JPH1088318A - Film formation of optical thin film and film forming device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form optical thin films which consist of MgF1 , have a high durability and absorb less light at a high speed by a sputtering method. SOLUTION: The film thickness at the time of forming the film on a substrate 2 by introducing gaseous O2 , into a vacuum vessel 1, supplying high-frequency electric power to a magnetron cathode 5 and sputtering a target 3 consisting of MgF2 is detected by a film thickness monitor meter 10a and the change in the film thickness with lapse of time is computed, by which a film forming speed is calculated. This film forming speed is fed back to a control circuit device 11. The high-frequency electric power is controlled by this control circuit device 11 via high-frequency power source 6. The optical thin films are thus formed while the film forming speed is kept constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and a method for forming an optical thin film, and more particularly to a method and an apparatus for forming an optical thin film made of magnesium fluoride by a sputtering method.

[0002]

2. Description of the Related Art Conventionally, when a thin film is formed,
From the viewpoint of easiness of the method and high deposition rate, the vacuum deposition method has been often used. This vacuum deposition method is also used when forming an optical thin film such as an antireflection film, a half mirror, and an edge filter. On the other hand, in recent years, there has been an increasing demand for coating of an optical thin film and other thin films by a sputtering method which is advantageous in terms of automation, labor saving, and applicability to a large-area substrate. However, the sputtering method has a disadvantage that the film formation rate is lower than that of the vacuum evaporation method. In the case of a metal film, it is still at a practical level, but in the case of other films, it has not been widely used industrially because the deposition rate is extremely slow.

When MgF ₂ (magnesium fluoride), which is a typical low refractive index material, is sputtered as an optical thin film, Mg and F are dissociated into Mg and F, and F is insufficient in the film. This has been a disadvantage that the sputtering method is applied to an optical thin film. An example in which a sputtering method is applied to an optical thin film is described in JP-A-4-223401.
There is a technique described in Japanese Patent Publication No. This publication discloses that a thin film formed when MgF ₂ is sputtered absorbs visible light, and that a low refractive index with almost no light absorption is obtained by sputtering with MgF ₂ added with Si as a target. And the like can be formed.

[0004]

However, in the invention described in Japanese Patent Application Laid-Open No. 4-223401, 50-inch targets commonly used industrially are used.
Even if 0 W (2.8 W / cm ² ) high-frequency power is applied,
The film formation rate is 10 nm / min or less at the maximum, and the drawback of the sputtering method that the film formation rate is low cannot be solved. At this film-forming speed, it takes 10 minutes or more to form a single-layer antireflection film applied in the visible region, for example, and it must be said that industrial dissemination is difficult. According to a follow-up experiment conducted by the inventor, a plate-shaped M
Sputtering with a target having a smaller diameter Si wafer mounted on gF ₂ as a target,
A film having no problem in light absorption in the visible region could not be formed.

The present invention has been made in view of the above problems, and a method for forming an optical thin film having low light absorption and excellent durability by a sputtering method at a high speed and a film forming method used in the method. It is intended to provide a device.

[0006]

According to the method of the present invention, a discharge gas is introduced into a vacuum chamber, the target is a granular magnesium fluoride, and the discharge gas is applied to the target by high frequency power controlled by control means. The plasma is generated, and the target is sputtered with positive ions while the temperature of the target is increased by the plasma to form an optical thin film on a substrate, and a temporal change in the thickness of the formed thin film is performed. Is calculated, and the high-frequency power is controlled by feeding back to the control means so that the film formation rate becomes constant.

The apparatus of the present invention comprises a cathode on which a target made of granular magnesium fluoride is mounted, a power supply for supplying high-frequency power to the cathode to form an optical thin film on a substrate, and detecting the film thickness. Means for calculating a film forming rate by calculating a temporal change of the film thickness detected by the film thickness detecting means, and the high frequency power via the power supply so that the film forming rate is constant. And control means for controlling.

In the present invention, granular magnesium fluoride is used as a film raw material, a target is formed from the film raw material, a discharge gas is introduced, and a high-frequency current is applied to a cathode on which the target film raw material is mounted. In this way, the cathode is set to a negative potential, plasma is generated on the target by high-frequency power, and the temperature of the target is increased by the plasma to weaken the bonding force between the atoms in advance, so that the positive ions Collides with the target, so that most of the accelerated positive ion energy is used for sputtering, thereby increasing the sputtering yield. As a result, the deposition rate can be significantly increased as compared with the conventional method. .

[0009] The film material to be a target is made into a granule because it is a granule and has poor heat conduction, and an electric field and a magnetic field are concentrated on a large amount of an edge portion so that the film can be easily heated. Because you can. Due to the thermal vibration caused by this heating, particles jump out of the film raw material, and the particles reach the substrate, forming a thin film on the substrate.

[0010] In the present invention, there may be a case where a portion having a strong bonding force and a portion having a weak bonding force are formed due to thermal vibration, and the form of particles jumping out by sputtering becomes a molecule. The molecule herein includes not only a single molecule but also a multi-molecule that forms an aggregate in a cluster. It can be considered that the form of the particles jumping out of the target composed of the film material is almost the same as the molecules evaporated by heat. As described above, in the present invention, since at least a part of the film raw material jumps out in the form of molecules, F of MgF ₂ is less likely to dissociate. This is suitable for an optical thin film having low absorption.

In the present invention, unlike the simple evaporation phenomenon in the vacuum evaporation method, the film material is sputtered by the energy of ions, so that the energy of particles generated by sputtering is higher than that of molecules generated by a normal evaporation phenomenon. The durability of the obtained film will be higher than that of a vapor-deposited film if the film forming conditions are properly adjusted. This tendency is particularly remarkable when the temperature of the substrate is low, for example, from room temperature to 100 ° C.

As a result of intensive studies conducted by the present inventors, it has been found that the film formation rate should be kept constant in order to form a durable film without light absorption while maintaining the film formation rate at a high speed. did. That is, it has been found that if the film formation rate is kept constant, it is possible to form an optical thin film having excellent durability without lowering the packing density of the film. Therefore, in the present invention,
By monitoring the film forming speed and keeping the film forming speed constant, a film of good quality can be obtained at high speed.

Here, the deposition rate depends on the positional relationship between the substrate and the target, the gas pressure, etc., but has the strongest correlation with the high-frequency power. By controlling the power, the deposition rate is kept constant. As an apparatus used in the film forming method, a unit functioning as a sensor for detecting a film thickness, a unit calculating a temporal change in the film thickness, and a film forming speed calculated as a calculation result are fed back, and the power is supplied via a power supply. Control means for controlling the high-frequency power by forming a feedback loop,
The deposition rate is kept constant.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment Mode 1 A film forming apparatus used in this embodiment mode is shown in FIG.
Shown in A substrate 2 is provided above the vacuum chamber 1 so that it can rotate. M is a granule having a particle size of 0.5 mm to 1 mm
A target 3 made of gF ₂ is placed in a quartz dish 4 and a magnetron cathode 5 having a diameter of 4 inches (about 100 mm) is placed.
Is placed on top. The distance between the lower surface of the substrate 2 and the upper surface of the target 3 is set to 75 mm, and the amount of eccentricity between the center of the substrate 2 and the center of the target 3 is set to 75 mm.
The cathode 5 is connected to a high-frequency power source 6 for sputtering. A discharge gas inlet 7 is provided on a side surface of the vacuum chamber 1, and a gas flow rate is controlled by a mass flow controller 8 so that a gas pressure in the vacuum chamber 1 can be set.

After the substrate 2 which is an optical glass made of BK7 (refractive index 1.52) is set in the vacuum chamber 1, the inside of the vacuum chamber 1 is evacuated to 1 × 10 ^-4 Pa by a vacuum pump (not shown). At this time, the substrate 2 was not particularly heated. Thereafter, O ₂ gas as a discharge gas is introduced from the gas inlet while controlling the flow rate by the mass flow controller 8,
The gas pressure is set to 2 × 10 ⁻¹ Pa. High-frequency power is supplied from a high-frequency power source 6 to the cathode 5 and the plasma 9
Generate. This plasma 9 generates granular MgF.
_The target 3 made of ₂ is heated, and at the same time, is sputtered and scattered by positive ions of the discharge gas in the plasma. A shutter 13 is provided below the substrate so as to cover only the substrate, and can be opened and closed by a shutter opening and closing device 12.

Further, the film thickness detecting means is located at a position beside the substrate 2 and horizontally symmetrical with respect to the substrate 2 with respect to the center axis of the target 3 in the vertical direction. A commercially available quartz-type film thickness monitor (trade name: IC4, manufactured by Leibold Inficon Co., Ltd.) 10a is installed. The quartz-type film thickness monitor 10a detects the film thickness and calculates the time-dependent change of the film thickness according to the following equation 1 to calculate the film forming speed. However, in a few 1, f is the resonant frequency, t is the physical thickness, P in the density of the film (MgF ₂ 3.
0 g / cm ³ ), Pθ is the density of the crystal, and N is a constant determined by the cutting direction of the crystal.

[0017]

(Equation 1)

The crystal type film thickness monitor 10a is a control circuit device 1
1, and the control circuit device 11 is connected to the high-frequency power supply 6 and the shutter opening / closing device 12. A signal output of the film formation rate calculated by the crystal film thickness monitor 10a is input to the control circuit device 11. Since the deposition rate tends to increase as the high-frequency power increases, the control circuit device 11 increases the high-frequency power when the input deposition rate is lower than the target deposition rate, The high-frequency power of the high-frequency power supply 6 can be controlled. For this reason, the film forming speed is fed back to the control circuit device 11 and the high-frequency power is controlled, so that the film forming speed can be kept constant.

In the first embodiment, the high-frequency power is increased until the deposition rate at the physical film thickness becomes 100 nm / min, and the high-frequency power is controlled by the control circuit device 11 so that the deposition rate becomes constant. Was controlled. At this time, the high frequency power was controlled to about 470 to 530 W. here,
When the substrate 2 is rotated and the shutter 13 is opened by the shutter opening / closing device 12, an MgF ₂ film is formed on the substrate 2. Since the deposition rate of MgF ₂ was 100 nm / min, the shutter was closed for 60 seconds in order to make the physical thickness of the MgF ₂ film 100 nm. Although the film thickness may be controlled by the time as described above, the output of the physical film thickness obtained by the quartz-type film thickness monitor 10a is output from the control circuit device 1.
1, the shutter opening / closing device 12 can be operated through the control circuit device 11, so that the physical film thickness is directly obtained from the quartz film thickness monitor 10a to open and close the shutter 13. May be used to control the film thickness.

The refractive index of the thus obtained MgF ₂ film is 1.385, which is as low as that obtained by the vacuum evaporation method, and the optical film thickness, that is, the product of the physical film thickness and the refractive index is also 138.5.
Since it is about nm, the reflectance is 2.5% or less in the visible region (wavelength 400 to 700 nm), as shown by the spectral reflectance characteristics in FIG. 2, and was extremely effective as an antireflection film. The light absorption of the film in the visible region was 0.5% or less, which was as low as that obtained by the vacuum deposition method, and there was no optical problem.

A so-called tape peeling test was conducted in which an adhesive tape was attached to the obtained film and then strongly peeled in a 90 ° direction, but no peeling occurred. After a strong rubbing with a lens cleaning paper moistened with alcohol, a durability test was conducted by observing the film surface with the naked eye. As a result, no practically problematic scratch was found. As described above, the film obtained in the first embodiment had practically sufficient mechanical strength such as adhesion and durability even though the substrate was not heated. (Comparative Example 1) In Comparative Example 1, a film was formed using the same film forming apparatus and material as in Embodiment 1 and was fixed at 500 W without feeding back the film forming speed and thus controlling the high frequency power. In this comparative example, the film formation rate was 50 to 200.
It was not stable at nm / min, and there were practical problems such as a slight increase in the light absorption of the film and a slight deterioration in the mechanical strength of the film. (Embodiment 2) In this embodiment, a plastic amorphous polyolefin resin (refractive index: 1.52) is used as the substrate 2. The distance between the lower surface of the substrate 2 and the upper surface of the target 3 was 100 mm, and the amount of eccentricity between the center of the substrate 2 and the center of the target was 100 mm. Except for the above, the same film forming apparatus and material as in Embodiment 1 were used. As in the first embodiment, the quartz-type film thickness monitor 10 a was set at the same position as the substrate 2 with respect to the target 3.

The film forming conditions in this embodiment will be described below. After evacuating the vacuum chamber 1, the O ₂ gas was pumped to a pressure of 5 × 1.
The gas was introduced from the gas inlet 7 so that the pressure became 0 ^-1 Pa. Under the above conditions, the high-frequency power was controlled so that the film formation rate was constant at 50 nm / min. At this time, the high frequency power is 4
It was controlled to about 20 to 480 W. In order to make the physical thickness of the MgF ₂ film 100 nm, the shutter 13 was closed 120 seconds after opening.

The refractive index of the MgF ₂ film thus obtained is 1.385, which is as low as that obtained by the vacuum deposition method, and the optical film thickness is about 138.5 nm. Met. The light absorption of the film in the visible region was 0.5% or less, which was as low as that obtained by the vacuum evaporation method, and there was no optical problem. Further, an adhesion test and a durability test were performed in the same manner as in Embodiment 1, but the mechanical strength of the film was sufficiently high for practical use even though the substrate was not heated. (Comparative Example 2) In Comparative Example 2, MgF was formed using the same film-forming apparatus, material, and film-forming conditions as in Embodiment 2 except that the film-forming speed was not fed back and the high-frequency power was fixed at 450 W.
₂ was formed. In this comparative example, the deposition rate of the film was not stable at 30 nm to 100 nm / min.
There are practical problems such as a slight increase in the light absorption of the film and a slight deterioration in the mechanical strength of the film. (Embodiment 3) FIG. 3 shows a film forming apparatus used in this embodiment.
The same elements as those in FIG. 1 are denoted by the same reference numerals and corresponded. A substrate 2 is provided above the vacuum chamber 1 so that it can rotate. M is a granule having a particle size of 1 mm to 10 mm
A target 3 made of gF ₂ is placed in a quartz dish 4 and a magnetron cathode 5 having a diameter of 4 inches (about 100 mm) is placed.
Is placed on top. The distance between the lower surface of the substrate 2 and the upper surface of the target 3 is set to 100 mm, and the amount of eccentricity between the center of the substrate 2 and the center of the target 3 is set to 100 mm. The cathode 5 is connected to a high-frequency power source 6 for sputtering. A discharge gas inlet 7 is provided on a side surface of the vacuum chamber 1, and a gas flow rate is controlled by a mass flow controller 8 so that a gas pressure in the vacuum chamber 1 can be set.

After the substrate 2 which is an optical glass made of synthetic quartz (refractive index: 1.46) is set in the vacuum chamber 1, the inside of the vacuum chamber 1 is evacuated to 1 × 10 ^-4 Pa by a vacuum pump (not shown). . At this time, the substrate 2 was not particularly heated. Thereafter, O ₂ gas as a discharge gas is introduced from the gas inlet while controlling the flow rate by the mass flow controller 8 so that the gas pressure becomes 1 Pa. High frequency power supply 6
Supplies high frequency power to the cathode 5 to generate plasma 9. The target 9 made of granular MgF ₂ is heated by the plasma 9, and is simultaneously sputtered and scattered by positive ions of the discharge gas in the plasma.
A shutter 13 is provided below the substrate so as to cover just the substrate, and a shutter opening / closing device 12 is provided.
Can be opened and closed.

A commercially available optical film is located at a position beside the substrate 2 and horizontally symmetrical with respect to the substrate 2 with respect to the vertical center axis of the target 3, so that the distance is the same with respect to the target. A thickness monitor 10b (manufactured by SYNCHRON, trade name OPM8) is installed. Optical film thickness monitor 10
b is the transparent glass substrate 10b1 (refractive index N = 1.52)
A film having a refractive index different from that of the transparent glass substrate 10b1 (Mg)
When the F ₂ at n = 1.385) is formed, which reflectance of the transparent glass substrate 10b1 is by utilizing the change periodically, continuously changes in the reflectance of the transparent glass substrate 10b1 By measuring, the optical film thickness, that is, the product of the physical film thickness and the refractive index, and its temporal change, that is, the film forming speed can be obtained from the following equation (2). In Equation 2, R is the reflectance, n is the refractive index of the film, N is the refractive index of the transparent glass substrate, nd is the optical film thickness, and λ is the wavelength.

[0026]

(Equation 2)

The optical film thickness monitor 10b is a control circuit device 1
1, and the control circuit device 11 is connected to the high-frequency power supply 6 and the shutter opening / closing device 12. The signal output of the film formation rate calculated by the optical film thickness monitor 10b is input to the control circuit device 11. In the control circuit device 11,
When the input deposition rate is lower than the target deposition rate, the high-frequency power is increased, and when the deposition rate is high,
The high frequency power of the high frequency power supply 6 can be controlled by lowering the high frequency power. For this reason, the film forming speed is fed back to the control circuit device 11 and the high-frequency power is controlled, so that the film forming speed can be kept constant.

In the third embodiment, the high-frequency power is increased until the film formation speed at the optical film thickness becomes 70 nm / min.
Further, the control circuit device 11 controls the film forming speed to be constant.
To control the high frequency power. At this time, the high frequency power was controlled to be about 420 to 480 W. Here, when the substrate 2 is rotated and the shutter 13 is opened by the shutter opening / closing device 12, an MgF ₂ film is formed on the substrate 2. Since the deposition rate of MgF ₂ is 70 nm / min, M
The shutter was closed for 120 seconds to make the optical thickness of the gF ₂ film 140 nm. Although the film thickness may be controlled by time as described above, the optical film thickness monitor 1
The output of the optical film thickness obtained at 0b is input to the control circuit device 11 and the shutter opening / closing device 12 can be operated through the control circuit device 11, so that the optical film thickness monitor 10b The film thickness may be controlled by directly obtaining the optical film thickness and opening and closing the shutter 13.

The MgF ₂ film thus obtained has a refractive index of 1.385, which is as low as that obtained by the vacuum evaporation method, and has an optical film thickness of 140 nm. Therefore, as shown by the spectral reflectance characteristics in FIG. The reflectance was 2.5% or less in the region (wavelength 400 to 700 nm), and was extremely effective as an antireflection film. The light absorption of the film in the visible region was 0.5% or less, which was as low as that obtained by the vacuum evaporation method, and there was no optical problem.

A so-called tape peeling test was conducted in which an adhesive tape was applied to the obtained film and then strongly peeled in a 90 ° direction, but no peeling occurred. Also,
After a strong rubbing with a lens cleaning paper moistened with alcohol, a durability test was conducted by observing the film surface with the naked eye. As a result, no practically problematic scratches were found. As described above, the mechanical strength such as adhesion and durability was sufficiently high for practical use, although the film obtained in the third embodiment was not heated. (Comparative Example 3) In Comparative Example 3, MgF was formed using the same film-forming apparatus, material, and film-forming conditions as in Embodiment 2 except that the film-forming speed was not fed back and the high-frequency power was fixed at 450 W.
₂ was formed. In this comparative example, the deposition rate of the film was not stable at 40 nm to 150 nm / min.
There are practical problems such as a slight increase in the light absorption of the film and a slight deterioration in the mechanical strength of the film.

In the above description, the thickness of the thin film formed on the film thickness detecting means is measured. However, the present invention is not limited to this, and the thickness of the thin film being formed on the surface of the optical element can be directly measured. It is of course possible to do this.

[0032]

According to the present invention, a high-frequency power is used to generate plasma on a target, and sputtering is performed while heating the target with the plasma, whereby a granular magnesium fluoride as a target is formed on a substrate. Because it forms a film, it is possible to form a film at high speed.
By feeding back the deposition rate, controlling the high frequency power,
By keeping the film formation rate constant, it is possible to provide a method and an apparatus for forming an optical thin film made of magnesium fluoride, having low light absorption and excellent in durability.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a film forming apparatus used in Embodiment 1 of the present invention.

FIG. 2 is a spectral reflectance characteristic diagram of the optical thin film obtained in the first embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of a film forming apparatus used in Embodiment 3 of the present invention.

FIG. 4 is a spectral reflectance characteristic diagram of an optical thin film obtained in Embodiment 3 of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Empty tank 2 Substrate 3 Target 5 Magnetron cathode 6 High frequency power supply 7 Gas inlet 10a Quartz film thickness monitor 10b Optical film thickness monitor 11 Control circuit device

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tadashi Watanabe 2-43-2 Hatagaya, Shibuya-ku, Tokyo Inside Olympus Optical Industrial Co., Ltd. (72) Norika Urata 2-43-2 Hatagaya, Shibuya-ku, Tokyo Within Olympus Optical Co., Ltd. (72) Nobuyoshi Toyohara 2-43-2 Hatagaya, Shibuya-ku, Tokyo Inside Olympus Optical Co., Ltd. Toshiaki Shimizu 2-43 Hatagaya, Shibuya-ku, Tokyo No. 2 Inside Olympus Optical Co., Ltd.

Claims (2)

[Claims] 1. A discharge gas is introduced into a vacuum chamber, and a target is a granular magnesium fluoride. A plasma of the discharge gas is generated on the target by high-frequency power controlled by control means. While increasing the temperature of the target, the target is sputtered with positive ions to form an optical thin film on a substrate, and the temporal change in the thickness of the optical thin film to be formed is calculated to reduce the deposition rate. A method for forming an optical thin film, comprising: calculating and feeding back to a control unit such that the film forming speed is constant to control the high-frequency power. 2. A cathode on which a target made of granular magnesium fluoride is mounted; a power supply for supplying high-frequency power to the cathode to form an optical thin film on a substrate; Means for calculating a time-dependent change in the film thickness detected by the film thickness detecting means to calculate a film forming speed; and control for controlling the high-frequency power via the power supply so that the film forming speed is constant. Means for forming an optical thin film.