Dependence of Set, Reset and Breakdown Voltages of a MIM Resistive Memory Device on the Input Voltage Waveform
Owing to its excellent scaling potential, low power consumption, high switching speed, and good retention, and endurance properties, Resistive Random Access Memory (RRAM) is one of the prime candidates to supplant current Nonvolatile Memory (NVM) based on the floating gate (FG) MOSFET transistor, which is at the end of its scaling capability. The RRAM technology comprises two subcategories: 1) the resistive phase change memory (PCM), which has been very recently deployed commercially, and 2) the filamentary conductive bridge RAM (CBRAM) which holds the promise of even better scaling potential, less power consumption, and faster access times. This thesis focuses on several aspects of the CBRAM technology. CBRAM devices are based on nanoionics transport and chemo-physical reactions to create filamentary conductive paths across a dielectric sandwiched between two metal electrodes. These nano-size filaments can be formed and ruptured reliably and repeatedly by application of appropriate voltages. Although, there exists a large body of literature on this topic, many aspects of the CBRAM mechanisms and are still poorly understood. In the next paragraph, the aspects of CBRAM studied in this thesis are spelled out in more detail.
CBRAM cell is not only an attractive candidate for a memory cell but is also a good implementation of a new circuit element, called memristor, as postulated by Leon Chua. Basically, a memristor, is a resistor with a memory. Such an element holds the promise to mimic neurological switching of neuron and synapses in human brain that are much more efficient than the Neuman computer architecture with its current CMOS logic technology. A memristive circuitry can possibly lead to much more powerful neural computers in the future. In the course of the research undertaken in this thesis, many memristive properties of the resistive cells have been found and used in models to describe the behavior of the resistive switching devices.
The research performed in this study has also an immediate commercial application. Currently, the semiconductor industry is faced with so-called latency scaling dilemma. In the past, the bottleneck for the signal propagation was the time delay of the transistor. Today, the transistors became so fast that the bottleneck for the signal propagation is now the RC time delay of the interconnecting metal lines. Scaling drives both, resistance and parasitic capacitance of the metal lines to very high values.
In this context, one observes that resistive switching memory does not require a Si substrate. It is therefore an excellent candidate for its implementation as an o n-chip memory above the logic circuits in the CMOS back-end, thus making the signal paths between logic and memory extremely short. In the framework of a Semiconductor Research Corporation (SRC) project with Intel Corporation, this thesis investigated the breakdown and resistive switching properties of currently deployed low k interlayer dielectrics to understand the mechanisms and potential of different material choices for a realization of an RRAM memory to be implemented in the back-end of a CMOS process flow.