JP2005248276A - Method for manufacturing optical multilayer film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing an optical multilayer film which can acquire a desired transmittance with thoroughly low optical absorption, because of causing no optical absorption due to an interface transition layer formed by an interface reaction occurring among multilayers, while keeping an adequate film-forming speed, and to provide the optical multilayer film manufactured with the method. <P>SOLUTION: The method for manufacturing the optical multilayer film with a sputtering technique includes, when forming another film on one previously-formed film with the sputtering technique, in a process of forming two or more films consisting of different metallic oxides with the sputtering technique, forming another film in such a condition that particles sputtered from a target for forming another film can not extract oxygen from metallic oxide on the surface of the one film when having deposited on the surface of the one film, at least in an early period of sputtering. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing an optical multilayer film using a sputtering method and an optical multilayer film produced by the production method.

  Conventionally, electron beam evaporation has been widely used as a method for producing optical multilayer films used for dielectric thin films for optical elements, but recently, the accuracy of film thickness control is high, and the film is stable. The use of sputtering methods that can be formed is increasing.

In addition, recent optical elements, in particular, various filters such as thin film interference filters used for optical communication, light emitting elements, and the like are required to have higher functions than conventional ones. As the multilayer film, a film in which a high refractive index film and a low refractive index film are alternately laminated has been mainly used.
Specifically, the low refractive index film (refractive index: 1.46) composed of widely SiO ₂ which is used as a laminate material of the optical multilayer film and Ta ₂ O ₅ comprising a high refractive index film (refractive index: 2. 18) using a sputtering method, a silicon (Si) target and a tantalum (Ta) target are used as targets, and (1) the target is sputtered using a mixed gas of an inert gas and an oxidizing gas. Reactive sputtering (hereinafter simply referred to as “reactive sputtering”), or (2) Sputtering the target with an inert gas to form a metal film, and oxidizing the metal film at another location A film forming method (hereinafter simply referred to as “semi-reactive sputtering method”) that repeats an oxidation process using a gas is known (see, for example, Patent Documents 1 and 2).
In general, the sputtering method is often inferior to the vapor deposition method in terms of film formation speed, and therefore, the film formation speed is increased by increasing the input power to the cathode as much as possible.

JP-A-3-229870 JP-A-11-279757

  However, when the input power to the cathode is increased, the film formation speed is improved, and the optical absorption is sufficiently small as a single layer film. However, if the conditions are applied as they are to the production of an optical multilayer film, the resulting optical multilayer film has a desired transmittance due to optical absorption by the interface transition layer caused by the interface reaction of the layers accompanying multilayering. There is a problem of not.

  Therefore, the present invention provides an optical that can achieve a desired transmittance such that the optical absorption by the interface transition layer caused by the interface reaction accompanying multilayering is not present and the light absorption is sufficiently small while maintaining a good film formation rate. An object of the present invention is to provide an optical multilayer film manufacturing method for manufacturing a multilayer film and an optical multilayer film manufactured by the manufacturing method.

As a result of intensive studies to achieve the above object, the present inventor speculated that the above-described problems may be caused by the following mechanism. That is, by increasing the input power to the cathode, the energy of the sputtered particles ejected from the target increases, and the metal oxide film is a layer formed immediately before the sputtered particles are different from the atoms of the sputtered particles. In order to reduce the metal oxide when adhering to the surface, specifically, when a SiO ₂ film is formed on the Ta ₂ O ₅ film, a part of the Ta ₂ O ₅ surface is reduced. It was speculated that this may be caused by the formation of a very thin Ta ₂ O _x (eg, x = 4.5, 4.8, etc.) film on at least a part of the interface.
Therefore, the present inventor, based on the above estimation, at least in the initial stage of sputtering when forming a low refractive index film (eg, SiO ₂ film) on a high refractive index film (eg, Ta ₂ O ₅ film) by sputtering. in conditions where the sputtered particles of Si are not withdrawn oxygen from Ta ₂ O ₅ is a metal oxide of said adhesion surface when attached to the Ta ₂ O ₅ film surface, i.e., by sputtering under the condition that does not reduce the Ta ₂ O ₅ However, it was found that the optical absorption by the interface transition layer caused by the interface reaction accompanying the multi-layering disappears while maintaining a good film forming speed, and the present invention was achieved. That is, this invention provides the manufacturing method of the optical multilayer film as described in the following (1)-(7) and the optical multilayer film as described in (8).

(1) A method for producing an optical multilayer film using a sputtering method,
When two or more films made of different metal oxides are formed by sputtering, when another film is formed by sputtering on one of the previously formed films, the other film is at least in the initial stage of sputtering. A method for producing an optical multilayer film (first embodiment) in which, when sputtered particles of a target for forming a film adhere to one film surface, oxygen is not extracted from the metal oxide on the one film surface.

(2) A method of manufacturing an optical multilayer film using a sputtering method,
The high refractive index film and the low refractive index film constituting the optical multilayer film are formed from different metal oxides, and when the low refractive index film is formed on the high refractive index film by sputtering, at least in the initial stage of sputtering. In the above (1), the film is formed under the condition that oxygen is not extracted from the metal oxide on the surface of the high refractive index film when the sputtered particles of the target forming the low refractive index film adhere to the surface of the high refractive index film. The manufacturing method of the optical multilayer film as described in any one of.

  (3) The high refractive index film is an oxide film of at least one metal selected from the group consisting of Ta, niobium (Nb), and titanium (Ti), and the low refractive index film is an oxide of Si The manufacturing method of the optical multilayer film as described in said (2) which is a physical film.

  (4) The surface of the high refractive index film when sputtered particles of the target forming the low refractive index film adhere to the surface of the high refractive index film at least in the initial stage of sputtering when the low refractive index film is formed by sputtering. The method for producing an optical multilayer film according to (2) or (3), wherein the power density applied to the target is set so as not to extract oxygen from the metal oxide.

  (5) The method for producing an optical multilayer film according to (4), wherein the power density in the initial stage of sputtering is lower than the power density in the sputtering after the initial stage of sputtering.

  (6) The metal oxide on the surface of the high refractive index film when the sputtered particles of the target forming the low refractive index film adhere to the surface of the high refractive index film before the low refractive index film is formed by sputtering. The method for producing an optical multilayer film according to any one of (2) to (5), wherein pre-sputtering is performed by setting an oxidizing gas amount so as not to extract oxygen from the substrate.

  (7) The method for producing an optical multilayer film according to (6), wherein the amount of oxidizing gas in pre-sputtering is larger than the amount of oxidizing gas in sputtering after pre-sputtering.

  (8) An optical multilayer film produced by the production method according to any one of (1) to (7) (second aspect).

  As will be described below, according to the present invention, a desired transmission rate such that the optical absorption by the interface transition layer caused by the interface reaction accompanying the multi-layering is not present and the light absorption is sufficiently small while maintaining a good film formation rate. This is useful because an optical multilayer film can be manufactured.

The present invention is described in detail below.
The method for producing an optical multilayer film according to the first aspect of the present invention (hereinafter simply referred to as “the production method of the present invention”) is a method for producing an optical multilayer film that produces an optical multilayer film using a sputtering method. When two or more films made of different metal oxides are formed by sputtering, when another film is formed by sputtering on one of the previously formed films, at least in the initial stage of sputtering, This is a method for producing an optical multilayer film, in which, when sputtered particles of a target forming the film adhere to the surface of one film, the film is formed under the condition that oxygen is not extracted from the metal oxide on the surface of the one film.
Specifically, the high refractive index film and the low refractive index film constituting the optical multilayer film are formed from different metal oxides, and the low refractive index film is formed on the high refractive index film by sputtering. Further, at least in the initial stage of sputtering, when the sputtered particles of the target for forming the low refractive index film adhere to the surface of the high refractive index film, the film is formed on the condition that oxygen is not extracted from the metal oxide on the surface of the high refractive index film. A method for producing an optical multilayer film is preferably exemplified.

In the present invention, the sputtering method refers to the semi-reactive sputtering method or the reactive sputtering method described above, and may be based on direct current discharge or alternating current discharge. The target is not limited to the above-described Si target and Ta target.
Next, a manufacturing method using a manufacturing method using a semi-reactive sputtering method and a reactive sputtering method will be described in detail.

<Manufacturing method using semi-reactive sputtering method>
As described above, the sputtering by the semi-reactive sputtering method repeats the process of forming a metal film by sputtering a target using an inert gas and oxidizing the metal film using an oxidizing gas at another location. This is a film forming method, which is sputtering in which the surface of a target to be used is ideally not oxidized or has a very low degree of oxidation.

Here, sputtering of a target using an inert gas and oxidation using an oxidizing gas will be described with reference to FIG. 1, but sputtering by a semi-reactive sputtering method is not limited to this. FIG. 1 is a schematic plan view showing the configuration of the sputtering apparatus.
In FIG. 1, a sputtering apparatus 1 has a cylindrical substrate holder 3 and a substrate 4 provided on the outer peripheral surface of the substrate holder 3 in a vacuum chamber 2, and each substrate 4 is the center of the substrate holder 3. It has a structure that is supported rotatably about an axis.
The vacuum chamber 2 includes a first sputtering source 7 including a first cathode 5 and a first target (for example, a target for forming a high refractive index film) 6 disposed in front of the first cathode 5. A second sputtering source 10 comprising a second cathode 8 and a second target (for example, a target for forming a low refractive index film) 9 installed in front of the second cathode 8 and plasma oxidation separated from both sputtering sources. A source 11 is installed.
Further, a first shutter 12 and a second shutter 13 for starting and stopping the film formation are provided in the film forming directions of the first sputtering source 7 and the second sputtering source 10, and each sputtering source and Partition plates (shielding plates) 14 and 15 for separating the plasma oxidation source 11 are provided.

Sputtering of the target using an inert gas will be described with reference to FIG. 1. Only the material of the first target 6 is formed on the substrate with the first shutter 12 opened and the second shutter 13 closed. Sputtering or sputtering in which a metal film made of only the material of the second target 9 is formed on the substrate with the second shutter 13 opened and the first shutter 12 closed.
Similarly, an oxidation process using an oxidizing gas will be described with reference to FIG. 1. After sputtering of the target, a metal film formed on the substrate is oxidized with an oxidizing gas from the plasma oxidation source 11, and metal oxidation is performed. It is a process to make a physical film. A metal oxide film having a desired film thickness can be formed by repeating such a process of forming a metal film and oxidizing the formed metal film to form a metal oxide.

  Sputtering by the semi-reactive sputtering method is performed at different locations in film formation and oxidation, so even if the target surface is not ideally oxidized or is oxidized by the inclusion of oxidizing gas, the extent is very high. Sputtering is performed in a low state.

In the present invention, specific examples of the inert gas include helium, neon, argon, krypton, and xenon. Among these, argon is preferable from the viewpoint of economy and ease of discharge. These may be used alone or in admixture of two or more.
Specific examples of the oxidizing gas include oxygen, ozone, carbon dioxide gas, nitrogen oxide gas (for example, N ₂ O), water vapor, and a mixed gas thereof (for example, a mixed gas of oxygen and ozone). Etc.). Of these, oxygen is preferable from the viewpoint of economy.

  In the present invention, a known material can be used as a target for forming the high refractive index film. Specifically, for example, at least one metal selected from the group consisting of Ta, Nb and Ti is used. Can be mentioned.

The high refractive index film formed by these targets is a metal oxide film in which sputtered particles ejected from the target are oxidized by an oxidizing gas.
Specifically, Ta ₂ O ₅ (2.10 to 2.20), Nb ₂ O ₅ (2.35), and TiO ₂ (2.40) have a sufficiently large refractive index and are refracted over a wide wavelength range. It is preferably exemplified because the extinction coefficient (k) of the rate is small. The number in parentheses is the refractive index.

  Moreover, as a target for forming the low refractive index film, a known material can be used, and specifically, for example, metal Si or the like can be used. Strictly speaking, Si is a semiconductor, but pure Si is often referred to as metallic silicon conventionally.

The low refractive index film formed by these targets is a metal oxide film in which sputtered particles ejected from the target are oxidized by an oxidizing gas.
Specifically, SiO ₂ (1.46) is preferable because the refractive index is sufficiently small, the extinction coefficient (k) of the refractive index is small over a wide wavelength range, and is excellent in weather resistance and inexpensive. Is exemplified. The number in parentheses is the refractive index.

  In the manufacturing method of the present invention, the metal oxide film forming the high refractive index film and the low refractive index film described above may be a metal oxide film containing nitrogen.

In the manufacturing method of the present invention, sputtering for forming another film on one film formed in advance, specifically, forming the low refractive index film on the high refractive index film is performed. Sputtering is performed under the condition that oxygen is not extracted from the metal oxide on the surface of the high refractive index film when the target sputtered particles forming the low refractive index film adhere to the surface of the high refractive index film at least in the initial stage of sputtering. Is called.
Here, “the initial stage of sputtering” means that the film thickness of the other film (low refractive index film) formed on the one film (high refractive index film) previously formed is the other film thickness. This refers to sputtering until the thickness of the monomolecular film of metal oxide constituting the film (low refractive index film) (for example, about 2 nm for SiO ₂ film) is reached.

  In addition, “the first film formed when the sputtered particles of the target for forming another film (low refractive index film) adhere to the surface of the first film (high refractive index film) previously formed ( High refractive index film) The conditions for not extracting oxygen from the surface metal oxide include, for example, 1) the power density applied to the target for forming the low refractive index film and / or 2) sputtering. Suitable examples include those that perform pre-sputtering in which an oxidizing gas amount has been previously set.

Specific examples of the power density setting include a condition in which the power density in the initial stage of sputtering is lower than the power density in the sputtering after the initial stage of sputtering. More specifically, the power density in the initial stage of sputtering is 1 and ~6W / cm ^2, condition and the like that the power density at the sputtering after sputter initial and 8~14W / cm ^2.
If the value of the power density is within this range, it is preferable because the optical absorption by the interface transition layer caused by the interface reaction of the obtained optical multilayer film can be surely suppressed and the desired transmittance can be obtained. The reason is that it is possible to suppress the energy of the sputtered particles ejected from the sputtered particles of the target forming the low refractive index film, thereby suppressing the reduction of the metal oxide of the high refractive index film by the sputtered particles. It is thought that it is possible.
At the same time as setting the power density, it is often necessary to reset the amount of oxidizing gas in the sputtering atmosphere when forming the metal film in order to stably maintain the film formation region. In particular, it is preferable to reduce the amount of oxidizing gas at a rate lower than the power density reduction rate (power density at the initial stage of sputtering / power density at subsequent sputtering) at the same time as reducing the power density. Usually, when a metal film is formed, an oxidizing gas is not included in the sputtering atmosphere. However, since the oxidizing gas from the plasma oxidation source inevitably flows into the sputtering atmosphere in which the metal film is formed, it is necessary to reduce the amount of oxidizing gas in the oxidation process at the initial stage of sputtering. At this time, the amount of the oxidizing gas in the oxidation process at the initial stage of sputtering differs depending on the type of the apparatus with respect to the total amount of the inert gas in the sputtering atmosphere for forming the metal film and the oxidizing gas in the oxidation process. The amount of oxidizing gas in the oxidation process during sputtering after the initial stage of sputtering is preferably 60 to 80% by volume. By adjusting the oxidizing gas amount in this way, it is preferable for the reason that sputtering proceeds stably without changing the film formation region from the initial stage of sputtering to the subsequent sputtering.

In addition, “pre-sputtering” refers to preliminary sputtering performed for the purpose of removing a contamination layer (including a natural oxide film) on the target surface or stabilizing the target surface before film formation by normal sputtering after pre-sputtering. In general, sputtering is performed with the shutter installed on the upper surface (substrate side) of the target surface closed.
Therefore, specific examples of pre-sputtering in which the amount of oxidizing gas is set include pre-sputtering that is performed under the condition that the amount of oxidizing gas in pre-sputtering is higher than the amount of oxidizing gas in sputtering after pre-sputtering. More specifically, although depending on the type of apparatus, the amount of oxidizing gas in the oxidation process during pre-sputtering is 69 to 75% by volume, and the amount of oxidizing gas in the oxidation process during sputtering after pre-sputtering is 60. Examples include pre-sputtering performed under a condition of ˜68% by volume.
If the amount of the oxidizing gas is within this range, it is preferable because optical absorption by the interface transition layer caused by the interface reaction of the obtained optical multilayer film can be reliably suppressed and a desired transmittance can be obtained. The reason is considered to be that the reducing ability of the sputtered particles can be reduced by increasing the contact opportunity between the sputtered particles of the target forming the low refractive index film and the oxidizing gas.

  In the manufacturing method of the present invention, as shown in the examples described later, sputtering under such conditions is at the initial stage of sputtering, in other words, the film thickness of the low refractive index film to be formed is about 2 nm. It may be done. However, it is preferably 0.5 nm or more from the viewpoint of reducing the interface transition layer and obtaining a film as designed, and is preferably 3 nm or less from the viewpoint of economy and optical properties.

<Manufacturing method using reactive sputtering method>
As described above, sputtering by the reactive sputtering method involves sputtering a target using a mixed gas of an inert gas and an oxidizing gas. In the present invention, a film obtained as a single layer film is transparent. Even in the metal mode film formation region, the region near the transition region, the transition region, or the oxide mode (oxidation mode) can be used.
The conditions for setting the inert gas, oxidizing gas, target and film, power density and pre-sputtering are basically the same as those of the manufacturing method using the semi-reactive sputtering method described above.

Here, the film formation region of the reactive sputtering method will be described with reference to FIG. FIG. 2 shows the oxidation in the case where a single layer film is manufactured by changing the flow rate of the oxidizing gas in the atmosphere while keeping the flow rate of the inert gas in the sputtering atmosphere constant under a constant input power. It is a schematic diagram (voltage change curve) which shows the relationship between property gas flow volume and a voltage.
In FIG. 2, the region where the voltage is high and the sputtering rate is high, which is obtained by reducing the oxidizing gas flow rate, is the film formation region in the metal mode.
On the other hand, a region where the voltage is low and the sputtering rate is low, which is obtained by increasing the oxidizing gas flow rate, is a film formation region in the oxide mode.
A region where the voltage between these two modes changes greatly is a transition region.

The optical multilayer film according to the second aspect of the present invention (hereinafter simply referred to as “optical multilayer film of the present invention”) is an optical multilayer film manufactured by the above-described manufacturing method of the present invention, and has a higher functionality. From the above viewpoint, as described above, the multi-layer film has two or more films made of different metal oxides on the substrate. Specifically, each of the high-refractive index film and the low-refractive index film includes one or more layers. , Preferably, it is a multilayer film formed by alternately laminating each so that there are 4 to 40 layers in total.
The film thickness of the high refractive index film is preferably 50 to 200 nm, and the film thickness of the low refractive index film is preferably 50 to 400 nm.

Here, the substrate is not particularly limited, and a conventionally known substrate can be used. For example, quartz, a hard plate, a glass plate, a plastic plate, a plastic film, or the like can be used.
Moreover, it is preferable that the thickness of such a base | substrate is 0.2-6.0 mm from a viewpoint of intensity | strength and the transmittance | permeability.

Since the optical multilayer film of the present invention is manufactured by the above-described manufacturing method of the present invention, there is an effect that there is no optical absorption by the interface transition layer generated by the interface reaction accompanying the multilayering while maintaining a good film forming speed. Have.
The use of the optical multilayer film of the present invention is not particularly limited, but for example, it can be suitably used for a thin film interference filter, a rugate filter, an antireflection film, a dichroic mirror, an ultraviolet / infrared cut filter, and a bandpass filter. This is useful.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

(Example 1)
A high-refractive-index Ta ₂ O ₅ film (average film thickness of 90 nm) is formed on a quartz substrate having a thickness of 1.0 mm by a semi-reactive sputtering method using alternating current discharge. An optical multilayer having a total of 15 Ta ₂ O ₅ films and SiO ₂ films alternately on a glass substrate by repeating the operation of forming a refractive index film SiO ₂ film (average film thickness of each layer: 135 nm). A membrane was prepared. Deposition conditions for the Ta ₂ O ₅ film and the SiO ₂ film are shown below.
In the formation of the SiO ₂ film, between the film thickness of the SiO ₂ film deposited by sputtering until the 2 nm, amount of oxygen gas 50 vol%, performed on ac power 3 kW, in subsequent sputtering Was performed with an oxygen gas amount of 64 vol% and an AC power supply power of 6 kW.

<Ta ₂ O ₅ film formation conditions>
-Sputter target: Metal Ta target (Ta99.99 mass%)
・ Atmosphere: Argon gas 80sccm
-Pressure during film formation: 0.22 Pa
・ AC frequency: 50 kHz
AC power supply power: 3 kW (Power density: 5 W / cm ² )
・ Cathode voltage: 520V
・ Cathode current: 6.9A
・ Deposition rate: 0.32 nm / sec
・ Plasma oxidation source: Oxygen gas 120sccm

<Deposition conditions for SiO ₂ film>
Sputter target: Metal B-doped Si target (Si 99.99 mass%)
・ Atmosphere: Argon gas 80sccm
-Pressure during film formation: 0.25 Pa
・ AC frequency: 50 kHz
AC power supply power: 3 kW → 6 kW (power density: 5 W / cm ² → 10 W / cm ² )
・ Cathode voltage: 630V
・ Cathode current: 10.8A
・ Deposition rate: 0.43 nm / sec
・ Plasma oxidation source: Oxygen gas 80sccm → 140sccm

(Comparative Example 1)
An optical multilayer film was produced under the same conditions as in Example 1 except that in the formation of the SiO ₂ film, sputtering was carried out constantly at an oxygen gas amount of 64 vol% and an AC power supply power of 6 kW.

  With respect to each of the manufactured substrates with optical multilayer films, the spectral transmittance of a transmission band having a wavelength of about 650 nm (transmission band of about 40 nm) was examined. The result is shown in FIG. The spectral transmittance refers to the maximum transmittance of about 650 nm, and the theoretical value of the spectral transmittance in this transmission band is about 96.5%.

  From the results shown in FIG. 3, the substrate with an optical multilayer film manufactured in Comparative Example 1 is about 95% including the back reflection of the quartz substrate, whereas the substrate with an optical multilayer film manufactured in Example 1 Similar to the theoretical value, it was about 96.5%. Thereby, it turned out that the light absorption in the interface of a multilayer film is suppressed.

FIG. 1 is a schematic plan view showing the configuration of the sputtering apparatus. FIG. 2 is a schematic diagram (voltage change curve) showing the relationship between the oxidizing gas flow rate and the voltage when manufacturing the optical multilayer film. FIG. 3 is a graph showing the spectral transmittance of the optical multilayer film obtained in Example 1 and Comparative Example 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sputtering apparatus 2 Vacuum chamber 3 Substrate holder 4 Substrate 5 First cathode 6 First target 7 First sputter source 8 Second cathode 9 Second target 10 Second sputter source 11 Plasma oxidation source 12 First shutter 13 Second shutter 14 , 15 Partition plate

Claims (8)

An optical multilayer film manufacturing method for manufacturing an optical multilayer film using a sputtering method,
When two or more films made of different metal oxides are formed by sputtering, when another film is formed by sputtering on one of the previously formed films, the other film is at least in the initial stage of sputtering. A method for producing an optical multilayer film, wherein sputtered particles of a target for forming a film are deposited on a surface of one film under a condition that oxygen is not extracted from a metal oxide on the surface of the one film. An optical multilayer film manufacturing method for manufacturing an optical multilayer film using a sputtering method,
The high refractive index film and the low refractive index film constituting the optical multilayer film are formed from different metal oxides, and when the low refractive index film is formed on the high refractive index film by sputtering, at least in the initial stage of sputtering. 2, wherein the target sputtered particles for forming the low refractive index film are deposited under conditions that do not extract oxygen from the metal oxide on the surface of the high refractive index film when attached to the surface of the high refractive index film. The manufacturing method of the optical multilayer film of description.   The high refractive index film is an oxide film of at least one metal selected from the group consisting of tantalum (Ta), niobium (Nb), and titanium (Ti), and the low refractive index film is silicon (Si The method for producing an optical multilayer film according to claim 2.   When the sputtered particles of the target forming the low refractive index film adhere to the surface of the high refractive index film at least in the initial stage of sputtering when the low refractive index film is formed by sputtering, metal oxidation of the surface of the high refractive index film is performed. The method for producing an optical multilayer film according to claim 2 or 3, wherein a power density applied to the target is set so as not to extract oxygen from the object.   The method for producing an optical multilayer film according to claim 4, wherein the power density in the initial stage of sputtering is lower than the power density in the sputtering after the initial stage of sputtering.   Before forming the low refractive index film by sputtering, when sputtered particles of the target forming the low refractive index film adhere to the surface of the high refractive index film, oxygen is removed from the metal oxide on the surface of the high refractive index film. The method for producing an optical multilayer film according to any one of claims 2 to 5, wherein pre-sputtering is performed by setting an oxidizing gas amount so as not to be pulled out.   The method for producing an optical multilayer film according to claim 6, wherein the amount of oxidizing gas in pre-sputtering is larger than the amount of oxidizing gas in sputtering after pre-sputtering.   An optical multilayer film produced by the production method according to claim 1.

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