Abstract:
A phase-change random access memory (PRAM) device including a plurality of nanowires and a method of manufacturing the same include: a lower structure including a plurality of contact plugs; the nanowires extending into the contact plugs from surfaces defining a respective terminal end of the contact plugs; and a phase-change layer formed on top of the nanowires. Therefore, a reset or a set current consumed by the PRAM device is significantly reduced.

Description:
This application claims priority to Korean Patent Application No. 10-2005-0038224, filed on May 7, 2005, and all the benefits accruing therefrom under 35 U.S.C. §119, and the contents of which in its entirety are herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a phase-change random access memory (“PRAM”) and a method of manufacturing the same, and more particularly, to a PRAM including nanowires and a method of manufacturing the PRAM, the PRAM having improved electrical characteristics by forming metal nanowires in a contact plug formed at a lower portion of a phase change layer. 
   2. Description of the Related Art 
   Phase-change materials undergo a structural transformation between crystalline and amorphous phases. The crystalline phase exhibits lower resistance relative to the amorphous phase and has an orderly atomic arrangement. The crystalline phase can be converted to the amorphous phase and vice versa. Phase-change random access memories (“PRAMs”) are devices based on reversible phase change between crystalline and amorphous phases, which have distinctly different resistances. 
   Generally, PRAMs have a structure in which a contact plug is formed in a source region or a drain region of a transistor, and a phase-change layer and an upper electrode are sequentially formed on top of the contact plugs. 
   A method of storing data in a PRAM having the general structure described above is as follows. When a current is supplied through an electrode at a lower portion of the phase-change layer, Joule heat is generated in a region where a lower electrode and the phase-change layer contact each other. When the generated Joule heat becomes higher than a re-crystallization temperature, the structure of the phase-change layer is intentionally changed from a crystalline structure to an amorphous phase by appropriately controlling the supplied current. Since a resistance of the phase-change layer varies as it is changed to the amorphous phase, previously stored data values can be distinguished. To change the phase-change layer from the amorphous phase to the crystalline phase, crystallization occurs when a predetermined period of time passes at a temperature lower than a melting point. To change the phase-change layer from the crystalline phase to the amorphous phase, the temperature is increased to approximately the melting point (Tm) of the phase-change layer and then drastically cooled. 
   Various materials may be used in memory devices. An example of a commonly used material is a material from the chalcogenide family, that is, a GeSbTe (GST) family alloy. In order to improve the quality of PRAM memory devices, current that is supplied should be reduced. In particular, in a case of the most commonly used GST PRAM, a reset current needed to change from the crystalline phase to the amorphous phase is high. 
     FIGS. 1A and 1B  are views of a conventional phase-change memory device disclosed in U.S. Pat. No. 6,800,563. 
   Referring to  FIG. 1A , a lower electrode  11  is formed on top of a substrate (not shown). A phase-change layer  12  and an upper electrode  13  are sequentially formed on top of the lower electrode  11 . The width of the lower electrode  11  narrows towards the top of the lower electrode  11  to reduce the area of the lower electrode  11  contacting the phase-change layer  12 . That is, by decreasing the area to which current is supplied, more heat is generated. 
   Referring to  FIG. 1B , a lower portion of a GST phase-change layer does not directly contact a lower electrode (BE). Instead, a TiN layer having a narrow width is separately formed so that more heat can be generated from the supplied current. 
   The structure as illustrated in  FIGS. 1A and 1B  is for reducing the contact area between the phase-change layer  12  and the area of the lower electrode  11  beneath the phase-change layer  12  to which current is supplied. However, there is a limit to how much the contact area can be reduced due to technical limitations of a semiconductor process. Therefore, a PRAM device with a new structure is required. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a PRAM device having a structure capable of minimizing a contact area of an electrode contacting a phase-change layer to reduce power consumption. 
   According to an exemplary embodiment of the present invention, a phase-change random access memory (PRAM) device includes a plurality of nanowires. The PRAM device further includes a lower structure having a plurality of contact plugs; the nanowires extending downwards from surfaces of the contact plugs; and a phase-change layer formed on the contact plugs. 
   The lower structure may include a semiconductor substrate; a source region and a drain region formed at sides of the semiconductor substrate; a gate insulating layer and a gate electrode layer sequentially formed on the semiconductor substrate, the gate insulating layer contacting the source region and the drain region; and the contact plugs respectively formed on the source region and the drain region. 
   Each of the contact plugs may be formed of at least one selected from the group consisting of Si, SiO 2 , SiGe, GaAs, GaN and SiC. 
   Each of the nanowires may be formed of at least one selected from the group consisting of Au, Ag, Al, Cu, Cr, Co, Ni, Ti, Sb, V, Mo, Ta, Nb, Ru, W, Pt, Pd, Zn and Mg. 
   The phase-change layer may be formed of a GeSbTe. 
   An upper electrode may be further formed on the phase-change layer. 
   A metal layer may be further formed beneath the contact plugs. 
   According to another exemplary embodiment of the present invention, a method of manufacturing a PRAM device includes forming a lower structure including a plurality of contact plugs; injecting a metal into the contact plugs; forming a plurality of nanowires extending downwards from surfaces of the contact plugs by thermally processing the contact plugs; and forming a phase-change layer on the contact plugs. 
   According to the present invention, the forming of the lower structure may include sequentially forming a gate insulating layer and a gate electrode layer on a semiconductor substrate; exposing both sides of the semiconductor substrate by etching sides of the gate insulating layer and the gate electrode layer; forming a source region and a drain region by doping impurities into both exposed sides of the semiconductor substrate; and forming contact plugs on the source region and the drain region. 
   According to the present invention, the forming of the contact plugs may include forming an intermediate insulating layer by depositing an insulating material on the gate electrode layer, the source region and the drain region; forming holes in the intermediate insulating layer to expose the source region and the drain region by etching the intermediate insulating layer; and depositing a contact plug material inside the holes. 
   According to the present invention, the forming of the contact plugs may include forming an intermediate insulating layer by depositing an insulating material on the gate electrode layer, the source region and the drain region; forming holes in the intermediate insulating layer to expose the source region and the drain region by etching the intermediate insulating layer; and forming metal layers in the holes by depositing a metal material inside the holes and depositing a contact plug material on the metal layers. 
   In the forming of the nanowires, a laser beam having an energy density in the range of about 300 mJ/cm 2  to about 2,000 mJ/cm 2  may be emitted onto the surfaces of the contact plugs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIGS. 1A and 1B  are cross-sectional views of a conventional phase-change memory device; 
       FIG. 2  is a cross-sectional view of an exemplary embodiment of a phase-change memory device according to the present invention; 
       FIGS. 3A through 3H  are cross-sectional views illustrating an exemplary embodiment of a method of manufacturing a phase-change memory device according to the present invention; 
       FIGS. 4A through 4I  are cross-sectional views illustrating another exemplary embodiment of a method of manufacturing a phase-change memory device according to the present invention; 
       FIG. 5  is a TEM image of a region of a nanowire in the phase-change memory device illustrated in  FIG. 3H ; 
       FIG. 6  is a cross-sectional view of a nanowire and a phase-change layer of a PRAM manufactured according to an exemplary embodiment of the present invention; 
       FIGS. 7A  is a graph illustrating temperatures of a phase-change layer after a predetermined current is supplied to a contact plug of a conventional PRAM; 
       FIG. 7B  is a graph illustrating temperatures of a phase-change layer after a predetermined current is supplied to a contact plug of a PRAM sample of the present invention, as illustrated in  FIG. 6 ; and 
       FIG. 7C  is a graph illustrating a temperature ratio of the phase-change layer  29  of  FIGS. 6 and 7B  to the phase-change layer in  FIG. 7A  at the same temperature when the same current is supplied using the results illustrated in  FIGS. 7A and 7B . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary 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 invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity. 
   It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or”includes any and all combinations of one or more of the associated listed items. 
   It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
   Spatially relative terms, such as “below” or “lower” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
   Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. 
   For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention. 
   Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     FIG. 2  is a cross-sectional view of an exemplary embodiment of a PRAM according to the present invention. Referring to  FIG. 2 , a source region  22   a  and a drain region  22   b  are formed on each side of a substrate  21 . A gate structure is formed on the substrate  21 , contacting the source region  22   a  and the drain region  22   b . An insulating layer  23  and a gate electrode layer  24  are sequentially formed as the gate structure. Such a structure is a common transistor structure. An intermediate insulating layer  26  is formed on the transistor structure, and contact plugs  27  are formed in the intermediate insulating layer  26  on top of the source region  22   a  and the drain region  22   b , respectively. Phase-change layers  29  are respectively formed on top of the contact plugs  27 . Nanowires  28  are respectively formed on bottom surfaces of the phase-change layers  29 , extending vertically downward as illustrated, inside the contact plugs  27 . 
   The substrate  21  may be formed of Si, SiC, etc., which is used in a common substrate of a semiconductor device. The gate insulating layer  23  in the gate structure may be formed of an insulating material such as SiO 2 , and the gate electrode layer  24  may be formed of commonly used metallic or non-metallic conductive materials. 
   The contacts plugs  27  formed on top of the source region  22   a  and the drain region  22   b  may be formed of at least one of Si, SiO 2 , SiGe, GaAs, GaN and SiC. The nanowires  28  are formed by ion injecting a metal material into the contact plugs  27 . The metal material may be, but is not limited to, Au, Ag, Al, Cu, Cr, Co, Ni, Ti, Sb, V, Mo, Ta, Nb, Ru, W, Pt, Pd, Zn or Mg. 
   A method of manufacturing the PRAM illustrated in  FIG. 2  according to another exemplary embodiment of the present invention is described below.  FIGS. 3A through 3H  are cross-sectional views illustrating an exemplary embodiment of a method of manufacturing the PRAM illustrated in  FIG. 2  according to the present invention. 
     FIG. 3A  is a cross-sectional view of the transistor structure of a lower portion of the PRAM illustrated in  FIG. 2 . Such a structure can be manufactured using a general semiconductor process as follows. First, a gate insulating layer  23  is formed by depositing an insulating material, such as SiO 2 , on top of a semiconductor substrate formed of Si, for example. Thereafter, a metallic or non-metallic material is formed on the gate insulating layer  23  to form a gate electrode layer  24 . The gate insulating layer  23  and the gate electrode layer  24  form a gate structure. Both sides of the gate insulating layer  23  and the gate electrode layer  24  are removed, thereby exposing the top surface of the substrate  21 . Thereafter, a dopant is injected into the substrate  21  through the top surface of the exposed substrate  21  using, for example, an ion injection process. When forming sides of the gate structure to be very narrow in a thermal process performed after the dopant is injected, a source region  22   a  and a drain region  22   b  formed in the substrate  21  beneath the gate insulating layer  23  can overlap due to diffusion caused by the thermal process. Therefore, generally, a low concentration doping process is performed. Side walls  25  are formed on both sides of the gate structure, as illustrated in  FIG. 3A , and then a high concentration doping process, which is optional, is performed. As a result, the transistor structure is formed. 
   Referring to  FIG. 3B , an insulating material is deposited on top of the transistor structure to form an intermediate insulating layer  26 . The insulating material is not limited to a certain material, and may typically be SiO 2 . Referring to  FIG. 3C , materials such as Si, SiO 2 , SiGe, GaAs, GaN or SiC are deposited inside holes h to form contact plugs  27   a  illustrated in  FIG. 3D . In the resultant structure, the contact plugs  27   a  formed on top of the source region  22   a  and the drain region  22   b  are in an amorphous phase since they have not gone through the thermal process. 
   Referring to  FIG. 3E , photoresist (“PR) is deposited on top of the intermediate insulating layer  26 , and the PR on top of the contact plugs  27   a  is removed so that the contact plugs  27   a  are exposed. Metal ions formed of Au, Ag, Al, Cu, Cr, Co, Ni, Ti, Sb, V, Mo, Ta, Nb, Ru, W, Pt, Pd, Zn or Mg are injected into the contact plugs  27   a  using, for example, an ion injection process. The energy required for the ion injection process may range from about 1 keV to about 1 MeV. The doping amount of the injected metal ions may range from about 10 10  atoms/cm 2  to about 10 17  atoms/cm 2 . The metal ions penetrate into the contact plugs  27   a  by the ion injection process of the metal ion. 
   Referring to  FIG. 3F , after removing the PR, a laser beam is emitted onto the contact plugs  27   a  from above the contact plugs  27   a  to perform an annealing process to form nanowires inside the contact plugs  27   a . The energy density of the emitted laser beam may range from about 300 mJ/cm 2  to about 2,000 mJ/cm 2 . 
   Referring to  FIG. 3G , the contact plugs  27   a  in the amorphous phase are converted into contact plugs  27  in a crystalline phase having a plurality of grains using the annealing process performed using the laser beam described above. Nanowires  28  extending from top surfaces of the contact plugs  27  are respectively formed inside the contact plugs  27 . The following is a more detailed description. 
   For example, when the contact plugs  27  are formed of Si, the insides of the contact plugs  27  go through nucleation and grain growth processes when performing the thermal process using the laser beam. Therefore, grain boundaries, caused by crystal growth, are formed inside the contact plugs  27 , and the metal ions, for example, silicide, stably grow nano-scaled nanowires from the grain boundaries. The nanowires  28  can also be grown at an angle, besides growing perpendicular to top surfaces of the contact plugs  27 . 
   Referring to  FIG. 3H , phase-change layers  29  such as GSTs are respectively formed on top of the contact plugs  27  in which the nanowires  28  are formed perpendicular to the phase-change layers  29 , thus completing the formation of the PRAM. An upper electrode (not shown) may be further formed on top of the PRAM. An image of the cross-section of a region N illustrated in  FIG. 3H  taken with a TEM is illustrated in  FIG. 5 . Referring to  FIG. 5 , after Mo ions are ion injected into a Si contact plug, a laser beam is emitted to perform a thermal process. The light portions indicate the Si layer and the dark portions indicate the silicide nanowires. Here, it can be seen that the width of the nanowires is about 2 nm or more. 
   Another exemplary embodiment of a method of manufacturing a PRAM according to the present invention will be described with reference to  FIGS. 4A through 4I  below. 
     FIGS. 4A through 4C  illustrate the same manufacturing process as illustrated in  FIGS. 3A through 3C . Therefore, the description regarding  FIGS. 3A through 3C  also applies to  FIGS. 4A through 4C . 
   Referring to  FIG. 4D , metal layers  27   b  are formed on top of a source region  22   a  and a drain region  22   b  by depositing a metal such as Au, Ag, Al, Cu, Cr, Co, Ni, Ti, Sb, V, Mo, Ta, Nb, Ru, W, Pt, Pd, Zn or Mg. Surfaces of the source region  22   a  and the drain region  22   b  are exposed via holes h ( FIG. 4C ) formed in an intermediate insulating layer  26 . The metal layers  27   b  are deposited up to a middle portion of the holes h. 
   Referring to  FIG. 4E , contact plugs  27   a  are formed by depositing a material such as Si, SiO 2 , SiGe, GaAs, GaN or SiC on top of the respective metal layers  27   b  inside the holes h. The contact plugs  27  are in an amorphous phase since the source region  22   a  and the drain region  22   b  have not gone through the thermal process. 
   Referring to  FIG. 4F , a PR is deposited on top of the intermediate layer  26  and the contact plugs  27   a , and the PR on top of the contact plugs  27   a  is removed so that the contact plugs  27   a  are exposed. Metal ions such as Au, Ag, Al, Cu, Cr, Co, Ni, Ti, Sb, V, Mo, Ta, Nb, Ru, W, Pt, Pd, Zn or Mg are doped into the exposed contact plugs  27   a  using, for example, an ion injection process. The energy required for the ion injection process may range from about 1 keV to about 1 MeV. The doping amount of the injected metal ions may range from about 10 10  atoms/cm 2  to about 10 17  atoms/cm 2 . The metal ions penetrate into the contact plugs  27   a  by the ion injection process of the metal ion. 
   Referring to  FIG. 4G , after removing the PR, a laser beam is emitted onto the contact plugs  27   a  to perform an annealing process to form nanowires inside the contact plugs  27   a . The energy density of the emitted laser beam may range from about 300 mJ/cm 2  to about 2,000 mJ/cm 2 . 
   Referring to  FIG. 4H , the contact plugs  27   a  in the amorphous phase are converted into contact plugs  27  in a crystalline phase having a plurality of grains when using the laser beam to perform the annealing process. Nanowires  28  extending from top surfaces of the contact plugs  27  are respectively formed inside the contact plugs  27 . 
   Referring to  FIG. 4I , phase-change layers  29 , such as GSTs for example, are respectively formed on top of the contact plugs  27  in which the nanowires  28  are formed perpendicular to the phase-change layers  29 , thus completing the formation of the PRAM. Generally, an upper electrode (not shown) is further formed on top of the PRAM. 
   Below, a PRAM having a structure as illustrated in  FIG. 6  was manufactured to check the effects of the present invention. The simulation conditions are as shown in Table 1. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               (Analysis Condition: Real Scale, Heat Insulating Condition, Analysis 
             
             
               Time: 25 ns) 
             
           
        
         
             
                 
               Electrical 
                 
                 
                 
             
             
                 
               Resistivity 
               Thermal Conductivity 
               Density 
               Heat Capacitor 
             
             
                 
               (□m) 
               (W/m K) 
               (kg/m 3 ) 
               (J/kg K) 
             
             
                 
                 
             
           
        
         
             
               GST 
               0.0001 
               1.8 
               6200 
               193.5 
             
             
               TiN 
                 1E−6 
               13 
               5400 
               599 
             
             
               MoSi 2   
               2.15E−7 
               53.92 
               6240 
               418.4 
             
             
               Si 
               13 
               149 
               2330 
               702.2 
             
             
                 
             
           
        
       
     
   
     FIG. 7A  is a graph illustrating temperatures of a phase-change layer when currents of 1.0 mA, 1.4 mA and 2.0 mA are supplied to a conventional PRAM sample where a contact plug is formed of TiN and nanowires are not formed. Referring to  FIG. 7A , the temperature of the phase-change layer gradually decreased as the distance from the contact plug increased from 0 to 100 nm. Also, the temperature of the phase-change layer drastically increased as current supplied to the contact plug was increased. 
     FIG. 7B  is a graph illustrating the temperatures of a phase-change layer  29  obtained after currents of 1.0 mA, 1.4 mA and 2.0 mA are supplied to a PRAM sample of the present invention having the structure as illustrated in  FIG. 6  where a contact plug  27  is formed of TiN and a nanowire  28  is formed of MoSi 2 . Referring to  FIG. 7B , the temperature of the phase-change layer  29  of  FIG. 6  gradually decreased as the distance from the contact plug  27  increased from 0 to 100 nm. Also, the temperature of the phase-change electrode  29  drastically increased as current supplied to the contact plug was increased. However, while the temperature of the phase-change layer  29  increased to a maximum of about 2,300° C. when a current of 2.0 mA was supplied in  FIG. 7A , the maximum temperature to which the phase-change layer  29  increased was 1.3×10 5 ° C. in  FIG. 7B . 
     FIG. 7C  is a graph illustrating a temperature ratio of the phase-change layer ( 29  of  FIG. 6 ) in  FIG. 7   b  to the phase-change layer in  FIG. 7A  at the same temperature when the same current is supplied using the results illustrated in  FIGS. 7A and 7B . Referring to  FIG. 7C , it can be seen that there is a maximum temperature increase 60 times higher than the conventional PRAM obtained when using the PRAM of the present invention. Using this result, the temperature of the phase-change layer  29  of the PRAM of the present invention can be increased significantly while significantly reducing the supplied current necessary to increase the phase-change layer  29  to this temperature compared to the conventional PRAM. 
   According to the present invention, a PRAM device has been described in which consumption of a supply current (e.g., reset and set current) required for a phase change of a phase-change layer of the PRAM device can be minimized by forming nanowires on the bottom of the phase-change layer so that a contact area with the phase-change layer is minimized. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.