WO2018076308A1 - Optical device and method for fabricating the same - Google Patents

Optical device and method for fabricating the same Download PDF

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Publication number
WO2018076308A1
WO2018076308A1 PCT/CN2016/103878 CN2016103878W WO2018076308A1 WO 2018076308 A1 WO2018076308 A1 WO 2018076308A1 CN 2016103878 W CN2016103878 W CN 2016103878W WO 2018076308 A1 WO2018076308 A1 WO 2018076308A1
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WO
WIPO (PCT)
Prior art keywords
layer
substrate
waveguide
groove
forming
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PCT/CN2016/103878
Other languages
French (fr)
Inventor
Susumu HIMI
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Huawei Technologies Co., Ltd.
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Application filed by Huawei Technologies Co., Ltd. filed Critical Huawei Technologies Co., Ltd.
Priority to CN201680090388.6A priority Critical patent/CN110192134B/en
Priority to PCT/CN2016/103878 priority patent/WO2018076308A1/en
Priority to CN202110200975.9A priority patent/CN112987170A/en
Publication of WO2018076308A1 publication Critical patent/WO2018076308A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4219—Mechanical fixtures for holding or positioning the elements relative to each other in the couplings; Alignment methods for the elements, e.g. measuring or observing methods especially used therefor
    • G02B6/4228—Passive alignment, i.e. without a detection of the degree of coupling or the position of the elements
    • G02B6/423—Passive alignment, i.e. without a detection of the degree of coupling or the position of the elements using guiding surfaces for the alignment
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/122—Basic optical elements, e.g. light-guiding paths
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12083—Constructional arrangements
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/36—Mechanical coupling means
    • G02B6/3628—Mechanical coupling means for mounting fibres to supporting carriers
    • G02B6/3632—Mechanical coupling means for mounting fibres to supporting carriers characterised by the cross-sectional shape of the mechanical coupling means
    • G02B6/3636—Mechanical coupling means for mounting fibres to supporting carriers characterised by the cross-sectional shape of the mechanical coupling means the mechanical coupling means being grooves

Definitions

  • the present invention described herein in general, relates to photo-electric field, and in particularly, to an optical device wherein a waveguide and an optical fiber are aligned with high accuracy, and to a method for fabricating the same.
  • an optical device always concludes a structure on a substrate, and the structure concludes a waveguide and a groove in which an optical fiber is disposed. Further, the optical fiber is aligned with the waveguide.
  • the objective of the present invention is to provide an optical device wherein the light coupling between a waveguide and an optical fiber is improved by accurately aligning the waveguide and the optical fiber, and a method for fabricating the same.
  • the present invention provides an optical device comprising: a substrate having a groove on a first surface; a waveguide disposed on the first surface of the substrate; and an optical fiber disposed in the groove; wherein a difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber is about 0.5 ⁇ m or less.
  • the waveguide and the optical fiber have excellent light coupling resulting from accurately aligning the optical axis of the waveguide and the optical axis of the optical fiber.
  • the substrate may be a silicon substrate.
  • the waveguide may comprise an underclad, a core, and an overclad.
  • the underclad may have a thickness of about 3 ⁇ m.
  • the overclad may have a thickness of about 6 ⁇ m.
  • the core may have a thickness of about 3 ⁇ m.
  • the underclad and the overclad may comprise SiO 2 .
  • a waveguide having a small dimension of micrometer size is advantageous for integration.
  • the core may be flat. Since the core is flat, the optical axis of the waveguide comprising the core and the optical axis of the optical fiber are accurately aligned.
  • the core may comprise a semiconductor core and an insulator core.
  • the core may comprise a silicon layer and an SiO x layer (0 ⁇ x ⁇ 2) .
  • the semiconductor core may comprise silicon, and the insulator core may comprise SiO x (0 ⁇ x ⁇ 2) .
  • the semiconductor core may have a width of about 400 nm and a thickness from about 200 nm to about 250 nm in a plane perpendicular to the longitudinal direction of the waveguide.
  • the semiconductor core may be completely covered by the insulator core. Since the waveguide can be fabricated by a complementary metal oxide semiconductor (CMOS) compatible process, manufacturing efficiency and cost-effectiveness are high.
  • CMOS complementary metal oxide semiconductor
  • the groove may have a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the optical fiber.
  • the groove having the V-shaped cross-section that is, a V-shaped groove, may have a depth of about 60 ⁇ m.
  • the error of the depth of the V-shaped groove may be about 0.5 ⁇ m or less, preferably about 0.4 ⁇ m or less. Since the V-shaped groove has a small error, the optical fiber disposed thereon is accurately aligned with the waveguide.
  • a part of the waveguide may protrude above the groove along with the first surface.
  • the groove may have a U-shaped or rectangular cross-section in a plane perpendicular to the longitudinal direction of the optical fiber. Since the groove having the U-shaped or rectangular cross-section can be formed by the CMOS compatible process, manufacturing efficiency and cost-effectiveness are high.
  • a commercially available optical fiber can be used as the optical fiber disposed in the groove.
  • the diameter of the optical fiber may be about 125 ⁇ m.
  • the diameter of the optical fiber is selected so that the optical fiber and the waveguide are accurately aligned.
  • An auxiliary underclad that is disposed under the waveguide and is adjacent to the groove may be included in the substrate. Due to the auxiliary underclad, reflection by the substrate is suppressed. Thus, the light coupling between the waveguide and the optical fiber is improved.
  • the substrate may comprise a trench, and a light source may be disposed in the trench.
  • the depth of the trench may be about 10 ⁇ m.
  • a commercially available light source may be used as the light source disposed in the trench.
  • the present invention provides a fabricating method of an optical device comprising: a step (S1) of preparing a first substrate having a groove in a first surface; a step (S2) of providing a waveguide layer in the first surface by bonding; a step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer; and a step (S4) of disposing an optical fiber in the groove.
  • the groove of the first substrate can be formed by any method.
  • the error of the depth of the groove can be accurately controlled within about 0.5 ⁇ m or less.
  • the step (S2) of providing a waveguide layer in the first surface by bonding may comprise: a step (S2-1) of preparing a second substrate having a first insulating layer on a second substrate, a semiconductor core layer on the first insulating layer, and an underclad layer on the semiconductor core layer; a step (S2-2) of bonding the first substrate and the second substrate through the first surface and the underclad layer; a step (S2-3) of removing the second substrate and the first insulating layer; a step (S2-4) of etching a part of the semiconductor core layer to form a semiconductor core; and a step (S2-5) of forming an insulator core layer which covers the semiconductor core on the underclad layer, and forming an overclad layer which covers the insulator core layer on the underclad layer.
  • the thickness of the first insulating layer may be about 0.2 ⁇ m, the thickness of the semiconductor core layer may be from about 200 nm to about 250 nm.
  • the step (S2-1) of preparing a second substrate may comprise: a step (S2-1-1) of bonding the second substrate and a third substrate through a second insulating layer on the semiconductor core layer and a third insulating layer on the third substrate; a step (S2-1-2) of removing the third substrate and forming the underclad layer which includes the second insulating layer and the third insulating layer.
  • the thickness of the second insulating layer may be about 0.2 ⁇ m, the thickness of the third insulating layer may be about 3 ⁇ m.
  • the step (S1) of preparing a first substrate may include a step (S1-1) of forming the groove in the first surface by wet etching.
  • the step (S1) of preparing a first substrate may include a step (S1-2) of forming an auxiliary underclad in the first substrate.
  • the auxiliary underclad may have a thickness from about 2 ⁇ m to about 12 ⁇ m.
  • the auxiliary underclad may be formed of the same material as the underclad layer.
  • the step (S1) of preparing a first substrate may include a step (S1-3) of forming a trench in the first surface by etching.
  • the step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer may include a step (S3-1) of etching a part of the waveguide layer so that the waveguide partially covers above the groove.
  • the step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer may include a step (S3-2) of etching a part of the waveguide layer so that the semiconductor core is not exposed.
  • the step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer may include a step (S3-3) of exposing the trench by etching a part of the waveguide layer.
  • the first substrate, the second substrate, and the third substrate may be silicon substrates, the first insulating layer, the second insulating layer, and the third insulating layer may be SiO 2 layers, the semiconductor core layer may be a silicon layer, the insulator core layer may be an SiO x layer (0 ⁇ x ⁇ 2) , and the underclad layer and the overclad layer may be SiO 2 layers.
  • the groove may have a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the optical fiber.
  • a difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber may be about 0.5 ⁇ m or less.
  • the present invention provides a method for fabricating an optical device, comprising: a step (P1) of preparing a first substrate having a third substrate through an underclad layer in a first surface, and having a second substrate through an etching-stop layer in a second surface; a step (P2) of providing a waveguide layer in the first surface of the first substrate; a step (P3) of forming a waveguide and a groove by etching parts of the waveguide layer and the first substrate so that the etching-stop layer is exposed; and a step (P4) of disposing an optical fiber in the groove.
  • the depth of the groove formed in the first substrate is accurately controlled when the first substrate is etched.
  • the optical fiber disposed in the groove and the waveguide have excellent light coupling.
  • the etching-stop layer may comprise SiO 2 .
  • the etching-stop layer may have a thickness of about 1 ⁇ m.
  • the step (P1) of preparing a first substrate may include a step (P1-1) of bonding the first substrate and the second substrate through the etching-stop layer on the second substrate.
  • the step (P1) of preparing a first substrate includes: a step (P1-2) of bonding the first substrate and the third substrate through the underclad layer on the first surface; and a step (P1-3) of removing a part of the first substrate.
  • the step (P1-3) of removing a part of the first substrate may be performed by grinding and fine chemical mechanical polishing (CMP) .
  • the obtained thickness (H) of the first substrate may be controlled with an error of about 0.5 ⁇ m or less, preferably about 0.4 ⁇ m or less.
  • the step (P2) of providing a waveguide layer may include: a step (P2-1) of forming a semiconductor core layer by thinning the third substrate; a step (P2-2) of forming a semiconductor core by etching a part of the semiconductor core layer; and a step (P2-3) of forming an insulator core layer which covers the semiconductor core on the underclad layer, and forming an overclad layer which covers the insulator core layer on the underclad layer.
  • the thickness of the semiconductor core layer may be from about 200 nm to about 250 nm.
  • the step (P1) of preparing a first substrate may include: a step (P1-4) of forming a smart-cut line by implanting impurities into the third substrate, and the step (P2-1) of forming a semiconductor core layer may include: a step (P2-1-1) of cutting the third substrate along the smart-cut line.
  • the step (P3) of forming a waveguide and groove may include: a step (P3-1) of etching a part of the waveguide layer so that the semiconductor core is not exposed.
  • the step (P3) of forming a waveguide and groove may include: a step (P3-2) of forming the waveguide and the groove by dry etching.
  • the step (P3) of forming a waveguide and groove may include: a step (P3-3) of forming a trench by etching parts of the waveguide layer and the first substrate.
  • the step (P1) of preparing a first substrate may include: a step (P1-5) of forming an auxiliary underclad within the first substrate.
  • the step (P3) of forming a waveguide and groove may include: a step (P3-4) of etching the first substrate so that the auxiliary underclad is exposed.
  • the first substrate, the second substrate, and the third substrate may be silicon substrates
  • the semiconductor core layer may be a silicon layer
  • the insulator core layer may be an SiO x layer (0 ⁇ x ⁇ 2)
  • the etching-stop layer the underclad layer
  • the overclad layer may be SiO 2 layers.
  • the groove may have a U-shaped or rectangular cross-section in a plane perpendicular to the longitudinal direction of the optical fiber.
  • the difference between the distance from the first surface to an optical axis of the waveguide and the distance from the first surface to an optical axis of the optical fiber may be about 0.5 ⁇ m or less.
  • the present invention can provide an optical device wherein light coupling between the waveguide and the optical fiber is improved by accurately aligning the waveguide and the optical fiber, and provides a method for fabricating the same.
  • Figs. 1 to 12 show a fabricating process for the optical device according to a first embodiment of the present invention.
  • Fig. 13 shows the optical device according to a first variant of the first embodiment of the present invention.
  • Fig. 14 shows the optical device according to a second variant of the first embodiment of the present invention.
  • Figs. 15 to 25 show a fabricating process for the optical device according to a second embodiment of the present invention.
  • Figs. 26 to 36 show a fabricating process for the optical device according to a variant of the second embodiment of the present invention.
  • Fig. 37 shows a perspective view of the optical device of the present invention.
  • Fig. 38 shows a sectional view of the optical device of the present invention.
  • the present invention provides an optical device wherein light coupling between a waveguide and an optical fiber is improved by accurately aligning the waveguide and the optical fiber, and provides a method for fabricating the same.
  • Fig. 12 is a sectional view of the optical device 10 along an optical axis 121 of an optical fiber 120 and an optical axis 111 of a waveguide 110.
  • Fig. 37 is a perspective view of the optical device 10.
  • Fig. 38 is a sectional view showing the positional relationships of a substrate 100 and the optical fiber 120 included in the optical device 10.
  • the optical device 10 comprises the substrate 100 having a V-shaped groove 102 and a trench 104 on a first surface 101, the waveguide 110 provided on the first surface 101 of the substrate 100, the optical fiber 120 disposed in the V-shaped groove 102, and a light source 105 disposed in the trench 104.
  • the waveguide 110 comprises an underclad 112, a core 113, and an overclad 114 provided in sequence on the first surface 101 of the substrate 100.
  • the core 113 comprises a semiconductor 115 and an insulator core 116 covering the semiconductor core 115.
  • the waveguide 110 has an accurate thickness because it is formed by using the conventional CMOS technique.
  • the V-shaped groove 102 of the substrate 100 has a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the substrate 100.
  • the longitudinal direction of the substrate 100 corresponds to the y-axis in Fig. 37.
  • a sectional view of the V-shaped groove 102 in which the optical fiber 120 is disposed is shown in Fig. 38.
  • the V-shaped groove 102 is formed by using anisotropic wet etching.
  • An etchant including potassium hydroxide (KOH) is capable of selectively etching a (100) plane of silicon. It is known that the etching rate of the (100) plane is about 100 times faster than the etching rate of a (111) plane.
  • the V-shaped groove 102 whose (111) plane is exposed is formed by anisotropic etching of the substrate 100 of which the first surface 101 is a (100) plane.
  • the V-shaped groove 102 formed by wet etching has an accurate depth. Specifically, the V-shaped groove 102 formed by wet etching is accurately controlled with an error of about 0.5 ⁇ m or less.
  • V-shaped groove 102 may also have an inclined plane 108 in a plane parallel to the longitudinal direction of the optical fiber 120 as shown in Fig. 12.
  • the optical fiber 120 is disposed in the V-shaped groove 102 whose depth is accurately controlled.
  • the optical fiber 120 is disposed in the V-shaped groove so that the optical axis 121 of the optical fiber 120 is set to the desired height. Accordingly, the optical axis 121 of the optical fiber 120 disposed in the V-shaped groove 102 and the optical axis 111 of the waveguide 110 are accurately aligned. As a result, the optical fiber 120 and the waveguide 110 have excellent light coupling.
  • the light source 105 is disposed in the trench 104.
  • the light source 105 is connected to a predetermined circuit (not shown) through a solder 107.
  • the optical axis 111 of the waveguide 110 and the optical axis 121 of the optical fiber 120 are accurately aligned.
  • Fig. 13 is a sectional view of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110. A part of the waveguide 110 protrudes above the V-shaped groove 102 along the first surface 101 of the Si substrate 100 (Fig. 13) .
  • the V-shaped groove 102 is formed by anisotropic wet etching.
  • the V-shaped groove 102 has a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the optical fiber 120.
  • the V-shaped groove 102 may have an inclined plane 108 in a plane parallel to the longitudinal direction of the optical fiber 120.
  • the inclined plane 108 of the V-shaped groove 102 may induce a problem in which the distance between the waveguide 110 and the optical fiber 120 is widened thereby degrading the light coupling therebetween.
  • the first variant can solve said problem which may be generated in the first embodiment. In the first variant, since a part of the waveguide 110 protrudes above the inclined plane 108 or over the inclined plane 108, above the V-shaped groove 102, the distance between the waveguide 110 and the optical fiber 120 is shortened thereby improving the light coupling there between.
  • Fig. 14 is a sectional view of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110.
  • a thick region of the underclad 112 in an edge portion of the waveguide 110 proximal to the optical fiber 120 is formed by forming an auxiliary underclad 103.
  • This region is called a spot size converter (SSC) 142, and can enlarge the spot size of the waveguide 110.
  • SSC spot size converter
  • the auxiliary underclad 103 may be formed of the same material as the underclad 112.
  • the thickness of only the underclad 112 might not be enough.
  • a signal from the light source 105 may be reflected by the silicon substrate 100 in the edge portion of the waveguide 110, and may not reach the optical fiber 120.
  • the underclad 112 can have a sufficient thickness in the edge portion of the waveguide 110. Thereby, since the reflection by the silicon substrate 100 is suppressed in the edge portion of the waveguide 110, the light coupling between the waveguide 110 and the optical fiber 120 is improved.
  • Fig. 25 is a sectional view of an optical device 20 along an optical axis 221 of an optical fiber 220 and an optical axis 211 of a waveguide 210.
  • the description regarding the same constitution as the optical device 10 of the first embodiment is omitted.
  • the optical device 20 of the second embodiment is different from the optical device 10 of the first embodiment in the point that the optical device 20 comprises a first substrate 200, an etching-stop layer 231, a second substrate 230, and an SiO 2 layer 232. Also, the optical device 20 of the second embodiment is different from the optical device 10 of the first embodiment in the point that a U-shaped groove or rectangular groove 202 is formed in the first substrate 200.
  • the first substrate 200 is provided on the etching-stop layer 231.
  • the waveguide 210 is provided on the first surface 201 of the first substrate 200.
  • the U-shaped groove or rectangular groove 202 is formed in the first substrate 200 by completely removing a part thereof in the depth direction. Since the distance from a first surface 201 of the first substrate 200 to the etching-stop layer 231, that is, the thickness of the first substrate 200, is accurately controlled, the U-shaped groove or rectangular groove 202 formed by dry etching has an accurate depth. Also, the U-shaped groove or rectangular groove 202 respectively has a U-shaped or rectangular cross-section in a plane perpendicular to the longitudinal direction of the substrate 200.
  • the optical fiber 220 is disposed in the U-shaped groove or rectangular groove 202.
  • the optical axis 221 of the optical fiber 220 disposed in the U-shaped groove or rectangular groove 202 and the optical axis 211 of the waveguide 210 are accurately aligned.
  • the optical fiber 220 and the waveguide 210 have excellent light coupling.
  • a light source 205 is disposed in a trench 204.
  • the light source 205 is connected to a predetermined circuit (not shown) through a solder 207.
  • Fig. 36 is a sectional view of the optical device 20 along the optical axis 221 of the optical fiber 220 and the optical axis 211 of the waveguide 210.
  • a thick region of an underclad 212 in the edge portion of the waveguide 210 proximal to the optical fiber 220 is formed by forming an auxiliary underclad 203.
  • This region is called a spot size converter (SSC) 242, and can enlarge the spot size of the waveguide 210.
  • the auxiliary underclad 203 may be formed of the same material as the underclad 212.
  • the thickness of only the underclad 212 may not be enough.
  • a signal from the light source 205 may be reflected by the first substrate 200 in the edge portion of the waveguide 210, and may not reach the optical fiber 220.
  • the underclad 212 can have a sufficient thickness in the edge portion of the waveguide 210. Thereby, since the reflection by the silicon substrate 200 is suppressed in the edge portion of the waveguide 210, the light coupling between the waveguide 210 and the optical fiber 220 is improved.
  • FIG. 1 to 12 are sectional views of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110.
  • the structured SOI is fabricated as shown in Figs. 1 to 5.
  • An Si substrate 130 having a thin BOX (SiO 2 ) layer 131, a thin Si layer 115’, and a thin SiO 2 layer 117 is prepared (Fig. 1) .
  • an Si substrate 140 having a conventional thick thermally oxidized SiO 2 layer 118, 119 on at least one surface thereof is prepared.
  • the thickness of the thermally oxidized SiO 2 layer 118, 119 may be about 3 ⁇ m.
  • the Si substrate 130 and the Si substrate 140 are bonded through the thin SiO 2 layer 117 and the thermally oxidized SiO 2 layer 118 by a conventional bonding technique.
  • the bonding strength of the thin SiO 2 layer 117 and the thermally oxidized SiO 2 layer 118 is increased by annealing at high temperature (Fig. 2) .
  • the thick SiO 2 layer 119 and the Si substrate 140 are removed by grinding, etching, or fine chemical mechanical polishing (CMP) , and the thick SiO 2 layer 118 is maintained (Fig. 3) .
  • CMP chemical mechanical polishing
  • the V-shaped groove 102 and the trench 104 are formed in the first surface 101 thereof by conventional wet etching.
  • the V-shaped groove 102 is formed by selectively etching a (100) plane of silicon, for example, by using an etchant including potassium hydroxide (KOH) . It is known that the etching rate of the (100) plane is about 100 times faster than the etching rate of a (111) plane. Thus, the V-shaped groove 102 whose (111) plane is exposed is formed by anisotropic etching of the substrate 100 in which the first surface 101 is a (100) plane.
  • the Si substrate 100 and the Si substrate 130 are bonded through the first surface 101 and the thick SiO 2 layer 118 by a conventional bonding technique (Fig. 4) .
  • the Si substrate 130 is then removed by grinding, etching, or fine chemical mechanical polishing (CMP) .
  • CMP chemical mechanical polishing
  • the thin SiO 2 layer 131 is removed by etching using hydrofluoric acid (HF) or the like (Fig. 5) .
  • Figs. 1 to 5 The process for fabricating the structured SOI having the V-shaped groove 102 has been described referring to Figs. 1 to 5.
  • the subsequent processes shown in Figs. 6 to 10 are conventional Si waveguide forming processes which are compatible with CMOS.
  • the thick SiO 2 layer 118 and the thin SiO 2 layer 117 constitute the underclad layer 112’ (Fig. 6) .
  • a part of the semiconductor core layer 115’ is etched to form the semiconductor core 115 (Fig. 7) .
  • An insulator core layer 116’which covers the semiconductor core 115 is formed on the underclad layer 112’, and the side part of the insulator core layer 116’ is etched.
  • An overclad layer 114’which covers the insulator core layer 116’ is formed on the underclad layer 112’ (Fig. 8) .
  • the underclad layer 112’, the semiconductor core 115, the insulator core layer 116’, and the overclad layer 114’ constitute a waveguide layer 110’.
  • a part of the waveguide layer 110’ is removed by dry etching, and the trench 104 is exposed (Fig. 9) .
  • the waveguide 110 includes the underclad 112, the semiconductor core 115, the insulator core 116, and the overclad 114.
  • a solder 107 is formed in the trench 104, in order to dispose a light source 105 (Fig. 11) .
  • the optical fiber 120 is disposed in the V-shaped groove 102 so that the optical axis 111 of the waveguide 110 and the optical axis 121 of the optical fiber 120 have light coupling.
  • the light source 105 is disposed in the trench 104 by the solder 107 so that the waveguide 110 and the light source (LD) 105 have light coupling (Fig. 12) .
  • the exposed V-shaped groove 102 is pre-formed by wet etching.
  • the optical fiber 120 formed in the V-shaped groove 102 is disposed at the desired position.
  • the waveguide 110 is formed with high accuracy by a conventional Si waveguide forming process. Therefore, the connection between the waveguide 110 and the optical fiber 120 is achieved with high accuracy by passive alignment, and high efficiency and low coupling loss are realized.
  • the first embodiment is a method for fabricating the optical device 10 shown in Fig. 12.
  • a fabricating method of the present invention can provide the structured SOI by using wafer bonding technique.
  • the structured SOI has a conventional V-shaped groove and a trench under a BOX layer of SOI.
  • This kind of SOI can be used for a conventional Si waveguide forming process which is compatible with CMOS, because the surface of the SOI is perfectly flat.
  • the BOX layer on the V-shaped groove can be easily removed. Since the V-shaped groove is formed with high accuracy by a wet etching process, the alignment of the optical fiber and the waveguide is easily achieved.
  • FIG. 13 is a sectional view of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110.
  • the waveguide layer 110’ is etched so that the waveguide 110 does not protrude above the groove 102 as shown in Fig. 10.
  • the waveguide 110’ is etched so that the waveguide 110 protrudes above the V-shaped groove 102 along the first surface 101, as shown in Fig. 13.
  • the waveguide 110 can protrude above the inclined plane 108 or passed the inclined plane 108, above the V-shaped groove 102. Since the V-shaped groove 102 is pre-formed in the substrate 100, the optical device such as shown in Fig. 13 can be obtained.
  • FIG. 14 A second variant for fabricating the optical device 10 is shown in Fig. 14.
  • the optical device 10 shown in Fig. 14 is similarly fabricated according to the variant of the second embodiment which will be described below.
  • Figs. 15 to 25 are sectional views of the optical device 20 along the optical axis 221 of the optical fiber 220 and the optical axis 211 of the waveguide 210.
  • the processes shown in Figs. 15 to 19 provide a SOI comprising double BOX layers 212’, 231.
  • a smart-cut line 241 is formed by implanting impurities such as hydrogen into a silicon substrate 240 (Fig. 15) .
  • a silicon substrate 200 having thick SiO 2 layer (s) 212’, 212” on both surfaces or one surface (not shown) is prepared.
  • the silicon substrate 240 and the silicon substrate 200 are bonded through the thick SiO 2 layer 212’ (Fig. 16) .
  • the SiO 2 layer 212’ corresponds to an underclad layer 212’of a waveguide layer 210’.
  • the thickness of the thick SiO 2 layers 212’, 212” of the silicon substrate 200 may be about 2 ⁇ m to about 3 ⁇ m.
  • a silicon substrate 230 having thin SiO 2 layer (s) 231, 232 on both surfaces or one surface (not shown) is prepared.
  • the thin SiO 2 layers 231, 232 may have a thickness of about 1 ⁇ m.
  • the silicon substrate 200 and the silicon substrate 230 are bonded through the SiO 2 layer 231 (Fig. 18) .
  • the SiO 2 layer 231 corresponds to the etching-stop layer 231.
  • the silicon substrate 240 is then cut along the pre-formed smart-cut line 241 to obtain a thin silicon layer 215’ (Fig. 19) .
  • the thin silicon layer 215’ corresponds to the semiconductor core layer 215’of a waveguide layer 210’.
  • a semiconductor core 215 is formed by etching a part of the semiconductor core layer 215’ (Fig. 20) .
  • An insulator core layer 216’which covers the semiconductor core 215 is formed on the underclad layer 212’, and the side part of the insulator core layer 216’is etched.
  • the underclad layer 212’, the semiconductor core 215, the insulator core layer 216’, and the overclad layer 214’ constitute the waveguide layer 210’.
  • a part of the waveguide layer 210’a nd a part of the silicon substrate 200 are etched to form a trench 204 (Fig. 22) .
  • a part of the waveguide layer 210’and a part of the silicon substrate 200 are etched to form a U-shaped groove or rectangular groove 202 (Fig. 23) .
  • the U-shaped groove or rectangular groove 202 may be formed by dry etching. Since the SOI shown in Fig. 23 has the SiO 2 layer 231 therein, etching is stopped by this SiO 2 layer. Thereby, the U-shaped groove or rectangular groove 202 having a controlled depth (H) with an error of about 0.5 ⁇ m or less is obtained. Thus, passive alignment between the optical fiber 220 disposed in the U-shaped groove or rectangular groove 202 and the waveguide 210 is easily achieved.
  • a solder 207 is formed in the trench 204 in order to dispose a light source 205 (Fig. 24) .
  • the light source 205 is disposed in the trench 204, and the optical fiber 220 is disposed in the U-shaped groove or rectangular groove 202 (Fig. 25) .
  • the etching-stop layer 231 is provided between the silicon substrate 200 and the silicon substrate 230.
  • the depth of the groove 202 formed in the silicon substrate 200 is accurately defined.
  • the second embodiment is a method for fabricating the optical device 20 shown in Fig. 25.
  • the optical device 20 comprises two BOX layers 212’, 231 as shown in Fig. 19.
  • the distance between the two BOX layers 212’, 231, that is, the thickness of the first substrate 200 is accurately controlled with an optimized value for forming the U-shaped groove or rectangular groove 202.
  • Such SOI can be used for the conventional Si waveguide forming process which is compatible with CMOS because the surface of the SOI is perfectly flat.
  • a high-accuracy U-shaped groove or rectangular groove 202 is easily fabricated by a dry etching process because the BOX layer 231 rules as an etching-stop layer.
  • Figs. 26 to 36 are sectional views of the optical device 20 along the optical axis 221 of the optical fiber 220 and the optical axis 211 of the waveguide 210.
  • FIGs. 26 to 30 provide an SOI comprising double BOX layers 212’, 231.
  • a smart-cut line 241 is formed by implanting impurities such as hydrogen into a silicon substrate 240 (Fig. 26) .
  • a silicon substrate 200 having thick SiO 2 layer (s) 212’, 212” on both surfaces or one surface (not shown) is then prepared.
  • the silicon substrate 240 and the silicon substrate 200 are bonded through the thick SiO 2 layer 212’of the silicon substrate 200 (Fig. 27) .
  • an auxiliary underclad layer 203’ is formed adjacent to the SiO 2 layer 212’.
  • the SiO 2 layer 212” and the silicon substrate 200 are removed by grinding and fine chemical mechanical polishing (CMP) so that the auxiliary underclad layer 203’is not exposed (Fig. 28) .
  • CMP chemical mechanical polishing
  • a silicon substrate 230 having thin SiO 2 layer (s) 231, 232 on both surfaces or one surface (not shown) is prepared.
  • the silicon substrate 200 and the silicon substrate 230 are bonded through the SiO 2 layer 231 (Fig. 29) .
  • the silicon substrate 240 is then cut along the pre-formed smart-cut line 241 to obtain a thin silicon layer 215’ (Fig. 30) .
  • the processes shown in Figs. 31 to 35 are the same processes as the integration processes which are compatible with the CMOS process.
  • the processes shown in Figs. 31 to 33 are the same processes as those shown in Figs. 20 to 22.
  • a part of the waveguide layer 210’, a part of the auxiliary underclad layer 203’, and a part of the silicon substrate 200 are etched until the etching-stop layer 231 is exposed thereby forming a waveguide 210 having the auxiliary underclad 203 and a groove 202 (Fig. 34) .
  • the processes shown in Figs. 35 and 36 are the same processes as those shown in Figs. 24 and 25.
  • the waveguide 210 has the auxiliary underclad layer 203 in the edge portion proximal to the optical fiber 220, reflection by the silicon substrate 200 is suppressed thereby improving the light coupling between the waveguide 210 and the optical fiber 220.
  • the fabricating method disclosed in the present invention can provide an optical device comprising a high-accuracy V-shaped groove or U-shaped groove or rectangular groove, and waveguide. Therefore, the positional error between the optical axis of the waveguide and the optical axis of the optical fiber can be set to be on sub-micron order thereby improving the light coupling between the waveguide and the optical fiber.

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Abstract

An optical device and a method for fabricating an optical device are disclosed. The optical device (10) includes: a substrate (100) having a groove (102) on a first surface (101); a waveguide (110) disposed on the first surface (101) of the substrate (100); and an optical fiber (120) disposed in the groove (102); wherein a difference between a distance from the first surface (101) to an optical axis of the waveguide (110) and a distance from the first surface (101) to an optical axis of the optical fiber (120) is about 0.5 μm or less.

Description

AN OPTICAL DEVICE AND A METHOD FOR FABRICATING THE SAME BACKGROUND
Field
The present invention described herein, in general, relates to photo-electric field, and in particularly, to an optical device wherein a waveguide and an optical fiber are aligned with high accuracy, and to a method for fabricating the same.
Background
In the prior art, an optical device always concludes a structure on a substrate, and the structure concludes a waveguide and a groove in which an optical fiber is disposed. Further, the optical fiber is aligned with the waveguide. However, in the prior art, it has been difficult to obtain a structure which can realize an accurate alignment between the optical fiber and the waveguide.
SUMMARY
The objective of the present invention is to provide an optical device wherein the light coupling between a waveguide and an optical fiber is improved by accurately aligning the waveguide and the optical fiber, and a method for fabricating the same.
The present invention provides an optical device comprising: a substrate having a groove on a first surface； a waveguide disposed on the first surface of the substrate； and an optical fiber disposed in the groove； wherein a difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber is about 0.5 μm or less.
The waveguide and the optical fiber have excellent light coupling resulting from accurately aligning the optical axis of the waveguide and the optical axis of the optical fiber.
The substrate may be a silicon substrate. The waveguide may comprise an underclad, a core, and an overclad. The underclad may have a thickness of about 3μm. The overclad may have a thickness of about 6μm. The core may have a thickness of about 3 μm. The underclad and the overclad may comprise SiO2. A waveguide having a small dimension of micrometer size is advantageous for integration.
The core may be flat. Since the core is flat, the optical axis of the waveguide comprising the core and the optical axis of the optical fiber are accurately aligned.
The core may comprise a semiconductor core and an insulator core. The core may comprise a silicon layer and an SiOx layer (0<x<2) . The semiconductor core may comprise silicon, and the insulator core may comprise SiOx (0<x<2) . The semiconductor core may have a width of about 400 nm and a thickness from about 200 nm to about 250 nm in a plane perpendicular to the longitudinal direction of the waveguide. The semiconductor core may be completely covered by the insulator core. Since the waveguide can be fabricated by a complementary metal oxide semiconductor (CMOS) compatible process, manufacturing efficiency and cost-effectiveness are high.
The groove may have a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the optical fiber. The groove having the V-shaped cross-section, that is, a V-shaped groove, may have a depth of about 60 μm. The error of the depth of the V-shaped groove may be about 0.5 μm or less, preferably about 0.4 μm or less. Since the V-shaped groove has a small error, the optical fiber disposed thereon is accurately aligned with the waveguide.
A part of the waveguide may protrude above the groove along with the first surface. Thereby, the distance between the waveguide and the optical fiber can be shortened, and thus the light coupling between them can be improved.
The groove may have a U-shaped or rectangular cross-section in a plane perpendicular to the longitudinal direction of the optical fiber. Since the groove having the U-shaped or rectangular cross-section can be formed by the CMOS compatible process, manufacturing efficiency and cost-effectiveness are high.
A commercially available optical fiber can be used as the optical fiber disposed in the groove. The diameter of the optical fiber may be about 125 μm. The diameter of the optical fiber is selected so that the optical fiber and the waveguide are accurately aligned.
An auxiliary underclad that is disposed under the waveguide and is adjacent to the groove may be included in the substrate. Due to the auxiliary underclad, reflection by the substrate is suppressed. Thus, the light coupling between the waveguide and the optical fiber is improved.
The substrate may comprise a trench, and a light source may be disposed in the trench.
The depth of the trench may be about 10 μm. A commercially available light source may be used as the light source disposed in the trench.
Also, the present invention provides a fabricating method of an optical device comprising: a step (S1) of preparing a first substrate having a groove in a first surface； a step (S2) of providing a waveguide layer in the first surface by bonding； a step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer； and a step (S4) of disposing an optical fiber in the groove.
Since the groove can be provided in the first substrate before the waveguide layer is provided in the first surface of the first substrate, the groove of the first substrate can be formed by any method. When the groove is formed by wet-etching, the error of the depth of the groove can be accurately controlled within about 0.5 μm or less.
The step (S2) of providing a waveguide layer in the first surface by bonding may comprise: a step (S2-1) of preparing a second substrate having a first insulating layer on a second substrate, a semiconductor core layer on the first insulating layer, and an underclad layer on the semiconductor core layer； a step (S2-2) of bonding the first substrate and the second substrate through the first surface and the underclad layer； a step (S2-3) of removing the second substrate and the first insulating layer； a step (S2-4) of etching a part of the semiconductor core layer to form a semiconductor core； and a step (S2-5) of forming an insulator core layer which covers the semiconductor core on the underclad layer, and forming an overclad layer which covers the insulator core layer on the underclad layer.
The thickness of the first insulating layer may be about 0.2 μm, the thickness of the semiconductor core layer may be from about 200 nm to about 250 nm.
The step (S2-1) of preparing a second substrate may comprise: a step (S2-1-1) of bonding the second substrate and a third substrate through a second insulating layer on the semiconductor core layer and a third insulating layer on the third substrate； a step (S2-1-2) of removing the third substrate and forming the underclad layer which includes the second insulating layer and the third insulating layer.
The thickness of the second insulating layer may be about 0.2 μm, the thickness of the third insulating layer may be about 3 μm.
By bonding two insulating layers, a thick layer can be prepared which is difficult in the CMOS process.
The step (S1) of preparing a first substrate may include a step (S1-1) of forming the groove in the first surface by wet etching.
The step (S1) of preparing a first substrate may include a step (S1-2) of forming an auxiliary underclad in the first substrate.
The auxiliary underclad may have a thickness from about 2 μm to about 12 μm. The auxiliary underclad may be formed of the same material as the underclad layer.
The step (S1) of preparing a first substrate may include a step (S1-3) of forming a trench in the first surface by etching.
The step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer may include a step (S3-1) of etching a part of the waveguide layer so that the waveguide partially covers above the groove.
The step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer may include a step (S3-2) of etching a part of the waveguide layer so that the semiconductor core is not exposed.
The step (S3) of forming a waveguide and exposing the groove by etching a part of the waveguide layer may include a step (S3-3) of exposing the trench by etching a part of the waveguide layer.
The first substrate, the second substrate, and the third substrate may be silicon substrates, the first insulating layer, the second insulating layer, and the third insulating layer may be SiO2 layers, the semiconductor core layer may be a silicon layer, the insulator core layer may be an SiOx layer (0<x<2) , and the underclad layer and the overclad layer may be SiO2 layers.
The groove may have a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the optical fiber.
A difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber may be about 0.5 μm or less.
Furthermore, the present invention provides a method for fabricating an optical device, comprising: a step (P1) of preparing a first substrate having a third  substrate through an underclad layer in a first surface, and having a second substrate through an etching-stop layer in a second surface； a step (P2) of providing a waveguide layer in the first surface of the first substrate； a step (P3) of forming a waveguide and a groove by etching parts of the waveguide layer and the first substrate so that the etching-stop layer is exposed； and a step (P4) of disposing an optical fiber in the groove.
Due to the etching-stop layer, the depth of the groove formed in the first substrate is accurately controlled when the first substrate is etched. Thus, the optical fiber disposed in the groove and the waveguide have excellent light coupling.
The etching-stop layer may comprise SiO2. The etching-stop layer may have a thickness of about 1 μm.
The step (P1) of preparing a first substrate may include a step (P1-1) of bonding the first substrate and the second substrate through the etching-stop layer on the second substrate.
The step (P1) of preparing a first substrate includes: a step (P1-2) of bonding the first substrate and the third substrate through the underclad layer on the first surface； and a step (P1-3) of removing a part of the first substrate.
The step (P1-3) of removing a part of the first substrate may be performed by grinding and fine chemical mechanical polishing (CMP) . The obtained thickness (H) of the first substrate may be controlled with an error of about 0.5 μm or less, preferably about 0.4 μm or less.
The step (P2) of providing a waveguide layer may include: a step (P2-1) of forming a semiconductor core layer by thinning the third substrate； a step (P2-2) of forming a semiconductor core by etching a part of the semiconductor core layer； and a step (P2-3) of forming an insulator core layer which covers the semiconductor core on the underclad layer, and forming an overclad layer which covers the insulator core layer on the underclad layer.
The thickness of the semiconductor core layer may be from about 200 nm to about 250 nm.
The step (P1) of preparing a first substrate may include: a step (P1-4) of forming a smart-cut line by implanting impurities into the third substrate, and the step  (P2-1) of forming a semiconductor core layer may include: a step (P2-1-1) of cutting the third substrate along the smart-cut line.
The step (P3) of forming a waveguide and groove may include: a step (P3-1) of etching a part of the waveguide layer so that the semiconductor core is not exposed.
The step (P3) of forming a waveguide and groove may include: a step (P3-2) of forming the waveguide and the groove by dry etching.
The step (P3) of forming a waveguide and groove may include: a step (P3-3) of forming a trench by etching parts of the waveguide layer and the first substrate.
The step (P1) of preparing a first substrate may include: a step (P1-5) of forming an auxiliary underclad within the first substrate.
The step (P3) of forming a waveguide and groove may include: a step (P3-4) of etching the first substrate so that the auxiliary underclad is exposed.
The first substrate, the second substrate, and the third substrate may be silicon substrates, the semiconductor core layer may be a silicon layer, the insulator core layer may be an SiOx layer (0<x<2) , the etching-stop layer, the underclad layer, and the overclad layer may be SiO2 layers.
The groove may have a U-shaped or rectangular cross-section in a plane perpendicular to the longitudinal direction of the optical fiber.
The difference between the distance from the first surface to an optical axis of the waveguide and the distance from the first surface to an optical axis of the optical fiber may be about 0.5 μm or less.
The present invention can provide an optical device wherein light coupling between the waveguide and the optical fiber is improved by accurately aligning the waveguide and the optical fiber, and provides a method for fabricating the same.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1 to 12 show a fabricating process for the optical device according to a first embodiment of the present invention.
Fig. 13 shows the optical device according to a first variant of the first embodiment of the present invention.
Fig. 14 shows the optical device according to a second variant of the first embodiment of the present invention.
Figs. 15 to 25 show a fabricating process for the optical device according to a second embodiment of the present invention.
Figs. 26 to 36 show a fabricating process for the optical device according to a variant of the second embodiment of the present invention.
Fig. 37 shows a perspective view of the optical device of the present invention.
Fig. 38 shows a sectional view of the optical device of the present invention.
DETAILED DESCRIPTION
The embodiments of the present invention will be described in detail as follows.
The present invention provides an optical device wherein light coupling between a waveguide and an optical fiber is improved by accurately aligning the waveguide and the optical fiber, and provides a method for fabricating the same.
(First embodiment) Referring to Figs. 12, 37, and 38, an optical device 10 of the first embodiment is described. Fig. 12 is a sectional view of the optical device 10 along an optical axis 121 of an optical fiber 120 and an optical axis 111 of a waveguide 110. Fig. 37 is a perspective view of the optical device 10. Fig. 38 is a sectional view showing the positional relationships of a substrate 100 and the optical fiber 120 included in the optical device 10.
The optical device 10 comprises the substrate 100 having a V-shaped groove 102 and a trench 104 on a first surface 101, the waveguide 110 provided on the first surface 101 of the substrate 100, the optical fiber 120 disposed in the V-shaped groove 102, and a light source 105 disposed in the trench 104.
The waveguide 110 comprises an underclad 112, a core 113, and an overclad 114 provided in sequence on the first surface 101 of the substrate 100. The core 113 comprises a semiconductor 115 and an insulator core 116 covering the semiconductor core 115. The waveguide 110 has an accurate thickness because it is formed by using the conventional CMOS technique.
As shown in Fig. 37, the V-shaped groove 102 of the substrate 100 has a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the substrate 100. The longitudinal direction of the substrate 100 corresponds to the y-axis in Fig. 37. A sectional view of the V-shaped groove 102 in which the optical fiber 120 is disposed is shown in Fig. 38. The V-shaped groove 102 is formed by  using anisotropic wet etching. An etchant including potassium hydroxide (KOH) is capable of selectively etching a (100) plane of silicon. It is known that the etching rate of the (100) plane is about 100 times faster than the etching rate of a (111) plane. Thus, the V-shaped groove 102 whose (111) plane is exposed is formed by anisotropic etching of the substrate 100 of which the first surface 101 is a (100) plane. The V-shaped groove 102 formed by wet etching has an accurate depth. Specifically, the V-shaped groove 102 formed by wet etching is accurately controlled with an error of about 0.5 μm or less.
Also, the V-shaped groove 102 may also have an inclined plane 108 in a plane parallel to the longitudinal direction of the optical fiber 120 as shown in Fig. 12.
The optical fiber 120 is disposed in the V-shaped groove 102 whose depth is accurately controlled. Thus, the optical fiber 120 is disposed in the V-shaped groove so that the optical axis 121 of the optical fiber 120 is set to the desired height. Accordingly, the optical axis 121 of the optical fiber 120 disposed in the V-shaped groove 102 and the optical axis 111 of the waveguide 110 are accurately aligned. As a result, the optical fiber 120 and the waveguide 110 have excellent light coupling.
The light source 105 is disposed in the trench 104. The light source 105 is connected to a predetermined circuit (not shown) through a solder 107.
In the optical device 10 of the first embodiment, since the optical fiber 120 is disposed in the V-shaped groove 102 formed by wet etching, the optical axis 111 of the waveguide 110 and the optical axis 121 of the optical fiber 120 are accurately aligned.
A first variant of the first embodiment mentioned above will be described as follows.
(First variant) The first variant of the first embodiment is shown in Fig. 13. Fig. 13 is a sectional view of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110. A part of the waveguide 110 protrudes above the V-shaped groove 102 along the first surface 101 of the Si substrate 100 (Fig. 13) . In the first embodiment, the V-shaped groove 102 is formed by anisotropic wet etching. The V-shaped groove 102 has a V-shaped cross-section in a plane perpendicular to the longitudinal direction of the optical fiber 120. Furthermore, the V-shaped groove 102 may have an inclined plane 108 in a plane parallel to the longitudinal direction of the optical fiber 120. The inclined plane 108 of the V-shaped groove 102 may induce a problem in which the distance between the waveguide 110 and the optical fiber 120 is widened thereby degrading the  light coupling therebetween. The first variant can solve said problem which may be generated in the first embodiment. In the first variant, since a part of the waveguide 110 protrudes above the inclined plane 108 or over the inclined plane 108, above the V-shaped groove 102, the distance between the waveguide 110 and the optical fiber 120 is shortened thereby improving the light coupling there between.
(Second variant) A second variant of the first embodiment is shown in Fig. 14. Fig. 14 is a sectional view of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110. In the second variant, as shown by a circle 142 in Fig. 14, a thick region of the underclad 112 in an edge portion of the waveguide 110 proximal to the optical fiber 120 is formed by forming an auxiliary underclad 103. This region is called a spot size converter (SSC) 142, and can enlarge the spot size of the waveguide 110. The auxiliary underclad 103 may be formed of the same material as the underclad 112. Without the auxiliary underclad 103, the thickness of only the underclad 112 might not be enough. In this case, a signal from the light source 105 may be reflected by the silicon substrate 100 in the edge portion of the waveguide 110, and may not reach the optical fiber 120. By forming the auxiliary underclad 103, the underclad 112 can have a sufficient thickness in the edge portion of the waveguide 110. Thereby, since the reflection by the silicon substrate 100 is suppressed in the edge portion of the waveguide 110, the light coupling between the waveguide 110 and the optical fiber 120 is improved.
(Second embodiment) Next, referring to Fig. 25, an optical device 20 of a second embodiment will be described. Fig. 25 is a sectional view of an optical device 20 along an optical axis 221 of an optical fiber 220 and an optical axis 211 of a waveguide 210. The description regarding the same constitution as the optical device 10 of the first embodiment is omitted.
The optical device 20 of the second embodiment is different from the optical device 10 of the first embodiment in the point that the optical device 20 comprises a first substrate 200, an etching-stop layer 231, a second substrate 230, and an SiO2 layer 232. Also, the optical device 20 of the second embodiment is different from the optical device 10 of the first embodiment in the point that a U-shaped groove or rectangular groove 202 is formed in the first substrate 200.
In the optical device 20 of the second embodiment, the first substrate 200 is provided on the etching-stop layer 231. The waveguide 210 is provided on the first surface 201 of the first substrate 200. The U-shaped groove or rectangular groove 202 is formed in the first substrate 200 by completely removing a part thereof in the depth  direction. Since the distance from a first surface 201 of the first substrate 200 to the etching-stop layer 231, that is, the thickness of the first substrate 200, is accurately controlled, the U-shaped groove or rectangular groove 202 formed by dry etching has an accurate depth. Also, the U-shaped groove or rectangular groove 202 respectively has a U-shaped or rectangular cross-section in a plane perpendicular to the longitudinal direction of the substrate 200. The optical fiber 220 is disposed in the U-shaped groove or rectangular groove 202. Thus, the optical axis 221 of the optical fiber 220 disposed in the U-shaped groove or rectangular groove 202 and the optical axis 211 of the waveguide 210 are accurately aligned. As a result, the optical fiber 220 and the waveguide 210 have excellent light coupling.
A light source 205 is disposed in a trench 204. The light source 205 is connected to a predetermined circuit (not shown) through a solder 207.
(Variant) A variant of the second embodiment is shown in Fig. 36. Fig. 36 is a sectional view of the optical device 20 along the optical axis 221 of the optical fiber 220 and the optical axis 211 of the waveguide 210. In the variant, as shown by a circle 242 in Fig. 36, a thick region of an underclad 212 in the edge portion of the waveguide 210 proximal to the optical fiber 220 is formed by forming an auxiliary underclad 203. This region is called a spot size converter (SSC) 242, and can enlarge the spot size of the waveguide 210. The auxiliary underclad 203 may be formed of the same material as the underclad 212. Without the auxiliary underclad 203, the thickness of only the underclad 212 may not be enough. In this case, a signal from the light source 205 may be reflected by the first substrate 200 in the edge portion of the waveguide 210, and may not reach the optical fiber 220. By forming the auxiliary underclad 203, the underclad 212 can have a sufficient thickness in the edge portion of the waveguide 210. Thereby, since the reflection by the silicon substrate 200 is suppressed in the edge portion of the waveguide 210, the light coupling between the waveguide 210 and the optical fiber 220 is improved.
Next, a method for manufacturing the optical devices described in the embodiments and variants mentioned above is described as follows.
(First embodiment) A method for fabricating the optical device 10 of the present invention in which a structured SOI and Si photonics are combined is shown in Figs. 1 to 12. Figs. 1 to 12 are sectional views of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110.
The structured SOI is fabricated as shown in Figs. 1 to 5. An Si substrate 130 having a thin BOX (SiO2) layer 131, a thin Si layer 115’, and a thin SiO2 layer 117 is prepared (Fig. 1) .
Next, an Si substrate 140 having a conventional thick thermally oxidized SiO2 layer 118, 119 on at least one surface thereof is prepared. The thickness of the thermally oxidized SiO2 layer 118, 119 may be about 3 μm. The Si substrate 130 and the Si substrate 140 are bonded through the thin SiO2 layer 117 and the thermally oxidized SiO2 layer 118 by a conventional bonding technique. The bonding strength of the thin SiO2 layer 117 and the thermally oxidized SiO2 layer 118 is increased by annealing at high temperature (Fig. 2) .
The thick SiO2 layer 119 and the Si substrate 140 are removed by grinding, etching, or fine chemical mechanical polishing (CMP) , and the thick SiO2 layer 118 is maintained (Fig. 3) .
Another Si substrate 100 is prepared, and the V-shaped groove 102 and the trench 104 are formed in the first surface 101 thereof by conventional wet etching. The V-shaped groove 102 is formed by selectively etching a (100) plane of silicon, for example, by using an etchant including potassium hydroxide (KOH) . It is known that the etching rate of the (100) plane is about 100 times faster than the etching rate of a (111) plane. Thus, the V-shaped groove 102 whose (111) plane is exposed is formed by anisotropic etching of the substrate 100 in which the first surface 101 is a (100) plane. Next, the Si substrate 100 and the Si substrate 130 are bonded through the first surface 101 and the thick SiO2 layer 118 by a conventional bonding technique (Fig. 4) .
The Si substrate 130 is then removed by grinding, etching, or fine chemical mechanical polishing (CMP) . Finally, the thin SiO2 layer 131 is removed by etching using hydrofluoric acid (HF) or the like (Fig. 5) .
The process for fabricating the structured SOI having the V-shaped groove 102 has been described referring to Figs. 1 to 5. The subsequent processes shown in Figs. 6 to 10 are conventional Si waveguide forming processes which are compatible with CMOS.
The thick SiO2 layer 118 and the thin SiO2 layer 117 constitute the underclad layer 112’ (Fig. 6) .
A part of the semiconductor core layer 115’is etched to form the semiconductor core 115 (Fig. 7) .
An insulator core layer 116’which covers the semiconductor core 115 is formed on the underclad layer 112’, and the side part of the insulator core layer 116’ is etched. An overclad layer 114’which covers the insulator core layer 116’is formed on the underclad layer 112’ (Fig. 8) .
The underclad layer 112’, the semiconductor core 115, the insulator core layer 116’, and the overclad layer 114’constitute a waveguide layer 110’.
A part of the waveguide layer 110’is removed by dry etching, and the trench 104 is exposed (Fig. 9) .
A part of the waveguide layer 110’is removed by dry etching, and the V-shaped groove 102 is exposed and a waveguide 110 is formed (Fig. 10) . The waveguide 110 includes the underclad 112, the semiconductor core 115, the insulator core 116, and the overclad 114.
A solder 107 is formed in the trench 104, in order to dispose a light source 105 (Fig. 11) .
The optical fiber 120 is disposed in the V-shaped groove 102 so that the optical axis 111 of the waveguide 110 and the optical axis 121 of the optical fiber 120 have light coupling. Also, the light source 105 is disposed in the trench 104 by the solder 107 so that the waveguide 110 and the light source (LD) 105 have light coupling (Fig. 12) .
In the first embodiment, the exposed V-shaped groove 102 is pre-formed by wet etching. Thus, the optical fiber 120 formed in the V-shaped groove 102 is disposed at the desired position. Also, the waveguide 110 is formed with high accuracy by a conventional Si waveguide forming process. Therefore, the connection between the waveguide 110 and the optical fiber 120 is achieved with high accuracy by passive alignment, and high efficiency and low coupling loss are realized.
The first embodiment is a method for fabricating the optical device 10 shown in Fig. 12. A fabricating method of the present invention can provide the structured SOI by using wafer bonding technique. The structured SOI has a conventional V-shaped groove and a trench under a BOX layer of SOI. This kind of SOI can be used for a conventional Si waveguide forming process which is compatible with CMOS, because the surface of the SOI is perfectly flat. After a process for forming the waveguide and the trench for LD platform is finished, the BOX layer on the V-shaped groove can be easily removed. Since the V-shaped groove is formed with high accuracy by a wet etching process, the alignment of the optical fiber and the waveguide is easily achieved.
(First variant) A first variant of the first embodiment is shown in Fig. 13. Fig. 13 is a sectional view of the optical device 10 along the optical axis 121 of the optical fiber 120 and the optical axis 111 of the waveguide 110. In the first  embodiment, the waveguide layer 110’is etched so that the waveguide 110 does not protrude above the groove 102 as shown in Fig. 10. Meanwhile, in the first variant, the waveguide 110’is etched so that the waveguide 110 protrudes above the V-shaped groove 102 along the first surface 101, as shown in Fig. 13. The waveguide 110 can protrude above the inclined plane 108 or passed the inclined plane 108, above the V-shaped groove 102. Since the V-shaped groove 102 is pre-formed in the substrate 100, the optical device such as shown in Fig. 13 can be obtained.
(Second variant) A second variant for fabricating the optical device 10 is shown in Fig. 14. The optical device 10 shown in Fig. 14 is similarly fabricated according to the variant of the second embodiment which will be described below.
(Second embodiment) The detailed process flow of the second embodiment is shown in Figs. 15 to 25. Figs. 15 to 25 are sectional views of the optical device 20 along the optical axis 221 of the optical fiber 220 and the optical axis 211 of the waveguide 210. The processes shown in Figs. 15 to 19 provide a SOI comprising double BOX layers 212’, 231.
A smart-cut line 241 is formed by implanting impurities such as hydrogen into a silicon substrate 240 (Fig. 15) .
Then, a silicon substrate 200 having thick SiO2 layer (s) 212’, 212” on both surfaces or one surface (not shown) is prepared. The silicon substrate 240 and the silicon substrate 200 are bonded through the thick SiO2 layer 212’ (Fig. 16) . The SiO2 layer 212’corresponds to an underclad layer 212’of a waveguide layer 210’. The thickness of the thick SiO2 layers 212’, 212” of the silicon substrate 200 may be about 2 μm to about 3 μm.
Subsequently, all of the SiO2 layer 212” and a part of the silicon substrate 200 are removed by grinding and fine chemical mechanical polishing (CMP) (Fig. 17) . Since the thickness (H) of the remaining silicon substrate 200 corresponds to the depth of the groove 202 in which the optical fiber 220 is disposed, this process is carefully conducted.
Next, a silicon substrate 230 having thin SiO2 layer (s) 231, 232 on both surfaces or one surface (not shown) is prepared. The thin SiO2 layers 231, 232 may have a thickness of about 1 μm.
The silicon substrate 200 and the silicon substrate 230 are bonded through the SiO2 layer 231 (Fig. 18) . The SiO2 layer 231 corresponds to the etching-stop layer 231.
The silicon substrate 240 is then cut along the pre-formed smart-cut line 241 to obtain a thin silicon layer 215’ (Fig. 19) . The thin silicon layer 215’corresponds to the semiconductor core layer 215’of a waveguide layer 210’.
The processes shown in the Figs. 20 to 24 below are the same processes as the integration processes which are compatible with the CMOS process. A semiconductor core 215 is formed by etching a part of the semiconductor core layer 215’ (Fig. 20) .
An insulator core layer 216’which covers the semiconductor core 215 is formed on the underclad layer 212’, and the side part of the insulator core layer 216’is etched. An overclad layer 214’which covers the insulator core layer 216’is formed on the underclad layer 212’ (Fig. 21) . The underclad layer 212’, the semiconductor core 215, the insulator core layer 216’, and the overclad layer 214’constitute the waveguide layer 210’.
Next, a part of the waveguide layer 210’a nd a part of the silicon substrate 200 are etched to form a trench 204 (Fig. 22) .
Until the etching-stop layer 231 is exposed, a part of the waveguide layer 210’and a part of the silicon substrate 200 are etched to form a U-shaped groove or rectangular groove 202 (Fig. 23) . In the process shown in Fig. 23, the U-shaped groove or rectangular groove 202 may be formed by dry etching. Since the SOI shown in Fig. 23 has the SiO2 layer 231 therein, etching is stopped by this SiO2 layer. Thereby, the U-shaped groove or rectangular groove 202 having a controlled depth (H) with an error of about 0.5 μm or less is obtained. Thus, passive alignment between the optical fiber 220 disposed in the U-shaped groove or rectangular groove 202 and the waveguide 210 is easily achieved.
Next, a solder 207 is formed in the trench 204 in order to dispose a light source 205 (Fig. 24) .
The light source 205 is disposed in the trench 204, and the optical fiber 220 is disposed in the U-shaped groove or rectangular groove 202 (Fig. 25) .
In the second embodiment, in order to solve the problem wherein the accuracy of the dry etching is not high, the etching-stop layer 231 is provided between the silicon substrate 200 and the silicon substrate 230. By accurately controlling the depth from the first surface 201 of the substrate 200 to the etching-stop layer 231, the depth of the groove 202 formed in the silicon substrate 200 is accurately defined. Thereby, an optical device can be provided in which the positional error between the optical axis 211 of the waveguide 210 on the substrate 200 and the optical axis 221 of the optical fiber 220 disposed in the groove 202 is about 0.5 μm or less.
The second embodiment is a method for fabricating the optical device 20 shown in Fig. 25. The optical device 20 comprises two  BOX layers  212’, 231 as shown in Fig. 19. The distance between the two  BOX layers  212’, 231, that is, the thickness of the first substrate 200 is accurately controlled with an optimized value  for forming the U-shaped groove or rectangular groove 202. Such SOI can be used for the conventional Si waveguide forming process which is compatible with CMOS because the surface of the SOI is perfectly flat. After the forming of the waveguide and the forming of the trench for the LD platform are finished, a high-accuracy U-shaped groove or rectangular groove 202 is easily fabricated by a dry etching process because the BOX layer 231 rules as an etching-stop layer.
(Variant) A variant of the second embodiment is described referring to Figs. 26 to 36. A description of portions that overlap the second embodiment is omitted. Figs. 26 to 36 are sectional views of the optical device 20 along the optical axis 221 of the optical fiber 220 and the optical axis 211 of the waveguide 210.
The processes shown in Figs. 26 to 30 provide an SOI comprising double BOX layers 212’, 231. A smart-cut line 241 is formed by implanting impurities such as hydrogen into a silicon substrate 240 (Fig. 26) .
A silicon substrate 200 having thick SiO2 layer (s) 212’, 212” on both surfaces or one surface (not shown) is then prepared. The silicon substrate 240 and the silicon substrate 200 are bonded through the thick SiO2 layer 212’of the silicon substrate 200 (Fig. 27) . As shown in Fig. 27, an auxiliary underclad layer 203’is formed adjacent to the SiO2 layer 212’.
Subsequently, the SiO2 layer 212” and the silicon substrate 200 are removed by grinding and fine chemical mechanical polishing (CMP) so that the auxiliary underclad layer 203’is not exposed (Fig. 28) .
Next, a silicon substrate 230 having thin SiO2 layer (s) 231, 232 on both surfaces or one surface (not shown) is prepared. The silicon substrate 200 and the silicon substrate 230 are bonded through the SiO2 layer 231 (Fig. 29) .
The silicon substrate 240 is then cut along the pre-formed smart-cut line 241 to obtain a thin silicon layer 215’ (Fig. 30) .
The processes shown in Figs. 31 to 35 are the same processes as the integration processes which are compatible with the CMOS process. The processes shown in Figs. 31 to 33 are the same processes as those shown in Figs. 20 to 22. After the trench is formed, a part of the waveguide layer 210’, a part of the auxiliary underclad layer 203’, and a part of the silicon substrate 200 are etched until the etching-stop layer 231 is exposed thereby forming a waveguide 210 having the auxiliary underclad 203 and a groove 202 (Fig. 34) . The processes shown in Figs. 35 and 36 are the same processes as those shown in Figs. 24 and 25.
Since the waveguide 210 has the auxiliary underclad layer 203 in the edge portion proximal to the optical fiber 220, reflection by the silicon substrate 200 is  suppressed thereby improving the light coupling between the waveguide 210 and the optical fiber 220.
The fabricating method disclosed in the present invention can provide an optical device comprising a high-accuracy V-shaped groove or U-shaped groove or rectangular groove, and waveguide. Therefore, the positional error between the optical axis of the waveguide and the optical axis of the optical fiber can be set to be on sub-micron order thereby improving the light coupling between the waveguide and the optical fiber.

Claims (34)

  1. An optical device comprising:
    a substrate having a groove on a first surface；
    a waveguide disposed on the first surface of the substrate； and
    an optical fiber disposed in the groove；
    wherein a difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber is about 0.5 μm or less.
  2. The optical device according to claim 1, wherein the waveguide includes an underclad, a core, and an overclad.
  3. The optical device according to claim 2, wherein surface of the core is flat.
  4. The optical device according to claim 2 or 3, wherein the core includes a silicon layer and an SiOx layer (0<x<2) .
  5. The optical device according to any one of claims 1 to 4, wherein the groove has a V-shaped cross-section in a plane perpendicular to a longitudinal direction of the optical fiber.
  6. The optical device according to any one of claims 1 to 5, wherein a part of the waveguide protrudes above the groove along a longitudinal direction of the optical fiber.
  7. The optical device according to any one of claims 1 to 4, wherein the groove has a U-shaped or rectangular cross-section in a plane perpendicular to a longitudinal direction of the optical fiber.
  8. The optical device according to claim 7, comprising an auxiliary underclad which is disposed under the waveguide and is adjacent to the groove in the substrate.
  9. The optical device according to any one of claims 1 to 8, wherein the substrate includes a trench, and a light source is disposed in the trench.
  10. A method for fabricating an optical device, comprising:
    preparing a first substrate having a groove in a first surface；
    providing a waveguide layer in the first surface by bonding；
    forming a waveguide and exposing the groove by etching a part of the waveguide layer； and
    disposing an optical fiber in the groove.
  11. The method according to claim 10, wherein the providing a waveguide layer in the first surface by bonding comprises:
    preparing a second substrate having a first insulating layer on a second substrate, a semiconductor core layer on the first insulating layer, and an underclad layer on the semiconductor core layer；
    bonding the first substrate and the second substrate through the first surface and the underclad layer；
    removing the second substrate and the first insulating layer；
    etching a part of the semiconductor core layer to form a semiconductor core； and
    forming an insulator core layer which covers the semiconductor core on the underclad layer, and forming an overclad layer which covers the insulator core layer on the underclad layer.
  12. The method according to claim 11, wherein the preparing a second substrate comprises:
    bonding the second substrate and a third substrate through a second insulating layer on the semiconductor core layer and a third insulating layer on the third substrate；
    removing the third substrate and forming the underclad layer which includes the second insulating layer and the third insulating layer.
  13. The method according to claim 12, wherein the preparing a first substrate includes: forming the groove in the first surface by wet etching.
  14. The method according to claim 12 or 13, wherein the preparing a first substrate includes: forming an auxiliary underclad in the first substrate.
  15. The method according to any one of claims 12 to 14, wherein the preparing a first substrate includes: forming a trench in the first surface by etching.
  16. The method according to any one of claims 12 to 15, wherein the forming a waveguide and exposing the groove by etching a part of the waveguide layer includes: etching a part of the waveguide layer so that the waveguide partially covers above the groove.
  17. The method according to any one of claims 12 to 16, wherein the forming a waveguide and exposing the groove by etching a part of the waveguide layer includes: etching a part of the waveguide layer so that the semiconductor core is not exposed.
  18. The method according to claim 15, wherein the forming a waveguide and exposing the groove by etching a part of the waveguide layer includes: exposing the trench by etching a part of the waveguide layer.
  19. The method according to any one of claims 12 to 18,
    wherein the first substrate, the second substrate, and the third substrate are silicon substrates,
    wherein the first insulating layer, the second insulating layer, and the third insulating layer are SiO2 layers,
    wherein the semiconductor core layer is a silicon layer,
    wherein the insulator core layer is a SiOx layer (0<x<2) , and
    wherein the underclad layer and the overclad layer are SiO2 layers.
  20. The method according to any one of claims 10 to 19, wherein the groove has a V-shaped cross-section in a plane perpendicular to a longitudinal direction of the optical fiber.
  21. The method according to any one of claims 10 to 20, wherein a difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber is about 0.5 μm or less.
  22. A method for fabricating an optical device, comprising:
    preparing a first substrate having a third substrate through an underclad layer in a first surface, and having a second substrate through an etching-stop layer in a second surface；
    providing a waveguide layer in the first surface of the first substrate；
    forming a waveguide and groove by etching parts of the waveguide layer and the first substrate so that the etching-stop layer is exposed； and
    disposing an optical fiber in the groove.
  23. The method according to claim 22, wherein the preparing a first substrate includes: bonding the first substrate and the second substrate through the etching-stop layer on the second substrate.
  24. The method according to claim 22 or 23, wherein the preparing a first substrate includes:
    bonding the first substrate and the third substrate through the underclad layer on the first surface； and
    removing a part of the first substrate.
  25. The method according to claim 24, wherein the providing a waveguide layer includes:
    forming a semiconductor core layer by thinning the third substrate；
    forming a semiconductor core by etching a part of the semiconductor core layer； and
    forming an insulator core layer which covers the semiconductor core on the underclad layer, and forming an overclad layer which covers the insulator core layer on the underclad layer.
  26. The method according to claim 25,
    wherein the preparing a first substrate includes: forming a smart-cut line by implanting impurities into the third substrate, and
    wherein the forming a semiconductor core layer includes: cutting the third substrate along the smart-cut line.
  27. The method according to claim 25 or 26, wherein the forming a waveguide and groove includes:
    etching a part of the waveguide layer so that the semiconductor core is not exposed.
  28. The method according to any one of claims 25 to 27, wherein the forming a waveguide and groove includes:
    forming the waveguide and the groove by dry etching.
  29. The method according to any one of claims 25 to 28, wherein the forming a waveguide and groove includes:
    forming a trench by etching parts of the waveguide layer and the first substrate.
  30. The method according to any one of claims 25 to 29, wherein the preparing a first substrate includes:
    forming an auxiliary underclad within the first substrate.
  31. The method according to claim 30, wherein the forming a waveguide and groove includes:
    etching the first substrate so that the auxiliary underclad is exposed.
  32. The method according to any one of claims 25 to 31,
    wherein the first substrate, the second substrate, and the third substrate are silicon substrates,
    wherein the semiconductor core layer is silicon layer,
    wherein the insulator core layer is SiOx layer (0<x<2) , and
    wherein the etching-stop layer, the underclad layer, and overclad layer are SiO2 layers.
  33. The method according to any one of claims 22 to 32, wherein the groove has a U-shaped or rectangular cross-section in a plane perpendicular to a longitudinal direction of the optical fiber.
  34. The method according to any one of claims 22 to 29, wherein a difference between a distance from the first surface to an optical axis of the waveguide and a distance from the first surface to an optical axis of the optical fiber is about 0.5 μm or less.
PCT/CN2016/103878 2016-10-29 2016-10-29 Optical device and method for fabricating the same WO2018076308A1 (en)

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