# Patent

## Title
Memory cell with memory element contacting ring-shaped upper end of bottom electrode

## Abstract

A memory cell includes a bottom electrode, a top electrode and a memory element switchable between electrical property states by the application of energy. The bottom element includes lower and upper parts. The upper part has a generally ring-shaped upper end surrounding a non-conductive central region. The lateral dimension of the lower part is longer, for example twice as long, than the lateral dimension of the ring-shaped upper end. The lower part is a non-perforated structure. The memory element is positioned between and in electrical contact with the top electrode and the ring-shaped upper end of the second part of the bottom electrode. In some examples the ring-shaped upper end has a wall thickness at the memory element of 2-10 nm. A manufacturing method is also discussed.

## Background
CROSS-REFERENCE TO OTHER APPLICATIONS
This is related to U.S. patent application Ser. No. 11/375,816 filed 15 Mar. 2006 entitled Manufacturing Method for Pipe-Shaped Electrode Phase Change Memory.
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 to high density memory devices based on memory materials, for example resistor random access memory (RRAM) devices, and to methods for manufacturing such devices. The memory material is switchable between electrical property states by the application of energy. The memory materials may be phase change based memory materials, including chalcogenide based materials, and other materials.
2. Description of Related Art
Phase change based memory materials are widely used in read-write optical disks. 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.
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; this difference in resistance 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 by reducing the size 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 using small quantities of programmable resistive material, particularly in 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.
In phase change memory, data is stored by causing transitions in the phase change material between amorphous and crystalline states using current. Current heats the material and causes transitions between the states. 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. 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 active phase change material element in the cell. One problem associated with phase change memory devices arises because the magnitude of the current required for reset operations depends on the volume of phase change material that must change phase. Thus, cells made using standard integrated circuit manufacturing processes have been limited by the minimum feature size of manufacturing equipment. Thus, techniques to provide sublithographic dimensions for the memory cells must be developed, which can lack uniformity or reliability needed for large scale, high density memory devices.
One approach to controlling the size of the active area in a phase change cell is to devise very small electrodes for delivering current to a body of phase change material. This small electrode structure induces phase change in the phase change material in a small area like the head of a mushroom, at the location of the contact. See, U.S. Pat. No. 6,429,064, issued Aug. 6, 2002, to Wicker, “Reduced Contact Areas of Sidewall Conductor;” U.S. Pat. No. 6,462,353, issued Oct. 8, 2002, to Gilgen, “Method for Fabricating a Small Area of Contact Between Electrodes;” U.S. Pat. No. 6,501,111, issued Dec. 31, 2002, to Lowrey, “Three-Dimensional (3D) Programmable Device;” U.S. Pat. No. 6,563,156, issued Jul. 1, 2003, to Harshfield, “Memory Elements and Methods for Making Same.”
Accordingly, an opportunity arises to devise methods and structures that form memory cells with structures that have small active regions of programmable resistive material using reliable and repeatable manufacturing techniques.
BRIEF SUMMARY OF THE INVENTION
A first example of a memory cell, of the type including a memory material switchable between electrical property states by the application of energy, includes a bottom electrode, a top electrode and a memory element. The bottom element includes a first, lower part having a first lateral dimension and a second, upper part. The second, upper part has a generally ring-shaped upper end surrounding a non-conductive central region, the ring-shaped upper end having a second lateral dimension. The first lateral dimension is longer than the second lateral dimension. The lower part is a non-perforated structure. The memory element comprises a memory material switchable between electrical property states by the application of energy. The memory element is positioned between and in electrical contact with the top electrode and the ring-shaped upper end of the second part of the bottom electrode. In some examples the ring-shaped upper end has a wall thickness at the memory element of 2-10 nm. In some examples of the first lateral dimension is about twice as long as the second lateral dimension. In some examples the non-conductive central region comprises a first dielectric material and the second, upper part of the top electrode is surrounded by and is in direct contact with a second dielectric material, the first and second dielectric materials having different etching properties.
One example of a method for manufacturing a memory cell device, of the type including a memory material switchable between electrical property states by the application of energy, proceeds as follows. A memory cell access layer, having a top surface and an electrically conductive element at the top surface, is provided. A first electrode material layer is formed on the top surface. A first dielectric material layer is formed on the first electrode material layer. A mask is formed over the first dielectric material layer and the electrically conductive element. Portions of the first dielectric material layer and the first electrode material layer not covered by the mask are removed. The mask is removed to leave a dielectric/electrode stack on the electrically conductive element, the dielectric/electrode stack comprising a dielectric spacer element on a bottom electrode element. The dielectric/electrode stack is covered with a second electrode material layer to create a bottom electrode structure. The bottom electrode structure comprises the dielectric spacer element surrounded by the bottom electrode element and the second electrode material layer. The second electrode material layer is covered with a second dielectric material layer. Portions of the second dielectric material layer and the second electrode material layer covering the dielectric spacer element are removed thereby creating a dielectric spacer from the dielectric spacer element while leaving portions of the second dielectric material layer to surround the remaining portions of the second electrode material layer, and also creating a bottom electrode. The bottom electrode comprises a first, lower part having a first lateral dimension and a second, upper part. The second, upper part has a generally ring-shaped upper end surrounding the exposed dielectric spacer element, the ring-shaped upper end having a second lateral dimension. A memory element is formed on the ring-shaped upper end. A top electrode is formed over the memory element. In some examples the portions removing step is carried out so that the ring-shaped upper end has a wall thickness of 2-10 nm. In some examples the portions removing step comprises an etching step followed by a planarization step, and further comprises selecting different first and second dielectric materials for the first and second dielectric material layers having different etching properties so that during the etching step, the dielectric spacer element is not etched to any substantial degree.
Other features, aspects and advantages of the present invention can be seen on review the figures, the detailed description, and the claims which follow.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an integrated circuit device in accordance with the present invention;
FIG. 2 is a partial schematic diagram of a representative memory array as shown in FIG. 1;
FIG. 3 is a simplified side view of one example of a memory cell made according to the invention; and
FIGS. 5-15 illustrate stages in the manufacture of the memory device of FIG. 15.



DETAILED DESCRIPTION OF THE INVENTION
The following description of the invention will typically be with reference to specific structural embodiments and methods. It is to be understood that there is no intention to limit the invention to the specifically disclosed embodiments and methods but that the invention may be practiced using other features, elements, methods and embodiments. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. Like elements in various embodiments are commonly referred to with like reference numerals.
Referring to FIG. 1, shown is a simplified block diagram of an integrated circuit 10 in which the present invention may be implemented. Circuit 10 includes a memory array 12 implemented using phase change memory cells (not shown) on a semiconductor substrate, discussed more fully below. A word line decoder 14 is in electrical communication with a plurality of word lines 16. A bit line decoder 18 is in electrical communication with a plurality of bit lines 20 to read data from, and write data to, the phase change memory cells (not shown) in array 12. Addresses are supplied on bus 22 to word line decoder and drivers 14 and bit line decoder 18. Sense amplifiers and data-in structures in block 24 are coupled to bit line decoder 18 via data bus 26. Data is supplied via a data-in line 28 from input/output ports on integrated circuit 10, or from other data sources internal or external to integrated circuit 10, to data-in structures in block 24. Other circuitry 30 may be included on integrated circuit 10, such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by array 12. Data is supplied via a data-out line 32 from the sense amplifiers in block 24 to input/output ports on integrated circuit 10, or to other data destinations internal or external to integrated circuit 10.
A controller 34 implemented in this example, using a bias arrangement state machine, controls the application of bias arrangement supply voltages 36, such as read, program, erase, erase verify and program verify voltages. Controller 34 may be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, controller 34 comprises a general-purpose processor, which may be implemented on the same integrated circuit to execute a computer program to control the operations of the device. In yet other embodiments, a combination of special-purpose logic circuitry and a general-purpose processor may be utilized for implementation of controller 34.
As shown in FIG. 2 each of the memory cells of array 12 includes an access transistor (or other access device such as a diode), four of which are shown as 38, 40, 42 and 44, and a phase change element, shown as 46, 48, 50 and 52. Sources of each of access transistors 38, 40, 42 and 44 are connected in common to a source line 54 that terminates in a source line termination 55. In another embodiment the source lines of the select devices are not electrically connected, but independently controllable. A plurality 16 of word lines including word lines 56 and 58 extend parallel along a first direction. Word lines 56 and 58 are in electrical communication with word line decoder 14. The gates of access transistors 38 and 42 are connected to a common word line, such as word line 56, and the gates of access transistors 40 and 44 are connected in common to word line 58. A plurality 20 of bit lines including bit lines 60 and 62 have one end of phase change elements 46 and 48 connected to bit line 60. Specifically, phase change element 46 is connected between the drain of access transistor 38 and bit line 60, and phase change element 48 is connected between the drain of access transistor 48 and bit line 60. Similarly, phase change element 50 is connected between the drain of access transistor 42 and bit line 62, and phase change element 52 is connected between the drain of access transistor 44 and bit line 62. It should be noted that four memory cells are shown for convenience of discussion and in practice array 12 may comprise thousands to millions of such memory cells. Also, other array structures may be used, e.g. the phase change memory element is connected to source.
FIG. 3 is an example of a mushroom-type memory cell 68 made according to the invention. Memory cell 68 includes a bottom electrode 70, a top electrode 72 and a memory element 74 therebetween. Bottom electrode 70 can be made of a suitable electrode material, such as TiN, TaN or WN. Top electrode 72 is typically made of TiN while memory element 74 is typically made of resistive memory materials such as GST, discussed in more detail below. Bottom electrode 70 includes a first part 76 having a first lateral dimensions 78 and a second part 80 having a ring-shaped upper end 82. Upper end 82 surrounds a central region 84 has a second lateral dimension 86 and a wall thickness 87 at memory element 74. Wall thickness 87 preferably 2-10 nm. First lateral dimensions 78 is longer than second lateral dimension 86. In one example first lateral dimensions 78 is 50 to 90 nm, typically about 65 nm, and second lateral dimension 86 is 20 to 45 nm, typically about 32 nm. First lateral dimension 78 is typically about twice as long as second lateral dimensions 86. Providing first part 76 with extra width compared to second part 80 results in better mechanical stability for bottom electrode 70 than would be achieved if first part 76 were the same width as second part 80.
A dielectric spacer 88 is contained within central region 84 and a dielectric spacer 89 surrounds second part 80. Dielectric spacer 89 has a wall thickness 91 at memory element 74. Wall thickness 91 is typically 10-30 nm. Dielectric spacers 88, 89 are typically made of different dielectric materials having different etching properties so that during the etching that creates dielectric spacer 89, discussed below with reference to FIGS. 10-12, dielectric spacer 88 is not etched to any substantial degree. The presence of dielectric spacer 89 protects second part 80 of bottom electrode 70 during etching procedures, discussed below. This is important because of the relatively thin wall thickness 87 of upper end 82. Typical materials for dielectric spacers 88, 89 include SiNx, silicon oxynitride and tantalum oxide. A dielectric material 90, such as SiOx, surrounds dielectric spacer 88 and first part 76. First part 76 rests against a plug 92.
The construction of memory cell 68 provides several advantages. Having a ring-shaped upper end 82 contacting memory element 74 reduces the contact area between bottom electrode 70 and memory element 74 thus concentrating the electrical current thereby reducing the required reset power and reset current. In addition, the ring-shaped upper end 82 provides better process uniformity after trimming when compared with a cylindrical bottom electrode. This is because the lithographic critical dimension (CD) variation and the trimming process used in creating a cylindrical bottom electrode and a ringed-shaped bottom electrode results in a relatively large variation in the radius for each bottom electrode. However, the variation in the radius of the cylindrical bottom electrode influences the area of the cylindrical bottom electrode more than it influences the ring-shaped bottom electrode. This is because the area of the cylindrical bottom electrode uses the radius squared in determining the area (pi R2) while the contact area of a bottom electrode with a ring-shaped upper end is calculated using only the radius (2 pi RT, where T is the wall thickness), not the radius squared.
Methods for making mushroom-type phase change memory cells 68 will be described with reference to FIGS. 4-15. Referring now to FIG. 4, a memory cell access layer 94 is shown formed on a substrate 96. Substrate 96 is typically SiO2. Access layer 94 typically comprises access transistors (not shown); other types of access devices, such as diodes, may also be used. Access layer 94 comprises first and second plugs 92, 98 and a source line 100 all within a dielectric film layer 102. First and second plugs 92, 98 and source line 100 are typically made of tungsten. Memory cell access layer 94 also contains polysilicon word lines 106, 108. Memory cell access layer 94 has an upper surface 104.
FIG. 5 illustrates the results of the deposition of a layer 110 of an electrode material, such as TiN, on upper surface 104, followed by the deposition of a layer 112 the same material as dielectric spacer 88. Photoresist masks 114, see FIG. 6, are then formed on the structure of FIG. 5 to be generally aligned with plugs 92, 98. Photo resist masks 114 preferably have a minimum lithographic lateral feature size, such as 30 to 65 nm, typically about 45 nm. FIG. 7 illustrates the results of a photoresist trimming step resulting in trimmed photoresist masks 116. Each trimmed photoresist mask 116 preferably has a sublithographic lateral feature size, such as 15 to 32 nm, typically about 22 nm. The portions of layers 110 and 112 not covered by trimmed photoresist masks 116 are etched away and photoresist masks 116 are removed as illustrated in FIG. 8, leaving a dielectric/electrode stack 115 including a bottom electrode element 117 and a dielectric spacer element 119. Dielectric spacer element 119 has an outer end 121. An electrode material 118, typically the same material as layer 110, is deposited on stack 115 as shown in FIG. 9 to create a bottom electrode structure 123. FIG. 10 illustrates the result of depositing a dielectric material 120 onto the structure of FIG. 9 with dielectric material 120 being the same as the dielectric material of dielectric spacer 89. The depositions of FIGS. 9 and 10 are typically accomplished using chemical vapor deposition (CVD) techniques. Dielectric material 120 is then etched followed by etching of exposed electrode material 118 covering outer end 121 of dielectric spacer element 119 creating the structure of FIG. 11. This etching is carried out to leave a dielectric material layer 125 laterally surrounding the remainder 127 of second electrode material layer 118 surrounding dielectric spacer element 119.
FIG. 12 illustrates the results of depositing a dielectric material 122, typically the same material as substrate 96, onto the structure of FIG. 11 followed by a chemical mechanical polishing (CMP) step. Doing so creates the bottom electrode 70 and dielectric spacers 88, 89 of FIG. 3 with dielectric material 122 acting as the dielectric material 90 of FIG. 3. FIG. 13 illustrates the results of depositing a memory material 124, typically GST, onto the structure of FIG. 12 and then depositing a top electrode material 126 onto memory material 124. FIG. 14 shows results of patterning memory material 124 and top electrode material 126 creating the memory element 74 and top electrode 72 of FIG. 3. The completed memory device 128 is shown in FIG. 15 after a metallization step in which a bit line 130 is electrically connected to top electrode 72 through conductive vias 132.
Dielectric materials 88, 89 may comprise an electrical insulator including one or more elements selected from the group consisting of Si, Ti, Al, Ta, N, O, and C. In preferred devices, dielectric materials 88, 89 have a low thermal conductivity, less than about 0.014 J/cm*K*sec. In other preferred embodiments, when memory element 74 is made from a phase change material, one or both of the thermally insulating dielectric materials 88, 89 have a thermal conductivity less than that of the amorphous state of the phase change material, or less than about 0.003 J/cm*K*sec for a phase change material comprising GST. Representative thermally insulating materials include 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 for the thermally insulating dielectric materials 88, 89 include SiO2, SiCOH, polyimide, polyamide, and fluorocarbon polymers. Other examples of materials which are candidates for use for the thermally insulating dielectric materials 88, 89 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. In other embodiments, the thermally insulating structure comprises a gas-filled void for thermal insulation. A single layer or combination of layers within dielectric materials 88, 89 can provide thermal and electrical insulation.
Useful characteristics of a programmable resistive type of memory material, like a phase change material, include the material having a resistance which is programmable, and preferably in a reversible manner, such as by having at least two solid phases that can be reversibly induced by electrical current. These at least two phases include an amorphous phase and a crystalline phase. However, in operation, the programmable resistive material may not be fully converted to either an amorphous or crystalline phase. Intermediate phases or mixtures of phases may have a detectable difference in material characteristics. The two solid phases should generally be bistable and have different electrical properties. The programmable resistive material may be a chalcogenide material. A chalcogenide material may include GST. In following sections of the disclosure, the phase change or other memory material is often 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 memory cell as described herein is Ge2Sb2Te5.
A memory device 128 as described herein is readily manufacturable using standard lithography and thin film deposition technologies, without requiring extraordinary steps to form sub-lithographic patterns, while achieving very small dimensions for the region of the cell that actually changes resistivity during programming. In embodiments of the invention, the memory material may be a programmable resistive material, typically a phase change material, such as Ge2Sb2Te5 or other materials described below. The region in the memory element 74 that changes phase is small; and accordingly, the magnitude of the reset current required for changing the phase is very small.
Embodiments of memory device 128 include phase change based memory materials, including chalcogenide based materials and other materials, for memory element 74. 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 (Sn). 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), where a and b represent atomic percentages that total 100% of the atoms of the constituent elements. 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. (Ovshinsky '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 '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 phase change 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 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 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, 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 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 empirically, without undue experimentation, specifically adapted to a particular phase change material and device structure.
The following are short summaries describing four types of resistive memory materials.
1. Chalcogenide Material

    
    
        GexSbyTez 
        x:y:z=2:2:5
        Or other compositions with x: 0 ̃5; y: 0 ̃5; z: 0 ̃10
        GeSbTe with doping, such as N—, Si—, Ti—, or other element doping may also be used.
        Formation method: By PVD sputtering or magnetron-sputtering method with reactive gases of Ar, N2, and/or He, etc chalcogenide @ the pressure of 1 mtorr ̃100 mtorr. The deposition is usually done at room temperature. The collimator with aspect ratio of 1 ̃5 can be used to improve the fill-in performance. To improve the fill-in performance, the DC bias of several ten to several hundred volts is also used. On the other hand, the combination of DC bias and the collimator can be used simultaneously.
        The post deposition annealing treatment with vacuum or N2 ambient is sometimes needed to improve the crystallize state of chalcogenide material. The annealing temperature typically ranges 100 C to 400 C with an anneal time of less than 30 minutes.
        The thickness of chalcogenide material depends on the design of cell structure. In general, a chalcogenide material with thickness of higher than 8 nm can have a phase change characterization so that the material exhibits at least two stable resistance states.
    
    


2. CMR (Colossal Magneto Resistance) Material

    
    
        PrxCayMnO3 
        x:y=0.5:0.5
        Or other compositions with x: 0 ̃1; y: 0 ̃1
        Another CMR material that includes Mn oxide may be used
        Formation method: By PVD sputtering or magnetron-sputtering method with reactive gases of Ar, N2, O2, and/or He, etc. at the pressure of 1 mtorr ̃100 mtorr. The deposition temperature can range from room temperature to  ̃600 C, depending on the post deposition treatment condition. A collimator with an aspect ratio of 1 ̃5 can be used to improve the fill-in performance. To improve the fill-in performance, the DC bias of several ten to several hundred volts is also used. On the other hand, the combination of DC bias and the collimator can be used simultaneously. A magnetic field of several ten gauss to 10,000 gauss may be applied to improve the magnetic crystallized phase.
        The post deposition annealing treatment with vacuum or N2 ambient or O2/N2 mixed ambient may be needed to improve the crystallized state of CMR material. The annealing temperature typically ranges 400 C to 600 C with an anneal time of less than 2 hours.
        The thickness of CMR material depends on the design of cell structure. The CMR thickness of 10 nm to 200 nm can be used to be the core material.
        A buffer layer of YBCO (YBaCuO3, a kind of high temperature superconductor material) is often used to improve the crystallized state of CMR material. The YBCO is deposited before the deposition of CMR material. The thickness of YBCO ranges 30 nm to 200 nm.
    
    


3. 2-Element Compound

    
    
        NixOy; TixOy; AlxOy; WxOy; ZnxOy; ZrxOy; CuxOy; etc
        x:y 0.5:0.5
        Other compositions with x: 0 ̃1; y: 0 ̃1
        Formation method:
        1. Deposition: By PVD sputtering or magnetron-sputtering method with reactive gases of Ar, N2, O2, and/or He, etc. at the pressure of 1 mtorr ̃100 mtorr, using a target of metal oxide, such as NixOy; TixOy; AlxOy; WxOy; ZnxOy; ZrxOy; CuxOy; etc. The deposition is usually done at room temperature. A collimator with an aspect ratio of 1 ̃5 can be used to improve the fill-in performance. To improve the fill-in performance, the DC bias of several ten to several hundred volts is also used. If desired, they combination of DC bias and the collimator can be used simultaneously.
        The post deposition annealing treatment with vacuum or N2 ambient or O2/N2 mixed ambient as sometimes needed to improve the oxygen distribution of metal oxide. The annealing temperature ranges 400 C to 600 C with an anneal time of less than 2 hours.
        2. Reactive deposition: By PVD sputtering or magnetron-sputtering method with reactive gases of Ar/O2, Ar/N2/O2, pure O2, He/O2, He/N2/O2 etc. at the pressure of 1 mtorr ̃100 mtorr, using a target of metal oxide, such as Ni, Ti, Al, W, Zn, Zr, or Cu etc. The deposition is usually done at room temperature. A collimator with an aspect ratio of 1 ̃5 can be used to improve the fill-in performance. To improve the fill-in performance, a DC bias of several ten to several hundred volts is also used. If desired, the combination of DC bias and the collimator can be used simultaneously.
        The post deposition annealing treatment with vacuum or N2 ambient or O2/N2 mixed ambient is sometimes needed to improve the oxygen distribution of metal oxide. The annealing temperature ranges 400 C to 600 C with an anneal time of less than 2 hours.
        3. Oxidation: By a high temperature oxidation system, such as furnace or RTP system. The temperature ranges from 200 C to 700 C with pure O2 or N2/O2 mixed gas at a pressure of several mtorr to 1 atm. The time can range several minute to hours. Another oxidation method is plasma oxidation. An RF or a DC source plasma with pure O2 or Ar/O2 mixed gas or Ar/N2/O2 mixed gas at a pressure of 1 mtorr to 100 mtorr is used to oxidize the surface of metal, such as Ni, Ti, Al, W, Zn, Zr, or Cu etc. The oxidation time ranges several seconds to several minutes. The oxidation temperature ranges room temperature to 300 C, depending on the degree of plasma oxidation.
    
    


4. Polymer Material

    
    
        TCNQ with doping of Cu, C60, Ag etc.
        PCBM-TCNQ mixed polymer
        Formation method:
        1. Evaporation: By thermal evaporation, e-beam evaporation, or molecular beam epitaxy (MBE) system. A solid-state TCNQ and dopant pellets are co-evaporated in a single chamber. The solid-state TCNQ and dopant pellets are put in a W-boat or a Ta-boat or a ceramic boat. A high electrical current or an electron-beam is applied to melt the source so that the materials are mixed and deposited on wafers. There are no reactive chemistries or gases. The deposition is done at a pressure of 10-4 torr to 10-10 torr. The wafer temperature ranges from room temperature to 200 C.
        The post deposition annealing treatment with vacuum or N2 ambient is sometimes needed to improve the composition distribution of polymer material. The annealing temperature ranges room temperature to 300 C with an anneal time of less than 1 hour.
        2. Spin-coat: By a spin-coater with the doped-TCNQ solution @ the rotation of less than 1000 rpm. After spin-coating, the wafer is put to wait the solid-state formation @ room temperature or temperature of less than 200 C. The waiting time ranges from several minutes to days, depending on the temperature and on the formation conditions.
    
    


For additional information on the manufacture, component materials, use and operation of phase change random access memory devices, see U.S. patent application Ser. No. 11/155,067, filed 17 Jun. 2005, entitled Thin Film Fuse Phase Change Ram And Manufacturing Method.
It is preferred that all or part of the portions of bottom and top electrodes 70, 72 contacting memory element 74 comprise an electrode material, such as TiN, or another conductor selected for compatibility with the phase change material of memory element 74. Other types of conductors can be used for the plug structures and the top and bottom electrodes structures, including for example aluminum and aluminum alloys, TiN, TaN, TiAlN or TaAlN. Other conductors that might be used comprise one or more elements selected from the group consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, Ru and O. TiN may be preferred because it makes good contact with GST (discussed above) as memory element 74, it is a common material used in semiconductor manufacturing, and it provides a good diffusion barrier at the higher temperatures at which GST transitions, typically in the 600-700° C. range.
The above descriptions may have used terms such as above, below, top, bottom, over, under, et cetera. These terms are used to aid understanding of the invention are not used in a limiting sense.
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 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.
Any and all patents, patent applications and printed publications referred to above are hereby incorporated by reference.

## Claims

1. A memory cell of the type including a memory material switchable between electrical property states by the application of energy, the memory cell comprising:
a substrate with a contact for an access circuit;
an electrically conductive plug extending through an interlayer dielectric within the substrate to the contact;
a ring-shaped bottom electrode comprising a first, ring-shaped lower part having a first contact area with a first lateral dimension in contact with the plug, and a second, upper part, the second, upper part having a generally ring-shaped upper end surrounding a non-conductive central region, the ring-shaped upper end with a second contact area between an inside perimeter and an outside perimeter of the ring-shaped upper end, the outside perimeter having a second lateral dimension;
the first lateral dimension being longer than the second lateral dimension;
the lower part being a non-perforated structure;
a top electrode; and
a memory element comprising a memory material switchable between electrical property states by the application of energy, the memory element positioned between and in electrical contact with the top electrode and the second contact area on the ring-shaped upper end of the second part of the bottom electrode.

2. The memory cell according to claim 1 wherein the generally ring-shaped upper end is a continuous ring-shaped element.

3. The memory cell according to claim 1 wherein the ring-shaped upper end has a wall thickness at the memory element of 2-10 nm.

4. The memory cell according to claim 1 wherein the first lateral dimension is about twice as long as the second lateral dimension.

5. The memory cell according to claim 1 wherein the non-conductive central region comprises a first dielectric material and the second, upper part of the bottom electrode is surrounded by and is in direct contact with a second dielectric material.

6. The memory cell according to claim 5 wherein the first and second dielectric materials have different etching properties.

7. The memory cell according to claim 5 further comprising a third dielectric material surrounding the second dielectric material.

8. The memory cell according claim 7 wherein:
the second dielectric material has an outer surface;
the third dielectric material contacts the outer surface of the second dielectric material; and
the second dielectric material overlies and is aligned with the first, ring-shaped lower part.

9. The memory cell according to claim 8 wherein the second dielectric material has a dielectric material lateral dimension about equal to the first lateral dimension.

10. The memory cell according to claim 1 wherein the memory element comprises a programmable resistive memory material.

11. A memory cell of the type including a memory material switchable between electrical property states by the application of energy, the memory cell comprising:
a ring-shaped bottom electrode comprising a first, ring-shaped lower part having a first lateral dimension and a second, upper part, the second, upper part having a generally ring-shaped upper end, the upper end being completely hollow to define an opening that surrounds a non-conductive central region, the ring-shaped upper end having a second lateral dimension;
the second, upper part having an outer sidewall;
the first lateral dimension being longer than the second lateral dimension;
the lower part being a non-perforated structure;
a top electrode;
a memory element comprising a memory material switchable between electrical property states by the application of energy, the memory element positioned between and in electrical contact with the top electrode and the ring-shaped upper end of the second part of the bottom electrode;
a dielectric film layer;
a spacer positioned between and contacting the outer sidewall and the dielectric film layer;
the spacer having an outer surface;
the dielectric film layer contacting the outer surface of the spacer; and
the spacer overlying and aligned with the first, ring-shaped lower part.

12. The memory cell according to claim 11 wherein the spacer has a spacer lateral dimension about equal to the first lateral dimension.

13. A memory cell of the type including a memory material switchable between electrical property states by the application of energy, the memory cell comprising:
a ring-shaped bottom electrode comprising a first, ring-shaped lower part having a first lateral dimension, a second, upper part, and a third, middle part between the first, upper part and the second, lower part;
the second, upper part having a generally ring-shaped upper end, the upper end being completely hollow to define an opening that surrounds a non-conductive central region, the ring-shaped upper end having a second lateral dimension;
the non-conductive central region comprising a first dielectric material;
the second, upper part of the bottom electrode being surrounded by and in direct contact with a second dielectric material, the second dielectric material having an outer surface;
a third dielectric material surrounding the second dielectric material and contacting the outer surface of the second dielectric material;
the second dielectric material overlying and aligned with the first, ring-shaped lower part;
the second dielectric material having a dielectric material lateral dimension about equal to the first lateral dimension;
the third, middle part having a lateral dimension substantially equal to the second lateral dimension;
the first lateral dimension being longer than the second lateral dimension;
the lower and middle parts being non-perforated structures;
a top electrode; and
a memory element comprising a memory material switchable between electrical property states by the application of energy, the memory element positioned between and in electrical contact with the top electrode and the ring-shaped upper end of the second part of the bottom electrode.