Abstract:
A phase change memory device is disclosed. A first columnar electrode and a second columnar electrode are provided, both arranged horizontally. A phase change layer is interposed between the first columnar electrode and the second columnar electrode, electrically connecting both thereof, wherein the entirety of the phase change layer is disposed on a plane. A bottom electrode electrically connects the first columnar electrode. A top electrode electrically connects the second columnar electrode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to memory devices, and in particular to a phase change memory device and fabrication thereof. 
         [0003]    2. Description of the Related Art 
         [0004]    Phase change memory devices have many advantages, such as high speed, low power consumption, large capacity, endurance, improved process integrity, and lower cost, with resulting suitability for use as independent or embedded memory devices with high integrity. Phase change memory devices can efficiently replace volatile memory devices, such as SRAM and DRAM, or non-volatile memory devices, such as flash memory devices. 
         [0005]      FIG. 1A  shows a conventional T shaped phase change memory device. Referring to  FIG. 1A , a conventional T-shaped phase change memory device sequentially comprises a bottom electrode  102 , a bottom via  104 , a phase change layer  106 , a top via  108  and a top electrode  110 , wherein the columnar bottom via  104  is a heating electrode, connecting the phase change layer  106 . Contact area of the bottom via  104  and the phase change layer  106  is determined by size of the bottom via  104 . The size of the bottom via  104 , however, is determined according to the limits of photolithography, rendering reduction of dimensions difficult. 
         [0006]      FIG. 1B  shows another conventional phase change memory device, in which a heating electrode  112  is disposed horizontally. Contact area of the heating electrode  112  and a phase change materials layer  114  is determined according to thickness of the heating electrode  112 , and is thus not subject to process limits of photolithography. Phase change materials layer  114  of the phase change memory device, however, is formed by gap filling, negatively affecting endurance and uniformity of contact between the phase change materials layer  114  and the heating electrode  112  of the phase change memory device are not good enough. In addition, the heating electrode  112  must comprise highly resistant materials to increase heating efficiency. Due to longer current path of the heating electrode  112  and the phase change material layer  114 , the phase change memory device consumes more power. Further, the horizontal phase change material layer  114  requires more lithography steps than conventional T-shaped phase change memory devices, increasing costs. 
         [0007]    As shown in FIG. IC, U.S. Pat. No. 6,867,425 discloses a lateral phase change memory device. A conductive material is formed on a substrate  150  and then patterned to form two electrodes  152  and  153 , supplying current to a phase change material layer  154 . A dielectric layer  156  is interposed between the phase change material layer  154  and the electrode  152  and  153 , and a protective layer  158  comprising dielectric materials covers the phase change material layer  154 . However, the phase change material layer  154  of the phase change memory device is still formed by gap filling, with accompanying deterioration of endurance and uniformity of contact between the phase change material layer  154  and the heating electrodes  152  and  153 . In addition, filling phase change materials into the gap between the electrodes  152  and  153  becomes more difficult with the reduced distance therebetween. Further, current path in the heating electrode, comprising highly resistant materials to achieve good heating efficiency, is longer than that of the conventional phase change memory device, consuming more power. In addition to the heating electrode, two additional conducing electrodes  152  and  153  are required on both sides of the phase change material layer  154 , increasing area occupied. The lateral phase change memory device, finally, still requires more photolithography steps than conventional T-shaped phase change memory devices, with correspondingly increased cost. 
       BRIEF SUMMARY OF INVENTION 
       [0008]    A detailed description is given in the following embodiments with reference to the accompanying drawings. According the issues above, Example of the present invention may provide a phase change memory device with shorter current path and fewer defects than conventional phase change memory device with a phase change layer formed in a trench. In addition, example of the present invention may provide a phase change memory device, in which area of a contacting region between a phase change layer and an electrode is determined by a thickness of the phase change layer, such that not limited to photolithography. 
         [0009]    In an embodiment of the invention, a phase change memory device comprises a first columnar electrode and a second columnar electrode, both arranged horizontally. A phase change layer is interposed between the first columnar electrode and the second columnar electrode, electrically connecting both thereof, wherein the entirety of the phase change layer is disposed on a plane. A bottom electrode electrically connects the first columnar electrode. A top electrode electrically connects the second columnar electrode. 
         [0010]    The invention further provides a method for forming a phase change memory device. A substrate comprising a source and a drain is provided. A plurality of interconnects and vias are formed with at least one of the interconnects and vias electrically connecting to the drain. A bottom electrode and a first dielectric layer are formed overlying the interconnects or the vias, wherein the bottom electrode is disposed in the first dielectric layer. Lower portions of a first columnar electrode and a second columnar electrode and a second dielectric layer are formed overlying the bottom electrode and the first dielectric layer, wherein the lower portions of the first columnar electrode and the second columnar electrode are disposed in the second dielectric layer, and the lower portion of the first columnar electrode electrically connects to the bottom electrode. A patterned phase change layer is formed overlying portions of the lower portions of the first columnar electrode and the second columnar electrode, and the second dielectric layer. Upper portions of the first columnar electrode and the second columnar electrode, and a third dielectric layer are formed overlying the lower portions of the first columnar electrode and the second columnar electrode, and a portion of the patterned phase change layer to form entireties of the first columnar electrode and the second columnar electrode, wherein the patterned phase change layer extends into the first columnar electrode and the second columnar electrode. A top electrode is formed to electrically connect a portion of the second columnar electrode. 
         [0011]    The invention provides another method for forming a phase change memory device. A substrate comprising a source and a drain is provided. A plurality of interconnects and vias are formed with at least one of the interconnects and vias electrically connecting the drain. A bottom electrode and a first dielectric layer are formed overlying the interconnects or the vias, wherein the bottom electrode is disposed in the first dielectric layer. A second dielectric layer is formed overlying the bottom electrode and the first dielectric layer. A phase change layer is formed overlying the second dielectric layer. A third dielectric layer is formed overlying the phase change layer and the second dielectric layer. A patterned photoresist layer is formed overlying the third dielectric layer. The second dielectric layer and the third dielectric layer are etched using the patterned photoresist layer as a mask to form at least two openings, wherein the openings penetrate portions of the phase change layer. A conductive material is filled into the openings to form at least two columnar electrodes. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIG. 1A  shows a conventional T-shaped phase change memory device. 
           [0014]      FIG. 1B  shows another conventional phase change memory device. 
           [0015]      FIG. 1C  shows yet another phase change memory device. 
           [0016]      FIG. 2A˜FIG .  2 E are intermediate cross sections of fabrications of a phase change memory device of an embodiment of the invention. 
           [0017]      FIG. 3  is a plan view of a phase change memory device of an embodiment of the invention. 
           [0018]      FIG. 4A˜FIG .  4 E are intermediate cross sections of fabrications of a phase change memory device of another embodiment of the invention. 
           [0019]      FIG. 5A˜FIG .  5 E are intermediate cross sections of fabrications of a phase change memory device of further another embodiment of the invention. 
           [0020]      FIG. 6  is a plan view of a phase change memory device of this embodiment. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0021]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. Embodiments of the invention, which provides a phase change memory device, will be described in greater detail by referring to the drawings that accompany the invention. It is noted that in the accompanying drawings, like and/or corresponding elements are referred to by like reference numerals. 
         [0022]      FIG. 2A˜FIG .  2 E show intermediate cross sections of fabrications of a phase change memory device of an embodiment of the invention. Referring to  FIG. 2A , a substrate  200  comprising an active  202  is provided, wherein a gate  204  is formed on the active area  202 . Source  206  and drain  208 , which both are doped regions, are formed on opposite sides of the gate  204 . The gate  204 , source  206  and drain  208  respectively connects a first interconnect  210 . A second interconnect  212  is connected to the first interconnect  210  through first vias  214 . A third interconnect  216  is connected to the second interconnect  212  through second vias  218 . A plurality of third vias  220  is formed on the third interconnect  216 . The vias  214 ,  218  and  220  are disposed in interlayer dielectric layers  222  separating the interconnects  210 ,  212  and  216 . 
         [0023]    A first dielectric layer  224 , comprising silicon nitride, silicon oxide, or silicon oxynitride, is formed on the third vias  220 . Next, the first dielectric layer  224  is patterned by a first photolithography step with a first mask to form an opening, and the opening is filled with conductive materials, such as TiN, Tan or TiW, to form a bottom electrode  226 . 
         [0024]    Referring to  FIG. 2B , a second dielectric layer  228 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the first dielectric layer  224  and the bottom electrode  226 . Next, the second dielectric layer  228  is patterned by a second photolithography step with a second mask to form at least two openings. Thereafter, refractory metals, such as W or TiAlN, metals with low heat conducting coefficient, phase change materials or chalcogenide are filled into the openings to form lower portions of columnar electrodes  230 . 
         [0025]    Referring to  FIG. 2C , a phase change layer (not shown), comprising Ag, In, Te, Sb, Ge or combinations thereof, is blanketly deposited on the lower portions of the columnar electrodes  230  and the second dielectric layer  228 . The phase change layer can be ternary chalcogenide compound, such as GeTe—Sb 2 Te 3 , or binary chalcogenide compound, such as combinations of Sb and Te in various percentages. The chalcogenide compound can comprise Cr, Fe, Ni, or combinations thereof, or Bi, Pb, Sn, As, S, Si, P, O, or combinations thereof. 
         [0026]    Next, the phase change layer is patterned by a photolithography step with a third mask to form a patterned phase change layer  232 , bridging the lower portions of the columnar electrodes  230 . 
         [0027]    Referring to  FIG. 2D , a third dielectric layer  234 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the patterned phase change layer  232 , second dielectric layer  228  and the lower portions of the columnar electrodes  230 . Next, the third dielectric layer  234  is patterned by photolithography using the second mask, the same mask used when forming the lower portions of the columnar electrodes  230 , to form at least two openings, each respectively exposing the corresponding lower portion of the columnar electrode  230 . Note that the patterned phase change layer  232  is substantially unaffected during the etching step. Etching ratio between the third dielectric layer  234  and the patterned phase change layer  232  is substantially more than  10 . Further note that deviations between positions of the openings and the corresponding lower portions of the columnar electrodes  230  cannot be too large. 
         [0028]    Next, refractory metals, such as W or TiAlN, metals with low heat conducting coefficient, phase change materials or chalcogenide, are filled into the openings to form upper portions of the columnar electrodes  236 , wherein the lower portions of the columnar electrodes  230  and the upper portions of the columnar electrodes  236  constitute columnar electrodes  240 . The two columnar electrodes  240  are located on the same layer, and the patterned phase change layer  232  extends into both the columnar electrodes  240 . 
         [0029]    Referring to  FIG. 2E , a fourth dielectric layer  242 , of silicon nitride, silicon oxide, or silicon oxynitride, is formed on the columnar electrodes  240 . Next, the fourth dielectric layer  242  is patterned by photolithography using a fourth mask to form an opening. A conductive material, such as TiN, TaN or TiW, is deposited into the opening, and then etched back to form a top electrode  244 . Thus, a major portion of the phase change memory device is fabricated. Note that fabrications of the phase change memory of the embodiment may use only four photolithography mask and process steps, one step and mask less than that of conventional planar phase change memory device. 
         [0030]      FIG. 3  is a plan view of a phase change memory device of an embodiment of the invention. Referring to  FIG. 2E  and  FIG. 3 , the patterned phase change layer  232  is formed on a plane, such that the entirety of the pattern phase change layer  232  is planar, having short current path and fewer defects than conventional phase change memory devices. In addition, area of the contact region of the electrode  240  and the patterned phase change layer  232  can be determined by thickness of the patterned phase change layer  232 , not being limited by photolithography technology. 
         [0031]      FIG. 4A˜FIG .  4 E are intermediate cross sections of fabrications of a phase change memory device of another embodiment of the invention, wherein portions of the structure of the device under the bottom electrode are similar to the device of  FIG. 2A˜FIG .  2 E. Elements of this portion use the same symbol numbers as the device of  FIG. 2A˜FIG .  2 E. Referring to  FIG. 4A , a substrate  200  comprising an active area  202  is provided, wherein a gate  204  is formed on the active area  202 . Source  206  and drain  208 , both doped regions, are formed on opposite sides of the gate  204 . The gate  204 , source  206  and drain  208  respectively connect to a first interconnect  210 . A second interconnect.  212  is connected to the first interconnect  210  through first vias  214 . A third interconnect  216  is connected to the second interconnect  212  through second vias  218 . A plurality of third vias  220  is formed on the third interconnect  216 . The vias  214 ,  218  and  220  are disposed in interlayer dielectric layers  222  separating the interconnects  210 ,  212  and  216 . 
         [0032]    A first dielectric layer  404 , comprising silicon nitride, silicon oxide and silicon oxynitride, is formed on the third vias  220 . Next, the first dielectric layer  404  is patterned by a first photolithography step with a first mask to form an opening. Conductive materials, such as TiN, TaN or TiW, are deposited into the opening to form a bottom electrode  402 . 
         [0033]    Referring to  FIG. 4B , a second dielectric layer  406 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the first dielectric layer  404  and the bottom electrode  402 . Next, a phase change layer (not shown), comprising Ag, In, Te, Sb, Ge or combinations thereof, is blanketly deposited on the second dielectric layer  406 . The phase change layer can be ternary chalcogenide compound, such as GeTe—Sb 2 Te 3 , or binary chalcogenide compound, such as combination of Sb and Te in various percentages. The chalcogenide compound can comprise Cr, Fe, Ni or combinations thereof, or Bi, Pb, Sn, As, S. Si, P, O or combinations thereof. Thereafter, the phase change layer is patterned to by photolithography using a second mask to form a patterned phase change layer  408 . 
         [0034]    Referring to  FIG. 4C , a third dielectric layer  410 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the patterned phase change layer  408  and the second dielectric layer  406 . Next, a photoresist layer  412  is coated on the third dielectric layer  410 , and then defined by exposure with a third mask. 
         [0035]    Referring to  FIG. 4D , the second dielectric layer  406  and the third dielectric layer  410  are etched using the defined photoresist layer  412  as a mask to form at least two openings, each penetrating a portion of the patterned phase change layer  408  at one side, in which the top electrode  402  or the first dielectric layer  404  are exposed. Next, refractory metals, such as W or TiAlN, metals with low heat conducting coefficient, phase change materials or chalcogenide, are filled into the openings to form two columnar electrodes  414 . The two columnar electrodes  414  are on the same level, and the patterned phase change layer  408  contacts sidewalls of the columnar electrodes  414 . 
         [0036]    Referring to  FIG. 4E , a fourth dielectric layer  416 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the columnar electrodes  414 . Next, the fourth dielectric layer  416  is patterned by photolithography using a fourth mask to form an opening. A conductive material, such as TiN, TaN or TiW, is deposited into the opening, and then etched back to form a top electrode  418 . Note that fabrication of the phase change memory of the embodiment may use only four masks and three photolithography steps, one mask and two photolithography steps less than a conventional planar phase change memory device. 
         [0037]      FIG. 5A˜FIG .  5 E are intermediate cross sections of fabrication of a phase change memory device of another embodiment of the invention, wherein portions of the structure of the device of this embodiment under the bottom electrode are similar to the device of  FIG. 2A˜FIG .  2 E. Elements of this portions use the same symbol numbers as the device of  FIG. 2A˜FIG .  2 E. Referring to  FIG. 5A , a substrate  200  comprising an active area  202  is provided, wherein a gate  204  is formed on the active area  202 . Source  206  and drain  208 , both doped regions, are formed on opposite sides of the gate  204 . The gate  204 , source  206  and drain  208  respectively connect to a first interconnect  210 . A second interconnect  212  is connected to the first interconnect  210  through first vias  214 . A third interconnect  216  is connected to the second interconnect  212  through second vias  218 . A plurality of third vias  220  are formed on the third interconnect  216 . The vias  214 ,  218  and  220  are disposed in interlayer dielectric layers  222  separating the interconnects  210 ,  212  and  216 . 
         [0038]    A first dielectric layer  502 , comprising silicon nitride, silicon oxide or silicon oxynitride, is formed on the third vias  220  and patterned by a first photolithography step with a first mask to form an opening. Conductive materials, such as TiN, TaN or TiW, are deposited into the opening to form a bottom electrode  504 . 
         [0039]    Referring to  FIG. 5B , a second dielectric layer  506 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the first dielectric layer  502  and the bottom electrode  504 . Next, a phase change layer  508 , comprising Ag, In, Te, Sb, Ge or combinations thereof, is blanlcetly deposited on the second dielectric layer  506 , wherein the phase change layer  508  can be ternary chalcogenide compound, such as GeTe—Sb 2 Te 3 , or binary chalcogenide compound, such as a combination of Sb and Te of various percentages. The chalcogenide compound can comprise Cr, Fe, Ni or combinations thereof, or Bi, Pb, Sn, As, S, Si, P, O or combinations thereof. Thereafter, a third dielectric layer  510 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the phase change layer  508 . 
         [0040]    Referring to  FIG. 5C , a photoresist layer  512  is coated on the third dielectric layer and defined by exposure with a third mask. Next, an etching process is utilized using the defined photoresist layer  512  as an etching mask to form at least two openings  514  in the second and third dielectric layers  506  and  510 , the openings  514  penetrating the phase change layer  508 , and the top electrode  504  or the first dielectric layer  502  are exposed. 
         [0041]    Next, referring to  FIG. 5D , refractory metals, such as W or TiAlN, metals with low heat conducting coefficient, phase change materials or Chalcogenide, are filled into the openings to form two columnar electrodes  516 . Phase change layer  508  contacts both sidewalls of each columnar electrode  516 . 
         [0042]    Referring to  FIG. 5E , a fourth dielectric layer  518 , such as silicon nitride, silicon oxide or silicon oxynitride, is formed on the columnar electrodes  516 . Next, the fourth dielectric layer  518  is patterned by photolithography using a third mask to form an opening. A conductive material, such as TiN, TaN or TiW, is deposited into the opening, and then etched back to fonn a top electrode  520 .  FIG. 6  is a plan view of a phase change memory device of this embodiment. Referring to  FIG. 6  and  FIG. 5E , the entire phase change layer  508  is disposed on a plane, surrounding and contacting the columnar electrodes  516  in their entirety. 
         [0043]    Note that fabrication of the phase change memory of this embodiment may use only three masks and photolithography steps, two mask and photolithography steps less than conventional planar phase change memory device. 
         [0044]    In addition, the phase change memory device can be connected to a driving device, such as a MOSFET device, a BJT device or a diode. 
         [0045]    According to the embodiments, since the patterned phase change layer is formed on a plane, the entirety of the patterned phase change layer can be planar, containing fewer defects and providing shorter current path than conventional phase change memory with phase change layer formed in/on a trench. In addition, area of a contact region between the phase change layer and the electrode can be determined by thickness of the phase change layer, not limited by a photolithography process. Further, fabrication of the phase change memory device of an embodiment of the invention requires fewer photolithography steps and/or masks than that of conventional phase change memory device. 
         [0046]    While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.