Abstract:
A recess gate of a semiconductor device includes: a substrate having a bulb-shaped recess pattern formed therein, wherein the bulb-shaped recess pattern includes a first ball pattern and a second ball pattern formed therein, the first ball pattern having a different diameter than the second ball pattern; a gate insulation layer formed over the bulb-shaped recess pattern and the substrate; and a conductive layer formed over the gate insulation layer and filling the bulb-shaped recess pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims the benefit of priority to Korean patent application number 10-2006-0096525, filed on Sep. 29, 2006, which is incorporated by reference in its entirety. 
   BACKGROUND 
   The present invention relates to a semiconductor device and a method for fabricating the same; and more particularly, to a bulb-shaped recess gate and a method for fabricating the same. 
   Typically, a recess gate of a semiconductor device is considered as a special structure which cannot be excluded from the fabrication of the semiconductor device. The recess gate increases electric properties including a threshold voltage, and a refresh time which may be generated due to a decreased channel area of a gate as a device pattern becomes densified. The recess gate also increases a length of the gate undergoing a gate patterning process, resulting in an increased channel area, thereby improving a device property. 
   However, as a device size has been reduced, patterns become smaller and a distance between the devices becomes reduced. Accordingly, it is required to increase the channel area. 
   Recently, a bulb-shaped recess gate increasing the channel area by increasing an area of a bottom portion of the recess gate has been suggested. 
     FIG. 1  illustrates a typical method for fabricating a bulb-shaped recess gate. 
   A bulb-shaped recess pattern  12  includes a neck pattern  12 A and a ball pattern  12 B, both formed over a substrate  11 . 
   A gate oxide layer  13  is formed over surfaces of bulb-shaped recess pattern  12  and substrate  11 . Then, a polysilicon layer  14  filling bulb-shaped recess pattern  12 , and used as a gate electrode, is formed over gate oxide layer  13 . 
   As for bulb-shaped recess pattern  12 , during forming polysilicon layer  14  which is the gate electrode, the inside of neck pattern  12 A is filled with polysilicon layer  14  before ball pattern  12 B is filled with the polysilicon layer  14 . As a result, a void V 1  may be generated. 
     FIGS. 2A and 2B  are micrographs illustrating a void V 2  typically generated during forming a polysilicon layer. 
   Void V 2  generated during the formation of the polysilicon layer does not typically affect a device property. However, if a width of a neck pattern of a bulb-shaped recess gate is small while that of a ball pattern of the bulb-shaped recess gate is large, a size of void V 2  may be increased. The increased size of void V 2  may then reduce a thickness of the polysilicon layer. Thus, an electric property of the device may be decreased. 
   A method for increasing the width of the neck pattern may be suggested to reduce the size of void V 2 . However, increasing the width of the neck pattern reduces an overlay margin between the neck pattern and a gate electrode formed over the neck pattern, thereby producing a mis-alignment, resulting in difficulties in device fabrication. 
   SUMMARY 
   Embodiments consistent with the present invention provide a bulb-shaped recess gate capable of minimizing a size of a void, usually generated inside a ball pattern of a bulb-shaped recess pattern, during formation of a polysilicon layer and a method for fabricating the same. 
   Consistent with the present invention, there is provided a recess gate of a semiconductor device, including: a substrate having a bulb-shaped recess pattern formed therein, wherein the bulb-shaped recess pattern includes a first ball pattern and a second ball pattern formed therein, the first ball pattern having a different diameter than the second ball pattern; a gate insulation layer formed over the bulb-shaped recess pattern and the substrate; and a conductive layer formed over the gate insulation layer and filling the bulb-shaped recess pattern. 
   Further consistent with the present invention, there is provided a method for fabricating a recess gate of a semiconductor device, including: forming a bulb-shaped recess pattern in a substrate, wherein forming the bulb-shaped recess pattern comprises: forming a first ball pattern to have a first diameter in the substrate; and forming a second ball pattern to have a second diameter in the substrate; forming a gate insulation layer over the bulb-shaped recess pattern and the substrate; and forming a conductive layer over the gate insulation layer such that the conductive layer fills the bulb-shaped recess pattern. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a typical method for fabricating a bulb-shaped recess gate; 
       FIGS. 2A and 2B  are micrographs illustrating a void typically generated during forming a polysilicon layer; 
       FIG. 3  illustrates a bulb-shaped recess gate in accordance with a first embodiment consistent with the present invention; 
       FIGS. 4A to 4F  illustrate a method for fabricating the bulb-shaped recess gate in accordance with the first embodiment; 
       FIG. 5  illustrates a bulb-shaped recess gate in accordance with a second embodiment consistent with the present invention; 
       FIGS. 6A to 6F  illustrate a method for fabricating the bulb-shaped recess gate in accordance with the second embodiment; and 
       FIGS. 7A and 7B  are micrographs illustrating the bulb-shaped recess pattern in accordance with the first and second embodiments consistent with the present invention and a result obtained forming a polysilicon layer. 
   

   DETAILED DESCRIPTION 
   According to embodiments consistent with the present invention which will be explained hereinafter, when a bulb-shaped recess gate is formed, a top portion of a neck pattern has substantially the same line width as that of the typical neck pattern. However, a bottom portion of the neck pattern contacting a ball pattern is formed in a large ball-shape to facilitate the formation of polysilicon. As a result, a bulb-shaped recess gate pattern can be formed in a gourd-shape. 
   A void cannot be generated in a region where the ball pattern and the neck pattern meet during the formation of the polysilicon. As a result, a size of the void can be reduced, thereby increasing an area of polysilicon and uniformly maintaining the line width of the top portion of the neck pattern. Accordingly, an overlay margin between the neck pattern and a gate electrode is not reduced. 
     FIG. 3  illustrates a bulb-shaped recess gate in accordance with a first embodiment consistent with the present invention. 
   A plurality of bulb-shaped recess patterns  100 , each including a first ball pattern  25 A and a second ball pattern  27  are formed in a silicon substrate  21 . A gate oxide layer  28  is formed over surfaces of bulb-shaped recess patterns  100  and silicon substrate  21 . 
   A polysilicon layer  29  is then formed to fill the inside of bulb-shaped recess patterns  100 . 
   The shapes and diameters of first ball pattern  25 A and second ball pattern  27  may be different from each other. For example, first ball pattern  25 A may have an elliptical shape while second ball pattern  27  may have a shape similar to a sphere. The diameter of first ball pattern  25 A may be smaller than that of second ball pattern  27 . Depths of first ball pattern  25 A and second ball pattern  27  may be approximately the same with each other. Both of first ball pattern  25 A and second ball pattern  27  are formed to the depths ranging from approximately 200 Å to approximately 500 Å. 
   Accordingly, bulb-shaped recess patterns  100 , each including first ball pattern  25 A and second ball pattern  27  are formed in gourd-shapes. Since first ball pattern  25 A, constituting a neck portion of corresponding bulb-shaped recess pattern  100 , is formed in an elliptical shape, polysilicon layer  29  fills the inside of second ball pattern  27  such that a small sized void V 3  is produced. 
     FIGS. 4A to 4F  illustrate a method for fabricating a bulb-shaped recess gate in accordance with the first embodiment consistent with the present invention. Herein, the same reference numerals used in  FIG. 3  are also used to denote the same elements in  FIGS. 4A to 4F . 
   As shown in  FIG. 4A , trenches are formed in a silicon substrate  21 . Then, an oxide layer fills the trenches to form a plurality of field oxide layers  22 . Field oxide layers  22  may be formed from a high density plasma oxide. 
   A hard mask layer  23  is formed over silicon substrate  21 . A photoresist layer is formed over hard mask layer  23 . Then the photoresist layer is patterned, and a photo-exposure process and a developing process are performed thereon to form a recess gate mask  24 . The hard mask layer  23  may include polysilicon. 
   As shown in  FIG. 4B , hard mask layer  23  is etched using recess gate mask  24  as an etch barrier. A reference numeral  23 A denotes a hard mask pattern. Since hard mask layer  23  includes polysilicon, hydrogen bromide (HBr), chlorine (Cl 2 ), or a combination thereof may be used as an etch gas when the hard mask layer  23  is etched. 
   After hard mask layer  23  is etched, exposed portions of silicon substrate  21  are etched to certain depths using the same etch gas used to etch hard mask layer  23 . As a result, a plurality of first neck patterns  25  of bulb-shaped recess patterns are formed. Depths of first neck patterns  25  range from approximately 200 Å to approximately 500 Å, and widths of first neck patterns  25  range from approximately 100 Å to approximately 200 Å. The etching process to form first neck patterns  25  includes performing a plasma etch using a mixture gas of HBr and Cl 2 . A flow rate of the HBr gas ranges from approximately 30 sccm to approximately 150 sccm, and a flow rate of the Cl 2  gas ranges from approximately 10 sccm to approximately 60 sccm. 
   When the formation of first neck patterns  25  is completed, recess gate mask  24  is removed. Accordingly, the hard mask pattern  23 A serves a role as an etch barrier. 
   First neck patterns  25  have vertically shaped sidewalls. However, according to this embodiment consistent with the present invention, the following method is used to transform the vertically shaped sidewalls of first neck patterns  25  to bulb-shaped sidewalls to increase widths of the sidewalls of first neck patterns  25 . 
   As shown in  FIG. 4C , after first neck patterns  25  having the vertically shaped sidewalls are formed, an isotropic etching process is additionally performed. The isotropic etching process is performed in-situ in the same chamber used to form first neck patterns  25 . For instance, the etching process to form first neck patterns  25  and the isotropic etching process may be performed using an inductively coupled plasma (ICP) type apparatus. 
   According to this embodiment consistent with the present invention, the isotropic etching process includes using a power ranging from approximately 1 W to approximately 20 W, and a mixture gas of tetrafluoromethane (CF 4 ), oxygen (O 2 ), and helium (He) as an etch gas. A flow rate of the CF 4  gas ranges from approximately 20 sccm to approximately 80 sccm. A flow rate of the O 2  gas ranges from approximately 5 sccm to approximately 10 sccm. A flow rate of the He gas ranges from approximately 100 sccm to approximately 200 sccm. 
   If the isotropic etching process is performed using a power ranging from approximately 1 W to approximately 20 W, a characteristic of an isotropic etch in which a radical chemically etches a surface, can be increased more than that of an etch in which an ion etches a surface in a straight line manner. Accordingly, the isotropic etch changes the vertically shaped sidewalls of first neck patterns  25  to elliptically shaped sidewalls of first neck patterns  25 . 
   As the sidewalls of first neck patterns  25  are formed into elliptical shapes, surface areas of first neck patterns  25  are increased to form second neck patterns  25 A. Inside portions of second neck patterns  25 A have surface areas larger than top portions thereof. Accordingly, second neck patterns  25 A formed performing the isotropic etching process is transformed into a ball pattern formed with a smooth curved line similar to a subsequent ball pattern. Hereinafter, second neck patterns  25 A will be referred to as first ball patterns  25 A. If the isotropic etching process is performed to form first ball patterns  25 A, line widths of the top portions of the first ball patterns  25 A can be maintained to be about the same as those of the top portions of first neck patterns  25 . As a result, an overlay margin between first ball patterns  25 A and a subsequent gate electrode is not reduced. 
   As shown in  FIG. 4D , passivation sidewalls  26  are formed over the sidewalls of first ball patterns  25 A. Passivation sidewalls  26  may comprise an oxide layer formed through a thermal oxidation process, an oxide layer formed through a deposition process, a nitride layer, or a nitride layer containing a large amount of silicon is formed to a depth ranging from approximately 50 Å to approximately 100 Å. Then, certain portions thereof are etched to remain only on the sidewalls of first ball patterns  25 A. Passivation sidewalls  26  protect the sidewalls of first ball patterns  25 A from being damaged during a subsequent etching process. 
   Bottom surfaces of first ball patterns  25 A are subjected to an isotropic etching process to form a plurality of second ball patterns  27  having sphere-shaped sidewalls. The isotropic etching process includes using the above described conditions. Second ball patterns  27  have sizes, i.e., diameters, larger than those of first ball patterns  25 A. Depths of second ball patterns  27  may be larger than or the same as those of first ball patterns  25 A. For instance, both of first ball patterns  25 A and second ball patterns  27  may be formed to a depth ranging from approximately 200 Å to approximately 500 Å. Since the line widths of the vertically shaped first neck patterns  25  range from approximately 100 Å to approximately 200 Å, the diameters of first ball patterns  25 A range from approximately 300 Å to approximately 500 Å, and the diameters of second ball patterns  27  range from approximately 500 Å to approximately 700 Å. 
   As shown in  FIG. 4E , hard mask pattern  23 A and passivation sidewalls  26  are removed. 
   After hard mask pattern  23 A and passivation sidewalls  26  are removed, bulb-shaped recess patterns  100  including first ball patterns  25 A and second ball patterns  27  are formed. As described above, first ball patterns  25 A and second ball patterns  27  have different shapes and diameters from each other. Reference letter D 1  denotes the diameter of each of first ball patterns  25 A, and reference letter D 2  denotes the diameter of each of second ball patterns  27 . As shown, diameter D 2  of second ball pattern  27  is larger than D 1  of first ball pattern  25 A, i.e., D 2 &gt;D 1 . As a result, bulb-shaped recess gate patterns  100  can have large surface areas. Particularly, because first ball patterns  25  are formed in an elliptical shape, bulb-shaped recess patterns  100  according to this embodiment consistent with the present invention have surface areas which are larger than the typical bulb-shaped recess pattern. As described above, bulb-shaped recess patterns  100  including first ball patterns  25 A and second ball patterns  27  have different shapes and diameters from each other, and thus can be formed in gourd-shapes. 
   The top portions of first ball patterns  25 A of bulb-shaped recess patterns  100  maintain the line width initially defined and thus, an overlay margin between first ball patterns  25 A and a subsequent gate electrode cannot be reduced. 
   As shown in  FIG. 4F , a gate oxide layer  28  is formed over surfaces of bulb-shaped recess patterns  100 . Then, a polysilicon layer  29  used as a gate electrode is formed over an entire surface of the above resulting structure until bulb-shaped recess patterns  100  are filled. 
   During the formation of polysilicon layer  29 , void generation may be minimized by first ball patterns  25 A of bulb-shaped recess patterns  100  formed in the gourd-shape. Although polysilicon layer  29  is formed until the inside of second ball patterns  27  is completely filled, the top portions of bulb-shaped recess patterns  100  cannot be blocked by first ball patterns  25 A having the increased sizes. As a result, a size of a void V 3  is minimized. 
   First ball patterns  25 A formed in elliptical shapes increases an area of the polysilicon layer  29 , thereby increasing a channel length. 
   Although not shown, a tungsten silicide layer and a gate hard mask nitride layer are formed over polysilicon layer  29  and then, a gate patterning process is performed. 
     FIG. 5  illustrates a bulb-shaped recess gate in accordance with a second embodiment consistent with the present invention. 
   A plurality of bulb-shaped recess patterns  200 , each including a first ball pattern  35 A and a second ball pattern  37  are formed in a silicon substrate  31 . A gate oxide layer  38  is formed over surfaces of bulb-shaped recess patterns  200  and silicon substrate  31 . 
   A polysilicon layer  39  fills the inside of bulb-shaped recess patterns  200 . 
   Shapes and diameters of first ball pattern  35 A and second ball pattern  37  may be different from each other. For example, first ball pattern  35 A has an elliptical shape while second ball pattern  37  has a shape similar to a sphere. The diameter of first ball pattern  35 A may be smaller than that of second ball pattern  37 . Depths of first ball pattern  35 A and the second ball pattern  37  may be the same. 
   Accordingly, bulb-shaped recess patterns  200 , each including first ball pattern  35 A and second ball pattern  37  are formed in gourd-shapes. Since first ball pattern  35 A is formed into an elliptical shape, a polysilicon layer  39  can fill the inside of second ball pattern  37  and only produce a small sized void V 4 . 
     FIGS. 6A to 6F  illustrate a method for fabricating a bulb-shaped recess gate in accordance with the second embodiment consistent with the present invention. Herein, the same reference numerals used in  FIG. 5  are also used to denote the same elements in  FIGS. 6A to 6F . 
   As shown in  FIG. 6A , trenches are formed in a silicon substrate  31 . Then, an oxide layer fills the trenches to form a plurality of field oxide layers  32 . Field oxide layers  32  may be formed of a high density plasma oxide. 
   A hard mask layer  33  is formed over silicon substrate  31 . A photoresist layer is formed over hard mask layer  33 . Then, the photoresist layer is patterned, and a photo-exposure process and a developing process are performed thereon to form a recess gate mask  34 . Hard mask layer  33  may include polysilicon. 
   As shown in  FIG. 6B , hard mask layer  33  is etched using recess gate mask  34  as an etch barrier. Reference numeral  33 A denotes a hard mask pattern. Since hard mask layer  33  includes polysilicon, hydrogen bromide (HBr), chlorine (Cl 2 ), or a combination thereof may be used as an etch gas when hard mask layer  33  is etched. 
   After hard mask layer  33  is etched, exposed portions of silicon substrate  31  are etched to certain depths using the same etch gas used to etch hard mask layer  33 . As a result, a plurality of first neck patterns  35  of bulb-shaped recess patterns are formed. Depths of first neck patterns  35  range from approximately 200 Å to approximately 500 Å, and widths of first neck patterns  35  range from approximately 100 Å to approximately 200 Å. The etching process to form first neck patterns  35  includes performing a plasma etch using a mixture gas of HBr and Cl 2 . A flow rate of the HBr gas ranges from approximately 30 sccm to approximately 150 sccm, and a flow rate of the Cl 2  gas ranges from approximately 10 sccm to approximately 60 sccm. 
   When the formation of first neck patterns  35  is completed, recess gate mask  34  is removed. Accordingly, hard mask pattern  33 A serves a role as an etch barrier in a subsequent etch process. 
   First neck patterns  35  have vertically shaped sidewalls. However, according to this embodiment consistent with the present invention, the following method is used to transform the vertically shaped sidewalls of first neck patterns  35  to ball-shaped sidewalls to increase widths of the sidewalls of first neck patterns  35 . 
   As shown in  FIG. 6C , after first neck patterns  35  having the vertically shaped sidewalls are formed, an isotropic etching process is additionally performed. The isotropic etching process is performed in a chamber different from that used to form first neck patterns  35 . For example, first neck patterns  35  may be etched by using an inductively coupled plasma (ICP) type apparatus. 
   According to this embodiment consistent with the present invention, the isotropic etching process includes using a microwave dry etching apparatus. A mixture gas of tetrafluoromethane (CF 4 ), oxygen (O 2 ), and helium (He) is used as an etch gas during the isotropic etching process. A flow rate of the CF 4  gas ranges from approximately 20 sccm to approximately 80 sccm. A flow rate of the O 2  gas ranges from approximately 5 sccm to approximately 10 sccm. A flow rate of the He gas ranges from approximately 100 sccm to approximately 200 sccm. A microwave power rages from approximately 500 W to approximately 2,500 W. If the isotropic etching process includes using the microwave dry etching apparatus, the microwave removes an ion from a plasma which contributes to the etching of a surface in a straight line. As a result, the ion cannot reach bottom portions of first neck patterns  35  but a chemical etch (e.g., radicals) may be used to perform the isotropic etching process. 
   As the sidewalls of first neck patterns  35  are formed into elliptical shapes, surface areas of first neck patterns  35  are increased to form second neck patterns  35 A. Inside portions of second neck patterns  35 A have surface areas larger than top portions thereof. Accordingly, second neck patterns  35 A formed by performing the isotropic etching process is transformed to a ball pattern formed with a smooth curved line similar to a subsequent ball pattern. Hereinafter, second neck patterns  35 A will be referred to as first ball patterns  35 A. If the isotropic etching process is performed to form first ball patterns  35 A, line widths of the top portions of the first ball patterns  35 A may be the same as those of the top portions of first neck patterns  35 . As a result, an overlay margin between the first ball patterns  35 A and a subsequent gate electrode is not be reduced. 
   As shown in  FIG. 6D , passivation sidewalls  36  are formed over the sidewalls of first ball patterns  35 A. Passivation sidewalls  36  may include an oxide layer formed through a thermal oxidation process, an oxide layer formed through a deposition process, a nitride layer, or a nitride layer containing a large amount of silicon, and may be formed to a depth ranging from approximately 50 Å to approximately 100 Å. Then, certain portions thereof are etched to remain only on the sidewalls of first ball patterns  35 A. Passivation sidewalls  36  protect the sidewalls of first ball patterns  35 A from being damaged during a subsequent etching process. 
   Bottom surfaces of first ball patterns  35 A are subjected to an isotropic etching process to form second ball patterns  37 . The isotropic etching process includes using the above described conditions. Second ball patterns  37  have sizes, i.e., diameters and depths larger than those of first ball patterns  35 A. Depths of second ball patterns  37  are larger than or the same as those of first ball patterns  35 A. For example, both of first ball patterns  35 A and the second ball patterns  37  may be formed to depths ranging from approximately 200 Å to approximately 500 Å. Since the line widths of the vertically shaped first neck patterns  35  range from approximately 100 Å to approximately 200 Å, the diameters of first ball patterns  35 A range from approximately 300 Å to approximately 500 Å, and the diameters of second ball patterns  37  range from approximately 500 Å to approximately 700 Å. 
   As shown in  FIG. 6E , hard mask pattern  33 A and passivation sidewalls  36  are removed. 
   After hard mask pattern  33 A and passivation sidewalls  36  are removed, bulb-shaped recess patterns  200  including first ball patterns  35 A and second ball patterns  37  are formed. As described above, first ball patterns  35 A and second ball patterns  37  have different shapes and diameters from each other. Reference letter D 3  denotes the diameter of each of first ball patterns  35 A, and reference letter D 4  denotes the diameter of each of second ball patterns  37 . As shown, diameter D 4  of second ball pattern  37  is larger than diameter D 3  of first ball pattern  35 A, i.e., D 4 &gt;D 3 . As a result, bulb-shaped recess gate patterns  200  can have large surface areas. Particularly, because first ball patterns  35  are formed in an elliptical shape, bulb-shaped recess patterns  200  according to this embodiment consistent with the present invention have surface areas larger than the typical bulb-shaped recess pattern. As described above, bulb-shaped recess patterns  200  including first ball patterns  35 A and second ball patterns  37  may have the different shapes and diameters from each other, and can be formed in gourd-shapes. 
   The top portions of first ball patterns  35 A of bulb-shaped recess patterns  200  maintain the line widths initially defined and thus, an overlay margin between first ball patterns  35 A and a subsequent gate electrode cannot be reduced. 
   As shown in  FIG. 6F , a gate oxide layer  38  is formed over surfaces of bulb-shaped recess patterns  200 . Then, a polysilicon layer  39  used as a gate electrode is formed over an entire surface of the above resulting structure until bulb-shaped recess patterns  200  are filled. 
   During the formation of polysilicon layer  39 , void generation can be minimized by first ball patterns  35 A of bulb-shaped recess patterns  200 . Although polysilicon layer  39  is formed until the inside of second ball patterns  37  are completely filled, the top portions of bulb-shaped recess patterns  200  can not be blocked by first ball patterns  35 A with the increased sizes. As a result, a size of a void V 4  can be minimized. 
   First ball patterns  35 A formed in the elliptical shapes thus increase an area of polysilicon layer  39 , thereby more increasing a channel length. 
   Although not shown, a tungsten silicide layer and a gate hard mask nitride layer are formed over polysilicon layer  39  and then, a gate patterning process is performed. 
     FIGS. 7A and 7B  are micrographs illustrating the bulb-shaped recess pattern according to the first and second embodiments of the present invention and a result obtained forming a polysilicon layer. 
   A size of a void V 5  generated after forming the polysilicon layer is very small. Particularly, the size of void V 5  shown in  FIGS. 7A and 7B  is much smaller than that of void V 2  shown in  FIGS. 2A and 2B . According to the first and second embodiments consistent with the present invention, by forming a first ball pattern in an elliptical shape, a size of a void V 5  can be reduced. 
   According to the embodiments consistent with the present invention, a bulb-shaped recess pattern including ball patterns having different shapes and diameter from each other is formed in a gourd-shape. Accordingly, a size of a void generated during forming a polysilicon layer can be minimized. 
   The bulb-shaped recess pattern formed in the gourd shape can increase an area of the polysilicon layer and uniformly maintain a line width of a top portion thereof. Accordingly, an overlay margin between the top portion of the bulb-shaped recess pattern and a gate electrode cannot be reduced. 
   While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.