Abstract:
A method for fabricating a phase change memory (PCM) cell includes forming a dielectric layer over an electrode, the electrode comprising an electrode material; forming a via hole in the dielectric layer such that the via hole extends down to the electrode; and growing a single crystal of a phase change material on the electrode in the via hole. A phase change memory (PCM) cell includes an electrode comprising an electrode material; a dielectric layer over the electrode; a via hole in the dielectric layer; and a single crystal of a phase change material located in the via hole, the single crystal contacting the electrode at the bottom of the via hole.

Description:
FIELD 
       [0001]    This disclosure relates generally to the field of phase change memory (PCM) fabrication. 
       DESCRIPTION OF RELATED ART 
       [0002]    Phase-change memory (PCM) is a type of non-volatile computer memory. PCM stores data in cells comprising a phase change material, which can be switched between two distinct states, i.e., crystalline and amorphous, with the application of heat. The phase change material may be deposited and patterned to form individual PCM cells. However, as PCM cells become smaller, it becomes difficult to pattern the cells using etching techniques such as reactive ion etching (RIE), as RIE may change the chemical makeup of the phase change material within a region of about 10 nm from the feature&#39;s edge, which may preclude following the scaling road map since the damaged region would constitute all the material in left in the cell for small dimensions. 
         [0003]    Alternately, a small amount of phase change material may be deposited in a small hole, or via, to form an individual PCM cell. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods may be used to deposit the phase change material. However, these methods may produce polycrystalline phase change material with crystals larger than the size of the via hole, which may not properly fill the via hole, or amorphous phase change material which may form voids and loose contact with an electrode located at the bottom of the via hole upon crystallization, as the phase change material may shrink as it changes from the amorphous state to the crystalline state. 
       SUMMARY 
       [0004]    In one aspect, a method for fabricating a phase change memory (PCM) cell includes forming a dielectric layer over an electrode, the electrode comprising an electrode material; forming a via hole in the dielectric layer such that the via hole extends down to the electrode; and growing a single crystal of a phase change material on the electrode in the via hole. 
         [0005]    In one aspect, a phase change memory (PCM) cell includes an electrode comprising an electrode material; a dielectric layer over the electrode; a via hole in the dielectric layer; and a single crystal of a phase change material located in the via hole, the single crystal contacting the electrode at the bottom of the via hole. 
         [0006]    In one aspect, a phase change memory (PCM) array comprising a plurality of cells, each cell including an electrode comprising an electrode material; a dielectric layer over the electrode; a via hole in the dielectric layer; and a single crystal of a phase change material located in the via hole, the single crystal contacting the electrode at the bottom of the via hole. 
         [0007]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0009]      FIG. 1  illustrates an embodiment of a method for formation of single crystal phase change material. 
           [0010]      FIG. 2  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material. 
           [0011]      FIG. 3  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after formation of a recess in the electrode. 
           [0012]      FIG. 4  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after formation of a recess in the oxide region. 
           [0013]      FIG. 5  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after formation of a dielectric layer and a keyhole. 
           [0014]      FIG. 6  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after formation of a via hole. 
           [0015]      FIG. 7  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after deposition of a single crystal phase change material in the via hole. 
           [0016]      FIG. 8  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after polishing. 
           [0017]      FIG. 9  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after formation of a recess in the polycrystalline phase change material, formation of a conductive layer in the recess, and polishing. 
           [0018]      FIG. 10  illustrates a cross-section of an embodiment of a process for formation of single crystal phase change material after formation of oxide and transparent electrical conductor layers. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments of systems and methods for formation of single crystal phase change material are provided, with exemplary embodiments being discussed below in detail. 
         [0020]    A single crystal of a phase change material may be grown on an electrode inside a via hole, filling the via hole and preventing void formation between the phase change material crystal and the electrode. The single crystal phase change material may be formed using CVD or ALD methods. The electrode material and the CVD/ALD precursors used to form the phase change material may be chosen such that the precursors used to form the phase change material react with the electrode material, and selective crystalline growth of the phase change material occurs directly on the electrode. The phase change material may also be selected such that the precursors do not react with a dielectric layer in which the via hole is formed. In some embodiments, the electrode may comprise tungsten (W) or titanium nitride (TiN), and the phase change material may comprise a combination of germanium (Ge), antimony (Sb), tellurium (Te) or selenium (Se). 
         [0021]    A phase change material has a typical crystal size that will vary depending on the material and temperature on which the crystal is grown. In a via hole larger than the typical crystal size for a chosen phase change material, electrode material, and temperature, a poly crystal may be formed, which may not properly fill the via hole. However, in a via hole smaller than the typical crystal size for the chosen phase change material and electrode material, a single crystal may be formed. Therefore, the via hole may be formed such that it is smaller than a typical crystal size of the chosen phase change material when that phase change material is grown on the chosen electrode material at a chosen temperature. For instance, for Ge 2 Sb 2 Te 5 (GST) deposited by CVD inside a via of a 200 nm CD with a W bottom electrode, at about 300° C., the typical crystal size is about 80 nm. For similar conditions, the typical crystal size for GeTe is about 120 nm. 
         [0022]      FIG. 1  illustrates an embodiment of a method  100  for formation of single crystal phase change material.  FIG. 1  is discussed with reference to  FIGS. 2-10 . As shown in cross-section  200  of  FIG. 2 , PCM word lines  205  are located in oxide regions  203 , which are underneath nitride regions  204 . Electrodes  201  may comprise tungsten (W) or titanium nitride (TiN), and are located in between oxide regions  203 . Electrodes  201  connect to the front end of line (FEOL) portion of the PCM. Protection bar  202  protects electrode  206  (which connects to a terminal of a row of selecting transistors, not shown). In block  101 , recesses  301  (as shown in cross-section  300  of  FIG. 3 ) are formed in electrodes  201 , and recesses  401  including overhang  411  (as shown in cross-section  400  of  FIG. 4 ) are extended into oxide regions  203 . Recesses  301  and  401  may be formed by any appropriate etching technique. 
         [0023]    In block  102 , dielectric layer  502  is formed by a conformal deposition, as shown in cross-section  500  of  FIG. 5 . Dielectric layer  502  fills recesses  401 , and comprises keyholes  501 . In some embodiments, dielectric layer  502  may comprise one of conformal oxide or silicon. Keyholes  501  may have a maximum width about equal to twice the width of overhang  411  in recess  401 . 
         [0024]    In block  103 , via holes  601  and via hole collars  602  are formed in dielectric layer  502 , as shown in cross-section  600   FIG. 6 . Via holes  601  and via hole collars  602  may be formed by RIE of dielectric layer  502  through keyholes  501 . Keyholes  501  act as a hard mask during RIE. The diameter of keyholes  501  determine the diameter of via holes  601 . The via holes  601  extend down to electrodes  201 , and have a diameter smaller than a typical crystal size of a phase change material (discussed in further detail with respect to block  104  below) when it is grown on the material that comprises electrodes  201 . Because the current density is higher in the via holes  601 , switching of the phase change material between the amorphous and crystalline states due to Joule heating will occur within via holes  601 . 
         [0025]    In block  104 , a phase change material  701  is deposited in the via holes  601 , as shown in cross-section  700  of  FIG. 7 . Phase change material  701  comprises a single crystal of a phase change material, which may comprise a combination of germanium (Ge), antimony (Sb), tellurium (Te) or selenium (Se). Phase change material  701  may be formed using CVD or ALD methods. The CVD/ALD precursors used to form phase change material  701  and the material comprising electrode  201  are chosen such that selective crystalline growth of phase change material  701  occurs directly on electrode  201  in via hole  601 . Polycrystalline phase change material  702  (comprising the same material as phase change material  701 ) is also formed in the via hole collars  602 . 
         [0026]    In block  105 , the surface comprising nitride  204  and polycrystalline phase change material  702  is polished, as is shown in cross-section  800  of  FIG. 8 . A recess is then formed in polycrystalline phase change material  702 , and conductive layer  901  is formed in the recess, and the surface comprising nitride  204  and conductive layer  901  is polished, as is shown in cross-section  900  of  FIG. 9 . Conductive layer  901  may comprise titanium nitride in some embodiments. Polishing may be performed using chemical mechanical polishing (CMD). 
         [0027]    In block  106 , oxide layers  1001  and electrical conductor layers  1002  are formed, as is shown in PCM cross-section  1000  of  FIG. 10 . PCM  1000  comprises single crystal phase change material  701  contacting electrodes  201 , which prevents formation of voids between the phase change material  701  and electrodes  201  when the phase change material  701  switches between the amorphous state and the crystalline state. 
         [0028]    The technical effects and benefits of exemplary embodiments include formation of relatively small PCM cells while preventing void formation between the phase change material and the electrodes that comprise the PCM cells within the switching region. 
         [0029]    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. 
         [0030]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.