Abstract:
A method for fabricating metal-oxide-semiconductor devices is provided. The method includes forming a gate dielectric layer on a substrate; depositing a polysilicon layer on the gate dielectric layer; forming a resist mask on the polysilicon layer; etching the polysilicon layer not masked by the resist mask, thereby forming a gate electrode; etching a thickness of the gate dielectric layer not covered by the gate electrode; stripping the resist mask; forming a salicide block resist mask covering the gate electrode and a portions of the remaining gate dielectric layer; etching away the remaining gate dielectric layer not covered by the salicide block resist mask, thereby exposing the substrate and forming a salicide block lug portions on two opposite sides of the gate electrode; and making a metal layer react with the substrate, thereby forming a salicide layer that is kept a distance “d” away from the gate electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. application Ser. No. 10/908,784 filed May 26, 2005 now U.S. Pat. No. 7,118,954. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the fabrication of semiconductor integrated circuits and, more particularly, to an improved process for fabricating high-voltage devices. According to the present invention, the salicide process is integrated with the high-voltage process, thereby reducing the resistance of high-voltage metal-oxide-semiconductor transistor devices. 
   2. Description of the Prior Art 
   Integrated circuits (ICs) containing both high-voltage and low-voltage devices such as high/low voltage MOS transistor devices are known in the art. For example, the low-voltage device may be used in the control circuits as the high-voltage device may be used in electrically programmable read only memory (EPROM) or the driving circuits of the liquid crystal display devices. 
   It is also known that self-aligned silicide (also referred to as “salicide”) process is typically utilized to form metal silicide layer such as cobalt silicide or titanium silicide on the gates, source or drain regions in order to reduce sheet resistances. However, the salicide process is merely performed on the low-voltage devices. Considering hot carrier effects, the conventional high-voltage process cannot integrate with the salicide process. As a result, the sheet resistance of the high-voltage devices is high. 
   In light of the above, there is a need to provide an improved method for reducing the sheet resistance of the high-voltage devices. 
   SUMMARY OF THE INVENTION 
   It is the primary object of the present invention to provide an improved high-voltage process for fabricating high-voltage metal-oxide-semiconductor (MOS) devices, thereby reducing the sheet resistance thereof. 
   According to the claimed invention, a method for fabricating metal-oxide-semiconductor (MOS) devices is disclosed. A gate dielectric layer having a first thickness is formed or grown on a semiconductor substrate. A polysilicon layer is deposited on the gate dielectric layer. A resist mask is formed on the polysilicon layer. The polysilicon layer not masked by the resist mask is etched away, thereby forming a gate electrode. The gate dielectric layer not covered by the gate electrode is then etched such that remaining gate dielectric layer not covered by the gate electrode has a second thickness that is smaller than the first thickness. The resist mask is stripped. A spacer is formed on the sidewalls of the gate electrode and on remaining gate dielectric layer. A salicide block resist mask is formed to cover the gate electrode, the spacer and a portions of remaining the gate dielectric layer laterally protruding an offset “d” from bottom of the gate electrode. The remaining gate dielectric layer not covered by the salicide block resist mask is completely removed, thereby exposing the semiconductor substrate and forming a salicide block lug portions on two opposite sides of the gate electrode with the offset “d” from sidewalls of the gate electrode. The spacer has a maximum thickness that is smaller than the offset “d” such that the salicide block lug portions laterally protruding from bottom of the spacer and forms a step thereto. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  to  FIG. 9  are schematic cross-sectional diagrams showing major intermediate stages in the process of fabricating high- and low-voltage MOS transistor devices in accordance with one preferred embodiment of the present invention. 
       FIG. 10  to  FIG. 18  are schematic cross-sectional diagrams showing major intermediate stages in the process of fabricating high- and low-voltage MOS transistor devices in accordance with another preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 1  to  FIG. 9 .  FIG. 1  to  FIG. 9  are schematic cross-sectional diagrams showing major intermediate stages in the process of fabricating high- and low-voltage MOS transistor devices in accordance with one preferred embodiment of the present invention. As shown in  FIG. 1 , a semiconductor substrate  10  is prepared. The semiconductor substrate  10  comprises a low-voltage device area  102  and a high-voltage device area  104 . Within the low-voltage device area  102 , low-voltage devices such as low-voltage (5V, 3.3V or lower) MOS transistors are fabricated. Within the high-voltage device area  104 , high-voltage devices such as high-voltage (12V or even higher) MOS transistors are fabricated. Initially, isolation structures  12  such as shallow trench isolation (STI) and active areas are defined on the semiconductor substrate  10  both in the low-voltage device area  102  and high-voltage device area  104 . 
   As shown in  FIG. 2 , a low-voltage gate dielectric  22  and a high-voltage gate dielectric  24  are formed on the surface of the semiconductor substrate  10  within the low-voltage device area  102  and high-voltage device area  104 , respectively. Techniques of forming gate dielectrics with two different thicknesses are known in the art, and are not discussed further. According to the preferred embodiment, the low-voltage gate dielectric  22  has a thickness that is less than 200 angstroms, preferably less than or equal to 100 angstroms, while the high-voltage gate dielectric  24  has a thickness that is thicker than 300 angstroms, preferably thicker than 600 angstroms. 
   As shown in  FIG. 3 , a polysilicon layer  30  is deposited on the low-voltage gate dielectric  22  and on the high-voltage gate dielectric  24 . A photoresist mask  42  and photoresist mask  44  are defined on the polysilicon layer  30 , wherein the photoresist mask  42  defines the gate pattern of a low-voltage MOS transistor device within the low-voltage device area  102 , while the photoresist mask  44  defines the gate pattern of a high-voltage MOS transistor device within the high-voltage device area  104 . 
   Subsequently, as shown in  FIG. 4 , using the photoresist masks  42  and  44  as an etching hard mask, a plasma dry etching is carried out to etched away the polysilicon layer  30  that is not covered by the photoresist masks  42  and  44 , thereby forming a gate electrode  32  of the low-voltage MOS transistor device and gate electrode  34  of the high-voltage MOS transistor device. The low-voltage dielectric  22  outside the gate electrode  32  is etched away to expose the semiconductor substrate  10 . The aforesaid plasma dry etching is not terminated until a predetermined thickness of the thicker high-voltage dielectric  24  is removed. At this phase, the remaining high-voltage dielectric  24  still covers the high-voltage device area  104 . 
   As shown in  FIG. 5 , a layer of photoresist (not explicitly shown) is coated over the semiconductor substrate  10 , and is then exposed and developed using conventional lithography to form photoresist mask  52  and photoresist mask  54 . The photoresist mask  52  covers the entire low-voltage device area  102 , while the photoresist mask  54  merely masks the gate electrode  34  and a portions of the remaining high-voltage dielectric  24  laterally protruding an offset “d” from the bottom of the gate electrode  34 . The offset “d” is substantially equal to the distance between the gate electrode  34  and the source/drain salicide formed in the subsequent processes. 
   As shown in  FIG. 6 , using the photoresist mask  52  and photoresist mask  54  as a hard mask, a plasma dry etching is carried out to etch away the remaining high-voltage dielectric  24  that is not covered by the photoresist mask  54 . Thereafter, the photoresist mask  52  and photoresist mask  54  are stripped off. The remaining high-voltage dielectric  24  that is not directly under the gate electrode  34  is hereinafter referred to as lug portions  24   a  that are formed on two opposite sides of the gate electrode  34  with an offset “d” from the gate sidewalls. According to the preferred embodiment, the lug portions  24   a  have a thickness of about 100˜600 angstroms, and the offset “d” is in a range of about 0.4˜2.0 micrometers. 
   As shown in  FIG. 7 , a spacer dielectric layer  60  such as silicon nitride is deposited over the semiconductor substrate  10 . Next, as shown in  FIG. 8 , an isotropic dry etching is carried out to etch the spacer dielectric layer  60 , thereby forming spacers  62  and  64  on sidewalls of respective gate electrodes  32  and  34 . Conventional ion implantation process is then performed to form source/drain regions  72  within the low-voltage device area  102  and source/drain regions  74  within the low-voltage device area  104 . After the implantation of source/drain regions, a typical salicide process is carried out. A metal layer  80  such as cobalt or titanium is deposited over the semiconductor substrate  10 . The metal layer  80  covers both the low-voltage device area  102  and high-voltage device area  104 . It is one feature of the present invention that the lug portions  24   a  function as a salicide block that keeps the metal layer  80  from contacting the substrate within the offset area directly under the lug portions  24   a.    
   Finally, as shown in  FIG. 9 , a thermal process is performed. The source/drain regions  72  and  74  that are in contact with the metal layer  80  react with the overlying metal layer  80  to form metal salicide layers  82   a  and  84   a . Simultaneously, metal salicide layers  82   b  and  84   b  are formed on the exposed gate electrodes  32  and  34 . 
     FIG. 10  to  FIG. 18  are schematic cross-sectional diagrams showing major intermediate stages in the process of fabricating high- and low-voltage MOS transistor devices in accordance with another preferred embodiment of the present invention. As shown in  FIG. 10 , likewise, the semiconductor substrate  10  comprises a low-voltage device area  102  and a high-voltage device area  104 . Within the low-voltage device area  102 , low-voltage devices such as low-voltage (5V, 3.3V or lower) MOS transistors are fabricated. Within the high-voltage device area  104 , high-voltage devices such as high-voltage (12V or even higher) MOS transistors are fabricated. Initially, isolation structures  12  such as shallow trench isolation (STI) and active areas are defined on the semiconductor substrate  10  both in the low-voltage device area  102  and high-voltage device area  104 . 
   As shown in  FIG. 11 , a low-voltage gate dielectric  22  and a high-voltage gate dielectric  24  are formed on the surface of the semiconductor substrate  10  within the low-voltage device area  102  and high-voltage device area  104 , respectively. According to the preferred embodiment, the low-voltage gate dielectric  22  has a thickness that is less than 200 angstroms, preferably less than or equal to 100 angstroms, while the high-voltage gate dielectric  24  has a thickness that is thicker than 300 angstroms, preferably thicker than 600 angstroms. 
   As shown in  FIG. 12 , a polysilicon layer  30  is deposited on the low-voltage gate dielectric  22  and on the high-voltage gate dielectric  24 . A photoresist mask  42  and photoresist mask  44  are defined on the polysilicon layer  30 , wherein the photoresist mask  42  defines the gate pattern of a low-voltage MOS transistor device within the low-voltage device area  102 , while the photoresist mask  44  defines the gate pattern of a high-voltage MOS transistor device within the high-voltage device area  104 . 
   Subsequently, as shown in  FIG. 13 , using the photoresist masks  42  and  44  as an etching hard mask, a plasma dry etching is carried out to etched away the polysilicon layer  30  that is not covered by the photoresist masks  42  and  44 , thereby forming a gate electrode  32  of the low-voltage MOS transistor device and gate electrode  34  of the high-voltage MOS transistor device. The low-voltage dielectric  22  outside the gate electrode  32  is etched away to expose the semiconductor substrate  10 . The aforesaid plasma dry etching is not terminated until a predetermined thickness of the thicker high-voltage dielectric  24  is removed. At this phase, the remaining high-voltage dielectric  24  still covers the high-voltage device area  104 . 
   As shown in  FIG. 14 , spacer  62  and spacer  64  are formed on sidewalls of gate electrodes  32  and  34 , respectively. One difference between this embodiment and previous embodiment is that in this embodiment the spacers  62  and  64  are formed prior to the formation of the lug portions  24   a.    
   As shown in  FIG. 15 , a layer of photoresist (not explicitly shown) is coated over the semiconductor substrate  10 , and is then exposed and developed using conventional lithography to form photoresist mask  52  and salicide block photoresist mask  54 . The photoresist mask  52  covers the entire low-voltage device area  102 , while the salicide block photoresist mask  54  merely masks the gate electrode  34 , spacer  64  and a portions of the remaining high-voltage dielectric  24  laterally protruding an offset “d” from the bottom of the gate electrode  34 . The offset “d” is substantially equal to the distance between the gate electrode  34  and the source/drain salicide to be formed in the subsequent processes. 
   As shown in  FIG. 16 , using the photoresist mask  52  and salicide block photoresist mask  54  as a hard mask, a plasma dry etching is carried out to etch away the remaining high-voltage dielectric  24  that is not covered by the salicide block photoresist mask  54 , thereby forming lug portions  24   a . The spacer  64  has a maximum thickness that is smaller than the offset “d” such that the salicide block lug portions  24   a  laterally protruding from bottom of the spacer  64  and forms a step thereto. 
   Thereafter, the photoresist mask  52  and photoresist mask  54  are stripped off. The lug portions  24   a  are formed on two opposite sides of the gate electrode  34  and protruding with an offset “d” from bottom of the gate sidewalls. According to the preferred embodiment, the lug portions  24   a  have a thickness of about 100˜600 angstroms, and the offset “d” is in a range of about 0.4˜2.0 micrometers. 
   As shown in  FIG. 17 , ion implantation processes are performed to form source/drain regions  72  within the low-voltage device area  102  and source/drain regions  74  within the low-voltage device area  104 . After the implantation of source/drain regions, likewise, a typical salicide process is carried out. A metal layer  80  such as cobalt or titanium is deposited over the semiconductor substrate  10 . The metal layer  80  covers both the low-voltage device area  102  and high-voltage device area  104 . The lug portions  24   a  function as a salicide block that keeps the metal layer  80  from contacting the substrate within the offset area directly under the lug portions  24   a.    
   Finally, as shown in  FIG. 18 , a thermal process is performed. The source/drain regions  72  and  74  that are in contact with the metal layer  80  react with the overlying metal layer  80  to form metal salicide layers  82   a  and  84   a . Simultaneously, metal salicide layers  82   b  and  84   b  are formed on the exposed gate electrodes  32  and  34 . 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.