Abstract:
Bonding of substrates including metal-dielectric patterns on a surface with the metal raised above the dielectric is disclosed. One method includes providing a first substrate having a metal-dielectric pattern on a surface thereof; providing a second substrate having a metal-dielectric pattern on a surface thereof; performing a process resulting in the metal being raised above the dielectric; cleaning the metal; and bonding the first substrate to the second substrate. A related structure is also disclosed. The bonding of raised metal provides a strong bonding medium, and good electrical and thermal connections enabling creation of three dimensional integrated structures with enhanced functionality.

Description:
GOVERNMENT INTEREST 
     This invention was made with government support under contract numbers: N66001-00-C-8003 and N66001-04-C-8032 of the Defense Advanced Research Projects Agency (DARPA). The government has certain rights to this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention generally relates to the integrated circuit fabrication and, more specifically, to bonding of substrates having metal-dielectric patterns. 
     2. Background Art 
     In the integrated circuit fabrication industry, substrate bonding is used to join various parts of an integrated circuit (IC) together. Thermal-compression bonding is one type of bonding that is advantageous because it allows the use of metals at the bonding interface rather than just dielectrics. The metallic surface serves as a good bonding medium, and also provides additional functionality such as electrical signal propagation and thermal spreading. Unfortunately, thermal compression bonding presents many challenges in order to achieve a good quality bond. 
     One challenge is optimizing process parameters. Process parameters may include, for example, surface preparation before bonding such as cleans and wetting treatments, process conditions during bonding such as temperature, pressure, force, time, etc., and post-bonding treatments such as thermal cycles. 
     Another challenge is addressing patterned metal-dielectric mating surfaces that typically include topography, which prevents a reliable bond. In particular, metal-dielectric patterning is conventionally formed using damascene processing. Damascene processing involves fabricating interconnect metal lines by forming and filling trenches in a dielectric, and attempting to planarize the surface by chemical-mechanical polishing (CMP). Bonding of patterned surfaces takes place after the CMP step. CMP, however, leaves the metal surface non-uniform, e.g., the metal is typically concave and non-planar relative to the dielectric. As a result, the topographical differences between bonding metallic surfaces presents a challenge. Conventionally, the heating process during the bonding, which causes the reflow of the metallic pattern, has been considered sufficient to cause bonding. Unfortunately, the non-uniformity and the difference in the distances between bonding metallic surfaces does not always allow for adequate bonding via reflow, resulting in low yields. 
     SUMMARY OF THE INVENTION 
     Bonding of substrates including metal-dielectric patterns on a surface with the metal raised above the dielectric is disclosed. One method includes providing a first substrate having a metal-dielectric pattern on a surface thereof; providing a second substrate having a metal-dielectric pattern on a surface thereof; performing a process resulting in the metal being raised above the dielectric in at least one of the substrates; cleaning the metal; and bonding the first substrate to the second substrate. A related structure is also disclosed. The bonding of raised metal provides a strong bonding medium, and good electrical and thermal connections enabling creation of three dimensional integrated structures with enhanced functionality. 
     A first aspect of the invention includes a method comprising: providing a first substrate having a metal-dielectric pattern on a surface thereof; providing a second substrate having a metal-dielectric pattern on a surface thereof; performing a process resulting in the metal being raised above the dielectric in at least one of the substrates; cleaning the metal; and bonding the first substrate to the second substrate. 
     A second aspect of the invention includes a structure comprising: a substrate including a dielectric having a metal therein, the metal extending above a surface of the dielectric and including an upper surface having at least a portion thereof in a substantially convex form. 
     A third aspect of the invention includes a method comprising: providing a first substrate having a metal-dielectric pattern on a surface thereof; providing a second substrate having a metal-dielectric pattern on a surface thereof; recessing the dielectric in at least one of the substrates by performing chemical mechanical polishing of the dielectric, and performing an etch; cleaning the metal; bonding the first substrate to the second substrate; and annealing the first substrate and the second substrate. 
     A fourth aspect of the invention includes a method comprising: providing a first substrate including a dielectric having a metal-dielectric pattern on a surface thereof with metal recessed; providing a second substrate having a metal-dielectric pattern on a surface thereof with dielectric recessed below metal by performing chemical mechanical polishing of the dielectric, and performing an etch; providing the lock-n-key ability, then cleaning the metal surface; bonding the first substrate to the second substrate; and annealing the bonded stack. 
     The illustrative aspects of the present invention are designed to solve the problems herein described and/or other problems not discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
         FIGS. 1-3  and  5  show one embodiment of raising a metal above a dielectric for bonding according to the invention. 
         FIGS. 4A-C  show graphs illustrating metal profiles according to various embodiments of raising a metal above a dielectric according to the invention. 
         FIG. 6  shows an optional process according to one embodiment of the invention. 
         FIGS. 7-8  show one embodiment of bonding according to the invention. 
         FIG. 9  shows an optional anneal according to one embodiment of the invention. 
         FIGS. 10-11  show an optional bonding according to an alternative embodiment of the invention. 
         FIGS. 12-13  show an alternative embodiment of raising a metal above a dielectric for bonding according to the invention. 
         FIGS. 14-15  show one embodiment of bonding the  FIGS. 12-13  embodiment according to the invention. 
         FIG. 16A  shows an optional anneal according to one embodiment of the invention applied to the embodiment of  FIGS. 12-15 . 
         FIG. 17A  shows an optional bonding according to an alternative embodiment of the invention applied to the embodiment of  FIGS. 12-13 . 
         FIGS. 16A-17B  show an optional anneal and bonding, respectively, according to an alternative embodiment of invention applied to the embodiment of  FIGS. 12-13 . 
         FIGS. 18-22  show an alternative embodiments of roughening a raised metal for bonding and bonding according to the invention. 
         FIGS. 23-24  show an alternative embodiment of providing an interlocking option to enable bonding with good alignment accuracy. 
     
    
    
     It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION 
     Turning to the drawings,  FIGS. 1-9  show one embodiment of a method of bonding according to the invention.  FIG. 1  shows a substrate  100  having a metal-dielectric pattern  102  on a surface  104  thereof. As will be described herein, two substrates  100  having metal-dielectric patterns  102  will ultimately be provided and bonded together. Metal-dielectric pattern  102  includes a metal  110  such as a wire or contact positioned within a dielectric  112 . It is understood that the layout of metal  110  within the different substrates may vary according to the functioning of the integrated circuit (IC) formed thereby. Metal  110  may include any now known or later developed metallic material  114 , e.g., copper, tungsten, aluminum, etc., or combination thereof. In addition, metal  110  may include an oxide layer  116  thereover, e.g., where it has been exposed to the environment. Dielectric  112  may include any now known or later developed insulator material usable in bonded substrates  100 , e.g., silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ) or low-k dielectrics. 
     Substrate  100  may be formed using any now known or later developed techniques such as damascene processing, chemical mechanical polishing (CMP), etc. As shown in  FIG. 1 , metal  110  has a concave upper surface  118  caused by CMP, which may exist in oxide layer  116  and/or metallic material  114 . Each substrate  100  may be exposed to CMP. Other layers  120  such as other interconnect layers, a silicon substrate, etc., may be provided under metal-dielectric pattern  102 . 
     Next, a process is performed resulting in metal  110  being raised above dielectric  112  in at least one of substrates  100  that will be bonded together.  FIGS. 2-3  show one embodiment of this process in which dielectric  112  is recessed. This embodiment may include one or more processes.  FIG. 2  shows one process in which dielectric  112  is chemical mechanical polished (CMP)  130  to recess dielectric  112 .  FIG. 4A  shows a graph illustrating a height of metal  110  compared to dielectric  112  according to one illustrative structure exposed to the CMP of  FIG. 2  alone.  FIG. 3  shows another process in which dielectric  112  is exposed to a wet etch  134  to recess dielectric  112 . In one embodiment, where dielectric  112  includes silicon dioxide (SiO 2 ), wet etch  134  may include a diluted hydrofluoric acid (HF) etch; however, other etch process are also considered within the scope of the invention.  FIG. 4B  shows a graph illustrating a height of metal  110  compared to dielectric  112  according to one illustrative structure exposed to wet etch  134  of  FIG. 3  alone. In another embodiment, substrate  100  is exposed to both CMP  130  of  FIG. 2  and wet etch  134  of  FIG. 3 .  FIG. 4C  shows a graph illustrating a height of metal  110  compared to dielectric  112  according to one illustrative structure exposed to CMP  130  of  FIG. 2  and wet etch  134  of  FIG. 3 .  FIGS. 4A-4C  show that metal  110  is raised above dielectric  112 , and may include an upper surface  135  having at least a portion  136  thereof in a substantially convex form, e.g., rounded off. (Upper surface  135  is labeled in  FIGS. 2 and 3  also, but the substantially convex form is not as recognizable due to limitations of the drawings.) In some instances, upper surface  135  may have a substantially domed shape  138 , as shown best in  FIG. 4C . In either case, any concave upper surface  118  ( FIG. 1 ) is removed in this process, which fosters better metal-to-metal bonding. 
       FIG. 5  shows cleaning  140  of metal  110 , e.g., etching of any oxide layer  116  ( FIGS. 1-3 ) off of metal  110 . The etching may include any now known or later developed etching technique for removing any dielectric used (e.g., silicon dioxide, silicon nitride) from a metal, e.g., a reactive ion etch, a wet etch, etc. 
       FIG. 6  shows an optional process of performing a metal surface neutralization process. In one embodiment, this process may include forming a layer  142  of benzotriazole (BTA) on metal  110 . BTA layer  142  prevents further oxidation. 
       FIGS. 7-8  show bonding a first substrate  100 A to a second substrate  100 B. As shown in  FIG. 7 , each of substrates  100 A,  100 B have been exposed to the above described process. However, it is understood that it may be possible to carry out the bonding with only one of substrates  100 A,  100 B exposed to the process to raise metal  110  above dielectric  112 , described herein. Bonding may include any now known or later developed thermal compression techniques. BTA layer  142  ( FIG. 6 ) dissolves at high temperature.  FIG. 8  shows a completed bonded structure  160  including first substrate  100 A coupled to second substrate  100 B by metal  110  of each respective substrate. In this case, bonding includes bonding only metal  110  of first substrate  100 A and second substrate  100 B because dielectric  112  is substantially recessed. As a result, a gap  162  is present between dielectric  112  of each respective substrate. 
       FIG. 9  shows a bonded structure  168  after an optional annealing  166 . As shown in  FIG. 8 , typically, a low temperature (&lt;400° C.) quick (&lt;1 hr) thermo-compression bonding ( FIG. 7 ) results in a clearly visible interface (or seam)  164  between metal  110  of each respective substrate  100 A,  100 B. An optimized post-bond annealing  166  provides the time and temperature needed for the interfacial grain growth enabling seam free interface ( FIG. 9 ). In addition, annealing  166  allows for lower thermo-compression bond temperatures ( FIG. 7 ) and higher process load/through-put through the bonding process ( FIG. 7-8 ) because substrates  100 A,  100 B can be processed in a batch (parallel processing) through annealing  166 . Annealing  166  may therefore improve the yield of the bonding process and can be utilized to improve manufacturability of bonded structures  160 ,  168 . 
     Turning to  FIGS. 10-11 , in an alternative embodiment, substrates  100 A,  100 B may be processed, as described above, in an optimized manner such that, when bonded, metal  110  (with or without BTA layer  142  ( FIG. 6 )) and dielectric  112  bond together, i.e., gap  162  ( FIGS. 8-9 ) is eliminated. That is, the bonding includes bonding metal  110  of first substrate  100 A and second substrate  100 B, and dielectric  112  of first substrate  100 A and second substrate  100 B. In this case, metal  110  is raised only high enough such that when bonded, dielectrics  112  bond also. Bonded structure  170  ( FIG. 11 ) exhibits enhanced bonding strength across the whole substrates  100 A,  100 B compared to that of  FIGS. 7-9 , and prevents potential contamination in gap  162  ( FIGS. 8-9 ).  FIG. 11  shows bonded structure  170  after an optional anneal, i.e., without an interface  164  ( FIG. 8 ). It is understood, however, that the annealing may not be necessary. 
     Referring to  FIGS. 12-13 , in an alternative embodiment, the process resulting in metal  110  being raised above dielectric  112  in at least one of substrates  100  may be provided by raising metal  110 , rather than recessing dielectric  112 . In this case, processing begins with substantially the same structure as shown in  FIG. 1 , and as shown in  FIG. 12 , metal  110  is cleaned (similar to cleaning  140   FIG. 5 ), e.g., by an etching of any oxide layer  116  ( FIG. 1 ) off of metal  110 . The etching may include any now known or later developed etching technique for removing oxide layer  116  ( FIG. 1 ) from a metal, e.g., a reactive ion etch, a wet etch, etc. 
       FIG. 13  shows forming a metal cap  182  over metal  110 . Metal  182  may be any metal or metal alloy compatible with metallic material  114 . As known to those with skill in plating, if metallic material  114  includes copper (Cu), metal cap  182  may include any thermal compression bondable metal such as copper or a copper alloy such as tin-copper (SnCu), titanium (Ti), etc. Metal cap  182  may be formed using any now known or later developed techniques, e.g., chemical vapor deposition, atomic layer deposition, etc. 
       FIG. 14-16A  show bonding of substrates  100 A,  100 B formed using the process of  FIGS. 12-13 . The bonding is substantially similar to that described relative to  FIGS. 7-9 .  FIG. 15  shows a bonded structure  184  after thermo-compression bonding, with an interface  186  therein.  FIG. 16A  shows a bonded structure  188  after an optional anneal  190 . While  FIGS. 15-16  show a gap  192 , it is understood that this embodiment may also employ the optimization of  FIGS. 10-11  to arrive at a bonded structure  194 , as shown in  FIG. 17A , without a gap  192  ( FIG. 16 ). Note,  FIG. 17A  also includes the optional anneal  190  ( FIG. 16A ) to remove interface  186  ( FIG. 15 ). As shown  FIG. 16B-17B , if metal  110  ( FIG. 15 ) used easily diffuses into copper (Cu), then no interface may exist between the different metals, and they may meld into a unitary copper alloy. 
       FIGS. 18-22  show another alternative embodiment in which a metal  210  is roughened to have a roughened surface  298  prior to bonding. In some cases, where metal  210  is sufficiently high, a substantially domed shape  138  ( FIG. 4C ) upper surface  135  ( FIG. 2 ) is advantageous compared to concave upper surface  118  ( FIG. 1 ) which has raised edges. However, if metal  210  is not sufficiently raised, roughening of metal  210  to have a roughened surface  298  may aid in strengthening bonding.  FIG. 18  shows roughening of metal  210  to include roughened surface  298 , e.g., by etching such as RIE or a wet etch.  FIG. 18  may be after the process to raise metal  210  according to any of the above-describe embodiments to raise metal  210 .  FIG. 18  shows metal  210  including an oxide layer  216 , and  FIG. 19  shows metal  210  after cleaning, e.g., etching to remove oxide layer  216  ( FIG. 18 ) as described above relative to  FIGS. 5 and 12 . It is understood, however, that metal  210  may be roughened in the absence of oxide layer  216 .  FIG. 19  shows optional performing of a metal surface neutralization process. As described above relative to  FIG. 6 , this process may include forming a layer  242  of benzotriazole (BTA) on metal  210 , which prevents further oxidation. 
       FIGS. 21-22  show bonding of substrates  200 A,  200 B formed according to the embodiments of  FIGS. 18-20 , resulting in a bonded structure  300  ( FIG. 22 ). It is understood that while bonded structure  300  includes a gap  302  and an interface  304 , it may be formed as described above relative to  FIGS. 10-11  without gap  302 , and/or, as described above relative to  FIG. 9  without interface  304 . If an optional anneal is performed, then interface  304  may be removed. 
       FIGS. 23-24  show another alternative embodiment in which substrates  300 A,  300 B may be bonded in an interlocking fashion. In this case, two substrates  300 A,  300 B each having substantially similar structure to that shown in  FIG. 1  may be provided. Here, as shown in  FIG. 23 , one substrate  300 A is processed according to one of the embodiments above to raise metal  110  above dielectric  112 . A BTA layer  142  may or may not be provided on metal  110 . In addition, a substrate  300 B having a structure substantially similar to  FIG. 1  may be provided. As shown in  FIG. 24 , substrate  300 B is processed to raise dielectric  112  above metal  110 , which may or may not include oxide layer  116 . Dielectric  112  may be raised by deposition of more dielectric in any manner, e.g., masking metal  110  and deposition, or by recessing of metal  110  in any manner, e.g., masking dielectric  112  and etching metal  110 . In one embodiment, metal  110  of substrate  300 B may be wider than metal  110  of substrate  300 A to accommodate mating, i.e., the opening in dielectric  112  is larger in substrate  300 B. In one alternative embodiment, where necessary, dielectric  112  and/or metal  110  of substrate  300 B may form an angled opening  103  to assist in mating with metal of substrate  300 A. However, this may not be necessary in all instances. Substrates  300 A,  300 B may then be bonded according to any one of the above described embodiments such that metals  110  thereof interlock between dielectrics  112  thereof. 
     Returning to  FIGS. 3 and 4C , in another embodiment, the invention may include a structure including substrate  100  including dielectric  112  having metal  110  therein. As described above, metal  110  extends above surface  104  of dielectric  112  and include an upper surface  135  having at least a portion  136  thereof in a substantially convex form. As shown in  FIG. 4C , upper surface  135  ( FIG. 3 ) may be substantially dome shaped  138 . 
     The method as described above may be used in the fabrication of integrated circuit chips. In addition, the method may be employed in bonding of mechanical components, e.g., micro-electrical mechanical systems (MEMS), or optical components, such as a micro-optical bench. In the case of IC chips, the resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). Regardless of the method, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.