Abstract:
In recent years, as electronic equipment becomes thinner, an area for mounting a semiconductor device used in the electronic equipment is required to be smaller, and a thickness of an encapsulating resin for encapsulating a semiconductor substrate having a circuit formed thereon and the like also becomes smaller. The encapsulating resin is marked with a product number, a manufacturer name, or the like. There arises a problem in that, in the marking, an infrared laser beam applied to the encapsulating resin passes through the encapsulating resin, generates heat in the semiconductor substrate, and destructs the formed circuit. By providing a thin film for refracting the infrared laser beam on a rear surface of the semiconductor substrate, the optical path of the infrared laser beam is made longer to reduce heat generated in the semiconductor substrate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor device, and more particularly, to a semiconductor device which has a semiconductor substrate on a package substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, electronic equipment using a semiconductor device, for example, a cellular phone, is required to be thinner. In order to fulfill the demand for thinner electronic equipment, it is important to reduce an area for mounting a semiconductor device used in the electronic equipment. Therefore, in a semiconductor device of recent years, during the process of manufacturing the semiconductor device, a semiconductor substrate is ground to be thinner or an encapsulating resin for encapsulating the semiconductor substrate and the like is made thinner to reduce the area for mounting, for example. 
         [0005]    Japanese Patent Application Laid-Open No. 2002-076206 discloses an exemplary configuration of a semiconductor device, which is illustrated in  FIG. 13 . In  FIG. 13 , a semiconductor device  1300  includes balls  1301 , a substrate  1302  disposed on the balls  1301 , projections  1303  disposed on the substrate  1302 , a resin  1304  for covering the projections  1303 , a chip  1305  disposed on the projections  1303 , and an epoxy resin  1306  for encapsulating the chip  1305 , the projections  1303 , and the resin  1304 . Japanese Patent Application Laid-Open No. 2002-076206 discloses a technology which may enhance the effect of exhausting heat generated by the chip  1305  by directly bringing the epoxy resin  1306  into contact with the chip  1305 . 
         [0006]    Japanese Patent Application Laid-Open No. Hei 9-040756 discloses a technology which adjusts the ingredients of an epoxy resin which is a resin for encapsulating a semiconductor device. 
         [0007]    During the process of manufacturing a semiconductor device, it is necessary to mark an encapsulating resin for encapsulating a semiconductor substrate having a circuit for implementing a desired function formed thereon with the product number of the manufactured semiconductor device, the manufacturer name of the semiconductor device, or the like. The marking is carried out by irradiating the encapsulating resin with an infrared laser to engrave the product number, the manufacturer name, or the like. However, as described above, the encapsulating resin is getting thinner and thinner in semiconductor devices of recent years, and hence the applied infrared laser may passes through the encapsulating resin to reach the semiconductor substrate having the circuit formed thereon. Thus, there arises a problem in that the semiconductor device malfunctions under the influence of the infrared laser applied to the semiconductor substrate. The inventor of the present invention has confirmed by experiments that a semiconductor device, which operated normally before marking, malfunctioned after the marking. The inventor of the present invention thinks that, because of the infrared laser applied to the semiconductor substrate, heat is generated in the semiconductor substrate, the generated heat disconnects wiring of the circuit formed on the semiconductor substrate, and thus, the semiconductor device malfunctions. 
       SUMMARY 
       [0008]    The present invention has been made in view of the above-mentioned problem, and therefore has an object to provide a semiconductor device which may prevent malfunction thereof under the influence of an infrared laser applied to an encapsulating resin when marking is made. 
         [0009]    A semiconductor device according to the present invention includes: a package substrate; a connecting portion provided on the package substrate; a semiconductor substrate having a main surface and a surface opposite to the main surface, the main surface having connecting terminals formed so as to correspond to the connecting portion and electrically connected to the connecting portion, and the surface opposite to the main surface having a thin film formed thereon; and an encapsulating resin for encapsulating the connecting portion, the semiconductor substrate, and the thin film, in which the thin film has an index of refraction which is smaller than an index of refraction of the encapsulating resin. When the marking is made, an infrared laser which passes through the encapsulating resin is refracted on an interface of the thin film to make longer the optical path of the infrared laser, thereby making heat generated by the infrared laser less likely to be conducted to the semiconductor substrate. 
         [0010]    A semiconductor device according to the present invention may prevent malfunction thereof under the influence of an infrared laser applied to an encapsulating resin when the marking is made. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  illustrates a semiconductor device according to an embodiment of the present invention; 
           [0013]      FIG. 2  is a top view of a semiconductor substrate of the semiconductor device according to the embodiment; 
           [0014]      FIG. 3  illustrates refraction of an infrared laser beam; 
           [0015]      FIG. 4  is a flow chart illustrating a method of manufacturing the semiconductor device according to the embodiment; 
           [0016]      FIG. 5  illustrates a thin film formed on a rear surface of a semiconductor wafer; 
           [0017]      FIG. 6  illustrates a manufacturing process step of the semiconductor device according to the embodiment; 
           [0018]      FIG. 7  illustrates the manufacturing process step of the semiconductor device according to the embodiment; 
           [0019]      FIG. 8  illustrates the manufacturing process step of the semiconductor device according to the embodiment; 
           [0020]      FIG. 9  illustrates the manufacturing process step of the semiconductor device according to the embodiment; 
           [0021]      FIG. 10  illustrates the manufacturing process step of the semiconductor device according to the embodiment; 
           [0022]      FIG. 11  illustrates the manufacturing process step of the semiconductor device according to the embodiment; 
           [0023]      FIG. 12  illustrates the manufacturing process step of the semiconductor device according to the embodiment; and 
           [0024]      FIG. 13  illustrates a conventional semiconductor device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
         [0026]    The best mode for carrying out the present invention is described in the following with reference to the attached drawings.  FIG. 1  illustrates a semiconductor device  100  according to an embodiment of the present invention. The semiconductor device  100  is mounted on a mounting substrate  101 . A plurality of solder balls  102  provided on the mounting substrate  101  so as to be spaced apart from one another electrically connect the mounting substrate  101  and a package substrate  103  via connecting terminals  109  provided on an upper surface of the mounting substrate  101  and connecting terminals  110  provided on a lower surface of the package substrate  103 . The package substrate  103  is a substrate for disposing a semiconductor substrate  106  having a circuit which performs desired operation formed thereon. The package substrate  103  is electrically connected to the semiconductor substrate  106  via connecting terminals  111  provided on an upper surface of the package substrate  103 , a plurality of solder balls  104  provided so as to be spaced apart from one another, and connecting terminals  112  provided on a main surface of the semiconductor substrate  106 . As described above, the package substrate  103  is also electrically connected to the mounting substrate  101  via the connecting terminals  110  and  109  and the solder balls  102 . A resin  105  fixes the solder balls  104 . The semiconductor substrate  106  has wiring on a lower surface thereof in  FIG. 1 , that is, a surface thereof on the side of the solder balls  104  (i.e., front surface), and has the circuit which performs desired operation formed thereon. In other words, in this embodiment, a flip-chip (FC) BGA structure is adopted. A surface which is opposite to the main surface (that is, rear surface) has a thin film  107  of, for example, SiO 2  formed thereon. The semiconductor substrate  106  is formed of, for example, silicon (Si). It is to be noted that the thickness of the semiconductor substrate  106  is, for example, 150 μm. The thin film  107  is provided for the purpose of refracting a laser beam in marking. An epoxy resin  108  which is an encapsulating resin for encapsulating the solder balls  104 , the resin  105 , the semiconductor substrate  106 , the thin film  107 , and the like is provided on the package substrate  103 . The epoxy resin  108  is irradiated with an infrared laser to mark the epoxy resin  108  with the product number, the manufacturer name, or the like of the semiconductor device  100 .  FIG. 2  is a top view of the semiconductor substrate  106 . As illustrated in  FIG. 2 , the thin film  107  is provided over the entire rear surface of the semiconductor substrate  106 . 
         [0027]    Here, the relationship between the thin film  107  and the infrared laser in marking is described with reference to  FIG. 3 .  FIG. 3  is a schematic sectional view of the semiconductor substrate  106 , the thin film  107  which is SiO 2 , and the epoxy resin  108  of the semiconductor device  100 . As illustrated in  FIG. 3 , the epoxy resin  108  is irradiated with the infrared laser to mark the epoxy resin  108  with the product number, the manufacturer name, or the like. Here, the infrared laser is a light beam, and thus, the infrared laser is refracted when entering into a medium having a different index of refraction. The index of refraction of the epoxy resin  108  is, for example, on the order of 1.55 to 1.61. Therefore, the wavelength of the laser beam which enters into the epoxy resin  108  becomes shorter, and is refracted at an angle as illustrated in  FIG. 3 . The laser beam which enters into SiO 2  through the epoxy resin  108  is again refracted. The index of refraction of SiO 2  is on the order of 1.46, which is smaller than that of the epoxy resin  108 . Therefore, the wavelength of the laser beam is longer in SiO 2  than in the epoxy resin  108 , and the laser beam is refracted as illustrated in  FIG. 3 . 
         [0028]    In the semiconductor device  100  according to this embodiment, the thin film  107  which is SiO 2  is provided on the rear surface of the semiconductor substrate  106 , and hence the infrared laser beam which passes through the epoxy resin  108  is refracted on an interface of SiO 2 , and the optical path of the infrared laser beam to the semiconductor substrate  106  becomes longer than that when SiO 2  is not provided. The longer optical path to the semiconductor substrate  106  reduces heat conducted to the semiconductor substrate  106 . Heat conduction is, in the case of a solid, based on energy propagation by lattice vibration of atoms forming the solid (phonons). The laser beam vibrates the atoms of the epoxy resin  108 , heat as energy is generated, and the vibration of the atoms is transferred to the side of the semiconductor substrate  106  via atoms. In other words, heat is propagated. Here, by providing SiO 2 , the laser beam is refracted to make longer the optical path of the laser beam, thereby making longer the propagation path of the energy. As the propagation path of the energy to the semiconductor substrate  106  is longer, the vibration, that is, the energy which reaches the semiconductor substrate  106  is more damped, and thus, heat generated in the semiconductor substrate  106  may be reduced. 
         [0029]    It is to be noted that, in the above-mentioned embodiment, SiO 2  is used as the thin film, but other materials may be used as the thin film insofar as it may make longer the optical path to the semiconductor substrate  106 . In other words, a material having the index of refraction which is smaller than that of the epoxy resin  108  may be used. This is because, if the index of refraction is smaller than that of the epoxy resin  108 , the infrared laser is refracted in the direction in which the optical path thereof becomes longer. Further, instead of employing the thin film  107  formed of a single material, a thin film formed by laminating a plurality of materials having different indices of refraction may also be used. In that case, a configuration may be adopted in which a first thin film located immediately below the epoxy resin  108  has an index of refraction which is smaller than that of the epoxy resin  108 , and a second thin film located below the first thin film has an index of refraction which is smaller than that of the first thin film. If such a configuration is adopted, the optical path of the infrared laser beam which passes through the epoxy resin  108  becomes still longer. As a matter of course, a third thin film having the index of refraction which is smaller than that of the second thin film may be further provided below the second thin film to make still longer the optical path of the laser beam. 
         [0030]    Further, the thickness of the thin film  107  for refracting the laser beam is preferably on the order of 10 μm. If the thin film  107  is excessively thick, it is against the demand of recent years for a reduced area for mounting the semiconductor device. If the thin film  107  is excessively thin, the rear surface of the semiconductor substrate  106  may not be coated uniformly with the thin film  107 , and thus, the rear surface of the semiconductor substrate  106  has a portion on which the thin film  107  is not provided, which causes the laser beam not to be refracted. 
         [0031]    Next, a method of manufacturing the semiconductor device  100  according to this embodiment is described with reference to  FIGS. 4 to 12 .  FIG. 4  is a flow chart illustrating the method of manufacturing the semiconductor device  100  according to the embodiment. A circuit pattern for performing desired operation is formed on a surface of a semiconductor wafer  500  before the semiconductor substrate  106  is cut out from the semiconductor wafer  500  (S 401 ). Here, the connecting terminals  112  and the plurality of solder balls  104  provided so as to be spaced apart from one another are also formed. Then, the semiconductor wafer  500  is ground (S 402 ). The grinding is carried out for the purpose of decreasing the area for mounting the entire semiconductor device, and, for example, the semiconductor wafer  500  having a thickness of 500 μm is ground to a thickness of 150 μm. 
         [0032]    After the rear surface of the semiconductor wafer  500  is ground, the thin film  107  for refracting the infrared laser is formed on the rear surface of the semiconductor wafer  500  (S 403 ). 
         [0033]      FIG. 5  illustrates the semiconductor wafer  500 . The thin film  107  which is, for example, SiO 2 , is provided on the entire rear surface of the semiconductor wafer  500 . The thin film  107  is formed on the entire rear surface of the semiconductor wafer  500  by, for example, sputtering. 
         [0034]    Then, the semiconductor wafer  500  is diced to cut out the semiconductor substrate  106  (S 404 ). 
         [0035]    The semiconductor substrate  106  which is cut out is mounted on the package substrate  103  (S 405 ), which is illustrated in  FIG. 6 . The plurality of solder balls  104  located on the surface having the circuit formed thereon are mounted on the package substrate  103 . In this step, as illustrated in  FIG. 7 , there exist a plurality of semiconductor substrates  106  on the entire package substrate  103 . 
         [0036]    Then, heat treatment is carried out with regard to the semiconductor substrate  106  mounted on the package substrate  103  to make the adhesion satisfactory between the solder balls  104  and the connecting terminals  111  (S 406 ). 
         [0037]    After cleaning for removing impurities generated by the heat treatment is carried out (S 407 ), the resin  105  for filling space between the solder balls  104  is injected (S 408 ). The state of the semiconductor device of this step is illustrated in FIG.  8 . 
         [0038]    After that, baking is carried out to cure the resin  105  and fix the solder balls  104  (S 409 ). 
         [0039]    Further, the semiconductor substrate  106  and the like are encapsulated by the epoxy resin  108  (S 410 ). After that, marking is carried out (S 411 ). The state of the semiconductor device of this step is illustrated in  FIGS. 9 and 10 . The epoxy resin  108  is irradiated with an infrared laser to carry out marking. Here, the infrared laser is refracted by the thin film  107  to make longer the optical path of the infrared laser. The longer optical path of the infrared laser prevents thermal destruction of the circuit formed on the semiconductor substrate  106 . 
         [0040]    Then, after impurities generated in the epoxy resin  108  are removed (S 412 ), the package substrate  103  is placed on the solder balls  102  provided so as to be spaced apart from one another (S 413 ). The state of the semiconductor device of this step is illustrated in  FIG. 11 . Then, heat treatment is carried out to enhance the adhesion between the solder balls  102  and the connecting terminals  110  (S 414 ). 
         [0041]    Further, in order to cut out individual semiconductor devices formed in the processes up to this step, the package substrate  103  is cut (S 415 ), which is illustrated in  FIG. 12 . Finally, operation of the individual semiconductor devices which are cut out is checked and a selection is made (S 416 ). 
         [0042]    When a customer mounts the semiconductor device which is cut out on the mounting substrate  101 , manufacture of the semiconductor device  100  according to this embodiment is completed. It is to be noted that the above describes a mere embodiment of the present invention and the scope of the present invention should not be construed to be limited thereto. 
         [0043]    It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.