JP6339965B2 - Optical waveguide fabrication method - Google Patents

Description

  The present invention relates to a method for manufacturing an optical waveguide, and more particularly to a method for manufacturing an optical waveguide having a spot-size converter (SSC) capable of controlling the spread of the electromagnetic field distribution of light in the optical waveguide.

  In many cases, an optical device represented by a semiconductor laser or the like has an optical circuit constituted by an optical waveguide structure. The mode electromagnetic field distribution of light propagating through the optical waveguide is determined by the material constituting the optical waveguide and the structure of the waveguide. The spread of the mode distribution may be called a spot size, and in many cases, it is preferable that the spot size is small. For example, if the spot size is small, the power density of the light guided through the waveguide increases, and the interaction between the light and the waveguide material becomes stronger. For this reason, it is necessary to reduce the power consumption of light control devices such as lasers and modulators. A waveguide with a small spot size is important. In addition, the minimum value of the bending radius of the bending waveguide, which is often a limiting factor for the size of the device, is generally smaller when light is strongly confined in the waveguide, so a waveguide with a small spot size may be preferable. Many.

  However, a small spot size may be a problem. One of them is a problem of coupling with an external optical system at the input / output end face of the optical device. If the spot size is small, the spread angle of the light emitted from the device to the free space becomes large due to the Fourier transform. Lens used to form optical coupling from optical device to other components such as optical fiber when the spread angle of light distribution in free space, that is, the far-field pattern (FFP) spread angle is large There is a problem that the diameter of the lens, that is, the size of the lens becomes large. The size of the lens often becomes a limiting factor for reducing the size of the entire optical module. In addition, when the spot size is small, there is a problem that the mounting tolerance is reduced in the lens mounting process for forming an optical coupling with the outside by a lens.

Although the above problem is the case of light emission from an optical device, since the optical system composed of passive elements is generally reciprocal, the same problem occurs when light is incident on the optical device. Therefore, it is preferable that the spot size is small in the waveguide in the device and large in the end face of the waveguide. In addition to external optical coupling, there are functions that can be realized on an optical device with a large spot size. To realize these functions, the spot size is converted at a specific part of the same optical device. The structure to be used is SSC.
As described above, in many cases, it is preferable that the optical device has a small spot size. Therefore, it is necessary to form an SSC on the optical waveguide as necessary to increase the spot size only at a local portion. Various structures and methods for manufacturing SSCs that locally increase the spot size in an optical waveguide type optical device exist.

  In order to convert from a small spot size to a large spot size in an optical waveguide, the most direct method is to enlarge the core region in which light is confined in the waveguide to increase the mode shape (in this specification, “method” A ") and a technique (referred to as" Method B "in this specification) that conversely weakens mode confinement and widens the spot size. In general, since the eigenmode of light approaches the cutoff in method B, the spot size changes sensitively to perturbation of the waveguide shape, so that the manufacturing tolerance is lower than that of method A. However, the advantage of Method B is that the amount of change in the geometric size of the waveguide required to obtain a certain amount of change in spot size can be reduced.

  When SSC is discussed, the spot size in the horizontal direction and the spot size in the vertical direction are often discussed separately. In most cases, the SSC in the horizontal direction can be formed by a tapered waveguide whose waveguide width changes in a tapered shape. That is, the waveguide width is gradually changed so that the mode shape of the guided light changes adiabatically with the propagation of light. Here, the adiabatic change means that the energy of the guided light is not scattered because the structure changes slowly. Tapered waveguides can be fabricated by a general photolithography process using a tapered mask pattern, so it is possible to use Method A, which narrows the waveguide width at small spot sizes and widens the areas that you want to increase. It is very stable and easy to produce. On the other hand, regarding the vertical direction, it is difficult to change the waveguide shape in the vertical direction in a normal photolithography process, so when considering SSC, how to control the spot size in the vertical direction in general Discussions are often concentrated. For the SSC in the vertical direction, in order to make the change in shape in the vertical direction as small as possible, the method B in which the amount of change in shape is small as described above is often adopted. For example, Non-Patent Document 1 is a report on SSC in a laser device using a compound semiconductor, and the SSC is formed by thinly growing only the core material of the waveguide end face by using a semiconductor regrowth technique.

  The method disclosed in Non-Patent Document 1 uses a semiconductor growth technique that can be controlled in the order of nm, and therefore has an advantage that SSC by Method B having a low manufacturing tolerance can be manufactured with good controllability. On the other hand, this method has a drawback in that the number of semiconductor crystal growth steps, which require a large manufacturing time and cost, is increased in the manufacturing process.

  The technique disclosed in Non-Patent Document 2 realizes weak mode confinement by extremely narrowing the width of the waveguide that can be easily controlled by the photolithography process, rather than the thickness of the core of the waveguide, As a result, a method of increasing the spot size in the vertical direction is taken. In this method, after forming a narrow thin wire waveguide, additional semiconductor growth is necessary in the sense that the thin wire waveguide is covered with an insulating semiconductor by semiconductor regrowth. However, this additional growth process of an insulating semiconductor is often performed in the optical device manufacturing process in general regardless of the presence or absence of SSC. Therefore, unlike the method of re-growing the core material as in Non-Patent Document 1, there is an advantage that the number of steps of semiconductor regrowth is not substantially increased. However, as described above, the present technique for extremely narrowing the waveguide width is a technique classified as technique B, which has a low manufacturing tolerance and requires high accuracy in the photolithography process. In addition, in addition to the core of the original optical waveguide, in addition to the core of the original optical waveguide, another core is pre-deposited on the substrate in order to reduce the amount of change in the geometric structure of the waveguide, regardless of Method A or Method B. There are various report examples, such as a method of forming an SSC using a coupling mode with this additional core layer (Non-patent Document 3).

Yasumasa Suzaki, Ryuzu Iga, Kenji Kishi, Yoshihiro Kawaguchi, Shin-ichi Matsumoto, Minoru Okamoto, and Mitsuo Yamamoto “Temperature- and Polarization-Insensitive Responsivity of a 1.3-μm Optical Transceiver Diode with an Integrated Spot-Size Converter”, IEEE J Quantum Electron., Vol. 34, no. 4, pp. 686-690, 1998. Masaki Kohtoku, Satoshi Oku, Yoshiaki Kadota, and Yuzo Yoshikuni, “Spotsize Converter With Improved Design for InP-Based Deep-Ridge Waveguide Structure”, J. Lightw., Technol, vol. 23, no. 12, pp. 4207-4214 , 2005. Andreas Beling and Joe C. Campbell, “InP-Based High-Speed Photodetectors”, J. Lightw., Technol, vol. 27, no. 3, pp. 343-355, 2009. K. Prosyk, A. Ait-Ouali, J. Chen, M. Hamacher, D. Hoffmann, R. Kaiser, R. Millett, A. Pirastu, M. Totolo, K. Velthaus, I. Woods, “Travelling Wave Mach -Zehnder Modulators ”Proc. Of IPRM2013, MoD3-1, 2013

  However, since it is necessary to deposit another core layer in addition to the core layer that is originally required, the manufacturing process of the multilayer film may be complicated, and such an additional layer may not be inserted depending on the type of optical device. Yes, and lacks versatility in manufacturing. Furthermore, there are reports of SSCs based on waveguide structures other than simple taper structures, but in any case, high precision is required for the photolithography process, or excessive loss may occur due to the complexity of the structure. is there.

  The present invention has been proposed in order to solve the above-described conventional problems, and an object of the present invention is important in coupling an optical system between a waveguide and the outside. Optical waveguide that can easily produce an optical waveguide having an optical spot size converter without requiring a complicated structure for an optical spot size converter that changes the mode distribution of the field, that is, converts the spot size at a specific part of the waveguide. A method for manufacturing a waveguide is provided.

  In order to solve the above-mentioned problem, the invention described in one embodiment includes a step of forming an opening mask whose opening width varies along the longitudinal direction of the optical waveguide with respect to a material substrate forming the optical waveguide; Etching the material substrate using the formed opening mask to form an inclined structure in which the thickness of the material layer forming the optical waveguide changes corresponding to the change in the opening width of the opening mask And a step of further etching the material substrate having an inclined structure with an optical waveguide pattern to produce an optical waveguide.

  The invention described in another embodiment includes a multilayer film on a multilayer substrate including a substrate and a layer serving as a core layer of an optical waveguide formed of a material having a refractive index different from that of the substrate. Forming a first mask layer serving as a mask for etching the substrate; processing the mask layer into an opening mask pattern whose opening width varies along the longitudinal direction of the optical waveguide; and the opening mask Etching the multilayer substrate using a pattern changes the etching depth of the layer that becomes the core layer, reflecting the change in the opening width along the longitudinal direction of the optical waveguide of the mask pattern. A step of forming an inclined structure on the multilayer substrate, and a second mask layer as a mask for further etching the multilayer substrate is formed on the multilayer substrate having the inclined structure. Processing the second mask layer into a waveguide mask pattern capable of forming a waveguide structure at a position where the inclined structure exists in the multilayer substrate; and forming the inclined structure using the waveguide mask pattern. Forming an optical waveguide at a location where the inclined structure is present on the multilayer substrate by etching the multilayer substrate having the optical waveguide substrate.

  According to the present invention, since a high-performance SSC can be easily produced, for example, an improvement in coupling loss of an optical device in general can be achieved, which can contribute to further spread of optical communication.

6 is a diagram illustrating a manufacturing process of Example 1. FIG. 6 is a diagram illustrating a manufacturing process of Example 1. FIG. 6 is a diagram illustrating a manufacturing process of Example 1. FIG. 6 is a diagram illustrating a manufacturing process of Example 1. FIG. 6 is a diagram illustrating a manufacturing process of Example 1. FIG. 6 is a diagram illustrating a manufacturing process of Example 1. FIG. 6 is a diagram illustrating a manufacturing process of Example 1. FIG. It is a figure which shows the coupling loss with respect to the change of the waveguide width | variety of VT-SSC. It is a figure which shows the coupling loss with respect to the change of the core layer thickness of the waveguide of VT-SSC. It is sectional drawing of the waveguide of a buried type VT-SSCT. It is sectional drawing parallel to the gap waveguide which formed the gap in the middle of VT-SSC.

  Hereinafter, embodiments of the present invention will be described in detail. The method for fabricating a waveguide according to the present invention uses a micro-loading effect (MLE) in an etching process of a material, thereby reducing the core region in the vertical direction of the substrate and thereby reducing the mode. The SSC is formed by a technique that weakens confinement and widens the spot size. MLE means that the etching rate changes as the width of the processed pattern changes. Since the etching rate is changed by MLE by changing the width of the processing pattern, the etching depth can be changed as a result.

  Specifically, in the method for manufacturing a waveguide according to the present invention, an opening mask whose opening width changes in a specific direction is formed on a material substrate on which the waveguide is formed, and the formed opening mask is used. Etching is performed on the target material substrate. When etching using an opening mask is performed, a so-called microloading effect allows a structure having a different etching depth in a specific direction to reflect the width of the opening mask, that is, an etching structure having an inclined structure in the vertical direction. Further, an optical waveguide is manufactured using a material substrate having an inclined etching shape. In the optical waveguide manufactured in this way, the thickness of the material constituting the optical waveguide is changed in the vertical direction in a specific direction. It has a part that functions as a continuously changing spot size converter.

[Example 1]
In this example, an MSC is used to produce an SSC having a VT (vertical taper) structure (hereinafter referred to as VT-SSC) from an InP-based material, and a coupling efficiency characteristic with external light by calculation of the produced VT-SSC. Was measured. 1 to 7 are diagrams for explaining a manufacturing process of the first embodiment. In each figure, (a) is a plan view, (b) is a yy ′ cross-sectional view, and (c) is an xx ′ cross-sectional view. In this embodiment, as shown in FIG. 1, a multi-quantum well (multi-quantum well) formed on an InP substrate which is the base substrate layer 1 and formed from a group III-V material of In, Al, Ga, As. A multilayer substrate 10 composed of a core layer 2 by Quantum well (MQW) and an InP over clad layer 3 further formed on the core layer 2 is employed. Here, an InP-based material is selected as a processing target. However, any material can be used as long as the waveguide can be formed by etching, such as Si or glass.

As the start of VT-SSC fabrication, SiO ₂ is deposited on the multilayer substrate 10 as a mask layer. As shown in FIG. 2, this SiO ₂ mask layer is processed into an opening mask pattern 11 in which the opening width w changes along a certain direction. In this embodiment, the opening width is set to change along the x direction.

  The length in the y direction of the region where the opening width w of the opening mask pattern 11 changes is preferably 100 μm to 300 μm. This is because if the length is too short, the gradient of the core becomes too steep, and if the length is too long, the length of the chip for SSC becomes long. The opening width w is preferably 3 μm to 20 μm. The narrowest width is 3 μm because, in order to lower the etching rate, the opening should be as narrow as possible, but it is necessary to form a waveguide later in this location. In general, the width of a semiconductor waveguide is about 2 μm, so this is set to allow for a margin. Further, the reason why the widest width is 20 μm is that when the aperture ratio is increased to some extent, the etching rate does not change with respect to the change in the aperture width. Although it depends on the etching material and the conditions of the dry etching apparatus, since MLE is a phenomenon that generally appears after a few μm, if the aperture width is changed at most 10 μm, it becomes meaningless. Considering other process margins, the maximum value is about 20 μm.

  Further, the opening mask pattern 11 is not limited to the shape in which the opening width w is linearly changed as in the present embodiment, and may be any shape such as a shape in which the opening width w is increased in a trigonometric function.

Using this SiO ₂ opening mask pattern, an opening pattern etching process is performed on the multilayer film substrate by a dry etching apparatus using reactive plasma. As a result, as shown in FIG. 3, an inclined structure 12 is formed in which the etching depth D changes along the x direction, reflecting the opening width w of the opening mask pattern 11. In this example, the plasma generation conditions and etching time used for etching were adjusted so that the etching stopped in the middle of the MQW core layer 2 at the position where the etching depth D was the deepest.

By optimizing the etching conditions, the etching depth D can be set to a depth that does not reach the MQW core layer 2 in a portion where the opening width w of the opening mask pattern 11 is narrow. In the present embodiment, the opening pattern etching process is performed so that the MQW core layer 2 remains at 100 nm in a portion where the opening width w is wide. Here, a pattern in which the opening width w changes linearly along the x direction is used as the SiO ₂ opening mask pattern 11. However, a pattern in which the opening width w changes depending on a desired geometric shape as the inclined structure 12. It may be used.

After the inclined structure 12 is obtained, SiO ₂ is again deposited as a mask material, processed into a waveguide mask pattern, and a dry etching process is performed, so that the MQW layer 2 is a core and an over clad is formed. A VT-SSC structure that becomes air can be obtained. However, when the VT-SSC is formed by using an over clad of InP having a high refractive index rather than an over clad of air, the spot size is likely to be widened. In addition, since the optical device body often has an InP overcladding, it is often desirable that the VT-SSC also has an InP overcladding for optical consistency with this part. . Therefore, in this embodiment, after the inclined structure 12 is formed, InP is regrown on the inclined structure 12 as the regrowth layer 13, and then the waveguide structure is formed.

  As shown in FIG. 4, InP is deposited as a regrowth layer 13 on the multilayer substrate 10 on which the inclined structure 12 is formed by a semiconductor regrowth process. InP of the regrowth layer 13 is preferably an intrinsic semiconductor that is neither p-type nor n-type from the viewpoint of light loss, but the process of re-growing InP of the intrinsic semiconductor in the optical device manufacturing process is performed regardless of the presence or absence of SSC. In many cases, this is essentially necessary, so that in this case, an additional semiconductor regrowth step just for SSC is substantially unnecessary.

After InP as the regrowth layer 13 is regrown, as shown in FIG. 5, SiO ₂ is deposited as a mask material and processed into a waveguide mask pattern 14 having a waveguide structure pattern. Thereafter, as shown in FIG. 6, when a waveguide etching process is performed in which etching is performed according to the waveguide mask pattern 14, a waveguide structure 15 including VT-SSC is obtained.

  In this embodiment, the waveguide etching process is performed at such a depth that the etching depth reaches the InP substrate, the waveguide structure 15 is changed to a so-called high mesa structure, and the taper structure in which the waveguide width is changed from 5 μm to 2 μm. To do. This is because the VT tip coupled with the external optical system increases the MFD in the lateral direction, while the device body side with a thick core needs to narrow the waveguide width due to the single mode condition. Furthermore, as shown in FIG. 7, the multilayer substrate 10 is cleaved across the waveguide structure 15 having VT-SSC, so that an optical device having SSC at the waveguide end face is obtained.

  FIG. 8 shows a change in coupling efficiency (coupling loss) between the end face of the VT-SSC obtained in this example and a Gaussian mode having a mode-field diameter (MFD) of 3.5 μm. As a waveguide etching process, the coupling loss with respect to the waveguide width of the end face of VT-SSC is shown when the thickness t of the MQW core layer 2 is 50, 100, 150, 200, 300, 400, 500 nm. The reason why the MFD is set to 3.5 μm is that the FFP angle of the spot size of this size is about 20 °, which is one reference for assembling an optical device and an optical system of external light (non-patent document). 4). A finite difference beam propagation method using imaginary axis propagation was used for mode analysis of VT-SSC. For the calculation, light having a wavelength of 1550 nm was used. The reflection loss due to Fresnel reflection at the waveguide end face is ignored.

  As shown in FIG. 8, even if the width of the waveguide structure 15 fluctuates ± 1 μm, the fluctuation of the coupling loss is 0.5 dB or less. Even if the accuracy of the current photolithography process is poor, the fluctuation is about ± 0.2 μm. Considering that the VT-SSC according to the manufacturing method of the present embodiment is considered, it can be said that the photolithography process for forming the waveguide mask pattern 14 is not burdened at all.

  FIG. 9 shows a change in coupling efficiency with a Gaussian mode having a mode-field diameter (MFD) of 3.5 μm with respect to the variation of the thickness t of the MQW core layer 2 when the waveguide width is fixed to 5 μm. (Coupling loss). That is, FIG. 9 shows the result of evaluating the accuracy required for the etching depth D in the waveguide etching process. When the thickness t of the core of the target is 100 nm, the coupling loss is about ± 0.5 dB assuming that t varies ± 50 nm. In the opening pattern etching process using reactive plasma, a relatively slow etching rate of about 50 nm / min can be obtained by reducing high-frequency energy for obtaining plasma. Therefore, the opening pattern etching process in this embodiment allows a fluctuation of about 1 minute in terms of etching time, which can be said to be a sufficiently stable process in view of general etching. That is, it can be said that the VT-SSC obtained by the manufacturing method of this example has an advantage of high manufacturing tolerance even in the vertical spot size.

[Example 2]
In Example 1, after the inclined structure 12 was obtained, InP was regrown as the regrowth layer 13, but in this example, after processing the waveguide structure 15 on the multilayer film substrate 10 including the inclined structure 12, By regrowing the regrowth layer 13, a buried type VT-SSC is obtained. FIG. 10 shows a yy′-direction cross section of the fabricated embedded VT-SSC.

[Example 3]
The first and second embodiments are examples in which the SSC is formed on the end face of the optical device to measure the improvement of the coupling efficiency with the external optical system. Can be achieved. In Examples 1 and 2, the waveguide 15 was cut by cleavage at a location where VT-SSC existed to produce an optical device having SSC on the end face. In this example, as shown in FIG. The SSC is formed inside the optical device by cutting the portion where the SSC exists by etching. FIG. 11 is a cross-sectional view illustrating a waveguide of a gap waveguide in which a gap is formed in the middle of VT-SSC.

  In this way, the waveguide having a gap can function as a small waveguide reflector, or the polarization state of light guided through the waveguide can be controlled by inserting a wave plate between the gaps. When the spot size of light propagating through the waveguide is small, there is a problem that light is emitted at a large angle in the gap portion, and loss when coupled to the waveguide again increases. Therefore, a gap waveguide with a small excess loss can be formed by forming a gap at the location of VT-SSC as in this embodiment.

DESCRIPTION OF SYMBOLS 1 Base substrate layer 2 Core layer 3 Over clad layer 10 Multilayer substrate 11 Opening mask pattern 12 Inclined structure 13 Regrown layer 14 Waveguide mask pattern 15 Waveguide structure

Claims (6)

Forming an opening mask whose opening width varies from 3 μm to 20 μm along the longitudinal direction of the optical waveguide with respect to the material substrate forming the optical waveguide;
Etching the material substrate using the formed opening mask to form an inclined structure in which the thickness of the material layer forming the optical waveguide changes in accordance with the change in the opening width of the opening mask; ,
And etching the material substrate having an inclined structure with an optical waveguide pattern to produce a high mesa optical waveguide in a tapered structure in which the waveguide width on the thick core side is narrow. Manufacturing method. A first mask serving as a mask for etching a multilayer film substrate on a multilayer film substrate including a substrate and a layer serving as a core layer of an optical waveguide formed of a material having a refractive index different from that of the substrate. Forming a mask layer of:
Processing the mask layer into an opening mask pattern whose opening width varies from 3 μm to 20 μm along the longitudinal direction of the optical waveguide;
Etching the multilayer film substrate using the opening mask pattern reflects the change in the opening width along the longitudinal direction of the optical waveguide of the mask pattern, and the etching depth of the layer serving as the core layer Forming a gradient structure on the multilayer substrate, wherein:
Forming a second mask layer on the multilayer substrate having the inclined structure as a mask for further etching the multilayer substrate;
Processing the second mask layer into a waveguide mask pattern capable of forming a waveguide structure at a location where a tilt structure exists in the multilayer substrate;
A tapered structure in which the waveguide width on the thicker side of the core becomes narrower at the location where the inclined structure exists on the multilayer film substrate by etching the multilayer film substrate having the inclined structure using the waveguide mask pattern. Forming an optical waveguide having a high mesa structure on the substrate .   The method of manufacturing an optical waveguide according to claim 2, wherein the inclined structure functions as a spot size converter that continuously changes a spread of an electromagnetic field mode of light propagating through the optical waveguide.   The method further includes depositing a regrowth layer made of at least one material on the multilayer substrate having the inclined structure after the step of forming the inclined structure on the multilayer substrate. The method for producing an optical waveguide according to claim 2 or 3.   After the step of forming the optical waveguide, a step of obtaining an optical waveguide in which the optical waveguide is buried in the portion where the inclined structure exists by depositing a regrowth layer made of at least one material. The method for producing an optical waveguide according to claim 2, further comprising:   The optical waveguide according to any one of claims 2 to 5, wherein a gap is formed in the optical waveguide by etching the optical waveguide including the inclined structure in a short direction of the optical waveguide. A method for manufacturing a waveguide.

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