Abstract:
A semiconductor device includes at least one bump electrode for inputting and outputting signals to and from the semiconductor device. The bump electrode is positioned above a semiconductor substrate with an electrode pad and metal layer disposed therebetween. A resin film covers a surface of the semiconductor substrate except at a top area of the bump electrode. The bump electrode projects a sufficient distance above a top surface of the resin film so that heat induced defects are reduced and pressure exerted on a top area of the bump electrode is absorbed to suppress occurrence of cracks in the resin film.

Description:
This application is a continuation, of application Ser. No. 08/075,373 filed Jun. 14, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device having a bump electrode as an electrode for inputting and outputting signals to and from a semiconductor element and a method for manufacturing the same, and in particular to a semiconductor device which can suppress occurrence of cracks at an area below the bump electrode upon the bonding of an inner lead and can achieve an enhanced heat resistance cycle property after the inner lead bonding has been carried out and a method for manufacturing the same. 
     2. Description of the Related Art 
     Conventionally bump electrodes have been employed as those electrodes for a TAB type and a COB (chip on board) type semiconductor device. 
     FIGS. 1 to  3 , each, are a cross-sectional view showing a major section of a semiconductor device having a conventional bump electrode. This structure is disclosed, for example, in Published Unexamined Japanese Patent Application H-1-1640401, but involves drawbacks as will be set out below. 
     As shown in FIG. 1, a bump electrode  102  is provided on a semiconductor substrate  101 . A resin film  103  is covered over the surface of the semiconductor substrate  101  except at the bump electrode  102 . The surface of the resin film  103  is flush with that of the bump electrode  102  at an area  105  adjacent the bump electrode  102 . The resin film  103  serves as a film for protection against any external mechanical damage due to a dicing process, etc., and can prevent a lead shorting whereby direct lead-to-substrate contact occurs. However, the aforementioned semiconductor structure has the following drawbacks. To be specific, upon the bonding of the lead to the device shown in FIG. 2, the lead  107  is pushed with a pressure P by a bonding tool  106 , causing the lead  107  to be pushed into the bump electrode  102 . At this time, the bump electrode  102  is deformed so that a crack occurs in the resin film  103  as indicated by reference numeral  108 . There is a fear that, upon the occurrence of such a crack in the resin film  103 , water and harmful impurities will intrude into the substrate (device body)  101  via the crack  108 . This adversely affects the device reliability. 
     FIG. 3 is a cross-sectional view, as taken along line III—III in FIG.  2 . As shown in FIG. 3, the lead  107  makes direct contact with the resin film  103  in the semiconductor device of FIG.  1 . In this state, upon the thermocompression bonding of the lead  107  to the device, heat is transmitted via the lead  107  to the resin film  103  to cause the resin film  103  to be affected by the heat at an area (heat-affected area) indicated by reference numeral  109  in FIG.  3 . The resin film  103 , being so heat-affected, gives a bad effect to the device reliability. In addition to the aforementioned drawbacks encountered upon the pushing of the lead onto the bump electrode and upon the thermocompression bonding, an eutectic alloy, i.e., an alloy of tin (Sn) plated to the surface of the lead  107  and gold (Au) contained in the bump electrode  102 , is not adequately formed due to the lead  107  contacting with the resin film  103  upon the pushing of the lead  107  into the bump electrode  102 . As a result, the lead  107  is liable to be separated off the bump electrode  102 . 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the present invention to provide a semiconductor device which can suppress occurrence of cracks at an area below a bump electrode upon the bonding of an inner lead and can achieve an enhanced heat resistance cycle property after the bonding of the inner lead and a method for manufacturing the same. 
     According to one aspect of the present invention there is provided a semiconductor device comprising a semiconductor substrate, at least one bump electrode provided over the semiconductor substrate to allow signals to be input and output to and from a semiconductor device, and a resin film covered on a surface of the semiconductor substrate except at a top area of the bump electrode, wherein the top area of the bump electrode is more projected than a top area of the resin film. 
     According to another aspect of the present invention there is provided a method for manufacturing a semiconductor device comprising the steps of forming at least one electrode pad over a semiconductor substrate, providing an opening in a surface portion of the electrode pad and forming a passivation film over a surface of the semiconductor substrate and that of the electrode pad, forming a metal film over the surface of the electrode pad at an area of the opening and over the passivation film near the opening, providing a bump electrode over the metal layer, forming an organic insulating film over the bump electrode and passivation film, and etching back the organic insulating film to allow a top area of the bump electrode to be more projected than a top area of the organic insulating film. 
     The present device and method can suppress occurrence of cracks at an area below a bump electrode upon the bonding of an inner lead and can achieve an enhanced heat resistance cycle property after the bonding of the inner lead. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particuarly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a cross-sectional view showing a major portion of a conventional semiconductor device equipped with a bump electrode; 
     FIG. 2 is a cross-sectional view showing a major portion of the semiconductor device at a step of connecting an inner lead to the bump electrode; 
     FIG. 3 is a cross-sectional view, taken along line III—III in FIG. 2; 
     FIGS. 4 to  7  are cross-sectional views showing the steps of manufacturing a semiconductor device of the present invention; 
     FIG. 8 is a vertical cross-sectional view, as taken along a longitudinal direction, showing a lead and semiconductor device at the manufacturing step shown in FIG. 7; and 
     FIG. 9 is a graph showing the result of a thermal cycle test (TCT) against a semiconductor device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 4, aluminum electrode pads  2   a ,  2   b  and  2   c  are formed on a silicon substrate  1  having semiconductor elements, not shown, formed therein and are electrically connected to the semiconductor elements. The electrode pads  2   a ,  2   b  and  2   c  are formed, in actual practice, over the substrate  1  with an insulating film provided therebetween, but in the present specification the substrate  1  is intended to mean a semiconductor integrated circuit (IC) with such an insulating film, etc., provided therebetween in which case the details of the substrate inner structure are omitted for brevity&#39;s sake. A passivation film  3 , such as phosphosilicate glass (PSG) or silicon nitride, is formed on the substrate  1  including the electrode pads  2   a ,  2   b  and  2   c . The passivation film  3  includes openings leading to the electrode pads  2   a ,  2   b  and  2   c . Metal layers  4   a ,  4   b  and  4   c  are provided on the passivation layer  3  at those areas near these openings such that they provide a Pd(palladium)-Ni(nickel)-Ti(titanium)-stacked layer structure each. The metal layers  4   a ,  4   b  and  4   c  are of the order of 4000 angstroms and electrically connected to the electrode pads  2   a ,  2   b  and  2   c , respectively, via the corresponding openings. About 18±2 μm-thick bump electrodes  5   a ,  5   b  and  5   c  electroplated with gold are provided on the metal layers  4   a ,  4   b  and  4   c , respectively. 
     A polyimide resin solution of about 1000 cP is dropwise applied to the device shown in FIG. 4 by sequentially spinning the substrate structure, including the bump electrodes  5   a ,  5   b  and  5   c , at 500 rpm for 10 seconds and 1000 to 1200 rpm for 10 seconds. Then the polyimide resin solution on the semiconductor structure is dried in an N 2  gas atmosphere at 150° C. for 60 minutes and solidified to provide a polyimide resin film  6 , specifically, about 1 to 2 μm thick (t 1  in FIG. 5) over the bump electrodes  5   a ,  5   b  and  5   c  and 7 to 8 μm thick (t 2  in FIG. 5) over the passivation film  3 . 
     Then a choline solution is spin-sprayed on the substrate structure at a 450 rpm for 30 seconds with an ordinary scan nozzle type developing apparatus using a positive resist. By so doing, the polyimide resin film  6  is wholly etched back as shown in FIG. 6 to expose top areas  7  of the bump electrodes  5   a ,  5   b  and  5   c . Through a further etch-back process the top areas  7  of the bump electrodes  5   a ,  5   b  and  5   c  are more projected than those top areas  8  of the polyimide resin film  6 . At this time, the choline solution does not substantially etch the gold but only the polyimide resin film  6  is effectively etched away. It is preferred that the etch-back time period be so set as to achieve a film thickness t 2  (FIG. 5) of about 5 to 6 μm for instance. The polyimide resin film  6  is finally solidified in an N 2  gas atmosphere at 320° C. to obtain a polyimide resin film  6  as a protective film having a thickness of 4 to 5 μm (t 3  in FIG. 6) over the passivation film  6 . At that time, if the height (t 4  in FIG. 6) of the polyimide resin film  6  surrounding the bump electrode ( 5   a ,  5   b ,  5   c ) is at least about 20% that of the bump electrode, there is no risk that the humidity resistance, as well as the heat resistance cycle property as will be set out below, will be impaired. As an upper limit, the top area  7  is more projected than the top area  8  to an extent that the lead is pushed in, taking into consideration the fact that deformation occurs at a subsequent step as will be set out below. The push-in amount of the lead, variously varying in the precision of the bonding tool, is about 4 μm on a present ordinary tool. In order to satisfy such a condition the height t 4  of the polyimide resin film  6  was set to be about 4 μm in the present embodiment. In this case, if the bump electrodes  5   a ,  5   b  and  5   c  are 18±2 μm high, the bump electrodes are surrounded with the polyimide resin film  6  at about their 20% height level and the top area  7  is about 12 to 16 μm as high as the top area  8 . 
     As shown in FIG. 7, TAB type leads  10   a ,  10   b  and  10   c are thermocompression-bonded to the bump electrodes  5   a ,  5   b  and  5   c  by applying a pressure P and heat to a bonding tool  9 . FIG. 8 is a side, cross-sectional view taken in a longitudinal direction of the lead  10   a  in FIG.  7 . The lead  10   a  as shown in FIG. 8 is arranged, as an inner lead, on a film carrier tape  11  of a polyimide resin. 
     With the aforementioned semiconductor device, even if the leads  10   a ,  10   b  and  10   c  are pushed into the bump electrodes  5   a ,  5   b  and  5   c  by the pressure P of the bonding tool  9  and deformed, it is possible to suppress occurrence of cracks in the polyimide resin film  6  because the top area  7  is more projected than the top area  8  of the polyimide resin film  6 . 
     Since the top area  7  is more projected than the top area  8 , the leads  10   a ,  10   b  and  10   c  are not contacted with the polyimide resin film  6  even under the application of that pressure and the polyimide resin film  6  is prevented from being heat-affected even upon the thermocompression bonding of the leads  10   a ,  10   b  and  10   c  to the bump electrodes. 
     According to the aforementioned embodiment, the polyimide film  6  serving as a surface protective film is composed of the same-based polyimide resin as that of the film carrier tape  11 . As one form of application, for example, the polyimide resin film  6  is formed of CRC-6061 (trade name) manufactured by Sumitomo Bakelite Co., Ltd. and the film carrier tape  11  is formed of UPILEX (trade name) manufactured by Ube Industries Ltd. 
     Since the polyimide resin film  6  and film carrier tape  11  are made of the same-based resin, a better heat resistance cycle property can be obtained in a semiconductor device. To be specific, the thermal expansion coefficient of the tape  11  and that of the resin film (surface protective film)  6  become substantially the same and, even if the tape  11  is repeatedly expanded and contracted, the resin film  6  is correspondingly expanded and contracted in repeated fashion. To put it in another way, there is almost no difference between the thermal expansion coefficient of the resin film  6  and that of the tape  11  and hence any stress is hard to involve in the resin film  6  near the bump electrodes  5   a ,  5   b  and  5   c , the passivation film  3 , the substrate  1 , etc. 
     The resin film (surface protective film)  6  is formed of the polyimide resin in an TAB-FP and hence the polyimide resin is particularly effective because it serves as a buffer member against a shearing stress the molded resin applies to the chip. 
     FIG. 9 is a graph showing the result of a TCT (thermal cycle test) carried out under the MIL Standard. In the graph of FIG. 9 the ordinate and abscissa represent the percent of defects and the number of heat cycles applied, respectively. 
     The line I in the graph of FIG. 9 denotes TAB-FP as explained in connection with the embodiment. As will be seen from the line I in FIG. 9, there are almost no defective products in TAB-FP, to which the present invention is applied, even if 1000 heat cycles are applied. The line II in the graph of FIG. 9 shows TAB-FP where no polyimide resin film  6  is formed. From the graph it will be seen that defective products tend to increase with an increase in the number of thermal cycles applied. 
     From the result of tests it has been confirmed that the heat resistance cycle property of the device is improved. The improved heat resistance cycle property obtained is probably due to the fact that (1) added strength is imparted to the bump electrodes  5   a ,  5   b  and  5   c , around which the resin film  6  is provided, and that (2) an enhanced heat compatibility is achieved through the use of substantially the same type of resin of which the resin film  6  and tape  11  are formed. 
     The improvement (1) as set out above is probably ascribable largely to the presence of the resin film  6  with which the bump electrodes  5   a ,  5   b  and  5   c  are firmly surrounded. To be specific, the polyimide resin film  6  around which the bump electrodes  5   a ,  5   b  and  5   c  are provided serves as an adequate resistance both to a residual stress resulting from a thermal expansion coefficient difference upon the bonding of the leads  10   a ,  10   b  and  10   c  and to a variation of the bump electrodes  5   a ,  5   b  and  5   c  under a bonding load. Further, the polyimide resin film  6  also serves as a resistance to a heat stress the bump electrodes  5   a ,  5   b  and  5   c  receive from the leads  10   a ,  10   b  and  10   c . Therefore, cracks are hard to produce in the oxide film (not shown) provided below the electrode pads  2   a ,  2   b  and  2   c  and in the passivation film  3 , etc., provided around the electrode pads  2   a ,  2   b  and  2   c . This improves the heat resistance cycle property of the device obtained. 
     Although the present invention has been explained as being applied to the TAB-FP structure, it is not restricted to this type of device. The present invention can also apply to a bump electrode-equipped device, such as a TCP (tape carrier package), a COB (chip on board), a COG (chip on glass) and an SOB (silicon on wafer) structure. 
     According to the present invention it is possible to obtain the following advantages in actual practice. It is only necessary to use a resin coating, a resin drying and a resin eliminating step and it is not necessary to make any large-scaled investment in a plant and equipmment and to use any photoetching step. A high yield, a low cost and an enhanced microfabrication can be achieved, according to this invention, without involving any lead-shorting problem. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.