Abstract:
A highly reliable stack type semiconductor package, which does not have a problem of interconnection areas becoming disconnected due to thermal expansion. The semiconductor package includes a second die adhesive, which is formed between a first semiconductor chip and a second semiconductor chip, applied to the upper surface of the first semiconductor chip, and extends to the wire forming units. The second die adhesive is selected to have a bulk modulus greater than 1 GPa to prevent electric disconnection due to breakage of wires in the stack type semiconductor package during thermal stress.

Description:
[0001]     This application claims the priority of Korean Patent Application No. 2003-84732, filed on Nov. 26, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor package, and more particularly, to a stack type semiconductor package in which a plurality of semiconductor chips are mounted.  
         [0004]     2. Description of the Related Art  
         [0005]     Semiconductor manufacturers have developed methods to increase integration and reduce the size of semiconductor devices. However, since research has to be carried out and investment in equipment must be made to increase integration of the semiconductor devices, the overall manufacturing cost of the semiconductor devices increases. For example, for manufacturing semiconductor memory devices, a large number of technical problems must be solved in the wafer manufacturing process and new equipment must be developed to increase from 64 MDRAM to 256 MDRAM.  
         [0006]     The development of semiconductor packages has provided a method of increasing the integrity without requiring technical development and investment in equipment since a semiconductor package includes a plurality of semiconductor chips without increased integration. Manufacturing the semiconductor package by mounting a plurality of semiconductor chips requires less effort to increase integration than to increase integration during the wafer manufacturing process. For example, it is possible to manufacture a 256 MDRAM by assembling a semiconductor package that includes four 64 MDRAM semiconductor chips.  
         [0007]     Initially, methods of manufacturing a semiconductor package included horizontally arranging the semiconductor chips so that the size of the semiconductor package was not reduced. However, most multi-chip type semiconductor packages are now manufactured by vertically arranging the semiconductor chips.  
         [0008]     Micron Technology, Inc. developed a method of manufacturing a semiconductor package by vertically stacking single semiconductor chips, which is included in U.S. Pat. No. 6,569,709, entitled “Assemblies Including Stacked Semiconductor Devices Separated a Distance Defined by Adhesive Material Interposed Therebetween, Packages Including the Assembly, and Method”.  
         [0009]      FIGS. 1 and 2  are sectional views of conventional stack type semiconductor packages.  
         [0010]     Referring to  FIG. 1 , a sectional view of a ball grid array (BGA) package  10  using solder balls  14  as external connection terminals is shown. Here, first and second semiconductor chips  30   a  and  30   b  are vertically stacked on a substrate  20  using a conventional die adhesive  36 . To manufacture the BGA package  10 , the first semiconductor chip  30   a  is mounted on the substrate  20  using an adhesive tape  26 , and bond pads  34  on the first semiconductor chip  30   a  are electrically connected by first wires  38   a  to bond fingers which are contact units  24  on the substrate  20 . Thereafter, the conventional die adhesive  36  is sprayed, and the second semiconductor chip  30   b  is adhered to the conventional die adhesive  36 . Then, the bond pads  34  on the second semiconductor chip  30   b  and the contact units  24  on the substrate are connected by second wires  38   b . Finally, the resultant structure is sealed using an epoxy mold compound (EMC) as a sealing resin  40 .  
         [0011]     Conventionally, when the sizes of the first semiconductor chip  30   a  and the second semiconductor chip  30   b  are the same, the first semiconductor chip  30   a  and the second semiconductor chip  30   b  are adhered using the die adhesive  36 , which has a bulk modulus less than 1 GPa. However, since the die adhesive  36  covers the interconnection areas of the first wires  38   a  on the first semiconductor chip  30   a , the reliability of the BGA package  10  is lowered, as explained below.  
         [0012]     Since the coefficients of thermal expansion of the die adhesive  36 , the first wires  38   a , and the first and second semiconductor chips  30   a  and  30   b  are different, the reliability is lowered when the temperature of electric equipment included in the BGA package  10  changes. Thus, the first wires  38   a  are broken at the bond pads on which the first wires  38   a  are connected to the first semiconductor chip  30   a . When the first wires  38   a  break, electrical connections are broken, and the BGA package  10  cannot operate properly.  
         [0013]     A temperature cycle test is a test for determining the reliability of a semiconductor package. In the test, the temperature of the semiconductor package fluctuates between a temperature of −55° C. and 125° C. during a time span of  30  minutes a predetermined number of times. As a result, the operation of the semiconductor package over a range of temperatures is determined.  
         [0014]     In a study of 126 BGA packages with the die adhesive having a bulk modulus less than 1 GPa, when the 126 units of BGA packages were temperature cycle tested 150 times, none of the BGA packages failed, 2 units failed after 300 times of temperature cycle tests, 13 units of BGA packages failed after 600 times of temperature cycle tests, and  56  units of BGA packages failed after 1,000 times of temperature cycle test.  
         [0015]     Semiconductor packages to be used in special situations, such as space engineering or military operations, should not fail, even when the temperature cycle test is performed more than 1,000 times. However, about 46% of the BGA packages using the conventional die adhesive failed after 1000 times. Accordingly, the BGA package using the conventional die adhesive cannot be used in situations in which the temperature has large fluctuations.  
         [0016]     Referring to  FIG. 2 , in order to improve the reliability of the BGA package  10 , the die adhesive  36  is not extended to the first wire interconnection areas, which is denoted by A in  FIG. 2 . Instead, the first wire interconnection areas are filled with the sealing resin  40 , such as the EMC, which has excellent adhesive strength and hardness. However, in this case, it is difficult to precisely control the amount, the viscosity, and the expansion of the die adhesive  36  on the first semiconductor chip. Accordingly, additional processes are required, and it is difficult to manufacture the BGA package  10 ′. Furthermore, when the bond pads are formed at the center of the semiconductor chip, as illustrated in  FIG. 3 , it is difficult to apply the die adhesive while avoiding the first wire interconnection areas.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention provides a highly reliable stack type semiconductor package which prevents electric disconnection at wire interconnection areas.  
         [0018]     According to an aspect of the present invention, there is provided a highly reliable stack type semiconductor package, comprising a basis frame of the semiconductor package, a first semiconductor chip mounted on the basis frame by using a first die adhesive, first wires, which connect bond pads on the first semiconductor chip to contact units on the basis frame, a second die adhesive having a bulk modulus greater than 1 GPa, which is formed on the first semiconductor chip having the first wires while being expanded to the edges of the first semiconductor chip, a second semiconductor chip attached to the first semiconductor chip by using the second die adhesive, second wires, which connect bond pads on the second semiconductor chip to the contact units on the basis frame, and a sealing portion, which seals the upper portion of the basis frame on which the second semiconductor chip and the second wires are formed.  
         [0019]     The basis frame may be one selected from a lead frame and a printed circuit board, and the first semiconductor chip may be one selected from a semiconductor chip on which bond pads are formed at the center and a semiconductor chip on which bond pads are formed at the edges.  
         [0020]     The type of the stack type semiconductor package may be one selected from a small outline package (SOP), a quad flat package (QFP), a ball grid array (BGA) package, and a chip scale package (CSP), and the stack type semiconductor package may further include a third semiconductor chip mounted on the second semiconductor chip while having the same structure as the second semiconductor chip. In addition, the stack type semiconductor package may further include a heat sink, which efficiently radiates heat to the outside.  
         [0021]     According to another aspect of the present invention, there is provided a stack type semiconductor package having high reliability comprising a substrate used as a basis frame of the semiconductor package; a first semiconductor chip mounted on the substrate by using a first die adhesive; first wires, which connect bond pads on the first semiconductor chip to contact units on the substrate; a second die adhesive having the bulk modulus greater than 1 GPa, which covers first wire interconnection areas on the first semiconductor chip; a third die adhesive, which completely covers the surface of the first semiconductor chip on which the second die adhesive is coated, while having a height greater than the height of the first wires; a second semiconductor chip mounted on the first semiconductor chip by using the third die adhesive; second wires, which connect bond pads on the second semiconductor chip to contact units on the substrate; and a sealing resin, which completely seals the second wires and the second semiconductor chip on the substrate.  
         [0022]     The size of the second semiconductor chip may be the same as or greater than that of the first semiconductor chip. In addition, the bulk modulus of the second die adhesive may be measured at a temperature of  0 C.  
         [0023]     Accordingly, the stack type semiconductor package has high reliability by using the die adhesive with a bulk modulus greater than 1 GPa, so that the first wires are prevented from being electrically disconnected, even during extreme temperature changes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0025]      FIGS. 1 and 2  are sectional views of a conventional stack type semiconductor package;  
         [0026]      FIG. 3  is a sectional view of a stack type semiconductor package according to a first embodiment of the present invention;  
         [0027]      FIG. 4  is a sectional view of a stack type semiconductor package according to a second embodiment of the present invention;  
         [0028]      FIG. 5A  is a plan view of a semiconductor chip included in the stack type semiconductor package of  FIG. 3 ;  
         [0029]      FIG. 5B  is a plan view of a semiconductor chip included in the stack type semiconductor package of  FIG. 4 ;  
         [0030]      FIG. 6  is a sectional view of a stack type semiconductor package according to a third embodiment of the present invention;  
         [0031]      FIG. 7  is a sectional view of a stack type semiconductor package according to a fourth embodiment of the present invention;  
         [0032]      FIG. 8  is a sectional view of a stack type semiconductor package according to a fifth embodiment of the present invention; and  
         [0033]      FIG. 9  is a sectional view of a stack type semiconductor package according to a sixth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.  
         [0035]      FIG. 3  is a sectional view of a stack type semiconductor package  100 A according to a first embodiment of the present invention.  
         [0036]     Referring to  FIG. 3 , a stack type semiconductor package  100 A includes a basis frame  102 , which is formed of a lead frame and a substrate, a first semiconductor chip  120   a , first wires  110   a , a second die adhesive  140 , a second semiconductor chip  120   b , second wires  110   b , and a sealing portion  130 . The first semiconductor chip  120   a  is mounted on the basis frame  102  using a first die adhesive  106 . The first wires  110   a  connect bond pads  122 , which are formed near the center of the first semiconductor chip  120   a , with contact units  104  on the basis frame  102 . The second die adhesive  140  is formed on the first semiconductor chip  120   a  on which the first wires  110   a  are formed, and the second die adhesive  140  is expanded to the edges of the first semiconductor chip  120   a . Here, the bulk modulus of the second die adhesive  140  is greater than 1 GPa. The second semiconductor chip  120   b  is mounted on the first semiconductor chip  120   a  using the second die adhesive  140 . The second wires  110   b  connect bond pads  124  on the second semiconductor chip  120   b  with the contact units  104  on the basis frame  102 . The sealing portion  130  seals the second semiconductor chip  120   b  and the second wires  110   b  on the upper surface of the basis frame  102 .  
         [0037]     The semiconductor package  100 A can be used as a small outline package (SOP), a quad flat package (QFP), and a chip scale package (CSP) as well as a BGA package that uses solder balls  150  as external connection terminals.  
         [0038]     The basis frame  102  of the semiconductor package can be a lead frame or a printed circuit board (PCB). In addition, the basis frame  102  can be a substrate used in the BGA package, which is either a flexible substrate including circuit patterns that is made of polyimide or a rigid substrate including circuit patterns that is made of FR-4 resin. Adhesive tapes or an epoxy may be used as the first die adhesive  106 . The first and second wires  110   a  and  110   b  are ball bonded to the bond pads  122 ,  124  of the first and second semiconductor chips  120   a  and  120   b  and are stitch bonded to the contact units  104  on the basis frame  102 . However, the first and second wires  110   a  and  110   b  may also be stitch bonded to the bond pads  122 ,  124  of the first and second semiconductor chips  120   a  and  120   b , respectively, and ball bonded to the contact units  104  on the basis frame  102 .  
         [0039]     The bulk modulus of the second die adhesive  140  is greater than 1 GPa at a temperature of 0° C., and the second die adhesive  140  is expanded to the edges of the first semiconductor chip  120   a  to fill the interconnection areas of the first wires  110   a . Here, the bulk modulus is the value representing the coefficient of elasticity against tensile force. In addition, the modulus characteristic represents the ratio of tensile force to transformation.  
         [0040]     If the second die adhesive was made of the same material as the die adhesive included in the conventional semiconductor package which has a bulk modulus less than 1 GPa, the second die adhesive  140  could not absorb the stress caused by thermal expansion and thermal contraction of the first wires  110   a , the second die adhesive  140 , and the first and second semiconductor chips  120   a  and  120   b . However, the second die adhesive  140  used in the first embodiment has a bulk modulus greater than 1 GPa, and sufficiently absorbs the stress. Accordingly, the first wires  110   a  are not removed from the bond pads  122  of the first semiconductor chip  120   a  when the temperature fluctuates.  
         [0041]     It is preferable that the size of the second semiconductor chip  120   b  is the same as or greater than the size of the first semiconductor chip  120   a . The sealing portion  130  can be substituted by a ceramic, an encapsulant, or a metal cap instead of the epoxy mold compound (EMC), which can seal the substrate  102  on which the second semiconductor chip  120   b  and the second wires  110   b  are formed. Thus, even if the bond pads  122  on the first semiconductor chip  120   a  are formed near the center of the first semiconductor chip  120   a , the first and second semiconductor chips  120   a  and  120   b  can be easily stacked.  
         [0042]      FIG. 4  is a sectional view of a stack type semiconductor package  100 B according to a second embodiment of the present invention.  
         [0043]     Referring to  FIG. 4 , the semiconductor package  100 B is similar to the semiconductor package  100 A, except that bond pads  122 ′ are formed at the edges of a first semiconductor chip  120   a ′. Accordingly, further description of the semiconductor package  100 B will be omitted.  
         [0044]      FIGS. 5A and 5B  are plan views of the semiconductor chips used in the semiconductor packages  100 A and  100 B of  FIGS. 3 and 4 , respectively.  
         [0045]     Referring to  FIG. 5A , the semiconductor chip  120   a  includes the bond pads  122  disposed near the center. In  FIG. 5B , the semiconductor chip  120   a ′ includes the bond pads  122 ′ near the edges. Both the semiconductor chips  120   a  and  120   a ′ include an active region on which circuits are formed.  
         [0046]      FIG. 6  is a sectional view of a stack type semiconductor package  100 C according to a third embodiment of the present invention.  
         [0047]     Referring to  FIG. 6 , the semiconductor package  100 C additionally includes a heat sink  160 , which is not included in the semiconductor package  100 B, below the first die adhesive  106  in order to efficiently extract heat from the first and second semiconductor chips  120   a ′ and  120   b . The material included in, the location of, and the shape of the heat sink  160  can be varied.  
         [0048]      FIG. 7  is a sectional view of a stack type semiconductor package  100 D according to a fourth embodiment of the present invention.  
         [0049]     Referring to  FIG. 7 , the semiconductor package  100 D is identical to the semiconductor package  100 B, except that the semiconductor package  100 D further includes a third semiconductor chip  120   c . The third semiconductor chip  120   c  is stacked by the same method as the second semiconductor chip  120   b . Only three semiconductor chips,  120   a , 120   b , and  120   c , are stacked in the semiconductor package  100 D, but the number of the semiconductor chips can be greater.  
         [0050]      FIG. 8  is a sectional view of a stack type semiconductor package according to a fifth embodiment of the present invention.  
         [0051]     Referring to  FIG. 8 , the semiconductor package  100 E is an SOP type semiconductor package. Accordingly, a lead frame  102  that includes a die pad  164  and a lead  162  is used as a basis frame. The remaining structure, including the mounted first and second semiconductor chips  120   a  and  120   b , the first and second wires  110   a  and  110   b , and the sealing of the first and second semiconductor chips  120   a  and  120   b  and the first and second wires  110   a  and  110   b  using the sealing portion  130  is the same as that in the semiconductor package  100 B. The structure of the semiconductor package  100 B can be applied to a QFP or a CSP semiconductor package.  
         [0052]      FIG. 9  is a sectional view of a stack type semiconductor package  200  according to a sixth embodiment of the present invention.  
         [0053]     Referring to  FIG. 9 , the stack type semiconductor package  200  includes a substrate  202 , a first semiconductor chip  220   a , first wires  210   a , a second die adhesive  240 , a third die adhesive  270 , a second semiconductor chip  220   b , second wires  210   b , and a sealing resin  230 . Here, the substrate  202  is used as the basis frame of the semiconductor package  200 . The first semiconductor chip  220   a  is mounted on the substrate  202  using a first die adhesive  206 . The first wires  210   a  connect bond pads  222  on the first semiconductor chip  220   a  with contact units  204  on the substrate  202 . The second die adhesive  240  has a bulk modulus greater than 1 GPa and covers the interconnection areas of the first wires  210   a  on the first semiconductor chips  220   a . The third die adhesive  270  completely covers portions of the first semiconductor chip  220   a  on which the second die adhesive  240  is not coated, with the height of the third die adhesive  270  greater than the height of the first wires  210   a . The second semiconductor chip  220   b  is stacked on the first semiconductor chip  220   a  using the first and the third die adhesives  240 ,  270 . The second wires  210   b  connect the bond pads  224  on the second semiconductor chip  220   b  with the contact units  204  on the substrate  202 . The sealing resin  230  seals the second wires  210   b  and the second semiconductor chip  220   b  onto the substrate  202 .  
         [0054]     In the semiconductor package  200 , the second die adhesive  240  with the bulk modulus greater than 1 GPa, which prevents the first wires from breaking, is applied to the first wire interconnection areas while not being applied to the entire surface of the first semiconductor chip  220   a . Here, the height of the second die adhesive  240  should be such that the interconnection areas of the first wires  210   a  (i.e., ball bonds) are covered. In addition, a die adhesive with a bulk modulus less than 1 GPa can be used as the third die adhesive  270 .  
         [0055]     The substrate  202  may be formed by a flexible substrate or a rigid substrate. The bond pads  222  may be formed at the center of the first semiconductor chip  220   a  or at the edges of the first semiconductor chip  220   a , as shown in  FIGS. 5A and 5B . It is preferable that the size of the second semiconductor chip  220   b  is the same as or greater than the size of the first semiconductor chip  220   a . The sealing resin  230  can be a ceramic, an encapsulant, or a metal cap, as well as the EMC. The semiconductor package  200  may further include a heat sink, as included in the semiconductor package  100 C and may include a third semiconductor chip, as included in the semiconductor package  100 D. In addition, the semiconductor package  200  may be part of a SOP, QFP or CSP package as shown in the fifth embodiment of the present invention, instead of the BGA package. It is preferable that the bulk modulus of the second die adhesive  240  be measured at a temperature of 0° C. The stack type semiconductor package  200  may include solder balls  250 , which are attached to the lower portion of the substrate  202 , as external connection terminals.  
         [0056]     In order to determine the effectiveness of the semiconductor package according to the embodiments, the BGA package  100 B according to the second embodiment was used as a sample in a temperature cycle test. The conditions of the temperature cycle test were the same as the conditions of the temperature cycle test described in connection with the conventional semiconductor package.  
         [0057]     The test performed on the BGA package  100 B found that no defects were detected, even when temperature fluctuated between extreme temperatures 150 times, 300 times, 600 times, and 1,000 times.  
         [0058]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the  25  spirit and scope of the present invention as defined by the following claims.