Abstract:
A method of encapsulating metal lines ( 130, 132, 134, 136, 138 ) by implantation of dopants to form surface regions ( 131, 133, 135, 137, 139 ) after the metal lines have been fabricated. The surface regions may act as passivation layers and electromigration inhibitors and so forth.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates. to semiconductor devices, and, more particularly, to integrated circuit interconnects and methods of fabrication. 
     Integrated circuits typically include field effect transistors with source/drains formed in a silicon substrate and insulated gates on the substrate together with multiple overlying metal (or polysilicon) interconnects formed in levels. An insulating layer lies between the gates/sources/drains and the interconnects formed from the first metal level (premetal dielectric) and also between successive metal levels (intermetal-level dielectric). Vertical vias in the insulating layers filled with metal (or polysilicon) provide connections between interconnects formed in adjacent metal levels and also between the gate/source/drain and the first metal level interconnects. 
     Typically the metal interconnects are made of aluminum due to its high conductivity and ease of fabrication. However, poor electromigration and stress voiding characteristics have driven the need to improve the reliability of aluminum interconnects. Alloying, dopant precipitation and intermetallic reaction layer formation affect electromigration, stress-migration robustness and mechanical properties of aluminum films. The simplest approach is to dope the aluminum with small amounts (0.5-4% by weight) of copper. Alloys with high copper content are difficult to etch and also have a tendency for whisker formation depending on the nature of the intermetallic precipitates that are formed. Excessive copper leads to etch residues and an increased tendency for metallic corrosion. These typically constrain the copper doping to below 2%. 
     Reliability of aluminum interconnects is improved by sandwiching the aluminum between other materials such as TiN and Ti. TiN reduces the tendency for hillock/void formation, serves as an anti-reflection coating to ease patterning, provides a current shunting path if voids are indeed formed, improves the texture of the aluminum, prevents stress migration at vias, and acts as a barrier to prevent interaction between the aluminum and other materials in the device structure. Titanium improves the texture of the aluminum and reacts with aluminum to form an intermetallic with improved electromigration characteristics. Preferred crystallographic orientation has long been recognized to playt an important role in interconnect reliability (i.e., thermal hillocks, grain collapse, stress voiding, and electromigration). A strong (111) texture improves electromigration lifetime. Deposition conditions and the presence of underlayers have the greatest influence on texture. As linewidths are scaled to below 0.5 μm, the percentage of Al surface area that is encapsulated with Ti and TiN gets smaller, and the effectiveness of this approach diminishes. For a 0.35 μm wide line, only the top and bottom surfaces can be protected with Ti/TiN layers, while the sides (0.6 μm tall) remain bare. Metal stacks are also harder to etch since a multiple step etch. with high selectivity to the underlayers is necessary. 
     Deposition conditions can also improve interconnect reliability. Higher deposition temperatures incrase grain size (electromigration performance is typically superior for larger grain sizes), but contact/via reliability can degrade. for high thermal budget processes. Thus there is a need for a method that provides complete Al encapsulation. One approach treats aluminum lines with a wet chemical bath of H 2 O 2  to oxidize the surfaces for passivation. 
     In general, addition of dopants (e.g., arsenic, antimony, metals, oxygen, nitrogen, and fluorine) into aluminum can modify electrical and mechanical properties. For example, arsenic improves elecrotromigration, and fluorine reduces stress-induced hillock formation. 
     SUMMARY OF THE INVENTION 
     The present invention provides doping of interconnect surface regions to encapsulate the interconnect and improve properties such as electromigration and corrosion resistance and also serves as a barrier. In particular, preferred embodiments include implants of dopants into the sidewalls of clad or unclad aluminum interconnects and dopant implants into sidewalls of trenches formed in dielectrics followed by diffusion into copper interconnects filling the trenches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings are schematic for clarity. 
     FIGS. 1 a-c  illustrate in cross sectional elevation views the steps of a first preferred embodiment and method. 
     FIGS. 2 a-b  show implant angles. 
     FIGS. 3 a-b  illustrate a second preferred embodiment. 
     FIGS. 4 a-c  show repeated use of the preferred embodiments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
     FIG. 1 a-c  illustrate in cross sectional elevation view the steps of a first preferred embodiment method of metal interconnect formation during integrated circuit fabrication. In particular, start with the partially fabricated circuit of FIG. 1 a  which includes polysilicon gate  104  and field oxide  106  on silicon substrate  102  and overlying premetal level dielectric (PMD)  110  (which may be silicon dioxide). Metal (e.g., tungsten) filled vias  122 - 124  extending through PMD  110  and blanket clad aluminum metal layer including TiN  112 , aluminum  114 , and TiN  116  covers PMD  110 . TiN  112  and  116  may each be about 50 nm thick and aluminum  114  about 0.6 μm thick. 
     Next, photolithographically define interconnect patterns on the clad aluminum layer and anisotropically etch the clad aluminum to form interconnects  130 - 138 . Clean up after the anisotropic etch; the etch may be a chlorine based plasma etch. The interconnects may be 0.35-0.5 μm wide and 0.7 μm high with only 0.35-0.5 μm spacing between interconnects  130 - 132 - 134  and between interconnects  136 - 138 . Thus the area of exposed aluminum exceeds the area of clad aluminum at the surfaces of the interconnects, and the cladding effectively covers less than half of the aluminum surface. 
     Then modify the unclad vertical aluminum sidewall surface by ion implantation. See FIG. 1 c  illustrating dopant implantation at angle θ tilted from the normal to the substrate; the implants form modified surface regions  131 ,  133 ,  135 ,  137 , and  139  in the vertical sidewalls of the aluminum interconnects  130 ,  132 ,  134 ,  136 , and  138 , respectively. Because interconnects tend to be laid out generally parallel to an edge of the integrated circuit&#39;s rectangular die, four angled implants with one implant angled in each of the four directions parallel the die edges suffice for good sidewall coverage. FIG. 1 c  shows just one of the four implants; a second implant would have ions directed generally from upper righthand to lower lefthand in FIG. 1 c ; and the other two implants would be directed at angles into and out of the plane of the figure. Alternatively, the wafer may be rotated to implant all exposed surfaces. 
     The implant dose for each of the four angled implants will be on the order of 10 15  atoms/cm 2  at a low energy such as 5-40 KeV. Dopants such as As, Sb, Si, Ti, O, N, F, . . . will have projected ranges of roughly 1-30 nm with the lighter ions having greater projected ranges for equal energies. This yields a surface region of doping concentration roughly 4×10 20 /cm 3 , or about 1% atomic. Such a concentration suffices to significantly modify the electrical and mechanical characteristics of aluminum and forms an encapsulation for the aluminum interconnect but has little effect on dielectric  110 , cladding on the aluminum, or the net resistance of the interconnect. Further, the top cladding on the aluminum could be omitted and the implanted dopants also provide the aluminum top surface encapsulation. 
     A thermal anneal (e.g., 200-450° C.) may be necessary after implantation to induce chemical reaction between the dopant and the metal in the modified surface layer and form the encapsulation of the interconnect. In particular, implanting As into aluminum yields a surface aluminum arsenide type region which helps suppress electromigration. 
     Similarly, implanting oxygen or nitrogen into aluminum gives a surface region of aluminum oxides or nitrides which passivate the surface. 
     Implantations of two or more types of dopants yield more complex modified surface layers which may provide more involved characteristics. 
     FIGS. 2 a-b  show the determination of the implant angle: (1) first, increasing the tilt angle θ increases the fraction of the implant lodging in (unshadowed) vertical sidewalls instead of horizontal surfaces, thus the greater the tilt angle the better the implant efficacy; and (2) most (e.g, at least 75%) of the exposed aluminum sidewall area should be implanted to achieve the desired effects, but this imposes an upper limit on the implant angle tilt due to shadowing by adjacent interconnects. For example, with a spacing between interconnects of about 0.35 μm and interconnects of height about 0.7 μm, the tilt angle for the implant should be about arctan(0.35/0.7) (about 26.5°) for full sidewall coverage as illustrated in FIG. 2 a . If only a portion of the sidewall need be implanted, then the angle is determined from the fraction of the sidewall implanted: the fraction equals Wcotθ/H with W the spacing between the sidewall and the adjacent shadowing structure of height H; see FIG. 2 b . These larger angle implants can be performed in special large angle ion implanters. 
     Second Preferred Embodiment 
     FIGS. 3 a-b  illustrate a second preferred embodiment method of tilted angle implant to affect metal surface properties, although the ions are originally implanted into the dielectric and then migrate into the metal after the metal deposition and anneal. In particular, form grooves of depth 0.7 μm and width 0.35 μm in dielectric  302 , and then implant Mg ions of dose 10 5  atoms/cm 2  at a low energy such as 5-20 KeV in each of the four tilt dirctions plus also implant with zero tilt to cover the bottom of the grooves. Again, the implant tilt angle for this height-to-width ratio is about 26.5; the illustration of implant angles in FIGS. 2 a-b  still applies. FIG. 3 a  shows sidewall implanted regions  310 , bottom implanted regions  312 , and top surface implanted regions  314 . Note that the angled implants also contribute to the surface regions  314  and to the bottom regions  312  if the groove is along the plane of the tilt direction. Thus very roughly the surface regions  314  have five times the dose of the sidewall regions  310 , and the bottom regions  312  have about three times the dose of the sidewall regions  310 . In fact, the zero tilt implant may be omitted if the angled implant sufficiently covers the groove bottoms. 
     After the Mg implant, fill the grooves with copper  320  by a blanket copper deposition followed by etchback. Then anneal at 200-350° C. to diffuse the Mg out of dielectric  302  into surface regions  322  of copper  320  to form Cu:Mg which encapsulates the copper surface; see FIG. 3 b . Dopants tend to segregate at interfaces; this phenomenom aids dopant diffusion and subsequent reaction at the dielectric-metal interface. The encapsulation serves as a diffusion, oxidation and corrosion barrier for the copper interconnects. 
     Multiple Metal Layers Preferred Embodiment 
     FIGS. 4 a-c  show two successive applications of the preferred embodiments for encapsulation of two successive metal levels. In particular, FIG. 4 a  shows implanting dopants into the surfaces of dielectric  410  which contains trenches  412 - 416 . Next, a blanket metal deposition followed by chemical mechanical polishing (CMP) fills the trenches with metal and planarizes the surface to yield interconnects  422 - 426  in trenches  412 - 416  as illustrated in FIG. 4 b . An anneal (temperature depending upon the metal used) diffuses the dopants from the dielectric to react with and encapsulate metal interconnects  422 - 426 . Then deposit planar dielectric  430  and form metal-filled vias  432 , deposit a metal layer and pattern it to form interconnects  442 - 446 . Lastly, implant dopants to encapsulate interconnects  442 - 446  as shown in FIG. 4 c.    
     Modifications 
     Modifications of the preferred embodiments feature of surface region modification after interconnect formation to encapsulate the interconnect include: 
     Use of different interconnect metals and dopants; application to structures other than interconnects; implantation of the surface region dopants through a thin layer deposited on the metal surface; various implant doses such as from low doses about 10 4 /cm 2  to high doses about 10 6 /cm 2 ; multiple implants for multiple dopants; different interconnect. sizes and aspect ratios; and so forth.