Patent Publication Number: US-2010122456-A1

Title: Integrated Alignment and Bonding System

Description:
TECHNICAL FIELD 
     This invention relates generally to integrated circuit manufacturing processes, and more particularly to apparatuses and methods for bonding semiconductor dies onto wafers. 
     BACKGROUND 
     With the evolving of semiconductor technologies, semiconductor dies are becoming increasingly smaller. However, more functions need to be integrated into the semiconductor dies. Accordingly, the semiconductor dies need to have increasingly greater numbers of I/O pads packed into smaller areas, and the density of the I/O pads rises quickly. As a result, the packaging of the semiconductor dies becomes more difficult, which causes the yield to be adversely affected. 
     Packaging technologies can be divided into two categories. One category is typically referred to as a wafer level package (WLP), wherein dies on a wafer are packaged before they are sawed. The WLP technology has some advantages, such as a greater throughput and a lower cost. Further, less under-fill and/or molding compound are needed. However, the WLP suffers from drawbacks. As aforementioned, the sizes of the dies are becoming increasingly smaller, and the conventional WLP can only be fan-in type packages, in which the I/O pads of each die are limited to a region directly over the surface of the respective die. With the limited areas of the dies, the number of the I/O pads is limited due to the limitation of the pitch of the I/O pads. If the pitch of the pads is to be decreased to incorporate more I/O pads on a die, solder bridges may occur. Additionally, under the fixed-ball-size requirement, solder balls must have a certain size, which in turn limits the number of solder balls that can be packed on the surface of a die. 
     In the other category of packaging, dies are sawed from wafers before they are packaged onto other wafers, and only “known-good-dies” are packaged. An advantageous feature of this packaging technology is the possibility of forming fan-out chip packages, which means the I/O pads on a die can be redistributed to a greater area than the die, and hence the number of I/O pads packed on the surfaces of the dies can be increased. 
     The bonding of dies to wafers includes dielectric-to-dielectric bonding (also referred to fusion bonding) and copper-to-copper bonding.  FIG. 1  illustrates a dielectric-to-dielectric bonding scheme, in which top die  100  is bonded onto bottom die  200 , wherein bottom die  200  may be a part of a wafer. Dielectric layer  102  in top die  100  is bonded to dielectric layer  202  in bottom die  200 . In the case, top die  100  and bottom die  200  have thickness variations, when top die  100  is bonded onto bottom die  200 , one side of top die  100  may be applied with a greater force then other sides, and hence the side(s) applied with the smaller force may not be bonded properly. 
     The similar situation may also occur with copper-to-copper bonding. Referring to  FIG. 2 , top die  300  is bonded onto bottom die  400  through the bonding between bond pads  304  and  404 , which may contact each other directly, or bonded through a very thin layer of solder. It is realized that with the increasing down-scaling of integrated circuits, the gap G between dielectric layers  302  and  402  becomes increasingly smaller, and the surface total thickness variation becomes increasingly greater. This applies a stricter requirement to the uniformity in the application of the bond force. When top die  300  is bonded onto bottom die  400 , the total thickness variation may cause one side of die  300  or  400  to be thicker than other sides. One side of top die  300  may thus be applied with a greater force than other sides, and hence the side applied with the smaller force again may not be bonded properly. 
     Conventionally, the above-discussed problems were solved by performing a post-contact leveling after the top die is bonded onto the bottom wafer. However, this incurs additional process steps and longer process time, and results in the reduction in throughput. Accordingly, what is needed in the art is a bonding system and methods for the bonding with a high throughput. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a method for bonding includes providing a first die and a second die; scanning at least one of the first die and the second die to determine thickness variations of the at least one of the first die and the second die; placing the second die facing the first die with a first surface of the first die facing a second surface of the second die; aligning the first surface and the second surface parallel to each other using the thickness variations; and bonding the second die onto the first die. The step of aligning the first surface and the second surface includes tilting one of the first die and the second die. 
     In accordance with another aspect of the present invention, a method for bonding dies includes providing a bottom wafer including first dies; providing second dies; scanning the bottom wafer to determine first thickness variations of the first dies; placing the second dies in a die tray; scanning the second dies in the die tray to determine second thickness variations of the second dies; picking up one of the second dies from the die tray; and moving the one of the second dies to over one of the first dies. The method further includes tilting at least one of the bottom wafer and the one of the second dies, so that the first surface of the one of the first dies is parallel to a second surface of the one of the second dies, wherein the first surface faces the second surface. The method further includes, after the first surface and the second surface are parallel to each other, bonding the one of the second dies to the one of the first dies. 
     In accordance with yet another aspect of the present invention, a method for bonding dies includes providing a first die and a second die; placing the first die on a stage; moving the second die to face the first die; tilting at least one of the first die and the second die to make a first surface of the first die facing and parallel to a second surface of the second die; moving the second die toward the first die while keeping the first surface of the first die parallel to the second surface of the second die; and bonding the second die to the first die. 
     In accordance with yet another aspect of the present invention, an apparatus for bonding dies includes a scanning system configured to scan thickness variations of a die; a control unit connected to the scanning system, the control unit being configured to collect the thickness variations; a bond head connected to the control unit; and a stage for mounting the die thereon. The control unit is configured to control at least one of the bond head and the stage to tilt. 
     In accordance with yet another aspect of the present invention, an apparatus for bonding a first die with a second die includes a control unit; a stage for mounting a wafer thereon, wherein the wafer includes the first die; and a bond head configured to pick up the second die. The control unit is connected to and configured to tilt at least one of the bond head and the stage to make a first surface of the first die parallel to a second surface of the second die. 
     The advantageous features of the present invention include greater throughput and improved reliability in the bonding of dies onto dies or wafers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a conventional dielectric-to-dielectric bonding, wherein dies are not properly bonded due to thickness variations in dies; 
         FIG. 2  illustrates a conventional copper-to-copper bonding, wherein dies are not properly bonded due to thickness variations in dies; 
         FIGS. 3A through 6  are top views and cross-sectional views of intermediate stages in a bonding process of the present application; and 
         FIG. 7  illustrates a scanning system deployed under a wafer to be scanned. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     A novel integrated aligning and bonding system and the methods for the bonding are provided. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements. 
       FIG. 3A  schematically illustrates a part of bonding system  20 , which includes control unit  22 , stage  24 , scanning system  26  and bond head  28  (not shown in  FIG. 3A , refer to  FIG. 5A ). Preferably, stage  24 , bond head  28 , and scanning system  26  are in a controlled environment (not shown), which is capable of being filled with desirable gases including, for example, clean air, nitrogen, and/or the like. The controlled environment may also be a bonding chamber that can be vacuumed. Stage  24  may be an electro-static chuck (e-chuck), which is capable of mounting a wafer thereon, and increasing the temperature of the wafer to a desirable temperature for bonding. 
     In  FIG. 3A , bottom wafer  32  is loaded on stage  24 . Scanning system  26  is then used to scan the surface of bottom wafer  32 . In an embodiment, scanning system  26  is a laser system, which measures the distances between scanning system  26  and the scanned points on wafer  32 , so that the thicknesses of bottom wafer  32  at the scanned points may be determined. The scan may be conducted line by line or point by point, with each bottom die  34  in bottom wafer  32  having multiple points scanned.  FIG. 4  illustrates a top view of bottom die  34 . In an exemplary embodiment, bottom die  34  has three rows and three columns of points scanned. The exemplary scanned points are indicated as P 1  through P 9 , wherein each edge and each corner of bottom die  34  has at least one, and preferably more points scanned. With the thicknesses of points P 1  through P 9  of bottom die  34  being known, the thickness variations (topography) of bottom die  34  and bottom wafer  32  are known. The scanned data are stored in control unit  22  for subsequent bonding. 
       FIG. 3B  illustrates the scanning of dies  36  that are to be bonded onto bottom wafer  32 . Throughout the description, dies  36  are referred to as top dies although they may actually on the top or bottom when bonded. In an embodiment, top dies  36  are placed in die tray (or frame)  38 . Die tray  38  is designed so that the surfaces contacting top dies  36  are leveled, and hence die tray  38  will not introduce thickness variations, which may be mistakenly construed as the thickness variations of top dies  36 . Again, scanning system  26  scans top dies  36  to determine the surface topology, and hence the thickness variations of top dies  36 . In an exemplary embodiment, the surface topologies may be determined, for example, by scanning nine points on each of top dies  36 , which points are similar to what are shown in  FIG. 4 . Alternatively, scanning system  26  may perform a blanket scan line by line, and then extracting the thicknesses of dies from the blanket scan result. Top dies  36  and die tray  38  may also be placed over stage  24  to perform the scanning. The scanned data are stored in control unit  22 . 
       FIG. 5A  illustrates the bonding of one of the top dies  36  onto one of bottom dies  34 . Bond head  28  is used to pickup one of top dies  36  from die tray  38 , and move it to over bottom die  34 . It is appreciated that the thickness variations of top die  36  and/or the thickness variation of bottom die  34  results in surface  40  of die  36  and surface  42  of die  34  to be unparallel to each other. As a result, if bond head  28  moves top die  36  down straightly, one side or one corner of top die  36  may touch the respective side or corner of bottom die  34  before other sides or corners. Accordingly, the side or corner that made contact first will be applied with a greater force than other sides or corners that make contact, and hence cold joints points are not bonded together) will be formed. 
     Since the thickness variation data of top die  36  and bottom die  34  are known to control unit  22 , control unit  22  may compensate for the thickness variations in top die  36  and bottom die  34 . In an embodiment, control unit  22  controls bond head  28  to slightly tilt by a small angle α, so that surface  40  of top die  36  is aligned parallel to surface  42  of bottom die  34 . In alternative embodiments, stage  24 , instead of bond head  28 , is tilted by an angle β equal to angle α. In yet other embodiments, both stage  24  and bond head  28  are tilted to make surfaces  40  and  42  parallel to each other.  FIG. 5B  illustrates a tilted bond head  28 . It is noted that the non-uniform thickness on bottom wafer  32  and the resulting tilt angle α may have been exaggerated in order to clearly show the concept of the present invention. 
     Next, bond head  28  is moved down (with surfaces  40  and  42  parallel to each other); so that surfaces  40  of top die  36  touch surface  42  of bottom die  34 . It is appreciated that the tilting of stage  24  and/or bond head  28  may be performed any time before top die  36  touches bottom die  34 . For example, the tilting of bond head  28  may be performed simultaneously with the downward motion of bond head  28 . Due to the alignment action, all sides and corners of surfaces  40  and  42  make contact substantially simultaneously. For the bonding, a force is applied to press top die  36  and bottom die  34  against each other. With the non-uniform topology compensated for, the force applied to all sides and corners of top die  36  is substantially uniform. During the bonding, the temperature of bottom wafer  32  and/or top die  36  may be increased to desirable temperatures. The resulting structure after bonding is shown in  FIG. 6 . After the illustrated top die  36  is bonded onto bottom wafer  32 , the remaining ones of dies  36  in die tray  38  ( FIG. 3B ) are bonded one by one onto bottom wafer  32 , wherein the bonding of each of top dies  36  may adopt the above-discussed process. Since the thickness variations of all top dies  36  and bottom dies  34  are known by control unit  22 , the compensation in the thickness variations may be performed for the bonding of each of the dies. 
     In the above-discussed embodiments, die-to-wafer bonding is discussed. In alternative embodiments, die-to-die bonding may be performed. The bonding process is essentially the same as discussed in the preceding paragraphs, except bottom dies may also be scanned when placed in a corresponding die tray. In yet other embodiments, wafer-to-wafer bonding is performed, in which both the bottom wafer and the top wafer are pre-scanned, and the topology of the bottom and top wafers are used for the alignment purpose. 
       FIG. 7  illustrates an alternative embodiment, in which wafer  32  is mounted facing down, while scanning system  26  faces up to scan wafer  32 . The bonding may be performed with wafer  32  facing down. Alternatively, after the scanning, wafer  32  is placed as shown in  FIG. 3A , and then bonded. 
     The embodiments of the present invention have several advantageous features. With the thickness variation compensated for, the cold joint problem is at least reduced, and possibly substantially eliminated. By determining the surface topography of the dies and/or wafers to be bonded, the leveling and bonding may be performed at the same time, and hence there is no need to perform an additional leveling after the bonding process. The throughput is thus improved. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.