Research On Fabrication And Secure Storage Of Physically Transient Memristors

The advent of big data era not only prompts more requirements for data processing speed,but also makes the secure storage of valuable information a big challenge.Memristor possesses simple structure,concise preparation process,low energy consumption and high integration density,which is of great significance to achieve massive data storage,overcome Von Neumann bottleneck and build in-memory computing.In the meantime,physically transient form of electronic devices are able to achieve physical and functional disappearance in whole or in part after triggered by external stimuli,which hold tremendous value in green electronics,implantable medical therapy and secure electronics.Therefore,building physically transient form of memristors provides a high-efficiency solution for high-speed processing of mass data and secure storage of valuable information in the era of big data.However,some nonnegligible reliability issues still exist in the high-density memristor arrays.How to solve the sneak-path problem in the physically transient memristor arrays,and how to exploit the in-memory computing advantage of memristors to realize secure neuromorphic computing and secure logic operations based on transient materials also need to be further explored.Here,we study the electric crosstalk phenomenon in the memristor arrays;and physically transient form of threshold switching devices are fabricated,which are integrated with transient resistive random access memory（RRAM）devices in series to mitigate the sneak-path leakage problem in the high-density memristor arrays;meanwhile,short-term plasticity（STP）features of bio-synapse are vividly emulated by the transient memristors;in addition,implication（IMP）gate logic function is achieved based on physically transient RRAM devices.The main research contents and results are summarized as follows.1.Based on the finite element method,the electric crosstalk phenomenon in cross-point valence change mechanism（VCM）-type RRAM device arrays has been studied in depth.The effects of distance between memory cells,electrode width and the height/shape of conductive filaments on the distribution of the disturbed electric field are investigated.According to the calculation results:the disturbed electric field increases gradually with the decrease of the distance between devices and the electrode width;a smaller height of conductive filament in high-resistance devices would be devoted to weakening the disturbed electric field.In addition,the retention time and endurance performance of the VCM-type RRAM devices decrease exponentially under the influence of disturbed electric field.Besides,selecting switching materials with a smaller field accelerated parameter will be beneficial to reduce the negative effects of the disturbed electric field,improving the data storage stability of the memristors in the cross-point arrays.2.Physically transient threshold switching devices W/Ag/MgO/Ag/W with cross-point structures are fabricated by sputtering,which are capable of achieving a high selectivity up to 10⁷ at threshold voltage of±1 V with a turn-on slope of 8 m V/dec.With read voltages of0.6 V and 1.2 V,the devices can be switched on and off repeatedly for more than 10000cycles;in addition,the devices are able to withstand DC voltage stress for more than 10000s.Immersing the devices into deionized water at room temperature to trigger the transient behaviors of devices,the threshold switching performances of the devices completely disappear after 8 min.To solve the sneak-path leakage problem in transient RRAM device arrays,we integrate the transient threshold switching devices and Mg/MgO/W transient RRAM devices in series,which effectively decreases the leakage current of OFF states in device arrays and is capable of achieving a memory array of 10⁷ Gb theoretically.3.Based on the volatile characteristics of the cross-point W/Ag/MgO/Ag/W transient memristors,typical features related to STP of bio-synapse have been vividly emulated in pulse mode tests.The switching dynamics of Ag ingredient in the memristor under the pulsed excitations are consistent with the behaviors of Ca²⁺in bio-synapse,establishing an one-to-one corresponding behavioral relations.Besides,the frequency of input pulses is able to modulate the variation trends of the current responses in the devices,and the typical reactions of short-term synaptic plasticity including pair-pulse facilitation and pair-pulse depression could be emulated with the device by modulating the frequency of input pulses.The synaptic emulator arrays that are transferred onto water-soluble polyvinyl alcohol（PVA）substrate fully disintegrate in deionized water at room temperature after 30 min immersion.4.The physically transient RRAM devices Mg/Si O₂/W are prepared by sputtering.The devices exhibit stable bipolar resistive switching characteristics with a compliance current of 10 m A,which can withstand consecutive DC voltage stress for 10000 s,possessing good data retention ability.The resistive switching characteristics completely disappear by soaking the devices into deionized water for 4 min,achieving the triggered failure.Taking the original resistance states of two RRAM devices as inputs and taking the final resistance states of devices after the IMP double-pulse operations as outputs,and we achieve IMP logic function by building device circuit.The RRAM device arrays that are transferred onto PVA transient substrate achieve physical disappearances in deionized water at room temperature after 20 min immersion.5.As further extension of flexible applications for transient electronics,we develop water-assisted transfer printing method for transient electronic devices.In this method,the rapidly hydrophilic separation can be achieved between the sacrificial layer Ni and Si O₂/Si substrate in water.With the assistance of external stress,we transfer the W/Ag/MgO/Ag/W physically transient memristor arrays with supporting layer of polyimide onto flexible polydimethylsiloxane（PDMS）substrate,and the device arrays could still maintain their electrical performances under a bending radius of 2 mm,exhibiting great potential in flexible applications.In the meantime,the W/Ag/MgO/Ag/W short-term synaptic plasticity emulator arrays and the Mg/Si O₂/W in-memory computing IMP logical operation arrays are transferred onto PVA transient substrates with supporting layers of water-soluble Si N_x,building fully transient systems.