Abstract:
A CMOS integrated circuit includes an NMOS transistor and a PMOS transistor in an integrated circuit substrate. The NMOS transistor and the PMOS transistor each include a gate, and a source/drain on opposing sides of the gate. An insulating layer is located on the integrated circuit substrate. The insulating layer includes a contact hole therein which exposes a portion of a corresponding one of the source/drains. A source/drain plug is formed in the corresponding one of the source/drains. The source/drain plug is of opposite conductivity from the corresponding one of the source/drains. The source/drain plug is centered about the portion of the corresponding one of the source/drains. The source/drain plug may be formed by ion implantation through the contact hole and is thereby self-aligned to the contact hole. The source/drain plug can compensate for misalignment and the diffusion for highly integrated CMOS devices.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to microelectronic devices and fabrication methods therefor, and more particularly to CMOS integrated circuits and fabrication methods therefor.  
         BACKGROUND OF THE INVENTION  
         [0002]    CMOS integrated circuits are widely used for logic, memory, microprocessor and other microelectronic devices. As is well known to those having skill in the art, CMOS integrated circuits include insulated gate field effect transistors of complementary conductivity types, often referred to as NMOS transistors and PMOS transistors, on a single integrated circuit. Although the acronym MOS actually stands for metal oxide silicon, it will be understood that this term as used herein refers to any insulated gate field effect transistor.  
           [0003]    As the integration density of integrated circuit devices continues to increase, it may become more difficult to ensure that CMOS devices can be reliably manufactured. For example, as the integration density of an integrated circuit device increases, and the design rules for fabricating the integrated circuit device continue to decrease, the size of a contact hole may extend into submicron dimensions. With such small dimensions, it may become difficult to ensure adequate overlap margins and reliable fabrication methods. Accordingly, there continues to be a need for CMOS integrated circuits and fabrication methods which can reduce the likelihood of overlap between regions thereof, notwithstanding decreasing device dimensions.  
           [0004]    [0004]FIG. 1 is a schematic plan view of a conventional CMOS integrated circuit. Referring to FIG. 1, reference numerals P 1  and P 2  denote mask patterns for forming N-type and P-type active regions respectively. P 3  and P 4  denote mask patterns for forming the gates of NMOS and PMOS transistors, respectively. P 5  and P 6  denote mask patterns for forming contact holes connecting the source/drains of the NMOS or PMOS transistors to a wiring layer, respectively. Reference numeral  1  denotes the distance between semiconductor devices, x 1  denotes the overlap margin between a gate and a contact hole, and x 2  and y denote overlap margins of an active region with respect to a contact hole in the direction of the x and y axis, respectively.  
           [0005]    The decrease in design rules due to high integration of integrated circuit devices may cause a decrease of the overlap margin between an active region and a contact hole. Moreover, the x 1 , x 2 , and y values may decrease to under 0.1 μm for a DRAM of over 64 Mbits.  
           [0006]    Techniques have been proposed in attempts to overcome these problems. For example, the contact hole size may be reduced to thereby improve overlap margin. The distance between integrated circuit devices may be decreased, allowing more space for contact holes. The overlap margin (x 1 ) between a gate and a contact hole or the overlap margins (x 2 , y) of the active region may also be decreased.  
           [0007]    Unfortunately, these techniques may require a misalignment of less than 0.1 μm, which may make it difficult to adopt these techniques for high volume fabrication. Moreover, decreasing contact hole size may also increase the contact resistance, which may deteriorate device operating speed. The aspect ratio of a contact hole may also increase, which may make it difficult to fill the contact hole.  
           [0008]    [0008]FIGS. 2A through 2D are cross-sectional views showing a conventional manufacturing method of a CMOS integrated circuit whose NMOS source/drain includes a lightly doped drain (LDD) structure. Referring to FIG. 2A, a field oxide  4  for separating active regions and inactive regions is formed on the surface of an integrated circuit substrate such as a semiconductor substrate  2 , and then an N-well  6  and a P-well are formed using a conventional well forming process. A gate insulating layer  8  is then formed on the substrate  2 , and impurity-doped polysilicon is deposited thereon and patterned to form a gate electrode  10 .  
           [0009]    Referring to FIG. 2B, a low concentration of N-type impurity ions are implanted into the semiconductor substrate  2  using the gate electrode  10  as a mask, in order to form an N −  source/drain  12 . This implant step generally is not limited to the NMOS transistor area in order to reduce the number of process steps. As a result, the N −  source/drain  14  in the N-well  6  can suppress short channel effects of the PMOS transistor.  
           [0010]    An insulating material then is deposited on the substrate  2  and is patterned in order to form an insulating layer  16 . Then, a high concentration of impurity ions are implanted into the NMOS and PMOS transistor areas of the semiconductor substrate using the insulating layer  16  and photoresist patterns for NMOS and PMOS (not shown in the FIG. 2B) as a mask, in order to form a N +  source/drain  18  and P +  source/drain  19 .  
           [0011]    Referring to FIG. 2C, an inter-layer insulating layer  20  having a predetermined thickness is formed by depositing an insulating material such as a high temperature oxide (HTO) on the substrate. Then, contact holes  22  for connecting the source/drains of the NMOS and PMOS transistors to a wiring layer are formed by selectively removing the inter-layer-insulating layer  20 .  
           [0012]    Referring to FIG. 2D, a wiring layer  24  is formed by depositing and patterning a conductive material on substrate. The wiring layer  24  electrically contacts the source/drain regions to complete the CMOS structure.  
           [0013]    The LDD structure has been used in NMOS transistors to improve reliability. The LDD structure is now often employed in PMOS transistors as well. When device dimensions were larger, it generally was easier to use the LDD structure in the N −  or P −  source/drains because the distance between CMOS devices was large enough. However, as CMOS devices become more highly integrated, and the margin between the N −  source/drain and the P-well, or between the P −  source/drain and the N-well, has generally decreased to under 1 μm, a mere 0.15 μm misalignment of the N-well or P-well can affect operation of the CMOS devices.  
           [0014]    Defects in CMOS devices due to the decreased distance between devices will be described below with reference to FIGS. 3, 3A,  4 , and  4 A. FIG. 3 is a cross-sectional view showing the N −  source/drain of a CMOS transistor electrically connected to the N-well thereof, which generally causes malfunction of the CMOS device. FIG. 3A is an enlarged magnified view of the marked portion shown in FIG. 3.  
           [0015]    Referring to FIG. 3, the N-type impurities implanted in the N −  source/drain  12  of the NMOS transistor, or in the N-well  6 , are diffused in the lateral direction by subsequent thermal processes. As a result, the N −  source/drain  12  may come into contact with the N-well  6 .  
           [0016]    In particular, if the impurity concentration in the N −  source/drain is 2.0×10 13  ion/cm 2  and the implantation energy is 30 KeV, annealing at 850° C. after forming the N −  source/drain may cause the impurities in the N −  source/drain to diffuse toward the N-well  6  by 0.25 μm. Thus, when the distance 1 between semiconductor devices (see FIG. 1) is 1.0 μm, such diffusion may encompass one fourth of the process margin. Moreover, if the N-well also includes impurities with concentration of 2.0×10 13  ion/cm 2  therein, the impurities in the N-well may diffuse toward the N −  source/drain by 0.25 μm during a subsequent anneal. Thus, when the impurities in the N −  source/drain  12  and the N-well  6  are diffused at the same time, causing the distance therebetween to decrease to 0.5 μm, the N −  source/drain and the N-well may come into contact with each other. When the N −  drain is the output terminal V OUT  and the voltage of the N +  region providing well bias to the N-well is V DD , the output voltage V out  is equal to V DD . Thus, improper operation may occur.  
           [0017]    [0017]FIG. 4 is a cross-sectional view showing misalignment of the P +  source/drain of a CMOS transistor which can cause malfunction of CMOS devices. FIG. 4A is a enlarged view of the marked portion shown in FIG. 4.  
           [0018]    Referring to FIGS. 4 and 4A, when P-type ions are implanted for forming the P +  source/drain at a high concentration after the implantation of N-type ions at a low concentration into the NMOS and PMOS transistors, P +  source/drain misalignment may prevent the P-type ions from completely compensating for the implanted N-type ions. Thus, when the wiring layer is formed thereon and voltage is supplied thereto, the remaining N −  source/drain  12  may contact the N-well  6 . Thus, the back bias voltage V BB . of the NMOS transistor may be combined with VDD of the PMOS transistor. A mere 0.1 μm misalignment of the P +  domain can cause malfunction of a CMOS device and degrade the reliability thereof. Accordingly, there is a need for CMOS integrated circuit structures and fabrication methods which can be highly integrated without undue reliability concerns.  
         SUMMARY OF THE INVENTION  
         [0019]    It is therefore an object of the present invention to provide improved CMOS integrated circuits and fabrication methods therefor.  
           [0020]    It is another object of the present invention to provide CMOS integrated circuits and fabricating methods which can allow highly integrated devices to be fabricated without undue reliability concerns.  
           [0021]    It is yet another object of the present invention to provide CMOS integrated circuit devices and fabricating methods which can provide adequate overlap margins notwithstanding shrinking device sizes.  
           [0022]    These and other objects are provided according to the present invention by CMOS integrated circuits which include at least one source/drain plug of opposite conductivity from a corresponding source/drain and centered about the corresponding source/drain. The source/drain plug can shield the source/drain from the adjacent well, notwithstanding process margins and shrinking device sizes.  
           [0023]    In particular, CMOS integrated circuits according to the invention include an integrated circuit substrate, and an NMOS transistor and a PMOS transistor in the integrated circuit substrate. The NMOS transistor and the PMOS transistor each include a gate, and a source/drain on opposing sides of the gate. An insulating layer is included on the integrated circuit substrate. The insulating layer includes a contact hole therein. The contact hole exposes a portion of a corresponding one of the source/drains. A source/drain plug is included in the corresponding one of the source/drains. The source/drain plug is of opposite conductivity from the corresponding one of the source/drains, and is centered about the portion of the corresponding one of the source/drains. A wiring layer is located in the contact hole and is electrically connected to the source/drain plug.  
           [0024]    A second contact hole may also be included in the insulating layer to expose a portion of the corresponding second one of the source/drains. A second source/drain plug may be included in the corresponding second one of the source/drains. The second source/drain plug is of opposite conductivity from the corresponding second one of the source/drains, and is centered about the portion of the corresponding second one of the source/drains. A second wiring layer is located in the second contact hole, electrically connected to the second source/drain plug.  
           [0025]    It will be understood that the two source/drain plugs may be formed in a source region and a drain region respectively of the NMOS transistor or the PMOS transistor. Alternatively, the source/drain plugs may be formed in a source/drain of the NMOS transistor and a source/drain of the PMOS transistor respectively. Alternatively, all of the source/drains of the NMOS and PMOS transistors may include a source/drain plug. Any or all of the source/drains may include lightly doped and heavily doped regions. For example, the corresponding one of the source/drains may comprise a first source/drain and a second source/drain within the first source/drain. The first and second source/drains are doped same conductivity type and the first source/drain is lightly doped relative to the second source/drain.  
           [0026]    It will also be understood that the PMOS and/or NMOS transistors may be formed in wells of opposite conductivity type from the corresponding source/drains. Thus, for example, the source/drain plug may be a P-type source/drain plug, which is formed in a source/drain of an NMOS transistor. The NMOS transistor may include an N − -type source/drain, and an N + -type source/drain within the N − -type source/drain.  
           [0027]    It will also be understood that not all of the source/drain plugs need be of opposite conductivity type from the corresponding source/drain. Rather, at least one of the source/drain plugs may be of opposite conductivity type from the corresponding source/drain, but other source/drain plugs may be of the same conductivity type as the corresponding source/drain.  
           [0028]    CMOS integrated circuit fabricating methods according to the invention form a source/drain plug through a contact hole in an insulating layer, into a corresponding source/drain. The source/drain plug is of opposite conductivity from the corresponding source/drain.  
           [0029]    In particular, an NMOS transistor and a PMOS transistor are formed in an integrated circuit substrate. The NMOS transistor and the PMOS transistor each include a gate, and a source/drain on opposing sides of the gate. An insulating layer is formed on the integrated circuit substrate, and includes a contact hole therein. The contact hole exposes a portion of a corresponding one of the source/drains. A source/drain plug is formed through the contact hole into the corresponding one of the source/drain. The source/drain plug is of opposite conductivity from the corresponding one of the source/drain. A wiring layer is then formed in the contact hole, electrically connected to the source/drain plug. The source/drain plug is preferably formed by implanting ions of opposite conductivity from the corresponding one of the source/drains through the contact hole into the corresponding one of the source/drains.  
           [0030]    The NMOS transistor and PMOS transistor may be fabricated by forming an N-well and a P-well in an integrated circuit substrate and forming a respective insulated gate electrode on the N-well and on the P-well. P-type source/drains are formed in the N-well on opposing sides of the insulated gate electrode on the N-well. N-type source/drains are formed in the P-well, on opposing sides of the insulated gate electrode on the P-well. The source/drains may include lightly doped and heavily doped source/drains. Multiple contact holes may be formed and multiple plugs may be formed, as was already described. Accordingly, the plugs may shield the source/drain from the adjacent well so that highly integrated devices may be formed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 is a schematic plan view of a conventional CMOS integrated circuit;  
         [0032]    [0032]FIGS. 2A through 2D are cross-sectional views showing a fabricating process for a conventional CMOS integrated circuit;  
         [0033]    [0033]FIG. 3 is a cross-sectional view showing the N −  source/drain of a conventional CMOS transistor and the N-well thereof which may cause improper operation;  
         [0034]    [0034]FIG. 3 a  is an enlarged view of the marked portion shown in FIG. 3;  
         [0035]    [0035]FIG. 4 is a cross-sectional view showing misalignment of the P +  source/drain which may cause improper device operation;  
         [0036]    [0036]FIG. 4 a  is a enlarged view of the marked portion shown in FIG. 4;  
         [0037]    [0037]FIG. 5 is a cross-sectional view of CMOS integrated circuits according to an embodiment of the present invention;  
         [0038]    [0038]FIG. 6 is a enlarged view of the marked portion shown in FIG. 5; and  
         [0039]    [0039]FIGS. 7A through 7F are cross-sectional views showing a manufacturing process for CMOS integrated circuits according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0040]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Moreover, each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well.  
         [0041]    [0041]FIG. 5 is a cross-sectional view of a CMOS integrated circuit according to an embodiment of the present invention. Reference numerals  100  and  52  denote an integrated circuit substrate such as a semiconductor substrate, and a field oxide which separates integrated circuit devices, respectively. Reference numerals  54  and  56  denote a P-well for an NMOS transistor and an N-well for a PMOS transistor, respectively. Reference numerals  58 ,  60  and  62  denote a gate insulating layer, a gate electrode of an NMOS transistor, and a gate electrode of a PMOS transistor, respectively. Reference numerals  64 ,  70  and  74  denote an N −  source/drain, an N +  source/drain, and a P +  source/drain, respectively. Reference numerals  66  and  76  denote inter-layer insulating layers. Reference numerals  80  and  82  denote a P-plug and a wiring layer, respectively.  
         [0042]    The P-well  54  and the N-well  56  are formed on the semiconductor substrate  100  which is separated into adjacent active and inactive regions by the field oxide  52 . The NMOS gate electrode  60  and the PMOS gate electrode  62  are formed on the substrate, with the gate insulating layer  58  therebetween.  
         [0043]    The LDD N −  source/drain  64  and N +  source/drains  70  are formed on opposing sides of the NMOS transistor gate electrode  60 , and P +  source/drains  74  are formed on opposing sides of the PMOS transistor gate electrode  62 . On the source/drains  64  and  74  of the NMOS and PMOS transistors, respectively, contact holes are formed by selectively etching the interlayer-insulating layer  76 . Wiring layer  82  fills the contact holes.  
         [0044]    [0044]FIG. 6 shows an enlarged view of the NMOS source/drain, N − , N +  and P −  impurity layers  64 ,  70  and  80 , respectively, which sequentially extend from the channel beneath the gate electrode  60 . The P −  plug  80  is preferably formed to completely shield the N −  source/drain  64  from the N-well  56 . The wiring layer  82 , shown in FIG. 5, is isolated from the NMOS gate electrode  60  by a predetermined distance, and is close to the field oxide  52 .  
         [0045]    A concentration of 1.0×10 13 ˜5.0×10 13  ions/cm 2  of N-type impurities, e.g., phosphorous ions (P), preferably is implanted into the N −  source/drain  64 . A concentration of 1.0×10 15 ˜5.0×10 15  ions/cm 2  of arsenic ions (As) preferably is implanted into the N −  source/drain  70 . P-type ions having one and one half the concentration of the impurities in the N −  source/drain  64  preferably are implanted into the P −  plug  80 .  
         [0046]    In a CMOS integrated circuit according to an embodiment of the present invention, the N −  source/drain  64  adjacent to the N-well may be completely shielded by the P −  plug  80 . Thus, even though the impurities in the N −  source/drain  64  and the N-well  56  may be diffused by a subsequent thermal annealing process, or the P +  source/drain is misaligned, the N −  source/drain  64  and the N-well  56  need not contact one another. Thus, improper operation of the CMOS devices can be reduced or prevented. The reliability of the devices thus may be improved.  
         [0047]    The present invention need not only be applied to a P −  plug  80  for shielding the N −  source/drain  64  adjacent to the N-well, as shown in above embodiment. Rather, the present invention may also be applied to the complementary case. That is, an N −  plug may shield a P +  source/drain. Also, plugs having reversed N −  and P +  regions on both sides of the NMOS and PMOS transistors may also be used according to the present invention.  
         [0048]    [0048]FIGS. 7A through 7F are cross-sectional views showing a fabrication process for CMOS integrated circuits according to an embodiment of the present invention.  
         [0049]    [0049]FIG. 7A shows the step of forming the field oxide  52  and the gate electrodes  60  and  62 . In particular, a field oxide defining the active regions is formed on the surface of the semiconductor substrate  100 . Then, the P-well  54  and N-well  56  are formed. The gate insulating layer  58  is formed thereon. A gate conductive layer is formed on the gate insulating layer  58 . Finally, gate electrodes  60  and  62  are formed by patterning the gate conductive layer and the gate insulating layer  58 .  
         [0050]    In more detail, a field oxide  52 , about 1,000˜5,000 Å thick, for separating the active and inactive regions, is formed on the substrate  100  using a conventional semiconductor device isolating process such as LOCal Oxidation of Silicon (LOCOS). Then, P-type impurities are implanted in a predetermined region of the substrate using conventional photolithography and ion implantation. Then, the impurities are diffused to a certain depth by a high-temperature treatment. Thus, the P-well  54  is formed. The N-well  56  is formed by implanting N-type impurities using the same process.  
         [0051]    The gate insulating layer  58  is formed by growing a thin thermal oxide layer to a thickness of about 60˜200 Å, on the substrate. The gate electrodes of the NMOS and PMOS transistors are formed by depositing an impurity doped-polysilicon layer on the gate insulating layer  58 , and anisotropically etching the polysilicon layer and gate insulating layer  58 .  
         [0052]    [0052]FIG. 7B shows a step of implanting ions to form the N −  source/drain  64 . In more detail, the N −  source/drain  64  is formed by implanting a concentration of 1.0×10 13 ˜5.0×10 13  ions/cm 2  of N − -type impurities, e.g., phosphorous ions, at an implantation energy of 20 KeV˜60 KeV. Normally, this step is not limited to the NMOS transistor area, in order to reduce the number of process steps. As a result, N −  region  65  of the N-well can suppress short channel effects in the PMOS transistor.  
         [0053]    The N −  source/drain  64  can be formed in the NMOS region by photolithography. However, when N −  impurities are implanted into the surface of the NMOS and PMOS transistors and then the P +  source/drain is formed later, short channel effects in the PMOS transistor can be reduced.  
         [0054]    [0054]FIG. 7C shows the step of forming the interlayer insulating layer  66  and the N +  source/drain  70 . The inter-layer insulating layer  66  is formed, surrounding the gate electrodes  60  and  62 . A first photoresist pattern is formed to expose the NMOS device and partially expose the N-well. The N +  source/drain  70  is formed by implanting impurity ions into the NMOS device.  
         [0055]    In more detail, the inter-layer insulating layer  66  is formed by depositing an insulating material such as a high temperature oxide (HTO), and anisotropically etching. The first photoresist pattern is  68  for exposing the NMOS device and partially exposing the N-well, is formed by depositing a photoresist on the inter-layer insulating layer  66  and performing mask exposure and development. Afterwards, the N +  source/drain  70  is formed by implanting N-type impurities, e.g. arsenic ions having a concentration of 1.0×10 15 ˜5.0×10 15  ions/cm 2  at an implant energy level of 20 KeV˜60 KeV, into the exposed NMOS device of the P-well and into the partial region of N-well, using the first photoresist pattern  68  as a mask.  
         [0056]    [0056]FIG. 7D shows the step of forming the P +  source/drain. The first photoresist pattern  68  is removed. A second photoresist pattern  72  is formed, exposing the PMOS device using photolithography. The P +  source/drain is formed by ion-implantation.  
         [0057]    In more detail, the first photoresist pattern  68  (see FIG. 7C) is removed. A second photoresist is deposited on the area that was not covered by the first photoresist pattern  68 . A mask exposure and developing process is performed, and, as a result, the second photoresist pattern  72 , exposing the PMOS device, is formed. Afterwards, P-type impurities, e.g. difluoric boride (BF 2 ) having a concentration of 1.0×10 15 ˜5.3 ×10 15  ions/cm 2 , are implanted at an implant energy level of 20 KeV˜60 KeV. As a result, the N −  impurity in the PMOS device is compensated completely, and thus the P +  source/drain  74  is formed. Next, annealing is performed at 800˜900° C. to activate the impurities in the N +  source/drain  70  and the P +  source/drain  74 .  
         [0058]    [0058]FIG. 7E shows the contact hole forming step and the plug ion implanting step. The second photoresist pattern  72  is removed. A second interlayer insulating layer  76  is formed. The contact holes  78  are formed, and the plug ions are implanted.  
         [0059]    In more detail, the inter-layer insulating layer  76  is formed by removing the second photoresist  72  and depositing an insulating material such as a high temperature oxide (HTO). The contact holes  78  for connecting the active domain on the substrate and the wiring layer are formed by partially etching the interlayer insulating layer  76  using conventional photolithography. Then, the P −  plugs  80  are formed in the exposed portions of the P-well and N-well by implanting P −  type impurities, e.g. difluoric boride (BF 2 ) or boron ions, therein through the contact holes  78 . It will be understood that the ion implantation for forming the P −  plugs can be performed only in the NMOS device by shielding the PMOS device using a mask.  
         [0060]    The impurity ion concentration for forming the P −  plug is preferably about 1.5 times that of the impurities implanted into the N −  source/drain  64 . Also preferably, the contact hole  78  is formed to be spaced apart from the gate electrode  60  by a predetermined distance and close to the field oxide  52 . Thus, the NMOS source/drain can be formed as an LDD structure extending from the gate electrode  60 , and the P-plug  80  can shield the N −  source/drain  64  completely. The N −  source/drain  64  thereby is prevented from contacting N-well  56 .  
         [0061]    [0061]FIG. 7F shows the step of forming the wiring layer  82 . In more detail, an annealing or a rapid thermal process (RTP) at 450˜900° C. is performed to diffuse the impurities implanted into the P −  plug  80 . Afterwards, the wiring layer  82  is formed by depositing and patterning a wiring metal such as aluminum (Al). Thus, the CMOS device is formed.  
         [0062]    It will be understood that the abovedescribed embodiment of the present invention forms the P −  plug in the source/drain of the NMOS transistor. However, the present invention can be applied to other cases. For example, N −  plugs can be formed in the source/drain of the PMOS transistor, or plugs having opposite conductivities to the impurities in the NMOS and PMOS source/drain can be formed therein.  
         [0063]    As described above, the present invention can reduce and preferably overcome problems of the prior art by forming plugs having the opposite polarities in at least one source/drain between the NMOS and PMOS transistors, to prevent the source/drain from contacting to the adjacent well. For example, if a P −  plug shields the N +  source/drain adjacent the N-well, even though the impurities in the source/drain or in the adjacent well are diffused, or misalignment of the active domain occurs, the source/drain and the well need not contact each other. Malfunction of semiconductor devices can be prevented.  
         [0064]    In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.