Ultra high resolution tomographic reconstruction using
Focused Ion Beam (FIB) and Scanning Electron Microscope (SEM) Techniques
AVS
51th
R. K. Bansal, R. Hull, and J. M. Fitz-Gerald
University of Virginia, Charlottesville, Virginia 22904
International Symposium & Exhibition, November 14-19, 2004 Anaheim, CA
CONTRAST NORMALIZATION
PIXE
TEM
SEM
X-Ray
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MOTIVATION AND OBJECTIVES
Figure 1 Comparison of Length Scales in
Tomographic Techniques
The FIB based serial sectioning allows us to reconstruct features with critical feature size as small as a few tens
of nanometers. The smallest ion probe that can be obtained in the FIB (8-10 nm) is one of the main factors
restricting the resolution. If a high resolution SEM was utilized for imaging, a further improvement in resolution is
possible, as has been observed with newer dual-beam FIB systems. It was also found during the course of this
work that when the distance between the slices is 10-20 nm it is difficult to consistently remove equidistant slices,
therefore a method to calculate the distance between adjacent slices with higher accuracy was developed.
Newer dual beam instruments The present work demonstrates a high resolution, three dimensional
reconstruction using the FIB along with the SEM to study the material structure with sub 10 nm resolution.
As-acquired image
Histogram
Contrast normalized image
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EXPERIMENTAL PROCEDURE
A schematic of the process used to obtain 3D information
about the sample using serial sectioning is shown in figure
2. The sample was first introduced into the FIB chamber
and Pt is deposited at the area of interest. A reference line
was milled into the Pt (shown by a darker shade) block to
determine the distance from the edge. The sample was
then turned by 90° such that the edge of the sample faces
upwards. Alignment marks (shown by rectangular trenches
in the sample) measuring 150 nm x 150 nm were made
using the FIB box milling. The sample was again turned by
90° to remove the first slice. The sample was then removed
from the FIB and inserted into the SEM chamber after
turning by 90°. This process was repeated between the FIB
and the SEM to obtain the desired number of slices. The
images were aligned and concatenated using the fiduciary
marks. MATLAB based routines were developed to perform
intensity based and shape based interpolation and 3D
visualization.
AL2Cu θ Precipitates
The process of creating and imaging multiple sections requires a continuous exchange
between the FIB and the SEM. In each session the brightness and contrast settings for image
acquisition have to be adjusted and it was found by visual inspection it was difficult to get the
exact same setting for all the slices. This factor assumes importance as subsequently the
gray scale intensity value of each pixel is used for 3D interpolation of the data set. Figure 6
below illustrates the as-acquired images of two different slices showing the cropped area of
interest along with the respective histograms. It can be seen that even though the two images
look similar, the highest grayscale value in the second image is roughly 40 gray levels higher
than the first one. Contrast normalization was therefore performed on every image to bring
the contrast to the same level. This is done by stretching the histogram to cover the entire
grayscale spectrum (0-255). Images obtained as a result of this normalization are shown in
figure 7 below along with the respective histograms.
Intensity
FIM
Intensity
SIMS
Intensity
Tomographic Technique
The ability to reconstruct a material or a system in three
dimensions is of great importance. Some of the available
techniques are shown in figure 1 with the range over which they
can be used. It is worth noting that the FIB based serial sectioning
technique covers the length scales from tens of nanometer to tens
of microns that are key to materials science.
FIB
Intensity
MRI
INTRODUCTION
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Epoxy
The sample used here has thin films of Si0.8Ge0.2
and Si epitaxially grown on a p-type Si 100 substrate
using MBE. The sample had a bi-layer pitch of 28nm
as measured from XTEM images. Due to the lattice
mismatch between Si0.8Ge0.2 and Si, crests and
troughs are formed which can be seen in the xsectional SEM and TEM images.
Si-Ge Multi-layers
004
(a)
Figure 2 Three-dimensional schematic illustration of the tomography
technique used for this work.
Si Substrate
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INSTRUMENTATION
The FIB used for these experiments is a FEI 200 using a Ga+ Liquid metal Ion Source (LMIS) primarily operated
at 30 KV. The SEM used for this research is a JEOL 6700F with a cold field emission source and an in-lens
detector. TEM studies were performed on a JEOL 2000FX. Imaging was performed with a JEOL 6700F FE-SEM
at 2KV accelerating voltage and 2mm working distance from the pole piece. These conditions were found to
yield the best resolution for the present set of samples. The resolution of the instrument under these conditions
is 2 to 3 nm
TEM SAMPLE PREPARATION
Cross-sectional transmission electron microscopy (XTEM) was performed to study the 2-D structure and
morphology of the sample used for study. Cross-sectional samples of Si-Ge multilayers and Al-Cu alloy were
prepared using conventional methods involving mechanical polishing and ion milling. Profile of the trenches
made by FIB was also studied by XTEM to calculate the distance between slices as shown in figures 3,4. The
sample was mechanically polished down to a thickness of about ~ 30 to 50 m and attached to a half Cu ring.
The sample was then mounted vertically on a sample holder and inserted into the FIB chamber. An area of
interest was chosen local to the center of the ring and an electron transparent TEM membrane (~250 nm) was
made by FIB milling. The trenches were then made on the TEM membrane using the same conditions as used
Etching area
for making fiduciary alignment
marks.
Alignment
trenches made
on TEM
membrane
FIB etching
TEM observation
Etching area
Alignment trenches made by FIB
Alignment marks
(c)
Figure 8
SEM micrograph showing the xsection of a MOSFET device with different
components labeled.
(a)
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B
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B
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B
x
FIB milled trenches
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B
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A
(a)
(b)
Figure 14 Perspective views of the ’ precipitates in an Al matrix
reconstructed using shape based interpolation and rendered in
Matlab
This technique can be applied to a variety of systems and materials with complex features and
provides an edge over the serial sectioning technique using only the FIB, in terms of higher
resolution. Based on the resolution of the SEM and the errors generated during alignment and
reconstruction, it is estimated that the resolution of the technique is 8 nm. The procedure
developed to estimate the inter-slice distance with high accuracy by studying the profile of zspacer trenches further improves the resolution and accuracy of the technique. It was found that
the inter-slice distance turns out to be unequal even though equidistant slices were attempted
while milling using the FIB.
This method requires the use of two specialized instruments in an alternating fashion. In order to
obtain every slice, the milling was performed in the FIB, the FIB chamber was vented and the
sample was inserted into the SEM chamber for imaging. This process consumes a significant
amount of time and since the two chambers have to be vented to atmosphere and pumped back
to operating pressure. In addition, the sample is exposed to atmosphere which can cause
contamination and surface oxidation. The stage also needs to be mechanically stabilized and the
beams aligned for every slice. Using a dual beam FIB/SEM system would make the process less
tedious and more attention can be paid to data analysis rather than data acquisition.
TEM Membrane
y
A
Images were acquired in a way similar to the
one described for the Si-Ge sample and aligned
using
fiduciary marks. Segmentation was
performed on the 2D images to delineate the
edges of the features and a distance
transformation was then applied to the image
such that the value of each pixel represents its
distance from the nearest edge. This value was
positive if the pixel was inside the feature and
negative when outside. These images were
then interpolated into a 3D volume. Since this
technique eliminates the noise from the images
before interpolation, the edges were clearly
defined and iso-surface plots were constructed
to visualize the volume of the reconstruction as
shown in figure 14. The plate like structure can
be clearly seen in figure 14 (a) oriented in three
different directions. The view in figure 14 (b)
has been adjusted to show the orthogonal
nature of these precipitates. Two of the
orientations appear as rods while the third can
be seen face on. In other words we are looking
along the <100> direction of the grain
reconstructed. It can also be seen from figure
14 that the face of the precipitates appears to
have a distinct rippling morphology. This was
not expected
Figure 13 image of section of Al-Cu alloy milled
using the FIB showing the θ’ precipitates and
alignment marks.
FUTURE WORK
Figure 4 Cross sectional TEM showing profile of trenches
The ability to mill away slices with precision is restricted by both the
imaging capabilities of the FIB and stage drift during milling. In
order to overcome the drift problem, the shape of the trenches for
alignment was used as a measurement tool. This was achieved by
milling additional trenches in the vicinity of area under investigation.
Trenches of size 150 nm x 150 nm were made in pairs (labeled “A”
and “B”) on a TEM membrane. All the “A” trenches were milled after
tilting the sample by 10° along the y-axis as shown in figure 5. The
“B” trenches were milled after tilting in the opposite direction by 10°
along the y-axis. In this way, pair are obtained as shown in figure 5.
The angle φ was found to be ~ 32° and the inside edge could be
identified in the SEM with 3 to 4 nm accuracy. This implies that the
distance between slices could be predicted with ~ 6 nm accuracy.
SHAPE BASED INETRPOLATION
Figure 10 Three-dimensional rendering showing how the slice
images were concatenated prior to interpolation.
500 nm
MEASUREMENT OF DISTANCE BETWEEN
SLICES
C
CONCLUSIONS
x
m
B
as the precipitates appear smooth in the SEM and TEM observation and the constituent images used
for the reconstruction comprise of manually-traced smooth rods. It is believed that this is due to the
way the edge finding algorithm operates and a slight angular misalignment between adjacent slices.
Another observation made from figure 14 (a) was that some of the precipitates appear discontinuous.
This is due to the fact that these precipitates were at an angle with the original 2D section and when
the inter-slice distance is large the precipitates do not overlap.
INTENSITY BASED
INTERPOLATION
TEM membrane
Figure 3 Schematic of the procedure used for studying
profile of alignment marls using XTEM
Figure 9 (a), (c) Bright field cross-sectional TEM images of the SiGe multilayered structure along the <210> zone axis and (b) the
corresponding SAED pattern on the Si substrate. The two images
were acquired at different sites on the same sample.
Slices obtained by the above technique were
concatenated and linearly interpolated to obtain
a tomographic reconstruction. Figure 10 shows a
perspective view of the 2D slices. It was
attempted to make slices 20 nm apart and it can
be seen that the actual distance between the
slices varied. We were able to reconstruct the
layers and see the modulation in the z direction
y
distinctly as shown in figure 11.
A
0
RESULTS AND DISCUSSION
Si / Si-Ge Multilayers
Figure 12 Bright field XTEM images along
<100> zone axis showing Al2Cu θ’ precipitates in
an Al matrix.
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Figure 7 SEM images showing post contrast normalization images
for two different slices along with the image histograms.
Figure 6 As acquired SEM images for two different
slices along with the image histograms.
It is well known that in this system, Al2Cu θ’ precipitates
form disc like structures along the orthogonal <100>
directions when aged. Figure 12 shows a TEM image
acquired along the <100> zone-axis. The orthogonal
alignment of the precipitates is clearly seen in the images
as expected. The precipitates appear as rods in two
directions as the projection is taken along the x-section of
the rod while the face of the plate can be seen in the third
direction. Since these precipitates are thin plates (10 to 20
nm), the diffraction contrast due to precipitates aligned
along the plane of the image is less than the other two
directions. Figure 13 shows an SEM image of a FIB milled
section from the same sample. Fiduciary alignment marks
are also shown in the image. In this case the section is cut
such that the precipitates along all three orthogonal
directions can be seen in cross-section as rods.
A
B
B
ACKNOWLEDGEMENTS
φ°
x
y
200 nm
z
Figure 5 Schematic top view of the z-spacer trenches and
the bright field cross-sectional TEM micrograph along the
<110> zone axis showing profile of the trenches.
z
x
Figure 11 Three-dimensional interpolated image of Si-Ge multilayered sample shown in a perspective
view.
The authors gratefully acknowledge support from Materials Structures and Devices (MSD) Focus
Centre under the MARCO grant by the semiconductor research corporation (SRC). At the
University of Virginia we would like to acknowledge: Professor John Bean for providing us with
the Si/SiGe multilayered sample; Professor Gary Shiflet and Dr. Brian Gable for providing the Al Cu alloy samples; Professor James Howe for insightful discussions.