Abstract:
A phase-change random access memory (PCRAM) device and a method of manufacturing the same. The PCRAM includes a heating electrode having an upper surface protruding in a stepped shape and a phase-change material layer formed in a phase-change space on the heating electrode, the phase-change material layer having a plurality of portions having thicknesses corresponding to the stepped shape of the heating electrode.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
       [0001]    The present application claims priority under 35 U.S.C. 119(a) to Korean application number 10-2012-0028379, filed on Mar. 20, 2012, in the Korean Patent Office, which is incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The inventive concept relates to a nonvolatile memory device and a method of manufacturing the same, and more particularly, to a phase-change random access memory (PCRAM) device and a method of manufacturing the same. 
         [0004]    2. Related Art 
         [0005]    A lot of effort has been placed in realizing nonvolatile memory devices, such as PCRAMs, having a multi-level structure, while minimizing changes in a memory cell structure . 
         [0006]    As part of this effort, technology for realizing the multi-levels by changing a degree of phase-change in a phase-change material by supplying a write voltage to a bit line of the PCRAMs step-by-step, has been suggested. 
         [0007]    However, it is difficult to read out an accurate value in the PCRAM when the write voltage has been supplied step-by-step, as described above. 
         [0008]    That is, in a high-integration PCRAM, adjacent memory cells, particularly cells connected to the same bit line, are formed close to one another. Thus, these cells are vulnerable to disturbance from a adjacent memory cells. Even when the write voltage is supplied step-by-step, a desired phase-change is not caused due to an effect of the adjacent memory cells. Thus, it is difficult to verify the memory cells and to realize a multi-level structure. 
       SUMMARY 
       [0009]    According to one aspect of an exemplary embodiment there is a provided a PCRAM device. The PCRAM device may include: a heating electrode having an upper surface protruding in a stepped shape and a phase-change material layer formed in a phase-change space on the heating electrode, the phase-change material layer having a plurality of portions having thicknesses corresponding to the stepped shape of the heating electrode. 
         [0010]    According to another aspect of an exemplary embodiment, there is a provided a PCRAM device. The PCRAM device may include: a semiconductor substrate including a word line extending in one direction; a first interlayer insulating layer formed on the substrate; a switching element formed in the first interlayer insulating layer, the switching element being electrically connected to the word line; a second interlayer insulating layer formed on the first interlayer insulating layer, the second interlayer insulating layer defining a phase-change space; a heating electrode formed in the phase-change space, the heating element being electrically connected to the switching element, and the heating element having a surface including a protrusion; a phase-change material layer disposed in the phase-change space on the heating electrode, the phase-change material layer having portions having different thicknesses corresponding to a height of the protrusion; and a bit line formed on the phase-change material layer. 
         [0011]    According to another aspect of an exemplary embodiment, there is a provided a method of manufacturing a PCRAM device. The method may include: defining a phase-change space; forming a heating electrode having a step-shaped stepped protrusion, where the step-shaped protrusion defines on a boundary of the phase-change space; and forming, in the phase-change space, a phase-change material layer in the phase-change space on the step-shaped protrusion of the heating electrode. 
         [0012]    These and other features, aspects, and embodiments are described below in the section entitled “DETAILED DESCRIPTION”. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above aspects, features, and 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: 
           [0014]      FIG. 1  is a cross-sectional view illustrating a heating electrode structure and a phase-change material layer structure in a PCRAM device according to an exemplary embodiment of the inventive concept; 
           [0015]      FIG. 2  is a cross-sectional view illustrating a heating electrode structure and a phase-change material layer structure in a PCRAM device according to another exemplary embodiment of the inventive concept; 
           [0016]      FIGS. 3A to 3C  are perspective views illustrating a method of manufacturing a PCRAM device according to an exemplary embodiment of the inventive concept; 
           [0017]      FIG. 4  is a cross-sectional view illustrating a PCRAM device according to an exemplary embodiment of the inventive concept; and 
           [0018]      FIG. 5  is a graph illustrating a phase-change curve of a PCRAM device according to the second exemplary embodiment of the inventive concept. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Hereinafter, exemplary embodiments will be described in greater detail with reference to the accompanying drawings. 
         [0020]    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, are to 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. 
         [0021]    Referring to  FIG. 1 , a PCRAM device includes a heating electrode  100 . The heating electrode  100  includes a body portion  100   a  and a stepped protrusion  100   b  formed on the body  100   a.    
         [0022]    Since the stepped protrusion  100   b  is formed in a phase-change space PS, the phase-change space PS has partially different heights. A phase-change material layer  120  is formed within the phase-change space PS having the partially different heights. The phase-change material layer  120  may include a single material and be formed through a single deposition process. The phase-change material layer  120  has partially different thicknesses by a structure of the heating electrode  100 . 
         [0023]    At this time, a portion {circle around (a)}, having a relatively small thickness, of the phase-change material layer  120  is first phase changed and portions {circle around (b)} and {circle around (c)} having a relatively small thickness are sequentially phase changed. Therefore, the phase-change material layer  120  is phase changed in a step-by-step manner, so that multi-levels are realized. 
         [0024]    To produce a more precise phase change of the phase-change material layer, the phase-change material layer  120  may be formed of partially different materials, as shown in  FIG. 2 . The phase-change material layer  120  may be formed of a first phase-change material layer  120   a  in edge portions, that is, the {circle around (c)} portions of the phase-change space PS. The phase-change material layer  120  may also be formed of a second phase-change material layer  120   b  in the {circle around (b)} portions of the phase-change space PS. The phase-change material layer  120  also may be formed of a third phase-change material layer  120   c  in the {circle around (a)} portion of the phase-change space PS. 
         [0025]    At this time, the multi-levels may be precisely defined by setting the first phase-change material layer  120   a  to have the highest phase-change temperature and setting the third phase-change material layer  120   c  to have the lowest phase-change temperature. 
         [0026]      FIGS. 3A to 3C  are perspective views illustrating a method of forming a heating electrode according to an exemplary embodiment of the inventive concept. 
         [0027]    Referring to  FIG. 3A , a disc-shaped first heating electrode  100   a  is formed within a phase-change space PS. A diameter of the first heating electrode  100   a  is substantially the same as that of the phase-change space PS. 
         [0028]    Referring to  FIG. 3B , a second heating electrode  100   b - 1  is formed on the first heating electrode  100   a.  The second heating electrode  100   b - 1  may have a disc shape and may have a smaller diameter than the first heating electrode  100   a.  The second heating electrode  100   b - 1  may be arranged on a central portion of the first heating electrode  100   a.    
         [0029]    Referring to  FIG. 3C , a third heating electrode  100   b - 2  is formed on the second heating electrode  100   b - 1 . The third heating electrode  100   b - 2  may have a smaller diameter than the second heating electrode  100   b - 1  and may be arranged on a central portion of the second heating electrode  100   b - 1 . Therefore, the stepped heating electrode may be formed. 
         [0030]      FIG. 4  is a cross-sectional view of a PCRAM device according to an exemplary embodiment. 
         [0031]    Referring to  FIG. 4 , a word line  210  is formed on a semiconductor substrate  200 . The word line  210  is electrically insulated from the semiconductor substrate  200  and includes a polysilicon layer including impurities or a metal layer. 
         [0032]    A first interlayer insulating layer  220  is formed on the semiconductor substrate  200 , in which the word line  210  is formed, and a diode D is formed in a predetermined portion of the interlayer insulating layer  220 . The diode D may be formed to be in contact with the word line  210 . The diode D may include a PN diode or a Schottky diode according to a material of the word line  210 . 
         [0033]    A second interlayer insulating layer  230  is formed on the first interlayer insulating layer  220 . A predetermined portion of the second interlayer insulating layer  230  is etched to form a phase-change space PS. A heating electrode  240  is formed on a bottom of the phase-change space PS through the above-described method. The heating electrode  240  may be formed through a stepped type etching method other than the above-described etching method. Although not shown, an ohmic contact layer may be interposed between the diode D and the heating electrode  240 . 
         [0034]    A phase-change material layer  250  is buried within the phase-change space PS on the heating electrode  240 . The phase-change material layer  250  may be formed of a single phase-change material. Alternatively, the phase-change material layer  250  may be formed of different materials for steps of the heating electrode  240 . 
         [0035]    A bit line  260  is formed on the phase-change material layer  250 . The bit line  260  may extend in a direction substantially perpendicular to the word line  210  and an upper electrode (not shown) may be interposed between the bit line  260  and the phase-change material layer  250 . 
         [0036]    When a predetermined current is applied to the PCRAM device, a plurality of temperature-induced phase-changes are caused based on a thickness , or composition, of the phase-change material layer  250 , as shown in  FIG. 5 . In  FIG. 5 , an x-axis indicates current and a y-axis indicates resistance. A symbol “X” denotes a phase-change period. 
         [0037]    Accordingly, the PCRAM may be precisely verified by supplying a constant write voltage, which is difficult to control in a conventional PCRAM. 
         [0038]    The inventive concept is not limited to the above-described exemplary embodiment. 
         [0039]    The exemplary embodiment has described that the phase-change space PS, in which the heating electrode and the phase-change material layer are formed, is configured in a cylindrical shape, but the structure of the phase-change space PS is not limited thereto. The phase-change space PS may be formed in various shapes, such as a square pillar or a trigonal prism. 
         [0040]    While certain embodiments have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the devices and methods described herein should not be limited based on the described embodiments. Rather, the systems and methods described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.