JP2010171265A - Forming method for diffraction grating and manufacturing method for distributed feedback semiconductor laser - Google Patents

Abstract

A method of forming a diffraction grating capable of controlling the depth of a concave portion with high accuracy and a method of manufacturing a distributed feedback semiconductor laser capable of suppressing variations in laser characteristics are provided.
A method of manufacturing a DFB laser 1 includes a step of forming an upper SCH layer 11, a first mask layer 13 and a second mask layer 15, and a hole 15h corresponding to a recess 11c of a diffraction grating 11d in a second mask. Forming the layer 15, forming the etching recess 14 including the upper SCH layer recess 11 c 1 formed in the upper SCH layer 11 so as to be shallower than the recess 11 c of the diffraction grating 11 d, and the depth of the etching recess 14. A calculation step of measuring HS and calculating an etching rate of the upper SCH layer 11, and additionally etching the upper SCH layer 11 to a desired depth H 11 c 2 based on the etching rate, thereby forming a diffraction grating 11 d in the upper SCH layer 11. And an additional etching step to be formed.
[Selection] Figure 4

Description

  The present invention relates to a method for forming a diffraction grating and a method for manufacturing a distributed feedback semiconductor laser.

The following Patent Document 1 discloses a method for forming a diffraction grating. In this method, first, an SiO ₂ layer (insulating layer) is formed on an InP substrate (semiconductor substrate), and a resist layer is formed on the SiO ₂ layer. Then, a predetermined mask pattern corresponding to the shape of the diffraction grating in the resist layer is formed, by etching the SiO ₂ layer by using the resist layer as a mask to transfer the mask pattern to the SiO ₂ layer. Thereafter, the resist layer is removed.

Subsequently, the InP substrate is dry-etched using the SiO ₂ layer as a mask to form a diffraction grating on the InP substrate. At this time, the SiO ₂ layer as a mask is also etched together with the InP substrate. Therefore, the thickness of the initial SiO ₂ layer is determined so that the SiO ₂ layer that is the mask is removed when the concave portion of the target diffraction grating is formed on the InP substrate.

JP 2003-75619 A

  In the conventional method for forming a diffraction grating as described above, it is necessary to grasp the etching rate of the InP substrate in order to control the depth of the concave portion of the diffraction grating. Therefore, prior to the step of etching the InP substrate to form the diffraction grating, a step of etching another InP substrate and calculating the etching rate of the InP substrate in advance is required.

  However, the etching rate may vary depending on the etching environment. The environment in which etching is performed differs depending on, for example, the type and amount of deposits attached in the chamber of the etching apparatus. For this reason, an error may occur between the actual etching rate of the InP substrate forming the diffraction grating and the etching rate of another InP substrate calculated prior to that. When such an error occurs, an error occurs in the depth of the concave portion of the diffraction grating.

  In order to prevent such an error due to the difference in etching environment, etching of the InP substrate forming the diffraction grating is interrupted halfway, and the depth of the recess formed in the InP substrate is measured. A method of calculating an etching rate, and performing additional etching based on this etching rate to form a diffraction grating, that is, calculating an etching rate of the InP substrate using the InP substrate itself that forms the diffraction grating, A method of forming a diffraction grating based on the etching rate is also conceivable.

However, in the conventional method for forming a diffraction grating as described above, when the InP substrate on which the diffraction grating is formed is etched, the SiO ₂ layer used as a mask is also etched at the same time. Therefore, in order to calculate the depth of the recess of the InP substrate that forms the diffraction grating, it is necessary to grasp the etching amount of the SiO ₂ layer. The etching amount of the SiO ₂ layer is calculated based on an etching rate of the SiO ₂ layer. However, since this etching rate is calculated by etching another SiO ₂ layer prepared in advance, an error in the etching rate due to the difference in the etching environment as described above occurs. Therefore, it is difficult to calculate an accurate etching rate of the InP substrate that forms the diffraction grating.

Further, in the conventional method for forming a diffraction grating, if the etching of the InP substrate on which the diffraction grating is to be formed is interrupted and then the SiO ₂ layer is removed, then the depth of the recess of the InP substrate is measured. However, since the SiO ₂ layer as a mask has been removed, additional etching was not performed thereafter to form the diffraction grating.

  As described above, the conventional method for forming a diffraction grating has a problem that it is difficult to control the depth of the concave portion of the diffraction grating with high accuracy. Therefore, for example, when a diffraction grating in a distributed feedback semiconductor laser is formed by a conventional diffraction grating formation method, the diffraction efficiency of the diffraction grating varies, and therefore laser characteristics such as light output and threshold current vary. was there.

  The present invention has been made in view of such problems, a method for forming a diffraction grating capable of controlling the depth of a recess with high accuracy, and a distribution capable of suppressing variations in laser characteristics. An object of the present invention is to provide a method for manufacturing a feedback semiconductor laser.

  In order to solve the above-described problem, a distributed feedback semiconductor laser manufacturing method according to the present invention is a distributed feedback semiconductor laser manufacturing method including a diffraction grating having a plurality of recesses, and the diffraction grating is formed on an active layer. A step of forming a semiconductor layer to be formed, a step of forming a first mask layer and a second mask layer on the semiconductor layer in this order, and selectively etching the second mask layer, A second mask layer processing step of forming a plurality of holes corresponding to the plurality of concave portions of the diffraction grating in the second mask layer; and after the second mask layer processing step, the first mask layer and the second mask layer as a mask By etching the semiconductor layer, a plurality of holes formed in the first mask layer so as to correspond to a plurality of recesses of the diffraction grating, a plurality of recesses of the diffraction grating, and a Multiple recesses A semiconductor layer etching step for forming a plurality of recesses formed in the semiconductor layer so as to be shallower, and a second mask layer is selectively removed after the semiconductor layer etching step. Thus, after the second mask layer removing step for exposing the surface of the first mask layer and the second mask layer removing step, the depth of at least one of the plurality of etching recesses is measured, thereby the semiconductor layer etching step. A calculation step of calculating an etching rate of the semiconductor layer in the step, and after the calculation step, the semiconductor layer is additionally etched using the first mask layer as a mask, thereby forming a diffraction grating having a plurality of recesses of a desired depth in the semiconductor layer An additional etching step to be formed, and a step of forming a cladding layer on the semiconductor layer after the additional etching step. The quenching step, based on the etching rate of the semiconductor layer which is calculated in the calculation step, characterized in that additional etching the semiconductor layer.

  According to the method of manufacturing a distributed feedback semiconductor laser of the present invention, the etching rate of the semiconductor layer is calculated using the semiconductor layer itself that forms the diffraction grating in the calculation step. Therefore, the semiconductor prepared separately from the semiconductor layer Compared to the case where the etching rate is calculated using a layer made of the same material as that of the layer, the etching rate calculation error due to the difference in the etching environment can be reduced. Furthermore, in the method for manufacturing a distributed feedback semiconductor laser according to the present invention, after removing the second mask layer, the depth of at least one of the plurality of etching recesses is measured. Therefore, since it is not influenced by the change in the thickness of the second mask layer before and after the semiconductor layer etching step, at least one depth of the plurality of etching recesses can be measured with high accuracy. Since the thickness of the first mask layer is the same as the initial thickness, the depth of the recess in the semiconductor layer, that is, the depth at which the semiconductor layer is etched in the semiconductor layer etching step, based on the depth of the etch recess. Can be calculated with high accuracy. As a result, the etching rate of the semiconductor layer can be calculated with high accuracy in the calculation step, so that in the additional etching step, the semiconductor layer is additionally etched based on the etching rate calculated with high accuracy to form a diffraction grating. be able to. As a result, the depth of the concave portion of the diffraction grating can be controlled with high accuracy, so that variations in laser characteristics can be suppressed.

  In the method for manufacturing a distributed feedback semiconductor laser according to the present invention, the second mask layer is preferably made of an insulator material. This facilitates selective etching of the second mask layer in the second mask layer processing step. Further, it becomes easy to selectively remove the second mask layer in the second mask layer removing step.

  Furthermore, in the distributed feedback semiconductor laser manufacturing method of the present invention, the first mask layer used as a mask in the additional etching step is made of a semiconductor material, and a part of the first mask layer constitutes a part of the cladding layer. It is preferable to do. Thereby, the process of removing the 1st mask layer which remains after an additional etching process becomes unnecessary.

  In order to solve the above-mentioned problem, a method for forming a diffraction grating according to the present invention is a method for forming a diffraction grating having a plurality of recesses, and a step of forming a first layer in which the diffraction grating is to be formed; Forming a first mask layer and a second mask layer in this order on the first layer; and selectively etching the second mask layer to thereby form a plurality of recesses corresponding to the plurality of concave portions of the diffraction grating. The second mask layer processing step for forming the hole in the second mask layer, and the first mask layer and the first layer are etched using the second mask layer as a mask after the second mask layer processing step. A plurality of holes formed in the first mask layer so as to correspond to the plurality of recesses, and a plurality of holes corresponding to the plurality of recesses of the diffraction grating and shallower than the plurality of recesses of the diffraction grating. A plurality of first layer recesses formed in one layer; A first layer etching step for forming a plurality of etching recesses, and a second mask layer exposing the surface of the first mask layer by selectively removing the second mask layer after the first layer etching step. A calculating step for calculating an etching rate of the first layer in the first layer etching step by measuring a depth of at least one of the plurality of etching recesses after the removing step and the second mask layer removing step; And an additional etching step of forming a diffraction grating having a plurality of recesses of a desired depth in the first layer by additionally etching the first layer using the first mask layer as a mask after the step. Then, the first layer is additionally etched based on the etching rate of the first layer calculated in the calculation step.

  According to the method for forming a diffraction grating of the present invention, since the etching rate of the first layer is calculated using the first layer itself forming the diffraction grating in the calculation step, the first prepared separately from the first layer. Compared to the case where the etching rate is calculated using a layer made of the same material as that of one layer, the etching rate calculation error due to the difference in the etching environment can be reduced. Furthermore, in the method for forming a diffraction grating according to the present invention, after removing the second mask layer, the depth of at least one of the plurality of etching recesses is measured. Therefore, since it is not affected by the change in the thickness of the second mask layer before and after the first layer etching step, at least one depth of the plurality of etching recesses can be measured with high accuracy. Since the thickness of the first mask layer is the same as the initial thickness, the depth of the first layer recess, that is, the first layer in the first layer etching step is determined based on the depth of the etching recess. The etched depth can be calculated with high accuracy. As a result, the etching rate of the first layer can be calculated with high accuracy in the calculation step, so that in the additional etching step, the semiconductor layer is additionally etched based on the etching rate calculated with high accuracy to form a diffraction grating. can do. As a result, the depth of the concave portion of the diffraction grating can be controlled with high accuracy.

  According to the present invention, there are provided a method of forming a diffraction grating capable of controlling the depth of the concave portion with high accuracy, and a method of manufacturing a distributed feedback semiconductor laser capable of suppressing variations in laser characteristics. The

It is sectional drawing of the distributed feedback semiconductor laser manufactured by the manufacturing method of the distributed feedback semiconductor laser which concerns on embodiment. It is sectional drawing which shows typically the process of the formation method of a diffraction grating, and the manufacturing method of a DFB laser laser. It is sectional drawing which shows typically the process of the formation method of a diffraction grating, and the manufacturing method of a DFB laser laser. It is sectional drawing which shows typically the process of the formation method of a diffraction grating, and the manufacturing method of a DFB laser laser. It is sectional drawing which shows typically the process of the formation method of a diffraction grating, and the manufacturing method of a DFB laser laser. It is sectional drawing which shows typically the process of the formation method of a diffraction grating, and the manufacturing method of a DFB laser laser.

  Hereinafter, a method for forming a diffraction grating and a method for manufacturing a distributed feedback semiconductor laser according to embodiments will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used for the same elements when possible. In addition, the dimensional ratios in the components in the drawings and between the components are arbitrary for easy viewing of the drawings.

  FIG. 1 shows a distributed feedback semiconductor laser manufactured by the distributed feedback semiconductor laser manufacturing method according to the present embodiment, which has a diffraction grating formed by the diffraction grating forming method according to the present embodiment. It is sectional drawing of a type semiconductor laser.

  As shown in FIG. 1, a distributed feedback semiconductor laser 1 (Distribution Feed Back Laser 1, hereinafter referred to as “DFB laser 1”) includes a semiconductor substrate 3 of a first conductivity type (for example, n-type). On the main surface of the semiconductor substrate 3, a first conductivity type lower clad layer 5, a first conductivity type lower SCH (Separate Confinement Heterostructure) layer 7, an active layer 9, and a first layer (or semiconductor layer) as a first layer (or semiconductor layer). A two-conductivity-type upper SCH layer 11, a second-conductivity-type upper clad layer 23, a contact layer 25, and an upper electrode 27 are provided in this order. A lower electrode 29 is provided on the back surface of the semiconductor substrate 3. Note that FIG. 1 and each drawing referred to below show an orthogonal coordinate system 30, and the Z axis is set parallel to the thickness direction of the semiconductor substrate 3.

  The semiconductor substrate 3 is a III-V group compound semiconductor substrate such as an InP substrate, for example, and is doped with Si, for example. The lower cladding layer 5 is made of a III-V group compound semiconductor such as InP, and is doped with, for example, Si. The lower SCH layer 7 is made of a III-V group compound semiconductor such as GaInAsP, and is doped with Si, for example.

  The active layer 9 has, for example, an MQW (multiple quantum well) structure or an SQW (single quantum well) structure. The active layer 9 is made of a III-V group compound semiconductor such as GaInAsP, for example. The upper SCH layer 11 is made of a III-V group compound semiconductor such as GaInAsP, for example, and doped with Zn, for example. In the upper SCH layer 11, a diffraction grating 11d is formed. The diffraction grating 11 d has a plurality of recesses 11 c, and the plurality of recesses 11 c are periodically arranged along one direction (Y-axis direction) parallel to the active layer 9. The upper cladding layer 23 is made of, for example, a III-V group compound semiconductor such as InP, and is doped with, for example, Zn.

  The semiconductor substrate 3 and / or the lower cladding layer 5 supplies carriers (electrons when the first conductivity type is n-type) to the active layer 9. The upper cladding layer 23 supplies carriers (in the case where the second conductivity type is p-type) to the active layer 9. In the active layer 9, electrons and holes are recombined to generate light. The diffraction grating 11d selectively diffracts and returns the light having the Bragg wavelength corresponding to the period λ11 of the array of the plurality of recesses 11c among the light generated in the active layer 9. As a result, the DFB laser 1 is a laser having high single wavelength characteristics.

  The semiconductor substrate 3, the lower cladding layer 5, and the lower SCH layer 7 are made of a material having a lower refractive index than the material of the active layer 9, and function to confine light generated in the active layer 9 in the vicinity of the active layer 9. do. Similarly, the upper SCH layer 11 and the upper cladding layer 23 are made of a material having a lower refractive index than the material of the active layer 9 and function to confine light generated in the active layer 9 in the vicinity of the active layer 9.

Next, a method for forming a diffraction grating and a method for manufacturing a DFB laser laser according to the present embodiment will be described with reference to FIGS. 2 to 6 are cross-sectional views schematically showing steps of a diffraction grating forming method and a DFB laser laser manufacturing method, respectively.
(Upper SCH layer (first layer, semiconductor layer) formation step)

  First, as shown in FIG. 2A, on a semiconductor substrate 3 such as an InP substrate, a lower cladding layer 5 made of InP or the like, GaInAsP, for example, by metal organic vapor phase epitaxy (hereinafter referred to as “OMVPE method”). The lower SCH layer 7, such as the active layer 9, and the upper SCH layer 11 (first layer, semiconductor layer) such as GaInAsP are grown in this order. In a later step, a diffraction grating is formed in the upper SCH layer 11.

(First mask layer and second mask layer forming step)
Subsequently, as shown in FIG. 2B, a first mask layer 13 and a second mask layer 15 are grown on the upper SCH layer 11, and a resist layer 17 is further formed on the second mask layer 15. Form. In the present embodiment, the first mask layer 13 is made of the same material as a buried layer 19 (see FIG. 5B) described later. Specifically, the first mask layer 13 is made of, for example, a III-V group compound semiconductor such as InP, and is doped with, for example, Zn or the like and has the second conductivity type. The first mask layer 13 can be grown by, for example, the OMVPE method. The thickness H13 of the first mask layer 13 is a thickness that remains after an additional etching step described later, and can be set to, for example, 50 nm to 80 nm.

The second mask layer 15 is made of an insulator material, and in this embodiment, is made of a silicon oxide material such as silicon oxide (SiO ₂ ). The second mask layer 15 can be grown by, for example, a chemical vapor deposition method (hereinafter referred to as “CVD method”). The thickness of the second mask layer 15 can be set to 30 nm to 50 nm, for example.

(Second mask layer processing step)
Subsequently, as shown in FIG. 3A, the resist layer 17 is patterned by an electron beam exposure method or the like. Specifically, a plurality of holes 17 h that extend along the X-axis direction and penetrate the resist layer 17 in the thickness direction are formed in the resist layer 17. At this time, the plurality of hole portions 17h are periodically arranged with a period λ17 along the Y-axis direction. The period λ17 is the same as the period λ11 (see FIG. 5A) of the diffraction grating 11d formed in a later step. For example, when the emission wavelength (or diffraction wavelength) of the DFB laser 1 is 1550 nm, the period λ17 is 240 nm. When the emission wavelength (or diffraction wavelength) of the DFB laser 1 is 1300 nm, it can be set to 200 nm. In addition, the plurality of hole portions 17h correspond to the plurality of concave portions 11c of the diffraction grating 11d formed in a later step (see FIG. 5A). Therefore, in the present embodiment, the width L17h of the hole 17h in the Y-axis direction is the same as the width L11c of the recess 11c in the Y-axis direction (see FIG. 5A), for example, the emission wavelength of the DFB laser 1 (or , The diffraction wavelength) is 120 nm, and when the emission wavelength (or diffraction wavelength) of the DFB laser 1 is 1300 nm, it can be 100 nm.

Next, as shown in FIG. 3B, the second mask layer 15 is selectively etched using the patterned resist layer 17 as a mask, and the resist layer 17 is removed. The selective etching of the second mask layer 15 can be performed, for example, by a reactive ion etching method (hereinafter referred to as “RIE method”) using CF ₄ gas as an etching gas. Thereby, the pattern of the resist layer 17 is transferred to the second mask layer 15. As a result, a plurality of holes 15 h extending along the X-axis direction are formed in the second mask layer 15. The plurality of hole portions 15h penetrate the second mask layer 15 in the thickness direction, and are periodically arranged with a period λ15 along the Y-axis direction. Since the plurality of hole portions 15h correspond to the plurality of concave portions 11c of the diffraction grating 11d formed in a later process, the period λ15 is the same as the period λ11 (see FIG. 5A). In the present embodiment, the width L15h in the Y-axis direction of the plurality of holes 15h is the same as the width L11c in the Y-axis direction of the plurality of recesses 11c (see FIG. 5A).

  In the present embodiment, since the second mask layer 15 is made of an oxide material, it is easy to selectively etch the second mask layer 15 in this step.

(Upper SCH layer (first layer, semiconductor layer) etching step)
Subsequently, as shown in FIG. 4A, by etching the first mask layer 13 and the upper SCH layer 11 using the second mask layer 15 as a mask, a plurality of holes are formed in the first mask layer 13. 13 h is formed, and a plurality of upper SCH layer recesses (semiconductor layer recesses, first layer recesses) 11 c 1 are formed in the upper SCH layer 11. This etching can be performed by, for example, an RIE method using a mixed gas of CH ₄ gas and H ₂ gas as an etching gas. In the present embodiment, the cross section in the YZ plane of the plurality of holes 13h and the plurality of recesses 11c is rectangular. The plurality of holes 13h are periodically arranged at a period λ13 along the Y-axis direction, and the plurality of upper SCH layer recesses 11c1 are respectively periodically arranged at a period λ11p along the Y-axis direction. In addition, since the second mask layer 15 is also etched in this step, the initial thickness of the second mask layer 15 is formed so that the second mask layer 15 remains on the first mask layer 13 after this step. Is determined (see FIG. 2B).

  Each of the plurality of holes 13h penetrates the first mask layer 13 in the thickness direction. The plurality of holes 13h correspond to the plurality of concave portions 11c of the diffraction grating 11d, which is formed in a later step, and the period λ13 is the same as the period λ11 (see FIG. 5A). In the present embodiment, the width L13h in the Y-axis direction of the plurality of holes 13h is the same as the width L11c in the Y-axis direction of the plurality of recesses 11c (see FIG. 5A).

  Each of the plurality of upper SCH layer recesses 11c1 is continuous with each of the plurality of holes 13h. The plurality of upper SCH layer recesses 11c1 respectively correspond to the plurality of recesses 11c of the diffraction grating 11d to be formed in a later step, and the period λ11p of the upper SCH layer recess 11c1 is the same as the period λ11 of the recess 11c. (See FIG. 5A). In this embodiment, the width L11cp in the Y-axis direction of the plurality of upper SCH layer recesses 11c1 is the same as the width L11c in the Y-axis direction of each of the plurality of recesses 11c (see FIG. 5A). Further, the depth H11c1 of the upper SCH layer recess 11c1 is shallower than the depth H11c of the plurality of recesses 11c of the diffraction grating 11d (see FIG. 5A), and the upper SCH layer recess 11c1 is the upper SCH layer. 11 is not penetrated in the thickness direction. The depth H11c1 can be set to, for example, 10 to 30 nm. The plurality of holes 13 and the plurality of upper SCH layer recesses 11c1 form a plurality of etching recesses 14.

(Second mask layer removal step)
Subsequently, as shown in FIG. 4B, the second mask layer 15 is selectively removed by wet etching, for example. Examples of the etchant used in this wet etching include buffered hydrofluoric acid. Thereby, the surface 13s of the first mask layer 13 is exposed. In the present embodiment, since the second mask layer 15 is made of an oxide material, it is easy to selectively remove the second mask layer 15 in this step.

(Calculation process)
Subsequently, as shown in FIG. 4B, at least one depth HS of the plurality of etching recesses 14, that is, at least one of the plurality of upper SCH layer recesses 11 c 1 from the surface 13 s of the first mask layer 13. The depth HS to one bottom 11b is measured. The depth HS can be measured with a scanning probe microscope such as the atomic force microscope 20 as shown in FIG. 4B, for example. The depth HS may be measured by measuring the depth HS of each of the plurality of etching recesses 14 and calculating the average value of the distances.

  Then, the value of the depth H11c1 of the plurality of upper SCH layer recesses 11c1 is calculated by subtracting the thickness of the first mask layer 13 from the value of the depth HS. At this time, since the thickness of the first mask layer 13 is the initial thickness and can be accurately grasped, the value of the depth H11c1 can be accurately calculated. Then, the etching rate of the upper SCH layer 11 in the upper SCH layer etching step is calculated from the value of the depth H11c1 and the etching time for forming the plurality of upper SCH layer recesses 11c1 in the upper SCH layer etching step.

(Additional etching process)
Subsequently, as shown in FIG. 5A, the upper SCH layer 11 is further etched by the depth H11c2 by further etching the upper SCH layer 11 using the first mask layer 13 as a mask. This etching can be performed by the same method as the etching in the upper SCH layer etching process, for example, the RIE method using a mixed gas of CH ₄ gas and H ₂ gas as an etching gas. Thereby, a diffraction grating 11d having a plurality of recesses 11c having a desired depth H11c is formed.

  In this step, when the upper SCH layer 11 is additionally etched by the depth H11c2, the upper SCH layer 11 is additionally etched based on the etching rate of the upper SCH layer 11 calculated in the above calculation step. Specifically, in order to form a diffraction grating 11d having a plurality of recesses 11c having a desired depth H11c in this step, it is necessary to additionally etch the upper SCH layer 11 by a depth H11c2. Therefore, by dividing the value of the depth H11c2 by the value of the etching rate of the upper SCH layer 11 calculated in the calculation step, the time required for additional etching of the upper SCH layer 11 by the depth H11c2 is calculated. Then, the upper SCH layer 11 is etched.

  In this step, the first mask layer 13 is also etched simultaneously with the additional etching of the upper SCH layer 11. Therefore, the initial thickness of the first mask layer 13 (see FIG. 2B) is determined so that the first mask layer 13 remains on the upper SCH layer 11 after this step.

(Upper clad layer forming process)
Subsequently, for example, the buried layer 19 is grown by the OMVPE method so as to fill the plurality of recesses 11 c and to cover the remaining upper SCH layer 11. The buried layer 19 is made of the same material as that of the first mask layer 13 in this embodiment. Specifically, the buried layer 19 is made of a III-V group compound semiconductor such as InP, for example, and is doped with Zn or the like to be of the second conductivity type. Then, the first mask layer 13 and the buried layer 19 form an upper clad layer 23. That is, in the present embodiment, the first mask layer 13 used as a mask in the above-described additional etching step, that is, the first mask layer 13 remaining on the upper SCH layer 11 after the additional etching step is the Part of it.

(Contact layer and electrode formation process)
Next, as shown in FIG. 6, the contact layer 25 and the upper electrode 27 are formed in this order on the upper cladding layer 23. Further, the lower electrode 29 is formed on the back surface of the semiconductor substrate 3. Thereby, the substrate product 1a is completed.

  Thereafter, the substrate product 1a is cleaved on two planes parallel to the XZ plane to form a semiconductor laser bar. Then, a reflectance control film is formed on each of the two cleaved surfaces, one being a light emitting surface and the other being a light reflecting surface. Further, the semiconductor laser bar is cut along two planes parallel to the YZ plane and separated into individual chips, thereby completing the DFB laser 1 as shown in FIG.

  According to the diffraction grating forming method of the present embodiment as described above, as shown in FIG. 4B, in the calculation step, the upper SCH layer 11 is formed using the upper SCH layer 11 itself that forms the diffraction grating 11d. Since the etching rate is calculated, compared to the case where the etching rate is calculated using a layer made of the same material as the upper SCH layer 11 prepared separately from the upper SCH layer 11, etching caused by the difference in etching environment The rate calculation error can be reduced.

  Furthermore, in the method for forming a diffraction grating of this embodiment, as shown in FIG. 4B, the measurement of the depth HS of at least one of the plurality of etching recesses 14 is performed after the second mask layer 15 is removed. ing. Therefore, since it is not affected by the change in the thickness of the second mask layer 15 before and after the upper SCH layer etching step (see FIG. 3B and FIG. 4A), at least of the plurality of etching recesses 14. One depth HS can be measured with high accuracy. Since the thickness of the first mask layer 13 is the same as the initial thickness, the depth H11c1 of the upper SCH layer recess 11c1, that is, the upper SCH layer etching step, based on the depth HS of the etching recess 14 The depth of etching the upper SCH layer 11 can be calculated with high accuracy.

  Thereby, since the etching rate of the upper SCH layer 11 can be calculated with high accuracy in the calculation step, the depth of the upper SCH layer 11 is accurately determined based on the etching rate calculated with high accuracy in the additional etching step. The diffraction grating 11d can be formed by additionally etching only H11c2 (see FIG. 5A). As a result, the depth H11c of the concave portion 11c of the diffraction grating 11d can be controlled with high accuracy. In addition, according to the distributed feedback semiconductor laser manufacturing method of the present embodiment as described above, the depth H11c of the concave portion 11c of the diffraction grating 11d can be controlled with high accuracy. As a result, variations in diffraction efficiency of the diffraction grating 11d are suppressed, and variations in laser characteristics such as light output and threshold current can be suppressed.

  In the method of manufacturing the distributed feedback semiconductor laser according to the present embodiment, the first mask layer 13 remaining on the upper SCH layer 11 after the additional etching step is formed on the upper cladding layer 23 as shown in FIG. Part of it. Thereby, the process of removing the 1st mask layer 13 which remains after an additional etching process is unnecessary.

  The present invention is not limited to the above-described embodiment, and various modifications can be made.

For example, in the above-described embodiment, the second mask layer 15 is made of silicon oxide (SiO ₂ ), but the second mask layer 15 is made of a nitride material such as silicon nitride (SiN) or silicon nitride oxide (SiON), for example. It may consist of.

  In the above-described embodiment, the first mask layer 13 is made of the same material as that of the buried layer 19. However, the first mask layer 13 is made of a material different from that of the buried layer 19 as long as the material can constitute a part of the upper cladding layer 23. It may be.

  Further, the first mask layer 13 remaining on the upper SCH layer 11 may be selectively removed after the additional etching step and before the upper clad layer forming step (FIGS. 5A and 5B). reference). In this case, only the buried layer 19 becomes the upper clad layer 23. In this case, since the first mask layer 13 does not constitute a part of the upper cladding layer 23, the selection range of the material constituting the first mask layer 13 is widened. Further, in this case, it is preferable to measure the depth H11c of the plurality of concave portions 11c of the diffraction grating 11d after selectively removing the first mask layer 13. This measurement can be performed by, for example, a scanning probe microscope such as an atomic force microscope. Thereby, it can be confirmed whether the depth H11c of the some recessed part 11c of the diffraction grating 11d is a value as a design value.

  In the above-described embodiment, the hole 13h after the upper SCH layer etching step, the upper SCH layer recess 11c1 (see FIG. 4A), and the recess 11c after the additional etching step (see FIG. 5A). ) Each has a rectangular cross section, but is not limited to such a shape. For example, it may be a cross section whose width becomes narrower as it approaches the semiconductor substrate 3, such as an inverted trapezoidal shape or an inverted triangular shape. Even in this case, the period λ17, the period λ15, the period λ13, and the period λ11p are the same as the period λ11 (see FIGS. 3 to 5). In this case, when measuring the depth HS in the calculation step, the distance from the surface 13s of the first mask layer 13 to the deepest part of the upper SCH layer recess 11c1 may be measured.

DESCRIPTION OF SYMBOLS 1 ... Distributed feedback type semiconductor laser (DFB laser), 3 ... Semiconductor substrate, 11 ... Upper SCH layer (1st layer, semiconductor layer), 11c ... Recessed part of diffraction grating, 11c1 ... Upper SCH layer recess (semiconductor layer recess, first layer recess), 11d ... diffraction grating, 11b ... bottom of recess in upper SCH layer, 13 ... first mask layer, 13h ... first mask Layer hole, 13 s... Surface of first mask layer, 14. Etching recess, 15... Second mask layer, 15 h.

Claims (4)

A method of manufacturing a distributed feedback semiconductor laser comprising a diffraction grating having a plurality of recesses,
Forming a semiconductor layer on which the diffraction grating is to be formed on the active layer;
Forming a first mask layer and a second mask layer in this order on the semiconductor layer;
A second mask layer processing step of forming, in the second mask layer, a plurality of holes corresponding to the plurality of recesses of the diffraction grating by selectively etching the second mask layer;
After the second mask layer processing step, the first mask layer and the semiconductor layer are etched using the second mask layer as a mask so as to correspond to the plurality of concave portions of the diffraction grating. A plurality of holes formed in the layer, and a plurality of holes formed in the semiconductor layer so as to correspond to the plurality of recesses of the diffraction grating and to be shallower than the plurality of recesses of the diffraction grating. A semiconductor layer etching step for forming a plurality of etching recesses comprising a semiconductor layer recess,
A second mask layer removing step of exposing the surface of the first mask layer by selectively removing the second mask layer after the semiconductor layer etching step;
A calculation step of calculating an etching rate of the semiconductor layer in the semiconductor layer etching step by measuring a depth of at least one of the plurality of etching recesses after the second mask layer removing step;
After the calculating step, an additional etching step of forming the diffraction grating having the plurality of concave portions of a desired depth in the semiconductor layer by additionally etching the semiconductor layer using the first mask layer as a mask;
Forming a cladding layer on the semiconductor layer after the additional etching step;
Including
In the additional etching step, the semiconductor layer is additionally etched based on the etching rate of the semiconductor layer calculated in the calculating step.   2. The distributed feedback semiconductor laser manufacturing method according to claim 1, wherein the second mask layer is made of an insulator material.   The first mask layer used as a mask in the additional etching step is made of a semiconductor material, and a part of the first mask layer constitutes a part of the clad layer. Manufacturing method of distributed feedback semiconductor laser. A method of forming a diffraction grating having a plurality of recesses,
Forming a first layer in which the diffraction grating is to be formed;
Forming a first mask layer and a second mask layer in this order on the first layer;
A second mask layer processing step of forming, in the second mask layer, a plurality of holes corresponding to the plurality of recesses of the diffraction grating by selectively etching the second mask layer;
After the second mask layer processing step, the first mask layer and the first layer are etched using the second mask layer as a mask, so as to correspond to the plurality of concave portions of the diffraction grating. A plurality of holes formed in the mask layer and the first layer so as to correspond to the plurality of concave portions of the diffraction grating and to be shallower than the plurality of concave portions of the diffraction grating. A first layer etching step for forming a plurality of etching recesses comprising a plurality of first layer recesses;
A second mask layer removing step of exposing the surface of the first mask layer by selectively removing the second mask layer after the first layer etching step;
A calculation step of calculating an etching rate of the first layer in the first layer etching step by measuring a depth of at least one of the plurality of etching recesses after the second mask layer removing step;
After the calculating step, an additional etching step of forming the diffraction grating having the plurality of concave portions having a desired depth in the first layer by additionally etching the first layer using the first mask layer as a mask. When,
Including
In the additional etching step, the first layer is additionally etched based on the etching rate of the first layer calculated in the calculating step.

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