Abstract:
An optical semiconductor apparatus composed of a cap and a base, includes: a metal package including a plurality of openings penetrating through the base from outside to inside, a lead with its end portion protruding to the inside of the base and an insulator covering a side surface of the lead being inserted into each of the openings, and the lead being insulated from the base; an insulating film with its backside bonded to the inside of the base; and a semiconductor component placed on the base or on the insulating film. The insulating film covers the opening up to the vicinity of the side surface of the lead.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-196184, filed on Jul. 30, 2008; the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to an optical semiconductor apparatus with an optical semiconductor chip built in a metal package. 
         [0004]    2. Background Art 
         [0005]    In a typical configuration of an optical semiconductor apparatus used for optical transmission/reception, an optical semiconductor chip, such as a semiconductor laser, light emitting diode, and photodiode, is placed in a metal package, and a lead for externally extracting an electrical signal is connected to the optical semiconductor chip by a metal wire in the metal package. As a metal package for an optical semiconductor apparatus, a TO (transistor outline) package is widely used, which is easily connected to an optical fiber and highly versatile. 
         [0006]    On the other hand, with the recent increase in demand for Internet communication, optical fiber communication apparatuses have become faster. Optical semiconductor apparatuses also require a technique for housing an optical semiconductor chip and a multifunctional semiconductor component in a single package in order to prevent degradation in S/N ratio due to increased speed and provide a high-performance optical semiconductor device. 
         [0007]    For instance, Patent Document 1 discloses an optical receiver with a photodiode and a preamplifier housed in a TO package. Patent Documents 2 and 3 disclose a technique for housing a plurality of semiconductor components in a single package using an insulative flexible substrate. 
         [0008]    However, the conventional technique disclosed in the above Patent Documents 2 and 3 is a technique applied to a large package, which is distinct from the package used for optical semiconductor apparatuses, and the technique is different from this invention in objects and problems. In the following, the problems to be solved by the invention are described with reference to the drawings. 
         [0009]      FIG. 1  is a schematic view illustrating the connection of an optical semiconductor apparatus  1  based on a TO package to an optical fiber  40 . The TO package is composed of a base  3  and a cap  2 . The cap  2  is provided with a lens  5  for optical transmission/reception to/from the optical fiber  40 . An optical semiconductor chip is placed inside the TO package and electrically connected to a lead  4  inserted through the base  3  from outside to inside. 
         [0010]      FIG. 2  is a schematic view showing the A-A cross section (see  FIG. 1 ) of an optical semiconductor apparatus for receiving an optical signal and converting it into an electrical signal. A photodiode  6  bonded onto a submount  7  is placed on a component placement surface  3   b  of the inside of the base  3 . A metal interconnect is provided on the surface of the submount  7  and electrically connected to the photodiode  6 . Furthermore, a semiconductor component  8 , such as a preamplifier and TIA (transimpedance amplifier), and a passive component  9 , such as a capacitor and resistor, are placed on the component placement surface  3   b.    
         [0011]    The photodiode  6  bonded onto the submount  7  is connected to the lead  4  by a metal wire  10  through the semiconductor component  8  or the passive component  9 . Light injected from the optical fiber  40  into the photodiode  6  is converted into an electrical signal by the photodiode  6 , amplified by the semiconductor component  8 , and externally extracted by the lead  4 . The lead  4  is inserted into an opening  4   b  provided through the base  3  from outside to the component placement surface  3   b . The lead  4  is insulated from the base  3  by an insulative glass sintered body  12 . 
         [0012]      FIG. 3  is a schematic view showing the component layout of the TO package shown in  FIG. 1 . Five leads  4 , including the leads not shown in  FIGS. 1 and 2 , are placed in the component placement surface  3   b  of the base  3 . Five openings  4   b  provided in the base  3  are each sealed with the lead  4  and the glass sintered body  12  surrounding the lead  4 . Furthermore, the submount  7  with the photodiode  6  bonded thereto, the semiconductor component  8 , and the passive component  9  are placed and connected by metal wires  10 . 
         [0013]    In order to allow the above optical semiconductor apparatus to receive a fast optical signal, it is useful to replace the semiconductor component  8  by a multifunctional semiconductor component to eliminate loss due to transmission of electrical signals between a plurality of functional devices. When a multifunctional semiconductor component is housed in a TO package, it is necessary to place a semiconductor component having a large chip size due to its multifunctionality, and a newly required passive component. Thus, one problem is to obtain space for placing these components in the limited space of the component placement surface  3   b  of the base  3 . 
         [0014]    In a small TO package, the area of the openings  4   b  sealed with the glass sintered body  12  accounts for a large proportion of the component placement surface  3   b . On the other hand, no component can be placed on the glass sintered body  12 . Hence, the effective space for placing components is limited. Furthermore, if the number of leads  4  is increased to provide new input terminals in order to house a multifunctional semiconductor component, the space is further decreased. 
         [0015]    Furthermore, increase in the number of housed components complicates interconnection by metal wires. Thus, component layout without intersection between metal wires is difficult, which is another problem. 
       Patent Document 1: US 2004/0264980 A1 
     Patent Document 2: JP-A-9-232500 
     Patent Document 3: JP-A-10-4510 
     SUMMARY OF THE INVENTION 
       [0016]    According to an aspect of the present invention, an optical semiconductor apparatus composed of a cap and a base including; a metal package including a plurality of openings penetrating through the base from outside to inside, a lead with its end portion protruding to the inside of the base and an insulator covering a side surface of the lead being inserted into each of the openings, and the lead being insulated from the base; an insulating film with its backside bonded to the inside of the base, the insulating film having a metal pattern formed on its frontside; and a semiconductor component placed on the base or on the insulating film, the insulating film covering the opening up to vicinity of the side surface of the lead. 
         [0017]    According to other aspect of the present invention, the optical semiconductor apparatus includes the metal pattern formed on the insulating film interconnecting between the semiconductor component and the lead, where the semiconductor component or the lead is connected to the metal pattern by a metal wire and impedance matching is achieved at the junction between the metal pattern and the metal wire. 
         [0018]    According to other aspect of the present invention, the optical semiconductor apparatus includes the insulating film including a backside metal pattern formed entirely or partly on the backside bonded to the inside of the base and at least one or more through holes electrically connecting between the backside metal pattern and the metal pattern formed on the frontside. 
         [0019]    According to other aspect of the present invention, the optical semiconductor apparatus includes the insulating film being a multilayer insulating film with a plurality of insulating films laminated therein, and containing an interlayer interconnect inserted between the insulating films and a through hole electrically connecting between the metal pattern provided on the frontside or backside of the multilayer insulating film and the interlayer interconnect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic view illustrating the connection of an optical semiconductor apparatus to an optical fiber; 
           [0021]      FIG. 2  is a schematic view showing the A-A cross section of an optical semiconductor apparatus according to a conventional technique; 
           [0022]      FIG. 3  is a schematic view showing the component layout of the optical semiconductor apparatus according to the conventional technique; 
           [0023]      FIG. 4  is a schematic view showing the component layout of an optical semiconductor apparatus according to the invention; 
           [0024]      FIG. 5  is a schematic view showing the B-B cross section of the optical semiconductor apparatus according to the invention; 
           [0025]      FIG. 6  is a schematic view showing the state in which an insulating film is bonded to a base of the optical semiconductor apparatus according to the invention; 
           [0026]      FIG. 7  is a schematic view showing the C-C cross section of the optical semiconductor apparatus according to the invention; 
           [0027]      FIG. 8  is a schematic view showing the component layout of the optical semiconductor apparatus according to the invention; 
           [0028]      FIG. 9  is a schematic view showing a calculation model for impedance matching according to the invention; 
           [0029]      FIG. 10  is a schematic view showing an alternative embodiment according to the invention; 
           [0030]      FIG. 11  is a schematic view showing the D-D cross section of the optical semiconductor apparatus according to the invention; and 
           [0031]      FIGS. 12 to 15  are schematic views showing insulating films used in alternative embodiments according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    Embodiments according to the invention will now be described in detail with reference to the accompanying drawings. 
         [0033]      FIG. 4  is a schematic view showing the component layout on the inside of the base  3  in an optical semiconductor apparatus of the invention. In this invention, the component placement surface  3   b  of the base  3  is entirely covered with an insulating film  14 . The insulating film  14  covers up to the vicinity of the leads  4  and overlies most of the glass sintered body  12 . The insulating film  14  can be bonded to the base  3  by using a resin-based adhesive, or can be bonded with conductive paste. Furthermore, as described later, a metal pattern can be formed on the backside, or the bonding surface, of the insulating film  14 , and bonded with solder. 
         [0034]    As shown, a photodiode  6  bonded onto a submount  7 , a semiconductor component  8   b  having TIA and LIA (limiting amplifier) functionality, and passive components  9  are placed on the insulating film  14 . Although the submount  7  with the photodiode  6  bonded thereto is also a semiconductor component, it is separately described in the following. 
         [0035]    In this invention, the insulating film  14  covers most of the area occupied by the openings  4   b  sealed with the glass sintered body  12 . This expands the region capable of placing components and allows placement of the semiconductor component  8   b  having a large chip size due to its multifunctionality and a plurality of passive components  9 . 
         [0036]      FIG. 5  schematically shows the B-B cross section of the base  3  shown in  FIG. 4 . The insulating film  14  is placed on the component placement surface  3   b  of the base  3 . The lead  4  penetrates through an opening  15  provided in the insulating film  14  and projects its end portion to the frontside of the insulating film  14 . 
         [0037]    In this embodiment, the opening  15  has a diameter of approximately 0.4 m, and the lead  4  has a diameter of approximately 0.2 mm. Hence, the gap formed between the lead  4  and the opening  15  is approximately 0.1 mm. On the other hand, the opening  4   b  provided in the base  3  has a diameter of approximately 1.0 mm. That is, the annular portion of the opening  4   b  having a width of 0.3 mm except the lead  4  having a diameter of 0.2 mm and a gap of 0.1 mm is covered with the insulating film  14 . 
         [0038]    Furthermore, as shown in  FIG. 5 , the semiconductor component  8   b , the photodiode  6  bonded onto the submount  7 , and the passive components  9  are placed on the insulating film  14 . 
         [0039]      FIG. 6  is a schematic view showing the state in which the insulating film  14  with metal patterns  17 ,  18  formed thereon is bonded to the component placement surface  3   b  of the base  3 . The metal pattern  17  in the figure is a GND pattern and electrically connected to a backside metal pattern  22  and the base  3  through a through hole  21  described later. On the other hand, the metal pattern  18  is an interconnect pattern provided to interconnect between the lead  4  and the output terminal of the semiconductor component  8   b.    
         [0040]      FIG. 7  is a schematic view showing the C-C cross section of the base  3  shown in  FIG. 6 . The insulating film  14  is bonded onto the component placement surface  3   b  of the base  3 . A backside metal pattern  22  is formed entirely on the backside of the insulating film  14 . The component placement surface  3   b  of the base  3  is bonded and electrically connected to the backside of the insulating film  14  with solder or conductive paste. A ground terminal  23  is attached to the outside of the base  3 . When the optical semiconductor apparatus is in use, the base  3  and the backside metal pattern  22  are held at a ground potential. 
         [0041]    The GND pattern  17  formed on the frontside of the insulating film  14  is electrically connected to the backside metal pattern  22  through a through hole  21 . Hence, when the optical semiconductor apparatus is in use, the GND pattern  17  is placed at the ground potential. On the other hand, no through hole  21  is provided between the interconnect pattern  18  formed on the frontside of the insulating film  14  and the backside metal pattern  22 . The interconnect pattern  18  is a microstrip line opposed to the backside metal pattern  22  across the insulating film  14 . 
         [0042]    The backside metal pattern  22  is formed throughout the backside of the insulating film  14 , and thereby the backside is entirely held at the ground potential. Consequently, the floating capacitance of the microstrip line formed at the frontside of the insulating film  14  has a definite value determined by the thickness and dielectric constant of the insulating film  14 . Thus, the electrical circuit formed on the frontside of the insulating film  14  has stable frequency characteristics, improving the performance of the optical semiconductor apparatus. 
         [0043]      FIG. 8  is a schematic view showing the state in which components are placed on the insulating film  14  shown in  FIG. 6  and interconnected by metal wires  10 . The photodiode  6  bonded onto the submount  7  and the semiconductor component  8   b  are placed. Furthermore, the passive components  9  are placed on the GND pattern  17  and connected to the lead  4 , the submount  7 , and the semiconductor component  8   b  by metal wires. These connections are interconnects for supplying a driving voltage from the lead  4  to the photodiode  6  and the semiconductor component  8   b.    
         [0044]    An optical signal injected from an optical fiber into the optical semiconductor apparatus is converted into an electrical signal in the photodiode  6 . The submount  7  and the semiconductor component  8   b  are interconnected by a metal wire  10  for transferring the electrical signal converted in the photodiode  6  to the semiconductor component  8   b . The signal output terminal  8   c  of the semiconductor component  8   b  and an end portion  18   a  of the interconnect pattern are also interconnected by a metal wire  10 . The other end portion  18   b  of the interconnect pattern  18  is connected to the lead  4 . 
         [0045]    The electrical signal outputted from the output terminal  8   c  of the semiconductor component  8   b  is transmitted through the interconnect pattern  18  and externally outputted from the lead  4 . Here, the interconnect pattern  18  can be impedance matched with the semiconductor component  8   b  and the lead  4  to reduce reflection of electrical signals. This serves to reduce the transfer loss of electrical signals and improve the performance of the optical semiconductor apparatus. 
         [0046]    For instance, if the semiconductor component  8   b  is an amplifier having TIA and LIA functionality, the characteristic impedance at its output terminal  8   c  is designed to be 50Ω. Hence, impedance matching can be achieved by designing the shape of the interconnect pattern  18  so that its characteristic impedance as a strip line is 50Ω at the end portion  18   a  of the interconnect pattern. On the other hand, the lead  4  has an impedance of approximately 35Ω. Hence, the shape of the interconnect pattern  18  can be designed so that its characteristic impedance at the other endpoint  18   b  is 35Ω. 
         [0047]      FIG. 9  is a schematic view showing a model used in impedance calculation. An interconnect pattern (microstrip line)  18  with line width W is placed on the frontside of the insulating film  14 , and a backside metal pattern  22  is provided entirely on the backside. The length of the microstrip line  18  is assumed infinite in the calculation. 
         [0048]    TABLE 1 shows the result of simulation based on the model shown in  FIG. 9 . The thickness of the insulating film  14  is 50 μm, and its dielectric constant is 3.2, that of polyimide film. The line width W of the microstrip line  18  giving a characteristic impedance Z (Ω) of 35Ω and 50Ω is 0.18 mm and 0.1 mm, respectively. This result shows that impedance matching can be achieved if the width of the interconnect pattern  18  on the lead  4  side is 0.18 mm and the width of the interconnect pattern  18  on the semiconductor component  8   b  (amplifier) side is 0.1 mm. 
         [0000]                                                                  TABLE 1                       Z (Ω)   Er   h (mm)   W (mm)                                        Lead side   35   3.2   0.05   0.18           Amp side   50   3.2   0.05   0.10                        
The above impedance matching is implemented in both the two interconnect patterns  18  shown in  FIG. 8 .
 
         [0049]    In the component layout shown in  FIG. 8 , it is necessary to shorten the metal wires  10  constituting the interconnect between the submount  7  and the semiconductor component  8   b , the interconnect between the semiconductor component  8   b  and the interconnect pattern  18 , and the interconnect between the interconnect pattern  18  and the lead  4 , where electrical signals are transferred. As the metal wire  10  becomes longer, its characteristic impedance increases, and the transfer loss of electrical signals increases. Furthermore, the metal wires connecting between the components have various loop configurations and increase variation in characteristic impedance. This causes the problem of unstable performance of the optical semiconductor apparatus in the high-frequency band. 
         [0050]    According to the invention, the high degree of freedom in component layout allows the submount  7  and the semiconductor component  8   b  to be placed always close to each other, and the aforementioned problems can be avoided. Furthermore, because the interconnect pattern  18  is interposed, a short metal wire can be used to connect between the end portion  18   a  of the interconnect pattern  18  and the semiconductor component  8   b  and between the other end portion  18   b  of the interconnect pattern  18  and the lead  4 . 
         [0051]    In the conventional technique where the output terminal  8   c  of the semiconductor component  8   b  is directly connected to the lead  4  by a metal wire  10 , the output terminal of the semiconductor component  8   b  needs to be placed close to the lead  4  in order to shorten the metal wire  10 . This requires expensive dedicated components designed in consideration of the layout of the lead  4  and the layout of the semiconductor component  8   b . In contrast, in this invention, the interconnect pattern  18  can be adapted to the semiconductor component  8   b  to avoid the problems, allowing use of general-purpose components. 
         [0052]      FIG. 10  is a schematic view showing an alternative embodiment of the invention. The description is omitted on elements labeled in the figure which are common to those in  FIG. 8  described above. Furthermore, illustration of passive components  9  to be placed on the GND pattern  17  is also omitted. 
         [0053]    In this embodiment, the insulating film  14  bonded onto the component placement surface  3   b  of the base  3  is a multilayer film with two insulating films  14   a ,  14   b  laminated therein. An interlayer interconnect  25  is provided at the interface between the insulating films  14   a  and  14   b . The interlayer interconnect  25  connecting between metal patterns  26  and  27  formed on the frontside of the insulating film  14   b  is shown by a dashed line in the figure. 
         [0054]    The semiconductor component  8   b  of this embodiment is a general-purpose component, and the voltage supply terminal is located at a position denoted by  31  in the figure. On the other hand, the driving voltage is supplied from the lead  4   c  shown in the figure. In such a case, no appropriate interconnect pattern can be formed on the frontside of the insulating film  14   b  because it is obstructed by the submount  7  or the semiconductor component  8   b . Connection using a metal wire is also obstructed by the submount  7  or the semiconductor component  8   b . This problem can be solved by interconnection using an interlayer interconnect  25  provided in the multilayer film. 
         [0055]      FIG. 11  is a schematic view showing the D-D cross section of the base  3  in the embodiment shown in  FIG. 10 . The multilayer film  14  with the insulating films  14   a ,  14   b  laminated therein is placed on the component placement surface  3   b  of the base  3 . A backside metal pattern  22  is provided on the backside of the multilayer film  14  and bonded to the base  3  using solder or conductive paste. Furthermore, the interlayer interconnect  25  is provided at the interface between the insulating films  14   a  and  14   b  and electrically connected to the metal patterns  26  and  27  formed on the frontside of the insulating film  14   b  through a through hole  21 . 
         [0056]    Furthermore, a passive component  9  is placed on the metal pattern  26 . (The passive component  9  in this case is a resistor.) The passive component  9  is connected to the lead  4   c  by a metal wire  10 . On the other hand, the metal pattern  27  is connected to the voltage supply terminal  31  of the semiconductor component  8   b  by a metal wire  10 . This allows the lead  4   c  to be connected to the voltage supply terminal  31  of the semiconductor component  8   b  irrespective of the layout of the submount  7  or the semiconductor component  8   b.    
         [0057]      FIGS. 12 to 15  are schematic views showing insulating films  14  used in alternative embodiments of the invention. The description is omitted on elements labeled in the figure which are common to those in the above embodiments. 
         [0058]    In  FIG. 12 , the insulating film  14  shown in  FIG. 6  further includes an opening  32  in a portion where the semiconductor component  8   b  is to be placed. The opening  32  is slightly larger than the outline of the semiconductor component  8   b , and is provided to directly bond the semiconductor component  8   b  to the component placement surface  3   b  of the base  3 . Such an insulating film  14  is used in the case where the backside of the semiconductor component  8   b  needs to be placed at the ground potential and, furthermore, the performance of the semiconductor component  8   b  is susceptible to variation in the ground potential. 
         [0059]      FIG. 13  is a schematic view of an insulating film  14  including an opening  33  so that both the submount  7  with the photodiode  6  bonded thereto and the semiconductor component  8   b  are bonded to the component placement surface  3   b  of the base  3 . In the case where the performance of the optical semiconductor apparatus may be varied due to the potential difference occurring between the ground potential to which the terminal of the photodiode  6  is connected and the ground potential of the backside of the semiconductor component  8   b , the insulating film  14  shown in  FIG. 13  is used in order to hold the terminal of the photodiode  6  and the backside of the semiconductor component  8   b  at the same ground potential. 
         [0060]      FIG. 14  is a schematic view showing an insulating film  14  in which interconnect patterns  18  and  35  are provided on the frontside of the insulating film  14 . It is used in order to interconnect between a plurality of components using the interconnect pattern  35 . This is useful in the case where the interconnect length of the metal wire is increased because the passive components  9  are distanced from the semiconductor component, and in the case where the number of components are increased, as shown by dashed lines in the figure. 
         [0061]      FIG. 15  is a schematic view showing an insulating film  14  including an alignment mark  37 . The alignment mark  37  placed therein serves as the target for alignment when an automated machine is used to perform placement of components and bonding of metal wires. 
         [0062]      FIG. 16  shows an insulating film  14  according to an embodiment of using only the interconnect pattern  18  for interconnecting between the semiconductor component  8   b  and the lead  4 . It can be conveniently used in the case where a short metal wire  10  is used to interconnect the semiconductor component  8   b  and the lead  4  to the interconnect pattern  18  and a desired performance is obtained by impedance matching of the semiconductor component  8   b  and the lead  4  with the interconnect pattern  18 . 
         [0063]    The material of the insulating film  14  according to the invention can be polyimide film, LCP (liquid crystal polymer) film and the like. The LPC film has low dielectric loss for high-frequency signals, and hence is favorable to electrical signals in the high-frequency band. However, it is inferior in heat resistance, and hence has the problem of limitations on available solders. 
         [0064]    The insulating film  14  can be manufactured by the following method. Through holes  21  are formed by a microdrill in a polyimide or other insulating film with copper foils laminated on both sides. TH (through hole) plating based on copper plating is performed thereon to electrically connect between the front and back copper foil. Next, patterns are formed by photolithography and etching. Furthermore, openings  15  for inserting leads  4  are processed by the microdrill. Finally, the contour is punched out. 
         [0065]    The metal wire described in the above embodiments is illustratively a gold wire. Although the embodiments have been described with reference to an optical semiconductor apparatus including a photodiode and a semiconductor component (amplifier), the invention can also be practiced in an optical semiconductor apparatus in which a surface emitting laser or light emitting diode is combined with a driver IC. 
         [0066]    The embodiments of the invention have been described in detail. However, the optical semiconductor apparatus according to the invention is not limited to the above embodiments, but can be variously modified and altered within the scope of the invention described in the claims.