JPS63157104A - Manufacture of band-pass filter - Google Patents

Abstract

PURPOSE:To easily hold a center wavelength value in a desired standard, after forming a film, by measuring a spectral characteristic after ending a film formation, and increasing or decreasing the film thickness of high and low refractive index substances of each layer in accordance with a shape of a ripple in a passing band and the center wavelength. CONSTITUTION:Two layers of a high refractive index substance H, and a low refractive index substance L are formed on one piece of monitor substrate, plural layers of BPFs are brought to a film formation, and thereafter, a ripple shape on its spectral characteristic and the center wavelength are decided. When the ripple shape is low in transmissivity of a long wavelength side, and the center wavelength is at a short wavelength side, an H layer is made constant and an L layer is increased, and when the center wavelength is at a long wavelength side, the L layer is made constant and the H layer is increased. Also, when the ripple shape is low in transmissivity of the short wavelength side, and the center wavelength is at the short wavelength side, the L layer is made constant and the H layer is increased, and when the center wavelength is at the long wavelength side, the H layer is made constant and the L layer is decreased. In such a way, after forming a film, the center wavelength value can be held easily in a desired standard value.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Field of Application) The present invention is directed to a band pass filter made of a dielectric multilayer film suitable for use in, for example, an optical multiplexer/demultiplexer in an optical wavelength division multiplexing transmission system. The present invention relates to a method for manufacturing a filter.

(Prior Art) Generally, a dielectric multilayer film is used in a band tube pass filter of an optical multiplexer/demultiplexer in an optical wavelength division multiplexing transmission system.

As a band/pass type filter made of such a dielectric multilayer film, when a semiconductor laser is used as the light source, a filter having 3 cavities and 23 layers as shown in the following configuration (1) is known. show.

Board IA-L-A-L-AI adhesive (or air)...
...Structure (1) Here, Ami H-L-H-2L-H-L-H However, H,
L is a high refractive index material and a low refractive index material each having an optical thickness of λ/4. λ is the center wavelength. 2L is a low refractive index material with an optical thickness of λ/2 and represents a cavity.

As an optical multiplexer/demultiplexer using such a band/pass filter, the wavelength is 810 nm to 890 nm.

FIG. 2 shows an example of use in four-wavelength multiplexing transmission of 1200 nm and 1300 nm. As shown in the figure, a band pass filter 1.2.3.4 and a low bass filter 5.5.5 are attached to a pre-optically processed glass block 6.6 using adhesive. has been done. In addition, the band and pass 9 filters 1.2.3.4 are made of two sheets of the same type, and the center wavelengths are 810 nm and 89 nm, respectively.
These are band-pass filters of 0 nm, 1200 nm, and 1300 nm. Also, low bass filter etc., 5
．． 5 have the same performance, and only one of each is used.

Now, to manufacture a band pass filter, an electron beam evaporation system is generally used, and the film thickness is controlled by using a monitor substrate and detecting the extreme value due to changes in transmittance or reflectance of the monitor substrate during evaporation. Optical monitoring method is used.

(Intermediate point to be solved by the invention) In order to satisfy the performance required for band pass filters generally used in optical multiplexers/demultiplexers, the deposition conditions during film formation, especially high-precision film thickness control. Accuracy is required. That is, errors in film thickness control during vapor deposition affect the ripple shape and center wavelength value in the passband of a single band pass filter. If large ripples occur, when using multiple stage band pass filters as optical multiplexers/demultiplexers,
The ripples are superimposed, and the overall reduction force in the passband increases. Before bonding the band pass filter with adhesive, that is, in configuration (1), light is transferred from the air to the band pass filter.
The spectral characteristics when incident on the filter are shown in FIG. 3 in terms of transmittance, and in FIG. 4 they are shown in terms of loss. Ideally, when there is almost no error in film thickness, the ripple shape in the pass band has spectral characteristics in which the transmittance has three maximum values and two minimum values, as shown in FIG.

However, the optical thickness of each layer can be controlled by detecting the extreme value of the change in transmittance or reflectance during the deposition of alternating layers of high refractive index material and low refractive index material formed on the monitor substrate. In the optical film thickness control method, as the number of layers on the monitor substrate increases, errors when capturing the extreme value of λ/4 or λ/2 in each layer are superimposed, and of course In particular, it is difficult to obtain reproducibility of film thickness accuracy at the position of the product.

In such a case, the spectral characteristics of the resulting band-pass filter will have large ripples, especially due to film thickness errors during evaporation, low transmittance at the minimum value, and low transmittance in the passband. This has the disadvantage that it no longer satisfies the standard values and at the same time the passband width becomes narrower. Therefore, the reproducibility of the spectral characteristics of the obtained Ballad Φ-pass filter was poor, and it was not possible to manufacture a band-pass filter for an optical multiplexer/demultiplexer with a high yield.

In the production of band pass filters for optical multiplexers/demultiplexers, it is important to ensure that the minimum value of ripple in the pass band is 93% or more (II loss is 0.3 dB or less), and that each The aim is to adjust the accuracy of the center wavelength of the wavelength band to a high precision, for example within a range of ±4 nm. In order to achieve these goals, the film thickness of each layer of the band pass filter must be highly accurate.
The film thickness accuracy must be within ±1% with good reproducibility.

This invention was made in view of the problems of the prior art described above, and is for use in optical multiplexer/demultiplexers that have a good ripple shape in the pass band, have high transmittance, and have a center wavelength value that falls within the standard value range with high precision. It is an object of the present invention to provide a method for manufacturing a band pass filter that can produce a band pass filter with good reproducibility and high yield.

[Structure of the Invention] (Means for Solving the Problems) This invention provides a high refractive index material and a low refractive index material that are formed on a substrate and control the optical film thickness of 1/4 wavelength or an integral multiple thereof. In the manufacturing method of a band pass filter consisting of a multilayer film mainly consisting of alternating layers, we focused on the ripples and center wavelength in the passband of the spectral characteristics of the film after film formation, and found that the shape of the ripple was on the short wavelength side. Depending on whether the transmittance is low, the transmittance is low on the longer wavelength side, or the center wavelength value is located on the shorter wavelength side or longer wavelength side than the desired value,
Either the high refractive index material or the low refractive index material of each layer has the same constant film thickness as the film that has already been formed, and the other types of constituent materials are uniformly increased or decreased at a constant rate for each layer. This method of manufacturing a band pass Q filter is characterized by gradually improving the ripple shape in the pass band to one with high transmittance and keeping the center wavelength value within a desired standard value range. .

(Function) According to the present invention, a band pass filter is formed by storing a dummy substrate in a vapor deposition apparatus in advance, and after the vapor deposition is completed, the spectral characteristics are measured, and the shape of the ripple in the pass band and the center wavelength are measured. By paying attention to the spectral characteristics and determining its spectral characteristics, in order to obtain a band pass filter with the desired ripple shape and center wavelength, it is necessary to determine the thickness of the high refractive index material and the low refractive index material in each layer. Thus, it is possible to judge whether to increase or decrease.

By performing the deposition several times gradually, it is easy to determine the optimum conditions, including the film thickness control method, to obtain a band pass filter with the desired ripple shape and center wavelength. , band pass filters that continuously satisfy predetermined standard values can be easily obtained with good reproducibility,
This contributes to improving yield.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings. The band pass filter to be formed has a film structure of 23 layers as mentioned in the above-mentioned structure (1), In the case of 3 cavities. As an example, titanium dioxide is used as the high refractive index material (H), and silicon dioxide is used as the low refractive index material (L).

In addition, the wavelengths required for optical multiplexer/demultiplexer are usually 2 to 4 wavelengths.
In the case of 4 waves, each wave is 810 nm.

Four types of band pass filters having four types of center wavelengths of 890 nm, 1200 nm, and 1300 nm are used. Therefore, in this embodiment, the center wavelength is 810 nm.
A case will be described in which the present invention is applied to a band pass filter having a band pass filter of m.

Now, the electron beam evaporation apparatus used in the manufacturing method of the present invention is constructed as shown in FIG.

That is, the crucible 33 that accommodates the vapor deposition material 30 provided in the vacuum chamber 20 is a disk-shaped crucible (not shown) that can contain a plurality of two types of titanium dioxide and silicon dioxide. When depositing a predetermined substance, it is rotated and brought to a position where the emission current from the electron gun 34 reaches.

The acceleration voltage of the electron gun 34 is, for example, 10 KV, and the emission current value is, for example, 280 mA in the case of titanium dioxide.
In the case of silicon dioxide, it is 60 mA. In the case of vapor deposition of titanium dioxide, in order to prevent titanium dioxide from being reduced and becoming a black colored film, oxygen is
Using reactive vapor deposition to introduce

Note that the substrate temperature was, for example, 300°C.

Now, the film thickness control method is an optical film thickness control method that detects an extreme value of 1/4 wavelength or an integral multiple thereof based on a change in transmittance during vapor deposition in the monitor section 11. In the monitor unit 11, 12 monitor boards 12, 13, . . .
. . . are used to control the optical thickness of each two layers and the optical thickness of one final layer from the substrate side among the 23 layers of the band pass filter.

That is, the vapor deposition film thicknesses of the high refractive index material (H) and the low refractive index material (L) (or 2L) are controlled on each monitor substrate 11,
Only the final layer made of a high refractive index material is controlled by one monitor substrate. These monitor substrates control the film thickness of the substrates 22, 23, and 24 that become products.

In the case of vapor deposition using 12 monitor substrates, FIG. 6 shows transmittance curves during vapor deposition for each monitor substrate. When a band pass filter is formed using λ□ as the control filter 35, the portion of the H1L2 layer in FIG. 6 is shown in the upper part of Fig. 7, and the enlarged view near its extreme value is shown in the lower part of Fig. 7. . 1st
The figure shows a case where the optical film thickness is λ1/4. However, as shown in the lower part of FIG. 7, in reality, it is not possible to control λ exactly to /4, and the deposition is stopped when the extreme value has passed to a certain extent. This is shown in FIG. 5, but when the light from the light source 31 is received by the light receiving element 32 through the control filter 35 and the change in transmittance is detected, noise etc. are superimposed, so it stops at exactly the extreme value of λ1/4. This poses a problem in terms of reproducibility. Changes in the amount of light during vaporization are counted, for example, at intervals of 200 m5ec, and in order to distinguish extreme values from noise, after a certain number of counts have elapsed, vapor deposition is stopped after confirming that the extreme value has passed. Therefore, the optical thickness of H formed on the monitor substrate is slightly thicker than λ,/4. Regarding L formed on H, as shown in FIG. 7, the vapor deposition starts at the point where the extreme value of H has passed, and the vapor deposition ends at the point where the extreme value of L has passed. Net L
It is unknown whether the optical thickness of H is thicker or thinner than λ1/4.
It depends on the magnitude relationship between λ of L, the amount of passage at the 74 points, and λ of L, the amount of passage at the 74 points.

By the way, the nearly ideal spectral characteristics of a band pass filter, before it is bonded to a glass substrate via an adhesive, for example, with a center wavelength of 810 nm, are as shown in Figures 3 and 4. (In the figure, the shaded area indicates that it is outside the standard range).

For example, the standard values after lamination are as follows.

・Center wavelength -・-811 (+4.0, -3.0)
nm・Bandwidth (90% or more)・・・37nm or more・Ripple minimum value・・・93% or more (0,31
dB or less) Stopband attenuation...700-75
15 dB or more at 0 nm, 16 dB or more at 870-950 nm The incident angle of light after lamination is not normal incidence as shown in Fig. 2, but is an incident angle of 15''. Therefore, according to Snell's law, Before lamination, the angle of incidence from air to the multilayer film is 23@.

As shown in Fig. 7, if the "count numbers" from passing the extreme value to stopping vapor deposition are set to constant values for H and L, each layer of H in the band pass filter is shown in Fig. 6. Since it is controlled as the first layer of each monitor board, all of them are monitored under the same conditions. Therefore, each H layer has the same optical thickness, which is slightly thicker than λ1/4. Similarly, each layer in the band/bass filter has the same optical thickness. In other words, HS with one monitor board
Method of controlling only two L layers The optical thicknesses of all H layers are uniform to a constant value, and the optical thicknesses of all L layers are uniform to another constant value.

However, even if this method is simply used, it is not easy to obtain spectral characteristics that satisfy the standards shown in Figures 3 and 4, and usually the minimum transmittance value of the ripple in the passband is There are many cases where the center wavelength value falls short of the standard. The present inventors have found that those that are out of specification are classified into six types, Type I to Type II shown in Table 2. In other words, regarding the center wavelength, there are two possibilities: either it is located on the short wavelength side or it is located on the long wavelength side with respect to the standard value, and regarding the ripple shape, the minimum transmittance is on the long wavelength side of the passband. shape (type 11 type ■), shape with minimum transmittance on the short wavelength side (type ■, type ■), and good three-peak shape (type V1 type ■). Furthermore, depending on how the ripple collapses, it collapses a little.
It is classified into ``two mountains,'' which are landslides and large collapses. If the ripple shape is good, it will be out of specification depending on whether the center wavelength value is located on the shorter wavelength side (type V) or on the longer wavelength side (type ■) than the standard range. be.

Table 3 Countermeasures for Kutsual type IK/medium uj wavelength (model number (corresponding to Figure 1) Two layers of H%L are formed on one monitor board, and a total of 23 layers of band・After forming the pass filter, perform the spectral M1 constant, and if the relationship between the ripple shape and the center wavelength in the spectral characteristics corresponds to type I to type The present inventors have discovered the relationship between the relative film thicknesses of each H layer and each sublayer to form "three peaks" in the shape of a ripple.

That is, when the spectral characteristics of type 1 or type H are obtained after the film formation is completed, the thickness of each H layer is decreased at a constant rate, and the thickness of each layer is increased at a constant rate. Then, the minimum transmittance value of the ripple on the long wavelength side increases, and "three peaks" are obtained. However, if the thickness of each H layer is decreased too much and the thickness of each sublayer is increased too much, the minimum value of the ripple will appear on the short wavelength side.
After going through the "three-mountain collapse" shown in Types ■ and Type ■, it becomes a "two-mountain collapse." If the spectral characteristics of type ■ or type ■ are obtained after the completion of film formation, there is an inverse relationship to that of type I or type H.
If the thickness of each H layer is increased by a certain percentage and the thickness of each layer is decreased by another certain percentage, the minimum transmittance of ripples on the short wavelength side will increase, and "three peaks" will be obtained. It will be done. However, if the thickness of each H layer is increased too much and the thickness of each layer is decreased too much, the minimum value of the ripple will appear on the long wavelength side, and the "triple collapse" shown in type I and type II will occur. There will be two mountains. In other words, the reason why the ripple shape becomes out of specification after film formation is because the balance between the thickness of each HJi and each layer is lost, and each H layer has a thickness (
This is a case where each layer is too thin (thick). Table 1 shows the relationship between the ripple shape after film formation and the relative film thickness of each layer.

In order to satisfy the standard only for the shape of the ripple, each H
It is sufficient to relatively increase or decrease the film thickness of each layer as shown on the left side of Table 3.However, if you try to match the center wavelength within the standard, it is necessary to increase or decrease the film thickness of each H layer and each layer. Varying two of the layers would be very complicated. In this case, depending on whether the center wavelength value after film formation is on the long wavelength side or short wavelength side from the standard value, the film thickness of either each H layer or each L layer should be kept at the same value as the previous one. value, and increase or decrease the film thickness of each other layer. For example, if the shape of the ripple shown in Type I is a downward-sloping J, and the center wavelength value is on the shorter wavelength side than the standard value, the thickness of each H layer should be kept constant as before. , the thickness of each layer is uniformly increased.

In this case, as the film thickness of each layer increases, the shape of the ripple changes from the left side to the right side in the figure shown in Table 1, and the center wavelength also moves from the short wavelength side to the long wavelength side. In this way, the thickness of the L layer that satisfies the specifications for both the ripple shape and center wavelength is determined.

The right side of Table 3 shows a simple countermeasure for creating "three peaks" for each type of ripple shape and center wavelength, and also matching the center wavelengths. In other words, the same is true for type ■ and type ■, for example, type ■
In order to correct the spectral characteristics of something like .

In this way, the optimal layer thickness that satisfies both the ripple shape and center wavelength is determined.

The spectral characteristics after film formation obtained by the method of stopping the vapor deposition of each H layer at a certain count number after reaching the extreme value shown in FIG. 7, and stopping the vapor deposition of each layer at another certain count number, are as follows: We will explain how to correct the problem during the next film formation using an actual optical film thickness control method. In other words, if type I spectral characteristics are obtained after film formation, the thickness of each H layer remains constant as shown in Figure 8, and each layer changes by increasing the "count number". The film thickness can be increased. Figures 8 to 13 show the type! For types (3) to (3), an optical film thickness control method is presented when the same control wavelength λ1 is used for the two layers of HL.

In addition, when forming the second layer of L on the monitor substrate, λ
It is also possible to control the optical film thickness using a desired control wavelength of λ2 that is slightly different from λ2. In this case, the passing amount (count number) of the first layer H and second layer L on the monitor substrate at the time of vapor deposition stop may not be much different from (or be exactly the same as) the previous film formation. It has characteristics. For example, in the case of type (2) shown in FIG. 4, the method is to form a film using a control wavelength λ2 that is slightly larger than λ1. Figures 14 to 1
Figure 7 shows the first part on each monitor board for types I to ■.
An optical film thickness control method using slightly different control wavelengths for layer H and second layer L will be described. In addition, in Figures 18 and 19, in order to match only the center wavelength, the first and second layers on the monitor substrate are different from λ1, but the same control wavelength is used, and the evaporation stops are almost the same passing through. Indicates when the amount ends.

By the above method, after setting the conditions, in the case of a band 0 pass filter (81Qnm), using one wavelength of 770% m as the control wavelength, a film was formed with a count number of titanium dioxide of 22 and a count number of silicon dioxide of 12, Products having the spectral characteristics shown in FIGS. 3 and 4 have been obtained with good repeatability. As the center wavelength becomes longer and the number of silicon dioxide counts increases, the spectral characteristics become better. For example, in the case of a band 0 pass-filter (1300 nm), the count number for titanium dioxide is the same 22, but the count number for silicon dioxide is 15.

In addition, when using two different control wavelengths for each H layer and each sublayer, for example, for a band pass filter (810% m), after setting the conditions, for titanium dioxide, use 770 nm for the count number. 22, in the case of silicon dioxide, use 778 nm and the count number is 11,
Similarly, what is shown in FIGS. 3 and 4 was obtained.

In an actual optical multiplexer/demultiplexer, a band pass filter is bonded to a prism-shaped glass substrate using an adhesive (for example, Norland's 61).

An example of the spectral characteristics of the band/bass filter (810%m) after lamination is shown in Figure 20 in transmittance (%), and the same is shown in loss (dB) in Figure 21. . still,
FIG. 20 and FIG. 21 show the results of measurement after calibrating the transmittance of the configuration of substrate:adhesive:substrate (c) to 100% as a reference.

(Modified Example) In the above embodiment, the case where the band pass filter has 23 layers and 3 cavities has been described in detail, but band pass filters with other film configurations, such as 37 layers and 5 cavities, have been described in detail. Of course, the present invention can also be applied to a band-pass filter having a cavity having the following configuration (2).

Substrate IB-AφLφA・LA-Bl Adhesive (or air)...Structure (2) Here, A
Mi H, L-H, 2L, H-L, HB3 = Yoyomi L-H-
L・2H・LΦH fist L [Effect of the invention] According to this invention, for example, a band pass for an optical multiplexer/demultiplexer
Like a filter, pass band ripple and center wavelength value, etc.
For band pass filters that have strict standards for spectral characteristics and require high film thickness accuracy, the control can be easily re-adjusted after film formation and can be easily kept within the standard range. As a result, the reproducibility of film formation is good, and troubles resulting from non-standardization can be easily countered, contributing to an improvement in yield.

[Brief explanation of the drawing]

Figure 1 is a sectional view showing a general band pass filter, Figure 2 is a configuration diagram showing an optical multiplexer/demultiplexer using a band pass filter, Figures 3, 4, 20, FIG. 21 is a characteristic curve diagram showing the spectral characteristics of the band pass filter manufactured according to the present invention, FIG. 5 is a schematic sectional view showing the electron beam evaporation apparatus used in the present invention, and FIGS.
FIG. 9 is a characteristic curve diagram showing a transmittance curve of a monitor substrate during vapor deposition according to the present invention. 1.2.3.4...Band pass filter, 6...
・Glass φ block, 11.12.13... Monitor substrate, 20... Vacuum chamber, 22.23.24... Substrate to become product, 30... Evaporation material, 31... Light source, 32
... Light receiving element, 33... Crucible, 34... Electron gun. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Wavelength (nm) Figure 3! '&, (nm) Figure 4 Figure 5 'a 6 II Figure 8 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 16 Figure 18 Figure f519 Liquid length (nm) w20 Figure 2 Skin length (nm) Figure 21

Claims (5)

[Claims] (1) A band pass filter consisting of a multilayer film mainly consisting of alternating layers of high refractive index material and low refractive index material that controls the optical film thickness of 1/4 wavelength or an integral multiple thereof formed on a substrate. In the manufacturing method, the spectral characteristics of the film after the film formation are ripples in the passband and the center wavelength. When the film is located at (b) If the ripple shape has low transmittance on the longer wavelength side and the center wavelength is located on the longer wavelength side than the predetermined value, compared to the one after the above film formation, The optical thickness of the low refractive index material in each layer remains constant, and the optical thickness of the high refractive index material in each layer is uniformly decreased at a constant rate. (c) The ripple shape is formed on the short wavelength side. When the transmittance is low and the center wavelength is located on the shorter wavelength side than the predetermined value, the optical film thickness of the low refractive index material of each layer remains constant compared to the film after the above film formation, (d) The ripple shape has low transmittance on the short wavelength side and the center wavelength is on the longer wavelength side than a predetermined value. In the case where the optical film thickness of the high refractive index material in each layer remains constant, and the optical film thickness of the low refractive index material in each layer is uniformly constant, compared to the film formed after completing the above film formation. manufacturing a band pass filter characterized in that the ripple shape in the transmission band is gradually corrected to increase the transmittance and the center wavelength value is kept within a predetermined range. Method. (2) Using multiple monitor substrates, a multilayer film is formed by forming two layers of a high refractive index material and a low refractive index material on each monitor board in this order, and all layers of this multilayer film are Claim 1, wherein the optical thickness of each layer is controlled using the same monochromatic light to correct the ripple shape and center wavelength.
2. Method for manufacturing the band pass filter described in Section 1. (3) A multilayer film is formed by using a plurality of monitor substrates and forming two layers of a high refractive index material and a low refractive index material on each monitor substrate in this order, but each layer of the high refractive index material A certain monochromatic light λ_1 is used to control the optical thickness of each layer of the low refractive index material using another monochromatic light λ_2, which has a value close to λ_1 but different from λ_1, and the shape of the ripples is determined. 2. A method of manufacturing a band pass filter according to claim 1, wherein the center wavelength is corrected. (4) The method for manufacturing a band pass filter according to any one of claims 1 to 3, wherein the basic membrane structure has 23 layers and 3 cavities. (5) The method for manufacturing a band pass filter according to any one of claims 1 to 3, wherein the high refractive index substance is titanium dioxide, and the low refractive index substance is silicon dioxide.