Abstract:
A method of fabricating an interconnect for a semiconductor device is disclosed. The method comprises: forming a dielectric layer on a semiconductor substrate; forming a trench in the dielectric layer; placing the semiconductor substrate in a plasma deposition chamber having a tantalum target; initiating a plasma in the presence of nitrogen in the plasma deposition chamber; and depositing an ultra-thin layer comprising tantalum and nitrogen in the trench.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor interconnect metallurgy; more specifically, it relates to a conformal barrier layer for copper interconnect metallurgy and methods of fabricating the layer. 
     BACKGROUND OF THE INVENTION 
     Advanced semiconductor technology utilize copper interconnect metallurgy for wiring of active devices into integrated circuits. Typically interconnect metallurgy is formed by a damascene or dual damascene process. Damascene processes allow for very narrow, high (greater than 1), aspect ratio (height divided by width) high-density wiring features. In a damascene process, a trench is etched into a dielectric layer. A layer of conductor of sufficient thickness to fill the trench is deposited and then a chemical-mechanical-polish (CMP) process performed to remove the conductor deposited on the surface of the dielectric layer. After CMP, only the layers filling the trench remain, the surface of the filled trench being flush with the surface of the dielectric layer. In damascene technology, various interconnect levels are connected by vias independently formed in intervening dielectric layers. In dual damascene technology, the vias are integrally formed in the same dielectric as the conductive wire. 
     Copper has become the conductor metallurgy of choice because its high conductivity allows for higher currents in the narrower wiring features than could be achieved with older, aluminum based interconnect metallurgy. While copper provides improved electro-migration and mechanical stress reliability, copper is usually used with a redundant conductor, such as tantalum in the form of a thin layer lining the bottom and sidewalls of the trench for improved reliability. However, tantalum is not deposited directly on such dielectrics as silicon oxides because it will form beta-phase tantalum. Beta-phase tantalum has a resistivity of about 200 micro-ohms per centimeter, much too high to be of use as a redundant conducing layer. Further, copper and tantalum generally require an adhesion promotion layer when used with silicon oxide dielectrics. One material used with tantalum and copper as an adhesion promoter is tantalum nitride. When tantalum is deposited on top of tantalum nitride, alpha-phase tantalum is formed. Alpha-phase tantalum has a resistivity of about 12 to 20 micro-ohms per centimeter. Additionally, tantalum nitride acts as a copper diffusion barrier. Copper can change the characteristics of active silicon devices and its migration through the dielectric layers into the silicon must be prevented. This is not only a concern with silicon oxide dielectrics, but is a very strong concern when low-K dielectrics, such as SILK™ (Dow Corning, Midland, Mich.) are used because of the porous nature of low-K materials. 
     However, by having a resistivity of about 250 to 500 micro-ohm per centimeter, tantalum nitride not a very good conductor. This high resistively becomes increasingly important as the density of interconnects increases and the wire size decreases with 0.25 micron and sub 0.25 micron groundrules. For a trench 0.225 microns wide and deep and a tantalum nitride layer of 25 to 50 nanometers, the tantalum nitride accounts for 30% to 56% of the cross-sectional area of the wire, partially negating the advantages gained due to the increased conductivity of copper. Further, as the aspect ratio of the trench increases the point is reached where only a narrow strip of copper can fit between the sidewalls of the trench, if indeed, the copper can be made to fill the remaining opening at all. 
     Therefore, there is a need for very thin tantalum nitride layer to be used as a liner in tantalum and tantalum/copper interconnect metallurgy and for a method of fabricating such very thin tantalum nitride layers. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is an interconnect for a semiconductor device, comprising: a conductive core having sidewalls and a bottom; and an ultra-thin layer disposed on the sidewalls and the bottom of said conductive core. 
     A second aspect of the present invention is a method of forming an ultra-thin tantalum nitride layer comprising: providing a tantalum target; initiating an inert gas plasma and flowing nitrogen into the plasma for a predetermined period of time to sputter tantalum nitride onto a substrate; and after expiration of the fixed period of time, stopping the flow of nitrogen. 
     A third aspect of the present invention is a method of forming an ultra-thin tantalum nitride layer comprising: providing a tantalum target; precharging the tantalum target with nitrogen by flowing nitrogen over the tantalum target; and initiating an inert gas plasma to sputter tantalum nitride onto a substrate. 
     A fourth aspect of the present invention is a method for fabricating an interconnect for a semiconductor device, comprising: forming a dielectric layer on a semiconductor substrate; forming a trench in the dielectric layer; placing the semiconductor substrate in a plasma deposition chamber having a tantalum target; initiating a plasma in the presence of nitrogen in the plasma deposition chamber; and depositing an ultra-thin layer comprising tantalum and nitrogen in the trench. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1A through 1D are partial cross-sectional views illustrating fabrication of a tantalum nitride/tantalum/copper interconnect according to the present invention; 
     FIG. 2 is a schematic cross-sectional view of a first tool for depositing an ultra-thin tantalum nitride layer according to the present invention; 
     FIG. 3 is a schematic cross-sectional view of a second tool for depositing an ultra-thin tantalum nitride layer according to the present invention; 
     FIG. 4 is a flowchart illustrating a first method for depositing an ultra-thin tantalum nitride layer according to the present invention; and 
     FIG. 5 is a flowchart illustrating a second method for depositing an ultra-thin tantalum nitride layer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A through 1D are partial cross-sectional views illustrating fabrication of a tantalum nitride/tantalum/copper interconnect according to the present invention. In FIG. 1A, a dielectric layer  100  is formed on a top surface  105  of a substrate  110 . In one example, substrate  110  is a semiconductor substrate. Formed in dielectric layer  100  is a trench  115  having sidewalls  120  extending from a top surface  125  of dielectric layer  100  to top surface  105  of substrate  110 . Trench  115  further has a bottom  130 . Dielectric layer  100  may be silicon oxide or a low-K dielectric material. In one example, the low-K dielectric material is SILK™ (Dow Corning, Midland, Mich.). Trench  115  may be formed by a reactive ion etch (RIE) process. 
     In FIG. 1B, an ultra-thin layer of tantalum nitride  135  is deposited by a plasma deposition process. The fabrication tooling for tantalum nitride layer  135  is illustrated in FIGS. 2 and 3 and described below. The process for tantalum nitride layer  135  is illustrated in FIGS. 4 and 5 and described below. Tantalum nitride layer  135  is about 0.5 to 3 nanometers thick and is a conformal coating, covering sidewalls  120  and bottom  130  of trench  115  as well as top surface  125  of dielectric layer  100 . Since the molecular diameter of tantalum nitride is about 0.42 nanometers, tantalum nitride layer  135  comprises one to six monolayers. 
     In FIG. 1C, a layer of tantalum  140  is deposited on top of tantalum nitride layer  135 . Tantalum layer  140  is deposited in the same tool and chamber used for deposition of tantalum nitride layer  135  after tantalum nitride deposition. In one example, tantalum layer  140  is about 5 to 30 nanometers thick. Tantalum layer  140  forms a conformal layer over tantalum nitride layer  135 . 
     In FIG. 1D, a conformal copper seed layer  145  is formed over tantalum layer  140 . Copper seed layer  145  is sputter deposited or evaporated to a thickness of about 10 to 200 nanometers. Copper seed layer may be deposited or evaporated in a second chamber of a load-lock tool, the first chamber used to form tantalum nitride layer  135  and tantalum layer  140 . Copper core conductor  150  is then formed over copper seed layer  145  by electroplating to a thickness sufficient to fill in trench  115  completely. A CMP step is performed to remove tantalum nitride layer  135 , tantalum layer  140 , copper seed layer  145  and copper core conductor  150  from over top surface  125  of dielectric layer  100  leaving a conductive wire  155  having a top surface  160  essentially coplanar the top surface of the dielectric layer. While copper seed layer  145  is illustrated in FIG. 1D, for all practical purposes the copper seed layer becomes part of copper core conductor  150  and is indistinguishable from the copper core conductor. 
     FIG. 2 is a schematic cross-sectional view of a first tool for depositing an ultra-thin tantalum nitride layer according to the present invention. Chamber  165  is an ion metal plasma chamber such as the IMP Vectra™ Chamber available from Applied Materials Inc. of Santa Clara, Calif. Chamber  165  includes sidewalls  170 , a lid  175  and a bottom  180 . A target  185  comprising tantalum is disposed in chamber  165  on lid  175 . A substrate support member  190  is movably disposed in chamber  165  and provides an upper support surface  195  for supporting a substrate  200 . In one example, substrate  200  is a semiconductor substrate. Substrate support member  190  is mounted on a stem connected to a lift motor  205  that raises and lowers substrate support  190  between a lowered loading/unloading position and a raised processing position. An opening  210  in chamber  165  provides access for a robot (not shown) to deliver and retrieve substrates  200  to and from the chamber while substrate support member  190  is in the lowered loading/unloading position. A lift plate  215  connected to a lift motor  220  is mounted in chamber  165  and raises and lowers pins  225  mounted in substrate support member  190 . Pins  225  raise and lower substrate  200  to and from the upper support surface  195  of substrate support member  190 . A shield  235  is disposed in chamber  165  to shield sidewalls  170  from sputtered material. A coil  230  is mounted to shield  235  via supports  240  between substrate support member  190  and target  195 . Coil  230  provides RF energy to assist in initiating and maintaining the plasma as well as increasing plasma density in order to increase the quantity of ionized species within the plasma. Supports  240  electrically insulate the coil  230  from the shield  235  and chamber  165 . Three power supplies are used in the chamber  165 . A DC power source  250  delivers DC power to target  185  to cause the processing gas to form a plasma. Spinning magnets  252  disposed behind target  185  form magnetic field lines at the target surface which trap electrons and increase the density of the plasma adjacent to the target in order to increase the sputtering efficiency. A first RF power source supplies RF power to coil  230  through a first matching network  255 B to increase the density of the plasma. A second RF power source  260 A, biases substrate support member  190  with respect to the plasma through a second matching network  260 B and provides directional attraction of the ionized sputtered material toward the substrate  200 . Two plasma gases are supplied to chamber  165  through a gas inlet  265  from gas sources  270 ,  275  as metered by respective mass flow controllers  280  and  285 . In the present example, the first gas is nitrogen and the second gas is an inert gas such as argon, helium, neon or krypton or a combination thereof. One or more vacuum pumps  290  are connected to chamber  165  at an exhaust port  295  to exhaust the chamber and maintain the desired pressure in the chamber. In one example vacuum pump  290  is a cryopump or any pump capable of sustaining a low pressure of about 10 −8  Torr. A controller  300  controls the functions of the power supplies  250 ,  255 A and  260 A, matching networks  255 B and  260 B, lift motors  205  and  220 , mass flow controllers  280  and  285 , vacuum pump  290  and other associated chamber components and functions. Controller  300  executes system control software stored in a memory, which in the in one example is a hard disk drive, and can include analog and digital input/output boards, interface boards and stepper motor controller boards. Optical and/or magnetic sensors are generally used to move and determine the position of movable mechanical assemblies. 
     In operation, a robot delivers a substrate  200  to chamber  165  through opening  210 . Pins  225  are extended upward to lift substrate  200  from the robot. The robot then retracts from chamber  165  and opening  210  is sealed. Pins  225  lower substrate  200  to upper surface  195  of substrate support member  190 . Substrate support member  190  raises substrate  200  into processing position under target  185 . One or more plasma gases are then introduced into chamber  165  to stabilize the chamber at a process pressure. A plasma is generated between target  185  and substrate support member  190  with power from DC power source  250 . The first RF power source  255 A delivers power to the coil  230  to create a plasma sufficiently dense to ionize the flux of sputtered target material from the target  185 . The ions are accelerated toward substrate  200 , which is biased, by second RF power source  260 A. After deposition, substrate support member  190  is lowered, pins  225  are raised to lift substrate  200 , the robot enters chamber  165  to retrieve substrate  200 , and if desired, delivers another substrate for processing. 
     FIG. 3 is a schematic cross-sectional view of a second deposition tool for depositing an ultra-thin tantalum nitride layer according to the present invention. FIG. 3 represents a generic DC magnetron plasma deposition tool. Chamber  305  includes sidewalls  170 , lid  175  and bottom  180 . Target  185  comprising tantalum is disposed in chamber  305  on lid  175 . Substrate support member  190  is movably disposed in chamber  305  and provides upper support surface  195  for supporting substrate  200 . Substrate support member  190  may comprise an electrostatic wafer chuck. Substrate support member  190  is mounted on a stem connected to lift motor  205  that raises and lowers substrate support member  190  between a lowered loading/unloading position and a raised processing position. Opening  210  in chamber  305  provides access for a robot (not shown) to deliver and retrieve substrates  200  to and from the chamber while substrate support member  190  is in the lowered loading/unloading position. Lift plate  215  connected to lift motor  220  is mounted in chamber  305 , raises, and lowers pins  225  mounted in substrate support member  190 . Pins  225  raise and lower substrate  200  to and from upper support surface  195  of substrate support member  190 . Shield  235  is disposed in chamber  305  to shield sidewalls  170  from sputtered material. Two power supplies are used in chamber  305 . DC power source  250  delivers DC power to target  185  to cause the processing gas to form a plasma. Spinning magnets  252  disposed behind target  185  form magnetic field lines at the target surface which trap electrons and increase the density of the plasma adjacent to the target in order to increase the sputtering efficiency. RF power source  260 A, biases substrate support member  190  with respect to the plasma through matching network  260 B and provides directional attraction of the ionized sputtered material toward substrate  200 . Optionally, no bias is applied to substrate support member  190  and FR power source  260 A and matching network are not used or not included as part of chamber  305 . Two plasma gases are supplied to chamber  305  through a gas inlet  265  from gas sources  270 ,  275  as metered by respective mass flow controllers  280  and  285 . In the present example, the first gas is nitrogen and the second gas is an inert gas such as argon, helium, neon or krypton or a combination thereof. One or more vacuum pumps  290  are connected to the chamber  305  at exhaust port  295  to exhaust the chamber and maintain the desired pressure in the chamber. In one example vacuum pump  290  is a cryopump or any pump capable of sustaining a low pressure of about 10 −8  Torr. Controller  300  controls the functions of power supplies  250  and  260 A, matching network  260 B, lift motors  205  and  220 , mass flow controllers  280  and  285 , vacuum pump  290  and other associated chamber components and functions. Controller  300  executes system control software stored in a memory, which in the in one example is a hard disk drive, and can include analog and digital input/output boards, interface boards and stepper motor controller boards. Optical and/or magnetic sensors are generally used to move and determine the position of movable mechanical assemblies. 
     In operation, a robot delivers a substrate  200  to chamber  305  through opening  210 . Pins  225  are extended upward to lift substrate  200  from the robot. The robot then retracts from chamber  305  and opening  210  is sealed. Pins  225  lower substrate  200  to upper surface  195  of substrate support member  190 . Substrate support member  190  raises substrate  200  into processing position under target  185 . One or more plasma gases are then introduced into chamber  305  to stabilize the chamber at process pressure. A plasma is generated between target  185  and substrate support member  190  with power from DC power source  250  and RF power source  260 A to create a plasma sufficiently dense to ionize the flux of sputtered target material from the target  185 . The ions are accelerated toward substrate. Additionally, non-ionized metal species dislodged from target  185  will deposit on the substrate. After deposition, substrate support member  190  is lowered, pins  225  are raised to lift substrate  200 , the robot enters chamber  305  to retrieve substrate  200 , and if desired, delivers another substrate for processing. 
     FIG. 4 is a flowchart illustrating a first method for depositing an ultra-thin tantalum nitride layer according to the present invention. The first method may be used with either the first tool illustrated in FIG. 2 or the second tool illustrated in FIG.  3  and described above. In step  310 , a substrate  200  is loaded into the deposition chamber and the chamber is pumped down to about 10 −8  Torr. In step  315 , an inert gas such as argon, helium, neon, krypton or a combination thereof is allowed to flow into the deposition chamber at a rate of about 5 to 200 sccm. The following steps,  320  and  325  occur simultaneously. In step  320 , DC and RF power is turned on initiating an inert gas plasma and tantalum starts to sputter from target  185 . With chamber  165 , about 500 to 3000 watts applied is to target  185  by DC power supply  250 , about 500 to 5000 watts is applied to coil  230  by RF power supply  255 A and about 10 to 500 watts is applied to substrate support member  190  by RF power supply  260 A. With chamber  305  about 500 to 3000 watts is applied to target  185  by DC power supply  250  and about 10 to 500 watts is applied to substrate support member  190  by RF power supply  260 A. With either chamber  165  or  305 , substrate  200  is maintained at a temperature of about 0° C. to 200° C. and the tantalum-sputtering rate is controlled to between about 2 to 50 Å per second for all subsequent process steps. In step  325 , nitrogen is allowed to flow as soon as tantalum sputtering begins. Nitrogen is allowed to flow at the rate of about 5 to 200 sccm for about 1 to 15 seconds while tantalum sputtering continues. During step  325 , two processes take place. In the first process, tantalum atoms react with nitrogen ions in the plasma to form tantalum nitride, which deposits on substrate  200 . In the second process, a tantalum nitride layer is formed on target  185 , which is then sputtered off to deposit on substrate  200 . In step  330 , nitrogen flow is turned off and tantalum continues to be sputtered. Tantalum sputtering is allowed to continue until a pre-determined thickness of tantalum is deposited on top of the tantalum layer deposited in step  325 . In step  335 , the RF and DC power is turned off, the plasma collapses, tantalum sputtering and deposition stop. Finally, in step  340 , the inert gas flow is turned off and substrate  200  is removed from the deposition chamber. 
     FIG. 5 is a flowchart illustrating a second method for depositing an ultra-thin tantalum nitride layer according to the present invention. The second method may be used with either the first tool illustrated in FIG. 2 or the second tool illustrated in FIG.  3  and described above. In step  345 , a substrate  200  is loaded into the deposition chamber and the chamber is pumped down to about 10 −8  Torr. In step  350 , nitrogen is turned on and allowed to flow at a rate of about 50 to 200 sccm. After a predetermined amount of time, in step  355 , nitrogen flow is stopped. Nitrogen is allowed to flow long enough to be absorbed on the surface of target  185  or to react with the tantalum of the target to form a thin tantalum nitride layer on the surface of the target. Additionally, nitrogen may be absorbed by coil  230  and sputtered from the coil as well. Step  350  effectively precharges the surface of target  185  with nitrogen. In step  360 , an inert gas such as argon, helium, neon, krypton or a combination thereof is allowed to flow into the deposition chamber at a rate of about 5 to 200 sccm. In step  365 , DC and RF power is turned on initiating an inert gas plasma and tantalum nitride starts to sputter from target  185 . It is important that the plasma be struck between about 0 to 2 seconds of turning on of the inert gas flow in order that the nitrogen absorbed on the surface of target  185  is not flushed off. During step  365 , as tantalum and nitrogen are sputtered off target  185 , tantalum nitride is formed which deposits on substrate  200 . With chamber  165 , about 500 to 3000 watts applied is to target  185  by DC power supply  250 , about 500 to 5000 watts is applied to coil  230  by RF power supply  255 A and about 10 to 500 watts is applied to substrate support member  190  by RF power supply  260 A. With chamber  305  about 500 to 3000 watts is applied to target  185  by DC power supply  250  and about 10 to 500 watts is applied to substrate support member  190  by RF power supply  260 A. With either chamber  165  or  305 , substrate  200  is maintained at a temperature of about 0° C. to 200° C. In step  370 , the absorbed nitrogen is used up and tantalum sputtering begins. Tantalum sputtering is allowed to continue until a pre-determined thickness of tantalum is deposited on top of the tantalum layer deposited in step  360 . In step  375 , the RF and DC power is turned off, the plasma collapses, tantalum sputtering and deposition stop. Finally, in step  380 , the inert gas flow is turned off and substrate  200  is removed from the deposition chamber. 
     A requirement of an ultra-thin tantalum nitride film is that it be continuous to avoid formation of beta-phase tantalum when tantalum is used a redundant conductor. As was noted above, beta-phase tantalum has a much higher resistivity than alpha phase tantalum. If the tantalum nitride layer is not continuous in a via formed in silicon oxide, beta-phase tantalum will be formed when a tantalum liner is formed. The presence of beta-phase tantalum can be determined by a simple resistivity measurement and forms the basis for determining if the ultra-thin tantalum layer of the present invention is continuous. 
     EXAMPLE 
     Two pairs of copper via chain structures were fabricated. One via chain in the pair comprised 68,000 vias in series and the second via chain comprised 100 vias in series. Each via in each chain was one micron deep by 0.4 micron in diameter and formed in a silicon oxide dielectric. The first pair was fabricated using a thick tantalum nitride liner having a thickness of 10 nanometers and a tantalum liner having a thickness of 40 nanometers. The second pair was fabricated using an ultra-thin tantalum nitride according to the present invention, the liner having a thickness of 1 to 3 nanometers and a tantalum liner having a thickness of 40 nanometers. Forty chains on each of forty locations on four wafers were measured. Table I lists the measured resistivity in ohms per link and standard deviation of each via chain for both the thick and the ultra-thin tantalum nitride lined via chains. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                                                    TABLE I 
               
               
                   
                   
               
               
                   
                                                                Thick 
                 Ultra-Thin 
               
               
                   
                 TaN 
                 TaN 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 First via chain resistivity 
                 0.618 
                 0.594 
               
               
                   
                 Standard deviation 
                 0.014 
                 0.001 
               
               
                   
                 Second via chain resistivity 
                 0.858 
                 0.792 
               
               
                   
                 Standard deviation 
                 0.013 
                 0.001 
               
               
                   
                   
               
             
          
         
       
     
     Since the resistivity of the ultra-thin tantalum nitride cells are virtually the same as the resistivity of the thick tantalum nitride cells, no beta phase tantalum was formed and it can be concluded that the ultra-thin tantalum nitride layer was continuous. 
     The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.