JPH1154829A - Manufacture of semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the positional deviation of a diffraction grating by protecting the contact layer of a stripe-like projecting section from dry etching, when a recessed section is formed in an upper clad layer by dry etching by only covering the top of the stripe-like projecting section with a protective layer. SOLUTION: When a diffraction grating is formed in a semiconductor layer, the grating is formed by etching the part of an upper clad layer 15 which is other than the part of the layer 15 under a stripe-like projecting section 16A, composed of a protective layer 30 and a contact layer 16 by forming the projecting section 16A on the clad layer 15 in advance. When this method is adopted, the positional deviation of the diffraction grating can be eliminated at the time of forming the grating. Therefore, a distributed feedback semiconductor laser having good characteristics can be obtained, without causing deviations, etc., of the emitted laser light caused by the decline of the coupling coefficient associated with the positional deviation of the diffraction grating and the increase in the threshold current and asymmetrical light intensity distribution associated with the decline in the coupling coefficient.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor laser, and more particularly to a method for manufacturing a distributed feedback semiconductor laser that enables stable longitudinal single mode oscillation.

[0002]

2. Description of the Related Art As application fields of semiconductor lasers, optical memory, optical communication, optical applied measurement, hologram scanner and the like are known. Among these application fields, in the field of optical communication, a semiconductor laser in the infrared wavelength band is preferable in relation to the waveguide characteristics of an optical fiber. From such a background, a Fabry-Perot type semiconductor laser is generally used as a conventional semiconductor laser.
There is _one variation, has the disadvantage that the oscillation wavelength is varied further by the use temperature, the oscillation of the single longitudinal mode is assumed impossible during high-speed modulation. Therefore, distributed feedback semiconductor lasers have been receiving attention.

FIGS. 6A to 7G are views showing an example of a manufacturing process of a conventional distributed feedback semiconductor laser.
These figures are all viewed from the end face of the semiconductor laser. First, as shown in FIG.
An InP lower cladding layer 52, an active layer 53, an etching stop layer 54, an InP upper cladding layer 55,
The GaAs contact layers 56 are sequentially stacked. Next, FIG.
As shown in (B), by patterning the InGaAs contact layer 56, the stripe-shaped convex portions 56 are formed.
Form A. The portions on both sides of the stripe-shaped convex portion 56A are regions where a diffraction grating is formed later.

[0006] Next, as shown in FIG. 6 (C), an anti-reflection layer (not shown) and a photoresist 57 are formed on the InP upper cladding layer 55 and the stripe-shaped convex portions 56 A.
Is applied. Next, as shown in FIG. 6 (D), a photomask 58 having a window conforming to the shape of the stripe-shaped protrusion 56A is brought into close contact with the photoresist 57 and irradiated with light to form the stripe-shaped protrusion 56A. Only the photoresist 57 in the portion corresponding to the upper part is exposed first. Next, as shown in FIG. 7E, two-beam interference exposure is performed on the entire surface.
Then, since the photoresist 57 in the portion above the stripe-shaped convex portion 56A has already been exposed, a sine-wave pattern is formed only in the unexposed photoresist portions on both sides of the stripe-shaped convex portion. You.

[0007] Next, as shown in FIG. 7 (F), a SiO ₂ film is obliquely deposited on the surface of the sine-waveform photoresist 57, and the photoresist 57 and the anti-reflection layer are formed using the deposited SiO ₂ film as a mask. Is etched by oxygen plasma. Thereafter, the InP upper cladding layer 55 is dry-etched. Further, as shown in FIG. 7 (G), the antireflection layer and the photoresist layer 57 are peeled off, and the etched portion of the InP upper cladding layer 55 is further wet-etched until reaching the etching stop layer 54 so that the concave portion 5 is formed.
9 is formed. The upper cladding layer 55 is covered with an insulating layer, and an upper electrode layer and a lower electrode layer are formed on the lower surface of the semiconductor substrate 51 and the upper surface of the stripe-shaped convex portion 56A, respectively. A distributed feedback semiconductor laser having a grating can be obtained.

[0006]

However, in the manufacturing process of the above-mentioned conventional distributed feedback semiconductor laser, as shown in FIG. 6 (D), there is provided a window portion conforming to the shape of the stripe-shaped convex portion 56A. When the photomask 58 is brought into close contact with the photoresist 57 and irradiated with light, the photomask 58 having the same width of the window is placed on the stripe-shaped convex portion 56A having a width of only a few μm so as not to be displaced. Alignment is very difficult. If the photo mask 58
8 is shifted in the width direction of the stripe-shaped convex portion, as shown in FIG. 8, a diffraction grating may be formed on a portion 55A of the InP upper cladding layer 55 where the diffraction grating should have been formed. A diffraction grating is formed on a part 56B of the stripe-shaped convex portion 56A on which a diffraction grating cannot be formed.

When such misalignment occurs, the laser characteristics of the obtained distributed feedback semiconductor laser include a decrease in the coupling coefficient, a corresponding increase in the threshold current value,
A problem arises, such as the direction of the emitted light, due to the asymmetric light intensity distribution. In view of the above, it is an object of the present invention to provide a method of manufacturing a distributed feedback semiconductor laser capable of obtaining good characteristics without causing displacement of a diffraction grating.

[0008]

According to the present invention, there is provided a method of manufacturing a semiconductor laser, comprising: forming a lower cladding layer on a semiconductor substrate;
An active layer, an upper clad layer, a contact layer, a current stop layer having a stripe groove forming a current path, and an electrode layer are sequentially laminated, and a plurality of concave portions having a rectangular cross section reaching a bottom portion to the upper clad layer are formed. In a method of manufacturing a semiconductor laser having diffraction gratings arranged at predetermined intervals in the direction of the stripe groove, a method of forming a contact layer on the upper clad layer and then protecting the upper clad layer with corrosion resistance during dry etching. Forming a layer on the contact layer, etching the protective layer and the contact layer to form on the upper cladding layer a stripe-shaped protrusion having an outer shape conforming to the outer shape of the stripe groove; The mask for forming the plurality of concave portions is formed on the stripe-shaped convex portions and the cladding layer, and is dry-etched. Forming a plurality of concave portions, removing the mask and the protective layer portion forming the surface layer of the stripe-shaped convex portions, and then forming the current stop layer on the clad layer and removing the electrode layer from the remaining stripe-shaped portions. It is characterized in that it is formed on a convex part.

In the conventional manufacturing method, as a means for arranging the diffraction grating on both sides of the stripe-shaped convex portion, a photomask having a window having substantially the same width as the stripe-shaped convex portion is used. In this method, only the photoresist corresponding to the above is exposed in advance, so that the two-beam interference exposure action is not applied to this portion. This step has caused the displacement of the diffraction grating with respect to the stripe-shaped convex portions.

Therefore, in the present invention, instead of providing this step, a mask for forming a concave portion forming a diffraction grating is formed on the entire surface, while only the stripe-shaped convex portion is covered with a protective layer. When the concave portion is formed by dry-etching the clad layer, the contact layer in the stripe-shaped convex portion is protected from the dry etching to form a diffraction grating only on both sides of the stripe-shaped convex portion. Therefore, in the method of the present invention, since there is no exposure step itself using a photomask as in the related art, there is no displacement of the diffraction grating with respect to the stripe-shaped convex portions. The protective layer may be made of any material that can withstand the etching of the upper clad layer for forming the diffraction grating, and may be made of SiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ or the like. Further, as a specific means for forming a mask for forming a concave portion, a method of forming a sine wave resist pattern by two-beam interference exposure after applying a photoresist, a resist pattern having a window by electron beam direct writing A forming method or the like can be adopted.

In the method of manufacturing a semiconductor laser according to the present invention, it is preferable that the thickness of the protective layer is in the range of 100 to 4000 °. The reason is that the thickness of the protective layer is 1
When the thickness is less than 00 °, the etching resistance becomes insufficient, and the contact layer of the stripe-shaped protrusion may be etched. Conversely, when the thickness of the protective layer exceeds 4000 °, the height of the stripe-shaped protrusion is increased. If the two-beam interference exposure is used, for example, when forming a mask for forming a plurality of concave portions, a shadow may be generated near the stripe-shaped convex portion due to the inclination of the entire substrate, and the concave portion may not be formed in the shadow portion. That's why.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings, but the present invention is not limited to only these embodiments. FIG. 1 is a perspective view showing an embodiment of a distributed feedback semiconductor laser manufactured by the method of the present invention, and FIG. 2 is a sectional view taken along line aa of FIG. As shown in FIGS. 1 and 2, the distributed feedback semiconductor laser 1 of the present embodiment has a lower electrode layer 11 on a lower surface of a semiconductor substrate 12.
Are formed. On the semiconductor substrate 12, a lower cladding layer 13, an active layer 14, and an upper cladding layer 15 are stacked, and a contact layer 16 having a predetermined width is formed at the center of the upper surface of the upper cladding layer 15. Further, an etching stop layer 10 is provided between the active layer 14 and the upper cladding layer 15.

Further, a current stop layer 17 made of an insulating layer is formed on the upper cladding layer 15, and a stripe groove 19 having a predetermined width is formed in the center of the current stop layer 17. An upper electrode layer 18 is formed on the upper surfaces of the current stop layer 17 and the stripe groove 19 so as to be in contact with the contact layer 16. The semiconductor epitaxial substrate composed of the above layers can be manufactured by epitaxial crystal growth using a MOCVD (metal organic chemical vapor deposition) apparatus.

In this semiconductor epitaxial substrate,
A large number of concave portions 20 each having a rectangular cross section from the upper cladding layer 15 to the etching stop layer 10 are arranged in parallel in the stripe groove direction at predetermined intervals. Are formed between the adjacent concave portions 20, 20,..., And the concave portions 20 and these convex portions 21 constitute a diffraction grating. In the embodiment of FIG. 1, these diffraction gratings are formed in two rows on both sides of the stripe groove 19. Each bottom of the recess 20 has reached the surface of the etching stop layer 10, the bottom of each recess 20 has a uniform depth, and the bottom of each recess 20 has been formed simultaneously with the formation of the current stop layer 17. An insulating layer 22 is provided.

In this embodiment, the semiconductor substrate 12 is an n-type InP,
The lower cladding layer 13 is n-type InP, and the active layer 14 is InG.
aAsP, the upper cladding layer 15 is made of p-type InP, and the contact layer 16 is made of InGaAs or InGaAsP. Therefore, the semiconductor laser 1 having this configuration has:
Since the cladding layer is formed of InP and the active layer is formed of InGaAsP, the oscillation wavelength is basically in the infrared wavelength band. The etching stop layer 10 is made of InGaAs or InGaAsP. When an AC drive voltage of a predetermined frequency is applied to the upper electrode layer 18 and the lower electrode layer 11 in the active layer 14 within the width dimension of the stripe groove 19, a concave portion 20 and a convex portion The diffraction grating composed of the portion 21 serves as a resonator, and resonance (feedback) occurs, and a longitudinal single mode laser beam having a resonance wavelength λ ₁ is generated.

Next, an example of a method of manufacturing the distributed feedback semiconductor laser 1 having the structure shown in FIGS. 1 and 2 will be described. In order to manufacture the distributed feedback semiconductor laser 1, first, an n-type InP is formed on a semiconductor substrate 12 made of n-type InP.
P, a lower cladding layer 13 of 1 μm thickness and InGa
An active layer 14 of AsP having a thickness of 0.2 μm and a P-type In
0.005 μm-thick etching stop layer 10 made of GaAsP, 0.7 μm-thick upper cladding layer 15 made of P-type InP, and 0.2 μm made of P-type InGaAs.
An element substrate on which an m-thick contact layer 16 is laminated is manufactured. This elementary substrate can be manufactured by forming each layer on the semiconductor substrate 12 by an epitaxial crystal growth technique using an MOCVD apparatus or the like.

Next, as shown in FIG. 3A, a protective layer 30 made of SiO ₂ and having a thickness of 100 to 4000 ° is laminated on the upper surface of the substrate by a film forming method such as sputtering. Next, as shown in FIG. 3B, after forming a resist pattern (not shown) covering the stripe convex portion to be formed, the protective layer 30 is wet-etched (etched) using the resist pattern as a mask. The liquid is HF: N
Patterning is performed by using H ₄ F = 1: 10, and then the contact layer 16 is patterned by wet etching (using an etching solution of H ₂ SO ₄ : H ₂ O ₂ : H ₂ O = 1: 1: 10). Thus, a stripe-shaped convex portion 16A including the protective layer 30 and the contact layer 16 is formed.
Next, as shown in FIG. 3C, an organic material film (ARC)
After the anti-reflection layer 31 is formed, a photoresist 32 is applied using a coating device such as a spin coater, and prebaked using a heating device such as a clean oven to cure the photoresist 32.

Next, as shown in FIG. 4D, after exposing the entire surface of the photoresist 32 by the two-beam interference exposure method, the photoresist 32 is developed and post-baked by a heating device to remove moisture. Thus, the surface of the photoresist 32 is formed into a waveform. Next, a protective film 33 having a thickness of 10 nm, such as a SiO ₂ film, is obliquely deposited on the photoresist 32 by a film forming method such as a vacuum oblique evaporation method.

Next, as shown in FIG. 4 (E), using the protective film 33 as a mask, the photoresist layer 32 and the anti-reflection layer 31 which are not covered with the protective film 33 are reacted with oxygen-based reactive ions. It is removed by etching (dry etching). Next, H _{2 is used} as an etching gas.
The upper clad layer 15 is etched using the mixed gas of CH ₄ and Ar as a mask, and the upper clad layer 15 is etched using the etching remaining portion of the protective film 33 and the photoresist layer 32 and the etching remaining portion of the antireflection layer 31 as a mask. Many concave portions 20 are formed in the cladding layer 15.

Next, as shown in FIG. 4F, the remaining portions of the photoresist 32 and the antireflection layer 31 are removed with acetone, and then the protection layer 30 is wet-etched (the etching solution is HF: NH ₄ F). = 1:10). Next, a mixed acid of hydrochloric acid and acetic acid (HCl: CH
₃ COOH = 1: 4 or HCl: CH ₃ C
The bottom of the recess 20 formed in the upper cladding layer 15 is etched down to the etching stop layer 10 by wet etching using OOH: H ₂ O = 1: 4: 0.5.

Next, to 1500 thickness made of SiO ₂ in the film forming means such as sputtering, as shown in FIG. 5 (G) 2
A current stop layer 17 of 000 ° is formed. Thereby, the insulating layer 22 at the bottom of the concave portion 20 is also formed at the same time.
Next, as shown in FIG. 5H, in order to form a stripe groove 19, a photoresist having a window corresponding to the groove shape at the center is formed on the upper surface of the current stop layer 17.
Prebaking with a heating device, exposure and development. Then, etching (using HF: NH ₄ F = 1: 10 as an etching solution) removes the window portion of the current stop layer 17 to form a stripe groove 19. Then, the photoresist is removed with an organic solvent, for example, acetone.

Further, the bottom surface of the substrate 12 is polished by mechanical polishing and chemical polishing to adjust the thickness of the substrate 12, and the lower electrode layer 11 mainly composed of Au is formed on the bottom surface of the substrate 12. Then, the current stop layer 17 and the stripe groove 1
9 on the upper electrode layer 18 mainly made of Au by vacuum evaporation
Is obtained, the semiconductor laser 1 shown in FIG. 5I is obtained.

In the method of manufacturing a distributed feedback semiconductor laser according to the present embodiment, when a diffraction grating is formed in a semiconductor laser as in the prior art, a portion where the diffraction grating is not formed is not protected by masking, but is formed in advance. In this method, a stripe-shaped protrusion 16A including the protective layer 30 and the contact layer 16 is formed on the upper clad layer 15, and a portion other than the stripe-shaped protrusion 16A is etched to form a diffraction grating. By adopting this method, the displacement of the diffraction grating at the time of manufacturing is eliminated, so that the coupling coefficient decreases due to the displacement of the diffraction grating, the threshold current value increases, and the light intensity distribution becomes asymmetric. A distributed feedback semiconductor laser with good characteristics can be obtained without causing any deviation of emitted light.

The technical scope of the present invention is not limited to the above embodiment, and various changes can be made without departing from the spirit of the present invention. For example, in the present embodiment, the semiconductor substrate 12 and the lower cladding layer 13 are n-type InP, and the active layer 14 is InGaAs.
P, the upper cladding layer 15 is made of p-type InP, and the etching stop layer 10 and the contact layer 16 are made of InGaAs or InGaAsP, but applicable raw materials are not limited to these. Further, specific numerical values of each layer, specific processing methods and processing conditions of each step used in the present embodiment can be appropriately changed.

[0025]

As described above, according to the method of manufacturing the distributed feedback laser according to the present invention, since the displacement of the diffraction grating does not occur at the time of manufacturing, the coupling coefficient decreases due to the displacement of the diffraction grating, and A distributed feedback semiconductor laser with good characteristics can be obtained without causing an increase in the threshold current value and a change in the emitted light due to an asymmetric light intensity distribution.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a distributed feedback semiconductor laser according to the present invention.

FIG. 2 is a sectional view taken along line aa of FIG.

FIG. 3 is a process flow chart showing a manufacturing method of the semiconductor laser in a step order.

FIG. 4 is a continuation of the process flow diagram.

FIG. 5 is a continuation of the process flow diagram.

FIG. 6 is a process flow chart showing a conventional method of manufacturing a distributed feedback semiconductor laser in the order of steps.

FIG. 7 is a continuation of the process flow diagram.

FIG. 8 is a partially enlarged view showing an example of a distributed feedback semiconductor laser manufactured by a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Etching stop layer 11 Lower electrode layer 12 Semiconductor substrate 13 Lower clad layer 14 Active layer 15 Upper clad layer 16 Contact layer 16A Stripe convex part 17 Current stop layer 18 Upper electrode layer 19 Stripe groove 20 Concave part 21 Convex part 22 Insulating layer

Claims (2)

[Claims] A first cladding layer, an active layer, an upper cladding layer, a contact layer, a current stop layer having a stripe groove forming a current path, and an electrode layer are sequentially stacked on a semiconductor substrate; A method of manufacturing a semiconductor laser having a diffraction grating arranged in the stripe groove direction with a plurality of rectangular recesses reaching the upper cladding layer at predetermined intervals, wherein the contact layer is formed on the upper cladding layer. After formation, a protective layer having corrosion resistance during dry etching of the upper clad layer is formed on the contact layer, and the protective layer and the contact layer are etched to have an outer shape conforming to the outer shape of the stripe groove. A stripe-shaped protrusion is formed on the upper clad layer, and the plurality of stripe-shaped protrusions and the plurality of Forming a plurality of concave portions by forming a mask for forming a portion and dry-etching, removing the mask and the protective layer portion forming a surface layer of the stripe-shaped convex portions, and then replacing the current stop layer with the cladding layer And forming the electrode layer on the remaining stripe-shaped protrusions. 2. The protective layer has a thickness of 100 to 400.
2. The method for manufacturing a semiconductor laser according to claim 1, wherein the angle is in the range of 0 [deg.].