Patent Publication Number: US-8110456-B2

Title: Method for making a self aligning memory device

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/552,487 filed on 24 Oct. 2006, entitled, “PHASE CHANGE MEMORY CELLS WITH DUAL ACCESS DEVICES,” which application is incorporated herein by reference. 
    
    
     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 phase change based memory materials, including chalcogenide based materials and other materials, and to methods for operating such devices. 
     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 that differ in the two phases. 
     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 break down the crystalline structure, after which the phase change material cools quickly, quenching the phase change process and 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. Even with small devices, the reset current remains a design limitation for high density and low voltage integrated circuits. 
     As the phase change memory cell structures are made small, a limiting factor on the density of a device incorporating an array of phase change memory cells is the array architecture, including access transistors, word lines and bit lines through which individual memory cells are accessed for read, set and reset operations. Typical array architectures are shown in Lung, “Spacer Chalcogenide Memory Method and Device,” U.S. Pat. No. 6,864,503; and Wu, “Self-Aligned Resistive Plugs for Forming Memory Cell with Phase Change Material,” U.S. Pat. No. 6,545,903. In the &#39;503 patent, the architecture for an array of phase change memory cells is shown in  FIG. 3 , and includes access transistors (called isolation transistors in the &#39;503 patent) that are formed on a semiconductor substrate, and a conductive plug formed in a contact via is formed between the drain of each access transistor and an electrode in the corresponding phase change memory cell. The array size is limited by the need to space the access transistors apart from one another in the semiconductor substrate, or to otherwise isolate adjacent access transistors. One array architecture that provides for high density is shown in Kang, et al., “A 0.1 mM 1.8V 256 Mb 66 MHz Synchronous Burst PRAM”, ISSCC, February, 2006. 
     It is desirable therefore to provide devices with an array architecture supporting high-density devices, and to facilitate applying relatively high currents to selected devices for reset operations at low voltages. 
     SUMMARY OF THE INVENTION 
     A first embodiment of the present invention is a self aligning memory device, the memory device of the type comprising a memory element switchable between electrical property states by the application of energy. The memory device includes a substrate and first, second, third and fourth word lines positioned over the substrate and oriented in a first direction. The word lines have tops and sides, at least the sides covered with a dielectric material. The dielectric material defines first and second and third gaps therebetween. The memory device also includes terminals of access devices formed within the substrate, a first terminal located directly under the second gap and a second terminal formed directly under each of the first and third gaps. First and second source lines are positioned within the first and third gaps and are electrically connected to respective ones of the second terminals. A first electrode is positioned within the second gap and is electrically connected to the first terminal. A memory element is within the second gap and is positioned over and electrically connected to the first electrode. A second electrode is positioned over and in contact with the memory element. The second electrode is oriented in a second direction, the second direction being perpendicular to the first direction. The first contact, the first electrode, the memory element and the second electrode are self aligning. In some embodiments the access devices comprise first and second transistors with a common drain. In some embodiments the word lines are separated by a distance equal to a minimum lithographic distance so that at least a portion of the gap is a sub lithographically dimensioned gap with at least a portion of the memory element having a sub lithographically dimensioned width. 
     One example of a method for making a self aligning memory device, the memory device of the type comprising a memory element switchable between electrical property states by the application of energy, carried out according to the present invention includes the following. First, second, third and fourth word line conductors are formed on a substrate, each word line conductor having a word line top and word line sides. Dielectric sidewall spacers are formed on the word line sides, the sidewall spacers spaced apart from one another by first, second and third gaps with the substrate exposed at the gaps. First and second terminals of an access device are formed within the substrate at the first, second and third gaps. A first terminal is located directly under the second gap and a second terminal is formed directly under each of the first and third gaps. A source line is formed in the first and third gaps and is electrically connected to respective ones of the second terminals. A first electrode is formed in the second gap and is electrically connected to the first terminal. A memory material is deposited in the second gap to form a memory element in electrical contact with the first electrode. A second electrode is formed to be in electrical contact with the memory element. In some embodiments the dielectric sidewall spacers forming step is carried out so that the word line conductors are separated by a distance equal to a minimum lithographic distance with the dielectric sidewall spacers defining the first, second and third gaps with at least a portion of the gaps being a sub lithographically dimensioned gaps; in such embodiments the memory element has a width defined by the second gap with at least a portion of the width being a sub lithographically dimensioned width. 
     The memory cell and access control device structure described enables dense, high capacity memory arrays using phase change memory cells operating at low voltages. 
     Other aspects and advantages of the invention are described below with reference to the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram for a memory array comprising phase change memory cells with dual source lines and dual word lines. 
         FIG. 2  is a block diagram of an integrated circuit device including a phase change memory array with dual source lines and dual word lines, and other circuitry. 
         FIG. 3  is a cross-sectional view of a memory device may according to the invention, the view taken parallel to and through a bit line; 
         FIG. 4  is top plan view of the structure of  FIG. 3 ; 
         FIGS. 5 ,  6  and  7  are side cross-sectional views taken along lines  5 - 5 ,  6 - 6  and  7 - 7  of  FIG. 3 ; 
         FIG. 8  illustrates word lines, covered by dielectric layers, on a substrate; 
         FIG. 8A  shows a nitride layer deposited on the structure of  FIG. 8 ; 
         FIG. 8B  illustrates the result of etching the structure of  FIG. 8A , leaving sidewall spacers on the sides of the word lines and creating sidewall gaps between the sidewall spacers, and the formation of sources and drains aligned with the sidewall gaps; 
         FIG. 9  shows a conductive layer deposited on the structure of  FIG. 8 ; 
         FIG. 10  shows result of etching back the conductive layer of  FIG. 9 ; 
         FIG. 11  shows a dielectric layer deposited on the structure of  FIG. 10 ; 
         FIG. 12  shows the structure of  FIG. 11  after chemical mechanical polishing; 
         FIG. 13  shows the mask formed on the structure of  FIG. 12 ; 
         FIG. 14  illustrates the result of etching the structure of  FIG. 13 ; 
         FIG. 15  shows a memory material deposit on the structure of  FIG. 14 ; 
         FIG. 16  shows the results of a memory material chemical mechanical processing step; 
         FIG. 17  shows a metal bit line layer deposited on the structure of  FIG. 16 ; 
         FIGS. 18 and 19  show the structure of  FIG. 17  with a second mask formed on the metal bit line layer of  FIG. 17 ; 
         FIG. 20  shows result of etching the structure of  FIG. 19  down to the substrate creating bit line stacks; 
         FIG. 21  is a side view of the structure of  FIG. 20  after removal of the second mask; 
         FIG. 22  is a schematic diagram for an alternative memory array comprising phase change memory cells with dual word lines which also act as source lines; and 
         FIG. 23  is a flow chart for a method of operating memory devices as described herein. 
     
    
    
     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. 
     A detailed description of phase change memory cells with dual access devices, arrays of such memory cells, and methods for manufacturing and operating such memory cells, is provided with reference to  FIGS. 1-23 . 
       FIG. 1  is a schematic illustration of a memory array, with dual word lines and dual source lines, and self-aligned contact/memory element between memory cell electrodes and the access array, which can be implemented as described herein. Four memory cells including memory elements  35 ,  36 ,  45  and  46  are illustrated, representing a small section of an array that can include millions of memory cells. Memory cells including memory elements  35  and  36  are representative. As can be seen, the memory cell including memory element  35  includes a top electrode  34  and a bottom electrode  32 , with the memory element  35  comprising a phase change material in electrical communication with the top and bottom electrodes  34 ,  32 . Likewise, the memory cell including memory element  36  includes a top electrode  37  and a bottom electrode  33 . The top electrodes  37 ,  34  are coupled to a bit line  41 . The memory cells including memory elements  45  and  46  are connected in a similar fashion. As shown in  FIG. 3 , each memory element has a phase change region  38  near its bottom electrode. 
       FIG. 1 , the common source lines  28   a ,  28   b  and  28   c , the word lines  23   a ,  23   b ,  23   c  and  23   d  are arranged generally parallel in the Y-direction (as opposed to the normal convention of illustrating word lines arranged in the X-direction). Bit lines  41  and  42  are arranged generally parallel in the X-direction. Thus, a Y-decoder and a word line driver  24 , having set, reset and read modes, are coupled to the word lines  23   a ,  23   b ,  23   c ,  23   d . Bit line current sources  43  for set, reset and read modes, a decoder and sense amplifiers (not shown) are coupled to the bit lines  41  and  42 . The common source lines  28   a ,  28   b  and  28   c  are coupled to source line termination circuit  29 , such as a ground terminal. The source line termination circuits may include bias circuits such as voltage sources and current sources, and decoding circuits for applying bias arrangements, other than ground, to the source lines in some embodiments. 
     Each memory cell in the illustrated array is coupled to first and second access transistors. Thus, the bottom electrode  32  of the memory cell including memory element  35  is coupled to the drain D 53  of access transistor  53  and to the drain D 52  of access transistor  52 . The source terminals S 52 , S 53  of access transistors  52  and  53  are coupled to respective source lines  28   a  and  28   b . The gate G 52  of access transistor  52  is coupled to the word line  23   a . The gate G 53  of access transistor  53  is coupled to the word line  23   b . In a similar fashion, the bottom electrode  33  of the memory cell including memory element  36  is coupled to the drain of access transistor  50  and to the drain of access transistor  51 . The source terminals of access transistors  50  and  51  are coupled to respective source lines  28   b  and  28   c . The gate of access transistor  50  is coupled to the word line  23   c . The gate of access transistor  51  is coupled to the word line  23   d.    
     It can be seen that the common source line  28   b  is shared by two rows of memory cells, where a row is arranged in the Y-direction in the illustrated schematic. 
     In operation, current sources  43  and the word line drivers  24  operate in a lower current read mode, one or more intermediate current set modes, and a higher current reset mode. During the higher current reset mode, a current path  51   a  through the selected memory cell (e.g. memory cell including element  35 ) is established by applying a current to the bit line  41 , and voltages on the word line conductors  23   a  and  23   b  sufficient to turn on the access transistors  52  and  53 , so that the current flows through both source line  28   a  and source line  28   b . The dual word line conductors  23   a ,  23   b  and the dual source line conductors  28   a  and  28   b , establish a lower resistance path to ground than can be established using only a single source line conductor. Therefore, the current source used during the higher current reset mode can operate at lower voltages, and more efficiently couple power to the memory element needed to achieve the reset state. 
     In contrast, during the lower current read mode, a current path  51   b  through the selected memory cell (see the memory cell including memory element  36 ) is established by applying a current to the bit line  41 , and a voltage on the word line conductor  23   d  sufficient to turn on the access transistor  51  and provide for current flow to the source line conductor  28   c . The voltage on the word line conductor  23   c  is kept at a level that is sufficient to turn off the access transistor  50 , and block current flow to the source line conductor  28   b . This provides for a lower capacitance in the circuitry used in the lower current read mode, and allows for faster operation of the read mode. 
     During the set mode, used for one or more intermediate current levels, only one access transistor is enabled, as just described with respect to the read mode. Alternatively, during the set mode, two access transistors may be used as described above with respect to the reset mode, depending on the design goals of the particular implementation. 
     Embodiments of the memory cell include phase change based memory materials, including chalcogenide based materials and other materials, for the memory element  35 ,  36 ,  45 ,  46 . 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 group IV 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 Te a Ge b Sb 100-(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 Ge 2 Sb 2 Te 5 , GeSb 2 Te 4  and GeSb 4 Te 7 . (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 empirically or by modeling, and 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 Ge 2 Sb 2 Te 5 . 
     The following are short summaries describing four types of resistive memory materials. 
     1. Chalcogenide Material 
     
         
         
           
             Ge x Sb y Te z    
             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, N 2 , 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
 
             Pr x Ca y MnO 3    
             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, N 2 , O 2 , 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 N 2  ambient or O 2 /N 2  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
 
             Ni x O y ; Ti x O y ; Al x O y ; W x O y ; Zn x O y ; Zr x O y ; Cu x O y ; 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, N 2 , O 2 , and/or He, etc. at the pressure of 1 mtorr˜100 mtorr, using a target of metal oxide, such as Ni x O y  Ti x O y ; Al x O y ; W x O y ; Zn x O y ; Zr x O y ; Cu x O y ; 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 N 2  ambient or O 2 /N 2  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/O 2 , Ar/N 2 /O 2 , pure O 2 , He/O 2 , He/N 2 /O 2  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 N 2  ambient or O 2 /N 2  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 O 2  or N 2 /O 2  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 O 2  or Ar/O 2  mixed gas or Ar/N 2 /O 2  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, C 60 , 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 ton. The wafer temperature ranges from room temperature to 200 C. 
             The post deposition annealing treatment with vacuum or N 2  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. 
           
         
       
    
     An exemplary method for forming chalcogenide material uses the PVD-sputtering or magnetron-sputtering method with source gas(es) of Ar, N 2 , and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. 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 tens of volts to several hundreds of volts is also used. On the other hand, the combination of DC bias and the collimator can be used simultaneously. 
     A post-deposition annealing treatment in vacuum or in an N 2  ambient is optionally performed to improve the crystallize state of chalcogenide material. The annealing temperature typically ranges from 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. It is expected that some materials are suitable with even lower thicknesses. 
       FIG. 2  is a simplified block diagram of an integrated circuit according to an embodiment of the present invention. The integrated circuit  75  includes a memory array  60  implemented using phase change memory cells with self-aligned contacts and isolation lines, on a semiconductor substrate. A row decoder  61  having read, set and reset modes is coupled to a plurality of pairs of word lines  62 , and arranged along rows in the memory array  60 . A column decoder  63  is coupled to a plurality of bit lines  64  arranged along columns in the memory array  60  for reading, setting and resetting memory cells in the memory array  60 . Addresses are supplied on bus  65  to column decoder  63  and row decoder  61 . Sense amplifiers and data-in structures in block  66 , including current sources for the read, set and reset modes, are coupled to the column decoder  63  via data bus  67 . Data is supplied via the data-in line  71  from input/output ports on the integrated circuit  75  or from other data sources internal or external to the integrated circuit  75 , to the data-in structures in block  66 . In the illustrated embodiment, other circuitry  74  is included on the integrated circuit  75 , such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by the phase change memory cell array. Data is supplied via the data-out line  72  from the sense amplifiers in block  66  to input/output ports on the integrated circuit  75 , or to other data destinations internal or external to the integrated circuit  75 . 
     A controller implemented in this example using bias arrangement state machine  69  controls the application of bias arrangement supply voltages and current sources  68 , such as read, set, reset and verify voltages and or currents for the word lines and bit lines, and controls the dual word line/source line operation using an access control process such as described below with reference to  FIG. 23 . The controller can be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, the controller comprises a general-purpose processor, which may be implemented on the same integrated circuit, which executes 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 the controller. 
       FIGS. 3-7  depict structure for a phase change random access memory (PCRAM) cells and self-aligned contacts/memory elements/source lines with access transistors, manufactured as described with reference to  FIGS. 8-21 . The cells are formed on a semiconductor substrate  20 . Access transistor pairs  52 ,  53  are formed by n-type terminals acting as source regions S 52 , S 53  and an n-type terminal acting as a common drain region D 52 /D 53  in a p-type substrate  20 . Polysilicon word lines  23   a ,  23   b  form the gates G 52 , G 53  of the access transistors  52 ,  53  for memory element  35 . The pattern repeats along the column of access transistors. The terminals S 52 , S 53  and D 52 /D 53  of the access transistors  52 ,  53  comprise doped regions in the substrate which can be implemented by self-aligned implant processes using the polysilicon word lines  23   a ,  23   b  as masks. The self-alignment arises in the embodiment described herein by using the word line structures to define gaps therebetween, the gaps used to locate the access transistor terminals over which plugs, constituting source lines and bottom electrodes, are formed, with memory elements and a bit lines formed over the bottom electrodes without requiring an additional lithographic mask to locate the elements. 
     A gate oxide layer  84  is formed in substrate  20  with word lines  23  formed on layer  84 . Each word line  23  is covered by a cap  80  surrounded on either side by a spacer  82 . Cap  80  and spacer  82  are typically nitride. A dielectric  86 , typically SiO 2 , is formed on top of the common source lines  28 . Bit lines  41 ,  42  are formed on top of memory elements  35 ,  36 , dielectric  86  and nitride cap  80 . Dielectric-filled trenches  88  are formed perpendicular to the bit lines and thus separate the bit lines as well as the memory elements, bottom electrodes and drains as shown in  FIG. 5 . The dielectric material within trenches  88  is omitted from  FIGS. 4-7  for clarity. Dielectric-filled trenches  88  are seen in  FIG. 6  to also separate nitride caps  80  and bit lines  41  but not the common source lines  28  or the sources of the access transistors. Similarly, dielectric-filled trenches  88  are seen in  FIG. 7  to separate bit lines  41  and dielectric  86  but not the word lines  23  or the gate oxide layer  84 . Providing dielectric-filled trenches  88  having different depths may be achieved in a conventional or unconventional manner, such as by the use of appropriate etch stop layers. 
       FIG. 8  illustrates result of having deposited gate oxide layer  84  on substrate  20  followed by a layer of polysilicon followed by a layer of a hard mask dielectric material, such as SiN, after which suitable etching procedures create word lines  23  and dielectric layers  81  covering the tops of the word lines. Word lines  23  and their associated dielectric layers  81  are separated by a distance  79 , distance  79  preferably being equal to the minimum lithographic features size for the lithographic procedures used to create the word lines and dielectric layers. 
       FIG. 8A  shows a nitride layer  80  having been deposited on the structure of  FIG. 8 .  FIG. 8B  illustrates the result of etching the structure of  FIG. 8A . The etching process causes nitride layer  80  to be removed down to dielectric layers  81  and down to substrate  20  leaving sidewall spacers  82  on the sides of word lines  23 . Sidewall spacers  82  are separated by sidewall gap distances  14  and create sidewall gaps  12  between sidewall spacers  82 . This is followed by the formation of source and drain terminals S, D for the access transistors using nitride-covered word lines as masks aligned with the sidewall gaps. When distance  79  is a minimum lithographic feature size, at least a portion of the resulting gap  12  is a sub lithographically dimensioned gap with the corresponding sidewall gap distance  14  being a sub lithographically dimensioned distance. The use of gaps  12  created by sidewall spacers  82  to guide the formation of sources and drains for the access transistors, as well as additional device elements created beneath gaps  12 , enables the process to be a fully self aligning process. Being a fully self aligning process reduces the need for additional masks thereby simplifying manufacturing procedures. The resulting array of memory devices is a contact electrode-free array, meaning it eliminates the need for additional lithographic steps to create additional contacts. The elimination of the need for such additional contacts helps to reduce the overall size of the memory cell. In addition, because the least a portion of each memory element  35  has a sub lithographically dimensioned width, the cell size can be reduced from a typical cell size of about 16F 2  to a cell size as small as 8F 2  to 4F 2 , typically about 6 F 2 , F referring to the minimum features size for the manufacturing procedure used. 
     Thereafter a conductive layer  90 , typically tungsten, or some other appropriate conductor such as Ti or TiN, is deposited on the structure of  FIG. 8  creating the structure of  FIG. 9 . Conductive layer  90  is etched back as shown in  FIG. 10  to create conductor elements  91 . An isolation dielectric layer  92 , typically silicon dioxide, is deposited on the structure of  FIG. 10  to create the structure of  FIG. 11 . This is followed by chemical mechanical polishing resulting in the polished dielectric layer  93  of  FIG. 12 . 
       FIG. 13  shows a mask  94  formed over the structure of  FIG. 12 , the mask aligned with what will be the sources for the access transistors.  FIG. 14  shows results of etching away those portions of polished dielectric layer  93  not covered by mask  94  leaving etched dielectric  96  in the region above the sources while exposing those conductor elements  91  above the drains and not covered by dielectric material.  FIG. 15  shows the result of depositing a phase change memory material  98  on the structure of  FIG. 14  with the memory maternal contacting the conductor elements  91  overlying him in contact with the drains.  FIG. 16  illustrates result of a chemical mechanical polishing step of the memory material  98  of  FIG. 15  to create a first subassembly. 
       FIG. 17  shows the result of depositing a metal bit line layer  100  onto the structure of  FIG. 16  to create a second subassembly.  FIGS. 18 and 19  illustrate the structure of  FIG. 17  with a second mask  102  formed on metal bit line layer  100 .  FIG. 20 , similar to  FIG. 5 , shows result of etching the structure of  FIG. 19  down to substrate  20  using second mask  102  thereby creating bit line stacks  104  separated by trenches  106 . As indicated in  FIGS. 4 ,  6  and  7 , this etching stops at dielectric layers  81  overlying word lines  23  and dielectric  86  overlying source lines  28 .  FIG. 21  is a side view of the structure of  FIG. 20  after removal of second mask  102 . This is followed by a dielectric fill-in procedure and chemical mechanical polishing resulting in the structure of  FIGS. 3-7 . 
       FIG. 22  illustrates an alternative implementation, in which the access devices comprise diodes.  FIG. 22  shows a first memory cell  350 , comprising a top electrode  334 , a memory element  335  and a bottom electrode  332 . A second memory cell  351  is also shown in the figure. A first word line conductor  321  and a second word line conductor  322  are coupled to word line drivers  320  which are operable in the set, reset and read modes as described above. A first bit line  341  and a second bit line  342  are coupled to biasing sources  340  for the set, reset and read modes, as well as sense amplifiers and data-in structures (not shown). A diode  315  and a diode  316  act as dual access devices for memory cell  350  while diodes  317  and  318  act as dual access devices for memory cell  351 . The bottom electrode  332  of the memory cell  350  is coupled to the anode of the diode  315 , and to the anode of the diode  316 . The cathode of diode  315  is coupled to the word line conductor  321 , and the cathode of the diode  316  is coupled to the word line conductor  322 . In this embodiment, the word line conductors  321  and  322  act as both word lines and a source lines, in contrast to the embodiment of  FIG. 1  which includes separate conductors as the source lines ( 28   a ,  28   b ,  28   c ) and the word lines ( 23   a ,  23   b ,  23   c ,  23   d ). 
     In operation, during the reset mode both word lines  321  and  322  are set to a low voltage such as ground, or other voltage sufficient cause the diodes  315 ,  316  to be conductive. The word lines  321  and  322  act as source lines in this mode, and current flows through the memory cell  350  along both word line conductors  321 ,  322  to establish a relatively low resistance path for the reset mode. During a read mode, only one of the word lines  321 ,  322  is set to a low voltage. During a set mode, only one of the word lines  321 ,  322  is set to a low voltage. As mentioned above, in some embodiments both of the word lines  321 ,  322  can be set to the low voltage during a set mode. 
       FIG. 23  illustrates a basic method of operating a memory device such as that illustrated in  FIG. 3  having dual word line/source line structures. The process illustrated in  FIG. 23  is executed under control of the state machine  69  of  FIG. 2 , for the illustrated embodiment. The process is executed upon a command to access a selected memory cell ( 500 ). Upon receiving the command, the process determines the mode of access ( 501 ). If the mode of access is the read mode, then the control logic enables the word line drivers to drive to the left word line with the voltage sufficient to enable current flow through the left access device and the selected memory cell, while leaving the right word line with a voltage sufficient to prevent current flow through the right access device ( 502 ). Next a read bias pulse is applied to the bit line corresponding with the selected memory cell ( 503 ). Finally, the data of the selected memory cell is sensed ( 504 ). 
     If the mode of access is the set mode, then the operation is similar. The control logic enables the word line drivers to drive to the left word line with the voltage sufficient to enable current flow through the left access device and the selected memory cell, while leaving the right word line with a voltage sufficient to prevent current flow through the right access device ( 505 ). Next, a set bias pulse is applied to the bit line corresponding with the selected memory cell ( 506 ). Finally, the data of the selected memory cell is verified ( 507 ). 
     If the mode of access is the reset mode, then the control logic enables the word line drivers to drive both the left word line and the right word line with a voltage sufficient to enable current flow through the left and right access devices in parallel ( 508 ). Next, a reset pulse is applied to the bit line ( 509 ). Finally, the data of the selected memory cell is verified ( 510 ). 
     As mentioned above, in an alternative embodiment, during the set mode, or during one or more of the set modes for multilevel cells, the control logic can enable driving both word lines in the manner described for the reset mode. 
     It will be understood that a wide variety of materials can be utilized in implementation of the structure illustrated in  FIG. 3 . For example, copper metallization can be used. Other types of metallization, including aluminum, titanium nitride, and tungsten based materials can be utilized as well. Also, non-metal conductive material such as doped polysilicon can be used. The electrode material in the illustrated embodiment is preferably TiN or TaN. Alternatively, the electrodes may be TiAlN or TaAlN, or may comprise, for further examples, one or more elements selected from the group consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, and Ru and alloys thereof. The various dielectric materials comprise silicon dioxide, a polyimide, silicon nitride or other dielectric fill materials. In embodiments, the fill layer comprises a relatively good insulator for heat as well as for electricity, providing thermal and electrical isolation for the phase change memory elements. 
     The process may involve formation of a low-temperature dielectric, such as a silicon nitride layer or silicon oxide layer, using a process temperature less than about 200 degrees C. One suitable process is to apply silicon dioxide using plasma enhanced chemical vapor deposition PECVD. 
     In some situations that may be desirable to deposit dielectric material using a higher temperature process such as high-density plasma chemical vapor deposition HDP CVD. Also, various masks may be formed and patterned in a mask lithographic process where the minimum feature size may be on the order of 0.2 microns (200 nm), 0.14 microns, or 0.09 microns in current mask lithographic processes. Embodiments of the process can be adapted to narrower minimum feature sizes as lithographic processes advance. Also, sub-lithographic processes may be employed, achieving line widths on the order of 40 nm or less. 
     In some embodiments a structure for thermally insulating memory elements  35  is provided, in addition to or instead of the disclosed dielectric materials. Representative materials for a layer of thermally insulating material 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 layer include SiO 2 , SiCOH, polyimide, polyamide, and fluorocarbon polymers. Other examples of materials which are candidates for use for the thermally insulating layer include fluorinated SiO 2 , 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 in the dielectric fill formed adjacent to the memory elements for thermal insulation. A single layer or combination of layers can provide thermal and electrical insulation. 
     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 incorporated by reference. What is claimed is: