Abstract:
A layer of nanoparticles having a dimension on the order of 10 nm is employed to form a current constricting layer or as a hardmask for forming a current constricting layer from an underlying insulator layer. The nanoparticles are preferably self-aligning and/or self-planarizing on the underlying surface. The current constricting layer may be formed within a bottom conductive plate, within a phase change material layer, within a top conductive plate, or within a tapered liner between a tapered via sidewall and a via plug contains either a phase change material or a top conductive material. The current density of the local structure around the current constricting layer is higher than the surrounding area, thus allowing local temperature to rise higher than surrounding material. The total current required to program the phase change memory device, and consequently the size of a programming transistor, is reduced due to the current constricting layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor structures, and particularly to phase change memory element structures having a current constricting layer and methods of manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     A phase change memory (PCM) device is a non-volatile memory device employing a change in resistivity of a phase change material. The PCM device is also called phase-change random access memory (PRAM). Typically, a chalcogenide material capable of switching between an amorphous state and a crystalline state is employed in PCM devices. 
     Depending on the cooling rate from a liquid state, the chalcogenide material may form an amorphous chalcogenide glass or a chalcogenide crystal. The difference between the two states is physically characterized by the presence or absence of a long range order. Further, the crystalline and amorphous states of chalcogenide material have drastically different resistivity values. By manipulating the phase of the chalcogenide material, a binary data bit may be written into a PCM device. By detecting the phase of the chalcogenide material, typically in the form of a resistivity measurement, the binary data bit stored in the PCM device may be read. Many types of PCM devices employing these methods are known in the art. 
     A typical chalcogenide material used in PCM devices is a germanium, antimony and tellurium compound commonly called GST (Ge 2 Se 2 Te 5 ). Along with oxygen, sulfur, selenium, and polonium, tellurium belongs to the chalcogen group, hence the name chalcogenide material. In a typical PCM device, a chalcogenide glass having a high resistivity value may be formed upon melting and rapid cooling of a chalcogenide material. Alternatively, a chalcogenide crystal having a low resistivity value may be formed by raising the temperature to a crystallization temperature, which is below the melting temperature, followed by a slow cooling of the chalcogenide material. The chalcogenide becomes liquid at a relatively high temperature, e.g., above 600° C. 
     Referring to  FIG. 1 , a prior art phase change memory (PCM) element structure comprises a stack of a bottom conductive plate  10 , a phase change material layer  20 , and a top conductive plate  30 . By passing current through the PCM element structure, the phase change material in the phase change material layer  20  is heated to a temperature that can induce a phase change, i.e., either above the melting temperature or the crystallization temperature. 
     One of the challenges of present day PCM element structures is to generate sufficient heat to reach the melting temperature of the phase change material. While not every portion of the phase change material needs to melt to encode data in the PCM element, at least a portion of the phase change material capable of significantly affecting the overall resistance of the PCM element structure needs to reach the phase transition temperatures, i.e., the melting temperature and/or the crystallization temperature, in order for the phase change material to change its state between a crystalline structure and an amorphous structure. To induce such melting or recrystallization, a relatively large amount of current is typically required. However, application of such a large current requires a large transistor, and consequently a large semiconductor area, making it difficult to increase the density of PCM devices. 
     A method of reducing current demand on a PCM element structure by mixing a phase change material with an inactive dielectric material has been disclosed in the U.S. Pat. No. 5,825,046 to Czubatyj et al.  FIG. 2  schematically represents the prior art structure by Czubatyj et al., in which a mixed phase change material layer  20 ′ comprises a phase change material  21  intermixed with the inactive dielectric material  26 . The mixing of the phase change material  21  with the inactive dielectric material  26  decreases the cross-sectional area for the electrical current path between the bottom conductive plate  10  and the top conductive plate  30 , thus increasing the current density within the mixed phase change material layer  20 ′. For a given level of current between the bottom conductive plate  10  and the top conductive plate  30 , the mixed phase change material layer  20 ′ in  FIG. 2  provides higher local temperature than the phase change material layer  20  in  FIG. 1 . 
     While Czubatyj et al. provides a structure that enhances the local temperature of a mixed phase change material layer, the mixing process is stochastic, i.e., statistical variations in the mixing process produces non-uniform mixing, resulting in significant variations in the resistance of the mixed phase change material layer. 
     Therefore, there exists a need for a phase change memory element structure capable of achieving phase transition temperatures in a phase change material layer consistently with less programming current and methods of manufacturing the same. 
     Further, there exists a need for a phase change memory element structure with a higher resistance in the phase change material layer, in which the resistance values have a tight distribution, and methods of manufacturing the same. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the needs described above by providing phase change memory structures having a current path that is constricted by insulating nanoparticles between a bottom conductive plate and a top conductive structure. 
     Specifically, a layer of nanoparticles having a dimension on the order of about 10 nm is employed to form a current constricting layer or as a hardmask for forming a current constricting layer from an underlying insulator layer. The nanoparticles are preferably self-aligning and/or self-planarizing on the underlying surface. The current constricting layer may be formed within a bottom conductive plate, within a phase change material layer, within a top conductive plate, or within a tapered liner between a tapered via sidewall and a via plug contains either a phase change material or a top conductive material. The current density of the local structure around the current constricting layer is higher than the surrounding area, thus allowing local temperature to rise higher than surrounding material. The total current required to program the phase change memory device, and consequently the size of a programming transistor, is reduced due to the current constricting layer. Further, the self-assembly feature of the nanoparticles provide a tight resistance distribution for the phase change memory device. 
     According to an embodiment of the present invention, a semiconductor structure comprises:
         a phase change material layer;   a current constricting layer abutting the phase change material layer and containing a monolayer of insulating nanoparticles embedded in and separated by a volume of a conductive material;   a first conductive plate abutting the current constricting layer and comprising another volume of the conductive material; and   a second conductive plate abutting the phase change material layer and disjoined from the first conductive plate.       

     The first conductive plate may be a top conductive plate and the second conductive plate may be a bottom conductive plate. Alternatively, the first conductive plate may be a bottom plate and the second conductive plate may be a top plate. 
     According to another embodiment of the present invention, a semiconductor structure comprises:
         a first conductive plate comprising a first conductive material;   a phase change material layer abutting the first conductive plate and comprising a phase change material;   a current constricting layer comprising a set of disjoined insulating layer islands in a planar arrangement, wherein each of the disjoined insulating layer islands is embedded in and separated by a volume of material selected from the phase change material and the first conductive material, has a lateral dimension of a nanoparticle, and directly contacts the first phase change material layer;   a second conductive plate comprising a second conductive material and resistively connected to the phase change material layer through the current constricting layer.       

     The second conductive plate may abut the current constricting layer. Alternatively, the semiconductor structure may further comprise another phase change material layer consisting of another volume of the phase change material and abutting the current constricting layer and the second conductive plate. 
     The disjoined insulating layer islands has a thickness ranging from about 3 nm to about 60 nm and comprises a material selected from the group consisting of silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, silicon carbide, titanium oxide, tantalum oxide, ruthenium oxide, tungsten oxide, zinc oxide, silicon, germanium, germanium oxide, carbon, and a combination thereof. 
     According to yet another embodiment of the present invention, the semiconductor structure comprises:
         a dielectric layer containing a tapered via hole sidewall;   a tapered via liner abutting the tapered via hole sidewall and having a liner bottom surface and containing a monolayer of insulating nanoparticles that are embedded in and separated by a volume of a conductive material;   a conductive via plug abutting the tapered via liner and comprising the conductive material;   a phase change material layer abutting the liner bottom surface and the dielectric layer; and   a bottom conductive plate abutting the phase change material layer.       

     According to a further embodiment of the present invention, the semiconductor comprises:
         a top conductive plate;   a dielectric layer abutting the top conductive plate and containing a tapered via hole sidewall;   a tapered via liner abutting the tapered via hole sidewall and having a liner bottom surface and containing insulating nanoparticles that are embedded in and separated by a volume of a phase change material;   a via plug consisting of another volume of the phase change material and encapsulated by the tapered via liner and the top conductive plate; and   a bottom conductive plate resistively connected to the top plate through the tapered via.       

     The bottom conductive plate may abut the liner bottom surface and the dielectric layer. Alternatively, the semiconductor structure may further comprise a phase change material layer consisting of yet another volume of the phase change material, wherein the phase change material layer abuts the liner bottom surface, the dielectric layer, and the bottom conductive plate. 
     Within the various exemplary semiconductor structures, the insulating nanoparticles may comprise a dielectric material selected from the group consisting of silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, silicon carbide, titanium oxide, tantalum oxide, ruthenium oxide, tungsten oxide, zinc oxide, silicon, germanium, germanium oxide, carbon, and a combination thereof. 
     Alternatively, each of the nanoparticles may be an organic molecule confined within a monolayer or a multilayer and having a characteristic dimension in the range from about 3 nm to about 24 nm, the characteristic dimension being selected from the group consisting of a full length, a full width, and a diameter. 
     A phase change material that can be employed herein includes, but is not limited to, a chalcogenide alloy, which is an alloy of a chalcogen element with at least one non-chalcogen element, wherein the chalcogen element is selected from the group consisting of Te, Se, and S, and the at least one non-chalcogen element is selected from the group consisting of Ge, Sb, Bi, Pb, Sn, As, Si, P, Ga, In, and Ag. 
     The various conductive plates and the conductive plug may comprise a material selected from the group consisting of Ti, Ta, W, Mo, Al, Cu, Pt, Ir, La, Ni, Ru, another elementary metal, and alloys thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are cross-sectional views of prior art phase change memory element structures. 
         FIGS. 3A-3F  are sequential cross-sectional views of a first exemplary phase change memory element structure according to a first embodiment of the present invention. 
         FIGS. 4A-4D  are sequential cross-sectional views of a second exemplary phase change memory element structure according to a second embodiment of the present invention. 
         FIGS. 5A-5F  are sequential cross-sectional views of a third exemplary phase change memory element structure according to a third embodiment of the present invention. 
         FIGS. 6A-6G  are sequential cross-sectional views of a fourth exemplary phase change memory element structure according to a fourth embodiment of the present invention.  FIG. 6H  is a horizontal cross-sectional view of the fourth exemplary structure in  FIG. 6G  in the plane H-H′ in  FIG. 6G . 
         FIGS. 7A-7F  are sequential cross-sectional views of a fifth exemplary phase change memory element structure according to a fifth embodiment of the present invention. 
         FIGS. 8A-8H  are sequential cross-sectional views of a sixth exemplary phase change memory element structure according to a sixth embodiment of the present invention. 
         FIGS. 9-13  are seventh through eleventh exemplary phase change memory element structures according to seventh through eleventh embodiments of the present invention, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As stated above, the present invention relates to phase change memory (PCM) element structures having a current constricting layer and methods of manufacturing the same, which are now described in detail with accompanying figures. It is noted that like and corresponding elements are referred to by like reference numerals. 
     Sequential cross-sectional views of a first exemplary structure according to a first embodiment of the present invention are shown at various stages of a manufacturing process in  FIGS. 3A-3F . Referring to  FIG. 3A , a bottom conductive plate  40  is formed on an underlying semiconductor structure (not shown). The bottom conductive plate  40  may be formed by chemical vapor deposition or sputtering of a metal layer. Methods of placing the bottom conductive plate  40  in a semiconductor structure for the purpose of forming a PCM memory element is well known in the art. The bottom conductive plate  40  comprises a conductive material that may be, for example, selected from Ti, Ta, W, Mo, Al, Cu, Pt, Ir, La, Ni, Ru, another elemental metal, and alloys thereof. The thickness of the bottom conductive plate  40  is typically in the range from about 10 nm to about 80 nm. 
     Referring to  FIG. 3B , a monolayer of insulating nanoparticles  70  is applied to a top surface of the bottom conductive plate  40 . Each of the insulating nanoparticles  70  may be an organic molecule confined within the monolayer and may be substantially spherical having a diameter in the range from about 1 nm to about 24 nm. The organic molecule may be, for example, a protein molecule. The organic molecule may also be ellipsoidal, superellipsoidal, or polyhedral and have a characteristic dimension in the range from about 1 nm to about 24 nm. The characteristic dimension may a full length, a full width, or a diameter of the insulating nanoparticles  70 . Yamashita, “Big Nano Process: Fabrication of Nanoelectronic Devices using Protein Supermolecules,” IEEE International Electron Devices Meeting, 1-4244-0439-Aug. 6, 2006 demonstrates that protein nanoparticles may be formed on a surface in self-alignment, thus forming a layer with regular arrangement in a monolayer. In other words, the density of the organic nanoparticles may be constant in a self-aligned monolayer. The organic nanoparticles are insulating and, preferably, self-planarizing as well. 
     Alternatively, the insulating nanoparticles  70  may comprise a dielectric material such as silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, silicon carbide, titanium oxide, tantalum oxide, ruthenium oxide, tungsten oxide, zinc oxide, silicon, germanium, germanium oxide, carbon, and a combination thereof. The insulating nanoparticles  70  may be substantially spherical, ellipsoidal, superellipsoidal, or polyhedral and have a characteristic dimension ranging from 1 nm to about 24 nm. The characteristic dimension may be a diameter, a full width, or a full length. The insulating nanoparticles  70  may be formed, for example, by sputtering of a dielectric material target in a sputtering chamber. 
     Preferably, the insulating nanoparticles  70  are self-aligning and/or self-planarizing on the top surface of the bottom conductive plate  40 . The monolayer of insulating nanoparticles  70  are separated among one another so that gaps are present among the insulating nanoparticles  70  when viewed from above. The gaps may be contiguous or many be disjoined among one another. 
     Referring to  FIG. 3C , a conductive material layer  88  is deposited on the monolayer of the insulating particles  70  and on the portion of the bottom conductive plate  70  underneath the gaps between the insulating nanoparticles  70 . The conductive material layer  88  comprises a conductive material, and preferably the same material as the conductive material of the bottom conductor plate  40 . A volume of the conductive material layer  88  fills the gaps between the insulating nanoparticles  70 . The thickness of the conductive material layer  88  is greater than the height of the monolayer of the insulating particles  70 , and may be from about 6 nm to about 60 mm. 
     Referring to  FIG. 3D , the conductive material layer  88  is etched to remove the portion above the monolayer of insulating nanoparticles  70 . The monolayer of insulating nanoparticles  70  and the remaining volume  41  of the conductive material layer  88  form a current constricting layer  90 . The current constricting layer  90  abuts the bottom conductive plate  40  and contains the monolayer of insulating nanoparticles  90  which are embedded in and separated by the remaining volume  41  of the conductive material layer  88 . The composition of the current constricting layer  90  is substantially uniform, providing a uniform resistance across the current constriction layer  90 , if the insulating nanoparticles  70  are uniformly distributed in the monolayer at the time of application. 
     Referring to  FIG. 3E , a phase change material layer  50  is formed directly on the current constricting layer  90 , for example, by chemical vapor deposition or sputtering. The phase change material layer  50  comprises a phase change material, which includes, but is not limited to, a chalcogenide alloy, or an alloy of a chalcogen element with at least one non-chalcogen element, wherein the chalcogen element is selected from the group consisting of Te, Se, and S, and the at least one non-chalcogen element is selected from the group consisting of Ge, Sb, Bi, Pb, Sn, As, Si, P, Ga, In, and Ag. Illustrative examples of such chalcogenide alloys are GaSb, InSb, InSe, Sb 2 Te 3 , GeTe, Ge 2 Sb 2 Te 5 , InSbTe, GaSbTe, SnSb 2 Te 4 , InSbGe, AgInSbTe, (GeSn)SbTe, GeSb(SeTe), Te 81 Ge 15 Sb 2 S 2 , and compounds with modified atomic ratios thereof. The thickness of the phase change material layer  50  is from about 10 nm to about 100 nm, and may be optimized for optimal performance of the PCM element. 
     Referring to  FIG. 3F , a top conductive plate  60  is formed directly on the phase change material layer  50 , for example, by chemical vapor deposition or sputtering. The top conductive plate  60  comprises a conductive material that may be, for example, a material selected from Ti, Ta, W, Mo, Al, Cu, Pt, Ir, La, Ni, Ru, another elementary metal, and alloys thereof. The thickness of the top conductive plate  60  is typically from about 10 nm to about 80 nm. The top conductive plate  60  and the bottom conductive plate  40  may comprise the same, or different material. 
     Sequential cross-sectional views of a second exemplary structure according to a second embodiment of the present invention are shown at various stages of a manufacturing process in  FIGS. 4A-4D . Referring to  FIG. 4A , a bottom conductive plate  40  is formed on an underlying semiconductor structure (not shown) as in the first embodiment. 
     Referring to  FIG. 4B , a phase change material layer  50  is formed directly on the bottom conductive plate  40 . The physical characteristics of the phase change material layer  50  are the same as in the first embodiment. 
     Referring to  FIG. 4C , a monolayer of insulating nanoparticles  70  is applied to a top surface of the phase change material layer  50 . The same type of insulating nanoparticles  70  may be employed as in the first embodiment. Preferably, the insulating nanoparticles  70  are self-aligning and/or self-planarizing on the top surface of the phase change material layer  50 . The monolayer of insulating nanoparticles  70  are separated among one another so that gaps are present among the insulating nanoparticles  70  when viewed from above. The gaps may be contiguous or many be disjoined among one another. 
     Referring to  FIG. 4D , a conductive material is deposited on the monolayer of the insulating particles  70  and on the portion of the phase change material layer  50  in the gaps between the insulating particles  70 . The conductive material may be any of the material listed for the bottom conductive plate  40  and the top conductive plate  60  in the first embodiment. A volume  65  of the conductive material fills the gaps between the monolayer of insulating nanoparticles  70 . The volume  65  of the conductive material and the monolayer of insulating particles  70  form a current constriction layer  90 . The thickness of the current constriction layer  90  is the same as the height of the monolayer of insulating particles  70 . 
     A top conductive plate  60  comprises another volume of the conductive material formed directly above the current constriction layer  90 . The thickness of the top conductive plate  60  is typically from about 10 nm to about 80 nm. The top conductive plate  60  and the bottom conductive plate  40  may comprise the same, or different, material. The current constriction layer  90  abuts both the phase change material layer  50  and the top conductive layer  60 . The volume  65  of the conductive material between the monolayer of insulating nanoparticles  70  and the another volume of the conductive material in the top conductive plate  60 , which is located above a top surface of the monolayer of insulating particles  70  represented by a dotted line in  FIG. 4D , are inherently adjoined. 
     Sequential cross-sectional views of a third exemplary structure according to a third embodiment of the present invention are shown at various stages of a manufacturing process in  FIGS. 5A-5F . Referring to  FIG. 4A , a bottom conductive plate  40  is formed on an underlying semiconductor structure (not shown) as in the first embodiment. 
     Referring to  FIG. 5B , an insulating layer  80  is formed on the bottom conductive layer  40 , for example, by chemical vapor deposition. The insulating layer  80  comprises an insulating material such as silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, silicon carbide, titanium oxide, tantalum oxide, ruthenium oxide, tungsten oxide, zinc oxide, silicon, germanium, germanium oxide, carbon, or a combination thereof. The insulating layer  80  may be formed by deposition such as chemical vapor deposition. The thickness of the insulating layer  80  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 5C , a monolayer of insulating nanoparticles  70  is applied to a top surface of the insulating layer  80 . The same type of insulating nanoparticles  70  may be employed as in the first embodiment. Preferably, the insulating nanoparticles  70  are self-aligning and/or self-planarizing on the top surface of the insulating layer  50 . The monolayer of insulating nanoparticles  70  are separated among one another so that gaps are present among the insulating nanoparticles  70  when viewed from above. The gaps may be contiguous or many be disjoined among one another. 
     Referring to  FIG. 5D , an anisotropic reactive ion etch is performed on the insulating layer  80  employing the monolayer of insulating nanoparticles  70  as hardmasks. During the anisotropic reactive ion etch, the portions of the insulating layer  80  under the gaps between the insulating nanoparticles  70  are etched and removed while the remaining portions of the insulating layer  80  that are located directly underneath the insulating nanoparticles  70  are shadowed from reactive ions by the insulating nanoparticles  70  during the anisotropic reactive ion etch. Consequently, a set of disjoined insulating layer islands  81  are formed by the anisotropic reactive ion etch. The insulating nanoparticles are preferably consumed during the anisotropic reactive ion etch. The set of the disjoined insulating islands  81  is in a planar arrangement. The thickness of the disjoined insulating islands  81  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 5E , a phase change material is deposited on the set of the disjoined insulating islands  81  and on the portion of the bottom conductive plate  40  underneath the gaps between the insulating particles  70 . The phase change material may be any of the material listed for the phase change material layer  50  in the first embodiment. A volume  55  of the phase change material fills the gaps among the disjoined insulating islands  81 . The volume  55  of the phase change material and the set of the disjoined insulating islands  81  form a current constriction layer  100 . The thickness of the current constriction layer  100  is the same as the height of the set of the disjoined insulating islands  81 . 
     Another volume of the phase change material forms a phase change material layer  50  directly above the current constriction layer  100 . The thickness of the phase change material layer  50  is typically from about 10 nm to about 100 nm. The current constriction layer  100  abuts both the phase change material layer  50  and the bottom conductive layer  40 . The volume  55  of the phase change material between the disjoined insulating islands  81  and the another volume of the phase change material in the phase change material layer  50  are inherently adjoined. The boundary between the current constriction layer  100  and the phase change material layer  50  is represented by a dotted line in  FIG. 5E . 
     Referring to  FIG. 5F , a top conductive plate  60  is formed by depositing a conductive material directly on the phase change material layer  50 . The conductive material may be any of the material listed for the bottom conductive plate  40  and the top conductive plate  60  in the first embodiment. The thickness of the top conductive plate  60  is typically in the range from about 10 nm to about 80 nm. The top conductive plate  60  and the bottom conductive plate  40  may comprise the same, or different, material. 
     Sequential cross-sectional views of a fourth exemplary structure according to a fourth embodiment of the present invention are shown at various stages of a manufacturing process in  FIGS. 6A-6H . Referring to  FIG. 6A , a bottom conductive plate  40  is formed on an underlying semiconductor structure (not shown) as in the first embodiment. 
     Referring to  FIG. 6B , a first phase change material is deposited on the bottom conductive layer  40  to form a first phase change material layer  51 . The first phase change material may be any of the material listed for the phase change material layer  50  in the first embodiment. The thickness of the first phase change material layer  51  is typically in the range from about 5 nm to about 50 nm. 
     Referring to  FIG. 6C , an insulating layer  80  is formed on the first phase change material layer  51 , for example, by chemical vapor deposition. The insulating layer  80  is formed by the same method as and comprises the same material as the insulating layer  80  according to the third embodiment. The thickness of the insulating layer  80  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 6D , a monolayer of insulating nanoparticles  70  is applied to a top surface of the insulating layer  80 . The same type of insulating nanoparticles  70  may be employed as in the first embodiment. Preferably, the insulating nanoparticles  70  are self-aligning and/or self-planarizing on the top surface of the insulating layer  80 . The monolayer of insulating nanoparticles  70  are separated among one another so that gaps are present among the insulating nanoparticles  70  when viewed from above. The gaps may be contiguous or many be disjoined among one another. 
     Referring to  FIG. 6E , an anisotropic reactive ion etch is performed on the insulating layer  80  employing the monolayer of insulating nanoparticles  70  as hardmasks. A set of disjoined insulating layer islands  81  are formed by the anisotropic reactive ion etch in the same fashion as in the third embodiment of the present invention. The insulating nanoparticles  70  are preferably consumed during the anisotropic reactive ion etch. The set of the disjoined insulating islands  81  is in a planar arrangement. The thickness of the disjoined insulating islands  81  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 6F , a second phase change material is deposited on the set of the disjoined insulating islands  81  and on the portion of the first phase change material layer  51  underneath the gaps between the insulating particles  70 . The second phase change material may be any of the material listed for the phase change material layer  50  in the first embodiment. A volume  95  of the second phase change material fills the gaps among the disjoined insulating islands  81 . The volume  95  of the second phase change material and the set of the disjoined insulating islands  81  form a current constriction layer  100 . The thickness of the current constriction layer  100  is the same as the height of the set of the disjoined insulating islands  81 . 
     Another volume of the second phase change material forms a second phase change material layer  51  directly above the current constriction layer  100 . The thickness of the second phase change material layer  52  is typically from about 5 nm to about 50 nm. The current constriction layer  100  abuts both the first phase change material layer  51  and the second phase change material layer  52 . The volume  95  of the second phase change material between the disjoined insulating islands  81  and the another volume of the second phase change material in the second phase change material layer  52  are inherently adjoined since both are formed during the same process step. The boundary between the current constriction layer  100  and the second phase change material layer  52  is represented by a dotted line in  FIG. 6F . The first phase change material and the second phase change material may comprise the same or different material. 
     Referring to  FIG. 6G , a top conductive plate  60  is formed by depositing a conductive material directly on the second phase change material layer  52 . The conductive material may be any of the material listed for the bottom conductive plate  40  and the top conductive plate  60  in the first embodiment. The thickness of the top conductive plate  60  is typically from about 10 nm to about 80 nm. The top conductive plate  60  and the bottom conductive plate  40  may comprise the same, or different, material. 
     Referring to  FIG. 6H , a horizontal cross-sectional view of the fourth exemplary structure along the plane H-H′ in  FIG. 6G  shows the set of the disjoined insulating islands  81  within the current constriction layer  100  are disjoined, or separated among one another by the volume  95  of the second phase change material within the current constriction layer  100 . 
     Sequential cross-sectional views of a fifth exemplary structure according to a fifth embodiment of the present invention are shown at various stages of a manufacturing process in  FIGS. 7A-7F . Referring to  FIG. 7A , a bottom conductive plate  40  is formed on an underlying semiconductor structure (not shown) as in the first embodiment. 
     Referring to  FIG. 7B , a phase change material is deposited on the bottom conductive layer  40  to form a phase change material layer  50 . The phase change material may be any of the material listed for the phase change material layer  50  in the first embodiment. The thickness of the phase change material layer  50  is typically from about 10 nm to about 100 nm. 
     Referring to  FIG. 7C , an insulating layer  80  is formed on the phase change material layer  50 , for example, by chemical vapor deposition. The insulating layer  80  is formed by the same method as and comprises the same material as the insulating layer  80  according to the third embodiment. The thickness of the insulating layer  80  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 7D , a monolayer of insulating nanoparticles  70  is applied to a top surface of the insulating layer  80 . The same type of insulating nanoparticles  70  may be employed as in the first embodiment. Preferably, the insulating nanoparticles  70  are self-aligning and/or self-planarizing on the top surface of the insulating layer  80 . The monolayer of insulating nanoparticles  70  are separated among one another so that gaps are present among the insulating nanoparticles  70  when viewed from above. The gaps may be contiguous or many be disjoined among one another. 
     Referring to  FIG. 7E , an anisotropic reactive ion etch is performed on the insulating layer  80  employing the monolayer of insulating nanoparticles  70  as hardmasks. A set of disjoined insulating layer islands  81  are formed by the anisotropic reactive ion etch in the same fashion as in the third embodiment of the present invention. The insulating nanoparticles  70  are preferably consumed during the anisotropic reactive ion etch. The set of the disjoined insulating islands  81  is in a planar arrangement. The thickness of the disjoined insulating islands  81  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 7F , a conductive material is deposited on the set of the disjoined insulating islands  81  and on the portion of the phase change material layer  50  underneath the gaps between the insulating particles  70 . The conductive material may be any of the material listed for the bottom conductive plate  40  and the top conductive plate  60  in the first embodiment. A volume  65  of the conductive material fills the gaps among the disjoined insulating islands  81 . The volume  65  of the conductive material and the set of the disjoined insulating islands  81  form a current constriction layer  100 . The thickness of the current constriction layer  100  is the same as the height of the set of the disjoined insulating islands  81 . 
     Another volume of the conductive material forms a top conductive plate  60  directly above the current constriction layer  100 . The thickness of the top conductive plate  60  is typically from about 10 nm to about 80 nm. The top conductive plate  60  and the bottom conductive plate  40  may comprise the same, or different, material. The current constriction layer  100  abuts both the phase change material layer  50  and the top conductive plate  60 . The volume  65  of the conductive material between the disjoined insulating islands  81  and the another volume of the conductive material in the top conductive plate  60  are inherently adjoined since both are formed during the same process step. The boundary between the current constriction layer  100  and the top conductive plate  60  is represented by a dotted line in  FIG. 7F . 
     Sequential cross-sectional views of a sixth exemplary structure according to a sixth embodiment of the present invention are shown at various stages of a manufacturing process in  FIGS. 8A-8H . Referring to  FIG. 5A , a bottom conductive plate  40  is formed on an underlying semiconductor structure (not shown) as in the first embodiment. 
     Referring to  FIG. 8B , a phase change material is deposited on the bottom conductive layer  40  to form a phase change material layer  50 . The phase change material may be any of the material listed for the phase change material layer  50  in the first embodiment. The thickness of the phase change material layer  50  is typically from about 10 nm to about 100 nm. 
     Referring to  FIG. 5C , a conductive material layer  62  is formed on the phase change material layer  50 , for example, by chemical vapor deposition. The conductive material layer  62  comprises the same material as the bottom conductive plate  40  and the top conductive plate  60  according to the first embodiment. The thickness of the conductive material layer  62  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 8D , a monolayer of insulating nanoparticles  70  is applied to a top surface of the conductive material layer  62 . The same type of insulating nanoparticles  70  may be employed as in the first embodiment. Preferably, the insulating nanoparticles  70  are self-aligning and/or self-planarizing on the top surface of the conductive material layer  62 . The monolayer of insulating nanoparticles  70  are separated among one another so that gaps are present among the insulating nanoparticles  70  when viewed from above. The gaps may be contiguous or many be disjoined among one another. 
     Referring to  FIG. 5E , an anisotropic reactive ion etch is performed on the conductive material layer  62  employing the monolayer of insulating nanoparticles  70  as hardmasks. During the anisotropic reactive ion etch, the portions of the conductive material layer  62  under the gaps between the insulating nanoparticles  70  are etched and removed while the remaining portions of the conductive material layer  62  that are located directly underneath the insulating nanoparticles  70  are shadowed from reactive ions by the insulating nanoparticles  70  during the anisotropic reactive ion etch. Consequently, a set of disjoined conductive layer islands  64  are formed by the anisotropic reactive ion etch. The insulating nanoparticles are preferably consumed during the anisotropic reactive ion etch. The set of the disjoined conducting islands  64  is in a planar arrangement. The thickness of the disjoined conducting islands  64  may be from about 3 nm to about 60 nm. 
     Referring to  FIG. 5F , an insulating material layer  82  is deposited over the exposed portions of the phase change material layer  50  and on the top surfaces and sidewalls of the disjoined conducting islands  64 . The thickness of the insulating material layer  82  is thicker than the disjoined conducting islands  64  and may be from about 6 nm to about 100 nm. 
     Referring to  FIG. 8G , the portion of the insulating material layer  82  above the top surface of the disjoined conducting islands  64  is removed, for example, by a reactive ion etch or by other means of planarization such as chemical mechanical planarization (CMP). The disjoined conducting islands  64  and the remaining insulting material  84  between the disjoined conducting islands  64  collectively form a current constriction layer  100 ′. 
     Referring to  FIG. 8H , a top conductive plate  60  is formed by depositing a conductive material directly on the current constriction layer  100 ′. The conductive material may be any of the material listed for the bottom conductive plate  40  and the top conductive plate  60  in the first embodiment. The thickness of the top conductive plate  60  is typically from about 10 nm to about 80 nm. The top conductive plate  60  and the bottom conductive plate  40  may comprise the same, or different, material. 
     Referring to  FIG. 9 , a seventh exemplary structure according to a seventh embodiment of the present invention comprises a bottom conductive plate  40 , a phase change material layer  50 , a dielectric layer  77  containing a tapered via hole sidewall  78 , a tapered via liner  110  having a liner bottom surface  79 , and a conductive via plug  60 ′. The phase change material layer  50  abuts the bottom conductive plate  40  located underneath, the liner bottom surface  79 , and the dielectric layer  77 . The tapered via liner  110  abuts the tapered via hole sidewall  78  and contains a monolayer of insulating nanoparticles  70  that are embedded in and separated by a volume  65  of a conductive material. The conductive via plug  60 ′ comprises the same conducive material as the volume  65  of the conductive material within the tapered via liner  110 . 
     The bottom conductive plate  40  and the conductive via plug  60 ′ comprise a conductive material that may be, for example, a material selected from Ti, Ta, W, Mo, Al, Cu, Pt, Ir, La, Ni, Ru, another elementary metal, and alloys thereof. The thickness of the bottom conductive plate  40  is typically in the range from about 10 nm to about 80 nm. 
     The phase change material layer  50  comprises a phase change material, which includes, but is not limited to, a chalcogenide alloy, or an alloy of a chalcogen element with at least one non-chalcogen element, wherein the chalcogen element is selected from the group consisting of Te, Se, and S, and the at least one non-chalcogen element is selected from the group consisting of Ge, Sb, Bi, Pb, Sn, As, Si, P, Ga, In, and Ag. Illustrative examples of such chalcogenide alloys are GaSb, InSb, InSe, Sb 2 Te 3 , GeTe, Ge 2 Sb 2 Te 5 , InSbTe, GaSbTe, SnSb 2 Te 4 , InSbGe, AgInSbTe, (GeSn)SbTe, GeSb(SeTe), Te 81 Ge 15 Sb 2 S 2 , and compounds with modified atomic ratios thereof. The thickness of the phase change material layer  50  is in the range from about 10 nm to about 100 nm, and may be optimized for optimal performance of the PCM element. 
     The dielectric layer  77  comprises a dielectric material that may be, for example, a silicon oxide, silicon nitride, or other dielectric material that can withstand the temperature needed for melting the phase change material in the phase change material layer  50 . The dielectric layer may have a thickness from about 50 nm to about 400 nm. A tapered via hole is formed on the dielectric layer  77  by lithographic patterning and etching of the dielectric layer  77 . The tapered via hole has a tapered via hole sidewall  78  that may have a circular, elliptical, or a polygonal cross-sectional area. The opening at the bottom of the tapered via hole may have a lithographic dimension or a sub-lithographic dimension. 
     The conductive material of the conductive via plug  60 ′ and the volume  65  of the conductive material within the tapered via liner  110  comprises a conductive material that may be, for example, a material selected from Ti, Ta, W, Mo, Al, Cu, Pt, Ir, La, Ni, Ru, another elementary metal, or alloys thereof. The conductive material of the conductive via plug  60 ′ and the volume  65  of the conductive material within the tapered via liner  110  comprise the same material and are inherently adjoined because both are formed during the same processing step by deposition of the same conductive material. The boundary between the conductive material of the conductive via plug  60 ′ and the volume  65  of the conductive material within the tapered via liner  110  is marked by a dotted line in  FIG. 9 . 
     Referring to  FIG. 10 , an eighth exemplary structure according to an eighth embodiment of the present invention comprises a bottom conductive plate  40 , a phase change material layer  50 , a dielectric layer  77  containing a tapered via hole sidewall  78 , a tapered via liner  120  having a liner bottom surface  79 , a phase change material plug  53 , and a top conductive plate  60 . The phase change material layer  50  abuts the bottom conductive plate  40  located underneath, the liner bottom surface  79 , and the dielectric layer  77 . The tapered via liner  120  abuts the tapered via hole sidewall  78  and contains a monolayer of insulating nanoparticles  70  that are embedded in and separated by a volume  55 ′ of a phase change material. The phase change material plug  53  comprises the same phase change material as the volume  55 , of the phase change material within the tapered via liner  120 . The top conductive plate  60  abuts the dielectric layer  77 , the tapered via liner  120 , and the phase change material plug  53 . 
     The bottom conductive plate  40  and the top conductive plate  60  may comprise the same material as the bottom conductive plate  40  according to the seventh embodiment. The bottom conductive plate  40  and the top conductive plate  60  may or may not, have the same composition. The thickness of the bottom conductive plate  40  is typically from about 10 nm to about 80 nm. Also, the thickness of the top conductive plate  60  is typically in the range from about 10 nm to about 80 nm. 
     The phase change material layer  50  and the phase change material plug  53  comprise the same phase change material as the phase change material layer  50  according to the seventh embodiment. The thickness of the phase change material layer  50  is from about 10 nm to about 100 nm, and may be optimized for optimal performance of the PCM element. 
     The dielectric layer  77 , the tapered via hole therein, and the tapered via hole sidewall  78  have the same structural characteristics as in the seventh embodiment. 
     Referring to  FIG. 11 , a ninth exemplary structure according to a ninth embodiment of the present invention comprises a bottom conductive plate  40 , a dielectric layer  77  containing a tapered via hole sidewall  78 , a tapered via liner  120  having a liner bottom surface  79 , a phase change material plug  53 , and a top conductive plate  60 . The bottom conductive plate  40  abuts the liner bottom surface  79  and the dielectric layer  77 . The tapered via liner  120  abuts the tapered via hole sidewall  78  and contains a monolayer of insulating nanoparticles  70  that are embedded in and separated by a volume  55 , of a phase change material. The phase change material plug  53  comprises the same phase change material as the volume  55 , of the phase change material within the tapered via liner  120 . The top conductive plate  60  abuts the dielectric layer  77 , the tapered via liner  120 , and the phase change material plug  53 . 
     The bottom conductive plate  40  and the top conductive plate  60  may comprise the same material as the bottom conductive plate  40  according to the seventh embodiment. The bottom conductive plate  40  and the top conductive plate  60  may or may not, have the same composition. The thickness of the bottom conductive plate  40  is typically from about 10 nm to about 80 nm. Also, the thickness of the top conductive plate  60  is typically from about 10 nm to about 80 nm. 
     The phase change material plug  53  comprises the same phase change material as the phase change material layer  50  according to the seventh embodiment. 
     The dielectric layer  77 , the tapered via hole therein, and the tapered via hole sidewall  78  have the same structural characteristics as in the seventh embodiment. 
     Referring to  FIG. 12 , a tenth exemplary structure according to a tenth embodiment of the present invention comprises the same structural elements and structural relationships as the eighth exemplary structure except that a monolayer of insulating nanoparticles in the eighth embodiment is replaced with a multilayer of nanoparticles in the tenth embodiment. The multiplayer of nanoparticles may be formed be a repeated formation of monolayers containing insulating particles  70 , or alternatively, may be formed by a sputtering of an insulator material during deposition of a phase change material. Alternatively, oxygen maybe supplied during formation of a phase change material to form a multiplayer containing a mixture of volumes of the phase change material and insulating nanoparticles. For example, germanium oxide is readily formed intermixed with a germanium containing phase change material in the presence of sufficient oxygen during the deposition process. 
     Referring to  FIG. 13 , an eleventh exemplary structure according to an eleventh embodiment of the present invention comprises the same structural elements and structural relationships as the ninth exemplary structure except that a monolayer of insulating nanoparticles in the ninth embodiment is replaced with a multilayer of nanoparticles in the eleventh embodiment. The same method for formation of a multilayer of insulating nanoparticles intermixed with a volume of a phase change material as in the tenth embodiment may be employed in the eleventh embodiment. 
     While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.