Characteristics of spin-on ferroelectric SrBi 2 Ta 2 O 9 thin film capacitors for ferroelectric random access memory app
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Characteristics of spin-on ferroelectric SrBi2 Ta2 O9 thin film capacitors for ferroelectric random access memory applications Peir Y. Chu, Robert E. Jones, Jr., Peter Zurcher, Deborah J. Taylor, Bo Jiang, and Sherry J. Gillespie Advanced Materials Group, Materials Research and Strategic Technologies, Motorola, Austin, Texas 78721
Y. T. Lii Advanced Products Research and Development Laboratory, Motorola, Austin, Texas 78721
Mike Kottke, Peter Fejes, and Wei Chen Materials Characterization Laboratories, Materials Research and Strategic Technologies, Motorola, Mesa, Arizona 85202 (Received 27 October 1995; accepted 27 December 1995)
We report on the properties and characterization of SrBi2 Ta2 O9 (SBT, or Y 2 1) thin film capacitors for ferroelectric random access memory (FERAM) applications. The films were prepared by spin-coating from carboxylate precursors. The remanent polarization sPr d was 5–6 mCycm2 and the coercive field was ,55 kVycm. Excellent fatigue endurance was observed up to 1011 cycles. Auger analysis indicates bismuth diffusion through the Pt electrode after capacitor anneal which might require excess Bi in the precursor solution for stoichiometry control. No detectable amount of a emission was found from SBT films, which reduces the possibility of soft error when used in the memory devices.
The use of ferroelectric thin films for nonvolatile memory applications has been under intensive investigation. Lead zirconate titanate (PZT) has been the most investigated material.1–3 However, recently, bismuth oxide layered perovskite materials based on SrBi2 Ta2 O9 (SBT) or SrBi2 Nb2 O9 (SBN) have become increasingly important due to their high fatigue resistance.4–7 These layered perovskites belong to a group of compounds which were initially described by Aurivillius with the formula Bi2 O2 (Me0 m21 Rm O3m11 ).8 The Me0 element can be Bi31 , Pb21 , Ca21 , Sr21 , and Ba21 , and the R element usually has a higher valence such as Ti41 , Nb51 , Ta51 , Mo61 , W61 , and Fe31 . These types of Bi-layered materials have m RO6 octahedra between two neighboring Bi2 O21 2 layers, which results in m 2 1 perovskite-like units within that layer.8,9 In the case of SBT, m equals 2 and there is one perovskite-like unit between adjacent Bi2 O221 layers. The perovskite-like units in neighboring layers are not aligned perfectly in the c-direction. They are actually offset by half the lattice parameter in both a and b directions between adjacent Bi2 O221 layers. The unit cell parameter c of the SBT compound is, therefore, twice the distance between the Bi2 O221 layers. The lattice parameters for the SBT ˚ and compound were reported to be a b 5.509 A 8 ˚ c 25.06 A with a tetragonal structure. Ferroelectric properties were first found in these layered materials in PbBi2 Nb2 O9 .10 Early attention was, J. Mater. Res., Vol. 11, No. 5, May 1996
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however, focused on Bi4 Ti3 O12 , which has a monoclinic point group m. It is, ho
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