JP2005129833A - Method of manufacturing semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a semiconductor laser in which shallowing of the depth of a diffraction grating due to the growth of a denatured layer by the activation of mass transport in crystal-growing a semiconductor layer on the diffraction grating by a metal-organic vapor-phase growth method is eliminated. <P>SOLUTION: The method of manufacturing the semiconductor laser includes a step of stacking a silicon-oxide film 102 and a photoresist film 11 on an n-type InP substrate 2, a step of patterning the photoresist film 11 to form a photoresist mask 11a, a step of patterning the silicon-oxide film 102 using the photoresist mask 11a to form a silicon-oxide-film mask 102a, a step of forming a diffraction grating 3a on the surface of the n-type InP substrate 2 using the silicon-oxide-film mask 102a by dry etching, a step of crystal-growing a first semiconductor layer 4a up to the thickness of filling the diffraction grating 3a while leaving the silicon-oxide-film mask 102a as a protection mask for crystal growth inhibition, and a step of crystal-growing a second semiconductor layer 4b to form a guiding layer 4 after removing the silicon-oxide-film mask 102a. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a semiconductor laser, and in particular, a mass transport generated when a semiconductor layer is crystal-grown on a diffraction grating formed on the surface of a semiconductor substrate by metal organic chemical vapor deposition (MOVPE). The present invention relates to a method for manufacturing a semiconductor laser capable of suppressing the activation of.

  Wavelength division multiplexing that can greatly expand communication capacity without adding new optical fibers by transmitting optical signals of different wavelengths in one optical fiber in order to respond to the rapidly increasing communication demand in recent years (WDM: wavelength division multiplexing) Optical communication systems have been developed. Since this WDM optical communication system requires a light source having high oscillation wavelength (oscillation spectrum) purity, a distributed feedback (DFB) semiconductor laser capable of outputting a single wavelength is used. The single wavelength output operation by the distributed feedback operation of this distributed feedback semiconductor laser is realized by introducing a diffraction grating that generates a periodic refractive index change inside the semiconductor optical waveguide. And, it is very important to optimize the shape and depth of the concave and convex portions of the diffraction grating (the step size of the concave and convex portions) in order to further stabilize the single wavelength oscillation.

  Note that the diffraction grating has a characteristic of selectively reflecting only light having a wavelength corresponding to the period, and only a specific wavelength determined by the period of the diffraction grating is reflected by the diffraction grating. It interacts (feeds back) within it and is amplified to form a standing wave. That is, the wavelength of the output light is determined by the period of the diffraction grating having a predetermined depth, thereby realizing a single wavelength output.

  FIG. 5 shows a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) as an example of a conventional semiconductor laser. FIG. 5A is a longitudinal sectional view of the DFB laser cut along a plane horizontal to the optical waveguide direction, and FIG. 5B is a perspective view of a diffraction grating formed on the surface of the semiconductor substrate.

  The DFB laser 1 is composed of a diffraction grating 3 composed of a large number of concave and convex line patterns formed on the surface of an n-type InP substrate 2 and n-type InGaAsP embedded with the diffraction grating 3 and further grown to a certain thickness. A guide layer 4 formed thereon, a multi-well active layer 5 formed thereon, a clad layer 6 formed of p-type InP formed thereon, a cap layer 7 formed of InGaAsP formed thereon, Ti / The p-side electrode 8 and the n-side electrode 9 are formed on the front and back surfaces made of Au or the like, respectively.

  The diffraction grating 3 has an irregular shape with a predetermined period Λ having a predetermined depth d (step size of the irregularities; hereinafter referred to as the diffraction grating depth d). In this case, as will be described later, wet etching (etc. The cross-sectional shape is a wave shape (substantially V-shaped) with a relatively round corner.

  In addition, a metamorphic layer 10 made of InAsP having a different composition from the guide layer 4 made of n-type InGaAsP is deposited in the concave portion of the diffraction grating 3. This metamorphic layer 10 is generated when the guide layer 4 is formed by metal organic vapor phase epitaxy (MOVPE) described later.

  Here, there is the following relationship between the period Λ of the diffraction grating 3 for causing Bragg reflection and the oscillation wavelength λ.

(Equation 1)
Λ = m · λ / (2 · _ne )
(M: positive integer, _ne : equivalent refractive index of optical waveguide inside DFB laser)

In order to accurately obtain the equivalent refractive index (n _e ) in the above formula, the diffraction grating depth d is an important dimension.

However, metamorphic layer 10 is formed on the refractive index different from that of the guide layer 4, for varying the diffraction grating depth d by deposition of the recess (shallowing), a factor to vary the equivalent refractive index (n _e) became.

  Next, a method for manufacturing the DFB laser 1 will be described with reference to FIGS. 6 and 7 are sectional views showing the order of steps of the manufacturing method.

  First, a diffraction grating is formed on the surface of the n-type InP substrate 2. The diffraction grating is formed by coating and forming a positive photoresist film 11 on an n-type InP substrate 2 as shown in FIG.

  Next, as shown in FIG. 6B, a diffraction grating pattern having a predetermined period Λ is directly drawn on the photoresist film by an electron beam by an electron beam exposure method, and then developed to obtain a photoresist mask 11a.

  Here, the electron beam exposure method has been described as a method for forming the photoresist mask 11a. However, as another exposure method, an interference exposure method, a method of transferring a pattern of a mask (not shown), or the like is employed. Also good. The electron beam exposure method can form the most precise and fine period and is suitable for manufacturing a DFB laser. However, since the drawing speed is slow and the throughput is inferior, the diffraction grating is not formed on the entire surface of the n-type InP substrate 2. Alternatively, the drawing distance and the drawing time may be shortened by locally forming only the portion corresponding to the active layer capable of laser oscillation and the periphery thereof.

  Next, as shown in FIG. 6C, using the photoresist mask 11a as an etching mask, the n-type InP substrate 2 is etched by wet etching to form the diffraction grating 3.

  At this time, since the wet etching isotropically proceeds, the etching solution inevitably flows under the photoresist mask 11a, and the concavo-convex shape of the diffraction grating 3 has a wave shape (substantially V-shaped) with a relatively round corner. Become.

  Next, as shown in FIG. 7 (d), after removing the photoresist mask, the diffraction grating 3 is embedded, and InGaAsP is further grown to a constant thickness by metal organic vapor phase epitaxy (MOVPE). 4 is formed.

At this time, the mass transport of the element (In) included in the diffraction grating 3 is activated by the temperature rise, and reacts with the source gas (AsH ₃ , PH ₃ ) to generate the modified layer 10 made of InAsP. As a result, the convex portions of the diffraction grating 3 were consumed and the height thereof was lowered, while the concave portions were filled with the generated metamorphic layer 10 and the diffraction grating depth d was shallow. (For example, refer to Patent Document 1).
Japanese Patent Laid-Open No. 11-112098 (page 2, paragraph 0004, FIG. 7)

  Further, the phenomenon that the diffraction grating depth d becomes shallow due to the mass transport is more remarkable as the cross-sectional shape of the diffraction grating 3 is closer to a wave shape (substantially V-shaped), and the effect is closer to a rectangular shape (substantially U-shaped). Few.

  One possible reason for this is that if the cross-sectional shape of the diffraction grating 3 is a corrugated shape (substantially V-shaped), the protrusions / recesses each have a tapered shape. This is because it is easy to be embedded. In addition, the InAsP crystal generated in the convex portion moves (drops) along the slope of the diffraction grating 3 into the concave portion, and a new diffraction grating 3 surface is exposed from below by the movement, and the mass transport is This is because activated and new InAsP crystals are formed one after another.

  On the other hand, if the cross-sectional shape of the diffraction grating 3 is rectangular (substantially U-shaped), the convex portions / concave portions have a flat shape having a flat surface with a constant area, so that the wide convex portions are difficult to reduce and the wide concave portions are embedded. It's hard to get it. Further, since the InAsP crystal generated in the convex portion does not move and remains in the flat convex portion, the growth of new InAsP is suppressed.

  That is, it can be said that the amount of change (shallowing) of the diffraction grating depth d due to the deposition of the metamorphic layer 10 in the recesses is affected by the cross-sectional shape of the diffraction grating, and the cross-sectional shape is desirably closer to a rectangular shape.

  Next, as shown in FIG. 7E, the active layer 5, the clad layer 6 made of p-type InP 6 and the cap layer 7 made of InGaAsP are successively grown on the guide layer 4 in order.

  Next, as shown in FIG. 7F, the p-side electrode 8 and the n-side electrode 9 made of Ti / Au or the like are formed on the front surface and the back surface, respectively, by vapor deposition or sputtering, and the DFB laser 1 is completed.

As described above, in the conventional method for manufacturing the DFB laser 1, when the guide layer 4 is formed on the diffraction grating 3 formed on the surface of the n-type InP substrate 2 by metal organic vapor phase epitaxy (MOVPE), the increase is made. The mass transport of the element (In) contained in the diffraction grating 3 is activated by the temperature, and the metamorphic layer 10 having a composition and refractive index different from that of the guide layer 4 is generated, and the convex portions of the diffraction grating 3 are consumed and the height is increased. On the other hand, since the concave portion is buried in the resulting metamorphic layer 10, the diffraction grating depth d becomes shallow as a result, and there is a possibility that a desired equivalent refractive index ( _ne ) cannot be obtained. Further, the amount of change (shallowing) of the diffraction grating depth d becomes more prominent as the cross-sectional shape of the diffraction grating 3 is closer to a wave shape (substantially V-shaped).

  An object of the present invention is to produce a metamorphic layer by activating mass transport when a semiconductor layer is crystal-grown on a diffraction grating formed on the surface of a semiconductor substrate by metal organic vapor phase epitaxy (MOVPE). The present invention provides a method of manufacturing a semiconductor laser in which the diffraction grating depth does not become shallow.

The method for producing a semiconductor laser of the present invention comprises:
at least,
Forming a diffraction grating on a semiconductor substrate;
When a semiconductor layer is grown on a diffraction grating by metal organic vapor phase epitaxy, the convex part of the diffraction grating is covered with a protective mask for preventing crystal growth so that the mass transport of elements contained in the diffraction grating is not activated. A method of manufacturing a semiconductor laser, comprising: covering and crystal-growing a semiconductor layer.

Furthermore, the manufacturing method of the semiconductor laser of the present invention includes:
at least,
Forming a protective film serving as a protective mask for preventing crystal growth on a semiconductor substrate;
A step of laminating a photoresist film on the protective film;
Exposing and developing the photoresist film to a diffraction grating pattern to form a photoresist mask;
Etching the protective film using a photoresist mask to form a protective mask having a diffraction grating pattern;
After removing the photoresist mask, etching the semiconductor substrate using a protective mask to form a diffraction grating;
A step of crystal-growing the first semiconductor layer to a thickness for embedding the diffraction grating by metal organic vapor phase epitaxy while leaving the protective mask on the convex portion of the diffraction grating;
And a step of crystal growth of a second semiconductor layer made of the same material as the first semiconductor layer with a constant thickness after removing the protective mask.

According to the semiconductor laser manufacturing method of the present invention, when a semiconductor layer is formed on a diffraction grating formed on the surface of a semiconductor substrate by metal organic chemical vapor deposition (MOVPE), mass transport of elements contained in the diffraction grating is performed. In order to prevent undesired metamorphic layers from being activated, and as a result, the diffraction grating is covered with a protective mask for preventing crystal growth so that the diffraction grating is not shallow. The depth can be maintained, and a desired equivalent refractive index ( _ne ) can be obtained. In addition, when the diffraction grating is formed in a rectangular shape by dry etching (anisotropy), the amount of change (shallowing) of the diffraction grating depth d can be further reduced.

  As an example of the semiconductor laser of the present invention, a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) is shown in FIG. FIG. 1A is a longitudinal sectional view of a DFB laser cut along a plane horizontal to the optical waveguide direction, and FIG. 1B is a perspective view of a diffraction grating formed on the surface of a semiconductor substrate. The same parts as those in FIG.

  The DFB laser 101 is a first semiconductor composed of a diffraction grating 3a formed of a large number of uneven line patterns formed on the surface of an n-type InP substrate 2 and n-type InGaAsP in which the diffraction grating 3a is embedded so as to be flat. A guide layer 4 composed of a layer 4a and a second semiconductor layer 4b made of n-type InGaAsP, on which a crystal of a certain thickness is further grown, and an active layer 5 having a multi-well structure formed thereon, A cladding layer 6 made of p-type InP formed thereon, a cap layer 7 made of InGaAsP formed thereon, and a p-side electrode 8 and an n-side electrode 9 formed on the front and back surfaces made of Ti / Au or the like, respectively. It consists of and.

  The diffraction grating 3a has an irregular shape with a predetermined period Λ having a predetermined depth d (a step size of the irregularities; hereinafter referred to as a diffraction grating depth d). In this case, as will be described later, dry etching (different Therefore, the cross-sectional shape is a rectangular shape (substantially U-shaped) with relatively square corners.

  In addition, no undesired metamorphic layer is deposited in the recesses of the diffraction grating 3a.

  Here, there is the following relationship between the period Λ of the diffraction grating 3a for causing Bragg reflection and the oscillation wavelength λ.

(Equation 1)
Λ = m · λ / (2 · _ne )
(M: positive integer, _ne : equivalent refractive index of optical waveguide inside DFB laser)

In order to accurately obtain the equivalent refractive index (n _e ) in the above formula, the diffraction grating depth d is an important dimension.

  Next, a method for manufacturing the DFB laser 101 will be described with reference to FIGS. 2-4 is sectional drawing which shows the process order of a manufacturing method. 6 and 7 are denoted by the same reference numerals.

  First, a diffraction grating is formed on the surface of the n-type InP substrate 2. As shown in FIG. 2A, the diffraction grating is formed on the n-type InP substrate 2 as a protective mask for preventing crystal growth when metal organic vapor phase epitaxy (MOVPE) is used later. A planned silicon oxide film 102 is formed by a CVD method.

  Next, as shown in FIG. 2B, a positive photoresist film 11 is formed on the silicon oxide film 102 by coating.

  Next, as shown in FIG. 2C, a diffraction grating pattern having a predetermined period Λ is directly drawn on the photoresist film by an electron beam by an electron beam exposure method, and then developed to obtain a photoresist mask 11a.

  Here, the electron beam exposure method has been described as a method for forming the photoresist mask 11a. However, as another exposure method, an interference exposure method, a method of transferring a pattern of a mask (not shown), or the like is employed. Also good. The electron beam exposure method can form the most precise and fine period and is suitable for manufacturing a DFB laser. However, since the drawing speed is slow and the throughput is inferior, the diffraction grating is not formed on the entire surface of the n-type InP substrate 2. Alternatively, the drawing distance and the drawing time may be shortened by locally forming only the portion corresponding to the active layer capable of laser oscillation and the periphery thereof.

  Next, as shown in FIG. 3D, the silicon oxide film 102a is formed by etching the silicon oxide film by dry etching (anisotropy) using the photoresist mask 11a as an etching mask.

  At this time, since the silicon oxide film is formed by dry etching (anisotropy), the pattern edge of the silicon oxide film mask 102a becomes sharp, and the n-type InP substrate 2 is etched later to form a diffraction grating. It is suitable for obtaining a diffraction grating having a sharp cross-sectional shape.

  Next, as shown in FIG. 3E, the n-type InP substrate 2 is etched by a predetermined depth d by dry etching (anisotropy) using the silicon oxide film mask 102a as an etching mask, so that the cross-sectional shape is rectangular ( A substantially U-shaped) diffraction grating 3a is formed.

  Next, as shown in FIG. 3 (f), the metal oxide film 102a is left just as thick as the diffraction grating 3a is embedded, leaving the silicon oxide film mask 102a as a protective mask for preventing crystal growth of the convex portions of the diffraction grating 3a. The first semiconductor layer 4a made of n-type InGaAsP is crystal-grown by phase growth (MOVPE) to form part of the guide layer.

  In this case, since the convex portion of the diffraction grating 3a is covered with the silicon oxide film mask 102a as a protective mask for preventing crystal growth, which is a feature of the present invention, the undesired metamorphic layer is formed. Is unlikely to occur. Therefore, the convex part of the diffraction grating 3a is not consumed, and the concave part is not filled with the metamorphic layer, and the diffraction grating depth d can be maintained.

  Further, the amount of change (shallowing) of the diffraction grating depth d can be reduced also in that the diffraction grating 3a is formed in a rectangular shape (substantially U-shaped) by dry etching (anisotropy).

  Next, after the burying of the diffraction grating 3a is completed, the silicon oxide film mask is removed with hydrofluoric acid, and then the second semiconductor layer 4b made of n-type InGaAsP is further fixed to a certain thickness as shown in FIG. Then, crystal growth is performed by metal organic vapor phase epitaxy (MOVPE) to complete the guide layer 4 composed of the first semiconductor layer 4a and the second semiconductor layer 4b.

  Next, as shown in FIG. 4H, the active layer 5, the clad layer 6 made of p-type InP 6 and the cap layer 7 made of InGaAsP are sequentially grown on the guide layer 4 in order.

  Next, as shown in FIG. 4 (i), the p-side electrode 8 and the n-side electrode 9 made of Ti / Au or the like are formed on the front and back surfaces, respectively, by vapor deposition or sputtering to complete the DFB laser 101.

  Thus, when the guide layer 4 is formed on the diffraction grating 3a formed on the surface of the n-type InP substrate 2 by metal organic vapor phase epitaxy (MOVPE), the convex portion of the diffraction grating 3a is formed by silicon oxide for preventing crystal growth. When the crystal is grown while being protected by the film mask 102a, activation of the mass transport can be suppressed and a metamorphic layer is hardly formed, and as a result, the diffraction grating depth d can be maintained. Further, since the cross-sectional shape of the diffraction grating 3a is rectangular (substantially U-shaped), the amount of change (shallowing) of the diffraction grating depth d can be further reduced.

  In the above description, the diffraction grating 3a is formed by dry etching (anisotropic) and the cross-sectional shape is rectangular (substantially U-shaped). However, for example, the diffraction grating is wet-etched (isotropic). Even when the cross-sectional shape is formed into a wave shape (substantially V-shaped), the effect of suppressing mass transport by covering the convex part of the diffraction grating with a protective mask for preventing crystal growth and growing the crystal is It goes without saying that it can be obtained as such.

  In the above description, the silicon oxide film has been described as the material for the protective mask for preventing crystal growth. However, the present invention is not limited to this. For example, a silicon nitride film may be used. In this case, however, it must be removed with hot phosphoric acid or the like.

  When the semiconductor layer is formed on the diffraction grating formed on the surface of the semiconductor substrate by metal organic vapor phase epitaxy (MOVPE), the convex portion of the diffraction grating is covered with a protective mask for preventing crystal growth to grow the crystal. The present invention can be applied to a method for manufacturing a semiconductor laser that does not generate an undesired metamorphic layer and as a result can maintain the diffraction grating depth.

Sectional view of principal part of DFB laser as an example of semiconductor laser of the present invention and perspective view of diffraction grating Sectional drawing which shows the process order of the manufacturing method of the DFB laser of this invention Sectional drawing which shows the process order of the manufacturing method of the DFB laser of this invention Sectional drawing which shows the process order of the manufacturing method of the DFB laser of this invention Cross-sectional view of a main part of a DFB laser as an example of a conventional semiconductor laser and a perspective view of a diffraction grating Sectional drawing which shows the process order of the manufacturing method of the conventional DFB laser Sectional drawing which shows the process order of the manufacturing method of the conventional DFB laser

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Conventional DFB laser 2 n-type InP substrate 3 Conventional diffraction grating 3a Diffraction grating of the present invention 4 Guide layer 4a First semiconductor layer 4b Second semiconductor layer 5 Active layer 6 Clad layer 7 Cap layer 8 P-side electrode 9 N-side Electrode 10 Metamorphic layer
11 Photoresist film 11a Photoresist mask 101 DFB laser of the present invention 102 Silicon oxide film 102a Silicon oxide film mask
Λ Grating period

Claims (6)

at least,
Forming a diffraction grating on a semiconductor substrate;
When the semiconductor layer is crystal-grown on the diffraction grating by metal organic vapor phase epitaxy, the convex portion of the diffraction grating is used to prevent crystal growth so that the mass transport of the elements contained in the diffraction grating is not activated. A method of manufacturing a semiconductor laser, comprising: covering with a protective mask and growing the crystal of the semiconductor layer. The method for producing a semiconductor laser of the present invention comprises:
at least,
Forming a protective film serving as a protective mask for preventing crystal growth on a semiconductor substrate;
Laminating a photoresist film on the protective film;
Exposing and developing the photoresist film to a diffraction grating pattern to form a photoresist mask;
Etching the protective film using the photoresist mask to form a protective mask having a diffraction grating pattern;
After removing the photoresist mask, etching the semiconductor substrate using the protective mask to form a diffraction grating;
Crystal growth of the first semiconductor layer to a thickness for embedding the diffraction grating by metal organic vapor phase epitaxy while leaving the protective mask on the convex portions of the diffraction grating;
And a step of crystal growth of a second semiconductor layer made of the same material as that of the first semiconductor layer after removing the protective mask.   The semiconductor layer constituted by the first semiconductor layer and the second semiconductor layer is a guide layer of a distributed feedback semiconductor laser, and further includes a step of forming an active layer on the guide layer. A method of manufacturing a semiconductor laser according to claim 1 or 2.   4. The method of manufacturing a semiconductor laser according to claim 1, wherein the protective mask is made of a silicon oxide film or a silicon nitride film.   5. The method of manufacturing a semiconductor laser according to claim 2, wherein an etching method for forming the protective mask is dry etching, and a pattern edge of the protective mask is sharply formed. 6.   The etching method for forming the diffraction grating on the surface of the semiconductor substrate is dry etching, and is formed so that a cross-sectional shape cut by a plane horizontal to the optical waveguide direction of the diffraction grating is rectangular. A method of manufacturing a semiconductor laser according to claim 2.

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