Abstract:
A memory device with improved thermal isolation. The memory cell includes a first electrode element, having an upper surface; an insulator stack formed on the first electrode element, including first, second and third insulating members, all generally planar in form and having a central cavity formed therein and extending therethrough, wherein the second insulator member is recessed from the cavity; a phase change element, generally T-shaped in form, having a base portion extending into the cavity to make contact with the first electrode element and making contact with the first and third insulating members, and a crossbar portion extending over and in contact with the third insulating member, wherein the base portion of the phase change element, the recessed portions of the second insulating member and the surfaces of the first and third insulating members define a thermal isolation void; and a second electrode formed in contact with the phase change member.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 11/408,598 filed on 21 Apr. 2006, now U.S. Pat. No. 7,829,876; which application claims the benefit of U.S. Provisional Patent Application No. 60/739,079 entitled “Improved Thermal Isolation for an Active-Sidewall Phase Change Memory Cell” filed on Nov. 21, 2005, each of which is incorporated by reference as if fully set forth herein. 
    
    
     PARTIES TO A JOINT RESEARCH AGREEMENT 
     International Business Machines Corporation, a New York corporation; Macronix International Corporation, Ltd., a Taiwan corporation, and Infineon Technologies A.G., 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 memory devices and more specifically to memory devices employing phase change element to store information. 
     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 Chalogenide [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 the phase change memory and 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 claimed invention is a memory device with improved thermal isolation. The memory cell includes a first electrode element, having an upper surface; an insulator stack formed on the first electrode element, including first, second and third insulating members, all generally planar in form and having a central cavity formed therein and extending therethrough, wherein the second insulator member is recessed from the cavity; a phase change element, generally T-shaped in form, having a base portion extending into the cavity to make contact with the first electrode element and making contact with the first and third insulating members, and a crossbar portion extending over and in contact with the third insulating member, wherein the base portion of the phase change element, the recessed portions of the second insulating member and the surfaces of the first and third insulating members define a thermal isolation void; and a second electrode formed in contact with the phase change member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view that illustrates an embodiment of a phase change memory element employing a vacuum cell thermal isolation element. 
         FIG. 1   a  illustrates a current path in a phase change memory element as shown in  FIG. 1 . 
         FIGS. 2   a - 2   f  illustrate alternative embodiments to the phase change memory element of  FIG. 1 . 
         FIGS. 3   a - 3   f  illustrate a process for fabricating the phase change memory element as shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion describes embodiments of the invention with particular reference to  FIGS. 1-3 . It will be understood that the examples and features shown are exemplary and illustrative in nature and not intended to limit the scope of the invention. That scope is defined solely by the claims appended hereto. 
     The present invention concerns memory elements and memory cells. As used herein, and as is well known in the art, a memory cell is a circuit device designed to hold a charge or state to indicate the logic level of a single data bit. Memory cells are arrayed to provide, for example, the random access memory for a computer. Within certain memory cells, a memory element performs the function of actually holding the charge or state. In a conventional dynamic random access memory cell, for example, a capacitor indicates the logic level of the cell, with a fully charged state indicating a logic 1, or high, state, and fully discharged indicating a logic 0, or low, state. 
     A memory element  10 , an embodiment of the present invention, is illustrated generally in  FIG. 1 . As seen there, the memory element  10  is shown as a single unit, for purposes of clarity. In practice, each element is a part of a memory cell, which in turn is part of a larger memory array, as discussed more fully below. The structure of a memory element will be discussed first, followed by a description of the process for fabricating the same. 
     The memory element is formed on a substrate  12 , which is preferably a dielectric fill material such as silicon dioxide. Substrate material surrounds and extends over the lower electrode  14 , including an upper ledge  15  lying over the top of the lower electrode  14 . The lower electrode  14  preferably is formed from a refractory metal such as tungsten, and it is formed in the oxide layer. Other suitable 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. 
     Above the lower electrode and in contact with it is a phase change element  16 , generally having a T-shape, with the upright portion in contact with and extending vertically from the lower electrode, through the substrate upper ledge  15  overlying the lower electrode. An upper electrode  26  is formed atop the crossbar portion of the phase change element. 
     The phase change element  16  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 combinations denote chalcogenides amalgamated with other materials such as transition metals. A chalcogenide combination usually contains one or more elements from group IV of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide combinations include 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 combinations 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 materials, a wide range of compositions may be workable. The compositions can be characterized as Te a Ge b Sb 100−(a+b) . 
     One researcher has described the most useful combinations 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 combinations 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 combinations thereof may be combined with Ge/Sb/Te to form a phase change material 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 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 its local order in the active channel region of the cell. These materials 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 detectable characteristics, such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a structure relatively more ordered than in an amorphous state, 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 materials 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 combination. 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 phase change element described herein is Ge 2 Sb 2 Te 5 . 
     Between the lower electrode and the crossbar of the phase change element lie three dielectric layers. Immediately above the lower electrode is the substrate upper ledge  15 , as described above. Atop and formed on the upper ledge is an intermediate dielectric layer  18 . Formed on the intermediate dielectric layer, and extending into contact with the lower surface of the phase change element, is an upper dielectric layer  24 . The upper dielectric layer is preferably composed of the same, or similar, material as the substrate, such as silicon dioxide, while the intermediate dielectric is preferably formed of SiN or a similar material. 
     The upper ledge and upper dielectric layer make contact with the upright portion of the phase change element. The intermediate dielectric layer, however, does not extend to the phase change element, but rather that layer is recessed, so that vacuum isolation cell  22  is defined by the end of the intermediate dielectric layer and the upright portion of the phase change element, on one axis, and the upper ledge and upper dielectric layer on the other. This chamber preferably contains a vacuum, and it provides improved thermal isolation for the phase change element. In the area adjacent the vacuum isolation cell, the upright portion of the phase change element is pinched to form neck  20 , a zone of reduced cross-sectional area. 
     In operation, current flows through the memory element from lower electrode  14 , into the phase change element  16 , and out through the upper electrode  26 . Of course, the current direction could be altered by changes in element geometry, as will be understood by those in the art. In either event, the phase change material is subject to joule heating as current flows, as discussed above, resulting in a temperature rise in the center of the GST material. When the temperature exceeds the level required for phase change, a portion of the phase change material changes state. Temperature is not uniform throughout the phase change element, with changing values of current density producing significant variations. The temperature of the phase change material determines the effect produced, so the current is chosen to produce a temperature sufficient to create the desired result—either an amorphous state or a crystalline state—in the GST material. If it is desired to read the element status, a low current is employed for sensing purposes. The read operation is non-destructive, as the element temperature is kept below the threshold for a phase change. 
     The vacuum isolation cell  22  functions to contain heat within the phase change element, which has several positive effects. First, by preventing the migration of heat away from the phase change element, this design reduces the total heat required to effect phase changes, which in turn reduces the current required for each SET or RESET operation. At the same time, retaining heat within the phase change element reduces the heat transferred to the remainder of the memory array, which translates directly into increased lifespan for the device. Given the vast numbers of memory elements within a complete integrated circuit—at least eight billion elements for a 1 GB memory device, for example—it can be appreciated that the effects of such a heat reduction will be significant. The efficiency of the phase change element is further enhanced by concentrating the current within the phase change element, in the area of neck  20 . This effect is shown in the detail view of  FIG. 1   a , in which arrows enter from electrode  14  (not shown) with a relatively uniform current density, but are concentrated at the contact with contact element  30 , producing an area  27  of high temperature, which with proper current selection leads to a phase change in that area. The illustrated design leads to reduced current consumption by the memory element. 
     Memory element  10  is subject to a number of variations as seen in  FIGS. 2   a - 2   f . These variations do not affect the overall performance, function or design of memory element  10 , but they do produce changes in specific performance parameters or manufacturability, as will be understood by those of skill in the art. Discussions of the following variations each take the memory element  10  of  FIG. 1  as a starting point. 
     The variation shown in  FIG. 2   a , removes the crossbar portion of the phase change element, leaving an hourglass-shaped element that extends to the upper surface of the upper dielectric layer. The advantage of this design is improved fabrication, as the layer of GST material above the upper dielectric layer is dispensed with. Better thermal isolation, 
     The variation of  FIG. 2   b  adds a bottom flange  17  to the phase change element, producing an element resembling an I-beam in outline. This design provides improved thermal isolation between the central portion of the phase change element and the bottom electrode. 
     In  FIG. 2   c  the neck  20  is not formed, leaving the upright portion of the phase change element as a straight member. This change clearly simplifies the manufacturing process. 
     The variation of  FIG. 2   d  moves in the opposite direction. Here the neck  20  is held straight, but a void  21  is formed inside the upright member. It can clearly be seen that this design constricts the current flow path even more than does the design of  FIG. 1 , producing more concentrated heating. Also, the void  21  also increases the degree of thermal isolation, serving the concentrate the heat even more. It is preferred to deposit this material by a sputtering process, and the internal void  21  is formed by controlling sputtering conditions, as is known in the art. 
     The variation of  FIG. 2   e  includes a neck  20  formed the opposite of the neck shown in  FIG. 1 . Rather than having a reduced cross-sectional area, the neck here has an increased cross-section, protruding into the vacuum isolation cell, as result of the deposition process. 
     Similarly,  FIG. 2   f  depicts a design in which the neck  20  protrudes into the vacuum isolation cell, but the protrusion is notched. 
     An embodiment of a process for fabricating the memory element  10  depicted in  FIG. 1  is illustrated in  FIGS. 3   a - 3   f . The process begins with a substrate  12 , as shown in  FIG. 3   a . A lower electrode  14  is formed in the substrate, using known techniques. The materials for both the substrate and the electrode are discussed above. 
     Next, as seen in  FIG. 3   b , a substrate upper ledge  15 , plus layers of an intermediate dielectric  18  and an upper dielectric  24  are formed across the width of the substrate. Each of these layers has a thickness of from about 10 nm to about 50 nm, preferably 30 nm. 
     Removal of material to prepare for the phase change element is seen in  FIG. 3   c . It is preferred to follow the conventional lithographic process of applying a photoresistive film; imprinting a pattern, as through a mask or reticle; exposing the pattern to visible light or other radiation; stripping the undesired portions of resist material to form an etch mask; and etching the material layers. It is preferred to employ a dry, anisotropic etch, which may be controlled with an optical device that senses arrival at the lower electrode layer. 
     The vacuum isolation cell is formed in the step shown in  FIG. 3   d , where a wet etch, preferably phosphorous acid, is employed to selectively etch the SiN of the intermediate dielectric layer  18  a controlled distance back from the surface formed by the ends of the substrate upper ledge  15  and the upper dielectric layer  24 . 
     Phase change element  16  is added in the following step, illustrated in  FIG. 3   e . As shown, a conventional deposition step, preferably employing a sputtering or plasma enhanced sputtering process, is employed to form the phase change element  16 . The width of the upright portion of the phase change element (which is also the width of the etched area) should be from about 10 nm to about 100 nm, preferably 50 nm. 
     Finally, the upper electrode  26  is deposited in the final step, shown in  FIG. 3   f . That element and the phase change element are patterned to the desired lateral dimension, using conventional lithographic techniques as discussed above. 
     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.