JPH087288B2 - Method for manufacturing hybrid optical integrated circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a hybrid optical integrated circuit in which an optical waveguide and an optical element such as a semiconductor laser are combined and integrated on a silicon substrate, and a manufacturing method thereof. .

[Problems to be solved by the prior art and the invention]

Hybrid optical integration in which optical waveguides formed on a Si substrate and various optical elements are combined and integrated in order to reduce the size, increase reliability, and reduce the cost of various optical circuits required in the fields of optical communication and optical information processing. Realization of the circuit is expected. In order to realize a hybrid optical integrated circuit, it is essential to efficiently optically couple an optical waveguide and an optical element on the same substrate.

FIG. 8 is a prototype of this kind of hybrid optical integrated circuit, which is an example in which a silica-based optical waveguide formed on a Si substrate and a semiconductor laser are integrated (H. Terui, Y. Yamad.
a, M.Kawachi and M.Kobayashi; “Hybrid Integration o
fa Laser Diode and High-silica Multimode Optical
Channel Waveguide on Silicon ", Electron.Lett.vol2
1, pp646-648, 1985).

In FIG. 8, 1 is a Si substrate, 2 is an optical waveguide, and a core layer
It has a three-layer structure of 2a, a buffer layer 2b and a cladding layer 2c. 3 is a laser guide, 4 is a semiconductor laser, and 4a is its active layer. Reference numeral 6 is a power supply wire, and 7 is a conductive film. The semiconductor laser 4 is pressed against the laser guide 3 and positioned at an appropriate position with respect to the optical waveguide 2. The height l ₀ from the surface of the Si substrate 1 to the center of the active layer 4a of the semiconductor laser 4 when the semiconductor laser 4 is mounted on the substrate 1 is equal to the height l ₀₁ of the center of the core 2a of the optical waveguide 2. L ₀ to be equal
= L ₀₁ is set. Therefore, the semiconductor laser 4
Positioning between the laser and the waveguide can be realized simply by mounting on the i substrate and aligning the waveguide and the laser active layer in the lateral direction. Further, at this time, since the semiconductor laser 4 is in direct contact with the Si substrate, the Si substrate also functions as a heat sink for the laser. By the way, in the above structure, in order to make the height l ₀ of the laser active layer 4a coincide with the height l _{01 of the} center of the waveguide core, it is necessary to polish the substrate of the semiconductor laser 4 to a required thickness. Therefore, the following problems occur. The thickness of each layer of the optical waveguide 2 is, in the multimode system 2, a core layer to 50 μm, a buffer layer to 15 μm, a clad layer to 5 μm, and a single mode system, the core layer to 10 μm and the buffer layer to 20 μm.
m, clad layer to about 10 μm. Therefore, l ₀ = l
To set ₀₁ , set the thickness of the semiconductor laser substrate to
〜40μm for multi-mode system, for single-mode system〜
Must be 25 μm. Moreover, the required accuracy for the thickness is strict within ± 3 μm for the multimode system and within 1 μm for the single mode system. GaAs or InP, which is a constituent material of the semiconductor laser, is fragile, and it is difficult to thinly polish it with the above-mentioned accuracy. In particular, it is necessary to bond a thin semiconductor laser such as a single mode system on a Si substrate. However, it has been a serious problem in the hybrid optical integrated circuit configuration. As a method for realizing precise alignment in the height direction without polishing the substrate of the semiconductor laser 4, as shown in FIG. 9, the active layer 4a side of the semiconductor laser 4 is faced down to the p-side. There is down bonding. In this method, the active layer 4a of the semiconductor laser 4 is
The thickness l of the epitaxially grown layer on the substrate and the height l ₁ at the center of the waveguide core layer may be matched. Since the thickness l of the epitaxial growth layer is at most 5 μm, this thickness is ± 1
It is easy to determine the accuracy within μm. However, in this case, contrary to FIG. 8, l ₁ must be set to a height of about 5 μm, so the waveguide core layer thickness is also 10 μm.
This method is applicable only to a single mode system. Moreover, if the core layer thickness is about 10 μm, the buffer layer thickness must be 10 μm or more. Therefore, as shown in FIG. 9, when p-side down bonding is performed, the semiconductor laser 4 cannot be brought into direct contact with the Si substrate 1, and the Si substrate cannot function as a heat sink. In this case, it is necessary to separately provide the heat sink 8, and it is difficult to reduce the size and the density of the optical integrated circuit.

It is an object of the present invention to eliminate the need for thinning a semiconductor laser, which has been a problem when hybridizing an optical waveguide and a semiconductor laser, and reduce the size and density by using the Si substrate as a heat sink. To provide a hybrid optical integrated circuit and a method for manufacturing the same.

[Means for solving problems]

The present invention is a method for manufacturing an optical integrated circuit in which an optical element such as a semiconductor laser and an optical waveguide are optically coupled on a silicon substrate, and a concavo-convex forming step of forming a concave portion and a convex portion on the surface of the silicon substrate, A buffer layer forming step of forming a buffer layer on the concave portion of the silicon substrate, a flattening step of flattening the surface of the buffer layer, and an optical waveguide forming step of forming a core layer and a clad layer on the buffer layer, And a step of mounting an optical element on the convex portion of the substrate with the active layer facing downward.

〔Example〕

Embodiment 1 FIGS. 1 (a) and 1 (b) are views for explaining the first embodiment of the present invention. 10 is Si having concave portions 10a and convex portions 10b
Substrate 2 is a silica-based optical waveguide formed on the recess, and 2a
Is a core layer, 2b is a buffer layer, and 2c is a cladding layer. Four
Is a semiconductor laser, 4a is its active layer, and 7 is a conductive film.
In this embodiment, the buffer layer 2b is formed so as to fill the concave portion 10a of the Si substrate 10, and the height of the upper surface of the buffer layer 2b is made to match the height of the upper surface of the Si substrate convex portion 10b. Therefore, the distance l ₁ from the upper surface of the buffer layer 2b to the center of the core layer 2a and the upper surface of the convex portion 10b when the semiconductor laser 4 is p-side down bonded onto the Si substrate to the active layer 4a.
By setting the height l up to the same, the precision alignment of the semiconductor laser 4 and the optical waveguide 2 in the height direction can be performed without polishing the substrate of the semiconductor laser 4. Moreover, in this case, since the semiconductor laser 4 is directly bonded onto the Si substrate 10, the Si substrate 10 can be used as a heat sink.

An optical integrated circuit having such a structure is shown in FIG.
It can be manufactured as in (e). This process will be explained step by step. (A) The figure shows a concave portion on the Si substrate 10.
This is a step of forming the protrusion 10a and the protrusion 10b. To do this,
For example, dry etching of the Si substrate with CBrF ₃ gas or the like or wet etching (anisotropic etching) of the Si substrate with an alkaline etching solution can be applied. When dry etching is used, the step on the Si substrate becomes nearly vertical, while in wet etching the step becomes oblique.
FIG. 3B shows a step of forming the buffer layer 2b on the Si substrate 10 on which the concave portions 10a and the convex portions 10b are formed. For this, for example, a raw material gas such as SiCl ₄ , TiCl ₄ is hydrolyzed in an oxyhydrogen flame, deposited on the Si substrate 10, and then heated to a high temperature in an electric furnace to form a transparent glass (flame. Deposition method) [Japanese Patent Application No. 58-
1473, M. Kawachi et al, Electron, Lett, 19 (1983) 583)
To use. (C) In the figure, the excess buffer layer is removed and Si
In this step, the surface of the substrate convex portion 10b is exposed and the substrate is flattened to this height. For this purpose, mechanical polishing may be used, or dry etching of the silica glass film with a fluorocarbon gas such as C ₂ F ₆ may be performed. (D)
The figure shows a step of forming the core layer 2a and the clad layer 2c on the planarized optical integrated circuit substrate. For this purpose, the flame deposition method used in FIG. Figure (e) shows the photolithography of silica-based waveguides in unnecessary parts and their continuation.
This is a process of removing by dry etching using a C ₂ F ₆ or other fluorocarbon gas. As a result, the Si substrate surface of the convex portion of the waveguide is exposed again. Finally, the exposed Si substrate protrusion 10
Bonding is completed by, for example, forming an An-Sn alloy film as a conductive film on the surface of b by vapor deposition and then mounting and fixing the semiconductor laser by p-side down. At this time, as shown in FIG. 1, the total height l of the distance from the active layer 4a of the semiconductor laser 4 to the upper surface of the laser and the height of the conductive film 7 is the height l _{1 at the} center of the core of the conductive path 2. The thickness of the epitaxial growth layer of the semiconductor laser 4 or the thickness of the conductive film may be controlled so as to be equal to.

Second Embodiment FIG. 3 is a diagram for explaining a second embodiment of the present invention. This embodiment is different from the first embodiment in that the Si substrate convex portion 10
The case where the heights of the upper surface of b and the upper surface of the optical waveguide buffer layer 2b do not match is shown. In FIG. 3, the height l of the active layer 4a of the semiconductor laser 4 mounted via the conductive layer 7 from the convex portion 10b of the Si substrate is the height l at the center of the waveguide core layer 2a.
It is smaller than ₁ . Since the step l ₃ (= l ₁ −l) is provided between the upper surface of the buffer layer 2b and the upper surface of the Si substrate convex portion 10b, the semiconductor laser 4 is p-side down on the upper surface of the Si substrate convex portion 10b. The semiconductor laser 4 and the optical waveguide 2 can be aligned in height by bonding. For example,
When the core layer thickness is thicker than the epitaxial layer grown on the upper surface of the active layer of the semiconductor laser like the multimode optical waveguide, the optical circuit structure of this embodiment is effective.

The optical circuit having such a structure can be manufactured by the following process, for example. First, in the same manner as in FIGS. 2 (a), (b) and (c), a concave portion and a convex portion are formed on a Si substrate, and then a quartz optical waveguide buffer layer is formed to flatten the substrate surface. After that, as shown in FIG. 4A, the silica-based optical waveguide buffer layer is etched so that a step having a desired size is formed. As this method, dry etching with a chlorofluorocarbon-based etching gas (for example, C ₂ F ₆ ) is used.
At this time, a step can be formed due to the difference in etching rate between the silica-based glass film and the Si substrate. Next, as shown in FIG. 4B, a core layer 2a is formed on the core layer 2a, and a cladding layer is further formed on the core layer 2a to form a silica-based optical waveguide. At this time, unlike the first embodiment, in this embodiment, a slight step is formed on the substrate surface. This step reflects the step between the upper surface of the Si substrate protrusion 10b and the buffer layer upper surface 2b in FIG. 4 (a). When this step is large and affects the subsequent optical waveguide patterning process, the surface is flattened by mechanical polishing as shown in FIG. 2 (c). Finally, the silica-based optical waveguide in an unnecessary portion may be removed by the step of FIG. 2 (e).

Third Embodiment FIG. 5 shows a third embodiment of the present invention in which a plurality of semiconductor lasers 4 are mounted on one substrate. When p-side down bonding a plurality of semiconductor lasers and operating each element independently, the p-side electrode of the semiconductor laser must be insulated for each element. For this reason, in this embodiment, the Si substrate is
A method of forming a pn junction and applying a reverse bias thereto (for example, JDCrow et al, “Galium arsenide laser-ar”).
ray-on-silicon package ", Appl, Opt., vol 17,479, (1
978)) has been adopted. That is, as the Si substrate 10,
A p-type substrate is used, and a dopant is diffused into the Si substrate convex portion 10b to form an n-type region 10n. Further, in this example, the Si substrate convex portion 10b was formed independently for each element. In the figure, 7b is a conductive film. Since the Si substrate convex portion 10b is manufactured as described above, the semiconductor laser 4 is p
-When side down bonding is performed, insulation of each element can be maintained as shown in FIG. That is, a forward bias is applied to the semiconductor laser 4. At this time, Si
The substrate is in a reverse bias state where the potential of the n-type region 10n is higher than that of the p-type region 10p. As a result, the depletion layer 10i having no dopant spreads in the pn junction, so that the p-type region and the n-type region on the Si substrate are insulated from each other. Furthermore, Si
Grooves deeper than the depth of the pn junction layer are formed between the convex portions 10b on the substrate. Therefore, the n regions 10n formed on the upper surface of each protrusion 10b are electrically insulated from each other. In the present embodiment, the dimensions of the optical waveguide 2 and the semiconductor laser 4 satisfy the relationship of the first embodiment or the second embodiment. Therefore, the semiconductor laser is mounted on the convex portion of the Si substrate and the optical coupling with the optical waveguide 2 is performed. Can be done. Moreover, as described above, since the Si substrate acts as a heat sink, it is not necessary to separately prepare a heat sink for hybrid integration. Therefore, it is possible to mount the elements on the substrate with high density.

Fourth Embodiment FIG. 7 is a fourth embodiment of the present invention. While the first to third embodiments described above are examples relating to the ridge-shaped waveguide, the present embodiment relates to a waveguide having a buried structure. In FIG. 7, the silica-based optical waveguide buffer layer 2b is formed in the Si substrate recess 10a, the core layer 2a is formed thereon, and finally the buried clad layer 2d is formed. The heights of the upper surface of the buffer layer 2b and the upper surface of the Si substrate convex portion 10b are the same. The height from the upper surface of the buffer layer 2b to the center of the core layer 2a is l ₁ . The semiconductor laser 4 is a p-side down conductive film (for example, An-Sn) on the Si substrate convex portion 10b.
When mounted on 7, the distance from the upper surface of the convex portion 10b to the active layer 4a is l. By setting l ₁ = l, if the semiconductor laser is mounted on the substrate, the semiconductor laser 4 and the optical waveguide 2
The height of the core layer can be matched with that of the core layer. Also, 3a
Is a laser guide that allows lateral alignment of the laser and the waveguide. As described above, hybrid integration of optical elements similar to that for the ridge-shaped waveguide is possible for the buried structure.

In Examples 1 to 3 above, a silica-based waveguide was mainly used as the optical waveguide, but it is a material that can be formed on a Si substrate and that can be etched, and further when an optical element is bonded. The waveguide is not limited to quartz as long as the waveguide can withstand the high temperature (-350 ° C.). The optical element to be mounted is not limited to the semiconductor laser. For example, a photodiode may be used, or an active element (for example, modulator) made of LiNbO ₃ or a semiconductor may be used. In addition, among the above-described examples, in 1 to 3, the guide for positioning the optical element is not used, but also for these,
A guide can be used as in the fourth embodiment.

Unlike the conventional optical integrated circuit, the hybrid optical integrated circuit of the present embodiment can effectively use not only the waveguide but also the Si substrate.
-48081) is expected to be applied to the field of large-scale opto-electronic integrated circuits that integrate optical waveguide circuits, semiconductor optical devices, and electronic circuits.

〔The invention's effect〕

According to the hybrid optical integrated circuit of the present invention, there is provided an optical integrated circuit for optically coupling an optical element and an optical waveguide on a silicon substrate, the silicon substrate having concave and convex portions formed on the surface, and the silicon substrate of the silicon substrate. A buffer layer formed on the concave portion; an optical waveguide formed on the buffer layer; and an optical element mounted on the convex portion of the silicon substrate and having an active layer-side surface facing downward, Since the optical waveguide and the optical element are coupled to each other, the alignment accuracy between the optical element and the optical waveguide becomes extremely high, and the coupling loss between the optical element and the optical waveguide can be reduced. Moreover, there is an advantage that the optical elements can be hybrid-integrated without thinning the substrate of the optical element, and it is particularly effective for a single-mode hybrid optical integrated circuit. Furthermore, in the present invention, since the Si substrate can function as a heat sink for the optical element,
It is not necessary to attach a heat sink to the optical element, and therefore, there is an advantage that high-density optical element mounting is possible.

A method for manufacturing a hybrid optical integrated circuit according to the present invention comprises a step of forming concaves and convexes on a surface of a silicon substrate, a step of forming a buffer layer on the concaves of the silicon substrate, and a step of forming the buffer layer. Since a flattening step of flattening the surface of the core and an optical waveguide forming step of forming a core layer and a clad layer on the buffer layer are provided, the thickness of the core layer is required to combine the optical element with the optical waveguide. Therefore, it is sufficient to control only the position of the optical element and the optical waveguide in the height direction, and
It can be done very easily. Furthermore, it is possible to manufacture a high-density hybridized optical integrated circuit by using a device used for usual semiconductor manufacturing without requiring a special device.

[Brief description of drawings]

1 (a) and 1 (b) are views showing a first embodiment of the present invention, wherein FIG. 1 (a) is a perspective view, FIG. 1 (b) is a side sectional view,
2 (a) to 2 (e) are views for explaining a method of manufacturing the circuit of FIGS. 1 (a) and 1 (b), FIG. 3 is a side sectional view of a second embodiment of the present invention, and FIG. 3 (a) and 3 (b) are views for explaining the method of manufacturing the circuit of FIG. 3, FIG. 5 is a perspective view for explaining the third embodiment of the present invention, and FIG. 6 is the principle of the third embodiment. FIG. 7 is a perspective view illustrating a fourth embodiment of the present invention, FIG. 8 is a perspective view of a conventional optical integrated circuit, and FIG. 9 is a side sectional view of a conventional optical integrated circuit. 2 ... Optical waveguide, 2a ... Core layer, 2b ... Buffer layer, 2c
...... Clad layer, 4 ...... Optical device (semiconductor laser), 4a ...
… Active layer, 10… Silicon substrate, 10a… Recess, 10b…
Convex part.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Morio Kobayashi 162 Shirahane, Shirahoji, Tokai-mura, Naka-gun, Ibaraki Prefecture Nippon Telegraph and Telephone Corporation, Ibaraki Telecommunications Research Institute (56) Reference JP-A-57-118686 (JP, A) JP 61-238018 (JP, A) JP 61-238020 (JP, A)

Claims (2)

[Claims] 1. A concavo-convex forming step of forming a concave portion and a convex portion on a surface of a silicon substrate, a buffer layer forming step of forming a buffer layer on the concave portion of the silicon substrate, and a flattening step of flattening the surface of the buffer layer. A forming step, an optical waveguide forming step of forming a core layer and a clad layer on the buffer layer, and an optical element mounting step of mounting an optical element on the convex portion of the substrate with the active layer facing downward. A method for manufacturing a hybrid optical integrated circuit having the characteristics. 2. The planarizing step is a step of planarizing a surface of the buffer layer so that an upper surface of the buffer layer and an upper surface of a convex portion of the silicon substrate are flush with each other. A method of manufacturing a hybrid optical integrated circuit according to claim 1.