Abstract:
Disclosed herein is a fabrication method of a semiconductor device to order to increase an operation liability of the semiconductor device. A method for fabricating a semiconductor device comprises forming a recess in a semiconductor substrate, forming a word line in a lower part of the recess, oxidizing a top portion of the word line, and depositing an insulating material in a remained part of the recess.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Priority to Korean patent application number 10-2009-0010685, respectively filed on Feb. 10, 2009, which is incorporated by reference in its entirety, is claimed. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method of manufacturing a high integration semiconductor memory device, and more specifically, to a method of forming a buried word line included in a cell array of a semiconductor memory device. 
         [0003]    A type of semiconductor memory device (e.g., DRAM) includes a plurality of unit cells each including a capacitor and a transistor. A double capacitor has also been used to temporarily store data. A transistor has been used to transmit data (i.e., read and write) between a bit line and a capacitor corresponding to a control signal (word line). The transistor includes a gate, a source and a drain. Charges between the source and drain move in response to a control signal inputted to the gate. The charges move through a channel region in accordance with the properties and operation of the transistor. 
         [0004]    When a transistor is formed in a semiconductor substrate, a gate is formed in the semiconductor substrate, and impurities are doped at both sides of the gate to form a source and a drain. Due to an increase in data capacity and integration of a semiconductor memory device, the size of each unit cell is driven to be smaller. That is, the design rule (i.e., critical dimension) of the capacitor and the transistor included in the unit cell is decreased. As a result, a channel length of the cell transistor is made shorter, which causes a short channel effect and a drain induced barrier lowering (DIBL) effect that hinders a normal operation. For preventing the short channel effect and the DIBL effect, the doping concentration of the channel region has been increased to obtain a threshold voltage required in the cell transistor. However, as the design rule is decreased to less than 100 nm, the increase of the doping concentration in the channel region increases an electric field of a storage node (SN) junction degrading a refresh characteristic of the semiconductor memory device. In order to prevent the degradation of the refresh characteristic, a cell transistor having a three-dimensional structure is used. As a result, it is possible to secure a long channel length of the cell transistor vertically even as the design rule is decreased. Moreover, if the channel length of the cell transistor is secured, the doping concentration is decreased to prevent the degradation of the refresh characteristic. 
         [0005]    Further, as a semiconductor memory device is more highly integrated, a distance between a word line and a bit line which are connected to the cell transistor becomes narrower. Thus, a parasitic capacitance between the word line and the bit line increases, and an operation margin of a sense amplifier which senses and amplifies data transmitted through the bit line is deteriorated. Finally, an operation reliability of the semiconductor memory device is degraded. 
         [0006]    For reducing the parasitic capacitance between the word line and the bit line, a buried word line structure has been suggested. The buried word line structure means that a word line is completely buried in a semiconductor substrate. A conductive material made of the word line is deposited in only lower part of a recess formed in the semiconductor substrate, and an insulating material is formed in an upper part of the recess. Accordingly, the word line is formed at a lower level than the bit line formed on top of the semiconductor substrate surface. Thus, the word line may be clearly separated from the bit line by the insulating material. 
         [0007]      FIGS. 1   a  to  1   g  are cross-sectional diagrams illustrating a method for fabricating a semiconductor device that has a buried word line structure in a general semiconductor apparatus. 
         [0008]    Referring to  FIG. 1   a , a device isolation film  104  that defines an active region is formed over a semiconductor substrate  102  by a shallow trench isolation (STI) method. After an insulating film  105  is formed over the active region and the device isolation film  104 , a recess  106  is formed. The recess  106  is used for forming a gate pattern after forming the device isolation film  104 . Two recesses  106  are formed over one active region, and one recess  106  is formed over the device isolation film  104 . 
         [0009]    As shown in  FIG. 1   b , a conductive material is deposited with a given thickness so that the recess  106  may be filled with a conductive layer  108 . The conductive layer  108  may include a metal material such as TiN. Unlike a recess gate, in the buried word line structure, the inside of the recess  106  does not include polysilicon but a metal material such as TiN, thereby securing a gate threshold voltage margin because of the different physical properties related to charge movement between metal and silicon. 
         [0010]    Referring to  FIG. 1   c , a chemical mechanical polishing (CMP) process is performed on the conductive layer  108  to expose the upper portion of the insulating film  105 . 
         [0011]    Referring to  FIG. 1   d , an etch-back process is performed using an etching selectivity difference between the insulating film  105  and the conductive layer  108  to remove the upper portion of the conductive layer  108  in the recess  106 . After the etch-back process, the insulating film  105  is removed. 
         [0012]    As shown in  FIG. 1   e , a nitride film  110  is deposited over the semiconductor substrate  102  and the upper portion of the recess  106  where the conductive layer  108  is removed. An interlayer insulating film  112  is deposited over the nitride film  110 . A patterning process is then performed using a mask that defines a bit line contact to etch a portion of the nitride film  110  and the interlayer insulating film  112 . A contact hole  114  for forming a bit line contact is formed between the neighboring recesses  106  formed in the active region, that is, over a drain region. 
         [0013]    Referring to  FIG. 1   f , after a conductive material is buried in the contact hole  114 , a planarization process is performed to form a bit line contact  116 . 
         [0014]    Referring to  FIG. 1   g , a metal barrier film  118  is formed over the bit line contact  116 , and a bit line  120  is formed over the metal barrier film  118 . The bit line  120 , the metal barrier film  118  and the bit line contact  116  are surrounded by an insulating film  122  so that they may be electrically separated from a storage node contact  124  formed over a source region. 
         [0015]    The semiconductor device fabricated by the above process increases a physical distance between the bit line  120  and the conductive layer  108  used as a word line buried in the recess  106 , thereby reducing a parasitic capacitance between the word line and the bit line that may degrade the reliability of the semiconductor device. However, due to the characteristic of the etch-back process performed on the conductive layer  108  shown in  FIG. 1   d , the conductive layer  108  disposed over the upper portion of the recess  106  is not completely removed and remains at the sidewalls of the recess  106 . The conductive layer  108  disposed in the center of the recess  106  is removed corresponding to the etching depth. When the conductive layer  108  remains at the sidewalls of the upper portion of the recess  106 , an electrical field may be concentrated in the conductive layer  108  that remains at the sidewalls of the recess  106  when a voltage is applied to the word line. When the semiconductor device is used as a cell transistor, the electrical field degrades the operating characteristic of the unit cell, and a leakage current generated by the electrical field shortens a data storage time. 
         [0016]    If the conductive layer  108  disposed at the sidewalls of the upper portion of the recess  106  is not completely removed, the conductive layer  108  may be exposed when the contact hole  114  for forming the bit line contact  116  is formed. In this case, the bit line contact  116  and the conductive layer  108  may be electrically connected. This phenomenon may occur while the storage node contact  124  as well as the bit line contact  116  is formed. The generation of the junction defect (i.e., electrically connecting the conductive layer  108  used as a word line with the bit line contact  116  or the storage node contact  124 ) degrades the operating reliability of the semiconductor device. 
       BRIEF SUMMARY OF THE INVENTION 
       [0017]    Various embodiments of the present invention are directed to providing a method for fabricating a semiconductor device to insulate a word line and remove an electrical short between a contact and a word line by oxidizing a partial portion of the word line throughout an ion-implantation, to order to increase and enhance an operation liability of the semiconductor device. 
         [0018]    According to an embodiment of the present invention, a method for fabricating a semiconductor device comprises: forming a recess in a semiconductor substrate; forming a word line in a lower part of the recess; oxidizing a top portion of the word line; and depositing an insulating material in a remained part of the recess. 
         [0019]    Preferably, the method further comprises forming a device isolation layer defining an active region in the semiconductor substrate. 
         [0020]    Preferably, the method further comprises forming a source/drain region in the active region at both sides of the recess; forming a contact connected to the source/drain region; and forming either a bit line or a capacitor on the contact. 
         [0021]    Preferably, the oxidizing-a-top-portion-of-the-word-line includes an ion-implantation process for injecting oxide-ion in the recess at tilt. 
         [0022]    Preferably, the ion-implantation process is rotatively performed with a tilt angle of about 35 degree and an energy of under 1 to 5 keV. 
         [0023]    Preferably, the word line includes TiN and an oxidized top portion of the word line includes TiON. 
         [0024]    Preferably, the oxidized top portion of the word line has a thickness of 50˜100 Å. 
         [0025]    Preferably, a depth of the recess is about 1100 Å to about 1300 Å and a height of the word line is about 800 Å to about 900 Å. 
         [0026]    Preferably, the forming-a-word-line-in-a-lower-part-of-the-recess includes: depositing a conductive material in the recess; and performing an etch-back process for removing the conductive material with a predetermined depth to expose an upper part of the recess. 
         [0027]    Preferably, the method further comprises performing a wet etch process to remove a partial portion of an oxidized top portion of the word line. 
         [0028]    According to another embodiment of the present invention, a semiconductor device comprises: a buried word line including a top portion oxidized with a predetermined thickness in a recess of a semiconductor substrate; an insulating layer deposited on the buried word line in the recess; and a source/drain region located at both sides of the insulating layer in the semiconductor substrate. 
         [0029]    Preferably, the semiconductor device further comprises a contact connected to the source/drain region; a bit line connected to the contact; and a capacitor connected to the contact. 
         [0030]    Preferably, the word line includes TiN and an oxidized top portion of the word line includes TiON. 
         [0031]    Preferably, an oxidized top portion of the word line has a thickness of 50˜100 Å. 
         [0032]    Preferably, a depth of the recess is about 1100 Å to about 1300 Å and a height of the word line is about 800 Å to about 900 Å. 
         [0033]    Preferably, the semiconductor device further comprises a device-isolation layer defining an active region in the semiconductor substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIGS. 1   a  to  1   g  are cross-sectional diagrams illustrating a conventional method for fabricating a semiconductor device that has a buried word line structure in a general semiconductor apparatus. 
           [0035]      FIGS. 2   a  to  2   h  are cross-sectional diagrams illustrating a method for fabricating a semiconductor device according to an embodiment of the present invention. 
           [0036]      FIG. 3  is a cross-sectional diagram illustrating a method for fabricating a semiconductor device according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    The present invention can be applied for fabricating a cell transistor of the semiconductor memory device, as well as forming any semiconductor device including a buried word line structure. Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. 
         [0038]      FIGS. 2   a  to  2   h  are cross-sectional diagrams illustrating a method for fabricating a semiconductor device according to an embodiment of the present invention. 
         [0039]    Referring to  FIG. 2   a , a device isolation film  204  that defines an active region is formed over a semiconductor substrate  202  by a shallow trench isolation (STI) method. After an insulating film  205  is formed over the active region and the device isolation film  204 , a recess  206  is formed. The recess  206  is for forming a gate pattern and formed after the device isolation film  204 . The depth of the recess  206  ranges from 1100 to 1300 Å. Two recesses  206  are formed over one active region, and one recess  206  is formed over the device isolation film  204 . 
         [0040]    As shown in  FIG. 2   b , a conductive material is deposited with a given thickness so that the recess  206  may be filled with a conductive layer  208 . The conductive layer  208  may include a metal material such as TiN. Unlike a recess gate, in the buried word line structure, the inside of the recess  206  does not include polysilicon but a metal material such as TiN, thereby securing a gate threshold voltage margin using the different physical properties related to charge movement between metal and silicon. 
         [0041]    Referring to  FIG. 2   c , a chemical mechanical polishing (CMP) process is performed on the connective layer  208  to expose the upper portion of the insulating film  205 . 
         [0042]    Referring to  FIG. 2   d , an etch-back process is performed to remove the upper portion of the conductive layer  208  filling in the recess  206 . This is done by choosing an etchant with a different etching selectivity between the insulating film  205  and the conductive layer  208 . When the upper portion of the conductive layer  208  is removed by the etch-back process, the conductive layer  208  disposed in the recess  206  is not completely planarized but removed with an etched depth only in the center of the recess  206 . The conductive layer  208  may remain at the upper sidewalls of the recess  206 . 
         [0043]    As shown in  FIG. 2   e , the upper portion of the conductive layer  208  is oxidized. In order to oxidize the upper portion of the conductive layer  208 , the substrate  202  including the conductive layer  208  is rotated with a given tilt angle and implanted with oxide-ions to form an oxide film  209  on the conductive layer  208 . For example, the oxide-ion-implantation process is performed with a tilt angle of about 35° and energy of under 1 to 5 keV, thereby obtaining an oxide film  209  that has a thickness ranging from 50 to 100 Å. In one embodiment, the tilt angle is no more than 450. As mentioned above, the conductive layer  208  includes TiN, and the oxide film  209  includes TiON. After the oxidization process, the insulating film  205  that remains on the semiconductor substrate  202  is removed. 
         [0044]    Although the conductive layer  208  remains at the upper sidewalls of the recess  206  after the etch-back process, the upper portion of the conductive layer  208  becomes oxidized and is changed into the oxide film  209 . The oxide film  209  is an insulating material, thereby preventing the shortcomings of the conventional art. Particularly, it is possible to prevent the conductive layer  208  that remains at the sidewalls of the recess  206  from being electrically connected to a contact formed in a subsequent process between the neighboring conductive layers  208 . It is also possible to inhibit the electric field from being concentrated on the sidewall of the conductive layer  208 . When the ion-implantation process is performed to oxidize the upper portion of the conductive layer  208 , the tilt angle and the implantation energy of the ion-implantation process are controlled so that it is possible to adjust the depth of the implant oxide-ions. As a result, it is possible to adjust the height of the conductive layer  108  more accurately using the ion-implantation process than to adjust it by a conventional etch-back process. For example, when the height of the conductive layer  208  can be adjusted accurately in the present invention, it is possible to form the conductive layer  208  with a height ranging from 800 to 900 Å in the recess that has a depth ranging from 1100 to 1300 Å. 
         [0045]    When a portion of the conductive layer  208  formed over the recess  206  is etched, the amount etched by an etch-back process is reduced and the amount of the oxide-ion-implantation is increased so as to adjust the height of the conductive layer  208 . In this case, since the etch-back process reduces the etched amount, the thickness of the insulating film  205  that protects the rest of the region in the etch-back process is not required to be deposited thickly. 
         [0046]    As shown in  FIG. 2   f , a nitride film  210  is deposited over the upper portion of the oxide film  209  and the semiconductor substrate  202 . A first interlayer insulating film  212  is deposited over the nitride film  210 . A patterning process is performed using a mask that defines a bit line contact to etch a portion of the nitride film  210  and the first interlayer insulating film  212 . As a result, a contact hole  214  for forming a bit line contact is formed between the neighboring recesses  206  formed in the active region, that is, over a drain region. 
         [0047]    Referring to  FIG. 2   g , a conductive material is filled in the contact hole  214 , and a planarization process is performed to form a bit line contact  216 . 
         [0048]    Referring to  FIG. 2   h , a metal barrier film  218  is formed over the bit line contact  216 , and a bit line  220  is formed over the metal barrier film  218 . A second interlayer insulating film  222  is deposited over the first interlayer insulating film  212  including a metal barrier film  218  and a bit line  220 . The insulating material is deposited over the bit line  220  for electric insulation. The first interlayer insulating film  212  and the second interlayer insulating film  222  are etched using a mask that defines the location of the storage node contact  224  to expose the active region. A conductive material is filled therein to obtain a storage node  224 . The bit line  220 , the metal barrier film  218  and the bit line contact  216  are surrounded by the first insulating film  212  and the second insulating film  222  so that the storage node contact  224  formed over a source region is electrically separated. A capacitor (not shown) is further formed over the storage node contact  224  so that a unit cell of a semiconductor memory apparatus may be obtained. 
         [0049]      FIG. 3  is a cross-sectional diagram illustrating a method for fabricating a semiconductor device according to another embodiment of the present invention. 
         [0050]    A semiconductor device fabricated by another embodiment of the present invention is different from that shown in  FIG. 2   h . A wet etching process is further performed after the oxidation process. If the oxidation process such as a tilted oxide-ion-implantation is performed for a longer time, an oxide film  309  becomes thicker. Particularly, the oxide film  309  formed at a side portion of a conductive layer  308  is thicker than those at a center portion of the conductive layer  308 . Thus, an upper portion of the conductive layer  308  has a convex shape. The wet etching process uses an etching selectivity difference between the conductive layer  308  and an oxide film  309  after the upper portion of the conductive layer  308  is oxidized as shown in  FIG. 2   e . That is, when the conductive layer  308  includes TiN and the oxide film  309  includes TiON, the wet etching process is performed using an etching selectivity difference between TiN and TiON to remove the oxide film  309  that remains at the sidewalls of the upper portion of the recess. As a result, while the upper portion of the oxide film  209  shown in  FIG. 2   h  has a concave shape, the upper portion of the oxide film  309  shown in  FIG. 3  can be flat. 
         [0051]    Except for the wet etching process, the same procedure for forming a device isolation film  304  and the conductive layer  308  in a semiconductor substrate  302 ; depositing a nitride film  310  and a first interlayer insulating film  312  to form a bit line contact  316 ; and forming a bit line and a storage node contact is repeated in this embodiment of the present invention. 
         [0052]    As mentioned above, the semiconductor device of the present invention comprises a word line buried in a semiconductor substrate that has an upper portion oxidized with a given thickness, an insulating film buried over the word line in the semiconductor substrate, and a source/drain region. When a unit cell of a semiconductor memory apparatus is formed, the semiconductor device comprises a storage node contact and a bit line contact connected to a source/drain region, a bit line formed over the bit line contact, and a capacitor formed over the storage node contact. 
         [0053]    A method for fabricating a semiconductor device according to the present invention comprises: forming a recess in a semiconductor substrate; forming a word line in a lower part of the recess; oxidizing a top portion of the word line; and forming an insulating material over the upper portion of the recess. The method may further comprise removing a part of the upper portion of the word line which is oxidized by a wet etching process using an etching selectivity difference between the word line and the oxidized upper portion of the word line. The method can reduce a leakage current as well as a junction defect which may be generated by a conductive material (i.e., a residual metal) that may remain at the sidewalls of the recess while forming a buried word line structure. 
         [0054]    The semiconductor device that has a buried word line structure is fabricated so that a conductive material does not remain at sidewalls of the upper portion of the recess buried with a word line. As a result, the semiconductor device prevents an electric junction defect between the word line and the bit line contact or the storage node contact, and improves uniformity of the word line formed in the recess to improve operating characteristics of the semiconductor device. When the semiconductor device is used as a cell transistor, it improves the device reliability in its refresh characteristic. 
         [0055]    In the present invention, since the ion-implantation process transforms the upper portion of the word line (gate) into an oxide film, the amount of dry etching for removing the word line disposed over the recess can be reduced in comparison with the conventional art. As a result, it is unnecessary to thickly deposit a hard mask film used as an etching barrier film in the etching process. Furthermore, it is possible to reduce a void generated by a defect when a conductive material for forming a word line is buried. 
         [0056]    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 type of deposition, etching polishing, and patterning steps describe herein. Nor is the invention limited to any specific type of semiconductor device. For example, the present invention may be implemented in a dynamic random access memory (DRAM) device or non volatile memory device. Other additions, subtractions, or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.