WO2022124030A1

WO2022124030A1 - Optical filter

Info

Publication number: WO2022124030A1
Application number: PCT/JP2021/042289
Authority: WO
Inventors: 啓一佐原; 正明伊村; 誉子東條
Original assignee: 日本電気硝子株式会社
Priority date: 2020-12-11
Filing date: 2021-11-17
Publication date: 2022-06-16
Also published as: JP7528762B2; JP2022092825A

Abstract

Provided is an optical filter with which transmittance can be effectively increased.　An optical filter 1 according to the present invention has a film containing a hydrogenated silicon, and is characterized by the hydrogenated silicon having a spin density of not more than 1.0 × 1018/cm2.

Description

Optical filter

The present invention relates to an optical filter capable of selectively transmitting light in a specific wavelength range.

Conventionally, an optical filter capable of selectively transmitting light in a specific wavelength range has been widely used in applications such as infrared sensors. As such an optical filter, for example, a bandpass filter using a multilayer film is known. As the multilayer film, a high-refractive index film having a relatively high refractive index and a low-refractive index film having a relatively low refractive index are alternately and repeatedly laminated (for example, Patent Document 1). .. Patent Document 1 describes silicon hydride as a material for a high refractive index film. Further, silicon nitride is described as a material for the low refractive index film. It is described that the silicon hydride film is formed by vapor deposition by plasma CVD (PECVD) using silane gas.

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

In recent years, with the advancement of automatic driving of automobiles and the like, it has been considered to equip automobiles and the like with a large number of sensors such as a laser radar, especially a sensor called LiDAR (Light Detection and Ringing) that detects an object by laser light. ing. For such a sensor, for example, an optical filter for near infrared rays as described in Patent Document 1 is used.

However, when 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.

An object of the present invention is to provide an optical filter capable of effectively increasing the 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 silicon hydride is 1.0 × 10 ¹⁸ pieces / cm ² or less.

A filter unit consisting of a transparent substrate and a multilayer film provided on one main surface of the transparent substrate and having a high refractive index film having a relatively high refractive index and a low refractive index film having a relatively low refractive index. It is preferable that the high-refractive index film is a silicon hydride-containing film. In this case, it is more preferable that the low refractive index film is a silicon oxide-containing film.

It is preferable that an antireflection film provided on the other main surface of the transparent substrate and containing silicon hydride is further provided.

According to the present invention, it is possible to provide an optical filter capable of effectively increasing the transmittance.

FIG. 1 is a schematic cross-sectional view showing an optical filter according to the first embodiment of the present invention. 2 (a) and 2 (c) are schematic views showing an example of a bonding state between a Si atom and an H atom, and FIG. 2 (d) is a schematic diagram showing a Si dangling bond. FIG. 3 is a diagram showing the relationship between the spin density of hydrogenated silicon and the absorbance k. FIG. 4 is a diagram showing the relationship between the spin density of hydrogenated silicon and the transmittance. FIG. 5 is a schematic cross-sectional view showing an optical filter according to a second embodiment of the present invention.

Hereinafter, preferred embodiments will be described. However, the following embodiments are merely examples, and the present invention is not limited to the following embodiments. Further, in the drawings, members having substantially the same function may be referred to by the same reference numeral.

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

The shape of the transparent substrate 2 is not particularly limited, but in the present 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 transparent substrate 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 and resin. Further, if the wavelength range used is an infrared region, it may be Si, Ge, or the like. Examples of the glass include soda-lime glass, borosilicate glass, non-alkali glass, crystallized glass, quartz glass and the like. Further, the glass used for the transparent substrate 2 may be aluminosilicate glass used as tempered glass.

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

The filter unit 3 is a multilayer film having a high refractive index film 4 having a relatively high refractive index and a low refractive index film 5 having a relatively low refractive index. In the present embodiment, the low-refractive index film 5 and the high-refractive index film 4 are alternately provided on the first main surface 2a of the transparent substrate 2 in this order to form a multilayer film.

In the present embodiment, the high refractive index film 4 is a film containing hydrogenated silicon. The high refractive index film 4 is preferably 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 Al, Ti, Nb, Ta, Zr, N, C and the like as elements. On the other hand, the low refractive index film 5 is made of silicon oxide. The material of the low refractive index film 5 is not limited to the above, and may be aluminum oxide, titanium oxide, niobium oxide, tantalum oxide, zirconium oxide, tin oxide, silicon nitride and the like. However, the low refractive index film 5 is preferably a silicon oxide-containing film.

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

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

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

The optical filter 1 of the present embodiment is a bandpass filter designed to selectively transmit light in a specific wavelength range by light interference by providing a filter unit 3 composed of such a multilayer film. .. In this embodiment, the center wavelength of the pass band (transmission band) is designed to be 800 nm to 1000 nm. However, the central wavelength of the transmission band may be outside the range of 800 nm to 1000 nm.

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

2 (a) to 2 (c) are schematic views showing an example of a bonded state between a Si atom and an H atom. FIG. 2D is a schematic diagram showing a Si dangling bond. In the part omitted in FIGS. 2 (a) and 2 (d), the Si atom is bonded to another Si atom. In FIG. 2D, the portion showing an electron is schematically shown as an electron orbital.

The silicon hydride-containing film includes the portion in the bonded state shown in FIGS. 2 (a) and 2 (d). In the case shown in FIG. 2A, one H atom is bonded to the Si atom. In the case shown in FIG. 2B, two H atoms are bonded to the Si atom. In the case shown in FIG. 2 (c), three H atoms are bonded to the Si atom. In the case shown in FIGS. 2 (a) to 2 (c), the Si atom is bonded to an H atom or another Si atom, and all the valence electrons of the Si atom are used for the bond. The silicon hydride-containing film also includes a portion in which all 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. 2D, the H atom is not bonded to the Si atom, and the unpaired electron e exists. Such a portion not used for bonding is a Si dangling bond. The spin density of silicon hydride is a value that quantitatively indicates Si dangling bonds. The smaller the spin density of silicon hydride, the less Si dangling bonds.

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

The spin density of silicon hydride may be measured, for example, for each layer of the silicon hydride-containing film. Alternatively, if a film or the like other than the silicon hydride-containing film in the optical filter does not affect the measurement of the spin density of the silicon hydride, the filter unit or the like may collectively perform the measurement by ESR. In this case, the spin density of silicon hydride in all the silicon hydride-containing films in the optical filter can be obtained at once.

[Production method]
Hereinafter, an example of a method for manufacturing the optical filter 1 will be described in detail.

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

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

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

The flow rate ratio (Ar / H ₂ ) of the argon (Ar) gas to the hydrogen (H ₂ ) gas is preferably 0.83 or more, more preferably 0.96 or more, and 1.4 or more. It is even more preferable, and it is even more preferable that it is 2.4 or more. Thereby, the spin density of the hydrogenated silicon in the obtained silicon hydride-containing film can be further lowered. The upper limit of the flow rate ratio (Ar / H ₂ ) is not particularly limited, but may be, for example, 10.

The temperature of the transparent substrate 2 when the high refractive index film 4 is formed 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 pieces / cm ² or less, the transmittance of the optical filter can be increased. This is shown by comparing Examples and Comparative Examples.

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

The silicon hydride film, which is a high refractive index film, was formed by forming a silicon film by a sputtering method and then hydrogenating the silicon film using RF plasma. In forming the silicon hydride film, a silicon target was used in the film formation of the silicon film, the flow rate of the argon gas sputtered was set to 300 sccm, and the applied power of the target was set to 10 kW. In the hydrogenation of the silicon film, the flow rate of argon gas as a 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 (SiO ₂ film) was formed as a low refractive index film. Argon gas and oxygen gas were used as the sputtering gas, and the silicon target was sputtered to form a SiO ₂ film. At this time, the flow rate of argon gas was set to 300 sccm, and the flow rate of oxygen gas was set to 120 sccm. The target applied power was set to 10 kW. The temperature of the transparent substrate was set to 25 ° C. when the SiO ₂ film and the hydrogenated silicon film were formed.

By repeating these operations, the SiO ₂ film and the hydrogenated silicon film were alternately laminated on the transparent substrate. As a result, a filter portion having a total of 29 layers of film was formed. From the above, the optical filter of Example 1 was obtained.

(Example 2)
An optical filter was produced in the same manner as in Example 1 except for the flow rate of the sputtering gas in the formation of the silicon hydride film. Specifically, in the formation of the silicon hydride film, the flow rate of the argon gas as the sputtering gas was set to 290 sccm in the film formation of the silicon film. In the hydrogenation of the silicon film, the flow rate of argon gas as a sputtering gas was 120 sccm, and the flow rate of hydrogen gas was 10 sccm.

(Example 3)
An optical filter was produced in the same manner as in Example 1 except for the flow rate of the sputtering gas in the formation of the silicon hydride film. Specifically, in the formation of the silicon hydride film, the flow rate of the argon gas as the sputtering gas was set to 330 sccm in the film formation of the silicon film. In the hydrogenation of the silicon film, the flow rate of argon gas as a sputtering gas was 84 sccm, and the flow rate of hydrogen gas was 6 sccm.

(Example 4)
An optical filter was produced in the same manner as in Example 1 except for the flow rate of the sputtering gas in the formation of the silicon hydride film. Specifically, in the formation of the silicon hydride film, the flow rate of the argon gas as the sputtering gas was set to 360 sccm in the film formation of the silicon film. In the hydrogenation of the silicon film, the flow rate of argon gas as a sputtering gas was set to 56 sccm, and the flow rate of hydrogen gas was set to 4 sccm.

(Comparative Example 1)
An optical filter was produced in the same manner as in Example 1 except for the flow rate of the sputtering gas in the formation of the silicon hydride film. Specifically, in the formation of the silicon hydride film, the flow rate of the argon gas as the sputtering gas was set to 370 sccm in the film formation of the silicon film. In the hydrogenation of the silicon film, the flow rate of argon gas as a sputtering gas was 47 sccm, and the flow rate of hydrogen gas was 3 sccm.

(Comparative Example 2)
An optical filter was produced in the same manner as in Example 1 except for the flow rate of the sputtering gas in the formation of the silicon hydride film. Specifically, in the formation of the silicon hydride film, the flow rate of the argon gas as the sputtering gas was set to 380 sccm in the film formation of the silicon film. In the hydrogenation of the silicon film, the flow rate of argon gas as a sputtering gas was 38 sccm, and the flow rate of hydrogen gas was 2 sccm.

Table 1 shows the layer structure of the filter unit in each Example and each Comparative Example. In Table 1, SiO ₂ indicates a low refractive index film made of a SiO ₂ film, Si: H indicates a high refractive index film made of a silicon hydride film, and glass indicates a transparent substrate.

(evaluation)
The spin density of hydrogenated silicon in the optical filters of each example and each comparative example was measured by ESR. Further, the refractive index n, the absorbance k and the transmittance of each optical filter at a wavelength of 940 nm were measured. These results are shown in Table 2. Further, the relationship between the spin density of hydrogenated silicon and the absorbance k and the transmittance is shown in FIGS. 3 and 4. Each plot of FIGS. 3 and 4 shows the results of each example and each comparative example.

FIG. 3 is a diagram showing the relationship between the spin density of hydrogenated silicon and the absorbance k. FIG. 4 is a diagram showing the relationship between the spin density of hydrogenated silicon and the transmittance.

As shown in FIG. 3, it can be seen that the lower the spin density of the hydrogenated silicon, the lower the absorbance k. Further, as shown in FIG. 4, it can be seen that the lower the spin density of the hydrogenated silicon, the higher the transmittance. In particular, when the spin density of hydrogenated silicon is 1.0 × 10 ¹⁸ pieces / cm ² or less, it can be seen that the transmittance is higher than 90%.

From the results shown in FIGS. 3 and 4, it can be seen that the lower the spin density of silicon hydride, the more difficult it is for light to be absorbed by the silicon hydride film, and the easier it is for light to pass through. It is considered that this is because the Si dangling bond contributed to the absorption of light. It is probable that the introduction of hydrogen reduced the Si dangling bonds, resulting in a decrease in the absorbance k and an increase in the transmittance.

From the results of Examples 1 to 4, when the flow rate ratio (Ar / H ₂ ) in hydrogenation of the silicon film is 0.96 or more, 1.44 or more and 2.43 or more, the hydrogenated silicon is used. It can be seen that the spin density is further lowered and the transmittance is further increased.

(Second embodiment)
FIG. 5 is a schematic cross-sectional view showing an optical filter according to a second embodiment of the present invention. In the optical filter 21, the antireflection film 6 is provided on the second main surface 2b of the transparent substrate 2. Other points are the same as those of the first embodiment.

The antireflection film 6 is a multilayer film having a high refractive index film 7 having a relatively high refractive index and a low refractive index film 8 having a relatively low refractive index. In the present embodiment, the low refractive index film 8 and the high refractive index film 7 are alternately provided on the second main surface 2b of the transparent substrate 2 in this order to form a multilayer film. In the present 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. Further, the low refractive index film 8 is made of silicon oxide. As the material of the low refractive index film 8, aluminum oxide, tantalum oxide, niobium oxide, titanium oxide, hafnium oxide, silicon nitride, zirconium oxide, and tin oxide may be used.

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

Also in the optical filter 21 of the second embodiment, in the high refractive index film 4 of the filter unit 3, the spin density of silicon hydride is 1.0 × 10 ¹⁸ pieces / cm ² or less. Therefore, the transmittance in the optical filter 21 can be increased as in the first embodiment.

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

1 ... Optical filter 1a ... Main surface 2 ... Transparent substrate 2a ... First main surface 2b ... Second main surface 3 ... Filter unit 4 ... High refractive index film 5 ... Low refractive index film 6 ... Antireflection film 7 ... High Refractive index film 8 ... Low refractive index film 21 ... Optical filter

Claims

An optical filter having a silicon hydride-containing film.
An optical filter having a spin density of silicon hydride of 1.0 × 10 18 pieces / cm 2 or less.
With a transparent board
A filter unit provided on one main surface of the transparent substrate and composed 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
The optical filter according to claim 1, wherein the high-refractive index film is the silicon hydride-containing film.
The optical filter according to claim 2, wherein the low refractive index film is a silicon oxide-containing film.
The optical filter according to claim 2 or 3, which is provided on the other side main surface of the transparent substrate and further includes an antireflection film containing silicon hydride.