Patent Publication Number: US-2005127459-A1

Title: Novel gate structure and method of forming the gate dielectric with mini-spacer

Description:
BACKGROUND OF THE INVENTION  
      (1) Field of the Invention  
      The present invention relates generally to methods of fabricating a gate dielectric and a mini-spacer, and more particularly, to methods of patterning gate structures with high-k gate dielectric films and mini-spacers in the fabrication of semiconductor devices and integrated circuits.  
      (2) Description of the Prior Art  
      The current generation ultra large-scale integrated (ULSI) circuits use predominantly field effect transistor (FET) devices, having poly-silicon gate electrode, silicon dioxide gate dielectric, and self-aligned source/drain regions. Typical process of fabricating an FET consists of growing a thin gate silicon dioxide on a silicon substrate and then forming the poly-silicon gate electrode. Source/drain (S/D) regions are then formed adjacent to the gate electrode, which then defines the FET gate length as the distance under the poly-silicon gate between the S/D regions. The gate length and the control of gate length is critical, particularly as channel lengths continue to decrease to achieve high device densities, since short channel effects (SCE) can occur in the device. SCE effects become predominant since band-gap and junction potentials cannot be scaled with channel length, being dependent on the substrate material (i.e. silicon).  
      Fabrication of the gate electrode structure is critical in achieving optimum performance of the MOSFET (metal-oxide-silicon field effect transistor) device. The gate stack, consisting of the gate electrode and the gate dielectric, essentially determines the operating speed and reliability of the device.  
      Although silicon dioxide has been the dielectric of choice for many years because of ease of forming the film and patterning it, high-k refractory metal oxides are being increasingly experimented in recent years. The motivation is that high-k gate dielectric films reduce the equivalent gate oxide thickness between the gate metal and the substrate, thereby increasing device performance. If silicon dioxide has to be used in ULSI devices with short channel length, it needs to be quite thin to achieve large capacitance values and formation of nanometer range, high quality oxides without defects is quite difficult if not impossible. Several gate structures with high-k gate dielectric have been proposed in prior art. However, patterning of high-k films, unlike the conventional silicon oxides or nitrided oxides, is difficult for two main reasons: i) plasma etching of these films that do not easily form volatile reaction products requires high self-developed or applied bias voltages will cause device damage of the silicon surface of the transistor and ii) alternatively, non-damaging wet etching processes result in poor dimensional control of the FET channel length and are prone to etch residues that are hard to remove during post-processing steps.  
      U.S. Pat. No. 5,447,874 describes a method of producing devices with improved gate length control, eliminating contamination induced surface damage, leakage problems without increasing processing steps. A gate opening is anisotropically etched in an oxide layer, creating a reverse gate metal image that has low gate length variability. Dual metal gate is then deposited and the excess gate metal is removed and the top surface of the gate planarized using chemical mechanical polishing. The remaining oxide is then removed. The patent refers to a dual metal gate with a conventional silicon dioxide gate dielectric film.  
      In U.S. Pat. No. 5,940,698, a device with a high performance gate structure and a fabrication process is described. According to the process disclosed, a gate insulating film is deposited over a substrate, a diffusion barrier layer is then formed over the gate-insulating layer, and a trench is etched in the diffusion barrier layer. In the trench, a metal gate electrode is formed. Although the high-k materials like cobalt niobate, barium strontium nitrate (BST), and tantalum oxide are proposed, the process described is quite complex and it is very likely that etch residues will be left on the silicon substrate when etching the high-k film within the gate trenches and around the gate structure, as previously discussed.  
      U.S. Pat. No. 5,960,270 describes a process of forming a metal-gate/metal-oxide/semi-conductor MOS transistor with self-aligned source and drain electrodes formed before defining the metal gate. Although high-k metal oxides like TiO 2  and Ta 2 O 5  are mentioned as a part of a composite dielectric gate (e.g. grown oxide/deposited high-k material), no specific process is described or taught on how to pattern high-k films without the problems associated with post-etch residues.  
      U.S. Pat. No. 5,766,998 describes a method of forming reverse self-aligned FETs (field effect transistors) with gate electrode of sub-quarter micrometer dimensions that exceed the lithographic resolution limit with the use of polymer sidewall spacers. The process described includes forming a stack of titanium layer, and N +  doped first poly-silicon layer, and a silicon nitride layer over the device regions. Non-volatile polymer sidewall spacers are then formed on the sidewalls of the first openings. The sidewalls and the photo-resist together act as a mask for selectively etching the said stack down to the substrate and form the second opening to define the FET channel. A gate oxide is grown on the substrate in the channel opening. The gate dielectric film is a conventional silicon dioxide grown by thermal oxidation.  
      In U.S. Pat. No. 6,033,963, a method is described to form a metal gate for a CMOS device, using a replacement gate process. According to the process, a dummy gate oxide and a poly-silicon gate electrode layer are formed and patterned to form a a dummy gate. Lightly doped source and drain regions are formed using dummy gate as implant mask. After forming sidewall spacers, source and drain regions are formed and annealed Tungsten layer is then selectively deposited on the exposed silicon surfaces. Blanket dielectric layer is then deposited and planarized, stopping on the tungsten layer. The tungsten overlying the dummy gate are removed, thereby forming a gate opening. A gate oxide layer and a metal gate electrode layer are then deposited in the gate opening and planarized, stopping on the blanket oxide layer. The structure uses a conventional oxide as the gate dielectric.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a primary object of the invention to describe a simple method to form a gate structure with high-k dielectric gate and mini-spacer surrounding the gate.  
      Another object of the invention is to describe a gate structure with high-k gate dielectric and high-k mini-spacer as an integral part of the device.  
      It is yet another object of the invention to describe a damage-free and residue-free process for forming a high-k gate dielectric and simultaneously form high-k mini-spacer as part of the gate forming process.  
      Yet another object of the invention is to describe a process for forming high-k gate dielectric and mini spacer structure, using a disposable dummy trench structure.  
      A further object of the invention is to provide an FET device with a gate structure having a high-k gate dielectric with improved control of gate length and an integral mini-spacer.  
      In accordance with these objectives, a method is described to fabricate a gate structure with high-k dielectric material, a mini-spacer of the same high-k material, and improved gate length. The high-k gate film is patterned damage-free and without leaving any process residues. A trench pattern is formed in a disposable dummy film that is deposited over a silicon substrate. A high-k dielectric film liner is then deposited within said trench pattern. The trench, with high-k film as the liner, is then filled with poly-silicon gate conductor. The structure is then planarized preferably using chemical mechanical polishing method or other etch-back processes, such that said high-k film on the planar surfaces of the dummy film is also removed during the planarizing step, thereby leaving the high-k film liner only inside the trenches. The dummy film around the gate is then removed to leave a structure with mini-spacer of high-k material surrounding the gate. Because the process uses a deposition method to form the high-k gate structure within a dummy trench, the gate length is much more precisely defined as compared to methods using etching process to pattern high-k gate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The objects, advantages, and details of fabricating a semiconductor device according to this invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.  
       FIG. 1  is a cross-sectional view of the invention, showing a dummy film and oxide film deposited over a silicon substrate.  
       FIG. 2  is a cross-sectional view of the invention showing the patterned dummy trench etched in dummy film and oxide film.  
       FIG. 3  is a cross-sectional view of the invention showing the trench pattern with a high-k dielectric film deposited on all exposed surfaces.  
       FIG. 4  is a cross-sectional view of the invention showing a structure wherein the trench pattern is filled with poly silicon which also shows a dimple over the trench.  
       FIG. 5  is a cross-sectional view of the invention showing the wafer after planarizing the poly-silicon and with high-k film removed from dummy film surface.  
       FIG. 6  is a cross-sectional view of the invention showing the final gate structure after the dummy film has been removed. The entire gate structure shows the poly gate conductor, high-k mini spacer, and high-k gate dielectric.  
       FIG. 7  is a cross-sectional view of an FET device showing source/drain regions and the high-k gate structure and mini-spacers of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The process described in the present invention provides a simplified method to fabricate a gate structure in an FET semiconductor device with a high-k gate dielectric and a mini-spacer, without producing any device damage or leaving any process residues. Although in the present invention a dummy trench is etched in nitride or organic BARC film, any material that can be easily patterned using conventional wet or plasma etching process can be used.  
      Referring now to  FIG. 1 , there is shown a substrate  10  that is silicon of desired crystallographic orientation in this embodiment having other parts of the FET device already formed (not shown) in it. A disposable dummy film  20 , is then deposited over substrate  10  using standard deposition methods such as: chemical vapor deposition (CVD), plasma enhanced CVD (PE-CVD), or spin coating known in prior art. The dummy film can be of inorganic type selected from a group of inorganic materials comprising silicon nitride, silicon oxy-nitride, silicon dioxide, inorganic arc film, amorphous silicon, doped or un-doped poly-silicon, silicon-germanium; or of organic type selected from a group of materials comprising resist, organic anti-reflection film, a polymer, and polyimide. If silicon nitride is used as the dummy film of choice to form the trenches in, a thin silicon dioxide film  21  is first deposited over substrate  10  to reduce the stress created by the thick silicon nitride film. The dummy film thickness is kept approximately at the thickness of the gate structure, of between about 1000-5000° A. Thickness of silicon dioxide used to reduce the stress of nitride film is normally in the range of about 50-150° A.  
      Referring now to  FIG. 2 , there is shown a trench pattern  22  etched in the dummy films  20  and  21 , using standard lithographic techniques and plasma etching process well known in prior art. A photo-resist mask is first formed (not shown) over the dummy film and the trench pattern formed using a selective plasma etching process such that etching stops on the substrate surface  11 . If an inorganic film is used as the dummy film, the etching process typically uses gaseous plasma containing predominantly fluorine species. Examples of such gases are: CF 4 , CHF 3 , C 2 F 6 , C 4 F 8 , NF 3 , SF 6 . If organic type films are used, the etching process uses gaseous plasma containing predominantly oxygen species. Examples of these gases are: O 2 , NO 2 , CO 2 .  
      Referring now to  FIG. 3 , there is shown said trench pattern  22  with high-k gate dielectric film  30  deposited on all of the exposed surfaces. High-k film  30  is selected from a group of materials comprising Zr x O y , Hf x O y , Ta x O y , Ti x O y , Al x O y , Hf x Si y O z , and/or Zr x Si y O z . Alternatively, gate dielectric film may also be selected from a groups of comprising silicon dioxide, silicon dioxide nitrided by decoupled plasma or nitridation process, silicon nitride, and/or silicon oxy-nitride. The high-k film, equal to the equivalent gate oxide thickness of about 20-100° A, in the range of about 50-200° A, is then deposited using chemical vapor deposition processes or other deposition methods such as sputter deposition, known in prior art such that conformal deposition takes place. Conformal deposition implies near-equal thickness of the film on vertical planes  23 , horizontal planes  24 , and surface  11 . Although in the preferred embodiment, conformal film of the high-k gate material is used, the film can also be deposited in a partially conformal manner (i.e. thickness on all the surfaces is not equal) as long as the high-k film adequately covers the step of the gate trench pattern.  
      Referring now to  FIG. 4 , there is shown the trench pattern with the preferred doped poly silicon gate conductor material  40  deposited inside the trench and over the entire planar surface. The poly-silicon thickness is so adjusted as to achieve adequate overburden so that the whole wafer is covered with poly-silicon and the dimples  41  over the trench patterns are well above the plane of the high-k film on surface  24 . The thickness of poly-silicon is normally greater than about 2000° A. Although doped poly-silicon is used as the gate conductor in the preferred embodiment, other silicon-based materials selected from the group comprising single crystal silicon, poly silicon, doped silicon, doped poly-silicon, amorphous silicon, and/or silicon-germanium and metal-based gate conductors such as, tungsten, tungsten nitride, aluminum, Ti x N y , Ta x N y , copper, WSi x , and CoSi x  can be successfully used.  
      Referring now to  FIG. 5 , there is shown the partial structure after the poly-silicon over the trench pattern has been planarized, preferably using chemical mechanical planarization (CMP) or plasma etch back processes well know in prior art. Planarization is done such that the high-k film on the horizontal surface  24  of the dummy film  20  is completely removed during this step. Incomplete removal of high-k film or residues left on surface  24  transfer themselves on the substrate and result in process residues when the dummy film is stripped off in a later and final step.  
      Referring now to  FIG. 6 , there is shown the final gate structure  50  after the dummy film  20  adjacent to the gate structure has been stripped off using plasma or wet etching process. When the dummy film is of the inorganic type, wet or suitable plasma etching processes selective to the substrate  10  are used. When the dummy film is of the organic type, organic solvent strip or suitable ashing processes such as oxygen plasma, UV/ozone, or microwave plasma are used. The final structure in  FIG. 6 , shows the gate conductor of poly silicon  40 , high-k gate dielectric  30   b  and high-k mini-spacer  30   a  surrounding the gate conductor, with the entire gate structure fabricated on silicon substrate  10 .  
       FIG. 7  shows the FET device, showing the source  42  and drain  44  regions and the gate structure of the invention: high-k gate dielectric  30   b , poly-silicon gate conductor  40 , and high-k mini-spacer  30   a.    
      The advantages of the aforementioned invention are: 
          a) A gate structure is formed with high-k gate dielectric and high-k mini spacer, both formed simultaneously without the use of additional steps.     b) A non-etching process is used for forming the high-k gate dielectric pattern and the mini-spacers, thereby not producing any etch residues.     c) High-k gate structure is patterned without any etch damage since plasma etching, which is inherently prone to producing device damage, is not directly used to etch the high-k film. Only the dummy film patterning uses the plasma etching process.     d) High-k gate pattern is formed by a deposition process and not by plasma etching.        

      While the invention has been particularly shown and described with reference to the preferred embodiment 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 concept, spirit, and the scope of the invention.