Patent Publication Number: US-9412665-B2

Title: Semiconductor device and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 13/719,017 filed on Dec. 18, 2012, which claims priority to Korean patent application number 10-2012-0096486, filed on 31 Aug. 2012, which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     1. Technical Field 
     The inventive concept relates to a semiconductor device and a method of fabricating the same, and more particularly, to technology to improve gate-induced drain leakage (GIDL) of a buried gate and to improve device characteristic and reliability with reduction in a gate resistance. 
     2. Related Art 
     In recent years, although demands on large capacity of dynamic random access memories (DRAMs) have been increasing, there is a limit to increase in the capacity of the DRAMs due to increased chip size. With the increase of the chip size, the number of chips per a wafer is reduced and yield of the device is reduced. Therefore, in recent years, there are studies to reduce a cell area through change of a cell layout and thus to integrate as many memory cells as possible on one wafer. 
     According to this, buried gate structures have been developed. In the buried gate, leakage current due to gate induced drain leakage (GIDL) of a semiconductor device, is increased between a conductive material (gate electrode) and an N type junction of an active region, or between the conductive material and a storage node contact. Refresh time (tREF) of the whole semiconductor device may be reduced due to degradation of GIDL. 
     To prevent the leakage current from being increased due to GIDL, a conductive material (gate electrode) of the buried gate is overetched to minimize an overlapping area between the storage node contact and the conductive material (gate electrode). 
     However, although the conductive material (gate electrode) of the buried gate is over etched to prevent the leakage current from being increased due to GIDL, resistance of the buried gate may be increased to degrade operation speed, current drivability, and a write-recovery time (tWR) in the semiconductor device. 
     SUMMARY 
     One or more exemplary embodiments are provided to a semiconductor device capable of improving GIDL in a buried gate, and preventing degradation of device characteristics and reliability due to reduction in gate resistance. Methods of fabrication are also disclosed. 
     According to one aspect of an exemplary embodiment, there is provided a semiconductor device. The semiconductor device may include: a semiconductor substrate having a trench, the trench having first and second side; junction regions formed at the semiconductor substrate deposed in the first and second side of a trench formed; a first gate electrode formed in a lower portion of the trench; a second gate electrode formed over at least one inner sidewall of the trench, which overlaps one of the junction regions, and formed over the first gate electrode; and a third gate electrode formed over one side of the second gate electrode and over the first gate electrode. 
     The device further includes a barrier layer disposed between the second gate electrode and the third gate electrode. The second gate electrode is formed over both inner sidewalls of the trench. The third gate electrode is formed between the second gate electrodes. The first gate electrode and the third gate electrode include metal and the second gate electrode includes polysilicon. 
     The second gate electrode is formed over the one inner sidewall of the trench adjacent to a storage node contact. The third gate electrode is formed in the trench at the one side of the second gate electrode over the first gate electrode. The second gate electrode has a height to overlap the junction regions. The semiconductor device further includes a sealing layer formed over the second gate electrode and the third gate electrode. 
     According to another aspect of an exemplary embodiment, there is provided a method of fabricating a semiconductor device. The method may include: etching a gate region of a semiconductor substrate to form a trench; forming a first gate electrode filling the trench; etching the first gate electrode to a depth in which the first gate electrode is not overlapped with junction regions of the semiconductor substrate; forming a second gate electrode over at least one inner sidewall of the trench and over the first gate electrode; and forming a third gate electrode in the trench over one side of the second gate electrode over the first gate electrode. 
     The forming the second gate electrode includes forming second gate electrode over the first and second inner sidewalls of the trench adjacent to the junction regions and over the first gate electrode. The forming the second gate electrode includes forming the second gate electrode over the one inner sidewall of the trench adjacent to a storage node contact. The forming the second gate electrode includes forming the second gate electrode to have a height to overlap the junction regions. 
     The etching the first gate electrode to the depth in which the first gate electrode is not overlapped with the junction regions, includes: etching the first gate electrode to have a height in which an upper surface of the first gate electrode is not overlapped with the junction regions. 
     The method further includes forming a barrier layer over the one side of the second gate electrode before the forming the third gate electrode. The barrier layer is formed using at least one of titanium (Ti) and titanium nitride (TiN). The method further includes forming a gate insulating layer along a contour of the trench before forming the first gate electrode. 
     The forming the first gate electrode includes: forming a barrier metal layer over the gate insulating layer in the trench; and depositing a first gate material over the barrier metal layer to form the first gate electrode. 
     The etching the first gate electrode to the depth in which the first gate electrode is not overlapped with the junction regions, includes: partially etching the first gate electrode and simultaneously partially etching the barrier metal layer. 
     The first gate electrode and the third gate electrode are formed of a metal material and the second gate electrode is formed of a polysilicon material. 
     The forming the second gate electrode includes: depositing a second gate material along a contour of the trench; performing spacer-etching on the second gate material to remain over the first and second inner sidewalls of the trench; and etching the second gate material remaining over the both sidewalls of the trench to a fixed height, to form the second gate electrode. 
     The forming the second gate electrode includes: depositing a second gate material along a contour of the trench; etching the second gate material through spacer-etching to remain over the first and second inner sidewalls of the trench; etching remaining second gate material over the both sidewalls of the trench to a fixed height; forming a photoresist layer covering the second gate material at a storage node contact side; and removing the second gate material formed over another sidewall of the trench and exposed by the photoresist layer. 
     spacer-etching on the According to the exemplary embodiment of the inventive concept, GIDL is improved and gate resistance is reduced to prevent device characteristics and reliability such as a refresh time tREF from being degraded. 
     These and other features, aspects, and embodiments are described below in the section entitled “DETAILED DESCRIPTION”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the subject matter of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a plan view illustrating a 6F 2  structure to which a semiconductor device according to exemplary embodiments of the inventive concept is applied; 
         FIG. 2  is a cross-sectional view illustrating a semiconductor device according to a first exemplary embodiment of the inventive concept taken along line A-A′ of  FIG. 1 ; 
         FIGS. 3A to 3H  are cross-sectional views illustrating a method of fabricating a semiconductor device according to a first exemplary embodiment; 
         FIG. 4  is a cross-sectional view illustrating a semiconductor device according to a second exemplary embodiment of the inventive concept taken along line A-A′ of  FIG. 1 ; 
         FIGS. 5A to 5E  are cross-sectional views illustrating a method of fabricating a semiconductor device according to a second exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments will be described in greater detail with reference to the accompanying drawings. 
     Exemplary embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of exemplary embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, may be expected. Thus, exemplary embodiments should not be construed as limited to the particular shapes of regions illustrated herein but may be to include deviations in shapes that result, for example, from manufacturing. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. It is also understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other or substrate, or intervening layers may also be present. 
     Particular embodiments relate to technology in which a buried gate is configured of a first and third gate electrodes using metal and a second gate electrode using N+ polysilicon to improve GIDL and to prevent degradation of device characteristic and reliability due to reduction in a gate resistance, while an overlapping region between a junction region and a gate electrode is deeply formed. The technical principle may be applied to any semiconductor apparatus including a semiconductor device. 
     Hereinafter, exemplary embodiments will be described with reference to  FIGS. 1 to 5E . 
       FIG. 1  is a plan view illustrating a structure of a semiconductor device according to exemplary embodiments of the inventive concept. 
       FIG. 1  is a plan view illustrating a 6F 2  structure to which a semiconductor device according to an exemplary embodiment is applied.  FIG. 2  is a cross-sectional view illustrating a semiconductor device according to a first exemplary embodiment taken along line A-A′ of  FIG. 1 . Referring to  FIGS. 1 and 2 , a device isolation layer  14  defining an active region  102  is formed in a predetermined region of a semiconductor substrate  101  and a gate  16  has a buried gate structure buried in the active region and the device isolation region  14 . The active region  102  is disposed not to orthogonally cross the gate  16 , but instead to obliquely cross the gate  16 . A gate insulating layer  109  is formed between the gate  16  and the active region  102 . The gate insulating layer  109  may comprise a silicon oxide (SiO 2 ) layer or a high dielectric (high-k) layer having a higher dielectric constant than a silicon oxide (SiO 2 ) layer. 
     As shown in  FIG. 2 , in the semiconductor device according to the first exemplary embodiment, the buried gate  16  is formed to be buried in the semiconductor substrate  101 . The buried gate  16  includes the gate insulating layer  109  and a barrier metal layer  111  sequentially stacked in a trench, a first gate electrode  113  formed on the barrier metal layer  111 , second gate electrodes  117  formed on both side walls of the trench on the first gate electrode, and a third gate electrode  121  formed between the second gate electrodes  117 . 
     The first gate electrode  113  has a height not to overlap the junction region  103 , and the second gate electrode  117  has a height to overlap the junction region  103 . A barrier layer  118  is formed between the second gate electrode  117  and a third gate electrode  121  to minimize a contact resistance between the second gate electrode  117  and the third gate electrode  121 . The barrier layer  118  may comprise titanium (Ti), titanium nitride (TiN) and the like. 
     The first gate electrode  113  and the third gate electrode  121  may comprise a metal material such as tungsten (W). The second gate electrode  117  may comprise N+ polysilicon material. 
     In the first exemplary embodiment described above, the buried gate  16  has a structure such that the N+ polysilicon gate electrode (second gate electrode) having a work function lower than the first gate electrode  113 , is formed on both sidewalls of the trench which are overlapping regions with the junction regions  103 . The second gate electrode is not limited to the N+ polysilicon gate electrode but may be formed of any material having a work function lower than the first gate electrode  113  and the barrier layer  118 . Therefore the overlapping region between the junction region  103  and the buried gate  16  can be sufficiently deeply formed, and GIDL can be improved. 
     Hereinafter, a method of fabricating a semiconductor device according to an exemplary embodiment is described with reference to  FIGS. 3A to 3H . 
     First, as shown in  FIG. 3A , a hard mask layer  105  for a trench  107  is formed on a semiconductor substrate  101  in which an active region  102  is defined by a device isolation layer (not shown). The trench  107  is formed in the semiconductor substrate  101  using the hard mask layer  105  as a mask. The trench  107  may be formed to have a depth of about 1500 Å. 
     As shown in  FIG. 3B  a gate insulating layer  109  and a barrier metal layer  111  are sequentially deposited in the trench along a step of the hard mask layer  105  and etched back, so that the gate insulating layer  109  and the barrier metal layer  111  remain on an inner surface of the trench  107 . A first gate material  112  is deposited on the barrier metal layer  111  to be filled within the trench. 
     Here, the gate insulating layer  109  protects a surface of the semiconductor substrate  101  comprising a silicon material. The gate insulating layer  109  is formed by depositing a high-k material such as silicon oxide (SiO 2 ), oxide-nitride-oxide (ONO), hafnium oxide (HfO 2 ), zirconium oxide (ZrO), or a lead zirconate titanate (PZT) material using a chemical vapor deposition (CVD) method or heating the semiconductor substrate using a furnace. Alternatively, the gate insulating layer  109  may be formed by depositing material such as zirconium (Zr) or hafnium (Hf) on the inner surface of the trench  107  using an atomic layer deposition (ALD) method and naturally-oxidizing the material to convert the material into a high-k material. 
     The barrier metal layer  111  is formed between the gate insulating layer  109  and the first gate material  112  to ensure a bonding between the gate insulating layer  109  and the first gate material  112 . The first gate material  112  may comprise tungsten (W), titanium (Ti), aluminum (Al), tantalum (Ta), tungsten nitride (WNx), aluminum nitride (AlNx), titanium nitride (TiNx), tungsten silicide (WSix), titanium silicide (TiSix), cobalt silicide (CoSix), or combinations thereof. 
     As shown in  FIG. 3C  the first gate material  112  is planarized, and then an etch back process is performed to partially remove the barrier metal layer  111  and the first gate material  112 . Thus formed is a trench  114  in which the first gate material  112 , a barrier metal layer  111 , and a first gate electrode  113  are formed in a lower portion thereof and only the gate insulating layer  109  remains in an upper portion thereof. The first gate electrode  113  may have a height of about 700 Å, and the trench  114  on the first gate electrode  114  may have a height of about 800 Å. 
     As shown in  FIG. 3D , a second gate material  115  is deposited on an inner surface of the trench  114  and an entire surface of the hard mask layer  105  along a step. The second gate material  115  may include a polysilicon material doped with N+ type dopants. Next, a barrier metal layer  116  (Ti, TiN, and the like) is deposited on the second gate material  115 . 
     As shown in  FIG. 3E , a spacer-etching process is performed to remove the second gate material  115  and the barrier metal layer  116  on a bottom and an upper inner sidewall of the trench  114  and the hard mask layer  105 . Therefore, a second gate electrode  117  and a barrier layer  118  are formed on both inner sidewalls of the trench  114 . The second gate electrode  117  has a height to overlap the junction regions (see  103  of  FIG. 1 ). 
       FIGS. 3D and 3E  illustrate a process of sequentially depositing the second gate material  115  and the barrier metal layer  116  (Ti, TiN, and the like) and etching back the gate material  115  and the barrier metal layer  116 . Alternatively, the second gate material  115  is deposited and then etched back, and the barrier metal layer  116  is deposited and etched back. 
     As shown in  FIG. 3F  a third gate material  119  is deposited on the inner surface of the trench  114  of  FIG. 3E  and a surface of the hard mask layer  105 , and planarized. The third gate material  119  may include the same metal-based material as the first gate material  112 , or may include a metal-based material different from the metal-based material for the first gate material  112 . Preferably, the third gate material  119  has a work function higher than the second gate electrode  117 . For example, when the first gate electrode  113  is formed of tungsten (W) and the second gate electrode  117  is formed of doped polysilicon, the third gate material  119  may be formed of tungsten (W). 
     As shown in  FIG. 3G , the planarized third gate material  119  is etched back to form a third gate electrode  121  between the second gate electrodes  117 . 
     As shown in  FIG. 3H , a sealing layer  140  is formed on the third gate electrode  121  and the surface of the hard mask layer  105 . The sealing layer  140  may comprise a nitride layer. 
     Subsequently, the sealing layer  140  may be planarized. A storage node contact (not shown), a bit line contact (not shown), and the like may be formed on the sealing layer  140 . Then, the junction regions  103  are formed as shown in  FIG. 1 . 
     In the buried gate of the above-described exemplary embodiment, the second gate electrodes  117  are formed in inner sidewalls of the trench using polysilicon to prevent degradation of refresh characteristic due to GIDL. Accordingly, a gate resistance is reduced and tWR characteristics are improved. 
     In other words, according to the present invention, the buried gate  16  includes a lower gate structure and an upper gate structure. The lower gate structure includes the first gate electrode  113  and does not overlap with any of the junction regions  103 . The upper gate structure includes the second the third gate electrodes  117 ,  121  and overlaps with any of the junction regions  103 . 
     Compared to a conventional device where a space between the junction region  103  and over the lower gate structure is filled with an insulating material, the present invention where the space between the junction region  103  is filled with the upper gate structure having a work function lower than the insulating material is advantageous in lowering resistance between the buried gate  16  and the junction regions  103  and improving tWR characteristics. 
       FIG. 4  is a cross-sectional view of a semiconductor device according to a second exemplary embodiment of the inventive concept taken along A-A′ of  FIG. 1 . 
     As shown in  FIG. 4 , a semiconductor device according to the second exemplary embodiment includes a buried gate  16  in a semiconductor substrate  101 . The buried gate  16  includes a gate insulating layer  109  and a barrier metal layer  111  sequentially stacked on an inner surface of a trench, a first gate electrode  123  formed on the barrier metal layer  111 , a second gate electrode  127  on one sidewall of the trench on the first gate electrode  123 , and a third gate electrode  131  formed on one side of the second gate electrode  127  on the first gate electrode  123 . 
     Here the second gate electrode  127  is formed on the one sidewall of the trench adjacent to a storage node contact  133 , and the third gate electrode  131  is formed on the other sidewall of the trench adjacent to a bit line contact  135 . A nitride layer  137  is formed between the storage node contact  133  and the bit line contact  135 . 
     The first gate electrode  123  has a height not to overlap a junction region  103 . The second gate electrode  127  has a height to overlap the junction region  103 . 
     A barrier layer  118  is formed between the second gate  127  and the third gate electrode  131  to reduce a contact resistance between the second gate electrode  127  and the third gate electrode  131 . The barrier layer  118  may comprise Ti, TiN, and or the like. 
     The first gate electrode  123  and the third gate electrode  131  may comprise a metal material such as tungsten (W). The second gate electrode  127  adjacent to the junction region  103  may comprise doped polysilicon, e.g., an N+ polysilicon material. 
     In the above-described second exemplary embodiment, the buried gate has a structure such that the N+ polysilicon gate electrode (second gate electrode) having a work function lower than the first gate electrode  113 , is applied to an overlapping region with the junction region  103  adjacent to the storage node contact  133  to form the overlapping region between the junction regions  103  to improve GIDL. Hereinafter, a method of fabricating a semiconductor device according to the second exemplary embodiment will be described with reference to  FIGS. 5A to 5E . 
     First, as shown previously in  FIGS. 3A to 3E , a first gate insulating layer  109 , a barrier metal layer  111 , and a first gate electrode  113  are formed in a lower portion of a trench  114  in a semiconductor substrate  101 . Then, a second gate material  115  is deposited on an inner surface of the trench  114  and an entire surface of a hard mask layer  105  along a step. The second gate material  115  may include a polysilicon material doped with an N+ type ion. Next a barrier metal (Ti, TiN, and the like)  116  is deposited on the second gate material  115  and then spacer-etched, so that the second gate material  115  and the barrier metal layer  116  remain on both sidewalls of the trench. 
     As shown in  FIG. 5A , a photoresist layer  122  is formed to cover the second gate material  115  and the barrier metal  116  at a storage node contact (not shown) side, and expose the second gate material  115  and the barrier metal layer at a bit line contact  116  (not shown) side. 
     As shown in  FIG. 5B , an etch back process is performed using the photoresist layer  122  to form a second gate electrode  127  and a barrier layer  118  which are the second gate material  115  and the barrier metal  116  remaining on one sidewall of the trench. The second gate electrode  127  and the barrier layer  118  are formed on the one sidewall of the trench  114  at the storage node contact (not shown) side. The second gate electrode  127  and the barrier layer  118  have a height to overlap the junction region. 
     As shown in  FIG. 5C , a third gate material  119  is deposited on the second gate electrode  127  and the barrier layer  118 . The second gate material  119  may include the same metal-based material as the first gate material. 
     As shown in  FIG. 5D , the third gate material  119  is planarized and then etched back to form a third gate electrode  131 . The third gate electrode  131  may be formed to have the same height as the second gate electrode  127 , or to have a height smaller than the second gate electrode  127 . A sealing layer  140  is formed on the second gate electrode  127  and the third gate electrode  131 . 
     As shown in  FIG. 5E , the sealing layer  140  is planarized. A bit line contact  135  is formed on the substrate  101  at one side of the buried gates  16 . A bit line  139  is formed on the bit line contact  135 , and a storage node contact  133  is formed on the substrate  101  at the other side of the buried gate  16 . When the storage node contact  133  is formed, a junction region  103  is formed on the substrate  101 . A nitride layer  137  is formed between the bit line contact  135  and the storage node contact  133  and on the bit line  139 . The junction region  103 , the bit line contact  135 , storage node contact  133 , and the bit line may be formed in the usual process. 
     In the buried gate according to an exemplary embodiment, the N+ polysilicon gate (second gate electrode) is formed between the third gate electrode  131  and the junction region  103  which is coupled to the storage node contact  133 . 
     In the second exemplary embodiment described above the buried gate has (i) a lower gate structure including a metal gate electrode (the first gate electrode) and (ii) an upper gate structure including the third gate electrode each having low work function values. The upper gate structure further includes the N+ polysilicon gate electrode (the second gate electrode) between the third gate electrode and the junction region  103 . Therefore, resistance in the overlapping region between the junction region  103  and the buried gate  16 , more specifically the upper gate structure, becomes low and thus tWR characteristics can be improved. 
     The above embodiments are illustrative and not limitative. Various alternatives and equivalents are possible. Embodiments are not limited to any specific type of semiconductor device. Other additions, subtractions, or modifications may fall within the scope of the appended claims.