Thermal stability of atomic layer deposited Zr:Al mixed oxide thin films: An in situ transmission electron microscopy st
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O. Richard, C. Zhao, and H. Bender Interuniversity MicroElectronics Center, B-3001 Leuven, Belgium
G. Van Tendeloo Electron Microscopy for Materials Science, University of Antwerp, B-2020 Antwerpen, Belgium (Received 9 November 2004; accepted 2 February 2005)
The thermal stability of amorphous Zr:Al mixed oxide films of different composition, produced on (001) silicon wafers by the atomic layer deposition method is studied by transmission electron microscopy during in situ heating experiments. The temperatures at which phase separation and crystallization occur are composition dependent. The crystallization of thick films (55–70 nm), deposited on HF-treated silicon surfaces covered with a 15 cycles Al2O3 layer, results in the formation of cubic ZrO2 and cubic ␥–Al2O3. In very thin films (5 nm), deposited on silicon surfaces covered with a 0.5 nm SiO2 thin film, the formation of tetragonal zirconium disilicide (ZrSi2) is observed in the microscope vacuum, at temperatures above 900 °C. This effect depends on the thickness of the as deposited thin film. I. INTRODUCTION
As the technology of metal-oxide-semiconductor devices evolves with a continuous scaling down and compacting, it becomes necessary to replace the ultrathin SiO2 gate dielectric used at present by other materials with higher dielectric constant k, which might prevent too high leakage currents in films with sub-nanometer equivalent oxide thickness.1 Besides having a permittivity higher than the one of SiO2, these thin dielectric films should be stable both to crystallization and to silicide formation, while heated during the subsequent integrated circuit (IC) processing treatments.2 It is preferred for the film to remain amorphous during the full thermal cycling as applied in complementary metal-oxide semiconductor (CMOS) device processing.3,4 Crystallization might induce thickness non-uniformity. There is also a general concern that leakage currents will increase due to the presence of grain boundaries. Recent studies on HfO2 layers indicate that this is not a major issue.5,6 From many points of view, ZrO2 seems to be an attractive material, since it has a high-dielectric constant (k ⳱ 25), and it is reported stable in contact with silicon at 1000 K.2 However, ZrO2 does not remain amorphous at high temperatures.7 It crystallizes according to in situ
DOI: 10.1557/JMR.2005.0217 J. Mater. Res., Vol. 20, No. 7, Jul 2005
http://journals.cambridge.org
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x-ray diffraction (XRD) studies already below 500 °C.8 ZrO2 thin films formed by atomic layer deposition (ALD) at 275 °C are reported to be nanocrystalline.9 Our transmission electron microscopy (TEM) studies also showed the ALD ZrO2 thin films deposited at 300 °C to be partially crystalline. Al2O3 has a lower k value (k ⳱ 9) but a higher crystallization temperature than ZrO2. To obtain amorphous high k thin films, mixed Zr:Al oxides with different composition have been explored.7,10–14 Zrrich Zr4AlO9 layers are shown by XRD to remain amorphous during vacuum or O2 annealing at 600 °
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