Ch. Turquat

Microstructure and resistance switching in NiO binary oxide films obtained from Ni oxidation

Oxide Resistive Random Access Memories (OxRRAM) are discussed for future high density non volatile memory chips. NiO and other simple binary transition metal oxides (such as TiO2, HfO2 or ZrO2) have recently attracted much attention. In most cases, polycrystalline oxide films are deposited by reactive sputtering on conductive substrates to form bi-stable Metal/Resistive oxide/Metal (MRM) structures. In this paper, an alternative way is explored to obtain NiO films from the controlled oxidation of a Ni metallic film. Different thermal treatments were evaluated to oxidize the metallic film with conditions preventing the complete consumption of Ni film used as bottom electrode. Process parameters of Rapid Thermal Annealing (RTA) route were adjusted to achieve controlled oxidation. Microstructural and electrical analyzes were performed to apprehend the influence of the process parameters on the switching behavior. Reproducible resistive switching phenomena have been demonstrated in Pt/NiO/Ni structures with threshold voltage varying from 2 to 5 V depending on oxidizing conditions.

Sidewalls contribution in integrated three-dimensional Sr0.8Bi2.2Ta2O9-based ferroelectric capacitors

As compared to usual planar capacitors, three-dimensional (3D) ferroelectric capacitors display a large polarization increase due to the additional electrical contribution of the capacitor sidewalls. However, in 3D capacitors the polarization increase with respect to the planar geometry is lower than expected. To uncover the origin of this discrepancy, the microstructure of 3D Sr0.8Bi2.2Ta2O9-based (SBT) capacitors was studied. First, high-resolution synchrotron x-ray diffraction has found a unit cell expansion of the SBT phase in the capacitor sidewalls. From local chemical analyses, this lattice distortion has been correlated to composition variations in this region. These microstructural changes are due to the peculiar topography of the etched bottom electrode and to the variety of layers in contact with the SBT film, which can explain the nonoptimized polarization in 3D capacitors.

Composition control and ferroelectric properties of sidewalls in integrated three-dimensional SrBi2Ta2O9-based ferroelectric capacitors

The difficult scaling of ferroelectric random access memories with the complementary metal-oxide semiconductor technology roadmap requires integration of three-dimensional (3D) ferroelectric capacitors (FeCAP’s). In this work the unusual electrical behavior of 3D FeCAP sidewalls was studied by comparing the electrical properties of two-dimensional and 3D integrated FeCAP structures. We evidenced composition variations of the SrBi2Ta2O9 (SBT) film in the sidewalls with marked bismuth segregation during metal-organic chemical-vapor deposition (MOCVD) of the SBT film. The segregation was reduced after decreasing the deposition temperature from 440°C, whereby the Bi-rich phase in the sidewalls does not contribute to polarization, down to 405°C, whereby sidewall SBT contributes to polarization. After further optimization of the MOCVD conditions at 405°C, the segregation is minimized and the ferroelectric contribution of the sidewall SBT is almost the same as the contribution of the planar SBT. As a result, 3D ...

Integration of resistive switching NiO in small via structures from localized oxidation of nickel metallic layer

Conventional memories approaching their scaling limit, reversible resistance switching effects attract considerable attention because of the potential for high density non volatile memory devices. These resistive switching phenomena have been reported in many simple transition metal oxide films such as TiO2 or NiO deposited by standard sputtering techniques. This paper is investigating the feasibility of emerging resistive-switching stacks enabling integration of the memory element in interconnect structures resulting in very small memory cells. Indeed, we have developed innovative process steps leading to localized formation of bi-stable NiO at the bottom of via structures. Thickness-controlled NiO layers were formed from the partial oxidation of blanket Ni metallic layer through via holes opened in SiO2. Reversible and repetitive switching was demonstrated on arrays of vias with diameter down to 150 nm. Besides, encouraging reliability performances in terms of endurance and retention were obtained.

Improvement of resistance switching characteristics in NiO films obtained from controlled Ni oxidation

Bi-stable resistive switching phenomena controlled by external voltages has drawn much attention for high-density nonvolatile memory devices, the resistive memory materials ranging from organic to inorganic materials, with either metallic or conductive oxide electrodes. These resistive switching phenomena have been reported in many simple transition metal oxide films such as CuOx, TiO2, NiO or ZrO2. The switching mechanism in Metal/Resistive oxide/Metal (MRM) structures has been attributed to the reversible formation/rupture of filamentary conducting paths at the interfaces between metal electrodes and bi-stable oxide film. Recently, we have explored the possibility to form memory cells from the partial oxidation of blanket Ni metallic bottom electrode. The switching characteristics were progressively improved by optimizing annealing conditions and selecting appropriate substrates on which nickel film was sputtered. Reversible and repetitive resistive switching phenomena with set/reset voltages around plusmn1 V were demonstrated in NiO/NiO/Ni structures fabricated from Ni/TiN/Ti/SiO2/Si2N4/Si substrates.

Radiation effects on switching kinetics of three-dimensional ferroelectric capacitor arrays

Effects of x-ray irradiation combined with either bias voltage or bipolar electrical cycling were analyzed on three-dimensional ferroelectric capacitor arrays developed for highly reliable ferroelectric random access memories. Using dedicated experimental setup enabling in situ measurements, switching kinetics were followed over several hours of exposure. Polarization changes and voltage shifts were interpreted in considering interactions between trapping of photoinduced charges and ferroelectric domain structure, which depends upon the capacitor geometry and external field. In memorylike arrays in “written” or “writing” states, high dose of x rays accelerates fatiguelike (polarization reduction) and/or imprintlike (voltage shift) mechanisms that may perturb normal memory operations.

Enhanced oxidation of TiAlN barriers integrated in three-dimensional ferroelectric capacitor structures

The oxidation characteristics of TiAlN films integrated in the bottom electrode (BE) stack of three-dimensional SrBi2Ta2O9-based (SBT) ferroelectric capacitors are investigated in the range of 650–800°C. The patterned TiAlN\Ir\IrO2\Pt BE is encapsulated by a thin ferroelectric SBT film deposited by metal organic chemical vapor deposition and then crystallized ex situ at temperatures higher than 650°C in oxygen. During this annealing step the TiAlN film oxidizes from the lateral side of the patterned BE mesas. Compared to the vertical oxidation in blanket TiAlN layers, the lateral oxidation rate in our capped patterned films is much larger for similar oxidation conditions. This lateral oxidation of the TiAlN is strongly correlated with the in-film SBT stress that depends upon the deposition temperature and the thickness of the SBT film: the higher the tensile stress in the SBT, the larger the TiAlN oxidation length induced. From a kinetic study, the lateral oxygen diffusion was found to be a self-limited p...

Study of Ferroelectric Bi3.25La0.75Ti3O12 Thin Films Deposited by Sol-Gel Method

Powders and thin films of ferroelectric Bi3.25La0.75Ti3O12 (BLT) have been prepared from a sol-gel precursor heated at various temperatures, up to 600°C. The BLT precursor and the BLT powders have been characterized by thermal analyses, X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM). Then, the precursor has been used to deposit thin films of ferroelectric BLT on specific silicon substrates, using a specific spin coating process. The thin films have been characterized by XRD, SEM and ellipsometry. Electrical polarization analyses have been finally performed on these films, and performing remnant polarization has been obtained.

From micrometric to nanometric scale switching of CuTCNQ-based non-volatile memory structures

Resistance switching is an interesting alternative to conventional charge storage devices for future high density storage media. Cu/CuTCNQ(300 nm)/Al memory structures with pad size ranging from 1000 down to 150 ¿m were carefully studied to apprehend their switching behavior. Electrical testing revealed bipolar resistive switching and a shrink of the memory window as the pad-size decreased. Current in low resistance state (LRS) was demonstrated as top pad area dependent. Besides, to investigate their potential scaling, local current measurements performed by conductive-AFM demonstrated the switching of CuTCNQ films at a nanometer scale with a discrimination window of 3 decades between high and low resistance states. Current mappings and local current-voltage characteristics were measured to study the statistical and spatial distribution of switching regions over the CuTCNQ thin film.