Patent Publication Number: US-10763374-B2

Title: Crystallized silicon vertical diode on BEOL for access device for confined PCM arrays

Description:
BACKGROUND 
     Technical Field 
     The present invention relates generally to semiconductor devices, and more specifically, to forming a crystallized silicon vertical diode on back end of the line (BEOL) processing for an access device combined with a phase change material (PCM) memory. 
     Description of the Related Art 
     In order to increase density of memory technologies (both volatile and nonvolatile), a crosspoint design can be employed. In such an optimized design, wordlines and bitlines run at a minimum pitch=2F, where F refers to a lithographic minimum feature size, and storage elements are placed between these perpendicularly oriented memory lines at their crosspoints. Two designs are conventionally employed in such memory technologies. One is a nano-crossbar design, which refers to a design where memory lines run at sub-lithographic pitches. The other design is a 3D design, which refers to a design where memory lines run at lithographic pitches, with multiple layers of memories being provided. In either design case, two device components are needed at the intersection of the memory lines. That is, a memory element, which refers to an element that is used to store data/information and a rectifying element or access device. 
     SUMMARY 
     In accordance with an embodiment, a method is provided for integrating an electronic component in back end of the line (BEOL) processing. The method includes forming a first electrode over a semiconductor substrate, forming a first electrically conductive material over a portion of the first electrode, forming a second electrically conductive material over the first electrically conductive material, where the first and second electrically conductive materials define a p-n junction, depositing a second electrode between a set of spacers and in direct contact with the p-n-junction, depositing a phase change material over the p-n junction and in direct contact with the second electrode, and forming a third electrode over a portion of the phase change material. 
     In accordance with another embodiment, a method is provided for integrating an electronic component in back end of the line (BEOL) processing. The method includes forming a first electrode over a semiconductor substrate, forming a first electrically conductive material over the first electrode, forming an undoped intrinsic semiconductor layer over the first electrically conductive material, and forming a second electrically conductive material over the undoped intrinsic semiconductor layer, where the first electrically conductive material, the undoped intrinsic semiconductor layer, and the second electrically conductive material define a p-i-n junction. The method further includes depositing a second electrode between a set of spacers and in direct contact with the p-i-n-junction, depositing a phase change material over the p-i-n junction and in direct contact with the second electrode, and forming a third electrode over a portion of the phase change material. 
     In accordance with yet another embodiment, a semiconductor structure is presented for integrating an electronic component in back end of the line (BEOL) processing. The semiconductor structure includes a first electrode formed over a semiconductor substrate, a first electrically conductive material formed over a portion of the first electrode, a second electrically conductive material formed over the first electrically conductive material, where the first and second electrically conductive materials define a p-n junction, a second electrode deposited between a set of spacers and in direct contact with the p-n-junction, a phase change material deposited over the p-n junction and in direct contact with the second electrode, and a third electrode formed over a portion of the phase change material. 
     It should be noted that the exemplary embodiments are described with reference to different subject-matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject-matter, also any combination between features relating to different subject-matters, in particular, between features of the method type claims, and features of the apparatus type claims, is considered as to be described within this document. 
     These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
         FIG. 1  is a cross-sectional view of a semiconductor structure having an opening formed through the interlayer dielectric (ILD) to expose a top surface of a first metal line, in accordance with an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the semiconductor structure of  FIG. 1  where doped silicon (Si) is deposited and converted to polysilicon (poly-Si) by an excimer laser, in accordance with an embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of the semiconductor structure of  FIG. 2  where the polysilicon is etched, in accordance with an embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of the semiconductor structure of  FIG. 3  where a doped poly-Si layer is deposited, in accordance with an embodiment of the present invention; 
         FIG. 5  is a cross-sectional view of the semiconductor structure of  FIG. 4  where spacers are formed within the opening and over the doped poly-Si layer, in accordance with an embodiment of the present invention; 
         FIG. 6  is a cross-sectional view of the semiconductor structure of  FIG. 5  where an electrode is deposited between the spacers, in accordance with an embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of the semiconductor structure of  FIG. 6  where a phase change material (PCM) is deposited over the structure, in accordance with an embodiment of the present invention; 
         FIG. 8  is a cross-sectional view of the semiconductor structure of  FIG. 7  where a second metal line is deposited crosswise with respect to the first metal line, in accordance with an embodiment of the present invention; 
         FIG. 9  is a cross-sectional view of the semiconductor structure having an amorphous silicon (a-Si) doped layer deposited over a first metal line, in accordance with another embodiment of the present invention; 
         FIG. 10  is a cross-sectional view of the semiconductor structure of  FIG. 9  where the doped a-Si is converted to polysilicon (poly-Si) by an excimer laser, in accordance with an embodiment of the present invention; 
         FIG. 11  is a cross-sectional view of the semiconductor structure of  FIG. 10  where a Si layer is deposited and exposure to an excimer laser takes place, in accordance with an embodiment of the present invention; 
         FIG. 12  is a cross-sectional view of the semiconductor structure of  FIG. 11  where the Si is etched to form a recess, in accordance with an embodiment of the present invention; 
         FIG. 13  is a cross-sectional view of the semiconductor structure of  FIG. 12  where doped poly-Si is deposited, in accordance with an embodiment of the present invention; 
         FIG. 14  is a cross-sectional view of the semiconductor structure of  FIG. 13  where spacers are formed within the recess and over the doped poly-Si, in accordance with an embodiment of the present invention; 
         FIG. 15  is a cross-sectional view of the semiconductor structure of  FIG. 14  where an electrode is deposited between the spacers, in accordance with an embodiment of the present invention; 
         FIG. 16  is a cross-sectional view of the semiconductor structure of  FIG. 15  where a phase change material (PCM) is deposited over the structure and a second metal line is deposited crosswise with respect to the first metal line, in accordance with an embodiment of the present invention; 
         FIG. 17  is a perspective view of a crossbar array with access devices and memory elements formed between the metal lines, in accordance with an embodiment of the present invention; and 
         FIG. 18  is a schematic block diagram of an embodiment of a computing system adapted for performing the method for employing a vertical diode with a phase change memory array, in accordance with an embodiment of the present invention. 
     
    
    
     Throughout the drawings, same or similar reference numerals represent the same or similar elements. 
     DETAILED DESCRIPTION 
     Embodiments in accordance with the present invention provide methods and devices for creating a memory architecture incorporated or integrated with a cross-point array architecture. A cross-point array architecture has the potential for achieving a fast, random access, nonvolatile memory with the highest density and smallest cell size possible. Nonvolatile resistance memory elements that can be placed at a cross-point of word-lines and bit-lines helps realize this potential. If this memory element is no larger than the pitch of these lines (2F), the cell is 2F×2F (4F 2 ) in area. 
     Embodiments in accordance with the present invention provide methods and devices for creating a memory architecture by employing a poly-silicon (poly-Si) diode compatible with back end of the line (BEOL) processing. The poly-Si diode can be incorporated or integrated in the cross-point array architecture. The poly-Si diode can be a vertically integrated p-n junction with confined phase change memory. Alternatively, the poly-Si diode can be a vertically integrated p-i-n junction with confined phase change memory. Thus, various structures of diodes with vertically integrated memory elements are introduced for BEOL processing. 
     Embodiments in accordance with the present invention provide methods and devices for employing phase change based materials in nonvolatile random access memory cells. Phase change materials, such as chalcogenides, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data. 
     Phase change materials are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in the active region of the cell. The term “amorphous” is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has detectable characteristics such as higher electrical resistivity than the crystalline phase. The term “crystalline” is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material can be switched into either different solid phases or mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. 
     The change from the amorphous to the crystalline state is generally a lower current operation, requiring a current that is sufficient to raise the phase change material to a level between a phase transition temperature and a melting temperature. The change from crystalline to amorphous, referred to as “reset,” is generally a higher current operation, which includes a short high current density pulse to melt or break down the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, thus allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of the phase change material from a crystalline state to an amorphous state. The magnitude of the needed reset current can be reduced by reducing the volume of the active region in the phase change material element in the cell. 
     Examples of semiconductor materials that can be employed in forming such structures include silicon (Si), germanium (Ge), silicon germanium alloys (SiGe), silicon carbide (SiC), silicon germanium carbide (SiGeC), III-V compound semiconductors and/or II-VI compound semiconductors. III-V compound semiconductors are materials that include at least one element from Group III of the Periodic Table of Elements and at least one element from Group V of the Periodic Table of Elements. II-VI compound semiconductors are materials that include at least one element from Group II of the Periodic Table of Elements and at least one element from Group VI of the Periodic Table of Elements. 
     It is to be understood that the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials and process features and steps/blocks can be varied within the scope of the present invention. It should be noted that certain features cannot be shown in all figures for the sake of clarity. This is not intended to be interpreted as a limitation of any particular embodiment, or illustration, or scope of the claims. 
       FIG. 1  is a cross-sectional view of a semiconductor structure having an opening formed through the interlayer dielectric (ILD) to expose a top surface of a first metal line, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, a semiconductor structure  5  includes an interlayer dielectric (ILD)  12  deposited over a substrate  10 . A first metal layer or first metal line  14  is formed over the ILD  12 . A second ILD layer  16  is formed over the metal line  14 . An opening or recess  18  is formed through the ILD  16  to expose a top surface  15  of the first metal line  14 . 
     The semiconductor substrate  10  that is employed in the present application can include any semiconductor material. Illustrative examples of semiconductor materials that can be employed for the semiconductor substrate  10  include, but are not limited to, Si, SiGe alloys, SiGeC, SiC, Ge alloy, GaSb, GaP, GaN, GaAs, InAs, INP, AlN and all other III-V or II-VI compound semiconductors. In one embodiment, the semiconductor substrate  10  can include a multilayered stack of such semiconductor materials. In some embodiments, the semiconductor substrate  10  can include a bulk semiconductor substrate. By “bulk” it is meant the entirety of the semiconductor substrate  10  from one surface to an opposite surface is composed of a semiconductor material. In other embodiments, the semiconductor substrate  10  can include a semiconductor-on-insulator (SOI) substrate. 
     In various embodiments, the height of the ILD oxide  16  can be reduced by chemical-mechanical polishing (CMP) and/or etching. Therefore, the planarization process can be provided by CMP. Other planarization process can include grinding and polishing. 
     In one or more embodiments, the ILDs  12 ,  16  can have a thickness in the range of about 10 nm to about 100 nm, or in the range of about 10 nm to about 50 nm. 
     The ILDs  12 ,  16  can be selected from the group consisting of silicon containing materials such as SiO 2 , Si 3 N 4 , SiO x N y , SiC, SiCO, SiCOH, and SiCH compounds, the above-mentioned silicon containing materials with some or all of the Si replaced by Ge, carbon doped oxides, inorganic oxides, inorganic polymers, hybrid polymers, organic polymers such as polyamides or SiLK™, other carbon containing materials, organo-inorganic materials such as spin-on glasses and silsesquioxane-based materials, and diamond-like carbon (DLC), also known as amorphous hydrogenated carbon, α-C:H. Additional choices for the ILDs  12 ,  16  include any of the aforementioned materials in porous form, or in a form that changes during processing to or from being porous and/or permeable to being non-porous and/or non-permeable. 
       FIG. 2  is a cross-sectional view of the semiconductor structure of  FIG. 1  where doped silicon (Si) is deposited and converted to polysilicon (poly-Si) by an excimer laser, in accordance with an embodiment of the present invention. 
     Amorphous silicon (a-Si)  22  is deposited within the opening  18  and over the ILD portions  16 . Radiation is applied to the a-Si  22  via an excimer laser  20 . The a-Si  22  can be doped before application of the excimer laser  20 . The excimer laser  20  converts the a-Si to, e.g., polysilicon (poly-Si) layer  22 . Poly-Si layer  22  can be, e.g., a doped poly-Si layer  22 . 
     A laser is an electronic-optical device that emits coherent radiation. In some embodiments, a laser emits light in a narrow, low-divergence beam and with a defined wavelength (corresponding to a particular color if the laser operates in the visible spectrum). In some embodiments, the laser type that is employed in the laser annealing method is an excimer laser  20 . Excimer lasers can be powered by a chemical reaction involving an excited dimer, or excimer, which is a short-lived dimeric or heterodimeric molecule formed from two species (atoms), at least one of which is in an excited electronic state. Commonly used excimer molecules include F 2  (fluorine, emitting at 157 nm), and noble gas compounds (ArF (193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm)). 
     The excimer laser  20  can be generated from at least one of the excimers selected from the group consisting of Ar 2 , Kr 2 , F 2 , Xe 2 , ArF, KrF, XeBr, XeCl, XeCl, XeF, CaF 2 , KrCl, and Cl 2  wherein the wavelength of the excimer laser is in the range from about 50 to about 300 nm. 
       FIG. 3  is a cross-sectional view of the semiconductor structure of  FIG. 2  where the polysilicon is etched, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, the poly-Si  22  is etched such that a poly-Si portion or section  24  remains between the ILDs  16 . A top surface  17  of the ILDs  16  is also exposed. Moreover, an opening or recess  26  is formed over the poly-Si layer  24  and in between the ILDs  16 . The layer  24  can be a p-doped or an n-doped Si layer. 
       FIG. 4  is a cross-sectional view of the semiconductor structure of  FIG. 3  where a doped poly-Si layer is deposited, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, a doped poly-Si layer  28  is deposited or formed over the doped poly-Si section  24 . In an alternative embodiment, Si can be implanted and then activated. The layer  28  can be a p-doped or an n-doped layer. In one embodiment, layers  24 ,  28  are oppositely doped. Layers  24 ,  28  can be referred to as electrically conductive material layers. 
     The poly-Si layer  24  and the poly-Si layer  28  together form a p-n junction  29 . The p-n junction  29  is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The “p” (positive) side includes an excess of holes, while the “n” (negative) side includes an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction  29  can be created by doping, for example by ion implantation or diffusion of dopants. The p-n junction  29  can also be referred to as a p-n junction diode. The diode conducts current in only one direction, and it is made by joining a p-type semiconducting layer (e.g., layer  24 ) to an n-type semiconducting layer (e.g., layer  28 ). 
       FIG. 5  is a cross-sectional view of the semiconductor structure of  FIG. 4  where spacers are formed within the opening and over the doped poly-Si layer, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, spacers  30  can be formed adjacent the ILDs  16  and over the doped poly-Si layer  28  or over the p-n junction  29 . The formation of the spacers  30  results in gap  32  between the spacers  30 . 
     Spacers  30  can be, e.g., a nitride film (i.e., nitride layer). In an embodiment, the spacers  30  can be an oxide, for example, silicon oxide (SiO), a nitride, for example, a silicon nitride (SiN), or an oxynitride, for example, silicon oxynitride (SiON). In an embodiment, the spacers  30  can be, e.g., silicon oxycarbonitride (SiOCN), SiBCN, or similar film types. In some exemplary embodiments, the spacers  30  can include a material that is resistant to some etching processes such as, for example, hydrogen fluoride (HF) chemical etching or chemical oxide removal etching. 
       FIG. 6  is a cross-sectional view of the semiconductor structure of  FIG. 5  where an electrode is deposited between the spacers, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, the electrode  34  contacts an upper surface of the p-n junction  29 . The height of the electrode  34  can be reduced by chemical-mechanical polishing (CMP) and/or etching. Therefore, the planarization process can be provided by CMP. Other planarization process can include grinding and polishing. 
     The electrode  34  can be, e.g., formed from Ta, Ti/TiN, W, WN, TaN, polysilicon, doped polysilicon, amorphous silicon, doped amorphous silicon, or any other suitable material, or any other conductive material. Alternatively, the electrode  34  can be, e.g., any suitable conductive material or materials, e.g., Ag, Al, Cu, Ta, TaN, Ti, TiN, Al, W or any other suitable material, and may be deposited or formed in any suitable manner. 
       FIG. 7  is a cross-sectional view of the semiconductor structure of  FIG. 6  where a phase change material (PCM) is deposited over the structure, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, a phase change material  36  is deposited over the structure. The phase change material  36  contacts an upper surface of the ILDs  16 , an upper surface of the spacers  30 , and an upper surface of the electrode  34 . The phase change material  36  extends a length of the ILD  12  and extends a length of the first metal line  14 . 
     Phase change material  36  can be changed from one phase state to another by application of electrical pulses. A shorter, higher amplitude pulse tends to change the phase change material  36  to a generally amorphous state, and is referred to as a reset pulse. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state, and is referred to as a program pulse. The energy in a shorter, higher amplitude pulse is high enough to melt the material in the active volume, and short enough to allow the material to solidify in the amorphous state. 
     The phase change material  36  can include chalcogenide based materials and other materials. Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VI of the periodic table. Chalcogenides include compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys include combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually includes one or more elements from column six of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Phase change based memory materials can include alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. 
     Chalcogenides and other phase change materials are doped with impurities in some embodiments to modify conductivity, transition temperature, melting temperature, and other properties of memory elements using the doped chalcogenides. Representative impurities employed for doping chalcogenides include nitrogen, silicon, oxygen, silicon dioxide, silicon nitride, copper, silver, gold, aluminum, aluminum oxide, tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide. 
       FIG. 8  is a cross-sectional view of the semiconductor structure of  FIG. 7  where a second metal line is deposited crosswise with respect to the first metal line, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, a second metal layer or metal line or top electrode  38  is deposited on a top surface  37  of the phase change material  36 . Thus, a p-n junction  29  with a phase change material  36  are sandwiched between the bottom electrode  14  and the top electrode  38  of a cross-point array. The p-n junction  29  can be referred to as the access device and the phase change material  36  can be referred to as the memory element. The p-n junction  29  is fabricated in series with the PCM  36  to be compatible with a BEOL wafer stack. 
     The first metal line or bottom electrode  14  can be, e.g., a boron-doped or other p-type polysilicon electrode that is in contact with a lower end face of the doped poly-Si switching medium. The second metal line or top electrode  38  can be, e.g., a conductive layer containing silver (Ag). Although silver can be employed, it will be understood that the top electrode  14  can be formed from various other suitable metals, such as gold (Au), nickel (Ni), aluminum (Al), chromium (Cr), iron (Fe), manganese (Mn), tungsten (W), vanadium (V), and cobalt (Co). It is preferred that all or part of the portions of bottom electrode  14  contacting the access device include an electrode material, such as TiN, or another conductor selected for compatibility with the phase change material. Other types of conductors can be employed for the top and bottom electrodes  38 ,  14 , including for example aluminum and aluminum alloys, TiN, TaN, TiAlN or TaAlN. Other conductors that can be employed include one or more elements selected from the group consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, Ru and O. 
       FIG. 9  is a cross-sectional view of the semiconductor structure having an amorphous silicon (a-Si) doped layer deposited over a first metal line, in accordance with another embodiment of the present invention. 
     In an alternative embodiment, a semiconductor structure  40  includes an interlayer dielectric (ILD)  12  deposited over a substrate  10 . A first metal layer or metal line  14  is formed over the ILD  12 . A doped amorphous silicon (a-Si) layer  42  is then deposited or formed over the metal line  14 . 
       FIG. 10  is a cross-sectional view of the semiconductor structure of  FIG. 9  where the doped a-Si is converted to polysilicon (poly-Si) by an excimer laser, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, the doped a-Si layer  42  is converted to a poly-Si doped layer  44 . An excimer laser, as described above, can be employed to convert the doped a-Si layer  42  to the poly-Si doped layer  44 . 
       FIG. 11  is a cross-sectional view of the semiconductor structure of  FIG. 10  where a Si layer is deposited and exposure to an excimer laser takes place, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, an ILD  46  is formed over the poly-Si doped layer  44 . An opening is formed within the ILD  46  and filled with silicon (Si) material  50 . The Si material  50  is exposed to radiation  48  from an excimer laser. The Si material  50  can be an undoped intrinsic semiconductor layer or region. 
       FIG. 12  is a cross-sectional view of the semiconductor structure of  FIG. 11  where the Si is etched to form a recess, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, the Si material  50  is etched such that a Si material section  52  remains between the ILDs  46 . Also, a top surface  47  of the ILDs  46  is exposed. The etching of the Si material  50  also results in opening or recess  54  formed between the ILDs  46 . Section  52  can be referred to as the undoped intrinsic semiconductor layer or region. 
       FIG. 13  is a cross-sectional view of the semiconductor structure of  FIG. 12  where doped poly-Si is deposited, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, doped poly-Si  56  is deposited over the Si material section  52 . This forms a p-i-n junction  59  over the bottom electrode  14 . The p-i-n junction  59  includes three differently doped regions. Namely, there is an intrinsic or undoped layer  52  sandwiched between a p-doped region (e.g., layer  44 ) and an n-doped region (e.g., layer  56 ). The p-i-n junction  59  can be fabricated from, e.g., amorphous silicon with a band gap of about 1.8 eV. The p-i-n junction  59  can be referred to as a p-i-n junction diode. A p-i-n diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. Therefore, a crystallized silicon vertical diode can be constructed in combination with a phase change material ( FIG. 16 ) for BEOL processing. 
       FIG. 14  is a cross-sectional view of the semiconductor structure of  FIG. 13  where spacers are formed within the recess and over the doped poly-Si, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, spacers  58  are formed within the recess  54  and over the doped poly-Si  56  or over the p-i-n junction  59 . The formation of the spacers  58  results in gap  60  between the spacers  58 . 
       FIG. 15  is a cross-sectional view of the semiconductor structure of  FIG. 14  where an electrode is deposited between the spacers, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, the electrode  62  contacts an upper surface of the p-i-n junction  59 . The height of the electrode  62  can be reduced by chemical-mechanical polishing (CMP) and/or etching. Therefore, the planarization process can be provided by CMP. Other planarization process can include grinding and polishing. 
     The electrode  62  can be, e.g., formed from Ta, Ti/TiN, W, WN, TaN, polysilicon, doped polysilicon, amorphous silicon, doped amorphous silicon, or any other suitable material, or any other conductive material. Alternatively, the electrode  62  can be, e.g., any suitable conductive material or materials, e.g., Ag, Al, Cu, Ta, TaN, Ti, TiN, Al, W or any other suitable material, and may be deposited or formed in any suitable manner. 
       FIG. 16  is a cross-sectional view of the semiconductor structure of  FIG. 15  where a phase change material (PCM) is deposited and a second metal line is deposited crosswise with respect to the first metal line, in accordance with an embodiment of the present invention. 
     In various exemplary embodiments, a phase change material  64  is deposited over the structure. The phase change material  64  contacts an upper surface of the ILDs  16 , an upper surface of the spacers  30 , and an upper surface of the electrode  62 . 
     In various exemplary embodiments, a second metal layer or metal line  66  is deposited over the phase change material  64 , as well as over the spacers  58  and over a portion of the ILDs  46 . The second metal layer  66  can be referred to as the top electrode. The p-i-n junction  59  is fabricated in series with the PCM  64  to be compatible with a BEOL wafer stack. 
     Therefore, the methods and structures describe herein ( FIGS. 1-16 ) enable a silicon p-n diode or a silicon p-i-n diode constructed from low temperature deposited materials that are annealed with laser exposure. This results in fabrication of large polycrystalline materials. During this laser exposure, silicon layers reach a partial to complete melt, thus ensuring a large polycrystalline material is formed. Another advantage of employing an excimer laser is that heat remains confined in the silicon layers and does not spread through the device. The excimer laser also enables crystallization of silicon on a BEOL wafer. Stated differently, methods and structures are provided for fabricating a cross-point array of p-n and p-i-n or Schottky silicon diodes in series with PCM memories with processes compatible with BEOL wafer stack and scalable to small pitches. 
       FIG. 17  is a perspective view of a crossbar array with access devices and memory elements formed between the metal lines (top and bottom electrodes), in accordance with an embodiment of the present invention. 
     In various example embodiments, the semiconductor structure  76 ,  78  represents a memory cell incorporated between a plurality of bit lines  72  and a plurality of word lines  74 . Thus, the array  70  is obtained by perpendicular conductive wordlines (rows)  74  and bitlines (columns)  72 , where a cell structure  76 ,  78  with a memory element exists at the intersection between each row and column. The cell structure  76 ,  78  with the memory element can be accessed for read and write by biasing the corresponding wordline  74  and bitline  72 . The cell structure includes an access device  76  and a memory element  78  (or mushroom memory element). The access device can be the p-n junction  29  or p-i-n junction  59  described above or a Schottky diode. The mushroom memory element can be the phase change materials  36 ,  64  described above and can be a single unit or single continuous component that extends over each of the individual access devices  29 ,  59 . 
     In order to construct a large scale crossbar array, each cross point needs to have a high resistance (or low leakage current). Otherwise, a voltage drop across the metal lines becomes an issue. Resistive random access memory devices (ReRAM) devices usually have low switching resistance (˜kOhm) due to a filamentary nature. This demands line resistance reduction beyond the conventional back end of line (BEOL) to enable large crossbar array structures. The exemplary embodiments of the present invention alleviate this issue by lodging or wedging an access device having a phase change material at the intersection of the wordlines and bitlines (or top and bottom electrodes). Moreover, emerging memories can be fabricated in the BEOL at relatively low temperatures, which allows for easy integration with CMOS devices and stacking in 3D. For all these reasons, vertical diodes integrated with PCM are promising not only for nonvolatile memories, but also for computing memories, thus allowing for fast data access and for computing architectures blurring a distinction between memory and computing circuits, such as nonvolatile memristive logic computation or neuromorphic networks. 
       FIG. 18  is a schematic block diagram of an embodiment of a computing system adapted for performing the method for employing a vertical diode with a phase change memory array, in accordance with an embodiment of the present invention. 
     The system  800  schematically represents a computerized unit  801 , e.g., a general-purpose computer. In exemplary embodiments, in terms of hardware architecture, the unit  801  includes a processor  805 , memory  810  coupled to a PCM memory controller  815 , and one or more input and/or output (I/O) devices  840 ,  845 ,  850 ,  855  (or peripherals) that are communicatively coupled via a local input/output controller  835 . The input/output controller  835  can be, but is not limited to, one or more buses or other wired or wireless connections. The input/output controller  835  can have additional elements, which are omitted for simplicity, such as controllers, buffers (caches), drivers, repeaters, and receivers, to enable communications. Further, the local interface can include address, control, and/or data connections to enable appropriate communications among the components. 
     The processor  805  is a hardware device for executing software, particularly that stored in PCM memory  810 . The processor  805  can be any custom made or commercially available processor, a central processing unit (CPU), an auxiliary processor among several processors associated with the computer  801 , a semiconductor based microprocessor (in the form of a microchip or chip set), or generally any device for executing software instructions. 
     The PCM memory  810  can include any one or combination of volatile memory elements (e.g., random access memory) and nonvolatile memory elements. The PCM memory  810  can incorporate electronic, magnetic, optical, and/or other types of storage media. Note that the PCM memory  810  can have a distributed architecture, where various components are situated remote from one another, but can be accessed by the processor  805 . 
     The software in memory  810  can include one or more separate programs, each of which includes an ordered listing of executable instructions for implementing logical functions. The software in the memory  810  includes methods described herein in accordance with exemplary embodiments and a suitable operating system (OS)  811 . The OS  811  essentially controls the execution of other computer programs, and provides scheduling, input-output control, file and data management, memory management, and communication control and related services. 
     The methods described can be in the form of a source program, executable program (object code), script, or any other entity including a set of instructions to be performed. When in a source program form, then the program needs to be translated via a compiler, assembler, interpreter, or the like, as known per se, which can or cannot be included within the memory  810 , so as to operate properly in connection with the OS  811 . The methods can be written as an object oriented programming language, which has classes of data and methods, or a procedure programming language, which has routines, subroutines, and/or functions. 
     A conventional keyboard  850  and mouse  855  can be coupled to the input/output controller  835 . Other I/O devices  840 - 855  can include sensors (especially in the case of network elements), e.g., hardware devices that produce a measurable response to a change in a physical condition like temperature or pressure (physical data to be monitored). The analog signal produced by the sensors is digitized by an analog-to-digital converter and sent to controllers  835  for further processing. Sensor nodes are ideally small, consume low energy, are autonomous and operate unattended. 
     The I/O devices  840 - 855  can further include devices that communicate both inputs and outputs. The system  800  can further include a display controller  825  coupled to a display  830 . In the exemplary embodiments, system  800  can further include a network interface or transceiver  860  for coupling to a network  865 . 
     The network  865  transmits and receives data between the unit  801  and external systems. The network  865  can be implemented in a wireless fashion, e.g., using wireless protocols and technologies, such as IEEE 802.15.4 or similar. The network  865  can be a fixed wireless network, a wireless local area network (LAN), a wireless wide area network (WAN) a personal area network (PAN), a virtual private network (VPN), intranet or other suitable network system and includes equipment for receiving and transmitting signals. 
     When the unit  801  is in operation, the processor  805  can be configured to execute software stored within the PCM memory  810 , to communicate data to and from the PCM memory  810 , and to generally control operations of the computer  801  pursuant to the software. The methods described herein and the OS  811 , in whole or in part are read by the processor  805 , usually buffered within the processor  805 , and then executed. When the methods described herein are implemented in software, the methods can be stored on any computer readable medium, such as storage  820 , for use by or in connection with any computer related system or method. 
     Regarding  FIGS. 1-15 , deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies include, but are not limited to, thermal oxidation, physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others. As used herein, “depositing” can include any now known or later developed techniques appropriate for the material to be deposited including but not limited to, for example: chemical vapor deposition (CVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), semi-atmosphere CVD (SACVD) and high density plasma CVD (HDPCVD), rapid thermal CVD (RTCVD), ultra-high vacuum CVD (UHVCVD), limited reaction processing CVD (LRPCVD), metal-organic CVD (MOCVD), sputtering deposition, ion beam deposition, electron beam deposition, laser assisted deposition, thermal oxidation, thermal nitridation, spin-on methods, physical vapor deposition (PVD), atomic layer deposition (ALD), chemical oxidation, molecular beam epitaxy (MBE), plating, evaporation. 
     It is to be understood that the present invention will be described in terms of a given illustrative architecture; however, other architectures, structures, substrate materials and process features and steps/blocks can be varied within the scope of the present invention. 
     It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements can also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements can be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     The present embodiments can include a design for an integrated circuit chip, which can be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer can transmit the resulting design by physical mechanisms (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer to be etched or otherwise processed. 
     Methods as described herein can be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     It should also be understood that material compounds will be described in terms of listed elements, e.g., SiGe. These compounds include different proportions of the elements within the compound, e.g., SiGe includes Si x Ge 1-x  where x is less than or equal to 1, etc. In addition, other elements can be included in the compound and still function in accordance with the present embodiments. The compounds with additional elements will be referred to herein as alloys. Reference in the specification to “one embodiment” or “an embodiment” of the present invention, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
     It is to be appreciated that the use of any of the following “/”, “and/or”, and “at least one of”, for example, in the cases of “A/B”, “A and/or B” and “at least one of A and B”, is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of both options (A and B). As a further example, in the cases of “A, B, and/or C” and “at least one of A, B, and C”, such phrasing is intended to encompass the selection of the first listed option (A) only, or the selection of the second listed option (B) only, or the selection of the third listed option (C) only, or the selection of the first and the second listed options (A and B) only, or the selection of the first and third listed options (A and C) only, or the selection of the second and third listed options (B and C) only, or the selection of all three options (A and B and C). This can be extended, as readily apparent by one of ordinary skill in this and related arts, for as many items listed. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. 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,” “comprising,” “includes” and/or “including,” when used herein, 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. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, can be used herein for ease of description to describe one element&#39;s or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the FIGS. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the FIGS. For example, if the device in the FIGS. is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device can be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein can be interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers can also be present. 
     It will be understood that, although the terms first, second, etc. can be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the scope of the present concept. 
     Having described preferred embodiments of a method for integrating an electronic component in back end of the line (BEOL) processing (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments described which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.