JP2006284964A - Manufacturing method of optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of an optical element using a thin plate having a rugged part formed on the surface of a substrate by dry etching and the like, wherein the surface of the substrate which lacks Li by its external diffusion is repaired and breakage of the substrate is suppressed. <P>SOLUTION: In the manufacturing method of the optical element having the substrate formed by using a material having an electrooptical effect and having the rugged part formed on the surface thereof and ≤30 μm thickness, (2) the rugged part is formed on the surface of the substrate in the state that the substrate has ≥200 μm thickness, (3) then thermal treatment is performed and (4) then the rear surface of the substrate is polished so that the substrate may have ≤30 μm thickness. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing an optical element, and more particularly to a substrate formed of a material having an electro-optic effect, where uneven portions are formed on the surface of the substrate and the thickness of the substrate is 30 μm or less. The present invention relates to a method for manufacturing an optical element having a substrate.

Conventionally, in an optical communication field and an optical measurement field, an optical element such as a waveguide type optical modulator in which an optical waveguide or a modulation electrode is formed on a substrate having an electro-optic effect has been widely used.
In order to realize a wider optical modulation frequency, it is important to match the speed of the modulation signal microwave and the light wave, and various methods have been devised so far. Specific examples include thicker buffer layers, higher aspect ratios of electrodes, and ridge structures.

In Patent Document 1 or 2 below, an optical waveguide and a modulation electrode are incorporated in an extremely thin substrate (hereinafter referred to as “first substrate”) having a thickness of 30 μm or less, and has a lower dielectric constant than the first substrate. Other substrates are bonded to reduce the effective refractive index with respect to the microwave, thereby achieving speed matching between the microwave and the light wave.
JP-A 64-18121 JP 2003-215519 A

By using the thinned first substrate like this, the design flexibility of the modulator is dramatically increased, and for example, an optical modulator having a wide band and a low driving voltage can be manufactured without using a buffer layer. Become. Furthermore, from the viewpoint of reducing the propagation loss of microwaves, it is synonymous with the use of a material having a low dielectric constant for the substrate. Specifically, the first substrate is specifically set to 150 μm or less, particularly in the region of 26 GHz or more. The fact that the influence of the dielectric loss (tan δ) of the wave itself can be reduced is disclosed by the following non-patent document and applied to widening the bandwidth of the modulator.
Y. Yamane et.al., “Investigation of sandblast machining techniques for broadband LN modulators”, Sumitomo Osaka Cement Technical report 2002, pp49-54 (2003)

On the other hand, conventionally, when an optical waveguide is formed in an optical element such as a waveguide type optical modulator, a refractive index of the diffusion portion is made higher than that of other portions by thermally diffusing a metal such as Ti into the substrate at a high temperature. Raised and confined light. Alternatively, a metal mask is applied to a predetermined portion, and proton ions are exchanged in phosphoric acid at 200 to 300 ° C. to change the refractive index, thereby forming a waveguide.
For this reason, the cross-sectional shape of the optical waveguide has an inverse semicircular shape diffused from the surface, whereas the cross-sectional shape of the optical fiber for introducing and deriving the light wave into the optical modulator is circular. There arises a problem that the coupling loss with the optical fiber becomes large.
In order to solve these problems, in Patent Document 3 below, as shown in FIG. 1, a single crystal dielectric lithium niobate (hereinafter referred to as “LN”) having an electro-optic effect and having a refractive index different depending on crystal orientation. And a substrate (thickness 600 μm) 4 and an LN thin plate (thickness 7 μm) 1 are joined by a silicon oxide film 3 while changing the crystal orientation to form an optical waveguide 2 using a ridge on the LN thin plate. Has been.
JP-A-6-289341

  As an optical waveguide using a ridge as shown in Patent Document 3, as shown in FIG. 1 (a), a rib-type optical waveguide in which an optical waveguide portion is protruded and other substrate regions are formed thin, As shown in FIG. 1B, there is a ridge type optical waveguide having a groove formed around the optical waveguide portion. These are referred to as a rib type or a ridge type when referring to individual features, but these are generally collectively referred to as a ridge type optical waveguide.

In forming the ridge-type optical waveguide, a region other than the region to be etched on the substrate surface is protected with a photoresist film or the like, and processing by dry etching or chemical etching, laser processing, or the like is performed.
However, in such processing by dry etching, Li on the substrate surface such as lithium niobate or lithium tantalate diffuses out of the substrate due to heat during etching or laser processing, sputtering, etc. It changes into a substrate lacking Li on the surface. As a result, there has been a problem that the DC drift of the optical modulator increases, or that the optical loss increases due to the change in the refractive index of the optical waveguide and the change in the surface shape.

  As a method for improving such a deficiency state of Li, the substrate is also heat-treated at a high temperature around 1000 ° C., but in the optical element using the thin plate having a thickness of 30 μm or less as described above, When a thin plate is heated at a high temperature, there is a risk that the thin plate may be damaged due to thermal strain or the like. In addition, when a thin plate is normally used, a reinforcing plate is bonded to the back surface of the thin plate via an adhesive layer in order to increase the mechanical strength of the substrate. Stress due to the difference in thermal expansion coefficient between the layer and the reinforcing plate is generated, and the thin plate is damaged.

  The problem to be solved by the present invention is to solve the above-mentioned problems, and in a method for manufacturing an optical element using a thin plate having a concavo-convex portion formed on the substrate surface by dry etching or the like, a substrate deficient due to out-diffusion of Li An object of the present invention is to provide a method of manufacturing an optical element that repairs a surface and suppresses breakage of a substrate.

  In order to solve the above-mentioned problems, the invention according to claim 1 is a substrate formed of a material having an electro-optic effect, and has a concavo-convex portion formed on the substrate surface and a thickness of the substrate of 30 μm or less. In the method of manufacturing an optical element having a substrate, a concavo-convex portion is formed on the surface of the substrate in a state where the thickness of the substrate is 200 μm or more, and after heat treatment, the substrate is made 30 μm or less in thickness. Therefore, the back surface of the substrate is polished.

  The invention according to claim 2 is characterized in that, in the optical element manufacturing method according to claim 1, after the substrate is polished, a reinforcing plate is attached to the back surface of the substrate through an adhesive layer. The “adhesive layer” according to the present invention is not limited to one using an adhesive, but includes one obtained by joining a substrate and a reinforcing plate by a direct joining method.

  According to a third aspect of the present invention, in the method for manufacturing an optical element according to the first or second aspect, the concavo-convex portion of the substrate surface forms at least a part of an optical waveguide.

  The invention according to claim 4 is the method for manufacturing an optical element according to any one of claims 1 to 3, wherein the substrate includes at least one of lithium niobate or lithium tantalate.

  The invention according to claim 5 is the method for manufacturing an optical element according to any one of claims 1 to 4, wherein the heat treatment is performed at a temperature of 900 to 1200 ° C.

  In the invention according to claim 6, in the method of manufacturing an optical element according to claim 4, the Li / Nb or Li / Ta composition ratio of the substrate surface at the end of manufacture is equal to or higher than the composition ratio at the start of manufacture. It is 0.7 or more.

  In the invention according to claim 7, in the substrate formed of a material having an electro-optic effect and an uneven portion formed on the surface of the substrate, and the thickness of the substrate is 30 μm or less, the substrate The uneven portion on the surface is characterized in that the thickness of the substrate is formed in a state of 200 μm or more, and the substrate is made 30 μm or less after heat treatment.

  In the invention according to claim 8, in the optical element according to claim 7, the composition ratio of Li / Nb or Li / Ta on the surface of the substrate is equal to or higher than the initial composition ratio of the substrate used for production or 0.7. It is the above.

  According to the first aspect of the present invention, in order to reduce the thickness of the substrate to 30 μm or less after forming a concavo-convex portion on the surface of the substrate in a state where the thickness of the substrate is 200 μm or more and then performing heat treatment. Since the back surface is configured to be polished, Li deficiency due to dry etching or the like can be repaired by heat treatment, and the thickness of the substrate can also withstand the heat treatment, so that the substrate can be prevented from being damaged. .

  According to the invention of claim 2, after the substrate is polished, the reinforcing plate is adhered to the back surface of the substrate through the adhesive layer, and therefore the reinforcing plate is bonded after the heat treatment according to claim 1, Further, the thermal stress is generated due to the difference in thermal expansion coefficient between the adhesive layer and the reinforcing plate, and the substrate is also prevented from being damaged.

  According to the invention of claim 3, since the uneven portion on the surface of the substrate forms at least a part of the optical waveguide, it is possible to provide an optical waveguide in which the deficiency of Li is repaired by heat treatment subsequent to dry etching or the like. It becomes possible. Thereby, an optical element with improved DC drift and optical loss can be provided.

  According to the invention of claim 4, since the substrate contains at least one of lithium niobate or lithium tantalate, the present invention effectively eliminates the Li deficiency even when Li outdiffusion occurs particularly during heat treatment. It can be repaired and the damage to the substrate can be suppressed.

  According to the invention of claim 5, since the heat treatment is performed at a temperature of 900 to 1200 ° C., Li outdiffusion is likely to occur. However, by applying the present invention, Li deficiency is effectively reduced from the inside of the substrate. It can be repaired and the damage to the substrate can be suppressed.

  According to the invention of claim 6, since the composition ratio of Li / Nb or Li / Ta on the substrate surface at the end of production is equal to or higher than the composition ratio at the start of production or 0.7 or more, DC drift due to Li deficiency In addition, it is possible to provide an optical element in which an increase in light loss is suppressed.

  According to the seventh aspect of the present invention, there is provided a substrate formed of a material having an electro-optic effect, and an optical element in which uneven portions are formed on the surface of the substrate, and the thickness of the substrate is 30 μm or less. The surface irregularities are formed in a state where the thickness of the substrate is 200 μm or more, and after the heat treatment, the substrate is made 30 μm or less in thickness. Nevertheless, it is possible to provide an optical element in which the deficiency of Li is repaired and the substrate is less damaged.

  According to the invention according to claim 8, since the composition ratio of Li / Nb or Li / Ta on the substrate surface is equal to or higher than the initial composition ratio of the substrate used for manufacturing or 0.7 or more, the DC drift due to the lack of Li In addition, it is possible to provide an optical element in which an increase in light loss is suppressed.

Hereinafter, the present invention will be described in detail using preferred examples.
The present invention is a substrate formed of a material having an electro-optic effect, wherein an uneven portion is formed on the surface of the substrate, and the optical element includes a substrate having a thickness of 30 μm or less. In order to reduce the thickness of the substrate to 30 μm or less after forming a concavo-convex portion on the substrate surface by dry etching, chemical etching, laser processing, or the like in a state where the thickness of the substrate is 200 μm or more, and then performing heat treatment The back surface of the substrate is polished.

FIG. 2 is a diagram showing a method for manufacturing an optical element according to the present invention.
FIG. 2 (1) shows a substrate 1 made of a material having an electro-optic effect. The thickness of the substrate 1 is set so that sufficient Li can be replenished in the subsequent heat treatment and the substrate is prevented from being damaged during the heat treatment. A substrate of 200 μm or more is used.
As materials having an electro-optic effect, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used, particularly during heat treatment and dry etching. The present invention can be suitably used for lithium niobate and lithium tantalate in which Li outdiffusion or deficiency occurs due to collision of high energy particles.

For example, dry etching is preferably used to form an uneven portion such as a ridge on the surface of the substrate.
A film intended to protect a region other than etching, called an etching mask, is formed on the surface of the substrate with a material such as photoresist, Ni, Ti, etc., and is then placed in a dry etching apparatus (RIE or ECR) using plasma. Fluorocarbon-based gas such as CF ₄ and Ar gas are introduced into the apparatus, plasma is generated in the apparatus to generate CF-based radicals and Ar ions, and physical sputtering and CF-based by collision with the substrate with Ar ions. Chemical etching using radicals is performed to etch the regions not protected by the etching mask to form uneven portions.
In order to form the optical waveguide with the ridge 2 or the like, it is necessary to form a convex portion or a concave portion of about 1 μm to several tens of μm (see FIG. 2B).

  Next, the substrate on which the concavo-convex portion is formed is placed in a heater, and the substrate is heated in the range of 900 to 1200 ° C. (see FIG. 2 (3)). The heating time depends on the heating temperature, but is preferably about 3 to 10 hours. When the heating temperature is less than 900 ° C., internal diffusion of Li is difficult to proceed, and it takes a long time to compensate for the lack of Li on the substrate surface, leading to an increase in production cost, which is not realistic. . In addition, when heat treatment is performed at a temperature higher than 1200 ° C., outdiffusion of Li becomes remarkable, so that it is difficult to sufficiently compensate for the Li deficiency state.

  In addition, by heating the substrate in a Li atmosphere or an atmosphere containing water vapor at the time of heat treatment, it becomes possible to suppress the outdiffusion of Li due to heating, and the deficiency of Li can be effectively repaired. Furthermore, it is also possible to prevent out-diffusion of Li by accommodating the substrate in a Pt box and performing heat treatment.

  Normally, when dry etching treatment is performed, the composition ratio of Li / Nb or Li / Ta on the substrate surface is reduced to about 0.2 to 0.3. It is possible to manufacture an optical element in which the composition ratio of the substrate at the time is restored to more than that, preferably 0.7 or more, and an increase in DC drift and light loss due to lack of Li is suppressed.

Next, as shown in FIG. 2 (4), the back surface of the substrate is polished to adjust the thickness d of the substrate to 30 μm or less.
As a substrate polishing method, a thermoplastic resin is applied to the substrate surface, a polishing jig is attached, and the back surface of the substrate is polished by a lapping machine polishing machine.

The reinforcing plate 4 is bonded to the thinned substrate 1 through the adhesive layer 3 (see FIG. 2 (5)).
Various materials can be used as the reinforcing plate 4. For example, in addition to using the same material as the thin plate, a material having a lower dielectric constant than the thin plate such as quartz, glass, alumina, etc. It is also possible to use a material having a crystal orientation different from that of the thin plate. However, it is preferable to select a material having a linear expansion coefficient equivalent to that of the thin plate in order to stabilize the modulation characteristics of the optical modulator with respect to temperature changes.

As the adhesive layer 3, various adhesive materials such as epoxy adhesives, thermosetting adhesives, ultraviolet curable adhesives, solder glass, thermosetting, photocurable or photothickening resin adhesive sheets are used. It is possible to use.
Furthermore, adhesion by a direct joining method is also possible. The direct bonding method utilizes the fact that the bonded surfaces are washed with acid or alkali chemicals and the cleaned surfaces are bonded together under an appropriate load, so that they are adsorbed via hydrogen bonds. In general, the bonding force is improved by a subsequent heat treatment, and a temperature of 300 ° C. or higher is preferably used. Further, the direct bonding method can be performed by bonding the bonding surfaces after plasma cleaning, and in this case, a practically sufficient strength is exhibited even at room temperature.

Finally, if necessary, a buffer layer made of SiO ₂ necessary for an optical modulator or the like is formed, and a modulation electrode such as a signal electrode or a ground electrode is formed by a Ti / Au electrode pattern formation or a gold plating method. .

Based on the method for manufacturing an optical element of the present invention, an optical modulator was prepared as follows.
A Z-cut LN substrate having a thickness of 500 μm was dry-etched using ECR-310E (manufactured by Anerva Co., Ltd.) to form a 3 μm ridge portion shown in FIG. As the dry etching conditions, a resist was used as an etching mask, and Ar and C ₂ F ₆ were introduced at a mixing ratio of 9: 1 as an etching gas.

  Next, using a heater (horizontal system furnace: manufactured by Koyo Thermo System Co., Ltd.), the substrate was heated to 990 ° C., maintained for 5 hours, and then naturally cooled. The heating conditions are as follows: from room temperature to 990 ° C. in 3 hours, held for 5 hours, and then naturally cooled. Water vapor (dew point temperature of 30 ° C.) was introduced into the atmosphere at 990 ° C. holding.

Thereafter, a thermoplastic resin is used on the substrate surface, a polishing jig is joined, and a lapping machine polishing machine (carrier: epoxy resin containing glass fiber, lapping agent: GC # 1200 20 wt% aq), speed 35 min ⁻¹ , lapping pressure. Polishing is performed under conditions of 12.75 to 9.81 kPa until the thickness of the substrate becomes approximately 50 μm or [finished thickness] of approximately 5 μm to 10 μm. Thereafter, precision mirror polishing was performed to a set thickness by mechanochemical polishing (CMP) using a nonwoven fabric as the pad material and colloidal silica as the processing liquid.

As a reinforcing plate, a Z-cut LN substrate having a thickness of 500 μm was bonded to the back surface of the substrate using an adhesive, and then the polishing jig was removed.
Finally, a SiO ₂ film having a thickness of 0.5 μm was formed on the substrate surface, and a modulation electrode and a ground electrode were formed on the substrate surface with a height of 5 μm.

The characteristics of the optical modulator were measured. The DC drift characteristics were evaluated with temperature acceleration. The measurement conditions are a temperature of 85 ° C. and an initial applied voltage of 3.5V. In this measurement, the operating point variation ΔVt is about 1/5 of the heat treatment untreated product (unfinished product ΔVt: 8.5V, product ΔVt: 1.5V), which is equivalent to a flat plate type (planar type) that is not a ridge type. The characteristics were obtained. Moreover, the optical propagation loss was as excellent as 0.5 dB / cm.
From this, it can be understood that the method for producing an optical element of the present invention is particularly useful for an optical element using a thin plate in which an uneven portion is formed on the substrate surface by dry etching or the like.

  According to the method of manufacturing an optical element according to the present invention, in the method of manufacturing an optical element that uses a thin plate in which a concavo-convex portion is formed on the substrate surface by dry etching or the like, while repairing the substrate surface deficient due to Li outdiffusion, It is possible to provide a method for manufacturing an optical element in which damage to the substrate is suppressed.

It is the schematic of the optical element which has a rib type | mold (a) and a ridge type (b) optical waveguide. It is a figure which shows the manufacturing method of the optical element which concerns on this invention.

Explanation of symbols

1 Substrate 2 Ridge 3 Adhesive layer 4 Reinforcing plate

Claims (8)

In a method for manufacturing an optical element, which is a substrate formed of a material having an electro-optic effect, and has an uneven portion formed on the substrate surface, and the substrate has a thickness of 30 μm or less.
In the state where the thickness of the substrate is 200 μm or more, an uneven portion is formed on the surface of the substrate, and after performing heat treatment, the back surface of the substrate is polished in order to make the substrate have a thickness of 30 μm or less. A method for manufacturing an optical element.   2. The method of manufacturing an optical element according to claim 1, wherein after the substrate is polished, a reinforcing plate is attached to the back surface of the substrate through an adhesive layer.   3. The method of manufacturing an optical element according to claim 1, wherein the uneven portion on the surface of the substrate forms at least a part of an optical waveguide.   4. The method of manufacturing an optical element according to claim 1, wherein the substrate includes at least one of lithium niobate or lithium tantalate.   5. The method of manufacturing an optical element according to claim 1, wherein the heat treatment is performed at a temperature of 900 to 1200 ° C. 6.   5. The method of manufacturing an optical element according to claim 4, wherein the composition ratio of Li / Nb or Li / Ta on the surface of the substrate at the end of production is equal to or higher than the composition ratio at the start of production or 0.7 or more. A method for manufacturing an optical element. In an optical element in which a substrate formed of a material having an electro-optic effect, and an uneven portion is formed on the substrate surface, and the thickness of the substrate is 30 μm or less,
The optical element is characterized in that the uneven portion on the surface of the substrate is formed in a state where the thickness of the substrate is 200 μm or more, and after the heat treatment, the substrate is made 30 μm or less in thickness. 8. The optical element according to claim 7, wherein a composition ratio of Li / Nb or Li / Ta on the surface of the substrate is equal to or higher than an initial composition ratio of the substrate used for manufacturing or 0.7 or higher. Optical element.

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