Abstract:
A memory device. The device includes first and second electrode members, in spaced relation on a substrate. A phase change element lies in electrical contact with the first and second electrode elements and spans the space separating them. The phase change element includes two segments, each in contact with one of the electrode elements. The segments are fused together at a location between the two electrodes such that the fused area has a smaller cross-sectional area than does the remainder of the phase change element. The electrodes, the substrate and the phase change element define a chamber containing a vacuum.

Description:
PARTIES TO A JOINT RESEARCH AGREEMENT 
     International Business Machines Corporation, a New York corporation; Macronix International Corporation, Ltd., a Taiwan corporation; and Infineon Technologies AG, a German corporation, are parties to a Joint Research Agreement. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to non-volatile memory devices, and more specifically to memory devices employing a phase-change memory element. 
     2. Description of Related Art 
     Phase change based memory materials are widely used in read-write optical disks, and such materials are seeing increasing use in computer memory devices. These materials have at least two solid phases, including for example a generally amorphous solid phase and a generally crystalline solid phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change, and electrical pulses are employed in the same manner in computer memory devices. 
     Phase change based memory materials, like chalcogenide based materials and similar materials, also can be caused to change phase 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. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access. 
     The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, 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 phase change material from crystalline state to amorphous state. The magnitude of the reset current needed for reset can be reduced by reducing the size of the phase change material element in the cell and of the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element. 
     One direction of development has been toward forming small pores in an integrated circuit structure, and using small quantities of programmable resistive material to fill the small pores. Patents illustrating development toward small pores include: Ovshinsky, “Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat. No. 5,687,112, issued Nov. 11, 1997; Zahorik et al., “Method of Making Chalcogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued Aug. 4, 1998; Doan et al., “Controllable Ovonic Phase-Change Semiconductor Memory Device and Methods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued Nov. 21, 2000, and Reinberg, “Chalcogenide Memory Cell with a Plurality of Chalcogenide Electrodes,” U.S. Pat. No. 5,920,788, issued Jul. 6, 1999. 
     A specific issue arising from conventional phase change memory structures is the heat sink effect of conventional designs. Generally, the prior art teaches the use of metallic electrodes on both sides of the phase change memory element, with electrodes of approximately the same size as the phase change member. Such electrodes act as heat sinks, the high heat conductivity of the metal rapidly drawing heat away from the phase change material. Because the phase change occurs as a result of heating, the heat sink effect results in a requirement for higher current, in order to effect the desired phase change. 
     One approach to the heat flow problem is seen in U.S. Pat. No. 6,815,704, entitled “Self Aligned Air-Gap Thermal Insulation for Nano-scale Insulated Chalcogenide Electronics (NICE) RAM”, in which an attempt is made to isolate the memory cell. That structure, and the attendant fabrication process, is overly complex, yet it does not promote minimal current flow in the memory device. 
     It is desirable therefore to provide a memory cell structure having small dimensions and low reset currents, as well as a structure that addresses the heat conductivity problem, and a method for manufacturing such structure that meets tight process variation specifications needed for large-scale memory devices. It is further desirable to provide a manufacturing process and a structure, which are compatible with manufacturing of peripheral circuits on the same integrated circuit. 
     SUMMARY OF THE INVENTION 
     An important aspect of the invention is a memory device. The device includes first and second electrode members, in spaced relation on a substrate. A phase change element lies in electrical contact with the first and second electrode elements and spans the space separating them. The phase change element includes two segments, each in contact with one of the electrode elements. The segments are fused together at a location between the two electrodes such that the fused area has a smaller cross-sectional area than does the remainder of the phase change element. The electrodes, the substrate and the phase change element define a chamber containing a vacuum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view that illustrates an embodiment of a phase change memory element according to the present invention. 
         FIG. 2  is a cross-sectional view of the phase change memory element of  FIG. 1 , showing the device in greater detail. 
         FIG. 3  illustrates the operation of the phase change memory element of  FIG. 1 . 
         FIGS. 4   a - 4   h  illustrate an embodiment of a process for fabricating a phase change memory element shown in  FIG. 1 . 
         FIG. 5  depicts the phase change memory element of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined solely by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. 
       FIG. 1  illustrates the basic layout of a phase change memory element  10  according to the present invention. As is known in the art, phase change random access memory (PCRAM) cells  10  include a phase change element  20 , formed from a material having two solid phases. Preferably, such material changes phase from amorphous to crystalline and back again, upon application of suitable current pulses. General details of such memory cells are disclosed in the references cited above, and details of a phase change material itself are set out below. 
     The structural and functional aspects of the memory cell will be discussed first, after which there will follow a detailed discussion of the process for forming the same. The cell is preferably formed on a dielectric layer or substrate  12 , preferably consisting of silicon oxide or a well-known alternative thereto, such as a polyimide, silicon nitride or other dielectric fill material. In embodiments, the dielectric layer comprises a relatively good insulator for heat as well as for electricity, providing thermal and electrical isolation. First and second electrodes  14  and  16 , preferably formed from a refractory metal such as tungsten, are formed in the oxide layer. Other refractory metals include Ti, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, and Ru, as well as oxides and nitrides of such materials. For example, materials such as TiN, RuO or NiO are known and effective refractory metals. The two electrodes should be spaced slightly apart, by a distance between about 30 nm to 70 nm, most preferably about 50 nm. It should be noted that, for purposes of reference only, the direction from the bottom toward the top of the drawings herein is designated “vertical,” and the side-to-side direction is “lateral” or “horizontal.” Such designations have no effect on the actual physical orientation of a device, either during fabrication or during use. 
     Phase change element  20  consists generally of a strip of phase change material positioned on the two electrodes and bridging the gap between them. The element is preferably between about 10 nm and 30 nm, most preferably about 20 nm wide, and about 10 nm thick. Dielectric fill material  26  (see  FIG. 4   h ) overlies the electrodes and the phase change element. This material is preferably identical to, or selected from the same class of materials as that employed to form the substrate  12 . This material preferably has a thermal conductivity value “kappa” of less than that of silicon dioxide which is 0.014 J/cm*K*sec. Representative materials for dielectric fill material  26  include low permittivity (low-K) materials, including materials that are a combination of the elements silicon Si, carbon C, oxygen O, fluorine F, and hydrogen H. Examples of thermally insulating materials which are candidates for use as dielectric fill material  26  include SiCOH, polyimide, polyamide, and fluorocarbon polymers. Other examples of materials which are candidates for use for dielectric fill material include fluorinated SiO2, silsesquioxane, polyarylene ethers, parylene, fluoro-polymers, fluorinated amorphous carbon, diamond like carbon, porous silica, mesoporous silica, porous silsesquioxane, porous polyimide, and porous polyarylene ethers. The dielectric fill closes off the top of the gap between the two electrodes, so that the electrodes and the two dielectric layers define a vacuum spacer  24  between the electrodes. 
     The phase change element  20  can be formed from a class of materials preferably including chalcogenide based 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 comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from column six of the periodic table of elements, such as germanium (Ge) and tin (Si). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including 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. The compositions can be characterized as TeaGebSb100−(a+b). One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky &#39;112 patent, cols. 10-11.) Particular alloys evaluated by another researcher include Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7. (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky &#39;112 at columns 11-13, which examples are hereby incorporated by reference. 
     Phase change alloys 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 its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the 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. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly. 
     Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. In following sections of the disclosure, the phase change material is referred to as GST, and it will be understood that other types of phase change materials can be used. A material useful for implementation of a PCRAM described herein is Ge2Sb2Te5. 
     Other programmable resistive memory materials may be used in other embodiments of the invention, including N2 doped GST, GexSby, or other material that uses different crystal phase changes to determine resistance; PrxCayMnO3, PrSrMnO, ZrOx, or other material that uses an electrical pulse to change the resistance state; TCNQ, PCBM, TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ, C60-TCNQ, TCNQ doped with other metal, or any other polymer material that has bistable or multi-stable resistance state controlled by an electrical pulse. 
     A more detailed view of the memory element, seen in  FIG. 2 , reveals that the phase change element is actually composed of two segments  20   a  and  20   b , each of which has a rounded tip. The two segments are joined over the vacuum spacer  24 , such that the cross-sectional area at the zone of contact is less than the cross sectional area of the rest of the phase change element. The discussion concerning the formation of this element, below, will clarify this point. 
     Operation of the memory cell of the present invention is shown in  FIG. 3 . In the figure, arrows labeled I indicate current flowing from electrode  14 , and through the phase change element  20   a / 20   b , and out through electrode  16 . It should be noted that the current directions are chosen arbitrarily, for purposes of illustration, and could be directed in an opposite direction in practice. 
     As shown, the electric field and current density in the two phase change members are relatively low compared to the values seen in the contact zone  25 , where the two phase change element segments meet. The relatively smaller cross sectional area in the contact zone produces current and field densities much higher than those in the rest of the phase change element. As a result, the contact zone will experience much higher value of heating than will the phase change members, and in fact the phase change will be restricted to the area of the contact zone (shown as an elliptical area in the drawing). 
     In addition, the low heat conductivity of the vacuum spacer  24  reduces the heat transfer from the contact zone area, effectively increasing the amount of heat generated within the phase change material per unit value of current. The thermal isolation of the contact zone area allows for memory cell design having lower currents than those permitted by the prior art, which in turn allows for reducing the size of the memory cell itself. 
     A process for fabricating a phase change memory element as shown in  FIG. 1  is illustrated in  FIGS. 4   a - 4   h . The first step, shown in  FIG. 4   a , consists of providing a substrate, preferably formed of dielectric material, such as silicon dioxide, as discussed above. The substrate is initially patterned and etched, preferably employing lithographic techniques known in the art, to reduce the thickness overall, leaving an upright member  13  in the central area of the substrate block as seen in  FIG. 4   b.    
     The two electrode elements are formed in the next two steps.  FIG. 4   c  illustrates the results of depositing electrode material  15  (discussed above) onto the substrate, to a depth greater than that of upright member  13 . The electrode material is then planarized, preferably employing chemical-mechanical polishing (CMP), down to a depth at which the upper end of upright member  13  is exposed as seen in  FIG. 4   d . The etching forms the two electrode members  14  and  16 . 
     A selective etch process to remove dielectric material from upright member  13 , leaving interelectrode space  23 , is seen in  FIG. 4   e . Here, assuming the dielectric material is SiO2, it is preferred to employ a wet etch process, preferably buffered HF. Alternatively, a dry etch, such as a fluorine-based plasma chemistry, could also be employed here. It will be understood that a different dielectric material will require a different etch chemistry. The conclusion of this step leaves the two electrodes  14  and  16  in spaced relationship on the substrate, separated by the interelectrode space  23 . 
     Deposition of the phase change element  20  is shown in  FIG. 4   f , forming phase change memory element  10 . This deposition is preferably a sputtering process. The manner in which the reduced-cross-section central area  15  is produced is best seen in the close-up view of  FIG. 4   g , which shows the deposition in process. As known in the art, a sputtering process will produce a deposit that “wraps” around a surface corner as shown. Addition of more material causes the deposit to grow from both sides, until finally the two sides meet in the middle of the gap. Just as newly applied material bonds to material already deposited, material from the two sides fuses when the two sides meet in the middle of the gap, as shown in  FIG. 4   f . The joining of the two sides closes off the interelectrode space  23 , thereby defining vacuum spacer  24 . Finally, the phase change element  20  is trimmed to an appropriate length, as it does not need to extend the width of the electrodes, and additional dielectric fill material  26  is deposited, as shown in  FIG. 4   h . That material seals vacuum spacer  24 , allowing that element to maintain a vacuum. 
     It will be understood that the illustrations are somewhat idealized.  FIG. 5  is a bit more realistic, as it depicts the fact that the ends of electrode elements  14  and  16  are most likely not actually vertical, given the likely undercutting to be expected during the course of etching the upright member  13  to form vacuum spacer  24 . Also, it is to be expected that some small amount of GST material will be deposited at the bottom of the vacuum spacer during the sputtering process, but the presence of a small quantity of material there will not affect the operation of the device. 
     While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.