Abstract:
A silicide method for integrated circuit and semiconductor device fabrication wherein a layer of nickel is formed over at least one silicon region of a substrate and a layer of cobalt is formed over the nickel layer. The cobalt/nickel bi-layer is then annealed to transform the bi-layer to a bi-silicide film having a cobalt-rich silicide portion and a nickel-rich silicide portion.

Description:
RELATED APPLICATIONS 
   This application is a divisional of application Ser. No. 10/607,496, filed Jun. 26, 2003 now U.S. Pat. No. 7,105,439. The entire disclosure of application Ser. No. 10/607,496 is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to integrated circuit and semiconductor device fabrication and, more particularly, to a cobalt/nickel bi-layer silicide process which utilizes a cobalt/nickel stack to create a bi-silicide CoNiSi x  film that provides better chemical and plasma resistance than pure nickel silicide. 
   BACKGROUND OF THE INVENTION 
   Self-aligned silicide (salicide) technology is required in modem integrated circuit and semiconductor device fabrication to lower the resistance of polysilicon gates, sources and drains to reduce RC delay, i.e., the gate speed performance index wherein less delay produces increased gate speed performance. On example of a well known silicide technology is cobalt silicide (CoSi 2 ). CoSi 2  process are commonly used for sub quarter micron and beyond technology. Unfortunately, the agglomeration effect of CoSi 2  on very narrow line polysilicon gates that are less than 65 nanometers, often limits its extendibility to the fabrication of shorter gates. 
   Nickel silicide (NiSi) technology appears to be emerging as a dominant solution to very narrow line polysilicon gates because it provides better sheet resistance (Rs) for narrow line polysilicon gates, less junction leakage, less silicon (Si) consumption, and can even improve the drive current (Idsat) of an NFET or PFET. 
   With respect to the NiSi, there are still some concerns about monosilicide phase stability, film thermal stability, and plasma damage resistance of NiSi while implementing this process. 
   Accordingly, a method or process is needed which allows NiSi to be implemented successfully in very narrow line polysilicon gate technology. 
   SUMMARY OF THE INVENTION 
   A first aspect of the invention is a silicide method for integrated circuit and semiconductor device fabrication. The method comprises: forming a layer of nickel over at least one silicon region of a substrate; forming a layer of cobalt over the nickel layer to form a cobalt/nickel bi-layer; and annealing the cobalt/nickel bi-layer to transform the bi-layer to a bi-silicide film including a cobalt-rich silicide portion and a nickel-rich silicide portion. 
   A second aspect of the invention is a method for fabricating a semiconductor device. The method comprises: forming a gate of the semiconductor device over a substrate; forming a layer of nickel over the gate; forming a layer of cobalt over the nickel layer to form a cobalt/nickel bi-layer; and annealing the cobalt/nickel bi-layer to transform the bi-layer to a bi-silicide film including a cobalt-rich silicide portion and a nickel-rich silicide portion. 
   A third aspect of the invention is a bi-silicide film. The film comprises: a cobalt-rich portion; and a nickel-rich portion. 
   A fourth aspect of the invention is a semiconductor device. The device comprises: a gate; source/drains regions formed on opposite sides of the gate; and a bi-silicide film formed over the gate and the source/drain regions, the bi-silicide film including a cobalt-rich portion and a nickel-rich portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1H  are sectional views showing the steps of the cobalt/nickel bi-layer silicide process of the invention. 
       FIG. 2  a graph plotting phase transformation curves (RTA temperature versus sheet resistance Rs) for pure Co/Si and pure Ni/Si. 
       FIG. 3  is a table comparing the sheet resistance of pure NiSi films and bi-silicide CoSi x /NiSi films. 
       FIG. 4  shows a TEM and EDX analysis of films made according to the Co/Ni bi-layer silicide process of the invention. 
       FIG. 5  is an Auger depth profile of a bi-silicide CoSi x /NiSi film made according to the invention. 
       FIG. 6  is a table comparing chemical resistance for Co, Ni and metal silicides. 
       FIG. 7  shows a TEM and diffraction pattern analysis for pure a NiSi film under a contact hole with made with a conventional backend CoSi 2  process. 
       FIG. 8  shows a F2p XPS spectra of a pure NiSi film and a bi-silicide CoSi x /NiSi film made according to the invention post plasma Fluorine gas treatment. 
       FIG. 9  is a graph comparing the contact chain for a pure NiSi film to a bi-silicide CoSi x /NiSi film made according to the invention. 
       FIGS. 10A-10C  are graphs comparing the sheet resistance ( FIG. 10A ), junction leakage ( FIG. 10B ), and isolation ( FIG. 10C ), of films made according to the Co/Ni bi-layer silicide process of the invention and of films made according to a conventional NiSi process. 
       FIG. 11  shows the Idsat/Ioff characteristics for films made according to the bi-layer Co/Ni silicide process of the invention and films made according to a conventional NiSi silicide process on a NFET. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention relates to a cobalt/nickel bi-layer silicide (salicide) process for 90 nanometer gate technology (and down to 20 nanometer gate technology) that combines the advantages of both cobalt salicide and nickel salicide, to provide low gate sheet resistance, low silicon (Si) consumption, and high plasma resistance. The bi-layer process of the invention is designed so that the top Co-rich silicide (CoSi x ) portion of the resulting bi-silicide film CoNiSi x  is the thinner and operative as a protection layer, and the bottom portion (which is close to pure NiSi) of the CoNiSi x  silicide film is thicker with a low resistance and operative as main gate and source/drain conductive material. Although this process is especially suited for 90 nanometer gate technology, it is contemplated that the invention will be useful down to 20 nanometers and below. 
   Referring now to  FIG. 1A , there is shown a sectional view of a semiconductor transistor  20 . The transistor  20  is formed on a semiconductor substrate  10 , composed, for example, from monocrystalline silicon. The transistor  20  comprises a gate  21  which may be conventionally formed, for example, from a conductive polycrystalline silicon (polysilicon) material which is dielectrically spaced above the substrate  10  by a gate oxide film  22  conventionally formed over a first surface  11  of the substrate  10 . A spacer  23  extends laterally outward from each of the sidewall surfaces of the gate  21 . The spacers  23  may be conventionally formed, for example, from a nitride material. The substrate  10  includes an active region which receives dopants that are self-aligned to the sidewall surfaces of the gate  21  and the spacers  23 . The dopants are those which comprise the source/drain dopants as well as the LDD dopants. More specifically, the active region may include lightly doped source  12  and drain  13  regions which are self-aligned to the sidewalls of the gate  21 , and heavily doped source  14  and drain  15  regions which are self-aligned to the sidewalls of the spacers  23 . The lightly doped source  12  and drain  13  regions and the heavily doped source  14  and drain  15  regions may be formed in the substrate  10  using conventional methods. A channel  16  region exists between the lightly doped source  12  and drain  13  regions. 
   The process of the invention commences with a pre-salicidation native oxide removal step. Native oxide removal may be accomplished by dipping the substrate  10  of  FIG. 1  in an etchant, such as hydrofluoric acid. The etchant removes any native oxide (SiO 2 ) on the silicon surfaces of the substrate  10 . 
   In  FIG. 1B , a layer  30  of nickel (Ni) is conformally formed over the substrate  10 . The Ni layer  30  can be formed, for example, using any conventional metal deposition technique, such as sputtering. The Ni layer  30  is typically deposited to a thickness of between about 10 and 20 nanometers. 
   In  FIG. 1C , a layer  40  of cobalt (Co) is conformally formed over the substrate  10 . The Co layer  40  may also be formed by sputtering or by any other suitable metal deposition technique. The Co layer  40  is typically deposited to a thickness of between about 5 and 10 nanometers. 
   In order to prevent oxidation, a cap layer  50  of titanium nitride (TiN) is conformally formed over the substrate  10  as shown in  FIG. 1D . The TN cap layer  50  may be deposited using any conventional deposition technique such as physical vapor deposition or chemical vapor deposition. The TiN cap layer  50  is typically deposited to a thickness of between about 5 and 30 nanometers. 
   A rapid thermal annealing (RTA) step is now performed on the substrate  10 . The RTA may be performed in a rapid thermal process (RTP) system, which provides a fast ramp-up/ramp-down for the anneal process. The RTA may be performed at a temperature of between about 300° C. and 700° C., for a time period of between about 10 and 120 seconds. As shown in  FIG. 1E , the RTA step transforms the Ni and Co layers  30 ,  40  overlying the source  14  and drain  15  regions of the silicon substrate  10  and overlying the polysilicon gate  21  to a bi-silicide film  60  of CoSi x /NiSi. The portions of the Ni and Co, and TiN layers  30 ,  40 ,  50  overlying the nitride spacers  23  remain unreacted after RTA. 
   In  FIG. 1F , the process is completed by removing unreacted portions of the Ni, Co, and TiN layers  30 ,  40 ,  50  from the substrate  10  using any conventional etching process, such as wet etching with a conventional silicide selective etch (SPM) solution, which typically comprises a H 2 SO 4 /H 2 O 2  (within H 2 O) mixture. After etching, an optional second RTA step may be performed to control the overall bi-silicide film thickness and the CoSi x /NiSi ratio. The optional RTA step may be performed in the RTP system at a temperature of between about 300° C. and 700° C., for a time period between about 10 and 120 seconds. 
   After completing the process of the invention, contacts to the gate  21 , and the source  14  and drain  15  regions may be formed using conventional methods. For example, the contacts may be formed by depositing a dielectric layer  80  of over the substrate  10  and etching contact openings  81  in the dielectric layer  80  above the gate  21 , and the source  14  and drain  15  regions as shown in  FIG. 1G . The contact openings  81  may then be filled with a conductive material  82  to complete the contacts  90  as shown in  FIG. 1H . 
   Referring still to  FIG. 1H , bi-silicide CoNiSi x  film  60  formed by the process of the invention is comprised of a relatively thinner top CoSi x  portion  61  that is operative as a protection layer and a relatively thicker, low resistance NiSi portion  62  underneath that is operative as gate and source/drain conductive material. The individual thickness of the CoSi x  and NiSi portions  61 ,  62  can be easily adjusted by altering the Co/Ni sputter ratio and/or the RTA process, to meet the required function of different high performance IC products. 
   EXPERIMENTAL RESULTS AND DISCUSSION 
   Referring to  FIG. 2 , there is shown a graph plotting phase transformation curves (RTA temperature versus sheet resistance Rs) for pure Co/Si and pure Ni/Si. The RTA temperatures to maintain low sheet resistance were selected in the range of 300° C. to 700° C. Within this temperature range, NiSi 2  with a high Rs does not form. Also with this temperature range, CoSi x  does not agglomerate, but does have a high Rs similar to that of Co 2 Si or CoSi, which might increase the overall Rs of the bi-layer silicide (CoNiSi x ). Hence, the ratio of CoSi x /NiSi in the bi-silicide film of the invention is very critical and should be carefully controlled to form the thinner CoSi x  portion at the top of the film. 
   In accordance with the optimized RTA process and Co/Ni thickness ratio of the invention, the Rs of the bi-silicide CoSi x /NiSi film of the invention can be lower than that of the pure NiSi process as shown in the Table of  FIG. 3 . 
   As shown in the X-ray transmission electron microscope (TEM) picture of  FIG. 4 , the method of the invention only forms one smooth metallic CoNiSi x  silicide film, without any interfacial boundary. An x-ray energy dispersive spectrometer (EDX) analysis shows that Co-rich silicide is at the top of the film and forms approximately 15% of the overall thickness of the film. The EDX analysis shows that the bottom of the film is substantially pure NiSi. The depth profile analysis obtained with Auger electron spectroscopy (AES), as shown in  FIG. 5 , reconfirms the composition distribution of the bi-silicide CoNiSi x  film of the invention, showing a high Co concentration at the top of the film. 
   Chemical acids such as H x SO 4 , H 3 PO 4 , and HCl have been tested and data shows a high etching rate on Co and Ni films, but low degradation on silicide films. The etching selectivity of silicide films such as CoSi x , NiSi, and CoNiSi x , and metal films such as Co and Ni, according to the Rs variation percentage after chemical attack, have been examined to be greater than 200 (see the Table of  FIG. 6 ). This indicates that silicide films such as CoSi x , NiSi, and CoNiSi x  show excellent chemical resistance to the chemical acids listed above. 
   On the other hand for plasma stability, a X-ray photoelectron study (XPS) shows that a pure NiSi film is easily attacked by plasma, as the Fluorine gas used in the plasma process (i.e., etch process) forms a Ni—F bond. Further, a chain reaction among a NiSi film, F 2  and O 2  destroys the NiSi film integrity and dramatically increases the Rs. An amorphous-like layer is formed on the damaged region as shown by the X-ray TEM picture of  FIG. 7 . This phenomena implies that the pure NiSi film exhibits poor plasma stability which would be an issue in manufacturing for a post salicide process similar to a CoSi 2  process. 
   The CoNiSi x  film produced by the Co/Ni bi-layer silicide process of the invention was treated with the same plasma and Fluorine gas, and showed significantly reduced Ni—F (or Co—F) bonding signal intensity, which provided the CoNiSi x  film of the invention with very good plasma resistance as shown in  FIG. 8 . Electrical performance on contact resistance by using the bi-layer Co/Ni silicide process of the invention also shows no failure site, which is much better than with the pure NiSi process as shown in  FIG. 9 . 
   As mentioned earlier, compared to the CoSi 2  process, the NiSi process has a lower Rs without agglomeration, which may allow gates to shrink down to 20 nanometers and below, and better junction leakage performance is due to less Si consumption. Furthermore, the NiSi process of less thermal budget, provides about an 8% improvement over the CoSi 2  process on both NFET and PFET Idsat/Ioff (100 nA). Films made according to the Co/Ni bi-layer silicide process of the invention exhibit no degradation in terms of the Rs ( FIG. 10A ), junction leakage ( FIG. 10B ), and isolation ( FIG. 10C ), which occurs in films made according to a conventional NiSi process. In addition,  FIG. 11  shows that the NFET Idsat/Ioff (100 nA) for films made according to the bi-layer Co/Ni silicide process of the invention measured 680 μA, which is an additional 5% gain over films made according to a conventional NiSi silicide process (640 μA). It is suspected that the reason for this improvement might be lower R SD  (source or drain R value) which is due to silicide lateral diffusion in the source/drain area. Moreover, no degradation was observed on PFET performance for the Co/Ni bi-layer silicide process of the invention. 
   While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.