Resistance variable 'on' memory

[0001] The invention relates to the field of random access memory (RAM) devices formed using a resistance variable material, and in particular to the operation of resistance variable memory elements used in memory devices, for instance, programmable conductor random access memory (PCRAM) devices.

[0002] A well known semiconductor component is semiconductor memory, such as a random access memory (RAM). RAM permits repeated read and write operations on memory elements. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Non-limiting examples of RAM devices include dynamic random access memory (DRAM), synchronized dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMS and SDRAMS also typically store data in capacitors which require periodic refreshing to maintain the stored data.

[0003] In recent years, the number and density of memory elements in memory devices have been increasing. Accordingly, the size of each element has been shrinking, which in the case of DRAMs also shortens the memory element's data holding time. Typically, a DRAM memory device relies on element capacity for data storage and receives a refresh command in a conventional standardized cycle, about every 100 milliseconds. However, with increasing element number and density, it is becoming more and more difficult to refresh all memory elements at least once within a refresh period. In addition, refresh operations consume power.

[0004] Recently programmable conductor memory elements have been investigated for suitability as semi-volatile and non-volatile random access memory elements. Kozicki et al. in U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796, discloses a programmable conductor memory element including an insulating dielectric material formed of a chalcogenide glass disposed between two electrodes. A conductive material, such as silver, is incorporated into the dielectric material. The resistance of such material can be changed between high resistance and low resistance states. The programmable conductor memory is normally in a high resistance state when at rest. A write operation to a low resistance state is performed by applying a voltage potential across the two electrodes. The mechanism by which the resistance of the memory element is changed is not fully understood. In one theory suggested by Kozicki et al., the conductively-doped dielectric material undergoes a structural change at a certain applied voltage with the growth of a conductive dendrite or filament between the electrodes, effectively interconnecting the two electrodes and setting the memory element in a low resistance state. The dendrite is thought to grow through the resistance variable material in a path of least resistance.

[0005] No matter what the mechanism, the low resistance state will remain in tact for days or weeks after the voltage potentials are removed. Such material can be returned to its high resistance state by applying a reverse voltage potential between the electrodes of at least the same magnitude as used to write the memory element to the low resistance state. Again, the highly resistive state is maintained once the voltage potential is removed. This way, such a device can function, for example, as a resistance variable memory element having two resistance states, which can define two logic states.

[0006] Since the typical non-programmed state of a resistance variable memory element is the high resistance state, the memory element is programmed by an applied voltage to place the memory element into a low resistance state. To provide added memory design flexibility, it would be desirable to have a resistance variable memory element that is in a low resistance state when not programmed and which switches to a high resistance state when programmed.

[0007] The invention relates to a resistance variable memory element and a method and apparatus for stabilizing the resistance variable memory element in a non-programmed low resistance state. This is achieved in one embodiment by providing a high work function first electrode, such as a platinum electrode, on one side of a chalcogenide glass and a lower work function second electrode on an opposite side of the glass. The electrodes cause the memory element to automatically write to a predetermined state in the absence of an applied voltage. In one preferred embodiment, a platinum electrode and a silver electrode are used as the first and second electrodes to initiate and stabilize the resistance variable memory element in a non-programmed low resistance state.

[0008] These and other features and advantages of the invention will be better understood from the following detailed description, which is provided in connection with the accompanying drawings.

[0009] FIG. 1 is a cross-sectional view of a resistance variable memory element according to an embodiment of the invention.

[0010] FIG. 2 is a diagram of a stabilizing circuit according to the invention.

[0011] FIG. 3 is a diagram of a program circuit according to the invention.

[0012] FIG. 4 is a diagram of an erase circuit according to the present invention.

[0013] FIG. 5 illustrates a computer system having one or more memory devices that contains resistance variable memory elements according to the invention.

[0014] In the following detailed description, reference is made to various specific embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made without departing from the spirit or scope of the invention.

[0015] The term substrate used in the following description may include any supporting structure, including, but not limited to, a semiconductor substrate that has an exposed substrate surface. A semiconductor substrate should be understood to include silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. When reference is made to a semiconductor substrate or wafer in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation.

[0016] The term silver is intended to include not only elemental silver, but silver with other trace metals or in various alloyed combinations with other metals as known in the semiconductor industry, as long as such silver alloy is conductive, and as long as the physical and electrical properties of the silver remain unchanged. The term platinum is also intended to include not only elemental platinum, but platinum with other trace metals or in various alloyed combinations with other metals as known in the semiconductor industry, as long as such platinum alloy is conductive, and as long as the physical and electrical properties of the platinum remain unchanged.

[0017] The term silver-selenide is intended to include various species of silver-selenide, including some species which have a slight excess or deficit of silver, for instance, Ag₂Se, Ag_2xSe, and Ag_2xSe.

[0018] The term semi-volatile memory element is intended to include any memory element which is capable of maintaining its memory state after power is removed from the memory element for a prolonged period of time. Thus, semi-volatile memory elements are capable of retaining stored data after the power source is disconnected or removed. The term semi-volatile memory as used herein includes not only semi-volatile memory elements, but also non-volatile memory elements.

[0019] The term chalcogenide glass is intended to include glasses that comprise an element from group VIA (or group 16) of the periodic table. Group VIA elements, also referred to as chalcogens, include sulfur (S), selenium (Se), tellurium (Te) and polonium (Po), and oxygen (O).

[0020] The term resistance variable material is intended to include chalcogenide glasses, and chalcogenide glasses comprising a metal, such as silver. For instance the term resistance variable material includes silver doped chalcogenide glasses, silver-germanium-selenide glasses, and chalcogenide glasses comprising a silver selenide layer.

[0021] The term resistance variable memory element is intended to include any memory element, including programmable conductor memory elements, semi-volatile memory elements, and non-volatile memory elements which exhibit a resistance change in response to an applied voltage.

[0022] The invention relates to a resistance variable memory element and a method and apparatus for turning the resistance variable memory element on to a low resistance state and stabilizing the resistance variable memory element in the on state. The invention more particularly relates to a resistance variable memory element, which incorporates a galvanic element that provides a contact potential across the resistance variable memory element to initiate the resistance variable memory element to write to the low resistance state. The electrode materials on opposite sides of the resistance variable memory element are chosen to create a contact potential between two electrodes which turn the resistance variable memory element on and stabilize the resistance variable memory element in an on or low resistance state. The resistance variable memory element of the invention may be used in memory applications as well as in creating various CMOS type circuits which may require complementary on and off devices.

[0023] The invention will be described as set forth in an exemplary embodiment illustrated in FIGS. 1-6 in the context of a random access memory. However, it should be understood that the invention may be used in other types of memory devices, in CMOS circuits and in other areas when a normally on device may be used. Also, other embodiments may be used and structural or logical changes may be made to the described and illustrated embodiment without departing from the spirit or the scope of the invention.

[0024] FIG. 1 illustrates an exemplary construction of a resistance variable memory element. Resistance variable memory elements 100 in accordance with the invention are generally fabricated over a substrate 62, which may be a semiconductor substrate and comprise a first insulating layer 60 formed over the substrate 62. The substrate 62 may be used to fabricate access circuitry for operating a memory device containing an array of resistance variable memory elements. A first metal electrode 52 is formed over the first insulating layer 60 and within a second insulating layer 53, a third insulating layer 68 is formed over the first electrode 52 and second insulating layer 53, and a resistance variable material 58 is disposed in an opening in the third insulating layer 68 such that the resistance variable material 58 is in contact with the first electrode 52. A second metal electrode 54 is formed over the resistance variable material 58.

[0025] The resistance variable material 58 may be formed over the first electrode 52 to dimensions (i.e., length, thickness, and width) suitable to produce desired electrical characteristics of the memory element 100. In an exemplary embodiment, the resistance variable material 58 comprises a silver-germanium-selenide chalcogenide glass. Preferably the silver-germanium-selenide chalcogenide glass composition has a GeSe stoichiometry of from about Ge₂₀Se₈₀ to about Ge₄₀Se₆₀ and more preferably has a stoichiometry of Ge₂₅Se₇₅. The silver concentration may vary widely, depending on the GeSe stoichiometry. Generally the silver concentration is represented by the formula (Ge_xSe_100x)_100yAg_y, wherein y typically falls somewhere in the range of about 15 to 34, and is more preferably 33.

[0026] One method of incorporating the silver into the germanium selenide material is to initially form the germanium-selenide glass and then deposit a thin layer of silver upon the glass, for example by sputtering, physical vapor deposition, or other known technique in the art. The layer of silver is irradiated, preferably with electromagnetic energy at a wavelength less than 600 nanometers, so that the energy passes through the silver and to the silver/glass interface, to break a chalcogenide bond of the chalcogenide material such that the glass is doped with silver. Alternatively, the silver can be co-sputtered with the glass composition to produce a ternary glass.

[0027] The first and second insulating layers 60, 53 may be formed of a conventional insulating oxide, such as silicon dioxide (SiO₂), or a low dielectric constant material such as, for example, BPSG polyimide, spin-on-polymers (SOP), parylene, flare, polyarylethers, polytetrafluoroethylene, benzocyclobutene (BCB), SILK, fluorinated silicon oxide (FSG), NANOGLASS or hydrogen silsesquioxane, among others. The invention is not limited, however, to the above-listed materials and other insulating and/or dielectric materials known in the industry may be used. The first insulating layer 60 may be formed by any known deposition methods, such as sputtering by chemical vapor deposition (CVD), plasma enhanced CVD (PECVD) or physical vapor deposition (PVD), among others.

[0028] The third insulating layer may be formed, for example, between the first electrode 52 and the second electrode 54. The second insulating layer 68 may be formed by any known deposition methods, such as sputtering, chemical vapor deposition (CVD), plasma enhanced CVD (PECVD) or physical vapor deposition (PVD), among others. The third insulating layer 68 may comprise any suitable insulating material that provides a diffusion barrier for metals, such as silver. A preferred insulating material is silicon nitride, but those skilled in the art will appreciate that there are numerous suitable insulating materials. The thickness T of the third insulating layer 68 and chalcogenide glass 58 is in the range of from about 100 Angstroms to about 10,000 Angstroms and is preferably about 500 Angstroms.

[0029] The first electrode 52 may also be electrically connected to a source/drain region 81 of an access transistor 83 which is fabricated within and on substrate 62. Another source/drain region 85 may be connected by a bit line plug 87 to a bit line of a memory array. The gate of the transistor 83 may be part of a word line which is connected to a plurality of resistance variable memory elements just as the bit line 93 may be coupled to a plurality of resistance variable memory elements through respective access transistors. This bit line 93 may be formed over a fourth insulating layer 91 and may be formed of any conductive material, for example, a metal. As shown, the bit line 93 connects to the bit line plug 87 which in turn connects with access transistor 83.

[0030] In accordance with the invention, a differential work function is used to produce an electrochemical potential across the resistance variable memory element which causes the resistance variable memory element to write to an on state. The differential work function is achieved by providing a first electrode 52 comprising a metal of a first work function value V₁, in conjunction with a second electrode 54 having a second work function value V₂, such that a difference in work function _V between V₁ and V₂ is greater than the write potential of the resistance variable memory element V_W. Accordingly, _V(V₁V₂)>V_W. Preferably, any suitable metal having a high work function may be used for the first electrode, for example platinum and iridium. The second electrode 54 preferably comprises a low work function material, such as tungsten, tantalum, or silver. The only constraint is that the difference in work functions is of sufficient magnitude to produce an electrochemical potential across the resistance variable memory element, which is sufficient to cause the resistance variable memory element to write to a low resistance on state.

[0031] In accordance with one preferred embodiment of the invention, the first electrode 52 is a platinum containing metal and the second electrode 54 is a silver containing metal, or vice versa, so long as a platinum electrode is used in conjunction with a silver electrode. The silver containing electrode is more preferably silver. Accordingly, silver has an affinity for platinum, which may also cause the silver to migrate towards the platinum, as represented by arrows 95, in response to the difference in work function values, which is greater than the write potential of the resistance variable memory element and sufficient to cause the resistance variable memory element to write to an on state.

[0032] In FIG. 2, silver migration is represented by arrows 95 indicating that the conductor having a higher work function potential V₁, e.g., platinum, is provided for the first electrode 52 and the material having a lower work function potential V₂, e.g., silver, is provided for the second electrode 54; however, as noted these can be reversed.

[0033] As also shown in FIG. 2 , when an electrode 52, having a high work function value V₁ is coupled with an electrode 54, having a low work function value V₂, the resistance variable memory element may be spontaneously stabilized in the on state. In other words, the resistance variable memory element 100 automatically turns to an on or low resistance state. In its on state, the resistance variable memory element is undisturbed and typically has a low resistance of 20,000 ohms or less. Unless disturbed by application of a voltage across the electrodes 52, 54, the resistance variable memory element will stay in this on state.

[0034] Work function is the amount of energy required to move an electron from the bulk of a material to an infinite distance away (the vacuum). The work function is also defined as the energy distance between the vacuum level and the Fermi level of a material. The work function value can be used to gauge the resistance of a particular material to current flow. The direction in which silver tends to cross the resistance variable material 58 is controlled by the electrochemical difference between the high work function electrode and the low work function electrode. While not wishing to be bound to any specific theory, it is believed that because the work functions of the electrodes are different, there is a preferred direction of electrode tunneling, which causes the resistance variable memory element to write to a predetermined state. Thus, the differential electrochemical potential draws silver from the low work function metal towards the high work function metal.

[0035] Referring again to FIG. 2, a resistance variable memory element 100 is turned on by the difference in work functions of the materials for electrodes 52 and 54. The difference in work functions is sufficient to generate a voltage V₃ across the resistance variable memory element that causes the resistance variable memory element to self-write to an on state. It has been observed that a voltage V₃ as low as 0.15 volts may be sufficient for this purpose.

[0036] Referring now to FIG. 3, a resistance variable memory element may be erased to an off, high resistance state, by applying a reverse voltage to the resistance variable memory element 100 which has an absolute magnitude which exceeds that of V₃. It has been observed that a negative voltage, more negative than 0.15 volts, is typically sufficient. The negative bias causes the resistance variable memory element 100 to erase to a high resistance state, i.e., greater than 20,000 ohms, and preferably greater than 100,000 ohms. An exemplary method of erasing the resistance variable memory element 100 would include applying a voltage V_E of 0.15 volts to cause the resistance variable memory element to erase as shown in FIG. 3. In this way a resistance variable memory element may store a binary value as one of the two on and off resistance states of the resistance variable memory element.

[0037] Referring again to FIG. 3, since the resistance variable memory element 100 self-writes to a low resistance on state, all resistance variable memory elements in an array will naturally erase to the on state. Any resistance variable memory elements which are written to an off state by the application of a negative voltage having an absolute value greater than that of the electrochemical potential V₁ must be maintained in the off state by a periodic refresh voltage.

[0038] Although, a resistance variable memory element constructed in accordance with the invention can retain a stable on state without application of an external voltage, an external voltage is required to both write a resistance variable memory element to the off state and to maintain a resistance variable memory element in the off state. However, since it is sufficient to merely set up a negative voltage electric field to cause an off resistance variable memory element to maintain a high resistance off state, negligible current flows during a refresh or write operation, so less current or power is required to refresh a resistance variable memory element than is typically encountered to refresh a DRAM element. As shown in FIG. 4, a refresh can be obtained by reverse biasing an off resistance variable memory element at a voltage V_R, and/or duration, below the self-on threshold voltage V₃ of the resistance variable memory element. Thus, a resistance variable memory element written to the high resistance state by about 0.15 volts would be refreshed using 0.12 volts. Since the refresh voltage is supplied to all resistance variable memory elements simultaneously, the 0.12 volts are insufficient to turn off an on resistance variable memory element, but is sufficient to prevent an off resistance variable memory element from reverting to an on state.

[0039] FIG. 5 is a block diagram of an exemplary processor system 20 that includes a memory circuit 140. The memory circuit 140 includes one or more integrated circuit memory devices containing resistance variable memory elements. A processor system, such as a computer system, generally comprises a central processing unit (CPU) 22, such as a microprocessor, a digital signal processor, or other programmable digital logic device, which communicates directly or indirectly with one or more input/output (I/O) devices 26 over one or more buses 30. The memory circuit 140 communicates with the central processing unit 22 directly or through a memory conductor and over one or more busses 30.

[0040] In the case of a computer system, the processor system may include peripheral devices such as a floppy disk drive 26 and a compact disk read only memory (CD ROM) drive 28, which also communicate with CPU 22 over the bus 30. Memory circuit 140 is preferably constructed as one or more integrated circuit memory devices, each of which includes one or more resistance variable memory elements 100 (FIG. 2) and which is coupled to CPU 22. If desired, the memory circuit 140 may be combined with the processor, for example CPU 22, in a single integrated circuit. Although one architecture for a computer system is illustrated in FIG. 5, it should be understood that this is merely exemplary of many computer architectures which may employ a memory circuit 140 employing resistance variable memory elements in accordance with the invention.

[0041] Although the embodiments described above refer to the formation of only one resistance variable memory element 100, it should be understood that the invention contemplates the formation of any number of such resistance variable memory elements, which can be formed as one or more resistance variable memory element arrays.

[0042] The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.