Abstract:
A semiconductor device having a buried gate line with a shaped gate trench and a method of fabricating the same are disclosed. The semiconductor device includes a trench isolation layer provided in a semiconductor substrate to define a multi-surfaced active region/channel. A gate line extending to the trench isolation layer fills a portion of the gate trench. The gate trench is formed with a series of depressions to accommodate peaks in the channel. The combination of depressions/peaks operate to increase the effective area of the channel, thereby enabling smaller channel semiconductor devices to be formed without increasing the width thereof.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates generally to a semiconductor device and a method of fabricating same. More particularly, this invention relates to a semiconductor device having a buried gate line and a method of fabricating same. 
     This application claims priority to Korean Patent Application No. 10-2006-0097267, filed Oct. 2, 2006, the contents of which are hereby incorporated herein by reference in its entirety. 
     2. Description of Related Art 
     Semiconductor devices widely employ a discrete device such as a field effect transistor as a switching device. In the transistor, the operating speed of the switching device is significantly determined by the on current flowing through a channel between the source and drain regions. In general, a planar-type transistor may be formed by fabricating a gate electrode and the source and drain regions in a device-forming region of a substrate, i.e., an active region. Common planar-type transistors have a planar channel between the source and drain regions. The on-current of the planar-type transistor is proportional to the width of the active region, and inversely proportional to the distance between the source region and the drain region, i.e., a gate length. Thus, in order to increase the on-current and thus the operating speed of the device, the gate length has to be reduced, and the width of the active region has to be increased. However, the increase in the width of the active region in the planar-type transistor is contrary to the trend of reducing the size of highly integrated devices. Also, in the planar-type transistor, as the distance between the source and drain regions becomes short, a short channel effect may occur. Accordingly, because a conventional planar-type transistor contains a flat channel on the surface of the semiconductor substrate and is a planar-type channel device; it is structurally disadvantageous to downsize the device, without also incurring the short channel effect. 
     A transistor having a recess channel has been disclosed to overcome the short channel effect for a downsized transistor. The recess channel transistor contains a depressed channel region and an insulated gate electrode. The insulated gate electrode is disposed in the depressed channel region, i.e., a recess channel region. Accordingly, the recess channel transistor may ensure a relatively greater effective channel length than the planar-type transistor. That is, the recess channel transistor provides a structure capable of overcoming problems caused by the short channel effect. However, the recess channel transistor has another problem that makes subsequent processes such as formation of a contact plug and planarization difficult when the gate electrode is disposed to project upward from the semiconductor substrate. Also, an upper corner part of the depressed channel region may result in leakage current from a field crowding effect. Further, a highly difficult patterning process is required to form the projected gate electrode. 
     Another approach using a buried word line is disclosed in U.S. Pat. No. 6,770,535 B2 entitled “Semiconductor Integrated Circuit Device and Process for Manufacturing the Same” by Yamada et al. According to Yamada et al., a trench is formed across a channel region and an isolation layer. A word line filling a part of the trench is then formed. An insulating pattern filling the rest of the trench is formed thereafter. As a result, the world line is buried under the surface of the semiconductor substrate. The buried word line provides a relatively greater effective channel length. However, the effective channel width of the semiconductor device is determined by the channel region and the buried word line. Accordingly, this semiconductor device has substantially the same effective channel width as a planar-type MOS transistor. Consequently, the buried word line semiconductor device has a relatively lower current drivability than the planar-type MOS transistor. Also, the recess channel MOS transistor has a relatively adverse body effect as well as a difficult structure as compared to the planar-type MOS transistor. 
     Therefore, there is a need in the semiconductor arts for a semiconductor device capable of substantially overcoming the deficiencies described above and a method of fabrication, thereof. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide systems and methods for increasing the effective channel length and width of a semiconductor device. 
     In one aspect of the invention, a semiconductor device is provided having a trench isolation layer disposed on a semiconductor substrate defining an active region, a gate trench extending from the trench isolation layer is disposed in the active region, the gate trench having a first region proximal to the trench isolation layer and a second region proximal to the first region, wherein a bottom of the second region is at a level lower than a bottom of the first region, a gate line across the active region fills a portion of the gate trench, and an insulating pattern is provided on the gate line. 
     In another embodiment of the present invention, an upper width of the active region is smaller than a lower width of the active region, the widths being substantially defined by an inclined sidewall of the trench isolation layer. An upper portion of the first region may be at a greater height than an upper portion of the second region. The upper width of the active region may be greater than the lower width of the active region. 
     In other embodiments of the present invention, the gate trench has a third region projecting from a central region of the second region, wherein an upper portion of the third region is at a greater height than the upper portion of the second region. The gate line may also cover a sidewall of the active region. 
     In still other embodiments, the invention may further comprise a gate dielectric layer interposed between the gate line and the active region. It may also further comprise source and drain regions provided in the active region adjacent to both sides of the gate line, upper surfaces of the source and drain regions being disposed at a higher level than the gate line. The upper surfaces of the source and drain regions and the insulating pattern may be disposed at the same level. Also, it may further comprise a data storage element electrically connected to one of the source and drain regions. 
     In another aspect, methods for fabricating a semiconductor device is provided, the method comprising, forming a trench isolation layer defining an active region in a semiconductor substrate, forming a gate trench extending from the trench isolation layer in the active region, the gate trench being formed with a first region proximal to the trench isolation layer and a second region proximal to the first region, wherein a bottom of the second region is at a lower level than a bottom of the first region, forming a gate line across the active region, and filling a portion of the gate trench, and forming an insulating pattern on the gate line. 
     In another embodiment of the present invention, the forming of the trench isolation layer may comprise, forming an isolation trench having an inclined sidewall to define the active region, an upper width of the active region being smaller than a lower width of the active region, forming an isolation insulating layer on the substrate with the isolation trench, and planarizing the isolation insulating layer. Additionally, forming the gate trench may comprise, forming a mask having an opening extending to the trench isolation layer across the active region on the substrate having the trench isolation layer, etching the active region using the mask and the trench isolation layer as etch masks, etching the trench isolation layer using the mask as an etch mask, and removing the mask. 
     In other embodiments, the gate trench in the active region is formed with an inclined sidewall, a lower region of the gate trench being narrower than an upper region thereof. Additionally, the gate trench can be formed to expose a sidewall of the active region. 
     In still other embodiments, the gate trench is formed to have a third region projecting from a central portion of the second region. 
     In yet another embodiment, the gate trench is formed by forming a mask having an opening extending to the trench isolation layer across the active region on the substrate having the trench isolation layer, partially etching the trench isolation layer using the mask and the active region as etch masks, etching the active region using the mask and the partially etched trench isolation layer as etch masks, and removing the mask. 
     In other embodiments of the present invention, the method of fabrication includes etching the partially etched trench isolation layer, after etching the active region, by using the mask as an etch mask to expose the sidewall of the active region. Additionally, the method may include forming a gate dielectric layer, before forming the gate line, on the substrate. The method may further include forming source and drain regions in the active region adjacent to both sides of the gate line, upper surfaces of the source and drain regions being disposed at a higher level than the gate line. 
     In yet another embodiment, an interlayer insulating layer on the substrate having the source and drain regions is formed and a contact plug passing through the interlayer insulating layer is formed, wherein the contact plug is in contact with at least one of the source and drain regions, and a data storage element is formed above the contact plug on the interlayer insulating layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of exemplary embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a plan view of a semiconductor device having a buried gate line according to an exemplary embodiment of the invention. 
         FIGS. 2A ,  3 A,  4 A and  5 A are cross-sectional views taken along line I-I′ of  FIG. 1  which illustrate a method of fabricating the semiconductor device having a buried gate line according to the exemplary embodiment of the invention. 
         FIGS. 2B ,  3 B,  4 B and  5 B are cross-sectional views taken along line II-II′ of  FIG. 1  which illustrate the method of fabricating the semiconductor device having a buried gate line according to the exemplary embodiment of the invention. 
         FIG. 6  is a plan view of a semiconductor device having a buried gate line according to another exemplary embodiment of the invention. 
         FIGS. 7 to 11  are cross-sectional views illustrating a method of fabricating the semiconductor device having a buried gate lined according to another exemplary embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the invention will be described in some additional detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to only the embodiments set forth herein. Rather, these embodiments are presented as teaching examples. Throughout the written description and drawings, like reference numbers and symbols refer to like or similar elements. 
     Certain drawing dimensions, particularly those related to elements, layers and regions of the exemplary interconnects described below may have been exaggerated for clarity. It will also be understood that when a layer is referred to as being ‘on’ another layer, element, or region, it may be “directly on” the other layer, element, or region, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being ‘under’ another layer, it may be “directly under”, or one or more intervening layers may be present. In addition, it will also be understood that when a layer is referred to as being ‘between’ two layers, elements, or regions, it may be the only layer there between, or one or more intervening layers may also be present. 
     Moreover, terms such as “first,” and “second” are used to describe various layers, elements, and regions in various embodiments of the invention, but such terms do not temporally or sequentially limit (e.g., in an order of formation) the related layers, elements, and regions. Rather, these terms are used merely to distinguish one layer, element or region from another. 
       FIG. 1  is a plan view of a semiconductor device having a buried gate line according to an exemplary embodiment of the invention,  FIGS. 2A ,  3 A,  4 A and  5 A are cross-sectional views taken along line I-I′ of  FIG. 1  which illustrate a method of fabricating the semiconductor device having a buried gate line according to an exemplary embodiment of the invention, and  FIGS. 2B ,  3 B,  4 B and  5 B are cross-sectional views taken along line II-II′ of  FIG. 1  which illustrate a method of fabricating the semiconductor device having a buried gate line according to an exemplary embodiment of the invention.  FIG. 6  is a plan view of a semiconductor device having a buried gate line according to another exemplary embodiment of the invention,  FIGS. 7 to 10  are cross-sectional views taken along line III-III′ of  FIG. 6 , and  FIG. 11  is a cross-sectional view taken along line IV-IV′ of  FIG. 6 . 
     First, a semiconductor device according to an exemplary embodiment of the invention will be described with reference to  FIGS. 1 ,  5 A and  5 B. 
     Referring to  FIGS. 1 ,  5 A and  5 B, a trench isolation layer  109  defining an active region  103   a  is provided in a semiconductor substrate  100 . The semiconductor substrate  100  may be a single crystal silicon wafer or any material that is exhibits the appropriate semiconductor-like characteristics, whether single crystal or otherwise. The trench isolation layer  109  may have an inclined sidewall. For example, the trench isolation layer  109  may have the inclined sidewall to define the active region  103   a  having an area gradually becoming larger from top to bottom. The trench isolation layer  109  may be, for example, a silicon oxide layer. That is, as illustrated in  FIG. 1 , an upper region of the active region  103   a  may have an area of A 1 , and a lower region of the active region  103   a  may have an area of A 2  larger than A 1 . 
     Further, a buffer oxide layer  105  and an insulating liner  106  which are sequentially stacked may be provided between the trench isolation layer  109  and the semiconductor substrate  100 . The buffer oxide layer  105  may be, for example, a thermal oxide layer. The insulating liner  106  may be, for example, a silicon nitride layer. 
     A gate trench  116  extending to the trench isolation layer  109  across the active region  103   a  may be provided. The gate trench  116  may have a first bottom region M 1  in the active region  103   a  adjacent to the trench isolation layer  109 , and a second bottom region M 2  disposed at a lower level than the first bottom region M 1  in the active region  103   a  adjacent to the first bottom region M 1 . As illustrated in  FIG. 5B , the gate trench  116  may have an inclined sidewall in order to have a lower region having a smaller area than the upper region in the active region  103   a.  Thus, a first region B 1  of the active region  103   a  corresponding to the first bottom region M 1  of the gate trench  116  may be more projected with respect to a particular direction than a second region B 2  of the active region  103   a  corresponding to the second bottom region M 2 . 
     The gate trench  116  may have a bottom region disposed at a lower level than the first bottom region B 1  in the trench isolation layer  109 . 
     A buried gate line  124  partially filling the gate trench  116  is provided. The gate line  124  may include one selected from the group comprising a poly silicon layer, a metal silicide layer, a metal layer, and a combination thereof. Other materials or combinations or groups may be used as deemed appropriate. The gate line  124  may cover a sidewall of the active region  103   a.    
     A gate dielectric layer  121  may be interposed between the gate line  124  and the active region  103   a.  The gate dielectric layer  121  may be a silicon oxide layer or a high-k dielectric layer. 
     Source and drain regions  130  may be provided in the active region  103   a  adjacent to both sides of the gate line  124 . Upper surfaces of the source and drain regions  130  may be disposed at a higher or elevated level than the gate line  124 . An insulating pattern  127  may be provided on the gate line  124 . The insulating pattern  127  may be, for example, a silicon oxide layer. Upper surfaces of the source and drain regions  130  and insulating pattern  127  may be disposed at substantially the same level. 
     The gate line  124  in the active region  103   a  may function as a gate electrode of a MOS transistor. Thus, a MOS transistor having the gate line  124 , the gate dielectric layer  121  and the source and drain regions  130  may be provided. Such a MOS transistor may have a greater effective channel length than a planar-type transistor since it has a recessed channel. Also, the gate trench  116  in the active region  103   a  covered by the gate line  124  has the first and second bottom regions M 1  and M 2  disposed at different levels from each other, and thus may have a double-fin structure. That is, the MOS transistor may have a greater effective channel width than a planar-type transistor since it has a fin structure. As a result, according to an exemplary embodiment of the invention, a MOS transistor which can improve on-current characteristics, body effect, and sub-threshold swing characteristics of a transistor as well as a short channel effect may be realized. 
     An interlayer insulating layer  132  may be provided on the substrate having the MOS transistor. The interlayer insulating layer  132  may be, for example, a silicon oxide layer. A contact plug  133  passing through the interlayer insulating layer  132  and electrically connected to one of the source and drain regions  130  may also be provided. The contact plug  133  may be formed of a conductive material such as, for example, polysilicon or tungsten. A data storage element  136  covering the contact plug  133  may be provided on the interlayer insulating layer  132 . The data storage element  136  may be a data storage unit of a volatile memory device such as a DRAM or a data storage unit of a nonvolatile memory device such as a PRAM. For example, when the data storage element  136  is employed in the DRAM, the data storage element  136  may be a cell capacitor of the DRAM. Other elements, whether storage-capable or not may be implemented according to design preference. 
     Next, a semiconductor device according to another exemplary embodiment of the invention will be described with reference to FIGS:  6 ,  10  and  11 . 
     Referring to  FIGS. 6 ,  10  and  11 , a trench isolation layer  209  defining an active region  203   a  is provided on a semiconductor substrate  200 . The semiconductor substrate  200  may be a single crystal silicon wafer or any material that is exhibits the appropriate semiconductor-like characteristics, whether single crystal or otherwise. The trench isolation layer  209  may have an inclined sidewall. For example, the trench isolation layer  209  may have an inclined sidewall to define the active region  203   a  having an area gradually becoming larger from top to bottom. That is, as illustrated in  FIG. 6 , an upper region of the active region  203   a  may have an area of D 1 , and a lower region of the active region  203   a  may have an area of D 2  larger than D 1 . 
     The trench isolation layer  209  may be, for example, a silicon oxide layer. Here, a buffer oxide layer  205  and an insulating liner  206  may be sequentially stacked between the trench isolation layer  209  and the semiconductor substrate  200 . 
     A gate trench  216  crossing the active region  203   a  and extending to the trench isolation layer  209  may be provided. 
     The gate trench  216  may have a first bottom region N 1  in the active region  203   a  adjacent to the trench isolation layer  209 , a second bottom region N 2  disposed at a lower level than the first bottom region N 1  in the active region  203   a  adjacent to the first bottom region N 1 , and a third bottom region N 3  disposed at a higher level than the second bottom region N 2 . Thus, a first region C 1  of the active region  203   a  corresponding to the first bottom region N 1  of the gate trench  216  may be more projected than a second region C 2  of the active region  203   a  corresponding to the second bottom region N 2 , and a third region C 3  of the active region  203   a  corresponding to the third bottom region N 3  may be more projected than the second region C 2 . 
     However, the first bottom region N 1  and the third bottom region N 3  of the gate trench  216  may be disposed at the same level, or at different levels as according to design preference. 
     The gate trench  216  may have an inclined sidewall so that a lower region has a smaller area than an upper region in the active regions  203   a  as illustrated in  FIG. 11 . 
     The gate trench  216  may have a bottom region disposed at a lower level than the first bottom region N 1  in the trench isolation layer  209 . 
     A buried gate line  224  partially filling the gate trench  216  may be provided. The gate line  224  may cover all or a portion of the sidewalls of the active region  203   a.    
     Meanwhile, a gate dielectric layer  221  may be interposed between the gate line  224  and the active region  203   a.  The gate dielectric layer  221  may be a silicon oxide layer or a high-k dielectric layer. 
     Source and drain regions  230  may be provided in the active region  203   a  adjacent to both sides of the gate line  224 . Upper surfaces of the source and drain regions  230  may be disposed at a higher level than the gate line  224 . An insulating pattern  227  may be provided on the gate line  224 . The insulating pattern  227  may be, for example, a silicon oxide layer. The upper surfaces of the source and drain regions  230  and insulating layer  227  may be disposed at substantially the same level. The gate line  224  in the active region  203   a  may function as a gate electrode of a MOS transistor. Thus, a MOS transistor having the gate line  224 , the gate dielectric layer  221  and the source and drain regions  230  may be provided. The gate trench  216  in the active region  203   a,  which is covered by the gate line  224 , may have the first to third bottom regions, N 1  to N 3 , respectively, disposed at different levels from each other, and thus form a triple-fin structure. That is, the MOS transistor may have a greater effective channel width than a planar-type transistor due to its fin structure. Thus, according to an exemplary embodiment of the invention, a MOS transistor which can improve on-current characteristics, body effect, and sub-threshold swing characteristics of a transistor as well as a short channel effect may be provided. Although not shown in this figure, a data storage element may be provided like in  FIG. 5B , and electrically connected to one selected from the source and drain regions  230 . 
     It should be apparent that while the exemplary embodiments of the invention described herein detail the use of a fin structure as illustrated, for example, in  FIGS. 5A and 10 , a different fin-like structure may be used according to design preference. For example, the upper and lower portions of the fins may be connected with an irregular taper to enable a contour that is reverse-tapered, hump-like or ball-like, or any variation thereof. Accordingly, one of ordinary skill in the art who has read and understood the aspects of the invention described herein will understand that various fin contours may be implemented without departing from the spirit and scope of this invention. 
     Methods of fabricating the semiconductor device according to the exemplary embodiments described above will be described below. 
     First, a method of fabricating a semiconductor device according to an exemplary embodiment of the invention will be described with reference to  FIGS. 1 ,  2 A,  2 B,  3 A,  3 B,  4 A,  4 B,  5 A and  5 B. 
     Referring to  FIGS. 1 ,  2 A and  2 B, a semiconductor substrate  100  is prepared. The semiconductor substrate  100  may be a single crystal silicon wafer or any material that is exhibits the appropriate semiconductor-like characteristics, whether single crystal or otherwise. An isolation trench  103   s  having an inclined sidewall may be formed on the semiconductor substrate  100  to define an active region  103   a  having an area gradually becoming larger from top to bottom. The isolation trench  103   s  may be formed to have a slope of Θ degrees. For example, the Θ may be 70 to 85 degrees. When the semiconductor substrate  100  is, for example, a single crystal silicon wafer, the isolation trench  103   s  may be formed by etching the semiconductor substrate  100  in a gas atmosphere including any one or more of Cl 2  gas, HBr gas and oxygen. Of course, other gases may be used according to design preference. 
     A buffer oxide layer  105  and an insulating liner  106  may be sequentially formed on an inner wall of the isolation trench  103   s.  The buffer oxide layer  105  is formed to cure etching damage to the semiconductor substrate  100  during the formation of the isolation trench. The buffer oxide layer  105  may be formed by thermally oxidizing the substrate having the isolation trench  103   s.  The insulating liner  106  may be formed, for example, of a silicon nitride layer by chemical vapor deposition or other suitable deposition method. The insulating liner  106  is formed to prevent oxidation of the semiconductor substrate contacting the inner wall of the isolation trench  103   s  by a subsequent thermal process for forming the semiconductor substrate. Also, the insulating liner  106  may prevent a reduction in area of the active region  103   a  due to oxidation by the subsequent thermal process. 
     A trench isolation layer  109  filling the isolation trench  103   s  whose inner wall is covered by the insulating liner  106  is formed. The formation of the trench isolation layer  109  may include forming an isolation insulating layer on the substrate having the isolation trench  103   s,  and planarizing the isolation insulating layer. The trench isolation layer  109  may be, for example, a silicon oxide layer. 
     Referring to  FIGS. 1 ,  3 A and  3 B, a mask  112  having an opening  112   a  extending to the trench isolation layer  109  across the active region  103   a  may be formed on the substrate having the trench isolation layer  109 . The mask  112  may be formed of a photoresist layer or a hard mask pattern. For example, the hard mask pattern may include a silicon nitride layer. 
     The active region  103   a  may be etched using the mask  112  and the trench isolation layer  109  as etch masks to form an active trench  115   a.  Here, the active trench  115   a  may have an inclined sidewall resulting in a lower region being narrower than an upper region thereof. The active trench  115   a  may be formed, for example, by the etching process used for forming the isolation trench  103   s,  as described above. 
     Referring to  FIGS. 1 ,  4 A and  4 B, the trench isolation layer  109  may be etched using the mask  112  as an etch mask to form a field trench  115   f.  The field trench  115   f  may expose the sidewall of the active region  103   a.  When the trench isolation layer  109  is formed, for example, of a silicon oxide layer, the field trench  115   f  may be formed by etching the trench isolation layer  109  in a gas atmosphere including, for example, C 4 F 6  gas, oxygen gas, CHF 3  gas and Ar gas. Other gases deemed suitable may be used according to design preference. As a result, a gate trench  116  having the active trench  115   a  and the field trench  115   f  may be formed. 
     The gate trench  116  may be formed to have a first bottom region M 1  in the active region  103   a  adjacent to the trench isolation layer  109 , and a second bottom region M 2  disposed at a lower level than the first bottom region M 1  in the active region  103   a  adjacent to the first bottom region M 1 . Thus, a first region B 1  of the active region  103   a,  corresponding to the first bottom region M 1  of the gate trench  116 , may be more projected than a second region B 2  of the active region  103   a,  corresponding to the second bottom region M 2 . Accordingly, the mask  112  may be removed. 
     Referring to  FIGS. 1 ,  5 A and  5 B, a gate dielectric layer  121  may be formed on the substrate having the gate trench  116 . The gate dielectric layer  121  may be a silicon oxide layer or a high-k dielectric layer. A gate line  124  partially filling the gate trench  116  may be formed on the substrate having the gate dielectric layer  121 . The gate line  124  may include one selected, for example, from a group comprising of a poly silicon layer, a metal silicide layer, a metal layer and a combination thereof. 
     An insulating pattern  127  filling the rest of the gate trench  116  may be formed on the gate line  124 . The insulating pattern  127  may be, for example, formed of a silicon oxide layer. The formation of the insulating pattern  127  may include forming an insulating layer filling the rest of the gate trench  116  and covering the semiconductor substrate  100 , and planarizing the insulating layer. The insulating layer may be planarized by an etch-back process, if desired. 
     Source and drain regions  130  may be formed in the active region  103  adjacent to both sides of the gate line  124 . Upper surfaces of the source and drain regions  130  may be disposed at a higher level than the gate line  124 . 
     An interlayer insulating layer  132  may be formed on the substrate having the source and drain regions  130 . A contact plug  133  passing through the interlayer insulating layer  132  and electrically connected to one of the source and drain regions  130  may be formed. A data storage element  136  covering the contact plug  133  may be formed on the interlayer insulating layer  132 . The data storage element  136  may include a lower electrode, a capacitor dielectric layer and an upper electrode to be employed in a volatile memory device such as a DRAM, or may include a lower electrode, a phase change material layer and an upper electrode to be employed in a non-volatile memory device such as a PRAM. Other elements, whether storage-capable or not may be implemented according to design preference. 
     Next, a method of fabricating a semiconductor device according to another exemplary embodiment of the invention will be described with reference to  FIGS. 6 to 11 . 
     Referring to  FIGS. 6 and 7 , a semiconductor substrate  200  is prepared, which may be a single crystal silicon wafer. An isolation trench  203   s  having an inclined sidewall may be formed to define an active region  203   a  having an area gradually becoming larger from top to bottom on the semiconductor substrate  200 . The isolation trench  203   s  may be formed by the same process as that of forming the isolation trench  103   s  according to the above described exemplary embodiment(s) of the invention. 
     A buffer oxide layer  205  and an insulating liner  206  may be sequentially formed on an inner wall of the isolation trench  203   s.  A trench isolation layer  209  filling the isolation trench  203   s  whose inner wall is covered by the insulating liner  206  is formed. The trench isolation layer  209  may be, for example, a silicon oxide layer. Thus, the trench isolation layer  209  may have an inclined sidewall. 
     A mask having an opening extending to the trench isolation layer  209  across the active region  203   a  may be formed. The mask is formed by substantially the same process as that of forming the mask  112  according to the above described exemplary embodiment(s) of the invention. The trench isolation layer  209  may be partially etched using the mask and the active region  203   a  as etch masks. As a result, an upper region of the active region  203   a  may be exposed. 
     Referring to  FIGS. 6 and 8 , the active region  203   a  may be etched using the mask and the partially etched trench isolation layer  209  as etch masks to form an active trench  215   a.  When seen along line III-III′ of  FIG. 6 , the active region  203   a  may be formed to be projected in the middle thereof. This projection arises because if the etching process is performed while the upper region of the active region  203   a  is exposed by the mask and the partially etched trench isolation layer  209 , as illustrated in  FIG. 7 , there is a difference in depth between the depth obtained by etching the active region  203   a  adjacent to the upper surface of the partially etched trench isolation layer  209  and the depth obtained by etching the upper region of the active region  203   a  exposed by the mask and the partially etched trench isolation layer  209 . That is, the depth obtained by etching the active region  203   a  adjacent to the upper surface of the partially etched trench isolation layer  209  may be deeper. Since the trench isolation layer  209  may have an inclined sidewall, the active region  203   a  overlapping the inclined sidewall of the trench isolation layer  209  may remain.  FIG. 8  is a cross-sectional view taken along line III-III′ of  FIG. 6 , and shows a bottom region of the active trench  215   a.  Accordingly, as illustrated in  FIG. 8 , the bottom region of the active trench  215   a  in the active region  203   a  may have a “W” shaped profile. 
     Referring to  FIGS. 6 and 9 , the partially etched trench isolation layer  209  may be etched using the mask as an etch mask to form a field trench  215   f.  Thus, a gate trench  216  having the field trench  215   f  and the active trench  215   a  may be formed. 
     The sidewall of the active region  203   a  may be exposed by the field trench  215   f.    
     The surface of the active region  203   a  exposed by the gate trench  216  is cleaned, and a pointed part of the active region  203   a  may be etched to have a more rounded appearance. 
     Consequently, the gate trench  216  may have a first bottom region N 1  in the active region  203   a  adjacent to the trench isolation layer  209 , a second bottom region N 2  disposed at a lower level than the first bottom region N 1  in the active region  203   a  adjacent to the first bottom region N 1 , and a third bottom region N 3  disposed at a higher level than the second bottom region N 2 . Thus, a first region C 1  of the active region  203   a,  corresponding to the first bottom region N 1  of the gate trench  216 , may be more projected than a second region C 2  of the active region  203   a,  corresponding to the second bottom region N 2 , and a third region C 3  of the active region  203   a,  corresponding to the third bottom region N 3 , may be more projected than the second region C 2 . 
     The first and third bottom regions N 1  and N 3  of the gate trench  216  may be disposed at substantially the same level, or at different levels. For example, the levels of the first bottom region N 1  and the third bottom region N 3  may be determined by the depth of the trench isolation layer  209 , which may be controlled by partially etching the trench isolation layer  209 , and using a mask and the active region  203   a  as etch masks to expose the upper region of the active region  203   a.  Then, the mask(s) may be removed. 
     Referring to  FIGS. 6 ,  10  and  11 , a gate dielectric layer  221  may be formed on the substrate having the gate trench  216 . The gate dielectric layer  221  may be, for example, a silicon oxide layer or a high-k dielectric layer. A gate line  224  partially filling the gate trench  216  may be formed on the substrate having the gate dielectric layer  221 . 
     An insulating pattern  227  filling the rest of the gate trench  216  may be formed on the gate line  224 . The insulating pattern  227  may be formed of a silicon oxide layer. The formation of the insulating pattern  227  may include forming an insulating layer filling the rest of the gate trench  216  and covering the semiconductor substrate  200 , and then planarizing the insulating layer. The insulating layer may be planarized, for example, by an etch-back process. 
     Source and drain regions  230  may be formed in the active region  203   a  adjacent to both sides of the gate line  224 . The upper surfaces of the source and drain regions  230  may be disposed at a higher level than the gate line  224 . 
     A data storage element, which is not illustrated, electrically connected to one selected from the source and drain regions  230  may be formed as necessary. 
     Consequently, a buried gate line is provided using a gate trench extending to a trench isolation layer across an active region. The gate trench in the active region may have bottom regions disposed at different levels from each other. Thus, a semiconductor device having an effective channel width and length which are relatively greater than conventional transistors can be realized. 
     Exemplary embodiments of the invention have been disclosed herein and, although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as set forth in the following claims.