Abstract:
An interconnect structure in the back end of the line of an integrated circuit forms contacts between successive layers by removing material in the top surface of the lower interconnect in a cone-shaped aperture, the removal process extending through the liner of the upper aperture, and depositing a second liner extending down into the cone-shaped aperture, thereby increasing the mechanical strength of the contact, which then enhance the overall reliability of the integrated circuit.

Description:
BACKGROUND OF INVENTION 
   1. Technical Field 
   The field of the invention is that of fabricating integrated circuits, in particular forming the interconnect structures in the back end of the line (BEOL). 
   2. Background of the Invention 
   In the course of developing copper interconnects for use in the back end of silicon, integrated circuits, it became necessary to encapsulate the copper interconnect material with a diffusion barrier in order to prevent the copper from escaping from the damascene channel. Because of the significant diffusion mobility of Cu atoms in most insulators, a small amount of Cu can cause serious short circuit problems in an integrated circuit. A small amount of Cu can further damage the function of devices completely when it diffuses into the well-defined implanted device areas. 
   Extensive work was done that resulted in a compatible system in which the copper was encased with a liner formed of a material having good diffusion blocking properties, such as one of the refractory metals, e.g. Ti, Ta or nitrides of those and similar metals. 
   However, these compounds have a high resistivity compared with copper and also have presented an electrical problem at the interface between adjacent back end layers. 
   A recurring problem with these interfaces is electromigration, in which atoms move under the influence of the electric field applied to the material. This material transport causes a significant number of circuit failures. The back end structures are built to carry electrical signals, of course, so that the existence and magnitude of the applied fields cannot be changed. 
     FIG. 1A  illustrates a dual damascene structure in the prior art, in which a lower level  100 , containing a copper interconnect  110  extending perpendicular to the plane of the paper has a cap layer  120 . In the next layer above, interlevel dielectric (ILD)  130  has had a dual damascene aperture formed in it with a lower part  148  and a broader upper part  150 . Illustratively, the ILD is silicon oxide or a low k dielectric such as SiLK, SiCOH, or any other kind of hybrid related materials. Aperture  148  is illustratively a via making contact with the lower structure and will be located at only a few locations. Upper portion  150  will extend over a significant distance to make contact with other elements of the circuit as required by the layout. In this figure, the portion of the cap layer  120  at the bottom of aperture  148  has been removed, usually by a different etching chemistry from that used to etch through the ILD. 
   Since the open surface of the metal underneath aperture  148  is a potential weak point, it is conventional to deposit a liner  160 , shown in  FIG. 1B , over the entire interior of the upper structure. Liner  160  may be TiN or a similar diffusion barrier, optionally including an adhesion layer that bonds well to the ILD. The liner is needed to confine the copper in the patterned ILD, i.e. both  150  and  148 . 
     FIGS. 2A and 2B  show similar figures for the case of a single damascene interconnect structure. 
   Problems with this prior art approach have been failure to make a good mechanical contact (leading to an open circuit or to a high resistance joint) at a chip operating temperature. Failures also include a poor electromigration resistance at the joint because of high resistivity that slows down the rise time of a signal or otherwise interferes with the electrical properties of the circuit. 
   SUMMARY OF INVENTION 
   The invention relates to an interconnect structure in the back end in which the material in a via penetrates for a considerable distance into the material of the underlying interconnect member. 
   A feature of the invention is the removal of the liner layer at the bottom of the via. 
   Another feature of the invention is the removal of material by forming a cone-shaped aperture in the lower material. 
   Yet another feature of the invention is the deposition of a first liner layer and, after removal of the liner at the bottom of the aperture, the deposition of a second liner layer. 
   Yet another feature of the invention is the simultaneous deposition of a material with the ion bombardment of the horizontal surfaces, so that the upper horizontal surfaces remain covered, while the surface at the bottom of the aperture is exposed. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1A and 1B  illustrate a prior art dual damascene structure. 
       FIGS. 2A and 2B  illustrate a prior art single damascene structure. 
       FIG. 3  illustrates a dual damascene structure after etching the damascene apertures. 
       FIG. 4  illustrates the same structure after depositing a first liner material. 
       FIG. 5  illustrates the same structure undergoing a sputter process. 
       FIG. 6  illustrates the same structure after the underneath material removal. 
       FIG. 7  illustrates the same structure after the deposition of a second liner layer. 
       FIGS. 8–12  illustrate corresponding steps in an alternative embodiment of the invention. 
       FIG. 13  illustrates a joint construction according to the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows a cross section of a portion of the back end of an integrated circuit. Material  100  is illustratively an interlevel dielectric (ILD), e.g. oxide or a low-k material such as SiLK™, SiCOH, or any kinds of hybrid related materials. The region illustrated may be any level of interconnect, from the lowest, in which case, the lower metal piece  110  will rest on the substrate or any of the higher ones, in which case, there will be an interface between two levels of dielectric that each contain a set of interconnect lines. 
   Embedded in the ILD  100 , is an interconnect member  110 , e.g. a copper, aluminum, tungsten, line extending perpendicular to the plane of the paper. The process used to place the interconnect lines is the well known damascene process, in which an aperture is etched in the ILD filled with a conductor that is then polished level with the surface. 
   In the case of copper conductors, it is necessary that the copper be confined and a liner is formed around the underneath copper conductor. In  FIG. 3 , a portion of a cap layer  120  is shown that covers the top surface of the copper. Cap layer  120  has been etched through in the course of forming an aperture denoted  150  for the upper portion and  148  for the lower portion. A hard mask layer  115  that was used to define the lower metal interconnect member has been left in place. 
   Above the cap layer  120 , an upper ILD  130  has had an aperture, denoted with numeral  150  for the upper portion and  148  for the lower portion, etched in it. This aperture is a dual damascene aperture, having an upper portion  150  that extends to the right at the top of the Figure and a via portion  148  that extends down to make contact with lower interconnect member  110 . In this Figure, the via portion  148  exposes the top surface of interconnect member  110 . 
     FIG. 4  shows the result of depositing a liner  140 , illustratively TaN. In other versions of the process, the liner layer may include an adhesion layer such as Ti that makes a good bond with the ILD and/or with the copper interconnect material. The liner material is preferably TaN, Ta, Ti, TiN, TiN(Si) or W, although other materials may be used. 
   A significant problem in the process of forming a back end interconnect structure such as that illustrated in  FIG. 3  is the mechanical and electrical connection between the bottom of the upper via and the top of the lower interconnect. 
   The thermal mismatch between Cu and the ILD material is significant, so that the thermal steps in the process tend to pull the joint between the interconnect levels apart. SiLK and other low-k ILDs tend to have a coefficient of thermal expansion that is greater than the copper. At one extreme, there can be an open circuit caused by mechanical separation between the lower interconnect and the bottom surface of the via. 
   In addition to mechanical separation, the liner materials have a higher resistivity than copper and impurities may be deposited in the via bottom, both of which increase the electrical resistance of the joint. 
     FIG. 5  shows the process of directing an ion bombardment, illustratively Ar + , at the structure. The ions impinge preferentially on horizontal surfaces. It is desired to remove the liner  140  at the bottom of the via. Necessarily, the horizontal surface at the bottom of the horizontal portion of aperture  150  is also affected by the ion bombardment, and the liner material there may be removed also. The gas for ion bombardment or sputter-etching, can be selected from the group consisting of Ar, He, Ne, Xe, N 2 , H 2 , NH 3 , N 2 H 2 . 
     FIG. 6  shows the structure after the ion bombardment, showing that the ions have not only removed the liner  140  at the bottom of the via, but may have also removed some material from lower interconnect member  110 . 
   In an alternative embodiment of the invention, there is an option of performing an in-situ metal deposition during the ion bombardment. Although the etching rate is almost the same between the trench bottom and the via bottom, the deposition rate can be higher at the trench sites than the via sites, which then results in higher “net” etching rate at the via bottom. Thus, the desired result of opening the via&#39;s bottom while still protecting the extensive horizontal surface is preserved in a single step. Those skilled in the art will readily be able to select materials for deposition. 
   Aperture  152 , at the bottom of the via, exposes new surface area and penetrates into the lower interconnect member. This physical penetration has resulted in a significant improvement in joint quality. Experimental results of a thermal stress test have shown that joints constructed according to the invention have shown no failures after 1000 hours, while joints with only a slight penetration in to the lower material, such as might occur during an overetch, exhibited a lower failure rate after 500 hours. In contrast, joints constructed according to the prior art, with the via not penetrating into the lower member showed extensive and commercially unacceptable failure rates after 250 hours test. 
     FIG. 7  shows the structure after deposition of a second liner  145 . The second liner covers the extensive horizontal surface of upper trench  150  in order to properly confine the copper. The bottom surface of the via is shown as being only partially covered with the second liner to illustrate that complete coverage here is not essential. Complete coverage at this via bottom surface is not required, i.e . . . it can be either fully covered with the second liner material, or partially. The second liner material is preferably selected from the group comprising TaN, Ta, Ti, TiN, TiN(Si) or W, although other material may be used. 
   If desired, the circuit designer could specify a process in which the via is filled with a temporary material such as photoresist or TEOS oxide that would block the deposition of the second liner layer at the bottom of aperture  152 . The temporary material would be chosen so that it can easily be removed before filling the aperture  150  with the final conductive material. This approach would provide a metal to metal contact between the via and the lower interconnect. It has been found that this degree of contact is not required for the present technology and the process described, in which the bottom of aperture  152  is fully or partially coated with the second liner, is satisfactory. 
     FIGS. 8 through 12  illustrate a similar set of steps in an alternative process. 
     FIG. 8  shows the structure at the same step as  FIG. 3 , differing in that the cap layer  120  has not been etched by the step that formed the damascene aperture,  150  for upper portion and  148  for lower portion. Those skilled in the art will appreciate that the cap layer is formed from a diffusion barrier such as TaN and the ILD is formed from oxide on a low-k material. The etching process can readily be specified with chemistry that stops on the cap layer, producing the result shown in  FIG. 8 . 
     FIG. 9  shows the same structure with the liner  140  in place. 
     FIG. 10  shows the same bombardment with ions as in the previous method. In this case, the bombardment will remove not only the liner  140  at the bottom of the via, but also the cap layer  120 . 
     FIG. 11  shows the result of removing the liner  140  at bottom of the via  148 , cap layer  120  and a cone of material at the bottom of the via, producing a lower aperture  152 , similar to the previous method. It is possible that hard mask  122  at the top of the Figure has been reduced in thickness by the ion bombardment, although it may not always be true. There is an option to have an in-situ metal deposition during the ion bombardment process, which may result in only the liner  140  at the via bottom being removed away after the bombardment process. 
   As compared to the first approach, the major advantage of the second approach is to prevent the exposed underneath metal surface  110  from contamination and oxidation during the process transition between the ILD etch and the liner deposition, i.e. a surface contamination may occur in the first approach between the step shown in  FIG. 3  and the step shown in  FIG. 4 . 
     FIG. 12  shows the result of the second method, which produces the same final structure as the first method. An aperture  150  for the upper portion and  148  for the lower portion, is prepared for the deposition of the (copper) conductor in the damascene trench. A second liner  145  has been deposited, which thickens the liner on the vertical surfaces of the via. 
   It has been found that there is a significant dependence on the depth of the aperture  152  that penetrates into the material of the lower interconnect. 
   In the prior art, the goal of the processing was to avoid penetrating lower interconnect  110 . There might be some slight penetration caused by overetching when forming the lower part of the aperture for the via, but proper procedure was considered to be stopping the etch on the cap layer. In fact, all the current tool manufacturing companies strongly disagree using the process of ion bombardment on the lower interconnect  100 , because of the resputtering of the underneath material can significantly contaminate the processing chamber. The philosophy of the current invention disclosure here is totally different and opposite from all the current commercial processes. 
   It has been found that the penetration provided by the invention provides a significant advantage. Experimental results have shown that interconnect structures made according to the invention provide considerably better reliability and significantly lower failure rates than structures constructed according to the prior art. 
   Referring to  FIG. 13 , there is shown a simplified drawing of a joint constructed according to the invention. Via  148 , extends down to meet lower interconnect member  110 . 
   Numeral  153  denotes the surface of a cone that penetrates into member  110 . The actual shape may be irregular, and the  FIG. 13  shows a version, which is very close to a real case, of a penetrating aperture that has the general shape of a cone. The shape will be described generally as one having substantially no horizontal surfaces. 
   The cone has a radius r, (or half the diameter of the via). A distance h represents the height of the cone and is the primary variable that may be adjusted according to the invention, since the diameter of the via is fixed by the design rules for the current generation of processing. 
   It has been found that there is a threshold below which the penetration into the lower interconnect has no significant effect. It has been found that when h is less than r, i.e. half the diameter of the via, the failure rate is not significantly different from that of the prior art. 
   In general, the area of the cone-shaped surface  153  is given by πr(r 2 +h 2 ) 1/2 −πr 2 . This quantity should be greater than the area (πr 2 ) of a flat-bottomed cylinder or radius r. 
   Further, it is preferable that h is X&gt;(3) 0.5  times the radius in order for the benefits of the invention to become fully realized. Thus, the preferred embodiment of the invention is a generally cone-shaped penetration of the via into the lower interconnect that extends by the (3) 0.5  times the radius of the via. 
   The invention has been explained in the context of a metal interconnect with specified liner material, but those skilled in the art will appreciate that other materials having the same or similar properties may be used. 
   While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.