Abstract:
A semiconductor device manufacturing system includes: a PL evaluation apparatus that evaluates wavelengths of photoluminescent light produced by individual optical modulators on a single semiconductor wafer; an electron beam drawing apparatus that draws patterns of diffraction gratings of laser sections that adjoin respective optical modulators on the wafer; and a calculation section that receives the wavelengths of the photoluminescent light from the PL evaluation apparatus, calculates densities of respective diffraction gratings so that differences between the wavelengths of the photoluminescent light and oscillating wavelengths of the laser sections become a constant, and sends the densities calculated to the electron beam drawing apparatus for drawing respective diffraction grating patterns on the respective laser sections.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method for manufacturing a semiconductor device used, for example, for a light source of optical fiber communication, the semiconductor device manufactured using the manufacturing method and a system for manufacturing the semiconductor device. 
     Background Art 
     Japanese Patent Laid-Open No. 2001-91913 discloses a semiconductor device in which a laser section including an active layer and an optical modulator including a light absorption layer are monolithically formed. This semiconductor device is manufactured, for example, by forming an optical waveguide of the optical modulator first and then forming an optical waveguide of the laser section using a butt joint method. 
     To obtain a satisfactory current-optical output characteristic and high frequency characteristic, the difference value between a photoluminescence wavelength of the optical modulator and an oscillating wavelength of the laser section preferably has a predetermined value. An ideal difference value is determined in consideration of various characteristics. 
     As disclosed in Japanese Patent Laid-Open No. 2001-91913, by forming the optical modulator before the laser section, the optical modulator can be formed on a flat surface. This has an effect of making uniform the composition of the light absorption layer and stabilizing the photoluminescence wavelength of the optical modulator. However, there may be a case where a tolerable variation from the ideal difference value is only ±2 nm. In this case, the method disclosed in Japanese Patent Laid-Open No. 2001-91913 alone cannot reduce the variation in the difference value sufficiently. 
     SUMMARY OF THE INVENTION 
     The present invention has been implemented to solve the above-described problem and it is an object of the present invention to provide a method for manufacturing a semiconductor device, the semiconductor device and a system for manufacturing the semiconductor device capable of sufficiently reducing a variation in a difference value. 
     The features and advantages of the present invention may be summarized as follows. 
     According to one aspect of the present invention, a method for manufacturing a semiconductor device, includes a first step of forming a lower light confinement layer on a substrate, a light absorption layer on the lower light confinement layer and an upper light confinement layer on the light absorption layer and removing parts of the lower light confinement layer, the light absorption layer and the upper light confinement layer to thereby form an optical modulator, a second step of forming a laser section having a diffraction grating in a portion of the substrate where the optical modulator is not formed, a step of forming a diffusion constraining layer that constrains diffusion of a dopant on the upper light confinement layer, and a third step of forming a contact layer on the laser section and the diffusion constraining layer. The same type of dopant is used for the contact layer and the upper light confinement layer. 
     According to another aspect of the present invention, a method for manufacturing a semiconductor device includes a first step of forming a lower light confinement layer on a wafer, a light absorption layer on the lower light confinement layer and an upper light confinement layer on the light absorption layer and removing parts of the lower light confinement layer, the light absorption layer and the upper light confinement layer to thereby form a plurality of optical modulators, an evaluation step of evaluating photoluminescence wavelengths of individual optical modulators of the plurality of optical modulators, a second step of forming a plurality of laser sections on the wafer so that the laser sections each having a diffraction grating are connected to the plurality of optical modulators respectively, and a third step of forming a contact layer on the plurality of optical modulators and the plurality of laser sections. In the second step, the individual diffraction gratings of the plurality of laser sections are formed so that difference values between the photoluminescence wavelength obtained in the evaluation step and the oscillating wavelengths of the laser sections become predetermined values. 
     According to another aspect of the present invention, a semiconductor device includes a substrate, an optical modulator having a lower light confinement layer formed on the substrate, a light absorption layer formed on the lower light confinement layer and an upper light confinement layer formed on the light absorption layer, a diffusion constraining layer that constrains diffusion of a dopant formed on the upper light confinement layer, a laser section formed on the substrate so as to adjoin the optical modulator, and a contact layer formed on the laser section and the diffusion constraining layer. The light absorption layer has a uniform overall composition, and the same type of dopant is used for the contact layer and the upper light confinement layer. 
     According to another aspect of the present invention, a semiconductor device manufacturing system includes a PL evaluation apparatus that evaluates photoluminescence wavelengths of individual optical modulators of a plurality of optical modulators formed on a wafer, an electron beam drawing apparatus that forms diffraction gratings of laser sections provided so as to adjoin the plurality of optical modulators, and a calculation section that receives information of the photoluminescence wavelengths from the PL evaluation apparatus, calculates densities of the diffraction gratings so that difference values between the photoluminescence wavelengths and oscillating wavelengths of the laser sections become predetermined values and sends the result to the electron beam drawing apparatus. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a semiconductor device according to a first embodiment; 
         FIG. 2  shows a semiconductor device in which an optical modulator is formed; 
         FIG. 3  shows a semiconductor device in which a laser section is formed; and 
         FIG. 4  is a diagram illustrating a semiconductor device manufacturing system according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method for manufacturing a semiconductor device, the semiconductor device and a system for manufacturing the semiconductor device according to embodiments of the present invention will be described with reference to the accompanying drawings. The same or corresponding components will be assigned the same reference numerals and duplicate description may be omitted. 
     First Embodiment 
       FIG. 1  is a cross-sectional view of a semiconductor device  10  according to a first embodiment of the present invention. The semiconductor device  10  is provided with a substrate  12  made of, for example, n-type InP. A laser section  14  is formed on the substrate  12 . The laser section  14  will be described. The laser section  14  is provided with an n-type clad layer  16  formed on the substrate  12 . An active layer  18  which is a MQW (Multi Quantum Well) made, for example, of InGaAsP is formed on the n-type clad layer  16 . A p-type clad layer  20  is formed on the active layer  18 . A diffraction grating  22  is formed on the p-type clad layer  20 . A p-type embedded layer  24  is formed on the p-type clad layer  20 . 
     An optical modulator  30  adjoining to the laser section  14  is formed on the substrate  12 . The optical modulator  30  will be described. The optical modulator  30  is provided with a lower light confinement layer  32  formed on the substrate  12 . The lower light confinement layer  32  is doped, for example, with S. A light absorption layer  34  is formed on the lower light confinement layer  32 . The light absorption layer  34  as a whole has a uniform composition. The light absorption layer  34  is formed, for example, of an MQW made of InGaAsP. The light absorption layer  34  is not doped with any dopant. An upper light confinement layer  36  is formed on the light absorption layer  34 . The upper light confinement layer  36  is doped with Be. 
     A diffusion constraining layer  38  is formed on the upper light confinement layer  36 . The diffusion constraining layer  38  is made, for example, of i-type InP. A contact layer  40  is formed on the laser section  14  and the diffusion constraining layer  38 . The contact layer  40  is doped with Be. Therefore, the type of dopant (Be) of the contact layer  40  is the same as the type of dopant (Be) of the upper light confinement layer  36 . 
     An insulating film  42  is formed on the contact layer  40 . A first p-side electrode  44  used as a p-side electrode of the laser section  14  and a second p-side electrode  46  used as a p-side electrode of the optical modulator  30  are formed on the insulating film  42 . A common n-side electrode  48  is formed on the back of the substrate  12 . 
     The semiconductor device  10  is an apparatus in which the laser section  14  and the optical modulator  30  are monolithically formed. The output light of the CW (continuous wave) driven laser section  14  is absorbed by the light absorption layer  34  of the RF (radio frequency) driven optical modulator  30 , and it is thereby possible to realize fast response, large-volume transmission and long-distance communication. 
     The method of manufacturing the semiconductor device  10  will be described. First, the optical modulator  30  is formed. The step of forming the optical modulator  30  is called a “first step.” In the first step, the lower light confinement layer  32 , the light absorption layer  34  on the lower light confinement layer  32 , and the upper light confinement layer  36  on the light absorption layer  34  are formed on the whole surface of the substrate  12  by means of epitaxial growth. Then, a mask is formed and part of the lower light confinement layer  32 , the light absorption layer  34 , and the upper light confinement layer  36  is removed by means of dry etching or wet etching. In this way, the lower light confinement layer  32 , the light absorption layer  34  and the upper light confinement layer  36  shown in  FIG. 2  are formed. The upper light confinement layer  36  is doped with Be. Then, the diffusion constraining layer  38  is formed on the upper light confinement layer  36  and the structure shown in  FIG. 2  is completed. Note that the lower light confinement layer, the light absorption layer, the upper light confinement layer and the diffusion constraining layer may be formed on the entire surface of the substrate  12  and etched to complete the structure shown in  FIG. 2  or the diffusion constraining layer  38  may be formed after the second step. 
     Next, the laser section  14  is formed. The step of forming the laser section  14  is called a “second step”. In the second step, the laser section  14  including the diffraction grating  22  is formed in a portion of the substrate  12  where the optical modulator  30  is not formed. The diffraction grating  22  is formed by forming a pattern in the p-type clad layer  20  using an electron beam drawing apparatus, forming periodic steps by means of etching and embedding a crystal having a refractive index different from that of the p-type clad layer  20  into the steps.  FIG. 3  shows a cross-sectional view of the semiconductor device after the second step. The optical modulator  30 , the diffusion constraining layer  38  and the laser section  14  are preferably formed using a butt joint method. 
     Next, the contact layer  40  is formed on the laser section  14  and the diffusion constraining layer  38 . The step of forming the contact layer  40  is called a “third step.” The contact layer  40  is doped with Be. 
     In the method for manufacturing a semiconductor device according to the first embodiment of the present invention, the optical modulator  30  is formed before forming the laser section  14 , the same type of dopant (Be) is used for the contact layer  40  and the upper light confinement layer  36  and the diffusion constraining layer  38  is formed. All of these are intended to set the photoluminescence wavelength of the optical modulator  30  to a predetermined value (first predetermined value). 
     If the optical modulator is formed after the formation of the laser section, the composition of the portion adjoining the laser section of the light absorption layer deteriorates. This prevents the photoluminescence wavelength of the optical modulator from being set to the first predetermined value. Thus, the first embodiment of the present invention forms the optical modulator  30  before forming the laser section  14  to allow the optical modulator  30  to be formed on the flat surface of the substrate  12  without any side of the laser section  14 . Compared to the case where the laser section is formed before forming the optical modulator, it is possible to make uniform the composition of the light absorption layer  34 , and thereby set the photoluminescence wavelength of the optical modulator  30  to the first predetermined value. 
     In the first embodiment of the present invention, heat produced when the laser section  14  and the contact layer  40  are formed affects the optical modulator  30 . Thus, the p-type dopant (e.g., Zn, Be) of the contact layer or the like may be diffused into the light absorption layer  34 . When the p-type dopant is diffused from outside the light absorption layer to the light absorption layer, the photoluminescence wavelength of the optical modulator deviates from the first predetermined value. 
     Thus, in the first embodiment of the present invention, the same type of dopant (Be) is used for the contact layer  40  and the upper light confinement layer  36 . This can prevent the dopant of the contact layer  40  from being diffused into the light absorption layer  34 . Furthermore, it is also preferable to use the same type of dopant for the p-type clad layer  20  and the p-type embedded layer  24  as the dopant for the upper light confinement layer  36  so as to prevent the dopant of the p-type clad layer  20  and the p-type embedded layer  24  from being diffused into the light absorption layer  34 . 
     The diffusion constraining layer  38  can suppress diffusion of the dopant from the contact layer  40  to the optical modulator  30 . When the diffusion constraining layer  38  is too thin, diffusion of the p-type dopant cannot be suppressed, whereas when the diffusion constraining layer  38  is too thick, an electric field for driving the optical modulator  30  cannot be applied. Therefore, the diffusion constraining layer  38  preferably has an optimum thickness that suppresses diffusion of the p-type dopant and allows a sufficient electric field to be applied to the optical modulator  30 . For example, the thickness of the diffusion constraining layer  38  is preferably set to within a range of 100 to 400 nm and concentration of the dopant of the diffusion constraining layer is preferably set to ≦1E+16 cm −3 . 
     Using the same type of dopant for the contact layer  40  and the upper light confinement layer  36  and forming the diffusion constraining layer  38  contribute not only to stabilization of the photoluminescence wavelength of the optical modulator  30 , but mainly to stabilization of the amount of light absorption. Therefore, according to the semiconductor device  10 , it is possible to stabilize the amount of light absorption while stabilizing the photoluminescence wavelength of the optical modulator  30  to the first predetermined value. 
     The oscillating wavelength of the laser section  14  is determined by spacing of the diffraction grating  22 . Since the diffraction grating  22  is formed using an electron beam drawing apparatus, it is easy to set the oscillating wavelength of the laser section  14  to a predetermined value (second predetermined value). 
     Thus, according to the method for manufacturing a semiconductor device according to the first embodiment of the present invention, it is possible to set the photoluminescence wavelength of the optical modulator  30  to the first predetermined value and set the oscillating wavelength of the laser section  14  to the second predetermined value. This makes it possible to sufficiently reduce a variation in the difference value between the photoluminescence wavelength and the oscillating wavelength. 
     When an InP-based material such as InGaAsP is used for the light absorption layer  34 , the contact layer  40  and the upper light confinement layer  36  are preferably doped with Be. When an Al-based material such as AlGaInAs is used for the light absorption layer, the contact layer and the upper light confinement layer  36  are preferably doped with Zn. The diffraction grating  22  may be formed in the n-type clad layer  16 . These modifications are applicable to a method for manufacturing a semiconductor device according to the following embodiment. 
     Second Embodiment 
     A second embodiment will be described focusing on differences from the first embodiment. The second embodiment relates to a method for manufacturing a semiconductor device and a semiconductor device manufacturing system whereby a plurality of semiconductor devices are manufactured on one wafer.  FIG. 4  is a diagram illustrating a semiconductor device manufacturing system  100  according to the second embodiment of the present invention. The semiconductor device manufacturing system  100  (hereinafter simply referred to as “system  100 ”) is provided with a PL evaluation apparatus  102 . A calculation section  120  is connected to the PL evaluation apparatus  102 . An electron beam drawing apparatus  130  is connected to the calculation section  120 . The electron beam drawing apparatus  130  is an apparatus that draws a diffraction grating of the laser section, provided so as to adjoin each of a plurality of optical modulators. 
     The method for manufacturing a semiconductor device according to the second embodiment will be described. First, the lower light confinement layer, the light absorption layer, and the upper light confinement layer are formed in a wafer  104 , and parts of these layers are removed to form a plurality of optical modulators. This step is a first step. 
     After the first step, photoluminescence wavelengths of individual optical modulators of the plurality of optical modulators formed on the wafer  104  are evaluated using the PL evaluation apparatus  102 . This step is called an “evaluation step.” In the evaluation step, the wafer  104  is moved to the PL evaluation apparatus  102  and photoluminescence wavelengths of 36 optical modulators formed on the wafer are measured. The 36 optical modulators are formed, one for each region enclosed by a broken line. 
     A distribution of photoluminescence wavelengths within the surface of the wafer is shown on the wafer  104  of the PL evaluation apparatus  102 . The photoluminescence wavelength at a peripheral part  106  has is λ1. The photoluminescence wavelength at an intermediate part  108  inside the peripheral part  106  is λ2 which is greater than λ1. The photoluminescence wavelength at a central part  110  inside the intermediate part  108  is λ3 which is greater than λ2. That is, the photoluminescence wavelength decreases toward the outer circumferential side of the wafer  104 . Data of photoluminescence wavelengths is transmitted to the calculation section  120 . 
     After the evaluation step, the process moves to a second step. In the second step, a plurality of laser sections are formed on the wafer so that laser sections each having a diffraction grating are connected to a plurality of optical modulators. The individual diffraction gratings of the plurality of laser sections are formed so that the difference value between the photoluminescence wavelength obtained in the evaluation step and the oscillating wavelength of the laser section becomes a predetermined value. 
     Specific processing will be described. First, the calculation section  120  receives information of photoluminescence wavelengths from the PL evaluation apparatus  102  and calculates a density of the diffraction grating so that the difference value between the photoluminescence wavelength and the oscillating wavelength of the laser section becomes a predetermined value. That is, an optimum density of the diffraction grating is calculated for the individual optical modulators. Note that the “density of the diffraction grating” represents number of diffraction grating cycles per unit length of a diffraction grating pattern. If the density of the diffraction grating is increased, the oscillating wavelength of the laser element decreases and if the density of the diffraction grating is decreased, the oscillating wavelength of the laser element increases. 
     Since the photoluminescence wavelength of the optical modulator at the wafer central part  110  is large, the density of the diffraction grating is decreased so as to set the difference value to a predetermined value. On the other hand, since the photoluminescence wavelength of the optical modulator at the peripheral part  106  is small, the density of the diffraction grating is increased so as to set the difference value to a predetermined value. The calculated result is then sent to the electron beam drawing apparatus  130 . 
     The electron beam drawing apparatus  130  radiates an electron beam onto the wafer  104  to form the diffraction grating based on calculation results of the calculation section  120 . That is, the density of the diffraction grating is increased at a peripheral part  132  of the wafer, the density of the diffraction grating is decreased at an intermediate part  134  compared to the peripheral part  132  and the density of the diffraction grating is decreased at a central part  136  compared to the intermediate part  134 . 
     Next, the contact layer is formed on the plurality of optical modulators and the plurality of laser sections. This step is a third step. Thus, 36 semiconductor devices are formed which have diffraction gratings optimized from the viewpoint of the difference value. 
     As described above, the photoluminescence wavelength has a certain variation within the surface of the wafer. Thus, when diffraction gratings of all semiconductor devices within the surface of the wafer are formed with uniform spacing, the difference value between the photoluminescence wavelength and the oscillating wavelength of the laser sections varies within the surface of the wafer. As a result, a semiconductor device is formed which has a difference value deviated from a predetermined difference value. 
     However, according to the method for manufacturing a semiconductor device using the system  100  according to the second embodiment of the present invention, an optimum diffraction grating is formed so that the difference value is set to a predetermined value in each semiconductor device based on the photoluminescence wavelength obtained in the evaluation step. Thus, the difference values can be set to predetermined values for all of the 36 semiconductor devices formed on the wafer. 
     The features of the first embodiment and the features of the second embodiment may be combined as appropriate. For example, in the method for manufacturing a semiconductor device according to the second embodiment, if the same type of dopant is used for the contact layer and the upper light confinement layer and a diffusion constraining layer is formed, it is possible to enhance the effect of reducing a variation of the difference value. 
     According to the present invention, it is possible to prevent diffusion of the dopant into the light absorption layer, optimize the density of the diffraction grating and thereby reduce a variation of the difference value. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.