Patent Publication Number: US-7897508-B2

Title: Method to eliminate Cu dislocation for reliability and yield

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to Patent Application No. 200510111640.0 of the People&#39;s Republic of China, filed Dec. 14, 2005, commonly assigned herewith and incorporated by reference herein for all purposes. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a method and device for manufacturing a metal inter-connect structure exhibiting reduced defects. Merely by way of example, the invention has been applied to a copper metal damascene structure such as a dual damascene structure for advanced signal processing devices. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to microprocessor devices, memory devices, application specific integrated circuit devices, as well as various other interconnect structures. 
     Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across. 
     Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility. Making devices smaller is very challenging, as each process used in IC fabrication has a limit. That is to say, a given process typically only works down to a certain feature size, and then either the process or the device layout needs to be changed. An example of such a limit is the ability to form interchanging metal and dielectric layers, where the metal layers to not interact with each other in the form of noise. 
     As merely an example, aluminum metal layers have been the choice of material for semiconductor devices as long as such layers have been used in the first integrated circuit device. Aluminum had been the choice since it provides good conductivity and sticks to dielectric materials as well as semiconductor materials. 
     Most recently, aluminum metal layers have been replaced, in part, by copper interconnects. Copper interconnects have been used with low k dielectric materials to form advanced conventional semiconductor devices. Copper has improved resistance values of aluminum for propagating signals through the copper interconnect at high speeds. 
     As devices become smaller and demands for integration become greater, limitations in copper and low k dielectric materials include unwanted migration of Cu material into other portions of the integrated circuit. Accordingly, conducting copper features are typically encased within barrier materials such as silicon nitride, which impede the diffusion of the copper. 
     Cu dislocation at post-CMP copper surface and SiN cap is one of top killer mechanisms affecting copper backend reliability failures as well as electric failures. One example of such a failure is local bridging of two or multiple metal lines by HTOL stress. 
     Cu dislocation includes copper mass migration, void formation during grain growth, and grain boundary reorganization. Controlling Cu dislocation is a key solution to improve reliability and yield issues due to such related fail modes. 
       FIG. 1A  shows simplified cross-sectional view of a copper feature  2  formed within dielectric  4  and sealed by overlying silicon nitride barrier layer  6 .  FIG. 1A  shows that the presence of topography such as hillocks  8  and voids  10  in the copper, can produce uneven thickness and passivation in the overlying SiN barrier layer. As a result, upon exposure of the copper-containing structure to a thermal cycle, stress release along grain boundaries of the copper can result in unwanted migration, breaking the SiN barrier. 
       FIG. 1B  is a TEM micrograph showing a cross section of metal bridging after stress due to copper dislocation.  FIG. 1B  shows the electrically stressed metal lines fabricated without copper dislocation control, where bulk copper migration outside of trench is seen. This migration caused an electric short and destroyed the functionality of the die. 
     From the above, it is seen that an improved technique for processing semiconductor devices is desired. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, the deleterious effects of migration of copper or other conducting material is avoided by briefly exposing metallization to a thermal anneal prior to chemical mechanical polishing and formation of an overlying capping/barrier layer. This brief thermal anneal intentionally provokes migration of the conducting material and formation of hillocks and voids therein, which may then be removed by a subsequent CMP step. The capping/barrier layer may subsequently be formed over a defect-free surface, which has already experienced release of stress along grain boundaries and migration in response to thermal treatment. 
     An embodiment of a method in accordance with the present invention for manufacturing integrated circuit devices including metal interconnect structures, comprises, providing a first dielectric material overlying a surface of a semiconductor substrate, the first dielectric material defining a trench. A conductor is formed within the trench and over the first dielectric material. The conductor is removed outside of the trench. The conductor is subjected to thermal energy to provoke migration of the conductor, and then the conductor is planarized. 
     An embodiment of a method in accordance with the present invention of fabricating a metallization structure in an integrated circuit, comprises, applying thermal energy to conducting material to provoke migration of the conductor prior to formation of an overlying capping layer, and then removing at least one of hillocks and voids resulting from the migration prior to forming the capping layer. 
     An embodiment of a semiconductor integrated circuit device structure in accordance with the present invention, comprises, a dielectric material, a conducting line formed in the dielectric material, and a capping layer overlying the conducting line. The conducting line is substantially free of voids or hillocks as a result of migration followed by chemical mechanical polishing prior to formation of the capping layer. 
     Various additional objects, features and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a simplified cross-sectional view of a copper structure experiencing unwanted copper migration in response to a thermal cycle. 
         FIG. 1B  is a TEM micrograph showing a cross section of metal bridging after stress due to copper dislocation. 
         FIGS. 2A-2E  show simplified cross-sectional views of a process flow for forming a Damascene structure. 
         FIG. 3  shows a simplified flow diagram of the embodiment of the process flow in accordance with the present invention utilized to form the copper structure of  FIG. 2 . 
         FIG. 4  shows an electron micrograph of a cross-section of a metal interconnect structure formed by a process flow according to an embodiment of the present invention. 
         FIG. 4A  shows an enlarged electron micrograph of a portion of the structure of  FIG. 4  that is free of Cu dislocation defects. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, techniques including methods for the manufacture of semiconductor devices are provided. More particularly, the invention provides a method of forming a metal interconnect structure avoiding unwanted migration of copper material by exposing a conducting feature to a brief thermal anneal prior to chemical mechanical polishing and subsequent formation of an overlying barrier layer. This thermal anneal intentionally provokes migration of the conductor and formation of hillocks and voids therein, which are then removed by a subsequent CMP step. The capping/barrier layer may thus subsequently be formed over a defect-free surface, which has already experienced release of stress at grain boundaries and migration in response to thermal treatment. 
     Merely by way of example, embodiments in accordance with the present invention have been applied to a copper metal damascene structure such as a dual damascene structure for advanced signal processing devices. But it would be recognized that the invention has a much broader range of applicability. For example, alternative embodiments in accordance with the present invention can be applied to form metallization structures comprising other than copper, for example aluminum, Al/Cu alloys, and gold. Other alternative embodiments in accordance with the present invention may be utilized in the fabrication of microprocessor devices, memory devices, and application specific integrated circuit devices, as well as various other interconnect structures. 
     A method according to an embodiment of the present invention can be briefly provided as follows:
     1. Form interlayer dielectric layer overlying substrate;   2. Pattern the interlayer dielectric layer;   3. Form liner layer within the patterned interlayer dielectric layer;   4. Form copper metal layer within the liner layer of the patterned interlayer dielectric layer to a height above a surface of the interlayer dielectric layer;   5. Recrystallize copper metal layer;   6. Remove copper metal layer outside of interlayer dielectric;   7. Apply thermal energy to provoke migration in planarized copper metal layer;   8. Planarize copper metal layer to remove voids/hillocks resulting from migration;   9. Form capping layer overlying the metal layer; and   10. Perform other steps as desired.   

     The above sequence of steps provides a method according to an embodiment of the present invention. Other alternatives can also be provided where steps are added, one or more steps are removed, or one or more steps are provided in a different sequence without departing from the scope of the claims herein. Further details of the present method can be found throughout the present specification and more particularly below. 
       FIGS. 2A-E  show simplified cross-sectional views of the steps of an embodiment of a process flow in accordance with the present invention. The steps shown in  FIGS. 2A-E  are part of a metallization process in a copper backend process flow. 
     The dielectric material may comprise a material sold under the trademark of SILK, or a material called BLACKDIAMOND of Applied Materials, Inc. 
       FIG. 2B  shows the next step in the process, wherein after electroplating, optimal anneal conditions of low-temperature (about 180-220° C.) and duration (about 5-30 minutes in furnace) result in formation of stabilized copper microstructure  208 . Conditions for annealing conditions during this recrystallization step to create mature copper, generally take place for longer periods and at lower temperatures than the subsequent migration-provoking step described below in connection with  FIG. 2C . 
       FIG. 2C  shows the next step, wherein mature copper is removed outside of the trench. In one particular embodiment, three-platen copper CMP with selective slurry combinations achieved good planarity of the structure within the die and over the wafer level. Backside (BS) polish on platen number three has been adjusted to appropriate down-force, slurry, and pad conditions to achieve minimal surface damage during this step. 
       FIG. 2C  also shows application of the brief annealing step in accordance with embodiments of the present invention, to the metallization structure. This short annealing may be performed on a hot plate after the initial CMP, in order to relieve metal stress between grain boundaries, and to provoke copper migration in accordance with the present invention.  FIG. 2C  accordingly shows formation of void  210  and hillock  212  in stabilized copper microstructure  208 . The short annealing during this step can be conducted at the temperature range of about 250-350° C. for about 30-90 seconds, in an inert gas ambient like N 2 , Ar, He or others. The anneal performed during this step is in general higher in temperature and of shorter duration than that which takes place during the previous recrystallization step to form mature copper described in  FIG. 2B . 
       FIG. 2D  shows the subsequent reworking of the metal surface with backside polishing in copper CMP, with low downforce and RPM to remove the hillocks and humps created during the previous brief thermal anneal in accordance with the present invention. The result is the formation of copper metal lines  214 . 
       FIG. 2E  shows formation of SiN by chemical vapor deposition (CVD) to encapsulate the copper metal lines resulting from the step shown in  FIG. 2D . Conditions for pretreatment of wafers with hydrogen gas (H 2 ) and hot chuck stabilization may be optimized to achieve optimal gap fill during this step. 
       FIG. 3  shows a simplified flow diagram of the embodiment of the process flow  300  in accordance with the present invention utilized to form the copper structure of  FIG. 2E . In step  302 , a copper metal material is provided, typically within a lined trench in a dielectric material. In step  304 , the copper metal is subjected to thermal annealing to relieve stress between grain boundaries and provoke migration and consequent hillocks and/or void formation. In step  306 , the copper material subjected to the brief thermal annealing is again planarized, thereby removing any void/hillock topography resulting from the brief thermal annealing process. 
     In step  308 , the planarized copper material is exposed to subsequent processing, including thermal treatment. Because the copper has already been subjected to annealing and experienced the resulting copper migration, however, this subsequent processing results in little or no hillock formation. 
     Accordingly,  FIG. 4  shows an electron micrograph of a cross-section of a metal interconnect structure formed by a process flow according to an embodiment of the present invention.  FIG. 4A  shows an enlarged electron micrograph of a portion of the structure of  FIG. 4  that is free of Cu dislocation defects. 
     Embodiments in accordance with the present invention are able to significantly reduce or even eliminate Cu dislocation phenomena, and consequently improve device reliability and virgin yield. While embodiments in accordance with the instant invention are not limited to any particular design rule of material composition, particular embodiments have provided successful in fabricating general interconnect damascene metallurgy where either 0.13 μm fluorosilicate glass (FSG) or 90 nm low K dielectrics are used. 
     Many benefits are achieved by way of the present invention over conventional techniques. For example, utilizing Cu dislocation control in accordance with embodiments of the present invention, clean and smooth Cu/SiN interfaces may be obtained free of voids or the need to leave room for copper migration and diffusion. Thus according to embodiments of the present invention, Cu could be well encapsulated to create a reliable chip. 
     Moreover, embodiments in accordance with the present technique provides an easy to use process that relies upon conventional technology. In some embodiments, the method provides higher device yields in dies per wafer. Additionally, the method provides a process that is compatible with conventional process technology without substantial modifications to conventional equipment and processes. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more throughout the present specification and more particularly below. 
     While the specific embodiment described above has utilized copper metallization, the present invention is not limited to the use of this or any other particular electrically conducting material. Various different materials may be subjected to brief thermal annealing to provoke migration followed by planarization, and would remain within the scope of the present invention. For example, alternative embodiments in accordance with the present invention may be utilized to provoke migration of conducting metals other than copper, including but not limited to Aluminum (Al), Al(Cu) alloys, and gold. Certain of these alternative conducting materials would be formed by deposition techniques, rather than by the electroplating processes utilized to form copper. 
     It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.