M. Porti

Metal oxide resistive memory switching mechanism based on conductive filament properties

By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament (CF) features controlling TiN/HfO2/TiN resistive memory (RRAM) operations. The leakage current through the dielectric is found to be supported by the oxygen vacancies, which tend to segregate at hafnia grain boundaries. We simulate the evolution of a current path during the forming operation employing the multiphonon trap-assisted tunneling (TAT) electron transport model. The forming process is analyzed within the concept of dielectric breakdown, which exhibits much shorter characteristic times than the electroforming process conventionally employed to describe the formation of the conductive filament. The resulting conductive filament is calculated to produce a non-uniform temperature profile along its length during the reset operation, promoting preferential oxidation of the filament tip. A thin dielectric barrier resulting from the CF tip oxidation is found to control filament resistance in the high resistive state. Field-driven dielectric breakdown of this barrier during the set operation restores the filament to its initial low resistive state. These findings point to the critical importance of controlling the filament cross section during forming to achieve low power RRAM cell switching.

Electrical characterization of stressed and broken down SiO2 films at a nanometer scale using a conductive atomic force microscope

A conductive atomic force microscope (C-AFM) has been used to investigate the degradation and breakdown of ultrathin (<6 nm) films of SiO2 at a nanometric scale. Working on bare gate oxides, the conductive tip of the C-AFM allows the electrical characterization of nanometric areas. Due to the extremely small size of the analyzed areas, several features, which are not registered during macroscopic tests, are observed. In particular, before the oxide breakdown, switchings between different conduction states and sudden changes of conductivity have been measured, which have been related to the prebreakdown noise observed in conventional metal–oxide–semiconductor structures. Moreover, similar switchings have been also measured after the oxide breakdown, which have been related to the opening or closure of conduction channels between the electrodes. The C-AFM has also allowed the determination of the areas in which the degradation and breakdown take place. The results have shown that, although degradation takes...

Resistive switching in hafnium dioxide layers: Local phenomenon at grain boundaries

Overcoming challenges associated with implementation of resistive random access memory technology for non-volatile information storage requires identifying the material characteristics responsible for resistive switching. In order to connect the switching phenomenon to the nano-scale morphological features of the dielectrics employed in memory cells, we applied the enhanced conductive atomic force microscopy technique for in situ analysis of the simultaneously collected electrical and topographical data on HfO2 stacks of various degrees of crystallinity. We demonstrate that the resistive switching is a local phenomenon associated with the formation of a conductive filament with a sufficiently small cross-section, which is determined by the maximum passing current. Switchable filament is found to be formed at the dielectric sites where the forming voltages were sufficiently small, which, in the case of the stoichiometric HfO2, is observed exclusively at the grain boundary regions representing low resistant...

Grain boundaries as preferential sites for resistive switching in the HfO2 resistive random access memory structures

Resistive switching (RS) phenomenon in the HfO2 dielectric has been indirectly observed at device level in previous studies using metal-insulator-metal structures, but its origin remains unclear. In this work, using the enhanced conductive atomic force microscope (ECAFM), we have been able to obtain in situ direct observation of RS with nanometric resolution. The ECAFM measurements reveal that the conductive filaments exhibiting the RS are primarily formed at the grain boundaries, which were shown exhibiting especially low breakdown voltage due to their intrinsic high density of the oxygen vacancies.

Metal oxide RRAM switching mechanism based on conductive filament microscopic properties

By combining electrical, physical, and transport/atomistic modeling results, this study identifies critical conductive filament features controlling TiN/HfO2/TiN resistive memory operations. The forming process is found to define the filament geometry, which in turn determines the temperature profile and, consequently, the switching characteristics. The findings point to the critical importance of controlling filament dimensions during the forming process (polarity, max current/voltage, etc.).

Correlation between the nanoscale electrical and morphological properties of crystallized hafnium oxide-based metal oxide semiconductor structures

The relationship between electrical and structural characteristics of polycrystalline HfO2 films has been investigated by conductive atomic force microscopy under ultrahigh vacuum conditions. The results demonstrate that highly conductive and breakdown (BD) sites are concentrated mainly at the grain boundaries (GBs). Higher conductivity at the GBs is found to be related to their intrinsic electrical properties, while the positions of the electrical stress-induced BD sites correlate to the local thinning of the dielectric. The results indicate that variations in the local characteristics of the high-k film caused by its crystallization may have a strong impact on the electrical characteristics of high-k dielectric stacks.

Nanometer-scale analysis of current limited stresses impact on SiO/sub 2/ gate oxide reliability using C-AFM

A conductive atomic force microscope (C-AFM) has been used to analyze at a nanometer scale the impact of the current limitation on the breakdown (BD) of thin (<6 nm) SiO/sub 2/ gate oxides of metal-oxide-semiconductor (MOS) structures. The high-lateral resolution of the technique (/spl sim/10 nm) allows to get more insight in the BD phenomenology and to study, independently, the effect of the current limit on different post-BD oxide properties such as the oxide conductivity at the primary location where the event is triggered (S/sub 0/) and the size of the broken-down region (S/sub BD/). The results show that the conductivity at S/sub 0/, the total area affected by the BD and the structural damage of the oxide increase when a current limitation is not imposed during the electrical stress, leading to harder BD events. The results demonstrate that the C-AFM is a very suitable tool to perform a complete analysis of the BD phenomenology at such reduced scale.

Current limited stresses of SiO/sub 2/ gate oxides with conductive atomic force microscope

Current limitation effects on the breakdown (BD) of ultrathin SiO/sub 2/ layers have been analyzed at a nanometric scale with a conductive atomic force microscope (C-AFM). Bare oxide regions have been stressed and broken down using the tip of the C-AFM as the metal electrode of a metal-oxide-semiconductor (MOS) structure. BD induced negative charge (BINC) has been observed at the BD location, which has been related to the structural damage generated by the BD event. Moreover, BD, although triggered at one point, is electrically propagated to neighbor regions. The area affected by BD and the amount of BINC (the structural damage) depend on the breakdown hardness. In particular, it is shown that both magnitudes are smaller when the current through the structure is limited during BD transient. Based on the results, a qualitative picture of the breakdown process is presented, which accounts for the current limitation effects.

Nanometer-scale electrical characterization of stressed ultrathin SiO2 films using conducting atomic force microscopy

A conductive atomic force microscope has been used to electrically stress and to investigate the effects of degradation in the conduction properties of ultrathin (<6 nm) SiO2 films on a nanometer scale (areas of ≈100 nm2). Before oxide breakdown, switching between two states of well-defined conductivity and sudden changes of conductivity were observed, which are attributed to the capture/release of single charges in the defects generated during stress.

Degradation of polycrystalline HfO2-based gate dielectrics under nanoscale electrical stress

The evolution of the electrical properties of HfO2/SiO2/Si dielectric stacks under electrical stress has been investigated using atomic force microscope-based techniques. The current through the grain boundaries (GBs), which is found to be higher than thorough the grains, is correlated to a higher density of positively charged defects at the GBs. Electrical stress produces different degradation kinetics in the grains and GBs, with a much shorter time to breakdown in the latter, indicating that GBs facilitate dielectric breakdown in high-k gate stacks.