Mihaela Ioana Popovici

Impact of different dopants on the switching properties of ferroelectric hafniumoxide

The wake-up behavior of ferroelectric thin film capacitors based on doped hafnium oxide dielectrics in TiN-based metal–insulator–metal structures is reported. After field cycling a remanent polarization up to 40 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54 pm (Si) to 132 pm (Sr) was evaluated. In all cases, an improved polarization–voltage hysteresis after wake-up cycling is visible. For smaller dopant atoms like Si and Al stronger pinching of the polarization hysteresis appeared with increasing dopant concentration and proved to be stable during cycling.

Atomic Layer Deposition of Strontium Titanate Films Using Sr ( #2#1Cp ) 2 and Ti ( OMe ) 4

Strontium titanate (STO) is a promising candidate as a high-k dielectric for dynamic random access memory application. STO thin films are deposited by atomic layer deposition using Sr( t Bu 3 Cp) 2 , Ti(OMe) 4 , and H 2 O as precursors. Growth and saturation behavior of STO and binary oxides are evaluated by ellipsometry thickness measurements. The precursor pulse ratio controls the amount of Sr and Ti incorporated in STO films. Stoichiometric SrTiO 3 is characterized by the lowest crystallization temperature and largest refractive index, density, and dielectric constant. An excess of Ti or Sr results in an increase in the crystallization onset temperature and contraction or expansion of the cubic cell constant of perovskite SrTiO 3 . Incorporation of more Sr in STO reduces the leakage current density but also increases the capacitance-equivalent thickness.

Electric field cycling behavior of ferroelectric hafnium oxide.

HfO2 based ferroelectrics are lead-free, simple binary oxides with nonperovskite structure and low permittivity. They just recently started attracting attention of theoretical groups in the fields of ferroelectric memories and electrostatic supercapacitors. A modified approach of harmonic analysis is introduced for temperature-dependent studies of the field cycling behavior and the underlying defect mechanisms. Activation energies for wake-up and fatigue are extracted. Notably, all values are about 100 meV, which is 1 order of magnitude lower than for conventional ferroelectrics like lead zirconate titanate (PZT). This difference is mainly atttributed to the one to two orders of magnitude higher electric fields used for cycling and to the different surface to volume ratios between the 10 nm thin films in this study and the bulk samples of former measurements or simulations. Moreover, a new, analog-like split-up effect of switching peaks by field cycling is discovered and is explained by a network model based on memcapacitive behavior as a result of defect redistribution.

Composition influence on the physical and electrical properties of SrxTi1−xOy-based metal-insulator-metal capacitors prepared by atomic layer deposition using TiN bottom electrodes

In this work, the physical and electrical properties of SrxTi1−xOy (STO)-based metal-insulator-metal capacitors (MIMcaps) with various compositions are studied in detail. While most recent studies on STO were done on noblelike metal electrodes (Ru, Pt), this work focuses on a low temperature (250 °C) atomic layer deposition (ALD) process, using an alternative precursor set and carefully optimized processing conditions, enabling the use of low-cost, manufacturable-friendly TiN electrodes. Physical analyses show that the film crystallization temperature, its texture and morphology strongly depends on the Sr/Ti ratio. Such physical variations have a direct impact on the electric properties of SrxTi1−xOy based capacitors. It is found that Sr-enrichment result in a monotonous decrease in the dielectric constant and leakage current as predicted by ab initio calculations. The intercept of the EOT vs physical thickness plot further indicates that increasing the Sr-content at the film interface with the bottom TiN...

Vacancy-modulated conductive oxide resistive RAM (VMCO-RRAM): An area-scalable switching current, self-compliant, highly nonlinear and wide on/off-window resistive switching cell

We report a novel self-compliant and self-rectifying resistive switching memory cell, with area-scalable switching currents, featuring a set current density of ~5nA/nm2 (<;9uA for a 40nm-size cell), high on-state half-bias nonlinearity of 102 and low reset current density of <;0.6nA/nm2 (<;1uA@40nm size). The cell can be operated at below ±4V/10ns, with a large on/off window of >102 and retention extrapolates to 10yr at 101°C. The switching stack is fully based on ALD processes, using common high-k dielectrics and has a thickness of <;10nm, meeting the 3D Vertical RRAM requirements. Moreover, we point out the nonlinearity-low-current operation interdependence and discuss the scaling potential of the areal switching RRAM for reliable sub-μA current operation in the 10nm-cell size realm.

Strontium doped hafnium oxide thin films: Wide process window for ferroelectric memories

Ferroelectricity in hafnium oxide has been reported for the incorporation of Al, Si, Y and Gd or in a solid solution with the chemically similar ZrO2. Here, we report strontium as the first bivalent and — so far — largest dopant in terms of atomic radius also inducing ferroelectric behavior. Besides the solid solution of HfO2/ZrO2 for Sr:HfO2, ferroelectricity is observed in the widest concentration range of all dopants used up to now. First results of ab initio simulations also suggest such a comparatively wide window for ferroelectricity. With a coercive field of about 2 MV/cm another figure exceeds the characteristics reported before. A maximum remanent polarization of 23 μC/cm2 also ranks among the highest values reported until now. The fabricated TiN-Sr:HfO2-TiN capacitors exhibit switching times in the nanosecond range and still retain 80 % of their initial remanent polarization after 106 endurance cycles. The 10 nm ferroelectric thin films prepared by atomic layer deposition are capable of integration into 3D capacitors or FinFETs.

Alternative metals for advanced interconnects

We discuss the selection criteria for alternative metals in order to fulfill the requirements necessary for interconnects at half pitch values below 10 nm. The performance of scaled interconnects using transition metal germanides and CoAl alloys as metallization are studied and compared to conventional Cu and W interconnects.

Leakage Control in 0.4-nm EOT Ru/SrTiO x /Ru Metal-Insulator-Metal Capacitors: Process Implications

Leakage currents as low as 10^-7 A/cm² at both 1 V and -1 V top electrode bias in the sub-0.4-nm equivalent SiO₂ thickness range are demonstrated in Ru/SrTiO_x/Ru metal- insulator-metal capacitors in which the 8.5-nm SrTiO_x layer is deposited by atomic layer deposition. The top electrode material and deposition technique as well as the postdeposition anneal are crucial parameters to control the leakage, not only at negative, but also at positive top electrode bias.

Direct physical evidence of mechanisms of leakage and equivalent oxide thickness reduction in metal-insulator-metal capacitors based on RuOx/TiOx/SrxTiyOz/TiN stacks

We present direct physical evidence supporting the mechanisms we proposed to explain low leakage (Jg) and low equivalent oxide thickness (EOT) in RuOx/TiOx/Sr-rich SrxTiyOz(STO)/TiN metal-insulator-metal capacitors which achieve Jg = 10−7 A/cm2 (1 V) at 0.45 nm EOT: (1) healing of STO traps by O incorporation from RuOx during STO crystallization anneal for low leakage and (2) TiOx/Sr-rich STO intermixing during STO crystallization anneal resulting in a higher Ti-content, higher k-value STO layer for low EOT. Proof of oxygen incorporation into the STO layer from the RuOx electrode as well as of TiOx/STO intermixing were obtained by secondary ion mass spectrometry performed on samples fabricated using O18 for Ru oxidation. We also show excellent match between measured leakage vs. voltage and simulations of trap-assisted leakage with asymmetric trap profiles due to STO trap healing next to RuOx, further supporting our model.

A-VMCO: A novel forming-free, self-rectifying, analog memory cell with low-current operation, nonfilamentary switching and excellent variability

We demonstrate a self-rectifying, compliance-free, BEOL CMOS-compatible, resistive switching memory device, with nonfilamentary switching mechanism, forming-free operation, analog switching behavior and excellent device to device operation uniformity, down to the smallest device size. The cells have a reset switching current density of ~0.3MA/cm2 (and ~10× lower set current). This corresponds to ~5uA reset current in a 40nm-size cell, projecting down to 1uA for a 20nm-size. The switching currents are tunable by process and structural cell design. The cells can be operated with pulses as short as 10ns, at below ±7V. Cycling for at least 106cy and retention of 55°C/3yr are demonstrated, with clear paths for further improvement. These key features are enabled by the use of an amorphous-Silicon (a-Si) barrier layer, which acts as a semi-insulating oxygen scavenger in a dual-layer a-Si/TiO2 active stack, being able to provide nonlinear IV cell characteristics, as well as to induce a large oxygen vacancy density in the switching layer.