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A collaborative project between IBM Watson researchers and Eric
Stach at the National Center for Electron Microscopy has
discovered a new type of
thin film texture. Coined “axiotaxy,” this texture is characterized
by the sharing of only one set of lattice spacings between the film and
its growth substrate. A report of the work appeared recently in Nature.
Thin metal films play
an important role in advanced semiconductor devices, and are used to interconnect
device elements on the chip. Often, these
metallic films are formed through solid-state reactions between a metallic
element and the silicon itself, resulting in compounds known as metal silicides.
One of the most fundamental structural properties of a metal silicide film
is its “texture,” the orientation of its individual grains
with respect to the deposition substrate. Texture is known to strongly
affect electronic and mechanical properties as well as the long term reliability
of these interconnects. Three types of texture are commonly observed: random,
where no single orientation is dominant; fiber-texture, where one orientation
of the film grains is parallel to the growth direction, but the distribution
is random about that direction; and epitaxial, where the film orientation
is fixed in three dimensions with respect to the substrate.
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Conventionally, texture is determined through the use of large area x-ray
diffraction “pole figures”which depict the relative orientation
of crystalline grains in a material In this work, researchers from IBM
Watson Research Lab utilized pole figures obtained at the National Synchrotron
Light source, where they have a dedicated beamline, to investigate the
texture of several silicide thin films of the type used as electrical contact
materials in integrated circuits. Surprisingly, in addition to evidence
of conventional epitaxial texture, complex arcs of intensity in specific
locations were seen. The geometrical signature of these arcs suggested
that the film and substrate shared a common plane orientation, resulting
from a lattice matching of just one plane between the two components. However,
this fixes only one crystallographic direction of the film grains, and
the presence of these arcs of intensity suggested that there was a non-random
distribution of orientations about this plane—i.e. an unusual fiber
texture.
High-resolution electron microscopy was required to confirm the existence of
this new type of texture. These samples presented several particular microscopy
challenges that required use of a unique microscope at the National Center for
Electron Microscopy, the 800 kV Atomic Resolution Microscope. This microscope
has sufficient point-to-point resolution (1.6 Å), sample tilting capabilities
(± 45ยบ along two orthogonal axes) and high voltage (for increased sample
penetration of the strongly scattering nickel silicide layers) to produce atomic
resolution images of these samples. Extensive microscopy confirmed the presence
of both conventional epitaxial grains, as well as grains with the new “axiotaxy” texture.
Additionally, the microscopy showed that those grains with an axitaxial relationship
showed strongly curved interfaces between the film and substrate. This observation
gives insight into the physical origin of this unique crystallographic behavior.
If there is nearly exact lattice matching between the film and substrate planes,
the one-dimensional periodicity of the interface can be preserved independent
of the curvature of the interface. If, however, the lattice matching is less
than perfect, and interfacial curvature results in increasing mismatch with curvature,
eventually leading to a breakdown in the possibility of plane alignment.
The appearance of axiotaxy was observed in many different thin film systems formed
by solid-state reactions and appears to be a robust and important new phenomenon
in thin film science.
Eric Stach (510 486-4634), National Center for Electron
Microscopy, Materials Sciences Division (510 486-4755), Berkeley Lab.
C. Detavernier, A.S. Özcan, J. Jordan-Sweet, E.A. Stach, J. Tersoff, F.M.
Ross, C. Lavoie; An off-normal fiber-like texture in thin films on single crystalline
substrates, Nature, 426, 641-5, 2003. . |
