JP2003215329A - Multilayer film optical filter, method for producing the same and optical parts using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer film optical filter which minimizes the effects of a measurement error of film thickness due to a temperature rise of a substrate in the initial stage of film deposition and has characteristics deviated hardly from design values. <P>SOLUTION: In an multilayer film optical filter 20 comprising a multilayer film 5 obtained on a substrate 1 by repeatedly stacking thin films of a plurality of dielectric materials different from each other in refractive index, a buffer layer 4 of 1 to 10 μm thickness comprising a single material for buffering the effects of a measurement error of film thickness due to a temperature rise of the substrate 1 in the initial stage of film deposition is disposed in the lower layer part of the multilayer film 5 present near the substrate 1. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer optical filter which transmits or reflects light of a specific wavelength and is used as a bandpass filter, a gain flattening filter or the like. More specifically, the present invention relates to a multilayer optical filter in which the effect of a film thickness measurement error caused by an increase in the substrate temperature at the initial stage of film formation is significantly reduced.

[0002]

2. Description of the Related Art A multilayer optical filter is intended to obtain a desired transmission or reflection characteristic in a target wavelength range by utilizing the interference phenomenon of light at each interface of laminated films. To do. FIG. 6 shows a high refractive index layer 1 as an example of a commonly used multilayer optical filter 10.
A tantalum oxide (hereinafter referred to as Ta ₂ O ₅ ) layer as 2,
Silicon oxide as the low refractive index layer 13 (hereinafter referred to as SiO ₂ )
An example using layers is shown. Dozens of layers to 10 layers
About 0 layers are alternately laminated on the glass substrate 1.

The film thickness of each layer is designed around the thickness at which the optical thickness is λ / 4 with respect to the wavelength λ of the light to be transmitted or reflected. Here, the optical thickness is a value defined by the product of the refractive index and the physical thickness (actual film thickness). For example, when the light having a wavelength of 1.55 μm for optical communication is targeted, the actual film thickness is 0.18 μm for the Ta ₂ O ₅ layer.
SiO _{2 in} the range of 0.05 to 0.8 μm mainly
In many cases, a layer having a thickness of 0.26 μm is mainly distributed in a range of 0.09 to 0.8 μm.

In such a multi-layer optical filter 10, in order to obtain a product having desired characteristics, the film thickness of each layer is accurately controlled so that the deviation from the design value falls within an allowable range. Becomes important. When it is used in the field of wavelength division multiplexing (WDM), which has been widely used recently, the requirement for film thickness accuracy is particularly strict, and the deviation from the design value must be kept within the range of 0.1 to 0.01%. I have to.

[0005]

The multi-layered film optical filter, which requires the film thickness control with high accuracy as described above, measures and monitors the film thickness during film formation by using an optical film thickness measurement system. Meanwhile, a method of forming a film is adopted. This is to measure the film thickness by the amount of light that is reflected at the interface after passing through each layer that has been formed and is incident on the measuring instrument. A small amount of light reflected and transmitted through the substrate is mixed.

If the thickness of the substrate is constant, accurate measurement is possible only by subtracting the light transmitted through the substrate.
In the initial stage of film formation, energy is supplied to the substrate, so that the temperature gradually rises, the substrate expands accordingly, and the thickness increases. For this reason, from the start of film formation until the substrate temperature stabilizes, the amount of transmitted light also changes as the substrate thickness increases. However, in the measurement of the film thickness, it is difficult to accurately correct the variation in the substrate thickness each time, and there is no choice but to employ a method of keeping the thickness constant and subtracting that amount. As a result, an error corresponding to the change in the amount of transmitted light occurs, and the film thickness of each layer in the initial stage of film formation near the substrate deviates greatly from the design value, and due to these effects, it deviates from the desired characteristic range as a product. There was a problem of being lost.

In order to solve the above problems, the present invention provides
A relaxation layer is provided in the lower layer near the substrate to absorb the temperature change and mitigate its effect, and the effect of the film thickness measurement error that occurs at the initial stage of film formation on the filter characteristics is negligibly small. It is intended to provide a membrane light filter.

[0008]

The present invention comprises a substrate and a multilayer film provided in contact with one surface of the substrate, in which thin films of a plurality of dielectric materials having different refractive indexes are repeatedly laminated, A multilayer film light having a relaxation layer made of a single material in a lower layer portion of the multilayer film near the substrate for alleviating an influence of a film thickness measurement error due to a temperature change. It is a filter. The influence of the temperature rise of the substrate at the initial stage of film formation is limited to the relaxation layer only, and the influence of the film thickness measurement error due to the temperature change on the filter characteristics is negligible. it can.

In the present invention, the thickness of the relaxation layer is 1 to 10 μm.
It is preferably in the range of. The lower limit of film thickness, 1 μm, is
In most film forming apparatuses, the thickness is sufficient to mitigate the effect of the initial temperature rise. Below this, it is expected that the substrate temperature is still in the rising period. The upper limit of 1
When the thickness exceeds 0 μm, the film formation time is lengthened accordingly, and the original characteristics of the optical filter are greatly affected, which is not preferable.

Further, it is preferable that the relaxation layer is located in the lowermost layer in contact with the substrate and is composed of either one of the high refractive index layer and the low refractive index layer. The filter can be manufactured easily by using an ordinary optical filter manufacturing apparatus without changing the manufacturing conditions, without using a special material for the relaxation layer.

The material of the high refractive index layer is either tantalum oxide (Ta ₂ O ₅ ) or titanium oxide (TiO ₂ ), and the material of the low refractive index layer is silicon oxide (Si).
O ₂ ). These materials have stable physical properties as a thin film layer and are suitable as constituent materials.

The present invention is characterized in that the optical film thickness d of the relaxation layer is d = 2m (λ / 4) (m is a positive integer) with respect to the wavelength λ of incident light. It can be a multilayer optical filter. Since the existence of the relaxation layer affects the filter characteristics to some extent, it is necessary to design it by weaving it. However, when this condition is satisfied, the relaxation layer hardly affects the filter characteristics. In other words, it is not necessary to consider the relaxation layer in the design, and the range of design is expanded accordingly. Especially when it is designed as a bandpass filter, its effect is great.

The method for manufacturing a multilayer optical filter according to the present invention comprises:
A first step of forming a relaxation layer on one surface of the substrate to mitigate the influence of a film thickness measurement error due to a temperature change, and two kinds of dielectric materials having different refractive indexes on the upper surface of the relaxation layer. A second step of alternately stacking thin films, and the first step is continuously performed until the temperature of the substrate rises from its initial value to a substantially steady value, and the second step is performed. The step is a method for manufacturing a multilayer optical filter, wherein the step is performed at the temperature of the substantially steady value subsequent to the first step. According to this manufacturing method, the measurement error of the film thickness due to the temperature change at the initial stage of film formation appears only in the relaxation layer that has little influence on the filter characteristics, and thus it becomes easy to obtain a multilayer optical filter having desired characteristics.

The optical component of the present invention uses a multilayer optical filter having a small deviation from the design value described above, and the performance as a component is stable.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to FIG.
It will be described with reference to FIGS. FIG. 1 shows a film structure of a multilayer optical filter 20 according to a first embodiment of the present invention, which is designed and manufactured as a gain flattening filter for optical communication for light having a wavelength of 1.55 μm. It is a thing. A relaxation layer 4 for mitigating the influence of the measurement error of the film thickness according to the present invention is formed on the lowermost layer in contact with the glass substrate 1 having a thickness of 6 μm.
Is provided. On the relaxation layer 4, two layers having different refractive indexes (low refractive index layer 3 and high refractive index layer 2), 35 layers each, 70 layers in total, are laminated to form the multilayer film 5. Here, Ta ₂ O ₅ (refractive index 2.05) is used for the high refractive index layer 2, and SiO ₂ (refractive index 1.46) is used for the low refractive index layer 3. The material of the relaxation layer 4 was Ta ₂ O ₅ which was the same as that of the high refractive index layer 2, and its thickness was 1.5 μm.

The multilayer optical filter 20 of this embodiment is
Each thin film layer was formed by ion beam sputtering (IBS). Tantalum (Ta) and silicon (Si), two targets are placed in a vacuum chamber, and oxygen is supplied as a reaction gas to deposit a thin film of each oxide on the substrate 1. A film thickness measuring means is arranged in the film forming apparatus to monitor the film thickness during film formation. One of the targets is selected to form a film, and when the film thickness reaches a predetermined value, the target is switched to the other target. By repeating this operation a predetermined number of times, the intended multilayer optical filter 20 is obtained.

2 (A) and 2 (B) are cross sections showing the film structure in the intermediate steps in the above manufacturing method. Figure 2
(A) is a relaxation layer 4 made of Ta ₂ O ₅ in the first step
It shows the stage where the film formation is completed. FIG. 2B is a step in which the first two layers are formed in the second step, which is a step of repeatedly laminating two layers (low refractive index layer 3 and high refractive index layer 2) having different refractive indexes. Indicates. Thereafter, the low-refractive index layers 3 and the high-refractive index layers 2 are alternately laminated under the same conditions, and finally the configuration shown in FIG. 1 is obtained.

It took about 5 hours to form the relaxation layer 4 in the film forming apparatus used for manufacturing the multilayer optical filter 20.
The substrate temperature rose from the initial temperature by about 14 ° C. during that time, and then stabilized. The movement of the substrate temperature rise is shown in FIG.
If the operating conditions (ion beam output, degree of vacuum, gas flow rate, etc.) of the film forming apparatus are the same,
The same applies when manufacturing the multilayer filter 10 of the conventional example. Since the substrate thickness increases by about 0.7 μm due to this temperature increase, it is expected that the film thickness measured by the amount of transmitted light will have an error of 5% at maximum. In this period, if each layer is laminated with a thickness of λ / 4 as in the conventional example, at least several layers will be present, and the state of interference at each interface will differ from the design value. The characteristics of will greatly deviate from the designed values.

The multilayer optical filter 20 of the present invention has a relaxation layer 4 having a thickness of 1.5 μm as the lowermost layer in contact with the substrate. Although there is a film thickness measurement error due to a change in the substrate thickness, the error decreases as the temperature approaches the steady value, and is about 0.1% for a film thickness of 1.5 μm. Further, since the interface does not exist, the interference phenomenon does not occur, and even if the layer having a thickness of 1.5 μm fluctuates by the ratio, it has almost no effect on the filter performance.

FIG. 4 shows an example of the transmission characteristics of the obtained multilayer filter in comparison with the conventional example. The dotted line is the transmission characteristic of the multilayer optical filter to be obtained, that is, the design value. Plots of ▲ and ◯ are transmission characteristics of the multilayer optical filter 20 of the conventional example and the present invention, respectively. Here, in the conventional example,
In the film structure of FIG. 1, there is not only the relaxation layer 4 but the other structure is exactly the same.

In the conventional example (▲), the wavelength shifts to the long wavelength side by about 5 nm and deviates from the design value due to the above-described measurement error of the initial film thickness. According to the simulation of the transmission characteristic, the line ▲ almost corresponds to the characteristic when it is assumed that the film thicknesses of the several layers formed in the initial stage fluctuate by about 5% and then the designed values are obtained. On the other hand, in the multilayer optical filter 20 (O) of the present invention, the wavelength shift is 0.1n.
It is within m and almost overlaps with the design value. From this result, it is estimated that the thickness error of each layer on the relaxation layer 4 is within 0.05%. That is, the error in the initial film thickness occurs only in the relaxation layer 4, and the error is set to a thickness that does not affect the transmission characteristics. Therefore, it is possible to determine that there is substantially no deviation from the design value. It is an optical filter 20.

FIG. 5 shows, as an example of an optical component using the multilayer optical filter 20 of the present invention, an optical fiber 32 and a lens 3.
An example in which the optical filter module 30 is configured in combination with 1 will be shown. The filter can be arranged in exactly the same manner as in the conventional example.

[0023]

As described above, in the multilayer optical filter 20 of the present invention, the measurement error of the film thickness occurring at the initial stage of film formation appears only in the relaxation layer 4. That is, the influence of the error is mitigated by the mitigation layer 4, and the film thickness thereof is set within the range that does not affect the filter characteristics. As a result, the film thickness of each layer laminated on the relaxation layer 4 is set within a predetermined range, and it is possible to realize the filter characteristic with almost no deviation from the design value.
Further, according to the method for manufacturing a multilayer optical filter of the present invention, it is possible to easily and reproducibly obtain the multilayer optical filter 20 having the characteristic that there is almost no deviation from the design value.

[Brief description of drawings]

FIG. 1 is a diagram showing a film configuration of a multilayer optical filter according to an embodiment of the present invention.

FIG. 2 is a process sectional view showing a method for manufacturing a multilayer optical filter of the present invention.

FIG. 3 is a diagram showing changes in substrate temperature in the method for manufacturing a multilayer optical filter of the present invention.

FIG. 4 is a transmission characteristic diagram of each multilayer optical filter according to an embodiment of the present invention and a conventional example.

FIG. 5 is a diagram showing a configuration of an optical filter module using the multilayer optical filter according to the embodiment of the present invention.

FIG. 6 is a cross-sectional view showing a film configuration of a conventional multilayer optical filter.

[Explanation of symbols]

1 substrate 2,12 High refractive index layer 3, 13 Low refractive index layer 4 Relaxation layer 5 Multi-layer film 10 Conventional multilayer optical filter 20 Multilayer optical filter of the present invention 30 Optical filter module 31 lens 32 optical fiber

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Claims (7)

[Claims] 1. A multi-layer film comprising a substrate and a multi-layer film, which is provided in contact with one surface of the substrate and in which a plurality of thin films of dielectric materials having different refractive indexes are repeatedly laminated, the multi-layer film being in the vicinity of the substrate. A multilayer optical filter having a relaxing layer made of a single material for relaxing an influence of a film thickness measurement error due to a temperature change in a lower portion of the film. 2. The multilayer optical filter according to claim 1, wherein the thickness of the relaxing layer is in the range of 1 to 10 μm. 3. The dielectric material is of two types, one of which forms a high-refractive index layer and the other of which forms a low-refractive index layer, and the layers are alternately laminated, and the relaxation layer contacts the substrate. 3. The multilayer optical filter according to claim 2, wherein the multilayer optical filter is in the lowermost layer and is made of either one of the two types of the dielectric materials. 4. The material of the high refractive index layer is either tantalum oxide (Ta ₂ O ₅ ) or titanium oxide (TiO ₂ ), and the material of the low refractive index layer is silicon oxide (Si).
4. The multilayer optical filter according to claim 3, which is O ₂ ). 5. The optical thickness d of the relaxing layer is d = 2m (λ / 4) (m is a positive integer) with respect to the wavelength λ of incident light. Item 4. The multilayer optical filter according to item 4. 6. A first relaxing layer is formed on one surface of the substrate to mitigate the influence of the measurement error of the film thickness due to the temperature change.
And a second step of alternately laminating thin films of two kinds of dielectric materials having different refractive indexes on the upper surface of the relaxation layer, the first step is that the temperature of the substrate is The multi-layer film is characterized in that it is continuously performed until it rises from its initial value to a substantially steady value, and the second step is performed at a temperature of the substantially steady value subsequent to the first step. Optical filter manufacturing method. 7. An optical component comprising the multilayer optical filter according to claim 1 as a constituent element.