Abstract:
A semiconductor device and a method for fabricating the same are disclosed that reduce short channel effects to improve device characteristics. The semiconductor device includes a gate insulating film formed on a semiconductor substrate, a gate electrode formed on the gate insulating film and a lightly doped region formed in the semiconductor substrate at both sides of the gate electrode. A sidewall insulating film is formed at both sides of the gate electrode and a heavily doped impurity region is formed in the semiconductor substrate extending from the sidewall insulating film. Further, an insulating film is formed at sides of the heavily doped impurity region. The insulating film prevents impurity ions from the heavily doped impurity region from diffusing into the channel region of the device.

Description:
This application is a Divisional of application Ser. No. 09/118,812 filed Jul. 20, 1998 U.S. Pat. No. 6,083,796. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device, and more particularly, to a semiconductor device and a method for fabricating the same that reduces a short channel effect. 
     2. Background of the Related Art 
     Generally, in fabricating a semiconductor integrated circuit, various efforts continue at reducing the dimensions of a metal oxide semiconductor field effect transistor (MOSFET) constituting a semiconductor chip integrated circuit that has excellent performance and high packing density. As a result of such efforts, a semiconductor integrated circuit and a method of making the same have been scaled down to a sub-micron size. 
     In reducing the dimensions of the semiconductor device, the vertical dimension as well as the horizontal dimension should be reduced to balance the characteristics of various devices. In other words for a transistor, if the distance between a source and a drain becomes close, a characteristic of the device is varied, which causes an undesired characteristic such as the short channel effect. To improve such short channel effects caused by high packing density, a lightly doped drain (LDD) structure is adopted in which a low density junction is formed below a gate sidewall. 
     A related art semiconductor device and a method for fabricating the same will now be described. FIG. 1 is a diagram showing a sectional view of the related art semiconductor device. 
     As shown in FIG. 1, a gate insulating film  12  is formed on a semiconductor substrate  11 . A gate electrode  13   a  is formed in a predetermined region on the gate insulating film  12 . A sidewall insulating film  16  is formed at both sides of the gate electrode  13   a.  A heavily doped impurity region  17  having an LDD structure is formed in a surface of the semiconductor substrate  11  at both sides of the gate electrode  13   a.    
     FIGS. 2 a  to  2   d  are diagrams showing sectional views of a method for fabricating the related art semiconductor device. As shown in FIG. 2 a,  a channel ion is implanted into the entire surface of the semiconductor substrate  1 . A gate insulating film  12  is formed on the semiconductor substrate  11  into which the channel ion is implanted. A polysilicon layer  13  for a gate electrode is formed on the gate insulating film  12 . Subsequently, a photoresist  14  is deposited on the polysilicon layer  13  and then patterned by exposure and developing processes to define a gate region. 
     As shown in FIG. 2 b,  the polysilicon layer  13  is selectively removed using the patterned photoresist  14  as a mask to form a gate electrode  13   a.  As shown in FIG. 2 c,  the photoresist  14  is removed, and an n type lightly doped impurity ion is implanted into the entire surface of the semiconductor substrate  11  using the gate electrode  13   a  as a mask to form a lightly doped impurity region  15  in the surface of the semiconductor substrate  11 . Thus, the lightly doped impurity region  15  is formed at both sides of the gate electrode  13   a.    
     As shown in FIG. 2 d,  an insulating film (not shown) is formed on the entire surface of the semiconductor substrate  11  including the gate electrode  13   a.  The insulating film is then etched back to form a sidewall insulating film  16  at both sides of the gate electrode  13   a.  Subsequently, an n type heavily doped impurity ion, which is used for a source and a drain, is implanted into the entire surface of the semiconductor substrate  11  using the sidewall insulating film  16  and the gate electrode  13   a  as masks. A heavily doped impurity region  17 , which is connected with the lightly doped impurity region  15 , is thereby formed in the surface of the semiconductor substrate  11  at both sides of the gate electrode  13   a.    
     However, the related art semiconductor device and method for fabricating the same have various problems. In the related art semiconductor device and the method for fabricating the same reliability deteriorates because of a short channel effect caused by diffusing impurity ions of the heavily doped impurity region into the channel region. 
     The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor device and a method for fabricating the same that substantially obviate one or more of the problems caused by limitations and disadvantages of the related art. 
     Another object of the present invention is to provide a semiconductor device and a method for fabricating the same having an insulating film is formed at sides of a heavily doped impurity region. 
     Another object of the present invention is to provide a semiconductor device and a method for fabricating the same that prevents diffusing of impurity ions of the heavily doped impurity region. 
     Another object of the present invention is to provide a semiconductor device and a method for fabricating the same that prevents impurity ions of the heavily doped impurity region from diffusing to a channel region. 
     Another object of the present invention is to provide a semiconductor device and a method for fabricating the same that increases reliability of the device. 
     To achieve at least these objects and other advantages in a whole or in parts and in accordance with the purpose of the present invention, as embodied and broadly described, a semiconductor device according to the present invention includes a gate insulating film on a prescribed portion of a semiconductor substrate, a gate electrode on the gate insulating film, a lightly doped region in the semiconductor substrate at both sides of the gate electrode, sidewall insulating films formed at both sides of the gate electrode and the gate insulating film, heavily doped impurity regions in the semiconductor substrate extending from a side of the sidewall insulating films opposite the gate electrode, and insulating films positioned at sides of the heavily doped impurity region. 
     To further achieve the above objects in a whole or in parts, a method for fabricating a semiconductor device is provided according to the present invention that includes the steps of forming insulating films on a semiconductor substrate having prescribed dimensions and separated by a prescribed distance, forming a semiconductor layer on an entire surface of the semiconductor substrate including the insulating films, forming, a gate on the semiconductor layer between pairs of the insulating films, forming a lightly doped impurity region in a surface of the semiconductor layer at both sides of the gate, forming a first sidewall insulating film at both sides of the gate, and forming, a heavily doped impurity regions in the semiconductor layer at both sides of the gate, the heavily doped impurity region being isolated from each other by the insulating film. 
     To further achieve the objects in a whole or in parts, a method for fabricating a semiconductor device is provided according to the present invention that includes the steps of forming a gate insulating film and a gate electrode on a semiconductor substrate, etching, the semiconductor substrate at both sides of the gate electrode to a predetermined depth from the top surface to form a trench, forming an insulating film on the surface of the semiconductor substrate in the trench, forming a first sidewall insulating film at both sides of the gate electrode, the gate insulating film, and the trench, selectively removing the insulating film using the gate electrode and the first sidewall insulating film as masks, removing the first sidewall insulating film, forming a semiconductor layer on the entire surface of the semiconductor substrate including the insulating film, and forming a heavily doped impurity regions in the semiconductor layer at both sides of the gate electrode, wherein the heavily doped impurity regions are isolated from each other by the insulating film. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 is a diagram illustrating a sectional view of a related art semiconductor device; 
     FIGS. 2 a  to  2   d  are diagrams illustrating sectional views of process steps of a method for fabricating the related art semiconductor device of FIG. 1; 
     FIG. 3 is a diagram illustrating a sectional view of a preferred embodiment of a semiconductor device according to the present invention; 
     FIGS. 4 a  to  4   h  are diagrams illustrating sectional views of a preferred embodiment of a method for fabricating a semiconductor device according to the present invention; and 
     FIGS. 5 a  to  5   f  are diagrams illustrating sectional views of another preferred embodiment of a method for fabricating a semiconductor device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 3 is a diagram showing a cross-sectional view of a first preferred embodiment of a semiconductor device according to the present invention. As shown in FIG. 3, the semiconductor device according to the first preferred embodiment includes a gate insulating film  26  formed on a semiconductor substrate  21 , a gate electrode  27   a  formed on the gate insulating film  26 , a lightly doped impurity region  29  formed in the semiconductor substrate  21  at both sides of the gate electrode  27   a  and a sidewall insulating film  30  formed at both sides of the gate electrode  27   a.  The first preferred embodiment of a semiconductor device further includes a heavily doped impurity region  31  in the semiconductor substrate  21  that extends from a lower portion of the sidewall insulating film  30  and a sidewall oxide film  24  formed at sides of the heavily doped impurity region  31 . 
     FIGS. 4 a  to  4   h  are diagrams illustrating sectional views of process steps of a method for fabricating a semiconductor device according to a second preferred embodiment of the present invention. The second preferred embodiment of a method for fabricating a semiconductor device can be used, for example, to form the first preferred embodiment according to the present invention. 
     As shown in FIG. 4 a,  a nitride film (Si 3 N 4 )  22  is formed on the semiconductor substrate  21 . A first photoresist  23  is deposited on the nitride film  22  and then patterned by exposure and developing processes. 
     As shown in FIG. 4 b,  the nitride film  22  is selectively removed using the patterned first photoresist  23  as a mask to form a nitride film pattern  22   a  having a prescribed size. 
     As shown in FIG. 4 c,  the first photoresist  23  is removed, and an oxide film is formed on the entire surface of the semiconductor substrate  21  including the nitride film pattern  22   a.  The oxide film is then etched back to form an sidewall oxide film  24  at sides of the nitride film pattern  22   a.    
     As shown in FIG. 4 d,  the nitride film pattern  22   a  is removed, and the semiconductor substrate  21  is epitaxially grown to form a p type silicon epitaxial layer  25  on the entire surface of the semiconductor substrate  21  including the sidewall oxide film  24 . The sidewall oxide film  24  is preferably completely buried by the silicon epitaxial layer  25 . 
     As shown in FIG. 4 e,  a gate insulating film  26  is formed on the silicon epitaxial layer  25 , and a polysilicon layer  27  for a gate electrode is formed on the gate insulating film  26 . Subsequently, a second photoresist  28  is deposited on the polysilicon layer  27  and then patterned by exposure and developing processes to define a gate region. 
     As shown in FIG. 4 f,  the polysilicon layer  27  is selectively removed using the patterned second photoresist  28  as a mask to form a gate electrode  27   a  on the gate insulating film  26  between each (e.g., pair) of the sidewall oxide film  24 . As shown in FIG. 4 g,  the second photoresist  28  is preferably removed. Then, an n type lightly doped impurity ion is implanted into the entire surface of the semiconductor substrate  21  using the gate electrode  27   a  as a mask to form a lightly doped impurity region  29  in the surface of the silicon epitaxial layer  25  at both sides of the gate electrode  27   a.    
     As shown in FIG. 4 h,  an insulating film (not shown) is formed on the entire surface of the semiconductor substrate  21  including the gate electrode  27   a.  The insulating film is then etched back to form a sidewall insulating film  30  at both sides of the gate electrode  27   a.  Subsequently, an n type heavily doped impurity ion, which is preferably used for source and drain regions, is implanted into the entire surface of the semiconductor substrate  21  using the sidewall insulating film  30  and the gate electrode  27   a  as masks to form a heavily doped impurity region  31  in the surface of the silicon epitaxial layer  25  at both sides of the gate electrode  27   a  extending from the sidewall insulating film  30 . The heavily doped impurity region  31  is connected with the lightly doped impurity region  29  and isolated from each other by the sidewall oxide film  24 . 
     FIGS. 5 a  to  5   f  are diagrams illustrating sectional views of process steps of a method for fabricating a semiconductor device according to a third preferred embodiment of the present invention. As shown in FIG. 5 a,  a gate insulating film  32  is formed on a semiconductor substrate  31 , and a polysilicon layer  33  for a gate electrode is formed on the gate insulating film  32 . Subsequently, a photoresist  34  is deposited on the polysilicon layer  33  and then patterned by exposure and developing processes to define a gate region. 
     As shown in FIG. 5 b,  the polysilicon layer  33  and the gate insulating film  32  are selectively removed using the patterned photoresist  34  as a mask to form a gate electrode  33   a.  At this time, the polysilicon layer  33  and the gate insulating film  32  are preferably overetched to form a trench  35  with a predetermined depth from a surface of the semiconductor substrate  31 . 
     As shown in FIG. 5 c,  the photoresist  34  is removed, and an oxide film  36  is formed on the surface of the semiconductor substrate  31  in which the trench  35  is formed. Subsequently, a first insulating film (not shown) is formed on the entire surface of the semiconductor substrate  31  including the gate electrode  33   a.  The first insulating film is then etched back to form a first sidewall insulating film  37  at sides of the gate electrode  33   a,  the gate insulating film  32 , and the trench  35 . 
     As shown in FIG. 5 d,  the oxide film  36  is selectively removed using the first sidewall insulating film  37  and the gate electrode  33   a  as masks to form an oxide film pattern  36   a.    
     As shown in FIG. 5 e,  the first sidewall insulating film  37  is removed, and a silicon epitaxial layer  38  is formed using the semiconductor substrate  31  as a seed. The silicon epitaxial layer  38  is preferably formed so that the oxide film pattern  36   a  is fully buried. Subsequently, an n type lightly doped impurity ion is implanted into the entire surface of the semiconductor substrate  31  using the gate electrode  33   a  as a mask to form a lightly doped impurity region  39  in a surface of the silicon epitaxial layer  38  at both sides of the gate electrode  33   a.    
     As shown in FIG. 5 f,  a second insulating film (not shown) is formed on the entire surface of the semiconductor substrate  31  including the gate electrode  33   a.  The second insulating film is then etched back to form a second sidewall insulating film  40  at both sides of the gate electrode  33   a.  Subsequently, an n type heavily doped impurity ion for a source and a drain is implanted into the entire surface of the semiconductor substrate  31  using the second sidewall insulating film  40  and the gate electrode  33   a  as masks to form a heavily doped impurity region  41  in the surface of the silicon epitaxial layer  38  at both sides of the gate electrode  33   a.  The heavily doped impurity region  41  is connected with the lightly doped impurity region  39  and isolated from each other by the oxide film pattern  36   a.    
     As described above, the preferred embodiments of a semiconductor device and a method for fabricating the same according to the present invention have various advantages. An insulating film according to the preferred embodiments is formed at sides of the heavily doped impurity region to prevent the impurity ion from being diffused into the channel region. The insulating film of the preferred embodiments improves a device short channel effect characteristic. 
     The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. For example, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.