MAG*I*CAL TEM Calibration Sample
Use Instructions
The MAG*I*CALŪ calibration sample for transmission electron microscopy (TEM)
has been developed to perform the three major calibrations of a transmission
electron microscope: the image magnification calibration, the camera
constant calibration for indexing diffraction patterns, and the
image/diffraction pattern rotation calibration. The sample consists of an
ion-milled cross-section of a silicon wafer on which a series of calibration
marks appear. The spacings between these calibration marks are very
accurately known.
On the MAG*I*CAL calibration sample, there are four regions that contain the
calibration marks. These are marked by arrows in Figure 1 below.
Figure 1: Diagram of the
Mag*I*Cal Calibration Sample
In the above diagram, the arrows indicate the four possible regions of
interest on the sample where the calibration marks may be found. Note that
this cross-sectional sample consists of two pieces of silicon wafer epoxied
face-to-face and held in a titanium grid. To find the areas of interest at
low magnification, look for the epoxy line on either side of the small ion-
milled perforation (between the arrows in the above diagram).
One of the regions of interest (marked by arrows in Figure 1) is found by
translating the sample in the microscope and focusing on the layered
structure (Figure 2 below).
Figure 2: TEM micrographs of the MAG*I*CALŪ
calibration sample.
The sample consists of a series of layered structures whose thicknesses are
accurately known, referenced to the (111) lattice spacing of Si. Inset shows
a higher magnification image of one of the layered structures.
Aligning the calibration sample:
The MAG*I*CALŪ calibration sample is made from a single crystal of silicon,
and therefore has many very useful attributes when used as a TEM sample. To
take advantage of these attributes, mount the sample in the TEM sample
holder as shown on the instruction sheet and translate the sample to the
region of interest. Now translate the sample a small distance away from this
region, but stay close to the thin edge of the sample. Put the TEM into
diffraction mode, choose the smallest camera length, and adjust the
intensity control (condenser control) back and forth to find the proper
value of over-focus that forms the striking pattern, called a Kikuchi
pattern, which should look similar to Figure 3-b below, but perhaps rotated.
These Kikuchi bands are formed by elastic Bragg scattering of previously
inelastically scattered electrons. Note that the intensity control can be
adjusted so that either a diffraction spot pattern or a Kikuchi pattern can
be observed. The sample should now be tilted to move this pattern so that
the central "intersection" in this diagram where the largest number of bands
intersect is centered on the brightest (zero order) diffraction spot in the
electron diffraction pattern. It may be necessary to translate the sample to
slightly thicker or thinner regions to get a clear Kikuchi pattern.
Figure 3-a: Calculated Kikuchi pattern of single crystal
silicon viewed down the <011> zone axis. The broad horizontal band in the center
of this figure is between the {200} Kikuchi lines.
Figure 3-b: Kikuchi pattern of the [011] zone axis of Si. The
arrows indicate the broad band between the {200} Kikuchi lines. When the
zero order beam in an electron diffraction pattern is centered anywhere
along this band, the electron beam will be parallel to the layered structure
, and result in accurate layer thickness values.
Image Magnification Calibration:
One of the regions of interest (marked by arrows in Figure 1) should be
found by translating the sample in the microscope and focusing on the
layered structure (Figure 2). The eucentric height of the microscope should
then be adjusted. On microscopes with no sample height adjustment, the
objective lens current must be monitored and kept constant at each
magnification value. Switching to diffraction mode, the microscope should be
adjusted to produce a Kikuchi pattern at the smallest available camera
length (see above, "Aligning the calibration sample").
When using a double-tilt sample holder, the sample should be tilted so that
the zero order diffracted beam (brightest diffracted beam) is at the center
of the [011] zone axis Kikuchi pattern (Figure 3). With a single tilt holder
, the sample should be mounted so that the epoxy line (between the arrows in
Figure 1) is parallel to the length of the sample holder rod and the sample
should be tilted so that the zero order diffracted beam is centered between
the two {200} Kikuchi lines, in the region indicated by the two arrows in
Figure 3-b. In either case, this will insure that the electron beam is
parallel to the layered structure. The eucentric height should again be
adjusted, and the microscope focused on the region shown in Figure 2 at the
highest magnification range available. When performing a magnification
calibration, a series of micrographs or digital images at all magnification
ranges should be taken starting at the highest magnification range and
working down to the lowest. Also, the microscope should be initially
overfocused, then brought into focus. These two precautions will help to
avoid lens hysteresis effects. Likewise, when making a measurement based on
this calibration, the image should be focused at a higher magnification,
then lowered to the magnification level of interest before producing the
micrograph or digital image.
The magnification calibration at the highest magnification ranges (>400,000)
can be accomplished by forming a lattice image of the Si below the layered
structure (Figure 4), measuring perpendicular to a number of (111) lattice
fringes then dividing by the number of fringes. This gives the measurement
on the micrograph or digital image that corresponds to the (111) lattice
spacing of Si (0.3135428 nm). Alternatively, one of the layered structures
could be imaged, and the measured value compared to the "Layer Thickness
Values". The microscope magnification can then be lowered through the
complete range of magnification steps, taking micrographs or digital images
and comparing these measurements with the actual thickness values, as given
in the "Layer Thickness Values" table.
mag5
Caption: Figure 4: Lattice image of Si viewed down the [011] zone axis.
Note the central diamond shape. The lines marked parallel to the upper two
sides of the diamond shape indicate (111) planes, with an interplanar
spacing of 0.3135428 nm. Note at the lower right of the diamond shape that
measurements must be made perpendicular to the (111) planes, since measuring
along a row of atomic columns will give an erroneous value.
Camera Constant Calibration:
After the sample is aligned parallel to the layered structure (preferably
down the zone axis), translate the sample to a defect-free region below the
layered structure, note the magnification value, and switch to diffraction
mode. Use a selected area diffraction aperture and record silicon
diffraction patterns at each magnification/camera length of interest. To
arrive at the value of the camera constant, measure the length of a series
of spots and divide by the total number of spacings (for example, measure on
the diffraction pattern micrograph the distance from the (222) spot to the
(222) spot and divide by 4; see fig. 5). This gives an accurate value of the
diffraction ring radius R in mm. By then using the equation:
W L = dR
and using the value for R from the above procedure as well as the
lattice spacing value of d. (in this case the (111) spacing of Si, 0.3135428 nm), one can
calculate the camera constant value WL in units of nm-mm. This value,
calculated at all camera lengths of interest, is essential for indexing electron diffraction patterns.
To now find an unknown atomic spacing from a diffraction pattern, measure
the unknown diffraction ring radius or a diffraction spot spacing in mm.
Then divide the appropriate camera constant value by this spacing. The
result is the unknown atomic spacing value in nm.
Figure 5: Electron diffraction pattern of Si viewed down the [011
] zone axis, with the diffraction spots indexed.
Image-Diffraction Pattern Rotation Calibration
As magnetic lenses in electron microscopes produce a rotation between the
image and the object itself and since the diffraction pattern is always
produced with the same lens excitation, it is useful to know the various
angles between the image and diffraction pattern at each magnification value
and camera constant. This is useful for identifying Burgers vectors of
dislocations in crystals and identifying the orientation of crystallites.
This calibration sample is designed so that the (200) planes are parallel to
the layered structure and to the surface. Silicon has the diamond cubic
crystal structure, so on this calibration sample the [200] direction is
perpendicular to the layered structure and to the surface. What is now
required is to measure the angle between the [200] direction on the image
and the (200) diffraction spot of the diffraction pattern on the micrograph
by means of a series of double exposures.
To produce the image-diffraction pattern rotation calibration with the
MAG*I*CALŪ calibration sample, align the sample so that the incident
electron beam is parallel to the layered structure (see the sheet titled
"Aligning the calibration sample..."), then take a series of double
exposures of the image and its diffraction pattern at each magnification
value and camera constant value of interest. Now identify the (200)
diffraction spot on the micrographs. On the diffraction pattern portion of
the double exposures, there are two possibilities for the (200) diffraction
spot 180 degrees from each other (ie: which is the (200) spot and which is
the (200) spot? See Figure 5.) To identify correctly the (200) diffraction
spot, slowly translate the sample through the layered structure and past the
top surface while viewing the diffraction pattern. As the sample is
translated past the top surface into empty space, the diffraction pattern
will fade and finally disappear first from the direction corresponding to
the (200) diffraction spot. By referring to Figure 5, all of the diffraction
spots can then be indexed.
To measure the actual rotation calibration values, draw a line from the zero
order diffraction spot up through the (200) spot. Also draw a line from the
zero order spot in a direction perpendicular to the layered structure. The
angle between these two lines is the rotation calibration value for this
magnification/camera length condition, and can now be used to identify
crystal direction in unknown samples, when the images and diffraction
patterns of the unknown material are taken at the same magnification/camera
length condition.
Some Warranty Limitations
The MAG*I*CAL TEM Calibration Sample is very useful and easy to use. Even
thought it has been ion milled and is therefore
presumed to be quite fragile, it is nevertheless surprisingly rugged, but if
the regions of interest are even slightly touched with
even the finest tweezers they may cleave off and destroy that part of the
sample. Please handle it carefully because SPI
Supplies will not replace free of charge samples damaged by improper
handling.
In some TEMs, where the grid cap is not so well aligned and fabricated to
fit over the grid holder, to secure the grid, it is
possible to over-tighten the grid holder cap, causing a level of compressive
stresses that prove fatal to the MAG*I*CAL.
Again, do exercise caution in this step as well because SPI Supplies will,
similarly, not replace, free of charge samples
damaged by over-tightening of the grid cap onto the grid holder.
SPI wishes you good luck in your work and using the MAG*I*CAL TEM
Calibration Sample!
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Tuesday December 02, 2008
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