Abstract:
A through-substrate via (TSV) structure includes at least two electrically conductive via segments embedded in a substrate and separated from each other by an electrically conductive barrier layer therebetween. The length of each individual conductive via segment is typically equal to, or less than, the Blech length of the conductive material so that the stress-induced back flow force, generated by each conductive barrier layer, cancels the electromigration force in each conductive via segment. Consequently, the TSV structures are immune to electromigration, and provide reliable electrical connections among a chips stacked in 3 dimensions.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of semiconductor structures, and particularly to electromigration immune through-substrate vias and methods of manufacturing the same. 
         [0002]    A metal structure includes a lattice of metal ions and non-localized free electrons. The metal ions are formed from metal atoms that donate some of their electrons to a common conduction band of the lattice, and the non-localized free electrons move with relatively small resistance within the lattice under an electric field. Normal metal lines, excluding superconducting materials at or below a superconducting temperature, have finite conductivity, which is caused by interaction of electrons with crystalline imperfections and phonons which are thermally induced lattice vibrations. 
         [0003]    When electrical current flows in the metal line, the metal ions are subjected to an electrostatic force due to the charge of the metal ion and the electric field to which the metal ion is exposed to. Further, as electrons scatter off the lattice during conduction of electrical current, the electrons transfer momentum to the metal ions in the lattice of the conductor material. The direction of the electrostatic force is in the direction of the electric field, i.e., in the direction of the current, and the direction of the force due to the momentum transfer of the electrons is in the direction of the flow of the electrons, i.e., in the opposite direction of the current. However, the force due to the momentum transfer of the electrons is generally greater than the electrostatic force. Thus, metal ions are subjected to a net force in the opposite direction of the current, or in the direction of the flow of the electrons. 
         [0004]    High defect density, i.e., smaller grain size of the metal, or high temperature typically increases electron scattering. The amount of momentum transfer from the electrons to the conductor material increases with electron scattering. Such momentum transfer, if performed sufficiently cumulatively, may cause the metal ions to dislodge from the lattice and move physically. The mass transport caused by the electrical current, or the movement of the conductive material due to electrical current, is termed electromigration in the art. 
         [0005]    In applications where high direct current densities are used, such as in metal interconnects of semiconductor devices, electromigration causes a void in a metal line or in a metal via. Such a void results in a locally increased resistance in the metal interconnect, or even an outright circuit “open.” In this case, the metal line or the metal via no longer provides a conductive path in the metal interconnect. Formation of voids in the metal line or the metal via can thus result in a product failure in semiconductor devices. Further, accumulation of electromigrated materials as extrusions or hillocks outside the volume of the original metal structures can result in electrical “shorts” with adjacent metal structures. 
         [0006]    Electromigration is a function of current density and temperature, and accelerates at high current densities and high temperatures. In addition, electromigration is a function of the grain size and the geometry of the metal line. Specifically, the width of the metal line relative to the grain size can have a significant effect on electromigration. If the width of the metal line becomes smaller than the grain size itself, all grain boundaries are perpendicular to the current flow. Such a structure is also known as a “bamboo structure.” Formation of a bamboo structure results in a longer path for mass transport, thereby reducing the atomic flux and electromigration failure rate. 
         [0007]    Further, the length of the metal line can have a significant effect on electromigration. If the length of the metal line is less than a critical length known as the “Blech length,” the metal line is immune to electromigration because the electromigration force is balanced by a stress-induced back-flow of atoms. 
         [0008]    In recent years, “three dimensional silicon” (3DSi) structures have been proposed to enable joining of multiple silicon chips and/or wafers that are mounted on a package or a system board. The 3DSi structures increase the density of active circuits that are integrated in a given space. Such 3DSi structures employ through-substrate vias (TSVs) to provide electrical connection among the multiple silicon chips and/or wafers. The length of the TSVs is substantially equal to the thickness of each silicon chip or wafer. Because the thickness of silicon chips and wafers are on the order of 100 microns, the length of the TSVs exceeds the Blech length. See I. A. Blech, J. Appl. Phys. 47, 1203 (1976). Thus, the TSVs as known in the prior art are inherently subject to electromigration, and can fail during operation of the semiconductor chips. 
       BRIEF SUMMARY 
       [0009]    A through-substrate via (TSV) structure is provided that includes at least two electrically conductive via segments embedded in a substrate. The at least two electrically conductive via segments are separated from each other by an electrically conductive barrier layer therebetween. The length of each individual conductive via segment is typically equal to, or less than, the Blech length of the conductive material so that the stress-induced back flow force, generated by each conductive barrier layer, cancels the electromigration force in each conductive via segment. Consequently, the TSV structures are immune to electromigration, and provide reliable electrical connections among chips stacked in 3 dimensions. 
         [0010]    According to an aspect of the present invention, a semiconductor structure includes a first substrate. The first substrate includes at least one through-substrate via (TSV) structure extending from a first surface located on one side of the first substrate to a second surface located on an opposite side of the first substrate. Each of the at least one TSV structure includes a plurality of conductive via segments that are vertically spaced from one another by at least one conductive liner portion. 
         [0011]    According to another aspect of the present invention, a method of forming a semiconductor structure is provided, which includes: forming at least one trench having substantially vertical sidewalls in a first substrate; filling the at least one trench with a conductive liner material and a conductive via segment material, wherein the conductive liner material constitutes a conductive liner located on sidewalls of the at least one trench and the conductive via segment material fills each cavity in the at least one trench; and performing at least once a set of processing steps. The set of processing steps may include: recessing a portion of a material that fills each upper portion of the at least one trench; depositing another conductive liner material on exposed sidewalls of each of the at least one trench; and depositing another conductive via segment material in each of the at least one trench. By performing the set of processing steps at least once, at least one through-substrate via (TSV) structure is formed in the first substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a vertical cross-sectional view of a first exemplary semiconductor structure before forming at least one trench in a first substrate according to a first embodiment of the present invention. 
           [0013]      FIG. 2  is a vertical cross-sectional view of the first exemplary semiconductor structure after forming at least one trench in the first substrate according to the first embodiment of the present invention. 
           [0014]      FIG. 3  is a vertical cross-sectional view of the first exemplary semiconductor structure after depositing a first conductive liner and a first conductive fill material layer according to the first embodiment of the present invention. 
           [0015]      FIG. 4  is a vertical cross-sectional view of the first exemplary semiconductor structure after removing the first conductive liner and the conductive fill material layer from above a first surface of the first substrate according to the first embodiment of the present invention. 
           [0016]      FIG. 5  is a vertical cross-sectional view of the first exemplary semiconductor structure after recessing the first conductive liner and at least one first conductive via fill portion from an upper portion of each of the at least one trench in the first substrate according to the first embodiment of the present invention. 
           [0017]      FIG. 6  is a vertical cross-sectional view of the first exemplary semiconductor structure after depositing a second conductive liner and a second conductive via fill portion according to the first embodiment of the present invention. 
           [0018]      FIG. 7  is a vertical cross-sectional view of the first exemplary semiconductor structure after removing the second conductive liner and the second conductive via fill portion from above the first surface of the first substrate according to the first embodiment of the present invention. 
           [0019]      FIG. 8  is a vertical cross-sectional view of the first exemplary semiconductor structure after formation of a back-end-of-line metal interconnect layer according to the first embodiment of the present invention. 
           [0020]      FIG. 9  is a vertical cross-sectional view of the first exemplary semiconductor structure after formation of an array of conductive bonding material portions on a second surface of the first substrate according to the first embodiment of the present invention. 
           [0021]      FIG. 10  is a vertical cross-sectional view of the first exemplary semiconductor structure after bonding a second substrate to the first substrate through the array of conductive bonding material portions according to the first embodiment of the present invention. 
           [0022]      FIG. 11  is a vertical cross-sectional view of a second exemplary semiconductor structure after bonding a second substrate to the first substrate through the array of conductive bonding material portions according to a second embodiment of the present invention. 
           [0023]      FIG. 12  is a vertical cross-sectional view of a third exemplary semiconductor structure after bonding a packaging substrate to the first substrate through the array of conductive bonding material portions according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    As stated above, the present invention relates to electromigration immune through-substrate vias and methods of manufacturing the same, which are now described in detail with accompanying figures. Throughout the drawings, the same reference numerals or letters are used to designate like or equivalent elements. The drawings are not necessarily drawn to scale. 
         [0025]    As used herein, a “conductive through-substrate via (TSV) structure” is a conductive structure that extends through a substrate, i.e., at least from a top surface of the substrate to a bottom surface of the substrate. 
         [0026]    As used herein, a surface is “substantially planar” if the surface is intended to be planar and the non-planarity of the surface is limited by imperfections inherent in the processing steps that are employed to form the surface. 
         [0027]    As used herein, a “mounting structure” is any structure to which a semiconductor chip can be mounted by making electrical connections thereto. A mounting structure can be a packaging substrate, an interposer structure, or another semiconductor chip. 
         [0028]    Referring to  FIG. 1 , a first exemplary semiconductor structure according to a first embodiment of the present invention includes a first substrate  100 . The first substrate  100  can include a first handle substrate  112  and a first semiconductor-device-containing layer  118  that can include at least one semiconductor device such as a field effect transistor, a bipolar transistor, a diode, a thyristor, and a memory cell. The first semiconductor-device-containing layer  118  can include semiconductor devices for a semiconductor chip. The first substrate  100  typically has a first substantially planar surface  111  on one side and a second substantially planar surface  119  on an opposite side. The thickness of the first substrate  100  is typically from 100 microns to 1,000 microns, although lesser and greater thicknesses can also be employed. 
         [0029]    A lower portion of the first semiconductor-device-containing layer  118  can include a semiconductor material layer  114 . The semiconductor material layer  114  includes a semiconductor material that can be selected from, but is not limited to, silicon, germanium, silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbon alloy, gallium arsenide, indium arsenide, indium phosphide, III-V compound semiconductor materials, II-VI compound semiconductor materials, organic semiconductor materials, and other compound semiconductor materials. The semiconductor material layer  114  can include a single crystalline material. For example, the semiconductor material layer  114  can be a single crystalline silicon layer. The semiconductor material layer  114  can be doped with dopants of a first conductivity type, which can be p-type or n-type. The dopant concentration of the semiconductor material layer can be from 1.0×10 14 /cm 3  to 1.0×10 17 /cm 3 , although lesser and greater dopant concentration can also be employed. An upper portion of the first semiconductor-device-containing layer  118  can include a lower level metal interconnect layer  116 , which includes at least one dielectric material layer embedding metal interconnect structures such as metal vias and metal lines as well as a gate structure of a field effect transistor and/or an emitter structure of a bipolar transistor. 
         [0030]    The first handle substrate  112  can be a semiconductor substrate including a semiconductor material or an insulator substrate including an insulator material. The first handle substrate  112  typically includes a material that can be removed from the second substantially planar surface  119 , for example, by polishing, grinding, and/or an etch. 
         [0031]    Referring to  FIG. 2 , a photoresist  117  is applied to the first substantially planar surface  111  and lithographically patterned to form at least one opening therein. Preferably, a plurality of openings is formed in the photoresist  117  by lithographic methods. The shape of each opening can be circular, oval, square, rectangular, or polygonal. The lateral dimensions of the shape of each opening in the photoresist  117  can be from about 0.3 microns to 100 microns, and typically from 1 micron to 10 microns, although lesser and greater dimensions can also be employed. 
         [0032]    The pattern in the photoresist  117  can be transferred through the first semiconductor-device-containing layer  118  and into an upper portion of the first handle substrate by an anisotropic etch employing the photoresist  117  as an etch mask. At least one trench  113  extends from the first substantially planar surface  111  into the first substrate  100  after the anisotropic etch. The at least one trench  113  can have substantially vertical sidewalls. The at least one trench can be a plurality of trenches arranged as an array. 
         [0033]    The depth d 0  of the at least one trench  113  as measured from the first substantially planar surface  111  to a horizontal bottom surface of the at least one trench  113  can be substantially equal to the final thickness of the first substrate  100  after planarization to be subsequently performed. The depth d 0  of the at least one trench  113  can be from 10 microns to 300 microns, and typically from 50 microns to 150 microns, although lesser and greater depths can also be employed. The photoresist  117  is subsequently removed, for example, by ashing. 
         [0034]    Referring to  FIG. 3 , a first conductive liner material is deposited on a bottom surface and sidewalls of each of the at least one trench  113 , for example, by electroplating, electroless plating, chemical vapor deposition (CVD), and/or physical vapor deposition (PVD) to form a first conductive liner  130 L. The first conductive liner  130 L includes a single piece of a contiguous metallic material that prevents diffusion of conductive materials that are subsequently deposited thereupon. The first conductive liner  130 L typically contacts the entirety of the bottom surfaces and sidewalls of the at least one trench  113 . The first conductive liner  130 L can be composed of an elemental transition metal, a nitride of an elemental transition metal, or an alloy thereof. For example, the first conductive liner  130 L can be composed of a material selected from titanium (Ti), tantalum (Ta), ruthenium (Ru), tungsten (W), titanium-nitride (TiN), tantalum-nitride (TaN), ruthenium nitride (RuN), tungsten nitride (WN), and alloys thereof. In one embodiment, the first conductive liner  130 L is substantially conformal, i.e., has the same thickness throughout. The thickness of the first conductive liner  130 L can be from 1 nm to 60 nm, and typically from 3 nm to 20 nm, although lesser and greater thicknesses can also be employed. 
         [0035]    A first conductive fill material is deposited on the first conductive liner  130 L, for example, by electroplating, electroless plating, chemical vapor deposition (CVD), and/or physical vapor deposition (PVD) to form a first conductive fill material layer  140 L. The first conductive fill material layer  140 L includes a single piece of a contiguous metallic material having high electrical conductivity. The first conductive fill material layer  140 L either completely or partially fills each of the remaining cavities in the at least one trench  113 . The first conductive fill material layer  140 L can be composed of an elemental metal, a nitride of an elemental metal, or an alloy thereof. For example, the first conductive fill material layer  140 L can be composed of a material selected from tungsten (W), copper (Cu), aluminum (Al), silver (Ag), gold (Au), and alloy thereof. 
         [0036]    The first conductive fill material and the first conductive liner material are selected such that the first conductive liner material prevents diffusion of the first conductive fill material therethrough. The depth d 0  can exceed the Blech length of the first conductive fill material for projected maximum current density therethrough in a vertical direction. 
         [0037]    Referring to  FIG. 4 , the portions of the first conductive liner  130 L from  FIG. 3  and the first conductive fill material layer  140 L from  FIG. 3  are removed from above the first substantially planar surface  111  of the first substrate  100 , for example, by chemical mechanical planarization (CMP), a recess etch, or a combination thereof. The first substantially planar surface  111  of the first substrate  100  is substantially coplanar with top surfaces of the remaining portions of the first conductive liner  130 L and the first conductive fill material layer  140 L in the case where the first conductive fill material layer  140 L completely fills each of the remaining cavities in the at least one trench  113 . If a plurality of trenches is present in the first substrate  100 , each trench includes a first conductive liner portion  130  and a first conductive via fill portion  140 , a portion of which becomes conductive via segment of a through-substrate via structure to be subsequently formed. Each first conductive liner portion  130  is a remaining portion of the first conductive liner  130 L, and each first conductive via fill portion  140  is a remaining portion of the first conductive fill material layer  140 L after the planarization. 
         [0038]    Referring to  FIG. 5 , a second photoresist  157  is applied to the topmost surface of the first substrate  100  and is lithographically patterned to form openings above each of the at least one trench  113  from  FIG. 2 . The at least one first conductive via fill portion  140  and the at least one first conductive liner portion  130  are removed by a recess etch, either concurrently or consecutively, to a first recess depth r 1  from the first substantially planar surface  111  from an upper portion of each of the at least one trench. In one case, the upper portion of a first conductive via fill portion  140  and an upper portion of a first conductive liner portion  130  are removed concurrently by a metal etch to the first recess depth r 1  in each of the at least one trench. Alternately, an upper portion of a first conductive via fill portion  140  is removed selective to a first conductive liner portion  130  to the first recess depth r 1  in each of the at least one trench, and exposed portions of the first conductive liner portion  130  can be subsequently removed by another metal etch. 
         [0039]    The recess depth r 1  is selected such that a first vertical distance d 1  of the remaining portions of the first conductive liner portion  130  and the first conductive via fill portion  140  is less than the Blech length of the material of the first conductive via fill portion  140  at a projected maximum current density that the first conductive via fill portion  140  is expected to be subsequently subjected to. 
         [0040]    Referring to  FIG. 6 , a second conductive liner material is deposited on the topmost surfaces of each of the first conductive liner portion  130 , the first conductive via fill portion  140 , the first substantially planar surface  111 , and exposed sidewalls of each of the at least one trench  113  from  FIG. 2 , for example, by electroplating, electroless plating, chemical vapor deposition (CVD), and/or physical vapor deposition (PVD) to form a second conductive liner  150 L. The second conductive liner  150 L includes a single piece of a contiguous metallic material that prevents diffusion of the material of the first conductive via fill portion  140  and additional conductive materials to be subsequently deposited thereupon. The second conductive liner  150 L contacts the entirety of the topmost surfaces of the first conductive liner portion  130  and the first conductive via fill portion  140 . The second conductive liner  150 L can be composed of the material that can be employed for the first conductive liner  130 L as described above. In one embodiment, the second conductive liner  150 L is substantially conformal. The thickness of the second conductive liner  150 L can be from 1 nm to 60 nm, and typically from 3 nm to 20 nm, although lesser and greater thicknesses can also be employed. 
         [0041]    A second conductive fill material is deposited on the second conductive liner  150 L, for example, by electroplating, electroless plating, chemical vapor deposition (CVD), and/or physical vapor deposition (PVD) to form a second conductive fill material layer  160 L. The second conductive fill material layer  160 L includes a single piece of a contiguous metallic material having high electrical conductivity. The second conductive fill material layer  160 L completely fills each of the remaining cavities in the at least one trench. The second conductive fill material layer  160 L can be composed of any material that can be employed for the first conductive fill material layer  140 L as described above. 
         [0042]    The second conductive liner material is selected such that the second conductive liner material prevents diffusion of the second conductive fill material and diffusion of the first conductive fill material of the first conductive via fill portion  140 . The depth, as measured from the horizontal plane of the first substantially planar surface  111 , of the interface(s) between a bottom surface of the second conductive liner  150 L and the top surfaces of the first conductive liner portion  130  and the first conductive via fill portion  140  is herein referred to as a second vertical distance d 2 . 
         [0043]    Referring to  FIG. 7 , the portions of the second conductive liner  150 L from  FIG. 6  and the second conductive fill material layer  160 L from  FIG. 6  are removed from above the first substantially planar surface  111  of the first substrate  100 , for example, by chemical mechanical planarization (CMP), a recess etch, or a combination thereof. The first substantially planar surface  111  of the first substrate  100  is substantially coplanar with top surfaces of the remaining portions of the second conductive liner  150 L and the second conductive fill material layer  160 L. If a plurality of trenches is present in the first substrate  100 , each trench includes a first conductive liner portion  130 , a first conductive via fill portion  140 , a second conductive liner portion  150 , and a second conductive via fill portion  160 . Each second conductive liner portion  150  is a remaining portion of the second conductive liner  150 L, and each second conductive via fill portion  160  is a remaining portion of the second conductive fill material layer  160 L after the planarization. 
         [0044]    If the second vertical distance d 2  is less than the Blech length of the material of the second conductive via fill portion  160  at the projected maximum current density that the second conductive via fill portion  160  is expected to be subsequently subjected to, each set of a first conductive liner portion  130 , a first conductive via fill portion  140 , a second conductive liner portion  150 , and a second conductive via fill portion  160  within a trench constitutes a through-substrate via (TSV) structure  180 . The first conductive via fill portion  130  is a first conductive via segment in each of the at least one TSV structure  180 , and the second conductive via fill portion  150  is a second conductive via segment in each of the at least one TSV structure  180 . Each of the at least one TSV structure  180  extends from the first substantially planar surface  111  through the first semiconductor-device-containing layer  118  and into an upper portion of the first substrate  100 . 
         [0045]    If the second vertical distance d 2  is greater than the Blech length of the material of the second conductive via fill portion  160  at the projected maximum current density that the second conductive via fill portion  160  is expected to be subsequently subjected to, the processing steps of  FIGS. 5-7  are repeatedly performed (N−1) more times until a TSV structure  180  is formed in each of the at least one trench to form a number (N+1) of vertically stacked set of a conductive liner portion and a conductive via fill portion embedded therein. N is an integer greater than 1. The vertical distance between a topmost surface and a bottommost surface of each set of a conductive liner portion and a conductive via fill portion embedded therein is less than the Blech length of the material of the conductive via fill portion of that set. 
         [0046]    In general, a set of processing steps corresponding to  FIGS. 5-7  can be employed to form each set of a conductive liner portion and a conductive via fill portion embedded therein. The set of processing steps typically includes recessing a portion of a material that fills each upper portion of the at least one trench, depositing a conductive liner material on exposed sidewalls of each of the at least one trench, and depositing a conductive via segment material in each of the at least one trench. The conductive liner material can be any material that can be employed for the first conductive liner portion  130 . The conductive via segment material can be any material that can be employed for the first conductive via fill portion  140 . Each remaining portion of a deposited and recessed conductive liner material constitutes a conductive liner portion that is structurally equivalent to the second conductive liner portion  150 . Each remaining portion of a deposited and receded conductive via segment material constitutes a conductive via segment, which is a conductive via fill portion that is structurally equivalent to the second conductive via fill portion  160  and embedded within one of the conductive liner portions. Each conductive via segment within one of the at least one TSV structure  180  has a vertical length that is less than the Blech length thereof for a maximum current density that the one of the at least one TSV structure  180  is configured to flow within the first substrate  100 . 
         [0047]    A Blech length, or L Blech , of a conductive material is generally determined by the equation of L Blech =(jL) th /j, wherein (jL) th  is the Blech threshold of the conductive material, and j is a current density passing through the conductive material in a direction where the Blech length is measured. For most conductive materials including Cu, Au, Ag, and Al, (jL) th  is a constant known in the art. For example, the Blech threshold (jL) th  is typically around 200 milliamp per micrometer (mA/μm) for copper. The current density can be measured, for example, in a unit of milliamp per micrometer square (mA/μm 2 ). Therefore, for a current density of for example 20 mA/μm 2  passing vertically through a TSV segment consisting essentially of copper, the Blech length may typically be found around 10 micrometers (μm). 
         [0048]    The first conductive via fill portion  140 , the second conductive via fill portion  160 , and the conductive via segment(s), if present, can be the same conductive material. In this case, the Blech lengths for each of the first conductive via fill portion  140 , the second conductive via fill portion  160 , and the conductive via segment(s), if present, are the same, and can be determined by circuit design of the semiconductor chip that is manufactured in the first substrate  100 . 
         [0049]    Specifically, the Blech length for each TSV structure  180  can be calculated by employing the following steps:
       (1) determining a cross-sectional area per through-substrate via from a design of a semiconductor chip that is manufactured in the first substrate  100  for each of the at least one TSV structure  180 ,   (2) determining a maximum current from the design for each of the at least one TSV structure  180 , and   (3) determining the Blech length for each of the at least one TSV structure  180 , wherein the Blech length is determined by L Blech =A TSV ×(jL) th /I TSV , wherein L Blech  is the Blech length, A TSV  is the cross-sectional area of the applicable TSV structure  180 , (jL) th  is a critical current density-length product for a material constituting the plurality of conductive via segments with the applicable TSV structure  180 , and I TSV  is the maximum current rated for the applicable TSV structure  180 .       
 
         [0053]    Because each of the at least one trench  113  (See  FIG. 2 ) has a substantially same depth d 0 , the minimal number of repetitions for performing of the set of processing steps can be determined. Specifically, min(L Blech ), i.e., the minimum of all L Blech , is determined by comparing the numbers for L Blech  among all of the at least one TSV structures  180  and selecting the smallest number among them. Then, d 0 /min(L Blech ) is calculated. If d 0 /min(L Blech ) is an integer, the integer N that satisfies the equation (N+1)=d 0 /min(L Blech ) is the minimal number of repetitions for performing of the set of processing steps. If d 0 /min(L Blech ) is not an integer, the value of d 0 /min(L Blech ) is rounded up to the next nearest integer, which is herein referred to INT {d 0 /min(L Blech )}. The integer N that satisfies the equation (N+1)=INT {d 0 /min(L Blech )} is the minimal number of repetitions for performing of the set of processing steps. Once the smallest integer (N+1) that is equal to or greater than d 0 /min(L Blech ) is determined, the integer N is the minimal number of repetitions for performing of the set of processing steps can be determined. In general, each of the first conductive via fill portion  140 , the second conductive via fill portion  160 , and the conductive via segment(s), if present, has a vertical length that is less than the min(L Blech ) of all of the at least one TSV structures  180 . 
         [0054]    The first conductive liner portion  130  and the first conductive via fill portion  140  collectively constitute a first TSV segment ( 130 ,  140 ). The second conductive liner portion  150  and the second conductive via fill portion  160  collectively constitute a second TSV segment ( 150 ,  160 ). Likewise, each pair of conductive liner portion and a conductive via segment embedded therein, if present, constitutes an i-th TSV segment, wherein i is an integer greater than 2 and is less than M, wherein M is the total number of TSV segments in each of the at least one TSV structure  180 . Each of the first, second, and i-th TSV segment has a vertical dimension (or height) that is smaller than a Blech length of electromigration for the material of the corresponding conductive via segment. 
         [0055]    Referring to  FIG. 8 , a first back-end-of-line (BEOL) metal interconnect layer  190  is formed on the first substantially planar surface  111  of the first substrate  100 . The first BEOL metal interconnect layer  190  typically includes at least one dielectric material layer and at least one metal interconnect structure embedded therein. The at least one metal interconnect structure can provide electrical connection between the at least one semiconductor device embedded in the first semiconductor-device-containing layer  118  and the at least one TSV structure  180  by providing a conductive path therebetween. The at least one metal interconnect structure embedded in the first BEOL metal interconnect layer  190  can include metal lines that provide conductive paths in horizontal directions, i.e., in directions parallel to the first substantially planar surface  111 , and metal vias that provide conductive paths in the vertical direction, i.e., in the direction perpendicular to the first substantially planar surface  111 . 
         [0056]    Referring to  FIG. 9 , the first substrate  100  can be thinned from the bottom. Specifically, a lower portion of the first substrate  100  can be removed from the second substantially planar surface  119  of the first substrate  100 , for example, by polishing or grinding of the material of the first substrate  100  until the at least one TSV structure  180  is exposed. The first TSV segment ( 130 ,  140 ) of the at least one TSV structure  180  is exposed on the second substantially planar surface  119 , which moves closer to the first substantially planar surface  111  during the thinning process. The vertical distance between the first substantially planar surface  111  and the second substantially planar surface  119  can be substantially the same as the depth d 0  of the at least one trench  113  at the processing step of  FIG. 2 . 
         [0057]    Each TSV structure  180  extends from the first substantially planar surface  111  located on one side of the first substrate  100  to the second substantially planar surface  119  located on the opposite side of the first substrate  100 . Each of the at least one TSV structure  180  includes a plurality of conductive via segments ( 140 ,  160 ) that are vertically spaced from one another by at least one conductive liner portion such as the second conductive liner portion  150 . The at least one TSV structure  180  can be an array of TSV structures, which can be arranged as a one-dimensional array, i.e., a linear array, or a two-dimensional array. 
         [0058]    Each vertically adjacent pair of the plurality of conductive via segments ( 140 ,  160 ) is spaced from each other by a conductive liner portion, e.g., the second conductive liner portion  150 , among the at least one conductive liner portion ( 130 ,  150 ). The bottom surface of one of the vertically adjacent pair of conductive via segments is in contact with an upper surface of the conductive liner portion, and a top surface of the other of the vertically adjacent pair of conductive via segments is in contact with a lower surface of the conductive liner portion. For example, the bottom surface of the second conductive via fill portion  160 , which is a second conductive via segment, is in contact with an upper surface of the second conductive liner portion  150 , and a top surface of the first conductive via fill portion  140 , which is a first conductive via segment, is in contact with a lower surface of the second conductive liner portion  150 . 
         [0059]    Further, such a conductive liner portion extends upward from a periphery of the upper surface and laterally surrounds, and contacts sidewalls of, the one of the vertically adjacent pair of conductive via segments. For example, the second conductive liner portion  150  extends upward from a periphery of the upper surface of the second conductive liner portion  150  and laterally surrounds, and contacts sidewalls of, the second conductive via fill portion  160 , which is the second conductive via segment. 
         [0060]    The lower surface of the conductive liner portion is in contact with another conductive liner portion that laterally surrounds, and contacts sidewalls of, the other of the vertically adjacent pair of conductive via segments. For example, the lower surface of the second conductive liner portion  150  is in contact with the first conductive liner portion  130  that laterally surrounds, and contacts sidewalls of, the first conductive via fill portion  140 , which is the first conductive via segment. 
         [0061]    As discussed above, each conductive via segment, such as the first and second conductive via fill portions ( 140 ,  160 ), within one of the at least one TSV structure  180  has a vertical length that is less than any Blech length for the maximum current density that the each of the at least one TSV structure  180  is configured to flow within the first substrate  100  in the vertical direction between the first substantially planar surface  111  and the second substantially planar surface  119 . 
         [0062]    If the at least one TSV structure  180  is an array of TSV structures  180 , an array of conductive bonding material portions  300  can be formed on the second substantially planar surface  119  of the first substrate  100 . The array of conductive bonding material structures  300  is located directly on the second substantially planar surface  119  of the first substrate  100 . Each of the array of conductive bonding material structures  300  contacts a bottom surface of a TSV structure  180  within the array of TSV structures. 
         [0063]    The array of conductive bonding material structures  300  can be a bonding layer including an array of C4 balls, which are solder balls that can be reflowed for the purposes of bonding. Alternately, the array of conductive bonding material structures  300  can be a bonding layer including an array of conductive bonding pads that are formed by patterning a metal layer of a constant thickness into isolated shapes, each underlying one of the at least one TSV structure  180 . 
         [0064]    Referring to  FIG. 10 , a mounting structure is bonded to the array of conductive bonding material structures  300 . The mounting structure can be an assembly of a second substrate  200  and a second BEOL metal interconnect layer  290  located thereupon. The array of conductive bonding material structures  300  provides a plurality of electrical connections between the at least one semiconductor device in the first semiconductor-device-containing layer  118  and the conductive structures located in the second substrate  200  and the second BEOL metal interconnect layer  290  to provide three-dimensional chip integration. 
         [0065]    The second substrate  200  can include a second handle substrate  212  and a second semiconductor-device-containing layer  218  that typically includes at least another semiconductor device. The second semiconductor-device-containing layer  218  can include semiconductor devices for another semiconductor chip. The thickness of the second substrate  200  is typically from 100 microns to 1,000 microns, although lesser and greater thicknesses can also be employed. 
         [0066]    A lower portion of the second semiconductor-device-containing layer  218  can include a second semiconductor material layer  214 . The second semiconductor material layer  214  can be any semiconductor material that can be used for the semiconductor material layer  114  in the first substrate  100 . The second semiconductor material layer  214  can include a single crystalline material. An upper portion of the second semiconductor-device-containing layer  218  can include a second lower level metal interconnect layer  216 , which includes at least one dielectric material layer embedding metal interconnect structures such as metal vias and metal lines as well as a gate structure of a field effect transistor and/or an emitter structure of a bipolar transistor. 
         [0067]    The second BEOL metal interconnect layer  290  typically includes at least one dielectric material layer and at least one metal interconnect structure embedded therein. The at least one metal interconnect structure can provide electrical connection between the at least another semiconductor device embedded in the second semiconductor-device-containing layer  218  and the array of conductive bonding material structures  300  by providing a conductive path therebetween. The at least one metal interconnect structure embedded in the second BEOL metal interconnect layer  290  can include metal lines that provide conductive paths in horizontal directions and metal vias that provide conductive paths in the vertical direction. 
         [0068]    Referring to  FIG. 11 , a second exemplary semiconductor structure according to a second embodiment of the present invention is derived from the first exemplary semiconductor structure by employing a second substrate  200  including at least another TSV structure  280 , which can be a second plurality of TSV structures. The second plurality of TSV structures  280  in the second substrate  200  can be formed employing the same processing steps as illustrated in  FIGS. 2-9 . Each of the at least another TSV structure  280  extends from the third substantially planar surface  211  to a fourth substantially planar surface  219  that are located on opposite sides second substrate  200 . Each of the at least another TSV structure  280  includes another plurality of conductive via segments that are vertically spaced from one another by at least another conductive liner portion. The at least another TSV structure  280  in the second substrate  200  is formed before forming the second BEOL metal interconnect layer  290  and bonding of the first substrate  100  to the assembly of the second substrate  200  and the second BEOL metal interconnect layer  290  through the array of conductive bonding material structures  300 . 
         [0069]    Another array of conductive bonding material structures (not shown) can be formed on the fourth substantially planar surface  219  to enable additional vertical electrical connection with another mounting structure (not shown). 
         [0070]    Referring to  FIG. 12 , a third exemplary semiconductor structure according to a third embodiment of the present invention is derived from the first exemplary semiconductor structure by employing a packaging substrate  400  instead of a second substrate  200 . The packaging substrate  400  can be a ceramic substrate or a laminar substrate comprising a dielectric material. 
         [0071]    The array of conductive bonding material structures  300  provides a plurality of electrical connections between the at least one semiconductor device in the first semiconductor-device-containing layer  118  and the conductive structures located in the packaging substrate  400  to provide three-dimensional chip integration. 
         [0072]    Further, any mounting structure can be employed instead of a second substrate  200  or a packaging substrate to provide a three-dimensional integration of semiconductor chips as needed. Because the length of each conductive via segment does not exceed the Blech length, each of the at least one TSV structure  180  and the at least another TSV structure (see  FIG. 11 ) is immune to electromigration. By eliminating electromigration from failure mechanisms of the at least one TSV structure  180  and the at least another TSV structure, the reliability of three dimensional integrated chips is increased significantly. 
         [0073]    While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details can be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.