TEM sample preparation by FIB for carbon nanotube interconnects Xiaoxing Ke

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Ultramicroscopy 109 (2009) 1353–1359
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Ultramicroscopy
journal homepage: www.elsevier.com/locate/ultramic
TEM sample preparation by FIB for carbon nanotube interconnects
Xiaoxing Ke a,, Sara Bals a, Ainhoa Romo Negreira b,c, Thomas Hantschel b,
Hugo Bender b, Gustaaf Van Tendeloo a
a
EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
c
Metallurgy and Materials Engineering Department, KU Leuven, Kasteelpark Arenberg 44, Leuven B-3001, Belgium
b
a r t i c l e in fo
abstract
Article history:
Received 22 January 2009
Received in revised form
4 May 2009
Accepted 30 June 2009
A powerful method to study carbon nanotubes (CNTs) grown in patterned substrates for potential
interconnects applications is transmission electron microscopy (TEM). However, high-quality TEM
samples are necessary for such a study. Here, TEM specimen preparation by focused ion beam (FIB) has
been used to obtain lamellae of patterned samples containing CNTs grown inside contact holes. A dualcap Pt protection layer and an extensive 5 kV cleaning procedure are applied in order to preserve the
CNTs and avoid deterioration during milling. TEM results show that the inner shell structure of the
carbon nanotubes has been preserved, which proves that focused ion beam is a useful technique to
prepare TEM samples of CNT interconnects.
& 2009 Elsevier B.V. All rights reserved.
Keywords:
Focused ion beam
Carbon nanotubes
Transmission electron microscopy
Sample preparation
Patterned nanostructures
1. Introduction
Interconnects play a key role in integrated circuits (ICs). Copper
interconnects are now being routinely used with minimum feature
sizes down to 45 nm [1]. However, the electrical resistivity of Cu
increases with decrease in dimensions due to electron surface
scattering and grain boundary scattering, which is difficult to
overcome [2,3]. Alternatives are therefore necessary and carbon
nanotubes (CNTs) are considered as a potential solution [3–5].
Calculations have shown that CNTs can potentially outperform
copper in contact holes [6,7]. Previous work of growing CNTs in
contact holes has been carried out [5,8,9], but its development and
integration processes are challenging and require the ability to
locally characterize CNTs. Recently a pick-and-place method is
reported for transfer of individual CNTs from a contact hole to a
copper grid with the assistance of a nanomanipulator in a scanning
electron microscope (SEM) for further study [10]. An earlier study
also succeeded in showing cross-section images of CNTs in one
contact hole [11]. Obviously, detailed knowledge is required on how
the CNTs are grown in situ, how catalysts perform during the growth
of CNTs and how the contact is formed and is related to the substrate.
A powerful method to acquire such information is transmission
electron microscopy (TEM). However, high-quality TEM samples are
necessary. An electron-transparent lamella needs to be prepared
from a well-defined zone with minimum damage. Specimen
preparation through a contact hole is not straightforward using
conventional TEM sample preparation techniques, which lack site
specificity. Therefore, focused ion beam (FIB) milling is proposed as
a possible technique to prepare TEM specimens from such
structures. Sample preparation by FIB has been extensively studied
for various applications including semiconductor materials and
devices [12,13]. Although the technique has several advantages, such
as the capability of preparing ultra-thin TEM lamella and a lower
preferential sputtering rate compared to other techniques, one
drawback of the technique is the formation of damaged and
amorphous layers at the surfaces of the sidewalls. This is mainly
caused by high-energy Ga+ ions during milling and redeposition of
sputtered material. For Si, this layer can be up to 20 nm or even
more, depending on the conditions applied [14]. For carbon material,
the situation may even be worse, since C does not exhibit a strong
contrast in TEM and the amorphous layers complicate TEM studies.
In the present study, it is shown that the application of a dual-cap
Pt protection layer to the specimen surface prior to milling avoids
the crush down of the CNTs. An extensive low kV cleaning method
by FIB is performed to minimize the damage induced by FIB. TEM
studies confirm that the CNTs are preserved inside the contact holes.
2. Experimental data
Corresponding author. Tel.: +32 3 265 33 05; fax: +32 3 265 32 57.
E-mail address: [email protected] (X. Ke).
0304-3991/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultramic.2009.06.011
The samples in this study consist of a layer of amorphous SiO2 on
a Si substrate. On top of it, W and SiC layers with nominal
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thicknesses of 70 and 30 nm, respectively, are deposited for contact
reasons. Then, amorphous SiO2 with nominal thickness of 300 nm is
deposited, in which arrays of so-called ‘‘contact holes’’ are etched.
The test structures consist of arrays of contact holes with nominal
diameters of 130–300 nm. CNTs are then grown inside the contact
holes after catalyst particles have been electrodeposited inside
[9,15]. Bundles of CNTs or single CNTs may grow in the contact holes,
as illustrated in Fig. 1. In order to avoid the difficulty of imaging
overlapping CNTs, as may happen in the case of a high density of
CNTs grown in one contact hole, 130 nm contact holes containing a
single CNT are investigated in the present study (Fig. 1c).
FIB sample preparation is performed using a FEI Nova 200
Nanolab DualBeam SEM/FIB system. A standard ion column is
installed, which allows Ga milling at 5–30 kV. The ion column axis
is positioned under an angle of 521 with respect to the electron
column axis. The system is also equipped with an in situ gas
injection system loaded with a Pt deposition needle and an
OMNIPROBETM extraction needle.
High resolution TEM (HRTEM) and energy filtered TEM
(EFTEM) are performed on a CM30 FEG microscope, operated at
300 kV, which is equipped with a GIF200 post column energy
filter. High-angle annular dark field scanning TEM (HAADF-STEM)
measurements are carried out on a JEOL3000F operated at 300 kV.
3. FIB sample preparation
Electron microscopy samples are prepared by standard FIB
preparation following the basic principles of this technique
documented before [16,17] but with a few modifications.
Deposition of a protective Pt layer is always applied to the area
of interest prior to milling. In general, the Pt protection layer is
deposited using ion beam induced deposition (IBID). This has the
Single CNT
Ni
advantage of depositing a thick protection layer within a short
time. The as-deposited protection layer is not pure Pt but contains
a considerable amount of C and Ga, of which the exact
composition may vary depending on deposition parameters [18].
Unfortunately, the IBID Pt layer can damage the surface of the
region of interest because of the energetic Ga+ ions used during
this procedure. In fact, the ion beam is not only inducing Pt
deposition but also milling away or amorphizing the sample
surface. The typical depth of damage caused by IBID is about
100–200 nm [19]. In the present study, CNTs may grow out of the
contact holes and are therefore exposed to IBID damage. One
solution is to perform the deposition of the protective layer in two
steps: first electron beam induced deposition (EBID) followed by
IBID [18,20]. EBID deposition requires a quite high electron dose
so that electron beam-sensitive materials might be damaged.
However, due to the low beam energy these effects can be
expected to be less important. Although it has been reported that
EBID can still induce damage to the surface, the depth of
amorphization damage is dramatically reduced to 10–20 nm
[21]. The deposition steps are illustrated in Fig. 2. An area of
interest is selected as shown in Fig. 2a. The first deposition process
consists of EBID Pt, approximately 1.2 mm wide by 8 mm long with
a total nominal deposition thickness of 300 nm (Fig. 2b). An
incident electron beam energy of 18 kV and a beam current
of 0.6 nA is applied to perform the deposition. It results in an
acceptable compromise between Pt content and deposition rate as
the atomic Pt content in EBID decreases as a function of the beam
energy whereas the vertical growth rate increases by the decrease
of beam energy and increase of beam current [22,23]. Then, an
IBID Pt of the same size is applied on top of the EBID Pt with an
additional thickness of 1 mm (Fig. 2c). In this manner, a dual-cap
protection layer is formed to protect the CNTs from ion beam
damage during subsequent milling.
Bundles of CNTs
SiO2
SiC
SiO2
W
Si
Fig. 1. (a) Illustration of contact holes; (b) SEM image of a certain area from the specimen’s surface; (c) enlargement of the boxed area in (b) shows that one single CNT can
grow in each contact hole.
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Fig. 2. Demonstrates two steps to deposit a dual-cap Pt protection layer. (a) Shows
the area of interest before deposition. After the EBID of Pt, the surface is shown as
(b). After the IBID of Pt, protection layer is finished as shown in (c).
Next, a staircase thinning at 3 nA at both sides of the region of
interest is performed until a lamella with a thickness of
approximately 1 mm is obtained. Directly after this step the
specimen is tilted over 71 with respect to the ion beam and an
undercut is made to release the sample by a needle. Next, the
specimen is welded to the needle and lifted out of the substrate;
the stage is then moved to a different holder that contains a TEM
grid onto which the specimen is welded. The TEM grid used in this
study is Omniprobes GRD-0001.01.01 Cu Lift-out Grid. After the
sample is attached using in situ Pt deposition, the needle is
separated from the specimen.
In the following step, the specimen is thinned from both sides
gradually using a 30 kV ion beam and a current of 30 pA. Since the
specimen contains contact holes approximately in the middle of
the lamella, a different contrast is present when the milling
approaches the contact holes. Fig. 3a shows this contrast when
imaging using SEM. This contrast indicates that thinning at 30 kV
should be stopped to prevent damage and the formation of an
amorphous layer onto the contact holes. Next, the stage is rotated
over 1801 and the same thinning procedure is repeated to the
other side of specimen until the contrast of contact holes shows
up. In practice, this last milling step can be carried out by small
steps using a ‘‘cleaning cross-section’’ pattern. A cleaning crosssection pattern approaches the area of interest line by line, which
minimizes redeposition effects onto the sample sidewalls [24].
This process is monitored by live SEM imaging.
Using FIB, a specimen thickness of approximately 100 nm is
often the final goal, but this rule does not apply to our particular
samples. The final thickness of the specimen depends on the size
of the contact holes to be investigated. For instance, the contact
holes selected in this study have a nominal diameter of 130 nm.
Therefore, when the thinning procedure as described above is
completed, the specimen is estimated to have a thickness of about
150 nm or even more. Nevertheless, TEM requires electrontransparent specimens of approximately 100 nm thick. HRTEM
and other advanced TEM studies require even higher quality
lamella with a thickness of approximately 50 nm. Therefore, an
additional low kV thinning procedure is required to prepare a fine
lamella suitable for advanced TEM studies.
Low kV thinning is carried out at 5 kV with a small current on
both sides of the specimen. First, a current of 29 pA is used to thin
Fig. 3. (a) Shows contrast of the contact holes when the milling is approaching;
(b) presents the lamella after the first step of low-kV milling.
the specimen down to about 90 nm, which can be monitored by the
SEM. A SEM image after this step of fine milling is shown in Fig. 3b.
During this process the stage is tilted 1.21 positive and negative with
respect to the incident ion beam in order to prepare parallel
sidewalls. The angle required depends on the ion–solid interaction
with the specimen. For slower sputtering materials such as CNTs
and a matrix of amorphous SiO2, a 1.21 incidence angle is often used
[24]. Finally, thinning and cleaning are carried out at 10 pA. This step
needs to be carried out with care by repeating approximately 1 min
milling for 3–5 times from both sides by using a cleaning crosssection. During this step, the dual-cap Pt protection layer starts to be
milled away and an electron-transparent lamella is made. This
process needs to be closely monitored by live SEM imaging.
4. Dual-cap investigation before the low kV cleaning
First, the effect of using a dual-cap protection layer will be
investigated. The as-prepared specimens are studied by SEM as
shown in Fig. 4. The contact hole, as measured on the image, has a
diameter of about 110 nm, whereas its nominal size is 130 nm. This
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Fig. 4. SEM images of the as-prepared lamella. (a) Shows a lamella with IBID of Pt whereas (b) and (c) with dual-cap protection layers. The advantage is shown when a CNT
is preserved better in (b) and (c) compared to that of (a). The lamella shown as (c) has a smaller thickness than that of (b).
indicates that the lamella is cut close to the middle of the contact
hole. In Fig. 4a, the specimen is prepared using only IBID Pt,
whereas Fig. 4b and c present similar specimens where dual-cap
protection layers are applied. For lamellae shown in Fig. 4b and c,
the CNTs growing above the holes are protected better compared
to the case in Fig. 4a. The dual-cap protection layers hardly alter
either the size or the shape of the covered CNTs. This clearly
illustrates the advantage of using a dual-cap protection layer.
The dual-cap protection layer of the lamella shown in Fig. 4b is
then studied by conventional TEM and HAADF-STEM. In Fig. 5 a
bright field (BF) TEM image and a HAADF-STEM image of the same
region are compared. The W contact layer and the SiO2 matrix
where contact holes are etched can be distinguished thanks to the
chemical sensitivity of HAADF-STEM, which is strongly dependent
on the atomic number Z. Note that at the bottom of the contact
hole the contrast of the catalysts is still present, as indicated by the
arrow. The CNT is growing out of the catalyst material and up
through the contact hole. The tip of the CNT, present above the
hole, is well embedded in the dual-cap protection layer, as the SEM
study also indicated. It is not obvious to see the contrast of the CNT
in this case, since C yields a low contrast in TEM. EFTEM is
performed with the carbon map shown as the inset in Fig. 5. The
carbon map shows the CNT tip, which has a strong carbon signal. In
addition, a certain intensity of carbon signal is present around the
CNT tip as well. This signal is due to the EBID Pt, which consists of
more than 70% of carbon and less than 30% of Pt. EBID is a process
by which a solid material is deposited onto a solid substrate by
means of an electron-mediated decomposition of a precursor
molecule [25], which is (CH3)3Pt(CpCH3) in the present study. As
the electron beam is only accelerated to 30 kV, the decomposition of the precursor is far from complete. Therefore EBID often
produces Pt nanoclusters embedded in amorphous matrix of
carbon. Using IBID, the decomposition of the precursor molecule
is performed by ions, which is more efficient since the ions carry
more energy. Therefore the IBID Pt contains less C. Note however
that the IBID Pt layer may contain an amount of Ga induced by the
ion beam. This fact also leads to a brighter contrast of IBID Pt and a
darker contrast of EBID Pt in the dual-cap layer as presented in the
HAADF-STEM image and vice versa in the BF-TEM image.
5. CNT investigation after low kV cleaning
A final milling/cleaning step was carried out as described in the
sample preparation section. During this step, the FIB milling
should be carried out carefully after the IBID Pt is milled away and
only the EBID Pt of the dual-cap protection layer is present. The
remains of the EBID Pt layer can be removed quite easily and
therefore the CNT may be completely exposed to the ion beam.
The main reason is that the tail of the 5 kV ion beam results in
much more rounding at the top edge of the lamella. A detailed
discussion of this enhanced rounding effect at low kV cleaning is
treated in the discussion section. The lamella at this step is
presented in Fig. 4c, which has a thickness of only 40–60 nm and
therefore is suitable for HRTEM investigation.
Fig. 6a presents a cross-section HRTEM image of the contact
holes. Embedded in an amorphous matrix, shells on both sides can
be distinguished, as indicated by red lines. Enlargements of these
shells from both sides are shown in Fig. 6b and c. The intensity
profile through the shells is shown in Fig. 6d; from this profile the
inter-distance between shells is measured to be 0.34 nm, which is
the distance expected for MWCNT walls. These experiments
clearly illustrate that the inner shell structure of the CNT is
preserved during the FIB preparation process.
Apart from the symmetric shells found at both sides of the
hollow core, a few pieces of discontinuous CNT walls are found to
be present as well, as indicated by yellow arcs in Fig. 6a. The
presence of these curled shells has several possible origins. It is
possible that they are artifacts/defects of the CNTs, which are
commonly observed in CVD-grown CNTs. It is also possible that
they are caused by FIB milling. When a CNT grows inside a contact
hole, it may not fill up the entire space in the hole. For the contact
holes with a nominal diameter of 130 nm, most as-grown CNTs have
a diameter of 40–60 nm, as measured by SEM. In addition, the CNTs
may not grow strictly vertical along the contact hole but wind their
way through. Therefore, it is virtually impossible to have the lamella
with the CNT in the centre over the full length of the CNT. Therefore
parts of the CNT can be milled away and only pieces of the CNT are
preserved, as illustrated in Fig. 7 for several possible situations. The
different cases are difficult to distinguish as the TEM image is a 2D
projection over the thickness of the TEM lamellae. Electron
tomography could be a solution to overcome this problem [26].
In Figs. 6 and 7, small areas of dark contrast are visible on all
HRTEM images. The contrast is attributed to small clusters of Pt
metal; this has been confirmed by EDX. The presence of Pt around
the CNTs inside the contact hole is mainly due to the deposition
of EBID Pt. This is difficult to avoid, as the CNTs do not fill up the
entire space in the contact holes and therefore the contact holes
are not fully covered by the CNTs (see Fig. 1c), part of the EBID Pt
fills in the space inside the contact holes.
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Fig. 5. BF and HAADF-STEM study of the same contact hole where the as-grown CNT is covered by dual-cap protection layer. The carbon map shown as inset provides the
distribution of carbon element at the CNT tip.
Fig. 6. Presents HRTEM study of the CNT preserved in contact holes in situ. The HRTEM image is shown as (a). The enlargements of the CNT shells are presented as (b) and
(c). An intensity profile through the CNT shells is shown as (d).
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mentioned in earlier studies [18,20], and we have demonstrated
how important this method is to protect the delicate surface
features like CNTs. When only IBID Pt is used, the sticking out
parts of the CNTs are undoubtedly destroyed and the contact hole
will also be inevitably filled. The IBID Pt contains more amount of
Pt than the EBID Pt and the high contrast of this heavy metal Pt
will certainly make the CNTs invisible in (S)TEM. Therefore the
use of a dual-cap layer is very important.
During the last steps of 5 kV cleaning, the remains of the
dual-cap Pt layer can be removed quite easily, as mentioned
before. On the one hand, as the accelerating voltage of the ion
beam decreases, the beam is less focused, and therefore the spot
size of the ion beam increases. When the current of 29 pA is
applied during the 5 kV thinning procedure, the spot size is
61.2 nm; when the current of 10 pA is applied during the last step
of the 5 kV thinning, the spot size is 46 nm. On the contrary, when
a current of 30 and 10 pA is applied for a 30 kV ion beam, the spot
size of the beam is only 16 and 12 nm. Obviously, the low kV
cleaning causes a broader and less-focused ion beam. One should
also realise that the incident FIB spot not only has a fixed size but
also presents an approximately Gaussian distribution of intensity
[27]. Moreover, an additional exponential tail of the beam has also
been reported [28]. Therefore, when a FIB spot is scanned to mill
the specimen, the side wall has a certain slope. As a consequence
the FIB milling cross-sections are not perfectly parallel to the
beam but result in a top rounded tapered shape, reported both
experimentally and theoretically [27]. Although we tilted the
sample away from the axis parallel to the incident beam, a
rounding near the top edge cannot be avoided in the cross-section
specimen. This is due to unwanted sputtering by the tails of the
FIB spot, especially in the last 5 kV cleaning where the spot size is
already broad and rounding effect can be worse. Since during this
last step of FIB preparation the specimen is already thin, the
enhanced rounding effect during FIB milling at both sides of the
lamella results in the removal of IBID as well as EBID Pt layer,
which may leave the CNT completely exposed to the ion beam.
Therefore, the protection layer has to be deposited thick enough in
the first place.
6.2. Preservation of CNTs in situ in the SiO2 matrix
Fig. 7. (a) Shows that only one side of the CNT shells is found from the contact hole
of an as-prepared lamella. (b) Presents a lamella containing pieces of CNTs.
6. Discussion
It has already been shown in the past that FIB is an excellent
technique to prepare TEM samples, especially in the field of
semiconductor research and industry. Here, a modified approach
is proposed to prepare samples of CNT interconnects with the
structure of CNTs grown in contact holes.
6.1. Dual-cap layer
A dual-cap protection layer is used to protect the CNTs from
destruction by ion milling. The idea of a dual-cap layer has been
A Ga+ beam is generally used to mill metal or semiconductor
materials, which are harder materials compared to CNTs. CNTs are
not only soft material but also quite beam-sensitive under the
exposure to either ions or electrons. Nevertheless, we have
successfully shown that the CNTs can be preserved from complete
amorphisation, although a slight modification and destruction of
the CNTs is inevitable. Furthermore, we have succeeded in
preserving the soft CNTs in situ within a matrix of SiO2, which is
the harder material.
Milling was carried out with a 5 kV ion beam, which is the
lower limit of the instrument. Recently, ultra-low kV FIB milling
became commercially available, e.g. 2 kV and even 1 kV in the
latest instruments. Since it has been successfully shown here that
5 kV milling can preserve the CNTs in situ in the contact holes,
lower energy FIB milling will certainly result in even higher
quality TEM samples.
In order to obtain a better cleaning of the sample, a posttreatment was carried out. Ar+ ion beam milling was performed exsitu at a voltage of 0.8 kV for 20 min at a small angle. However, the
sample did not show any obvious improvement. Plasma cleaning is
usually applied to clean the specimen surface before TEM
investigation. Nevertheless, in the general study of the CNTs, plasma
treatment is seldom applied as it may alter the as-grown structure.
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6.3. Limits of the technique and prospects
Although samples have been successfully prepared for qualitative HRTEM work, there are certain limits for this preparation
technique. First, it should be realised that the CNTs can be modified
by the energetic Ga+ ions. Despite the fact that a 5 kV ion beam is
applied to minimize the amorphous layers on the sample and to
preserve the CNT structure as good as possible, the energy carried
by Ga+ ions undoubtedly exceeds the theoretically calculated
knock-out energy of carbon atoms in CNTs. Therefore, destruction
of the outer shells of the CNTs is virtually impossible to avoid.
Moreover, a slight amount of Ga+ ion may have been implanted
into the sample, which is common to all FIB prepared lamella.
As the CNT does not grow strictly vertical in the contact hole, it
is virtually impossible to have the lamella with the CNT in the
centre over the full length of the CNT. Therefore, it is difficult to
guarantee that each lamella will have the cross-section of an
entire CNT. In Fig. 7a, e.g., only part of the CNT is present.
Also, the EBID Pt filling inside the contact hole during
protection layer deposition hampers the interpretation of the
CNTs. As it is inevitable that the EBID Pt fills up the space in the
contact hole, the Pt nanoclusters coat the outer shell of the CNT,
however without damaging the CNT.
Regarding the application of CNTs in contact holes as future
interconnects, it has now been shown that the FIB sample
preparation is able to preserve the CNTs in the contact holes, and
further investigations can be carried out including not only lowdensity CNTs but also high-density CNTs grown in situ in contact
holes. A high-density of CNTs in a contact hole will complicate CNT
analysis due to superposition effects in projection, but this may be
overcome using 3D electron tomography, which enables the
visualisation of the CNT network inside a contact hole [26].
7. Conclusions
FIB specimen preparation using a dual-cap Pt protection layer
and extensive low kV (5 kV) cleaning has been applied to prepare
TEM samples from contact holes containing single CNTs. By SEM,
HRTEM, HAADF-STEM, and EFTEM it has been demonstrated that
the milling and cleaning process yields samples in which the CNTs
inside contact holes have been preserved. The success of preparing
such specimens for low densities of CNTs in the contact holes has
opened ways for further application to high densities of CNTs.
Acknowledgements
The authors acknowledge financial support from the European
Union under the Framework 6 program under a contract for an
1359
Integrated Infrastructure Initiative. Reference 026019 ESTEEM. S.B.
is grateful to the Fund for Scientific Research-Flanders. This work
has been performed within the Interuniversity Attraction Poles
Programme – Belgian State – Belgian Science Policy.
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