Institute of Electroceramic Materials
The research areas of the institute comprise (1) technologies for the integration of electroceramic materials into microelectronics and microsystems, (2) dielectric and ferroelectric properties of oxide ceramics, and (3) the defect structure in the vicinity of internal and external interfaces in oxides. These areas are complementary to the research areas of the Institute for Materials in Electronics 2 (IWE 2) at the Aachen Technical University (RWTH). Project groups often comprise staff members and students from both institutes.
Research within the area of integration technologies is focussed on the fundamental understanding of dry processes. The deposition of oxide thin films is performed by means of MOCVD (MOCVD = Metal Organic Chemical Vapor Deposition) systems. In cooperation with AIXTRON AG, a multiwafer planetary reactor is used for the development and optimization of modified (Ba,Sr)TiO3 films as part of an ESPRIT project. In addition, a horizontal MOCVD reactor has been built for the deposition of new material systems and for the development of alternative vaporizer systems. For the patterning of the ceramic films and electrode / ceramic film stacks, Reactive Ion Beam Etching (RIBE) and Reactive Ion Etching (RIE) techniques will be employed. In contrast to the situation in the standard Si and compound semiconductor technologies, dry etching processes of oxide ceramics have hardly been investigated as yet and, hence, represent a research area in which basic studies and industrially funded applied research can be linked in a beneficial manner. The integration processes are complemented by metallization methods based on electron beam and sputter techniques. Within this area, our studies aim at a better understanding of the processes and material parameters which govern the adhesion, the mechanical stress, and the microstructure. The complementary methods at RWTH Aachen are solution-based deposition (CSD = Chemical Solution Deposition) and etch techniques with topics more strongly concentrated on the design and fabrication of integrated devices.
The second research area focuses on the dielectric and ferroelectric properties of oxide thin films and bulk materials, which are being investigated in Jülich as well as in Aachen. The material systems are based on compositions used for practical devices and model systems, e.g. SrTiO3, BaTiO3, SrBi2Ta2O9, Pb(Zr,Ti)O3 and (Ba1-xPbx)TiO3. One of the research topics is the microscopic understanding of ferroelectric hysteresis including new approaches for the separation of reversible and irreversible contributions to polarisation based on the analysis of frequency-dependent small and large signals. These studies are linked to the aging (imprint) phenomenon, i.e. the polarisation-dependent shift of the hysteresis curve with time and to the ferroelectric fatigue process, i. e. the reduction of the remanent polarisation by cycling. Both aging and fatigue processes play an important role in the operation of the novel non-volatile memories (Ferroelectric Random Access Memories, FeRAM). Up to now, they are only partially understood. In cooperation with external research groups, single domain processes in ferroelectric thin films are studied by means of scanning probe techniques. In addition, scaling effects (grain size effects in ceramics, thickness effects in films) on the transition from the ferroelectric to the superparaelectric state are investigated.
Impedance spectroscopy in the lower GHz regime is employed to determine the relaxation of the ferroelectric domain wall motion and to separate this contribution from the contribution of the crystal lattice. By varying the microstructure of the ceramics and by comparison between bulk ceramics and thin films, the model of Arlt will be extended with respect to the impact on 2D constraints imposed by mechanical stress due to the presence of substrates. For dielectric ceramics, impedance spectroscopy is used to elucidate the interrelation of extrinsic losses and lattice defects. This activity includes the development and characterization of new microwave ceramics and is embedded into a cooperation with the Institute for Microstructural Research (Urban) and Norbert Klein’s group within the framework of a BMBF-Leitprojekt (leading project for the Federal Ministry of Education, Science, Research and Technology (BMBF).
In the case of ferroelectric materials, existing theories will be further developed and extended towards a more quantitative description of the dielectric, piezoelectric, and elastic properties. Numerical finite-element-methods are used to describe the mutually coupled mechanical, thermal, and electrical properties of ceramic components such as multilayer capacitors and actuators. In a new activity, these methods will be extended to piezoelectric microactuators which will be designed and fabricated in cooperation with Prof. Mokwa’s group at Aachen.
The third research area comprises the lattice disorder in the vicinity of internal interfaces (grain boundaries) and external interfaces (surfaces and electrode interfaces) and their impact on electronic and ionic charge transport. In the case of acceptor and donor doped titanate ceramics, the studies are focused on the formation of space charge depletion layers at grain boundaries as well as the related potential barriers and the transport of charge carriers along and across the grain boundary barrier. A hot-pressing technology has been developed to decorate the grain boundary area with additional dopants and to study the influence of these artifical grain boundary states. In some material systems, such as titanate-zirconate solid solutions, it is necessary to determine the equilibrium constants of the defect reactions and the diffusion constants of the system in order to create the basis for the research on interfaces. In this respect, the comparison of bulk ceramics and thin films of the same composition is of vital interest. In thin film systems, the significant influence of the electrode metals, the unexpectedly high stability under conditions of dc-voltage-induced resistance degradation, as well as the tolerance of the lattice concerning the incorporation of non-stoichiometries represent current research topics.
Continuing the cooperation with the Research Center’s fuel cell project of the Research Center, proton-conducting ceramics are studied with respect to their suitability for solid oxide fuel cells (SOFCs). In addition, a new activity on thin film oxide conductors has been started as a part of a national priority program by the German Research Association (DFG).
Research on electronically conducting perovskites can be divided into studies on semiconducting and on metallically conducting oxides. In semiconducting, donor-doped SrTiO3, the emphasis is placed on the interrelationship between point defects and extended defects involved in oxidation and reduction processes using single crystals and ceramics. The metallically conducting manganates and cobaltates are investigated with respect to their magnetoresistive and ferromagnetic properties. By comparing epitaxial and polycrystalline thin films, the influence of the grain boundaries on charge transport and magnetic properties is studied.
Rainer Waser
Institute for Electroceramic Materials
Head: Prof. Dr.-Ing. Rainer Waser
Secretariat: Maria Garcia
Tel. (02461) 61 5811; Fax: (02461) 61 8209
e-mail: r. waser@fz-juelich.de/m.garcia@fz-juelich.de
SCIENTISTS:
| Arons, R.R.: | Structure of magnetoresistive and ferroelectric oxides; charge transport in proton conductors |
| Bohn; H.G.: | Mechanical and dielectric relaxation in solids. Impedance spectroscopy. Defect chemistry. Electrochemical characterization of ceramic conductors. |
| Ehrhart, P.: | MOCVD methods for electroceramical thin films; X-ray diffraction and optical spectroscopy. |
| Hoffmann, S.: | High-permittivity electroceramic thin films: MOCVD, dielectric properties, charge transport, defect chemistry |
| Kohlstedt, H.H.: | Reactive ion beam etching of ceramic and metallic materials, superconducting and magnetic multilayers |
| Krasser, W.: | Optical excitation-processes in electroceramic materials;light-annealing processes |
| Meuffels, P.: | Processing of electroceramic materials; defect chemistry of electroceramic materials |
| Otterstedt, R.: | Development of microwave ceramics; dielectric characterization; extrinsic losses in microwave ceramics |
| Schroeder, H.: | Technology and properties of (metal) electrodes of electroceramic thin films; mechanical properties and electromigration in thin films and interconnects. |
| Waser, R.: | Electronic ceramics and integration of ceramic thin films |
TECHNICAL ENGINEERS:
| Bierfeld, H.: | Ceramic technology and sputtering techniques |
| Dedek, U.: | Electrical characterization of electronic ceramics; design of measuring setups. |
| Haselier, H.: | Metallization and thin film technology as well as clean-room technology |
| Hermanns, B.: | MOCVD, RIBE, sputtering of magnetic materials |
| John, H.: | Clean-room technology, microlithography and optical laboratory; LRP |
| Speen, R.: | Relaxation spectroscopy and electrochemical characterization |
Ph.D. STUDENTS
| Baldus, O. (TH Aachen): | Laser annealing of CSD, MOCVD electroceramic thin films |
| Barton, M. (Uni Bonn): | Synthesis and characterization of single crystals of the high temperature protonic conductor Ba3(Ca1+xNb2-x)O9-d |
| Fitsilis, F. (TH Aachen): | Thin film capacitors for future DRAM applications using the MOCVD technique |
| Huck, H. (TH Aachen): | Characterization of oxide perovskites with optical methods |
| Rickes, J. (TH Aachen): | Reconfigurable multimedia processors based on ferroeletric RAM (FeRAM) |
| Schmitz, S. (TH Aachen): | Influence of the contact metal on leakage current and dielectric permittivity of electroceramic thin film capacitors. |
| Schmitz, R.: | Magnetic tunnel junctions, fabrication and experiments |
| Schneider, St. (TH Aachen): | Reactive ion etching (RIE) and reactive ion beam etching (RIBE) of ceramic thin films. |
| Schäfer, P. (TH Aachen): | Evaluation of novel MOCVD systems for the deposition of ferroelectric thin films. |
GRADUANTS:
| Hövelmann, J. (TH Aachen): | Computer control system for a laser annealing device |
| Ohly, Ch. (TH Aachen): | Investigations of the high-temperature conductivity of doped titanate thin films |
GUEST SCIENTISTS:
| Dr. St. Hwang (Univ. of California, USA): | Polarization switching models in polycrystalline ceramics |
| Dr. W. Ma (Southeast Univ., Nanjing, China): | Growth, microstructure and electrical property of ferro- electric thin films of perovskite-type oxides by MOCVD |
| Prof. J. Scott (Univ. of New South Wales, Australia): | Theory of charge distribution and leakage currents in BST type thin films |
| Dr. K. Szot (Univ. Katowice, Poland): | Study of surface layer of perovskite materials of ABO3 structure |