Abstract:
Disclosed are a transistor and a method for forming the same. The present transistor comprises: a groove formed in a semiconductor substrate; a couple of first sidewall spacers formed in inner sidewalls of the groove, protruding over the substrate; a gate electrode formed between the first sidewall spacers; a gate insulating layer interposed between the gate electrode and the substrate; and source and drain regions formed in the substrate beside the groove.

Description:
[0001]     This application claims the benefit of Korean Application No. 10-2005-0067896, filed on Jul. 26, 2005, which is incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor device and a fabrication method thereof. More specifically, the present invention relates to a transistor, as a semiconductor device or a component thereof (e.g., a chip or monolithic integrated circuit), in which a GIDL (Gate Induced Drain Leakage) phenomenon can be reduced or prevented, and a method forming the same.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, a semiconductor device is fabricated with a plurality of passive and active circuit elements functioning as logic circuits, data storage circuits, and the like. A transistor has been used as one of representative active circuit elements for various functions such as switching, distributing of voltage/current, reception and/or outputting of signals, and so on. Especially, a transistor generally exhibits its performance according to a given design rule. However, characteristics of the fabricated transistor often depart from the design rule, because processing variables and/or structural variations occur in fabrication thereof.  
         [0006]     FIGS.  1  to  3  are cross-sectional views illustrating problems in a conventional method for forming a transistor.  
         [0007]     Referring to  FIG. 1 , in the conventional fabrication method, a gate insulating layer  12  is formed on a semiconductor substrate  10 , and then a conductive layer  14  for a gate electrode is formed on the gate insulating layer  12 .  
         [0008]     Referring to  FIG. 2 , the conductive layer  14  is patterned to form a gate electrode  14   a . In this patterning process, the gate insulating layer  12  may remain or be patterned to form a gate insulating layer pattern  12   a . In a typical manufacturing method of semiconductor devices, a fine pattern such as a gate electrode can be formed using an anisotropic plasma etching process. During the formation of the gate electrode  14   a , sidewalls of the gate electrode  14   a  and the gate insulating layer  12  may be damaged by the plasma, thus resulting in defects therein. Especially, as shown in  FIG. 2 , the vicinities of the lower edge of the gate electrode  14   a  may be damaged by the plasma so that the gate insulating layer in such regions can deteriorate, resulting in a relatively high trap density and a transistor that may be vulnerable to charge leakage.  
         [0009]     Referring to  FIG. 3 , source/drain regions  20   a  and  20   d  are formed under and adjacent to opposed sides of the gate electrode  14   a . Here, the damaged gate insulating layer  12   a  in the vicinity of lower edges of the gate electrode  14   a  may act as trap-sites of hot carriers that generate in a channel near the drain region  20   d , and also may offer current leakage routes that may cause operational failures of transistors. Conventionally, source/drain regions  20   s  and  20   d  comprise low concentration regions which are formed by implantation of impurities in the substrate  50  closely to both sides of the gate electrode  14   a , and heavy concentration regions which are formed by implantation of impurities after forming sidewall spacers  18 . In such double junction structures of source/drain regions, the transistor can be protected from a hot carrier injection and a short channel effect. However, a GIDL (Gate Induced Drain Leakage) phenomenon may occur in regions  22 , indicated by circles in  FIG. 3 , where the source/drain diffusion regions  20   s  and  20   d  partially overlap with the gate electrode  14   a , which may result in an operational failure of the transistor.  
       SUMMARY OF THE INVENTION  
       [0010]     It is, therefore, an object of the present invention to provide a transistor and a method for forming the same, wherein sidewalls of a gate electrode and a gate insulating layer are rarely damaged during an anisotropic etching process, and that can reduce or prevent a GIDL phenomenon.  
         [0011]     To achieve the above object, an embodiment of a transistor according to the present invention, comprises: a trench or groove in a semiconductor substrate; first sidewall spacers formed in inner sidewalls of the trench or groove, extending over an uppermost surface of the substrate; a gate electrode between the first sidewall spacers; a gate insulating layer between the gate electrode and the substrate; and source and drain regions in the substrate beside the trench or groove.  
         [0012]     Because of the first sidewall spacers at both sides of the gate electrode, the source and drain regions can be separated from each other by a lower portion of the gate electrode. In addition, a silicide layer can be further formed on the source region, the drain region, and the gate electrode, respectively. Preferably, the source and drain regions comprise a low concentration diffusion region and a heavy concentration diffusion region. Second sidewall spacers can be formed on the low concentration diffusion regions, and at outer walls of the first sidewall spacers. Here, each portion of the silicide layer can be automatically separated by the second sidewall spacers.  
         [0013]     In addition, a method for forming a transistor according to the present invention may comprise the steps of forming a mask layer on a semiconductor substrate, the mask layer including an opening; forming a trench or groove having a predetermined depth by etching the substrate using the mask layer as an etching mask; forming sidewall spacers on inner sidewalls of the trench or groove and the mask layer; forming a gate insulating layer on a surface of the substrate exposed by the opening; forming a gate electrode on the gate insulating layer between the first sidewall spacers; removing the mask layer; and forming source and drain regions in the substrate adjacent to the trench or groove.  
         [0014]     The source/drain regions may each comprise a low concentration diffusion region and a heavy concentration diffusion region, wherein the low concentration diffusion region is formed by implantation of impurities in the substrate beside the trench or groove, after removing the mask layer; and the heavy concentration diffusion region is formed by implantation of impurities in the substrate after forming second sidewall spacers at outer walls of the first sidewall spacers.  
         [0015]     A silicide layer may be formed on the gate electrode and the heavy concentration diffusion regions, respectively. The silicide layer can be automatically formed adjacent to the second sidewall spacers on source/drain regions. Alternatively, the silicide layers can be automatically formed adjacent to the first sidewall spacers, after removing the second sidewall spacers. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]     FIGS.  1  to  3  are cross-sectional views illustrating problems in a conventional method for forming a transistor.  
         [0017]      FIG. 4  shows a cross-sectional view of a transistor according to a first embodiment of the present invention.  
         [0018]     FIGS.  5  to  8  are cross-sectional views illustrating a method for forming a transistor according to the first embodiment of the present invention.  
         [0019]      FIG. 9  shows a cross-sectional view illustrating another embodiment of a method for forming a transistor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Hereinafter, preferred embodiments of the present invention will be described in detail referring to the following drawings.  
         [0021]      FIG. 4  shows a cross-sectional view of a transistor according to a first embodiment of the present invention.  
         [0022]     Referring to  FIG. 4 , a gate insulating layer  60  is formed on a bottom surface of a groove or trench  56  that is formed by etching a portion of a substrate  50  to a predetermined depth, and a gate electrode  62   a  is formed on the gate insulating layer  60 . First sidewall spacers  58  are formed in the groove  56 , and spacers  58  extend along both sides of the gate electrode  62   a  in upper direction. Source/drain regions comprise low concentration diffusion regions  64  and heavy concentration diffusion regions  68  formed in the substrate  50  adjacent opposed sides of the groove  56 . Second sidewall spacers  66  are formed at outer walls of the first sidewall spacers  58 , covering the low concentration diffusion regions  64 . In addition, silicide layers  68   g ,  68   s , and  68   d  can be respectively formed on top portions of the gate electrode  62   a , the source region and the drain region. The silicide layers  68   s  and  68   d  are formed adjacent to the second sidewall spacers  66 .  
         [0023]     In the conventional structure, source and drain regions partially overlap with the gate electrode. As a result, currents through the transistor may leak due to a GIDL phenomenon. However, in the above-described transistor structure according to the invention, the gate electrode  62   a  is formed in and/or on the groove  56  of the substrate, and the first sidewall spacers  58  are formed at inner sidewalls of the groove  56 . Accordingly, source and drain regions can be formed not to overlap with the gate electrode  62   a.    
         [0024]     FIGS.  5  to  8  are cross-sectional views illustrating a method for forming a transistor according to the first embodiment of the present invention.  
         [0025]     Referring to  FIG. 5 , a buffer insulating layer  52  comprising an oxide material is formed on a substrate  50  (e.g., silicon dioxide formed by conventional chemical vapor deposition or thermal growth), and then a mask layer  54  is formed on the buffer insulating layer  52 . The mask layer  54  can comprise a silicon nitride layer. The buffer insulating layer  52  lessens or eliminates stresses applied to the substrate by the mask layer  54  (e.g., silicon nitride). In the case where the mask layer  54  comprises or consists essentially of an insulating material having a low stress to the substrate, such as silicon oxide, etc., the buffer insulating layer  52  can be omitted.  
         [0026]     Referring to  FIG. 6 , the mask layer  54  and the buffer insulating layer  52  are etched, and then a portion of the substrate  50  is etched, thus forming a groove or trench  56  having a predetermined depth, and preferably, substantially vertical sidewalls and a substantially horizontal lower surface having a width (or length) of at least a critical dimension (e.g., a minimum feature size of the technology used for manufacturing the device). The groove  56  can be formed in a suitable depth, considering thicknesses of the gate insulating layer  60  and source/drain regions; e.g., the predetermined depth may be equal to or greater than a depth of the source/drain regions (or the depth of the low concentration regions of the source and drain).  
         [0027]     Referring to  FIG. 7 , a spacer insulating layer is formed on the mask layer  54  by conformal deposition of an insulating material (e.g., a low-pressure chemical vapor deposition process using TEOS, or tetraethyl orthosilicate), and then it is anisotropically etched to form first sidewall spacers  58  extending along the inner walls of the groove  56  (i.e., the inner walls of the substrate  50  and the mask layer  54 ). In addition, the gate insulating layer  60  is formed on the exposed substrate between the first sidewall spacers  58 , generally by conventional thermal oxidation (e.g., when the exposed substrate surface comprises or consists essentially of silicon), although deposition of a suitable gate dielectric material and densification thereof (e.g., by rapid thermal annealing) may also be suitable (in which case a slightly thinner layer of insulating material may be deposited for the first sidewall spacers  58 ). Comparing with a conventional structure in which a gate insulating layer may be damaged by plasma during patterning of a gate electrode, the gate insulating layer  60  is formed on the exposed portion of the substrate between first sidewall spacers  58  so that etch damage can be prevented.  
         [0028]     Next, a conductive layer  62  for a gate electrode is formed on the gate insulating layer  60 , filling the groove  56  (e.g., an opening in the mask layer  54 ). The conductive layer  62  can comprise a polysilicon layer, and further a metal layer or a metal silicide layer can be formed thereon (preferably after subsequent planarization; see, e.g.,  FIG. 8  and the discussion thereof below).  
         [0029]     Subsequently, as shown in  FIG. 8 , the conductive layer  62  is planarized on the mask layer  54  (e.g., by conventional chemical mechanical polishing or conventional etch back) to form a gate electrode  62   a  on the gate insulating layer  60  in the trench  56 . The top surface of the mask layer  54  is exposed by the planarization, and then the exposed mask layer  54  beside the gate electrode  62   a  is removed (e.g., by selectively etching the mask layer  54  relative to the other exposed materials; when the mask layer  54  comprises or consists essentially of silicon nitride, conventional wet etching with phosphoric acid may be employed). Thus, portions of first sidewall spacers  58  and the gate electrode  62   a  protrude over the substrate  50  (e.g., its uppermost surface). Afterward, one or more n-type dopants (e.g., As) or p-type dopants (e.g., BF 2 ) are implanted in the substrate  50 , thus forming low concentration diffusion regions  64  adjacent to the first sidewall spacers  58 . Low concentration diffusion regions  64  can also be considered as lightly doped drain (LDD) structures. In the first embodiment, the first sidewall spacers  58  at both sides of the gate electrode  62   a  are partially buried in the substrate  50  by a depth corresponding to the depth of the groove  56 , which can prevent overlap of the low concentration diffusion regions  64  with the gate electrode  62   a.    
         [0030]     Thereafter, second sidewall spacers  66  can be further formed at exposed outer walls of the first sidewall spacers  58 , in order to form source/drain regions. Then, one or more n-type dopants (e.g., P) or p-type dopants (e.g., B) are implanted in the substrate, thus forming heavy concentration diffusion regions  68  aligned with the second sidewall spacers  66 .  FIG. 8  shows a source/drain structure where heavy concentration diffusion region  68  is formed deeper than low concentration diffusion region  64 . Alternatively, the source/drain regions can comprise a DDD structure where low concentration diffusion regions  64  are formed deeper than heavy concentration diffusion regions  68 . Low and heavy concentration diffusion regions functions as a source or drain region of a transistor.  
         [0031]     Continuously, the exposed buffer insulating layer  52  is removed, and then silicide layers  68   s ,  68   d , and  68   g  are respectively formed on the source region, the drain region, and the gate electrode  62   a  by a typical silicidation process, as shown in  FIG. 4 . The silicide layers  68   s  and  68   d  are automatically separated by second sidewall spacers  66 .  
         [0032]     The silicide layers can be respectively formed on a whole surface of source or drain region, in order to further reduce the electrical resistance of the source or drain region.  
         [0033]      FIG. 9  shows a cross-sectional view illustrating another embodiment of a method for forming a transistor according to the present invention. This embodiment is similar to the first embodiment; however, it provides a transistor structure without second sidewall spacers  66 .  
         [0034]     Referring to  FIG. 9 , after forming low and heavy concentration diffusion regions  164  and  168  on the substrate  150 , the second sidewall spacers  66  as shown in  FIG. 8  are removed. In order to remove the second sidewall spacers while retaining the first sidewall spacers  158 , it is preferable that the first sidewall spacers  158  comprise or consist essentially of a material having a high etching selectivity relative to the material for the second sidewall spacers.  
         [0035]     When the outer walls of the first sidewall spacers  158  are exposed by removal of the second sidewall spacers, a silicidation process may be performed to form silicide layers  168   s ,  168   d , and  168   g  on the source region, the drain region, and the gate electrode  162   a , respectively. The silicide layers  168   s  and  168   d  are formed on both the heavy and low concentration diffusion regions  168  and  164 , thus further reducing the electrical resistance of the source/drain regions relative to the transistor shown in FIGS.  4  and/or  8 .  
         [0036]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.