Integration of redox based resistive switching memory devices (2015)

The steadily growing market for consumer electronics and the rapid proliferationof mobile devices such as tablet computers, MP3 players and smart phones makehigh demands for the nonvolatile memory. Present FLASH memory technology approachesto the end due to physical scalability limits. Therefore, an alternativetechnology must be developed. For memory technology, not only the storage densityand cost are important factors but the power consumption and the writing/readingspeed must also be taken in account. Redox-based resistive memory (ReRAM) offersa potential alternative to the FLASH technology and presently is in the focus of researchactivities. The operating principle of the ReRAM is based on the non-volatilereversible change in resistance by electrical stimuli in a simple metal-insulator-metal(MIM) device architecture. This simple structure enables the integration of ReRAMin passive crossbar arrays, in which each crosspoint consumes only 4F² (F- featuresize) device area. This leads to an ultra-high storage density at reduced cost.Research on the ReRAM memory elements requires a technology platform that ensuresa cost-effective fabrication of the crossbar devices with nanometer feature size.In this thesis, the fabrication processes have been developed based on the nanoimprintlithography, which facilitates both the high resolution (<50 nm) and the highthroughput at low cost. The stamp for the UV-nanoimprinting is developed withplasma etching and electron-beam lithography. This process facilitates the fabricationof the ReRAM devices sizes ranging from 40x40 nm² to 100x100 nm². Thefabricated nano-crosspoint ReRAM of different switching layer thickness and differentdevice areas are electrically characterized. In order to toggle the resistance statein the ReRAM device, an electroforming step is generally required. In this work, asystematic analysis of the electroforming process is carried out on TiO2 and WO3-based ReRAM cells and the respective switching characteristics are investigated. Theswitching mechanism is explained by the filamentary conduction model. The formingvoltage decreases with decreasing oxide layer thickness whereas it increases for thesmaller device size. Due to overshoot phenomena during the electroforming process,these devices show a significant increased switching current, lower non-linearity, andlower endurance. The ReRAM device performance is improved by integration in thebackend of a 65nm CMOS process. In the integrated 1T-1R stack, the electroformingis performed by controlling the current flow with the gate electrode. By employingthis approach, the switching current in the ReRAM devices is reduced to 1 µA. Inorder to lower the sneak path current in the passive crossbar arrays, a high degree ofnonlinearity is required. This nonlinearity parameter has been investigated with 100ns transient pulses in the nano-crossbar devices and in the 1T-1R structures. Thisparameter depends on the switching current and switching material properties. Thelower switching current in the TiO2 ReRAM leads to the higher nonlinearity.Furthermore, the ReRAM nanodevices inherently exhibit open clamp voltage in theswitching characteristics. This phenomenon is explained by the electromotive force(EMF). The amplitude of the generated EMF voltage depends on the nature of theswitching materials and can be several hundred mV. This degrades the conductingfilament and thereby limits the ON state retention properties of the ReRAM devices.Additionally, the non-zero crossing of the I-V characteristics, caused by theEMF voltage demands the refinement of the memristor theory.