Abstract:
The present invention provides a stabilized fine textured metal microstructure that constitutes a durable activated surface usable for bonding a 3D stacked chip. A fine-grain layer that resists self anneal enables metal to metal bonding at moderate time and temperature and wider process flexibility.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation U.S. Ser. No. 14/485,674 filed Sep. 13, 2014 which is a continuation of U.S. Ser. No. 13/736,984 filed Jan. 9, 2013, now U.S. Pat. No. 8,916,448, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Embodiments of present invention relate generally to three dimensional (3D) integrated circuits, and more particularly to enabling the metal-to-metal bonding of integrated circuit substrates at low temperature and pressure. 
         [0003]    The semiconductor industry continues to drive toward greater functionality and speed of integrated circuits. For the most part, such improvements have been achieved by scaling the feature size such that more smaller devices can fit in a given area. But such scaling cannot continue indefinitely because devices are now approaching atomic dimensions. Furthermore, as the density has increased, so has the complexity and length of the interconnect circuitry, causing increases in both circuit resistance-capacitance (RC) delay and power consumption. Three-dimensional integrated circuits—that is, stacked chips bonded together—provides an opportunity to overcome these limitations. 
         [0004]      FIG. 1  illustrates a 3D stack  100  in which two semiconductor die are bonded together. Each die may be one of a plurality of dice on a whole or partial semiconductor wafer, such that 3D stack  100  can represent any combination such as wafer to wafer, die to wafer, or die to die. A first die  110  includes a semiconductor substrate  106  in which at least one semiconductor device  101  formed and interconnect wiring layer  102 , and a second die  120  includes one or more TSVs (through substrate vias)  121  for passing power or signals entirely through the second die  120  to the devices  101  of the first die. The interconnect wiring  102  is embedded in back end of the line (BEOL) dielectric layers  103 , formed on substrate  106 . The interconnect wiring and TSVs include a conductive core, which can be formed of copper. The core is typically separated from the surrounding materials (e.g., the substrate wafer or dielectric layers) by liner and barrier layers. 
         [0005]    The wiring and TSVs are conventionally formed by plating copper onto a seed layer that has been deposited such as by PVD or ALD. The grain size of the plated copper depends on the plating conditions and the thickness of the deposition. This microstructure is known to “self-anneal”, that is, the thermodynamics favor grain growth, even at room temperature, and even though this grain growth induces a tensile stress. See Lee and Wong, “Correlation of stress and texture evolution during self- and thermal annealing of electroplated Cu films”,  J. Appl. Phys  93:4 3796-3804. Such stresses can cause distortions such as warpage of a silicon wafer. Typical semiconductor processing includes a thermal anneal to accelerate the atomic rearrangement and bring the plated copper to an equilibrium state. 
         [0006]    Controlling the anneal enables further high fidelity processing such as alignment and bonding of two substrates to form the 3D stacked structure of  FIG. 1 . First die  110  (after completing BEOL processing to form layer  103  with interconnect wiring  102 ) and second die  120  can be bonded by thermocompression bonding between a plated metal (e.g.,  105 ) on the bonding surface  104  of the first die and a plated metal (e.g.,  125 ) on the bonding surface  124  of the second die. This method may be used in die to die, or die to wafer, as well as for wafer to wafer bonding for 3D applications. The facing surfaces of the two die can be formed with metal regions, for example, a mirror image pattern of copper regions as depicted in  FIG. 2 . These metal regions can constitute part of the circuitry of the final 3D stack, or they can be created strictly as bonding regions. Metal bonding to form the 3C integrated circuit stack can be achieved by holding the stack together at temperature of at least 350-400 C for at least 30 to 60 minutes. 
         [0007]    Unfortunately, such conditions can exceed the thermal budget of delicate integrated circuitry which cannot be exposed, or at least not for extended time, to high temperature. Farrens reports in “Wafer and Die Bonding Technologies for 3D Integration”, MRS Fall 2008 Proceedings E, that lower temperature atomic diffusion of all fcc metals is primarily along grain boundaries. Bonding a 3D stack at lower temperatures would permit a wider selection of devices and materials, but has not been possible because, as noted above, conventional processing promotes grain growth such that a plated copper surface has a very low concentration of grain boundaries. Even without a thermal anneal, thermodynamics drives grain growth and within a very short time converts the surface microstructure of plated copper such that metal to metal bonding at temperature below 350 C is impractical. 
         [0008]    A need remains to achieve reliable metal bonding in reasonable time at less stressful conditions. 
       BRIEF SUMMARY 
       [0009]    Embodiments of present invention enables metal to metal bonding at lower bond temperature and time combination, enabling a lower thermal budget. 
         [0010]    According to an embodiment, metal to metal bonding can be enabled by electroless plating a metal surface to form a bonding layer wherein an average grain size of said bonding layer is smaller than an average grain size of said metal surface. According to a further embodiment, the metal surface can be oxidized by exposure to H 2 O 2 , TEAH or TMAH to create a roughened surface prior to such electroless plating. 
         [0011]    A custom electroless plating solution may include a reducing agent poison. The custom electroless plating solution may include a deposition poison. The plating solution may include a contaminant species that inhibits grain growth in a deposited layer. 
         [0012]    According to another embodiment, a 3D stacked structure includes a first die bonded to a second die by a metallic bond. The first die includes metal wiring structure formed within a one or more dielectric layers disposed on a semiconductor substrate within which at least one semiconductor device has been formed, and the second die includes a semiconductor substrate having at least one contact pad. The metallic bond is at an interface between said metal structure and said contact pad. A species that inhibits plating other than oxygen is present at the interface at a concentration substantially higher than the concentration of such species within either of said metal structure or said contact pad. 
         [0013]    In another embodiment, a 3D structure includes a bonding layer upon a metal surface. In yet another embodiment, an interconnect for connecting a first semiconductor die to a second semiconductor die includes the bonding layer upon the metal surface. The average grain size of the bonding layer is smaller than an average grain size of the metal surface. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    Exemplary embodiments may best be understood by reference to the detailed description in conjunction with the accompanying figures. The Figures are provided for illustration and are not drawn to scale. 
           [0015]      FIG. 1  depicts a 3D integrated circuit. 
           [0016]      FIG. 2  illustrates the bonding surfaces of two substrates comprising a 3D integrated circuit structure wherein the surfaces have mirror image metal regions within a field of non-conductive material. 
           [0017]      FIGS. 3A and 3B  illustrate a discontinuous seed layer according to various embodiments of the invention. 
           [0018]      FIGS. 4A ,  4 B and  4 C illustrate pre-treating a bonding surface according to embodiments of the invention. 
           [0019]      FIG. 5  illustrates an incompletely polished surface amenable to treatment by embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    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 can 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. Similarly, when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Additionally, to the extent a feature is described to be horizontal or vertical, that orientation is with respect to a generally planar major surface of the substrate which can be in the form of a wafer or disk. 
         [0021]    The present invention promotes metal to metal bonding at lower temperature by forming a layer of fine textured structure on the bonding surface. Fine metal grains can be deposited and arranged to create a layer of fine textured structure capable of metal to metal surface bonding at a reduced temperature, that is, at a temperature lower than the temperature normally required for metal to metal bonding. A plating step ordinarily is conducted in such a manner so that clean surfaces are maintained to enable even metal deposition and growth. Even if a fine metal grain structure could be formed on such a plated surface, the microstructure is not durable because thermodynamics drives the fine grains to “self-anneal”, that is, to join together to form larger grains constituting a lower energy state. The present invention provides a stabilized fine textured metal microstructure that constitutes a durable activated surface even after significant passage of time. Such a fine textured structure can enable metal bonding at lower thermal budget and can be achieved in various ways. 
         [0022]      FIG. 2  illustrates a possible arrangement of the bonding surfaces of the 3D stack  100  of  FIG. 1 . The material of substrate  106  and  126  can be any semiconductor material including but not limited to group IV semiconductors such as silicon, silicon germanium, or germanium, a III-V compound semiconductor, or a II-VI compound semiconductor. Furthermore, substrates  106  and  126  can be bulk silicon or can comprise layers such as, silicon/silicon germanium, silicon on insulator (SOI), ETSOI (extremely thin semiconductor on insulator), PDSOI (partially-depleted semiconductor on insulator) or silicon germanium-on-insulator. The insulator layers of these can be referred to as a buried oxide (BOX) layer which can be any insulating oxide such as, e.g., silicon dioxide, or even an epitaxial oxide such as Gadolinium(III)-oxide (Gd 2 O 3 ). 
         [0023]    Bonding surface  104  of die  110  can be the top of BEOL layer  103 . As such, surface  104  can include exposed regions of metal  105  which could be the topmost portion of interconnect wiring  102 , or they could be pads formed exclusively as bonding structure which does not electrically connect to any devices. The remaining portion of surface  104  would be dielectric material such as oxide. Metal regions  105  can be laid out to align with metal regions  125  on the bonding surface  124  of die  120 . 
         [0024]    Surface  124  includes metal regions  125  surrounded by a field. It should be understood that  FIG. 2  can represent bonding surfaces for die to die, die to wafer, or wafer to wafer bonding. Metal regions of the bonding surfaces can be as depicted in  FIG. 2 , with small isolated metal regions in a dielectric (or other non-metal) field, or could be more of a checkerboard or mixed line pattern. The metal regions  105  and  125  can be made from a material selected from the group consisting of copper, nickel, copper/nickel, copper/gold and copper/nickel/gold. One or more layers, e.g. a barrier layer, may separate metal regions from surrounding material but are not shown. The field material surrounding regions  125  depends on the design of die  120  and stack  100 . For example, if vias  121  were etched into substrate  126  from the side opposite surface  124 , but not all the way through the substrate, and then exposed by thinning the substrate to expose the metal regions  125  of vias  121 , then the field could be the material of substrate  126  or it could be another material such as a passivation material. Another option is that die  120  has a BEOL layer (not shown), oriented “face to face” with BEOL layer  103 , in which case the field of surface  124  could be predominantly dielectric material. It should be understood, that first and second die  110  and  120  may have similar or different structure or function, and they can be formed of similar materials such as both having a bulk silicon substrate, but there is no requirement that any analogous structure be of the same material. The metal of regions  125  can be different from that of  105 , provided that they are capable of combining to permit electron delocalization, i.e., forming a metallic bond. A 3D stack can also have bonding surfaces that are primarily metal. For example U.S. Pat. No. 7,939,369, the specification of which is hereby incorporated by reference, teaches a 3D structure joined by metal to metal bonding, where a metal plane constitutes the last layer of both die to be bonded so that each bonding surface is almost completely metal. 
         [0025]    Electroless plating according to embodiments of the present invention can be used to activate a bonding surface having isolated bonding regions (such as depicted by  FIG. 2 ), or one comprised primarily by metal, which is to say it is applicable to joining any two structures by forming a metallic bond. Ordinarily, an electroless solution composition is carefully controlled to promote deposition of a smooth and continuous layer, and is designed to maintain clean interfaces as the plating process proceeds. The plating solution and the process controls are designed to enable and promote continuous films to the thickness required from the plating solution to produce the correct film to meet design characteristics. For example a 1 micrometer thick film may be required for a process and the electroless chemistry is designed to produce a continuous film that will achieve a 1 micrometer thickness without significant internal imperfection. The standard electroless plating solution typically includes a source of metal ions; a reducing agent, such as formaldehyde for copper deposition; additives such as to keep the metal ions in solution or a pH buffer; and a very small concentration, such as a few ppm, of a deposition poison, such as lead (Pb) ions, to stop unwanted plating on the tank or transfer equipment. 
         [0026]      FIG. 3B  illustrates the top or bonding surface  304  of a structure  300 , and  FIG. 3A  illustrates a sideview of structure  300  along a cut at  3 A. Structure  300  includes isolated metal regions  305  embedded within a non-metal material  303 . Isolated metal regions  305  could be the surface, for example, of BEOL metallization, a contact pad, or a TSV. Metal regions  305  may be annealed to have large average grain size of e.g., 2.0 micron or more. According to the present invention, a thermodynamically-stable fine-textured metal layer (having small average grain size such as &lt;1.0 or even &lt;0.6 micron or less) can be formed on a bonding surface by electroless plating. According to a first embodiment, an activated layer (such as a fine-textured layer)  310  can be formed on metal regions  305  by exposing bonding surface  304  to a customized electroless plating solution that includes, in addition to the usual components, an inhibitor to interfere with metal deposition. 
         [0027]    The inhibitor, which may also be referred to as a poison, can be selected from species known to inhibit or stop metal deposition. The selection and concentration of the inhibitor species depends on the resultant textured surface that is desired. For very fine grain textured surfaces, the inhibitor concentration is selected to surface adsorb and poison small regions  314  on which initial fine grains  310  will not form, at a rate comparable to deposition of fine grains  310  on to regions  315 . The chemistry can be tuned so that regions  315  and  314  have roughly equivalent surface area and have a mean diameter as small as tens of nanometers. The proper inhibitor concentration appreciates competitive surface adsorption rates of the inhibitor and metal which is desired to be plated, and can be tuned for plating onto a freshly plated surface or onto a stabilized or annealed metal surface. The electroless solution may include inhibitor at a concentration as low as 100 ppm to as high as several hundred thousand ppm dependent on the reactant constant of the inhibitor relative to the reactant constant of the plating system to deposit metal. For example, the solution could include cobalt (Co) ions in a concentration between 1000 and 10000 ppm, or about 5000 ppm. 
         [0028]    Selection of an inhibitor species depends on the particular electroless chemistry, and can be, for example, one or more of arsenic, cobalt, manganese, chromium, lead, silver, nickel or other metals, metal oxides of any of the foregoing, and can also be compounds such as, e.g., acetone, ammonium peroxydisulfate, cerium ammonium nitrate (CAN), 2-mercapto-5-benzimidazolesulfonic acid (MBIS), and bis-(3-sulfopropyl)-disulfide (SPS). 
         [0029]    According to another embodiment, uniform metal deposition can be disrupted by interfering with the activity of the plating reducing agent. In that case, an inhibitor to the reducing agent can promote unequal rates of metal ion reduction (ie, deposition) to produce a fine textured surface. The inhibitor concentration for such embodiment could be as low as 100 ppm or several hundred ppm to as high as tens of thousands ppm dependent on the reactant constant of the inhibitor relative to the reactant constant of the plating system to deposit metal. The reducing agent inhibitor could be a non-metal or a metal compound such as a metal ion oxide, or could be, for example, arsenic within the range of 100 to 1000 ppm, or at a concentration of about 500 ppm. 
         [0030]    According to yet another embodiment, activated layer  310  can be formed on a bonding surface  304  by forming a dispersed seed layer. Such dispersed seed layer could be formed by exposing surface  304  to a customized seed solution that includes palladium (Pd) or other seeding catalyst along with a slightly higher concentration of an inhibitor that poisons portions of the exposed surface of metal region  305  and preferentially prohibits uniform seeding. An example of such inhibitor could be, e.g. lead (Pb) or thallium (TI), at a concentration in the range of 50 to 500 ppm. In particular embodiments the inhibitor could be in the range of 350 to 500 ppm to form seeded regions  315  within a matrix of poisoned regions  314  constituting a discontinuous seed layer. Subsequent electroless deposition can form a fine textured and durable activated layer  310  on top of the discontinuous seed layer. 
         [0031]    The poison species will inhibit seed deposition on regions of the exposed metal, such that the seeded deposition that does occur will be a discontinuous layer. Optimally each region  315  is very small, such as just a few seed species, and region  314  has less total surface area than the aggregation of all regions  315 . In a preferred embodiment, the bonding surface constitutes a finely dispersed composition, such as where the mean diameter of regions  315  (ie, the average width of a seed region) is greater than or at least the same order of magnitude as the average edge to edge distance between adjacent seed regions. Electroless deposition of layer  310 , e.g., copper onto such ‘dispersed seed layer’ can maintain a fine grain structure for subsequent bonding because the seed layer constitutes dispersed particles of seed rather than a continuous film. When subsequently bonded, the fine grained layer  310  may anneal with or into the opposite bond surface, but the finely dispersed seed layer will remain as a very thin layer wherein the poison is at substantially higher concentration than in the bulk of the bonded material. In embodiments, the poison concentration at the interface could be more ten times or even several orders of magnitude greater than its concentration in the bulk of the bonded metal structures. In other words, the interface will include a detectible plating poison at a concentration at least one order of magnitude higher than in the adjacent metal structures. In a preferred embodiment, the poison concentration at the interface is at least three orders of magnitude greater than in the bulk of metal regions  305 . 
         [0032]    According to yet another embodiment,  FIG. 4A  depicts subjecting exposed metal  405  to surface pretreatment such as in H 2 O 2 , TEAH, TMAH, to promote non-uniform oxide growth  409 . For example, according to  FIG. 4B , a metal copper surface  406  can be roughened by pre-treatment such as by H 2 O 2 , followed by a partial surface clean with hydroxyl amine. As shown in  FIG. 4C , subsequent plating a film onto such treated surface can create a durable activated layer  408  with enhanced roughness and fine grain size. 
         [0033]      FIG. 5  illustrates yet another substrate  500  that can be treated according to any embodiment of this invention to form a stable activated bonding surface. Substrate  500  includes conductive pathways  522  plated to fill patterned openings in a top dielectric layer  503  where a layer  528  constitutes a current carrier for electrochemical deposition. CMP to remove excess metal can incompletely remove the barrier layer  528 , exposing only small regions of dielectric  503 . Surface pretreatment such as with H 2 O 2  can promote non-uniform oxide growth on exposed metal  522  as illustrated in  FIG. 4 . When subsequently subjected to electroless plating, the differential thickness of this oxide growth, which can preferentially form along copper grain boundaries, is removed and exposes a rough pristine metal surface. A fine-grained bonding layer can be deposited onto the rough surface. The roughness helps to stabilize an activated layer that can be formed with a high density of dislocations and fine grain structure. 
         [0034]    In yet a further embodiment, a contaminant such as tin or silver is included in the electroless solution. Such contaminant is selected to co-deposit with the particular metal to be deposited, so that e.g., tin dispersed in copper is deposited onto a prepared seed layer. Such deposition can be onto a seed layer formed by conventional processing or preferably onto a finely dispersed seed layer as described above. The contaminant, which may constitute from a few ppm up to several percent such as 200 ppm to 1.5%, or within the range of 0.01% to 1% of a thinly deposited layer, can pin the grain boundaries of the deposited metal and thereby enable formation of a stable fine-textured bonding layer. By inhibiting or delaying grain growth, the deposited bonding layer can maintain a fine grained microstructure and enable metal to metal bonding at lower temperature, or in less time, or both. 
         [0035]    It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.