Abstract:
A system and method to form a stacked barrier layer for copper contacts formed on a substrate. The substrate is serially exposed to first and second reactive gases to form an adhesion layer. Then, the adhesion layer is serially exposed to third and fourth reactive gases to form a barrier layer adjacent to the adhesion layer. This is followed by deposition of a copper layer adjacent to the barrier layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the processing of semiconductor substrates. More particularly, this invention relates to improvements in the process of forming contacts.  
           [0002]    Formation of contacts in multi-level integrated circuits poses many challenges to the semiconductor industry as the drive to increase circuit density continues, due to the reduction in size of the circuit features. Contacts are formed by depositing conductive interconnect material in an opening on the surface of insulating material disposed between two spaced-apart conductive layers. The aspect ratio of such an opening inhibits deposition of conductive interconnect material that demonstrates satisfactory step coverage and gap-fill, employing traditional interconnect material such as aluminum. In addition, diffusion between the aluminum and the surrounding insulating material often occurs, which adversely effects operation of the resulting electrical circuits.  
           [0003]    Barrier materials have been introduced to improve both the step coverage and gap-fill of aluminum, while limiting diffusion of the same. Barrier materials must also provide good adhesion properties for aluminum. Otherwise, the thermal and electrical conductance of the resulting contact may be compromised. Examples of barrier materials providing the aforementioned characteristics include TiN, TiW, TiB 2 , TiC and Ti 2 N.  
           [0004]    However, attempts have been made to provide interconnect material with lower electrical resistivity than aluminum. This has led to the substitution of copper aluminum. Copper, like aluminum, also suffers from diffusion characteristics and may form undesirable intermetallic alloys that reduce the availability of suitable barrier materials.  
           [0005]    Tungsten has proved to be a suitable barrier material that effectively prevents diffusion of copper. Typically deposited employing chemical vapor deposition (CVD) techniques, tungsten deposition is attendant with several disadvantages. Tungsten diffuses easily into surrounding dielectric material. In addition, tungsten has proven difficult to deposit uniformly. This has been shown by variance in tungsten layers&#39; thickness of greater than 1%. As result, it is difficult to control the resistivity of a tungsten layer.  
           [0006]    What is needed, therefore, are improved techniques to form barrier layers for copper interconnects that include tungsten.  
         SUMMARY OF THE INVENTION  
         [0007]    One embodiment of the present invention is directed to a method to form a stacked barrier layer on a substrate disposed in a processing chamber by serially exposing the substrate to first and second reactive gases to form an adhesion layer. The adhesion layer is then serially exposed to third and fourth reactive gases to form a barrier layer adjacent to the adhesion layer. A copper layer is disposed adjacent to the barrier layer. To that end, another embodiment of the invention is directed to a system to carry out the method. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a perspective view of a semiconductor processing system in accordance with the present invention;  
         [0009]    [0009]FIG. 2 is a detailed view of the processing chambers shown above in FIG. 1;  
         [0010]    [0010]FIG. 3 is a detailed cross-sectional view of a substrate shown above in FIG. 2 before deposition of a first refractory metal layer in accordance with one embodiment of the present invention;  
         [0011]    [0011]FIG. 4 is a detailed cross-sectional view of the substrate shown above in FIG. 3 after deposition of a first refractory metal layer in accordance with one embodiment of the present invention;  
         [0012]    [0012]FIG. 5 is a detailed cross-sectional view of a substrate shown above in FIG. 4 after deposition of a second refractory metal layer in accordance with one embodiment of the present invention;  
         [0013]    [0013]FIG. 6 is a detailed cross-sectional view of a substrate shown above in FIG. 2 after deposition of a copper contact in accordance with one embodiment of the present invention;  
         [0014]    [0014]FIG. 7 is a schematic view showing deposition of a first molecule onto a substrate during sequential deposition techniques in accordance with one embodiment of the present invention;  
         [0015]    [0015]FIG. 8 is a schematic view showing deposition of second molecule onto a substrate during sequential deposition techniques in accordance with one embodiment of the present invention;  
         [0016]    [0016]FIG. 9 is a graphical representation showing the concentration of gases introduced into the processing chamber shown above in FIG. 2, and the time in which the gases are present in the processing chamber to deposit the Titanium refractory metal layer shown above in FIG. 4, in accordance with one embodiment of the present invention; and  
         [0017]    [0017]FIG. 10 is a graphical representation showing the concentration of gases introduced into the processing chamber shown above in FIG. 2, and the time in which the gases are present in the processing chamber to deposit the Tungsten layer shown above in FIG. 4, in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 1, an exemplary wafer processing system includes one or more processing chambers  12 ,  13  and  14  disposed in a common work area  16  surrounded by a wall  18 . Processing chambers  12  and  14  are in data communication with a controller  22  that is connected to one or more monitors, shown as  24  and  26 . Monitors  24  and  26  typically display common information concerning the process associated with the processing chambers  12  and  14 . Monitor  26  is mounted to the wall  18 , with monitor  24  being disposed in the work area  16 . Operational control of processing chambers  12  and  14  may be achieved with use of a light pen, associated with one of monitors  24  and  26 , to communicate with controller  22 . For example, a light pen  28   a  is associated with monitor  24  and facilitates communication with the controller  22  through monitor  24 . A light pen  28   b  facilitates communication with controller  22  through monitor  26 .  
         [0019]    Referring both the to FIGS. 1 and 2, each of processing chambers  12  and  14  includes a housing  30  having a base wall  32 , a cover  34 , disposed opposite to the base wall  32 , and a sidewall  36 , extending therebetween. Housing  30  defines a chamber  37 , and a pedestal  38  is disposed within processing chamber  37  to support a substrate  42 , such as a semiconductor wafer. Pedestal  38  may be mounted to move between the cover  34  and base wall  32 , using a displacement mechanism (not shown), but is typically fixed proximate to bottom wall  32 . Supplies of processing gases  39   a,    39   b,    39   c,    39   d  and  39   e  are in fluid communication with the processing chamber  37  via a showerhead  40 . Regulation of the flow of gases from supplies  39   a,    39   b  and  39   c  is effectuated via flow valves  41 .  
         [0020]    Depending on the specific process, substrate  42  may be heated to a desired temperature prior to layer deposition via a heater embedded within pedestal  38 . For example, pedestal  38  may be resistively heated by applying an electric current from an AC power supply  43  to a heater element  44 . Substrate  42  is, in turn, heated by pedestal  38 , and can be maintained within a desired process temperature range of, for example, about 20° C. to about 750° C., with the actual temperature varying dependent upon the gases employed and the topography of the surface upon which deposition is to occur. A temperature sensor  46 , such as a thermocouple, is also embedded in the wafer support pedestal  38  to monitor the temperature of the pedestal  38  in a conventional manner. For example, the measured temperature may be used in a feedback loop to control the electrical current applied to heater element  44  by the power supply  43 , such that the wafer temperature can be maintained or controlled at a desired temperature the is suitable for the particular process application. Pedestal  38  is optionally heated using radiant heat (not shown). A vacuum pump  48  is used to evacuate processing chamber  37  and to help maintain the proper gas flows and pressure inside processing chamber  37 .  
         [0021]    Referring to FIGS. 1 and 3, one or both of processing chambers  12  and  14 , discussed above may operate to form, on substrate  42 , a contact in accordance with the present invention on substrate  42 . To that end, substrate  42  includes a wafer  50  that may be formed from any material suitable for semiconductor processing, such as silicon. One or more layers, shown as layer  52 , may be present on wafer  50 . Layer  52  may be formed from any suitable material, including dielectric or conductive materials. Layer  52  includes a void  54 , exposing a region  56  of substrate  42 .  
         [0022]    Referring to FIG. 4, formed adjacent to layer  52  and region  54  is a layer containing a refractory metal compound, such as titanium. In the present example, layer  58  is formed from titanium nitride, TiN, by sequentially exposing substrate  42  to processing gases to chemisorb monolayers of differing compounds onto the substrate, discussed more fully below. Layer  58  conforms to the profile of the void  54  so as to cover region  56  and layer  52 .  
         [0023]    Referring to FIG. 5, adjacent to layer  58  is formed an additional refractory metal layer  60 . In the present example, layer  60  is formed from tungsten in the manner discussed above with respect to layer  52 , except using different process gases. Layer  60  conforms to the profile of layer  58  and, therefore, conforms to the profile of void  54 .  
         [0024]    Referring to FIG. 6, shown is one example of a contact  62  formed in void  54  in accordance with the present invention by deposition of a layer of copper  64  that fills void  54 , using standard deposition techniques. With this configuration, a stacked barrier layer consisting of TiN layer  58  and W layer  60  surrounds contact  62 . TiN layer  58  serves as an adhesion layer to facilitate nucleation and deposition by W layer  60 . TiN layer also serves as a diffusion barrier to reduce, if not prevent, diffusion of W into the surrounding environs, such as region  56  and layer  52 . W layer  60  serves as a barrier layer for contact  62 , thereby preventing copper material from diffusing into or through TiN layer  58  and into the environs surrounding void  54 . Employing sequential deposition techniques, such as atomic layer deposition, provides superior thermal and conductive characteristics of the aforementioned stacked barrier layer. Specifically, the sequential deposition techniques described below enable precise control over the thickness of both layers  58  and  60 .  
         [0025]    Referring to FIGS. 1, 6 and  7 , one or both of processing chambers  12  and  14 , discussed above, may operate to deposit layers  58  and  60  on substrate  42  employing sequential deposition techniques. Specifically, the initial surface of substrate  42 , e.g., the surface of region  56  and the surface of layer  52 , presents an active ligand to the process region. A batch of a first processing gas, in this case Aa x , results in a layer of A being deposited on substrate  42  having a surface of ligand x exposed to the processing chamber  37 . Thereafter, a purge gas enters processing chamber  37  to purge the gas Aa x . After purging gas Aa x  from processing chamber  37 , a second batch of processing gas, Bb y , is introduced into processing chamber  37 . The a ligand present on the substrate surface reacts with the b ligand and B atom, releasing molecules ab and Ba, that move away from substrate  42  and are subsequently pumped from processing chamber  37 . In this manner, a surface comprising a monolayer of A atoms remains upon substrate  42  and exposed to processing chamber  37 , shown in FIG. 4. The process proceeds cycle after cycle, until the desired thickness is achieved.  
         [0026]    Referring to both FIGS. 2 and 8, although any type of processing gas may be employed, in the present example, the processing gas Aa x  is a titanium-containing gas selected from the group that includes TDMAT, TDEAT and TiCl 4 . The processing gas Bb y  functions as a reducing agent and is selected from the group including H 2 , B 2 H 6 , SiH 4  and NH 3 . Two purge gases were employed: Ar and N 2 . Each of the processing gases is flowed into processing chamber  37  with a carrier gas, which in this example, is one of the purge gases. It should be understood, however, that the purge gas may differ from the carrier gas, discussed more fully below. One cycle of the sequential deposition technique in accordance with the present invention includes flowing a purge gas into processing chamber  37  during time t 1  before the titanium-containing gas is flowed into processing chamber  37 . During time t 2 , the titanium-containing processing gas is flowed into the processing chamber  37 , along with a carrier gas. After t 2  has lapsed, the flow of titanium-containing gas terminates and the flow of the carrier gas continues during time t 3 , purging the processing chamber of the titanium-containing processing gas. During time t 4 , the processing chamber  37  is pumped so as to remove all gases. After pumping of process chamber  37 , a carrier gas is introduced during time t 5 , after which time the reducing process gas is introduced into the processing chamber  37  along with the carrier gas, during time t 6 . The flow of the reducing process gas into processing chamber  37  is subsequently terminated. After the flow of reducing process gas into processing chamber  37  terminates, the flow of carrier gas continues, during time t 7 . Thereafter, processing chamber  37  is pumped so as to remove all gases therein, during time t 8 , thereby concluding one cycle of the sequential deposition technique in accordance with the present invention. The aforementioned cycle is repeated multiple times until layer  58  reaches a desired thickness. After TiN layer  58  reaches a desired thickness, W layer  60  is deposited adjacent thereto employing sequential deposition techniques.  
         [0027]    Referring to FIG. 2 and  10  to form W layer  60 , processing gas Aa x  may be any known tungsten-containing gas, such a tungsten hexafluoride, WF 6 . The processing gas Bb y  functions as a reducing agent and is selected from the group including SiH 4 , B 2 H 6  and NH 3 . The same purge gases may be employed, as discussed above. Each of the processing gases is flowed into the processing chamber  37  with a carrier gas, as discussed above. One cycle of the sequential deposition technique to form W layer  60  in accordance with the present invention includes flowing a purge gas into the processing chamber  37  during time t 9 , before the tungsten-containing gas is flowed into the processing chamber  37 . During time t 10 , the tungsten-containing processing gas is flowed into the processing chamber  37 , along with a carrier gas. After time t 10  has lapsed, the flow of tungsten-containing gas terminates and the flow of the carrier gas continues during time t 11 , purging the processing chamber of the tungsten-containing processing gas. During time t 12 , processing chamber  37  is pumped so as to remove all gases. After pumping of the process chamber  37 , a carrier gas is introduced during time t 13 , after which time the reducing process gas is introduced into the processing chamber  37  along with the carrier gas, during time t 14 . The flow of the reducing process gas into processing chamber  37  is subsequently terminated. After the flow of reducing process gas into the processing chamber  37  terminates, the flow of carrier continues during time t 15 . Thereafter, the processing chamber  37  is pumped so as to remove all gases therein, during time t 16 , thereby concluding one cycle of the sequential deposition technique in accordance with the present invention. The aforementioned cycle is repeated multiple times until layer  60  reaches a desired thickness. After W layer  60  reaches a desired thickness, the contact  62 , shown in FIG. 6 may be deposited employing known techniques.  
         [0028]    The benefits of employing sequential deposition are manifold, including flux-independence of layer formation that provides uniformity of deposition independent of the size of a substrate. For example, the measured difference of the layer uniformity and thickness measured between of 200 mm substrate and a 32 mm substrate deposited in the same chamber is negligible. This is due to the self-limiting characteristics of chemisorption. Further, the chemisorption characteristics contribute to near-perfect step coverage over complex topography.  
         [0029]    In addition, the thickness of the layers  58  and  60  may be easily controlled while minimizing the resistance of the same by employing sequential deposition techniques. In one example of the present invention, layers  58  and  60 , as well as contact  62  may be deposited in a common processing chamber, for example chambers  12  and  14 . To provide added flexibility when depositing layers  58  and  60 , as well as contact  62 , a bifurcated deposition process may be practiced in which layer  58  is deposited in one process chamber, for example chamber  12 , and layer  60  is deposited in a separate chamber, for example chamber  14 . This may reduce the deposition time of each of layers  58  and  60  by, inter alia, having each processing chamber  12  and  14  preset to carry-out the process parameters necessary to deposit the requisite refractory metal layers.  
         [0030]    Referring again to FIG. 2, the process for depositing the tungsten layer may be controlled using a computer program product that is executed by the controller  22 . To that end, the controller  22  includes a central processing unit (CPU)  70 , a volatile memory, such as a random access memory (RAM)  72  and permanent storage media, such as a floppy disk drive for use with a floppy diskette, or hard disk drive  74 . The computer program code can be written in any conventional computer readable programming language; for example, 68000 assembly language, C, C++, Pascal, Fortran, and the like. Suitable program code is entered into a single file, or multiple files, using a conventional text editor and stored or embodied in a computer-readable medium, such as the hard disk drive  74 . If the entered code text is in a high level language, the code is compiled and the resultant compiler code is then linked with an object code of precompiled Windows® library routines. To execute the linked and compiled object code the system user invokes the object code, causing the CPU  70  to load the code in RAM  72 . The CPU  70  then reads and executes the code to perform the tasks identified in the program.  
         [0031]    Although the invention has been described in terms of specific embodiments, one skilled in the art will recognize that various changes to the reaction conditions, i.e., temperature, pressure, film thickness and the like can be substituted and are meant to be included herein and sequence of gases being deposited. For example, sequential deposition process may have different initial sequence. The initial sequence may include exposing the substrate the reducing gas before the metal-containing gas is introduced into the processing chamber. In addition, other stacked layers may be deposited, in addition to the refractory-metal layers described above and for purposes other than formation of a barrier layer. Tungsten and other deposition techniques may be employed in lieu of CVD. For example, physical vapor deposition (PVD) techniques, or a combination of both CVD and PVD techniques, may be employed. Therefore, the scope of the invention should not be based upon the foregoing description. Rather, the scope of the invention should be determined based upon the claims recited herein, including the full scope of equivalents thereof.