Patent Publication Number: US-9842785-B2

Title: Apparatus and method for verification of bonding alignment

Description:
PRIORITY CLAIM 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 14/184,402, filed on Feb. 19, 2014 and entitled “Apparatus and Method for Verification of Bonding Alignment,” which application is incorporated herein by reference. 
    
    
     BACKGROUND 
     Semiconductor devices are used in a variety of electronic applications, such as, for example, personal computers, cell phones, digital cameras, and other electronic equipment. The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area. In some devices, multiple dies are stacked vertically to reduce the footprint of a device package and permit dies with different processing technologies to be interconnected. 
     Devices may be stacked and interconnected by bonding dies directly to each other. In some cases, dies are bonded in bulk by bonding wafers having multiple dies on each wafer. Components on different wafers or different are interconnected with metal lines that are exposed at the bonding surface of the dies and brought into contact during bonding. After bonding, the metal lines provide electrical connectivity between the dies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to illustrate the relevant aspects of the embodiments and it should be noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is an orthogonal view of an alignment test structure according to some embodiments; 
         FIGS. 2A-2B  are cross-sectional views of an alignment test structure according to some embodiments; 
         FIG. 3A  is a top view diagram of a wafer illustrating placement of alignment test structures according to some embodiments; 
         FIGS. 3B-3C  are enlarged views of placement of alignment test structures according to some embodiments; 
         FIGS. 4A-4B  are wiring diagrams illustrating the use of an alignment test structure with a Vernier-type comb according to some embodiments; 
         FIGS. 5A-5C  are wiring diagrams illustrating the use of an alignment test structures with a binary comb according to some embodiments; and 
         FIG. 6  is a flow diagram illustrating a method for forming and using an alignment test structure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Units such as dies, chips, substrates, or other structures used in semiconductor production are sometimes bonded directly together. The units are formed on wafers, and in order to bond the dies on two different wafers together most efficiently, the wafers are bonded prior to singulating the dies. The wafers are bonded through wafer-to-wafer bonding, hybrid bonding or another bonding procedure. 
     Wafer-to-wafer bonding involves bonding the surface of two wafers directly together. A hybrid bond is formed where the wafer surfaces are bonded together, and metal interconnecting features extend to the surface of each wafer and are fused together to form a metal-to-metal bond in addition to the wafer surface bond. In order to ensure that the metal feature connect at the wafer bond interface, the wafers are aligned prior to bringing the wafers together for bonding. As described in greater detail below, a portion of an alignment verification test structure is formed in each wafer and after bonding the alignment of the wafers is verified by testing the conductivity of different portions of the test structure. 
       FIG. 1  is an orthogonal view of an alignment test structure  100  according to some embodiments. A common node  106  and a test node  118  are disposed in a first wafer, and a comb  112  is disposed in a second wafer. The common node  106 , test node  118  and comb  112  are formed from conductive materials. The alignment of the first wafer to the second wafer is verified by testing the connectivity of a signal though the common node  106  and through the comb  112  to a specific test pad  104  of the test node  118 . 
     The common node  106  has a common pad  102  contacting a plurality of common node lines  108 . Each of the common node lines  108  has a common line contact surface  110  that is exposed at the surface of the first wafer. The comb  112  has a plurality of comb lines  114 . Each of the comb lines  114  has a comb contact surface  116  on each end that is exposed at the surface of the second wafer. The test node  118  has a plurality of test pads  104  contacting a plurality of test node lines  120 . The test node lines  120  each have a test line contact surface  122  disposed at the surface of the first wafer. 
     When a particular comb line  114  is aligned and in contact with a respective test node line  120  and common node line  108 , an electrical signal will be conducted from the common node lines  108 , through an aligned comb line  114  and to a corresponding test node line  120 . The electrical signal can be detected at a test pad  104  connected to the corresponding test node line  120 . The comb lines  114  have a spacing that is different than the spacing of the test node lines  120 . Additionally, in some embodiments, the comb lines  114  will also have spacing different than the common node lines  108 . This permits different test node lines  120  to be connected by the comb lines  114  depending on the alignment of the comb lines  114 , which is dictated by the alignment of the wafers. The mismatch in spacing permits testing of which comb lines  114  are in contact with the test node lines  120  and common node lines  108 . The number or arrangement of connections by the comb lines  114  between the common node  106  and test node  118  indicates the alignment shift of the wafers. Determining which comb line  114  is aligned with the common node  106  and test node  118  permits calculation of how far the second wafer is misaligned with the first wafer. 
       FIG. 2A  is a cross-sectional view of the alignment test structure  100  as taken on the A-A plane illustrated in  FIGS. 1 and 2B  according to some embodiments.  FIG. 2A  shows the common node  106  and a portion of the comb  112 .  FIG. 2B  is a cross-sectional view of the alignment test structure  100  according to some embodiments as taken on the B-B plane illustrated in  FIGS. 1 and 2A . The common node  106  and test node  118  are disposed in a top wafer  202 , and in some embodiments, each have a portion formed in a redistribution layer (RDL)  210 . The RDL  210  comprises one or more intermetal dielectric layers (IMDs)  216 A . . .  216 N. The IMDs  216 A . . .  216 N are dielectric layers with metal lines and vias formed in a dielectric material. The RDL  210  is formed on a substrate  206 . The comb  112  is disposed in the RDL  212  on the bottom wafer  204 . In some embodiments, portions of the comb  112  extend into, or are formed in, the substrate  208 . In embodiment, the bottom wafer  204  has a similar structure, with an RDL  212  formed from one or more IMDs  214 A . . .  214 N on a substrate  208 . The substrates  206  and  208  of the top wafer  202  and bottom wafer  204  each, in some embodiments, have one or more active or passive devices such as transistors, resistors, capacitors or the like, forming, for example, an integrated circuit. While the alignment test structure  100  is described herein as being formed in a top wafer  202  and bottom wafer  204 , in other embodiments, the top wafer  202  or the bottom wafer  204  are dies, packages, carriers, or other structures that can be directly bonded. 
     In some embodiments, the metal lines and vias forming the common node  106 , comb  112  and test node  118  may are formed using, for example, a dual damascene technique. In such an embodiment, the dielectric layers of each of the IMDs  214 A . . .  214 N and  216 A . . .  216 N are dielectric materials such as an oxide, a nitride, a polymer or another electrically insulating material that is formed through, for example, a spin on glass procedure, chemical vapor deposition, thermal oxidation, or another process. The dielectric layer is etched once to create openings for vias and subsequently etched a second time to create trenches or openings for the metal lines. A metal layer is deposited in the openings formed in the dielectric layers through, for example, chemical vapor deposition, sputtering, atomic layer deposition, or another process. The metal layer is reduced through a chemical mechanical polish, plasma etch, chemical etch, or the like. Subsequent IMD layers are deposited over the metal liens in the dielectric layer. In some embodiments the lines or conductive elements of the common node  106 , comb  112  and test node  118  are formed from a metal such as copper, aluminum, nickel, an alloy, or another conductive material. 
     In some embodiments, the common pad  102  and each of the test pads  104  are exposed at the top surface of the top wafer, opposite the top wafer  202  from the bond interface  218 . In such an arrangement, the alignment of the wafers  202  and  204  can be tested from a single side. It should be understood that the alignment test structure  100  is not limited to such an arrangement. For example, the test pads  104  may be disposed on the comb  112  and exposed on the bottom surface of the bottom wafer  204 . 
     A bond interface  218  is created when the wafers  202  and  204  are bonded together. The bond interface  218  is formed by bonding the top wafer  202  to the bottom wafer  204  using a suitable wafer bonding technique, such as direct wafer surface bonding or hybrid bonding. In direct wafer surface bonding, wafers are brought together, with the wafer surfaces contacting each other. In an embodiment, the wafer surfaces themselves are bonded without an intermediate bonding layer through, for example, fusion boding of silicon, silicon germanium, gallium arsenide, or another semiconductor material. In another embodiment, a bonding layer such as an RDL, passivation layer or the like is formed on the wafer surfaces from a material such as a native oxide, deposited oxide, thermal oxide, nitride, or the like. In some embodiments, the wafer surfaces are bonded together using a chemical treatment or a combination of pressure and heat at the wafer surfaces to form a bond by interdiffusion of the wafer surface or by formation of covalent bonds between atomic structures of the wafers. Thus, where the wafers  202  and  204  have RDLs  210  and  212 , the surfaces of the RDLs  210  and  212  are directly bonded. Direct wafer surface bonding holds the contact surfaces  110 ,  116  and  122  in contact with each other. 
     In an embodiment, direct wafer surface bonding is achieved through, for example, oxide-to-oxide, dielectric-to-dielectric, or substrate-to-substrate bonding or by bonding any combination of substrate, semiconductor or dielectric bonding by washing the wafer surfaces with an RCA clean with distilled water and hydrogen peroxide (H 2 O 2 ) combined with ammonium hydroxide (NH 4 OH) or hydrochloric acid (HCl). In an embodiment, the wafer surfaces are plasma activated with, for example, a reactive ion etch or a non-etching plasma treatment. The wafer are be joined after cleaning and plasma activation and subsequently annealed at a relatively low temperature to bond the wafer surfaces at an atomic level. In such an embodiment, an anneal process is performed on the stacked wafer structure in a chamber with inert gases such as argon, nitrogen, helium and the like to bond the RDLs. 
     Additionally, hybrid bonding is achieved using metal-to metal bonding of the contact surfaces  110 ,  116  and  122  in addition to bonding of the wafer surfaces. In an embodiment, a thermo-compression process may be performed on the wafers during bonding to achieve metal-to-metal bonding of the contact surfaces  110 ,  116  and  122 . Such a thermo-compression process leads to metal inter-diffusion. In an embodiment, the contact surfaces  110 ,  116  and  122  are copper and the copper atoms acquire enough energy to diffuse between adjacent contact surfaces  110 ,  116  and  122  during bonding. As a result, a homogeneous copper interface is formed between the contact surfaces  110 ,  116  and  122  where they contact each other. Such a homogeneous copper interface helps the contact surfaces  110 ,  116  and  122  form a uniform bonded feature. 
     The common node  106  is illustrated as having a center common node line  108  aligned with the center comb line  114 . In such an alignment, the common line contact surface  110  and the comb contact surface  116  are in electrical contact. Additionally, due to lateral spacing of the comb lines  114  (along the X axis) that is greater than the common node lines  108 , the outer common node lines  108  are nonaligned with the respective outer comb line  114 . Thus, when the alignment of the top wafer  202  relative to the bottom wafer  204  is laterally misaligned, one of the outer common node lines  108  and outer comb lines  114  will come into contact, and the center common node line  108  will lose contact with the center comb line  114 . When a voltage is applied to the common pad  102 , the signal can be detected at one of the test pads  104  corresponding to a portion of the comb  112  that is aligned with the test node lines  120 . 
     The shape of the alignment test structure  100  is not limited to that illustrated. For example, the common node  106  may have a single common node line  108  that contacts all of the comb lines  114  through all alignments. In such an arrangement, the comb lines  114  will remain in contact with the continuous common node line  108  and will come into contact with, or become misaligned with, the test node lines  120  depending on the alignment of the wafers  202  and  204 . In another example, the depth (along the Y axis) of the common node line  108 , test node line  120  and comb lines  114  and the respective contact surfaces  110 ,  116  and  122  is shown to be about the same as the width of the contact surfaces  110 ,  116  and  122 . However, the size of the contact surfaces  110 ,  116  and  122  and lines  108 ,  114  and  120  may be adjusted to provide precise alignment analysis. In some embodiments the contact surfaces  110 ,  116  and  122  may have a length (in the Y direction) that is greater than the width (in the X direction) to prevent misalignment in a direction (such as the Y direction) other than the lateral test direction (where the lateral test direction is the X direction). Additionally, while the comb lines  114  are shown as having a center section below the top surface of the bottom wafer  204 , the comb lines  114  may be disposed at the surface of the bottom wafer  204  to prevent misalignment in the Y direction from preventing the comb lines  114  from contacting the common node  106  or the test node  118 . 
     It has been discovered that infrared alignment techniques used to align wafers are constrained by the wavelength of the infrared signals, which is from about 1 μm to about 15 μm. As alignment resolutions approach 0.1 μm, the alignment measurements provided by infrared alignment becomes ambiguous, preventing accurate alignment of wafers. The alignment test structure  100  provides testing and verification of alignment when aligning wafers with a resolution less than 0.1 μm. 
     Thus, some embodiments, the width of the comb contact surfaces  116  and the width of the common line contact surfaces  110  and the test line contact surfaces  122  each are between about 0.005 μm and about 0.2 μm, and in an embodiment, are about 0.01 μm. Such sizes permit a more precise analysis of alignment accuracy than purely infrared alignment techniques. 
       FIG. 3A  is a top view diagram of a wafer illustrating placement of alignment test structures  100  according to some embodiments.  FIG. 3B  is an enlarged view of placement of a comb  112  in a dicing street  304  according to some embodiments, and  FIG. 3C  is an enlarged view of placement of a common node  106  and test node  118  in a dicing street  304  according to some embodiments. The alignment test structure  100  is disposed in a wafer  300  such as a bottom wafer  204 . The wafer  300  has a plurality of dies  302  separated by dicing streets  304 . The common node  106  and test node are formed in a first wafer  300  and the comb  112  is formed in a second wafer  300  in a corresponding location. 
     In an embodiment, the alignment test structures  100  are formed in the dicing streets  304  so that the alignment can be tested after bonding, but without using die  302  area. In another embodiment, the alignment test structures  100  are formed in the dies  302 , or in both the dies  302  and dicing streets  304 , or in other locations, to permit testing of alignment of the individual dies  302  after bonding and dicing. 
     In some embodiment, multiple alignment test structures  100  are formed in the wafer  300 . For example, a first alignment test structure  100  is formed to test alignment in a first direction, and a second alignment structure  100  is formed with a different orientation to test alignment in a second direction. In such an arrangement, the alignment test structures  100  are oriented at substantially 90 degrees to each other, or at another angle. In other embodiment, multiple alignment test structures  100  in substantially the same direction, but spaced apart laterally. Such an arrangement permits testing of rotational misalignment, as multiple alignment test structures  100  that are laterally spaced apart will not all register the same alignment shift if the wafers  300  are rotated with respect to each other. 
       FIG. 3B  is a diagram illustrating an enlarged top view of a comb  112  disposed in a bottom wafer  204  according to some embodiments.  FIG. 3C  is a diagram illustrating an enlarged top view of a common node  106  and test node  118  disposed in a top wafer  202  according to some embodiments. In come embodiments the common node  106  and test node  118  are disposed in the bottom wafer  204  in the same location in the dicing street  304  and relative to the dies  302  as the location of the comb  112  in the bottom wafer  204  in relation to the dicing street  304  and dies  302 . Thus, the comb  112  will correspond to the test node  118  and the common node  106  when the top wafer  202  is bonded to the bottom wafer  204 . The arrangement of the comb  112  with respect to the dies  302  of the bottom wafer  204  and the corresponding arrangement of the test node  118  and common node  106  to the dies  302  to the top wafer  202  results in a portion the comb  112  contacting the common node  106  and test node  118  when the dies  302  of the top wafer  202  and bottom wafer  204  are in contact. Therefore, the contact of the comb  112  with the common node  106  and test node  118  gives an indication of the alignment of the dies  302  of the top wafer  202  with the dies  302  of the bottom wafer  204 . 
       FIGS. 4A-4B  are wiring diagrams illustrating the use of an alignment test structure with a Vernier-type comb  112  according to some embodiments.  FIG. 4A  illustrates a wiring diagram with a Vernier-type comb  112  in a reference or centered alignment according to some embodiments.  FIG. 4B  illustrates a wiring diagram with a Vernier-type comb  112  in an alignment with an alignment shift according to some embodiments. It should be noted that, while the common node  106  and the test node  118  are shown separated by the comb  112 , that the common node  106  and test node  118  are, in embodiments, disposed in the same wafer, but are shown separated for clarity. 
     The Vernier-type comb  112  provides an arrangement of connections between the common node  106  and test node  118  where at least one connection is made between the common node  106  and test node  118 . Where no connections are made, the Vernier-type comb  112  indicates that the alignment shift of the wafers is greater than can be measured by the alignment test structure  100 . Additionally, the comb  112  is shown with five comb lines  114  and the common node  106  and test node with corresponding lines  108  and  120 . However, the number of comb lines  114  is not limited to five as shown, or to three as shown in  FIGS. 1-2B , and in other embodiments, other numbers of comb lines  114  are used. 
     The comb lines  114  have a comb line spacing  406  that is, in some embodiments, greater than the test line spacing  404 . The test line spacing  404  is great enough that as the comb  112  does not connect non-corresponding common node lines  108  with the test node lines  120 . Thus, the test line spacing  404  is at least double the width of the test line contact surfaces  122  multiplied by the number of test line contact surfaces  122  to one side of the center test comb line. For example, where the comb  112  has five come lines  114 , there are two comb lines  114  to each side of the center comb line  114 . Where the width of the test line contact surfaces  122  is 0.01 μm, the test line spacing  404  is at least 0.04 μm. Additionally, the comb line spacing  406  is based on the test line spacing. In some embodiments, the comb line spacing  406  is the comb line contact width added to the test line spacing  404 . In the aforementioned example, the comb line spacing  406  is 0.05 μm. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Alignment 
               
               
                 Node A 
                 Node B 
                 Node C 
                 Node D 
                 Node E 
                 Shift 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0 
                 0 
                 1 
                 0 
                 0 
                 0 
               
               
                 0 
                 1 
                 0 
                 0 
                 0 
                 −0.01 
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 −0.02 
               
               
                 0 
                 0 
                 0 
                 1 
                 0 
                 +0.01 
               
               
                 0 
                 0 
                 0 
                 0 
                 1 
                 +0.02 
               
               
                   
               
            
           
         
       
     
     Table 1 illustrates the different alignment shifts  402  corresponding to a connection arrangement being detected at each test node line  120 . 
     In a reference or centered alignment, as shown in  FIG. 4A , the center comb line  114  is aligned with the center common node line  108  and test node line  120 . In such an alignment, a connection is made through the common pad  102  through Node C, indicating that the alignment shift  402  is zero, as shown in the first row of Table 1. 
     In a misaligned arrangement, as shown in  FIG. 4B , the comb  112  is shifted so that the center comb line  114  does not align with the center common node line  108  and test node line  120 . Instead, the comb  112  has a comb line  114  aligned with test node line  120  of Node B, indicating an alignment shift  402  of −0.01 μm, as shown in the second line of Table 2. 
     Notably, Table 1 illustrates a single comb line  114  contacting a single test node line  120 . The comb lines  114  are arranged and spaced to have a single comb line  114  aligning directly with a corresponding test node line  120  at predetermined alignment shifts. However, in a bonding arrangement where the alignment shift  402  falls between the discrete alignment shift measurements shown in Table 1, comb lines  114  may not align directly with corresponding test node lines  120 . The alignment shift  402  is interpolated by determining which two test node lines  120  produce a test signal through the common node. 
       FIGS. 5A-5C  are wiring diagrams illustrating the use of an alignment test structure with a binary comb  502  according to some embodiments. In some embodiments, the combination of connections between multiple test node lines  120  indicates the alignment shift. Instead of a single comb line  114  corresponding to each test node line  120  and common node line  108 , a binary comb  502  has multiple comb lines  114  corresponding to each test node line  120  and common node line  108 . The comb lines  114  are arranged so that different comb line  114  combinations come into contact with the test node line  120  and common node line  108  depending on the alignment shift  504 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Alignment 
               
               
                   
                 Node A 
                 Node B 
                 Node C 
                 Shift 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 1 
                 0 
                 0 
                 −0.01 
               
               
                   
                 0 
                 1 
                 0 
                 −0.02 
               
               
                   
                 0 
                 0 
                 1 
                 −0.03 
               
               
                   
                 1 
                 0 
                 1 
                 −0.04 
               
               
                   
                 1 
                 1 
                 1 
                 −0.05 
               
               
                   
                 0 
                 1 
                 1 
                 −0.06 
               
               
                   
                 1 
                 1 
                 0 
                 −0.07 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 illustrates the different alignment shifts  402  corresponding to a different number of connections or connection arrangements being detected at each test node line  120 . 
     In a reference or centered alignment, as shown in  FIG. 5A , none of comb lines  114  are aligned with the common node lines  108  and test node lines  120 . In such an alignment, the arrangement of connections with no connections made through the test node lines  120  indicates that the alignment shift  504  is zero, as shown in the first row of Table 2. As shown in  FIG. 5B , a connection made only through Node A indicates that the alignment shift  504  has shifted the binary comb  502  by the width of a single comb line  114 , as shown in the second row of Table 2. In an example where the comb lines  114  are 0.01 μm wide, the resulting alignment shift would be 0.01 μm. As shown in  FIG. 5C , a connection made through Nodes A, B and C indicates that the alignment shift  504  has shifted the binary comb  502  by 5 comb lines, or 0.5 μm, as shown in the sixth row of Table 2. It should be understood that the arrangement of comb lines  114  in the binary comb  502  is not limited to the arrangement shown, and that any arrangement of comb lines  114  may be provided, resulting in a different combination of comb line connections associated with each alignment shift  504 . 
       FIG. 6  is a flow diagram illustrating a method  600  for forming and using an alignment test structure. Test nodes and common nodes are firmed in a first wafer in block  602 , and combs are formed in a second wafer in block  604 . In an embodiment, the comb and test and common nodes may be formed during the formation of the dies or other structures on the wafers. The wafers are bonded in block  606 , and individual node connections are tested in block  608 . The information gathered from testing the connections in block  608  is used to determine the alignment offset of the bonded wafers on block  610 . 
     Thus, a device according to some embodiments comprises a common node disposed in a first wafer and a test node disposed in the first wafer, the test node having a plurality of test pads exposed at a first surface of the first wafer, the test node further having a plurality of test node lines separated by a first spacing and extending to a second surface of the first wafer and each connected to a respective one of the plurality of test pads. The device further comprises a comb disposed in a second wafer and having a plurality of comb lines having a second spacing different from the first spacing, each of the comb lines having a first surface exposed at a first side of the second wafer, with the comb lines configured to provide an indication of an alignment of the first wafer with the second wafer by a number or arrangement of connections made by the plurality of comb lines between the test node lines and the common node. In an embodiment, the second spacing is greater than the first spacing. The comb is a Vernier-type comb and the test node has a same number of test node lines as a number of comb lines. The comb lines are arranged to have a single comb line aligning directly with a corresponding test line at predetermined alignment shifts. In another embodiment, the comb is a binary comb. The comb lines are spaced laterally apart, and the first surface of each of the comb lines has a lateral width between about 0.005 μm and about 0.2 μm. A portion of the test node and a portion of the common node are disposed in a redistribution layer (RDL) on the first wafer, and the comb is disposed in an RDL on the second wafer. 
     A device according to some embodiments comprises a first wafer having a common node extending from a first side of the first wafer to a second side, the first wafer further having a test node having a plurality of test node lines each extending from the first side of the first wafer to the second side. The device further comprises a second wafer having a comb having a plurality of comb lines, each of the comb lines having a first portion and a second portion each disposed at a first side of the second wafer. The first wafer is bonded at a second side to a first side of the second wafer and the comb is configured to create a number of connections and connection arrangements between the common node and test node that indicate an alignment shift of the first wafer in relation to the bonded second wafer. The comb lines are configured to provide, at one of a plurality of predetermined alignment shifts, electrical connections between the common node and one or more test lines when the first wafer is bonded to the second wafer. In some embodiments, the comb is a Vernier-type comb, and in other embodiments, the comb is a binary comb. In some embodiments, the comb lines are spaced laterally apart, and the first portion of each of the comb lines has a lateral width between about 0.005 μm and about 0.2 μm. In some embodiments, wherein the comb lines are spaced laterally apart and the test lines have a first surface of the first potion disposed at the first side of the second wafer, and the first surface of each test node line has a lateral width of about 0.01 μm. A portion of the test node and a portion of the common node are disposed in a redistribution layer (RDL) on the first wafer, and the comb is disposed in an RDL on the second wafer. The test node and the common node are disposed in a dicing street on the first wafer, and wherein the comb is disposed in a dicing street on the second wafer. 
     A method according to an embodiment comprises providing a top wafer having a common node extending from a first side to a second side and a test node extending from the first side to the second side, the test node having a plurality of test node lines spaced apart by a first separation distance. The method further comprises providing a bottom wafer having a comb, the comb having a plurality of comb lines spaced apart by a second separation distance different than the first separation, each of the comb lines having a first portion and a second portion disposed at a first surface of the bottom wafer. A location of the comb in the bottom wafer corresponds to locations of the common node and the test node in the top wafer. The second side of the top wafer is bonded to the first side of the bottom wafer to form a wafer bond and an alignment shift of the wafer bond is determined by determining a number or arrangement of connections between the common node and the test node by the comb. In some embodiment, the comb is a Vernier-type comb, and the determining the alignment shift comprises checking for an electrical connection between the common node and each of the plurality of test node lines and determining the alignment shift based on which test node exhibits the electrical connection with the common node. In other embodiments, the comb is a binary comb, and the determining the alignment shift comprises checking for a combination of electrical connections between the common node and each of the plurality of test node lines and determining the alignment shift from the combination of electrical connections. 
     One general aspect of embodiments disclosed herein includes a device including: a common node disposed in a first wafer; a test node disposed in the first wafer and having a plurality of test pads exposed at a first surface of the first wafer, the test node further having a plurality of test node lines separated by a first spacing and exposed at a second surface of the first wafer and each connected to a respective one of the plurality of test pads; and a comb disposed in a second wafer and having a plurality of comb lines having a second spacing different from the first spacing, each of the comb lines having a first surface exposed at a first side of the second wafer 
     Another general aspect of embodiments disclosed herein includes a device including: a first wafer having a common node extending from a first side of the first wafer to a second side, the common node including a plurality of common node lines exposed at the second side of the first wafer, the first wafer further including a plurality of test node lines each test node line having a first end exposed at the first side of the first wafer and a second end exposed at the second side of the first wafer; and a second wafer bonded to the first wafer, the second wafer having a comb including plurality of comb lines, each of the comb lines having a first end adjacent a corresponding common node line and a second end adjacent to a corresponding test node line. 
     Yet another general aspect of embodiments disclosed herein includes a method, including bonding a first wafer to a second wafer, the first wafer having therein a first conductive structure having a repeating pattern with a first pitch, the second wafer having therein a second conductive structure having a repeating pattern with a second pitch different than the first pitch; and determining an alignment shift of the first wafer and the second wafer determining a number or arrangement of connections between the first conductive structure and the second conductive structure. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.