Abstract:
An apparatus and method bond a first wafer to a second wafer. The apparatus includes a first pressure application device configured to apply pressure at a central region of the first wafer in a direction toward the second wafer to initiate a bonding process between the first wafer and the second wafer. The apparatus also includes one or more second pressure application devices configured to apply pressure between the central region and an outer edge of the first wafer to complete the bonding process. The one or more second pressure application devices apply pressure on the first wafer after the first pressure application device has initiated the bonding process and while the first pressure application device continues to apply pressure at the central region. A controller controls the first pressure application device and the one or more second pressure application devices.

Description:
BACKGROUND 
     The present invention relates to three-dimensional (3D) device packaging fabrication, and more specifically, to wafer to wafer bonding. 
     Three-dimensional (3D) integrated circuits are chips with two or more layers of active electronic components integrated both vertically and horizontally. This stacking of components (wafers) can reduce cycle time, increase frequency and chip performance, allow more active components to be placed per unit volume, and require fewer pins to communicate among the layers, thereby simplifying the packaging. However, successful 3D integration depends on the strength of the bond between the wafers, because subsequent grinding and chemical mechanical planarization (CMP) steps put stress on the bonds. 
     Existing wafer bonding techniques apply pressure on the top or bottom or both wafers to press them together. One previous technique involves the application of pressure at the center of the top wafer, pushing it down onto the bottom wafer. Another involves applying pressure at the center of both the top and bottom wafers. Yet another involves applying uniform pressure (several pressure points along the surface) to the top and bottom. However, these techniques leave voids (trapped air) between the wafers that represent areas of incomplete bonding. This incomplete bonding increases the vulnerability of the 3D IC to high mechanical stresses. 
     SUMMARY 
     According to one embodiment, an apparatus to bond a first wafer to a second wafer includes a first pressure application device configured to apply pressure at a central region of the first wafer in a direction toward the second wafer to initiate a bonding process between the first wafer and the second wafer; one or more second pressure application devices configured to apply pressure between the central region and an outer edge of the first wafer to complete the bonding process, wherein the one or more second pressure application devices apply pressure on the first wafer after the first pressure application device has initiated the bonding process and while the first pressure application device continues to apply pressure at the central region; and a controller configured to control the first pressure application device and the one or more second pressure application devices. 
     According to another embodiment, a method of bonding a first wafer to a second wafer includes applying pressure, with a first pressure application device, at a central region of the first wafer in a direction toward the second wafer to initiate a bonding process between the first wafer and the second wafer, and applying pressure, with one or more second pressure application devices, between the central region and an outer edge of the first wafer to complete the bonding process, wherein applying pressure with the one or more second pressure application devices is after bond initiation with the first pressure application device and applying pressure with the first pressure application device continues at the central region during the applying pressure with the one or more second pressure application devices. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts a wafer bonding apparatus prior to initiation of wafer bonding according to an embodiment; 
         FIG. 2  depicts the wafer bonding apparatus of  FIG. 1  at a start of a wafer bonding process; 
         FIG. 3  depicts the wafer bonding apparatus of  FIG. 1  during the wafer bonding process; 
         FIG. 4  depicts the wafer bonding apparatus of  FIG. 1  during the wafer bonding process; 
         FIG. 5  depicts the wafer bonding apparatus of  FIG. 1  during the wafer bonding process; 
         FIG. 6  depicts the wafer bonding apparatus of  FIG. 1  at a completion of the wafer bonding process. 
         FIG. 7  depicts a wafer bonding apparatus prior to initiation of wafer bonding according to another embodiment; 
         FIG. 8  depicts the wafer bonding apparatus of  FIG. 7  at a start of a wafer bonding process; 
         FIG. 9  depicts the wafer bonding apparatus of  FIG. 7  during the wafer bonding process; 
         FIG. 10  depicts the wafer bonding apparatus of  FIG. 7  during the wafer bonding process; 
         FIG. 11  depicts a perspective top view of the wafer bonding apparatus of  FIG. 7 ; 
         FIG. 12  depicts a perspective top view of the wafer bonding apparatus of  FIG. 7  at a completion of the wafer bonding process; 
         FIG. 13  depicts a wafer bonding apparatus prior to initiation of wafer bonding according to an embodiment; 
         FIG. 14  depicts the wafer bonding apparatus of  FIG. 13  at a start of a wafer bonding process; 
         FIG. 15  depicts the wafer bonding apparatus of  FIG. 13  during the wafer bonding process; 
         FIG. 16  depicts the wafer bonding apparatus of  FIG. 13  during the wafer bonding process; 
         FIG. 17  depicts a perspective top view of the wafer bonding apparatus of  FIG. 13  during the wafer bonding process; 
         FIG. 18  depicts the wafer bonding apparatus of  FIG. 13  during the wafer bonding process; 
         FIG. 19  depicts the wafer bonding apparatus of  FIG. 13  at a completion of the wafer bonding process; 
         FIG. 20  depicts a perspective top view of the wafer bonding apparatus of  FIG. 13  at the completion of the wafer bonding process; 
         FIG. 21  depicts a wafer bonding apparatus prior to initiation of wafer bonding according to an embodiment; 
         FIG. 22  depicts the wafer bonding apparatus of  FIG. 21  at a start of a wafer bonding process; 
         FIG. 23  depicts the wafer bonding apparatus of  FIG. 21  during the wafer bonding process; 
         FIG. 24  depicts the wafer bonding apparatus of  FIG. 21  during the wafer bonding process; 
         FIG. 25  depicts the wafer bonding apparatus of  FIG. 21  during the wafer bonding process; 
         FIG. 26  depicts the wafer bonding apparatus of  FIG. 21  at a completion of the wafer bonding process; 
         FIG. 27  depicts a wafer bonding apparatus prior to initiation of wafer bonding according to an embodiment; 
         FIG. 28  depicts the wafer bonding apparatus of  FIG. 27  at a start of a wafer bonding process; 
         FIG. 29  depicts the wafer bonding apparatus of  FIG. 27  during the wafer bonding process; 
         FIG. 30  depicts the wafer bonding apparatus of  FIG. 27  during the wafer bonding process; 
         FIG. 31  depicts a perspective top view of the wafer bonding apparatus of  FIG. 27  during the wafer bonding process; 
         FIG. 32  depicts the wafer bonding apparatus of  FIG. 27  during the wafer bonding process; 
         FIG. 33  depicts a perspective top view of the wafer bonding apparatus of  FIG. 27  during the wafer bonding process; 
         FIG. 34  depicts the wafer bonding apparatus of  FIG. 27  at a completion of the wafer bonding process; 
         FIG. 35  depicts a perspective top view of a chuck used in wafer bonding according to an embodiment; 
         FIG. 36  depicts a perspective side view of the chuck shown in  FIG. 35  according to an embodiment; 
         FIG. 37  depicts a chuck with a heating coil according to an embodiment; 
         FIG. 38  depicts a chuck with heating rings and spikes according to an embodiment; 
         FIG. 39  depicts a chuck with heating rings and spikes according to another embodiment; 
         FIG. 40  depicts a chuck with randomly arranged heating elements according to an embodiment; 
         FIG. 41  depicts a chuck with randomly arranged heating elements according to another embodiment; 
         FIG. 42  depicts processes involved in a wafer bonding process according to various embodiments; and 
         FIG. 43  is a block diagram of a wafer bonding system according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Incomplete bonding (voids) between wafers of a 3D IC creates vulnerabilities to mechanical stresses. To address the problems created by incomplete bonding, several different embodiments of wafer bonding apparatus and techniques are detailed below. 
       FIGS. 1-6  depict a wafer bonding apparatus  100  at various stages of a wafer bonding process according to an embodiment. The apparatus  100  includes a pin  110  surrounded by concentric rings  120 . The pin  110 , disposed substantially near a center of a first wafer (not shown) (top wafer in the exemplary arrangement of  FIGS. 1-6 ), is first lowered ( FIG. 2 ) to apply pressure on the top wafer. The pin  110  applies force to the top wafer for single point bond initiation with another wafer (not shown) (bottom wafer in the exemplary arrangement of  FIGS. 1-6 ) beneath the wafer being pushed by the pin  110  shown in  FIGS. 1-6 . Each of the concentric rings  120  is then lowered, in turn, starting from the closest to the center to the farthest from the center ( FIGS. 3-6 ), to apply pressure sequentially on the top wafer. Each of the concentric rings  120  applies uniform pressure at a given radial distance from the center of the top wafer. In prior systems that apply uniform pressure to one or both of the wafers, the simultaneous application of that pressure allows air to remain trapped between pressure application points. By contrast, the sequential application of pressure using the concentric rings  120  allows air to be pushed to the edge of the wafers with the addition of each concentric ring  120  such that the air ultimately exits the gap between the top and bottom wafers. In alternate embodiments, the concentric rings  120  may simultaneously be brought into contact with the top wafer to apply uniform and continuous force resulting in a complete bond. The number and position of the concentric rings  120 , as well as the amount of force applied, may be varied to control the dynamics of the applied pressure. 
       FIGS. 1-6  show the pin  110  and concentric rings  120  being lowered ( FIGS. 2-5 ) and thereby imply an arrangement in which the pin  110  and concentric rings  120  push down a top wafer onto a bottom wafer. This is the arrangement discussed specifically above. However, in alternate embodiments, the pin  110  and concentric rings  120  may be arranged to alternately or additionally push up the bottom wafer to bond with the top wafer. Also, application of force directly on a wafer is discussed herein. However, preferred alternate embodiments contemplate the top wafer sitting on a flexible chuck and the pin  110  and concentric rings  120  applying pressure on top of the flexible chuck which is in contact with the top wafer (or pressure from below a flexible chuck in contact with the bottom wafer) or applying pressure to the top wafer (or bottom wafer) through grooves in a non-flexible chuck. 
       FIGS. 7-12  depict a wafer bonding apparatus  200  at various stages of a wafer bonding process according to an embodiment. The apparatus  200  includes a central pin  210  and outer pins  220 . The central pin  210  is lowered ( FIG. 8 ) onto a wafer  240  (top wafer according to the exemplary arrangement of  FIGS. 1-6 ) for single point bond initiation between the top wafer  240  and the bottom wafer  250 . Then, the outer pins  220  are simultaneously lowered onto the top wafer ( FIGS. 3-6 ). As shown by the perspective top view in  FIG. 11 , the outer pins  220  are lowered onto the top wafer  240  at a given radial distance r1 from the center of the top wafer  240  and the outer pins  220  are then moved radially outward, as indicated by the arrows in  FIG. 11 , to the outer edge of the top wafer  240  to the position shown at  FIG. 12 . The outer pins  220  are moved while continuing to apply pressure to the top wafer  240 . As such, the outer pins  220  push air to the outer edge of the wafer bond where it exits the gap between the top and bottom wafers rather than being trapped to form a void in the wafer bond. As noted with regard to the previous embodiment, the pins  210 ,  220  may alternately or additionally be arranged to push up the bottom wafer  250  in alternate embodiments. In the embodiment shown by  FIGS. 7-12 , the pins  210 ,  220  apply pressure to the top wafer  240  through grooves in a non-flexible chuck  230  on which the top wafer  240  sits. In alternate embodiments, the pins  210 ,  220  may apply pressure to a flexible chuck (not shown) in contact with the top or bottom wafer or may pressure directly to the top or bottom wafer without the presence of a chuck. The number and position (e.g., the initial radial distance from the central pin  210  when the outer pins  220  are lowered) of the outer pins  220 , as well as the amount of force applied, may be varied to control the dynamics of the applied pressure. 
       FIGS. 13-20  depict a wafer bonding apparatus  300  at various stages of a wafer bonding process according to an embodiment. The apparatus  300  includes a pin  310  and a spreading cone  320 . In the exemplary illustrations of  FIGS. 13-20 , the apparatus  300  is used to apply pressure on a flexible chuck  330  in contact with a top wafer  340 . In alternate embodiments, force may be applied to the top wafer  340  directly or through grooves in a non-flexible chuck. Also, force may additionally or alternately be applied to a bottom wafer  350  or a flexible chuck  360  on which the bottom wafer  350  is sitting. As shown in  FIGS. 13-20 , the pin  310  is lowered ( FIG. 14 ) onto a flexible chuck  330  in contact with the top wafer  340  to affect single point bond initiation ( FIG. 15 ) between the top wafer  340  and bottom wafer  350  near the center of the wafer  340 ,  350 . The spreading cone  320  is then lowered onto the chuck  330  ( FIG. 16 ) at a given radial distance (r) from the pin  310  and also applies pressure on the chuck  330  to continue the bonding process between the wafers  340 ,  350 . The top view ( FIG. 17 ) shows the spreading cone  320  initially touching down on the chuck  330  at a radial distance r from the pin  310 . As shown at  FIGS. 18-20 , the circular base of the spreading cone  320  that contacts the chuck  330  then spreads to expand the circular base to a radial distance (R,  FIGS. 19 and 20 ) greater than the initial radial distance (r) from the pin  310  and, ultimately, to the edge of the chuck  330 . The continuous application of pressure during the spreading of the circular base of the spreading cone  320  prevents de-bonding or inward movement of air or particles between the top wafer  340  and bottom wafer  350 . 
       FIGS. 21-26  depict a wafer bonding apparatus  400  at various stages of a wafer bonding process according to an embodiment. The apparatus  400  includes a central pin  410  and concentric shapes  420  that may be cones, cylinders, or trapezoids. The central pin  310  is lowered ( FIG. 22 ) onto a wafer (for example, a top wafer according to the exemplary arrangement of  FIGS. 21-26 ) for single point bond initiation between the top and bottom wafers. Then each of the shapes  420  is lowered onto the top wafer and then spread (for example,  FIGS. 23 and 24 ) sequentially. As indicated by the arrows at  FIGS. 25 and 26 , the lowering and spreading of each shape  420  are done successively for shapes  420  of increasing radial distance from the pin  410 . That is, the circular or other-shaped base of each shape  420  that contacts the top wafer as it contacts the bottom wafer in the bonding process spreads radially outward while continuing to apply pressure on the top wafer to continue the bonding process. The spread of each shape  420  may be limited to prevent interference with the next shape  420  (radially farther from the pin  410  than the previously lowered shape  420 ) that is lowered onto the top wafer. Although described as direct pressure on the top wafer, the pin  410  and shapes  420  may instead apply pressure on a flexible chuck in contact with the top wafer when the top wafer is sitting on a flexible chuck. Also, as discussed with regard to previous embodiments, the pin  410  and shapes  420  may alternately or additionally apply pressure on the bottom wafer or flexible chuck in contact with the bottom wafer. The number and positions of the shapes  420 , as well as the amount of force applied, may be varied to control the dynamics of the applied pressure. 
       FIGS. 27-34  depict a wafer bonding apparatus  500  at various stages of a wafer bonding process according to an embodiment. The apparatus  500  includes a pin  510  and chain links  520 . The pin  510  is lowered ( FIG. 28 ) onto a wafer (top wafer  530  according to the exemplary arrangement of  FIGS. 27-34 ) for single point bond initiation between the top and bottom wafers at substantially a center of the bond. Each of the chain links  520  has a spiral motion and can apply force to the top wafer  530 . The chain links  520  may be sequentially or simultaneously brought into contact with the top wafer  530  ( FIGS. 29-34  show sequential application of force by the chain links  520 ). When the chain links  520  spiral onto the top wafer  530  sequentially, the application of force by those chain links  520  as well as the continuing application of force by chain links  520  already on the top wafer  530  prevents de-bonding of already bonded portions. The perspective top views shown by  FIGS. 31 and 33  illustrate that the chain links  520  land on various parts of the top wafer  530  to collectively apply randomly distributed pressure on the top wafer  530 . Although described as direct pressure on the top wafer  530 , the pin  510  and chain links  520  may instead apply pressure on a flexible chuck in contact with the top wafer  530  when the top wafer  530  is sitting on a flexible chuck or in grooves of a non-flexible chuck when the top wafer is sitting on a non-flexible chuck. Also, as discussed with regard to previous embodiments, the pin  510  and chain links  520  may alternately or additionally apply pressure on the bottom wafer or on a flexible chuck in contact with the bottom wafer. The number of chain links  520  and the amount of force applied by each chain link  520  may be varied to control the dynamics of the pressure applied. 
       FIGS. 35 and 36  depict perspective top and side views, respectively, of a chuck  600  used in wafer bonding according to an embodiment. The chuck  600  includes concentric rings  610  used for temperature control of the chuck  600 . The chuck  600  represents both the top chuck  600  on which the top wafer sits and the bottom chuck  600  on which the bottom wafer sits. The concentric rings  610  control the temperature of the chuck  600  during bonding of the two wafers to prevent uneven expansion. That is, the temperature control enhances void-free bonding by expelling air from between the two wafers as discussed herein. The concentric rings  610  may be used to create a temperature gradient across the wafer surface as the bond propagates. That is, a higher temperature is applied to the chuck  600  at smaller radial distances from the center (the concentric ring  610  closest to the center of the chuck  600  has the highest temperature) and temperatures of the concentric rings  610  are lowered as radial distances from the center increase. In this way, the higher temperature at the bond initiation point (center) helps to heat and expand air outward toward the edge of the bond where relatively lower temperature areas are created by concentric rings  610  at increasing radial distances. The temperature of each concentric ring  610  may be varied as the bonding process propagates from the center out to the edges. That is, the temperature of each concentric ring  610  successively farther from the center may be increased during the bonding process. 
       FIGS. 37-41  depict various embodiments of a temperature module  700  including a chuck  710  and one or more heating elements  720 . Each of the embodiments may represent a top or bottom chuck  710 . The embodiments depicted by  FIGS. 37-39  enhance uniformity of heating of the chuck  710  and thus discourage air gaps between the wafers during bonding.  FIG. 37  includes heating elements  720  formed of a coiled temperature control filament. The coiled heating elements  720  mitigate uneven expansion during the bonding process which may pull air gaps to one side of the wafer bond. As discussed with reference to  FIGS. 35 and 36 , a temperature gradient may be created from the center (point of bond initiation) to the outer edge of the chuck  710 .  FIGS. 38 and 39  depict heating elements  720  that include concentric rings  720   a  and radial spikes  720   b  whose temperature may be individually controlled. The embodiment shown by  FIG. 38  includes fragments of radial spikes  720   b  while the embodiment shown by  FIG. 39  includes continuous radial spikes  720   b . By controlling the temperature of the radial spikes  720   b  to be the same or successively higher toward the center of the chuck  710 , a uniform temperature or a temperature gradient, respectively, may be created on the chuck  710  as needed to enhance the wafer bonding process.  FIGS. 40 and 41  depict heating elements  720  in random configurations. These heating elements  720  can be individually controlled to create a uniform or variable temperature across the chuck  710  as needed to expel air from between the wafers during bonding and ensure that air pockets (voids) are not created as a source of weakness in the bond. 
       FIG. 42  depicts processes  800  involved in wafer bonding according to various embodiments. At block  810 , the processes  800  may include disposing a first wafer on a first chuck and disposing a second wafer on a second chuck. Whether the wafers are attached to a chuck or not, disposing the first wafer and the second wafer for bonding at block  820  includes arranging the radial plane of the first and second wafers to be substantially parallel and also lined up such that they can be moved together to bond. The first wafer may be arranged on top of the second wafer, for example, such that pressure may be applied down on the top wafer while the second (bottom) wafer is held in place or moved up to meet the top wafer. When one or more of the first and second wafers is mounted on a chuck, the processes  800  may include controlling the temperature one or both of the chucks at block  830 . The temperature control can aid in void-free bonding by moving air out from between the wafers, as discussed with reference to  FIGS. 35-41 . At block  840 , bonding the first wafer and the second wafer to each other may be accomplished by any of the embodiments discussed with reference to  FIGS. 1-34 . 
       FIG. 43  is a block diagram of a wafer bonding system  900  according to various embodiments. The wafer bonding system  900  includes a first pressure application device  910 . In various embodiments discussed above, the first pressure application device is, for example, a pin. The wafer bonding system  900  also includes one or more second pressure application devices  920 . In various embodiments discussed above, the second pressure application devices include pins, cones, cylinders, trapezoids, and chain links, for example. The heating elements  930  discussed with reference to  FIGS. 35-41  above are disposed on one or both chucks on which one or both of the wafers being bonded sit. In various embodiments discussed above, the heating elements are disposed as concentric rings, with and without radial spikes, a spiral, and a random arrangement. The controller  940  controls the first and second pressure application devices and affects not only when the first and second pressure application devices begin applying force but also how much force is applied. The controller  940  also controls the heating elements  930  and controls whether uniform heat or a temperature gradient is applied and also determines the timing of any temperature gradient that is varied during bonding. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     The flow diagram depicted herein is just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.