Nonvolatile Ferroelectric Random Access Memory Research Articles

Large remanent polarization of (Bi,Nd)4Ti3O12 epitaxial thin films grown by metalorganic chemical vapor deposition

(104)-oriented Bi4Ti3O12, La-substituted Bi4Ti3O12[(Bi3.44La0.56)Ti3O12] and Nd-substituted Bi4Ti3O12[(Bi3.54Nd0.46)Ti3O12] films were epitaxially grown on (111)SrRuO3//(111)SrTiO3 substrates at 700 °C by metalorganic chemical vapor deposition. All deposited films showed strong (104) orientations. The values of the remanent polarization (Pr) and coercive field (Ec) of the (104)-oriented epitaxial (Bi3.54Nd0.46)Ti3O12 thin film were 25 μC/cm2 and 135 kV/cm, respectively. This Pr value was larger than that of the (104)-oriented (Bi3.44La0.56)Ti3O12 film: Pr and Ec values of the (Bi3.44La0.56)Ti3O12 were 17 μC/cm2 and 145 kV/cm, respectively. These good ferroelectric properties of (Bi3.54Nd0.46)Ti3O12 films can be explained by a large tilting of TiO6 octahedra induced by the substitution of Nd3+, the ionic radius of which is smaller than that of La3+. Moreover, this Pr value is almost equal to that of commercially used lead zirconate titanate (PZT) films for nonvolatile ferroelectric random access memory (FeRAM) applications. These (104)-oriented epitaxial (Bi3.54Nd0.46)Ti3O12 thin films also showed a fatigue-free character. As a result, lead-free Nd-substituted Bi4Ti3O12 films with good ferroelectric properties comparable with those of PZT films are useful candidates for FeRAM applications.

Ferroelectric Properties of Ta- and Mn-Doped Bi 3.3 La 1.0 Ti 3 O 12 Thin Films

Bi 3.3 LaTi 3 O 12 (BLT), Bi 3.3 LaTi 3 O 12 :Ta (BLT:Ta), and Bi 3.3 LaTi 3 O 12 :Mn (BLT:Mn) thin films are synthesized on Pt/Ti/SiO 2 /Si substrates using a pulsed laser deposition technique. The BLT:Ta and BLT:Mn films enhanced the ferroelectric properties, compared to BLT film. The remnant polarization (2P r ) and the coercive field (2E c ) of BLT:Ta film are 17 w C/cm 2 , and 78 kV/cm, respectively. All of the films show little polarization fatigue up to 5 2 10 10 switching cycles. I-V characteristics of the BLT films exhibit the mechanism of a space charge limited conduction (SCLC). However, BLT:Ta and BLT:Mn films show the Ohmic like behavior with no space-charge region up to the 170 kV:cm. This new BLT:Ta thin film seems to be very attractive for nonvolatile ferroelectric random access memory devices.

Ferroelectrics 1966-2001: An Overview

The number of publications and scientific meetings in the field of ferroelectricity has been increasing exponentially in the last 30 years. In addition to the classical fields of phase transitions and critical phenomena four new sub fields have emerged: a) ferroelectric liquid crystals, b) thin films and integrated ferroelectrics which led to the discovery of 2D ferroelectricity and the development of non-volatile ferroelectric random access memory, c) dipolar glasses and relaxors as well as d) incommensurate systems. Quantum effects have been observed as well.

Ferroelectrics 1966-2001: An Overview

The number of publications and scientific meetings in the field of ferroelectricity has been increasing exponentially in the last 30 years. In addition to the classical fields of phase transitions and critical phenomena four new sub fields have emerged: a) ferroelectric liquid crystals, b) thin films and integrated ferroelectrics which led to the discovery of 2D ferroelectricity and the development of non-volatile ferroelectric random access memory, c) dipolar glasses and relaxors as well as d) incommensurate systems. Quantum effects have been observed as well.

Ferroelectric Properties of Ta- and Mn-Doped Bi 3.3 La 1.0 Ti 3 O 12 Thin Films

Bi 3.3 LaTi 3 O 12 (BLT), Bi 3.3 LaTi 3 O 12 :Ta (BLT:Ta), and Bi 3.3 LaTi 3 O 12 :Mn (BLT:Mn) thin films are synthesized on Pt/Ti/SiO 2 /Si substrates using a pulsed laser deposition technique. The BLT:Ta and BLT:Mn films enhanced the ferroelectric properties, compared to BLT film. The remnant polarization (2P r ) and the coercive field (2E c ) of BLT:Ta film are 17 w C/cm 2 , and 78 kV/cm, respectively. All of the films show little polarization fatigue up to 5 2 10 10 switching cycles. I-V characteristics of the BLT films exhibit the mechanism of a space charge limited conduction (SCLC). However, BLT:Ta and BLT:Mn films show the Ohmic like behavior with no space-charge region up to the 170 kV:cm. This new BLT:Ta thin film seems to be very attractive for nonvolatile ferroelectric random access memory devices.

Preparation of SiNx passivation films for PZT ferroelectric capacitors at low substrate temperatures by catalytic CVD

The feasibility of SiNx films prepared by catalytic chemical vapor deposition (Cat-CVD) at low substrate temperatures was studied for passivation of ferroelectric non-volatile random access memories (FRAMs). First, the influence of exposure to active NH3 gas generated by the heated catalyzer on ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) capacitors was examined. Second, SiNx films were prepared by Cat-CVD at low substrate temperature, at which the ferroelectricity of PZT is not degraded. The ferroelectric degradation of PZT capacitors due to exposure to active NH3 gas strongly depended on the sample temperature and the ambient. However, no degradation occurred when keeping the sample temperature below 200°C at an ambient of 1.3 Pa by controlling the heat flow from the catalyzer. By adjusting the flow rate ratio of SiH4/NH3, the refractive index of SiNx films measured by ellipsometry was controlled to be 2.0 for various substrate temperatures. The dense SiNx films, which were resistive to oxidation in air exposure, were prepared at 200°C at an ambient of 1.3 Pa. The ferroelectric PZT capacitors were not degraded during SiNx film deposition using the Cat-CVD method. The results appeared to demonstrate the feasibility of application of Cat-CVD films to passivation of ferroelectric devices.

Studies of thin film growth and oxidation processes for conductive Ti–Al diffusion barrier layers via in situ surface sensitive analytical techniques

Conducting diffusion barrier layers play a critical role in high-density memory integration. We recently demonstrated that Ti–Al can be used as a diffusion barrier layer for the integration of ferroelectric capacitors with complementary metal–oxide semiconductor devices for the fabrication of nonvolatile ferroelectric random access memories (NVFRAMs). Here, we discuss results from systematic studies designed to understand Ti–Al film growth and oxidation processes using in situ ion beam sputter deposition in conjunction with complementary in situ atomic layer-resolution mass spectroscopy of recoil ion (MSRI) and surface sensitive x-ray photoelectron spectroscopy (XPS). The concurrent MSRI/XPS analysis revealed that amorphous Ti–Al layers produced by tailored sputter-deposition methods are resistant to oxidation to at least 600 °C, and that oxidation occurs only when the a-Ti–Al layers are exposed to oxygen at >600 °C, via the segregation of Ti species to the surface and TiO2 formation. The a-Ti–Al layers discussed in this letter could be used in the double functionality of a bottom electrode/diffusion barrier for the integration of ferroelectric capacitors with Si substrates for the fabrication of NVFRAMs and other devices.

Rhodium and Rhodium Oxide Thin Films Characterized by XPS

RhO2 belongs to the family of conducting platinum group metal oxides, which have attracted attention as new capacitor electrode materials for dynamic random access memories (DRAMs) and nonvolatile ferroelectric random access memories (FeRAMs). Rh, Rh2O3, and RhO2 thin films were prepared by sputtering and their XPS spectra were collected with a monochromatic Al Kα x-ray source. This report includes XPS spectra of Rh 3d and O 1s core regions for these films.

Strain induced (Ba,Sr)TiO3 thin films for Metal-Ferroelectric-Insulator-Semiconductor memory devices

Strain induced Metal-Ferroelectric-Insulator-Semiconductor (MFIS) has been proposed for non-volatile ferroelectric random access memory (NV-FRAM). This MFIS structure employs (Ba,Sr)TiO3 (BST) thin films as a ferroelectric layer. These BST thin films were at paraelectric composition and were transformed to have a tetragonality due to the strain induced into the BST films. X-ray diffraction study on the BST layer revealed that the in-plane and plane normal lattice parameters of BST films were different. This suggests that the strain induced into the BST thin films distorts the BST lattices. In order to obtain the epitaxial nature of the BST thin films as well as to prevent charge injection from silicon to the BST film, yttria stabilized zirconia (YSZ) thin films was used as a buffer layer. The MFIS structures with the strained BST thin films exhibited memory window, i.e., non-volatility of 0.6 V in C-V measurements at the maximum bias voltage of 5 V. We have reported the ferroelectric and structural properties in the strain-induced MFIS structure.

Fabrication and characteriztion of an all-perovskite ferroelectric field effect transistor based on Pb(Zr0.52Ti0.48)O3/LaSrMnO3 heteroepitaxial structures

Ferroelectric field effect transistor (FeFET) has shown great potential in nonvolatile ferroelectric random access memory (NVFRAM) applications. In the present study, we have fabricated an all-perovskite FeFET on (001) LaAlO3 (LAO) by pulsed laser deposition method. The perovskites used for the ferroelectric gate and the semiconducting channel are Pb(Zr0.52Ti0.48)O3 (PZT) and LaSrMnO3 (LSMO) respectively. X-ray diffraction studies in both the Bragg-Brentano and four-circle modes reveal high quality heteroepitaxy in the PZT/LSMO/LAO structures. Optimum channel modulation is obtained by matching the carrier density of the LSMO films with the induced charges at the interface by the ferroelectric PZT layer. This is achieved by controlling the oxygen content of the LSMO during deposition at various ambient oxygen pressures. High thermal stability of the LSMO films against post-deposition heat treatments is demonstrated. Results of the structural and electrical characterizations of the epitaxial PZT/LSMO deposited on MgO/TiN buffered Si for integrated device applications are described.

Development of PZT sputtering method for mass-production

Ferroelectric thin films have been widely expected for nonvolatile ferroelectric random access memory (FeRAM) application. Production process of depositing Pb(Zr,Ti)O 3 (PZT) thin films has been developed by using RF-magnetron sputtering method. It is very important to study stability of Pb content in the PZT films, uniformity of film thickness on large (8 in.) substrate and maintenance periodicity of the sputtering chamber (coping with particles) for the production. The stability for 1000 wafer deposition and the thickness uniformity of 5% were realized. Improving the sputtering equipment, the shield life until particle-break-out became long.

A 0.4-/spl mu/m 3.3-V 1T1C 4-Mb nonvolatile ferroelectric RAM with fixed bitline reference voltage scheme and data protection circuit

A 0.4-/spl mu/m 3.3-V 1T1C 4-Mb nonvolatile ferroelectric random access memory (FRAM) was developed. The FRAM relies on the use of a reference scheme optimally adapted to the entire cell population of an individual device. A simple voltage level detector protects the device against data loss during drops in supply voltage. Finally, a special test mode was implemented to optimize read pulse width. By using these techniques, a high-performance 1T1C 4-Mb FRAM was successfully developed.

Spatial variation of ferroelectric properties in Pb(Zr0.3, Ti0.7)O3 thin films studied by atomic force microscopy

Imaging of the phase and magnitude of the piezoelectric strain in Pb(Zr0.3, Ti0.7)O3 (PZT) capacitors is performed with an atomic force microscope. The imaging reveals a significant spatial dependence of the ferroelectric properties of both fatigued and unfatigued PZT films. We propose that the variation is related to the domain structure of the PZT. Through the measurement of local piezoelectric hysteresis loops and imaging of the piezoelectric strain, areas are observed in fatigued PZT that exhibit hysteresis loops shifted along the polarization axis. In some regions of fatigued samples, the hysteresis loops are shifted such that both remanent points of the hysteresis curve have the same polarization direction. These results have important implications for the scalability of nonvolatile ferroelectric random access memory to higher device densities.

Transmission electron microscopy study of hydrogen-induced degradation in strontium bismuth tantalate thin films

The fabrication of nonvolatile ferroelectric random access memories based on SrBi2Ta2O9 (SBT) or other ferroelectric capacitors require exposure of these capacitors to processing gases mixtures including hydrogen. This results in a strong degradation of the capacitor electrical properties, mainly due to the interaction of hydrogen with the ferroelectric layer. Using surface analysis methods, we previously determined that the hydrogen-induced degradation of SBT capacitors might be partially due to the degradation of the near surface region of the SBT layer. It was also demonstrated that oxygen annealing after the hydrogen exposure results in the recovery of the degraded SBT surface layer and the electrical properties of the capacitor. We have now performed detailed cross sectional transmission electron microscopy studies of virgin, hydrogen, and oxygen annealed SBT/Pt/TiO2/SiO2/Si heterostructures. These studies combined microstructural imaging with a nanoscale compositional analysis of the SBT layer as a function of depth from the free surface of the film and across grain boundaries. High resolution energy dispersive x-ray spectroscopy revealed that hydrogen annealed SBT layers have a deficiency of Bi up to a depth of ∼30 nm underneath the free surface, and that there is accumulation of Bi at the grain boundaries of pristine, hydrogen, and oxygen-recovery annealed SBT layers. This suggests that the diffusion of Bi is largely controlled by grain boundaries and they play a critical role in controlling the electrical properties of SBT capacitors.

Studies of ferroelectric heterostructure thin films and interfaces, via in situ analytical techniques

The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMs) are now incorporated in commercial products such as “smart cards”, while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and interface processes, which play critical roles in film microstructures and properties. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface processes, while SE permits the investigation of buried interfaces as they are being formed. Recent studies on SrBi2Ta2O9 (SBT) and BaxSr1-xTiO3 (BST) film growth and interface processes are discussed.

Lattice Vibrational Properties of SrBi2Ta2O9

AbstractSrBi2Ta2O9 is one of the layered perovskite ferroelectrics with excellent fatigue resistance and thus has gained great interest of semiconductor industries recently in non-volatile ferroelectric random access memory (FERAM) applications. The understanding of the lattice vibrational properties of this material can provide crucial information on its structure and dielectric behavior. In this work, we carried out Raman spectroscopy study of thin film, powder and single crystal SrBi2Ta2O9. At temperatures above the Curie temperature, all 12 Raman modes expected from the tetragonal (I4/mmm) structure have been observed and assigned to the 4 A1g, 6 Eg and 2 B1g modes based the polarized Raman spectra from the single crystal samples. At lower temperature, the Raman spectra have revealed all 22 A1 modes, but only 3 out of the 20 A2 modes and 5 out of the 42 B1/B2 modes were observed in the ferroelectric phase (A21am). The soft mode was found to be of A1 symmetry and appearing only when light polarization lay in the ab-plane.

Metallic bismuth on strontium-bismuth tantalate thin films for ferroelectric memory application

Strontium–bismuth tantalate (SBT) has been actively investigated as an attractive candidate for non-volatile ferroelectric random access memories because of its high resistance to fatigue. However, the ferroelectric property of SBT is easily affected by fabrication process parameters. The relationship between the surface chemistry of SBT thin films and the fabrication process parameters, such as crystallization temperature, upper Pt electrode, annealing in oxygen after Pt electrode fabrication (= the second annealing), hydrogen sintering and ion etching, were examined mainly by XPS analysis. In all specimens, metallic Bi, which is one of the main causes for deterioration of their performances, was observed in addition to oxidic Bi, Sr and Ta. The deposition of an upper Pt electrode resulted exclusively in an increase in metallic Bi content. Sintering in a hydrogen atmosphere and ion etching increased the ratio of metallic Bi to total Bi. The second annealing was effective in suppressing the metallic Bi content. Copyright © 2000 John Wiley & Sons, Ltd.

200 °C Preparation of SiNx Passivation Films for PZT Ferroelectric Capacitors by Catalytic CVD

AbstractFeasibility of SiNx passivation films at low substrate temperatures prepared by catalytic chemical vapor deposition (Cat-CVD) is studied for ferroelectric nonvolatile random access memories (FRAMs). SiNx films were prepared at low substrate temperatures of 100 °C, 175 °C and 200 °C using Cat-CVD. Adjusting on flow rate ratio of SiH4/NH3, the refractive index of SiNx film, deposited at 175 °C and 200 °C, measured by ellipsometry is controlled around 2.0. SiNx films, with the refractive index around 2.0, deposited at only 200 °C show the following properties. 1) No oxidation during air exposure for 3 months was observed for the films. 2) Etching rate of the films in buffered HF is 20 nm/min. The dense SiNx film, which is resistive for oxidation in air exposure and dissolution in buffered HF, is prepared at 200 °C and the film is suitable to the passivation of ferroelectric capacitors.

Studies of ferroelectric heterostructure thin films, interfaces, and device-related processes via in situ analytical techniques

The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMs) are now incorporated in commercial products such as “smart cards”, while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and device processes relevant to device development. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface processes, while SE permits the investigation of buried interfaces as they are being formed. Recent studies on SrBi2Ta2O9 (SBT) and BaxSr1-xTiO3 (BST) film growth and interface processes are discussed. Direct imaging of ferroelectric domains under applied electric fields can provide valuable information to understand domain dynamics in ferroelectric films. We discuss results of piezoresponse scanning force microscopy (SFM) imaging for nanoscale studies of polarization reversal and retention loss in Pb(ZrxTi1-x)O3 (PZT)-based capacitors. Another powerful technique suitable for in situ, real-time characterization of film growth processes and ferroelectric film-based device operation is based on synchrotron X-ray scattering, which is currently being implemented at Argonne National Laboratory.

Low temperature processed 0.7SrBi2Ta2O9–0.3Bi3TaTiO9 thin films fabricated on multilayer electrode-barrier structure for high-density ferroelectric memories

Thin films of solid-solution material 0.7SrBi2Ta2O9–0.3Bi3TaTiO9 (0.7SBT–0.3BTT) were fabricated on n+-polycrystalline (n+-poly) Si substrates by a metalorganic solution deposition technique at a low processing temperature of 650 °C using a Pt–Rh/Pt–Rh–Ox electrode-barrier structure. The Pt–Rh/Pt–Rh–Ox structure was deposited using an in situ reactive radio frequency sputtering process. The electrodes had a smooth and fine-grained microstructure and were excellent diffusion barriers between the 0.7SBT–0.3BTT thin film and Si substrate. The ferroelectric (0.7SBT–0.3BTT) test capacitors using these electrode-barrier grown directly on Si showed good ferroelectric hysteresis properties, measured through n+-poly Si substrate, with 2Pr and Ec values of 11.5 μC/cm2 and 80 kV/cm, respectively, at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics (<10% decay) under bipolar stressing up to 1011 switching cycles and the leakage current density was lower 10−7 A/cm2 at an applied electric field of 200 kV/cm. The good ferroelectric properties of 0.7SBT–0.3BTT solid-solution thin films at a low processing temperature of 650 °C and excellent electrode-diffusion barrier properties of a Pt–Rh/Pt–Rh–Ox structure are encouraging for the realization of high-density nonvolatile ferroelectric random access memories on silicon substrates.