Abstract:
Novel processes for the in-situ nitridation of metal layers particularly for the subsequent formation of metal salicides. In one embodiment, the nitridation process comprises connecting a remote plasma generator to a rapid thermal processing (RTP) chamber and introducing nitrogen plasma into the chamber as the metal layer is converted into a nitridated metal salicide layer in gate regions on a substrate. In a second embodiment, a remote plasma generator is connected to a physical vapor deposition (PVD) chamber and nitrogen plasma is introduced into the chamber during metal sputter formation of the metal layer. In a third embodiment, the metal layer is first deposited on the silicon or polysilicon and then nitrided using a decoupled plasma nitridation (DPN) process. The metal salicide is formed by subjecting the nitridated metal salicide to a thermal anneal process.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to salicides formed on semiconductor substrates and more particularly, to a process for in-situ nitridation of metal salicides to improve thermal stability and inhibit agglomeration of the salicides.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the fabrication of semiconductors, advanced lithography and etching processes have facilitated synthesis of integrated circuit devices with ever-decreasing dimensions and increasing integration densities. These scaled-down integrated circuits have higher processing speeds than their larger predecessors. However, this reduction in dimensions has caused a corresponding decrease in the cross-sectional area of the interconnect regions of the circuits, thus leading to an increase in sheet resistance and interconnection time delay. Approaches made in IC manufacturing to decrease the interconnection time delay includes formation of a metal silicide layer on the top of a doped polycrystalline silicon, or polysilicon, in order to lower the sheet resistance of the polysilicon interconnections and thus, facilitate increased circuit speed. A refractory metal silicide that has been reacted with the polysilicon is known as a polycide.  
           [0003]    A polycide process is carried out by initially depositing an amorphous silicide conductor, such as nickel or cobalt, on unpatterned doped polysilicon on the wafer substrate. An insulating layer is then deposited on the polycide, and the wafer is patterned and heated to form a crystalline polycide having low resistivity. After insulating sidewall spacers are deposited in the gate region, the source and drain regions are silicided.  
           [0004]    [0004]FIG. 1 schematically illustrates a polysilicon gate  20  formed between a source  16  and a drain  18  of a device  30  on a semiconductor wafer substrate  10 . A shallow trench  12  filled with oxide  14  separates devices from each other on the wafer substrate  10 . A polysilicon silicide, or polycide  22 , typically composed of nickel or cobalt, is deposited on the polysilicon gate  20 , and an insulating layer  28  is deposited on the polycide  22 . A source silicide  24  is deposited on the source  16 , and a drain silicide  26  is deposited on the drain  18 .  
           [0005]    As the device features on a wafer decrease in size, the junction between the source and drain regions on the wafer decreases as well, and this requires that a self-aligned silicide, or “salicide”, be used to reduce both the source/drain resistance and the gate resistance. In a salicide process, a metal is deposited over and reacts with the exposed silicon in the source and drain regions and the polysilicon in the gate region to form a silicide. The unreacted metal is removed by etching, which leaves the silicides on the respective source and drain regions and the polycide on the polysilicon gate. Since a masking step is not required for etching the unreacted metal from the reacted metal portions, the silicide process is termed, “self-aligned”.  
           [0006]    While titanium has been frequently used in the past to form titanium salicide (TiSi 2 ) in gate regions on substrates, titanium salicide manifests problems as the source/drain junction decreases to widths of less than 2000 angstroms. Because the silicide thickness may be only several hundred angstroms in an ultra-shallow junction, the etch selectivity of TiSi 2  to borophosphosilicate glass (BPSG) may not be high enough for the TiSi 2  source/drain to withstand the contact etch. Moreover, titanium atoms form compounds with boron (B), and this renders PMOS contact resistance very high. Cobalt silicide (CoSi 2 ) has been found to be a promising metal for forming ultra-shallow junctions in salicide processes, since CoSi 2  has exhibited excellent etch selectivity to BPSG and since cobalt atoms do not form tightly bonded compounds with arsenic (As) and boron (B) atoms.  
           [0007]    One of the problems encountered in the formation of silicide gates is agglomeration of the matal silicide during high-temperature annealing at temperatures of greater than approximately 800 degrees C. Agglomeration results when silicon within and under the metal silicide diffuses and coalesces to form large silicon grains which break the continuity of the silicide film. Consequently, a narrow gate constructed with an agglomerated silicide tends to manifest a significant increase in average sheet resistance. In this regard, localized breaks in the film can impart very high resistance if the silicide is completely severed across the width of the line. As such, in high speed circuit applications which require low-resistance silicide conductors, agglomeration can result in performance degradation or total functional failure.  
           [0008]    It has been found that doping nitrogen atoms into a polycide can improve silicide thermal stability and reduce S/D junction leakage during subsequent thermal processing of wafers. Current approaches include incorporating the nitrogen into the silicon substrate and polysilicon gate before or after deposition of the metal silicide to retard silicide agglomeration during subsequent RTA (rapid thermal anneal) processes. However, these approaches have been shown to adversely affect device performance and gate oxide integrity (GOI).  
           [0009]    U.S. Pat. No. 5,518,958, dated May 21, 1996, to Giewont, et al., describes a process by which conductors are fabricated by forming a layer of doped polysilicon on a semiconductor substrate, forming a nitrogen-enriched conductive layer on the layer of doped polysilicon, wherein nitrogen contained in the nitrogen-enriched conductive layer provides for improved thermal stability thereof, and patterning the nitrogen-enriched conductive layer and layer of doped polysilicon so as to form the conductors.  
           [0010]    U.S. Pat. No. 5,536,684, dated Jul. 16, 1996, to Dass, et al., describes a process wherein a refractory metal layer is deposited on a silicon substrate. On top of the refractory metal layer is deposited a group VIII metal layer. Then a first anneal is performed on the silicon substrate in an ambient comprising a nitrogen containing gas. During the first anneal a group VIII metal silicide layer is formed above the silicon substrate and a refractory metal nitride layer is formed above the group VIII metal silicide layer. After the first anneal is completed, the portion of the group VIII metal silicide layer is transformed into an amorphous group VIII metal silicon mixture. Finally, a second anneal is performed on the silicon substrate in a second ambient. During the second anneal an epitaxial group VIII metal silicide layer is formed.  
           [0011]    It has been found that nitridation of salicides during formation of salicide layers on polysilicon or silicon films, rather than nitridation of the polysilicon or silicon followed by salicide formation or nitridation following formation of the salicide, provides a salicide which is both thermally stable with less sheet resistance and does not adversely affect device performance or gate oxide integrity (GOI). The process is preferably performed using a remote plasma generator connected directly to a rapid thermal processing chamber or physical vapor deposition chamber, since this facilitates precise control over incorporation of nitrogen into the forming metal salicide layer.  
           [0012]    Accordingly, an object of the present invention is to provide a new and improved process for improving thermal stability and inhibiting agglomeration of salicides.  
           [0013]    Another object of the present invention is to provide a process for preventing excessive metal oxide formation on a salicide.  
           [0014]    Still another object of the present invention is to provide a process for the nitridation of salicides without sacrificing device performance or gate oxide integrity.  
           [0015]    Another object of the present invention is to provide novel processes for the nitridation of salicides.  
           [0016]    Still another object of the present invention is to provide novel processes for incorporating nitrogen into a metal film during a physical vapor deposition (PVD) process in a PVD chamber.  
           [0017]    Yet another object of the present invention is to provide novel processes for incorporating nitrogen into metal layers using remote plasma nitridation (RPN).  
           [0018]    A still further object of the present invention is to provide novel processes for nitridation of a metal film using decoupled plasma nitridation (DPN).  
           [0019]    Yet another object of the present invention is to provide a novel process for incorporating nitrogen into a metal film by connecting a remote plasma generator to a plasma vapor deposition (PVD) chamber and introducing nitrogen plasma into the PVD chamber during sputter deposition of the metal in the chamber.  
           [0020]    A still further object of the present invention is to provide a novel process for incorporating nitrogen into a metal salicide film by connecting a remote plasma generator to a rapid thermal processing (RTP) chamber and introducing nitrogen gas into the RTP chamber during formation of the metal salicide in the chamber.  
           [0021]    Yet another object of the present invention is to provide a novel process for the nitridation of a metal film by forming a metal layer on a polysilicon gate and subjecting the metal to a decoupled plasma nitridation process.  
         SUMMARY OF THE INVENTION  
         [0022]    According to these and other objects and advantages, the present invention comprises novel processes for the in-situ nitridation of metal layers particularly for the subsequent formation of metal salicides. In one embodiment, the nitridation process comprises connecting a remote plasma generator to a rapid thermal processing (RTP) chamber and introducing nitrogen plasma into the chamber as the metal layer is converted into a nitridated metal salicide layer in gate regions on a substrate. In a second embodiment, a remote plasma generator is connected to a physical vapor deposition (PVD) chamber and nitrogen plasma is introduced into the chamber during metal sputter formation of the metal layer. In a third embodiment, the metal layer is first deposited on the silicon or polysilicon and then nitrided using a decoupled plasma nitridation (DPN) process. The metal salicide is formed by subjecting the nitridated metal salicide to a thermal anneal process.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
         [0024]    [0024]FIG. 1 is a schematic view illustrating a typical standard gate electrode structure or device on a substrate;  
         [0025]    [0025]FIG. 2A is a schematic view of a silicon wafer substrate suitable for implementation of the present invention;  
         [0026]    [0026]FIG. 2B is a schematic view illustrating deposition of a gate oxide layer on a silicon wafer substrate according to the process of the present invention;  
         [0027]    [0027]FIG. 2C is a schematic view illustrating deposition of a nitridated metal layer on a gate oxide layer on a silicon substrate according to the process of the present invention;  
         [0028]    [0028]FIG. 3 is a schematic view illustrating a remote plasma generator attached to a rapid thermal processing (RTP) chamber in implementation of the present invention;  
         [0029]    [0029]FIG. 4 is a schematic view illustrating a remote plasma generator attached to a physical vapor deposition (PVD) chamber in implementation of the present invention;  
         [0030]    [0030]FIG. 5 is a schematic view illustrating a dual plasma source (DPS) chamber in implementation of the present invention;  
         [0031]    [0031]FIG. 6 is a graph illustrating sheet resistance as a function of line-width, comparing the sheet resistance of nitrogen-devoid salicides with the sheet resistance of salicides nitridated according to the processes of the present invention; and  
         [0032]    [0032]FIG. 7 is a graph illustrating sheet resistance as a function of processing temperature, comparing the sheet resistance of nitrogen-devoid salicides with the sheet resistance of salicides nitridated according to the processes of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Referring initially to FIGS. 2A-2C, fabrication of a gate electrode structure or device including a nitridated salicide on a silicon wafer substrate  35  begins with formation of a gate oxide layer  37  of selected thickness, typically in the range of about 10-100 angstroms, on the substrate  35 . Next, a polysilicon layer  39  is formed on the gate oxide layer  37  and typically has a thickness of about 100-200 nm. The gate oxide layer  37  and polysilicon layer  39  may be formed using conventional CVD (chemical vapor deposition) techniques, after which the polysilicon layer  39  may be implanted with ions. The implanted polysilicon layer  39  is next annealed at a temperature of about 900 degrees C., for example, to distribute and activate the dopants therein.  
         [0034]    According to a process of the present invention, a nitrogen-enriched metal layer  41  is next deposited on the annealed polysilicon layer  39  as hereinafter further described. Preferably, the nitrogen-enriched metal layer  41  has about 0.5%-15% nitrogen by atomic composition, and the metal used in forming the salicide layer  41  is typically nickel or cobalt. In a first embodiment of the invention, the nitrogen-enriched metal layer  41  is formed by physical vapor deposition and then annealed in a rapid thermal processing (RTP) chamber  60  to convert the metal layer  41  into a metal salicide layer. This is accomplished by initially forming the metal layer  41  on the polysilicon layer  39 , using conventional physical vapor deposition process parameters for metal layer formation in an RTP chamber, and then annealing the metal layer  41  in the RTP chamber  60  while simultaneously introducing an argon-nitrogen plasma  78  into the RTP chamber  60  through a remote plasma generator  43 . The RTP chamber  60  may be conventional and typically includes a base  68  on which is removably mounted a heater head  62  containing multiple halogen lamps  64 . A wafer support  70  is provided inside the RTP chamber  60  and supports the wafer substrate  35  thereon. Optical pyrometers  66  or other temperature-sensing elements extend through the base  68  for measuring the backside temperature of the wafer  35 . The base  68  further includes a gas inlet arm  72  for connection with the remote plasma generator  43  and a gas outlet  74  for escape of process gases from the RTP chamber  60 .  
         [0035]    The remote plasma generator  43  may be conventional and typically includes an applicator  45 , having an inlet arm  47  connected to a nitrogen source  76 ; an outlet arm  49  connected to a gas inlet arm  72  of the RTP chamber  60 ; a magnetron  55 ; an isolator  57 ; and an auto-tuner  53 . The magnetron  55  houses a magnetron tube (not shown) which produces microwave energy when supplied with DC power from a DC power supply  56 . An antenna (not shown) channels the microwaves from the magnetron  55  to an isolator  57 , which absorbs and dissipates reflected power to prevent damage to the magnetron  62 . The auto-tuner  53  minimizes the power reflected to the magnetron  62 . The applicator  45  of the remote plasma generator  43  is typically water-cooled and is capable of operating continuously at maximum power, and the magnetron  55  of the remote plasma generator  43  generates high frequency (3 kW) microwaves.  
         [0036]    Formation of the nitrogen-enriched metal layer  41  is carried out by initially positioning the wafer substrate  35 , having had the polysilicon layer  39  (FIG. 2C) previously deposited thereon and the nitrogen-devoid metal film  41  deposited on the polysilicon layer  39  by conventional physical vapor deposition techniques, on the wafer chuck  70  in the RTP chamber  60 . Next, as the metal salicide layer is formed from the metal layer  41  in the RTP chamber  60 , nitrogen gas  80  is distributed from the nitrogen source  76  and into the applicator  45  of the remote plasma generator  43 , which is programmed and operated according to the knowledge of those skilled in the art to generate a nitrogen plasma  78  in the applicator  45 . The nitrogen plasma  78  enters the RTP chamber  60 , and nitrogen atoms from the nitrogen plasma  78  are embedded in the metal silicide layer  41  as the metal salicide layer  41  is formed on the polysilicon layer  39 . The volume of nitrogen gas  80  used to form the nitrogen plasma  78  is selected such that the total nitrogen atom composition in the nitridated metal salicide layer  41  ranges from about 0.5% to about 15% by atomic composition. After formation of the nitrided metal salicide layer  41 , conventional process steps may follow to complete the device on the wafer substrate  35 .  
         [0037]    Referring next to FIG. 4, in a second embodiment of the invention, the nitrogen-enriched metal layer  41  (FIG. 2C) is formed on the polysilicon layer  39  using a metal sputtering process in a PVD chamber  82 , in conjunction with a remote plasma generator  43  connected to the PVD chamber  82 . The PVD chamber  82  may be conventional, and the chamber interior  84  thereof typically contains a cathode  86 , an anode  88  and a metal silicate target  90 . The wafer substrate  35  is supported on the anode  88 . The base chamber of the PVD chamber  82  is typically an Endura PVD chamber.  
         [0038]    The remote plasma generator  43  may be conventional and typically includes an applicator  45 , having an inlet arm  47  connected to a nitrogen source  76 ; an outlet arm  49  connected to a gas inlet (not illustrated) in the side of the PVD chamber  82 ; a magnetron  55 ; an isolator  57 ; and an auto-tuner  53 . The applicator  45  of the remote plasma generator  43  is typically water-cooled and is capable of operating continuously at maximum power, and the magnetron  55  of the remote plasma generator  43  generates high frequency (3 kW) microwaves.  
         [0039]    Argon plasma can be used as the sputter process plasma, and nitrogen gas  80  may be introduced from the nitrogen source  76  into the inlet arm  47  of the applicator  45  through a calibrated mass flow controller (not illustrated). Typical sputtering conditions may include 2220 Watts of DC power at a sputtering plasma pressure of approximately 6 milliTorr and a wafer temperature in the range of about 20 to 25 degrees C., and preferably, about 20 degrees C. Argon gas mixes with nitrogen gas  80  entering the applicator  45  from the nitrogen source  76 , and the microwaves generated by the magnetron  55  create an argon-nitrogen sputter process plasma  94  in the applicator  45 . A sufficient quantity of the nitrogen gas  80  is mixed with the argon gas in the applicator  45  to form a sputter process plasma  94  sufficient to incorporate between approximately 0.5% and 15%, and preferably, about 0.5% and 10%, of nitrogen by atomic composition in the metal layer  41 . After the sputter process plasma  94  exits the outlet arm  49  of the applicator  45  and enters the chamber interior  84 , the sputter deposition process then proceeds with bombardment of the metal silicate target  90 , with ions from the nitrogen-enriched sputter process plasma  94  displacing molecules from the metal target  90  to deposit the nitrogen-enriched metal layer  41  on the polysilicon layer  39  of the wafer substrate  35 . The wafer substrate  35  is typically rotated in the chamber interior  34  throughout the process. After formation of the nitrided metal layer  41 , the nitridated metal layer  41  may be annealed in a rapid thermal processing chamber, typically according to conventional process parameters, to convert the metal layer  41  into a metal salicide layer. Conventional process steps may follow to complete the device on the wafer substrate  35 .  
         [0040]    Referring next to FIG. 5 of the drawings, in a third embodiment the metal layer  41  is nitridated using a DPS (dual plasma source) chamber  1 , which may be conventional and typically includes a quasi-remote plasma source  2  located above a chamber interior  4 , which is typically a silicon etch DPS (dual plasma source) chamber. Plasma injection openings  99  facilitate  4 -point symmetric plasma flow into the chamber interior  4 . A cathode  5  is provided in the chamber interior  4  and supports the wafer substrate  35  for nitridation of a nitrogen-devoid metal layer  42  previously deposited on the polysilicon layer  39  typically using a standard PVD process. After processing, as hereinafter described, the nitrogen plasma is evacuated from the chamber interior  4  through a throttle valve  96  and gate valve  97  by operation of a turbo pump  98 . An RF source power  3  is connected to an RF match  7  and generates RF energy in the quasi-remote plasma source  2  through inductive coils  6 . An RF bias power  9  is connected to a second RF match  8  for applying a voltage bias to the wafer substrate  35 , as needed.  
         [0041]    In application, the initially nitrogen-devoid metal layer  41  is first formed on the polysilicon layer  39  on the wafer substrate  35  using conventional PVD techniques, tyically using nickel or cobalt as the metal, before the wafer substrate  35  is positioned on the cathode  5  in the chamber interior  4 . Typical process conditions include a wafer substrate temperature of less than about 100 degrees C.; source RF power  3  set at 12.56 MHz and 0-2000 Watts; and bias power  9  set at 13.56 MHz and 0-500 Watts. A nitrogen plasma is next generated inside the plasma source  2 , and the plasma flows through the plasma injection openings  99  and into the chamber interior  4 , where the neutral nitrogen atoms strike and are embedded in the initially nitrogen-devoid nickel or cobalt metal layer  41  to convert the nitrogen-devoid metal layer  41  to the nitridated metal layer  41  having from about 0.5% to about 15% nitrogen by atomic composition. Because the source plasma power  3  is decoupled from the bias power  9 , decoupled plasma nitridation of the metal layer  41  according to the process of the present invention permits enhanced control over ion density and ion energy of the nitrogen plasma, resulting in improved control over incorporation of nitrogen into the metal layer. After formation of the nitrided metal layer  41 , the nitridated metal layer  41  is annealed in a rapid thermal processing chamber, typically according to conventional process parameters, to convert the metal layer  41  into a metal salicide layer.  
         [0042]    Referring next to FIG. 6, a graph is illustrated wherein sheet resistance is plotted as a function of line width of a salicided polysilicon gate. Nitrogen-devoid salicided polysilicon is indicated by the connected diamonds, whereas polysilicon salicide nitridated according to a process of the present invention is indicated by the connected circles. It can be seen from the graph that nitridation of the salicide according to the process of the present invention substantially reduces sheet resistance at line widths of between 0.1 and about 0.25.  
         [0043]    Referring next to FIG. 7, a graph is illustrated wherein sheet resistance is plotted as a function of processing temperature. Nitrogen-devoid salicided polysilicon is indicated by the connected diamonds, whereas polysilicon salicide nitridated according to a process of the present invention is indicated by the connected circles. It can be seen from the graph that nitridation of the salicide according to the process of the present invention substantially enhances thermal stability of the salicide at temperatures exceeding about 700 degrees C., as indicated by the substantially lower sheet resistance of the nitridated salicide as compared to that of the nitrogen-devoid salicide at those temperatures. Thermal stability of the nitridated salicide remains stable up to about 800 degrees C.  
         [0044]    While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.  
         [0045]    Having described our invention with the particularity set forth above, we claim: