18-04-2011, 01:02 PM
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NON-VOLATILE OVONICS UNIFIED MEMORY (OUM)
ABSTRACT
Nowadays, digital memories are used in each and every fields of day-to-day life. Semiconductors form the fundamental building blocks of the modern electronic world providing the brains and the memory of products all around us from washing machines to super computers. But now we are entering an era of material limited scaling. Continuous scaling has required the introduction of new materials.
Current memory technologies have a lot of limitations. The new memory technologies have got all the good attributes for an ideal memory. Among them Ovonic Unified Memory (OUM) is the most promising one. OUM is a type of non-volatile memory, which uses chalcogenide materials for storage of binary data. The term “chalcogen” refers to the Group VI elements of the periodic table. “Chalcogenide” refers to alloys containing at least one of these elements such as the alloy of germanium, antimony, and tellurium, which is used as the storage element in OUM. Electrical energy (heat) is used to convert the material between crystalline (conductive) and amorphous (resistive) phases and the resistive property of these phases is used to represent 0s and 1s.
To write data into the cell, the chalcogenide is heated past its melting point and then rapidly cooled to make it amorphous. To make it crystalline, it is heated to just below its melting point and held there for approximately 50ns, giving the atoms time to position themselves in their crystal locations. Once programmed, the memory state of the cell is determined by reading its resistance.
INTRODUCTION
The use of phase-change Chalcogenide alloy films to store data electrically and optically was first reported in 1968 and in 1972, respectively. Early phase-change memory devices used tellurium-rich, multi-component Chalcogenide alloys with a typical composition of Te81Ge15Sb2S2. Both the optical and electrical memory devices were programmed by application of an energy pulse of appropriate magnitude and duration. A short pulse of energy was used to melt the material, which was then allowed to cool quickly enough to “freeze in” the glassy, structurally disordered state. To reverse the process, somewhat lower- amplitude, longer-duration pulse was used to heat a previously vitrified region of the alloy to a temperature below the melting point, at which crystallization could occur rapidly. Differences in electrical resistivity and the optical constants between the amorphous and polycrystalline phases were used to store data. During the 1970s and 1980s, significant research efforts by many industrial and academic groups were focused on understanding the fundamental properties of Chalcogenide alloy amorphous semiconductors. Prototype optical memory disks and electronic memory device arrays also were announced, beginning in the early 1970s. Rapidly crystallizing Chalcogenide alloys were later reported by several optical memory research groups. These new material compositions, derived from the Ge-Te-Sb ternary system, did not phase segregate upon crystallization like the earlier Te-rich alloys, but instead exhibited congruent crystallization with no large-scale atomic motion.
In the 1990s, researchers at Energy Conversion Devices Inc. and Ovonyx Inc. Developed new thermally optimized phase-change memory device structures that exploited rapidly crystallizing Chalcogenide alloy materials to achieve increased programming speed and reduced programming current.
These devices could be programmed in 20 ns—about six orders of magnitude faster than the early phase-change memory cells, and their much lower programming current requirements permitted the design of memory arrays using memory bit access devices (transistors or diodes) fabricated at minimum litho-graphic dimensions. Ovonyx is now commercializing its phase-change memory technology called Ovonics Unified Memory (OUM) through a number of license agreements and joint development programs with semiconductor device manufacturers.
CHALCOGENIDES
The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity, and this forms the basis by which data are stored. The amorphous, high resistance state is used to represent a binary 0, and the crystalline, low resistance state represents a 1. Chalcogenide is the same material used in re-writable optical media (such as CD-RW and DVD-RW). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as Chalcogenide’s refractive index also changes with the state of the material.
The term “chalcogen” refers to the Group VI elements of the periodic table. “Chalcogenide” refers to alloys containing at least one Group VI element such as the alloy of germanium, antimony, and tellurium discussed here. Energy Conversion Devices, Inc. has used this particular alloy to develop a phase-change memory technology used in commercially available re-writeable CD and DVD disks. This phase-change technology uses a thermally activated, rapid, reversible change in the structure of the alloy to store data. Since the binary information is represented by two different phases of material it is inherently non-volatile, requiring no energy to keep the material in either of its two stable structural states.
Used in a binary mode, the two structural states of the Chalcogenide alloy, as shown in Figure, are an amorphous state (no long-range order of atoms) and a polycrystalline state (composed of many crystals, each having atoms placed in a repetitive order). Relative to the amorphous state, the polycrystalline state shows a dramatic increase in free electron density (similar to a metal). This difference in free electron density gives rise to a difference in reflectivity and, more importantly, resistivity. In the case of the rewriteable CD and DVD disk technology, this difference in reflectivity is used to read the state of each memory bit by directing a low-power laser at the material and detecting the amount of light reflected.
Glassy materials are produced by rapidly super cooling a liquid below its melting point to a temperature at which the atomic motion necessary for crystallization cannot readily occur. Chalcogenide alloys - materials containing one or more elements from Group VI of the periodic table—are typically good glass formers, in large part because the Group VI elements form pre-dominantly twofold-coordinated covalent chemical bonds that can produce linear, tangled, polymer like clusters in the melt. This increases the viscosity of the liquid, inhibiting the atomic motion necessary for crystallization. Many amorphous Chalcogenide alloys have been reported in the literature. The Ge2Sb2Te5 (GST 225) Chalcogenide alloy currently used in OUM rmemory devices melts at approximately 610C۫ and has a glass-transition temperature of 350C۫. In order to crystallize an amorphous region of GST 225, the material must be heated to a temperature somewhat below the melting point and held at this temperature for a time sufficient to allow the crystallization to occur. The compositional dependence of crystallization kinetics in the GeSbTe ternary system has been extensively studied and reported in the literature. OUM cells based on GST 225 that can be programmed (crystallized) to the “SET” state in <20 ns have been reported
Figure 3. Schematic temperature–time relationship during programming, in a phase-change rewriteable memory device. Tm and Tx are the amorphization and crystallization temperatures, respectively. The SET and RESET states of the memory correspond to a stored binary 1 or binary 0
• For R/W CD’s and DVD’s heat is supplied by use of a laser
• For integrated circuits heat is supplied by resistors
ELECTRONIC PROPERTIES OF CRYSTALLINE AND AMORPHOUS GST ALLOYS
Two special electronic properties of chalcogenide amorphous semiconductor alloys are required for the operation of OUM memory—the strong dependence of electrical resistivity on the structural state of the material and the high-field threshold switching phenomenon. Polycrystalline GST 225 alloy has a resistivity of ~25 mΩcm, while resistivity in the vitreous state is three orders of magnitude higher—sufficient to enable good memory read capability. Both structural states of the alloy are semiconductors with comparable energy band gaps. The band gap Eg is 0.7 eV in the amorphous state and 0.5 eV in the poly-crystalline state. The conductivity activation energy Ea is ~0.3 eV for the amorphous state and 0.02 eV for the polycrystalline state. In addition, the amorphous phase exhibits a very low, trap-limited hole mobility of ~2 ×10-5cm2/V s, while the polycrystalline phase shows band-type mobility of ~10 cm2/ V s. These large differences come about because of disorder-induced localized electronic states as originally described by Mott and by Cohen, Fritzsche, and Ovshinsky (CFO) and later by Kastner, Adler, and Fritzsche.When chalcogenide alloy semiconductors are amorphized, electronic energy levels originating in the valence and conduction bands are pulled into the what was originally the empty energy band gap of the crystalline material. As described by Mott– CFO, these new gap states are localized spatially and do not extend throughout the material. Consequently, carriers move through the amorphous material either by hopping among the localized states or by being successively thermally excited to spatially extended band states and then being trapped into localized states. This gives rise to a mobility gap—a range of energy between the valence and conduction bands in which carriers have small, trap-limited mobility. The later work by Kastner, Adler, and Fritzsche explained the observation that Ea≈ Eg/2 in terms of a large density of special negatively and positively charged traps that also result from structural disorder in amorphous chalcogenide alloys. Kastner argued that charged traps (valence alternation pairs) act like compensating dopant levels in a conventional crystalline semiconductor, effectively forcing the Fermi level to lie near the mid gap between the energy levels of the two types of traps. In the polycrystalline state, crystal vacancies are proposed to give rise to acceptor-like states that move the Fermi level close to the valence-band edge. This Fermi level position, plus the loss of the disorder produced trapping states, gives rise to the nearly degenerate p-type high conductivity of the polycrystalline state. Thus, the phase-change memory cell uses a reversible change in long-range atomic order (the amorphous-to-crystalline phase change) to modulate both the Fermi level position in the Chalcogenide alloy and the carrier mobility to change the cell’s resistance.
NON-VOLATILE OVONICS UNIFIED MEMORY (OUM)
ABSTRACT
Nowadays, digital memories are used in each and every fields of day-to-day life. Semiconductors form the fundamental building blocks of the modern electronic world providing the brains and the memory of products all around us from washing machines to super computers. But now we are entering an era of material limited scaling. Continuous scaling has required the introduction of new materials.
Current memory technologies have a lot of limitations. The new memory technologies have got all the good attributes for an ideal memory. Among them Ovonic Unified Memory (OUM) is the most promising one. OUM is a type of non-volatile memory, which uses chalcogenide materials for storage of binary data. The term “chalcogen” refers to the Group VI elements of the periodic table. “Chalcogenide” refers to alloys containing at least one of these elements such as the alloy of germanium, antimony, and tellurium, which is used as the storage element in OUM. Electrical energy (heat) is used to convert the material between crystalline (conductive) and amorphous (resistive) phases and the resistive property of these phases is used to represent 0s and 1s.
To write data into the cell, the chalcogenide is heated past its melting point and then rapidly cooled to make it amorphous. To make it crystalline, it is heated to just below its melting point and held there for approximately 50ns, giving the atoms time to position themselves in their crystal locations. Once programmed, the memory state of the cell is determined by reading its resistance.
INTRODUCTION
The use of phase-change Chalcogenide alloy films to store data electrically and optically was first reported in 1968 and in 1972, respectively. Early phase-change memory devices used tellurium-rich, multi-component Chalcogenide alloys with a typical composition of Te81Ge15Sb2S2. Both the optical and electrical memory devices were programmed by application of an energy pulse of appropriate magnitude and duration. A short pulse of energy was used to melt the material, which was then allowed to cool quickly enough to “freeze in” the glassy, structurally disordered state. To reverse the process, somewhat lower- amplitude, longer-duration pulse was used to heat a previously vitrified region of the alloy to a temperature below the melting point, at which crystallization could occur rapidly. Differences in electrical resistivity and the optical constants between the amorphous and polycrystalline phases were used to store data. During the 1970s and 1980s, significant research efforts by many industrial and academic groups were focused on understanding the fundamental properties of Chalcogenide alloy amorphous semiconductors. Prototype optical memory disks and electronic memory device arrays also were announced, beginning in the early 1970s. Rapidly crystallizing Chalcogenide alloys were later reported by several optical memory research groups. These new material compositions, derived from the Ge-Te-Sb ternary system, did not phase segregate upon crystallization like the earlier Te-rich alloys, but instead exhibited congruent crystallization with no large-scale atomic motion.
In the 1990s, researchers at Energy Conversion Devices Inc. and Ovonyx Inc. Developed new thermally optimized phase-change memory device structures that exploited rapidly crystallizing Chalcogenide alloy materials to achieve increased programming speed and reduced programming current.
These devices could be programmed in 20 ns—about six orders of magnitude faster than the early phase-change memory cells, and their much lower programming current requirements permitted the design of memory arrays using memory bit access devices (transistors or diodes) fabricated at minimum litho-graphic dimensions. Ovonyx is now commercializing its phase-change memory technology called Ovonics Unified Memory (OUM) through a number of license agreements and joint development programs with semiconductor device manufacturers.
CHALCOGENIDES
The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity, and this forms the basis by which data are stored. The amorphous, high resistance state is used to represent a binary 0, and the crystalline, low resistance state represents a 1. Chalcogenide is the same material used in re-writable optical media (such as CD-RW and DVD-RW). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as Chalcogenide’s refractive index also changes with the state of the material.
The term “chalcogen” refers to the Group VI elements of the periodic table. “Chalcogenide” refers to alloys containing at least one Group VI element such as the alloy of germanium, antimony, and tellurium discussed here. Energy Conversion Devices, Inc. has used this particular alloy to develop a phase-change memory technology used in commercially available re-writeable CD and DVD disks. This phase-change technology uses a thermally activated, rapid, reversible change in the structure of the alloy to store data. Since the binary information is represented by two different phases of material it is inherently non-volatile, requiring no energy to keep the material in either of its two stable structural states.
Used in a binary mode, the two structural states of the Chalcogenide alloy, as shown in Figure, are an amorphous state (no long-range order of atoms) and a polycrystalline state (composed of many crystals, each having atoms placed in a repetitive order). Relative to the amorphous state, the polycrystalline state shows a dramatic increase in free electron density (similar to a metal). This difference in free electron density gives rise to a difference in reflectivity and, more importantly, resistivity. In the case of the rewriteable CD and DVD disk technology, this difference in reflectivity is used to read the state of each memory bit by directing a low-power laser at the material and detecting the amount of light reflected.
Glassy materials are produced by rapidly super cooling a liquid below its melting point to a temperature at which the atomic motion necessary for crystallization cannot readily occur. Chalcogenide alloys - materials containing one or more elements from Group VI of the periodic table—are typically good glass formers, in large part because the Group VI elements form pre-dominantly twofold-coordinated covalent chemical bonds that can produce linear, tangled, polymer like clusters in the melt. This increases the viscosity of the liquid, inhibiting the atomic motion necessary for crystallization. Many amorphous Chalcogenide alloys have been reported in the literature. The Ge2Sb2Te5 (GST 225) Chalcogenide alloy currently used in OUM rmemory devices melts at approximately 610C۫ and has a glass-transition temperature of 350C۫. In order to crystallize an amorphous region of GST 225, the material must be heated to a temperature somewhat below the melting point and held at this temperature for a time sufficient to allow the crystallization to occur. The compositional dependence of crystallization kinetics in the GeSbTe ternary system has been extensively studied and reported in the literature. OUM cells based on GST 225 that can be programmed (crystallized) to the “SET” state in <20 ns have been reported
Figure 3. Schematic temperature–time relationship during programming, in a phase-change rewriteable memory device. Tm and Tx are the amorphization and crystallization temperatures, respectively. The SET and RESET states of the memory correspond to a stored binary 1 or binary 0
• For R/W CD’s and DVD’s heat is supplied by use of a laser
• For integrated circuits heat is supplied by resistors
ELECTRONIC PROPERTIES OF CRYSTALLINE AND AMORPHOUS GST ALLOYS
Two special electronic properties of chalcogenide amorphous semiconductor alloys are required for the operation of OUM memory—the strong dependence of electrical resistivity on the structural state of the material and the high-field threshold switching phenomenon. Polycrystalline GST 225 alloy has a resistivity of ~25 mΩcm, while resistivity in the vitreous state is three orders of magnitude higher—sufficient to enable good memory read capability. Both structural states of the alloy are semiconductors with comparable energy band gaps. The band gap Eg is 0.7 eV in the amorphous state and 0.5 eV in the poly-crystalline state. The conductivity activation energy Ea is ~0.3 eV for the amorphous state and 0.02 eV for the polycrystalline state. In addition, the amorphous phase exhibits a very low, trap-limited hole mobility of ~2 ×10-5cm2/V s, while the polycrystalline phase shows band-type mobility of ~10 cm2/ V s. These large differences come about because of disorder-induced localized electronic states as originally described by Mott and by Cohen, Fritzsche, and Ovshinsky (CFO) and later by Kastner, Adler, and Fritzsche.When chalcogenide alloy semiconductors are amorphized, electronic energy levels originating in the valence and conduction bands are pulled into the what was originally the empty energy band gap of the crystalline material. As described by Mott– CFO, these new gap states are localized spatially and do not extend throughout the material. Consequently, carriers move through the amorphous material either by hopping among the localized states or by being successively thermally excited to spatially extended band states and then being trapped into localized states. This gives rise to a mobility gap—a range of energy between the valence and conduction bands in which carriers have small, trap-limited mobility. The later work by Kastner, Adler, and Fritzsche explained the observation that Ea≈ Eg/2 in terms of a large density of special negatively and positively charged traps that also result from structural disorder in amorphous chalcogenide alloys. Kastner argued that charged traps (valence alternation pairs) act like compensating dopant levels in a conventional crystalline semiconductor, effectively forcing the Fermi level to lie near the mid gap between the energy levels of the two types of traps. In the polycrystalline state, crystal vacancies are proposed to give rise to acceptor-like states that move the Fermi level close to the valence-band edge. This Fermi level position, plus the loss of the disorder produced trapping states, gives rise to the nearly degenerate p-type high conductivity of the polycrystalline state. Thus, the phase-change memory cell uses a reversible change in long-range atomic order (the amorphous-to-crystalline phase change) to modulate both the Fermi level position in the Chalcogenide alloy and the carrier mobility to change the cell’s resistance.
