JPH09258153A - Magneto-optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magneto-optical waveguide having a desired isolator characteristic and to make its distribution uniform by using a substrate having a prescribed thickness in accordance with the factor analysis of the degradation in the yield in a photolithography stage. SOLUTION: The min. line width in a contact exposure method suitable for simultaneously exposure of a large area with high resolution is practically 2μm in a photolithography stage at the time of manufacturing the magneto- optical waveguide and, therefore, the essential requirement for obtaining pattern formation down to 2μm width in the photolithography stage is to confine the deformation rate of the substrate to <=2μm. In such a case, the deformation rate of the magneto-optical waveguide is expressed by the warpage rate of the substrate added to the deformation rate by a stress. If the stress is zero, the warpage of the substrate eventually remains as it is even after the formation of the epitaxial film. Then, if the substrate is formed to >=0.8mm thickness, the total deformation rate of the substrate is confined to <=2μm. The thickness of the substrate is required to be 0.8 to 3mm from the points described above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical waveguide such as an optical isolator used for optical communication and optical signal processing.

[0002]

2. Description of the Related Art A conventional bulk type optical isolator using the magneto-optical effect is a solution of gadolinium gallium garnet (Gd ₃ Ga ₅ O ₁₂ , GGG, etc.) and its partial element substitution on a garnet crystal substrate. The garnet type magnetic thin film formed by the phase growth method is mainly used as a magneto-optical material,
After growing to a predetermined film thickness, the substrate was removed by polishing or the like and used as a Faraday rotator. In recent years, a waveguide type Faraday rotator capable of in-plane optical integration has been proposed. This is because the Faraday rotator is a waveguide having a light wave confinement structure, which enables connection with other waveguides without a lens. The direction of the magnetic field enables the applied magnetic field intensity to be reduced, and the volume of the external magnetic circuit can be reduced while the selectivity of the external magnetic circuit is increased, so that the external magnetic circuit can be incorporated into the planar optical circuit.

The manufacturing process of this waveguide type Faraday rotator consists of forming a lower clad layer and a core layer on a substrate, then forming a core ridge, forming an upper clad layer, cutting out, forming a mirror end face, antireflection processing and the like. However, in the core ridge formation process in this manufacturing process, in order to form a core ridge having a width and height of the order of several μm,
Usually, it is performed by combining photolithography and etching. The photolithography process includes a batch exposure method for exposing the entire surface of the sample and a step exposure method for exposing a part of the sample. The step exposure method is suitable for repeated formation of the same pattern in a minute area and is used for VLSI production, but is not suitable for optical circuit pattern formation that requires exposure of a large area compared to VLSI. The batch exposure method is mainly used for this. The collective exposure method also includes two methods, a contact method and a projection exposure method. In the contact exposure method, the pattern is transferred with a size of 1: 1 with respect to the pattern drawn on the mask, and the minimum drawable line width is said to be practically 2 μm. In the projection exposure method, a lens system is used for 1: 2, 1
Reducing projection exposure such as 5 is common, and the minimum drawable line width also becomes thinner as the reducing magnification increases. In the step exposure method,
Reduction projection exposure with a larger reduction ratio such as 1:10 is adopted. The core size of the waveguide to be patterned is on the order of several μm, and in the above various exposure methods, the required smoothness and flatness of the substrate are determined according to the size of the pattern to be transferred. The most lenient condition is the contact exposure method, and in the projection exposure method, the conditions for the smoothness and flatness of the substrate required according to the depth of focus of the lens system become tighter as the reduction magnification increases.

In general, the substrate often has some warp. Also, when a garnet-type magnetic thin film is formed on a substrate, it is compressed or pulled due to the difference in the coefficient of thermal expansion between the substrate and the garnet-type magnetic thin film, and if it is crystalline, due to the difference in the lattice constant between the substrate and the garnet-type magnetic thin film. It causes stress and causes deformation of the entire substrate. When the resist mask layer is patterned by such deformation of the entire substrate, an air gap is created between the waveguide substrate and the resist mask layer, and the line width of the transferred pattern is completely adhered according to the gap width of the air gap. It spreads compared to when it was done. Therefore, in a commonly used circular substrate, the substrate itself is deformed convexly around the center of the substrate,
In addition, the stress causes deformation around the center of the substrate. Then, depending on the distribution of the line width of this transfer pattern, the characteristics of the magnetic waveguide also cause a distribution in the substrate, which has been a cause of reducing the yield of the magneto-optical waveguide having the desired characteristics. Conventionally, the factor analysis of the yield reduction in such a photolithography process has been insufficient, and therefore the countermeasure against this factor has not been sufficient.

[0005]

An object of the present invention is to use a substrate having a predetermined thickness based on the factor analysis of the yield reduction in the photolithography process, and thereby to obtain the line width distribution of the transfer pattern in the photolithography process. Within the specified range, and has the desired isolator characteristics,
The present invention provides a magneto-optical waveguide whose distribution is uniform.

[0006]

According to the present invention, a magnetic crystal thin film is grown on a substrate, and a part of the thin film is processed into an optical waveguide to induce a magneto-optical effect in propagating light under an external magnetic field. In the optical waveguide, the thickness of the substrate is 0.8-3 mm
The gist of the present invention is a magneto-optical waveguide characterized by the following.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the photolithography process for manufacturing a magneto-optical waveguide, the minimum line width in the contact exposure method, which has high resolution and is suitable for large-area batch exposure, is practically 2 μm.
Therefore, in order to obtain the pattern formation up to the width of 2 μm in the photolithography process, it is a necessary condition that the deformation amount of the substrate is 2 μm or less.

The GGG substrate used for the magneto-optical waveguide is obtained by cutting a GGG single crystal obtained by the pulling method into a wafer shape with a wire saw or the like, lapping both surfaces of the wafer, and then performing mirror polishing processing. . The warp amount of the GGG substrate with a diameter of 50 mm thus obtained was adjusted to various thicknesses (0.
4, 0.5, 0.6, 0.8, 1.0, 1.4, 2.0, 3.0, and 3.3 mm) are shown, and the results are shown by the triangle marks in FIG. The numerical value marked with ▲ in Fig. 1 is the average value of the average warp amount of 100 substrates for each thickness. As shown in Fig. 1, a GG with a thickness of 0.4 mm, which was conventionally used to manufacture a waveguide type Faraday rotator, was used.
The G substrate has a problem that the warp amount of the substrate itself is larger than 2 μm. Therefore, it was necessary to manufacture the Faraday rotator by delicately controlling the difference in lattice constant between the liquid phase epitaxial thin film and the GGG substrate so as to cancel the warp of the substrate. Also, as shown in FIG. 1, the thinner the substrate, the greater the warp of the convex shape, and the thicker the substrate, the less the warp. However, when manufacturing the liquid phase epitaxial film, the thickness of the substrate is more than 3 mm.
It has been revealed from the present invention that when a GGG substrate is used, it is difficult to produce a thin film with a high yield because the substrate itself is heavy and it comes off from a holder that supports the substrate during thin film formation.

Next, the deformation of the substrate due to the stress caused by the difference in lattice constant between the substrate and the magnetic crystal thin film was examined. When the thickness of the substrate is t, the Poisson's ratio of the substrate material is ν, the stress generated between the substrate and the thin film is σ, the film thickness of the thin film is d, the substrate radius r, and the Young's modulus of the substrate material is E, the substrate strain amount δr Is expressed by equation (1). δr = 3 (1−ν) σdr ² / (Et ² ) ... (1) The relationship between the substrate strain amount δr derived from the equation (1) and the substrate thickness t is shown by the black squares in FIG. In the figure, the Poisson's ratio ν and Young's modulus E of the GGG substrate are shown as ν = 0.281 and E = 2.74x10 ¹² dyn.
e / cm ^2, and the stress σ generated on the substrate and the thin film is 0.
The substrate radius was 25 mm with 10 ⁸ dyne / cm ² corresponding to 001 Å. From the figure, it can be seen that the deformation amount Δr of the substrate is rapidly reduced as the thickness t 1 of the substrate is increased.

Therefore, the amount of deformation of the magneto-optical waveguide is expressed by adding the amount of substrate warp and the amount of deformation due to stress.
If the stress is 0, the warp of the substrate shown in FIG. 1 remains as it is even after the epitaxial film is formed. The difference in lattice constant between the substrate and the thin film when forming the garnet type magnetic thin film is
It can be controlled by precisely controlling the growth temperature of the thin film during liquid phase epitaxy, but even if the growth temperature is set to a target with a lattice constant difference of 0Å, the temperature control accuracy of the electric furnace for epitaxial film production is limited. Due to this, the current situation is that the target value deviates from 0.001Å. The total deformation amount of the substrate after thin film formation when the stress (10 ⁸ dyne / cm ² ) corresponding to the lattice constant difference of 0.001 Å was applied to the substrate warpage amount is shown by the ● mark in FIG. As a result, it can be seen that when the thickness of the substrate is 0.8 mm or more, the total amount of deformation of the substrate can be 2 μm or less. From the above points, the thickness of the substrate needs to be 0.8 to 3 mm. When a substrate with a diameter of 75 mm is used, the average warp amount of the substrate and the lattice constant difference of 0.00
When the stress corresponding to 1 Å (10 ⁸ dyne / cm ² ) is applied, the total warpage amount is 2 μm or less.
The thickness of the substrate is preferably 2 mm or more, and when the thickness of the substrate exceeds 3 mm, the substrate falls during the growth. Therefore, the thickness of the substrate is preferably 1.2 to 3 mm.

The magneto-optical waveguide is manufactured by the known method using the above substrate. As an example, a lower clad layer and a core layer are sequentially formed on the substrate by the liquid phase epitaxial growth method, and then a SiO ₂ film is formed on the core layer. Next, spin coat the photoresist, heat cure the photoresist, and apply a titanium thin film on the entire surface of the substrate with DC.
It is formed by magnetron sputtering. Next, a photoresist mask for processing the titanium thin film is formed by a photolithography method using a contact exposure device, and then the titanium thin film is processed by argon ion beam etching. Is processed to form three types of patterns of a two-layer laminated mask composed of a titanium thin film and a photoresist and having a width of about 2, 3, 4 μm and a height of about 5 μm. After that, the gas species is switched to argon again, the garnet film is processed by the ion beam, and the etching is performed to a depth of about 4 μm.
The remaining resist and SiO ₂ film are completely removed by oxygen plasma ashing, hot phosphoric acid and mild hydrofluoric acid. Further, an upper clad layer is formed thereon by a liquid phase epitaxial method to produce a waveguide substrate. From this waveguide substrate, strips were cut in the direction perpendicular to the waveguide using a dicing saw, the end faces of the waveguide were polished, and the rotation angle of the polarization plane was 45 °.
A waveguide array with a waveguide length of about 3 mm is produced. Then, an antireflection coating is applied to the end faces of the waveguide array to produce a magneto-optical waveguide.

[0012]

EXAMPLES The present invention will be described in detail below with reference to examples and comparative examples, but these do not limit the present invention. Example A lower clad layer having a thickness of 6 μm and a core layer having a thickness of 4 μm were sequentially formed on a GGG substrate having a substrate diameter of 50 mm and a thickness of 1 mm by a liquid phase epitaxial growth method. At this time, the melt composition at the time of forming the lower cladding layer is La ₂ O ₃ : Y ₂ O ₃ : Fe ₂ O ₃ : by weight.
_{Ga 2 O 3: PbO: B} 2 O 3 = 0.202: 0.690: 7.034: 0.255
: 100: 2.450, and the growth temperature was 899 ° C. The melt composition of the core layer is La ₂ O ₃ : Y ₂ O ₃ : Fe in weight ratio.
₂ O ₃ : Ga ₂ O ₃ : PbO: B ₂ O ₃ = 0.196: 0.701: 7.097:
0.170: 100: 2.451 and the growth temperature was 896 ° C.
The warp amount of the substrate at this time was 2 μm or less. After this,
A 0.2 μm-thick SiO ₂ film was formed on the core layer by a sputtering method, and then a photoresist having a thickness of about 4.5 μm was spin-coated. Next, after hardening the photoresist at about 200 ° C., a titanium thin film was formed on the entire surface of the substrate by DC magnetron sputtering to a thickness of about 1 μm while cooling the substrate.
After that, a photoresist mask for processing the titanium thin film was formed by a photolithography method using a contact exposure device, and subsequently the titanium thin film was processed by argon ion beam etching. Further, the gas species is switched to oxygen, and the resist under the titanium thin film is processed, and the width of the titanium thin film and the photoresist is 2, 3, 4 μm, and the height is about 5
Three types of patterns of a two-layer laminated mask of μm were prepared. Then, the gas species was switched to argon again, the garnet film was processed by an ion beam, and etching was performed to a depth of 4 μm. After the remaining resist and SiO ₂ film were completely removed by oxygen plasma ashing, hot phosphoric acid and mild hydrofluoric acid, an upper clad layer was further formed to a thickness of 6 μm by the LPE method. The melt composition and growth temperature of the upper clad layer were the same as those of the lower clad layer. A strip length is cut from the fabricated waveguide substrate in the direction perpendicular to the waveguide using a dicing saw, and the waveguide end face is polished to produce a waveguide length 3 with a rotation angle of the polarization plane of 45 °.
A millimeter-waveguide array was fabricated. An antireflection coating was applied to the end face of this waveguide array, the isolator characteristics of each waveguide were measured, and the distribution of the isolator characteristics was evaluated. 20 in FIG.
The distributions of the insertion loss value and the extinction ratio of the isolator characteristics measured for the example are shown. It can be seen from FIG. 2 that the distributions of the insertion loss value and the extinction ratio are uniform.

Comparative Example 1 The insertion loss value of the isolator characteristic and the distribution of the extinction ratio of the waveguide array fabricated under the same conditions as those of the example except that the substrate thickness was 0.4 mm and other fabrication conditions were the same. did. The results are shown in FIG. 3, and it can be seen that the distribution is non-uniform. The amount of deformation of the substrate after forming the core layer at this time was 5 μm.

Comparative Example 2 An attempt was made to manufacture a waveguide type isolator by setting the thickness of the substrate to 3.3 mm and the other manufacturing conditions being the same as those of the example. However, in the liquid phase epitaxial film manufacturing process, the formation of the epitaxial film was performed. It was not possible to manufacture an epitaxial film because the substrate was removed from the substrate supporting holder in the film.

[0015]

According to the present invention, the distribution of magneto-optical characteristics in the substrate can be made uniform, and the yield of the magneto-optical waveguide having desired characteristics can be easily improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a thickness of a substrate and a warp amount of the substrate when a substrate diameter is 50 mm.

FIG. 2 is a diagram showing a distribution of isolator characteristics measured in an example.

3 is a diagram showing a distribution of isolator characteristics measured in Comparative Example 1. FIG.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Naoto Sugimoto 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Masayuki Tanno 2-13-1 Isobe, Gunma Prefecture Issue Shin-Etsu Chemical Co., Ltd., Precision Materials Research Laboratory (72) Inventor Satoru Fukuda 2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu Chemical Industry Co., Ltd. Precision Materials Research Laboratory (72) Inventor Toshihiko Nagao 2-13-1 Isobe, Annaka-shi, Gunma Shin-Etsu Chemical Co., Ltd. Precision Materials Research Laboratory

Claims (1)

[Claims] 1. A magneto-optical waveguide in which a magnetic crystal thin film is grown on a substrate, a part of which is processed into an optical waveguide to induce a magneto-optical effect in propagating light under an external magnetic field.
A magneto-optical waveguide characterized in that the thickness of the substrate is 0.8 to 3 mm.