ICMCTF2000 Session H3-2: Ferroelectric DRAM's

Tuesday, April 11, 2000 8:30 AM in Room Sunrise

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	H3-2-1 Fundamental Understanding of Materials Issues in Ferroelectric Thin Films X.X. Xi (The Pennsylvania State University) The properties of ferroelectrics in the thin film form are often different from those of their bulk counterparts. Among various critical materials parameters, strain, oxygen vacancies, and interfacial effects have been recognized to play important roles in determining the thin films properties. In pulsed-laser-deposited SrTiO$_3$ thin films, we found that under strain, the thin films are in a tetragonal structure at room temperature instead of a cubic structure as observed in single crystals. The cubic-to-tetragonal structural phase transition, which occurs in single crystals at about 105 K, is shifted to above 800 K. In films with little strain, the transition occurs at 125 K. In both cases, Raman scattering shows the symmetry-forbidden optical phonons, indicating the reduction of symmetry, caused most likely by local defects such as oxygen vacancies. The line shape of the polar phonon shows a Fano asymmetry, indicating the interaction with a polar continuum of excitation, which we suggest to be due to the micro polar regions around the oxygen vacancies. The temperature dependence of the low-frequency complex dielectric constant shows markedly distinct behaviors from those of the single crystals, which is consistent with the 2D Ising model in transverse field if one assumes a stronger ferroelectric coupling with respect to the quantum fluctuations in the thin films. Besides the "dead layers" effect at the interface with the electrodes, we found that the dielectric constant decreases in the film is due to the soft mode hardening, in agreement with the LST relation. The phonon frequencies are also measured under the electric-field bias using Raman scattering, and a non-vanishing field-induced soft-mode hardening is observed up to room temperature. These lattice dynamics results are important for the understanding of the dielectric and ferroelectric properties in the thin films.
9:10 AM		H3-2-3 Structural and Electrical Properties of Sr_1-xBa_xBi2Ta209 Films R. Melgarejo, M.S. Tomar, P. Dobal, R.S. Katiyar (University of Puerto Rico) Bismuth based layered and perovskite tantalates, titanates and niobates have received special attention for ferroelectric and dielectric materials. We have synthesized Sr1-xBaxBi2Ta2O9 by a solution route for various Ba/Sr ratios. Thin films were deposited by spin coating. Films were characterized by x-ray diffraction, Raman spectroscopy, and dielectric spectroscopy. The results indicate that the films were polycrystalline and stoichiometric at the process temperature about 650 degree C. They make a complete solid solution for a complete range of Ba to Sr ratios. These films could be used for ferroelectric and microwave devices.
9:30 AM		H3-2-4 Influence of Excess Lead and Oxygen Contents on the Microstructure and the Feroroelectric Properties of Pb(Zr_0.52 Ti_0.48 )O₃ Thin Films H.H. Park, S.M. Ha, W.S. Kim (Yonsei University, Korea) There have been a lot of works to improve the ferroelectric properties of PZT by preparing PZT film with excess contents of Pb and O. Besides the prevention from creating defects by volatilization, excess Pb and O seems to contribute to lowering crystallization temperature and improving ferroelectric properties of PZT films, especially fatigue property. But there were few studies about the exact role of excess Pb and O on the improved properties. In this work, we investigated the effect of excess Pb and O contents on the ferroelectric properties of Pt/Pb(Zr_0.52Ti_0.48)O₃/Pt system. PZT films were sputter-deposited from various single-ceramic targets containing 50, 75, or 100 % excess Pb and O with varying substrate heating temperature and successive annealing temperature. Through the ferroelectric analyses, the effects of excess Pb and O contents on the microstructure and the ferroelectric properties of PZT films could be explained.
10:30 AM	Invited	H3-2-7 Oxidation Resistant Diffusion Barrier Layers for Integration of Ferroelectric Capacitors on Si S. Aggarwal, R. Ramesh (University of Maryland) Integration of ferroelectric capacitors on Si requires the development of electrically conducting diffusion barrier layers between the polycrystalline Si plug and the ferroelectric capacitor. This barrier layer must prevent the inter-diffusion of species from the two components namely the transistor and capacitor of the high-density 1T – 1C memory. Furthermore, the barrier layer should itself resist oxidation at the growth temperatures of the ferroelectric capacitor. The oxidation resistance can be enhanced using different approaches such as regulating the thermal budget and controlling the crystallinity. We have investigated the Ti-Al intermetallic material system for application as a conducting diffusion barrier. La-Sr-Co-O (LSCO) / Pb-Zr-Ti-Nb-O / La-Sr-Co-O ferroelectric capacitors were fabricated on Ti-Al/polycrystalline-Si / Si substrates, where the Ti-Al layer provides the double functionality of diffusion barrier and metallic component of a hybrid La-Sr-Co-O / metal bottom electrode. The crystallinity of the Ti-Al layer determines its oxidation resistance during fabrication of the ferroelectric heterostructure. The crystalline Ti-Al layer oxidizes during the fabrication of the ferroelectric capacitor and forms a Schottky-type contact to the LSCO electrode. In contrast, the amorphous Ti-Al barrier layer forms an Ohmic contact to the oxide electrode. For crystalline Ti-Al, X-ray photoelectron spectroscopy (XPS) shows the formation of an Al2O3 layer induced by the segregation of Al to the surface of the Ti-Al layer during annealing of the LSCO layer. For amorphous Ti-Al, XPS reveals that no Al2O3 layer is formed. In addition, Rutherford backscattering analysis shows almost no difference in the Ti-peak spectrum before and after deposition of LSCO. Finally, transmission electron microscopy studies show no reaction layer in the amorphous Ti-Al barrier, as compared to a ~ 20nm thick reaction zone in the crystalline Ti-Al layer. This work has been supported by the NSF-MRSEC grant #DMR96-32521 and Bellcore.
11:10 AM		H3-2-9 Leakage Current Mechanism in (Ba,Sr)TiO₃ Thin Films with Oxide Electrodes B. Nagaraj, S. Aggarwal, R. Ramesh (University of Maryland) Interfaces and hence electrodes determine the performance of (Ba,Sr)TiO3 (BST) capacitors for ultra large scale integration dynamic random access memories. Electrode materials forming a rectifying contact on BST drastically reduce the dielectric constant and hence the capacitance and charge storage density of the capacitor, when the dielectric thickness is reduced. This can limit the role of Pt as an electrode material for giga-bit dynamic random access memories (DRAM). The conducting oxide, La0.5Sr0.5CoO3 (LSCO) with its perovskite structure, has structural and chemical compatibility with BST. A 75nm BST film with LSCO electrodes shows a leakage current density of 1x10-7A/cm2 at 1V, 85°C. The dielectric constant at 1V, 105 Hz is 350, making LSCO a potential contact electrode for DRAM memories. A detailed investigation of the mechanism of conduction in LSCO/BST/LSCO capacitor will be presented. The preliminary investigation shows that the mechanism of conduction in these structures is bulk limited Poole Frenkel emission. This work has been supported by the NSF-MRSEC grant #DMR96-32521.
11:30 AM		H3-2-10 Studies of Hydrogen-Induced Degradation Processes inSrBi₂Ta₂O₉ Thin Films Using Complementary In Situ Mass Spectroscopy of Recoiled Ions and Cross Section TEM Analysis O. Auciello, J. Im, A.R. Krauss (Argonne National Laboratory); N. Poonawala, V.P. Dravid (Northwestern University) It is known that the forming gas (N₂}-H₂ mixture) annealing process required for microcircuit fabrication results in electrical degradation of SrBi₂Ta₂O₉ (SBT) ferroelectric capacitors, mainly due to the interaction of H₂ with the ferroelectric layer. We performed mass spectroscopy of recoiled ions (MSRI) analysis which revealed a depletion of Bi on the surface of SBT layers exposed to hydrogen at high temperatures (~500C), resulting in very high leakage currents and polarization degradation. Annealing in oxygen at 700-800 °C resulted in the replenishment of Bi in the SBT-depleted layer and recovery of ferroelectric properties. X-ray Diffraction (XRD) spectra did not show any significant change between the virgin, H₂ annealed and O₂ recovery-annealed samples. We have performed cross-section TEM studies on pristine, H₂ annealed, and O₂ annealed SBT/Pt/TiO₂/SiO₂/Si heterostructures to investigate the microstructural and microchemical changes induced by the annealing. We have exploited the high spatial resolution of a STEM-EDS probe (~2 nm diameter) to investigate compositional-microstructural-ferroelectric property relationships. Cross-section TEM samples were prepared using a Focused Ion Beam system. EDS line profiles for Bi/Sr, Bi/Ta and Sr/Ta ratios were obtained in the SBT layer as a function of distance from the surface and grain-grain distance across grain boundaries. The TEM data revealed a Bi depletion layer extending up to ~30 nm from the SBT surface of the hydrogen-annealed sample, and a slight excess of Bi at the grain boundaries. The oxygen-annealed sample revealed a replenishment of Bi in the previously depleted near surface region, and an accumulation of Bi at the grain boundaries. We believe that the displacement of Bi throughout the SBT layer during H- and O- annealing is largely controlled by grain boundaries. *Work supported by the U.S. Department of Energy, BES-Material Sciences, under Contract W-31-109-ENG-38 and NSF.
11:50 AM		H3-2-11 Recovery of Forming Gas Damaged Pb(Nb,Zr,Ti)0₃ Capacitors S. Aggarwal, R. Perusses, C.J. Kerr, R. Ramesh (University of Maryland); D.B. Romero (NIST) A low temperature forming gas (4% H2, balance N2) anneal is performed at the end of CMOS processing to passivate Si. However, ferroelectric capacitors become leaky and no longer exhibit ferroelectricity when subjected to hydrogen in forming gas anneals. This poses a challenge for the integration of ferroelectric materials on Si and demands a thorough understanding of interaction of hydrogen with ferroelectric materials. In an earlier publication we established that loss of ferroelectricity in Pb(Zr,Ti)O3-based capacitors is due to the incorporation of hydrogen and subsequent formation of [OH]-1 bonds between the ionized hydrogen and oxygen ions along the polarization axis in the octahedra. In this paper, we report on the recovery of fully integrated Pb(Nb,Zr,Ti)O3 ferroelectric capacitors damaged during forming gas (4% H2, balance N2) annealing. The capacitors were encapsulated using TiOx and SiO2 as inter-level dielectrics to prevent any loss of oxygen or lead. Hydrogen however diffused into the ferroelectric film leading to the loss of ferroelectricity. To recover the properties of the capacitor, the fully integrated structure was annealed in N2 ambient to drive the hydrogen out. Raman scattering experiments performed in the high frequency regime to detect the [OH–] stretching vibration mode confirmed the removal of hydrogen after annealing in N2. The ferroelectric properties, including polarization and resistivity of the capacitors, were restored to their initial values prior to damage. This shows that the process of hydrogen damage is reversible with the time to recovery being dependent on the amount of hydrogen in the forming gas. This work has been supported by the NSF-MRSEC grant #DMR96-32521.