Abstract:
A method of forming a silicided gate of a field effect transistor on a substrate having active regions is provided. The method includes the following steps: (a) forming a silicide in at least a first portion of a gate; (b) after step (a), depositing a metal over the active regions and said gate; and (c) annealing to cause the metal to react to form silicide in the active regions, wherein the thickness of said gate silicide is greater than the thickness of said silicide in said active regions.

Description:
FIELD OF THE INVENTION 
   The present invention relates to semiconductor fabrication and more particularly to methods of forming field effect transistors having silicided regions. 
   BACKGROUND OF THE INVENTION 
   The principle way of reducing contact resistance between polysilicon gates and source/drain regions and interconnect lines is by forming a metal silicide atop the source/drain regions and the gate electrodes prior to application of the conductive film for formation of the various conductive interconnect lines. Presently, the most common metal silicide materials are CoSi 2  and TiSi 2 , typically formed by the so called salicide (self-aligned silicide) process. In the salicide process, a thin layer of a metal, such as titanium, is blanket deposited over the semiconductor substrate, specifically over exposed source/drain and gate electrode regions. The wafer is then subjected to one or more annealing steps, for example at a temperature of 800° C. or higher for titanium. This annealing process causes the metal to selectively react with the exposed silicon of the source/drain regions and the gate electrodes, thereby forming a metal silicide (e.g., TiSi 2 ). The process is referred to as the self-aligned silicide process because the silicide layer is formed only where the metal material directly contacts the silicon source/drain regions and the polycrystalline silicon (polysilicon) gate electrode. Following the formation of the silicide layer, the unreacted metal is removed and an interconnect process is performed to provide conductive paths, such as by forming via holes through a deposited interlayer dielectric and filling the via holes with a conductive material, e.g., tungsten. 
   The thickness of the silicide layer is an important parameter because a thin silicide layer is more resistive than a thicker silicide layer of the same material. Therefore, a thicker silicide layer increases semiconductor speed. The formation of a thick silicide layer, however, may cause a high junction leakage current and low reliability, particularly when forming ultra-shallow junctions. The formation of a thick silicide layer consumes silicon from the underlying semiconductor substrate such that the thick silicide layer approaches and even shorts the ultra-shallow junction, thereby generating a high junction leakage current. 
   It is desirable to also lower the resistance of the gate electrode to increase the speed of the device. The greater the amount of silicon converted into silicide in the gate electrode, the lower the resistance will be in the gate electrode. However, formation of silicide on the gate electrode simultaneously with the source/drain regions leads to the risk of spiking in the source/drain regions if the complete silicidation of the gate electrode is attempted. This process, therefore, suffers from a very narrow processing window due to the strong likelihood that exposure of the metal and silicon to rapid thermal annealing conditions sufficient to completely silicidize a gate electrode will also cause the silicide in the source/drain region to spike and reach the bottom of the junction, undesirably causing leakage. 
   Various methods have been suggested for forming fully silicided gate electrodes. For example, B. Tavel et al. propose in “Totally Silicided (CoSi 2 ) Polysilicon: a novel approach to very low-resistive gate (˜2 Ω/sq) without metal CMP nor etching” (IEDM 01-825) (IEEE 2001) formation of a fully silicided gate electrode by the following steps: (a) polysilicon gate electrode formation; (b) simultaneous silicidation of the source/drain and gate area, with the gate only partially silicided with a Cobalt/Titanium silicide; (c) deposition of a nitride liner; (d) deposition of a dielectric coating layer over the nitride liner; (e) chemical mechanical polishing (CMP) the dielectric and liner layers to the top surface of the gate electrode; (f) deposition of a second Cobalt/Titanium layer over the polished dielectric layer and exposed gate structure; and (g) silicidation of the remaining portion of the gate electrode. 
   While the method of Tavel et al. provides a fully silicided gate electrode, it is very difficult to control the gate electrode height when a CMP step is employed. For example, the polishing rate is different at the wafer center and the wafer edge. Further, the CMP process tends to result in dishing and erosion, leaving concave gate top surfaces, i.e., individual gates with non-uniform heights. Because the gate electrode height is difficult to control, i.e., each wafer may include gates having different heights and individual gates may have non-uniform heights, control of the complete silicidation of the gate electrodes is also difficult. Further, if the gate heights are too low, bridging may occur between the gates and active regions. Still further, the device speed is very difficult to control by this method. 
   Therefore, there remains a need for a method of increasing silicide thickness at the gate area and fully silicidizing a gate electrode, such as a method that affords greater control over the gate electrode height. 
   SUMMARY OF THE INVENTION 
   A method of forming a silicided gate of a field effect transistor on a substrate having active regions is provided. The method includes the following steps: (a) forming a silicide in at least a first portion of a gate; (b) after step (a), depositing a metal over the active regions and said gate; and (c) annealing to cause the metal to react to form silicide in the active regions, wherein the thickness of said gate silicide is greater than the thickness of said silicide in said active regions. 
   The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate preferred embodiments of the invention, as well as other information pertinent to the disclosure, in which: 
       FIGS. 1–10  schematically illustrate sequential steps for forming a fully silicided gate in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The method steps described below do not form a complete process flow for manufacturing integrated circuits. The present embodiments can be practiced in conjunction with integrated circuit fabrication techniques currently used or proposed in the art or that may be developed later, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the described embodiments. The figures represent cross-section portions of a semiconductor chip or a substrate during fabrication and are not drawn to scale, but instead are drawn to illustrate the features of the described embodiments. 
   Sequential steps of an exemplary embodiment of the method of forming silicide regions in a semiconductor substrate are described below with respect to the schematic illustrations of  FIGS. 1–10 . Similar reference numerals denote similar features. Referring first to  FIG. 1 , a conventional transistor structure is shown comprising a substrate  10  doped with either an N-type impurity or P-type impurity, and source/drain regions  11  comprising shallow extension regions  11 A and heavily doped regions  11 B doped with either a P-type impurity or an N-type impurity. In an exemplary embodiment, the substrate comprises crystalline silicon, e.g., monocrystaline silicon. The substrate  10  may also be, for example, a silicon-germanium substrate, III–V compound substrate, silicon-on-insulator (SOI) substrate or other substrate. As is common, the source/drain regions  11  have a conductivity opposite to that of the substrate. The source/drain regions  11  are formed by first forming polysilicon gate electrode  13  on the substrate  10  with gate dielectric layer  12 , e.g., a gate oxide such as silicon dioxide or a high-K dielectric material, therebetween. In one embodiment, the gate electrode is formed to a thickness between about 500–2000 Å. The gate electrode  13  may also comprise amorphous silicon or silicon-germanium. Using the gate electrode  13  as a mask, shallow extension regions  11 A are formed. Dielectric sidewall spacers  14  are then formed on the side surfaces of the gate electrode  13 . Dielectric sidewall spacers  14  may comprise any suitable dielectric material, such as silicon dioxide, silicon nitride, or a composite of silicon dioxide and silicon nitride. Ion implantation is then conducted, using the gate electrode  13  and sidewall spacers  14  as a mask to form heavily doped regions  11 B. Although a conventional FET structure is shown, the method described herein is also applicable to raised source/drain, FinFET and other alternative FET designs. 
   Referring to  FIG. 2 , a shielding layer  16 , such as a layer of SiO 2 , SiN, SiON, SiC, SiCN or some other material that will not react with the subsequently deposited metal layer, is formed over the substrate, such as by a chemical vapor deposition process or furnace process. The shielding layer  16  is conformally deposited to cover the active regions  11  and the gate electrode  13  and preferably has a thickness between about 30–1000 Å, and more preferably a thickness of about 300 Å. 
   Referring to  FIG. 3 , a process is employed to selectively remove the shielding layer  16  over the gate electrode  13  to expose the top surface  15  of the gate electrode  13 . In an exemplary embodiment, an etch process is used to remove portions of layer  16  to expose gate  13 . Use of an etch process enables greater control of gate height and uniformity of gate heights across the wafer. The remaining portions of the shielding layer  16  serve to protect the source/drain regions  11  from silicidation when a subsequent metal layer is deposited over the substrate  10 . In one embodiment, the mask used in forming the polysilicon gate electrode  13  is used in the lithography/etch process employed in opening the shielding layer  16 , thereby ensuring good alignment with the gate electrode surface  16  in the etch process. In an exemplary process, the gate electrode  13  is exposed using a HF etch. For example, if the shielding layer  16  comprises SiO 2 , a 1:1–1 000:1 (HF/H 2 O ratio) HF etchant may be used. In one embodiment, a F −  dry plasma etch may be used as the etch process. 
   As shown in  FIG. 4 , metal layer  18 , which may comprise a pure metal, a metal alloy or a metal with additives (e.g., C, Al, Sc, Ti, V, Cr, Mn, Fe, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, In, Sn, La, Hf, Ta, W, Re, Ir, Pt, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof) that improve or change the thermal stability and/or salicide formation temperature is blanket deposited over at least the exposed portions of the upper surface  15  of the gate electrode  13 . In an exemplary embodiment, layer  18  comprises cobalt/titanium (Co/Ti) (i.e., a first deposited layer of cobalt and a titanium capping layer or vice versa), cobalt/titanium nitride (Co/TiN), nickel/titanium (Ni/Ti), or nickel/titanium nitride (Ni/TiN) deposited to a thickness between about 10–2000 Å. As shown in  FIG. 4 , the metal layer  18  is also deposited over the remaining portions of the shielding layer  16 . The metal layer  18  can be deposited in any manner, such as by chemical vapor deposition (CVD), atomic layer deposition (ALD) or by sputtering. 
   Referring to  FIG. 5 , an annealing step, preferably a rapid thermal annealing step, is performed. The annealing step may take place at a temperature between about 200° C. to 900° C. for about 10 to about 1000 seconds, depending upon the metal that is employed and the desired depth of the silicide layer  20 . Upon annealing, a metal silicide layer  20 , e.g., cobalt silicide, nickel silicide, etc. is formed in at least a portion of the gate electrode  13 , leaving a remaining portion unsilicided, or optionally, fully siliciding gate electrode  13  (as shown in, for example, gate  20 A of  FIG. 5A ). For some silicides, e.g., CoSi2, TiSi, etc., a two-step rapid thermal anneal process is utilized to form metal silicide layer  20 . 
   Referring to  FIG. 6 , any unreacted metal  18 A ( FIG. 5 ) is removed from the substrate  10 , leaving remaining portions of shielding layer  16 . The unreacted metal layer  18 A may be removed by a wet chemical etch, for example, or other process. In an exemplary embodiment, the unreacted metal is removed using an HNO 3 , HCl, NH 4 OH, H 2 SO 4  or other acid etchant, such as a mix of acids. In one embodiment, the etching is performed between about room temperature and 150° C. for between about 2–60 minutes. 
   Referring now to  FIG. 7 , the remaining portions of the shielding layer  16  disposed over the spacers  14  and active regions  11  are removed to expose the active regions, i.e., source/drain regions  11 , for silicidation. In an exemplary embodiment, the remaining portions of the shielding layer  16  are removed using a HF etch. 
   Referring to  FIG. 8 , a second layer of metal  22 , preferably, but not necessarily, the same metal deposited to form layer  18 , is deposited over the substrate  10  to cover the top surface of the silicide layer  20  of the gate electrode and the source/drain regions  11 . As noted above, exemplary metal layers may comprise cobalt/titanium (Co/Ti), cobalt/titanium nitride (Co/TiN), nickel/titanium (Ni/Ti), or nickel/titanium nitride (Ni/TiN). The metal layer  22  is deposited to a thickness sufficient to produce silicide layers having a desired thicknesses in the source/drain regions  11  and, optionally, to complete or partially complete the silicidation of the remaining unsilicided portions of the gate electrode  13 . In one embodiment, the silicide formed in the gate electrode  13  is thicker than the silicide that is formed in the active regions. In a further embodiment, the gate electrode is fully silicided. By “fully silicidize” or “fully silicided” it is meant that the gate electrode is substantially silicided, meaning, in one embodiment, silicide forms in at least 90–100 percent of the gate height, and more preferably at least 95–100 percent of the height of the gate. 
   Referring to  FIGS. 9 and 9A , a rapid thermal anneal process is again applied to the substrate causing the metal  22  to react with source/drain regions  11 . Silicide regions  26  are formed to a desired depth in active regions  11 . In one embodiment, the processing time and temperature of the second anneal is limited to prevent (in whole or in part) additional diffusion of metal atoms into the gate electrode  13 , leaving a partially silicided gate as shown in  FIG. 9A , but with the gate silicide  24 A that is thicker than the silicide  26  in the active regions. In essence, the silicide  20  formed from the first metal deposition and annealing serves as a barrier to further diffusion. In this embodiment, gate  13  is fully silicided by the first anneal (if a fully silicided gate is required) or initially, partially silicided to a thickness greater than the eventual thickness of silicides  26  in the active regions. In the embodiment shown in  FIG. 9 , metal layer  20  is processed for sufficient time and at a sufficient temperature to promote further metal diffusion into the gate to promote additional silicidation or, in one embodiment, full silicidation. 
   By controlling and limiting the thickness of the silicide in the active regions, junction shorts are prevented. By thickening the gate silicide, device speed is improved. Fully silicided gates allow for control of the work function of the device. 
   Although cobalt/titanium (Co/Ti), cobalt/titanium nitride (Co/TiN), nickel/titanium (Ni/Ti), and nickel/titanium nitride (Ni/TiN) have been described as a preferred metals for the embodiments described herein, other metals or alloys that form silicides and are predominant diffusion species may be used, such as Nickel (Ni), Palladium (Pd), Chromium (Cr), Cobalt (Co), Titanium (Ti), Tungsten (W), Molybdenum (Mo), etc. Annealing process parameters and metal thickness may change dependent on the metal selected for layer  22 . Assuming the example above where layer  22  is deposited to a thickness of about 20–150 Å and assuming layer  22  comprises nickel, the substrate is annealed at a temperature between about 200–700° C. for a time of about 10–500 seconds, thereby forming silicide regions  26  to a depth of about 40–300 Å, in some embodiments, completing silicidation of the gate. 
   Referring to  FIG. 10 , the unreacted portions of metal layer  22 A ( FIG. 9 ) are removed, thereby providing a silicidized gate electrode  24  and silicided source/drain regions  26  where the gate electrode silicidation is thicker than the silicidation of the active regions, thereby increasing device speed without shorting ultra-shallow junctions. A wet chemical etch that is highly selective to the unreacted metal layer  22  relative to the silicide may be employed to remove the unreacted metal  22 A. In an exemplary embodiment, the unreacted metal  22 A is removed using an HNO 3 , HCl, NH 4 OH, H 2 SO 4  or other acid etchant. 
   The manufacturing process described herein is adaptable to manufacturing any of the various types of semiconductor devices, particularly advanced deep-submicron CMOS devices, such as 0.1 micron devices with ultra-shallow junctions, e.g., above 500 Å to about 2000 Å, while significantly improving the reliability of ultra-shallow junctions. Parasitic, sheet and contact resistance between the active regions and the gate electrode and interconnects is achieved without increasing junction leakage current. Further, because no polishing or etch back process need be employed to expose the surface of the gate electrode for silicidation, the height of the gate electrode is more easily controlled, thereby facilitating greater control of the silicidation process itself in forming fully silicided gate electrode  24  and silicided active regions  26 . 
   The method described herein also provides for excellent control of the gate electrode height during the silicidation process. The process has an improved process window for exposing the top surface of the gate electrode in a two-step silicidation formation process, thereby facilitating improved control of the silicidation process, which provides improved silicided gates, and consequent benefits thereof, such as lower gate electrode resistance, improved device speed, prevention or reduction of boron migration into the gate electrode and reduction or elimination of the depletion effect, without high junction leakage current or spiking. 
   Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.