JP2827376B2 - Method for manufacturing semiconductor optical waveguide - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor optical waveguide used for connection between optical functional elements in a semiconductor optical integrated circuit or the like, and particularly relates to a semiconductor including a curved optical waveguide portion. The present invention relates to a method for manufacturing an optical waveguide.

[Prior art and its problems] With the progress of optoelectronics, research and development of integration of semiconductor optical devices have been actively pursued in recent years. In particular, semiconductor optical waveguides can be realized on a semiconductor substrate by applying the microfabrication technology cultivated in semiconductor electronic devices. Connections between switches in a semiconductor optical matrix switch, and semiconductor optical functional devices in the same substrate It is used for connection between them (for example, connection between a light source and a switch or an amplifier) and is considered as one of the important components of the semiconductor optical integrated circuit. Such a semiconductor optical waveguide requires not only a linear optical waveguide but also a curved optical waveguide in order to connect the semiconductor optical devices. conventionally,
In general, a method of forming the curved optical waveguide together with the linear optical waveguide using a single photolithography method is used.

Incidentally, in order to reduce the size of the integrated device, it is preferable to reduce the radius of curvature of the curved optical waveguide. However, since light originally has the property of straightness, if the radius of curvature of a curved optical waveguide is reduced excessively, there is a problem that radiation loss in the curved optical waveguide increases in the conventional rib forming method. Was. With a certain radius of curvature,
Radiation loss can be reduced by widening the waveguide and increasing light confinement by RJDERI et al. In Electronics Letters Vol. 23, p. 845 (ELE).
CTRONICS LETTERS Vol.23 p.845).
However, as the waveguide width is increased, the guided light approaches the multimode condition, and at the same time, the characteristics of an optical device composed of other integrated linear optical waveguides, such as a directional coupler type optical switch. Adversely affect In addition, when the radius of curvature is reduced to the order of mm or less, there is a problem that even if the width of the waveguide is increased to enhance the confinement of light, the radiation loss is not significantly reduced.

FIG. 3 (a) is an example showing the relationship between the radius of curvature and the radiation loss when only the waveguide width of the S-shaped curved optical waveguide is changed within the single mode condition.
In the case of the mm order, the radiation loss is reduced by increasing the waveguide width. However, in the case where the radius of curvature is less than the mm order, even if the waveguide width is increased, the radiation loss is hardly reduced. As described above, the conventional method has a problem to be solved regarding the reduction of the radiation loss of the curved waveguide portion.

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides a method for manufacturing a semiconductor optical waveguide, wherein a linear and curved rib-shaped optical waveguides are continuously formed, and a linear optical waveguide is formed. A method for manufacturing a semiconductor optical waveguide in which a rib height of a waveguide portion and a curved optical waveguide portion are different, wherein at least a semiconductor first cladding layer, a semiconductor waveguide layer, and a semiconductor second cladding layer are formed on a semiconductor substrate in this order. A step of laminating, a step of covering a portion other than the portion corresponding to the curved optical waveguide portion on the semiconductor substrate on which the semiconductor layer is laminated by the laminating step with a first mask, and a step of partially covering the first Forming a second mask having a shape of a continuous waveguide of the linear optical waveguide and the curved optical waveguide on the semiconductor substrate covered by the first mask; and a portion covered by the second mask. The second mask until the first mask is removed in a portion other than
A step of etching the semiconductor substrate covered with the mask and a step of removing the second mask.

[Operation] In general, light has the property of traveling straight, so that radiation loss occurs at the curved portion of the semiconductor optical waveguide.
Since the radiation loss increases as the curvature of the curved optical waveguide is reduced, conventionally, the curved optical waveguide must be manufactured with a curvature that is negligible compared to the waveguide loss. This was a major hindrance in reducing the overall length. Also, it is possible to increase the waveguide width to enhance the light confinement and reduce the radiation loss of the curved optical waveguide.However, when the curvature is on the order of mm or less,
Even with this method, the radiation loss is not reduced so much, and since it approaches the multi-mode condition, it is not preferable in that it adversely affects other optical devices on the same substrate.

In contrast, the present invention provides a manufacturing method for etching a curved optical waveguide deeper than a linear optical waveguide so that the rib height of the curved optical waveguide is higher than the rib height of the linear optical waveguide. When the rib height of the curved optical waveguide is higher than the rib height of the linear optical waveguide, the radiation loss of the curved optical waveguide when the radius of curvature is reduced can be reduced, and the single mode condition can be easily realized. Although the present invention has a feature that other optical devices on the substrate are not adversely affected, the present invention provides a manufacturing method capable of effectively making the rib height of the curved waveguide higher than that of the straight waveguide. . In the present invention, the first mask is formed on a portion other than the portion corresponding to the curved waveguide portion, and then the second mask having the waveguide shape, which is the original mask, is formed. Since the height is higher than the rib height of the linear optical waveguide portion, the number of times of the etching process is only one as compared with the case where two etchings are normally considered. Furthermore, by appropriately selecting the material of the first mask, the heights of the ribs in the linear portion and the curved portion can be controlled with high accuracy, and strict alignment accuracy is required compared to the usual two photolithography steps. However, the radiation loss of the curved optical waveguide can be reduced by a simple process.

[Example] The present invention will be described below in more detail with reference to the drawings.

FIG. 1 is a perspective view showing an outline of a GaAs / AlGaAs semiconductor optical waveguide according to one embodiment of the present invention.

An Al _0.5 Ga _0.5 As first cladding layer 2 is grown on a GaAs substrate 1, and a GaAs waveguide layer 3 is grown on the Al _0.5 Ga _0.5 As first cladding layer 2. On the GaAs waveguide layer 3, an Al _0.5 Ga _0.5 As second clad layer 4 having a rib portion is formed. The height of the rib portion is higher in the curved optical waveguide 5 than in the straight optical waveguide portion 6.

First, a method for manufacturing the semiconductor optical waveguide shown in FIG. 1 will be described below. On a GaAs substrate 1, a molecular beam epitaxial growth method (MBE method) or a metal organic chemical vapor deposition method (MO
-CVD method) using, Al _0.5 Ga _0.5 As first cladding layer 2, G
The aAs waveguide layer 3 and the Al _0.5 Ga _0.5 As second cladding layer 4 are grown. The thickness of each layer is 1 to _{0.5 for} the first cladding layer 2 of Al _0.5 Ga _0.5 As.
About 2 μm, GaAs waveguide layer 3 about 0.2 μm, Al _0.5 Ga _0.5 A
s The second cladding layer 4 has a thickness of about 1.2 μm. After the crystal is grown as described below, a semiconductor optical waveguide is formed in which the rib height of the linear optical waveguide and the rib height of the curved optical waveguide are different.

FIG. 2 is a perspective view showing an outline of a process of forming a semiconductor optical waveguide. First, as shown in FIG. 2 (a), a portion 15 corresponding to a curved optical waveguide and its peripheral portion is formed on a GaAs / AlGaAs wafer 10 by a normal photolithography method.
Mask. Next, as shown in FIG. 2B, a Ti metal 9 is deposited on the entire surface to a thickness of about 15 nm by an electron beam evaporation method (EB method) or a sputtering method. Since the photoresistor 11 can be easily removed using an organic solvent, only the portion 16 corresponding to the linear optical waveguide and its peripheral portion is covered with the Ti metal 9 as shown in FIG. As shown in FIG. 2 (d), a mask is formed on this with a photoresist 41 in the shape of the waveguide to be formed, as shown in FIG. 2 (d). Thereafter, portions other than those masked by the waveguide-shaped photoresist 41 are etched by a reactive beam etching method (RIBE method). As a result, as shown in FIG. 5 can be formed simultaneously. At this time, the etching time of the Al _0.5 Ga _0.5 As second cladding layer 4 in the peripheral portion of the linear optical waveguide portion 6 is reduced by the time required to completely remove the Ti metal layer 9, The rib height of the curved optical waveguide portion 5 can be made higher than the rib height of the rib 6. After that, the photoresist 41 is changed to GaAs / A
FIG. 2 (f) shows that when the substrate is removed with an organic solvent or the like which does not react with the lGaAs wafer and can remove only the photoresist.
As described above, a semiconductor optical waveguide in which the rib height of the linear optical waveguide portion 6 and the rib height of the curved optical waveguide portion 5 are different from each other is formed. At this time, the rib height of the linear optical waveguide 6 is 0.9 μm, and the rib height of the curved optical waveguide 5 is 1 μm.

The above is the embodiment of the method for manufacturing a semiconductor optical waveguide according to the present invention, in the semiconductor optical waveguide according to the above-described manufacturing method, the radiation loss of the curved optical waveguide is improved than before,
The principle by which the single mode condition can be easily realized will be described below.

In this embodiment, as shown in FIG. 1, the rib height of the straight optical waveguide portion 6 is normal, whereas the rib height of the curved optical waveguide portion 5 is slightly higher. Therefore,
In the curved optical waveguide section 5, the light is strongly confined, and the radiation loss is reduced.

FIG. 3A shows an example of calculation of the relationship between the radius of curvature and the radiation loss when the waveguide width is changed within the single mode condition. The radiation loss was calculated as the total radiation loss per one S-shaped waveguide as shown in the top view of FIG. 4 (a). The cross-sectional structure of the S-shaped waveguide is shown in FIG. Calculation was performed on the layer structure as shown in (b). Here, the thickness of the Al _0.5 Ga _0.5 As first cladding layer 70 is
1.5 μm, the thickness of the waveguide layer 71 is 0.2 μm, Al _0.5 Ga _0.5 As second
The thickness of the cladding layer 72 is 1.2 μm, and the etching depth t _e (7
4) is set to 0.9 μm, and the waveguide width W (73) is set to 2 μm and 2.5 μm.
μm and 3 μm. From FIG. 3 (a), when the radius of curvature is several mm, the radiation loss can be reduced by increasing the waveguide width. However, when the radius of curvature is less than the order of mm, the radiation loss does not change much even if the waveguide width is increased. I understand that.

FIG. 3 (b) shows an example of the relationship between the radius of curvature and the radiation loss when the rib height of the waveguide is changed within the single mode condition in the same structure as in FIG. 3 (a). Similar to FIG. 3 (a), the radiation loss is calculated as the total radiation loss per one S-shaped waveguide as shown in FIG. 4 (a), and calculated as the cross-sectional configuration of the S-shaped waveguide. Is calculated for a layer structure as shown in FIG. 4 (b).
The thickness of the Al _0.5 Ga _0.5 As first cladding layer 70 is 1.5 μm, the thickness of the waveguide layer 71 is 0.2 μm, and the thickness of the Al _0.5 Ga _0.5 As second cladding layer 72 is 72 μm.
Is assumed to be 1.2 μm and the waveguide width W (73) is 2 μm.
Etching depth t _e (74) is 0.9 μm, 0.95 μm, 1 μm
Was changed. FIG. 3 (b) shows that when the rib height of the waveguide is increased, the radiation loss can be effectively reduced even if the radius of curvature is on the order of several mm or less.

In this embodiment, since the height of the ribs is increased to confine light, even if the radius of curvature of the curved optical waveguide portion 5 is reduced to the order of mm or less, the width of the waveguide is increased to confine light. It is possible to effectively reduce the radiation loss as compared with the case where it is strengthened. Further, since the straight optical waveguide portion 6 has the same rib height as the conventional one, the single mode condition can be easily maintained in the straight optical waveguide portion 6, and the single mode condition of the curved optical waveguide portion 5 is set to be equal to the curved optical waveguide portion 6. Since a higher rib height is permissible, a single mode condition can be easily realized. Further, the linear optical waveguide section 6
And the width of the curved optical waveguide section 5 are the same, so that there is no particular problem regarding the connection loss between the straight optical waveguide section 6 and the curved optical waveguide section 5.

Note that the present invention is not limited to the above embodiment. Although a GaAs-based material was used in the embodiment, the present invention is not limited to this, and the present invention is applicable to other materials such as an InP-based material as long as the material is an optical waveguide material. In this embodiment, the etching is performed by dry etching by the RIBE method. However, other methods may be used as long as the rib portion is formed, for example, a reactive ion etching method (RIE method). Alternatively, wet etching may be used. Further, in order to increase the etching depth of the ribs of the curved optical waveguide portion 5 from the linear optical waveguide portion 6, a Ti metal 9 was deposited on the portion 16 corresponding to the linear optical waveguide portion and its periphery in the present embodiment. In particular, the material is not limited to Ti metal, and any other material may be used as long as the material has excellent layer thickness controllability and etching rate controllability. Further, in the present embodiment, the Ti metal layer 9 remains on the rib portion of the linear optical waveguide section 6, but it is not particularly necessary that the Ti metal layer 9 remains, and if necessary, the Ti metal layer 9 is removed. It does not matter.

[Effects of the Invention] As described above, the present invention provides a semiconductor optical waveguide in which the radiation loss is improved even in a curved optical waveguide section having a curvature of the order of mm or less and the realization of a single mode condition is easy. A manufacturing method is provided. Therefore, by employing the manufacturing method of the present invention, the length occupied by the curved optical waveguide of the semiconductor optical waveguide in the optical functional device such as an optical switch can be shortened, and the device can be shortened. As the optical path length is shortened by the development, the waveguide loss can be reduced, and a semiconductor optical waveguide that does not adversely affect other optical devices on the same substrate can be manufactured. In addition, in the present invention, the first mask is formed in a portion other than the portion corresponding to the curved waveguide portion, and then the second mask having the waveguide shape, which is the original mask, is formed. The rib height of the part is higher than the rib height of the linear optical waveguide part, so the method of providing a difference in the rib height normally requires two etchings, The method of the present invention requires only one etching step. Further, in the method of the present invention, the height of the ribs in the straight portion and the curved portion can be accurately controlled by appropriately selecting the material of the first mask. No alignment accuracy is required, and the radiation loss of the curved optical waveguide can be reduced by a simple process.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an outline of a GaAs / AlGaAs semiconductor optical waveguide according to one embodiment of the present invention. FIG. 2 is a perspective view showing an outline of a step of forming a semiconductor optical waveguide according to one embodiment of the present invention. FIG. FIG. 2B is a perspective view showing an outline of a state in which a corresponding portion 15 is masked with a photoresist 11, and FIG.
FIG. 3 is a perspective view showing an outline of a state where metal 9 is deposited, and FIG.
FIG. 2C is a perspective view showing an outline of a state where the photoresist 11 shown in FIG. 2B is removed, and FIG. AlGaA
It is a perspective view showing an outline of a state in which the s wafer is masked,
FIG. 2 (e) is a perspective view showing an outline of a state where the waveguide to be formed by the RIBE method is etched, and FIG. 2 (f) is a photo resist 41 shown in FIG. 2 (e). FIG. 3 is a schematic perspective view after removing with an organic solvent. FIG. 3 is a view showing the relationship between the radiation loss of the curved optical waveguide and the radius of curvature R;
FIG. 3A shows the relationship between the radiation loss and R when the waveguide width is changed within the single mode condition. FIG. 3B shows the same structure as in FIG. It is a figure which shows the relationship between radiation loss and R when the rib height is changed within the conditions. FIG. 4 is a diagram for supplementarily explaining the calculation of the radiation loss in FIG. 3. FIG. 4 (a) is a diagram of an S-shaped curve from which the radiation loss is obtained, and FIG. It is sectional drawing which shows the model of the waveguide structure of the curved optical waveguide used for loss calculation. 1 ... GaAs substrate, 2 ... Al _0.5 Ga _0.5 As first cladding layer,
3 ...... GaAs waveguide layer, 4 ...... Al _0.5 Ga _0.5 As second cladding layer, 5 ...... curved waveguide section, 6 ...... straight waveguide section, 9
... Ti metal layer, 10 ... GaAs / AlGaAs wafer, 11,41 ... Photoresist, 15 ... Part corresponding to curved optical waveguide and its periphery, 16 ... Linear optical waveguide and part corresponding to its periphery, 70 ...... Al _0.5 Ga _0.5 As first cladding layer, 71 ...... GaAs
Waveguide layer, 72 ...... Al _0.5 Ga _0.5 As second cladding layer, 73 ...... waveguide width W, 74 ...... etching depth t _e.

Claims (1)

(57) [Claims] 1. A method of manufacturing a semiconductor optical waveguide in which linear and curved rib-shaped optical waveguides are continuously formed and a rib height of a linear optical waveguide portion and a curved optical waveguide portion are different from each other. A step of stacking at least a semiconductor first cladding layer, a semiconductor waveguide layer, and a semiconductor second cladding layer in this order, and corresponds to a curved optical waveguide portion on the semiconductor substrate on which the semiconductor layers are stacked by the stacking step. A step of covering a part other than the part with a first mask, and a step of forming a waveguide in which the linear optical waveguide and the curved optical waveguide are continuous on the semiconductor substrate partially covered with the first mask by the masking step. Forming a second mask having a shape;
Etching the semiconductor substrate covered with the second mask until the first mask is removed in a portion other than the portion covered with the second mask; and removing the second mask. A method of manufacturing a semiconductor optical waveguide.