Patent Grant
9459388
The present invention relates to a multi-layered optical film which imparts to optical elements a high resistance against lights in the ultraviolet region including blue lasers in a high ambient temperature and a method for producing the same.
Generally, transmittance of plastics is very low for lights having a wavelength less than 300 nanometers, and therefore plastics cannot be used for such lights. On the other hand, deterioration of plastics due to lights having a wave length greater than 450 nanometers is negligible. Accordingly, a resistance of plastics against lights having a wavelength from 300 nanometers to 450 nanometers is significant. In the present text of specification, lights having a wavelength from 300 nanometers to 450 nanometers are also referred to as lights in the ultraviolet region including blue lasers.
Thus, plastics are likely to be damaged by lights having a wavelength from 300 nanometers to 450 nanometers. This means that they have a low resistance for such lights. Accordingly, in optical devices used with lights in the ultraviolet region including blue lasers, optical elements made of glass are mainly used.
On the other hand, a multi-layered optical film is often formed on a surface of an optical element of optical devices for the purpose of anti-reflection or the like. A method for producing an optical element provided with a multi-layered optical film which imparts to the element a resistance against lights in the ultraviolet region including blue lasers has been developed (JP4178190). However, a multi-layered optical film which imparts to an optical element a high resistance against lights in the ultraviolet region including blue lasers even in a high ambient temperature has not been developed.
Accordingly, there is a need for a multi-layered optical film which imparts to an optical element a high resistance against lights in the ultraviolet region including blue lasers even in a high ambient temperature.
A multi-layered optical film according to the first aspect of the present invention is formed on a plastic substrate. Each layer of the multi-layered optical film is made of an oxide or an oxynitride. Layers adjacent to each other are made of materials having different refractive indexes. Oxidation-reduction potential of elements constituting oxides or oxynitrides is −0.9 volts or less. Thickness of a first layer adjacent to the substrate is 10 nanometers or more. An absolute value of a difference in refractive index between a material of the substrate and a material of the first layer is 0.2 or less. An absolute value of a difference in refractive index between two kinds of materials of layers adjacent to each other is 0.45 or less. Total thickness of the multi-layered optical film is 3000 nanometers or less.
The multi-layered optical film according to the present aspect will restrain chemical reactions due to lights in the ultraviolet region including blue lasers and will enhance resistance against lights in the ultraviolet region including blue lasers of the optical element.
In a multi-layered optical film according to the first embodiment of the first aspect of the present invention, material with the highest refractive index among materials constituting the multi-layered optical film includes one of aluminium, lanthanum, gadolinium, zirconium, calcium, cerium, europium, hafnium, magnesium, niobium, scandium, yttrium and ytterbium.
In a multi-layered optical film according to the second embodiment of the first aspect of the present invention, material constituting the first layer of the multi-layered optical film includes silicon or aluminium.
A multi-layered optical film according to the third embodiment of the first aspect of the present invention includes two kinds of materials.
A multi-layered optical film according to the fourth embodiment of the first aspect of the present invention includes three kinds of materials.
In a multi-layered optical film according to the fifth embodiment of the first aspect of the present invention, total thickness of the multi-layered optical film is 240 nanometers or more.
A multi-layered optical film according to the second aspect of the present invention is formed on a plastic substrate, and has resistance against lights in the ultraviolet region including blue lasers in an ambient temperature of 75° C. Each layer of the multi-layered optical film is made of an oxide or an oxynitride. Layers adjacent to each other are made of materials having different refractive indexes. Oxidation-reduction potential of elements constituting oxides or oxynitrides is −0.9 volts or less. Thickness of a first layer adjacent to the substrate is 10 nanometers or more. An absolute value of a difference in refractive index between a material of the substrate and a material of the first layer is 0.2 or less. An absolute value of a difference in refractive index between two kinds of materials of layers adjacent to each other is 0.45 or less. Total thickness of the multi-layered optical film is 3000 nanometers or less.
The multi-layered optical film according to the present aspect will impart resistance against lights in the ultraviolet region including blue lasers in an ambient temperature of 75° C. to an optical element.
In a method for forming a multi-layered optical film on a plastic substrate according to the third aspect of the invention, the multi-layered optical film includes at least one kind of pair of layers in which a layer made of a material having a higher refractive index and a layer made of a material having a lower refractive index are alternately stacked, each layer of the multi-layered optical film is made of an oxide or an oxynitride, oxidation-reduction potential of elements constituting oxides or oxynitrides is −0.9 volts or less, thickness of a first layer adjacent to the substrate is 10 nanometers or more, an absolute value of a difference in refractive index between a material of the substrate and a material of the first layer is 0.2 or less, an absolute value of a difference between the higher refractive index and the lower refractive index is 0.45 or less and total thickness of the multi-layered optical film is 3000 nanometers or less. The method includes the steps of determining materials, thickness and the number of the layers in the above-described conditions in such a way that the multi-layered optical film has a required optical performance; and adjusting the determined materials, thickness and the number of the layers in the above-described conditions in such a way that the multi-layered optical film has a required resistance against lights in the ultraviolet region including blue lasers in an ambient temperature of 75° C.
In
In step S010 of
In step S020 of
In
M=M(L)·M(L−1) . . . M(j) . . . M(1) (1)
M is a matrix of 2 by 2 and the matrix of each layer is also a matrix of 2 by 2. M(j) represents the matrix of the j-th layer and can be represented as below.
where
δ(j)=(2π/λ)·(n(j)·d(j)·cos θ(j))
L represents the number of the layers of the multi-layered optical film as described above. Description of the symbols is below.
Further, reflectance (R) and transmittance (T) of the multi-layered optical film can be obtained by the following equations.
In designing a multi-layered optical film, the wavelength of light to be used, the angle of incidence of light and the optical characteristics (reflectance and transmittance) are determined by the required performance specifications. Accordingly, when designing a multi-layered optical film, a plurality of materials of the layers, thickness of each layer and the number of the layers are determined based on the theory described above and according to the required performance specifications in such a way that the optical characteristics (reflectance and transmittance) are realized.
A simple structure in which two kinds of layers having different refractive indexes are alternately stacked can advantageously reduce reflectance of the multi-layered optical film for anti-reflection as is evident from the above-described equation of reflectance.
In step S030 of
In step S040 of
The multi-layered optical film according to the present invention is formed in such a way that the predetermined resistance against lights in the ultraviolet region including blue lasers can be obtained. Accordingly, the multi-layered optical film has to be formed in such a way that it performs the desired anti-reflection functions as well as it has the predetermined resistance against lights in the ultraviolet region including blue lasers.
In general, in a multi-layered optical film for anti-reflection, when materials of layers of the multi-layered optical film are determined, the number of layers can be increased without having a significant influence on the anti-reflection functions provided that the design is optimized. Further, resistance against lasers will increase with the whole thickness of the multi-layered optical film for anti-reflection (the total thickness). Accordingly, resistance against lasers can be enhanced by increasing the number of layers.
In step S050 of
In step S060 of
The film-forming method, that is, a method for forming a multi-layered optical film will be described below. The multi-layered optical film is preferably formed by vacuum thin-film coating technologies such as an ion-plating method and a sputtering method. Multi-layered optical films formed by vacuum thin-film coating technologies are very dense and have a high resistance against lights. In the text of specification, by way of example, an ion-plating method will be described. The present invention can be applied to multi-layered optical films formed by any other methods, provided that the multi-layered optical films have similar properties.
A high-frequency power supply 401 is connected between the vacuum chamber 412 and the substrate holder 407 via a blocking capacitor 403 and a matching box 402 to apply a high-frequency voltage. A DC power supply 404 is connected between the vacuum chamber 412 and the substrate holder 407 via a choke coil 405 to apply a DC bias voltage such that the substrate holder 407 is on a negative side. By way of example, an output from the high-frequency power supply 401 is 500 W, and a voltage of the DC power supply 404 is 100 V.
An output from the high-frequency power supply 401 is preferably in the range from 300 to 900 W. In this range, an output value can be adjusted to enhance the denseness of the film.
The capacitor 406 operates together with the matching box 402 connected to the high-frequency power supply 401 which supplies a high-frequency voltage power into the vacuum chamber 412 to perform matching, and thus a stable electric field can be made and maintained between a material to be evaporated 409 on a resistor heating board 410 and the substrate 408. As a result, a thin film having high purity, high density, and high adhesiveness can be formed on the surface of the substrate 408.
An electron gun 4101 for electron beam heating is installed under a crucible including the resistor heating board 410.
Further, when forming the film, atmosphere gases such as oxygen gas and argon gas are fed into the vacuum chamber 412 thorough a valve which is not shown in
Layers of plural evaporated materials 409 are successively formed on the plastic substrate 408 to form the multi-layered optical film. When forming a film, a highly dense film can be formed by appropriately setting values of high-frequency voltage and of atmosphere gas pressure. Specific set values of atmosphere gas pressure will be shown later.
How to determine plural materials by which the multi-layered optical film is formed will be described below. According to the new finding of the inventors of the present application, resistance against lights in the ultraviolet region including blue lasers of the multi-layered optical film remarkably varies on properties, particularly refractive indexes, of plural materials which constitute the multi-layered optical film. Accordingly, the plural materials which constitute the multi-layered optical film should be determined in consideration of properties, particularly refractive indexes, of the plural materials in such a way that resistance against lights in the ultraviolet region including blue lasers of the multi-layered optical film is enhanced to the extent as high as possible. Among others, as described in detail later, it has been found that a combination of materials of adjacent layers, which have different values of refractive index plays an essential role. Thus, the inventors of the present application have focused attention on refractive indexes of the plural materials which constitute the multi-layered optical film, not from the standpoint of optical characteristics such as reflectance and transmittance of the multi-layered optical film, but from the standpoint of resistance against lights of the multi-layered optical film. In this point, the finding of the inventors of the present application is quite novel.
Tables 1 to 11 show structures and typical film-forming conditions of multi-layered optical films No. 1 to No. 11 in which adjacent layers are made of materials having different refractive indexes in such a way that the multi-layered optical films have anti-reflection function. The substrate of multi-layered optical film 2 is made of glass. The substrates of the other 10 multi-layered optical films are made of plastic (cyclo-olefin polymer). Multi-layered optical films No. 1 to No. 8 have a structure in which layers made of two kinds of materials having different refractive indexes are alternately stacked. Multi-layered optical films No. 9 to No. 11 have a structure in which layers made of three kinds of materials are stacked. In multi-layered optical films No. 1 to No. 11, the material having the lowest refractive index is silicon dioxide (SiO2). As the material having the lowest refractive index, silicon monoxide or a mixture including silicon dioxide such as SixAly Oz (x, y and z represent positive numbers), can be used.
In Tables 1 to 11, values of refractive index are those at wavelength of 400 nanometers. Values of refractive index which are less than 1.5, values of refractive index which are 1.5 or more and less than 1.9 and values of refractive index which are 1.9 or more are grouped respectively as a lower refractive-index group, a medium refractive-index group and a higher refractive-index group. Refractive index of the glass for the substrate is 1.52 at wavelength of 400 nanometers. Refractive index of cyclo-olefin polymer for the substrates is 1.52 at wavelength of 400 nanometers.
Table 12 shows resistance against lights in the ultraviolet region including blue lasers of multi-layered optical films No. 1 to No. 11. As an index of resistance against lasers, a difference in wave aberration measured before the laser irradiation under the predetermined conditions and that after the irradiation was measured. In Table 12 and the other tables described below, wave aberration is represented by standard deviation in unit of one thousandth (1/1000) of wavelength (mλ). Further, in Table 12, x, y and z of LaxAlyOz represent positive numbers.
Table 13 shows the above-described predetermined conditions The multi-layered optical film was irradiated with laser of wavelength of 405 nanometers at ambient temperature of 75° C. for 1000 hours. The irradiation was performed in such a way that energy density at the irradiated surface of the multi-layered optical film is 60 mW/mm2. The total energy of light is 60 Whr (watt-hours). This value of the total energy is great enough to evaluate resistance of optical elements for general use.
Similar results were obtained for light in the wavelength range from 300 nanometers to 450 nanometers.
Detailed description on ambient temperature of the irradiation test will be given below. By way of example, as to an optical device provided with multi-layered optical film No. 1 shown in Table 12, a difference between wave aberration before laser irradiation of 60 Whr performed under the conditions shown in Table 13 and that after the laser irradiation is 37 mλ. As to the same optical device provided with multi-layered optical film No. 1, the difference caused by laser irradiation of 60 Whr, is 2 mλ when ambient temperature alone is changed to 25° C. in the conditions shown in Table 13. Thus, some optical devices provided with a multi-layered optical film which show little difference in wave aberration when ambient temperature of the irradiation test is 25° C., show a remarkable difference in wave aberration when ambient temperature of the irradiation test is 75° C. Conventionally, an optical element provided with a multi-layered optical film which has a sufficient resistance against lights in the ultraviolet region including blue lasers even for ambient temperature of the irradiation test of 75° C. has not been developed. Ambient temperature of the irradiation test of 75° C. is high enough to evaluate resistance of optical elements for general use. In contrast to this embodiment, in the multi-layered optical films described in JP4178190, ambient temperature of the laser irradiation test is 25° C. and therefore resistance in a high ambient temperature such as 75° C. has not been taken into consideration.
The present invention is intended to provide a multi-layered optical film which shows little difference between wave aberration before blue laser irradiation and that after blue laser irradiation even when ambient temperature of the irradiation test is 75° C.
For example, as to some optical elements of optical pickup devices, resistance of total energy of light of 60 Whr at ambient temperature of 75° C. is required. On the other hand, as to many optical elements of optical pickup devices, wave aberration of transmitted light of 25 mλ or less is required. Since in optical elements, there exists wave aberration of approximately 5 mλ at the beginning, a difference between wave aberration before blue laser irradiation and that after blue laser irradiation is required to be 20 mλ or less.
As shown in Tables 1 and 2, multi-layered optical film No. 1 and multi-layered optical film No. 2 have the same structure except that the substrate of multi-layered optical film No. 1 is made of plastic while the substrate of multi-layered optical film No. 2 is made of glass. When in Table 12 a difference between wave aberration before blue laser irradiation and that after blue laser irradiation of multi-layered optical film No. 1 and that of multi-layered optical film No. 2 are compared with each other, the difference in wave aberration of multi-layered optical film No. 1 is much greater than that of multi-layered optical film No. 2. In general, resistance against lights in the ultraviolet region including blue lasers of a multi-layered optical film formed on a plastic substrate is remarkably lower than that of a multi-layered optical film formed on a glass substrate. The reason is presumed to be that chemical reactions such as catalytic reaction is generated through the interface between the plastic substrate and the multi-layered optical film. In other words, it is presumed that when chemical reactions are likely to occur when irradiated with laser, the multi-layered optical film will be heavily damaged by the laser.
Based on the results in Table 12, it will be described which factors affect resistance against laser of a multi-layered optical film.
Table 14 shows values of oxidation-reduction potential of elements.
As described above, a multi-layered optical film is presumed to be heavily damaged when the multi-layered optical film is likely to undergo chemical reactions by laser irradiation. Accordingly, when oxides of elements having lower oxidation-reduction potential are used as materials which constitute the multi-layered optical film, damage of the multi-layered optical film caused by laser is presumed to be lessened, because reaction rate of the elements having lower oxidation-reduction potential is small in the range in which the rate is determined by reduction.
Table 15A shows a relationship between oxidation-reduction potential of elements which constitutes oxides or oxynitrides besides silicon dioxide among the oxides or oxynitrides which constitute the multi-layered optical films and difference between wave aberration before laser irradiation and that after laser irradiation of the multi-layered optical films. In Table 15A, oxidation-reduction potential of elements which constitutes oxides besides silicon dioxide among the oxides which constitute multi-layered optical film 4, is set to an average of oxidation-reduction potential of La (lanthanum) and that of Al (aluminium).
Table 15 B shows other elements oxidation-reduction potential of which is −0.9 volts or less and their oxides. The elements are calcium, cerium, europium, hafnium, magnesium, niobium, scandium, yttrium and ytterbium. These oxides can be used as materials of medium refractive index or those of higher refractive index.
Table 16 shows a relationship between a difference (absolute value) between refractive index of the first layer and that of the substrate and a difference between wave aberration of transmitted light before laser irradiation and that after laser irradiation. The first layer means the layer which is adjacent to the substrate.
As shown in Table 16 and
Table 17 shows a relationship between a difference in refractive index (absolute value) between adjacent layers of the multi-layered optical film and a difference between wave aberration of transmitted light before laser irradiation and that after laser irradiation. As described above, in many cases, at least one kind of pair of layers in which a layer made of a material having a higher refractive index and a layer made of a material having a lower refractive index are alternately stacked is stacked to form the multi-layered optical film. The difference in refractive index (absolute value) between adjacent layers of the multi-layered optical film means a difference between the higher refractive index and the lower refractive index. In table 17, as to multi-layered optical film No. 11, a difference in refractive index (absolute value) between Al2O3 and SiO2 and that between ZrO2 and SiO2 are represented respectively as 0.20 and 0.36.
As shown in Table 17 and
The reason why the multi-layered optical film suffers severe deterioration when a difference in refractive index (absolute value) between adjacent layers of the multi-layered optical film is great is presumed as below. Interface state is generated at the interface between a layer of higher refractive index which is more likely to be crystallized and a layer of lower refractive index which is less likely to be crystallized. Accordingly, a possibility that the interface state releases or absorbs energy will increase. As a consequence, deterioration of the interface is accelerated particularly by ultraviolet radiation emission, and the interface is thermally damaged by the absorption.
Table 18 shows a relationship between total thickness of the multi-layered optical film and a difference between wave aberration of transmitted light before laser irradiation and that after laser irradiation.
As shown in Table 18 and
The conditions for enhancing resistance against lights in the ultraviolet region including blue lasers of a multi-layered optical film are summarized below.
Each layer of the multi-layered optical film is made of an oxide or an oxynitride.
Oxidation-reduction potential of elements which constitutes oxides or oxynitrides should preferably be −0.9 volts or less. Elements which satisfy the above-described condition include aluminium, lanthanum, gadolinium, zirconium, calcium, cerium, europium, hafnium, magnesium, niobium, scandium, yttrium and ytterbium.
A difference (absolute value) between refractive index of the first layer and that of the substrate should preferably be 0.2 or less. When a material of the substrate is cyclo-olefin polymer, refractive index of which is 1.52 at wavelength of 400 nanometers, materials for the first layer which satisfy the above-described condition include silicon dioxide (SiO2), aluminium oxynitride (AlON) and aluminium oxide (Al2O3).
A difference in refractive index (absolute value) between adjacent layers of the multi-layered optical film should preferably be 0.45 or less.
Total film thickness of the multi-layered optical film should preferably be 240 nanometers or more.
In general, when a multi-layered optical film in which a layer of a material having higher refractive index and a layer of a material having lower refractive index are arranged adjacent to each other is used to obtain predetermined optical characteristics, the greater the difference in refractive index (absolute value) between the adjacent layers, the smaller is the number of required layers. Accordingly, particularly when a multi-layered optical film is formed by a complicated vacuum forming method, a value of a difference in refractive index (absolute value) between the adjacent layers tends to be great in design.
However, a multi-layered optical film which has a high resistance against lights in the ultraviolet region including blue lasers even in a high ambient temperature can be obtained when differences in refractive index (absolute values) between a material of the substrate and materials of layers are set to values less than predetermined ones. This is based on the new finding of the inventors of the present application that differences in refractive index (absolute values) between a material of the substrate and materials of layers have a remarkable influence over resistance against lights in the ultraviolet region including blue lasers of a multi-layered optical film at a high ambient temperature.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/JP2012/050234 | Jan 2012 | WO | international |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3604784 | Louderback | Sep 1971 | A |
| 4988164 | Ichikawa | Jan 1991 | A |
| 5282084 | Hatano | Jan 1994 | A |
| 20020060848 | Mitsuishi | May 2002 | A1 |
| 20080192349 | Inoue | Aug 2008 | A1 |
| 20090219620 | Yamada | Sep 2009 | A1 |
| 20090252862 | Inoue | Oct 2009 | A1 |
| 20100271703 | Nakabayashi | Oct 2010 | A1 |
| 20110194181 | Takahashi | Aug 2011 | A1 |
| 20140285896 | Hisamitsu | Sep 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 101910872 | Dec 2010 | CN |
| 102029739 | Apr 2011 | CN |
| 2007-121391 | May 2007 | JP |
| 4178190 | Sep 2008 | JP |
| 2010-181902 | Aug 2010 | JP |
| WO 2008023802 | Feb 2008 | WO |
| Entry |
|---|
| International Search Report dated Feb. 7, 2012 corresponding to International Patent Application No. PCT/JP2012/050234. |
| International Search Report dated Apr. 28, 2011 corresponding to International Patent Application No. PCT/JP2012/050234. |
| Chinese Office Action dated May 4, 2015 corresponding to Chinese Patent Application Application No. 201280066085.2. |
| Number | Date | Country | |
|---|---|---|---|
| 20130176615 A1 | Jul 2013 | US |
