Abstract:
A method of manufacturing a semiconductor device which includes a semiconductor chip and a plastic package of a thermosetting polymer, including the steps of performing an ultraviolet cleaning process on the bottom surface of the semiconductor chip and, encapsulating the semiconductor chip through a molding process. The thermosetting polymer of the plastic package fully or partially covers the bottom surface of the semiconductor chip.

Description:
This application is a Divisional of application Ser. No. 08/919,170, filed Aug. 28, 1997, now U.S. Pat. No. 5,821,612, which was a continuation application of prior application Ser. No. 08/605,834, filed Feb. 22, 1996, now abandoned, which was a continuation of prior application Ser. No. 08/257,036 file Jun. 8, 1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention generally relates to a semiconductor device and, more particularly, to a semiconductor device in which a semiconductor chip is bonded to lead portions and encapsulated in plastic, the semiconductor chip having a bottom surface covered with the plastic, which plastic is likely to crack due to heat stresses. 
     (2) Description of the Prior Art 
     A semiconductor device in which a semiconductor chip is bonded to lead portions and enclosed in a plastic package is known. Before the plastic package is molded to enclose the semiconductor chip, the semiconductor chip is bonded onto a die pad, the semiconductor chip having a bottom surface covered with the die pad. When the semiconductor device of this type is bonded onto a printed circuit board, a portion of the plastic package on the bottom surface of the semiconductor chip is affected by heat. The portion of the plastic package on the bottom surface of the semiconductor chip is likely to crack due to heat stresses between the plastic package and corners of the die pad. To increase the reliability of the semiconductor device as a manufactured product, it is desirable that the plastic package does not crack if the heat stresses occur. 
     FIG. 1 shows a conventional semiconductor device in which a semiconductor chip is bonded to lead portions and enclosed in a plastic package. In FIG. 1, the semiconductor device  1  has an epoxy resin package  2  and a semiconductor chip  3  enclosed in the epoxy package  2 . A certain filler is added to the epoxy resin package  2  so as to increase the strength of the epoxy resin package  2 . 
     In the semiconductor device  1 , a bottom surface  3   a  of the semiconductor chip  3  is covered with a die pad, and the die pad adheres to the epoxy resin package  2 . However, the adhesion between the epoxy resin package  2  and the die pad is not sufficiently firm. 
     The semiconductor device  1  is subjected to heat when it is bonded onto a printed circuit board, and especially a bottom portion of the epoxy resin package  2 , covering corners of the die pad, is influenced by heat stresses. Moisture in the epoxy resin package  2  vaporizes in the heat. Since the volume of the vapor in the epoxy resin package  2  is much greater when the semiconductor device is subjected to heat than before it is heated, heat stresses between the semiconductor chip  3  and the die pad and heat stresses between the epoxy resin package  2  and the die pad are produced. The epoxy resin package  2  and the die pad at the corners are likely to be separated from each other. Cracks  4  in the bottom portion of the epoxy resin package  2  may be produced due to the heat stresses. If the epoxy resin package  2  cracks, the entry of humid air through the cracks into the epoxy resin package  2  will make the reliability of the semiconductor device  1  low, and will make the life of the semiconductor device  1  shorter. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide an improved semiconductor device in which the above described problems are eliminated. 
     Another, more specific object of the present invention is to provide a semiconductor device in which a semiconductor chip is bonded to lead portions and enclosed in a plastic package, adhesion between the semiconductor chip and the plastic package after the plastic package is molded around the semiconductor chip being increased to a level sufficient for preventing a portion of the plastic package on a bottom of the semiconductor chip from cracking due to heat stresses. 
     The above mentioned objects of the present invention are achieved by a semiconductor device which includes a semiconductor chip and a plastic package for enclosing the semiconductor chip in a plastic material through a molding process, the plastic package fully or partially covering a bottom surface of the semiconductor chip with the plastic material, wherein an ultraviolet cleaning process is performed for cleaning the bottom surface of the semiconductor chip prior to the molding process. 
     According to the present invention, it is possible to increase adhesion between the semiconductor chip and the plastic package after the molding process is performed, thereby preventing the plastic package from cracking due to heat stresses. The reliability of the semiconductor device thus manufactured can be increased, and the life of the semiconductor device can be made longer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a vertical cross-sectional view showing a conventional semiconductor device; 
     FIG. 2 is a vertical cross-sectional view showing a semiconductor device in a first embodiment of the present invention; 
     FIG. 3 is a plan view showing a semiconductor chip of the semiconductor device in FIG. 2 wherein a plastic package is omitted; 
     FIG. 4 is a diagram for explaining a molding step in which the semiconductor chip is enclosed in the plastic package in FIG. 2; 
     FIG. 5 is a flow diagram for explaining a sequence of manufacturing steps for manufacturing the semiconductor device in the first embodiment; 
     FIG. 6 is a diagram for explaining an ultraviolet cleaning step of the manufacturing steps in FIG. 5; 
     FIG. 7 is a sectional view showing a semiconductor device in a second embodiment of the present invention; 
     FIG. 8 is a plan view showing a semiconductor chip of the semiconductor device in FIG. 7 wherein a plastic package is omitted; 
     FIG. 9 is a diagram for explaining a molding step in which the semiconductor chip is enclosed in the plastic package of the semiconductor device in FIG. 7; 
     FIG. 10 is a flow diagram for explaining a sequence of manufacturing steps for manufacturing the semiconductor device in FIG. 7; and 
     FIG. 11 is a diagram for explaining an ultraviolet cleaning step of the manufacturing steps in FIG.  10 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will now be given, with reference to FIGS. 2 through 6, of a first embodiment of a semiconductor device according to the present invention. 
     FIG. 2 shows a semiconductor device of the first embodiment. FIG. 3 shows a semiconductor chip of the semiconductor device in FIG. 2 wherein a plastic package is omitted. As shown in FIGS. 2 and 3, the semiconductor device  10  generally has a semiconductor chip  11 , a stage  12 , a set of inner lead portions  14 , a set of wires  15 , a lead  17 , and a plastic package  16 . 
     The chip  11  is primarily made of silicon. A silver paste layer  13  is formed on the top of the stage  12 , and the semiconductor chip  11  is fixed to the stage  12  with the silver paste through a chip bonding process. The width of the stage  12 , indicated by the arrow “a” in FIG. 3, is smaller than the width of the semiconductor chip  11 , indicated by the arrow “b” in FIG.  3 . The peripheral portions of the semiconductor chip  11  extend outwardly from the periphery of the stage  12 . 
     An integrated circuit on the semiconductor chip  11  is connected to the inner lead portions  14  via the wires  15 . The plastic package  16  is primarily made of a thermosetting polymer such as epoxy resin. The chip  11 , the stage  12 , the wires  15 , and the inner lead portions  14  are enclosed in the plastic package  16 . The lead  17  outwardly projects from the periphery of the plastic package  16 . 
     The bottom peripheral portions  11   a - 1 ,  11   a - 2  of the bottom surface  11   a  of the semiconductor chip  11  directly come into contact with the thermosetting polymer of the plastic package  16 , as indicated in FIG.  2 . The bottom peripheral portions  11   a - 1 ,  11   a - 2  are subjected to radiation of ultraviolet rays, of wavelengths 2537 Å and 1849 Å for a given time period, during an ultraviolet cleaning process. Undesired organic compounds, which may be sticking to the semiconductor chip since the wafer process, are removed from the semiconductor chip  11  after the ultraviolet cleaning process is performed. 
     In the ultraviolet cleaning process described above, the radiation of the 1849 Å ultraviolet rays accelerates decomposition of oxygen gas (O 2 ) in the air to radical oxygen ions (O + ), and produces ozone (O 3 ) in a high energy state. At the same time as mentioned above, the radiation of the 2537 Å ultraviolet rays accelerates decomposition of organic compounds into radical carbon ions (C + ) and radical hydrogen ions (H + ) by cutting C-H bonds. The radical carbon ions (C + ) and the ozone (O 3 ) react together to produce carbon dioxide gas (CO 2 ). The radical hydrogen ions (H + ) and the ozone (O 3 ) react together to produce water (H 2 O). The carbon dioxide gas and the water can easily be removed. Accordingly, the undesired organic compound can be removed from the semiconductor chip  11  after the ultraviolet cleaning process described above is performed. 
     As described above, the bottom peripheral portions  11   a - 1 ,  11   a - 2  of the semiconductor chip  11  are modified by the ultraviolet cleaning process so as to create a clean and hydrophilic surface  18 . Thus, the surface  19  of the plastic package  16  firmly adheres to the bottom peripheral portions  11   a - 1 ,  11   a - 2  of the semiconductor chip  11 . The resultant adhesion (P 1 ) between the semiconductor chip  11  and the plastic package  16 , at the bottom peripheral portions  11   a - 1 ,  11   a - 2  after the ultraviolet cleaning process is performed, is stronger than the adhesion between the semiconductor chip and the plastic package when no ultraviolet cleaning is performed. 
     Therefore, the semiconductor device  10  has the semiconductor chip  11  with an increased adhesion to the plastic package  16  at the bottom peripheral portions  11   a - 1 ,  11   a - 2  because of the ultraviolet cleaning process described above. 
     The semiconductor device  10  in the first embodiment has the following advantages. The semiconductor device  10  is subjected to heat when it is mounted on a printed circuit board, and especially the bottom portion of the plastic package  16  is influenced by the heat stress. The plastic package surface and the chip surface are likely to be separated from each other due to the heat stress at this time. As the adhesion between the semiconductor chip  11  and the plastic package  16  at the portions  11   a - 1 ,  11   a - 2  is increased remarkably, when compared with that of a conventional semiconductor device, the separation of the plastic package surface and the chip surface, as described above, can be prevented. 
     Accordingly, in the semiconductor device of the first embodiment, the adhesion between the semiconductor chip and the plastic package after the molding process is performed can be remarkably increased, thereby preventing the plastic package from cracking due to the heat stresses. 
     The humidity resistance of the semiconductor device  10  according to the present invention is higher than that of the conventional device, and the life of the semiconductor device is longer than that of the conventional device. The reliability of the semiconductor devices thus manufactured is increased. 
     Next, a description will be given of a sequence of manufacturing steps for manufacturing the semiconductor device  10 . FIG. 5 shows the sequence of the manufacturing steps. FIG. 6 shows an ultraviolet cleaning step of the manufacturing steps in FIG.  5 . 
     Step  30  in the manufacturing steps in FIG. 5 is a chip bonding process wherein the semiconductor chip  11  is mounted on the stage  12  and the semiconductor chip is bonded to the stage  12  using the silver paste. 
     Step  31  is a curing process wherein the semiconductor chip  11 , bonded to the stage  12 , is heated to 150° C. for about 60 minutes, so as to cure the silver paste between the semiconductor chip  11  and the stage  12 . 
     Step  32  is a wire bonding process wherein the wires  15  are bonded to the semiconductor chip  11 , so as to connect the integrated circuit on the semiconductor chip  11  to the inner lead portions  14  via the corresponding wires  15 . 
     Step  33  is the ultraviolet cleaning process described above. FIG. 6 shows the manner in which the ultraviolet cleaning step of the manufacturing steps in FIG. 5 is performed. As shown in FIG. 6, at the ultraviolet cleaning step  33 , the ultraviolet rays  40  having the wavelengths 1849 Å and 2537 Å are radiated, for a given time period, to the bottom surface of the semiconductor chip  11 , which surface is disposed on and thus partially covered with the stage  12 . Only a portion of the bottom surface of the semiconductor chip  11 , covered with the stage  12 , is not subjected to the radiation of the ultraviolet rays. 
     After the ultraviolet cleaning step  33  is performed, the remaining organic compounds are removed from the semiconductor chip  11 , and the bottom peripheral portions  11   a - 1 ,  11   a - 2  of the semiconductor chip  11  are modified to be clean and hydrophilic. 
     Step  34  is a molding process wherein the plastic package  16  is molded, using epoxy resin through a transfer molding procedure, so that the semiconductor chip  11  is enclosed in the plastic package  16 . 
     Step  35  is a plating process wherein the lead  17 , outwardly extending from the plastic package  16 , is plated with metal. 
     Step  36  is an ultraviolet cleaning process wherein the ultraviolet rays, having the wavelengths 1849 Å and 2537 Å, are radiated to a marking surface of the plastic package  16  for a given time period. This process is similar to the above-described ultraviolet cleaning process at step  33 . After the ultraviolet cleaning process at step  36  is performed, remaining organic compounds are removed from the marking surface of the plastic package  16 , and the marking surface is modified to be clean. 
     Step  37  is a marking process wherein the plastic package  16  is marked with ink by using a stamper, so that a model number of the semiconductor device or the like is indicated on the marking surface of the plastic package  16 . As the marking surface of the plastic package  16  is modified to be clean at step  36 , a clear marking can be provided. 
     Step  38  is a press forming process wherein the lead  17  is bent by using progressive dies. 
     Accordingly, the semiconductor device  10  in FIG. 2 is produced after the manufacturing steps are performed as described above. 
     Next, a description will be given of a second embodiment of the semiconductor device according to the present invention. 
     FIG. 7 shows a semiconductor device  50  of the second embodiment. The semiconductor device  50  has a lead-on-chip (LOC) structure wherein the lead is mounted on the semiconductor chip. FIG. 8 shows a semiconductor chip of the semiconductor device in FIG. 7 wherein a plastic package is omitted. 
     As shown in FIGS. 7 and 8, the semiconductor device  50  comprises a semiconductor chip  51 , adhesive tapes  52 , a set of inner lead portions  53 , a set of wires  54 , a lead  56 , and a plastic package  55 . As the semiconductor device  50  has a LOC structure with no stage, the semiconductor chip  51  has a bottom surface  51   a,  the entire area of which directly comes in contact with the plastic package  55 . 
     The chip  51  is primarily made of silicon. The adhesive tapes  52  are attached to the top surface  51   b  of the semiconductor chip  51 , and the inner lead portions  53  are fixed to the semiconductor chip  51 , and the inner lead portions  53  are fixed to the semiconductor chip  51  with the adhesive tapes  52 . 
     The inner lead portions  53  are connected to a pad on the top surface of the semiconductor chip  51  via the wires  54 . The plastic package  55  is primarily made of a thermosetting polymer such as epoxy resin. The chip  51 , the adhesive tapes  52 , the inner lead portions  53 , and the wires  54  are enclosed in the plastic package  55 . The leads  56  outwardly project from the periphery of the plastic package  55 . 
     In the semiconductor device  50  described above, the bottom surface  51   a  of the semiconductor chip  51  is subjected to the radiation of the ultraviolet rays of the wavelengths 2537 Å and 1849 Å for a given time period in an ultraviolet cleaning process prior to a package molding process. Undesired organic compounds, which may stick to the semiconductor chip  51  in the course of the wafer process, are removed from the semiconductor chip  51  by performing the ultraviolet cleaning process prior to the package molding process. 
     As described above, the bottom surface  51   a  of the semiconductor chip  51  is modified to create a clean and hydrophilic surface  57 , by performing the ultraviolet cleaning process prior to the package molding process. 
     The chip  51  with the clean, hydrophilic bottom surface is enclosed in the plastic package  55  by performing the package molding process, as shown in FIG.  9 . The resulting adhesion (P 2 ) between the semiconductor chip  51  and the plastic package  55  on the bottom surface  51   a  after the package molding process is performed following the ultraviolet cleaning process, is tougher than the adhesion between the semiconductor chip and the plastic package when no ultraviolet cleaning process is performed. 
     Therefore, the semiconductor device  50  after the package molding process has the semiconductor chip  51  with an increased adhesion to the, plastic package  55 . The entire bottom surface  51   a  of the semiconductor chip  51  firmly adheres to the plastic package  55  because of the ultraviolet cleaning process. 
     The semiconductor device  50  in the second embodiment has the following advantages. The semiconductor device  50  is subjected to heat when it is mounted on a printed circuit board, and especially a bottom portion of the plastic package  55  is influenced by a heat stress. The package surface and the chip surface are likely to be separated from each other due to the heat stress. However, the adhesion between the semiconductor chip  51  and the plastic package  55  at the bottom surface  51   a  is remarkably increased in comparison with that of the conventional device. As the adhesion mentioned above is firm enough to resist against the heat stress, the separation of the package surface and the chip surface is prevented. Thus, in the semiconductor device  50  according to the present invention, it is possible to prevent the plastic package  55  from cracking due to the heat stress. 
     The humidity resistance of the semiconductor device  50  in the second embodiment is increased from that of the conventional device, and the life of the semiconductor device can be made longer than that of the conventional device. The reliability of the semiconductor devices manufactured is thus increased. 
     Next, a description will be given of a sequence of manufacturing steps for manufacturing the semiconductor device  50  in the second embodiment. FIG. 10 shows the sequence of the manufacturing steps. FIG. 11 shows an ultraviolet cleaning step of the manufacturing steps in FIG.  10 . 
     Step  60  in the manufacturing steps in FIG. 10 is a mounting process wherein the inner lead portions  53  are fixed to the top surface of the semiconductor chip  51  by using the adhesive tapes  52 . Thus, the semiconductor chip  51  is placed on the bottom of the inner lead portions  53 . 
     Step  61  is a wire bonding process wherein the wires  54  are bonded to the semiconductor chip  51 , so as to connect the inner lead portions  53  to an integrated circuit on the semiconductor chip  51  via the corresponding wires  54 . 
     Step  62  is the ultraviolet cleaning process described above. FIG. 11 shows the manner in which the ultraviolet cleaning step  62  is performed As shown in FIG. 11, at the ultraviolet cleaning step  62 , the ultraviolet rays  40  having the wavelengths 1849 Å and 2537 Å are radiated, for a given time period, to the bottom surface  51   a  of the semiconductor chip  51 . The entire bottom surface  51   a  of the semiconductor chip  51  is subjected to the radiation of the ultraviolet rays. 
     After the ultraviolet cleaning step  62  is performed, the remaining organic compounds are removed from the semiconductor chip  51  and the bottom surface  51   a  of the semiconductor chip  51  is modified to be clean and hydrophilic. 
     Step  63  is a molding process wherein the plastic package  55  is molded, using epoxy resin through a transfer molding procedure, so that the semiconductor chip  51  is enclosed in the plastic package  55 . The bottom surface  51   a  of the semiconductor chip  51  firmly adheres to the plastic package  55 . 
     Step  64  is a plating process wherein the lead  56  outwardly extending from the plastic package  55  is plated with metal. 
     Step  65  is an ultraviolet cleaning process wherein the ultraviolet rays having the wavelengths 1849 Å and 2537 Å are radiated onto a marking surface of the plastic package  55  for a given time period. This process is similar to the above-described ultraviolet cleaning process at step  62 . After the ultraviolet cleaning process at step  65  is performed, remaining organic compounds are removed from the marking surface of the plastic package  55 , and the marking surface is modified to be clean and hydrophilic. 
     Step  66  is a marking process wherein the plastic package  55  is marked with ink by using a stamper, so that a model number of the semiconductor device or the like is provided on the marking surface of the plastic package  55 . As the marking surface of the plastic package  16  is made clean and hydrophilic at step  66 , a clear marking can be provided. 
     Step  67  is a press forming process wherein the lead  56  is bent by using progressive dies. 
     Accordingly, the semiconductor device  50  in FIG. 7 is produced after the manufacturing steps are performed as described above. 
     In the semiconductor device in the second embodiment described above, the adhesion between the semiconductor chip and the plastic package after the molding process is performed can be increased, thereby preventing the plastic package from cracking due to the heat stresses. The reliability of the semiconductor device thus manufactured can be increased, and the life of the semiconductor device can be made longer. 
     Further, the present invention is not limited to the above described embodiments, and variations and modifications may be made without departing from the scope of the present invention.