Study On Device Characteristic And Modeling Of Resistive Random Access Memory(RRAM)

With the development of communication, computer and personal consumer electronics, the market demand for Non-Volatile Memory is exigent. As the semiconductor process into the 20 nm technology node, the Flash memory, which is based on the traditional floating-gate structure, has encountered its technical bottleneck. Therefore, the emerging memories with high speed, large capacity, high density, excellent reliability and low power become the research hotspots. Resistive random access memory(RRAM) attracts great attention of the academic and industrial fields because of its excellent characteristics. We investigated the performances of the Zr O2- and Si3N4- based RRAM devices, the main contents are as follows:1. The characteristics of Ti/Zr O2/Pt RRAM device are investigated from DC switching, resistive switching mechanism, and conductive mechanism. Capacitance studies have suggested different characters for high and low resistance states: an inductor dominates the LRS and a capacitive element dominates the HRS. The calculation value of the dielectric constant by experiment is significantly greater than actual value of the Zr O2. That means the oxygen vacancies exist in the film at the HRS, and don’t completely recombine with oxygen ions. The effective thickness of the film decreases. The Nyquist diagram of the impedance spectroscopy shows RRAM devices can be equivalent to the resistor and the capacitor in parallel at the HRS. The current driving mode is an efficient method to improve the uniformity of the resistances at HRS/LRS and the storage windows by one order of magnitude. The effects of SET compliance current and RESET stop voltage on the switching parameters are also studied. The device is potentially suitable for multilevel memory application by controlling the magnitude of the SET compliance current or the RESET stop voltage. In addition, the influence of top electrode material is also investigated. Due to the high work function of Pt, there is no stable switching phenomena but only hard breakdown of thin ?lm or very few switching cycles. For Cu or Ni as the device electrode, the films both form the metal filamentary path. However, the switching voltage of Ni device is higher than Cu device.2. The temperature characteristics of Ti/Zr O2/Pt RRAM device is investigated. We analyze the influences of temperature on parameters such as the device resistance,Forming process, SET/RESET voltage and so on. A parasitic capacitance may enable the current through the RRAM device during RESET process to exceed the current compliance limit in SET process. This phenomenon is called the current overshoot. High temperature forming method not only leads to better device uniformity, but also reduces the switching voltage value, and thus suppressing overshoot phenomenon. The scanning voltage stress and constant voltage stress(CVS) on the devices are also analyzed. As the increase of cycles of sweep voltage switching, the switching voltage increase, the on state current decreases, and the off state current increases, resulting in the storage window becoming smaller. In addition, the devices show multilevel transformation under CVS stress. If the CVS stress voltage increases, the time passed by the device to achieve the conversion is short. If the resistance of the LRS is small, RESET time is long by CVS stress.3. A 2-D axisymmetric electro-thermal coupling model is constructed to describe detailed electrical and thermal behavior of RRAM device by the COMSOL Multiphysics. The model takes the ion migration model,the electric conduction model and Joule heating model with material characteristics of dielectric and electrodes into account. The model can simulate the I-V characteristics during RESET/SET process accurately. By solving a set of partial differential equation, the internal parameters of device can be researched. On this basis, the influences of some key factors such as temperature, conductive filament size, thermal and electrical conductivity of electrode material on device transformation processes are simulated and analyzed. It provides a reference for device mechanism analysis and performance improvement.4. The Ti/Si3N4/Au RRAM devices are fabricated and investigated. The devices show stable bipolar resistive switching behavior and the conduction mechanism is dominated by TC-SCLC, with low operation voltages(SET/RESET voltage: ~0.6V/-0.5V) and switching current(as low as ~1m A). The power densities of SET and RESET process are 9.6×10-9 W/μm2 and 8×10-9 W/μm2 respectively, which are much smaller than previous reported results about Si3N4-based resistive switching cells. For this very low power, the HRS-LRS ratio is still ~102. Further, the Ti/Al2O3-Si3N4/Au RRAM devices are fabricated. The device is ohmic conduction mechanism at LRS. Because of the Al2O3 layer, the performance the Ti/Si3N4/Au RRAM devices are improved. Preparation of Si3N4 based RRAM devices and the study of stack structure, are of great significancefor improving the properties of RRAM devices based on nitrides films.