Memristor multi-valued programming method for improving retention characteristic of device

Abstract

The invention discloses a memristor multi-valued programming method for improving the retention characteristic of a device. According to the method, target conductance of memristor programming is divided into a low-conductance range or a high-conductance range based on a certain conductance threshold value, and programming schemes with different initial conductance states, programming pulse directions and de-noising pulse directions are adopted for different ranges. According to the multi-valued programming method, on the basis of the physical process of conductive filament growth and fracture in the resistance change process of the memristor, the state of the memristor after programming is kept in a more stable state as much as possible, and then the memristor has a better multi-valued maintaining characteristic. Compared with an existing memristor multi-valued programming scheme, the method has the advantages that the retention characteristic of data storage after multi-valued programming of the device can be improved under the condition that the programming time and the power consumption overhead of the device are not greatly increased, so that the device can be better applied to the field of data storage or in-memory computing, and the memory performance of the device is improved. Therefore, the storage and calculation reliability and the system performance of the memristor are improved.

Description

Technical Field

[0001] The invention belongs to the field of semiconductor technology, relates to a memristor, and in particular to a multi-value programming method for improving the retention characteristics of a memristor. Background Art

[0002] Memristors, as an emerging memory device, have been widely used as storage media for high-density storage and in-memory computing in recent years. Compared with traditional memory devices, such as SRAM, flash memory, etc., and other emerging memory devices, such as phase change memory, ferroelectric memory, and magnetic memory, memristors have the characteristics of non-volatile storage, small unit area, low power consumption, multi-value storage, and compatibility with CMOS processes, which makes them have great potential in edge computing.

[0003] The principle of memristor data storage is to use the migration of metal atoms or oxygen vacancy defects in the resistive layer to form conductive filaments for current to pass through, so that the memristor reaches a certain resistance value. By applying a certain voltage, the current resistance value of the memristor can be read. The characteristic of memristor that stores data through the resistance value of the conductive channel enables it to have the ability to store multi-bit data in a single device, which can greatly improve the density of data storage in memristor arrays. Especially in edge storage and in-memory computing that pursues system performance, the use of the multi-value characteristics of memristors is almost a factor that must be considered.

[0004] As the utilization of the multi-value characteristics of memristors continues to increase, some schemes for multi-value programming have also been gradually proposed, such as incremental step pulse program (ISPP) and repeated pulse programming. However, these multi-value programming schemes do not pay more attention to the retention characteristics of the device resistance after programming the RRAM device. These programming schemes are either derived from the original scheme migration for multi-value programming of flash memory, or only perform simple repeated pulse operations, without considering more about the influence of the conductive mechanism of the memristor itself on its storage data retention characteristics. In addition, some multi-value programming schemes, such as small pulse step programming, require more programming time or power consumption to achieve good multi-value programming effects. At the same time, more complex and diverse edge data storage, as well as edge computing devices that require a long standby time, have put forward higher requirements for the long-term multi-value storage retention characteristics of memristors, as well as the time and power consumption of multi-value programming storage. Therefore, the demand 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 the present invention is to solve the problem that the device resistance retention characteristic is poor after programming is completed in the current programming scheme for multi-value programming of a memristor.

[0006] The present invention includes a memristor multi-value programming method for improving device retention characteristics and a principle description thereof.

[0007] The present invention proposes to divide the programming target conductance of the memristor into a high conductance region and a low conductance region according to a certain threshold value, and select different programming schemes according to different ranges of the programming target conductance, so as to achieve better retention characteristics of the memristor after multi-value programming.

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

[0009] Step 110: Determine a conductance threshold according to the conductance range of the memristor normally used after forming, and divide the target conductance of the memristor programming into a high conductance range or a low conductance range.

[0010] Step 120: Initialize the memristor to different conductance states within the normal use range measured above according to the target conductance range of this programming. If the target conductance of this programming is in the high conductance range, the device is reset to the low conductance state; if the target conductance of this programming is in the low conductance range, the device is set to the high conductance state.

[0011] Step 130: According to the range of the target conductivity for programming, the device conductivity is programmed to near the target value using a pulse programming method that is as unidirectional as possible: if the target conductivity for this programming is in the high conductivity range, pulses in the SET direction are used as much as possible during the programming process; if the target conductivity for this programming is in the low conductivity range, pulses in the RESET direction are used as much as possible during the programming process.

[0012] Step 140: When the device conductivity reaches the vicinity of the set target conductivity value, a certain number of unidirectional small voltage pulses are applied to remove noise and stabilize the device conductivity. If the target conductivity of this programming is in the high conductivity range, a small voltage pulse in the SET direction is used for removal of noise and stabilization; if the target conductivity of this programming is in the low conductivity range, a small voltage pulse in the RESET direction is used for removal of noise and stabilization.

[0013] The multi-value programming method of the present invention utilizes the growth and fusing process of the conductive channel during the resistive switching process of the memristor, so that when programming different target conductances, the conductive channel 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 programming scheme principle adopted by the present invention when the target conductivity state is in the high conductivity range is as follows: Figure 2As shown. If a memristor wants to have a higher conductance, it can be achieved through the growth process of the conductive channel from the broken state or the melting process from the connected state. To achieve a higher conductance state through the melting process of the conductive filament, the conductive filament is first broken axially. At this time, the distance between the broken part of the conductive filament and the top electrode is small, and there are a large number of free defects in the gap, which makes it more likely that the conductive filament will reconnect with the bottom electrode and significantly change the device conductance, which has higher instability. To achieve a higher conductance state through the growth process of the conductive channel, the conductive filament in the resistive layer of the memristor needs to grow axially first to connect the top electrode of the memristor with the bottom electrode, and then grow radially. This conductive filament makes the device have a higher conductance and is not easily affected by external interference. At the same time, a small voltage pulse in the growth (SET) direction is used to denoise and stabilize the device, so that the conductive filament is not easily affected by the peripheral defects in the resistive layer.

[0015] The programming scheme principle adopted by the present invention when the target conductivity state is in the low conductivity range is as follows: Figure 3 As shown. Lower device conductance can be achieved by breaking or growing the conductive channel. A lower conductance state is achieved by the growth of conductive filaments. At this time, the conductive filaments mainly grow axially. The thinner conductive filaments connect the top electrode and the bottom electrode, which is easily affected by the outside world and breaks, causing a significant change in the device conductance. A lower conductance state is achieved by breaking the conductive filaments. The conductive filaments mainly break axially, making the distance between the conductive filaments and the bottom electrode farther and less susceptible to external influences. At the same time, a small voltage pulse in the break (RESET) direction is used to denoise and stabilize the device to prevent the conductive filaments from being affected by defects in the break area.

[0016] Preferably, in the above step 110, the middle value of the conductivity range normally used after forming of the memristor is set as the conductivity threshold for dividing the high conductivity range and the low conductivity range, the range below the threshold is the low conductivity range, and the range above the threshold is the high conductivity range.

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

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

[0019] The present invention uses the growth of conductive filaments to achieve a high conductivity target state, uses the breakage of conductive filaments to achieve a low conductivity target state, and uses small voltage pulses in different directions to reduce the influence of the defects in the peripheral area of ​​the conductive filaments on the device resistance value without causing interference to the programmed conductive filaments as much as possible, so that the memristor has better retention characteristics after the multi-value programming is completed. At the same time, the ISPP programming scheme with as much single direction as possible and a small number of small voltage pulses are used for noise stabilization, so that the programming scheme overhead of the present invention is not greatly increased.

[0020] Beneficial technical effect of the present invention: Compared with the traditional memristor multi-value programming method (such as ISPP), the memristor multi-value programming method proposed in the present invention can improve the data storage retention characteristics of the device after multi-value programming without significantly increasing the device programming time and power consumption overhead, so that the memristor can be better used in the field of data storage or in-memory computing, thereby improving the reliability and system performance of memristor storage and computing. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flow chart of the multi-value programming method of the memristor of the present invention.

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

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

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

[0025] Figure 5 The beneficial effects of the embodiments of the present invention are as follows: compared with the traditional multi-value programming scheme, the present scheme can improve the retention characteristics of the device after multi-value programming. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0027] In the embodiment of the present invention, the structure of the memristor is TiN / HfO x / TiN structure, in which the main defects constituting the conductive channel are oxygen vacancies, and the transistor process node is 28nm.

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

[0029] Step 210: Since the normal operating conductivity range of the memristor after forming is 0-100 μS, the threshold for dividing the programming target conductivity into high and low conductivity is selected as 50 μS. The target conductivity is higher than 50 μS, which is in the high conductivity range, and is lower than 50 μS, which is in the low conductivity range.

[0030] Step 220: According to 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 (20 μS) 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 100 μS.

[0031] Step 230: Program the device using an ISPP-based programming method according to the target conductance state of the device. In this embodiment, the ISPP programming parameters in the SET direction are as follows: the BL voltage amplitude is fixed at 1.6V, SL is connected to GND, the initial amplitude of the WL voltage is 0.5V, the continuous SET process will step the WL voltage amplitude with a step size of 0.01V, and 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, the WL voltage amplitude is fixed at 2V, the initial amplitude of the SL voltage is 1V, the continuous RESET process will step the SL voltage amplitude 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 a continuous RESET process as much as possible to reach the target conductance.

[0032] Step 240: When the device is programmed to within 20μS±1μS, the ISPP programming phase ends. Then, according to the range of the programming target conductivity, a unidirectional small voltage pulse is applied to stabilize the device conductive channel. If the programming 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 a SL connected to GND and a pulse width of 100ns will be applied, repeated 10 times, and the interval between each pulse is 100ns; if the target conductivity is less than 50μS, a RESET direction pulse with a BL connected to GND, a WL voltage amplitude of 0.9V, a SL voltage amplitude of 0.35V, and a pulse width of 100ns will be applied, repeated 10 times, and the interval between each pulse is 100ns.

[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 regular intervals using a read voltage setting with a WL voltage amplitude of 0.9V and a BL voltage amplitude of 0.1V.

[0034] The multi-value programming effect of this embodiment is compared with the programming effect of the traditional multi-value programming scheme. Figure 5 The present invention has a good improvement on the device retention characteristics of different programming target conductivity values, and does not significantly increase the number of pulses required for successful programming.

[0035] The embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above is only an 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 in the protection scope of the present invention.

Claims

1. A memristor multi-value programming method, comprising the following steps: 1) According to the normal conductance range of the memristor after forming, a conductance threshold is determined to divide the target conductance of the memristor programming into a high conductance range or a low conductance range; 2) Initializing the memristor to different conductance states according to the range of the programmed target conductance: if the programmed target conductance is in the high conductance range, RESET the device to the low conductance state; if the programmed target conductance is in the low conductance range, SET the device to the high conductance state; 3) According to the range of the target conductance to be programmed, use a pulse programming method that is as unidirectional as possible to program the device conductance to near the target value: if the target conductance to be programmed is in the high conductance range, use pulses in the SET direction as much as possible during the programming process; If the target conductance of programming is in the low conductance range, use pulses in the RESET direction as much as possible during the programming process; 4) When the device conductivity reaches near the set target conductivity value, a certain number of unidirectional small voltage pulses are applied to denoise and stabilize the device conductivity.

2. The memristor multi-value programming method according to claim 1, characterized in that: In step 1), the middle value of the conductance range normally used after forming the memristor is set as the conductance threshold for dividing the high conductance range and the low conductance range.

3. The memristor multi-value programming method according to claim 1, characterized in that: In step 2), if the programmed target conductance is in the high conductance range, the device is RESET to the lowest conductance state in the normal use conductance range; if the programmed target conductance is in the low conductance range, the device is SET to the highest conductance state in the normal use conductance range.

4. The memristor multi-value programming method according to 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 length, and the device conductivity is read for verification.

5. The memristor multi-value programming method according to claim 4, characterized in that: In step 3), preliminary experiments are performed on a small number of devices to obtain appropriate starting voltage pulse parameters and step lengths required for the ISPP programming method.

6. The memristor multi-value programming method according to claim 1, characterized in that: In step 4), if the target conductance of programming is in the high conductance range, a small voltage pulse in the SET direction is used for denoising and stabilization; if the target conductance of this programming is in the low conductance range, a small voltage pulse in the RESET direction is used for denoising and stabilization.

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

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