Abstract:
Provided is a LOCOS offset MOS field-effect transistor in which a first lightly-doped N-type drain offset region with a LOCOS oxide film and a second lightly-doped N-type drain offset region without a LOCOS oxide film are formed in a drain-side offset region, and both the regions are covered with a gate electrode. Provision of the first lightly-doped N-type drain offset region mitigates an electric field applied to the first lightly-doped N-type drain offset region to increase a breakdown voltage. Provision of the second lightly-doped N-type drain offset region increases carriers within the second lightly-doped N-type drain offset region to obtain a high current drivability.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor device. More specifically, the present invention relates to a LOCOS offset field-effect transistor having a high breakdown voltage and a high current drivability. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 2  illustrates an example of a conventional N-channel LOCOS offset MOS field-effect transistor having a high breakdown voltage structure. An N-channel LOCOS offset MOS field-effect transistor  101  includes a P-type silicon substrate  16 , a P-type well region  17 , a lightly-doped N-type source LOCOS offset region  18 , a lightly-doped N-type drain LOCOS offset region  19 , a heavily-doped N-type source region  20 , a heavily-doped N-type drain region  21 , a channel formation region  22 , a gate oxide film  23 , a gate electrode  24 , LOCOS oxide films  25 , a protective oxide film  26 , a source electrode  27 , a drain electrode  28 , and the like. As illustrated in  FIG. 2 , features of the MOS field-effect transistor  101  reside in that the lightly-doped N-type drain LOCOS offset region  19  is formed between the channel formation region  22  and the heavily-doped N-type drain region  21  for the purpose of increasing a breakdown voltage, and in that the LOCOS oxide films  25  are each formed to be as thick as 5,000 Å to 10,000 Å for the purpose of preventing a channel formation in a parasitic field transistor formed between elements. In general a drain breakdown voltage of a MOS field-effect transistor having a large channel length is determined as a voltage at which an avalanche breakdown occurs in a portion to which the largest electric field is applied in a depletion layer formed at a boundary between the channel formation region and the drain region, that is, a surface portion which is the most sensitive to a gate potential. The reason for a high drain breakdown voltage of the MOS field-effect transistor  101  is that a bird&#39;s beak of the LOCOS oxide film  25  is positioned in the vicinity of the boundary surface between the channel formation region  22  and the offset region  19 , alleviating the influence of the gate potential so that an avalanche breakdown may less likely occur. 
         [0005]    Further reduction of a dopant concentration of the offset region  19  to increase a width of the depletion layer to obtain a higher breakdown voltage leads to an increase of the resistance of the offset region  19 , causing a generation of Joule heat in the offset region  19  to break down the element at a turning on of the transistor to get a large drain current. There is a trade-off relationship between a high breakdown voltage and a current drivability. 
         [0006]    In view of the above-mentioned problem, Japanese Patent Application Laid-open No. H 11-26766 proposes the following method. Japanese Patent Application Laid-open No. H11-26766 discloses a method of optimizing a film thickness of a LOCOS oxide film to a film thickness satisfying the following two conditions. The first condition is a film thickness condition as to whether to suppress the above-mentioned influence of the gate potential on the avalanche breakdown. The second condition is a film thickness condition as to whether or not the gate potential may allow the surface of the lightly-doped drain LOCOS offset region to enter an accumulated state. If the film thickness of the LOCOS oxide film is set to an optimum film thickness, a high breakdown voltage element having a high current drivability may be produced. 
         [0007]    In a case where the above-mentioned conventional example is utilized to produce a high breakdown voltage element having a high current drivability, because the above-mentioned two conditions are inherently in a trade-off relationship, it is difficult to select an optimum film thickness satisfying the two conditions simultaneously. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a LOCOS offset MOS field-effect transistor having a high breakdown voltage in which a first lightly-doped drain offset region with a LOCOS oxide film and a second lightly-doped drain offset region without a LOCOS oxide film are formed in a drain-side offset region, and both the regions are covered with a gate electrode. Specifically, the following means is employed. 
         [0009]    The present invention provides a semiconductor device including: a first conductivity type semiconductor substrate; a first conductivity type well region formed in a surface of the first conductivity type semiconductor substrate; a second conductivity type well region formed in contact with the first conductivity type well region; a heavily-doped second conductivity type source region formed at a top of the first conductivity type well region; a channel formation region; a lightly-doped second conductivity type source offset region formed in contact with the heavily-doped second conductivity type source region so as to be spaced away from the second conductivity type well region by a length of the channel formation region; a heavily-doped second conductivity type drain region formed at a top of the second conductivity type well region; a second lightly-doped second conductivity type drain offset region formed in contact with the heavily-doped second conductivity type drain region on a side of the channel formation region; a first lightly-doped second conductivity type drain offset region formed at the top of the second conductivity type well region in contact with the channel formation region and the second lightly-doped second conductivity type drain offset region; a LOCOS oxide film formed in a surface portion of the first conductivity type semiconductor substrate except for the heavily-doped second conductivity type source region, the channel formation region, the second lightly-doped second conductivity type drain offset region, and the heavily-doped second conductivity type drain region; a gate oxide film which is formed on: a part of the LOCOS oxide film formed in contact with the channel formation region on a source side; the channel formation region; an entirety of the LOCOS oxide film formed in contact with the channel formation region on a drain side; and the second lightly-doped second conductivity type drain offset region; a gate electrode formed on the gate oxide film; a source electrode formed on the heavily-doped second conductivity type source region; a drain electrode formed on the heavily-doped second conductivity type drain region; and a protective oxide film formed over the surface of the first conductivity type semiconductor substrate except for the source electrode and the drain electrode. 
         [0010]    In the drain-side offset region, the first lightly-doped drain offset region with the LOCOS oxide film and the second lightly-doped drain offset region without the LOCOS oxide film are formed so that the first lightly-doped drain offset region may mitigate a magnitude of an electric field applied to the first lightly-doped drain offset region, to thereby produce a high breakdown voltage MOS field-effect transistor. In addition, the second lightly-doped drain offset region without the LOCOS oxide film is formed so that an electric field may be applied from the gate electrode formed above the second lightly-doped drain offset region to allow the second lightly-doped drain offset region to enter an accumulated state. As a result, carrier density of the second lightly-doped drain offset region may be increased with the gate voltage remaining large, to thereby enhance a current drivability as well. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the accompanying drawings: 
           [0012]      FIG. 1  is a cross-sectional view of a semiconductor device according to a first embodiment of the present invention; 
           [0013]      FIG. 2  is a cross-sectional view of a semiconductor device in a conventional MOS field-effect transistor; and 
           [0014]      FIG. 3  is a cross-sectional view of a semiconductor device according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Now, referring to the accompanying drawings, exemplary embodiments of the present invention are described. 
       First Embodiment 
       [0016]      FIG. 1  is a cross-sectional view of a semiconductor device  100  according to a first embodiment of the present invention. Herein, an N-channel MOS transistor is described by way of example. The semiconductor device  100  of  FIG. 1  has the following exemplary structure. In a surface of a P-type silicon substrate  1  having a resistance of 20 to 30 Ω·cm, a lightly-doped P-type well region  2  is formed at a depth of 20 μm with boron or the like doped at a concentration of approximately 1×10 16  cm −3 , and a lightly-doped N-type well region  3  is formed in contact with the P-type well region  2  at a depth of 20 μm with phosphorus or the like doped at a concentration of approximately 1×10 16  cm −3 . 
         [0017]    Next, using a resist pattern as a mask, ion implantation is performed to form a lightly-doped N-type source offset region  4  at a depth of 1 μm with phosphorus or the like doped at approximately 5×10 17  cm −3 . In addition, using a resist pattern as a mask, ion implantation is performed to form a lightly-doped N-type drain offset region  5  at a depth of 1 μm with phosphorus or the like doped at approximately 5×10 17  cm −3 . Then, selective oxidation is performed to form a thermal oxide film of approximately 8,000 Å thickness on each of the lightly-doped N-type source offset region  4  and the lightly-doped N-type drain offset region  5  so as to grow as a LOCOS oxide film  12 . Subsequently, using a resist pattern as a mask, ion implantation is performed to form another lightly-doped N-type drain offset region  6  at a depth of 1 μm with phosphorus or the like doped at approximately 5×10 17  cm −3 . 
         [0018]    Subsequently, thermal oxidation is performed to form a gate oxide film  10  of approximately 1,000 Å thickness on the silicon surface. Subsequently, chemical vapor deposition (CVD) is performed to form a polycrystalline silicon film of approximately 4,000 Å thickness over the gate oxide film  10 . Then, phosphorus or the like is doped and diffused into the polycrystalline silicon film at approximately 1×10 20  cm −3 . Then, a resist pattern is formed and dry etching is performed to form a gate electrode  11  so as to cover a range from a part of the LOCOS oxide film  12  formed on the lightly-doped N-type source offset region  4  to the lightly-doped N-type drain offset region  6  through a channel formation region  9  and the lightly-doped N-type drain offset region  5 . 
         [0019]    Subsequently, using a resist pattern as a mask, ion implantation is performed to dope the silicon surface with arsenic or the like at approximately 1×10 20  cm −3 , to thereby form a heavily-doped N-type source region  7  and a heavily-doped N-type drain region  8  at a depth of 0.4 μm. Subsequently, a protective oxide film  13  is formed at a thickness of approximately 7,000 Å by CVD or the like. Subsequently, an opening is formed in the protective oxide film  13  at a position on each of the heavily-doped N-type source region  7  and the heavily-doped N-type drain region  8 . Then, an aluminum alloy is deposited therein and pattered to form a source electrode  14  on the heavily-doped N-type source region  7  and a drain electrode  15  on the heavily-doped N-type drain region  8 . 
         [0020]    With the above-mentioned structure, in the drain-side offset region, the first lightly-doped drain offset region with the LOCOS oxide film and the second lightly-doped drain offset region without the LOCOS oxide film are formed so that the first lightly-doped drain offset region may mitigate a magnitude of an electric field applied to the first lightly-doped drain offset region, to thereby produce a high breakdown voltage MOS field-effect transistor. In addition, the second lightly-doped drain offset region without the LOCOS oxide film is formed so that an electric field may be applied from the gate electrode formed above the second lightly-doped drain offset region to allow the second lightly-doped drain offset region to enter an accumulated state. As a result, carrier density of the second lightly-doped drain offset region may be increased with the gate voltage remaining large, to thereby enhance a current drivability as well. 
       Second Embodiment 
       [0021]      FIG. 3  is a cross-sectional view of a semiconductor device  102  according to a second embodiment of the present invention. The semiconductor device  102  of  FIG. 3  has the following exemplary structure. In a surface of a P-type silicon substrate  29  having a resistance of 20 to 30 Ω·cm, a lightly-doped P-type well region  30  is formed at a depth of 20 μm with boron or the like doped at a concentration of approximately 1×10 16  cm −3 , and a lightly-doped N-type well region  31  is formed in contact with the P-type well region  30  at a depth of 20 μm with phosphorus or the like doped at approximately 1×10 17  cm −3 . Next, using a resist pattern as a mask, ion implantation is performed to form a lightly-doped N-type source offset region  32  at a depth of 1 μm with phosphorus or the like doped at approximately 5×10 17  cm −3  in a region at the top of the P-type well region  30  which is spaced away from the N-type well region  31  by a length of a channel formation region  43 . 
         [0022]    Subsequently, selective oxidation is performed to form a thermal oxide film of approximately 8,000 Å thickness on each of the lightly-doped N-type source offset region  32  and a first drain offset region  33  so as to grow as a LOCOS oxide film  35 . In this case, an available method of forming a second drain offset region  34  is as follows. First, selective oxidation is performed to form the thermal oxide film of approximately 8,000 Å thickness on each of the lightly-doped N-type source offset region  32 , the first drain offset region  33 , and the second drain offset region  34  so as to grow as the LOCOS oxide film  35 . Then, using a photoresist, wet etching is performed to remove the LOCOS oxide film on the second drain offset region  34 , and thermal oxidation is subsequently performed to form a gate oxide film  36  of approximately 1,000 Å thickness on the silicon surface. 
         [0023]    Subsequently, CVD is performed to form a polycrystalline silicon film of approximately 4,000 Å thickness over the gate oxide film  36 . Then, phosphorus or the like is doped and diffused into the polycrystalline silicon film at approximately 1×10 20  cm −3 . Then, a resist pattern is formed and dry etching is performed to form a gate electrode  37  so as to cover a range from a part of the LOCOS oxide film  35  formed on the lightly-doped N-type source offset region  32  to the second drain offset region  34 . Subsequently, using a resist pattern as a mask, ion implantation is performed to dope the silicon surface with arsenic or the like at approximately 1×10 20  cm −3 , to thereby form a heavily-doped N-type source region  38  and a heavily-doped N-type drain region  39  at a depth of 0.4 μm. 
         [0024]    Subsequently, a protective oxide film  40  is formed at a thickness of approximately 7,000 Å by CVD or the like. Subsequently, an opening is formed in the protective oxide film  40  at a position on each of the heavily-doped N-type source region  38  and the heavily-doped N-type drain region  39 . Then, an aluminum alloy is deposited therein and pattered to form a source electrode  41  on the heavily-doped N-type source region  38  and a drain electrode  42  on the heavily-doped N-type drain region  39 . 
         [0025]    It should be understood that the structure according to the second embodiment can also produce the same effect as in the first embodiment.