Abstract:
The invention relates to a semiconductor device, wherein a storage node contact hole is made large to solve any problem caused during etching a storage node contact hole with a small CD, a landing plug is formed to lower plug resistance. A semiconductor device according to the invention comprises: first and second active regions formed in a substrate, the first and second active being adjacent to each other, each of the first and second active regions including a bit-line contact region and a storage node contact region and a device isolation structure; a word line provided within a trench formed in the substrate; first and second storage node contact plugs assigned to the first and second active regions, respectively, the first and second storage node contact plugs being separated from each other by a bit line groove; and a bit line formed within the bit-line groove.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to Korean patent application number 10-2009-73818, filed on 11 Aug. 2009, which is incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a semiconductor device and a method of fabricating the same; more particularly, to a semiconductor device including a buried gate, a storage node and bit lines and a method of fabricating the same. 
         [0003]    In semiconductor memory devices, DRAM has a multiplicity of unit cells, each consisting of a capacitor and a transistor. Among them, the capacitor is used to store data temporarily, and the transistor is used to transmit data between a bit line and the capacitor in response to a control signal (word line), while using the nature of a semiconductor having variable conductivity. The transistor has a gate, a source and a drain. According to a control signal applied to the gate, the charged particles are allowed to move between the source and the drain. The movement of charged particles between the source and the drain is realized via a channel region defined by the gate. 
         [0004]    According to a method of fabricating a conventional transistor on a semiconductor substrate, a gate is first formed over the semiconductor substrate and impurities are doped into two sides of the gate to form a source and a drain. A region between the source and drain under the gate becomes a channel region for the transistor. The transistor having such a horizontal channel region occupies a certain area of the semiconductor substrate. A high density semiconductor memory device has numerous transistors formed therein so it is difficult to reduce the size of the semiconductor memory device (or the chip size). 
         [0005]    Decreasing the chip size allows a larger number of semiconductor memory chips to be produced per wafer, leading to an improved yield. Indeed, a number of different techniques have been used to reduce the chip size. One technique is to use a recess gate instead of a traditional planar gate having a horizontal channel region, where a recess is formed on the substrate and then forming a gate in that recess so as to obtain a channel region along the curved surface of the recess. Another technique uses a buried gate that is formed by burying the entire gate within the recess. 
         [0006]    In such a buried gate structure, an isolation gate has been used to form a bit-line contact as well as a storage node contact in a line type. However, in doing so, the cell area may become larger than the isolation gate structure and may experience a greater leakage current than for the existing trench-type device isolation film. 
         [0007]    Also, a buried gate structure using such a trench-type device isolation film has a disadvantage in that during the patterning of a bit-line contact, a contact hole is generally require to be patterned as a hole by dry etching. If a Critical Dimension (CD) becomes smaller in size, the contact hole pattern may not be defined on a mask. Moreover, when the contact hole needs to be etched in an active region during a subsequent etching process, the active region may not open. Increasing the CD to prevent this may cause a short problem with the storage node. 
         [0008]    Besides, there are other problems: for example, the storage node contact has to be formed as a Self Aligned Contact (SAC) after the formation of bit lines, and a reduced contact area between the active regions and the contact increases contact resistance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the present invention are directed to providing a semiconductor device and a method of fabricating the same, wherein a storage node contact hole is made sufficiently large to facilitate the etching of a storage node contact hole with a small CD. Also, a landing plug is formed to have resistance that is lower than the plug resistance, and the SAC process is performed at the time of the bit line formation. 
         [0010]    In one embodiment, a method of fabricating a semiconductor device, includes forming a device isolation film for defining a multiplicity of active regions in a semiconductor substrate; forming a multiplicity of buried word lines in the semiconductor substrate; forming a storage node contact hole for exposing a storage node contact region of two adjoining active regions; filling the storage node contact hole with a storage node contact plug material; forming a bit-line groove for exposing a bit-line contact region of the active region and splitting the storage node contact plug material into two; and forming a bit line into the bit-line groove. By making the storage node contact hole large, any problem caused during the etching of a storage node contact hole with a small CD can be solved, and the SAC process is no longer needed during the formation of bit lines. 
         [0011]    The method of fabricating a semiconductor device further comprises: forming a landing plug over the bit-line contact region and storage node contact region of the active region. 
         [0012]    The formation of the landing plug preferably comprises: before the formation of the word lines, forming a hard mask oxide and a hard mask layer over the surface of the active region; after the formation of the word lines, removing the hard mask layer and the hard mask oxide from the surface of the active region; forming a landing plug in a space of the active region from which the hard mask layer has been removed; and implanting ions into the landing plug. 
         [0013]    The formation of the storage node contact hole preferably comprises: forming an interlayer dielectric over the semiconductor substrate including the landing plug and the word lines; and etching the interlayer dielectric with the landing plug as an etch stop layer. 
         [0014]    The method further comprises: forming a selective epi growth (SEG) layer in the bit-line contact and storage node contact regions of the active region, so as to increase the height of a junction region (source/drain) of the active region by SEG layer to lower plug resistance and to reduce GIDL (Gate Induced Drain Leakage). 
         [0015]    The formation of the buried word lines comprises: forming a trench in the semiconductor substrate; filling the trench with a gate conductive layer; and forming a capping layer over the gate conductive layer within the trench. In this manner, buried word lines are formed on the lower portion of the surface of the silicon substrate. 
         [0016]    The method preferably further comprises: after the formation of the bit-line groove, forming a nitride spacer on the side walls of the bit-line groove. 
         [0017]    Preferably, the formation of the bit lines is performed by the damascene process comprising: forming a barrier metal layer on the side walls and bottom surface of the bit-line groove; and forming a bit-line conductive layer over the surface of the barrier metal layer. 
         [0018]    The method preferably further comprises: after the formation of the barrier metal layer, thermally oxidizing the barrier metal layer to form silicide; and removing the barrier metal layer to leave the silicide. 
         [0019]    The surface of the landing plug has preferably the same height with the surface of the device isolation film. 
         [0020]    The present invention method of fabricating a semiconductor device comprised of: active regions formed over a semiconductor substrate, each active region including a bit-line contact region and a storage node contact region, and a device isolation film for defining the active regions; a multiplicity of word lines buried in the semiconductor substrate; a storage node contact plug buried in a storage node contact hole that exposes the storage node contact region of the two adjoining active regions; a bit-line groove for exposing the bit-line contact region of the active region and splitting the storage node contact plug material into two; and bit lines buried in the bit-line groove, is characterized in that any problem caused during the etching of a storage node contact hole with a small CD can be solved by making the storage node contact hole large and that the SAC process is not required at the time of the formation of bit lines. 
         [0021]    With the semiconductor device further comprising a polysilicon plug formed over the bit-line contact and storage node contact regions of the active region, plug resistance can be lowered. 
         [0022]    With the semiconductor device further comprising a selective epi growth layer over the bit-line contact and storage node contact regions of the active region, the height of the junction region (source/drain) of the active region can be increased by the SEG layer, and this in turn lowers plug resistance and reduces GIDL. 
         [0023]    The semiconductor device preferably further comprises an interlayer dielectric formed over the word lines and device isolation film, and the word line is formed of a buried word line comprising: a gate conductive layer buried in a trench of the semiconductor substrate; and a capping layer formed over the gate conductive layer within the trench. 
         [0024]    The semiconductor device preferably further comprises a nitride spacer formed on the side walls of the bit-line groove, and the bit lines are formed by the damascene process comprising: forming a barrier metal layer on the side walls and bottom surface of the bit-line groove; and forming a bit-line conductive layer over the surface of the barrier metal layer. 
         [0025]    The semiconductor device preferably further comprises silicide formed between the barrier metal layer and the bit-line conductive layer, and the surface of the landing plug has the same height as the surface of the device isolation film. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  to  FIG. 13  are plan or sectional views sequentially illustrating a fabrication method of a semiconductor device according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Embodiments of a method of fabricating a semiconductor device according to the present invention will now be described in greater detail with reference to the accompanying drawings. 
         [0028]      FIG. 1  to  FIG. 13  are plan or sectional views sequentially illustrating a fabrication method of a semiconductor device according to the present invention. In  FIG. 2 , (a) is a sectional view taken along the Y direction in  FIG. 1 , (b) is a sectional view taken along the X direction in  FIG. 1 . Other figures follow the same orientations. 
         [0029]    Referring first to  FIG. 1 , active regions  12  and device isolation films  14  for defining the active regions  12  are formed over a semiconductor substrate. Each active region  12  is intersected by two word lines  20 . The active region defines an acute angle with respect to the word line  20  in the present embodiment. The word line  20  is formed within the semiconductor substrate as a buried word line, i.e., the upper surface of the word line  20  is lower than the upper surface of the semiconductor substrate. 
         [0030]    Referring to  FIG. 2 , a hard mask oxide  15  and a nitride or a hard mask layer made of a polysilicon material are sequentially formed over the surface of the semiconductor substrate. The hard mask layer is etched using a mask that defines a device isolation film  14  region to form a hard mask pattern  16 . The semiconductor substrate is etched using this hard mask pattern  16  as a mask to form a trench therein, and the trench is filled with an oxide to form the device isolation film  14 . 
         [0031]    In one embodiment, a liner nitride (not shown) and a liner oxide (not shown) are formed over the trench surface of the semiconductor substrate before the device isolation film  14  is formed thereon. The device isolation film  14  made of an oxide material is obtained by forming an oxide, e.g., using a Spin On Dielectric (SOD) method, over the entire surface of the semiconductor substrate having a trench formed therein and then removing the oxide from the surface of the hard mask pattern  16  by the CMP method to fill the device isolation film  14  into the trench. 
         [0032]    In a conventional method, the active regions  12  were formed to have their surfaces to be the same height as the surface of the device isolation film  14 . In the present embodiment, since the hard mask  16  resides on the surface of the active regions  12 , the surface of the hard mask  16  is formed to have the same height as the surface of the device isolation film  14 , i.e., the upper surface of the active region  12  is provided to be lower than the upper surface of the device isolation  14 . This height difference between the active region  12  and the device isolation  14  facilitates the formation of a landing plug because interlayer dielectric need not be formed over the active region  12  when the hard mask pattern  16  is made of a nitride and then the nitride material is removed, or when the hard mask  16  is made of polysilicon and then the polysilicon is patterned to be used as a landing plug. In another embodiment of the present invention, instead of forming the hard mask  16  over the active region  12 , a selective epi growth (SEG) process is performed on the active region to make the surface of an SEG layer (not shown) of the active region  12  to have substantially the same height as the surface of the device isolation film  14 . In this case, although a storage node contact is formed directly on the surface of the SEG layer (not shown) over the active region  12  without forming a landing plug, since the SEG layer increases the height of a junction region (source/drain) of the active region  12 , plug resistance can be lowered and Gate Induced Drain Leakage (GIDL) is reduced. 
         [0033]    Referring to  FIG. 3 , a trench  22  having a predetermined depth for forming a buried gate  20  is formed in the semiconductor substrate having the active regions  12  and the device isolation films  14  formed thereon. The surface of the trench  22  is oxidized to form a gate oxide  24 , and the trench  22  having the gate oxide  24  formed thereon is filled with a gate electrode  26  ( FIG. 4 ). The gate electrode  26  includes TiN and tungsten (W) in the present embodiment. A capping film  28  for protecting the gate electrode  26  is formed over the gate electrode  26  within the trench  22 . 
         [0034]    Referring to  FIG. 5 , the hard mask layer  16  is removed and a landing plug  30  is formed in a space created by removing the hard mask layer  16 . The landing plug  30  electrically connects the junction region (source/drain) of the semiconductor substrate to the bit-line contact plug and the storage node contact plug. The landing plug  20  is formed of conductive material (e.g., polysilicon). The landing plug formation process includes depositing a landing plug material such as polysilicon within the space created by the removal of the hard mask layer  16  and over the capping film  28 . Excess landing plug material that protrudes above the capping film  28  is removed using chemical mechanical polishing (CMP) or etch-back. The landing plug  30  can be formed by depositing doped polysilicon, or by depositing polysilicon and then implanting ions for doping. 
         [0035]    Rather than forming a bit-line contact plug and a storage node contact plug directly on the junction region of the semiconductor substrate, the landing plug  30  on the junction region of the semiconductor substrate since bit lines are to be formed by the damascene process (to be described) in the present embodiment. The landing plug  30  (or an SEG layer grown on the active region  12 ) couples the bit lines and the junction regions of the active region  12 . By the use of landing plug  30 , loss of the semiconductor substrate can be avoided during the damascene process used to form the bit lines. As a result, a junction region having a low depth can be utilized. In addition, since a metal plug is not formed directly on the active region, the landing plug  30  serves as a buffer, and the contact surface between the active regions  12  and the contact plug (landing plug) is increased, thereby lowering plug resistance. 
         [0036]    A sealing nitride  32  is formed over the landing plug  30 , buried gate  20  and device isolation film  14  to protect the buried gate  20  and the landing plug  30 . A gate is formed in a peripheral region (not shown), not in the cell region that has been described thus far. 
         [0037]    Referring to  FIG. 6 , an interlayer dielectric (ILD)  40  is formed over the sealing nitride  32 . 
         [0038]    A portion of the ILD  40  is etched to form a storage node contact hole  42  ( FIG. 7 ). Hole  42  exposes the landing plug  30  and is used to form a storage node contact plug.  FIG. 7(   b ) shows a cross sectional view along the X direction of the semiconductor device of  FIG. 8 . The storage node contact hole  42  includes a storage node region of two adjacent active regions  12  in the present embodiment. 
         [0039]    In a conventional method, a storage node contact hole  42  of each active region  12  is formed individually, i.e., one contact hole for each active region. In the present embodiment, one storage node contact hole  42  is formed for two adjacent active regions  12 . As a result, a bigger contact hole pattern may be used for the contact hole. The occurrence of a contact hole pattern not being defined on the mask produced during the formation of a contact hole pattern may be minimized. The storage node contact hole pattern including two storage node contact regions is split into two to electrically isolate the two, the process of which is described later. 
         [0040]    In the present embodiment, a storage node contact plug is formed before a bit line contact plug so that the bit line formation process may proceed in a similar way with the buried gate formation process. 
         [0041]    Referring to  FIG. 9 , the storage node contact hole  42  exposing the landing plug  30  is filled with a conductive material to form a storage node contact plug  44 . This storage node contact plug  44  is made of the same material as the landing plug  30  (e.g., polysilicon) in the present embodiment. For example, after a polysilicon layer is deposited on the entire surface of the semiconductor substrate having the storage node contact hole  42  formed therein, the polysilicon layer on the upper portion of the surface of the interlayer dielectric  40  is removed by CMP or etch-back. 
         [0042]    Referring to  FIG. 10 , a hard mask pattern  55  is formed to define a bit line region over the interlayer dielectric  40  having the storage node contact plug  44  formed therein. The hard mask pattern  55  may be a nitride. 
         [0043]    With the hard mask pattern  55  as a mask, a bit-line groove  51  for forming bit lines is etched to a predetermined depth so as to expose the surface of the landing plug  30  in a portion of the bit line region. 
         [0044]    The bit-line groove  51  is used to form bit lines by a damascene process. The bit-line groove  51  formed also splits the storage node contact plug  44  (see  FIG. 10  ( b )) that extends across two adjacent active regions  12  into two separate pieces, where each separated piece connecting the respective active region  12 . As a result, the SAC process does not need to be used at the time of the formation of a bit-line contact hole and a storage node contact hole. 
         [0045]    As shown in  FIG. 11 , as the storage node contact plug  44  is formed in each of a multiplicity of active regions  12 , bit lines are also formed in an intersecting manner with those active regions  12 . Therefore, by forming a bit-line groove  51 , one storage node contact plug  44  formed over two active regions can be split. 
         [0046]    Referring next to  FIG. 12 , dielectric spacers  52  for insulating the bit-line groove  51  are formed on the bottom and side walls of the bit-line groove  51 . This dielectric spacer  52  is preferably made of a material having a low dielectric constant, such as a nitride or oxide. 
         [0047]    A contact mask (not shown) opens only a bit-line contact node portion (the left side bit-line groove  51  in  FIG. 12(   b )) where a bit-line contact is supposed to be formed so as to remove the dielectric spacer  52  on the bottom surface of the bit-line groove  51 , thereby exposing the landing plug  30 . 
         [0048]    Referring to  FIG. 13 , bit lines  50  are formed in the bit-line grooves  51  having the dielectric spacer  52  formed thereon. To be more specific, a bit-line barrier metal layer  53  is formed first on the bottom and sidewall surfaces of the bit-line groove  51 . The barrier metal layer  53  includes Ti/TiN or Ti x Si x  material in one embodiment. 
         [0049]    In one embodiment, the barrier metal layer  53  is thermally treated for silicidification, where an interface region is converted to silicide while the bulk material (not shown) remains non-silicided. The upper portion is removed to leave the lower portion that has been converted to silicide remaining in the bit line groove  51 . Since the portion where the bit-line contact is to be formed is free of the dielectric spacer  52 , the bit lines are electrically connected with the landing plug  30 . 
         [0050]    A bit-line conductive layer  54  is formed over the surface of the barrier metal layer  53  (silicide in one embodiment), the bit-line conductive layer  54  may be made of tungsten (W). Meanwhile, to increase adhesion between the barrier metal layer  53  and the bit-line conductive layer  54 , an adhesive layer or TiN layer (not shown) may additionally be formed on the interface between these two layers. 
         [0051]    A bit-line hard mask  56  made of a nitride material is formed over the barrier metal layer  53  and bit-line conductive layer  54 . The bit-line hard mask formation process includes depositing a nitride over the entire surface, and leaving the bit-line hard mask  56  only inside the bit-line groove  52  using CMP or etch-back. 
         [0052]    Although subsequent processes are not shown, it should be understood that a capacitor consisting of a lower electrode, an upper electrode and a dielectric is formed over the storage node contact plug  44 . Additional layers are formed thereover to complete the fabrication process of a semiconductor device. 
         [0053]    The above embodiments of the present invention are illustrative and not limitative. Various alternatives and equivalents are possible. The invention is not limited by the embodiments described herein. Nor is the invention limited to any specific type of semiconductor device. Other additions, subtractions, or modification are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.