Pulsing of synaptic devices based on phase-change memory for improved linearity in weight updates

Abstract

According to one embodiment, a method, computer system, and computer program product are provided for improving linearity of weight updates of a phase change memory (PCM) cell, which may include applying a RESET pulse to amorphize a phase change material of the PCM cell, applying an incubation pulse to the PCM cell in response to applying the RESET pulse, and applying a plurality of partial SET pulses to incrementally increase the conductance of the PCM cell.

Description

Technical Field

[0001] The present invention generally relates to the field of computing, and more particularly to phase change memory.

Background Art

[0002] Phase change memory, or PCM, is a type of non-volatile random access memory that utilizes a semiconductor alloy that can rapidly change between an ordered crystalline phase with low electrical resistance and a disordered amorphous phase with high electrical resistance. The resistance of the current passing through the PCM cell can be measured to identify the phase of the cell, enabling the cell to store bits of information and function as memory. PCM is non-volatile because no power is required to maintain either phase of the material. PCM has the potential to be a strong candidate for storage class memory to bridge the performance gap between flash memory and DRAM, and can offer significant benefits in terms of speed and scalability, non-volatility, and power consumption.

Summary of the Invention

[0003] According to one embodiment, a method, computer system, and computer program product for improving the linearity of weight updates of phase change memory (PCM) cells are provided. The present invention may include applying a RESET pulse to amorphize the phase change material of the PCM cell, applying an incubation pulse to the PCM cell in response to applying the RESET pulse, and applying a plurality of partial SET pulses to incrementally increase the conductance of the PCM cell.

[0004] These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of exemplary embodiments to be read in conjunction with the accompanying drawings. The illustrative drawings, together with the detailed description, are provided to clarify the various features of the present invention to facilitate understanding by those skilled in the art. The various features of the drawings are not to scale. [Brief explanation of the drawing]

[0005] [Figure 1] This shows the phase change cycle of a phase change material used in a phase change memory (PCM) according to at least one embodiment of the present invention. [Figure 2] This is an electrical circuit diagram showing a PCM synaptic device according to at least one embodiment of the present invention. [Figure 3] This graph shows the actual weight update behavior of PCM cells using the current method. [Figure 4] This is a graph showing the target weight update behavior of a PCM cell according to at least one embodiment of the present invention. [Figure 5] This is an operation flowchart illustrating a phase change update process according to at least one embodiment of the present invention. [Figure 6A] This is a diagram showing the PCM "mushroom" cell after a RESET pulse according to at least one embodiment of the present invention. [Figure 6B] This is a diagram illustrating a PCM "mushroom" cell after a partial SET pulse without pre-annealing, according to at least one embodiment of the present invention. [Figure 6C] This is a diagram illustrating a PCM "mushroom" cell after a partial SET pulse with pre-annealing according to at least one embodiment of the present invention. [Figure 7] This is a graph showing the RESET and incubation pulses, respectively, according to at least one embodiment of the present invention. [Figure 8] This graph shows the change in conductance of a PCM cell during weight updating with and without an annealing step, according to at least one embodiment of the present invention. [Figure 9A] This is a graph showing a partial SET pulse according to at least one embodiment of the present invention. [Figure 9B] This is a graph showing a partial SET pulse with a pre-annealing component according to at least one embodiment of the present invention. [Modes for carrying out the invention]

[0006] Detailed embodiments of the claimed structure and method are disclosed herein. However, the disclosed embodiments should be understood to merely illustrate the claimed structure and method, which may be embodied in various forms. The invention may, however, be embodied in many different forms and should not be construed as being limited to the exemplary embodiments presented herein. In this description, well-known features and technical details may be omitted to avoid unnecessarily obscuring the presented embodiments.

[0007] Embodiments of the present invention relate to the field of computing, and more particularly to phase-change memory. The exemplary embodiments described below provide systems, methods, and program products for improving the linearity and uniformity of weight updates, among other things, by modifying the RESET voltage, applying a post-RESET annealing pulse, modifying the weight update pulse width and amplitude, and / or adding a pre-annealing pulse to the weight update pulse. Thus, these embodiments have the ability to improve the field of phase-change memory by improving the consistency and accuracy of the changes in the resistance / conductivity of the PCM by electrical pulses applied to the PCM, enabling the PCM cell to achieve an intermediate state between the amorphous and crystalline phases, thereby improving the amount of information that each PCM cell can store.

[0008] As previously mentioned, phase-change memory, or PCM, is a type of non-volatile random-access memory that utilizes a semiconductor alloy that can rapidly change between an ordered crystalline phase with low electrical resistance and a disordered amorphous phase with high electrical resistance. The resistance of the current passing through the PCM cell can be measured to identify the phase of the cell, enabling the cell to store bits of information and function as memory. PCM is non-volatile because it does not require power to maintain either phase of the material. PCM has the potential to be a strong candidate for storage-class memory to bridge the performance gap between flash memory and DRAM, and can offer significant advantages in terms of speed and scalability, non-volatility, and power consumption.

[0009] One advantage of phase-change memory is its promising application in neuromorphic computing. The human brain provides processing power exceeding the petaflop mark with power consumption of less than 20 watts, resulting in energy efficiency and volume that surpasses state-of-the-art supercomputers by several orders of magnitude. Therefore, building a cognitive computing system that replicates the functionality of the human brain could result in a computing system with comparable power and energy efficiency. However, current hardware implementations of deep neural networks are based on the conventional von Neumann architecture, which cannot replicate the area efficiency, in situ learning, and non-volatile synaptic behavior of the biological nervous system, and thus still cannot compete with the efficiency of the biological nervous system. Phase-change memory is a promising alternative hardware option to the conventional von Neumann architecture in mimicking the biological nervous system because it is small, power-efficient, and can store information in its resistive / conductance state. However, the central idea when building cognitive hardware based on devices like PCMs is to store synaptic weights as their conductance states and to perform the associated computational tasks in place. However, conventional PCMs can currently not store the weights of more than two possible synapses. While PCM implementations can currently store information in crystalline and amorphous states, the decomposition of conductance from crystalline to amorphous states and vice versa is currently highly inaccurate, making it impossible to reliably store information in intermediate states. Each pulse applied to a PCM cell changes the resistance by an inconsistent and unpredictable amount. Therefore, intermediate states cannot be reliably achieved. Thus, the field of PCMs needs a more accurate way to update the conductance of PCM cells before PCMs can realize the functionality and benefits of biological synapses.

[0010] PCMs also have the potential to function as variable resistors in crosspoint array architectures. A crosspoint array is a memory architecture in which non-volatile resistive memory elements are placed at the intersections of word lines and bit lines. Such arrays have the potential to achieve high-speed, random-access, non-volatile memory with stable, high-speed operation, high density, and small cell size. The use of PCMs in multilevel cell (MLC) non-volatile resistive memory elements within a crosspoint array has the potential to further improve the crosspoint array, particularly by adding the inherent advantages of PCMs, such as low power consumption and small size. However, PCMs are not suitable for use as MLCs in crosspoint arrays without a significant improvement in the range and granularity of the resistive states to which they can be set. Their use requires a more precise method for updating the conductance of PCM cells to enable PCM cells to reliably achieve intermediate resistive states.

[0011] Under conventional approaches, ensuring precise control of conductance expansion in PCMs is extremely difficult, hindering the reliable achievement of more than two states in PCMs and impeding their applications in neurological computing devices and crosspoint arrays. Thus, it may be advantageous to implement a system in which PCM cells are prepared and specially calibrated pulses are applied to ensure that each pulse produces a uniform change in the resistance of the PCM cell, thereby enabling precise control of conductance expansion and allowing the PCM cell to achieve several intermediate phase states, which may have a granularity similar to the amount of pulses used to transition the PCM cell from one state to another.

[0012] According to one embodiment, the present invention is a method for increasing the linearity and number of steps for updating the weights of a phase-change memory (PCM) cell by modifying the RESET voltage, applying a post-RESET forming annealing pulse, modifying the weight update pulse width and amplitude, and / or adding a pre-annealing pulse. There are no general-purpose optimal pulsing conditions, and the weight update behavior of any PCM cell is determined by several factors, including the design of the PCM cell and the phase-change material used. Thus, the linearity of the weight update behavior can only be improved for each PCM cell by modifying the individual width and amplitude of the RESET voltage, post-RESET forming annealing pulse, weight update pulse, and / or pre-annealing component of the weight update pulse, in accordance with the weight update behavior revealed through modeling or experimental results, in order to tune the weight update behavior to produce improved linearity.

[0013] According to at least one embodiment, the present invention is a method for modifying a RESET pulse applied to a PCM cell to improve the linearity of weight updates. The RESET pulse may be an electrical pulse applied to the PCM cell to melt and quench a programming region of the PCM cell, thereby returning it to its RESET level, where the RESET level may be the resistance of the PCM cell in its amorphous phase. The RESET pulse of the PCM cell may be modified by changing its width or amplitude. The pulse width is the amount of time between the leading and trailing edges of the pulse, while the amplitude is the maximum amount of current or voltage in the pulse. Increasing the amplitude of the RESET pulse results in a more amorphous material and increases the threshold voltage required for switching, which is the voltage required to induce a dramatically increasing programming current during programming of the PCM cell. Conversely, decreasing the amplitude of the RESET pulse results in a less amorphous material and decreases the threshold voltage required for switching. Increasing the RESET pulse width results in more amorphous material and more time for heat conduction, while decreasing the RESET pulse width results in less amorphous material and less time for heat conduction. Generally, a smaller dome of crystallized material may be desirable to improve linearity, however, if the RESET pulse amplitude is too small, melting will not occur. Thus, the RESET pulse amplitude must be as small as possible, but still induce melting.

[0014] The purpose of modifying the RESET pulse is to position the PCM cell in a state where subsequent weight update pulses will produce more consistent and uniform changes due to the cell's resistance. For example, reducing the amplitude of the RESET pulse can generate a smaller dome of amorphous material such that the paths of crystallized material generated by the weight update pulses are of equivalent size, allowing the domes to crystallize integrally and improving the uniformity of the changes.

[0015] The system may modify the RESET pulse through an iterative process of adjusting the width and amplitude of the RESET pulse, applying the RESET pulse to the PCM cell, measuring the resistance of the PCM cell after several weight update pulses have been applied, and making further modifications to the RESET pulse until the resistance change per weight update pulse falls below a threshold level of variance, depending on the resistance measured after those several pulses. Variance may be a value representing the uniformity of the resistance change in the PCM cell after each weight update pulse, averaged over all weight update pulses previously applied since the last RESET pulse, and the threshold level of variance may be the maximum level of variance between resistance changes considered acceptable to generate an intermediate state. Once the system has generated a RESET pulse that results in a level of variance below the threshold level of variance, the system may store the parameters of that RESET pulse for application to future RESET pulses.

[0016] According to at least one embodiment, the present invention is a method of applying incubation pulses after a RESET pulse to improve the linearity of weight update. The phase-change material used in the PCM is a material that changes phase in response to an electrical pulse and has an incubation time that represents the amount of energy that must be introduced into the phase-change material before crystal growth can occur. The incubation time correlates with the number of weight update pulses that must be applied to the phase-change material after the RESET before the energy barrier to crystal growth is overcome and an increase in conductance can be detected. In other words, the longer the incubation time, the more weight update pulses that must be applied to the PCM cell before there is any measurable change in conductance or resistance. The incubation time, therefore, increases the total number of pulses required to reach a given conductance / resistance by reducing the uniformity of how weight update pulses change the resistance / conductivity of the PCM cell, by introducing a period during which the weight update pulses do not have a measurable effect on the resistance / conductivity. By applying an incubation pulse after a reset pulse, the incubation time can be shortened, reducing the number of weight update pulses required before the resistance / conductivity changes measurably. The incubation pulse may be a long-duration, low-power pulse that anneals the PCM cell, inducing the formation of a large number of nuclei to overcome the energy barrier to crystal growth, allowing subsequent weight update pulses to immediately produce a measurable change in the PCM's conductance / resistance.

[0017] According to at least one embodiment, the present invention is a method for modifying the width and amplitude of a weight update pulse to improve the linearity of weight update. A weight update pulse may also be called a partial SET pulse, and a SET pulse is a long-duration, low-power electron pulse that changes a PCM from its amorphous state to its crystalline state by crystallizing the phase change material. However, a SET pulse can be divided into a series of smaller pulses, called partial SET pulses or weight update pulses, which increment the conductance of the PCM and its progression to the crystalline state in small increments rather than completing the transition in a single pulse. These weight update pulses can be modified by changing the pulse width and amplitude. Increasing the amplitude can increase the gradient of the resistance / conductivity change and reduce the incubation time. However, if the amplitude is too high, linearity will decrease. If the current is too high, quenching will occur following melting, reducing conductivity. If the current is too low, there will be no crystallization, and the resistance / conductivity will not change. Thus, to produce the highest linearity, the amplitude must fall in the middle. Similarly, if the width is too small, there will not be enough time for crystallization to occur, resulting in no change in resistance / conductivity, and if the width is too large, too much material will crystallize, causing a large spike in resistance / conductivity, reducing the number of weight update pulses before reaching the crystalline phase, which in turn reduces the number of possible intermediate states. Thus, the width must fall in the middle, not so large as to allow fewer intermediate states than desired, but just large enough to induce crystallization.

[0018] The system may modify the weight update pulse through an iterative process of adjusting the width and amplitude of the weight update pulse, applying the adjusted weight update pulse to the PCM cell, measuring the conductance / resistance of the PCM cell after several weight update pulses have been applied, and applying further corrections to the weight update pulse until the resistance change per weight update pulse exceeds a threshold level of uniformity or linearity. Uniformity may be a measure of the variance of the resistance change in the PCM cell after each weight update pulse, and the threshold level of uniformity may be the maximum level of variance between conductance / resistance changes considered sufficient to produce an intermediate state. Once the system generates a weight update pulse that results in a level of variance below the threshold level of variance, the system can store the parameters of that weight update pulse for future application to weight update pulses.

[0019] According to at least one embodiment, the present invention is a method of adding a pre-annealing component to a weight update pulse to improve the linearity of weight updates. Due to the nature of the weight update pulse being smaller and more incremental compared to the SET pulse, the weight update pulse may sometimes fail to initiate threshold switching of the material, and although a current path may be formed it does not grow, and as a result, some weight update pulses may fail to produce a measurable effect on the resistance / conductivity of the PCM cell. This reduces the uniformity / linearity of the conductivity change. By adding an annealing component to each weight update pulse, the first path to be generated will grow, reducing the likelihood that the weight update pulse will fail to produce a measurable effect on the resistance / conductivity of the PCM cell. The annealing component may be a long-duration, low-power pulse that precedes a short-duration, higher-power weight update pulse, and its width and amplitude are carefully selected, for example based on experimental results, to improve the linearity of weight updates.

[0020] One skilled in the art can understand that embodiments of the present invention may comprise any combination of a modified RESET pulse, a post-RESET incubation pulse, and / or a pre-annealing component added to a weight update pulse, which are applied to the general process of changing the phase of a PCM cell to improve the linearity of weight updates.

[0021] The present invention may be a system, method, and / or computer program product at any possible technically detailed level of integration. The computer program product may include one or more computer-readable storage media having thereon computer-readable program instructions for causing a processor to implement aspects of the present invention.

[0022] A computer-readable storage medium can be a tangible device capable of holding and storing instructions for use by an instruction-executing device. A computer-readable storage medium may be, for example, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of computer-readable storage media includes portable computer diskettes, hard disks, random-access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random-access memory (SRAM), portable compact disk read-only memory (CD-ROM), digital multipurpose disks (DVDs), memory sticks, flexible disks, mechanically coded devices such as punch cards or grooved raised structures having instructions recorded thereon, and any suitable combination of the foregoing. Computer-readable storage media should not be construed herein as themselves being transient signals, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses passing through optical fiber cables), or electrical signals transmitted through lines.

[0023] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network may include copper transmission cables, optical transmission fibers, wireless transmissions, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and transfers them for storage in the computer-readable storage medium within each computing / processing device.

[0024] The computer-readable program instructions for performing the operations of the present invention may be either source code or object code written in any combination of one or more programming languages, including assembler instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state setting data, configuration data for integrated circuits, or object-oriented programming languages ​​such as Smalltalk, C++, or similar, as well as procedural programming languages ​​such as the "C" programming language or similar. The computer-readable program instructions may run entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or it may be connected to an external computer (for example, via the Internet using an Internet service provider). In some embodiments, to perform the embodiments described herein, an electronic circuit including, for example, a programmable logic circuit, a field-programmable gate array (FPGA), or a programmable logic array (PLA) may execute a computer-readable program instruction by personalizing the electronic circuit using state information of the computer-readable program instruction.

[0025] Aspects of the present invention will be described with reference to flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to embodiments of the present invention. It will be understood that each block in the flowcharts and / or block diagrams, as well as combinations of blocks in the flowcharts and / or block diagrams, can be implemented by computer-readable program instructions.

[0026] These computer-readable program instructions may be provided to a computer or other programmable data processing device processor to produce a machine in which those instructions generate means for implementing a specified function / operation in one or more blocks of a flowchart and / or block diagram. These computer-readable program instructions may be stored on a computer-readable storage medium, and these program instructions can instruct a computer, programmable data processing device, and / or other device to function in a particular way such that the computer-readable storage medium having the instructions stored therein comprises a product containing instructions for implementing a mode of function / operation specified in one or more blocks of a flowchart and / or block diagram.

[0027] Computer-readable program instructions may be loaded onto a computer, another programmable device, or another device so that a series of operational steps are performed on the computer, another programmable device, or another device to produce a computer-executed process such that instructions executed on the computer implement a function / operation specified in one or more blocks of a flowchart and / or block diagram.

[0028] The flowcharts and block diagrams in the figures illustrate implementable architectures, functionalities, and operations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagram may represent a module, segment, or part of an instruction comprising one or more executable instructions for implementing a specified logical function(s). In some alternative implementations, the functions described in a block may occur in an order other than that shown in the figure. For example, depending on the functionality included, two consecutively shown blocks may actually be achieved as a single step, executed simultaneously, substantially simultaneously, partially or entirely in overlapping time, or multiple blocks may sometimes be executed in reverse order. It will also be noted that each block in the block diagram and / or flowchart diagram, as well as combinations of blocks in the block diagram and / or flowchart diagram, can be implemented by a dedicated hardware-based system that performs a specified function or operation, or implements a combination of dedicated hardware and computer instructions.

[0029] The exemplary embodiments described below provide systems, methods, and programmed products for improving the linearity and uniformity of weight updates by modifying the RESET voltage, applying a post-RESET annealing pulse, modifying the weight update pulse width and amplitude, and / or adding a pre-annealing pulse to the weight update pulse.

[0030] Next, referring to Figure 1, a phase change cycle 100 of a phase change material 102 used in a phase change memory (PCM) according to at least one embodiment of the present invention is depicted. Here, the phase change material 102 may be any material, such as chalcogenide glass, that can exist in at least two phase states with different electrical resistivity values ​​and can rapidly and repeatedly transition between those states. These states may include a crystalline state 104 and an amorphous state 106. The crystalline state 104 is a state having a symmetrical pattern, like a lattice, in which the microscopic arrangement of the constituent atoms of the phase change material repeats along the principal directions in three-dimensional space. The crystalline state 104 may be called the SET state and may represent 1 in binary. The crystalline state 104 has lower resistivity and higher conductance compared to the amorphous state 106, and the resistivity value of the phase change material 102 in the crystalline state 104 may be called the SET level or resistivity of the PCM. The amorphous state 106 is a state in which the constituent atoms of the phase change material are disordered and the long-range repeating pattern of atoms found in the crystalline state 104 is absent. The amorphous state 106 may be called the RESET state and may represent 0 in binary. The phase change material 102 in this amorphous state has a higher resistivity value compared to the crystalline state 104, and this resistivity value may be called the PCM's RESET level or resistivity.

[0031] The phase change material 102 may transition between phases using Joule heating, in which an electric current is passed through the phase change material 102 by electrodes to heat the phase change material 102. The phase change material 102 may transition from a crystalline state 104 to an amorphous state 106 through a process called melt-quenching 108. Melt-quenching 108 may involve applying a short-duration, high-power pulse, called a melt pulse or RESET pulse, to melt the phase change material 102 and reduce the order of its constituent atoms. The phase change material 102 may transition from an amorphous state 106 to a crystalline state 104 through a process called annealing 110. Annealing 110 may involve applying a long-duration, low-power pulse, called an annealing pulse or SET pulse, to gradually heat the phase change material 102 and crystallize it.

[0032] Unlike the melt-quench process 108, the annealing process 110 can be achieved by a series of small annealing pulses, referred to as partial SET pulses or weight update pulses, which may be smaller SET pulses that reduce the energy barrier of the phase-change material 102 or induce some amount of crystal growth, resulting in a measurable change in the resistance and conductivity of the phase-change material 102. The process of applying partial SET pulses to the phase-change material 102 is referred to herein as weight update. Each incremental increase in crystal growth between the amorphous state 106 and the crystalline state 104 of the phase-change material 102 following the partial SET pulses may represent a distinct intermediate phase state of the phase-change material 102 having a unique resistivity or conductivity value.

[0033] Next, referring to Figure 2, an electrical circuit diagram is drawn showing a PCM synapse device 200 according to at least one embodiment of the present invention. Here, a phase-change memory (PCM) cell 202 is depicted integrated into a memory array that functions, for example, as an array of synapses in a neural network. The PCM cell 202 is connected at one end to a bit line 204 and at the other end to a pair of access transistors, which in turn are connected to a word line 208 and an output line 210. The PCM cell 202 may be further shown below with reference to Figure 6A.

[0034] Next, referring to Figure 3, a graph 300 is drawn showing the actual weight update behavior of the PCM cell 202. Here, the Y-axis 302 represents the conductivity of the phase change material 102, while the X-axis 304 represents the number of partial SET pulses applied to the phase change material 102. Line 310 represents the relationship between the conductivity of the phase change material 102 between RESET level 306 and SET level 308 and the number of partial SET pulses applied to the phase change material 102. Line 310 represents the weight update behavior under the current weight update method, which is irregular and nonlinear, resulting in non-uniform changes in the conductivity of the PCM cell 202 after each pulse.

[0035] Next, referring to Figure 4, a graph is drawn showing the ideal weight update behavior 400 of a PCM cell 202 according to at least one embodiment of the present invention. Here, the Y-axis 302 represents the conductivity of the phase change material 102, while the X-axis 304 represents the number of partial SET pulses applied to the phase change material 102. Line 402 represents the relationship between the conductivity of the phase change material 102 between RESET level 306 and SET level 308 and the number of partial SET pulses applied to the phase change material 102. Line 402 represents the ideal weight update behavior that embodiments of the present invention may achieve or approach, in which the conductance of the phase change material 102 changes uniformly with each partial SET pulse, resulting in a smooth and linear weight update that supports intermediate phase states and, consequently, intermediate memory states.

[0036] Next, referring to Figure 5, an operational flowchart is drawn showing a phase change update process 500 according to at least one embodiment. In 502, the system applies a RESET pulse to amorphousize the phase change memory (PCM) cell 202. Here, the width and amplitude of the RESET pulse may be specifically formulated to prepare the PCM cell 202 so that future partial SET pulses produce a more consistent effect on the conductivity / resistivity of the PCM cell 202. The width and amplitude of the RESET pulse may be tuned to produce within the PCM cell 202 a certain volume of amorphous material, a threshold voltage required for switching, and a thermal conduction time that promotes uniform and consistent crystal growth. The amplitude of the RESET pulse may be tuned to reduce the incubation time of the PCM cell 202, thereby overcoming the energy barrier to crystal growth and reducing the number of partial SET pulses required to produce a measurable change in the conductance / resistivity of the phase change material 102.

[0037] In 504, the system applies a low-power, long-duration annealing pulse to the PCM cell. Here, the post-RESET annealing pulse, or incubation pulse, may be a long-duration, low-power pulse applied after the RESET pulse, the width and amplitude of which are specially calibrated based on the incubation time of the phase change material so that the incubation pulse introduces just enough energy into the phase change material to overcome the energy barrier to crystal growth. By overcoming the energy barrier and causing the formation of a large number of nuclei, the incubation pulse allows all the energy introduced by the subsequent partial SET pulse to contribute to crystal growth rather than overcoming the energy barrier, resulting in an immediately measurable change in the conductance / resistance of the PCM cell 202. The incubation pulse may be further shown below with respect to Figures 7 and 8.

[0038] In 506, the system applies several partial SET pulses to gradually increase the conductance of the PCM cell. The partial SET pulses may be short-duration pulses with lower power than the SET pulses. Here, the width and amplitude of the partial SET pulses may be specifically formulated to produce a consistent effect on the conductivity / resistivity of the PCM cell 202. The amplitude of the partial SET pulses may be tuned so that the current is neither too high to induce melting quenching nor too low to induce crystallization, and so that the pulses maintain a ratio of conductivity change to the number of pulses sufficient to produce a linear weight update as shown in Figure 4. The width of the partial SET pulses may be tuned so that the duration is neither too short to induce crystallization nor too long to crystallize too much material and reach saturation too rapidly. Partial SET pulses may be further shown below with respect to Figures 6B and 9A.

[0039] In step 508, the system may determine whether a desired conductance has been reached. The desired conductance may be a conductance associated with a specific state of the PCM cell, for example, corresponding to a crystalline state or a desired intermediate state. In some embodiments of the present invention, the system may rather determine whether a desired number of pulses has been reached, the desired number of pulses may be a number of pulses associated with a specific state of the PCM cell. The system may determine the conductance by applying a read pulse to the PCM cell after each pulse has been applied. According to one implementation, if the system determines that a desired conductance has not been reached (step 508, "NO" branch), the system may proceed to step 510 to determine whether the conductance corresponds linearly to the number of pulses. If the system determines that a desired conductance has been reached (step 508, "YES" branch), the PCM cell is then set to the desired conductance, and the system may complete the process.

[0040] In step 510, the system may determine whether the conductance corresponds linearly to the number of pulses. The system may compare the conductance measured in step 508 to the number of steps in order to determine the level of dispersion between resistance changes. The dispersion may be the dispersion averaged over all applied pulses since the last RESET pulse. The variation may then be compared to a threshold level of dispersion, which may be the maximum level of dispersion between resistance changes that is considered sufficiently uniform to allow the generation of intermediate states. If the variation exceeds the threshold level of dispersion (step 510, "NO" branch), the system may proceed to step 502 to apply a RESET pulse to amorphous the phase-change memory (PCM) cell, essentially resetting the PCM cell and restarting the process. In some embodiments of the present invention, the system may modify the width and / or amplitude of the RESET pulse, incubation pulse, and / or partial SET pulse to improve the uniformity of weight updates on subsequent passes. If the system determines that the fluctuation is below the threshold of dispersion (step 508, "YES" branch), then the system may proceed to step 506 to apply multiple partial SET pulses to the PCM in order to continue incrementally increasing the conductance of the PCM cell.

[0041] In some embodiments of the present invention, a pre-annealing component may be added to a partial SET pulse. The pre-annealing component may be a long-duration, low-power pulse that precedes a short-duration, relatively higher-power pulse of a typical partial SET pulse. The width and amplitude of the annealing component may be tuned to improve the chances that the partial SET pulse induces threshold switching and sufficient crystal growth to cause a measurable change in the resistance / conductivity of the PCM cell 202. Threshold switching may refer to the phenomenon in which a large electric field resulting from the application of a voltage higher than a certain threshold voltage to a phase-change material in an amorphous phase significantly increases its electrical conductivity. Due to this increase in conductivity, a larger current may be applied to the phase-change material, leading to a temperature increase in the phase-change material, which may then lead to either crystallization or melting. Partial SET pulses with the annealing component may be further shown below with reference to Figures 6C and 9B.

[0042] Next, referring to Figure 6A, a diagram 600 is drawn showing a PCM "mushroom" cell 202 after a RESET pulse according to at least one embodiment of the present invention. Here, the PCM cell 202 comprises a layer of phase-change material 102, which then comprises a programming region 602, the programming region 602 being a region of phase-change material 102 that changes phase in response to a programming pulse applied by an upper electrode 604 and a lower electrode 606. The programming pulse may be any pulse applied to the phase-change material 102 as part of a process to change the phase of the phase-change material 102, including but not limited to a SET pulse, a partial SET pulse, a RESET pulse, and an incubation pulse. The size of the programming region 602 and the volume of phase-change material 102 whose phase state is modified by the programming pulse may be increased or decreased based on the amplitude of the programming pulse.

[0043] Next, referring to Figure 6B, a diagram 600 is drawn showing a PCM "mushroom" cell 202 without pre-annealing according to at least one embodiment of the present invention. Here, the system applies a partial SET pulse to the PCM cell 202 to induce gradual crystallization, resulting in the formation of various narrow crystallized current paths 608 through the programming region 602. Without the pre-annealing component, the partial SET pulse has the opportunity to form new current paths through the programming region 602 rather than growing existing current paths 608, and forming new current paths versus growing existing current paths 608 causes different amounts of change in the resistance / conductivity of the PCM cell 202, resulting in inconsistent weight update behavior.

[0044] Next, referring to Figure 6C, a diagram 600 is drawn showing a PCM "mushroom" cell 202 with pre-annealing according to at least one embodiment of the present invention. Here, the system applies a partial SET pulse to the PCM cell 202 with a pre-annealing component, resulting in a pulse that generates a crystalline path 610 through the programming region 602, which is then expanded by subsequent pulses, resulting in more consistent weight update behavior, where the energy is not wasted in forming new paths but is mostly applied to grow the crystal structure.

[0045] Next, referring to Figure 7, a graph 700 is drawn showing a RESET pulse and an incubation pulse, respectively, according to at least one embodiment of the present invention. Here, the Y-axis 702 represents the programming current of the PCM cell 202, while the X-axis 704 represents time. Bar 706 represents the current of the PCM cell 202 during the RESET pulse, while bar 708 represents the current of the PCM cell 202 during the incubation pulse. The RESET pulse produces a higher current compared to the incubation pulse, but the incubation pulse is much longer than the RESET pulse. The incubation pulse may be based on the energy barrier to crystal growth, and different phase change materials have different energy barriers to crystal growth; therefore, the amplitude of the incubation pulse must be tuned to match the energy barrier of the phase change material.

[0046] Next, referring to Figure 8, a graph 800 is drawn showing the change in conductance of PCM cell 202 during weight update with and without the annealing step according to at least one embodiment of the present invention. Here, the Y-axis 802 represents the conductance of PCM cell 202, while the X-axis 804 represents the number of partial SET pulses applied to PCM cell 202. Line 806 shows the relationship between the conductance of PCM cell 202 and the number of pulses applied to PCM cell 202 without a previously applied incubation pulse. Line 808 shows the relationship between the conductance of PCM cell 202 and the number of pulses applied to PCM cell 202 with a preceding incubation pulse. From both lines 806 and 808, it can be seen that several pulses must be applied to PCM cell 202 before there is a measurable change in its conductivity. However, in the case where an incubation pulse is applied, the number of pulses required to generate a measurable change in the conductivity of PCM cell 202 is smaller because the incubation pulse omits several pulses that would not cause a measurable change in conductivity.

[0047] Next, referring to Figure 9A, a graph 900 is drawn showing a partial SET pulse according to at least one embodiment of the present invention. Here, the Y-axis 902 represents the current of the PCM cell 202, while the X-axis 904 represents time. Bar 906 represents the current of the PCM cell 202 during a partial SET pulse. All pulses applied to the PCM cell 202 during the weight update process may be partial SET pulses, which may have a uniform width and amplitude and / or be applied at a uniform time interval.

[0048] Next, referring to Figure 9B, a graph 900 is drawn showing a partial SET pulse with a pre-annealing component according to at least one embodiment of the present invention. Here, the Y-axis 902 represents the current of the PCM cell 202, while the X-axis 904 represents time. Bar 908 represents the current of the PCM cell 202 during the pre-annealing component of the partial SET pulse, while bar 906 represents the current of the original partial SET pulse and the corresponding component, the original partial SET pulse and the corresponding component may be pulses having the same width and amplitude as a partial SET pulse applied during a weight update process in a different context. The pre-annealing component and the original partial SET pulse and the corresponding component may be combined into a single composite partial SET pulse. All pulses applied to the PCM cell 202 during the weight update process may be composite partial SET pulses, which may be applied at uniform time intervals and / or may have pre-annealing components and original partial SET pulse components of uniform width and amplitude.

[0049] It should be noted that Figures 1-9 provide only illustrative diagrams of one implementation and do not imply any limitation regarding how different embodiments may be implemented. Many modifications may be made to the shown embodiments based on design and implementation requirements.

[0050] While various embodiments of the present invention have been presented for illustrative purposes, these descriptions are not intended to be exhaustive or to limit the disclosed embodiments. Many changes and modifications will be apparent to those skilled in the art without departing from the scope of the described embodiments. The terminology used herein has been chosen to best describe the principles of the embodiments, practical applications or technical improvements beyond the art found in the market, or to enable other those skilled in the art to understand the embodiments disclosed herein.

Claims

1. A processor implementation method for improving the linearity of weight updates of phase-change memory (PCM) cells, wherein the method is: In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, Modify the amplitude and / or width of at least one pulse of the RESET pulse, the incubation pulse, and the partial SET pulse in accordance with the variance of the weight update that is or exceeds a threshold. Methods that further include this.

2. A processor implementation method for improving the linearity of weight updates of phase-change memory (PCM) cells, wherein the method is: In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, The amplitude and / or width of the partial SET pulse are predetermined to produce weight update behavior below a threshold level of dispersion, in a method.

3. A processor implementation method for improving the linearity of weight updates of phase-change memory (PCM) cells, wherein the method is: In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, Applying a RESET pulse to amorphousize the PCM cell, depending on whether the weight update variance is at or exceeds a threshold. Methods that further include this.

4. The method according to any one of claims 1 to 3, wherein the amplitude and width of the incubation pulse are based on the incubation time of the phase change material comprising the PCM cell.

5. The method according to any one of claims 1 to 3, wherein the plurality of partial SET pulses further comprises a pre-annealing component.

6. The method according to claim 5, wherein the amplitude and width of the pre-annealing component are formulated to improve the crystal growth rate.

7. A computer system for improving the linearity of weight updates of phase-change memory (PCM) cells, wherein the computer system comprises: One or more PCM cells, one or more processors, one or more computer-readable memories, one or more computer-readable tangible storage media, and program instructions stored on at least one of the one or more tangible storage media for execution by at least one of the one or more processors via at least one of the one or more memories The computer system is equipped with, In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, Modify the amplitude and / or width of at least one pulse of the RESET pulse, the incubation pulse, and the partial SET pulse in accordance with the variance of the weight update that is or exceeds a threshold. It is possible to perform methods that further include, Computer system.

8. A computer system for improving the linearity of weight updates of phase-change memory (PCM) cells, wherein the computer system comprises: One or more PCM cells, one or more processors, one or more computer-readable memories, one or more computer-readable tangible storage media, and program instructions stored on at least one of the one or more tangible storage media for execution by at least one of the one or more processors via at least one of the one or more memories The computer system is equipped with, In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, The amplitude and / or width of the partial SET pulse can be predetermined to produce weight update behavior below a threshold level of variance. Computer system.

9. A computer system for improving the linearity of weight updates of phase-change memory (PCM) cells, wherein the computer system comprises: One or more PCM cells, one or more processors, one or more computer-readable memories, one or more computer-readable tangible storage media, and program instructions stored on at least one of the one or more tangible storage media for execution by at least one of the one or more processors via at least one of the one or more memories The computer system is equipped with, In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, Applying a RESET pulse to amorphousize the PCM cell, depending on whether the weight update variance is at or exceeds a threshold. It is possible to perform methods that further include, Computer system.

10. The computer system according to any one of claims 7 to 9, wherein the amplitude and width of the incubation pulse are based on the incubation time of the phase change material comprising the PCM cell.

11. The computer system according to any one of claims 7 to 9, further comprising a pre-annealing component, wherein the plurality of partial SET pulses further comprises a pre-annealing component.

12. The computer system according to claim 11, wherein the amplitude and width of the pre-annealing component are formulated to improve the crystal growth rate.

13. A computer-readable storage medium storing a computer program for improving the linearity of weight updates in phase-change memory (PCM) cells, wherein the computer-readable storage medium is One or more computer-readable tangible storage media, and program instructions stored on at least one of the one or more tangible storage media. The program instructions are provided to the processor, In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, Modify the amplitude and / or width of at least one pulse of the RESET pulse, the incubation pulse, and the partial SET pulse in accordance with the variance of the weight update that is or exceeds a threshold. To perform a method that further includes, which is executable by the processor, Computer-readable storage medium.

14. A computer-readable storage medium storing a computer program for improving the linearity of weight updates in phase-change memory (PCM) cells, wherein the computer-readable storage medium is One or more computer-readable tangible storage media, and program instructions stored on at least one of the one or more tangible storage media. The program instructions are provided to the processor, In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, The amplitude and / or width of the partial SET pulse is predetermined to produce weight update behavior below a threshold level of variance, and this method is executable by the processor to cause such behavior to occur. Computer-readable storage medium.

15. A computer-readable storage medium storing a computer program for improving the linearity of weight updates in phase-change memory (PCM) cells, wherein the computer-readable storage medium is One or more computer-readable tangible storage media, and program instructions stored on at least one of the one or more tangible storage media. The program instructions are provided to the processor, In response to applying a RESET pulse to make the PCM cell amorphous, an incubation pulse is applied to the PCM cell, and Applying multiple partial SET pulses to incrementally increase the conductance of the PCM cell. Includes, Applying a RESET pulse to amorphousize the PCM cell, depending on whether the weight update variance is at or exceeds a threshold. To perform a method that further includes, which is executable by the processor, Computer-readable storage medium.

16. The computer-readable storage medium according to any one of claims 13 to 15, wherein the amplitude and width of the incubation pulse are based on the incubation time of the phase change material comprising the PCM cell.

17. The computer-readable storage medium according to any one of claims 13 to 15, further comprising a pre-annealing component, wherein the plurality of partial SET pulses further comprises a pre-annealing component.

18. The computer-readable storage medium according to claim 17, wherein the amplitude and width of the pre-annealing component are formulated to improve the crystal growth rate.

Images