JP2004341045A - Optical waveguide element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide element which is strong in light confinement and is excellent also in productivity, and to provide a method of manufacturing such an optical waveguide element with sufficient productivity. <P>SOLUTION: This optical waveguide element is characterized in that a ridge structure 14 extended like a channel is formed on a LiNbO<SB>3</SB>substrate 11, forming an optical waveguide part consisting of ion diffusion layers 12 which are extended at both side ends of the ridge in the ridge structure 14 and whose refractive index is higher than that of the material of the substrate. To manufacture the optical waveguide element, for example, a method of forming two grooves 13 extending in one direction by machining, etc. after forming a proton diffusion layer 12 on the LiNbO<SB>3</SB>substrate 11, so as to allow a part between the grooves 13 to have the ridge structure 14, is used. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical waveguide device and a manufacturing method thereof.
[0002]
[Prior art]
Conventionally, lithium niobate (LiNbO) for forming an optical wavelength conversion element, an optical modulation element, etc. ₃ ) Various examples using proton exchange have been reported as methods for producing optical waveguide elements (see, for example, Non-Patent Documents 1 and 2). In this method, for example, a mask having an opening that defines an optical waveguide fabrication region is formed on a lithium niobate substrate, and the Li portion of the surface portion of the lithium niobate crystal is formed through the mask. ⁺ H in acid ⁺ In this method, the difference in refractive index between the depth direction and the width direction is controlled by the proton concentration. According to this method, the refractive index distribution can be obtained relatively easily by a semiconductor process.
[0003]
Further, as another method for manufacturing an optical waveguide element, a method for forming a ridge can be mentioned. In this method, a lithium niobate crystal is pasted on a substrate with an adhesive or the like, the surface of the lithium niobate crystal is polished and thinned, and then groove processing is performed on the crystal surface by dicing or the like to form a channel-shaped ridge structure. The optical waveguide part which consists of this is formed (for example, refer patent document 1, nonpatent literature 3). According to this method, since a step-type refractive index distribution is obtained, an optical waveguide element with strong light confinement can be obtained.
[0004]
[Patent Document 1]
JP 2002-250949 A
[0005]
[Non-Patent Document 1]
Journal of Applied Physics (Journal of Applied Physics) 54 (11) November 1983 p. 6218
[0006]
[Non-Patent Document 2]
Journal of Applied Physics 66 (11) December 1989 p. 5161
[0007]
[Non-Patent Document 3]
Laser Research September 2000 p. 600
[0008]
[Problems to be solved by the invention]
When an optical waveguide device is manufactured by applying the ion diffusion method including the proton exchange described above, the refractive index distribution becomes graded, so that not only light confinement becomes insufficient, but also the device is made light-sensitive. When used as a wavelength conversion device, the overlap between the fundamental wave and the wavelength conversion wave is poor, causing a problem that mode matching efficiency is lowered.
[0009]
On the other hand, when an optical waveguide device is manufactured by forming a ridge, a step-type refractive index distribution can be obtained in a direction parallel to the substrate surface (width direction of the ridge structure) in a plane perpendicular to the waveguide direction. However, in order to obtain a step-type refractive index distribution in the substrate thickness direction, a high-precision surface polishing control technique as shown in Patent Document 1 or Non-Patent Document 3 described above is required, which increases productivity. Difficulties are recognized.
[0010]
In view of the above circumstances, an object of the present invention is to provide an optical waveguide device that is strong in light confinement and excellent in productivity.
[0011]
Another object of the present invention is to provide a method capable of producing the above-described optical waveguide device with high productivity.
[0012]
[Means for Solving the Problems]
The optical waveguide device of the present invention is LiNbO. ₃ A ridge structure extending in a channel shape is formed on a substrate, and an optical waveguide portion formed of an ion diffusion layer having a higher refractive index than that of the substrate material is formed between the both ends of the ridge in the ridge structure. Is.
[0013]
The ion diffusion layer can be formed by, for example, proton exchange or Ti diffusion.
[0014]
Further, it is desirable that a clad portion having a refractive index lower than that of the optical waveguide portion is formed on the ion diffusion layer. It is desirable that such a clad portion has a refractive index substantially equal to that of the substrate. For example, the clad portion can be formed by performing Li diffusion on the ion diffusion layer, or the substrate is formed on the ion diffusion layer. Another member can be formed by bonding.
[0015]
On the other hand, the first method for producing an optical waveguide device according to the present invention for obtaining the optical waveguide device having the above-described structure is LiNbO. ₃ After forming an ion diffusion layer on the surface portion of the substrate, the substrate is etched to a depth extending from the ion diffusion layer to at least a part of the substrate portion below the ion diffusion layer, thereby forming the ridge structure. To do.
[0016]
In addition, the second method for producing an optical waveguide device according to the present invention is LiNbO. ₃ After forming an ion diffusion layer on the surface portion of the substrate, a groove having a depth extending from the ion diffusion layer to at least a part of the substrate portion below is formed by machining to form the ridge structure. It is a feature.
[0017]
In addition, as said machining, the cutting process by a dicing saw can be used suitably. In that case, after forming the ion diffusion layer, a member different from the substrate is bonded onto the ion diffusion layer, and cutting with a dicing saw is performed while the other member is bonded. desirable. The other member may be removed from the substrate after the cutting process, or may be left in a state of being bonded to the substrate.
[0018]
The separate member is desirably bonded onto the ion diffusion layer by direct bonding at room temperature. This room temperature direct bonding is a technique in which the bonding interface is cleaned at the atomic level and bonded together. Both cleaning by ion beam irradiation and the like and adhesion by a mechanical press or the like are performed in a vacuum chamber. Joining is by nuclear power, and different types of members can be joined together.
[0019]
【The invention's effect】
In the optical waveguide device of the present invention, LiNbO ₃ A ridge structure extending in a channel shape is formed on the substrate, and an optical waveguide made of an ion diffusion layer having a refractive index higher than that of the substrate material is formed between the both ends of the ridge in this ridge structure. Although the refractive index distribution in the direction is basically a graded type, the refractive index distribution in this direction can be easily controlled by controlling the ion diffusion concentration. Therefore, this optical waveguide device can be manufactured with high productivity as compared with a conventional optical waveguide device that requires high-precision surface polishing in order to provide a refractive index step in the substrate thickness direction. On the other hand, since the refractive index distribution in the width direction (the direction parallel to the substrate surface) of the ridge structure is a step type, the optical waveguide device of the present invention can have a strong light confinement.
[0020]
Therefore, when the optical wavelength conversion element is configured from the optical waveguide element of the present invention, the overlap between the fundamental wave and the wavelength conversion wave is increased, so that the mode matching efficiency is improved, whereby high wavelength conversion efficiency can be obtained. .
[0021]
When the ion diffusion layer is formed by proton exchange, it is possible to realize a refractive index step with good optical confinement in the substrate thickness direction by optimizing the proton exchange conditions.
[0022]
In the optical waveguide device of the present invention, the refractive index distribution in the substrate thickness direction is basically a graded type as described above, but the ion diffusion layer is formed on the ion diffusion layer (that is, this layer is subjected to ion diffusion). The clad part having a refractive index lower than that of the optical waveguide part is formed on the opposite side of the original substrate material as it is, and the optical waveguide part made of this ion diffusion layer is embedded into a waveguide. For example, symmetry can be achieved in the refractive index distribution in the substrate thickness direction, and the mode matching efficiency can be further increased. In particular, when the clad portion has the same or close refractive index as that of the substrate, the symmetry of the refractive index distribution becomes better.
[0023]
On the other hand, the first method for producing an optical waveguide device according to the present invention is LiNbO. ₃ After forming the ion diffusion layer on the surface portion of the substrate, the substrate is etched to a depth ranging from this ion diffusion layer to at least a part of the substrate portion below it to form the ridge structure. Thus, the above-described optical waveguide device can be manufactured with high productivity.
[0024]
In addition, the method for producing the second optical waveguide device according to the present invention also includes LiNbO. ₃ After forming the ion diffusion layer on the surface portion of the substrate, a groove having a depth extending from the ion diffusion layer to at least a part of the substrate portion below is formed by machining to form the ridge structure. Therefore, the above-described optical waveguide device can be manufactured with high productivity.
[0025]
Here, as machining in the second method, particularly when cutting with a dicing saw is applied, after forming an ion diffusion layer, a member different from the substrate is joined on the ion diffusion layer, When cutting with a dicing saw is performed while another member is joined, the ion diffusion layer is protected by the other member, and chipping (chips) is formed at the edge of the ion diffusion layer. Can be prevented.
[0026]
Moreover, when the above-mentioned normal temperature direct joining is applied to the joining of the separate members, the following effects can be obtained.
[0027]
(1) It is possible to strongly reinforce the entire edge portion where chipping (chips) easily occurs with another member.
[0028]
(2) In direct bonding at room temperature, the bonding force can be controlled, so that it is possible to remove another member after cutting or to commercialize the optical waveguide element with the other member in close contact.
[0029]
(3) Since bonding is performed at room temperature, it is not necessary to match the thermal expansion coefficients of the separate member and the workpiece.
[0030]
(4) By selecting an appropriate separate member, a desired refractive index step can be imparted to the interface of the workpiece (optical component).
[0031]
The optical waveguide device according to the present invention can avoid damage to the ridge-processed portion by carrying a protective material such as a resist resin on both sides of the ridge structure to enhance the strength and then carrying it. . Such a protective resin can be removed by dissolving in an organic solvent or the like for use. Furthermore, by selecting a resin having an appropriate refractive index, it is possible to perform the function as an optical waveguide without being removed.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0033]
FIG. 1 shows a process of manufacturing an optical waveguide device according to the first embodiment of the present invention. The optical waveguide device of this embodiment is, for example, a waveguide type optical wavelength conversion device. First, as shown in FIG. 4A, a 0.4 mm thick, Z-cut LiNbO doped with 5 mol% of MgO. ₃ A proton diffusion layer 12 having a refractive index higher than that of the substrate material is formed on a substrate (hereinafter referred to as an MgO-LN substrate) 11.
[0034]
In this example, the MgO-LN substrate 11 is formed with a periodic domain inversion structure (not shown) in which domain inversion portions are repeated at a predetermined period in a direction perpendicular to the drawing sheet. 11, a so-called quasi phase matching type optical wavelength conversion element is manufactured. The MgO-LN substrate 11 is prepared, for example, as a 3-inch wafer, and is processed in the state of this wafer until the cutting process described later. However, in order to avoid complication, the entire substrate 11 is not shown in the drawing. Only the portion constituting one element is shown (the same applies hereinafter).
[0035]
In the present embodiment, the proton diffusion layer 12 is formed as follows. First, the surface of the MgO-LN substrate 11 is washed with an organic solvent. Next, the cleaned substrate 11 is immersed in an acid solution to perform proton exchange. At this time, pyrophosphoric acid is used as the acid, the temperature is 200 ° C., and the immersion time is 5 hours. The atmosphere is an air atmosphere. Next, this MgO-LN substrate 11 is annealed to diffuse protons. The heat treatment conditions are 350 ° C. and 20 hours in the atmosphere.
[0036]
Next, as shown in FIG. 2B, the MgO-LN substrate 11 is linearly formed in one direction (in the direction perpendicular to the drawing sheet in the X axis direction) from the surface side where the proton diffusion layer 12 is formed. Two extending grooves 13 are formed to form a ridge structure 14 extending in a channel shape in the X-axis direction. In the present embodiment, the groove processing is performed using a dicing saw. Resin-bonded diamond abrasives are used for the blade, the conditions are a rotational speed of 30000 rpm, a feed rate of 1 mm / s (seconds), a groove interval of 10 μm, and a groove depth of MgO-LN below the proton diffusion layer 12. The depth reaching the substrate 11 is, for example, 0.1 mm.
[0037]
Next, the wafer-like MgO-LN substrate 11 is cut outside each of the two grooves 13 so that only one ridge structure 14 is included, and the ridge structure 14 is cut to a predetermined length. Then, by polishing both end faces in this length direction (surfaces that become the light incident surface and the output end surface, the X surface), the optical waveguide type optical wavelength conversion element 10 shown in FIG.
[0038]
As is clear from the above, in this optical wavelength conversion element 10, a ridge structure 14 extending in a channel shape is formed on the MgO-LN substrate 11, and the refractive index higher than that of the substrate material extending between both ends of the ridge in the ridge structure 14 is obtained. An optical waveguide portion made of the proton diffusion layer 12 is formed.
[0039]
In the optical wavelength conversion element 10 having the above configuration, the refractive index distribution in the substrate thickness direction is basically a graded type. The refractive index distribution in this direction is controlled by changing the proton exchange condition and controlling the proton diffusion concentration. It can be easily controlled. Therefore, the optical wavelength conversion element 10 can be manufactured with high productivity as compared with a conventional product that requires high-precision surface polishing in order to have a refractive index step in the substrate thickness direction. On the other hand, since the refractive index distribution in the width direction (the direction parallel to the substrate surface) of the ridge structure 14 is a step type, the optical wavelength conversion element 10 can have a strong light confinement.
[0040]
Therefore, in this optical wavelength conversion element 10, the overlap between the fundamental wave and the wavelength conversion wave is increased, and the mode matching efficiency is improved, whereby high wavelength conversion efficiency can be obtained. Further, by optimizing the proton exchange conditions, it is possible to realize a refractive index step having good optical confinement in the substrate thickness direction.
[0041]
In this embodiment, after forming the proton diffusion layer 12 on the surface portion of the MgO-LN substrate 11, a groove 13 having a depth extending from the proton diffusion layer 12 to a part of the substrate portion below it is machined. Since the ridge structure 14 is formed, the optical wavelength conversion element 10 can be manufactured with high productivity.
[0042]
Hereinafter, the result of evaluating the waveguide performance of the optical wavelength conversion element 10 according to the present embodiment will be described. FIG. 2 shows an apparatus for this evaluation. As shown in the drawing, a laser beam emitted from a tunable laser diode 20 having an oscillation wavelength of 15301600 nm is input to the polarization maintaining fiber 21, and an optical amplifier comprising an erbium-doped fiber. Amplified at 22 to obtain an optical output of 10 dBm. Further, the laser beam is input to the single mode optical fiber 23, the polarization direction is controlled by the polarization controller 24, and then input to the polarization maintaining fiber 21, and directly coupled (butt-coupled) to the polarization maintaining fiber 21. The light was made incident on the optical waveguide portion (proton diffusion layer 12 in the ridge structure 14) of the optical wavelength conversion element 10. Then, the emitted light from the optical waveguide portion was condensed by the lens 26 and led to an imaging tube made of vidicon, and a near-field image (Near Field Pattern) of the emitted light was observed. Thereby, the mode field of the guided light in the optical waveguide unit was observed, and the light was guided.
[0043]
It is possible to avoid damage to the ridge-processed portion by carrying a transport or the like after embedding a protective material such as a resist resin in the grooves 13 and 13 on both sides of the ridge structure 14 to increase the strength. Such a protective resin can be removed by dissolving with an organic solvent or the like when the light wavelength conversion element 10 is used. Furthermore, by selecting a resin having an appropriate refractive index, it is possible to perform the function as an optical waveguide without being removed. The same applies to each embodiment described below.
[0044]
Next, another embodiment of the present invention will be described. FIG. 3 shows a process of manufacturing an optical waveguide device according to the second embodiment of the present invention. The optical waveguide element of this embodiment is also a waveguide type optical wavelength conversion element as an example. First, as shown in FIG. 2A, a proton diffusion layer 12 having a higher refractive index than that of the substrate material is formed on a 0.4 mm thick, Z-cut MgO-LN substrate 11 doped with 5 mol% of MgO.
[0045]
In the present embodiment, the proton diffusion layer 12 is formed as follows. First, the surface of the MgO-LN substrate 11 is washed with an organic solvent. Next, the cleaned substrate 11 is immersed in an acid solution to perform proton exchange. At this time, pyrophosphoric acid is used as the acid, the temperature is 200 ° C., and the immersion time is 5 hours. The atmosphere is an air atmosphere. Next, this MgO-LN substrate 11 is annealed to diffuse protons. The heat treatment conditions are 350 ° C. and 20 hours in the atmosphere.
[0046]
Next, as shown in FIG. 2B, an etching mask composed of an Au vapor deposition film 31 and a Ni plating layer 32 is formed on the proton diffusion layer 12 by lithography and a lift-off process. This etching mask has a shape extending linearly in one direction of the MgO-LN substrate 11 (a direction perpendicular to the drawing sheet in the X-axis direction), its width is 10 μm, and the thickness of the Au vapor deposition film 31 is 50 nm. The thickness of the Ni plating layer 32 is 1.5 μm.
[0047]
Next, as shown in FIG. ₄ ECR (electron cyclotron resonance) dry etching is performed. The etch rate at this time is set to 800 nm / h, for example. As a result, as shown in FIG. 4D, the proton diffusion layer 12 is etched leaving a striped portion covered with the etching mask.
[0048]
Next, HNO ₃ Then, the Ni plating layer 32 and the Au vapor deposition film 31 that acted as an etching mask are removed. As a result, as shown in FIG. 5E, a ridge structure 34 made of the proton diffusion layer 12 and extending in the X-axis direction is formed on the MgO-LN substrate 11. Next, the wafer-like MgO-LN substrate 11 is cut so as to include only one ridge structure 34, and is cut so that the ridge structure 34 has a predetermined length. An optical waveguide type optical wavelength conversion element 30 having a periodic domain inversion structure is obtained by polishing the surface (the light incident surface, the surface serving as the emission end surface, and the X surface).
[0049]
As is clear from the above, in the optical wavelength conversion element 30, the ridge structure 34 composed of the proton diffusion layer 12 extending in a channel shape is formed on the MgO-LN substrate 11, and the entire ridge structure has a higher refractive index than the substrate material. It functions as a wave part.
[0050]
In the optical wavelength conversion element 30 configured as described above, the refractive index distribution in the substrate thickness direction is basically a graded type. The refractive index distribution in this direction is controlled by changing the proton exchange condition and controlling the proton diffusion concentration. It can be easily controlled. Therefore, the optical wavelength conversion element 30 can be manufactured with high productivity as compared with a conventional product that requires high-precision surface polishing in order to have a refractive index step in the substrate thickness direction. On the other hand, since the refractive index distribution in the width direction (the direction parallel to the substrate surface) of the ridge structure 34 is a step type, the light wavelength conversion element 30 can have a strong light confinement.
[0051]
Therefore, in this optical wavelength conversion element 30, the overlap between the fundamental wave and the wavelength conversion wave is increased, and the mode matching efficiency is improved, whereby high wavelength conversion efficiency can be obtained. Further, by optimizing the proton exchange conditions, it is possible to realize a refractive index step having good optical confinement in the substrate thickness direction.
[0052]
In the present embodiment, after the proton diffusion layer 12 is formed on the surface portion of the MgO-LN substrate 11, the proton diffusion layer 12 is etched to form the ridge structure 34. It can be manufactured with high productivity.
[0053]
The waveguide performance of this optical wavelength conversion element 30 was previously evaluated by the apparatus shown in FIG. 2. In this case as well, the mode field of the guided light is observed in the optical waveguide portion composed of the ridge structure 34, and the light guiding is performed. A wave was confirmed.
[0054]
Next, still another embodiment of the present invention will be described. FIG. 4 shows a process of manufacturing an optical waveguide device according to the third embodiment of the present invention. The optical waveguide element of this embodiment is also a waveguide type optical wavelength conversion element as an example. The third embodiment is different from the first embodiment in which the manufacturing process is shown in FIG. 1 in that the optical waveguide is embedded, and the other points are fundamental. The same as above.
[0055]
First, as shown in FIG. 2A, a proton diffusion layer 12 having a higher refractive index than that of the substrate material is formed on a 0.4 mm thick, Z-cut MgO-LN substrate 11 doped with 5 mol% of MgO. This proton diffusion layer 12 is also formed basically in the same manner as in the first embodiment.
[0056]
Next, as shown in FIG. 2B, a Li diffusion layer 41 having a lower refractive index than that of the proton diffusion layer 12 is formed on the surface of the proton diffusion layer 12. The Li diffusion layer 41 is formed by heat-treating the MgO-LN substrate 11 in a Li-rich atmosphere. The heat treatment conditions are, for example, a temperature of 330 ° C. and a treatment time of 10 hours.
[0057]
Next, as shown in FIG. 2C, two grooves 13 extending linearly in one direction (a direction perpendicular to the drawing sheet) from the surface side of the MgO-LN substrate 11 where the Li diffusion layer 41 is formed. 13 are formed to form a ridge structure 44 extending in a channel shape in the X-axis direction. Also in this embodiment, a groove process is performed using a dicing saw. Resin-bonded diamond abrasives are used for the blade, the conditions are a rotational speed of 30000 rpm, a feed rate of 1 mm / s (seconds), a groove interval of 10 μm, and a groove depth of MgO-LN below the proton diffusion layer 12. The depth reaching the substrate 11 is, for example, 0.1 mm.
[0058]
Next, the wafer-like MgO-LN substrate 11 is cut outside each of the two grooves 13 so that only one ridge structure 44 is included, and the ridge structure 44 is cut to a predetermined length. Then, by polishing both end faces in this length direction (surfaces that become a light incident surface and an output end surface, the X plane), an optical waveguide type optical wavelength conversion element 40 having a periodic domain inversion structure is obtained.
[0059]
As is clear from the above, in this optical wavelength conversion element 40, a ridge structure 44 extending in a channel shape is formed on the MgO-LN substrate 11, and the refractive index higher than that of the substrate material extending between both side ends of the ridge in this ridge structure 44. An optical waveguide portion made of the proton diffusion layer 12 is formed. In this example, in particular, the proton diffusion layer 12 is embedded in the Li diffusion layer 41 and the MgO-LN substrate 11 having a lower refractive index than that in the ridge structure 44, thereby forming an embedded waveguide. .
[0060]
Also in this embodiment, the same effects as in the first embodiment can be basically obtained. In addition to this, in the optical wavelength conversion element 40 of the present embodiment, since it is formed as a buried waveguide as described above, symmetry occurs in the refractive index distribution in the substrate thickness direction, and the fundamental wave and the wavelength conversion wave overlap. Becomes larger and the mode matching efficiency is further improved.
[0061]
With respect to the optical wavelength conversion element 40, the waveguide performance was previously evaluated by the apparatus shown in FIG. A wave was confirmed.
[0062]
Next, still another embodiment of the present invention will be described. FIG. 5 shows a process of manufacturing an optical waveguide device according to the fourth embodiment of the present invention. The optical waveguide element of this embodiment is also a waveguide type optical wavelength conversion element as an example. In the fourth embodiment, a different member is bonded on the proton diffusion layer 12 when the grooves 13 and 13 are processed as compared with the first embodiment shown in FIG. The points described above are different, and the other points are basically the same.
[0063]
First, as shown in FIG. 3A, a proton diffusion layer 12 having a higher refractive index than that of the substrate material is formed on a MgO-LN substrate 11 having a thickness of 0.4 mm doped with 5 mol% of MgO. In the present embodiment, the proton diffusion layer 12 is formed as follows. First, the surface of the MgO-LN substrate 11 is washed with an organic solvent. Next, the cleaned substrate 11 is immersed in an acid solution to perform proton exchange. At this time, pyrophosphoric acid is used as the acid, the temperature is 200 ° C., and the immersion time is 5 hours. The atmosphere is an air atmosphere. Next, this MgO-LN substrate 11 is annealed to diffuse protons. The heat treatment conditions are 350 ° C. and 20 hours in the atmosphere.
[0064]
Next, as shown in FIG. 4B, a Si (silicon) substrate 51 having a thickness of 0.2 mm is bonded onto the proton diffusion layer 12 of the MgO-LN substrate 11 as an example. In the present embodiment, the aforementioned room temperature direct bonding is applied to this bonding. This room temperature direct joining is performed as follows.
[0065]
Both the MgO-LN substrate 11 and the Si substrate 51 are supplied in wafer form. First, those wafers subjected to surface etching or organic cleaning using a strong acid such as hydrofluoric acid are mounted in the front chamber of the vacuum chamber of the cleaning apparatus. Vacuum the chamber (10 ^-1 The wafers are automatically transferred to the Ar fast atom beam irradiation area. Then, dirt such as an oxide film on the bonding interface of the wafer is removed at an atomic level by an Ar fast atom beam.
[0066]
The MgO-LN substrate 11 and the Si substrate 51 thus cleaned are automatically transferred to a mechanical press apparatus in the same chamber. The two wafers are pressed by the pressing device in a direction in which the cleaned interfaces come into close contact with each other, and the bonding is completed. Both bonded wafers are automatically transferred to the rear chamber. Then, only the rear chamber portion is returned to normal pressure, and the bonded wafer is taken out.
[0067]
The pressing pressure has an optimum value depending on the material and its crystal structure, or the thickness of the material. In the case of a roll press, the value is often about 1 to 100 kPa · m. The room temperature bonding strength is greatly affected by the surface roughness and flatness of the wafer. The required accuracy depends on the material and dimensions, but in the case of the MgO-LN substrate 11 having a diameter of 4 inches, the surface roughness Ry is preferably 10 nm or less (particularly desirably 1 nm or less) and the flatness is 1 μm or less.
[0068]
Thereafter, as shown in FIG. 2C, the bonded Si substrate 51 is surface-ground to a thickness of several tens of μm to reduce the burden on the blade during subsequent groove processing. In this case, the surface roughness can be further reduced by performing loose abrasive processing.
[0069]
Next, as shown in FIG. 4D, two grooves 13 and 13 extending linearly in one direction (in the direction perpendicular to the drawing sheet in the X-axis direction) from the surface-ground Si substrate 51 side are formed. Then, a ridge structure 54 extending in a channel shape in the X-axis direction is formed. In the present embodiment, the groove processing is performed using a dicing saw. Resin bond diamond abrasive grains are used for the blade, the rotation speed is 30000 rpm, the feed speed is 1 to 5 mm / s (seconds), the groove interval is 4 to 16 μm, the groove depth is the thickness of the Si substrate 51 +20 μm, The groove length is about 50 mm.
[0070]
Next, as shown in FIG. 5E, the Si substrate 51 is removed using a Si etchant such as fluorinated nitric acid or TMAH. And it arrange | positions to a washing | cleaning jig so that a groove process part may not touch directly, and washing | cleaning and ultrasonic cleaning with an organic solvent are performed. After this process, chipping on the surface of the optical waveguide was observed with a microscope and an SEM (scanning electron microscope). As a result, the upper corner of the ridge structure 54 (as indicated by a broken-line circle A in FIG. It was confirmed that the chipping generated in the portion (shown) was greatly reduced. If the surface roughness Ry of the wafer is large, the bonding strength between the two wafers may vary, and the occurrence of chipping may not be completely suppressed.
[0071]
Thereafter, the wafer-like MgO-LN substrate 11 is cut outside each of the two grooves 13 so that only one ridge structure 54 is included, and the ridge structure 54 has a predetermined length. The optical waveguide type optical wavelength conversion element 50 is obtained by cutting and polishing both end faces in the length direction (surfaces that become the light incident surface and the output end surface, the X surface).
[0072]
With respect to this optical wavelength conversion element 50, the waveguide performance was previously evaluated by the apparatus shown in FIG. 2. In this case as well, the mode field of the guided light is observed in the optical waveguide portion composed of the ridge structure 54, and the light guiding is performed. A wave was confirmed. Further, in this optical wavelength conversion element 50, it was confirmed that the propagation loss was reduced by 2 dB or more compared to the case where the Si substrate 51 was not grooved and grooved.
[0073]
Next, still another embodiment of the present invention will be described. FIG. 6 shows a process of manufacturing an optical waveguide device according to the fifth embodiment of the present invention. The optical waveguide element of this embodiment is also a waveguide type optical wavelength conversion element as an example. The fifth embodiment is different from the first embodiment shown in FIG. 1 in that a separate member is bonded on the proton diffusion layer 12 when the grooves 13 and 13 are formed. The other member is used as a clad portion, and the other points are basically the same.
[0074]
First, as shown in FIG. 3A, a proton diffusion layer 12 having a higher refractive index than that of the substrate material is formed on a MgO-LN substrate 11 having a thickness of 0.4 mm doped with 5 mol% of MgO. In the present embodiment, the proton diffusion layer 12 is formed as follows. First, the surface of the MgO-LN substrate 11 is washed with an organic solvent. Next, the cleaned substrate 11 is immersed in an acid solution to perform proton exchange. At this time, pyrophosphoric acid is used as the acid, the temperature is 200 ° C., and the immersion time is 5 hours. The atmosphere is an air atmosphere. Next, this MgO-LN substrate 11 is annealed to diffuse protons. The heat treatment conditions are 350 ° C. and 20 hours in the atmosphere.
[0075]
Next, as shown in FIG. 2B, as an example, a 0.2 mm thick LiNbO film is formed on the proton diffusion layer 12 of the MgO-LN substrate 11. ₃ A substrate (hereinafter referred to as an LN substrate) 61 is bonded. The LN substrate 61 is not doped with MgO and is not subjected to proton exchange, and has a lower refractive index than the proton diffusion layer 12 on the MgO-LN substrate 11. In the present embodiment, the aforementioned room temperature direct bonding is applied to this bonding. This room temperature direct joining is performed as follows.
[0076]
Both the MgO-LN substrate 11 and the LN substrate 61 are supplied in wafer form. First, those wafers subjected to surface etching or organic cleaning using a strong acid such as hydrofluoric acid are mounted in the front chamber of the vacuum chamber of the cleaning apparatus. Vacuum the chamber (10 ^-1 The wafers are automatically transferred to the Ar fast atom beam irradiation area. Then, dirt such as an oxide film on the bonding interface of the wafer is removed at an atomic level by an Ar fast atom beam.
[0077]
The MgO-LN substrate 11 and the LN substrate 61 thus cleaned are automatically transferred to a mechanical press apparatus in the same chamber. The two wafers are pressed by the pressing device in a direction in which the cleaned interfaces come into close contact with each other, and the bonding is completed. Both bonded wafers are automatically transferred to the rear chamber. Then, only the rear chamber portion is returned to normal pressure, and the bonded wafer is taken out.
[0078]
The pressing pressure has an optimum value depending on the material and its crystal structure, or the thickness of the material. In the case of a roll press, the value is often about 1 to 100 kPa · m. The room temperature bonding strength is greatly affected by the surface roughness and flatness of the wafer. The required accuracy depends on the material and dimensions, but in the case of the MgO-LN substrate 11 having a diameter of 4 inches, the surface roughness Ry is 1 nm or less and the flatness is 1 μm or less.
[0079]
Thereafter, as shown in FIG. 3C, the bonded LN substrate 61 is surface ground to a thickness of several tens of μm to reduce the burden on the blade during subsequent groove processing. In this case, the surface roughness can be further reduced by performing loose abrasive processing.
[0080]
Next, as shown in FIG. 6D, the two grooves 13 and 13 extending linearly in one direction (in the direction perpendicular to the drawing sheet in the X axis direction) from the surface-ground LN substrate 61 side are formed. Then, a ridge structure 64 extending in a channel shape in the X-axis direction is formed. In the present embodiment, the groove processing is performed using a dicing saw. Resin bond diamond abrasive grains are used for the blade, the rotation speed is 30000 rpm, the feed speed is 1 to 5 mm / s (seconds), the groove interval is 4 to 16 μm, the groove depth is the thickness of the LN substrate 61 +20 μm, The groove length is about 50 mm.
[0081]
Next, it arrange | positions to a washing | cleaning jig so that a groove process part may not touch directly, and washing | cleaning and ultrasonic cleaning with an organic solvent are performed. After this process, chipping on the surface of the optical waveguide was observed with a microscope and SEM (scanning electron microscope). As a result, a significant reduction in chipping was confirmed as compared with the case where the LN substrate 61 was not joined.
[0082]
Thereafter, the wafer-like MgO-LN substrate 11 is cut outside each of the two grooves 13 so that only one ridge structure 64 is included, and the ridge structure 64 has a predetermined length. The optical waveguide type optical wavelength conversion element 60 is obtained by cutting and polishing both end surfaces in this length direction (surfaces that become the light incident surface and the output end surface, the X surface). Note that the LN substrate 61 having a refractive index lower than that of the proton diffusion layer 12 is left as it is to form a cladding portion.
[0083]
With respect to this optical wavelength conversion element 60, the waveguide performance was previously evaluated by the apparatus shown in FIG. 2. In this case as well, the mode field of the guided light is observed in the optical waveguide portion composed of the ridge structure 64, and the light guide A wave was confirmed. Further, in this optical wavelength conversion element 60, it was confirmed that the propagation loss was reduced by 2 dB or more as compared with the case where the LN substrate 61 was not grooved and grooved.
[0084]
Instead of the LN substrate 61 as another member remaining on the proton diffusion layer 12 as described above, lithium tantalate (TaNbO) that is not ion-diffused is used. ₃ ) A substrate or the like can also be used. Also, congruent (coincidence melting) LiNbO is used as a substrate for ion diffusion to form an optical waveguide. ₃ While using a substrate, etc., stoichiometry (stoichiometric composition) LiNbO as another member as described above ₃ Or TaNbO ₃ May be used.
[0085]
In the fourth and fifth embodiments described above, the MgO-LN substrate 11 and the separate member are joined by the press means, but they can be joined by another means. For example, as shown in FIG. 7, the MgO-LN substrate 11 on which the proton diffusion layer 12 is formed is placed on a magnetic base plate 70, and another member 71 made of an iron-based material is formed on the proton diffusion layer 12. It is possible to process the grooves 13 and 13 with the dicing saw 72 while strongly joining the separate member 71 to the proton diffusion layer 12 by the magnetic force acting between the separate member 71 and the base plate 70. . This method has an advantage that the separate member 71 can be easily detached from the MgO-LN substrate 11 after the grooves 13 and 13 are processed.
[0086]
Furthermore, it is possible to join another member to the proton diffusion layer 12 on the MgO-LN substrate 11 by applying a mechanical clamping means using a screw or the like.
[0087]
In order to prevent chipping from occurring at the upper corner of the ridge structure, it is effective to perform cutting with another member joined to the MgO-LN substrate 11 as described above. Techniques can also be adopted. For example, as shown in FIG. 8, the groove processing portion of the MgO-LN substrate 11 on which the proton diffusion layer 12 is formed is irradiated with laser light 73, and stress on the MgO-LN substrate 11 due to heating / shrinkage by the laser light 73 is applied. It is effective to dice along. By doing so, the stress of the MgO-LN substrate 11 at the time of groove processing can be relaxed, and the occurrence of chipping in portions other than the laser light irradiation region can be suppressed.
[0088]
Also, as shown in FIG. 9 (a), after forming the proton diffusion layer 12 on the MgO-LN substrate 11, in the method of cutting the grooves 13 and 13 by dicing as shown in FIG. It is also effective to use a dicing saw 72 whose arc is chamfered. That is, by doing so, the edge portions of the grooves 13 and 13 become rounded obtuse as shown in FIG. 5C, and the occurrence of chipping can be suppressed.
[0089]
Furthermore, as described above, not only the occurrence of chipping can be suppressed, but also the chipping that has occurred can be removed. That is, for example, in the method of cutting the grooves 13 and 13 by dicing as shown in FIG. 10B after forming the proton diffusion layer 12 on the MgO-LN substrate 11 as shown in FIG. As shown in FIG. 3C, the paste-like filler 74 having a refractive index different from that of the MgO-LN substrate 11 is filled in the grooves 13 and 13 and then solidified. Next, as shown in FIG. If the surface of the filler 74 is ground or polished, the generated chipping is removed by grinding or polishing. As the paste-like filler 74, a protective resist or the like can be used. In this case, the optical waveguide element can be used with the filler 74 remaining in the grooves 13, 13, or the filler 74 is removed as shown in FIG. Elements can also be used.
[0090]
Further, the optical waveguide element having the proton diffusion layer 12 as the ion diffusion layer has been described above. However, the present invention is not limited to the proton diffusion layer, and the optical waveguide unit is not limited to the proton diffusion layer but may be an ion diffusion layer such as a Ti (titanium) diffusion layer. Similarly, the present invention can be applied to the optical waveguide element constituting the same, and in this case, the same effect can be obtained.
[0091]
Here, a description will be given of the result of estimating by simulation how much light conversion efficiency improvement is expected in the SHG (second harmonic generation) optical wavelength conversion element by introducing the ridge structure in the present invention. In this estimation, after slab-like proton diffusion was simulated, the part removed by ridge processing was replaced with air (refractive index = 1) as a calculation model. Then, the waveguide mode for the fundamental wave and the second harmonic was calculated, and the efficiency of mode matching was derived. The parameter S shown below was applied to the definition of mode matching efficiency. The conversion efficiency of SHG is proportional to the square of S.
[0092]
S = ∬FM × FM × SH × d (x, y) dxdy
Where FM (x, y) and SH (x, y) are the fundamental and second harmonic mode space distributions, d (x, y) is the in-plane normalized d-constant distribution, and the bulk d Let the constant be 1.
[0093]
As a premise, direct coupling between an optical fiber (beam diameter 10 μm) and an optical waveguide is assumed, and the coupling efficiency with the fiber was also verified. Further, as the ridge shape, a vertical ridge shape (width 10 μm, processing depth infinite, side wall vertical) as shown in FIG. 11 and a taper shape (upper end width 8 μm, processing depth t = 1, 2, as shown in FIG. 12). (4, 6, 10, 18 μm, side wall 75 ° taper shape). 12A to 12F schematically show ridge shapes in which the machining depth t is the above six values.
[0094]
FIG. 13 shows the mode matching efficiency with respect to the ridge processing depth for each optical waveguide shape. The mode matching efficiency of the conventional proton exchange optical waveguide (without the ridge structure: processing depth t = 0 μm) is about 4 in relative value, while the proton exchange ridge optical waveguide clearly shows an improvement in efficiency. It is shown that the deeper the shape, the higher the possibility of higher efficiency.
[0095]
Furthermore, from the calculation result of the coupling efficiency with the fiber, it is possible to guide sufficiently even if the processing depth of the ridge is about 1 to 2 μm, but since the optical confinement in the lateral direction is weak, the horizontally elongated waveguide mode shape Thus, it was confirmed that high coupling efficiency cannot be expected. If the processing depth of the ridge is about 4 μm or more, the waveguide mode shape becomes nearly circular, and there is room for expecting realistic coupling efficiency.
[0096]
From the above simulation results, it can be said that the groove depth of the proton exchange ridge optical waveguide having a width of 10 μm is appropriate to be 4 μm or more. That is, it has been found that when the ridge processing depth is 40% or more of the width of the optical waveguide, the effect of improving the wavelength conversion efficiency by increasing the mode matching efficiency is great.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a process of manufacturing an optical waveguide device according to a first embodiment of the present invention.
FIG. 2 is a side view showing an apparatus for evaluating the performance of the optical waveguide element.
FIG. 3 is a schematic view showing a process of manufacturing an optical waveguide device according to a second embodiment of the present invention.
FIG. 4 is a schematic view showing a process of manufacturing an optical waveguide device according to a third embodiment of the present invention.
FIG. 5 is a schematic view showing a process of manufacturing an optical waveguide device according to the fourth embodiment of the present invention.
FIG. 6 is a schematic view showing a process of manufacturing an optical waveguide device according to a fifth embodiment of the present invention.
FIG. 7 is a schematic view showing an example of a groove cutting method according to the present invention.
FIG. 8 is a schematic view showing another example of the groove cutting method according to the present invention.
FIG. 9 is a schematic view showing still another example of the groove cutting method according to the present invention.
FIG. 10 is a diagram illustrating a method for repairing chipping of a substrate that can be applied to the present invention.
FIG. 11 is a diagram illustrating a model for simulating the performance of the optical waveguide device according to the present invention.
FIG. 12 is a diagram illustrating a model for simulating the performance of the optical waveguide device according to the present invention.
FIG. 13 is a graph showing the results of the simulation
[Explanation of symbols]
10, 30, 40, 50, 60 Optical wavelength conversion element
11 MgO-LN substrate
12 Proton diffusion layer
13 groove
14, 34, 44, 54, 64 Ridge structure
70 Base plate
71 Separate parts
72 dicing saw
73 Laser light
74 Filler

Claims (12)

An optical waveguide element in which a ridge structure extending in a channel shape is formed on a LiNbO ₃ substrate, and an optical waveguide portion made of an ion diffusion layer having a higher refractive index than that of the substrate material is formed extending between both ends of the ridge in the ridge structure. . 2. The optical waveguide element according to claim 1, wherein the ion diffusion layer comprises a proton exchange layer. 3. The optical waveguide device according to claim 1, wherein a cladding portion having a refractive index lower than that of the optical waveguide portion is formed on the ion diffusion layer. 4. The optical waveguide device according to claim 3, wherein a refractive index of the cladding portion is substantially equal to a refractive index of the substrate material. 5. The optical waveguide element according to claim 3, wherein the clad portion is formed by performing Li diffusion on the ion diffusion layer. 5. The optical waveguide element according to claim 3, wherein the clad portion is formed on the ion diffusion layer by bonding a member different from the substrate. A method for producing an optical waveguide device according to any one of claims 1 to 6,
After forming an ion diffusion layer on the surface portion of the LiNbO ₃ substrate,
A method of manufacturing an optical waveguide device, comprising: etching the substrate to a depth extending from the ion diffusion layer to at least a part of a substrate portion thereunder to form the ridge structure. A method for producing an optical waveguide device according to any one of claims 1 to 6,
After forming an ion diffusion layer on the surface portion of the LiNbO ₃ substrate,
A method of manufacturing an optical waveguide device, wherein a groove having a depth extending from the ion diffusion layer to at least a part of a substrate portion thereunder is machined to form the ridge structure. 9. The method of manufacturing an optical waveguide element according to claim 8, wherein the machining is a cutting process using a dicing saw. After forming the ion diffusion layer,
A member different from the substrate is bonded on the ion diffusion layer,
10. The method for manufacturing an optical waveguide element according to claim 9, wherein the cutting process is performed by the dicing saw while the another member is joined. The method of manufacturing an optical waveguide element according to claim 10, wherein the another member is removed from the substrate after the cutting process. The method of manufacturing an optical waveguide element according to claim 10, wherein after the cutting process, the another member is kept bonded to the substrate.

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