The identification of unknown micron-sized phases
in the SEM has been limited by the lack of a robust and
simple way to obtain crystallographic information about
the unknown while observing the microstructure of the
specimen. A variety of techniques are available that can
provide some information about the identity of unknown
phases. For example, energy dispersive x-ray spectrometry
(EDS) is of some use but obviously cannot
distinguish between phases of similar compositions but
different crystal structures (an example of this is TiO2
that has two tetragonal forms with different atomic arrangements
and an orthorhombic form). Other techniques
can provide the required information but have
significant limitations. Micro area x-ray diffraction
techniques are capable of identifying crystalline phases,
but cannot be used to identify sub-micron sized areas.
Selected area electron diffraction in the transmission
electron microscope (TEM) can provide crystallographic
information from micron-sized regions of the
specimen, but TEM requires electron-transparent thin
specimens to be produced, which is time consuming and
can be very difficult. In this paper we will describe a
new charge coupled device (CCD) based camera and
then demonstrate that BEKP in the SEM using this
camera can provide crystallographic phase identification
of sub-micron sized areas with little or no difficult specimen
preparation.
The first backscattered electron Kikuchi patterns
were observed nearly 40 years ago before the development
of the SEM [1]. The patterns were recorded using
a special chamber and photographic film. The addition
of an appropriate camera system to an SEM for the
observation of BEKPs in a modern SEM was demonstrated
in the 1970s [2]. Since the late 1970s, BEKPs
have been used for the accurate determination of crystallographic
orientation of sub-micron crystals [3]. The
determination of crystallographic texture from BEKPs
does not require high quality patterns and can be performed
on-line with a conventional video camera system
[4].
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Journal of Research of the National Institute of Standards and Technology
BEKP has been used to identify crystal symmetry
elements and crystallographic point groups. This work
demonstrated that 27 of the 32 possible point groups
could be distinguished using BEKP [5–9]. The pattern
quality required for studies of this type dictated that the
patterns be recorded on photographic film with all of
the inherent difficulties of film use in a vacuum chamber.
Recent work has shown that on-line phase identification
can be achieved through the use of BEKP with a
CCD camera [11, 12].
The major advantage of BEKP over conventional
powder diffraction is the ability in the SEM to image a
feature of interest in either secondary electron or
backscattered electron imaging modes and then obtain
crystallographic phase information and compositional
information using EDS from this feature. BEKP can
provide phase information on specimens in reflection
with little or no sample preparation in very short periods
of time (i.e., on the order of minutes) on features below
the resolution limits of microdiffraction. A considerable
difficulty with microdiffraction is the problem of accurately
viewing the area of interest and assuring that the
x-ray beam is positioned on the area of interest. Microdiffraction
uses two separate sets of optics, a traveling
microscope for viewing the sample and positioning the
x-ray beam and a collimator that delivers x rays to the
specimen. These two sets of optics view the specimen
from separate directions and at different angles and thus
require very accurate alignment in order to ensure that
the light optics and the x-ray collimator are coincident
upon the specimen. BEKP uses one set of optics for both
viewing and analyzing the specimen. Another difficulty
with microdiffraction is the requirement that the material
be polycrystalline or that the sample be oscillated
and rotated in order to yield a pseudo-polycrystalline
x-ray pattern. As the specimen is rotated and oscillated
the x-ray beam can move off of the feature of interest.
BEKP does not require specimen motion to collect a
pattern. A detailed description of the technique and
comparisons to electron channeling patterns, transmission
electron diffraction patterns and micro x-ray diffraction
techniques is described elsewhere [11]. A disadvantage
of BEKP is its inability to distinguish
between very fine grained (< 0.1 mm) or amorphous
material.
2. Theory
Backscattered electron Kikuchi patterns are obtained
by illuminating a highly tilted specimen with a stationary
electron beam. The beam electrons are elastically
and inelastically scattered within the specimen with
some of the electrons scattered out of the specimen.
These backscattered electrons form the diffraction pattern.
There are currently two mechanisms used to describe
the formation of these patterns. In the first description,
Kikuchi patterns are formed by the elastic
scattering (diffraction) of previously inelastically scattered
electrons [1]. These backscattered electrons appear
to originate from a virtual point below the surface
of the specimen. Some of the backscattered electrons
which satisfy the Bragg condition (+ u and 2 u) are
diffracted into cones having a semi-angle of (908– u),
with the cone axis normal to the diffracting plane [4].
Since the wavelength for 20 kV–40 kV electrons is
small, the Bragg angle ( u) is less than 28. These pairs of
very flat cones intersect the detector and are imaged as
two nearly straight Kikuchi lines separated by 2 u [13].
An alternative description of the pattern formation requires
one high-angle scattering event that results in an
electron exiting the specimen. The channeling of these
electrons by the crystallographic planes of the specimen
results in the formation of cones of intensity in a manner
analogous to the channeling of electrons in electron
channeling patterns [14]. Fortunately, we do not require
a detailed understanding of the physics of electron scattering
to use these patterns for phase identification in
the SEM. The intensities of the Kikuchi lines are proportional
to the structure factor to second power (F2)
and do not vary significantly as the crystal orientation is
changed. This insensitivity of intensity to orientation is
an important property that make BEKPs easily applicable
to crystallographic phase analysis.
3. Experimental
In order to overcome the disadvantages of photographic
film and video rate recording of the patterns, a
camera was developed based on CCD technology. This
camera consists of a thin yttrium aluminum garnet
(YAG) scintillator that is fiber-optically coupled to a
scientific-grade thermoelectrically-cooled slow-scan
CCD. A CCD with 1024 3 1024 pixels was chosen for
this application to achieve high accuracy in the measurement
of the recorded BEKP. In order to achieve a large
collection angle the CCD, 2.54 cm 3 2.54 cm, was coupled
to the scintillator through a 2.5:1 fiber-optic reducing
bundle. The front surface of the YAG scintillation
was parallel to the electron beam and between 40 mm to
60 mm from the specimen resulting in a maximum
collection angle of over 708 [11, 12].
The CCD is a good replacement for photographic
film recording of BEKPs. Typically, high quality CCDs
have a higher dynamic range than photographic film,
but the resolution of the CCD still cannot approach the
resolution of photographic film [19, 20]. However, for
images collected at a resolution of 1024 3 1024, it is
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