JP2013205642A - Method for manufacturing spot size converter element and optical waveguide with spot size converter element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing spot size converter elements avoiding etching damage on a silicon wire waveguide.SOLUTION: The method for manufacturing spot size converter elements includes: forming a first core having a first core diameter on a first cladding layer on a substrate; forming a metal silicide on the first core other than a spot size conversion area via a silicone oxide film; depositing a second core material having a refractive index larger than that of the first cladding layer on the entire surface of the substrate; etching the second core material, with the metal silicide used as an etching stopper; and forming a second core on the spot size conversion area, the second core connected to the first core and having a second core diameter larger than the first core diameter.

Description

  The present invention relates to a method for manufacturing a spot size conversion element and an optical waveguide with a spot size conversion element.

  With the progress of silicon photonics, attention has been focused on the development of optical modules in which various optical elements are integrated on a substrate on which a silicon optical waveguide is formed. The silicon optical waveguide can be manufactured by a normal silicon CMOS process using an SOI (silicon-on-insulator) substrate. By using silicon with a large refractive index as a core, light in the 1.3 to 1.6 μm band can be propagated with low loss.

  The cross-sectional area of the silicon core is very small, and the height and width of the cross section are on the order of submicrons. On the other hand, the spot diameter in the optical fiber and laser end face used for optical communication is 10 times or more the spot diameter of the silicon core. If the silicon core optical waveguide is directly coupled to the input / output end face of the optical fiber or laser, the coupling loss increases due to mode mismatch.

  To solve this problem, a spot size converter is used. By interposing a core / clad having a waveguide mode with a large spot diameter in the spot size conversion region, the silicon core and the optical fiber or laser can be efficiently coupled.

  As a spot size converter, the configuration of FIG. 1A has been proposed (see, for example, Patent Document 1). The tip of the silicon core 403 on the silicon oxide film 402 is tapered and connected to the core 404 having a large diameter. By tapering the tip of the silicon core 403, reflection and loss of guided light generated at the interface between the silicon core waveguide and the spot size conversion region are reduced.

  However, the conventional method for manufacturing a spot size converter has a processing problem. As shown in FIG. 1B, when the silicon core 403 is connected to the silicon oxynitride (SiON) core 404, an unnecessary portion of the SiON film is removed by reactive ion etching (RIE) or the like to obtain a predetermined shape. Process. At this time, damage (or deformation) 410 occurs in the waveguide of the silicon core 403. This is because the selective ratio between the SiON film 404 and the silicon core 403 cannot be obtained.

  Even if the etching conditions are optimized, the thickness of the SiON film 404 of the spot size converter is as thick as about 3 μm, so that it is difficult to prevent damage only by optimization. In the case where the SiON film 404 is removed by wet etching, the silicon door 403 is less damaged because the selection ratio with respect to the silicon core 403 is secured to some extent. However, since wet etching is isotropic, side etching is included, and stable production of a spot size converter is hindered.

JP 2010-230982 A

  In one aspect, an object of the present invention is to provide a method for manufacturing a spot size conversion element that avoids etching damage on a silicon waveguide.

In a first aspect, a method for manufacturing a spot size conversion element includes:
Forming a first core having a first core diameter on the first cladding layer on the substrate;
Except for the spot size conversion region, a metal silicide is formed on the first core through a silicon oxide film,
Depositing a second core material having a refractive index larger than that of the first cladding layer on the entire surface of the substrate;
Using the metal silicide as an etching stopper, the second core material is etched, and the spot size conversion region has a second core diameter that is coupled to the first core and larger than the first core diameter. Forming a second core;
Removing the metal silicide;
After removing the metal silicide, a second cladding layer is formed to cover the first core and the second core.

In another aspect, a method for manufacturing a spot size conversion element includes:
Forming a first core having a first core diameter on the first cladding layer on the substrate;
A metal film is formed on the first cladding layer and the first core in the spot size conversion region via a silicon oxide film,
Forming a second cladding material film on the entire surface of the substrate;
Etching the second cladding material film using the metal film as an etching stopper, forming a second cladding layer on the first core excluding the spot size conversion region,
Removing the metal film,
Depositing a second core material having a higher refractive index than the first core on the entire surface of the substrate;
Processing the second core material into a predetermined shape, and in the spot size conversion region, a second core coupled to the first core and having a second core diameter larger than the first core diameter. Form.

  A stable spot size conversion element manufacturing method that avoids etching damage to the silicon waveguide is realized.

It is a figure which shows the structure and problem of manufacture of the conventional spot size converter. It is a figure which shows the method considered in the process leading to embodiment. It is a figure which shows the cross-sectional structure of the spot size conversion element by one Embodiment. It is a manufacturing-process figure of the spot size conversion element of Example 1 in a cross section perpendicular | vertical to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in a cross section perpendicular | vertical to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in a cross section perpendicular | vertical to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in a cross section perpendicular | vertical to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 1 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 2 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 2 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 2 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 2 in the cross section parallel to a light propagation direction. It is a manufacturing-process figure of the spot size conversion element of Example 2 in the cross section parallel to a light propagation direction.

  A method for manufacturing a spot size converter according to an embodiment will be described with reference to the drawings.

  FIG. 2 shows a possible method in the process leading to the present embodiment. Conventionally, RIE etching damage to a silicon core has been attributed to a low selectivity between the SiON film and silicon (see FIG. 1B). Therefore, as shown in FIG. 2, it is conceivable to form the SiO 2 hard mask 104 on the silicon core 13 and etch the SiON film 15 by RIE.

  Although this method can prevent etching damage to the silicon core 13, another problem arises. When the thickness of the SiO 2 hard mask 104 is large, the SiO 2 hard mask 104 functions as an optical confinement layer, and the spread of light from the silicon core 13 to the SiON film 15 is inhibited. In this case, the effect as a spot size conversion element is reduced. On the contrary, if the film thickness of the SiO2 hard mask 104 is small, the silicon core 13 is etched, causing a propagation loss.

  Therefore, the embodiment provides a method for manufacturing a spot size conversion element that avoids damage to the silicon core and does not hinder the spot size conversion function.

  FIG. 3 is a cross-sectional view along the light propagation direction of a spot size converter 10 manufactured by the method of the embodiment. The spot size conversion element 10 includes a buried oxide film 12 on a silicon substrate 11, a silicon core 13 on the buried oxide film 12, and a silicon oxynitride (SiON) core 15 connected to the silicon core on the buried oxide film 12. The silicon oxide film 21 covering the silicon core 13 and the SiON core 15 is included.

  The buried oxide film 12 functions as a lower clad of the optical waveguide including the spot size conversion element 10. The silicon oxide film 21 functions as an upper clad of the optical waveguide including the spot size conversion element 10. The silicon core 13 is transparent to light having a wavelength of 1.3 to 1.6 μm, and is effectively used for optical communication in the near infrared band. The silicon core 13, the buried oxide film (lower clad) 12, and the silicon oxide film (upper clad) 21 constitute a silicon optical waveguide.

  The SiON core 15 is connected to the silicon core 13 on the buried oxide film 12. The core diameter of the SiON core 15 is larger than the core diameter of the silicon core 13. The SiON core 15 covers a part of the silicon core 13 and forms an interface 15 a with the silicon oxide film 21 on the silicon core 13. The refractive index of SiON is larger than the refractive index of the buried oxide film 12 and is transparent to light having a wavelength of 1.3 to 1.6 μm. The center of the incident / exit end face 15b of the SiON core 15 is aligned with the center of an optical fiber (not shown) or the laser end face.

  As a feature of the spot size conversion element 10, at the interface 15 a between the SiON core 15 and the silicon oxide film 21 on the silicon core 13, either the SiON core 15 or the silicon oxide film 21 has a protrusion 22 with respect to the other. . As will be described later, this is due to the removal trace of the etching stopper used in the manufacturing process. The protrusion 22 has a height of 15 to 50 nm and a protrusion amount (length in the light propagation direction) of 20 to 40 nm. Protrusion 22 of this level does not hinder the spread of light to SiON core 15, so that propagation loss can be avoided.

  Next, a method for manufacturing the spot size conversion element 10 will be described.

  4A to 4C are views showing a manufacturing process of the spot size conversion element 10 of Example 1 in a cross section perpendicular to the light propagation direction.

  The upper side of FIG. 4A is a top view of the optical module 30, and the lower side is a cross-sectional view taken along the line AB. A rib-type waveguide 23 is formed on the silicon substrate 11 via the buried oxide film 12. Optical devices such as an optical modulator 31 and a ring resonator 32 are formed by the rib-type waveguide 23. One end 23a of the rib-type waveguide 23 becomes an incident end of laser light. The other end 23b of the rib waveguide 23 serves as a coupling portion with the optical fiber. Spot size conversion elements to be described later are formed at the end portions 23 a and 23 b of the rib-type waveguide 23. The ring resonator 32 is configured to determine the oscillation wavelength of the laser according to the resonance frequency. The optical modulator 31 is formed so as to modulate laser light in accordance with application of a drive signal or a bias voltage.

  In forming the rib-type waveguide 23, first, the SiO2 film 24 is formed on the entire surface of the SOI substrate by CVD. As the film forming conditions, for example, a source gas (SiH4: 20% / He: 80% + N2O) is supplied, and 50 nm is deposited at a temperature of 790 ° C. A resist mask (not shown) having a waveguide pattern including the modulator 31 and the resonator 32 is formed on the SiO 2 film 24. Using this resist mask, the SiO2 film and the surface silicon layer of the SOI substrate are etched. Etching conditions include, for example, etching the SiO2 film 24 at 100 mTorr and 150 W by RIE using CF4 gas, and then etching the surface silicon layer of the SOI substrate at 50 mTorr and 200 W by RIE using HBr gas. . The surface silicon layer is etched so that the slab regions 23s remain with a film thickness of about 40 nm on both sides of the silicon core 23c. The height of the core 23c of the rib-type waveguide 23 is 240 to 500 nm, and the width is 300 to 500 nm. Thereafter, the resist mask is peeled off.

  The upper side of FIG. 4B is a top view of the optical module 30, and the lower side is a CD sectional view thereof. A resist mask (not shown) having an opening at the end of the substrate including the spot size conversion region 50 is formed on the substrate on which the rib waveguide 23 is formed. Using this resist mask and the SiO 2 hard mask 24 on the silicon core, the silicon film 40 nm in the slab region 23 s is removed by etching. Etching of the silicon film in the slab region 23s is performed at 50 mTorr and 200 W by RIE of HBr gas. By this etching, the shape of the silicon core 13 of the channel type waveguide 5 is obtained. Further, the buried oxide film 12 on both sides of the silicon core 13 is exposed. Thereafter, the resist mask is peeled off.

  The upper side of FIG. 4C is a top view of the optical module 30, and the lower side is a CD cross-sectional view thereof. The SiO 2 hard mask 24 on the silicon 13 is stripped with hydrofluoric acid (HF), and the channel waveguide 5 is formed in the spot conversion region 50. The channel-type waveguide 5 can be formed continuously from the rib-type waveguide 23 by lithography. Thereby, the propagation light field shape of the rib-type waveguide 23 and the propagation light field shape of the channel-type waveguide 5 can be substantially matched.

  In FIG. 4D, the SiON core 15 of the spot size conversion element 10 is formed on the channel type waveguide 5. The process of forming the SiON core 15 will be described with reference to FIGS. 5A to 5F. 5A to 5F are manufacturing process diagrams in a cross section parallel to the light propagation direction.

  The upper side of FIG. 5A is a top view of the optical module 30, and the lower side is an EF cross-sectional view thereof. In FIG. 5A, after the end of the silicon core 13 in the spot size conversion region 50 is removed, the SiO 2 film 14 is formed on the silicon core 13 with a film thickness of 2 to 5 nm. In the case of forming by CVD, a source gas (SiH4: 20% / He: 80% + N2O) is supplied and deposited at a temperature of 790 ° C. for 2 to 5 nm. In the case of thermal oxidation, it is formed with a film thickness of 4 to 5 nm.

  Subsequently, a polysilicon film 16 is formed on the SiO 2 film 14 and the buried oxide film 12 with a thickness of 10 to 50 nm, preferably 20 to 30 nm. The thickness of the polysilicon film 16 is such a thickness that the metal silicide functions as an etching stopper when the SiON film is processed and does not hinder light diffusion in the spot size conversion portion. The polysilicon film 16 is formed at a temperature of 620 ° C. by supplying a source gas (SiH4: 20% / He: 80%) by CVD.

  In FIG. 5B, a resist mask (not shown) having an opening in the spot size conversion region 50 is formed by lithography, and the polysilicon film 16 is etched. Etching is, for example, RIE using HBr gas. The etching selectivity between the polysilicon film 16 and the underlying SiO 2 film 14 is large and does not damage the silicon core 13. Thereafter, the resist mask is peeled off.

  In FIG. 5C, a refractory metal film (not shown) such as Ni, Co, Ti, Pt, or Pd is formed on the entire surface. As an example, 20-30 nm of Ni is deposited. Then, nickel silicide (NiSi) 17 is formed by annealing at 400 ° C. for 120 minutes. The thickness of the NiSi layer 17 is 50 to 70 nm. Since silicide is formed only in the region where the polysilicon film 16 exists, unreacted Ni deposited in the region where the polysilicon film 16 does not exist can be peeled off with sulfuric acid / hydrogen peroxide (H 2 SO 2 and H 2 O 2). The NiSi layer 17 protrudes 20 to 40 nm in the spot size conversion region in consideration of the lithography alignment margin.

  In FIG. 5D, the SiO 2 film 14 on the silicon core 13 is optionally etched with a CF-based gas. Since the SiO2 film 14 is as thin as 2 to 5 nm, it may be left without being removed.

In FIG. 5E, a SiON film is deposited on the entire surface. Film formation conditions are as follows: SiH4 is supplied by CVD at 78 sccm (131.82 × 10 ⁻⁴ Pa · m ³ / sec), N 2 O is supplied at 500 sccm (845 × 10 ⁻⁴ Pa · m ³ / sec), and a temperature of 360 ° C. is 3 μm. accumulate. Further, a resist mask 18 is formed, and the SiON film 15 is etched by RIE. The etching gas is C4F8. At this time, the NiSi layer 17 functions as an etching stopper. The etching selectivity between SiON and nickel silicide (NiSi) is large, and damage to the silicon core 13 can be prevented. By this etching, the SiON core 15 in the spot size conversion region is formed. The SiON core 15 covers the end of the silicon core 13 and is connected to the silicon core 13. The SiOn core 15 slightly covers the NiSi layer 17. In this example, the protrusion amount of the NiSi layer 17 and the SiO2 film 14 to the SiON core 15 is 25 nm.

  In FIG. 5F, the NiSi layer 17 on the silicon core 13 is wet-etched with dilute hydrofluoric acid (HF). Since SiON and silicon (Si) are not etched with dilute hydrofluoric acid, only the NiSi layer 17 is removed. Thereafter, a TEOS film 21 is formed to a thickness of about 1 μm on the entire surface by CVD. By forming the TEOS film 21, the TEOS film enters the recess from which the NiSi layer 17 has been removed. As a result, the upper clad slightly enters the SiON core 15 and has a shape having protrusions 22. Thereby, the spot size conversion element 10 is completed.

  Next, with reference to FIGS. 6A to 6E, a manufacturing process of the spot size conversion element 40 of Example 2 will be described. 6A to 6E are cross-sectional views horizontal to the light propagation direction.

  In FIG. 6A, a SiO2 film 14 is formed to 5 to 10 nm on the channel-type waveguide (see FIG. 4C), and subsequently, Ti, Mo, V, W, Ta, Zr, Hf, nitrides thereof, etc. A melting point metal film 47 is formed to 10 to 50 nm. In this example, after the SiO2 film 14 is formed to a thickness of 5 nm by thermal oxidation, a titanium nitride (TiN) layer 47 is formed to a thickness of 20 nm by the CVD method. The TiN layer 47 functions as an etching stopper in a later process.

  In FIG. 6B, a resist mask (not shown) is formed, and the TiN layer 47 in the region to be the upper clad of the silicon waveguide is etched with chlorine gas. Thereafter, the resist mask is removed.

  In FIG. 6C, a TEOS film 41 is formed to a thickness of 1 μm on the entire surface, and the TEOS film 41 in a portion serving as a spot size conversion region is removed by etching using a resist mask 44. At this time, the TiN layer 47 serves as an etching stopper. In the state after the etching, the TiN layer 47 remains in a state protruding from 20 to 40 nm, in this example, about 25 nm, with respect to the TEOS film 41 serving as the upper clad.

  In FIG. 6D, the TiN layer 47 is wet etched with sulfuric acid peroxide (H 2 SO 2 + H 2 O 2), and the SiO 2 film 14 is further etched with dilute hydrofluoric acid (HF). A recess 51 is formed in the TEOS film 41 by etching.

  In FIG. 6E, a SiON film is formed to a thickness of about 3 μm on the entire surface by the CVD method. At this time, SiON enters the recess 51. After the formation of the SiON film, a resist mask (not shown) is formed at a predetermined position, and the SiON core 45 is formed by RIE. Since the silicon core 13 is covered with the TEOS film 41 having a sufficient thickness, the silicon core 13 is not damaged. Thereafter, a TEOS film (not shown) covering the entire surface may be further formed.

  By the method of Example 2, a spot size conversion element with little damage to the silicon core 13 can be formed. The TEOS film 41 as the upper clad slightly enters the SiON core 15, but does not hinder the spread of light from the silicon core 13 to the SiON core 45. This spot size conversion element 40 realizes low-loss optical coupling.

  In the first and second embodiments, the fabrication of a single spot size conversion element has been described for the sake of illustration. However, a plurality of spot size conversion elements are arranged in an array corresponding to a plurality of silicon fine wire optical waveguides. Even in this case, the method of the embodiment is effective.

  Further, the method of the embodiment can be applied to a case where a silica-based material having a refractive index larger than that of SiO 2 is used as the second core material in addition to SiON.

The following notes are presented for the above explanation.
(Appendix 1)
Forming a first core having a first core diameter on the first cladding layer on the substrate;
Except for the spot size conversion region, a metal silicide is formed on the first core through a silicon oxide film,
Depositing a second core material having a refractive index larger than that of the first cladding layer on the entire surface of the substrate;
Using the metal silicide as an etching stopper, the second core material is etched, and the spot size conversion region has a second core diameter that is coupled to the first core and larger than the first core diameter. Forming a second core;
Removing the metal silicide;
A method of manufacturing a spot size conversion element, comprising: forming a second cladding layer that covers the first core and the second core after removing the metal silicide.
(Appendix 2)
The second core covers an end of the first core and is formed to partially overlap the metal silicide on the first core,
By removing the metal silicide, a recess in the light propagation direction is formed between the second core and the first core,
The method for manufacturing a spot size conversion element according to appendix 1, wherein the second cladding layer enters the recess.
(Appendix 3)
The formation of the metal silicide is
A polysilicon film is formed on the entire substrate area excluding the spot size conversion area via the silicon oxide film,
A refractory metal film is disposed on the polysilicon film, and a metal silicide is formed by heat treatment;
The manufacturing method of the spot size conversion element of Additional remark 1 or 2 characterized by the above-mentioned.
(Appendix 4)
4. The method for manufacturing a spot size conversion element according to appendix 3, wherein the metal silicide is a material having a high etching selection ratio with SiON.
(Appendix 5)
Forming a first core having a first core diameter on the first cladding layer on the substrate;
A metal film is formed on the first cladding layer and the first core in the spot size conversion region via a silicon oxide film,
Forming a second cladding material film on the entire surface of the substrate;
Etching the second cladding material film using the metal film as an etching stopper, forming a second cladding layer on the first core excluding the spot size conversion region,
Removing the metal film,
Depositing a second core material having a higher refractive index than the first cladding layer and the second cladding layer on the entire surface of the substrate;
Processing the second core material into a predetermined shape, and in the spot size conversion region, a second core coupled to the first core and having a second core diameter larger than the first core diameter. Form,
A method for manufacturing a spot size conversion element.
(Appendix 6)
By removing the metal film, a depression along the light propagation direction is formed between the first core and the second cladding,
The method for manufacturing a spot size conversion element according to appendix 5, wherein the second core enters the recess. (Claim 4, FIG. 6D6E)
(Appendix 7)
The method for manufacturing a spot size conversion element according to appendix 5 or 6, wherein the metal film contains Ti, Mo, V, W, Ta, Zr, Hf, and nitrides thereof.
(Appendix 8)
A first cladding layer on the substrate;
A first core located on the first cladding layer and having a first core diameter;
A second core coupled to the first core on the first cladding and having a second core diameter greater than the first core diameter;
A second cladding layer covering the first core and the second core;
Including
The second core and the second clad layer are adjacent to each other on the first core, and at the interface between the second core and the second clad layer, either the second core or the second clad However, the optical waveguide with a light converting element has a protrusion protruding in the light propagation direction with respect to the other.

  It can be used for spot size conversion in optical communication. In particular, it can be used in a server system, an optical network, etc., which optically couples a photonics device using a silicon waveguide with an optical fiber or a laser.

2 Optical fiber 5 Channel type waveguide 10, 40 Spot size conversion element 11 Silicon substrate 12 Embedded oxide film 13 Silicon core or silicon waveguide (first core)
14 Silicon oxide films 15 and 45 SiON core (second core)
16 Polysilicon film 17 Metal silicide 21, 41 TEOS film (silicon oxide film)
22, 52 Protrusions 23 Rib type waveguide 30 Optical module 47 Refractory metal film

Claims (5)

Forming a first core having a first core diameter on the first cladding layer on the substrate;
Except for the spot size conversion region, a metal silicide is formed on the first core through a silicon oxide film,
Depositing a second core material having a refractive index larger than that of the first cladding layer on the entire surface of the substrate;
Using the metal silicide as an etching stopper, the second core material is etched, and the spot size conversion region has a second core diameter that is coupled to the first core and larger than the first core diameter. Forming a second core;
Removing the metal silicide;
A method of manufacturing a spot size conversion element, comprising: forming a second cladding layer that covers the first core and the second core after removing the metal silicide. The second core covers an end of the first core and is formed to partially overlap the metal silicide on the first core,
By removing the metal silicide, a recess in the light propagation direction is formed between the second core and the first core,
The second cladding layer enters the recess,
The method for manufacturing a spot size conversion element according to claim 1. Forming a first core having a first core diameter on the first cladding layer on the substrate;
A metal film is formed on the first cladding layer and the first core in the spot size conversion region via a silicon oxide film,
Forming a second cladding material film on the entire surface of the substrate;
Etching the second cladding material film using the metal film as an etching stopper, forming a second cladding layer on the first core excluding the spot size conversion region,
Removing the metal film,
Depositing a second core material having a higher refractive index than the first cladding layer and the second cladding layer on the entire surface of the substrate;
Processing the second core material into a predetermined shape, and in the spot size conversion region, a second core coupled to the first core and having a second core diameter larger than the first core diameter. Form,
A method for manufacturing a spot size conversion element. By removing the metal film, a recess along the light propagation direction is formed between the first core and the second cladding layer,
The method for manufacturing a spot size conversion element according to claim 3, wherein the second core enters the recess. A first cladding layer on the substrate;
A first core located on the first cladding layer and having a first core diameter;
A second core coupled to the first core on the first cladding and having a second core diameter greater than the first core diameter;
A second cladding layer covering the first core and the second core;
Including
The second core and the second clad layer are adjacent to each other on the first core, and at the interface between the second core and the second clad layer, either the second core or the second clad However, the optical waveguide with a light converting element has a protrusion protruding in the light propagation direction with respect to the other.

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