Abstract:
An optical semiconductor device includes: a photo detector section which includes: a first semiconductor layer of a first conductivity type formed on a surface of a semiconductor substrate of the first conductivity type, a second semiconductor layer of a second conductivity type formed on a surface of the first semiconductor layer, and an antireflection film formed on a surface of the second semiconductor layer and preventing reflection of incident light; and a circuit element section which includes: a circuit element formed on the second semiconductor layer on the semiconductor substrate, and a passivation film covering an uppermost electrode layer among electrode layers constituting the circuit element and formed out of a same material as a material of the antireflection film.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-63188, filed on Mar. 8, 2002, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an optical semiconductor device and a method for manufacturing the optical semiconductor device.  
           [0004]    2. Related Background Art  
           [0005]    A photodiode of a PIN structure is conventionally employed as a photo detector which converts an optical signal used in optical communication or a DVD and the like into an electrical signal.  
           [0006]    The PIN-type photodiode has a structure in which a so-called i (intrinsic) layer consisting of a semiconductor having a relatively low impurity concentration is put between p and n semiconductors having relatively high impurity concentrations.  
           [0007]    A bipolar transistor, a capacitor, a resistance, a MOSFET and the like are used as signal-processing circuit elements which process an electrical signal from the photo-detector.  
           [0008]    An optical semiconductor device is conventionally formed by hybridizing a PIN photodiode and signal-processing circuit elements formed on different semiconductor substrates or semiconductor chips, respectively (such optical semiconductor device will be referred to as “hybrid-optical semiconductor device” hereinafter).  
           [0009]    Further, there is known an optical semiconductor device which has a PIN photodiode and a signal-processing circuit formed on the same semiconductor substrate or semiconductor chip (such optical semiconductor device will be referred to as “single-substrate-type optical semiconductor device” hereinafter).  
           [0010]    The single-substrate-type optical semiconductor device has fewer parts than those of the hybrid-optical semiconductor device in an assembly process and requires fewer steps in the assembly process. Therefore, the single-substrate-type optical semiconductor device can reduce manufacturing costs more than the hybrid-optical semiconductor device. Further, the single-substrate-type optical semiconductor device does not require a bonding wire that connects from a semiconductor chip on which a PIN photodiode is formed to a semiconductor chip on which a signal-processing circuit is formed. Therefore, the single-substrate-type optical semiconductor device can resist external electromagnetic noise better than the hybrid-optical semiconductor device. As a consequence, the single-substrate-type optical semiconductor device is more advantageous than the hybrid-optical semiconductor device.  
           [0011]    [0011]FIG. 8 is a schematic enlarged cross-sectional view of a conventional single-substrate-type optical semiconductor device. As shown therein, a p − -type epitaxial layer  12  is formed on a p-type semiconductor substrate  10 . An n-type epitaxial layer  16  is formed on the epitaxial layer  12 . An insulating layer  18 , an insulating layer  20 , an electrode layer  22 , a passivation film  24  and a passivation film  26  are sequentially provided on the epitaxial layer  16  in this order.  
           [0012]    On the epitaxial layers  12  and  16 , various diffused layers  14 ,  40 ,  42  and  44  are provided to form a photodiode section  50  and a signal-processing circuit section  60 . In addition, electrodes  28  and  29  connected to the diffused layers through the insulating layer  18  are formed on the epitaxial layers  16 .  
           [0013]    The electrode layer  22  is a metal layer electrically connected to one of the electrodes formed on the epitaxial layer  16  and also functions as a light-shielding film which shields the signal-processing circuit section from light. Therefore, in the optical semiconductor device  200 , the electrode layer  22  is not formed in the photodiode section  50  and light is allowed to be incident only on this photodiode section  50 .  
           [0014]    However, the insulating layers  18  and  20  and the passivation films  24  and  26  used to manufacture the signal-processing circuit section  60 , the electrode  28  and the like are formed on the surface of the epitaxial layer  16  in the photodiode section  50 . Because of the presence of the insulating layers  18  and  20  and the passivation films  24  and  26 , most of the incident light incident on the photodiode section  50  is reflected. As a result, the quantity of light incident on portions below the surface of epitaxial layer  16  is decreased. Due to this, the photo sensitivity of the optical semiconductor device  200  disadvantageously deteriorates.  
           [0015]    Furthermore, the film formed on the surface of the epitaxial layer  16  in the photodiode section  50  is a multilayer film which consists of the insulating films  18  and  20  and the passivation films  24  and  26  different from one another in property and thickness. Since the respective films of this multilayer film are formed in different manufacturing steps from one another, the material, property and film thickness vary among these films. As a result, the reflectance of the incident light incident on the photodiode section  50  is not kept constant. Due to this, there occurs the problem that the photo sensitivity of the optical semiconductor device  200  has a disadvantageously large variation.  
           [0016]    As stated above, the reflectance for reflecting the incident light incident on the photodiode section  50  is largely influenced by the materials, properties and thicknesses of the films covering the surface of the epitaxial layer  16 . However, it is difficult to form the films having different materials, properties and thicknesses on the epitaxial layer  16  so as to minimize reflectance in view of the refractive index of the epitaxial layer (e.g., the refractive index of silicon≈3.44) and the wavelength of the incident light.  
           [0017]    In addition, Japanese Patent Application Publication No.4-271173 discloses an optical semiconductor device having a dielectric thin film and an antireflection film which have common properties and thickness, and which are manufactured in a common manufacturing step. The dielectric thin film is used between the electrodes of the capacitor of a peripheral circuit element. The antireflection film is used in a photo detector.  
           [0018]    In the optical semiconductor device disclosed in Publication No. HEI4-271173 (1992), however, the thickness of the antireflection film is a factor that determines the capacitance of the capacitor. Therefore, the thickness of the antireflection film is limited by the capacitance of the capacitor. If the thickness of the antireflection film is set at an optimum thickness in accordance with the wavelength of incident light, the areas of the electrodes of the capacitor have to be changed so as to obtain a desired capacitance.  
           [0019]    Furthermore, in the optical semiconductor device disclosed in Publication No. 4-271173, the antireflection film of the photo detector is formed when the dielectric thin film used between the electrodes of the capacitor is formed. Due to this, such films as passivation films are formed on the antireflection film of the photo detector. As a result, there occurs the problem that in order to control the reflectance in the photo detector, it is disadvantageously necessary to control not only the thickness of the antireflection film but also that of the passivation films on the antireflection film.  
           [0020]    Therefore, it is desired to provide an optical semiconductor device which has a relatively high photo sensitivity and which can reduce the variation of photo sensitivity even if a photodetector and a circuit element are formed on the same semiconductor substrate, and to provide a method for manufacturing the optical semiconductor device.  
           [0021]    It is also desired to provide an optical semiconductor device which can control a photo sensitivity relatively easily without influencing a circuit element even if a photo detector and a circuit element are formed on the same semiconductor substrate, and to provide a method for manufacturing the optical semiconductor device.  
           [0022]    It is further desired to provide a method for manufacturing an optical semiconductor device which enables a photo detector and a circuit element having relatively high photo sensitivity and small variation in photo sensitivity to be manufactured on the same semiconductor substrate, and to provide the optical semiconductor device.  
         SUMMARY OF THE INVENTION  
         [0023]    An optical semiconductor device according to an embodiment of the present invention, the optical semiconductor device comprises: a photodetector section including a first semiconductor layer of a first conductivity type formed on a surface of a semiconductor substrate of the first conductivity type, a second semiconductor layer of a second conductivity type formed on a surface of the first semiconductor layer, and an antireflection film formed on a surface of the second semiconductor layer and preventing reflection of incident light; and  
           [0024]    a circuit element section including a circuit element formed on the second semiconductor layer on the semiconductor substrate, and a passivation film covering the circuit element and having a passivation film formed out of a same material as a material of the antireflection film.  
           [0025]    A method for manufacturing the optical semiconductor device according to the embodiment of the present invention, is the method for manufacturing the optical semiconductor device constituted so that a photo detector section which receives light and generates a photocurrent and a circuit element section which processes a signal based on the photocurrent from at least the photo detector section are formed on a same semiconductor substrate, the method comprising: a step of forming a first semiconductor layer of a first conductivity type on a surface of the semiconductor substrate of the first conductivity type; a step of forming a second semiconductor layer of a second conductivity type on a surface of the first semiconductor layer; a diffused layer formation step of selectively forming diffused layers in the second semiconductor layer in the photo detector section and the circuit element section; an insulating film formation step of depositing a first insulating film on the second semiconductor layer; an exposure step of exposing the second semiconductor layer in a light-receiving region which receives the light in the photodetector section; and a passivation film formation step of forming an antireflection film which prevents reflection of incident light on the second semiconductor layer in the light-receiving region, and forming a passivation film which is made of a same material as a material of the antireflection film and covers the circuit element above the first insulating film. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a schematic partially enlarged cross-sectional view of an optical semiconductor device  100  in an embodiment according to the present invention;  
         [0027]    [0027]FIG. 2 is a partially enlarged cross-sectional view showing a manufacturing step of a method for manufacturing the optical semiconductor device in the embodiment according to the present invention;  
         [0028]    [0028]FIG. 3 is a cross-sectional view of the elements in a manufacturing step following the step shown in FIG. 2;  
         [0029]    [0029]FIG. 4 is a cross-sectional view of the elements in a manufacturing step following the step shown in FIG. 3;  
         [0030]    [0030]FIG. 5 is a cross-sectional view of the elements in a manufacturing step following the step shown in FIG. 4;  
         [0031]    [0031]FIG. 6 is a cross-sectional view of the elements in a manufacturing step following the step shown in FIG. 5;  
         [0032]    [0032]FIG. 7 is a cross-sectional view of the elements in a manufacturing step following the step shown in FIG. 6; and  
         [0033]    [0033]FIG. 8 is a schematic enlarged cross-sectional view of a conventional single-substrate-type optical semiconductor device  200 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    An embodiment according to the present invention will be described hereinafter with reference to the drawings. It is noted, however, that the embodiment is not intended to limit the present invention. In addition, in the embodiment described below, even if an n-type semiconductor is employed in place of a p-type semiconductor and a p-type semiconductor is employed in place of an n-type semiconductor, the same advantages as those of the present invention or the embodiment can be obtained.  
         [0035]    [0035]FIG. 1 is a schematic partially enlarged cross-sectional view of an optical semiconductor device  100  in an embodiment according to the present invention. The optical semiconductor device  100  includes a p-type semiconductor substrate  10 , a p − -type epitaxial layer  12  and an n-type epitaxial layer  16 . The p − -type epitaxial layer  12  is higher in specific resistance than the semiconductor substrate  10 . In this embodiment, the epitaxial layer  12  is a semiconductor layer formed by epitaxially growing silicon which contains p-type impurities. The epitaxial layer  16  is formed to provide a pn junction on the surface of the epitaxial layer  12 . In this embodiment, the epitaxial layer  16  is a semiconductor layer formed by epitaxially growing silicon which contains n-type impurities.  
         [0036]    In this embodiment, each of the semiconductor substrate  10 , the epitaxial layer  12  and the epitaxial layer  16  consist of silicon. However, alternatively, these may be semiconductors that contain germanium, carbon or gallium.  
         [0037]    The optical semiconductor device  100  may be divided into two sections, i.e., a photo detector section  52  which receives light and generates a photocurrent, and a signal-processing circuit section  60  which processes a signal based on the photocurrent generated by the photo detector section  52 . In the epitaxial layer  16 , a p + -type isolation  40  is formed between the photo detector section  52  and the signal-processing circuit section  60  so as to isolate the photo detector section  52  from the signal-processing circuit section  60 .  
         [0038]    The photo-detector section  52  includes an n + -type lead layer  42  and a cathode electrode  28  connected to the lead layer  42  in the epitaxial layer  16  in order to lead out the photocurrent generated by the photo-detector section  52 . An antireflection film  32  is formed on the surface of the epitaxial layer  16  in a light-receiving region  52   a  of the photo detector section  52  in order to prevent the reflection of incident light incident on the light-receiving region  52   a . The light-receiving region  52   a  is a region receiving the light in the photodetector section  52 . The antireflection film  32  is directly formed on the surface of the epitaxial layer  16  and no other film is present above the antireflection film  32 . FIG. 1 shows a PIN photodiode as one example of the photo detector section  52 . However, alternatively, a PN photodiode may be used as another example of the photo detector section  52 .  
         [0039]    The antireflection film  32  is formed out of a dielectric material such as a silicon-nitride film or a silicon-oxide film. The silicon-nitride film can resist water content better than the silicon-oxide film and has the effect of being a passivation film. Therefore, the silicon-nitride film is particularly preferable as the antireflection film  32 .  
         [0040]    The signal-processing circuit section  60  includes various semiconductor elements to process the signal from the photo detector. FIG. 1 shows one bipolar transistor as one example of the semiconductor element. The other examples of the semiconductor elements formed in the signal-processing circuit section  60  involve a resistance, a capacitor, a MOSFET and the like.  
         [0041]    Diffused layers necessary to form the signal-processing circuit section  60  are formed in the epitaxial layer  16 . In this embodiment, for example, a base layer  44   b , an emitter layer  44   e  and a collector layer  44   c  of the bipolar transistor are formed.  
         [0042]    On the epitaxial layer  16 , interlayer-insulating films, including insulating films  18  and  20 , are formed so as to form a base electrode  29   b , an emitter electrode  29   e  and a collector electrode  29   c  which have contact with the base layer  44   b , the emitter layer  44   e  and the collector layer  44   c , respectively. The base electrode  29   b , the emitter electrode  29   e  and the collector electrode  29   c  are each formed out of metal such as aluminum or copper. The insulating films  18  and  20  are each formed out of a silicon oxide film. The insulating films  18  and  20  are employed to insulate the electrodes  29   b ,  29   e  and  29   c  from one another and to insulate the electrodes  29   b ,  29   e  and  29   c  from an electrode layer  22  to be described later.  
         [0043]    Further, the electrode layer  22  is formed on the insulating film  20 . The electrode layer  22  is a metal layer electrically connected to one of the electrodes formed on the epitaxial layer  16  and also functions as a light-shielding film which shields the signal-processing circuit from light. It is thereby possible to prevent the semiconductor elements formed in the signal-processing circuit section  60  from malfunctioning. The electrode layer  22  is one electrode layer of a multilayer wiring made of metal. Passivation films  24  and  30  are further formed on the electrode layer  22 .  
         [0044]    On the epitaxial layer  16  in the photo detector section  52  except for the light-receiving region  52   a , the interlayer insulating films including the insulating films  18  and  20  are also formed to form a cathode electrode  28 . The cathode electrode  28  is made of metal such as aluminum or copper or the like. Further, the electrode layer  22  is formed on the insulating film  20 , and the passivation films  24  and  30  are formed on the electrode layer  22  as in the case of the epitaxial layer  16  in the signal-processing circuit section  60 .  
         [0045]    The passivation film  30  and the antireflection film  32  serve as passivation films which cover the outermost layers in the signal-processing circuit section  60  and the photo detector section  52 , respectively. In addition, the passivation film  30  is formed out of the same material as that of the antireflection film  32 . In this embodiment, the passivation film  30  and the antireflection film  32  are each formed out of a silicon nitride film. The passivation film  30  and the antireflection film  32  also cover a sidewall which consists of the insulating films  18  and  20  and the passivation film  24  and which is provided on a boundary between the signal-processing circuit section  60  and the photodetector section  52 . In other words, the passivation film  30  and the antireflection film  32  are continuous to each other and formed out of the same single layer film.  
         [0046]    The operation of the optical semiconductor device  100  in this embodiment as well as the advantages of the operation will now be described.  
         [0047]    Light is incident on the light-receiving region  52   a  of the photodetector section  52 . This incident light reaches a depletion layer formed in a pn junction between the p − -type epitaxial layer  12  and the n-type epitaxial layer  16  and generates a photocurrent. The photocurrent generated in the pn junction section is led out from the cathode electrode  28  through the lead layer  42  or an anode electrode (not shown) electrically connected to the epitaxial layer  12 . Then, the photocurrent is processed by the signal-processing circuit section  60  as an electrical signal.  
         [0048]    The response rate of the photodetector is restricted by a CR time constant which is the product of the capacitance (C) and the resistance component (R) of the photodetector, and is restricted by the running time of optically-excited carriers. In this embodiment, the photodetector section  52  is a PIN photodiode which has a low impurity concentration in the epitaxial layer  12 . The depletion layer, therefore, easily spreads in the epitaxial layer  12  at a low bias voltage. If the thickness of the epitaxial layer  12  is appropriately set, it is possible to suppress the capacitance and resistance element of the photo detector and to lower the CR time constant. Further, since the depletion layer spreads at a low bias voltage, it is possible to easily increase a field intensity in the depletion layer and to accelerate the running rate of the optically-excited carriers. As a result, the optical semiconductor device  100  can deal with a high frequency signal.  
         [0049]    In the conventional optical semiconductor device  200  shown in FIG. 8, the insulating films  18  and  20  and the passivation films  24  and  26  formed in the photodiode  50  reflect most of the incident light. That is, the reflectance of the multilayer which consists of the insulating films  18  and  20  and the passivation films  24  and  26  is high. In addition, because of the variation of the respective constituent films of the multilayer film, reflectance has great variation.  
         [0050]    Generally, if a dielectric film which has a thickness d satisfying an expression 1 is formed on a semiconductor material, the reflectance R of the surface of the dielectric film may possibly be a minimum reflectance. The minimum reflectance R is given by an expression 2.  
           d= (λ/4 n   1 )*(2 m +1)   (Expression 1)  
           R= ( n   0   n   2   −n   1   2 ) 2 /( n   0   n   2   +n   1   2 ) 2    (Expression 2)  
         [0051]    In the expressions 1 and 2, symbol λ denotes the wavelength of incident light in vacuum. Symbol m denotes an integer not smaller than 0. Symbol n 0  denotes the refractive index of a medium propagated by the light before the light is incident on the dielectric film. This medium is often nitrogen (N 2 ) and the refractive index n 0  is about 1. In this embodiment, it is assumed that n 0  is 1. Symbol n 1  denotes the refractive index of the dielectric film. Symbol n 2  denotes the refractive index of the semiconductor material.  
         [0052]    Conventionally, each of the insulating films  18  and  20  and the passivation films  24  and  26  consists of the combination of a silicon oxide film and a silicon nitride film. The reflectance of the multilayer film which consists of the films  18 ,  20 ,  24  and  26  experimentally varies in a range of 10% to 40%. That is, the reflectance of the conventional photodiode  50  is high and the variation of the reflectance thereof is large. In this case, the photo sensitivity of the conventional optical semiconductor device  200  varies from about 0.3 A/W (ampere/watt) to about 0.43 A/W for light having a wavelength of 650 nm. Here, the photo sensitivity is defined as a ratio of a photocurrent (A) to an incident light power (W).  
         [0053]    On the other hand, in the optical semiconductor device  100  in this embodiment according to the present invention, only the antireflection film  32  is formed on the epitaxial layer  16  in the photodetector section  52 . In this embodiment, the epitaxial layer  16  is a silicon layer and has a refractive index (n 2 ) of about 3.44. The antireflection film  32  is a silicon nitride film and has a refractive index (n 1 ) of about 2.05. The incident light has a wavelength k of 650 nm. The medium propagated by the light before the light is incident on the antireflection film  32  is nitrogen (N 2 ). Therefore, n 0  is about 1. In this case, the thickness of the antireflection film  32  which satisfies the expression 1 is about 79.3 nm (m=0).  
         [0054]    Using the expression 2, the reflectance R of the antireflection film  32  at this moment is calculated as about 0.99%. As can be seen, the reflectance of the antireflection film  32  of the photo detector in this embodiment is far lower than that of the conventional multilayer film. In addition, even if the thickness of the antireflection film  32  varies in a range of about 79.3 nm±10%, the reflectance R is always about 4% or less. Obviously, the variation of the reflectance is smaller than that in the conventional art.  
         [0055]    The antireflection film  32  which is formed out of a silicon nitride film of 70 nm to 90 nm is experimentally formed on the epitaxial layer  16  made of silicon. As a result, the photo sensitivity is in a range from about 0.49 A/W to about 0.50 A/W. Therefore, the photo sensitivity of the optical semiconductor device  100  in this embodiment improves beyond that of the conventional optical semiconductor device. This is because the reflectance is lower than that of the conventional reflectance and the quantity of light incident on the photodetector increases.  
         [0056]    Under the same conditions as those in this embodiment, when quantum efficiency is 100% in theory, the photo sensitivity is 0.524 A/W. That is, the optical semiconductor device  100  in this embodiment can obtain the photo sensitivity at quantum efficiency of about 95%. The quantum efficiency is the ratio of the number of charges which generate a photocurrent to the light quantum of light incident on a photo detector.  
         [0057]    Since no other film is present above the antireflection film  32  and since the antireflection film  32  is a single layer film, it is possible to control reflectance and photo sensitivity by controlling the thickness of the antireflection film  32 . In this embodiment, therefore, it is possible to easily control the reflectance and the photo sensitivity of the optical semiconductor device.  
         [0058]    Since the antireflection film  32  is a single layer film, it is possible to easily make the thickness of the antireflection film  32  thin. Normally, it is considered that the variation range of a film thickness is about 10% of a desired film thickness. The variation of the film thickness is made small because of the thin antireflection film  32 . As a result, the reflectance of the antireflection film  32  is stabilized without causing variation thereof. In other words, the photo sensitivity of the optical semiconductor device  100  is stabilized.  
         [0059]    The passivation film  30  and the antireflection film  32  are passivation films which cover the outermost layers in the signal-processing circuit section  60  and the photo detector section  52 , respectively. In addition, the passivation film  30  and the antireflection film  32  are formed out of the same single layer film (silicon nitride film in this embodiment) and continuous to each other. It is, therefore, possible to form the passivation film  30  and the antireflection film  32  in the same manufacturing step in the manufacturing process of the optical semiconductor device  100 .  
         [0060]    Furthermore, since the passivation film  30  and the antireflection film  32  are continuous to each other, the passivation film  30  and the antireflection film  32  serve as effective passivation films for semiconductor elements which are present below the passivation film  30  and the antireflection film  32 . That is, the antireflection film  32  has not only the antireflection function for preventing the reflection of the incident light, but also the function of a passivation film for the semiconductor elements.  
         [0061]    Next, a method for manufacturing the optical semiconductor device  100  in this embodiment according to the present invention will be described. FIGS.  2  to  6  are partially enlarged cross-sectional views showing the method for manufacturing the optical semiconductor device in this embodiment in the order of manufacturing steps.  
         [0062]    As shown in FIG. 2, first, the p − -type epitaxial layer  12  is formed on the surface of the p-type semiconductor substrate  10 . Next, the n-type epitaxial layer  16  is formed on the epitaxial layer  12 . The epitaxial layers  12  and  16  can be formed by a vapor phase epitaxial growth method, a solid phase epitaxial growth method or the like. Prior to the formation of the epitaxial layer  16 , the diffused layer  14  is formed in the epitaxial layer  12 . In this embodiment, the semiconductor substrate  10  is a silicon substrate and the epitaxial layers  12  and  16  are both formed out of silicon single crystal.  
         [0063]    As shown in FIG. 3, such diffused layers as the element isolation layer  40  for isolating the photodetector section  52  from the signal-processing circuit section  60 , the base layer  44   b , emitter layer  44   e  and collector layer  44   c  for forming a bipolar transistor, and the lead layer  42  are next formed in the epitaxial layers  12  and  16 . To form these diffused layers, impurities are selectively injected into the epitaxial layers  12  and  16  and a heat treatment is then conducted. The impurities are arsenic (As) or phosphorus (P) as n-type impurities, boron (B) as p-type impurities and the like.  
         [0064]    As shown in FIG. 4, the insulating film  18  is deposited by a CVD (Chemical Vapor Deposition) method or the like. The insulating film  18  is patterned by photolithography and etching. Further, a metal layer is formed by a sputtering method and then patterned. As a result, the electrodes  28 ,  29   b ,  29   e  and  29   c  are connected to the diffused layers  42 ,  44   b ,  44   e  and  44   c , respectively. In this embodiment, the insulating film  18  is a silicon oxide film and the electrode  28  is made of copper, aluminum or the like.  
         [0065]    As shown in FIG. 5, the insulating film  20  is deposited to cover these electrodes  28 ,  29   b ,  29   e  and  29   c  by the CVD method or the like. In addition, the electrode layer  22  is formed on the insulating film  20 . The electrode layer  22  shields the signal-processing circuit section  60  and the photo detector section  52  except for the light-receiving region  52   a  from incident light. The electrode layer  22  is often formed as one metal wiring layer of a multilayer wiring. The electrode layer  22  is formed by sputtering the metal film and removing the metal film from the light-receiving region  52   a  by the photolithography and etching. Further, the passivation film  24  is deposited on the electrode layer  22  and the insulating layer  20  by the CVD method or the like. In this embodiment, the insulating film  20  and the passivation film  24  are silicon oxide films and the electrode layer  22  is made of copper, aluminum or the like.  
         [0066]    As shown in FIG. 6, the passivation film  24  and the insulating films  20  and  18  are removed from the light-receiving region  52   a  by the photolithography and etching. As a result, the surface of the epitaxial layer  16  in the light-receiving region  52   a  is exposed.  
         [0067]    Alternatively, the epitaxial layer  16  in the light-receiving region  52   a  may be exposed by forming a plug in the light-receiving region  52   a  in advance, selectively depositing the insulating films  18  and  20 , the electrode layer  22  and the passivation film  24  on the epitaxial layer  16  and then removing the plug.  
         [0068]    As shown in FIG. 7, the passivation film  30  and the antireflection film  32  are deposited so as to cover the signal-processing circuit section  60  and the photodetector section  52 . The passivation film  30  covers the passivation film  24  in the signal-processing circuit section  60  and in the photo detector section  52  except for the light-receiving region  52   a . The antireflection film  32  covers the surface of the exposed epitaxial layer  16  in the light-receiving region  52   a . In this embodiment, the passivation film  30  and the antireflection film  32  are each formed out of a silicon nitride film. The passivation film  30  and the antireflection film  32  may be formed simultaneously in the same manufacturing step. The passivation film  30  and the antireflection film  32  may be deposited by, for example, an LP-CVD (Low-Pressure Chemical Vapor Deposition) method.  
         [0069]    According to the method for manufacturing the optical semiconductor device in this embodiment, the passivation film  30  and the antireflection film  32  are formed simultaneously in the same manufacturing step. This makes it possible to easily manufacture the optical semiconductor device according to the present invention. More specifically, only a photolithographic step and an etching step are added so as to remove the films  24 ,  20  and  18  from the light-receiving region  52   a . Therefore, according to the method for manufacturing the optical semiconductor device in this embodiment, it is possible to easily manufacture the optical semiconductor device according to the present invention and to hold down the manufacturing cost.  
         [0070]    Further, according to the method for manufacturing the optical semiconductor device in this embodiment, the passivation film  30  and the antireflection film  32  are formed in the final step of the optical semiconductor device manufacturing steps. Due to this, no other film but the antireflection film  32  is formed on the antireflection film  32 . In other words, the film formed on the epitaxial layer  16  is a single layer film consisting only of the antireflection film  32 . It is thereby possible to determine the reflectance of the optical semiconductor device by the thickness of the antireflection film  32 . In addition, by making only the antireflection film  32  thin, it is possible to decrease the variation of the reflectance.  
         [0071]    Furthermore, since the passivation film  30  and the antireflection film  32  are formed simultaneously in the same manufacturing step, the films  30  and  32  also cover a sidewall which consists of the films  18 ,  20  and  24  provided on a boundary between the signal-processing circuit section  60  and the photo detector section  52 . Therefore, the films  30  and  32  are formed to be continuous to each other. It is thereby possible to enable the passivation film  30  and the antireflection film  32  to function more effectively as passivation films.  
         [0072]    Meanwhile, Japanese Patent Application Publication No. 4-271173 discloses an optical semiconductor device having a dielectric thin film and an antireflection film which have common properties and thickness and which are manufactured in a common manufacturing step. The dielectric thin film is used between the electrodes of the capacitor of a peripheral circuit element. The antireflection film is used in a photo detector.  
         [0073]    In the optical semiconductor device disclosed in Publication No.4-271173, the thickness of the antireflection film is a factor which determines the capacitance of the capacitor. Therefore, the thickness of the antireflection film is limited by the capacitance of the capacitor. If the thickness of the antireflection film is set at an optimum thickness in accordance with the wavelength of incident light, the areas of the electrodes of the capacitor have to be changed so as to obtain a desired capacitance. For example, it is sometimes necessary to set the electrode areas of the capacitor to be larger than those in an ordinary case in order to obtain the desired capacitance while maintaining the thickness d of the antireflection film which satisfies the expression 1.  
         [0074]    In the optical semiconductor device disclosed in Publication No.4-271173, the antireflection film of the photo detector is formed when the dielectric thin film used between the electrodes of the capacitor is formed. Due to this, such films as passivation films are formed on the antireflection film of the photo detector. As a result, in order to control the reflectance in the photo detector, it is disadvantageously necessary to control not only the thickness of the antireflection film but also those of the passivation films on the antireflection film.  
         [0075]    In the optical semiconductor device  100  in the embodiment according to the present invention, by contrast, only the single layer film consisting of the antireflection film  32  exists on the epitaxial layer  16  in the photo detector section. Therefore, it suffices to control only the thickness of the antireflection film so as to control the reflectance of the photo detector. In addition, in the optical semiconductor device in the embodiment according to the present invention, the antireflection film  32  also functions as a passivation film, irrespectively of the capacitance of the capacitor. As a result, even if the antireflection film  32  maintains the thickness d which satisfies the expression 1, no problem occurs to the signal-processing circuit section  60  as the capacitor and the like.  
         [0076]    As stated so far, according to the optical semiconductor device and the method for manufacturing the optical semiconductor device in this embodiment, even if the photo detector and the signal-processing circuit elements are formed on the same semiconductor substrate, it is possible to set the photo sensitivity of the optical semiconductor device higher than that of the conventional optical semiconductor device and to make the variation of the photo sensitivity smaller than that of the conventional optical semiconductor device.  
         [0077]    According to the optical semiconductor device and the method for manufacturing the optical semiconductor device in this embodiment, even if the photo detector and the circuit elements are formed on the same semiconductor device, it is possible to control the photo sensitivity relatively easily without influencing the circuit elements.  
         [0078]    According to the optical semiconductor device and the method for manufacturing the optical semiconductor device in this embodiment, it is possible to form the photo detector and the signal-processing circuit elements which have relatively high photo sensitivity and smaller range of the variation of the photo sensitivity, on the same semiconductor substrate.