Papers

ICMCTF1999 Session H4: Ferroelectric and Piezoelectric Thin Films

Wednesday, April 14, 1999 8:30 AM in Sunrise Room

Wednesday Morning

Start	Invited?	Item
8:30 AM		H4-1 MOCVD of Ferroelectric Oxides by Liquid Delivery J.F. Roeder, T.H. Baum (Advanced Delivery and Chemical Systems); P.C. Van Buskirk (Advanced Technology Materials, Inc.) Ferroelectric thin films enable numerous applications, including high density DRAMs, uncooled IR detectors, piezoelectrically controlled microelectromechanical devices, ferroelectric gate transistors, and FeRAMs. This presentation restricts its attention the perovskite ferroelectrics: (Ba,Sr)TiO₃, Pb(Zr,Ti)O₃, and SrBi₂Ta₂O₉ (BST, PZT, and SBT). The materials properties relevant to each application are described, and integration issues are identified. Progress toward metalorganic chemical vapor deposition (MOCVD) processes is described for the three materials, with an emphasis on process - property relationships. In each case, high quality thin films have been produced using a liquid delivery approach in which ß-diketonate or mixed ß-diketonatealkoxide precursors are flash vaporized. Results are given for selected applications, highlighting microstructure and electrical properties.
9:10 AM		H4-3 Deposition of Thin Film Piezoelectric ZnO on Silicon Substrates Via rf-Magnetron Sputtering B.R. Stoner (University of North Carolina) Piezoelectric zinc-oxide is becoming an important material from both the standpoint of sensor and acoustic filtering applications. The integration of thin film piezoelectrics into existing device processing technologies relies on the ability to produce highly textured material with low defect density and relatively high piezoelectric coupling coefficient and electrical resistivity. This report will highlight the effects of various process factors on the above properties, specifically focusing on the role of background water vapor on both the microstructure and texture evolution of ZnO deposited on silicon via rf-magnetron sputtering. Background water vapor levels were varied over several orders of magnitude leading to variations in both grain size and surface roughness of the highly textured films. Background contaminant levels were measured with a quadrapole mass-spectrometer. The films were characterized via scanning electron and transmission electron microscopy in both planar and cross-sectional views. XTEM also indicated that a high degree of texture was present in the grains near the ZnO/Si interface, suggesting that the preferred texture is formed at the initial states of film growth. It was observed that the ZnO morphology was greatly dependent on the water vapor partial pressure, indicating that water vapor contamination affects the surface ad-atom mobility and reacitvity during deposition. We propose a mechanism whereby -OH radicals tie up dangling bonds at the grain boundaries, limiting coalescence of highly textured polycrystalline films.
9:50 AM		H4-5 Piezoelectric Thin Films for Volumetric Sampling of Materials P.M. Martin, M.S. Good, G.J. Posakony, J.W. Johnston (Pacific Northwest National Laboratory) Piezoelectric aluminum nitride (AlN) and zinc oxide (ZnO) thin films were deposited by reactive magnetron sputtering for use in high frequency transducers. Pulse-echo imaging ultrasonic transducers are being developed to image subsurface defects in nuclear fuel rods and metal structures. Operating frequencies range from 100 MHz to 300 MHz. The design and fabrication of the transducers will be presented. The KLM model is used to aid in the design of the transducer. Design criteria include piezoelectric material properties and thickness, electrode thickness and placement, substrate dielectric properties, and sensitivity requirements. Design performance will be compared to measured transducer performance. AlN was chosen because of its capability to generate high velocity ultrasonic sound waves. Strongly oriented (002) AlN coatings, up to 50 µm thick, were deposited in transducer structures with flat and curved surfaces. Fused quartz and sapphire substrates were used. Substrate temperatures were equal to or less than 200 °C. Crystalline structure was found to depend on deposition conditions, substrate placement, and substrate and electrode material. X-ray diffraction data will be presented describing the relationships between the dominant hexagonal crystalline orientation, which varied from (001), (103), and (002), and deposition conditions. The performance of the transducers was close to that of the design. Our first transducers on sapphire substrates resonated at 113 MHz with sensitivities better than 20 dB. Improvements in performance using curved substrates and alternate electrode designs will also be discussed. These coatings also have applications in microelectricalmechanical systems (MEMS).
10:30 AM		H4-7 High k Dielectric Thin Films by CVD J.W. Rogers, Jr. (University of Washington) In less than 5 years, the Roadmap for Semiconductors suggests that silicon dioxide will no longer be an acceptable material for the gate dielectric in MOSFET-based devices. As feature size shrinks, the films of silicon dioxide for gate dielectrics will be so thin (equivalent oxide thickness 1.5-2.0 nm) that growth becomes an issue and electron tunneling through the dielectric layer will prevent their use. The search for new "high k" dielectric materials suitable for these applications is underway. Candidate materials include aluminum nitride, tantalum oxide, titanium oxide, and perhaps perovskites such as the alkaline earth titanates. Chemical vapor deposition will likely be the preferred technique for depositing these films. In this talk, I will review our work in the area of aluminum nitride deposition using amine alanes and ammonia as precursors. High quality amorphous thin films have been grown on silicon(100) substrates at temperatures as low as 675 K. Recent results on the deposition of titanium and tantalum oxides using metal alkoxide precursors will also be presented. In addition, I will review relevant work on CVD of these materials from other laboratories.
11:10 AM		H4-9 High Density Thin Film Ferroelectric Nonvolatile Memories R. Ramesh, S. Aggarwal (University of Maryland) Over the past two years, we have focused considerable effort on understanding the growth and characterization of conducting barrier layers for the direct integration of ferroelectric capacitors on a poly-Si plug. Our specific focus has been on the materials science of the barrier layers to understand the role of crystallinity and process parameters on the structural and chemical integrity of the barrier layers during the subsequent growth of the ferroelectric capacitor stack. We are using the PZT system with conducting oxide electrodes as a prototypical test system for which at least two different conducting barrier materials systems have been successfully developed. Using both epitaxial and polycrystalline capacitors on these conducting barriers as test vehicles, we have been carrying out systematic studies on the effect of composition, point defect chemistry, strain and other processing variables on the structural integrity and ferroelectric properties. A novel aspect of our work is the use of scanning electric force microscopy techniques to understand the microscopic influence of film microstructure on the ferroelectric properties. In this presentation, we will present results of our progress on the process integration, device properties, specifically, polarization switching and relaxation dynamics and microscopic observations of ferroelectric properties and time dependent changes ; stress effects on fundamental properties. This work is supported by the NSF-MRSEC under Grant No. DMR- 96-32521.
11:50 AM		H4-11 The Effects of Lanthanide Dopants on the Ferroelectric Properties of Sol-gel Derived PZT Thin Film for FRAM Application H.H. Park, W.S. Kim, S.M. Ha, C.E. Kim (Yonsei University, KOREA) Donor-doped PZT thin films are more desirable for ferroelectric memory applications due to their lower coercive field, higher resistivity, and lower dielectric memory aging rate compared to undoped PZT thin films. The resulting effect of donor dopant can be explained on the basis of vacancy compensation in the perovskite lattice. In the study , La and Nd were chosen as dopants and their effect on the fatigue behavior and leakage properties of PZT thin films were investigated, La or Nd doped PZT thin films were deposited by alkoxide-alkanomine CSD method on Pt electrode, Zr/Ti ratio is fixed as 40/60 that shows tetragonal phase. The optimum doping concentration of La or Nd in PZT thin films could be obtained. The excess Pb added onto starting precursor was fixed to 15 wt%. Each sol-gel process condition and heating process was optimized based upon its thermal analysis result. The microstructure, crystallinity, ferroelectric properties, and leakage characteristics were investigated.