Abstract:
A metal edge seal ring is formed in a trench made up of a large number of short, connected legs in perpendicular relation. Metal is deposited in the trench, and because the metal is comprised of many short segments rather than several long, straight sections, the subsequent chemical-mechanical polishing step does not cause significant cupping of the metal in the trench.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to edge sealing for a semiconductor device, and more particularly, to an improved method for manufacturing an edge seal ring in a copper dual inlaid process, and the device produced thereby. 
     2. Discussion of the Related Art 
     The copper dual inlaid or dual damascene process is optimized for the formation of an array of small vias, which connect layers of metal separated by dielectric. With reference to FIG. 1, in the formation of such a via, a dielectric layer  20  is provided over a metal layer  22 , and an opening  24  is provided in the dielectric layer  20  exposing a portion of the metal layer  22 . After deposition of a diffusion barrier layer  26  such as tantalum or titanium nitride, copper  28  is deposited over the resulting structure, overfilling the opening  24  and having portions overlying the dielectric layer  20 . 
     Next, a chemical-mechanical polishing step is undertaken. In such chemical-mechanical polishing process, as is well-known (FIG.  1 ), a wafer is mounted on a rotating platen, an independently rotating polishing pad  30  is pressed against the wafer surface  32 , and a slurry  34  carrying small abrasive particles, usually colloidal, is flowed onto the platen. The particles attack and remove small pieces of the wafer surface  32  which are carried away by the movement of the slurry  34  across the surface  32 . Furthermore, a slurry chemistry is selected that dissolves or etches surface materials. The combined actions of the two rotations with the slurry provide for effective chemical-mechanical polishing of the wafer surface  32 , commonly known as planarization. 
     During such chemical-mechanical polishing, the metal is removed from the dielectric layer  20  and the level of copper  28 A in the opening  24  is generally brought down to the level of the upper surface  36  of the dielectric layer  20  (FIGS.  2  and  3 ), so that the resulting upper surface  29  of the copper  28  is substantially coplanar with the upper surface  36  of the dielectric layer  20 , forming an overall substantially planar surface ready for application of the next metal layer thereover. 
     Typically, the polishing pad  30  is made of somewhat compliant material, so that it is able to form itself generally to the surface  34  to be chemically-mechanically polished, meanwhile providing the greatest pressure on the highest surface area, so that as such polishing proceeds, surface planarity is achieved. 
     Such a polishing operation has proven effective in the formation of vias, which measure for example 0.2 μm by 0.2 μm. In via formation (FIGS.  2  and  3 ), the high (projecting) surfaces of the metal on the dielectric layer  20  are chemically-mechanically polished away first. Then, chemical-mechanical polishing proceeds until all metal is removed from the upper surface  36  of the dielectric layer  20  and the upper surface  29  of the copper via  28 A in the opening  24  is generally coplanar with the upper surface  36  of the dielectric layer  20 . 
     Even though pad  30  has a degree of flexibility, the upper surface  29  of the copper  28 A in the opening  24  is substantially coplanar with the upper surface  36  of the dielectric layer  20 , i.e., “dishing” of the copper (depression at center) is minimal. This is so because of the small dimensions as described above, i.e., the span across which the polishing pad  30  must extend in polishing the copper, from one edge of the dielectric to the other across the opening  24 , is very small, for example 0.2 μm as stated above. 
     However, in forming an edge seal ring for a device, i.e., typically a rectangular metal ring around the active area of the device, which metal ring contacts a lower silicon layer to form a seal therewith for keeping contaminants from entering the active area, a significant problem arises. Typically, in the dual inlaid process, the metal ring is formed in the same general manner as are the vias discussed above. That is (FIGS.  4 - 6 ), an opening in the form of a rectangular trench  40  is formed in a dielectric layer  42  overlying a silicon layer  44 . The trench  40  surrounds an active device area  46 , and includes four long, straight, continuous trench portions  48 ,  50 ,  52 ,  54 , connected by trench corner regions  56 ,  58 ,  60 ,  62 . 
     After deposition of a barrier layer  64  over the structure, copper  66  is deposited in the trench  40  and chemical-mechanical polishing is undertaken as described above. A long trench portion  48  with barrier metal  64  and copper  66  therein is shown in FIGS. 7 and 8. 
     During chemical-mechanical polishing, the polishing pad  68  is brought into contact with the exposed surface  70  of the copper  66 , and the higher portions thereof will be removed first. Chemical-mechanical polishing continues until the pad  68  is brought into contact with the edges of the dielectric layer  42  adjacent the trench portion  48 . Because the portion  48  of the trench  40  is quite long, for example, 10 mm, and for example 1 μm wide, such dimensions, coupled with the compliant nature of the pad  68 , cause a substantial degree of “dishing” or “cupping” to occur in the upper surface  65  of the copper  66 A as shown in FIG.  9 . That is, near the dielectric layer  42  edge adjacent the trench portion  48  (FIG.  10 ), the upper surface  65  of the copper  66  and upper surface  43  of the dielectric layer  42  are substantially coplanar, because the pad  68 , even though compliant, is supported in that area by the edge of the dielectric layer  42 . Meanwhile, in the center of the span, distant from the edges of the dielectric layer  42  (FIG.  11 ), removal of the copper will be significantly greater. That is, the vertical dimension (thickness) of the copper  66 A above the silicon layer  44  at that point is much less than it is adjacent the dielectric. This lack of planarity can clearly lead to problems during the further fabrication of the device. Furthermore, a trench portion  48  of such significant length has proven difficult to fill with copper  66  as is needed for device reliability. 
     Therefore, what is needed is a method for forming an edge seal ring in a semiconductor device, which avoids the problem of dishing or cupping of the top surface of the ring during its fabrication, meanwhile being properly functional and promoting device reliability. 
     SUMMARY OF THE INVENTION 
     In the present invention, a trench in the general form of a continuous ring is provided in a dielectric layer overlying a silicon layer, the trench defining a continuous opening communicating with the silicon layer. The trench has elongated trench portions connected by trench corner regions. The elongated portions of the trench are each configured to include a plurality of connected legs of substantially the same length, positioned perpendicular to each other. After deposition of a diffusion barrier layer in the trench, metal such as copper or copper alloy is deposited in the trench. A chemical-mechanical polishing step is undertaken. The metal in the trench takes the general form of a continuous ring and is comprised of a plurality of elongated sections connected by corner regions. Each elongated section is made up of a plurality of connected segments, positioned in precise linear (perpendicular in the preferred embodiments) relationship. By forming the ring in short sections, the problem of cupping of the metal during chemical-mechanical polishing is avoided. 
     The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as said preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a sectional view illustrating the step of chemical-mechanical polishing in the formation of a via; 
     FIG. 2 is a sectional view similar to that shown in FIG. 1, illustrating a further step in the chemical-mechanical polishing process; 
     FIG. 3 is a sectional view taken along the line  3 — 3  of FIG. 2; 
     FIG. 4 is a plan view of a device including a trench as known in the prior art; 
     FIG. 5 is a sectional view taken along the line  5 — 5  of FIG. 4; 
     FIG. 6 is a sectional view taken along the line  6 — 6  of FIG. 4; 
     FIG. 7 is a sectional view similar to that shown in FIG. 5, and illustrating a step in the chemical-mechanical polishing process; 
     FIG. 8 is a sectional view taken along the line  8 — 8  of FIG. 7; 
     FIG. 9 is a sectional view similar to that shown in FIG. 7, illustrating a further step in the chemical-mechanical polishing process; 
     FIG. 10 is a sectional view taken along the line  10 — 10  of FIG. 9; 
     FIG. 11 is a sectional view taken along the line  11 — 11  of FIG. 10; 
     FIG. 12 is a plan view of a device incorporating the present invention, illustrating a first configuration of the trench; 
     FIG. 12A is a plan view of a repeating cell used to build a substantial portion of the trench of FIG. 12; 
     FIG. 12B is a plan view of a repeating cell used to build the remaining portions of the trench of FIG. 12; 
     FIG. 13 is a plan view of a device incorporating the present in invention, illustrating a second configuration of the trench; 
     FIG. 14 is a sectional view taken along the line  14 — 14  of FIG. 12; 
     FIG. 15 is a sectional view taken along the line  15 — 15  of FIG. 12; 
     FIG. 16 is a sectional view similar to the shown in FIG. 14, illustrating a step in the chemical-mechanical polishing process; 
     FIG. 17 is a sectional view similar to that shown in FIG. 16, illustrating a step in the chemical-mechanical polishing process; 
     FIG. 18 is a sectional view similar to that shown in FIG. 16, illustrating a further step in the chemical-mechanical polishing process; 
     FIG. 19 is a sectional view similar to that shown in FIG. 17, illustrating a further step in chemical-mechanical polishing process; 
     FIG. 20 is a plan view similar to that shown in FIG. 12, illustrating the configuration of a first embodiment of inventive seal edge ring; and 
     FIG. 21 is a plan view similar to that shown in FIG. 13, illustrating the configuration of a second embodiment of the inventive sealing ring. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to specific embodiments of the present invention which illustrate the best mode presently contemplated by the inventors for practicing the invention. 
     FIG. 12 shows an opening in the form of a trench  100  in a dielectric layer  102  which overlies a silicon layer  104 , the trench  100  having a configuration in accordance with a first embodiment of the present invention. The trench  100  comprises generally elongated trench portions  106 ,  108 ,  110 ,  112  connected by trench corner regions  114 ,  116 ,  118 ,  120 . Each trench portion is made up of a plurality of connected trench legs  122  which are substantially perpendicular to each other at their connections. In addition, the legs  122  are of similar width and length. Cross-sections of a trench leg  122  are shown in FIGS. 14 and 15. A typical trench leg  122  would for example be 1 μm in width (FIG. 14) and 5 μm in length (FIG.  15 ). Thus, the trench  100  contains no long, continuous, straight portions. 
     The trench  100  may readily be fabricated by using cells  124 ,  126  in the form show in FIGS. 12A and 12B. A step-and-repeat process would be used to replicate the cell shown in FIG. 12A along substantially the entire length of each portion of the trench  100 . A step and repeat process would also be used to replicate the cell  126  shown in FIG. 12B to form the corners  114 ,  116 ,  118 ,  120  of the trench  100 . 
     FIG. 13 shows a trench  100 A in a dielectric layer  102 A and having a configuration in accordance with a second embodiment of the invention. The trench  100 A again comprises generally elongated trench portions  106 A,  108 A,  110 A,  112 A, connected by corner regions  114 A,  116 A,  118 A,  120 A. Each trench portion is again made up of a plurality of connected trench legs  122 A which are substantially perpendicular to each other at their connections. However, these legs  122 A are positioned in a zigzag configuration, rather than in the step configuration of FIG.  12 . 
     Again, the legs  122 A are of similar width and length. The cross-sections shown in FIGS. 14 and 15 are representative of the cross-sections of these trench legs  122 A. Similar to the embodiment of FIG. 12, a typical trench leg length would for example be 1 μm in width and 5 μm in length. It will be again seen that the trench  100 A contains no long, continuous portions. 
     The trench  100 A may readily be fabricated by using the cell  127  in the form shown in FIG. 13A. A step-and-repeat process would be used to replicate the cell shown in FIG. 13A about the entire trench  100 A. 
     In the following discussion, reference is made to the embodiment of trench  100  shown in FIG.  12 . However, and it will be readily seen that the following discussion and all method steps shown and described therein apply equally to the embodiment shown in FIG.  13 . 
     With reference to FIGS. 16 and 17, after deposition of a barrier diffusion layer  128  over the dielectric layer  102  and in the trench leg  122  and in contact with the silicon layer  104 , copper or copper alloy or other conductive metal  130  is electrolessly deposited thereover. Typically, the metal  130  in the opening  122  extends above the upper surface  132  of the dielectric layer  102 , and is even high over the upper surface  132  of the dielectric layer  102 . 
     A chemical-mechanical polishing step is undertaken using polishing pad  134  and slurry  136 . The pad  134 , being somewhat compliant, to an extent conforms to the configuration of the top surface  140  of the metal  130 , meanwhile applying maximum pressure to the highest portions of the metal  130 . 
     After sufficient chemical-mechanical polishing, all metal over the dielectric layer  102  is removed, and the level of metal  130 A is brought down so that the top surface  141  thereof is substantially coplanar with the top surface  132  of the dielectric  102  (FIGS.  18  and  19 ). Thus, an edge seal structure  142  generally in the form of a continuous ring, surrounding an active area  144 , is fabricated (FIG.  20 ). It will be seen that the edge seal structure  142  is made up of sections  146 ,  148 ,  150 ,  152  connected by corner regions  154 ,  156 ,  158 ,  160 , with each section made up of a plurality of connected segments  162 , which segments  162  are of similar length and which are perpendicular, i.e. at right angles, to each other at their connections. 
     Because the distance between the portions of dielectric layer  102  across the opening  122  is so small (for example approximately 5 μm as indicated in FIG. 15, and 1 μm as indicated in FIG.  16 ), the polishing pad  134 , when it reaches the dielectric layer  102 , is supported thereby over a relatively short span across the opening  122  so that significant cupping of the metal in the trench leg  122  is avoided. Furthermore, the short segments  162  of the present invention are readily filled with metal during fabrication of the edge seal structure  142 . This is to be compared with the prior art trench  40  (and thus the edge seal ring) having long, continuous, straight portions, leading to the problems discussed above. 
     When the method is applied to the device having the trench configuration of FIG. 13, a seal structure  142 A generally in the form of a continuous ring, surrounding an active area  144 A, is fabricated (FIG.  21 ). It will be seen that the edge seal structure  142 A is made up of a plurality of connected segments  162 A which are of similar length, and which are perpendicular, i.e., at right angles, to each other at their connections. These sections  162 A are in a zigzag configuration, rather than a step configuration as in the previous embodiment. However, all the features of the previous embodiment apply to this embodiment. 
     It will be understood that the drawing configurations have been chosen to more clearly illustrate the features of the invention. That is, for example, in reality, the number of segments of the edge seal ring are much greater than that illustrated, and the width and length of the segments are adjusted from their actual size for greater clarity. 
     The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications or variations are possible in light of the above teachings. 
     The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill of the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.