Improvement of Hydrogen Degradation in Pt/SrBi 2 Ta 2-x Nb x O 9 /Pt Capacitor
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electrical properties of the ferroelectric oxide by reducing elemental oxides. There have been reports on the improvement of the degradation: TiN as a barrier is effective in preventing hydrogen degradation 3 and the use of an electrode with catalysis property reduces degradation.' However, the relationship between the TiN film quality and the remaining degradation is yet unknown. The improvement of ferroelectric memory properties has not been investigated in depth and should be studied since H2 diffusion into the films is effected by the quality of the barrier and processing temperatures. The measured ferroelectric properties are manipulated by controlling the ratio of substitutional elements in a solid solution, such as Zr/Ti in PZT` or Ta/Nb in SBTN t' 7 depending on material performance for the required device design. Therefore, we expected that degradation of SBTN could be reduced and device performance improved by controlling the Ta/Nb ratio. This paper will present compositional dependence of H2 degradation in SBTN by controlling Ta/Nb ratio of the ferroelectric capacitors and using a TiN barrier metal to block hydrogen migration into the storage capacitor. EXPERIMENT Ta/Nb Ratio SBTN capacitors for evaluating the reduction effect of the Ta/Nb ratio were built on silicon substrates by the enhanced metal-organic decomposition (EMOD) method.' We experimentally controlled the Ta/Nb ratio by mixing solutions of SBT and SrBi2Nb 2O9 (SBN). The Nb concentration was varied from 0 to 100 % by synthesizing metal-hexanoate in xylene and diluting with n-butylacetate before spin coating the solution on Pt(300run)/Ti(20nm)/SiO 2(500nm)/Si substrates. The wafers were baked on a hot plate in air to evaporate excess solvent then cured in a rapid thermal processor at 725 'C for 30sec in an ambient of oxygen. This procedure was repeated twice to achieve a final film thickness of approximately 190nm. After the deposition procedures, the films were crystallized at 800 'C for lh under an ambient of oxygen in a diffusion furnace. For electrical characterization, 200nm Pt was sputter deposited as a top electrode contact and then patterned using common photolithography and ion milling techniques to form capacitors. After patterning, the samples were annealed at 800'C for 30min in oxygen so that the ferroelectric forms an ohmic contact with the top electrode. The top electrode geometry is circular which helps avoid concentration of the electric field at edge corners induced from processing damage. Subsequently, the substrates were annealed at 200 'C in 1% hydrogen forming gas for 10, 30 and 60min in a diffusion furnace. In parallel, films were annealed at 400 'C for 10min in 50 % hydrogen forming gas, however, these films mechanically failed at the top electrode / ferroelectric interface and could not be measured for electrical performance.' Therefore, subsequent 50 % hydrogen forming gas samples were annealed at a lower temperature to prevent peeling and allow observation of the degradation. TiN Barrier Metal In order to investigate
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