed by PEALD. That is, the physical film thickness of the optical film 5 formed on the main surface and end surface of the substrate 1 is approximately the same and is D. Here, the main surface refers to the effective surface of the optical member 6, and in the case of a lens, for example, it refers to a surface having a curvature.

In Example 3, the low refractive index layer was formed by repeating the required number of sets of a main material thin film of 4 nm and a molecular layer of 0.8 nm. At this time, the refractive indexes of the main material thin film 3 and molecular layer 4 forming the low refractive index layer were 1.46 and 1.62, respectively, and the refractive index of the low refractive index layer can be considered to be 1.487 according to formula (1). In addition, the high refractive index layer was formed by repeating the required number of sets of a main material thin film of 4 nm and a molecular layer of 0.5 nm. At this time, the refractive indexes of the main material thin film 3' and molecular layer 4' forming the high refractive index layer were 2.40 and 2.00, respectively, and the refractive index of the high refractive index layer can be considered to be 2.356 according to formula (1).

In the third embodiment, all the low refractive index layers have substantially the same refractive index, but the refractive index may be different for each layer in the low refractive index layer, that is, the thickness ratio of the main material thin film 3 to the molecular layer 4 may be different, or a specific low refractive index layer may be the optical layer 2, and the other layers may be layers made of, for example, SiO ₂ alone. Similarly. In the high refractive index layers, the refractive index (thickness ratio of the main material thin film to the molecular layer) may be different for each layer, or a specific high refractive index layer may be the optical layer 2', and the other layers may be layers made of, for example, TiO ₂ alone. Furthermore, at least one layer or all layers of either the low refractive index layer or the high refractive index layer forming the optical film 5 may be the optical layer 2, 2'.

Furthermore, a functional layer for improving, for example, adhesion or gas barrier properties may be provided in the optical film 5. Examples of the functional layer include Al ₂ O _3. The Al ₂ O ₃ layer may be formed to a desired thickness at the interface between the substrate 1 and the optical film 5, or may be inserted into the optical film 5. In this case, the functional layer may have a thickness at the so-called molecular layer level where optical interference does not occur, or may have a thickness where optical interference occurs. When inserting a functional layer with a thickness where optical interference occurs, the optical interference of the functional layer may be taken into consideration to perform an optimal film design for the optical film. Of course, a plurality of functional layers may be provided, for example, at the interface between the substrate 1 and the optical film 5 and in the optical film 5.

In this embodiment, a multi-AR film is used as the optical film 5, but the optical function is not limited to this.

Example 4
FIG. 7 shows a cross-sectional view of the optical film and the optical member according to the fourth embodiment cut along a plane passing through the optical axis. FIG. 7(b) shows a partially enlarged view of the optical layer 2 in FIG. 7(a), FIG. 7(c) shows a partially enlarged view of the optical layer 2', and FIG. 7(d) shows a partially enlarged view of the optical layer 2. The optical member 6 of the fourth embodiment uses a resin lens made of a fluorene structure polyester resin as the substrate 1, and a multi-AR film is formed thereon, which is an optical film 5 in which low refractive index layers and high refractive index layers are alternately laminated. Here, in the fourth embodiment, the low refractive index layer and the high refractive index layer are both optical layers 2, 2' made of the main material thin film 3, 3' and the molecular layer 4, 4', and the low refractive index layer uses SiO ₂ as the main material thin film 3 and Al ₂ O ₃ as the molecular layer 4, and the high refractive index material uses TiO ₂ as the main material thin film 3' and Si ₃ N ₄ as the molecular layer 4', respectively. In this embodiment, the main material thin film of the low refractive index layer and the high refractive index layer are formed by PECVD, and the molecular film is formed by PEALD. That is, the main surface of the substrate is formed with a physical film thickness D, and the end surface is formed with a physical film thickness of approximately ΣdA+ΣdA'. The end surface is configured by alternately laminating Al ₂ O ₃ molecular layers of the low refractive index layer and Si ₃ N ₄ molecular layers of the high refractive index layer. Here, the Al ₂ O ₃ molecular layers and Si ₃ N ₄ molecular layers formed on the end surface have stresses in opposite directions.

In Example 4, similarly to Example 3, a set of 4 nm main material thin film and 0.8 nm molecular layer was formed for each low refractive index layer, and the required number of sets were repeated. At this time, the refractive indexes of the main material thin film 3 and molecular layer 4 forming the low refractive index layer were 1.46 and 1.62, respectively, and the refractive index of the low refractive index layer can be considered to be 1.487 from formula (1). In addition, a set of 4 nm main material thin film and 0.5 nm molecular layer was formed for each high refractive index layer, and the required number of sets were repeated. At this time, the refractive indexes of the main material thin film and molecular layer forming the high refractive index layer were 2.40 and 2.00, respectively, and the refractive index of the high refractive index layer can be considered to be 2.356 from formula (1).

As in Example 3, in the low refractive index layer, each layer may have a different refractive index (film thickness ratio of the main material thin film to the molecular layer), or only some of the low refractive index layers may be optical layers 2. Similarly, in the high refractive index layer, each layer may have a different refractive index (film thickness ratio of the main material thin film to the molecular layer), or only some of the low refractive index layers may be optical layers 2'. Furthermore, at least one layer or all layers of either the low refractive index layer or the high refractive index layer forming the optical film 5 may be optical layers 2, 2'. Also, as in Example 3, a functional layer such as Al ₂ O ₃ may be inserted with any thickness and number of layers as appropriate.

In this embodiment, a multi-AR film is used as the optical film 5, but the optical function is not limited to this.

Figure 8 shows an optical device according to the present invention. The optical device 21 of the present invention includes a lens unit 20 consisting of a lens barrel 11, lenses 13 to 16, and an aperture 17, and further comprises a protective filter 12, a cover glass 18, and an image sensor 19. Light incident on the lens unit is focused on the image sensor 19 consisting of a CCD or CMOS sensor by a lens or the like. The light focused on the image sensor 19 is converted into an electrical signal and imaged as necessary. At least one of the lenses 13 to 16 mounted on the lens unit 20 is provided with the optical member 6 described in this embodiment. That is, at least one of the lenses constituting the lens unit has an optical film 5 including at least optical layers 2 and 2' formed on a substrate 1. The aperture 17 has a function of preventing unnecessary light from entering the image sensor 19, and may be provided between each lens, or may be provided only partially between the lenses.

When used in harsh temperature conditions such as in-vehicle cameras and surveillance cameras, the aperture is preferably made of a metal substrate. Metal apertures that are blackened by anodizing a metal plate or that have a film with light absorbing properties formed on a metal substrate by deposition or the like can be suitably used. The protective filter 12 is a filter provided to protect the lens unit 20 from ultraviolet rays, dirt, and damage, and has functions such as UV protection, water repellency, oil repellency, hydrophilicity, and hard coating. Furthermore, the protective filter 12 may have a function of cutting off part of the visible range.

When a synthetic resin material is used for the lens, light absorption may occur mainly in the short wavelength region of the visible light range due to yellowing. In order to reduce the variation in the transmittance incident on the imaging element 19 before and after the yellowing of the synthetic resin substrate, the light transmission in this region may be limited in advance. For example, by cutting light wavelengths of about 450 nm or less, which are significantly affected by yellowing, the variation in the transmittance incident on the imaging element 19 before and after yellowing can be effectively reduced. This type of cutting function can be provided, for example, by stacking multiple layers of low refractive index layers and high refractive index layers alternately and utilizing the light interference effect. Note that it is not necessary to provide the protective filter 12, and the function of the protective filter 12 may be provided on the lens 13 that is furthest from the imaging element 19.

The cover glass 18 is provided to prevent foreign matter from adhering to the imaging element 19, and in addition to general glass materials, a quartz substrate or the like can be suitably used. The cover glass 18 may be provided with a near-infrared cut function. When imaging the light incident on the imaging element 19, the sensitivity of the imaging element 19 in the near-infrared region may cause a difference from the color visible to the human eye. In such a case, an infrared cut film or the like may be formed on the cover glass 18 to prevent light in the near-infrared region from entering the imaging element 19. The infrared cut film can be provided, for example, by laminating multiple layers of low refractive index layers and high refractive index layers alternately and utilizing the light interference effect. In addition, infrared absorbing glass in which metal ions having absorption in the near-infrared region are kneaded into the cover glass 18 as necessary may be used.

In order to prevent the synthetic resin substrate from yellowing, the lens barrel 11 may be filled with an inert gas such as nitrogen or argon, or an oxygen scavenger made of iron or the like may be placed inside the lens barrel 11 so as not to be in the optical path. In this way, the synthetic resin substrate is prevented from coming into contact with oxygen, and discoloration due to oxidation, i.e., yellowing, can be prevented. The lenses mounted on the lens unit 20 may be a combination of glass lenses and synthetic resin lenses, but it is preferable that at least the synthetic resin lenses are formed with an optical film 5 including the optical layers 2 and 2' shown in this embodiment. As described above, ALD can obtain a film with good gas barrier properties, and can coat the edge of the substrate 1, so that oxygen, which causes yellowing, is less likely to reach the synthetic resin lens. In addition, when a glass lens is used, it is preferable that the lens 13 farthest from the image sensor 19 among the lenses constituting the lens unit 20 is a glass lens. This is because the lens farthest from the image sensor 19 is most likely to come into contact with oxygen and water vapor.

In FIG. 8, the lens unit 20 of the present invention is shown as being made up of four lenses, but there is no limit to the number of lenses, and it may be more or less than four. In the optical device of FIG. 8, a lens on which an optical film 5 having optical layers 2, 2' is formed is described, but the optical layers 2, 2' or optical film of the present invention may be formed on the aforementioned infrared cut film, UV cut film, etc., and not limited to lenses.

As described above, according to the present invention, an optical layer with low film stress and an optical film including the same can be obtained. By forming the optical layer or optical film of the present invention on a substrate, an optical member that is less susceptible to cracking can be provided. Furthermore, an optical device including the optical member of the present invention has excellent durability and can be particularly suitably used in applications exposed to harsh environments, such as in-vehicle cameras and surveillance cameras.

1 Substrate 2, 2' Optical layer 3, 3' Main material thin film (first thin film layer)
4, 4' molecular layer (second thin film layer)
5 Optical film 6 Optical member

11 Lens barrel 12 Protective filters 13 to 16 Lens 17 Aperture 18 Cover glass 19 Image sensor 20 Lens unit 21 Optical device

Claims (8)

An optical layer having an overall optical thickness that satisfies an interference condition for a predetermined wavelength,
The optical layer is
a first thin film layer having a physical thickness of 4 nm or less that mainly contributes to the refractive index of the optical layer;
a second thin film layer provided adjacent to the first thin film layer and having an internal stress opposite to that of the first thin film layer;
the optical thickness of the second thin film layer is an optical thickness at which no optical interference occurs at the predetermined wavelength,
a refractive index difference between the optical layer and the first thin film layer is ±3% or less;
The optical layer is characterized in that the optical layer is formed by repeating a set of the first thin film layer and the second thin film layer according to a predetermined film thickness ratio. 2. The optical layer according to claim 1, wherein the physical thickness of the second thin film layer is 0.05 nm or more, and the physical thickness of the optical layer occupied by the second thin film layer is 1/3 or less. The optical layer according to claim 1 or 2, characterized in that the refractive index difference between the first thin film layer and the second thin film layer is 0.5 or less. The optical layer according to any one of claims 1 to 3, characterized in that the second thin film layer is formed by atomic layer deposition (ALD). 5. The optical layer according to claim 1, wherein the first thin film layer is made of SiO ₂ and the second thin film layer is made of Al ₂ O ₃ . An optical member comprising a substrate and a plurality of optical layers according to any one of claims 1 to 5 formed thereon. The optical member according to claim 6, characterized in that only the second thin film layer is formed on the end surface of the substrate. The optical member according to claim 6 or 7,
an imaging element that captures an image of light transmitted through the optical member.

Images