TiN Top Electrode Research Articles

A Comprehensive Study on the Effect of TiN Top and Bottom Electrodes on Atomic Layer Deposited Ferroelectric Hf0.5Zr0.5O2 Thin Films.

The discovery of ferroelectricity in HfO2-based materials in 2011 provided new research directions and opportunities. In particular, for atomic layer deposited Hf0.5Zr0.5O2 (HZO) films, it is possible to obtain homogenous thin films with satisfactory ferroelectric properties at a low thermal budget process. Based on experiment demonstrations over the past 10 years, it is well known that HZO films show excellent ferroelectricity when sandwiched between TiN top and bottom electrodes. This work reports a comprehensive study on the effect of TiN top and bottom electrodes on the ferroelectric properties of HZO thin films (10 nm). Investigations showed that during HZO crystallization, the TiN bottom electrode promoted ferroelectric phase formation (by oxygen scavenging) and the TiN top electrode inhibited non-ferroelectric phase formation (by stress-induced crystallization). In addition, it was confirmed that the TiN top and bottom electrodes acted as a barrier layer to hydrogen diffusion into the HZO thin film during annealing in a hydrogen-containing atmosphere. These features make the TiN electrodes a useful strategy for improving and preserving the ferroelectric properties of HZO thin films for next-generation memory applications.

Physical chemistry of the TiN/Hf0.5Zr0.5O2 interface

Ferroelectric hafnia-based thin films are promising candidates for emerging high-density embedded nonvolatile memory technologies, thanks to their compatibility with silicon technology and the possibility of 3D integration. The electrode–ferroelectric interface and the crystallization annealing temperature may play an important role in such memory cells. The top interface in a TiN/Hf0.5Zr0.5O2/TiN metal–ferroelectric–metal stack annealed at different temperatures was investigated with X-ray photoelectron spectroscopy. The uniformity and continuity of the 2 nm TiN top electrode was verified by photoemission electron microscopy and conductive atomic force microscopy. Partial oxidation of the electrode at the interface is identified. Hf is reduced near the top interface due to oxygen scavenging by the top electrode. The oxygen vacancy (VO) profile showed a maximum at the top interface (0.71%) and a sharp decrease into the film, giving rise to an internal field. Annealing at higher temperatures did not affect the VO concentration at the top interface but causes the generation of additional VO in the film, leading to a decrease of the Schottky Barrier Height for electrons. The interface chemistry and n-type film doping are believed to be at the origin of several phenomena, including wake-up, imprint, and fatigue. Our results give insights into the physical chemistry of the top interface with the accumulation of defective charges acting as electronic traps, causing a local imprint effect. This may explain the wake-up behavior as well and also can be a possible reason of the weaker endurance observed in these systems when increasing the annealing temperature.

Role of an Interfacial Layer in Ta2O5‐Based Resistive Switching Devices for Improved Endurance and Reliable Multibit Operation

A comparative study of two electrode materials is conducted to demonstrate the effect of the electrode material on the resistive switching (RS) behavior of Ta2O5‐based devices. Compared with a Pt top electrode (TE), application of a TiN TE shows an improved endurance of up to 109 cycles and a highly reliable 4 bit (16 states) operation. Various structural analyses reveal that this distinctive RS performance originates from an oxygen‐deficient layer (TaOx) between the Ta2O5 and TiN TEs. This interfacial layer improves the endurance of the device by acting as an oxygen reservoir that prevents the severe consumption of oxygen during repetitive RS and makes the device compatible with electrical pulse–based operations for stable multibit operation by acting as an appropriate load resistor. The results herein suggest that the optimized interfacial layer is essential for the development of high‐performance RS devices for future nonvolatile memory applications.

Resistive switching in a LaMnO3 + δ/TiN memory cell investigated by operando hard X-ray photoelectron spectroscopy

Transition metal oxides are promising candidates in the development of valence change memories thanks to their ability to present the valence change mechanism. The resistive switching mechanism of TiN/LaMnO3 + δ (LMO)/Pt devices was investigated by operando hard X-ray photoelectron spectroscopy after careful in situ electrical characterization. The results presented here highlight the oxygen exchange process at the TiN/LMO interface. The active TiN top electrode acts as an oxygen getter, pumping O2− anions that are attracted by the positive bias and repelling them under negative bias. This drift of charged defects is correlated with variations of the interfacial resistance. Our results confirm the critical role of the TiN/LMO interface and oxygen drift in the resistive switching behavior of such devices.

Effect of hydrogen derived from oxygen source on low-temperature ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors

The ferroelectric (FE) properties of 10-nm-thick Hf0.5Zr0.5O2 (HZO) films deposited by an atomic layer deposition technique were improved by adopting O3 as an oxygen source instead of H2O. All HZO films were annealed at 400 °C for 1 min in an N2 atmosphere after TiN top electrode deposition. Regardless of the oxygen source, the HZO films exhibited the formation of a noncentrosymmetric orthorhombic phase, which is responsible for FE behavior with the suppression of the monoclinic phase. However, compared to the O3-based HZO film, it was confirmed that the H2O-based HZO film was more incorporated with hydrogen derived from H2O, thereby degrading FE polarization and leakage behavior. The results indicate that the strategy of using O3 as the oxygen source is useful for the fabrication and integration of FE HZO films for next-generation memory applications.

Exploring New Metal Electrodes for Ferroelectric Aluminum-Doped Hafnium Oxide

In this paper, we explore new metal electrodes for ferroelectric capacitors based on Al-doped HfO2. We find that Ti/Pd, Ti/Au, and W top electrodes can induce much higher remanent polarization as compared to the traditional TiN top electrode. The endurance of the capacitors with Ti/Pd electrodes is also much better than that with TiN and W electrodes. These results indicate that Ti/Pd is a very promising candidate for ferroelectric Al-doped HfO2. In addition, we find that the remanent polarization reaches maximum when the annealing temperature is around 900 °C–950 °C. At a given annealing temperature, the optimal Hf-to-Al cycle ratio corresponding to the highest remanent polarization is around 23:1. With optimized process conditions, we demonstrate high-performance Ti/Pd gated ferroelectric Al-doped HfO2 capacitors with remanent polarization up to $20~\mu \text{C}$ /cm2, endurance higher than 108 cycles, and retention over ten years at room temperature. Another interesting feature of the ferroelectric capacitors with Ti/Pd electrodes is high tunability of polarization by external pulses, which will be important for neurosynaptic computing applications.

Stress-Induced Crystallization of Thin Hf1- XZr XO2 Films: The Origin of Enhanced Energy Density with Minimized Energy Loss for Lead-Free Electrostatic Energy Storage Applications.

Increasing interest in the development of alternative energy storage technologies has led to efforts being taken to improve the energy density of dielectric capacitors with high power density. However, dielectric polymer materials still have low energy densities because of their low dielectric constant, whereas Pb-based materials are limited by environmental issues and regulations. Here, the energy storage behaviors of atomic layer-deposited Hf1- XZr XO2 ( X = 0-1) thin films (10 nm) and the phase transformation mechanism associated with an enhancement of their energy density are reported using unipolar pulse measurements. Based on electrical and material characterization, the energy density and energy efficiency are dependent on the Zr content, and stress-induced crystallization by the encapsulating Hf1- XZr XO2 films with TiN top electrodes prior to annealing can enhance the energy density (up to 47 J/cm3 at a small voltage value of 3.5 MV/cm) while minimizing energy loss even at low process temperatures (400 °C). This work will facilitate the realization of Hf1- XZr XO2-based capacitors for lead-free electrostatic energy storage applications.

Nucleation‐Limited Ferroelectric Orthorhombic Phase Formation in Hf0.5Zr0.5O2 Thin Films

AbstractVarious possibilities have been proposed as the cause of the doped‐ or undoped‐HfO2 thin film materials showing unusual ferroelectricity. These assumptions are based on empirical results, yet finding the origin of the unprecedented ferroelectricity within HfO2 has suffered from a serious gap between its theoretical calculation, mostly based on thermodynamic approach and the actual experimental results. To fill the gap, this study proposes to consider the kinetic energy, providing the evidence of the kinetic energy barrier upon a phase transformation from the tetragonal phase to the monoclinic phase affected by the TiN top electrode (capping layer). 10 nm thick Hf0.5Zr0.5O2 thin films are deposited and annealed with or without the TiN capping layer with subsequent annealing at different time and temperature. Arrhenius plot is constructed to obtain the activation energy for the tetragonal‐to‐monoclinic phase transformation by calculating the amount of the transformed phase using X‐ray diffraction pattern. Johnson–Mehl–Avrami and nucleation‐limited transformation models are utilized to describe the characteristic nucleation and growth time and calculate the activation energy for the monoclinic phase transformation of the Hf0.5Zr0.5O2 thin film. Both models demonstrate that the TiN capping layer provides a kinetic energy barrier for tetragonal‐to‐monoclinic phase transformation and enhances the ferroelectric property.

Low-voltage operation and high endurance of 5-nm ferroelectric Hf0.5Zr0.5O2 capacitors

In this letter, the ferroelectric (FE) properties of 5-nm-thick Hf0.5Zr0.5O2 (HZO) films deposited by atomic layer deposition have been investigated. By reducing the HZO film thickness to 5 nm, low-voltage operation (1.0 V) of the HZO-based capacitor was achieved while maintaining a remnant polarization (Pr) of about 10 μC/cm2 (i.e., 2Pr of 20 μC/cm2). Meanwhile, in order to form an orthorhombic phase, which is responsible for FE properties, a rapid thermal annealing process was performed after TiN top electrode deposition. The FE properties were realized after low temperature annealing (450 °C for 1 min), making them compatible with the back-end of the line. In addition, the low operating voltage and the suppression of an additional monoclinic phase formation by stress-induced crystallization induced a robust endurance (>1010 cycles at 1.2 V) of the 5-nm-thick HZO sample.

Compatibility of HfN Metal Gate Electrodes With Hf0.5Zr0.5O2 Ferroelectric Thin Films for Ferroelectric Field-Effect Transistors

Compatibility of hafnium nitride (HfN) electrodes with Hf0.5Zr0.5O2 (HZO) ferroelectric thin films was investigated for ferroelectric field-effect transistor (FeFET) applications. It was found that HZO thin films capped by HfN top electrodes offer improved ferroelectric properties and lower leakage current densities compared to HZO thin films capped by TiN top electrodes. Moreover, FeFETs with HfN/HZO (10 nm)/SiO2 gate-stacks showed a fast write speed of 100 ns, a large 10-year extrapolated memory window (MW) of 0.92 V, and a moderate MW of 0.5 V after 104 program/erase cycles. This letter represents a first attempt to fabricate high-performance FeFET memory devices with a fully hafnium-based gate-stack.

Electrical Properties of ZrO2/Al2O3/ZrO2‐Based Capacitors with TiN, Ru, and TiN/Ru Top Electrode Materials

To improve the electrical properties of metal/insulator/metal capacitors for dynamic random access memory, the effects of the top electrode materials and their structures on the capacitor performance are examined. Three kinds of top electrode types (TiN, Ru, and TiN/Ru) are sputter‐deposited on ZrO2/Al2O3/ZrO2 (ZAZ) dielectric layers grown via atomic layer deposition (ALD) on TiN bottom electrodes. The TiN/Ru top electrode samples show the highest capacitance density among the three types of top electrodes, and the Ru and TiN/Ru top electrodes show less leakage current density than the TiN top electrode. The interface property is optimized when the Ru directly contacts the insulating layer due to its higher work function. The TiN layer on the 2 nm‐thick Ru top electrode decreases the adverse interfacial reaction layer (TiOxNy) of the dielectric/TiN bottom electrode through the scavenging oxygen atoms.

Elimination of endurance degradation by oxygen annealing in bilayer ZnO/CeO2-x thin films for nonvolatile resistive memory

Effect of oxygen annealing on bipolar resistive switching (BRS) properties of TiN/ZnO/CeO2-x/Pt devices was investigated. Bilayer ZnO/CeO2-x thin films were fabricated by rf-magnetron sputtering. It was observed that the improvement in cycle-to-cycle endurance degradation and uniformity of the bilayer ZnO/CeO2-x thin film is optimum at 400 °C annealing temperature due to decrease in oxygen vacancies during annealing, as confirmed by x-ray photoelectron spectroscopy. The BRS could be caused by the formation of interfacial TiON layer, which is most likely to be accountable for creating an adequate quantity of oxygen vacancies necessary for the formation and rupture of conductive filaments. Smaller Gibbs free energy of the formation of interfacial TiON (−611kJmol−1) layer as compared to bilayer film ZnO (−650kJmol−1) and CeO2(−1024kJmol−1) results in an easier re-oxidation of the filaments through the oxygen exchange with TiN top electrode. The analysis of current–voltage characteristics shows that the charge transport mechanism is Schottky emission. Moreover, the temperature dependence of high resistance state (HRS) and low resistance state (LRS) revealed the physical origin of the RS mechanism, which entails the oxygen vacancies for the formation and rupture of conducting paths.

Effect of film thickness on the ferroelectric and dielectric properties of low-temperature (400 °C) Hf0.5Zr0.5O2 films

We report on the effect of the Hf0.5Zr0.5O2 (HZO) film thickness on the ferroelectric and dielectric properties using pulse write/read measurements. HZO films of thicknesses ranging from 5 to 20 nm were annealed at 400 °C for 1 min in a nitrogen ambient to be compatible with the back-end of the line thermal budget. As the HZO film thickness decreases, low-voltage operation (1.0 V or less) can be achieved without the dead layer effect, although switching polarization (Psw) tends to decrease due to the smaller grain size. Meanwhile, for 20-nm-thick HZO films prepared under the identical stress (similar TiN top electrode thickness and thermal budget), the Psw and dielectric constant are reduced because of additional monoclinic phase formation.

Comparisons of switching characteristics between Ti/Al2O3/Pt and TiN/Al2O3/Pt RRAM devices with various compliance currents

In this study, the influence of the Ti and TiN top electrodes on the switching behaviors of the Al2O3/Pt resistive random access memory devices with various compliance currents (CCs, 1–15 mA) has been compared. Based on the similar statistical results of the resistive switching (RS) parameters such as Vset/Vreset, RHRS/RLRS (measured at 0.10 V) and resistance ratio with various CCs for both devices, the Ti/Al2O3/Pt device differs from the TiN/Al2O3/Pt device mainly in the forming process rather than in the following switching cycles. Apart from the initial isolated state, the Ti/Al2O3/Pt device has the initial intermediate state as well. In addition, its forming voltage is relatively lower. The conduction mechanisms of the ON and OFF state for both devices are demonstrated as ohmic conduction and Frenkel–Poole emission, respectively. Therefore, with the combined modulations of the CCs and the stop voltages, the TiN/Al2O3/Pt device is more stable for nonvolatile memory applications to further improve the RS performance.

Large ferroelectric polarization of TiN/Hf0.5Zr0.5O2/TiN capacitors due to stress-induced crystallization at low thermal budget

We report on atomic layer deposited Hf0.5Zr0.5O2 (HZO)-based capacitors which exhibit excellent ferroelectric (FE) characteristics featuring a large switching polarization (45 μC/cm2) and a low FE saturation voltage (∼1.5 V) as extracted from pulse write/read measurements. The large FE polarization in HZO is achieved by the formation of a non-centrosymmetric orthorhombic phase, which is enabled by the TiN top electrode (TE) having a thickness of at least 90 nm. The TiN films are deposited at room temperature and annealed at 400 °C in an inert environment for at least 1 min in a rapid thermal annealing system. The room-temperature deposited TiN TE acts as a tensile stressor on the HZO film during the annealing process. The stress-inducing TiN TE is shown to inhibit the formation of the monoclinic phase during HZO crystallization, forming an orthorhombic phase that generates a large FE polarization, even at low process temperatures.

Ferroelectricity in Lu doped HfO2 layers

Doped HfO2 has become a promising candidate for non-volatile memory devices since it can be easily integrated into existing CMOS technology. Many dopants like Y, Gd, and Sr have been investigated for the stabilization of ferroelectric HfO2. Here, we report the fabrication of capacitors comprising ferroelectric HfO2 metal-insulator-metal structures with TiN bottom and top electrodes using the dopant Lu. Amorphous 5% Lu doped HfO2 was deposited by pulsed laser deposition and afterwards annealed to achieve the ferroelectric, orthorhombic phase (space group Pbc21). The polarization of the layers was confirmed by capacitance-voltage, polarization-voltage, and current-voltage measurements. Depending on the anneal temperature, the remanent polarization changes and the initial state of the oxide varies. The layer exhibits initially a pinched hysteresis up to an annealing temperature of 600 °C and an unpinched hysteresis at 700 °C. The maximum polarization is about 11 μC/cm2 which is measured after 104 cycles and stable up to 106 cycles. The influence of the layer thickness on the oxide properties is investigated for 10–40 nm thick HfLuO; however, a thickness dependence of the ferroelectric properties is not observed.

Preparation and characterization of ferroelectric Hf0.5Zr0.5O2 thin films grown by reactive sputtering

HfO2-ZrO2 solid-solution films were prepared by radio frequency sputtering, and the subsequent annealing process was optimized to render enhanced ferroelectric behavior. The target power, working pressure and O2 partial pressure ratios were varied, along with the annealing gas, time and temperature. Then, the film’s structural and electrical properties were carefully scrutinized. Oxygen-deficient conditions were necessary during the sputter deposition to suppress grain growth, while annealing by O2 gas was critical to avoid defects and leakage problems. It is expected that the grain size difference under various deposition conditions combined with the degree of TiN top and bottom electrode oxidation by O2 gas will result in different ferroelectric behaviors. As a result, Hf0.5Zr0.5O2 prepared by radio frequency sputtering showed optimized ferroelectricity at 0% of O2 reactive gas, with a doubled remnant polarization value of ∼20 μC cm−2 at a thickness of 11 nm. Film growth conditions with a high growth rate (4–5 nm min−1) were favorable for achieving the ferroelectric phase film, which feasibly suppressed both the grain growth and accompanying monoclinic phase formation.

Electron transport across ultrathin ferroelectric Hf0.5Zr0.5O2 films on Si

In this work, we report on the electron transport properties of ultrathin (~2.5nm) ferroelectric polycrystalline Hf0.5Zr0.5O2 (HZO) films grown by the Atomic Layer Deposition (ALD) technique directly on the highly doped Si substrate. On the devices with TiN top electrode we demonstrate the reversible electroresistance change R↑/R↓ up to 3 across HZO layer following the polarization reversal. We further correlate the electroresistance switching with the polycrystalline structure of the the HZO layer, the polarization measurements, the current lateral distribution over the HZO layer as well as DFT calculations of the electronic band structure. Such analysis reveals that the polarization switching in ferroelectric HZO grains contribute to the current transport in TiN/HZO/n+-(p+-)Si devices.

Atomic-scale quantification of interdiffusion and dopant localization in GeSbTe-based memory devices

Fabrication of phase-change memory devices at modest or ambient temperatures leads to nanoscale compositional variations in phase-transition layers, where amorphous-polycrystalline phase change takes place via electrical switching, and can alter the device's performances. Here, by transmission electron microscopy and atom probe tomography, we address that thermal annealing at 400 °C for 20 min induces an elemental interdiffusion in the devices consisting of TiN (top electrode), carbon-doped GeSbTe (phase-transition layer), and TiSiN (bottom heater). With respect to the employed annealing process, the Ge atoms of GeSbTe layer have diffused into TiSiN layer at a given sample volume, while the Ti atoms of TiSiN layer into GeSbTe layer. Furthermore, non-random nature of dopant distribution in the GeSbTe materials leads to a Ti-localization including dopants at the GeSbTe/TiSiN interfaces. Our findings have two important implications: First, the annealing-driven interdiffusion of Ge and Ti is a predominant mechanism responsible for nanoscale compositional variations in GeSbTe layer; second, such an interdiffusion and the resultant dopant localization play a crucial role on the driving force for amorphous-polycrystalline transition of GeSbTe-based memory devices.

Role of High-k Interlayer in ZrO2/High-k/ZrO2 Insulating Multilayer on Electrical Properties for DRAM Capacitor

For < 25 nm DRAM capacitors, large cell capacitance (20 fF/cell), low leakage current density (J ≤ 1.6×10-7 A/cm2) at the operation voltage (0.6 V), maximum process temperature < 650 oC and conformal films on a three-dimensional structure (pedestal structure) with a large aspect ratio over 30 are required. Recently, the ZrO2/Al2O3/ZrO2 (ZAZ) multilayer was widely used as insulator of DRAM.[1] Al2O3 has advantages of large band gap and amorphous structure, and disadvantage of low k-value. However, it is still not clear how amorphous structure and band gap of interlayer affect to leakage current property. Here, we pay attention to (Ta/Nb)Ox (TN) because of its high k-value (~29), amorphous structure and low band gap (4.3 eV).[2] In this paper, we studied role of various interlayer between ZrO2 layers on leakage current property for DRAM capacitors with TiN electrode. We fabricated four TiN capacitors with ZrO2/ZrO2 (ZZ), ZAZ, ZrO2/(Ta/Nb)Ox/ZrO2 (ZTNZ), and ZrO2/(Ta/Nb)Ox-Al2O3/ZrO2 (ZTNAZ) multilayer by atomic layer deposition (ALD) and post-deposition annealing (PDA) processes. The ZrO2, Al2O3, and TN layers were deposited by ALD using (C5H5)Zr[N(CH3)2]3, Al(CH3)3, and (Ta/Nb = 1/1)(NtAm)(NMe2)3 precursor, respectively, and H2O gas. The TNA nano-laminate layers were fabricated by alternately depositing every 1 cycle using TN and Al2O3. After multilayer fabrication, PDA was performed at 600 °C. Finally, TiN top electrode was fabricated by photo-litho. process. Fig. 1 shows relationship between CET and J at 0.6 V for all capacitors. The J values of the ZTNAZ and ZAZ capacitors decrease significantly by 10-7 ~ 10-8 A/cm2 order of magnitude while ZTNZ exhibits large J of ~ 10-5 A/cm2. Moreover, the J value of the ZTNAZ (CET = 1.2 nm) is lower than that of the ZAZ (CET = 1.3 nm). AFM images and RMS roughness of ZZ (CET = 1.1 nm), ZTNZ (CET = 1.2 nm), ZAZ (CET = 1.3 nm), and ZTNAZ (CET = 1.2 nm) are shown in Fig. 2. The RMS roughness of all samples exhibit almost the same value of around 1 nm. Based on these experimental data, we found that the leakage current property of capacitor is influenced by band gap width (conduction band offset of TiN) rather than amorphous structure. We conclude that the TNA is one of candidate material as interlayer for future DRAM. A part of this research is supported by CREST, JST. [1] D. Martin et al., J. Appl. Phys. 113, 194103 (2013). [2] T. Nabatame et al., J. Vac. Sci. Technol. A 33, 01A118 (2015). Figure 1