Exafs Studies of the Difference in Local Structure of Various Tantalum Oxide Capacitor Films
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229, Japan
ABSTRACT The extended x-ray absorption fine structure (EXAFS) above the Ta L3-edge on tantalum oxide capacitor films has been measured. Four kinds of tantalum oxide films were studied: as-deposited (amorphous), dry 02 annealed (crystalline) , 0 2-plasma annealed (amorphous) and From EXAFS analysis, differences in the 2-step (0 2-plasma + dry 02) annealed (crystalline). local structures of tantalum oxide capacitor films, in terms of oxygen deficiency around Ta, were observed in the various annealed films. The leakage current characteristics of tantalum oxide capacitors correspond with the differences in the local structures around Ta. The discussion looks at the relationship between the leakage current characteristics and the local structures around Ta.
INTRODUCTION In highly integrated memory devices, such as 256 mega-bit dynamic random access memories (DRAMs), the thickness of the storage dielectrics must be reduced below the effective 1 SiO 2 film thickness of 5nm for three-dimensional stacked or trenched capacitor structures. Conventional films such as Si3N/SiO2 film on polycrystalline silicon have reached their physical limits (~5un) of thinning.2 This is because the direct tunneling current greatly
increases below an effective Si0 2 film thickness of 5nm. On the other hand, CVD tantalum oxide is a potential film material, because of its high dielectric constane and its excellent step coverage characteristics.4 Unfortunately, however, tantalum oxide capacitors with sufficient leakage current characteristics have not yet been fabricated, even with as good characteristics as conventional silicon nitride capacitors. There have been many studies on the structure of crystallized tantalum oxide fims to
elucidate the relation to the leakage current characteristics."
Shinriki et al.5 reported that
oxygen addition and crystallization, which is a hexagonal phase, are key points for good 489 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
insulating properties. On the other hand, Oehrlein et al.6 concluded that polycrystalline Ta 20 5 films on Si, formed by thermal oxidation of deposited Ta, have worse dc conduction properties and lower dielectric breakdown strength than comparable amorphous Ta205 films. Thus the leakage current characteristics seemed to have no simple relationship to crystal structure based on the long-range order. In our previous studies we found that the leakage current characteristics for the 02 -plasma annealed film was markedly improved in comparison with any other films reported so far.1'0 n We found that the oxygen contents around Ta atom in 02 -plasma annealed film, which is amorphous, is the most among as-deposited (amorphous), dry 02 annealed (crystalline) and 0 2-plasma annealed films. As a result, we revealed that this characteristics depend on the oxygen contents around Ta atom, not on the long-range ordered crystal structure. 10 '11 In order to use the tantalum oxide capacitor films for memory devices, the crystalline film is believed to be better bec
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