Abstract:
A wafer bonding method includes placing a top wafer on a top bonding framework including a plurality of outlet holes around a periphery of the top bonding framework. A bottom wafer is placed on a bottom bonding framework that includes a plurality of inlet holes around a periphery of the bottom bonding framework. The top bonding framework is in overlapping relation to the bottom bonding framework such that a gap exist between the top wafer and the bottom wafer. A gas stream is circulated through the gap between the top wafer and the bottom wafer entering the gap through one or more of the plurality of inlet holes and exiting the gap through one or more of the plurality of outlet holes. The gas stream replaces any existing ambient moisture from the gap between the top wafer and the bottom wafer.

Description:
BACKGROUND 
       [0001]    The present invention generally relates to semiconductor manufacturing and more particularly to wafer bonding techniques as part of three-dimensional (3D) integration processes. 
         [0002]    In a 3D integration process individual wafers may be stacked and joined into a single package in order to reduce space. A common technique used in wafer-scale 3D integration is wafer bonding. In a wafer bonding process, the electronic devices on one wafer may be aligned with the electronic devices on another wafer, and then the wafers may be bonded together using, for example, an oxide-oxide fusion bonding process. 
       SUMMARY 
       [0003]    The ability to conduct wafer bonding processes with a reduced amount of defects formed in the bonded surface may facilitate advancing the capabilities of current wafer-scale 3D integration technology. 
         [0004]    According to one embodiment of the present disclosure, a wafer bonding method may include placing a top wafer on a top bonding framework including a plurality of outlet holes around a periphery of the top bonding framework, placing a bottom wafer on a bottom bonding framework including a plurality of inlet holes around a periphery of the bottom bonding framework. The top bonding framework may be in overlapping relation to the bottom bonding framework such that a gap may exist between the top wafer and the bottom wafer. A gas stream may be circulated through the gap between the top wafer and the bottom wafer. The gas stream may enter the gap through one or more of the plurality of inlet holes and may exit the gap through one or more of the plurality of outlet holes. The gas stream may replace any existing ambient moisture from the gap between the top wafer and the bottom wafer. 
         [0005]    According to another embodiment of the present disclosure, a wafer bonding method may include placing a top wafer on a top bonding framework including a first plurality of outlet holes and a second plurality of outlet holes around a periphery of the top bonding framework, placing a  bottom wafer on a bottom bonding framework including a plurality of inlet holes around a periphery of the bottom bonding framework. The top bonding framework may be in overlapping relation to the bottom bonding framework such that a gap may exist between the top wafer and the bottom wafer. A flow control device may be added to the first plurality of outlet holes to monitor flow conditions of a gas stream and to block the first plurality of outlet holes. A pumping device may be added to a second plurality of outlet holes to prevent back diffusion of moisture into the gap between the top wafer and the bottom wafer. The gas stream may be circulated through the gap between the top wafer and the bottom wafer. The gas stream may enter the gap through one or more of the plurality of inlet holes and may exit the gap through one of more of the second plurality of outlet holes. The gas stream may replace any existing ambient moisture from the gap between the top wafer and the bottom wafer. 
         [0006]    According to another embodiment of the present disclosure, a wafer bonding structure may include a top wafer on a top bonding framework of a top platform, the top bonding framework may include a first plurality of outlet holes and a second plurality of outlet holes around a periphery of the top bonding framework, and a bottom wafer on a bottom bonding framework of a bottom platform. The bottom bonding framework may include a plurality of inlet holes around a periphery of the bottom bonding framework. The top platform may be in an overlapping relation to the bottom platform. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a cross-sectional view of a wafer bonding structure, according to an embodiment of the present invention; 
           [0009]      FIG. 1A  is a bottom view of a top platform of the wafer bonding structure, according to  FIG. 1 ;  
           [0010]      FIG. 1B  is a top view of a bottom platform of the wafer bonding structure, according to  FIG. 1 ; 
           [0011]      FIG. 2  is a cross-sectional view of the wafer bonding structure depicting bringing the top platform and the bottom platform into contact, according to an embodiment of the present invention; 
           [0012]      FIG. 3  is a cross-sectional view of the wafer bonding structure depicting purging moisture between two wafers by introducing a gas stream, according to an embodiment of the present invention; 
           [0013]      FIG. 3A  is a bottom view of the top platform of the wafer bonding structure, according to  FIG. 3 ; 
           [0014]      FIG. 4  is a cross-sectional view of the wafer bonding structure depicting adding a pumping device, according to an embodiment of the present invention; 
           [0015]      FIG. 4A  is a bottom view of the top platform of the wafer bonding structure, according to  FIG. 4 ; 
           [0016]      FIG. 5  is a cross-sectional view of the wafer bonding structure depicting adding a trapping region, according to an embodiment of the present invention; 
           [0017]      FIG. 6  is a cross-sectional view of the wafer bonding structure depicting pinning the top wafer to the bottom wafer, according to an embodiment of the present invention; and 
           [0018]      FIG. 7  is a cross-sectional view of the wafer bonding structure depicting separating the top platform from the bottom platform, according to an embodiment of the present invention. 
       
    
    
       [0019]    The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention. In the drawings, like numbering represents like elements.  
       DETAILED DESCRIPTION 
       [0020]    Detailed embodiments of the claimed structures and methods are disclosed herein; however, it may be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this invention to those skilled in the art. 
         [0021]    In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps, and techniques, in order to provide a thorough understanding of the present invention. However, it will be appreciated by one of ordinary skill of the art that the invention may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the invention. It will be understood that when an element as a layer, region, or substrate is referred to as being “on” or “over” another element, it may be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “beneath,” “below,” or “under” another element, it may be directly beneath or under the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly beneath” or “directly under” another element, there are no intervening elements present. 
         [0022]    In the interest of not obscuring the presentation of embodiments of the present invention, in the following detailed description, some processing steps or operations that are known in the art may have been combined together for presentation and for illustration purposes and in some instances may have not been described in detail. In other instances, some processing steps or operations that are known in the art may not be described at all. It should be understood that the following description is rather focused on the distinctive features or elements of various embodiments of the present invention.  
         [0023]    In wafer-scale 3D integration technology, voids and other defects may form on or in the wafer surface during the wafer bonding process. The voids or defects may include un-bonded areas between the joined wafers. Various atmospheric and operational conditions may cause these voids to form during the wafer bonding process. Since voids may negatively impact device functionality, the presence of these un-bonded areas between the wafers may affect reliability and yield. Poor reliability and yield may affect performance and cost, respectively. 
         [0024]    Voids may form particularly on a periphery of the wafer during, for example, an oxide-oxide low-temperature fusion bonding process. The formation of voids may be caused by the condensation of pre-existing moisture in a gap located between the wafers to be bonded. The moisture present in this gap may generally come from the atmospheric humidity existent within the semiconductor fabrication plant. 
         [0025]    The present invention generally relates to semiconductor manufacturing and more particularly to wafer bonding techniques as part of three-dimensional (3D) integration processes. Moisture between the two wafers may be purged during bonding to produce a void free bond between the two wafers. One way to purge the moisture from between the two wafers may include purging the bonding tool with a gas. One way to purge the bonding tool with the gas is described in detail below by referring to the accompanying drawings  FIGS. 1-7 . 
         [0026]    Referring now to  FIGS. 1 ,  1 A and  1 B, a bonding structure  100  is shown. The bonding structure  100  may include a top platform  102  and a bottom platform  202 .  FIG. 1A  is a bottom view of the top platform  102  in  FIG. 1  and  FIG. 1B  is a top view of the bottom platform  202  in  FIG. 1 . The bonding structure  100  may alternatively be referred to as a bonding tool. 
         [0027]    The top platform  102  may include a top bonding framework  104  configured to hold or carry a top wafer  112 . The top bonding framework  104  may further include a pin structure  116  located at or near a center of the top bonding framework  104 . In addition, the top bonding framework  104  may also include a plurality of outlet holes  114  (hereinafter “outlet holes”). The outlet holes  114  may be arranged near a perimeter  12  of the top bonding framework  104 . In some embodiments, the outlet holes  114  may be uniformly distributed between the perimeter  12  of the top bonding framework  104  and a perimeter  10  of the top wafer  112 . The outlet holes  114  may  be spaced apart from each other by a distance ranging from of approximately 5 mm to approximately 50 mm. 
         [0028]    In the present embodiment, the bottom platform  202  may include a bottom bonding framework  204 , a bottom stage  206 , and a bottom bonding chuck  210 . In general, the bottom stage  206  may be recessed within an opening in a center of the bottom bonding framework  204 . Stated differently, the bottom bonding framework  204  may generally have a ring shape which surrounds the bottom stage  206 . Also, the bottom stage  206  may be movable in a vertical direction with respect to the bottom bonding framework  204 . The bottom bonding chuck  210  may be located on top of the bottom stage  206  to carry a bottom wafer  212 . The bottom bonding chuck  210  may be configured to hold or carry the bottom wafer  212 . 
         [0029]    Similar to the top bonding framework  104  described above, the bottom bonding framework  204  may include a plurality of inlet holes  214  (hereinafter “inlet holes”). The inlet holes  214  may be arranged near a perimeter  22  of the bottom bonding framework  204 . In some embodiments, the inlet holes  214  may be uniformly distributed between a perimeter  20  of the bottom stage  206  and the perimeter  22  of the bottom bonding framework  204 . The inlet holes  214  may be spaced apart from each other in a similar fashion to the outlet holes  114  described above. 
         [0030]    In general, the top platform  102  may be positioned above the bottom platform  202 , as illustrated. It should be noted that the relative position of the top platform  102  and the bottom platform  202  is merely one example and that other configurations may be considered and are expressly contemplated. Furthermore, the top platform  102  may further include a configuration similar to the bottom bonding platform  202 , and vice versa. 
         [0031]    With continued reference to  FIG. 1 , the bonding structure  100  may be used to bond or join the top wafer  112  to the bottom wafer  212 . During a wafer bonding process using a structure similar to the bonding structure  100 , the top wafer  112  and the bottom wafer  212  may be brought into close physical contact with one another without the presence of any adhesion materials. The two wafers ( 112 ,  212 ) may each have a hydrophilic surface with a substantially high density of OH −  groups attached to the surface.  
         [0032]    First, the top wafer  112  may be positioned in and held by the top platform  102 , and the bottom wafer  212  may be positioned in and held by the bottom platform  202 . Each of the top and bottom platforms  102 ,  104  may use any known system to hold the wafers  112 ,  212 , such as, for example, a pneumatic system which may create a vacuum to hold each of the wafers  112 ,  212 . In general, the bonding process may commence by bringing either of the wafers ( 112 ,  212 ) within a predetermine distance from the other such that a gap remains. Next, the pin structure  116  of the top bonding platform  102  may rapidly force a center portion of the top wafer  112  to contact the bottom wafer  212 . As the top wafer  112  releases from the top platform  102 , contact between the two wafers ( 112 ,  212 ) may continue to propagate from the center outwards towards the wafers edge. The progression of the contact between the top wafer  112  and the bottom wafer  212  may be referred to as a bonding wave. 
         [0033]    Referring now to  FIGS. 2-7 , exemplary process steps of bonding the top wafer  112  to the bottom wafer  212  using the boding structure  100  in accordance with one embodiment of the present invention are shown. 
         [0034]    Since cross-sectional views of the top platform  102  ( FIG. 1A ) and the bottom platform  202  ( FIG. 1B ) will be used to describe the processing steps, it should be understood that although only two outlet holes  114  and only two inlet holes  214  are shown, the following processing steps may equally apply to more than two outlet holes and more than two inlet holes. 
         [0035]    Referring now to  FIG. 2 , an initial alignment step may be performed between the top platform  102  and the bottom platform  202 . In this initial step, electronic devices (not shown) on the top wafer  112  may be aligned with electronic devices (not shown) on the bottom wafer  212 . After aligning the top platform  102  and the bottom platform  202 , either of the top platform  102  or the bottom platform  202  may move toward the other until they contact each other. Next, the bottom stage  206  may move within the bottom platform  202  bringing the bottom wafer  212  within a predetermined distance from the top wafer  112 , such that a wafer-wafer gap  302  (hereinafter “gap”) remains between the two wafers ( 112 ,  212 ). 
         [0036]    The gap  302  may have a height h ranging from approximately 0.01 mm to approximately 10 mm. In one embodiment, the height h of the gap  302  may vary between approximately 0.05 mm to approximately 0.3 mm.  
         [0037]    Referring now to  FIGS. 3 and 3A , a gas stream  310  may be introduced into the bonding structure  100  to purge excess moisture from between the top and bottom wafers  112 ,  212 . The gas stream  310  may be used to purge any ambient moisture from the bonding structure  100  in an effort to create a void free bond between the wafers ( 112 ,  212 ), as mentioned above. More specifically, the gas stream  310  may be supplied to the bonding structure  100  through the inlet holes  214  and exit the bonding structure  100  through the outlet holes  114 . The gas stream  310  may be distributed into and out of the bonding structure  100  in any fashion as long as it flows across the wafers ( 112 ,  212 ) sufficiently to purge the gap  302  ( FIG. 2 ) from any ambient moisture. As such, the gas stream  310  may be distributed through any number and/or combination of the inlet holes  214  and the outlet holes  114 . 
         [0038]    In one embodiment, the gap  302  ( FIG. 2 ) may contain air having a relative humidity of approximately 30% at room conditions. The gas stream  310  may circulate through the gap  302  ( FIG. 2 ) until any ambient moisture has been purged from the bonding structure  100 . In one exemplary embodiment, the gas purging time may range from approximately 30 sec to approximately 300 sec. 
         [0039]    The gas stream  310  may include any dehumidified gas suitable for purging any ambient moisture from the gap  302  ( FIG. 2 ). In one embodiment, the gas stream  310  may include dehumidified air. In another embodiment, the gas stream  310  may include nitrogen, argon, hydrogen, helium, neon or a mixture thereof with a negative Joule-Thomson coefficient. Although gases with positive Joule-Thomson coefficients may also be considered. 
         [0040]    In other embodiments, the gas stream  310  may include a pre-heated or hot dry gas. The use of a hot dry gas may ensure substantial moisture removal from the gap  302  ( FIG. 2 ) prior to bonding the top wafer  112  to the bottom wafer  212 . In embodiments where the outlet holes  114  may remain open, the hot dry gas may prevent back diffusion of moisture into the gap  302  ( FIG. 2 ). In one exemplary embodiment, the hot dry gas may include dry nitrogen, argon, hydrogen, helium or a mixture thereof. The purging time for the hot dry gas may range from approximately 10 sec to approximately 300 sec. 
         [0041]    In one embodiment, a flow control device  306  may be inserted into one or more of the outlet holes  114  of the top platform  102  to control the flow characteristics of the gas stream  310 .  The flow control device  306  may effectively prevent the gas stream  310  from exiting any outlet hole  114  in which it is located. More specifically, because the flow control device  306  may block one or more of the outlet holes  114 , the gas stream  310  may be directed towards other outlet holes  114  that remain unblocked. In one example, about half of the outlet holes  114  located on one side of the top bonding platform  102 , may be blocked with a flow control device  306 , as illustrated in  FIG. 3A . Doing so may improve the flow characteristics of the gas stream  302  to maximize flow across the wafers ( 112 ,  212 ) and through the gap  302 . 
         [0042]    The flow control device  306  may include any suitable shutter or plug structure capable of preventing the gas stream  302  from exiting one or more outlet holes  114 . In one embodiment, the flow control device  306  may be capable of monitoring properties such as, for example, pressure and flow rate of the gas stream  310 . In such embodiments, the flow control devices  306  may include a flow meter and/or a pressure sensor. 
         [0043]    Referring now to  FIGS. 4 and 4A , and according to one embodiment of the present invention, a pumping device  308  may be coupled to one or more of the outlet holes  114  of the top platform  102  to control the flow characteristics of the gas stream  310 . The pumping device  308  may be designed to lower the pressure at one or more of the outlet holes  114  and cause the gas stream  310  to flow towards the pumping device  308 . In one example, a single pumping device ( 308 ) may be coupled to about half of the outlet holes  114  located on one side of the top boding platform  102  as illustrated in  FIG. 4A . Doing so may result in a pressure differential between opposite sides of the gap  302  which may improve the flow characteristics of the gas stream  302  and maximize flow across the wafers ( 112 ,  212 ). 
         [0044]    The pumping device  308  may include, for example, a multi stage diaphragm pump, a molecular drag pump, a molecular screw pump, and/or a standard roots and claw dry pump. In one exemplary embodiment, the pumping device  308  may include a dry multi-stage vacuum pump. In addition to improving the flow characteristics of the gas stream  302 , the pumping device  308  may, among other things, reduce gas purging time, and prevent back diffusion of moisture into the gap  302  ( FIG. 2 ) during the wafer bonding process. 
         [0045]    It should be noted that the flow control device  306  and the pumping device  308  are mutually exclusive. More specifically, the flow control device  306  may be used without the  pumping device  308  and vice versa, or they may be used in combination to achieve the desired flow characteristics of the gas stream  310 . Furthermore, the configuration of inlet holes  214  ( FIG. 2 ) and outlet holes  114  may preferably maximize the flow of the gas stream  310  across the gap  302 . Therefore, the configuration of inlet holes  214  ( FIG. 2 ) and outlet holes  114  as depicted in the figures is intended to illustrate one particular configuration or one embodiment; however alternative configurations which achieve the desired flow characteristics are expressly envisioned. 
         [0046]    Referring now to  FIG. 5 , a trapping region  320  may be positioned in or near the outlet holes  114  ( FIG. 2 ), according to one embodiment of the present invention. The trapping region  320  may further enable control of the flow characteristic of the gas stream  310 . More specifically, the trapping region  320  may further assist preventing back diffusion or back flow of the moisture removed from the gap  302  ( FIG. 2 ) to the bonding area between the top wafer  112  and the bottom wafer  212 . The trapping region  320  may be used with or without either or both of the flow control device  306  and the pumping device  308 . In one exemplary embodiment, the trapping region  320  may include cryogenically cooled traps such as traps cooled by, for example, liquid nitrogen, and made of materials that may be easy to cool (i.e., metals) in order to promote the trapping of water molecules, hydrocarbon molecules, and the like. 
         [0047]    Referring now to  FIG. 6 , once the gas stream  310  ( FIG. 3 ) has purged all or substantially all of the existing moisture of the gap  302  ( FIG. 2 ), the bonding process may continue by reducing the amount of gas circulating within the gap  302  ( FIG. 2 ). The flow rate of the gas stream  310  ( FIG. 3 ) may be reduced to prevent misalignment between the wafers ( 112 ,  212 ) during the bonding process. It should be noted that the gas stream may continue to circulate at a reduced rate to prevent any backflow of moisture into the bonding structure  100 , as mentioned above. The process continues by moving the bottom stage  206  within the bottom platform  202  to bring the bottom wafer  212  into contact with the top wafer  112 . 
         [0048]    In embodiment in which the pumping device  308  may be coupled to some of the outlet holes  114  ( FIG. 4 ) of the top bonding framework  104 , the gas stream  310  may be completely stopped and the pumping device  308  ( FIG. 4 ) may be used to maintain a vacuum during bonding. Alternatively, the pumping device  308  ( FIG. 4 ) may be used to apply a vacuum to the  bonding structure  100  prior to bonding at which time all the inlet holes  214  and the outlet holes  114  may be sealed to maintain vacuum during bonding. 
         [0049]    After bonding, the top wafer  112  may then be released and pinned to the bottom wafer  212  by means of the pin structure  116  to initiate the propagation of the bonding wave between the top wafer  112  and the bottom wafer  212 . The bonding wave may rapidly propagate from the pinned area towards an edge region of the top wafer  112  and the bottom wafer  212 . Formation of voids and other defects may be linked to the bonding wave propagation and to fluid dynamics between the wafers. A gas pressure drop may take place at the wafer edge described by a Joule-Thomson expansion. This adiabatic process may result in a gas temperature change which may lead to the condensation of small water droplets close to the wafer edge which may result in un-bonded areas such as voids. 
         [0050]    Referring now to  FIG. 7 , once the top wafer  112  and the bottom wafer  212  are bonded together, the top bonding framework  104  and the bottom bonding framework  204  may return to their original positions. 
         [0051]    Since void formation may be attributed to the condensation of moisture contained in the existing fluid within the gap  302  ( FIG. 2 ), replacement of this fluid by a dehumidified gas may substantially reduce or eliminate any possible condensation of moisture occurring as a result of the adiabatic expansion that may take place during propagation of the bonding wave particularly in the edge region of the wafers. 
         [0052]    Therefore, a wafer bonding structure that may allow for gas flow control between the top wafer  112  and the bottom wafer  212  may enhance moisture removal from the gap  302  before initiating the wafer bonding process thereby reducing the formation of voids particularly in the edge region of the wafers. Gas flow conditions may be adjusted to maximize moisture removal from the gap and to minimize gas consumption in the system which may potentially increase device reliability and reduce device manufacturing costs. The bonding structure  100  may also be compatible with other existing bonding alignment processes. 
         [0053]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the  embodiments 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 described embodiments. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.