Synthesis of Zirconia and Hafnia Tubes by Atomic Layer - PDF

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					  Synthesis of Zirconia and Hafnia Tubes by Atomic              for integration into future integrated circuits (IC)
     Layer Deposition (ALD) Template Method                     technologies and photonic crystals.

                 Tarek M. Abdel-Fattah*
      Chemistry, Christopher Newport University
                Newport News,VA 23606
  Diefeng Gu, Helmut Baumgart and Gon Namkoong
    Electrical Engineering, Old Dominion University
         231 Kaufman Hall, Norfolk, VA 23529
The Applied Research Center-CNU& ODU-Jefferson Lab
                Newport News, VA 23606
                 *Email:                                        (a)                      (b)

Metal oxides tubes, synthesized in the nano-range, exhibit      Figure 1. SEM front image of AAO Template (a)
novel physical properties and play an important role in         Cross-sectional SEM image of AAO, insert is higher
fundamental research. In addition, they play a role in          magnification of tubes (b) SEM top view of tube
practical applications, because of their restricted size and    opening
high surface area of the one dimensional structure.
Hafnium oxide (hafnia, HfO2) and zirconium oxide
(zirconia, ZrO2) are important materials widely used in
ceramics, gas sensors, catalysts, opto-electronics and as
high-k dielectrics in microelectronics. Semiconductor
metal oxides tubes of hafnia and zirconia, with high
aspect ratio and a small size of nanotubes or nanowires,
are expected to improve the sensitivity of chemical
sensors and reinforce thermal stability and toughness of
the materials.
Anodic aluminum oxide (AAO) and other nanoporous
materials are very attractive as a template for                          (a)                                  (b)
nanofabrication [1]. Anodic aluminum oxide is being
formed by electrochemical oxidation of aluminum in              Figure 2. (a) Cross-sectional SEM image of hafnia
acidic solutions to form regular porous channels, which         tubes (b) EDS Hf mapping corresponding to (a)
are parallel to each other (Figure 1). The channel diameter
is mainly defined by the anodization voltage. Diameter of
the pore depends on the electrolyte nature, its temperature
and concentration, the current density and other
parameters of the anodization process. It is possible to
vary the diameters of the channels and the pore by
variations of the electrolyte composition and anodization
conditions. The pore diameter can also be enlarged by
selective etching of cell walls. Anodic alumina was used
as templates to fabricate the hafnia and zirconia tubes.
The diameters of these channels lie in the range of 200
nm and thickness of 60 µm (Figure1).                                     (a)                                  (b)
Atomic Layer Deposition (ALD) is the only method for
the deposition of hafnia and zirconium within AAO, in a         Figure 3. (a) Cross-sectional SEM image of zirconium
controlled fashion, to yield good composition control and       tubes (b) EDS Zr mapping corresponding to (a)
film uniformity within AAO, and excellent conformal
step coverage on complex nonplanar surface topographies
[2]. Conventional alkylamido precursors (tetrakis
(dimethylamido)        hafnium      (IV)    and      tetrakis
(dimethylamido) zirconium (IV)) were used in this study.
Highly ordered zirconia and hafnia tubes are prepared by
ALD within the pores of an anodic alumina oxide (AAO)
template. Scanning electron microscopy (SEM) and
energy dispersive spectroscopy (EDS) are used to
characterize the morphology and elemental compositions                   (a)                                  (b)
of the different tubes and EDS mapping of the entire
sample cross-section. The length and diameter of the            Figure 4. (a) SEM image top view of hafnia tubes
hafnia tubes are dependent upon the pore diameter, the          within AAO (b) SEM image top view of zirconia tubes
thickness of the applied AAO template and the deposition        within AAO
time. The results indicate that the tubes are very
uniformly assembled and parallel to each other in the           Reference:
pores of the AAO template (Figures 2and 3). Figure 4               [1] T. Abdel-Fattah; E. Siochie, and R. Crooks,
showed that the tube size for hafnia and zirconia are              Fullerenes, Nanotubes, and Carbon Nanostructures,
different, as well as the surface morphology. This new             14(4), 585-594 (2006).
method of preparing zirconia and hafnia tubes may be               [2] D. Gu; K. Tapily; P. Shrestha, M.Y. Zhu; G. Celler
important in many applications ranging from gas sensors            and H. Baumgart, J. Electrochem. Soc.,155(6), G129-
to various engineering materials such as high-k materials          G133 (2008).