JP7041424B2 - Thin film formation method and optical elements - Google Patents

Description

The present invention significantly reduces the number of layers of a multilayer optical thin film made of alternating layers of low-refractive index material and high-refractive index material, which not only makes it easy and low-cost to manufacture, but also cracks the vapor-deposited thin film. The present invention relates to a method for forming a thin film that is hard to be peeled off or broken, and an optical element manufactured by this method. Further, the present invention not only makes it possible to easily and inexpensively manufacture optical elements such as antireflection films, reflectors, deflectors, filters, etc., but also uses them in a wide wavelength range from the deep ultraviolet wavelength range to the infrared wavelength range. It is also suitable for optical elements such as cameras, displays, laser devices, and astronomical telescopes.

In addition to the chemical treatment method using a large amount of pure water, the vacuum vapor deposition method has been conventionally adopted as a main method for manufacturing an antireflection film, a reflector, a polarizing element, a filter, etc. using an optical thin film.
As an antireflection film by this vacuum vapor deposition method, a single-layer film in which a vapor deposition material having a refractive index lower than that of the vapor deposition substrate is vapor-deposited on the surface of the vapor deposition substrate with a film thickness of λ / 4 (λ: wavelength of incident light), or low. It has been known that the film is formed by a multilayer film formed by laminating two or more layers of a vapor-filmed material having a refractive index and a high refractive index. For reflectors such as high-reflecting mirrors, modulators, filters, etc., a low-refractive-index vapor-deposited material and a high-refractive-index vapor-deposited material are usually laminated in multiple layers of 10 or more on the surface of a thin-film deposition substrate by a vacuum vapor deposition method. It is manufactured by

However, the optical thin film produced by the vacuum vapor deposition method does not have a uniform refractive index (homogeneous film), but the refractive index changes in the thickness direction of the film (heterogeneous film). Since the optical thin film is exposed to the surrounding atmosphere during use, the optical characteristics of the optical thin film are greatly affected by the surrounding atmosphere.

Here, assuming that the packing density (packing density) of the optical thin film, which is an index when the refractive index changes in the thickness direction of the film (heterogeneous film), is p, the packing factor p is according to the following equation (1). Can be defined.
p = V ₁ / V ₂ ... (1) Equation V ₁ indicates the volume of the homogeneous film portion of the optical thin film, and V ₂ is the total volume of the optical thin film (volume of the homogeneous film portion and the inhomogeneous portion combined). ) Is shown. With this definition, the value of the packing factor p in the optical thin film is usually in the range of 0.7 to 1.0, most often 0.8 to 0.95, and very rarely 1.

Therefore, since the filling rate p is smaller than 1, the refractive index of the optical thin film is smaller than the refractive index of the homogeneous film portion of the optical thin film. The refractive index n of the optical thin film can be expressed by the following equation (2).
n = pn _s + (1-p) n _v ... (2) Here, n _s is the refractive index of the homogeneous film portion of the optical thin film, and n _v is the refractive index of the void layer.
From the above, it can be seen from the equation (2) that in order to reduce the refractive index n of the optical thin film, the void layer should be increased and the filling factor p should be decreased.

On the other hand, as the prior art, the following Patent Document 1 which the inventors have already applied for and published is known. That is, on the surface of the thin-film deposition substrate, two kinds of substances consisting of one of four kinds of oxides, fluorides, semiconductors, and metals and plastics are simultaneously put on the surface of the thin-film deposition substrate. Is deposited to form a mixed thin film. However, with the technique disclosed in Patent Document 1, although a high-refractive index material and a low-refractive index material could be laminated to obtain an optical thin film having a multilayer structure, the manufacturing cost could not be sufficiently reduced.

Japanese Unexamined Patent Publication No. 2016-69720

The conventional method for forming an optical thin film described above has the following problems.
That is, the multi-layered optical thin film produced by laminating a high-refractive index material and a low-refractive index material using only two types of dielectric materials by the conventional vacuum vapor deposition method is MgF ₂ having the minimum refractive index. Is as large as 1.38. Therefore, even if this MgF ₂ is adopted as a low refractive index material, it is necessary to have several tens or more layers in order to manufacture a highly reflective film, a polarizing film, various filters, etc. with excellent optical characteristics. There was a problem that the cost increased.

In addition, since the thermal expansion of the vapor-filmed substrate and the vapor-filmed material are significantly different, internal stress is generated in the formed optical thin film, and as a result, the optical thin film is cracked due to tensile stress or peeled off due to compressive stress. There was also.

The present invention has been made in view of the above background, and by significantly reducing the number of layers of the multilayer optical thin film manufactured by alternating layers of a low refractive index material and a high refractive index material, it is only easy and low cost to manufacture. It is an object of the present invention to provide a method for forming a thin film in which a thin-film vapor-deposited thin film is not easily cracked, peeled, broken, etc., and an optical element manufactured by this method.

In order to achieve the above object, the method for forming a thin film according to claim 1 is a multilayer optical thin film formed on a substrate by an alternating layer in which a high-dielectric-constant material and a low-dielectric-constant material are alternately stacked in a vacuum chamber. When the above is manufactured , an oxide or fluoride which is a dielectric material is used as a high refraction material, and a plastic material is used as a low refraction material, and these materials are sequentially heated by spatter, electron beam or resistance heating to evaporate. At the same time, the dielectric material is finally overcoated to form a multilayer optical thin film by a dry process of forming a film on the substrate.
At this time, according to claim 2, when the plastic material is evaporated to form a thin film, a halogen gas, an oxygen gas, or a hydrogen gas is introduced into the vacuum chamber, and the plastic material is heated by an electron beam or resistance. Is characterized by evaporating and forming a film.

A plastic thin film is formed by the plastic material according to the above claim, but this thin film can be manufactured at low cost, and the filling rate is very small because the heterogeneous film is formed. Therefore, although this plastic material is useful as a low refraction material, the plastic thin film has a weak point that it is very easily damaged. However, by finally overcoating the dielectric material on the surface of the plastic thin film, the hardness can be made hard to be scratched, and the weak points can be eliminated.

Further, according to the above claim, the materials are sequentially heated in a dry process in a vacuum chamber and vapor-deposited on a substrate to form a film. If the dry process is adopted in this way, a multilayer film can be formed in a vacuum without using a large amount of pure water as in the chemical treatment method, and the problem of waste liquid treatment does not occur.

Further, the plastic thin film formed by using a plastic material having a significantly lower refractive index as a low-refractive material as compared with the conventional low-refractive material made of a dielectric material is an inhomogeneous film and has a small filling rate as described above. The stress generated during thin film formation is a small value of 1/10 or less as compared with the conventional one.

Therefore, using a multi-layer optical thin film formed by alternating layers of thin films made of a plastic material with a low refractive index and thin films made of a dielectric material with a high refractive index, a high-refractive index mirror, a decoder, When manufacturing an optical element such as a bandpass filter, the number of thin film layers can be significantly reduced, and the problem of thin film damage due to stress can be solved.

On the other hand, the optical thin film may be damaged by irradiation with high-intensity laser light. This is because the absorbing substance in the optical thin film absorbs the laser beam and causes a rapid temperature rise in the optical thin film, which causes damage due to the pressure rise. On the other hand, since the thin film made of the plastic material according to claim 1 has voids inside, the pressure increase is alleviated and the laser damage threshold value of the thin film can be increased.

From the above, according to the method for forming a thin film according to claim 1, by using a plastic material having a low filling rate and an ultra-low refractive index as a low refractive index material, an alternating layer of a low refractive index material and a high refractive index material was produced. The number of layers of the multilayer optical thin film can be significantly reduced, and the production of the multilayer optical thin film becomes easy and low cost. Further, a high-strength multilayer optical thin film is obtained in which the vapor-filmed thin film is not easily cracked, peeled, broken, or the like.

In the optical element of claim 3, an oxide or fluoride is used as a high refraction material, a plastic material is used as a low refraction material, and a multilayer optical thin film is formed on a substrate by an alternating layer in which these materials are alternately stacked. It is characterized by being formed.

According to the optical element of claim 3, by adopting a plastic material as a low refraction material, the stress generated at the time of forming a thin film as described above is a small value of 1/10 or less as compared with the conventional one. Therefore, it can be applied to high-reflecting mirrors, deflectors, bandpass filters, etc., in which the number of thin film layers is significantly reduced while reducing thin film damage due to stress. Further, with the adoption of the thin film made of a plastic material, the pressure increase is alleviated as a result of the presence of voids in the layer, and the laser damage threshold of the thin film can be increased.

Based on the above, as in the method for forming the thin film according to claim 1, by using a plastic material having a low filling rate and an ultra-low refractive index as the low refractive index material, an alternating layer of a low refractive index material and a high refractive index material can be produced. The number of layers of the multi-layer optical thin film can be significantly reduced, and an optical element having the multi-layer optical thin film can be easily manufactured at low cost. Further, an optical element having a high-strength multilayer optical thin film in which the vapor-filmed thin film is not easily cracked, peeled, broken, etc. can be obtained.

In the optical element of claim 3, as the substrate, quartz glass, borosilicate crown glass, glass containing phosphate glass, fluorite, crystal, sapphire crystal, YAG _, Al ₂ O ₃ laser crystal, ceramics, etc. It is characterized by being composed of either semiconductor or metal .

In the optical element according to claim 3, as oxides , SiO _2, Al ₂ O ₃ , CeO ₂ , HfO ₂ , Ta ₂ O ₅ , ThO ₂ , TIO ₂ , ZrO ₂ , Sc ₂ O ₃ , Y ₂ O ₃ , La ₂ O ₃ , or Nd ₂ O ₃
The fluoride is one of MgF _2, AlF ₃ , CaF ₂ , LaF ₃ , NdF ₃ , YbF ₃ , and YF ₃ .

The optical element according to claim 3 is characterized in that any one of acrylic, polypropylene , polyamide, and Teflon (registered trademark) is used as the plastic material.

According to the method for forming a thin film according to the present invention and the optical element manufactured by this method, the production can be performed by significantly reducing the number of layers of the multilayer optical thin film manufactured by alternating layers of a low refractive index material and a high refractive index material. Not only is it easy and low cost, but it also has the excellent effect that the vapor-deposited thin film is less likely to crack, peel, or break.

Specifically, the number of layers can be reduced by about 30% by using a plastic material having a folding rate gradient and a refractive index much smaller than that of the existing low refractive index vapor deposition material as the low refractive index material. As a result, it becomes possible to easily and inexpensively manufacture optical elements such as an antireflection film, a reflecting mirror, a polarizing element, and a filter.

In order to explain the present embodiment, it is a figure which shows the state which the plastic thin film was formed on the substrate surface, (A) is the figure which shows the graph which showed the relationship between the thickness of a layer, and the refractive index (A). B) is a cross-sectional view showing the structure of the layer. The figure which shows the graph which showed the relationship between the thickness and the refractive index of each layer in the state which the multilayer film is formed on the substrate surface by the alternating layer of the dielectric material of high refractive index and the plastic of low refractive index which concerns on this embodiment. Is. Comparison of the reflectance of a 5-layer optical element formed by combining Al ₂ O ₃ and Teflon (registered trademark) with the reflectance of a 5-layer sample formed by combining Al ₂ O ₃ and Mg F ₂ . However, it is a figure which shows the graph which showed the relationship with the wavelength.

Hereinafter, a method for forming a thin film and an embodiment of an optical element according to the present invention will be described in detail with reference to the drawings.
First, a film formation of a plastic thin film made of a plastic material on a substrate surface will be described. FIG. 1 is a diagram showing a state in which a plastic thin film is formed on a substrate surface in order to explain the present embodiment. Of these, FIG. 1 (A) is a graph showing the relationship between the layer thickness and the refractive index, and FIG. 1 (B) is a cross-sectional view showing the structure of the layer.

When a plastic material is vapor-deposited on the surface of a thin-film deposition substrate 1 used for an optical element, the plastic thin film 3 shown in FIG. 1 (B) is formed. Here, 4 is an air layer, and the refractive index of the air layer 4 is 1.0. Further, n _s is the refractive index of the vapor deposition substrate 1, n _p is the refractive index of the bottom layer of the plastic thin film 3, and n ₁ is the refractive index of the surface layer of the plastic thin film 3.

It is conceivable that any of acrylic, polyethylene, polypropylene, polyamide, and Teflon (registered trademark) will be used as the vapor deposition substance when the plastic material is vapor-deposited as the plastic thin film 3. Then, in order to form the plastic thin film 3 on the vapor deposition substrate 1, a vacuum vapor deposition method is used in which the plastic material is evaporated by an electron beam or resistance heating in a vacuum.

Since the plastic thin film 3 formed on the vapor deposition substrate 1 is an inhomogeneous film, it has a refractive index gradient as shown in FIG. 1 (A). At this time, the filling rate p of the plastic thin film 3 is about 65%, and the reflectance when vapor-filmed on a quartz substrate corresponds to about 0.1%.

The stress generated in the thin film was about 400 to 700 kgf / cm ² when the quartz substrate was heated to 250 ° C. and a dielectric vapor deposition material as a comparative example was formed on the surface thereof. However, when the quartz vapor deposition substrate 1 was heated to 250 ° C. and a plastic material was vapor-deposited on the surface to form a plastic thin film 3, the amount was about 50 to 60 kgf / cm ² . That is, the stress generated inside the plastic thin film 3 was about 1/10 of the value of the comparative example.

On the other hand, the laser proof stress of this plastic thin film 3 was 213 J / cm ² with respect to irradiation with a pulse-oscillated YAG laser (wavelength: 1.06 μm, pulse width: 10 ns). This is a value comparable to the maximum value of a thin film formed of a dielectric thin-film deposition material.

Next, the optical element according to this embodiment will be described.
Five layers each of a dielectric thin film 2 made of Al ₂ O ₃ as a dielectric material of a high refractive material and a plastic thin film 3 made of Teflon (registered trademark) which is polytetrafluoroethylene as a plastic material of a low refractive material. A multilayer optical thin film is formed on the vapor deposition substrate 1 by alternating layers that are sequentially laminated to each other. However, in order to increase the mechanical strength, the dielectric thin film 2 such as SiO ₂ or Al ₂ O ₃ which is an oxide or Mg F ₂ which is a fluoride is finally overcoated, so that the dielectric thin film 2 is used. Has substantially 6 layers.

According to such an optical element, by adopting a plastic material as a low refraction material, the stress generated at the time of thin film formation as described above is as small as 1/10 or less as compared with the conventional one. became. Therefore, the number of layers of the thin film can be significantly reduced while reducing the damage of the thin film due to stress, and this optical element can be applied to a high-reflecting mirror, a splitter, a bandpass filter, and the like. Further, with the adoption of the plastic thin film 3 made of a plastic material, the pressure increase is alleviated as a result of the presence of voids in the layer, and the laser damage threshold of the thin film can be increased.

FIG. 2 is a graph of the refractive index when an alternating layer is manufactured by alternately depositing a dielectric thin film 2 made of a dielectric material and a plastic thin film 3 made of a plastic material on the surface of a vapor deposition substrate 1 and stacking them. Is. Here, it can be seen that the refractive index of the plastic thin film 3 indicates a refractive index gradient that changes along the thickness direction of the layer. Reference numeral 4 is an air layer, and the refractive index of the air layer 4 is 1.0. Further, n _s is the refractive index of the vapor deposition substrate 1, n _h is the refractive index of the dielectric thin film 2, n _p is the refractive index of the bottom layer of the plastic thin film 3, and n ₁ is the refractive index of the plastic thin film 3. The refractive index of the surface layer.

On the other hand, as the material of the above-mentioned vapor deposition substrate 1, quartz glass, borosilicate crown glass, glass containing phosphate glass, fluorite (CaF ₂ ), crystal (SiO ₂ ), sapphire (Al ₂ O ₃ ) and the like are used. It is conceivable to use any of crystals, YAG _, Al ₂ O ₃ laser crystals, ceramics, semiconductors, plastics, and metals. That is, for example, quartz glass, borosilicate crown glass, and phosphate glass are used as the glass, and YAG _and Al ₂ O ₃ are used as the laser crystals.

Further, an oxide or a fluoride is used as the above-mentioned dielectric material. Here, the oxides include SiO _2, Al ₂ O ₃ , CeO ₂ , HfO ₂ , Ta ₂ O ₅ , ThO ₂ , TiO ₂ , ZrO ₂ , Sc ₂ O ₃ , Y ₂ O ₃ , and La ₂ O ₃ . , Nd ₂ O ₃ can be used, and any of MgF _2, AlF ₃ , CaF ₂ , LaF ₃ , NdF ₃ , YbF ₃ , and YF ₃ can be used as the fluoride.

As described above, it is conceivable to use any one of acrylic, polyethylene, polypropylene, polyamide, and Teflon (registered trademark) for the above plastic material.

Next, a method for forming a thin film when producing such an optical element will be specifically described.
First, in order to form a thin film on the vapor deposition substrate 1, a dielectric material as a high refraction material and a plastic material as a low refraction material are installed in a vacuum chamber (not shown) in addition to the vapor deposition substrate 1. Further, the vapor deposition substrate 1 is set to a temperature in the range of 120 to 350 ° C., and these dielectric materials and plastic materials are sequentially heated in a dry process to form a film on the vapor deposition substrate 1 to form a multilayer optical thin film. When forming the alternating layers of the multilayer optical thin film, the film thickness is measured by an optical monitor for monitoring, and the evaporation rate is monitored by a crystal oscillator.

Here, in order to form the dielectric thin film 2 of the dielectric material as the high refractive index material, a vacuum vapor deposition method, a sputtering method, resistance heating, or an electron beam is used. Specifically, when the high refractive index material is an oxide, oxygen gas is introduced into the vacuum chamber to evaporate the oxide by an electron beam or spatter. When the high refractive index material is fluoride, the fluoride is evaporated by resistance heating or an electron beam. Further, in order to form the plastic thin film 3, a vacuum vapor deposition method is used in which the plastic material is evaporated by an electron beam or resistance heating in a vacuum.

At this time, when the plastic material is evaporated to form the plastic thin film 3, halogen gas, oxygen gas, or hydrogen gas is introduced into the vacuum chamber to form a film, but this plastic thin film is low cost. The filling rate is very small due to the formation of an inhomogeneous film. However, by finally overcoating the dielectric material on the surface of the plastic thin film, the hardness can be made hard to be scratched, and the weak points can be eliminated. Further, the same materials as described above can be adopted as the material, the dielectric material and the plastic material of the vapor deposition substrate 1 at this time.

Here, multi-layer optical coating is performed on the vapor deposition substrate 1 by alternating layers in which Al ₂ O ₃ as a dielectric material and Teflon (registered trademark) as a plastic material are combined and sequentially laminated with each other in five layers. An optical element provided with a thin film is manufactured. Further, as a comparative example, a sample in which a multilayer optical thin film is provided on a substrate is prepared by combining Al ₂ O ₃ and magnesium fluoride (MgF ₂ ) and alternately laminating 5 layers each in sequence. A graph comparing the reflectances with respect to these is shown in FIG. At this time, the refractive index of Teflon, which is a low refractive index material, is 1.25, and the refractive index of MgF ₂ is 1.38.

Then, the data of the high-reflecting mirror by the optical element of the present embodiment is referred to as the characteristic curve C, and the data of the high-reflecting mirror by the sample is referred to as the characteristic curve D. At the wavelength of 355 nm, which is the third harmonic of the YAG laser, the reflectance of the characteristic curve C was about 32%, whereas the reflectance of the characteristic curve D was about 21%. That is, since the reflectances differ greatly between the two at a wavelength of 355 nm, an optical element made of an alternating layer of a dielectric material having a high refractive index and Teflon having an ultra-low refractive index value can be referred to as a high-reflecting mirror. For example, the graph of FIG. 3 shows that the number of layers to be laminated may be reduced by about 30%.

Further, while the plastic material can maintain the refractive index even in a wide band, the optical element of the present invention is transparent and has an ultra-low refractive index over a wide wavelength range from 190 nm, which is a deep ultraviolet wavelength range, to 8000 nm, which is an infrared wavelength range. Not only that, a multilayer optical thin film can be formed by alternating layers of a transparent plastic material and a high refractive index dielectric material, and optimum optical characteristics can be provided for optical system equipment in these wavelength ranges. Along with this, according to the present invention, it is possible to significantly improve the performance and characteristics of an optical element for an optical device on which an optical thin film is vapor-deposited and an optical element for a laser including a high-power laser.

In the above embodiment, the dielectric thin film and the plastic thin film have five layers each, but it is conceivable that there are actually several tens of layers. However, in any case, the present invention can reduce the number of thin film layers by about 30% as compared with the conventional one. Furthermore, although there is a difference between a high-refractive index material and a low-refractive index material, it is not a matter of a specific upper limit or lower limit of the refractive index, but in manufacturing a multilayer optical thin film composed of alternating layers on a substrate. It suffices if there is a difference in the refractive index between the two types of materials.

Although the examples according to the present invention have been described above, the present invention is not limited to the above embodiment, and various modifications can be made based on the gist of the present invention, and these are excluded from the scope of the present invention. It's not something to do.

The optical element based on the multilayer optical thin film obtained by the method for forming a thin film according to the present invention is for a laser system including a high-power laser in the near-infrared region to the infrared region as well as in the deep ultraviolet wavelength region to the visible region. It is most suitable for optical elements, optical elements for astronomical observation and optical elements for optical equipment, such as digital cameras, video cameras, liquid crystal projectors, displays such as smartphones and Facebook, paintings, and protective glasses for displays.

1 Thin-film deposition substrate (substrate)
2 Dielectric thin film 3 Plastic thin film 4 Air layer n _s Refractive index of substrate for vapor deposition n _p Refractive index of bottom layer of plastic thin film n ₁ Refractive index of surface layer of plastic thin film n _h Refractive index of dielectric thin film

Claims (6)

When manufacturing a multilayer optical thin film on a substrate by alternating layers of high-refractive index materials and low-refractive index materials alternately stacked in a vacuum chamber.
An oxide or fluoride which is a dielectric material is used as a high refraction material, and a plastic material is used as a low refraction material. These materials are sequentially heated by spatter, electron beam or resistance heating to evaporate , and a dielectric material is used. A method for forming a thin film, which comprises forming a multilayer optical thin film by a dry process of forming a film on a substrate by finally overcoating . When the plastic material is evaporated to form a thin film, halogen gas, oxygen gas, or hydrogen gas is introduced into the vacuum chamber, and the plastic material is evaporated by electron beam or resistance heating to form a film. The method for forming a thin film according to the above item. Oxides or fluorides are used as the high refraction material, plastic materials are used as the low refraction material, and a multilayer optical thin film is formed on the substrate by alternating layers in which these materials are alternately stacked. Optical element. The substrate shall consist of quartz glass, borosilicate crown glass, glass containing phosphate glass, fluorite, crystal, sapphire crystals, YAG _, Al ₂ O ₃ laser crystals, ceramics, semiconductors, or metals. 3. The optical element according to claim 3. As oxides, SiO _2, Al ₂ O ₃ , CeO ₂ , HfO ₂ , Ta ₂ O ₅ , ThO ₂ , TIO ₂ , ZrO ₂ , Sc ₂ O ₃ , Y ₂ O ₃ , La ₂ O ₃ , Nd ₂ O It is considered to be one of ₃
The optical element according to claim 3 or 4, wherein the fluoride is any one of MgF _2, AlF ₃ , CaF ₂ , LaF ₃ , NdF ₃ , YbF ₃ , and YF ₃ . The optical element according to any one of claims 3 to 5, wherein any one of acrylic, polypropylene, polyamide, and Teflon (registered trademark) is used as the plastic material.

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