Integration of Redox-based Resistive Switching Memory Devices

Abstract

The steadily growing market for consumer electronics and the rapid proliferation of mobile devices such as tablet computers, MP3 players and smart phones make high demands for the nonvolatile memory. Present FLASH memory technology approaches to the end due to physical scalability limits. Therefore, an alternative technology must be developed. For memory technology, not only the storage density and cost are important factors but the power consumption and the writing/reading speed must also be taken in account. Redox-based resistive memory (ReRAM) offers a potential alternative to the FLASH technology and presently is in the focus of research activities. The operating principle of the ReRAM is based on the non-volatile reversible change in resistance by electrical stimuli in a simple metal-insulator-metal(MIM) device architecture. This simple structure enables the integration of ReRAM in passive crossbar arrays, in which each crosspoint consumes only 4F$^{2}$ (F- feature size) device area. This leads to an ultra-high storage density at reduced cost. Research on the ReRAM memory elements requires a technology platform that ensures a cost-effective fabrication of the crossbar devices with nanometer feature size. In this thesis, the fabrication processes have been developed based on the nanoimprint lithography, which facilitates both the high resolution (<50nm) and the high throughput at low cost. The stamp for the UV-nanoimprinting is developed with plasma etching and electron-beam lithography. This process facilitates the fabrication of the ReRAM devices sizes ranging from 40x40 nm$^{2}$ to 100x100 nm$^{2}$. The fabricated nano-crosspoint ReRAM of different switching layer thickness and different device areas are electrically characterized. In order to toggle the resistance state in the ReRAM device, an electroforming step is generally required. In this work, a systematic analysis of the electroforming process is carried out on TiO$_{2}$ and WO$_{3}$-based ReRAM cells and the respective switching characteristics are investigated. The switching mechanism is explained by the filamentary conduction model. The forming voltage decreases with decreasing oxide layer thickness whereas it increases for the smaller device size. Due to overshoot phenomena during the electroforming process, these devices show a significant increased switching current, lower non-linearity, and lower endurance. The ReRAM device performance is improved by integration in the backend of a 65nm CMOS process. In the integrated 1T-1R stack, the electroforming is performed by controlling the current flow with the gate electrode. By employing this approach, the switching current in the ReRAM devices is reduced to 1 $\mu$A. In order to lower the sneak path current in the passive crossbar arrays, a high degree of nonlinearity is required. This nonlinearity parameter has been investigated with 100ns transient pulses in the nano-crossbar devices and in the 1T-1R structures. This parameter depends on the switching current and switching material properties. The lower switching current in the TiO$_{2}$ ReRAM leads to the higher nonlinearity. Furthermore, the ReRAM nanodevices inherently exhibit open clamp voltage in the switching characteristics. This phenomenon is explained by the electromotive force(EMF). The amplitude of the generated EMF voltage depends on the nature of the switching materials and can be several hundred mV. This degrades the conducting filament and thereby limits the ON state retention properties of the ReRAM devices. Additionally, the non-zero crossing of the I-V characteristics, caused by the EMF voltage demands the refinement of the memristor theory