Abstract:
A process for forming a low resistance, titanium silicide layer, for use as a component of a narrow width, polycide gate structure, has been developed. The process features a combination of ion implantation procedures, performed prior to, and after, titanium deposition. The combination of ion implantation procedures restricts excessive movement of silicon, from a polysilicon gate structure, as well as from a source/drain region, into the forming titanium silicide layer, during subsequent anneal cycles used to form the titanium silicide layer. The ability to limit the amount of silicon, in the titanium silicide layer, allows a low resistance, titanium silicide layer to be used for polycide gate structures, with a width narrower than 0.20 micrometers.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to processes used to fabricate semiconductor devices, and more specifically to a process used to create a low resistance, metal silicide layer, on a polysilicon structure. 
     (2) Description of Prior Art 
     The use of polycide, (metal silicide—polysilicon), gate structures, has resulted in a decrease in word line resistance, for sub-micron, metal oxide semiconductor field effect transistors, (MOSFET), devices, when compared to word lines fabricated using only polysilicon. One version of a polycide layer, used for gate structures, has been a titanium silicide polysilicon gate structure. The ability to selectively form titanium silicide, on polysilicon lines: via deposition of a titanium layer;, annealing to convert the portions of the titanium layer, overlying silicon regions, to titanium silicide; and removing the portions of unreacted titanium; have made titanium silicide an attractive refractory silicide, for use in a polycide gate structure. However as micro-miniaturization, or the use of sub-quarter features, proceeds, conventional procedures, used to create titanium silicide, polycide gate structures, do not allow the needed titanium silicide sheet resistance to be obtained. For example, conventional procedures, comprised of titanium deposition, anneal, removal of unreacted titanium, result in the desired sheet resistance, for polycide gate structures, with widths greater than 0.20 micrometers. However when polycide gate structures, with widths less than 0.20 micrometers are used, the resulting titanium silicide sheet resistance does not satisfy the designed, or desired, word line resistance. 
     This invention will describe a process for fabricating a titanium silicide layer, in which the resistance of the metal silicide layer is reduced as a result of the unique set of processing steps used. A pre-amorphization, ion implantation procedure, is first used, prior to titanium deposition, to prepare the exposed silicon surfaces, such as the exposed polysilicon, of the gate structure, as well as the exposed, heavily doped, source/drain regions, for the titanium deposition, and subsequent anneal. The pre-amorphization step, retards the movement of silicon atoms, into the forming titanium silicide layer, during the anneal cycle, thus allowing a less silicon rich, and lower resistance, titanium silicide layer to be formed. A second procedure, used in combination with the pre-amorphization ion implantation step, is an ion mixing, or an ion implantation procedure, performed after titanium deposition, but prior to the anneal procedure used to form the metal silicide layer. This procedure places the implanted species, in the titanium layer, near the source/drain interface, again retarding the movement of silicon atoms into the titanium silicide layer, during the anneal procedure, but more importantly retarding movement of boron, if a P type, MOSFET device is used, into the titanium silicide layer. Prior art, such as Anjum et al, in U.S. Pat. No. 5,401,674, describes an ion implantation procedure, into titanium, prior to the anneal, but does not describe the combination of the pre-amorphization, ion implantation, pre-titanium, and the post-titanium, ion mixing, ion implantation procedure. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to form a low resistance, titanium silicide layer, for use as a component for narrow, less than 0.20 micrometer width, polycide word lines. 
     It is another object of this invention to perform a pre-amorphization, ion implantation procedure, prior to titanium deposition. 
     It is still another object of this invention to perform an ion mixing procedure, via ion implantation into the titanium layer, prior to the anneal procedure, used to form titanium silicide. 
     In accordance with the present invention, a process is described for forming a low resistance, titanium silicide layer, for a narrow width, polycide gate structure, via a combination of pre-titanium, and post-titanium, ion implantation procedures. After creation of: a narrow width, polysilicon line; a lightly doped source/drain region; insulator spacers on the sides of the narrow width, polysilicon line; and a heavily doped source/drain region; a pre-amorphization, ion implantation procedure is performed, to exposed polysilicon and silicon regions, using germanium or arsenic ions. After deposition of a titanium, or titanium nitride layer, an ion mixing, ion implantation procedure is used to place germanium or silicon ions, in the titanium, or titanium nitride—titanium layer, near the metal—silicon interface. A first anneal procedure, converts titanium , overlying silicon or polysilicon, to a first phase titanium silicide layer, while titanium overlying insulator layers, remain unreacted. After selective removal of the unreacted titanium, a second anneal is used to convert the first titanium silicide phase, to a more conductive second titanium silicide phase, resulting in a narrow width, polycide gate structure, featuring low word line resistance as a result of forming the low resistance, titanium silicide layer, using the combination of ion implantation procedures, described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and other advantages of this invention are best described in the preferred embodiments with reference to the attached drawings that include: 
     FIGS. 1-6, which schematically, in cross-sectional style, show the key stages of fabrication used to form a narrow width, polycide gate structure, featuring a low resistance titanium silicide layer, created using a pre-titanium, pre-amorphization, ion implantation procedure, and a post titanium, ion mixing, ion implantation procedure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of forming a low resistance, titanium silicide layer, via use of a pre-titanium, pre-amorphization, ion implantation procedure, and via use of a post-titanium, pre-anneal, ion mixing, ion implantation procedure, will now be described in detail. This invention will show the method of forming a low resistance titanium silicide layer, applied to narrow width, polycide gate structures, however this invention can also be applied to any dimension, gate structures, or to other elements of a semiconductor device. 
     A P type, semiconductor substrate  1 , comprised of single crystalline silicon, with a &lt;100&gt; crystallographic orientation, is used and shown schematically in FIG. 1. A gate insulator layer  2 , comprised of silicon dioxide, is thermally grown in an oxygen—steam ambient at a temperature between about 800 to 1000° C., to a thickness between about 50 to 200 Angstroms. A polysilicon layer is next deposited, using low pressure chemical vapor deposition, (LPCVD), procedures, to a thickness between about 1000 to 4000 Angstroms. The polysilicon layer can be doped in situ, during deposition, via the addition of arsine, or phosphine, to a silane ambient, or the polysilicon layer can be deposited intrinsically, then doped via an ion implantation procedure, using arsenic, or phosphorous ions. Conventional photolithographic and reactive ion etching, (RIE), procedures, using Cl 2  as an etchant, are used to pattern the polysilicon layer, creating polysilicon structure  3 , shown schematically in FIG. 1, with the width of polysilicon structure  3 , less than 0.20 um. Removal of the photoresist shape, used for definition of polysilicon structure  3 , is accomplished via plasma oxygen ashing and careful wet cleans. The portion of gate insulator  2 , not covered by polysilicon structure  3 , is removed during the wet clean procedures. 
     Lightly doped source/drain regions  4 , are next formed in regions of semiconductor substrate  1 , not covered by polysilicon structure  3 , via an ion implantation procedure. If the desired MOSFET device is an N type, (NFET), device, lightly doped source drain region  4 , are formed via ion implantation of arsenic, or phosphorous ions, at an energy between about 20 to 70 KeV, at a dose between about 1E13 to 5E14 atoms/cm 2 . However if the MOSFET device is a P type, (PFET), lightly doped source drain region  4 , is formed by an ion implantation procedure, using boron, or BF 2  ions, at an energy between about 20 to 70 KeV, at a dose between about 1E13 to 5E14 atoms/cm 2 . The lightly doped source/drain regions, are schematically shown in FIG.  1 . 
     Insulator spacers  5 , shown schematically in FIG. 2, are next formed on the sides of polysilicon structure  3 . A layer of silicon oxide is deposited via LPCVD or plasma enhanced chemical vapor deposition, (PECVD), procedures, at a thickness between about 1000 to 3000 Angstroms. An anisotropic RIE procedure, using CHF 3  as an etchant, is then employed, resulting in the formation of insulator spacers  5 . If desired insulator spacers  5 , can be comprised of silicon nitride. Heavily doped source/drain region  6 , is formed, via ion implantation procedures, in a region of semiconductor substrate  1 , not covered by polysilicon structure  3 , or by insulator spacers  5 . Heavily doped source/drain region  6 , if used for NFET devices, is obtained via ion implantation of arsenic, or phosphorous ions, at an energy between about 30 to 75 KeV, at a dose between about 5E14 to 1E16 atoms/cm 2 , while a ion implantation procedure, using boron, or BF 2  ions, is performed at an energy between about 30 to 75 KeV, at a dose between about 5E14 to 1E16 atoms/cm 2 , if PFET, MOSFET devices are used. The heavily doped source/drain region  6 , is schematically shown in FIG.  2 . 
     A critical ion implantation procedure, called a pre-amorphization procedure, is next performed. Subsequent formation of a low resistance, titanium silicide layer, will be in part, dependent on the amount of silicon moving from the polysilicon, or source/drain region, into the overlying, growing, titanium silicide layer. If to great a level of silicon is incorporated in the forming titanium silicide layer, the resistance of this metal silicide layer may be greater than desired. In addition, if the width of the gate structure is narrow, less than 0.20 um, the lack of a low resistance metal silicide layer, can result in a polycide gate structure, or word line, with non-acceptable resistance. Therefore a pre-amorphization, ion implantation procedure, near the top surface of polysilicon structure  3 , or near the top surface of heavily source/drain region  6 , will retard the movement of silicon, into the metal silicide layer, during a subsequent anneal procedure, that is used to form the metal silicide layer. The pre-amorphization procedure, is performed via ion implantation of germanium, or arsenic ions, at an energy between about 20 to 40 KeV, at a dose between about 2E14 to 5E14 atoms/cm 2 . The implanted ions  7 , are schematically shown in FIG.  2 . In addition, this procedure also fixes, or sets, the dopants in the source/drain region, reducing the risk of increasing the source/drain region from diffusion during subsequent anneal cycle, thus preserving the shallow junctions needed for the high performance, MOSFET devices. Regions of the MOSFET device, designed to not incorporate metal silicide layers, are protected by a patterned insulator layer, such as silicon oxide, at a thickness between about 500 to 1500 Angstroms,(not shown in the drawings). The patterned insulator layer is formed from a rapid thermal oxidation, (RTO), and defined via conventional photolithographic and dry etching procedures, prior to the pre-amorphization ion implantation procedure, and is used to protect regions of the MOSFET device, from subsequent metal silicide formation. 
     A titanium layer  8   a , shown schematically in FIG. 3, is next deposited, via R.F. sputtering, to a thickness between about 500 to 1000 Angstroms. If desired a titanium nitride—titanium composite layer, can also be used. A second critical ion implantation procedure, called an ion mixing procedure, is next employed to place implanted ions  9 , shown schematically in FIG. 4, in titanium layer  8   a , at a depth near the bottom of the titanium layer, in an effort to further decrease the risk of silicon atoms, diffusing from underlying silicon regions, such as polysilicon structure  3 , into the forming titanium silicide layer, during the subsequent anneal cycles. The ion mixing procedure is accomplished via ion implantation of germanium or silicon ions, at an energy between about 35 to 40 KeV, at a dose between about 7E14 to 1E15 atoms/cm 2 . These condition place the implanted ions at a depth between about 500 to 1000 Angstroms, in titanium layer  8   a.    
     A first rapid thermal anneal, (RTA), procedure, is performed at a temperature between about 650 to 850° C., for a time between about 10 to 120 sec., in a nitrogen ambient, resulting in the formation of a first titanium silicide layer  8   b , in regions in which titanium layer  8   a , resided on either polysilicon structure  3 , or heavily doped source/drain region  6 . This is schematically shown in FIG.  5 . The resistance of first titanium silicide layer  8   b , was not compromised by excessive silicon diffusion into the forming layer, during the first RTA procedure, as a result of the combination of the pre-amorphization, and ion mixing, ion implantation procedures. Regions of titanium layer  8   a , overlying insulator regions, such as insulator spacers  5 , remained unreacted. 
     Unreacted regions of titanium layer  8   a , are selectively removed using a solution containing H 2 O 2 —H 2 SO 4 —NHOH 4 —HCl, at a temperature between about 50 to to 80° C. First titanium silicide layer  8   b , is not soluble in the above solution, during the removal of unreacted titanium. A second RTA anneal, performed at a temperature between about 700 to 900° C., for a time between about 10 to 120 sec., in a nitrogen ambient, is used to convert first titanium silicide layer  8   b , to a more conductive, second titanium silicide layer  8   c , shown schematically in FIG.  6 . The ability of the resulting metal silicide layer to withstand RTA procedures, without experiencing excessive silicon diffusion, or without compromising the sheet resistance of the metal silicide layer, is a result of the combination of the pre-amorphization, and ion mixing procedures, used to retard silicon diffusion. The polycide gate structure, shown schematically in FIG. 6, comprised of second titanium silicide layer  8   c , on polysilicon structure  3 , offers a low resistance structure for the narrow width, less than 0.20 um, polycide gate structure. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.