Patent Document

This application is a continuation of application Ser. No. 09/735,566 filed Dec. 12, 2000 (U.S. Pat. No. 6,678,951 issued Jan. 20, 2004), which is a divisional application of Application Ser. No. 09/316,916 filed May 20, 1999 (Pat. No. 6,245,996 issued Jun. 12, 2001) which is a continuation of application Ser. No. 08/722,532 filed Sep. 27, 1996, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to metallization systems and methods and more particularly to metallization systems and methods suitable for use with very large scale integrated (VLSI) circuits. More particularly, the invention relates to metallization systems having increased electromigration (EM) resistance. 
     As is known in the art, electromigration (EM) in on-chip electrical interconnects is one of the wearout mechanisms which limit the lifetime of integrated circuits. On-chip interconnects are typically made of highly-conductive, polycrystalline metal films, such as aluminum, copper, or their alloys. In such films, electromigration typically proceeds along the network of grain boundaries. EM failures, in the form of voids or hillocks, usually occur at certain grain-boundary intersections, called “triple points”, where flux divergence exists, i.e., the flux of metallic atoms entering the intersection is different from the flux of atoms leaving this intersection. However, EM failure is even more likely to occur at the end of a metal conductor where it is attached to an interlevel contact or via. At the same time, as discussed in a paper entitled, “Electromigration in thin aluminum films on titanium nitride” by I. A. Blech, published in the Journal of Applied Physics, Vol. 47, No. 4, April 1976, pages 1203–1208, EM voids and hillocks cannot develop in metal lines or conductors which are shorter than a certain “critical length”. The “critical length” effect was observed in Al/W/Al via chains as reported in “Evidence of the electromigration short-length effect in aluminum-based metallurgy with tungsten diffusion barriers” by Ronald G. Filippi et al, Proceedings of MRS Symposium, Vol. 309, 1993 pages 141–148 and in a paper entitled “Permitted Electromigration of Tungsten-Plug vias in Chain for Test Structure with Short Inter-Plug Distance”, by T. Aoki et al., published in Proceedings of VMIC Conference, 1994 beginning at page 266. The critical length effect in all-aluminum lines with polycrystalline segments has been reported in a paper entitled “Two Electromigration Failure Modes in Polycrystalline Aluminum Interconnects”, by E. Atakov, J. J. Clement and B. Miner, published in the Proceeding of the IRPS, 1994, beginning at page 213. At typical operating conditions of silicon integrated circuits, the critical length is expected to be at least 100 um, as discussed in the above reference papers. 
     Prolongation of the lifetime of a contact to the silicon substrate by forming a gap in one layer of a multilayered metal line within the critical distance from the contact, and filling the gap with a refractory metal has been reported in a paper entitled “An Increase of the Electromigration Reliability of Ohmic Contacts by Enhancing Backflow Effects”, by Wei Zhang, et al., Proceedings of the IRPS, 1995, beginning at page 365. As described in the Zhang et al. paper, a 4000 Å thick Al-1% Si electrically conductive film is deposited over a 4700 Å thick dielectric layer and through a contact opening formed in a region of a dielectric layer to make electrical contact with an electric device formed in a semiconductor body, as shown in  FIG. 1  of the paper. The Al-1% Si layer is patterned to form a stripe which is attached to the contact and has a gap at a critical distance, L c , from the contact. A 3200 Å trilevel metallization layer made of 100 Å thick Ti, 3000 Å thick W, 100 Å thick Ti is deposited over the substrate, covering the Al-1% Si stripe and filling the gap. Next a 4000 Å thick Al-1% Si layer is deposited over the surface. Because the gap presumably has a depth of the thickness of the first Al-1% Si layer (i.e., a depth of 4000 Å), it appears that the resulting metal surface is non-planar. 
     The two top metallization layers are patterned to form a stacked stripe coincident with the first Al-1% Si stripe. The first stripe itself is non-planar, making it difficult to perform photolithography to align the stacked stripe. Because of non-planarity, the process described by Wei Zhang, et al. does not ensure the dimension control which is required to fabricate devices with submicron feature size. Particularly, it cannot easily be used to fabricate the conductors in high-performance, state-of-the-art Very Large Integrated Circuits (VLSI). 
     One of the requirements for metal interconnects in such circuits is that the equidistant conductors be spaced at submicron distance. Very tight dimensional control is required for the fabrication process to ensure such small distance without causing unintended electrical shorts between the conductors. 
     Also, the structure proposed by Wei Zhang et al., does not provide complete blocking of electromigration, because aluminum can migrate away from the contact in the top conducting layer of Al-1% Si. On the other hand, even though the gap can somewhat prolong the life of the nearby contact, the gap itself creates a flux divergence and is a likely site for an EM failure. 
     Interconnect structures with a plurality of high electrically conductive, electromigration-prone segments separated by very short, electromigration-resistant refractory metal segments were proposed in U.S. Pat. No. 5,439,731, entitled Interconnect Structures Containing Blocking Segments to Minimize Stress Migration and Electromigration Damage, by Li et al., issued on Aug. 8, 1995. 
     However, Li et al., propose that the high electrically conductive segments be formed first, and the gaps between the segments be filled with EM-resistant metal afterwards. Another photolithography/metal etch step is required to form the intended interconnect structure. This method has the same disadvantage as the method proposed by Wei Zhang, et al. 
     Conductors in high-performance VLSI are required to have as low electrical resistance as possible. The EM-resistant refractory metals are known to have a lower electrical conductivity than Al, Au, Cu, etc. For this reason, it is critical that the method which is used to form the interconnect structures allow for making the EM-resistant segments as short as possible. 
     Also for the purpose of reducing the overall resistance of segmented conductors, it is desirable that the high electrically conductive segments be as long as possible, without compromising the conductor reliability. Li et al., propose that the high electrically conductive segments be as short as 5 to 20 microns. However, it was shown that the high electrically conductive segments are immune to electromigration if they are no longer than the critical length, L c . As discussed by I. A. Blech, L c  is inversely proportional to the electrical current density in the conductor, and L c  depends on the physical characteristics of the conductor and the overlying dielectric. L c  can be determined using special experimental techniques. As shown by R. G. Filippi et al., and T. Aoki et al., L c  can be as long as 100 um or even longer for state-of-the-art VLSI conductors at typical VLSI operating currents. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a method is provided for forming at least one electrical conductor having a plurality of relatively high electrically conductive segments separated by, and electrically interconnected through, relatively short electromigration-inhibiting/electrically conductive segments, or plugs. The electromigration-inhibiting/electrically conductive segments are formed within a planar surface. More particularly, windows are formed in the planar surface. The windows are filled with electromigration-inhibiting/electrically conductive material to thereby form the plugs, upper portions of the electromigration-inhibiting/electrically conductive material extending above the planar surface. The upper portions of the electromigration-inhibiting/electrically conductive material extending above the planar surface are removed to form the plugs with surfaces co-planar with the aforementioned planar surface. The plugs are separated from each other by a distance less than, or equal to, a predetermined critical length, L c . Typically, L c  is at least 100 microns, and the electromigration-inhibiting/electrically conductive plugs are shorter, in length, than one micron. 
     The relatively high electrically conductive segments are formed within the same planar surface as the plugs, either before, or after the plug formation, in such a way that these segments are co-planar with, and abutting, the plugs. 
     With such method, such formed electrical conductors have improved electromigration resistance, low electrical resistance, and can be readily formed at submicron distance to each other, as required for metallization in high-performance VLSI. 
     According to one feature of the invention, an electrical conductor is produced by forming a plurality of windows within a planar surface. The windows are aligned along the desired path of the electrical conductor with a space, or distance, between adjacent windows of less than, or equal to, the critical length, L c . The number of windows is equal to or more than (L/L c )−1 where L is the desired length of the conductor. The dimension, W p , of each window along the path of the electrical conductor is the minimum width allowed by the given technology, and preferably should be less than, or equal to, one micron. This dimension is further referred to as the window width. The window dimension orthogonal to the path of the electrical conductor, L p , is at least as large as the desired width, W c , of the electrical conductor. This dimension is further referred to as the window length. The window depth, D p , is approximately the same as the desired thickness, D c , of the electrical conductor, and preferably less than, or equal to, one micron. 
     An electromigration-inhibiting/electrically conductive material is deposited over the planar surface and through the windows to fill the windows. Because of the small width of the windows, the material fills them up completely, with upper portions of such material extending above the planar surface and the windows, and the upper portions of the material deposited above the windows are nearly co-planar with the material deposited above the surrounding planar surface. The upper portion of the material above the windows and the surrounding planar surface is then removed, to form plugs in the windows with surfaces co-planar with the surrounding surface. The relatively high electrically conductive segments are formed within the same planar surface as the plugs, either before, or after, the plug formation with surfaces co-planar with the plugs, aligned with and abutting the plugs, and electrically interconnected through the plugs. 
     Such process sequence ensures a very short length, and, consequentially, low resistance of electromigration-inhibiting segments. With the conductive segments being relatively long, the overall resistance increase caused by the electromigration-inhibiting segmentation is very small. The improved electromigration-inhibiting resistance of the resulting electrical conductors is ensured by keeping the length of the electrically conductive segments equal to or less than the predetermined critical length, L c . 
     The method also ensures adequate control of the space between equidistant electrical conductors when this space is required to be less than 1 micron. A plurality of equidistant electrical conductors spaced at less than 1 micron can thereby be accurately formed within one layer of metallization using photolithography and dry etching, and multiple layers of metallization can be fabricated in the same way. 
     In one embodiment of the invention, the planar surface is formed by a relatively high electrically conductive film. The windows, which are at least as deep as the thickness of the relatively high conductive film, are formed in the surface. The electromigration-inhibiting/electrically conductive material is deposited over the conductive film and into the windows formed therein to provide, in such windows, the plugs, an upper portion of such electromigration-inhibiting/electrically conductive material extending above the planar surface and windows. Subsequently, the upper portion of the deposited material is removed to form the plugs with surfaces co-planar with a surface surrounding the plugs. The relatively high electrically conductive film is patterned to form relatively high electrically conductive segments electrically interconnected through the plugs. 
     In another embodiment of the invention, the planar surface is formed by a dielectric layer. The electromigration-inhibiting/electrically conductive material is deposited over the dielectric layer and into the windows formed therein to provide the plugs, an upper portion of the material extends above the dielectric layer. The upper portion of the deposited electromigration-inhibiting/electrically conductive material extending above the planar surface is removed to form the plugs with surfaces co-planar with the surface of the dielectric layer surrounding the plugs. Trenches are formed in the surface portions of the dielectric film between and aligned with, the plugs. A relatively high electrically conductive material is deposited over the dielectric layer and into the trenches. Subsequently, portions of the deposited electrically conductive material are removed from the dielectric layer to form, in each one of the trenches, corresponding relatively high electrically conductive segments with surfaces thereof co-planar with each other, with the surface of the plugs, and with the surface of the dielectric layer. 
     In accordance with another embodiment of the invention, the planar surface comprises a dielectric layer having electrical conductors disposed therein. Windows are formed in the electrical conductors thereby separating the electrical conductors into plurality of relatively high electrically conductive segments. The windows are at least as deep as the thickness of the electrical conductors. The electromigration-inhibiting/electrically conductive material is deposited over the dielectric layer, over the electrical conductors and into the windows to provide, in such windows, the plugs, an upper portion of the material extending above the electrical conductor segments and the dielectric layer. The upper portion of the deposited electromigration-inhibiting/electrically conductive material above the electrical conductive segments and dielectric layer is removed to form the plugs with surfaces co-planar with the surface of the dielectric layer and with surfaces of the relatively high electrically conductive segments. 
     In accordance with still another feature of the invention, windows are formed within a planar surface. An electromigration-inhibiting/electrically conductive liner and relatively high electrically conductive material are successively deposited into the windows and over the surrounding planar surface, an upper portion of such material extending above the windows and the planar surface. The upper portion of the material extending above the windows and the surrounding planar surface is removed to form plugs in the windows with surfaces co-planar with the surrounding surface. Relatively high electrically conductive segments are formed within the same planar surface as the plugs, either before, or after, the plug formation, so that the surfaces of said segments are co-planar with the plugs, aligned with and abutting the plugs, and electrically interconnected through the plugs. With such an arrangement, the plugs have even smaller resistance than the plugs consisting only of an electromigration-inhibiting/electrically conductive material. 
     In accordance with still another feature of the invention, a metallization system is provided comprising a plurality of equidistant electrical conductors separated by a distance smaller than 1 micron. Each of the electrical conductors includes a plurality of electrically conductive segments interconnected by much shorter electromigration-inhibiting segments. The conductive segments are co-planar with the electromigration-inhibiting-segments. The electromigration-inhibiting segments within each conductor are spaced at a distance less than, or equal to, L c . 
     In accordance with still another feature of the invention, a multilevel metallization system is provided. Electrical devices are formed in a semiconductor substrate. A dielectric layer is disposed over the semiconductor surface. Windows are formed to open contact regions of the devices. The windows are filled with an electrically conductive material to electrically connect the devices with the first metallization level. The first metallization level comprises first electrical conductors each having a plurality of first electromigration-inhibiting/electrically conducting plugs therein. The first plugs have a space, or distance between adjacent plugs, less than, or equal to, L c . The first plugs have co-planar surfaces. The first electrical conductors comprise pluralities of first electrically conductive segments electrically interconnected through the first plugs. The first electrically conductive segments are co-planar with each other and the first plugs. Electrically conductive vias pass through apertures in a dielectric layer disposed on the first metallization system to electrically interconnect the first metallization level and a second metallization level. The second metallization level includes electrical conductors having each a plurality of second electrically conductive segments electrically interconnected through a plurality of second electromigration-inhibiting/electrically conducting plugs. The second plugs have a space, or distance between adjacent ones thereof, less than, or equal to, L c . The second electrically conductive segments and the second plugs are co-planar. With such an arrangement, the distance between any region of relatively high electrically conductive segments which is near an interlevel via or near a contact to electrical devices, and the nearest electromigration-inhibiting segment never exceeds L c . Thus, electromigration is suppressed in the relatively high conductive segments, even if they are connected to interlevel vias or contacts to electrical devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features of the invention will become more readily apparent with reference to the detailed description below taken together with the accompanying drawings, in which: 
         FIGS. 1A  though  1 D are diagrammatic cross-sectional sketches of a semiconductor substrate with a metallization system at various stages in the fabrication thereof in accordance with the invention,  FIGS. 1A ,  1 B,  1 C being cross-sectional elevation view sketches and  FIG. 1D  being a cross-sectional perspective view sketch; 
       FIGS.  1 A′ and  1 B′ are plan views of the semiconductor structure of  FIGS. 1A and 1B , respectively, the cross sections of  FIGS. 1A and 1B  being taken along lines  1 A— 1 A and  1 B— 1 B in FIGS.  1 A′,  1 B′, respectively; 
       FIG.  1 D′ is a plan view of the semiconductor structure of  FIG. 1D  in accordance with one embodiment of the invention; 
         FIG. 2  is a diagrammatical cross-sectional sketch of a multilevel metallization system according to the invention; 
         FIGS. 3A through 3F  are diagrammatic cross-sectional sketches of a semiconductor structure at various stages in the fabrication thereof in accordance with an alternative embodiment of the invention; 
       FIGS.  3 A′,  3 C′, and  3 D′ are plan views of the semiconductor structure of  FIGS. 3A ,  3 C and  3 D, respectively, the cross sections of  FIGS. 3A ,  3 C and  3 D being taken along lines  3 A— 3 A,  3 C— 3 C, and  3 D— 3 D in FIGS.  3 A′,  3 C, and  3 D, respectively; 
         FIGS. 4A through 4E  are plan and cross-sectional view sketches of a semiconductor structure at various stages in the fabrication thereof in accordance with an alternative embodiment of the invention,  FIGS. 4A and 4E  being plan view sketches and  FIGS. 4B through 4D  being cross-sectional elevation view sketches; and 
         FIG. 5  is a diagrammatical cross-sectional elevation view of a metallization system according to an alternative embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1A , a semiconductor structure  10  is shown having a silicon layer  14 , and a silicon dioxide layer  16  as shown. A 0.6 micron thick film, or layer  24  of a relatively high electrically conductive material, here an aluminum-copper (Al—Cu) alloy is evaporated over the surface. Other material may be used for film  24 , such as Al, Cu, Au, Ag, or their alloys, i.e. the electrically conductive film  24  need not be immune to electromigration. The film  24  alternatively may be a multi-layer structure having one or more additional layers made of conductive materials, such as indicated above, and/or refractory metals or their compounds, such as Ti, W, TiN, TiW, Mo, Ta, or others, which are known to be immune to electromigration at typical operating conditions of silicon integrated circuits. It is noted that the upper surface of film  24  is a planar surface  21 . 
     Multiple equidistant rows of windows are formed so that they are aligned along the desired paths of conductors. Minimum-width (Wp) windows  25  (i.e. windows  25  formed with the minimum width practical within the photolithography and etch processes available) are opened in conductive film  24  by conventional photolithography and dry etching as shown in 
     FIG.  1 A′. Here, Wp=0.25 □ m. The depth, Dp, of windows is at least as large as the electrical conductor thickness, Dc, here Dp=Dc=0.6 □ m. Within each conductor path, the windows  25  are spaced at a distance less than, or equal to, a predetermined critical length, Lc, as shown in FIG.  1 A′. The length Lc is selected experimentally, as previously described, to prevent electromigration in the relatively high electrically conductive segments  34  to be patterned in conductive film  24 , as will be described in detail in connection with  FIG. 1D . The electromigration is prevented by creating a backflow in the relatively high electrically conductive segments  34  which counter-balances electromigration flow. In integrated circuits with submicron feature size, Lc&gt;&gt;Wp. Here, Lc is 100 to 300 microns. The number of windows in each of the desired conductor paths is at least (L/Lc)−1, where L is a desired conductor length. The length, Lp, (FIG.  1 A′) of each one of the windows  25  is selected so as to be at least as large as the desired width, Wc, (FIG.  1 D′) of relatively high electrically conductive segments  34  to be patterned in conductive film  24 , as will be described in detail in connection with  FIG. 1D . Here, Wc=0.5 □ m. The space Ws between windows belonging to neighboring conductors can be as small as allowed by photo-etch (FIG.  1 A′). Here, Ws=0.25 □ m. 
     Referring again to FIG.  1 A and  1 A′, after a layer of photoresist, not shown, deposited over the surface of the structure and used to form the windows  25  is stripped off, a refractory metal liner  28  ( FIG. 1B ) and a metal layer  30  are successively deposited over the structure, filling the windows  25  as shown in FIGS.  1 B and  1 B′ to provide electromigration-inhibiting/electrically conductive plugs  31 . Liner  28  is here sputter deposited or chemically vapor deposited, and metal layer  30  is here sputter deposited, chemically vapor deposited, electroplated or electroless plated. The specific resistivity, RHOp, of conductive layer  30  should preferably be equal to or less than, four times the specific resistivity, RHOo, of relatively high electrically conductive layer  24 . While conductive layer  30  does not have to be immune to electromigration, liner  28  does have to be immune to (i.e., act as a barrier against) electromigration, such as a refractory metal. In fact, conductive layer  30  need not be different from conductive layer  24 . Here, the conductive layer  30  is a 0.4 micron thick layer of tungsten and the liner  28  is here a 0.25 micron thick layer of titanium and titanium nitride. Here, the titanium is 0.01 microns thick and the titanium nitride is 0.15 microns. 
     Next, referring to  FIG. 1C , the conductive layer  30  is etched back using plasma etching, to form a surface co-planar with the surface of liner  28  surrounding plug  31 ; i.e., to form a planar surface over the plugs  31 . That is, portions of the electromigration-inhibiting/electrically conductive material filing the windows  25 , here an upper portion of the conductive layer  30 , is removed to form the plugs  31  with surfaces co-planar with each other and with the surface of the liner  28  surrounding the plugs  31 . Portions of the liner  28  may or may not be removed as well. Layer  30  and liner  28  may also be removed by chemical-mechanical polishing (CMP) techniques. 
     Electrical conductive segments  34 , are formed within the relatively high electrically conductive layer  24  and overlying refractory metal liner  28 , as shown in  FIG. 1D  using photolithography and plasma etching techniques. It is noted that the patterning is such that the patterned electrically conductive segments abut the corresponding plugs  31 , as shown in  FIG. 1D . Thus, the electrically conductive segments  34  are electrically interconnected through the plugs  31 . A top view of the structure is shown in FIG.  1 D′. It is noted that the length, L p , of plug  31  is equal to, or greater than, the width W c  of the conductor segments  34 , as shown in FIG.  1 D′. Here, L p =0.5 μm. 
     Thus, in summary, a method is provided for forming electrical conductors  35  with electromigration-inhibiting/electrically conductive plugs  31  disposed between electrically conductive segments  34 , as shown in FIGS.  1 D and  1 D′. The plugs  31  are formed by depositing the electromigration-inhibiting/electrically conductive material (i.e., liner  28  and conductor  30 ) into windows  25  and subsequently removing portions of the deposited material, here conductive material  30 , to form plugs  31  with surfaces co-planar with the surface of the liner  28  surrounding the plugs  31 . In accordance with such method, the windows  25  are formed within a planar surface  21  of film  24 . The electrically conductive segments  34  have surfaces co-planar with the plugs  31 , abut the plugs  31 , and are electrically interconnected through the plugs  31 . The plugs  31  are formed at a distance less than, or equal to, the predetermined critical length, L c , from each other. The length, L p  of the plug  31  is not less than the desired width, W c , of the electrically conductive segments  34 . The conductors formed in such a way have improved electromigration resistance, because the length of relatively high electrically conductive segments is less than, or equal to, L c . 
     The relative increase in conductor electrical resistance associated with the electromigration-inhibiting plugs is calculated as (R−Ro)/Ro=RHOpWp/RHOoLc, where R and Ro are, respectively, the resistances of conductor  35  and a same-length conductor without the plugs, and RHOp and RHOo are the specific resistivities of the electromigration-inhibiting conductive material  30  and the relatively high electrically conductive material  24 , respectively. Here, RHOp 8×10 −6  Ohm-cm, RHOo=3×10 −6  Ohm-cm, Wp=0.25□m, and Lc=10.0□m. Then, (R−Ro)/Ro=7×10 −3 =0.7%. So, the electrical conductors  35  formed by the described method have low electrical resistance, which does not exceed the resistance of solid relatively high electrically conductive conductors by more than 11. With the described method, a planar surface is provided along the conductor film  24  for accurately photolithographically forming equidistant conductors  15  at a distance smaller than a micron. 
     Referring now to  FIG. 2 , the semiconductor structure  10  is shown having an electrical device, here a metal oxide silicon (MOS) transistor, only the drain region  13  thereof being shown, formed in a silicon layer  14 , as shown. Disposed over the silicon layer  14  is a dielectric layer  16 , here silicon dioxide. A contact opening, or recess  26 , is etched into a portion of the dielectric layer  16  to expose a contact region  18  of the drain  13 . A thin layer  22  of a refractory metal, here titanium (Ti) and titanium nitride (TiN) is sputtered over the surface and into the recess  26  to a total thickness here of 0.025 microns. A layer  23  of a second metal, here tungsten, is deposited over the surface to fill the recess  26 , as indicated; excess tungsten being removed by etch-back or CMP. The Ti/TiN may or may not be removed as well. In this way, contacts to silicon, Si, devices are formed. Next, a 0.6 micron thick film, or layer  24  of a highly conductive material, here an aluminum-copper (Al—Cu) alloy is evaporated over the surface. Other material may be used for film  24 , such as Al, Cu, Au, Ag, or their alloys. The film  24  may be a multi-layer structure having one or more additional layers made of refractory metals or their compounds, such as Ti, W, TiN, TiW, Mo, Ta, or others, which are known to be immune to electromigration at typical operating conditions of silicon integrated circuits. It is noted that the upper surface of film  24  is a planar surface  21 . The first metallization level comprised of conductors  35  and described above in connection with  FIGS. 1A through 1D , is formed. Then, a second dielectric layer  50 , here silicon dioxide layer, is deposited over the surface of the structure, as shown. An opening  52  is formed therein to expose a portion of the electrically conductive segment  34   a  of electrical conductor  35 . A layer  54  of titanium and TiN followed by a layer  56  of tungsten are deposited in a manner similar to that described above in connection with layers  22  and  26 . The materials of layers  54 ,  56  are removed to form planar surface, by plasma etch or chemical-mechanical polishing (CMP). Next, a second relatively high electrically conductive film, or layer  60  is formed in the same manner as film, or layer  24 . It is noted that the bottom portion of conductive layer  60  is in electrical contact with the via  59  provided by titanium/TiN layer  54  and tungsten layer  56 . Here, the conductive layer  60  is electrically connected to conductive segment  34   a  of conductor  35 . The process sequence shown in  FIGS. 1B ,  1 B′,  1 C,  1 D and  1 D′ is then repeated. That is, film  60  has a planar upper surface  61 . Windows  62  are formed in the planar surface  61  of conductive film  60  at the space, or distance, L c , along the desired conductor path. The conductor is routed in such a way that it overlaps the via  59 . An electromigration-inhibiting/electrically conductive material (i.e., liner  64  and conductive material  66 ) is deposited over the planar surface  61  and through the windows  62  to fill the windows  62  and thereby provide, in such windows  62 , plugs  63  of the electromigration-inhibiting/electrically conductive material. Portions of the electromigration-inhibiting/electrically conductive material  66  are removed to form the plugs  63  with surfaces co-planar with the planar surface of the liner  64 . The film  60  and liner  64  are then patterned into electrical conductor segments  68  in the same manner film  24  was patterned into electrical conductor segments  34 . Electrical conductive segments  68 , of conductor  69  are formed with surfaces co-planar with the plugs  63 , and segments  68  are electrically interconnected through the plugs  63 . The plugs  63  are formed with a space, or distance between adjacent plugs  63  less than, or equal to, the predetermined critical length, L c , from each other. The number of plugs in each of conductors  69  is at least (L/L c )−1, where L is the length of conductor  69 . The length, L p , of the each plug  63  is not less than the desired width, W c , of the electrically conductive segments  68 . Equidistant conductors can be formed at a distance W s  smaller than 1 um. Here, W s =0.25 μm. The vias  26 ,  59  are within L c  distance from the nearest plug  31  in the first layer or plug  63  in second layer, respectively. The windows have minimum width, W p =0.25 μm, and length L p  no less than conductor width, W c =0.5 μm. Windows are as deep, D p , as desired electrical conductor thickness, D c . Here, D p =D c =0.6 μm. 
     Referring now to  FIGS. 3A through 3F , an alternative embodiment is shown. Multiple equidistant rows of minimum-width recessed areas are formed, so that they are aligned along the desired paths of conductors. The number of recessed areas in each row is equal to or more than (L/L c )−1 where L is the desired length of each respective conductor. Here, minimum-width recessed areas (i.e. windows  80 ) are formed in a planar surface  79  of a film  82 , here a dielectric layer  82 ,by photolithography and dry etching; the dielectric layer  82  having been deposited over the semiconductor layer  14 , as shown. The windows  80  are spaced at the predetermined critical distance, L c , described above in connection with  FIG. 1A , to inhibit, electromigration, as shown in FIGS.  3 A and  3 A′. 
     Referring to  FIG. 3B , a refractory metal, here titanium and TiN liner  28  and conductive, here tungsten, layer  30  are deposited over the structure as described above in connection with  FIG. 1B ; here, however the liner  28  and layer  28 ,  30  are deposited over silicon dioxide layer  82  rather than the relatively high electrically conductive layer  24  as described in connection with  FIG. 1B . More particularly, a refractory metal liner  28  ( FIG. 3B ) and a metal layer  30  are successively deposited over the structure, filling the windows  80  as shown in  FIG. 3B  to provide electromigration-inhibiting/electrically conductive plugs  31 . Liner  28  is here sputter deposited or chemically vapor deposited, and metal layer  30  is here sputter deposited, chemically vapor deposited, electroplated or electroless plated. While, as discussed above, conductive layer  30  does not have to be immune to electromigration, liner  28  does have to be immune to (i.e., act as a barrier against) electromigration, such as a refractory metal. In fact, conductive layer  30  may not be different from conductive layer  24  in  FIGS. 1A–1D . Here, the conductive layer  30  is a 0.4 micron thick layer of tungsten and the liner  28  is here a 0.025 micron thick layer of titanium and titanium nitride. Here, the titanium is 0.01 microns thick and the titanium nitride is 0.015 microns. 
     Next, referring to  FIG. 3C , the conductive layer  30  is etched back using plasma etching, or polished back to form a surface co-planar with the surface of dielectric layer  82  surrounding plugs  31 ; i.e., a planar surface over the plugs  31 . That is, portions of the electromigration-inhibiting/electrically conductive material filing the windows  80 , here an upper portion of the conductive layer  30  and liner  28  are removed to form the plugs  31  with surfaces co-planar with each other and with the surface of the dielectric layer  82  surrounding the plugs  31 . The conductive layer  30  and the portions of liner  28  disposed on the planar surface  79  of dielectric layer  82  are removed using plasma etch-back or chemical-mechanical polishing (CMP) so the surface of plugs  31  is co-planar with the upper surface  79  of the dielectric layer  82 , as shown in FIGS.  3 C and  3 C′. 
     Referring now to  FIG. 3D , trenches  90  are formed in the dielectric layer  82  using photo-lithography and dry etching. The trenches  90  are formed in such a way that they are aligned with, and abutting, the plugs of each separate row of the plugs. It is noted that the end-walls  92  of the trenches  90  abut the liner  28 . Trenches have width equal to desired conductor width, W c . Here, W c =0.5 μm. 
     Referring now to  FIG. 3E , a refractory, electromigration-inhibiting liner  98 , here titanium and TiN and a relatively high electrically conductive layer  100 , here Al(Cu), are deposited over the structure in a manner described above in connection with layers  28 ,  30  ( FIG. 1B ) (e.g., here such deposition being chemical vapor deposition (CVD), electroplating, reflow-sputtering, or other deposition process). Subsequently, an upper portion of liner  98  and layer  100  are removed (e.g., etch-back, lift-off, CMP, or other) to form a relatively high electrically conductive segments  102 , as shown in  FIG. 3F . The segments  102  have a surface which is co-planar with the surface of plugs  31 . 
     Thus, a method is provided for forming electrical conductors  103  with electromigration-inhibiting/electrically conductive plugs  31  disposed between electrically conductive segments  102 . Windows  80  are formed within a planar surface  79  of dielectric layer  82 . An electromigration-inhibiting/electrically conductive material (i.e., liner  28  and conductive material  30 ) is deposited over the planar surface  79  and through the windows  80  to fill the windows  80  and thereby provide, in such windows  80 , plugs  31  of electromigration-inhibiting/electrically conductive material. Portions of the electromigration-inhibiting/electrically conductive material  28 ,  30  are removed to form the plugs  31  with surfaces co-planar with the planar surface  79 . The electrical conductive segments  102  are formed with surfaces co-planar with the plugs  31 , and segments  102  are electrically interconnected through the plugs  31 . The plugs  31  are formed with a space, or distance between adjacent plugs  31  less than, or equal to, the predetermined critical length, L c . The length of the plug  31  L p  is approximately equal to the desired width of the electrically conductive segments  102 , W c  as shown in FIG.  3 D′. Here, W c =0.5 μm. It is noted that, here, L p  is approximately equal to W c  and D p  is approximately equal to D c . 
     Referring now to  FIGS. 4A through 4E , another method is provided for forming conductors  111  ( FIG. 4E ) with electromigration-inhibiting/electrically conductive plugs  31  disposed between electrically conductive segments  110 . 
     Referring to  FIG. 4A , conductor-length slots, or trenches,  120  are formed in the dielectric layer  112  by photolithography and dry etching. The slots  120  are filled with refractory metal liner  114  and relatively high electrically conductivity conductor  116 , as shown. The upper surfaces of the dielectric layer  112 , liner  114  and conductor  116  are formed to provide a planar surface  121 . Here, the slot width (i.e., electrical conductor width), W c , equals 0.5 μm. 
     Minimum-width windows  118  ( FIG. 4B ), (W p =0.25 μm) are formed in the planar surface  121 ; more particularly in liner  114  and conductor  116  at the predetermined critical distance, L c , as shown in  FIG. 4B . The windows  118  separate conductors  116  and liner  114  into segments  110 , as shown in  FIG. 4E . The window length, L p , is equal to, or greater than, W c ; and the window depth D p  is equal to, or greater than, D c , as shown in  FIG. 4B . The windows  118  are filled with a electromigration-inhibiting/electrically conductive material ( FIG. 4C ), here liner  28  and a conductor  30 , as described above in connection with  1 B. Thus, a refractory metal liner  28  and a metal layer  30  are successively deposited over the structure, filling the windows  118  as shown in FIGS.  1 B and  1 B′ to provide electromigration-inhibiting/electrically conductive plugs  31 . Liner  28  is here sputter deposited or chemically vapor deposited, and metal layer  30  is here sputter deposited, chemically vapor deposited, electroplated or electroless plated. While, as discussed above, conductive layer  30  does not have to be immune to electromigration, liner  28  does have to be immune to (i.e., act as a barrier against) electromigration, such as a refractory metal. In fact, conductive layer  30  may not be different from conductive layer  116 . Here, the conductive layer  30  is a 0.4 micron thick layer of tungsten and the liner  28  is here a 0.025 micron thick layer of titanium and titanium nitride. Here, the titanium is 0.01 microns thick and the titanium nitride is 0.015 microns. 
     Subsequently, conductive material  30  and liner  28  are etched back or polished back as shown in  FIG. 4D  to form the plugs  31  with surfaces co-planar with the surrounding surface  121 , as shown in  FIG. 4D . The plugs  31  provide electrical interconnection between abutting electrically conductive segments  110  forming electrical conductors  111 , as shown in  FIG. 4E . 
     Alternatively, liner  28  is not removed. Then, photomask, not shown, is used to remove liner  28  from the portions of the dielectric layer  112  surrounding the slots  120  to form electrically isolated parallel conductors  111 , as shown in  FIG. 4E . 
     Other embodiments are within the spirit and scope of the appended claims. For example, considering the embodiment described above in connection with  FIGS. 1A through 1D , if a conductive underlayer, such as conductive underlayer  200  in  FIG. 5 , is used beneath the relatively high electrically conductive layer  24 , the window  25  need only be etched through the layer  24  to the underlayer  200 , as shown in  FIG. 5 .

Technology Category: 5