Abstract:
A semiconductor chip packaging method ( 34 ) includes printing an adhesive ( 46 ) on a support surface ( 42 ), typically a surface of a semiconductor substrate ( 44 ), to create individual adhesive areas ( 40 ). Semiconductor chips ( 24 ) are individually placed on the individual adhesive areas thereby securing the semiconductor chips to the support surface to create first chip subassemblies ( 55 ). The semiconductor chip and the semiconductor substrate are electrically connected to create second chip subassemblies ( 62 ). At least a portion of each of at least some of the second chip subassemblies is encapsulated to create semiconductor chip packages ( 64 ). The adhesive at the individual adhesive areas is preferably a B-staged adhesive so that solvent is removed from the adhesive before the individually placing step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Application No. 60/555,058, filed Mar. 19, 2004, titled “Die to substrate attach using printed adhesive”. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to the fabrication of semiconductor packages, including single-chip and multi-chip modules.  
         [0003]     Semiconductor chip packaging process typically begins with wafer dicing, that is sawing a semiconductor wafer to separate the wafer into individual semiconductor devices or chips. Before sawing, a wafer mounting tape is typically attached to the backside of the wafer. The wafer mounting tape keeps the chips together after sawing.  
         [0004]     The semiconductor chip is typically adhered to a previously mounted chip or to the substrate with a paste (typically an epoxy paste adhesive) or a film adhesive. Generally, paste adhesives have been used more often than film adhesives. However, some multi-chip modules are more successfully fabricated using film adhesives because the thickness of adhesive film is uniform so that there is minimal or no tilt of the semiconductor chips and no fillet of adhesive encircling the semiconductor chip. Moreover, no resin is bled so that it is suitable for multi chip stacking and packages with tight design tolerances or thinner chip.  
         [0005]     In one method of fabricating a multi-chip module using film adhesive, an adhesive film is laminated directly to the backside of the semiconductor wafer and then the wafer is diced into individual semiconductor chips using conventional wafer dicing equipment. For stacking the semiconductor chips, each chip is lifted by a chip-bonding tool, which is usually mounted at the end of a pick-and-place device, and mounted onto the substrate or onto a semiconductor chip mounted previously. This method requires special film laminating equipment. However, it can shorten fabrication time and lower cost because the paste-dispensing process is not needed.  
         [0006]     After the chip mounting process, bonding pads of the chips are connected to bonding pads of the substrate with Au or Al wires during a wire bonding process to create an array of semiconductor chip devices. Finally, the semiconductor chips and their associated wires connected to the substrate are encapsulated, typically using an epoxy-molding compound, to create an array of encapsulated semiconductor devices. The molding compound protects the semiconductor devices from the external environment, such as physical shock and humidity. After encapsulation, the encapsulated devices are separated, typically using a laser saw, into individual semiconductor chip packages.  
         [0007]     See U.S. Pat. Nos. 5,776,799; 6,436,732 and 6,503,821.  
       SUMMARY  
       [0008]     A first aspect of the present invention is directed to a first semiconductor chip packaging method. Adhesive is printed on a support surface to create individual adhesive areas. The support surface may be a surface of a semiconductor substrate or a circuit-containing die or a spacer die. Semiconductor chips are individually placed on the individual adhesive areas thereby securing the semiconductor chips to the support surface to create first chip subassemblies. The first chip subassemblies comprise semiconductor substrates. The semiconductor chip and the semiconductor substrate of each of a plurality of the first chip subassemblies are electrically connected to create second chip subassemblies. At least a portion of each of at least some of the second chip subassemblies is encapsulated to create semiconductor chip packages. The adhesive at the individual adhesive areas may be B-staged, thereby removing solvent from the adhesive, before the individually placing step. The support surface may be a surface of the substrate or may be a surface of a circuit-containing die or a spacer die.  
         [0009]     A second aspect of the invention is directed to a second semiconductor chip packaging method. Adhesive is printed on a semiconductor substrate to create individual adhesive areas. The adhesive at the individual adhesive areas is B-staged, thereby removing solvent from the adhesive, before the individually placing step. Semiconductor chips are individually placed on the B-staged individual adhesive areas thereby securing the semiconductor chips to the support surface to create first chip subassemblies. The semiconductor chip and support surface of each of a plurality of the first chip subassemblies are electrically connected to create second chip subassemblies. The electrically connecting step is carried out using wires between wire bond positions on the semiconductor chip and the semiconductor substrate. At least a portion of at least some of the second chip subassemblies are encapsulated to create semiconductor chip packages. The semiconductor chips may be selected with an adhesive layer on a back side thereof and the individually placing step may include placing the semiconductor chips on the individual adhesive areas with the adhesive layer contacting the adhesive at the individual adhesive areas. Various features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a flowchart of a conventional semiconductor packaging process in which a paste is used as the semiconductor chip-bonding adhesive;  
         [0011]      FIG. 2  is a simplified view illustrating the wafer mounting tape lamination step of  FIG. 1 ;  
         [0012]      FIG. 3  is a simplified illustration of the result of the wafer dicing step of  FIG. 1 ;  
         [0013]      FIG. 4  illustrates a conventional diced semiconductor chip, or die, made according to the method of  FIG. 1 ;  
         [0014]      FIG. 5  is a flowchart of a semiconductor packaging process according to the invention;  
         [0015]      FIG. 6  illustrates a substrate, supported by a heater block, on which adhesive has been printed at spaced apart adhesive areas;  
         [0016]      FIG. 7  illustrates a die bonding tool mounting die to the B-staged printed adhesive at the adhesive areas to create first chip subassemblies;  
         [0017]      FIG. 8  is an enlarged simplified side view showing the die bonding tool of  FIG. 7  mounting die to the B-staged printed adhesive at the adhesive areas;  
         [0018]      FIG. 9  shows result of connecting wires between the die and substrate of a first chip subassembly of  FIG. 7  to create a second chip subassembly;  
         [0019]      FIG. 10  illustrates an alternative embodiment to the second chip subassembly of  FIG. 9  in which a second die has been mounted to the first die and wires have been mounted between the first and second die and the substrate to create a multiple chip subassembly;  
         [0020]     FIGS.  11  shows an alternative to the structure shown in  FIGS. 7 and 8  in which the die has a wafer backside laminated film adhesive secured thereto to provide enhanced adhesion between the die and the substrate;  
         [0021]      FIG. 12  shows result of connecting wires between the die and substrate of the structure of  FIG. 11  to create an alternative embodiment to the second chip subassembly of  FIG. 9 ;  
         [0022]      FIG. 13  shows a multiple chip subassembly similar to that of  FIG. 10  but in which each die has been mounted using a wafer backside laminated film adhesive as shown in  FIG. 11 ;  
         [0023]     and  
         [0024]      FIG. 14  illustrates a semiconductor chip package in which the second chip subassembly is encapsulated using a molding compound. 
     
    
     DETAILED DESCRIPTION  
       [0025]     The invention will now be described in further detail by reference to the drawings, which illustrate alternative embodiments of the invention. The drawings are diagrammatic, showing features of the invention and their relation to other features and structures, and are not made to scale. For improved clarity of presentation, in the FIGs. illustrating embodiments of the invention, elements corresponding to elements shown in other drawings are not all particularly renumbered, although they are all readily identifiable in all the FIGs.  
         [0026]      FIGS. 1-4  illustrate a conventional semiconductor packaging process  10  in which a wafer, not shown, is subjected to a wafer back grinding step  12  during which material is removed from the back or non-circuit side of the wafer. The ground wafer  14 , see  FIG. 2 , is then laminated to a wafer mounting tape  16  during a wafer mounting tape lamination step  17  with the second, back ground side  18  (also called backside  18 ) being adhered to the wafer mounting tape so that the first, circuit side  20  of ground wafer  14  is exposed. The wafer-dicing step  22  is carried out to create diced semiconductor chips  24 , also called die  24 . Die  24  are bonded to a substrate using a paste adhesive pursuant to the paste dispensing step  25 , semiconductor chip bonding step  26  and cure after bonding step  28  of  FIG. 1 . Thereafter the wire-bonding step  30  is carried out. Steps  25 - 30  may be repeated according to the number of layers of chips for the completed device. After the molding step  32 , which is typically carried out using an epoxy molding compound, appropriate sawing or other severing equipment is used to cut through the substrate and, when the entire substrate is covered with molding compound, the molding compound to create the individual semiconductor chip packages. It should be noted that a residue of wafer mounting tape  16  can adhere to backside  18  of die  24  after the die has been removed from the wafer mounting tape; however, such remaining adhesive is not sufficient to create an effective bond between die  24  and the substrate so that the paste adhesive is needed.  
         [0027]      FIG. 5  is a flowchart of a semiconductor packaging process  34  according to the present invention. Portions of process  34  are similar to process  10  of  FIG. 1  with like steps and elements referred to with like reference numerals.  
         [0028]     As indicated in  FIG. 5 , back grinding step  12  follows start  36 . At decision point  38 , the decision to use wafer backside lamination film adhesive is made. Depending on the answer, ground wafer  14  is, in this embodiment, either mounted for wafer dicing with mounting tape, as in step  17 , or with wafer backside lamination film adhesive, is in step  40 . One conventional wafer backside lamination film adhesive is a dielectric film adhesive available from Lintec Corporation as Lintec LE5000. An advantage of using a wafer backside lamination film adhesive instead of mounting tape will be discussed below.  
         [0029]     One of the main aspects of the invention is that the support surface  42 , see  FIG. 6 , typically a surface of a substrate  44 , has an adhesive  46  printed on the support surface at individual, spaced apart adhesive areas  48  as indicated in step  50  of  FIG. 5 . It is preferred that the printable adhesive  46  be a B-stageable adhesive. The main advantages of using B-stageable adhesives lie in their good bold line thickness and tilt control, resin bleedout/flow control, lower cost and long working life after B-staging without the disadvantages associated with film adhesive, such as higher costs and lower throughput, or the disadvantages associated with paste type adhesives, such as poor dimension control (bond line thickness), resin bleed out and flow out problems. An example of a suitable B-stageable adhesive is manufactured by Ablestik Laboratories of Rancho Dominguez, California as Able  6200 . Adhesive printing step  50  is carried out using, for example, a stencil printer such that the dimensions of adhesive  46  at adhesive areas  48  are substantially the same as or smaller than the size of die  24  to be attached on adhesive  46 . Other types of adhesive printing techniques, such as spraying, spin coating or syringe dispensing, can be used.  
         [0030]     Printed adhesive  46  is B-staged at step  52 , typically at an elevated temperature so that all the solvents, which control the rheology of the typically paste-type adhesive, are driven off. After B-staging, an essentially solid form of adhesive  46  remains. Die  24  are then bonded to adhesive areas  48  on substrate  44  in the die bonding step  54  of  FIG. 5  using a die bonding tool  53  of  FIGS. 7 and 8 . Substrate  44  is supported on a heater block  51 . This creates first chip subassemblies  55  (see  FIG. 7 ). The cure after bonding step  28 , which typically occurs at elevated pressure and temperature, then follows to promote complete adhesive cure and better bonding of die  24  to B-staged adhesive  46  and B-staged adhesive  46  to substrate  44 . When the wafer mounting tape lamination step  17  is used, see  FIG. 8 , an additional cure step  56  may be required or desirable. Additional cure step  56 , which typically involves an after press and/or autoclave, uses higher die attach pressures to promote better adhesion of die  24  to B-staged adhesive  46 . However, when step  40 , using wafer backside lamination film adhesive  57 , is followed, the adhesion between die  24  and B-staged adhesive  46  is promoted by polymer-to-polymer bonding. See  FIGS. 11 and 12 . In this case, additional cure decision point  58  may be answered in the negative so that additional cure step  56  may be skipped.  
         [0031]      FIGS. 9 and 12  illustrate wires  60  connecting bonding pads  59  on a single semiconductor chip  24  to bonding pads  61  on substrate  46  after wire bonding step  30  to create second semiconductor chip subassemblies  62 . An individual semiconductor chip package  64  is shown in  FIG. 14  after molding step  32  using an appropriate molding compound  66  and an appropriate sawing or other severing step.  
         [0032]     The invention may also be carried out with multichip packages. These multichip packages may include a combination of circuit-containing die and spacer die. The second or subsequent die  70 , see  FIGS. 10 and 13 , may be adhered using conventional adhesion techniques, such as paste type adhesives, wafer backside laminated adhesives or cut and placed film adhesives. However, the second or subsequent die  70  may also have a B-stageable adhesive printed onto, for example, the back side of a thinned wafer, after which the B-stageable adhesive is B-staged; thereafter the wafer and B-staged adhesive is appropriately mounted and diced to create individual die  70 . Die  70  to die  24  attachment can be done using conventional die attach equipment, such as available from Esec of Cham, Switzerland as Esec 2008 at an appropriately elevated pressure and temperature. Alternatively, or in addition, the wafer from which the first (or a subsequently placed) die  24  are created could have a B-stageable adhesive printed on the top or circuit side (being careful not to cover wire bonding pads  59 ,  61 ) prior to dicing.  
         [0033]     Other modification and variation can be made to the disclosed embodiments without departing from the subject of the invention as defined in following claims.  
         [0034]     Any and all patents, patent applications and printed publications referred to above are incorporated by reference.  
         [0035]     Other embodiments are within the scope of the invention.