JP7703214B2 - Manufacturing method for blue laser mirror - Google Patents

Description

The present invention relates to a method for manufacturing a mirror for blue laser light that can be used to reflect a blue laser, etc.

In the optical system of laser welding, a mirror that reflects a laser is used, as described in Patent Document 1 (JP 2015-79132 A).
This mirror has a multilayer structure of 60 layers on a quartz substrate, and reflects light with a wavelength of 1030 nm (Yb:YAG laser). As described in [0022] and Fig. 6, the transmittance of this mirror is almost 0 for light with a wavelength of 1030 nm and light with a wavelength of 30 nm around it.

JP 2015-79132 A

Recently, in the processing of automotive batteries and the like, there is a demand for high-quality welding of copper, aluminum (including alloys of each), and the like.
The absorptance of light with a wavelength of around 1000 nm in copper and aluminum is low, at around 10 to 20%, making it difficult to achieve high-quality, highly efficient laser welding for this wavelength.
On the other hand, the absorption rate of copper and aluminum in light of a shorter wavelength in the blue range (for example, a wavelength range of 400 nm or more and 500 nm or less), i.e., blue light, is higher at about 20 to 80%. Therefore, copper and aluminum can be welded with higher quality and efficiency by using a laser relating to this wavelength (blue laser).
A laser diode is used to oscillate a blue laser, and the model and individual differences in the oscillation wavelength are more pronounced than those of lasers with wavelengths of around 1000 nm. Therefore, mirrors used in blue laser optical systems need to have a wider wavelength range with high reflectance. However, in mirrors with multilayer film structures, the wider the wavelength range with high reflectance is, the more difficult it becomes to increase the reflectance, and the reflectance tends to decrease (trade-off between the width of the wavelength range and the reflectance). In addition, in a multilayer film structure with about 60 layers, the reflectance assumed at the time of design may not be obtained in the mirror that is actually formed. Therefore, there is room for improvement in the reflectance of mirrors with a wider wavelength range with high reflectance to accommodate blue laser optical systems.

Therefore, one of the main objects of the present invention is to provide a method for producing a mirror for blue laser light that has a wide wavelength range in which high reflectance is achieved in the blue region and has high reflectance in that wavelength range.

In order to achieve one of the above-mentioned main objects, the invention described in claim 1 is a manufacturing method for a mirror for blue laser light, which comprises directly forming an optical multilayer film using only dielectric materials or semiconductor materials that reflects laser light with a wavelength in the blue range on the surface of a glass, crystal, or ceramic substrate having a root-mean-square roughness of more than 1 Å and not more than 5 Å, and which is characterized in that the width of the reflection band in the wavelength range of 400 nm or more and 500 nm or less in the optical multilayer film is 70 nm or more, and the reflectance in the reflection band is 97.5% or more.
In order to achieve one of the above-mentioned main objects, the invention described in claim 2 is a manufacturing method for a mirror for blue laser light, which comprises directly forming an optical multilayer film using only dielectric materials or semiconductor materials that reflects laser light with a wavelength in the blue range on the surface of a glass, crystal, or ceramic substrate having a root-mean-square roughness of 3.5 Å or less and a root-mean-square roughness of more than 1 Å when filtered with a roughness period of 250 nm or more, and which is characterized in that the width of the reflection band of the optical multilayer film in the wavelength range of 400 nm or more and 500 nm or less is 70 nm or more, and the reflectance of the reflection band is 97.5% or more.

One of the main effects of the present invention is that a mirror having a wide wavelength range of high reflectance in the blue range and having high reflectance in that wavelength range is provided.
Another main object of the present invention is to provide a method for producing a mirror having a wide wavelength range of high reflectance in the blue range and high reflectance in that wavelength range.

1 is a schematic cross-sectional view of a mirror according to the present invention. 2 is a graph showing the refractive index dispersion of SiO ₂ and Ta ₂ O ₅ in Examples 1 and 2 and Comparative Example 1. 1 is an AFM image of the surface of the optical multilayer film in Example 1. 1 is an AFM image of the surface of the optical multilayer film in Example 2. 1 is an AFM image of the surface of an optical multilayer film in Comparative Example 1. FIG. 4 is an image in which the AFM image of FIG. 3 (Example 1) is color-coded according to height with the lowest point in the image as the reference point. The image in FIG. 3 (Example 1) is filtered to have a roughness period of 250 nm or less, and is color-coded according to the height based on the lowest point in the image. The image in FIG. 3 (Example 1) is filtered for a roughness period of 250 nm or more, and is color-coded according to the height based on the lowest point in the image. FIG. 5 is an image in which the AFM image of FIG. 4 (Example 2) is color-coded according to height with the lowest point in the image as the reference point. The image in FIG. 4 (Example 2) is filtered to have a roughness period of 250 nm or less, and is color-coded according to the height based on the lowest point in the image. The image in FIG. 4 (Example 2) is filtered for a roughness period of 250 nm or more, and is color-coded according to the height based on the lowest point in the image. 6 is an image in which the AFM image of FIG. 5 (Comparative Example 1) is color-coded according to height with the lowest point in the image as the reference point. The image in FIG. 5 (Comparative Example 1) is filtered to have a roughness period of 250 nm or less, and is color-coded according to the height based on the lowest point in the image. The image in FIG. 5 (Comparative Example 1) is subjected to filtering with a roughness period of 250 nm or more, and is color-coded according to the height based on the lowest point in the image. 1 is a graph showing the spectral reflectance for s-polarized light at an incident angle of 45° for Examples 1 and 2, Comparative Example 1, and for a case in which a conventional simulation was applied to the design values of each thickness of the optical multilayer film, and a case in which scattering theory was applied to the design values of each thickness of the optical multilayer film. 1 is a graph showing the spectral reflectance for p-polarized light at an incident angle of 45° for Examples 1 and 2, Comparative Example 1, and for a case in which a conventional simulation was applied to the design values of each thickness of the optical multilayer film, and a case in which scattering theory was applied to the design values of each thickness of the optical multilayer film. 1 is a graph showing the RMS values before filtering, the RMS values after filtering when the roughness period is 250 nm or more, and the RMS values after filtering when the roughness period is 250 nm or less in Examples 1 and 2 and Comparative Example 1. 1 is a graph showing the refractive index dispersion of SiO ₂ and Ta ₂ O ₅ in Example 3 and Comparative Example 2. 13 is an AFM image of the surface of the optical multilayer film in Example 3. 1 is an AFM image of the surface of an optical multilayer film in Comparative Example 2. FIG. 20 is an image in which the AFM image of FIG. 19 (Example 3) is color-coded according to height with the lowest point in the image as the reference point. This is an image obtained by filtering the image of FIG. 19 (Example 3) with a roughness period of 250 nm or less, and color-coding the image according to the height based on the lowest point in the image. This is an image obtained by filtering the image of FIG. 19 (Example 3) with a roughness period of 250 nm or more. 21 is an image in which the AFM image of FIG. 20 (Comparative Example 1) is color-coded according to height with the lowest point in the image as the reference point. This is an image obtained by filtering the image of FIG. 20 (Comparative Example 1) with a roughness period of 250 nm or less. This is an image obtained by filtering the image of FIG. 20 (Comparative Example 1) with a roughness period of 250 nm or more. 13 is a graph showing the spectral reflectance distribution for light with an incident angle of 5° for Example 3, Comparative Example 2, and a case in which a conventional simulation was applied to the design values of each thickness of the optical multilayer film, and a case in which scattering theory was applied to the design values of each thickness of the optical multilayer film. 13 is a graph showing the RMS values before filtering, the RMS values after filtering when the roughness period is 250 nm or more, and the RMS values after filtering when the roughness period is 250 nm or less in Example 3 and Comparative Example 2.

Below, examples of embodiments of the present invention will be described with reference to the drawings as appropriate. Note that the present invention is not limited to these examples.

As shown in FIG. 1, a mirror 1 according to the present invention has a substrate 2 and an optical multilayer film 4 .
The substrate 2 has a substrate surface Q on which an optical multilayer film 4 is directly formed. The mirror 1 reflects a blue laser light at the optical multilayer film 4 laminated on the substrate surface Q.
The optical multilayer coating 4 may be formed on the substrate surface Q, which is the surface of the substrate 2, via another film, i.e., indirectly. Also, another type of film, such as at least one of a protective film and an antifouling film, may be disposed on the outside (air side) of the optical multilayer coating 4. The other type of film may be a single layer film or a multilayer film. The configuration of the optical multilayer coating 4 may include the configuration of the other type of film.

The base material 2 may or may not have light-transmitting properties.
The material of the substrate 2 is not particularly limited, and may be, for example, a nonmetallic material such as glass, crystal, ceramics, or resin.
The shape of the substrate 2 is not particularly limited, and may be, for example, a parallel plate or a wedge.

The optical multilayer film 4 contributes to the reflection of a blue laser (blue light) and is, for example, an inorganic multilayer film using a dielectric material or a semiconductor material, such as a dielectric multilayer film or a semiconductor multilayer film.
The optical multilayer film 4 is formed on a part or the whole of at least one surface of the substrate 2 .
The optical multilayer film 4 includes a low refractive index layer and a high refractive index layer. In the optical multilayer film 4, the low refractive index layer and the high refractive index layer are preferably arranged alternately. The optical multilayer film 4 may further include a medium refractive index layer.
The design of the optical multilayer film 4 can be changed by changing design factors such as the number and materials of the high and low refractive index layers (as well as the medium refractive index layers), and increasing or decreasing the thickness of each layer (the physical film thickness or optical film thickness of the layer).
For example, a part or all of the structure in the optical multilayer film 4 may be replaced with another optically equivalent structure, such as replacing a medium refractive index layer with a combination of a high refractive index layer and a low refractive index layer that are optically equivalent thereto.

The high refractive index layer is _formed from _a high refractive index material such as zirconium oxide ( _ZrO2 ), titanium oxide ( _TiO2 ), tantalum oxide ( _Ta2O5 ), niobium oxide ( _Nb2O5 ), hafnium oxide ( _HfO2 ), lanthanum oxide ( _La2O3 ), silicon (Si), or praseodymium _oxide ( _Pr2O3 ), or _a mixture of two or more of these.
The low refractive index layer is formed from a low refractive index material such as silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), calcium fluoride (CaF ₂ ), magnesium fluoride (MgF ₂ ), a combination of aluminum oxide and praseodymium oxide (Al ₂ O ₃ -Pr ₂ O ₃ ), a combination of aluminum oxide and lanthanum oxide (Al ₂ O ₃ -La ₂ O ₃ ), or a combination of aluminum oxide and tantalum oxide (Al ₂ O ₃ -Ta ₂ O ₅ ), or a mixture of two or more of these.
The medium refractive index layer is made of a medium refractive index material such as Al ₂ O ₃ , Pr ₂ O ₃ , La ₂ O ₃ , Al ₂ O ₃ --Pr ₂ O ₃ , or Al ₂ O ₃ --La ₂ O ₃ .
For example, two materials may be selected from the above-mentioned high refractive index materials to form the optical multilayer film 4. In addition, a film having another function, such as an antifouling film, may be combined on the outside or inside of the optical multilayer film 4.

The low and high refractive index layers (as well as the medium refractive index layer) of the optical multilayer film 4 are formed by, for example, physical vapor deposition, more specifically, vacuum deposition, ion-assisted deposition, ion plating, sputtering or the like.
The optical multilayer film 4 may be formed on a plurality of surfaces of the substrate 2. That is, the substrate 2 may have a plurality of substrate surfaces Q on which the optical multilayer film 4 is formed. For example, the optical multilayer film 4 may be formed on both the front and back surfaces of the substrate 2 that is a parallel plate or a wedge, or has a concave or convex surface.

From the viewpoint of ensuring ease of design for the mirror 1 for a blue laser, which has a relatively wide bandwidth, and ensuring performance as designed as much as possible, it is preferable that the refractive index of the high refractive index material in the wavelength range of 400 nm to 500 nm is 2.0 or more and 2.35 or less. From the same viewpoint, it is preferable that the refractive index of the low refractive index material in the wavelength range of 400 nm to 500 nm is 1.45 or more and 1.5 or less.

In the mirror 1, the root mean square roughness (RMS) of the optical multilayer surface R, which is the surface of the optical multilayer 4, is 0.5 nm (5 Å) or less. There are multiple methods for measuring RMS, and even if the same object is measured using each method, the measured values may differ due to differences in resolution, etc. Here, the RMS is measured using an atomic force microscope (AFM). The sampling interval of the AFM is 11.7 nm.
The RMS (herein referred to as fRMS) of the optical multilayer surface R filtered at a spatial frequency of 250 nm or more is 0.35 nm (3.5 Å) or less. Such fRMS is calculated based on the above-mentioned RMS and is a calculated value based on the RMS. Incidentally, fRMS may be measured directly by AFM.
Since the optical multilayer film 4 is basically formed with the same film thickness at each location by physical vapor deposition or the like, the roughness of the optical multilayer film surface R depends on the roughness of the substrate surface Q. When it is desired to set the roughness of the optical multilayer film surface R to a predetermined level or less, it is preferable that the roughness of the substrate surface Q is the predetermined level or less.
Thus, if the roughness of the optical multilayer surface R of the mirror 1 is below a predetermined level, even if the width of the high reflectance region (reflection band) for the blue laser is set to be, for example, 70 nm or more, the decrease in reflectance is suppressed throughout the entire reflection band. If the width of the reflection band is 70 nm or more, it is possible to accommodate differences in the oscillation wavelength of the blue laser between different models and individual differences. If the minimum reflectance in the reflection band is 97.5% or more, it is possible to introduce the blue laser into the optical system with the output decrease being suppressed.

Generally, the surface of a substrate on which a multilayer film is formed is finished by general optical polishing from the viewpoint of the balance between cost and quality. In this study, the period T of the surface roughness was measured and found to be about 1000 nm (1 μm).
In the X-ray region, a scattering theory associated with surface roughness, which is related to the following formula (1), is known.
Here, R is the regular reflectance, R ₀ is the regular reflectance on an ideal flat surface, λ is the wavelength of light, n ₀ is the refractive index of the incident medium, σ is the root mean square roughness of the surface, and θ is the angle of incidence.

However, such a scattering theory is valid only when the period T of the surface roughness is sufficiently long with respect to the wavelength λ of interest (λ<<T).
In the mirror 1 for the blue laser, T (about 1 μm) in general optical polishing is close to λ (about 450 nm), so it cannot be said that λ<<T, and the above scattering theory does not hold.
Therefore, the reflectance in the reflection band cannot be precisely predicted from the above-mentioned scattering theory, and it is necessary to understand the tendency from an actual prototype sample.
A large number of samples were then produced, and the upper limits of the RMS and fRMS were found, mainly from the viewpoint of suppressing a decrease in reflectance.
As a method for finishing the substrate surface Q so that the RMS is 5 Å or less, there is a technique called super polishing, for example, which obtains a surface with an RMS of 1 Å or less.

The mirror 1 can be manufactured by forming an optical multilayer film 4 that reflects blue light on a substrate surface Q having an RMS of 5 Å or less (a first manufacturing method for a mirror).
Alternatively, the mirror 1 can be manufactured by forming an optical multilayer film 4 that reflects blue light on a substrate surface Q having an fRMS of 3.5 Å or less (a second manufacturing method for a mirror).

Next, examples according to the above-described embodiment of the present invention will be described.
However, the examples do not limit the scope of the present invention.
Furthermore, depending on how the present invention is understood, an Example may be essentially a Comparative Example outside the scope of the present invention, or a Comparative Example may be essentially an Example within the scope of the present invention.

[Examples 1 to 2, Comparative Example 1]
In Examples 1 and 2 and Comparative Example 1, mirrors having optical multilayer films 4 with the same film configuration were formed on one surface (substrate surface Q) of three types of parallel flat substrates 2 (base plates).
The substrates are made of synthetic quartz, and differ from each other only in the polishing accuracy of the base material surface Q.
The optical multilayer film 4 is an alternating film in which the layer closest to the substrate is the first layer, the odd-numbered layers are SiO ₂ (low refractive index layers made of low refractive index material L), and the even-numbered layers are Ta ₂ O ₅ (high refractive index layers made of high refractive index material H), and each layer has a physical film thickness (nm) as shown in Table 1 below. The refractive index dispersion of each of _{SiO 2} and Ta ₂ O ₅ is as shown in Figure 2. The total number of layers in the optical multilayer film 4 is 69. The optical multilayer film 4 was manufactured in the same batch for Examples 1-2 and Comparative Example 1 by ion-assisted deposition.
The difference in RMS of the optical multilayer film surface R is due to the difference in the polishing accuracy of the substrate surface Q. The RMS of the optical multilayer film surface R of the first type (Example 1) is 3.7 Å. The RMS of the optical multilayer film surface R of the second type (Example 2) is 4.8 Å. The RMS of the optical multilayer film surface R of the third type (Comparative Example 1) is 5.9 Å.

Fig. 3 is an AFM image of the optical multilayer film surface R in Example 1. Fig. 4 is an AFM image of the optical multilayer film surface R in Example 2. Fig. 5 is an AFM image of the optical multilayer film surface R in Comparative Example 1. The measurement regions for these AFM images are squares with sides of 3 μm.
These AFM images show that the roughness of the optical multilayer film surface R increases in the order of Example 1, Example 2, and Comparative Example 1.

6 is a color-coded image of the AFM image of FIG. 3 (Example 1) according to the height relative to the lowest point in the image. The RMS roughness of Example 1 obtained from this image is 3.7 Å.
FIG. 7 shows the image of FIG. 3 filtered with a roughness period (spatial frequency) of 250 nm or less, and color-coded according to the height based on the lowest point in the image. This filtering is performed by removing roughness periods exceeding 250 nm by Fourier transform. This filtering removes the influence of the global change in height of the optical multilayer film surface R caused by at least one of scratches and polishing marks seen in finishing to the extent of general optical polishing. The roughness period of polishing marks on the substrate surface Q seen in finishing to the extent of general optical polishing is often about 1 μm, and filtering with a roughness period of 250 nm or less sufficiently removes the influence of such polishing marks. The RMS roughness of the optical multilayer film surface R from which the influence of the global change of Example 1 obtained from this image has been removed is 2.9 Å.
8 is an image obtained by filtering the image of FIG. 3 with a roughness period of 250 nm or more, and color-coding the image according to the height based on the lowest point in the image. This filtering is performed by removing roughness periods of less than 250 nm using a Fourier transform. This filtering allows the global change in height of the optical multilayer film surface R to be grasped. The global RMS roughness (fRMS) of Example 1 obtained from this image is 1.7 Å.

9 is an image of the AFM image of FIG. 4 (Example 2) color-coded according to height relative to the lowest point in the image. The RMS roughness of Example 2 obtained from this image is 4.8 Å.
Fig. 10 is a color-coded image obtained by filtering the image in Fig. 4 with a roughness period of 250 nm or less in the same manner as described above. The RMS roughness obtained from this image, from which the influence of the global change in Example 2 has been removed, is 2.8 Å.
Fig. 11 is a color-coded image obtained by filtering the image in Fig. 4 with a roughness period of 250 nm or more in the same manner as described above. The global RMS roughness (fRMS) of Example 2 obtained from this image is 3.1 Å.

12 is an image in which the AFM image of FIG. 5 (Comparative Example 1) is color-coded according to height relative to the lowest point in the image. The RMS roughness of Comparative Example 1 obtained from this image is 5.9 Å.
Fig. 13 is a color-coded image obtained by filtering the image in Fig. 5 with a roughness period of 250 nm or less in the same manner as described above. The RMS roughness obtained from this image, from which the influence of the global change in Comparative Example 1 has been removed, is 3.2 Å.
Fig. 14 shows an image obtained by filtering the image of Fig. 5 with a roughness period of 250 nm or more in the same manner as described above. The global RMS roughness (fRMS) of Comparative Example 1 obtained from this image is 4.0 Å.

FIG. 15 is a graph showing the spectral reflectance for s-polarized light with an incidence angle of 45° for Examples 1 and 2, Comparative Example 1, and for a case in which a normal simulation was applied to the design values of each thickness of the optical multilayer coating 4, and a case in which the scattering theory according to the above-mentioned formula (1) was applied to the design values of each thickness of the optical multilayer coating 4.
FIG. 16 is a graph showing the spectral reflectance for p-polarized light with an incidence angle of 45° for Examples 1 and 2, Comparative Example 1, and for a case in which a normal simulation was applied to the design values of each thickness of the optical multilayer coating 4, and a case in which the scattering theory according to the above-mentioned formula (1) was applied to the design values of each thickness of the optical multilayer coating 4.
The wavelength range in FIG. 15 and FIG. 16 is set to 370 nm or more and 550 nm or less, taking into consideration that the wavelength range includes a wavelength range suitable for blue lasers.
In applying the scattering theory in Figures 15 and 16, the RMS is set to 5.9 Å.

According to FIGS. 15 and 16 , the spectral reflectances (theoretical) of the cases where a normal simulation was applied and where scattering theory was applied are similar across the entire range, and are 99.2% or more in the wavelength range of 390 nm or more and 480 nm or less (blue range).
On the other hand, the spectral reflectance of Comparative Example 1 is about 97% to 99% in the short wavelength half of the blue range, which is lower than the theoretical value by up to about 2 points.
On the other hand, the spectral reflectance of Example 2 is about 98% to 99.5% in the short wavelength half of the blue region, which is reduced by a maximum of about 1.5 points compared to the theoretical value.
Moreover, the spectral reflectance of Example 1 is about 98.5% to 99.5% in the short wavelength half of the blue region, which is reduced by a maximum of about 1 point compared to the theoretical value.
Therefore, in Examples 1 and 2 in which the RMS of the substrate is 5 Å or less, the decrease in reflectance in the blue region with respect to the reflection of the blue laser from the theoretical value is suppressed compared to Comparative Example 1.
The reflection band (minimum reflectance value of 97.5% or more) in Examples 1 and 2 is ensured to be 390 nm or more and 500 nm or less (width of 70 nm or more and 110 nm or less) for both s-polarized light and p-polarized light.

FIG. 17 is a graph showing the RMS values before filtering, the RMS values after filtering when the roughness period is 250 nm or more, and the RMS values after filtering when the roughness period is 250 nm or less in Examples 1 and 2 and Comparative Example 1.
In FIG. 17, the difference in each RMS value after filtering when the roughness period is 250 nm or more is larger than the difference in each RMS value after filtering when the roughness period is 250 nm or less.
Therefore, the difference in each RMS after filtering at a roughness period of 250 nm or more (fRMS difference) is more significantly related to the difference in characteristics between Examples 1 and 2 and Comparative Example 1.
Therefore, in Examples 1 and 2, in which the RMS (fRMS) after filtering at a roughness period of 250 nm or more on the optical multilayer film surface R is 3.5 Å or less, the decrease in reflectance in the blue range regarding the reflection of blue laser light from the theoretical value is suppressed compared to Comparative Example 1.
Therefore, in Examples 1 and 2, the wavelength range of high reflectance in the blue range is wide, and the mirror has high reflectance in that wavelength range. Therefore, in Examples 1 and 2, when used in an optical system of a blue laser necessary for high-precision processing of copper, aluminum, etc., a decrease in the intensity of the blue laser due to reflection is suppressed.

[Example 3, Comparative Example 2]
As Example 3 and Comparative Example 2, mirrors were formed that were similar to those of Example 1 and the like, except that the film configuration of the optical multilayer film 4 and the RMS of the optical multilayer film surface R involved in the film formation on the substrate were different.
The optical multilayer coating 4 of Example 3 and Comparative Example 2 is an alternating film in which the layer closest to the substrate is the first layer, the odd-numbered layers _{are Ta2O5} (high refractive index layers made of high refractive index material H ₎ , and the even-numbered layers are _SiO2 (low refractive index layers made of low refractive index material L), and each layer has a physical film thickness (nm) as shown in Table 2 below. The refractive index dispersion of _SiO2 and _Ta2O5 is as shown in Figure 18. The total number of layers of the optical multilayer coating 4 is 102. The optical multilayer coating 4 was manufactured in the same batch for Example 3 and Comparative Example 2 by ion-assisted deposition.
The RMS of the optical multilayer film surface R of Example 3 is 4.9 Å. The RMS of the optical multilayer film surface R of Comparative Example 2 is 8.6 Å. The measurement area for each RMS is a square with one side of 3 μm, as described above. The difference in RMS of the optical multilayer film surface R is due to the difference in the finishing accuracy of the substrate surface Q.

Fig. 19 is an AFM image of the optical multilayer film surface R in Example 3. Fig. 20 is an AFM image of the optical multilayer film surface R in Comparative Example 2. The measurement regions for these AFM images are squares with sides of 3 µm.
According to these AFM images, it can be seen that the substrate surface Q becomes rougher in the order of Example 3 and Comparative Example 2.

21 is an image of the AFM image of FIG. 19 (Example 3) color-coded according to height relative to the lowest point in the image. The RMS roughness of Example 3 obtained from this image is 4.9 Å.
Fig. 22 is a color-coded image obtained by filtering the image of Fig. 19 at a roughness period (spatial frequency) of 250 nm or less in the same manner as described above. The RMS roughness obtained from this image, from which the influence of the global change in Example 3 has been removed, is 3.6 Å.
Fig. 23 is a color-coded image obtained by filtering the image in Fig. 19 for roughness periods of 250 nm or more in the same manner as described above. The global RMS roughness of Example 3 obtained from this image is 2.3 Å.

24 is an image in which the AFM image of FIG. 20 (Comparative Example 2) is color-coded according to the height relative to the lowest point in the image. The RMS roughness of Comparative Example 2 obtained from this image is 8.6 Å.
Fig. 25 is a color-coded image of Fig. 20, which has been filtered with a roughness period of 250 nm or less in the same manner as described above. The RMS roughness of Comparative Example 2, from which the influence of global changes has been removed, is 4.3 Å.
Fig. 26 is a color-coded image obtained by filtering the image in Fig. 20 for roughness periods of 250 nm or more in the same manner as described above. The global RMS roughness of Comparative Example 2 obtained from this image is 6.1 Å.

FIG. 27 is a graph showing the spectral reflectance distribution for light with an incidence angle of 5° in Example 3, Comparative Example 2, and in a case where a conventional simulation was applied to the design values of each thickness of the optical multilayer coating 4, and in a case where the scattering theory according to the above-mentioned formula (1) was applied to the design values of each thickness of the optical multilayer coating 4.
The wavelength range in FIG. 27 is set to 370 nm or more and 550 nm or less, taking into consideration that the wavelength range includes a wavelength range suitable for blue lasers.
In applying the scattering theory in FIG. 27, the RMS is set to 8.6 Å.

According to FIG. 27, the spectral reflectances (theoretical) of the cases where a normal simulation was applied and where scattering theory was applied are similar across the entire range, and are 98.3% or more in the wavelength range of 390 nm or more and 470 nm or less (blue range).
On the other hand, the spectral reflectance of Comparative Example 2 is about 97% to 98.5% in the wavelength range of 390 nm to less than 450 nm in the blue range, which is lower than the theoretical value by up to about 1.5 points. Also, the spectral reflectance of Comparative Example 2 is about 95.3% to 99.3% in the wavelength range of 450 nm to 470 nm, which is lower than the theoretical value by up to about 2 points.
On the other hand, the spectral reflectance of Example 3 is almost the same as the theoretical one in the wavelength range of 390 nm or more and less than 450 nm in the blue range. Also, the spectral reflectance of Example 3 is about 97.6% or more and 99.6% or less in the wavelength range of 450 nm or more and 470 nm or less, which is suppressed to a maximum decrease of about 1 point compared to the theoretical one.
Therefore, in Example 3 in which the RMS of the optical multilayer film surface R is 5 Å or less, the decrease in reflectance in the blue range with respect to the reflection of the blue laser from the theoretical value is suppressed compared to Comparative Example 2.
The reflection band (minimum reflectance value of 97.5% or more) in Example 3 is ensured to be 380 nm or more and 550 nm or less (width is 70 nm or more and 170 nm or less).

FIG. 28 is a graph of the RMS before filtering, the RMS after filtering when the roughness period is 250 nm or more, and the RMS after filtering when the roughness period is 250 nm or less in Example 3 and Comparative Example 2.
In FIG. 28, the difference in each RMS value after filtering when the roughness period is 250 nm or more is larger than the difference in each RMS value after filtering when the roughness period is 250 nm or less.
Therefore, the difference in each RMS value after filtering at a roughness period of 250 nm or more is more significantly related to the difference in characteristics between Example 3 and Comparative Example 2.
Therefore, in Example 3, in which the RMS after filtering at a roughness period of 250 nm or more on the substrate is 3.5 Å or less, the decrease in reflectance in the blue range regarding reflection of the blue laser from the theoretical value is suppressed compared to Comparative Example 2.
Therefore, in Example 3, the wavelength range of high reflectance in the blue range is wide, and the mirror has high reflectance in that wavelength range. Therefore, in Example 3, when used in an optical system of a blue laser necessary for high-precision processing of copper, aluminum, etc., a decrease in the intensity of the blue laser due to reflection is suppressed.

1: mirror, 2: substrate, 4: optical multilayer film, Q: substrate surface, R: optical multilayer film surface.

Claims (2)

A method for manufacturing a laser device comprising the steps of: forming an optical multilayer film using only a dielectric material or a semiconductor material that reflects laser light having a wavelength in the blue region directly on a surface of a substrate made of glass, crystal, or ceramics, the surface having a root mean square roughness of more than 1 Å and not more than 5 Å;
The width of the reflection band in the wavelength region of 400 nm or more and 500 nm or less in the optical multilayer film is 70 nm or more;
A method for manufacturing a mirror for blue laser light, wherein the reflectance in the reflection band is 97.5% or more. A method for manufacturing a laser beam having a wavelength in a blue region, comprising: forming an optical multilayer film using only a dielectric material or a semiconductor material on a surface of a glass, crystal, or ceramic substrate having a root-mean-square roughness of 3.5 Å or less and a root-mean-square roughness of more than 1 Å when filtered with a roughness period of 250 nm or more;
The width of the reflection band in the wavelength region of 400 nm or more and 500 nm or less in the optical multilayer film is 70 nm or more;
A method for manufacturing a mirror for blue laser light, wherein the reflectance in the reflection band is 97.5% or more.

Images