JP3887733B2 - Method for manufacturing optical semiconductor element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an optical semiconductor device, and more particularly to a method for manufacturing an optical semiconductor device such as a semiconductor laser integrated with a spot size converter used for optical fiber communication.
[0002]
[Prior art]
In recent years, optical fiber communication can send a large amount of information with a single optical fiber, so it has been required to expand the range of applications from conventional trunk systems to networks such as subscriber systems or optical LANs. ing.
[0003]
In order to meet such a demand, it is essential to reduce the cost. In particular, optical coupling between the optical semiconductor element and the optical fiber in the optical module has become a big problem.
[0004]
In order to solve such a problem, an optical semiconductor element in which a spot size converter having a structure in which the thickness of the core layer for guiding light is changed in a tapered shape toward the emission end face is integrated. Semiconductor laser with converter (if necessary, H. Kobayashi et al., IEEE Photon. Tech. Lett., Vol. 6, 1994, pp. 1080-1081, Sugie et al., “Butt-Joint type selective growth spot size conversion. 1.3 μmL D ”, Proceedings of the 1995 IEICE General Conference, p. 465, SC-4-5, or Y. Tohmori et al., Electronics Letters, vol. 31, 1995, pp. 1069- 1070) has been proposed.
[0005]
Such a semiconductor laser with a spot size converter is expected as an element capable of simplifying optical coupling because optical coupling with high coupling efficiency is possible without using a lens and there is a large margin of positional accuracy. .
[0006]
As a semiconductor laser with such a spot size converter, a semiconductor laser with a ridge type tapered waveguide that does not use a buried structure has been proposed. In this semiconductor laser with a ridge type tapered waveguide, the core layer is mesa-etched. After that, since the process of regrowth of the buried layer is not required, it can be manufactured by a simpler process than the element of the buried structure.
[0007]
Here, a schematic configuration of a conventional semiconductor laser with a ridge-type tapered waveguide will be described with reference to FIG.
See FIG. 7. This semiconductor laser with a ridge-type taper waveguide is formed on the n-type InP substrate 31 so that the film thickness decreases in the taper waveguide section 36 toward the emission end face and the film thickness in the laser section 35. A certain MQW core layer 32 is provided, and a striped p-type InP ridge 34 is provided thereon via a thin p-type InP cladding layer 33. In this case, the width of the p-type InP ridge 34 is the laser portion. No. 35 is constant, and the taper waveguide portion 36 has a shape in which the film thickness increases in a flare shape toward the emission end face.
[0008]
As a method of forming a film thickness taper waveguide in such a semiconductor laser with a ridge type taper waveguide, a selective growth method using a dielectric mask (if necessary, see JP-A-8-46295), a shadow mask is used. Growth technology (if necessary, see Aoki et al., IEICE General Conference, C-372, 1996) or etching formation (if necessary, T. Brenner et al., Electronics Letters, vol. 31, pp. 1443-1445 (1995)).
[0009]
Of these three types of formation methods, the selective growth method using a dielectric mask, which is often used in the fabrication of buried-film-thickness tapered waveguides, provides shape controllability and reproducibility. Since this is excellent in terms, the selective growth method using this dielectric mask will be described with reference to FIG.
[0010]
8A. First, on the surface of the n-type InP substrate 31, in the region where the tapered waveguide portion is to be formed, a tapered opening having a width gradually increasing toward the emission end face is formed, while the laser portion is formed. In the region to be formed, an SiO ₂ mask 37 having a stripe-shaped opening 38 which is a uniform and narrow opening is provided.
[0011]
Next, referring to FIG. 8B, the MQW core layer 32 and the p-type InP cladding layer 33 are sequentially grown by a reduced pressure MOVPE method (a reduced pressure metal organic vapor phase growth method). In this case, the MQW core layer 32 is Upper and lower light guide layers composed of InGaAsP layers having a 1.1 μm wavelength composition, InGaAsP barrier layers having a 1.1 μm wavelength composition and InGaAsP well layers having a 1.35 μm wavelength composition are alternately deposited so that there are five well layers. It is constituted by a quantum well layer.
[0012]
At this time, due to the shape of the stripe-shaped opening 38 provided in the SiO ₂ mask 37, the film thickness of the MQW core layer 32 decreases in a tapered shape toward the emission end face in the tapered waveguide portion, and the laser portion. However, the abnormally grown portion 39 is generated at the end of the growth layer.
[0013]
Next, referring to FIG. 8C, a p-type InP ridge 34 surrounded by two stripe-shaped grooves 40 is formed by etching using a predetermined mask (not shown). The lower MQW core layer 32 is used as an active region and an optical waveguide region.
[0014]
Next, referring to FIG. 8D, a semiconductor laser with a ridge type taper waveguide is soldered on a mount substrate 41 on which an optical fiber (not shown) is mounted without monitoring light, that is, in a passive alignment method. Using the layer 42, junction down bonding is performed to complete the schematic configuration of the optical module.
[0015]
[Problems to be solved by the invention]
However, in the semiconductor laser with a ridge-type tapered waveguide using the selective growth method using such a dielectric mask, the surface becomes uneven due to the abnormal growth portion 39 at the end of the growth layer, so that stable junction-down bonding is possible. There is a problem that it becomes difficult.
[0016]
At the same time, it becomes difficult to perform passive alignment for aligning the optical fiber and the semiconductor laser without actually monitoring the light, and in order to facilitate the assembly of the module, a tapered waveguide portion, that is, a spot size converter. There is a problem that the meaning of providing is lost.
[0017]
Accordingly, an object of the present invention is to flatten the surface of a semiconductor laser with a ridge-type tapered waveguide and facilitate bonding.
[0018]
[Means for Solving the Problems]
FIG. 1 is an explanatory diagram of the principle configuration of the present invention. Means for solving the problems in the present invention will be described with reference to FIG.
1A to 1D. (1) In the method of manufacturing an optical semiconductor device, the present invention provides a mask 2 having an opening 3 on the surface of a one-conductivity type semiconductor substrate 1, and a core is formed only on the opening 3. A step of selectively growing a semiconductor layer 4 including a layer, a step of growing a reverse conductivity type semiconductor layer 6 over the entire surface after removing the mask 2, and a semiconductor layer 4 including a core layer of the reverse conductivity type semiconductor layer 6 The method includes a step of forming a convex portion 8 sandwiched between two stripe-shaped grooves 7 in an upper portion.
[0019]
In this manner, since the reverse conductivity type semiconductor layer 6 is grown on the entire surface after the semiconductor layer 4 including the core layer is selectively grown, the semiconductor layer to be selectively grown is thinned only by the semiconductor layer 4 including the core layer. Therefore, the abnormally grown portion 5 formed at the end portion can be made smaller, and the surface can be flattened by the thick reverse conductivity type semiconductor layer 6 without much influence of the abnormally grown portion 5. It is possible to bond at the junction down required for passive alignment.
[0020]
(2) Further, according to the present invention, in the method for manufacturing an optical semiconductor element, the mask 2 having the opening 3 is provided on the surface of the one-conductivity-type semiconductor substrate 1 and the one-conductivity-type semiconductor substrate 1 is etched using the mask 2. Thus, a step of forming a groove in the opening 3 and selectively growing the semiconductor layer 4 including the core layer only in the groove formed in the opening 3 while leaving the mask 2, the entire surface after removing the mask 2 And the step of growing the reverse conductivity type semiconductor layer 6 and the protrusion 8 sandwiched between the two stripe-shaped grooves 7 in the portion located on the semiconductor layer 4 including the core layer of the reverse conductivity type semiconductor layer 6. Including the step of:
[0021]
As described above, since the semiconductor layer 4 including the core layer is selectively grown in the groove, the abnormally grown portion 5 formed at the end portion of the semiconductor layer 4 including the core layer becomes very small, and then the entire surface is formed thereafter. Since the surface of the grown reverse conductivity type semiconductor layer 6 becomes flatter, bonding at the junction down required for passive alignment becomes more possible.
[0022]
(3) Further, according to the present invention, in the above (1) or (2), the semiconductor layer 4 including the core layer that has been selectively grown is gradually thinned toward the flat region and the light emitting end face. It is characterized by comprising a taper region that goes on.
[0023]
As described above, since the spot size converter can be configured by providing the tapered region where the film thickness is gradually reduced toward the light emitting end face, that is, the tapered waveguide portion, passive alignment is facilitated. .
[0024]
(4) Moreover, the present invention is characterized in that, in any one of the above (1) to (3), the core layer is constituted by a multiple quantum well layer and a light guide layer sandwiching the multiple quantum well layer. .
[0025]
In this way, by configuring the core layer with a multiple quantum well structure, the effective forbidden bandwidth due to the quantum level in the tapered waveguide portion can be made larger than the effective forbidden bandwidth of the laser portion, and in the tapered waveguide portion, Light absorption loss can be reduced.
[0026]
(5) Further, the present invention provides the reverse conductivity type having the same composition as the reverse conductivity type semiconductor layer 6 provided on the entire surface as the uppermost layer of the semiconductor layer 4 including the core layer in any one of the above (1) to (4). A semiconductor layer is provided.
[0027]
As described above, by providing the reverse conductive semiconductor layer having the same composition as the reverse conductive semiconductor layer 6 provided on the entire surface as the uppermost layer of the semiconductor layer 4 including the core layer, the regrowth after removing the mask 2 is accompanied. The influence of the roughening of the growth interface is not exerted on the core layer, and the heterojunction interface can be sharpened as compared with the case where the reverse conductivity type semiconductor layer 6 is grown directly on the core layer.
[0028]
(6) Moreover, the present invention includes a core layer in any one of the above (1) to (5) after removing the mask 2 and before growing the reverse conductivity type semiconductor layer 6 on the entire surface. The method includes a step of leaving the semiconductor layer 4 at a high temperature to which the constituent atoms of the semiconductor layer 4 move.
[0029]
In this way, by allowing the constituent atoms of the semiconductor layer 4 including the core layer to move before the reverse conductivity type semiconductor layer 6 is grown on the entire surface, it is left at a high temperature so that the abnormally grown portion 5 of the abnormally grown portion 5 is formed by the mass transport phenomenon. The protrusions can be reduced, and the surface of the reverse conductivity type semiconductor layer 6 grown on the entire surface can be made flatter.
[0030]
(7) Further, the present invention is characterized in that in any of the above (1) to (6), a step of forming a diffraction grating in at least a part of the opening 3 of the mask 2 is characterized.
[0031]
Thus, by forming a diffraction grating, a DFB (distributed feedback) semiconductor laser or a DBR (distributed Bragg reflection) semiconductor laser can be formed. Can be unified.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Here, a first embodiment of the present invention will be described with reference to FIGS.
2A, first, a tapered waveguide portion is formed on the surface of an n-type InP substrate 11 having an electron concentration of 1.0 to 3.0 × 10 ¹⁸ cm ⁻³ , for example, 2.0 × 10 ¹⁸ cm ^−3. In the region having a length of 200 μm, a tapered opening having a width gradually increasing toward the emission end face is formed. On the other hand, in the region in which the laser portion having a length of 300 μm is formed, a uniform and narrow stripe-shaped opening is formed. An SiO ₂ mask 12 serving as a selective growth mask having an opening 13 made of is provided.
[0033]
Next, referring to FIG. 2B, the MQW core layer 14 and the p-type InP clad layer 15 having a thickness of 50 to 300 nm, for example, 100 nm are sequentially selectively grown by using a low pressure MOVPE method.
This p-type InP clad layer 15 prevents the roughness of the growth interface during the subsequent regrowth after removal of the SiO ₂ mask 12 from reaching the MQW core layer 14, that is, sharpens the heterojunction interface. It is provided to do.
[0034]
At this time, the film thicknesses of the upper and lower light guide layers and the quantum well layers constituting the MQW core layer 14 are, for example, tapered to a thickness of 1/3 over 200 μm toward the emission end face in the tapered waveguide portion. In addition, a flat structure is formed in the laser portion.
[0035]
In the first embodiment, the upper and lower light guide layers are 50 to 150 nm thick, for example, 100 nm, for example, by an InGaAsP layer with a 1.1 μm wavelength composition in the flat region forming the laser portion. The quantum well layer is formed by depositing six 10 nm thick 1.1 μm wavelength InGaAsP barrier layers and five 6 nm thick 1.35 μm wavelength InGaAsP well layers.
[0036]
Next, after removing the SiO ₂ mask 12, the reduced pressure MOVPE method is used again to form a p-type InP cladding layer 16 having a thickness of 2000 to 6000 nm, for example, 4000 nm (4 μm) and a thickness of 200 to 1000 nm. For example, a 500 nm p-type InGaAs contact layer 17 is deposited.
[0037]
Next, referring to FIG. 3D, by using a resist pattern (not shown) as a mask, dry etching using ethane-based gas is performed so that the width is 10 μm and the surface of the MQW core layer 14 is 50 to 300 nm, for example, 100 nm. Two stripe-shaped grooves 18 having a depth in which the p-type InP clad layer 15 or the p-type InP clad layer 16 having a thickness remains are formed, and a convex portion therebetween is defined as a ridge 19.
[0038]
In this case, the resist pattern is formed so that the width of the ridge 19 is, for example, 2 μm at the laser portion, flares toward the emission end surface at the tapered waveguide portion, and 5 μm at the emission end surface.
[0039]
Next, referring to FIG. 3E, after removing the p-type InGaAs contact layer 17 in the region of 150 μm from the tip of the tapered waveguide portion 21 by etching, an SiO ₂ film is provided on the entire surface to form the top of the ridge 19 although not shown in the drawings. By providing an opening for contact only with respect to the p-type InGaAs contact layer 17, providing a p-side electrode so as to cover the opening, and providing an n-side electrode on the back surface of the n-type InP substrate 11, The basic structure of a semiconductor laser with a tapered waveguide is completed.
[0040]
In the first embodiment, the thickness of the semiconductor layer to be selectively grown is about 600 nm only with the MQW core layer 14 of 290 nm and the p-type InP cladding layer 15 of 300 nm. The thickness of the abnormally grown portion can be greatly reduced.
[0041]
In this case, since the thick p-type InP cladding layer 16 and the p-type InGaAs contact layer 17 are grown on the entire surface, the surface can be flattened. Therefore, after the ridge 19 is formed, bonding is performed by junction-down. Even so, accurate and stable passive alignment becomes possible.
[0042]
In order to make the abnormally grown portion smaller, before the p-type InP cladding layer 16 is grown, it is maintained at a high temperature of about 620 ° C. in a PH ₃ atmosphere in the MOVPE apparatus. The shape of the abnormally grown portion may be smoothed, whereby the flatness of the surface can be further increased.
[0043]
Next, a second embodiment of the present invention will be described with reference to FIGS.
4A. First, a tapered waveguide portion is formed on the surface of an n-type InP substrate 11 having an electron concentration of 1.0 to 3.0 × 10 ¹⁸ cm ⁻³ , for example, 2.0 × 10 ¹⁸ cm ^−3. In the region having a length of 200 μm, a tapered opening having a width gradually increasing toward the emission end face is formed. On the other hand, in the region in which the laser portion having a length of 300 μm is formed, a uniform and narrow stripe-shaped opening is formed. the selective growth mask having an opening 13 consisting of provided SiO ₂ mask 12, ethane gas depth corresponding to the thickness is selectively grown on the n-type InP substrate 11 by dry etching using, e.g., the depth A 400 nm trench 22 is formed.
[0044]
Next, referring to FIG. 4B, the MQW core layer 14 and the p-type InP clad layer 15 having a thickness of 50 to 300 nm, for example, 100 nm are sequentially and selectively grown inside the trench 22 by using a reduced pressure MOVPE method.
Note that the p-type InP cladding layer 15 in this case is also provided so that the roughness of the growth interface during the subsequent regrowth after removal of the SiO ₂ mask 12 does not reach the MQW core layer 14.
[0045]
At this time, the film thicknesses of the upper and lower light guide layers and the quantum well layers constituting the MQW core layer 14 are, for example, tapered to a thickness of 1/3 over 200 μm toward the emission end face in the tapered waveguide portion. In addition, a flat structure is formed in the laser portion.
[0046]
Also in this second embodiment, the upper and lower light guide layers are 50 to 150 nm thick, for example, 100 nm, for example, by an InGaAsP layer with a 1.1 μm wavelength composition in the flat region forming the laser portion. The quantum well layer is formed by depositing six 10 nm thick 1.1 μm wavelength InGaAsP barrier layers and five 6 nm thick 1.35 μm wavelength InGaAsP well layers.
[0047]
Next, after removing the SiO ₂ mask 12, using the reduced pressure MOVPE method, the p-type InP cladding layer 16 having a thickness of 2000 to 6000 nm, for example, 4000 nm (4 μm), and a thickness of 200 to 1000 nm are used. For example, a p-type InGaAs contact layer 17 of 500 nm is sequentially deposited.
[0048]
Next, referring to FIG. 5D, by using a resist pattern (not shown) as a mask, dry etching using ethane-based gas is performed so that the width is 10 μm and the surface of the MQW core layer 14 is 50 to 300 nm, for example, 100 nm. Two stripe-shaped grooves 18 having a depth in which the p-type InP clad layer 15 or the p-type InP clad layer 16 having a thickness remains are formed, and a convex portion therebetween is defined as a ridge 19.
[0049]
In this case, the resist pattern is formed so that the width of the ridge 19 is, for example, 2 μm at the laser portion, flares toward the emission end surface at the tapered waveguide portion, and 5 μm at the emission end surface.
[0050]
Next, after removing the p-type InGaAs contact layer 17 in the region of 150 μm from the tip of the tapered waveguide portion 21 by etching, an SiO ₂ film is provided on the entire surface to form the top of the ridge 19 although not shown in the drawings. By providing an opening for contact only with respect to the p-type InGaAs contact layer 17, providing a p-side electrode so as to cover the opening, and providing an n-side electrode on the back surface of the n-type InP substrate 11, The basic structure of a semiconductor laser with a tapered waveguide is completed.
[0051]
In the second embodiment, since the MQW core layer 14 and the p-type InP clad layer 15 are grown inside the trench 22, the protrusions in the abnormally grown portion are significantly larger than those in the first embodiment. It can be made smaller, which makes the surface flatness even better.
[0052]
Also in this second embodiment, since the thickness of the growth layer formed by selective growth is not constant, a step is inevitably generated, and the growth layer thickness of the laser unit 20 is the depth of the groove 22. If it is larger than that, an abnormally grown portion is generated although it is smaller than in the case of the first embodiment.
[0053]
In order to reduce this abnormal growth portion, before the p-type InP cladding layer 16 is grown, it is maintained at a high temperature of about 620 ° C. in a PH ₃ atmosphere in the MOVPE apparatus, thereby causing abnormal growth due to the mass transport phenomenon. What is necessary is just to make the shape of a part smooth.
[0054]
Next, a third embodiment of the present invention will be described with reference to FIG.
See FIG. 6. The semiconductor laser with a ridge type tapered waveguide shown in FIG. 6 is a DFB type semiconductor laser, and the process is the same as that of the first embodiment, but the SiO ₂ mask shown in FIG. After forming the diffraction grating 23 in the region where the tapered waveguide is not formed in the opening, a process similar to the process after FIG.
Note that the SiO ₂ mask may be formed after partially forming the diffraction grating 23 first.
[0055]
In this case, an AR coating (non-reflective coating) 24 is applied on the emission end face side, and an HR coating (high reflection coating) 25 is applied on the back side.
Such an HR coating 25 is also provided in the first and second embodiments.
[0056]
Further, such a diffraction grating 23 may be provided in the semiconductor laser with a ridge type tapered waveguide of the second embodiment, and in that case, as shown in FIG. After forming a groove having a width of 20 μm in the region where the laser part is to be formed, a diffraction grating may be formed in a region having a width of about 5 μm at the center.
[0057]
In such a DFB type semiconductor laser, a single oscillation wavelength can be obtained together with the effect of increasing the spot size by the tapered waveguide.
[0058]
The embodiments of the present invention have been described above. In the first and second embodiments described above, an InGaAsP barrier layer having a thickness of 1.1 μm and a thickness of 10 nm as a quantum well layer, 6 nm 1.35 μm wavelength composition InGaAsP well layers are alternately deposited to form five well layers. However, the present invention is not limited to such a configuration, and a required wavelength, for example, What is necessary is just to select arbitrarily the composition of each layer, thickness, and the number of layers according to a 1.5 micrometer zone | band and an output.
[0059]
In each of the above embodiments, the InGaAsP / InP optical semiconductor element is described. However, the present invention is not limited to the InGaAsP / InP system, and may be an InAlGaAs system, an InGaAs / GaAs / AlGaAs system, or The present invention can also be applied to InGaP / AlInGaP systems.
[0060]
【The invention's effect】
According to the present invention, since the clad layer for forming the ridge of the semiconductor laser with a ridge type tapered waveguide is formed by the entire surface growth, the surface can be flattened, and thereby a junction essential for passive alignment. Since down bonding is possible, the assembly of the optical module can be simplified, which greatly contributes to the development of optical fiber communication.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a basic configuration of the present invention.
FIG. 2 is an explanatory diagram of the manufacturing process up to the middle of the first embodiment of the present invention.
FIG. 3 is an explanatory diagram of the manufacturing process from FIG. 2 onward according to the first embodiment of the present invention.
FIG. 4 is an explanatory diagram of the manufacturing process up to the middle of the second embodiment of the present invention.
FIG. 5 is an explanatory diagram of the manufacturing process after FIG. 4 according to the second embodiment of the present invention.
FIG. 6 is an explanatory diagram of a third embodiment of the present invention.
FIG. 7 is a perspective view of a main part of a conventional semiconductor laser with a ridge type tapered waveguide.
FIG. 8 is an explanatory diagram of a manufacturing process of a conventional semiconductor laser with a ridge type tapered waveguide.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 One conductivity type semiconductor substrate 2 Mask 3 Opening part 4 Semiconductor layer 5 including a core layer Abnormal growth part 6 Reverse conductive type semiconductor layer 7 Striped groove 8 Protrusion part 11 n-type InP substrate 12 SiO ₂ mask 13 Opening part 14 MQW core Layer 15 p-type InP clad layer 16 p-type InP clad layer 17 p-type InGaAs contact layer 18 striped groove 19 ridge 20 laser part 21 taper waveguide part 22 groove 23 diffraction grating 24 HR coating 25 AR coating 31 n-type InP substrate 32 MQW core layer 33 p-type InP clad layer 34 p-type InP ridge 35 laser part 36 taper waveguide part 37 SiO ₂ mask 38 opening 39 abnormally grown part 40 stripe-like groove 41 mount substrate 42 solder layer

Claims (7)

A step of providing a mask having an opening on the surface of one conductivity type semiconductor substrate, and selectively growing a semiconductor layer including a core layer only on the opening; after removing the mask, a reverse conductivity type semiconductor layer is grown on the entire surface And a step of forming a convex portion sandwiched between two stripe-shaped grooves in a portion located on the semiconductor layer including the core layer of the reverse conductivity type semiconductor layer. Device manufacturing method. A mask having an opening is provided on the surface of the one-conductivity-type semiconductor substrate, and a groove is formed in the opening by etching the one-conductivity-type semiconductor substrate using the mask, and the opening is left while leaving the mask. A step of selectively growing a semiconductor layer including a core layer only in the groove formed in the step, a step of growing a reverse conductivity type semiconductor layer over the entire surface after removing the mask, and the step of forming the reverse conductivity type semiconductor layer. 2. The method of manufacturing an optical semiconductor element according to claim 1, further comprising a step of forming a convex portion sandwiched between two stripe-shaped grooves in a portion located on the semiconductor layer including the core layer. 3. The semiconductor layer including the selectively grown core layer is composed of a flat region and a tapered region in which the film thickness gradually decreases toward the light emitting end face. The manufacturing method of the optical-semiconductor element of description. 4. The method of manufacturing an optical semiconductor element according to claim 1, wherein the core layer includes a multiple quantum well layer and a light guide layer sandwiching the multiple quantum well layer. 5. 5. The reverse conductivity type semiconductor layer having the same composition as the reverse conductivity type semiconductor layer provided on the entire surface is provided as an uppermost layer of the semiconductor layer including the core layer. 6. Manufacturing method of optical semiconductor element. After the removal of the mask and before the growth of the reverse conductivity type semiconductor layer on the entire surface, the method includes a step of leaving the semiconductor layer including the core layer at a high temperature to which the constituent atoms of the semiconductor layer move. The manufacturing method of the optical-semiconductor element of any one of Claim 1 thru | or 5. 7. The method of manufacturing an optical semiconductor element according to claim 1, further comprising a step of forming a diffraction grating in at least a part of the opening of the mask.

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