# Patent

## Title
Semiconductor device with buried bit lines and method for fabricating the same

## Abstract

A semiconductor device includes: a semiconductor substrate configured to include a plurality of trenches therein; a plurality of buried bit lines each configured to fill a portion of each trench; a plurality of active pillars each formed in an upper portion of each buried bit line; a plurality of vertical gates each configured to surround each active pillar; and a plurality of word lines configured to couple neighboring vertical gates with each other.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
The present application claims priority of Korean Patent Application Nos. 10-2009-0117433 and 10-2010-0051196, filed on Nov. 30, 2009, and May 31, 2010, respectively, which are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
Exemplary embodiments of the present invention relate to a method for fabricating a semiconductor device, and more particularly, to a semiconductor device including buried bit lines, and a method for fabricating the semiconductor device.
To increase the integration degree, memory devices manufactured by under 40 nm process are useful. In case of planar transistors or recessed gate transistors used in cell architectures of 8F2 or 6F2, where F denotes a minimum feature size, it is difficult to perform scaling under 40 nm. Therefore, a cell having a cell architecture of 4F2 that may improve the integration degree by 1.5 to 2 times with the same scaling is useful. Accordingly, a vertical gate forming process is introduced.
Vertical gate forming process is a process for forming a surround-type gate electrode, which is referred to as a vertical gate, hereafter, surrounding an active pillar extended vertically from a semiconductor substrate and forming a source region and a drain region in the upper and lower portions of the active pillar with respect to the vertical gate, respectively. With the vertical gate forming process, channels are formed vertically and accordingly, although the area of a transistor on the semiconductor substrate is reduced, the length of channels is not restricted by the reduced area.
Through the vertical gate forming process, memory cells come to have buried bit lines for high integration, which are formed by in performing ion implantation using a dopant.
However, in shrinking a semiconductor device, reducing the resistance of the buried bit lines by performing the dopant ion implantation is difficult. Thus, the characteristics of the semiconductor device may deteriorate.
To alleviate such a concern, a metal silicidation method is suggested for forming buried bit lines.
The conventional technology, however, has the following drawbacks.
According to the metal silicidation method, a silicided portion is cut according to the size of each cell after the metal silicidation in order to separate neighboring bit lines from each other. When the silicided portion is cut, the cross-sectional area of a contact is reduced and the reduced cross-sectional area increases a resistance value. Also, since the metal silicidation is performed from the surface of the space between active pillars, the distance from the vertical gate becomes short and thus bridging may occur.
According to conventional methods, active pillars are formed first and then bit lines are formed later. The formation of bit lines after the formation of the active pillars may negatively affect resistance.
Therefore, it is required to develop a method that can resolve the problems of complicated fabrication process and resistance increasing due, to the reduced cross-sectional area when the bit lines in are formed, while using the vertical gate forming process.
SUMMARY OF THE INVENTION
An embodiment of the present invention is directed to a semiconductor device that may decrease the resistance of buried bit lines and simplify a fabrication process, and a method for fabricating the semiconductor device.
In accordance with an embodiment of the present invention, a semiconductor device includes: a semiconductor substrate configured to include a plurality of trenches therein; a plurality of buried bit lines each configured to fill a portion of each trench; a plurality of active pillars each formed in an upper portion of each buried bit line; a plurality of vertical gates each configured to surround each active pillar; and a plurality of word lines configured to couple neighboring vertical gates with each other.
In accordance with another embodiment of the present invention, a method for fabricating a semiconductor device, which includes: forming a plurality of trenches by etching a semiconductor substrate; forming a plurality of buried bit lines each filling a portion of each trench; forming a conductive layer gap-filling upper portions of the plurality of the buried bit lines; forming a plurality of active pillars by simultaneously etching the conductive layer and the semiconductor substrate; forming a plurality of vertical gates each surrounding each active pillar; and forming a plurality of word lines coupling neighboring vertical gates with each other.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view illustrating a semiconductor device in accordance with a first embodiment of the present invention.
FIG. 2A is a cross-sectional view of the semiconductor device of FIG. 1 along a line A-A′ and a line B-B′.
FIG. 2B is a perspective view of the semiconductor device shown in FIG. 1.
FIGS. 3A to 3H are cross-sectional views describing a method for fabricating the semiconductor device of FIG. 1 along the line A-A′ and the line B-B′ in accordance with the first embodiment of the present invention.
FIGS. 4A to 4H are perspective views describing the method for fabricating the semiconductor device in accordance with the first embodiment of the present invention.
FIG. 5 is a plan view illustrating a semiconductor device in accordance with a second embodiment of the present invention.
FIG. 6A is a cross-sectional view of the semiconductor device of FIG. 5 along a line A-A′ and a line B-B′.
FIG. 6B is a perspective view of the semiconductor device shown in FIG. 5.
FIGS. 7A to 7I are cross-sectional views describing a method for fabricating the semiconductor device of FIG. 5 along the line A-A′ and the line B-B′ in accordance with the second embodiment of the present invention.
FIGS. 8A to 8I are perspective views describing the method for fabricating the semiconductor device in accordance with the second embodiment of the present invention.



DESCRIPTION OF SPECIFIC EMBODIMENTS
Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention.
The drawings are not necessarily to scale and in some instances, proportions may have been exaggerated in order to clearly illustrate features of the embodiments. When a first layer is referred to as being “on” a second layer or “on” a substrate, it not only refers to a case where the first layer is formed directly on the second layer or the substrate but also a case where a third layer exists between the first layer and the second layer or the substrate.
An exemplary embodiment is directed to a semiconductor device that may reduce the resistance of buried bit lines as well as simplifying process for manufacturing a vertical gate semiconductor device, and a method for fabricating the semiconductor device. Here, buried bit lines are formed as a metal layer under active pillars before the formation of the active pillars.
1st Embodiment
FIG. 1 is a plan view illustrating a semiconductor device in accordance with a first embodiment of the present invention. FIG. 2A is a cross-sectional view of the semiconductor device of FIG. 1 along a line A-A′ and a line B-B′. FIG. 2B is a perspective view of the semiconductor device shown in FIG. 1.
Referring to FIGS. 1, 2A and 2B, a semiconductor substrate 11A includes a plurality of trenches 12, and a plurality of buried bit lines 14 are formed to fill a portion of each trench 12, individually. The buried bit lines 14 may be a metal layer, which has a lower resistance than the resistance of the buried bit lines 14 formed through an ion implantation process. A barrier layer pattern 13A may be disposed on the interface between the buried bit lines 14 and the semiconductor substrate 11A to prevent diffusion. The barrier layer pattern 13A may be a stacked layer where a titanium (Ti) layer and a titanium nitride (TiN) layer are stacked.
A plurality of active pillars 16 are formed over the buried bit lines 14. Each active pillar 16 includes a first active pillar 15B gap-filling the upper portion of a buried bit line 14 and a second active pillar 11B formed on both sidewalls of the first active pillar 15B. The second active pillar 11B is a structural element formed by etching the semiconductor substrate 11A, and the first active pillar 15B is a silicon epitaxial layer. These elements will be described in detail later. An inter-layer dielectric layer 17 is formed over the semiconductor substrate 11A around the active pillar 16. A conductive layer pattern 15A is disposed between the buried bit line 14 and the first active pillar 15B, and the conductive layer pattern 15A is of the same material as that of the first active pillar 15B. The conductive layer pattern 15A electrically connects the active pillar 16 to the buried bit line 14.
The active pillars 16 are disposed over the buried bit lines 14, respectively. In other words, each of the active pillars 16, which are disposed apart from each other by a desired space, is formed in the upper portion of each buried bit line 14. As a result, the active pillars 16 are arranged in a matrix form.
The external walls of each active pillar 16 are surrounded by a each vertical gate 19, and a gate insulation layer 18 is formed between the vertical gate 19 and the active pillar 16.
Neighboring vertical gates 19 are connected to each other by word lines 20. The word lines 20 are insulated from the buried bit lines 14 by the inter-layer dielectric layer 17. The word lines 20 and the buried bit lines 14 are arranged to cross each other at, for example, right angles. The word lines 20 may be a metal layer. For example, the word lines 20 include any one selected from the group consisting of tungsten silicide (WSix), titanium nitride (TiN), tungsten (W), aluminum (Al), cupper (Cu), gold (Au), and ruthenium (Ru).
FIGS. 3A to 3H are cross-sectional views describing a method for fabricating the semiconductor device of FIG. 1 along the line A-A′ and the line B-B′ in accordance with the first embodiment of the present invention. FIGS. 4A to 4H are perspective views describing the method for fabricating the semiconductor device in accordance with the first embodiment of the present invention.
Referring to FIGS. 3A and 4A, the trenches 12 are formed by etching a semiconductor substrate 11 to a desired depth. The semiconductor substrate 11 may include a silicon substrate. To form the trenches 12, the semiconductor substrate 11 may be etched using a hard mask layer (not shown). The trenches 12 are formed as a line type. The etching method for etching the semiconductor substrate 11 to form the trenches 12 may be an anisotropic dry etch process. When the semiconductor substrate 11 is a silicon substrate, chlorine (Cl2) gas or hydrogen bromide (HBr) gas may be used alone or a mixed gas of chlorine (Cl2) gas and hydrogen bromide (HBr) gas may be used. The depth of the trenches 12 depends on the height of the buried bit lines 14, which will be formed subsequently. For example, the trenches 12 may be formed with a depth of at least approximately 5,000 Å.
Referring to FIGS. 3B and 4B, a barrier layer 13 is formed on the surface of the semiconductor substrate 11 including the trenches 12. The barrier layer 13 may be formed by stacking a titanium (Ti) layer and a titanium nitride (TiN) layer. According to another embodiment, a spacer insulation layer instead of forming the barrier layer 13 may be formed using a nitride layer.
Subsequently, a metal layer is formed over the barrier layer 13 until the trenches 12 are gap-filled, and the buried bit lines 14 each filling a portion of each trench 12 are formed by performing an etch-back process. Here, the metal layer used as the buried bit lines 14 may include a tungsten layer.
Referring to FIGS. 3C and 4C, the sidewalls of each trench 12 (that is, the barrier layer 13 over the semiconductor substrate 11) are selectively removed, except the buried bit lines 14. Hereafter, the etched barrier layer 13 is referred to as a barrier layer pattern 13A.
The barrier layer 13 is etched through the etch-back process. Accordingly, the barrier layer pattern 13A remains only on the interface between the buried bit lines 14 and the semiconductor substrate 11 in the inside of the trenches 12. Hereafter, the sidewalls of the trench 12 after the barrier layer 13 is removed are referred to as top sidewalls for illustration purposes.
Referring to FIGS. 3D and 4D, a conductive layer 15 gap-filling the upper portion of each buried bit line 14 is formed. The conductive layer 15 gap-fills the upper-portion of each buried bit line 14 through epitaxial growth. Here, the epitaxial growth is performed on the top sidewalls. Since the semiconductor substrate 11 is a silicon substrate, it is called silicon epitaxial growth.
Through the epitaxial growth, the conductive layer 15 gap-filling the upper portion of the buried bit line 14 is grown, and the conductive layer 15 includes a silicon epitaxial layer.
Meanwhile, the conductive layer 15 may also grow on the upper surface of the semiconductor substrate 11, and a surface step height may occur due to the conductive layer 15 formed on the upper surface of the semiconductor substrate 11. Therefore, a planarization process may be performed to remove the surface step height on the upper surface of the semiconductor substrate 11. Here, the planarization process may be a Chemical Mechanical Polishing (CMP) process.
Referring to FIGS. 3E and 4E, the active pillars 16 separated from each other are formed by simultaneously etching the semiconductor substrate 11 and the conductive layer 15. Here, the active pillars 16 may have a cylindrical pillar structure. The active pillars 16 may be referred to as active regions as well. Hereafter, the semiconductor substrate 11 with the active pillars 16 is denoted with 11A.
The active pillars 16 have a structure of cylindrical pillars, each of which includes the first active pillar 15B and the second active pillar 11B. The first active pillar 15B is formed by steps including etching the conductive layer 15, and the second active pillar 11B is formed by etching the semiconductor substrate 11. Here, the first active pillar 15B is a silicon epitaxial layer, and the second active pillar 11B is a silicon layer. The first active pillar 15B according to an example may be the same epitaxial layer that forms the conductive layer pattern 15A and is etched to form the first active pillar 15B at the same time that the second active pillar 11B is formed by etching the semiconductor substrate 11. The active pillars 16 may be called silicon structures. Looking at the active pillars 16 from the top, the active pillars 16 are arranged in the form of matrix.
The active pillars 16 are formed by etching the semiconductor substrate 11 by using a photoresist pattern (not shown), and the etch depth is decided in such a manner that the bottom surface of the etched portion is at least higher than the upper surface of the buried bit lines 14. Accordingly, the conductive layer pattern 15A may remain with a desired thickness in the upper portion of the buried bit lines 14, and the height of the semiconductor substrate 11A may be lowered by lowering the second active pillar 11B. The conductive layer pattern 15A has a pattern of integrated lines connected to the first active pillars 15B in the upper portion of the conductive layer pattern 15A. The active pillars 16 functioning as active regions with the conductive layer pattern 15A are electrically connected to the buried bit lines 14.
Referring to FIGS. 3F and 4F, the inter-layer dielectric layer 17 in gap-filling the space between the active pillars 16 is formed, and then a recess process is performed by sequentially performing an etch-back process and a wet etch process. The inter-layer dielectric layer 17 may be formed of baro-phosphorous silicate glass (BPSG), which has excellent gap-filling characteristics. The inter-layer dielectric layer 17 covers the surfaces of the semiconductor substrate 11A around the active pillars 16 and the conductive layer pattern 15A.
Referring to FIGS. 3G and 4G, a gate insulation layer 18 is formed on the sidewalls of the active pillars 16. The gate insulation layer 18 may include a silicon oxide layer, and the gate insulation layer 18 may be formed with a thickness of approximately 50 Å through a deposition process or an oxidation process.
Subsequently, vertical gates 19 are formed by depositing the conductive layer to be used as gates and performing an etch-back process. The conductive layer used as the vertical gate 19 may be a metal layer or a polysilicon layer. The vertical gates 19 are of a surround gate type, where each vertical gate 19 surrounds the respective active pillar 16.
Referring to FIGS. 3H and 4H, the word lines 20 are formed to connect the neighboring vertical gates 19 to each other. The word lines 20 may be a metal layer. For example, the word lines 20 may include any one selected from the group consisting of tungsten silicide (WSix), titanium nitride (TiN), tungsten (W), aluminum (Al), cupper (Cu), gold (Au), and ruthenium (Ru). The word lines 20 are arranged to cross the buried bit lines 14 at, for example, right angles, and the word lines 20 are insulated from the buried bit lines 14 by the inter-layer dielectric layer 17.
The word lines 20 may be formed as follows.
First, a metal layer to be used as the word lines 20 is formed over the substrate structure including the vertical gates 19, and a partial etch-back process is performed onto the metal layer. Subsequently, the word lines 20 are formed by partially etching the metal layer in a direction crossing the buried bit lines 14.
According to the first embodiment of the present invention, the resistance of the buried bit lines 14 may be decreased by forming the buried bit lines 14 of a metal layer.
Also, with the buried bit lines 14 partially fill the trenches 12 under the active pillars 16, the dimensions of the buried bit lines 14 may be increased easily. Therefore, the resistance of the buried bit lines 14 may further be reduced.
In addition, since the active pillars 16, the vertical gate 19, and the word lines 20 are formed after the formation of the buried bit lines 14, the process for forming the buried bit lines 14 may be simplified, which leads to decreased complexity in a semiconductor device fabrication process.
Furthermore, according to the first embodiment of the present invention, an etch process for separating the buried bit lines 14 between neighboring cells may be avoided. Therefore, the process for forming the buried bit lines 14 may further be simplified.
2nd Embodiment
FIG. 5 is a plan view illustrating a semiconductor device in accordance with a second embodiment of the present invention. FIG. 6A is a cross-sectional view of the semiconductor device of FIG. 5 along a line A-A′ and a line B-B′, FIG. 6B is a perspective view of the semiconductor device shown in FIG. 5.
Referring to FIGS. 5, 6A and 6B, a semiconductor substrate 31A according to the second embodiment includes a plurality of trenches 32, and a plurality of buried bit lines 34 are formed to fill a portion of each trench 32, individually. The buried bit lines 34 may be a metal layer to reduce the resistance more than the resistance of the buried bit lines 34 formed through an ion implantation process. A barrier layer pattern 33A may be disposed on the interface between the buried bit lines 34 and the semiconductor substrate 31A to prevent diffusion. The barrier layer pattern 33A may be a stacked layer where a titanium (Ti) layer and a titanium nitride (TiN) layer are stacked.
The trenches 32 may have a form of bulb. More specifically, the trenches 32 include a first trench 32A and a second trench 32B which is connected to the bottom of the first trench 32A and has a greater line width than that of the first trench 32A. The buried bit lines 34 have a structure of filling the second trench 32B. Here, since the buried bit lines 34 have a structure of filling the second trench 32B which has a relatively greater line width, the total volume of the buried bit lines 34 may increase. Therefore, the resistance of the buried bit lines 34 may be decrease.
A plurality of active pillars 37 are formed over the buried bit lines 34. Each active pillar 37 includes a first active pillar 36B gap-filling the upper portion of each buried bit line 34 and a second active pillar 31B formed on both sidewalls of the first active pillar 36B. Here, the first active pillar 36B and the second active pillar 31B may be formed of the same material. The second active pillar 31B is a structural element formed by etching the semiconductor substrate 31A, e.g., a silicon substrate, and the first active pillar 36B is a silicon epitaxial layer. These elements will be described in detail later. An inter-layer dielectric layer 38 is formed over the semiconductor substrate 31A in the outside of the active pillar 37. A conductive layer pattern 36A is disposed between the buried bit line 34 and the first active pillar 36B, and the conductive layer pattern 36A is of the same material as that of the first active pillar 36B. In other words, a portion of the first active pillar 36B, which is the conductive layer pattern 36A, fills the remaining portion of each trench 32. The conductive layer pattern 36A electrically connects the active pillar 37 to the buried bit line 34.
Also, an ohmic contact layer 35A is interposed between the active pillars 37 and the buried bit lines 34. More specifically, the ohmic contact layer 35A is positioned between the first active pillar 36B and the buried bit line 34. To be more specific, the ohmic contact layer 35A is positioned between the conductive layer pattern 36A and the buried bit line 34. The ohmic contact layer 35A alleviates the potential barrier (or a work function difference) between the active pillars 37 including a semiconductor material such as a silicon layer, and the buried bit lines 34 including the metal layer as well as reducing contact resistance. Since a metal silicide used for the ohmic contact layer, for example, is a low-resistance substance, the contact resistance may be easily reduced. Also, since the metal silicide is a material formed by combining silicon and metal, the metal silicide may stably alleviate the potential barrier between the buried bit lines 34 including a metal layer and the active pillars 37 including a silicon layer. For example, at the junction between the metal layer and the silicon layer, a Schottky barrier may be formed due to the work function difference between them. In other words, a Schottky barrier may be formed between the buried bit lines 34 and the active pillars 37, and the contact resistance may be increased between them due to the Schottky barrier. Use of the metal silicide as the ohmic contact layer 35A prevents/reduces formation of Schottky barriers between the buried bit lines 34 and the active pillars 37.
The active pillars 37 are disposed over the buried bit lines 34, respectively. In other words, each of the active pillars 37, which are disposed apart from each other by a desired space, is formed in the upper portion of each buried bit line 34. As a result, the active pillars 37 are arranged in a matrix form.
The external walls of each active pillar 37 are surrounded by each vertical gate 40, and a gate insulation layer 39 is formed between the vertical gate 40 and the active pillar 37.
Neighboring vertical gates 40 are connected to each other by word lines 41. The word lines 41 are insulated from the buried bit lines 34 by the inter-layer dielectric layer 38. The word lines 41 and the buried bit lines 34 are arranged to cross each other at, for example, right angles. The word lines 41 may be a metal layer. For example, the word lines 41 include any one selected from the group consisting of tungsten silicide (WSix), titanium nitride (TiN), tungsten (W), aluminum (Al), cupper (Cu), gold (Au), and ruthenium (Ru).
FIGS. 7A to 7I are cross-sectional views describing a method for fabricating the semiconductor device of FIG. 5 along the line A-A′ and the line B-B′ in accordance with the second embodiment of the present invention. FIGS. 8A to 8I are perspective views describing the method for fabricating the semiconductor device in accordance with the second embodiment of the present invention.
Referring to FIGS. 7A and 8A, the trenches 32 are formed by etching a semiconductor substrate 31 to a desired depth. The semiconductor substrate 31 may include a silicon substrate. To form the trenches 32, the semiconductor substrate 31 may be etched using a hard mask layer (not shown). The trenches 32 are formed as a bulb type, where they extend as lines in one direction.
The etching method for etching the semiconductor substrate 31 to form the trenches 32 may include a primary etch process for forming the first trench 32A through an anisotropic dry etch process and a secondary etch process for forming a round second trench 32B having a greater line width than that of the first trench 32A by etching the semiconductor substrate 31 under the first trench 32A through an isotropic etch process. When the semiconductor substrate 31 is a silicon substrate, the primary and secondary etch processes may be performed using chlorine (Cl2) gas or hydrogen bromide (HBr) gas alone or a mixed gas of chlorine (Cl2) gas and hydrogen bromide (HBr) gas. The depth of the trenches 32 depends on the height of the buried bit lines 34, which will be formed subsequently. For example, the trenches 32 may be formed with a depth of approximately 5,000 Å.
Referring to FIGS. 7B and 8B, a barrier layer 33 is formed on the surface of the semiconductor substrate 31 including the trenches 32. The barrier layer 33 may be formed of a nitride layer or by stacking a titanium (Ti) layer and a titanium nitride (TiN) layer. According to another embodiment, a spacer insulation layer instead of forming the barrier layer 33 may be formed using a nitride layer.
Subsequently, a metal layer is formed over the barrier layer 33 until the trenches 32 are gap-filled, and the buried bit lines 34 each filling a portion of each trench 32 are formed by performing an etch-back process. The buried bit lines 34 may be formed to fill at in least the second trench 32B. Here, the metal layer used as the buried bit lines 34 may include a tungsten layer.
Referring to FIGS. 7C and 8C, the metal silicide layer 35 is formed over the substrate structure including the buried bit lines 34. Here, the thickness T1 of the metal silicide formed over the buried bit is lines 34 may be thicker than the thickness T2 of the metal silicide layer 35 formed on the sidewalls of each trench 32 (T1>T2). Here, the metal silicide layer 35 may include a tungsten silicide (WSi) layer.
The metal silicide layer 35 having different thickness for different locations as described above may be formed through a Physical Vapor Deposition (PVD) method which can control the deposition rate in the horizontal and vertical directions. The metal silicide layer 35 may be formed by sputtering a metal silicide target, or the metal silicide layer 35 may be formed by alternately sputtering a metal target and a silicon target multiple times. Here, the deposition speed (or deposition thickness) in the vertical direction may be controlled to be faster (or thicker) than the deposition speed (or deposition thickness) in the horizontal direction by controlling the voltage bias applied to a bottom electrode which accelerates sputtered elements to the semiconductor substrate 31.
Referring to FIGS. 7D and 8D, the metal silicide layer 35 remains only in the upper portion of the buried bit lines 34 by performing a selective etching process onto the metal silicide layer 35. In other words, the ohmic contact layer 35A is formed by leaving the metal silicide layer 35 over the buried bit lines 34. The etching method performed onto the metal silicide layer 35 to form the ohmic contact layer 35A may be a wet etch process or a dry etch process.
Subsequently, the sidewalls of each trench 32 (that is, the barrier layer 33 and the ohmic contact layer 34 over the semiconductor substrate 31) are selectively removed, except the buried bit lines 34 and the ohmic contact layer 35A. Hereafter, the etched barrier layer 33 is referred to as a barrier layer pattern 33A.
The barrier layer 33 is etched through the etch-back process. Accordingly, the barrier layer pattern 33A remains only on the interface between the buried bit lines 34, the ohmic contact layer 35A, and the semiconductor substrate 31 in the inside of the trenches 32. Hereafter, the sidewalls of the trench 32 after the barrier layer 33 is removed are referred to as top sidewalls for the illustration purposes.
Referring to FIGS. 7E and 8E, a conductive layer 36 gap-filling the other portion of the trench 32 is formed in the upper portion of each buried bit line 34. The conductive layer 36 gap-fills the upper-portion of each buried bit line 34 through epitaxial growth. Here, the epitaxial growth is performed on the top sidewalls. Since the semiconductor substrate 31 is a silicon substrate, it is called silicon epitaxial growth.
Through the epitaxial growth, the conductive layer 36 gap-filling the upper portion of the buried bit line 34 is grown, and the conductive layer 36 includes a silicon epitaxial layer.
Meanwhile, the conductive layer 36 may also grow on the upper surface of the semiconductor substrate 31, and a surface step height may occur due to the conductive layer 36 formed on the upper surface of the semiconductor substrate 31. Therefore, a planarization process may be performed to remove the surface step height on the is upper surface of the semiconductor substrate 31. Here, the planarization process may be a Chemical Mechanical Polishing (CMP) process.
Referring to FIGS. 7F and 8F, the active pillars 37 separated from each other are formed by simultaneously etching the semiconductor substrate 31 and the conductive layer 36. Here, the active pillars 37 may have a cylindrical pillar structure. The active pillars 37 may be referred to as active regions as well. Hereafter, the semiconductor substrate 31 with the active pillars 37 is denoted with 31A.
The active pillars 37 have a structure of cylindrical pillars, each of which includes the first active pillar 36B and the second active pillar 31B. The first active pillar 36B is formed by etching the conductive layer 36, and the second active pillar 31B is formed by etching the semiconductor substrate 31. Here, the first active pillar 36B is a silicon epitaxial layer, and the second active pillar 31B is a silicon layer. Thus, the active pillars 37 may be called silicon structures. Looking at the active pillars 37 from the top, the active pillars 37 are arranged in the form of matrix.
The active pillars 37 are formed by etching the semiconductor substrate 31 and the conductive layer 36 by using a photoresist pattern (not shown), and the etch depth is controlled in such a manner that the bottom surface of the active pillars 37 is at least higher than the upper surface of the buried bit lines 34. According to an embodiment, in the etch process for forming the active pillars 37, the etch depth is decided in such a manner that the bottom surface of the etched portion is higher than the surface of the ohmic contact layer 35A. Accordingly, the conductive layer pattern 36A may remain with a desired thickness in the upper portion of the buried bit lines 34, and the height of the semiconductor substrate 31A may be lowered by lowering the second active pillar 31B. The conductive layer pattern 36A has a pattern of integrated lines connected to the first active pillars 36B in the upper portion of the conductive layer pattern 36A. The active pillars 37 functioning as active regions by the conductive layer pattern 36A are electrically connected to the buried bit lines 34.
Referring to FIGS. 7G and 8G, the inter-layer dielectric layer 38 gap-filling the space between the active pillars 37 is formed, and then a recess process is performed by sequentially performing an etch-back process and a wet etch process. The inter-layer dielectric layer 38 may be formed of boro-phosphorous silicate glass (BPSG), which has excellent gap-filling characteristics. The inter-layer dielectric layer 38 covers the surfaces of the semiconductor substrate 31A around the active pillars 37 and the conductive layer pattern 36A.
Referring to FIGS. 7H and 8H, a gate insulation layer 39 is formed on the sidewalls of the active pillars 37. The gate insulation layer 39 may include a silicon oxide layer, and the gate insulation layer 39 may be formed with a thickness of approximately 50 Å through a deposition process or an oxidation process.
Subsequently, vertical gates 40 are formed by depositing the conductive layer to be used as gates and performing an etch-back process. The conductive layer used as the vertical gates 40 may be a metal layer or a silicon layer. The vertical gates 40 are of a surround gate type, where each vertical gate 40 surrounds the respective active pillar 37.
Referring to FIGS. 7I and 8I, the word lines 41 are formed to connect the neighboring vertical gates 40 to each other. The word lines 41 may be a metal layer. For example, the word lines 41 may include any one selected from the group consisting of tungsten silicide (WSix), titanium nitride (TiN), tungsten (W), aluminum (Al), cupper (Cu), gold (Au), and ruthenium (Ru). The word lines 41 are arranged to cross the buried bit lines 34 at, for example, right angles, and the word lines 41 are insulated from the buried bit lines 34 by the inter-layer dielectric layer 38.
The word lines 41 may be formed as follows.
First, a metal layer to be used as the word lines 41 is formed over the substrate structure including the vertical gates 40, and a partial etch-back process is performed onto the metal layer. Subsequently, the word lines 41 are formed by partially etching the metal layer in a direction crossing the buried bit lines 34.
According to the second embodiment of the present invention, any increase in the resistance of the buried bit lines 34 in that may have occurred in separating the buried bit lines 34 from one cell to another cell after the formation of the buried bit lines may be prevented by forming the buried bit lines 34 that fill the trenches 32, respectively, under the active pillars 37. Since the process of separating the buried bit lines 34 from one cell to another cell is not required, the process may be simplified and the process complexity may be decreased.
Also, since the buried bit lines 34 fill the second trench 32B having a relatively longer line width for the trenches 32 by having a bulb shape, the resistance of the buried bit lines 34 may be further reduced.
In addition, since the ohmic contact layer 35A is interposed between the active pillars 37 and the buried bit lines 34, the contact characteristics and contact resistance between the active pillars 37 and the buried bit lines 34 may be improved.
Furthermore, since the active pillars 37, the vertical gates 40, and the word lines 41 are formed after the formation of the buried bit lines 34, the complexity in the process for forming the buried bit lines 34 is decreased.
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.

## Claims

1. A method for fabricating a semiconductor device, comprising:
forming a plurality of trenches by etching a semiconductor substrate;
forming a plurality of buried bit lines each filling a portion of each trench;
forming a conductive layer gap-filling upper portions of the plurality of the buried bit lines;
forming a plurality of active pillars by simultaneously etching the conductive layer and the semiconductor substrate;
forming a plurality of vertical gates each surrounding each active pillar; and
forming a plurality of word lines coupling neighboring vertical gates with each other.

2. The method of claim 1, further comprising:
forming an ohmic contact layer over the plurality of the buried bit lines.

3. The method of claim 2, wherein the forming the ohmic contact layer comprises:
forming a metal silicide layer on a surface of the semiconductor substrate including the plurality of the buried bit lines, wherein a portion of the metal silicide layer formed over the plurality of the buried bit lines is thicker than a portion of the metal silicide layer on sidewalls of each trench; and
leaving the metal silicide layer on the plurality of the buried bit lines by selectively etching the metal silicide layer.

4. The method of claim 1, wherein the plurality of the active pillars are formed in upper portions of the plurality of the buried bit lines.

5. The method of claim 1, wherein the forming the plurality of the buried bit lines comprises:
forming a barrier layer over the semiconductor substrate including the plurality of the trenches;
forming a metal layer over the barrier layer until the plurality of the trenches are filled;
forming the plurality of the buried bit lines by performing an etch-back process onto the metal layer; and
performing an etch-back process onto the barrier layer until a surface of the plurality of the buried bit lines is exposed.

6. The method of claim 1, wherein the forming the conductive layer comprises:
performing an epitaxial growth on both sidewalls of each of the plurality of the trenches exposed by the plurality of the buried bit lines.

7. The method of claim 6, further comprising:
performing a planarization process onto the surface of the semiconductor substrate after the forming the conductive layer.

8. The method of claim 1, wherein the forming the plurality of the trenches comprises:
forming a plurality of first trenches by performing a primary etch process onto the semiconductor substrate; and
forming a plurality of second trenches having a longer line width than the plurality of the first trenches by performing a secondary etch process onto the semiconductor substrate under the plurality of the first trenches.

9. The method of claim 8, wherein the plurality of the buried bit lines are formed to fill the plurality of the second trenches.

10. The method of claim 1, further comprising:
forming an inter-layer dielectric layer over a substrate structure including the plurality of the active pillars before the forming the plurality of the vertical gates; and
recessing the inter-layer dielectric layer by sequentially performing an etch-back process and a wet etch process.

11. The method of claim 10, wherein the forming the plurality of the vertical gates comprises:
forming a gate conductive layer over the substrate structure including the plurality of the active pillars; and
performing an etch-back process onto the gate conductive layer.

12. The method of claim 1, wherein the forming the plurality of the word lines comprises:
forming a metal layer over a substrate structure including the plurality of the vertical gates;
performing an etch-back process onto the metal layer; and
etching the metal layer which is etched back in a direction crossing the plurality of the buried bit lines.