Abstract:
A transparent plate with fiducials for aligning and placing of dies on a panel with a high degree of accuracy is disclosed. The locations of the fiducials correspond to desired die locations. The transparent plate is arranged beneath the panel, with the fiducials aligned with cavities formed in the panel. The cavities have transparent bottoms. A die alignment mark on a die residing over the panel is used to establish the position of the die. Imaging a fiducial associated with a cavity is used to center the die with respect to the cavity. Accurate placement of the fiducials is accomplished by electron-beam lithography.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to integrated circuit (IC) packaging, and in particular relates to performing accurate die placement in IC packaging.  
         BACKGROUND OF THE INVENTION  
         [0002]    The manufacturing of electronic and optoelectronic integrated circuits (ICs) involves complex lithographic processes to form microscopic solid-state devices and circuits in semiconductor wafers. These lithographic processes typically include forming layers of material on the wafer, patterning the layers, doping the substrate and/or the patterned layers, and heat-treating (e.g., annealing) the resulting structures. These processes are repeated to build up the IC structure. The result is a wafer containing a large number of ICs.  
           [0003]    A “wafer sort” is then performed, wherein each IC chip on the wafer is electrically tested for functionality. The wafer is then separated (“diced”) into the individual IC chips, which are then “packaged” individually or in groups for incorporation onto a “panel,” e.g., a printed circuit board (PCB) or motherboard.  
           [0004]    The packaged dies (or “dies” for short) must be placed in specific locations on the panel to within a given accuracy so that interconnections between the dies can be successfully established. To this end, the panel includes alignment marks or “fiducials” to assist in achieving the desired placement accuracy. The dies are placed on the panel by a die placement machine, sometimes referred to as a “chip shooter.” The machine includes an optical vision system that locates and recognizes the fiducials as well as an alignment mark on the die. Information from the optical vision system relating to the position of the die relative the fiducials allows the die to be placed on the panel at a specific location.  
           [0005]    This die placement process provides a die placement accuracy no better than about 25 microns (3 sigma). The main factor limiting the accuracy of the die placement process is the error in the placement of fiducials on the panel. To date, a placement accuracy of 25 microns (3 sigma) has been satisfactory for most die placement applications. However, for certain new packaging applications, a die placement with much greater accuracy (e.g., 2 microns, 3 sigma) is required. For example, in bump lithography, once the dies are mounted to the panel, further lithography steps are carried out. In particular, a print solder resist layer is deposited, and then plate metal (e.g., copper) traces are formed to establish the electrical connections between the dies. Without highly accurate die placement on the panel, the subsequent lithography steps cannot be successfully performed.  
           [0006]    Accordingly, what is needed is a die placement apparatus and method that provides for greater die placement accuracy. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a combination cross-sectional and schematic view of a die placement apparatus according to the present invention, illustrating the use of the transparent plate with fiducials of the present invention;  
         [0008]    [0008]FIG. 2 is a face-on view of an example panel used in the die placement apparatus of FIG. 1;  
         [0009]    [0009]FIG. 3 is a face-on view of the transparent plate with fiducials of the present invention, illustrating an arrangement of fiducials on the upper surface;  
         [0010]    [0010]FIG. 4 is a face-on view of the panel of FIG. 2, with the transparent plate of FIG. 3 arranged beneath the panel such that the fiducials are aligned with the panel cavities;  
         [0011]    [0011]FIG. 5 is the apparatus of FIG. 1, shown placing a die into a select cavity in the panel and onto the bottom transparent surface; and  
         [0012]    [0012]FIG. 6 is the same as FIG. 5, but with the pick-up head retracted and the gaps in the cavity between the die and cavity walls filled with epoxy. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0014]    [0014]FIG. 1 shows a die placement apparatus  10  that includes a pick-up head  20  capable of holding, positioning and releasing a die  24 . Die  24  includes an alignment mark  26 . The die alignment mark may be any shape or size as long as it can be imaged by an optical vision system. Further, the die alignment mark need not be specialized marks, i.e., it can be a feature or other indicia that may not have been originally intended as an alignment mark. Also, a die alignment mark can consist of one or more identical or different features. Pick-up head  20  is electrically connected to a controller  30  that controls the movement (as indicated by arrows  34 ) and operation of the pick-up head.  
         [0015]    Apparatus  10  also includes a panel support system  40  with support arms  42  capable of movably supporting a thin rectangular die-bearing panel  50 . Support arms  42  support the panel at panel edges  52  and hold the panel in a panel support plane  54  beneath the pick-up head. Panel support system  40  is also electrically connected to controller  30 , which controls the movement of support arms  42  and thus the movement of the panel.  
         [0016]    With reference also to FIG. 2, panel  50  includes opposing upper and lower surfaces  62  and  64  and a number of cavities  70  formed in the upper surface. Each cavity includes walls  72  and a bottom  74 . The latter is formed by a portion of an upper surface  76  of a transparent segment  80  attached to bottom surface  64 . In an example embodiment, transparent segments  80  are formed from an adhesive member, such as adhesive tape, so that cavity bottom  74  is adhesive. Transparent segment  80  serves as a window at the bottom of the cavity.  
         [0017]    In an example embodiment, panel  50  is about 340 mm by 255 mm and includes a 5×7 array of square cavities  70  each 25 mm on a side. The positions of the cavities are accurate to about +/−0.1 mm on-center. However, the cavities are made larger enough (e.g., 1 to 2 mm wider than the width of the die) so that the accuracy of the placement of dies within the cavities is not limited by the cavity position error.  
         [0018]    Apparatus  10  also includes transparent plate support member  110  capable of supporting a transparent plate  120  directly beneath panel  50 . Transparent plate  120  has opposing upper and lower surfaces  122  and  124 . In an example embodiment, the separation between panel lower surface  64  and transparent plate upper surface  122  is anywhere between about 1 mm to about 3 mm. In an example embodiment, transparent plate  120  is made of glass. In a further example embodiment, the glass is quartz. In another example embodiment, the plate is a standard photolithography mask blank.  
         [0019]    [0019]FIG. 3 is a face-on view of transparent plate  120 . The transparent plate includes an array of fiducials  130 . Generally, the fiducials can be any shape and size, as long as they can be detected by an optical vision system. In an example embodiment, fiducials  130  are formed from an opaque material, such as chrome. In one example embodiment, a fiducial is a single mark or feature. In another example embodiment, a fiducial is made up of two or more features, such as for example a dot  132  and a square  134  as illustrated in FIG. 3. In yet another example embodiment, the fiducials have a spacing equal to twice that of the cavity width so that the fiducials can be aligned to the center of each cavity.  
         [0020]    Further in an example embodiment, the fiducials are formed using the same or similar techniques used to pattern photolithographic masks. One such technique utilizes electron-beam lithography, which can provide highly accurate placement of the fiducials, i.e., to about 1 to 2 microns (3 sigma). The fiducials are spaced apart to correspond to desired locations of dies within cavities  70  of panel  50 .  
         [0021]    Apparatus  10  further includes an optical vision system  150  arranged adjacent transparent plate lower surface  124  and electrically connected to controller  30 . In an example embodiment, optical vision system  150  is positioned along an axis A 0  joining the optical vision system with pick-up head  20 , and moves along with the pick-up head in the x-y plane, as indicated by arrows  152 . Optical vision system  150  is adapted to simultaneously image at least one of fiducials  130  and die alignment mark  26 , the latter being imaged through transparent segment  80 . The optical vision system  150  is further adapted to provide an electrical signal  156  to controller  30 . The electrical signal corresponds to the position of the die relative to the fiducial corresponding to the cavity through which the die alignment mark is imaged. The fiducial corresponding to a select cavity is referred to herein as a “local fiducial.” 
         [0022]    In another example embodiment, apparatus  10  includes an optical vision system  160  that is attached to or that moves with pick-up head  20 . Optical vision system is arranged so that it can image the fiducials through cavities  70 .  
         [0023]    Method of Operation  
         [0024]    With continuing reference to FIG. 1, the method of accurately placing dies within the panel proceeds as follows. First, transparent plate  120  is arranged in transparent plate support member  40  of apparatus  10  at a set location. Panel  50  is then moved so that each fiducial  130  is aligned with a corresponding cavity  70 , thereby establishing local fiducials for each cavity (FIG. 4). In an example embodiment, the local fiducials are aligned to the center of each cavity.  
         [0025]    Pick-up head  20  then picks up a die from a die bin  200  that contains a number of dies to be placed onto the panel. The pick-up head moves until optical vision system  150  views the die alignment mark through a select transparent cavity bottom (i.e., transparent segment  80 ) while also viewing the local fiducial through the transparent plate.  
         [0026]    Alternatively, the pick-up head moves until optical vision system  160  views one of the local fiducials. In this embodiment, the alignment mark on the die is at a known distance from optical vision system  160  so that determining the location of the local fiducial with optical vision system  160  provides the needed information for centering the die to the cavity center.  
         [0027]    Optical vision system  150  images the local fiducial and the alignment mark and generates electrical signal  156  corresponding to the position of the die alignment mark relative to the local fiducial. The electrical signal is provided to controller  30 , which moves the pick-up head until proper alignment of the die with respect to the local fiducial (and hence the select cavity) is attained.  
         [0028]    Once alignment of the die relative to the cavity is attained, controller  30  instructs the pick-up head to move toward the panel. The pick-up head then places the die on bottom surface  74  of the select cavity as illustrated in FIG. 5. As mentioned above, the bottom surface is adhesive in an example embodiment, so that the die is fixed in place. The pick-up head then releases the die, as shown in FIG. 6. The above process is repeated until each cavity designated to receive a die has received a die.  
         [0029]    An encapsulation step is then performed wherein an epoxy  220  is used to fill the gap  230  (FIG. 5) between each die  24  and cavity walls  72  (FIG. 6). When the epoxy cures, the dies are fixed within the cavities at very accurate positions relative to one another. In example embodiment, the placement accuracy of the dies is 2 microns (3 sigma) or less. Thus, further lithography steps requiring accurate alignment of the dies can be carried out. Such lithographic steps include steps for bump lithography, which includes forming a print solder resist layer and selectively exposing and processing the resist layer to form plate metal traces to electrically connect the dies.  
         [0030]    While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.