JP7528762B2 - Optical Filters - Google Patents

Description

The present invention relates to an optical filter that can selectively transmit light in a specific wavelength range.

Conventionally, optical filters that can selectively transmit light in a specific wavelength range have been widely used in applications such as infrared sensors. For example, a bandpass filter using a multilayer film is known as such an optical filter. A multilayer film is used in which a high refractive index film with a relatively high refractive index and a low refractive index film with a relatively low refractive index are alternately laminated (for example, Patent Document 1). Patent Document 1 describes silicon hydride as a material for the high refractive index film. It also describes silicon nitride as a material for the low refractive index film. It describes that the silicon hydride film is formed by deposition using plasma enhanced chemical vapor deposition (PECVD) using silane gas.

U.S. Pat. No. 5,398,133

In recent years, with the advancement of autonomous driving in automobiles and other vehicles, the installation of many sensors in automobiles and other vehicles is being considered, such as laser radar, particularly a sensor called LiDAR (Light Detection and Ranging) that detects objects using laser light. Such sensors use optical filters for near-infrared rays, such as those described in Patent Document 1.

However, when the silicon hydride described in Patent Document 1 is used as the high refractive index film, it is difficult to sufficiently increase the transmittance of the optical filter.

The object of the present invention is to provide an optical filter that can effectively increase transmittance.

The optical filter according to the present invention is an optical filter having a silicon hydride-containing film, and is characterized in that the spin density of the silicon hydride is 1.0×10 ¹⁸ /cm ² or less.

The filter comprises a transparent substrate and a filter section formed on one main surface of the transparent substrate and consisting of a multilayer film having a high refractive index film and a low refractive index film, the high refractive index film being preferably a silicon hydride-containing film. In this case, it is more preferable that the low refractive index film be a silicon oxide-containing film.

It is preferable that the transparent substrate further includes an anti-reflection film that is provided on the other main surface of the transparent substrate and contains silicon hydride.

The present invention provides an optical filter that can effectively increase transmittance.

1 is a schematic cross-sectional view showing an optical filter according to a first embodiment of the present invention. 1A to 1C are schematic diagrams showing examples of bonding states between Si atoms and H atoms, and FIG. 1D is a schematic diagram showing a Si dangling bond. FIG. 1 is a graph showing the relationship between the spin density of hydrogenated silicon and the absorbance k. FIG. 13 is a diagram showing the relationship between the spin density of hydrogenated silicon and the transmittance. FIG. 4 is a schematic cross-sectional view showing an optical filter according to a second embodiment of the present invention.

The following describes preferred embodiments. However, the following embodiments are merely examples, and the present invention is not limited to the following embodiments. In addition, in the drawings, components having substantially the same functions may be referred to by the same reference numerals.

[Optical Filter]
First Embodiment
Fig. 1 is a schematic cross-sectional view showing an optical filter according to a first embodiment of the present invention. As shown in Fig. 1, the optical filter 1 includes a transparent substrate 2 and a filter portion 3. The optical filter 1 is used, for example, in a sensor such as a LiDAR, although it is not particularly limited thereto.

The shape of the transparent substrate 2 is not particularly limited, but in this embodiment, it is a rectangular plate. The thickness of the transparent substrate 2 can be, for example, 30 μm or more and 2 mm or less.

The transparent substrate 2 is preferably a substrate that is transparent in the wavelength range used by the optical filter 1. The material of the transparent substrate 2 is not particularly limited, and examples thereof include glass, resin, etc. Furthermore, if the wavelength range used is in the infrared range, the transparent substrate 2 may be made of Si, Ge, etc. Examples of glass include soda-lime glass, borosilicate glass, alkali-free glass, crystallized glass, quartz glass, etc. Furthermore, the glass used for the transparent substrate 2 may be aluminosilicate glass used as reinforced glass.

The transparent substrate 2 has a first main surface 2a as one main surface and a second main surface 2b as the other main surface. The first main surface 2a and the second main surface 2b face each other. A filter section 3 is provided on the first main surface 2a of the transparent substrate 2.

The filter section 3 is a multilayer film having a high refractive index film 4 with a relatively high refractive index and a low refractive index film 5 with a relatively low refractive index. In this embodiment, the multilayer film is formed by alternately providing the low refractive index film 5 and the high refractive index film 4 in this order on the first main surface 2a of the transparent substrate 2.

In this embodiment, the high refractive index film 4 is a film containing silicon hydride. It is preferable that the high refractive index film 4 is a silicon hydride film. However, the material of the high refractive index film 4 is not limited to the above as long as it contains silicon hydride, and may contain elements such as Al, Ti, Nb, Ta, Zr, N, and C. On the other hand, the low refractive index film 5 is composed of silicon oxide. It is not limited to the above material, and may be aluminum oxide, titanium oxide, niobium oxide, tantalum oxide, zirconium oxide, tin oxide, silicon nitride, etc. However, it is preferable that the low refractive index film 5 is a silicon oxide-containing film.

The thickness of each layer of the high refractive index film 4 is preferably 10 nm or more, and more preferably 15 nm or more. On the other hand, the thickness of each layer of the high refractive index film 4 is preferably 1000 nm or less, and more preferably 750 nm or less.

The thickness of each layer of the low refractive index film 5 is preferably 10 nm or more, and more preferably 20 nm or more. On the other hand, the thickness of each layer of the low refractive index film 5 is preferably 500 nm or less, and more preferably 300 nm or less.

The number of layers constituting the multilayer film in the filter section 3 is preferably 16 or more, and more preferably 20 or more. On the other hand, the number of layers constituting the multilayer film in the filter section 3 is preferably 50 or less, and more preferably 40 or less.

The optical filter 1 of this embodiment is a bandpass filter that is designed to selectively transmit light in a specific wavelength range by using optical interference, by including a filter section 3 made of such a multilayer film. In this embodiment, it is designed so that the center wavelength of the passband (transmission band) is 800 nm to 1000 nm. However, the center wavelength of the transmission band may be outside the range of 800 nm to 1000 nm.

The feature of this embodiment is that the spin density of silicon hydride in the high refractive index film 4, which is a silicon hydride-containing film, is 1.0×10 ¹⁸ /cm ² or less. That is, the inventors of the present application have found that if there are few Si dangling bonds in the silicon hydride-containing film, the absorption of light in the silicon hydride-containing film can be effectively suppressed, and even when a silicon hydride-containing film is used, the transmittance of the optical filter 1 can be effectively increased. Details will be described below.

Figures 2(a) to 2(c) are schematic diagrams showing examples of the bond state between Si atoms and H atoms. Figure 2(d) is a schematic diagram showing a Si dangling bond. Note that in the omitted parts in Figures 2(a) to 2(d), Si atoms are bonded to other Si atoms. In Figure 2(d), the parts showing electrons are shown typically as electron orbitals.

The hydrogenated silicon-containing film includes portions in the bonded states shown in Figures 2(a) to 2(d). In the case shown in Figure 2(a), one H atom is bonded to the Si atom. In the case shown in Figure 2(b), two H atoms are bonded to the Si atom. In the case shown in Figure 2(c), three H atoms are bonded to the Si atom. In the cases shown in Figures 2(a) to 2(c), the Si atom is bonded to an H atom or another Si atom, and all of the valence electrons of the Si atom are used for bonding. Note that the hydrogenated silicon-containing film also includes portions in a state where all of the valence electrons of the Si atom are used for bonding with other Si atoms.

On the other hand, in the case shown in FIG. 2(d), no H atom is bonded to the Si atom, and an unpaired electron e exists. Such a portion not used in bonding is a Si dangling bond. The spin density of hydrogenated silicon is a value that quantitatively indicates the Si dangling bond. The smaller the spin density of hydrogenated silicon, the fewer the Si dangling bonds.

The spin density of hydrogenated silicon can be measured by electron spin resonance (ESR). Here, the g value of the Si dangling bond is 2.0049. Therefore, when quantifying the spin density of hydrogenated silicon, the signal intensity of the peak at g value = 2.0049 can be used.

The spin density of silicon hydride may be measured, for example, for each layer of silicon hydride-containing film. Alternatively, if films other than the silicon hydride-containing film in the optical filter do not affect the measurement of the silicon hydride spin density, the ESR measurement may be performed collectively in the filter section, etc. In this case, the spin density of silicon hydride in all of the silicon hydride-containing films in the optical filter can be obtained at once.

[Production method]
An example of a method for manufacturing the optical filter 1 will now be described in detail.

First, a transparent substrate 2 is prepared. Next, a filter portion 3 is formed as a multilayer film on the first main surface 2a of the transparent substrate 2. The filter portion 3 can be formed by alternately laminating a low refractive index film 5 and a high refractive index film 4 in this order on the first main surface 2a of the transparent substrate 2. The high refractive index film 4 and the low refractive index film 5 can each be formed by a sputtering method.

For example, the high refractive index film 4, which is a silicon hydrogenated film, may be formed by reactive sputtering using argon gas and hydrogen gas, or by forming a silicon film and then hydrogenating the formed silicon film. Specifically, a silicon film may be formed by a sputtering method, and then hydrogenating the silicon film using RF plasma to form a silicon hydrogenated film.

The silicon film can be formed, for example, by using a silicon target, setting the flow rate of the inert gas such as argon gas as the sputtering gas to 100 sccm to 500 sccm, and setting the target applied power to 2 kW to 10 kW. The silicon film can be hydrogenated by setting the flow rate of the inert gas such as argon gas as the sputtering gas to 100 sccm to 500 sccm, the flow rate of hydrogen gas to 5 sccm to 200 sccm, and setting the RF power to 1 kW to 5 kW.

The flow rate ratio (Ar/ _H2 ) of argon (Ar) gas to hydrogen ( _H2 ) gas is preferably 0.83 or more, more preferably 0.96 or more, even more preferably 1.4 or more, and even more preferably 2.4 or more. This can further reduce the spin density of silicon hydride in the resulting silicon hydride-containing film. The upper limit of the flow rate ratio (Ar/ _H2 ) is not particularly limited, but can be, for example, 10.

The temperature of the transparent substrate 2 when forming the high refractive index film 4 can be, for example, 15°C or higher and 300°C or lower.

As described above, by setting the spin density of silicon hydride to 1.0×10 ¹⁸ /cm ² or less, the transmittance of the optical filter can be increased. This will be shown by comparing the examples and the comparative examples.

[Example]
Example 1
First, a glass substrate was prepared as a transparent substrate. Next, a filter portion was formed on a first main surface of the transparent substrate. The filter portion was formed by alternately laminating a low refractive index film and a high refractive index film in this order on the first main surface of the transparent substrate. The high refractive index film and the low refractive index film were each formed by a sputtering method.

The high refractive index hydrogenated silicon film was formed by forming a silicon film by sputtering and then hydrogenating the silicon film using RF plasma. When forming the hydrogenated silicon film, a silicon target was used for the silicon film formation, the flow rate of argon gas as the sputtering gas was 300 sccm, and the target applied power was 10 kW. When hydrogenating the silicon film, the flow rate of argon gas as the sputtering gas was 170 sccm, the flow rate of hydrogen gas was 30 sccm, and the RF plasma power was 2.5 kW.

On the other hand, a silicon oxide film ( _SiO2 film) was formed as a low refractive index film. Argon gas and oxygen gas were used as sputtering gases to sputter a silicon target and form a _SiO2 film. At this time, the flow rate of argon gas was 300 sccm and the flow rate of oxygen gas was 120 sccm. The target application power was 10 kW. When forming the _SiO2 film and the silicon hydride film, the temperature of the transparent substrate was 25°C.

By repeating these operations, _SiO2 films and silicon hydride films were alternately laminated on the transparent substrate. As a result, a filter part having a total of 29 layers was formed. In this way, the optical filter of Example 1 was obtained.

Example 2
Except for the flow rate of the sputtering gas in forming the hydrogenated silicon film, the optical filter was fabricated in the same manner as in Example 1. Specifically, in forming the hydrogenated silicon film, the flow rate of argon gas as the sputtering gas in forming the silicon film was set to 290 sccm. In hydrogenating the silicon film, the flow rate of argon gas as the sputtering gas was set to 120 sccm, and the flow rate of hydrogen gas was set to 10 sccm.

Example 3
Except for the flow rate of the sputtering gas in forming the hydrogenated silicon film, the optical filter was fabricated in the same manner as in Example 1. Specifically, in forming the hydrogenated silicon film, the flow rate of argon gas as the sputtering gas in forming the silicon film was set to 330 sccm. In hydrogenating the silicon film, the flow rate of argon gas as the sputtering gas was set to 84 sccm, and the flow rate of hydrogen gas was set to 6 sccm.

Example 4
Except for the flow rate of the sputtering gas in forming the hydrogenated silicon film, the optical filter was manufactured in the same manner as in Example 1. Specifically, in forming the hydrogenated silicon film, the flow rate of argon gas as the sputtering gas in forming the silicon film was set to 360 sccm. In hydrogenating the silicon film, the flow rate of argon gas as the sputtering gas was set to 56 sccm, and the flow rate of hydrogen gas was set to 4 sccm.

(Comparative Example 1)
Except for the flow rate of the sputtering gas in forming the hydrogenated silicon film, the optical filter was fabricated in the same manner as in Example 1. Specifically, in forming the hydrogenated silicon film, the flow rate of argon gas as the sputtering gas in forming the silicon film was set to 370 sccm. In hydrogenating the silicon film, the flow rate of argon gas as the sputtering gas was set to 47 sccm, and the flow rate of hydrogen gas was set to 3 sccm.

(Comparative Example 2)
Except for the flow rate of the sputtering gas in forming the hydrogenated silicon film, the optical filter was fabricated in the same manner as in Example 1. Specifically, in forming the hydrogenated silicon film, the flow rate of argon gas as the sputtering gas in forming the silicon film was set to 380 sccm. In hydrogenating the silicon film, the flow rate of argon gas as the sputtering gas was set to 38 sccm, and the flow rate of hydrogen gas was set to 2 sccm.

The layer structure of the filter part in each example and each comparative example is shown in Table 1. In Table 1, _SiO2 indicates a low refractive index film made of _SiO2 film, Si:H indicates a high refractive index film made of silicon hydride film, and glass indicates a transparent substrate.

(evaluation)
The spin density of hydrogenated silicon in the optical filters of each Example and Comparative Example was measured by ESR. Furthermore, the refractive index n, absorbance k, and transmittance at a wavelength of 940 nm of each optical filter were measured. These results are shown in Table 2. Furthermore, the relationship between the spin density of hydrogenated silicon and the absorbance k and transmittance is shown in Figures 3 and 4. The plots in Figures 3 and 4 show the results of each Example and Comparative Example.

Figure 3 shows the relationship between the spin density of hydrogenated silicon and the absorbance k. Figure 4 shows the relationship between the spin density of hydrogenated silicon and the transmittance.

As shown in Fig. 3, it can be seen that the absorbance k is lower as the spin density of hydrogenated silicon is lower. Furthermore, as shown in Fig. 4, it can be seen that the transmittance is higher as the spin density of hydrogenated silicon is lower. In particular, it can be seen that the transmittance is higher than 90% when the spin density of hydrogenated silicon is 1.0 x ¹⁰¹⁸ / ^cm2 or less.

From the results shown in Figures 3 and 4, it can be seen that the lower the spin density of the hydrogenated silicon, the less light is absorbed by the hydrogenated silicon film and the easier it is for light to transmit through. This is thought to be because the Si dangling bonds contribute to the absorption of light. It is thought that the introduction of hydrogen reduces the Si dangling bonds, thereby decreasing the absorbance k and increasing the transmittance.

Furthermore, from the results of Examples 1 to 4, it can be seen that when the flow rate ratio (Ar/H ₂ ) in the hydrogenation of the silicon film is 0.96 or more, 1.44 or more, or 2.43 or more, the spin density of the hydrogenated silicon becomes lower and the transmittance becomes higher.

Second Embodiment
5 is a schematic cross-sectional view showing an optical filter according to a second embodiment of the present invention. In an optical filter 21, an anti-reflection film 6 is provided on a second main surface 2b of a transparent substrate 2. The other points are the same as those in the first embodiment.

The anti-reflection film 6 is a multilayer film having a high refractive index film 7 with a relatively high refractive index and a low refractive index film 8 with a relatively low refractive index. In this embodiment, the multilayer film is formed by alternately providing the low refractive index film 8 and the high refractive index film 7 in this order on the second main surface 2b of the transparent substrate 2. In this embodiment, the high refractive index film 7 is made of silicon hydride. However, the material of the high refractive index film 7 is not limited to this. The low refractive index film 8 is made of silicon oxide. Note that the material of the low refractive index film 8 may be aluminum oxide, tantalum oxide, niobium oxide, titanium oxide, hafnium oxide, silicon nitride, zirconium oxide, or tin oxide.

The number of layers constituting the multilayer film of the anti-reflection film 6 is preferably 10 or more. On the other hand, the number of layers constituting the multilayer film of the anti-reflection film 6 is preferably 40 or less.

In the optical filter 21 of the second embodiment, the spin density of silicon hydride is 1.0× ¹⁰ /cm ^or less in the high refractive index film 4 of the filter portion 3. Therefore, similarly to the first embodiment, the transmittance of the optical filter 21 can be increased.

It is also preferable that the silicon hydride spin density in the silicon hydride-containing film serving as the high refractive index film 7 of the antireflection coating 6 is 1.0×10 ¹⁸ /cm ² or less. In this case, the transmittance of the optical filter 21 can be increased more reliably and effectively.

REFERENCE SIGNS LIST 1...optical filter 1a...principal surface 2...transparent substrate 2a...first principal surface 2b...second principal surface 3...filter section 4...high refractive index film 5...low refractive index film 6...anti-reflection film 7...high refractive index film 8...low refractive index film 21...optical filter

Claims (3)

1. An optical filter having a hydrogenated silicon-containing film, comprising:
A transparent substrate;
a filter portion provided on one main surface of the transparent substrate and including the silicon hydride-containing film;
a high refractive index film provided on the other main surface of the transparent substrate and having a relatively high refractive index;
an anti-reflection film having a low refractive index film having a relatively low refractive index;
Equipped with
The high refractive index film is made of silicon hydride,
an optical filter, wherein the spin density of the silicon hydride in the high refractive index film and the silicon hydride in the silicon hydride-containing film is 1.0 x ¹⁰¹⁸ / ^cm2 or less. The transparent substrate;
the filter portion being provided on one main surface of the transparent substrate and being made of a multilayer film having a high refractive index film having a relatively high refractive index and a low refractive index film having a relatively low refractive index;
Equipped with
2. The optical filter according to claim 1, wherein the high refractive index film in the filter portion is the silicon hydride-containing film. The optical filter according to claim 2, wherein the low refractive index film in the filter portion is a silicon oxide-containing film.

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