A multi-value programming method for memristor to improve device retention characteristics

Abstract

The application discloses a multi-value programming method of a memristor for improving device retention characteristics. The method divides the target conductance of the programming of the memristor in a low conductance range or a high conductance range based on a certain conductance threshold, and adopts a programming scheme with different initial conductance states, programming pulse directions and denoising pulse directions for different ranges. The multi-value programming method of the application is based on the physical process of the growth and rupture of a conductive filament in the resistance change process of the memristor, so that the state of the memristor after the programming is as stable as possible, and the memristor device has better multi-value retention characteristics. Compared with the existing multi-value programming scheme of the memristor, the application can improve the retention characteristics of the data storage after the multi-value programming of the device without greatly increasing the programming time and power consumption of the device, so that the device can be better used in the data storage field or the in-memory computing field, thereby improving the reliability and system performance of the storage and calculation of the memristor.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor technology, and relates to memristors, specifically to a multi-value programming method for improving the retention characteristics of memristors. Background Technology

[0002] Memristors, as a new type of memory device, have been widely used in recent years as a storage medium for high-density storage and in-memory computing. Compared with traditional memory devices such as SRAM and flash memory, as well as other emerging memory devices such as phase-change memory, ferroelectric memory, and magnetic memory, memristors have the characteristics of non-volatile storage, small cell area, low power consumption, multi-value storage capability, and compatibility with CMOS technology, making them extremely promising for edge computing.

[0003] The principle behind memristor data storage is to utilize the migration of metal atoms or oxygen vacancy defects within the resistive switching layer to form conductive filaments through which current flows, allowing the memristor to reach a certain resistance value. By applying a specific voltage, the current resistance value of the memristor can be read. This characteristic of memristors in storing data through the resistance value of conductive channels enables them to store multiple bits of data in a single device, greatly increasing the data density of memristor arrays. Especially in edge-end storage and in-memory computing where system performance is paramount, utilizing the multi-value characteristics of memristors is almost an essential consideration.

[0004] As the utilization of the multi-value characteristics of memristors continues to increase, several multi-value programming schemes have been proposed, such as Incremental Step Pulse Programming (ISPP) and repetitive pulse programming. However, these multi-value programming schemes do not pay much attention to the retention characteristics of the device's resistance after programming. These schemes are either derived from schemes used for multi-value programming of flash memory or simply perform repetitive pulse operations, without considering the impact of the memristor's own conductivity mechanism on its data retention characteristics. Furthermore, some multi-value programming schemes, such as small-pulse step programming, require more programming time or power consumption to achieve good multi-value programming results. Meanwhile, more complex and diverse edge data storage, and edge computing devices requiring longer standby times, place higher demands on the long-term multi-value retention characteristics of memristors, as well as the time and power consumption during multi-value programming. Therefore, the need for a memristor programming scheme that can achieve better multi-value retention characteristics and lower programming time has become particularly urgent. Summary of the Invention

[0005] The purpose of this invention is to solve the problem of poor resistance retention characteristics of current multi-value programming schemes for memristors after programming.

[0006] This invention includes a memristor multi-value programming method for improving device retention characteristics and an explanation of its principles.

[0007] This invention proposes to divide the target conductance of a memristor into a high-conductance region and a low-conductance region according to a certain threshold. Different programming schemes are selected according to different ranges of the target conductance, so that the retention characteristics of the memristor after multi-value programming can be optimized.

[0008] The memristor multi-value programming method of the present invention is as follows: Figure 1 As shown, it includes the following steps:

[0009] Step 110: Based on the normal conductance range of the memristor after forming, determine a conductance threshold and classify the target conductance of the memristor programming into either the high conductance range or the low conductance range.

[0010] Step 120: Based on the target conductance range of this programming, initialize the memristor to different conductance states within the previously measured normal operating range. If the target conductance is in the high conductance range, reset the device to the low conductance state; if the target conductance is in the low conductance range, reset the device to the high conductance state.

[0011] Step 130: Based on the target conductance range, use a unidirectional pulse programming method to program the device conductance to near the target value: if the target conductance is in the high conductance range, use pulses in the SET direction as much as possible during programming; if the target conductance is in the low conductance range, use pulses in the RESET direction as much as possible during programming.

[0012] Step 140: Once the device conductance reaches near the set target conductance value, apply a certain number of unidirectional small voltage pulses to denoise and stabilize the device conductance. If the target conductance for this programming is in the high conductance range, use small voltage pulses in the SET direction for denoising and stabilization; if the target conductance for this programming is in the low conductance range, use small voltage pulses in the RESET direction for denoising and stabilization.

[0013] The multi-value programming method of the present invention utilizes the growth and melting process of conductive channels during the resistive switching process of memristors, so that when programming different target conductivities, the conductive channels in the resistive switching layer of the device can ultimately remain in a relatively stable state, thereby improving the retention characteristics of the device after programming.

[0014] The principle of the programming scheme used in this invention when the target conductivity state is in the high conductivity range is as follows: Figure 2As shown. To achieve high conductivity in a memristor, it can be achieved through either a growth process from a fractured state or a melting process from a connected state. Achieving higher conductivity through the melting process of the conductive filament involves first axially fractured the filament. At this point, the distance between the fractured filament and the top electrode is small, and there are numerous free defects within the gap. This makes it highly likely that the conductive filament will reconnect to the bottom electrode, significantly altering the device's conductivity and resulting in higher instability. Achieving higher conductivity through the growth process of the conductive channel requires the conductive filament within the memristor's resistive switching layer to first grow axially to connect the top and bottom electrodes, followed by radial growth. This type of conductive filament results in higher conductivity and is less susceptible to external interference. Simultaneously, a small voltage pulse is used in the growth (SET) direction to denoise and stabilize the device, making the conductive filament less susceptible to the influence of surrounding defects within the resistive switching layer.

[0015] The principle of the programming scheme used in this invention when the target conductivity state is in the low conductivity range is as follows: Figure 3 As shown. Lower device conductivity can be achieved through the fracture or growth of conductive channels. Achieving a lower conductivity state through the growth of conductive filaments involves primarily axial growth of the filaments. These thin filaments connect the top and bottom electrodes and are easily fractured by external influences, causing significant changes in device conductivity. Conversely, achieving a lower conductivity state through the fracture of conductive filaments involves primarily axial fracture, resulting in a greater distance between the filaments and the bottom electrode, making them less susceptible to external influences. Simultaneously, a small voltage pulse along the fracture (RESET) direction is used to denoise and stabilize the device, preventing the conductive filaments from being affected by defects within the fracture region.

[0016] Preferably, in step 110 above, the median value of the conductance range that the memristor can normally use after forming is set as the conductance threshold for dividing the high conductance range and the low conductance range. The range below the threshold is the low conductance range, and the range above the threshold is the high conductance range.

[0017] Preferably, in step 120 above, if the target conductance for programming is in the high conductance range, the device is reset to the lowest conductance state (LCS) of the normal operating conductance range; if the target conductance for programming is in the low conductance range, the device is set to the highest conductance state (HCS) of the normal operating conductance range. Preferably, in step 130 above, the device is programmed using the ISPP programming method with appropriate starting voltage pulse parameters and step size, and the device conductance is read for verification. The appropriate starting voltage pulse parameters and step size required for the ISPP programming method can be obtained by conducting preliminary experiments on a small number of devices.

[0018] Preferably, in step 140 above, the programming stage ends after the device is programmed to the target conductance range of ±1μS; then, according to the range of the programming target conductance, a unidirectional small voltage pulse is applied to stabilize the conductive channel of the device. The relevant parameters of the small voltage pulse need to be below the threshold of the electrical pulse parameter that causes the device conductance to undergo binary resistance change, and are usually less than the starting pulse parameter of the ISPP programming process. The specific parameters can be obtained by conducting preliminary experiments on a small number of devices.

[0019] This invention achieves a high-conductivity target state through the growth of conductive filaments and a low-conductivity target state through the breakage of conductive filaments. It also employs small voltage pulses in different directions to minimize interference with the programmed conductive filaments and reduce the impact of ionization defects in the surrounding area on the device resistance, resulting in good retention characteristics after multi-value programming of the memristor. Furthermore, the use of a predominantly unidirectional ISPP programming scheme and a smaller number of small voltage pulses for noise reduction and stabilization ensures that the programming overhead of this invention is not significantly increased.

[0020] The beneficial technical effects of the present invention are as follows: Compared with traditional memristor multi-value programming methods (such as ISPP), the memristor multi-value programming method proposed in this invention can improve the data storage retention characteristics after multi-value programming without significantly increasing the device programming time and power consumption. This allows memristors to be better used in the fields of data storage or in-memory computing, thereby improving the reliability of memristor storage and computing and the system performance. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the memristor multi-value programming method of the present invention.

[0022] Figure 2 This is a schematic diagram illustrating the programming principle of the memristor multi-value programming method of the present invention for a high-conductivity target state.

[0023] Figure 3 This is a schematic diagram illustrating the programming principle of the memristor multi-value programming method of the present invention for a low-conductivity target state.

[0024] Figure 4 This is a flowchart of a memristor multi-value programming method according to an embodiment of the present invention.

[0025] Figure 5 The beneficial effects of this embodiment of the invention are as follows: compared with the traditional multi-value programming scheme, this scheme can improve the retention characteristics of the device after multi-value programming. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0027] In this embodiment of the invention, the memristor uses a TiN / HfO structure. x The / TiN structure, in which the main defect constituting the conductive channel is oxygen vacancy, has a transistor process node of 28nm.

[0028] In this embodiment of the invention, the target conductance for programming the memristor is 20 μS, and the programming scheme is as follows: Figure 4 As shown.

[0029] Step 210: Since the normal operating conductance range of the memristor after forming is 0-100μS, the threshold for dividing the target conductance into high and low conductance is selected as 50μS. The target conductance is above 50μS and is in the high conductance range, and below 50μS and is in the low conductance range.

[0030] Step 220: Based on the programmed target conductance, use a single pulse to SET or RESET the device to the corresponding initial conductance state. In this embodiment, the programmed target conductance (20μS) is in the low conductance range, so the WL voltage is set to 0.6V, the BL voltage is set to 1.6V, and a SET pulse with a pulse width of 200ns is applied to SET the device to a high conductance state of about 100μS.

[0031] Step 230: Based on the target conductance state of the device, program the device using the ISPP-based programming method. In this embodiment, the ISPP programming parameters in the SET direction are as follows: BL voltage amplitude is fixed at 1.6V, SL is connected to GND, the initial amplitude of WL voltage is 0.5V, and the WL voltage amplitude is changed stepwise in the continuous SET process with a step size of 0.01V. The applied programming pulse width is fixed at 100ns. The ISPP programming parameters in the RESET direction are as follows: BL is connected to GND, WL voltage amplitude is fixed at 2V, the initial amplitude of SL voltage is 1V, the SL voltage amplitude is changed stepwise in the continuous RESET process with a step size of 0.01V, and the applied programming pulse width is fixed at 100ns. Since the target conductance in this embodiment is less than 50μS, the programming process uses continuous RESET processes as much as possible to reach the vicinity of the target conductance.

[0032] Step 240: Once the device is programmed to within 20μS ± 1μS, the ISPP programming phase ends. Subsequently, based on the target conductivity range, a small unidirectional voltage pulse is applied to stabilize the device's conductive path. If the target conductivity is greater than 50μS, a SET direction pulse with a BL voltage amplitude of 0.35V, a WL voltage amplitude of 0.9V, and SL connected to GND, with a pulse width of 100ns, is applied, repeated 10 times with a 100ns interval between each pulse. If the target conductivity is less than 50μS, a RESET direction pulse with BL connected to GND, a WL voltage amplitude of 0.9V, and SL voltage amplitude of 0.35V, with a pulse width of 100ns, is applied, repeated 10 times with a 100ns interval between each pulse.

[0033] After programming the memristor to different target conductance values ​​using the multi-value programming scheme and electrical parameter settings described in this embodiment, the device is placed and the conductance value of the device is read out at certain intervals using read voltage settings of WL voltage amplitude of 0.9V and BL voltage amplitude of 0.1V.

[0034] The multi-valued programming effect of this embodiment is compared with that of traditional multi-valued programming schemes, for example... Figure 5 As shown, this invention significantly improves the retention characteristics of devices with different target conductivity values ​​for programming, without substantially increasing the number of pulses required for successful programming.

[0035] The embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only one embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for multi-value programming of a memristor, comprising the following steps: 1) Based on the normal conductance range of the memristor after forming, determine a conductance threshold and classify the target conductance of the memristor programming into the high conductance range or the low conductance range. 2) Initialize the memristor to different conductance states according to the target conductance range of the programming: if the target conductance is in the high conductance range, reset the device to the low conductance state; if the target conductance is in the low conductance range, set the device to the high conductance state. 3) Based on the target conductance range, use a unidirectional pulse programming method to program the device conductance to near the target value: if the target conductance is in the high conductance range, use SET direction pulses for programming as much as possible during the programming process; If the target conductance for programming is in the low conductance range, then use pulses in the RESET direction as much as possible during the programming process; 4) When the device conductance reaches near the set target conductance value, apply a certain number of unidirectional small voltage pulses to denoise and stabilize the device conductance.

2. The memristor multi-value programming method as described in claim 1, characterized in that, In step 1), the median value of the conductance range that the memristor can normally use after forming is set as the conductance threshold that divides the high conductance range and the low conductance range.

3. The memristor multi-value programming method as described in claim 1, characterized in that, In step 2), if the target conductivity of the program is in the high conductivity range, the device is RESET to the lowest conductivity state of the normal operating conductivity range; if the target conductivity of the program is in the low conductivity range, the device is SET to the highest conductivity state of the normal operating conductivity range.

4. The memristor multi-value programming method as described in claim 1, characterized in that, In step 3), the device is programmed using the ISPP programming method with appropriate starting voltage pulse parameters and step size, and the device conductance is read for verification.

5. The memristor multi-value programming method as described in claim 4, characterized in that, In step 3), by conducting preliminary experiments on a small number of devices, the appropriate starting voltage pulse parameters and step size required by the ISPP programming method are obtained.

6. The memristor multi-value programming method as described in claim 1, characterized in that, In step 4), if the target conductance for programming is in the high conductance range, a small voltage pulse in the SET direction is used for noise reduction and stabilization; if the target conductance for programming is in the low conductance range, a small voltage pulse in the RESET direction is used for noise reduction and stabilization.

7. The memristor multi-value programming method as described in claim 6, characterized in that, In step 4), the programming stage ends after the device is programmed to the target conductance range of ±1μS; then, according to the range of the target conductance, a unidirectional small voltage pulse is applied to stabilize the conductive channel of the device, and the relevant parameters of the small voltage pulse are below the threshold of the electrical pulse parameter that causes the device conductance to undergo binary resistance change.

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