fib-based target preparations of complex material systems for

FIB-based target preparations of complex material systems for advanced

TEM investigations

U. Muehle1, S. Jansen

2, L.Hillmann

3, H.-J. Engelmann

2 and D. Rafaja

1

1 TU Bergakademie Freiberg, Inst. of Materials Science, Gustav-Zeuner-Str. 5, D-09599 Freiberg 2 Globalfoundries Dresden, Modul One LLC & Co. KG, Wilschdorfer Landstraße 101, D-01109 Dresden 3Infineon Technologies Dresden GmbH, Koenigsbruecker Str. 150, D-01099 Dresden

Increasing number of TEM-investigations and particularly the request for very special studies leads to a higher demand for

target preparations. The Focused Ion Beam technique (FIB) is able to cover a large amount of these requirements. In this

contribution the principles of FIB-based target preparations are demonstrated on some example from the semiconductor

technology. Using this technique several challenging sample preparations even for special techniques like TEM

holography and/or TEM tomography that requires a very high quality of the samples could be performed. Furthermore

possible consequences of the damage of the specimen by the ion beam and the possibilities, how to avoid this damage, are

discussed.

Keywords TEM; Preparation; Focused Ion Beam, Lift-Out

1.Introduction

In the last years the number of TEM investigations in material science strongly increased. This trend is caused by the

necessity of understanding the materials behaviour on the nanoscale and is supported by the vendors of the TEMs and

FIBs, who introduced more and more TEM-applications to reliable utilization [1, 2]. Furthermore several sectors of

industry are acting in dimensions, which require inspections by TEM. One of the key players in downgrading the valid

dimensions of the products is the semiconductor industry that went below 0.3µm in defined structures still in the early

1990’s [3], which required functional layers of less than 10nm thickness. Consequently, the dimensions of these

structures have to be controlled frequently by TEM, which led first to formation of TEM laboratories near to production

sites [4] and finally to the demand for more, faster and more reliable preparation techniques. Whereas a target

preparation of the single device down to 350nm is possible by a combination of the mechanical preparation and broad

ion milling [5], this approach fails in case of smaller sizes due to missing possibility of the optical control.

Fortunately in the early 1990’s Liquid Metal Ion Sources (LMIS) as the main feature of FIB machines were ready for

production [6]. These machines fulfill the demand of semiconductor industry for an exact control of the devices and

their features. The consequence was the dominance of semiconductors industry in development, technical design and

market share of FIB respectively FIB/SEM combinations mainly in the first decade of their availability on the market.

So a large amount of accessories was dedicated to handle wafers and pieces of them in this machines. The high

standards of industrial customers lead to a fast development of these machines to high quality and reliability, expressed

in uptimes rates larger than 90% and Mean Time between Failure (MTBF) values of several hundred hours.

One of the main utilizations of the FIB-machines from the beginning was the target preparation of TEM-samples [7].

The techniques for realization were improved very fast in terms of reliability and time to result [8], driven by the

expectations of industrial users, where the success of failure analysis directly influences the economical results. The

utilization of these experiences in other fields of application, especially in materials science, where composite materials

and the complex relations between their components and phases play a more and more important role, is the key for

TEM-characterizations at locations, which are hard or impossible to provide.

2.The H-bar technique

A very broadly applied kind of the TEM samples preparation by FIB is the so called H-bar technique, named according

to the appearance of the specimen from the top that is similar to the letter “H”. This technique enables any previously

defined target to be isolated in form of a slice, which will be attached to grid1, transferred in the preparation machine

and thin the requested location with a large amount of freedom in final geometry. [9]

The specified target on the wafer has to be isolated in a slice of about 1.5mm width a 50µm thickness using the

equipment of mechanical preparation like sawing and grinding (fig. 1). The availability of a commercial wafer saw

shortens this step down to about 15 minutes. The slice will be glued to a half moon grid of copper or a thrust ring half.

1 The expression “grid” does not refer to real grid with meshes. It is a more or less commonly used name for the part on which the specimen is mounted and is responsible to a safe handling in the TEM samples holder as well as in the preparation machine.

Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)

1704 ©FORMATEX 2010

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Some commercial equipment on the market overtakes all steps from the target definition to the optimized sample

geometry for FIB treatment [10]. The first step in the FIB preparation is to cover the target against ion beam damage by

deposition of a protective layer (fig. 2). This layer can be deposited using the Ion Beam Induced Deposition (IBID) or,

if the target of the analysis is situated close to the samples surface, by Electron Beam Induced Deposition (EDID) in a

FIB/SEM-combination and leads to sharp edges on the top during final cutting. The mostly used precursors for the

deposition contain platinum, tungsten or silicon oxides. The second step in the FIB preparation is the removal of larger

amounts of surrounding material using higher ion currents, creating the “H”. Utilization of gas injection systems (GIS)

for gas enhanced etching, e.g. I2 or XeF2, avoids redeposition and shortens the preparation time, but it can only be

applied if the sample contains no components, which are sensitive to the gases like Cu. For thinning of electron

transparent regions lower currents are to be used in two or three steps, where the option “polishing cross section” with a

slice by slice removal of material is recommended. In a FIB-SEM combination [11, 12] the achievement of the target

level can be controlled very exactly. Results of the TEM characterization can be supplemented by SEM images of both

sides of the foil, documenting the exact location of the lamella.

1.5 mm

Cu grid

50µm

Fig. 1 General method for isolation of a target (bright square) for the preparation of a H-bar specimen using a wafer saw and mount

it on a half moon grid.

Fig. 2 H-bar technique for preparation of a TEM-specimen: The target is protected by a platinum bar and large patterns using high

ion currents take care for a path for the TEM (left). Smaller ion current cut a fine lamella (centre). On the right part the whole

geometry of the sample is demonstrated.

TEM specimens prepared this way are usually very robust. Depending on the periodicity of the structures an

additional preparation of locations for comparison is possible as well as keeping parts as reserve for a 2nd

attempt (fig.

3, right). For challenging analysis a sequence of a first “rough” preparation, stopping at 100…150nm thick foil, primary

inspection in TEM and careful final thinning until 50nm or less improves the success rates. Restrictions are given by

allowed range of tilting in the TEM due to the channel, which the TEM beam has to pass for imaging.


©FORMATEX 2010 1705

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Fig. 3 Multilevel preparation of a plane view sample from an integrated circuit. The electron image (left) enables to control the

target levels. In the centre the different levels are demonstrated and the TEM-image (right) shows the functional details.

In order to be able to fulfill special requests like a preparation parallel to the surface (“plane view”) of the wafer a

mechanical polishing until a few µm to the target can be used. The milling direction parallel to the surface of the wafer

enables a very exact definition of the level of the TEM lamella in a complex structure. This can be done for several

levels at adjacent positions leading to a complete characterization of complex structures (fig. 3).

3.Lift-out techniques

The lift-out techniques do not require a special mechanical treatment of the sample to produce a geometry which is

suited for both the FIB and the TEM. The specimen can be extracted from the wafer (or any other material surface part)

using special equipment. So these approaches are preferred to extract a specimen out of the production line and keep the

wafer for further processing and possibly extracting additional specimen some production steps later [13]. A second

reason for development of these techniques was the intention to reduce the preparation time in order to realize the

demand to faster results and higher throughput of samples, required by technologists.

3.1 Ex-situ lift-out

These techniques are characterized by the involvement of a second machine for extraction of the specimen. The TEM

sample is completely prepared, including final thinning to electron transparency, while mounted with small bar at the

surroundings (fig. 4). The extraction of this specimen can be realized under an optical microscope with a large working

distance, enabling to manipulate between objective and sample. A second precondition is the availability of

manipulators with very reliable and sensitive movements. Former approaches used a needle of glass using a

combination of adhesion and electrostatic forces [14], which are strong enough to break the remaining attachment of the

lamella.

The specimen extracted this way will be put on a TEM-grid with a carbon film2. Whereas the glass needle put the

sample without any possibility of control to the grid, newer solutions use a hollow needle for picking up and putting

down the specimen [15]. This enables a high precision management during removing the sample out of the wafer as

well a defined positioning at the grid (fig. 5). If necessary this technique is qualified for working on a whole wafer in

the production line. Preparation of the specimen can be done by single beam instrument and extraction of the sample

takes few minutes only. The presence of a carbon foil overlaying the sample reduces the contrast but this is not critical

2 Here a real grid mainly consisting of copper with meshes is used. To keep the specimen it is overlaid with a thin carbon foil. The best parts available are regularly perforated, which gives the chance to transmit the sample without the influence of continuous carbon layer.

Fig. 4 TEM specimen, prepared for the Ex-situ lift-out method: The

specimen is completely thinned and separated from the surrounding

material. Two small connections, which will be destroyed during the

lift-out procedure keep the specimen in the right position.



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for imaging only. For elemental analytics it may be a detractor. It might be an issue, that after preparation no more

corrective actions like additional thinning, plasma cleaning or other can take place.

Fig. 5 Ex-situ lift-out preparation: The TEM specimen is focussed by a visible light microscope (left). A needle approaches and

breaks it out of the sample (centre) and lays it down to TEM grid with a perforated carbon foil (right).

3.2 In-situ lift-out

In contrary to the technique described before, all steps of the in-situ lift out preparation have to be done inside the FIB

machine. The main feature is a micromanipulator, carrying the specimen out of its origin location to a special TEM grid

where it has to be attached. Some early approaches realized this operation in a single beam instrument [16], but the

availability of FIB/SEM combinations gave raise to a broader distribution of this technique.

First the target has to be covered and the surrounding material has to be removed using high ion currents. In contrary

to the procedure described in section 3.1 the specimen keeps a thickness of 1µm or more and remains at the

surroundings by a stable connection, small enough for interruption by the ion beam in a short time. There are different

versions for a proper geometry of the specimen to be extracted (fig. 6). In a following step a micromanipulator

approaches to the specimen and is be attached by IBID with the GIS-system (fig. 7). Now the specimen can be cut

completely free and the micromanipulator picks it up and moves to a safe position.

The second part of the procedure starts with changing the sample against a special TEM grid3, containing “teeth” for

mounting the specimen [17]. The micromanipulator with the specimen has to be approached carefully to the edge

(alternatively the top) of one tooth and the specimen will be mounted using IBID again. After that the connection

between the micromanipulator and the specimen is cut. Final thinning using low ion currents can be done similarly the

H-bar technique (fig. 8). All degrees of freedom including second time thinning, SEM imaging of surface etc. are

available.

Fig. 6 Two options of the primary preparation of an In-situ lift-out preparation: On the left side the sample is completely released

by large FIB-patterning, on the right image cuts from different directions meet each other in a deeper level.

3 See note a: The expression grid is here the commercial used name of the part again.



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Fig. 7 Sequence of in-situ lift-out preparation: The micromanipulator approaches (top left), will be mounted on the separated

specimen (top centre), which is released (top right). Then the specimen will be moved away (low left), will be approached to the

TEM-grid (low centre) and is attached by Pt- or W-deposition (low right).

It should be noted, that so prepared lift-out samples are well suited for some special techniques in the TEM. In

comparison to H-bar samples, the absence of surrounding material leads to lower amounts of secondary excitation in

EDX-analysis [18]. For investigations using diffraction and diffraction contrast the free standing sample enables large

tilting angles to adjust different excitation conditions [19]. In comparison to the ex-situ lift-out method, where

frequently losses of samples have to be taken into account, a success rate near 100% for the preparation can be

achieved.

4. Non-semiconductor applications

4.1 H-bar technique for any geometry

For applications in the semiconductors industry the conditions for H-bar technique are favoured. The thickness of a

wafer fits very well to dimension which is allowed by the geometry of the grid and the target is located on the top. Only

the lateral dimensions have to be achieved by mechanical preparation. In case of any geometry these conditions should

be created. This may be realized by preparation of a piece in the allowed dimensions. Figure 9 demonstrates the

preparation of Ti-based sheet material, where the H-bar preparation enables to investigate the deformation mechanisms

Fig. 8 TEM-specimen after in-situ lift-out preparation: The region of

interest is final thinned for application of the TEM-technique of choice.



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depending on the distance to the surface, here in the central part of the sheet. A disadvantage of this method is the

possibility to define a target location, restricted to the options which are given by mechanical treatment.

Cross sections at layers systems can be prepared by FIB, when mechanical pretreatment until a proper thickness for

broad ion milling fails. Here grinding and polishing is only necessary until the dimensions of the slice to be mounted on

the half-moon grid. Especially in complex or thicker systems the FIB allows a very exact definition of the mostly

interesting location.

4.2 Lift-out of a metallographic section

In many cases the detachment of the target region as a slice with dimensions suited for FIB out of the system is

challenging or impossible. On the other hand metallography supports a lot of experiences in preparing metallographic

sections for Optical Microscopy (OM) or Scanning Electron Microscopy (SEM) of complex situations like small

bondings, tips of cracks etc. The desire of TEM investigations at several details after a characterization by visible light

microscopy, SEM or even X-ray diffraction (XRD) could be satisfied using the lift-out technique at the metallographic

section observed by the mentioned methods before. The preconditions for such an experiment are good due to the

excellent surface of a metallographic section.

Fig. 10 Metallographic section with a solder connection, prepared for in-situ lift-out preparation of a TEM sample at specified

location (left). The height of the sample is reduced to 3…5 mm, defined by the samples holder of the FIB tool. In a SEM-

characterization (right) the components solder material (1), intermetallic phase (2) and basic material (Ni - 3) is identified.

The first step for the realization is to reduce the samples dimensions to a value covered by the recipient, usually in

term of height and take care to a good conductivity to ground (figure 10). Sometimes it is recommended to mark the

region of interest by microhardness indentations because contrast behaviour in FIB may differ, complicating the

relocation. The following steps are equal to the described in-situ lift-out procedure and consist of (a) removing the

Fig. 9 Titanium based sheet material (1), mounted on a

TEM half moon grid (2). For the final preparation (3) the

location can be selected at any level of depth.



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surrounding material, (b) approaching and mounting the micromanipulator, (c) cutting and removing the specimen, (d)

approaching and mounting it to the grid and (e) final thinning to electron transparency. It should be noted, that the

sputtering behaviour of complex material systems may differ strongly from well known samples (e.g. silicon). This has

to be taken into account in definition of pattern dimensions and depths. Figure 11 [20] demonstrates the results of

complex investigations of solder joint. Here the primary metallographic preparation including SEM characterization

was the best choice to meet the target region before starting the TEM preparation.

Fig. 11 Result of complex TEM characterization after lift-out preparation: The specified grain of the intermetallic compound (1 in

left picture) was investigated by electron diffraction in three zone axis. In combination with an EDX-analysis the phase could be

recognized as Cu6Sn5, where Cu is partly substituted by Ni [20].

4.3 Applications at complex materials systems

Based on the experiences of chapter 4.2 a broad variety of materials and systems can be prepared by lift-out technique.

In many cases it is not required to prepare a perfect metallographic section. The existence of a well polished surface,

meeting the target region, is the only precondition. Examples for application are the cross sections of layers, where the

substrate was too brittle for the isolation of a TEM sample by sawing and/or grinding (fig. 12) or grain boundaries of a

solar cell with conspicuous behaviour in electrical tests (fig. 13). In general it should be noted that the lift-out

techniques enlarges the possibilities of TEM-target preparations to all cases, where a reliable preparation of an exact

metallographic preparation succeeds. The availability of in-situ lift-out techniques extends the TEM preparations to

almost every location like tips of cracks etc.

Fig. 12 TEM preparation of a brittle substrate (SiN) with a Si-layer for investigation of phase boundary behaviour and orientation

relations: Two parts of the sample were glued “face to face” and polished using metallographic techniques (left) for the extraction of

a TEM specimen (centre), thinning and TEM-observation (right).



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5. Requirements of special TEM techniques

The development of special techniques in the broad field of TEM requests special ways for specimen preparation taking

into account the special circumstances of these techniques. Due to the control by FIB and/or SEM imaging the

requirements of well defined thickness (e.g. for spectroscopic and diffraction methods) can be fulfilled, much better

than by other methods. Additional requirements are formulated by techniques like off-axis holography or 3D

investigations using tomography of multiple views.

5.1 Electron holography

The electron holography is a powerful tool for a more detailed microstructure analysis, as it allows not only the

intensities but also the phase shift to be observed [21, 22]. One application is the imaging of variations in the mean

inner potential of doped regions in semiconductor devices [23], leading to a better understanding of the technological

steps. The FIB technique is indispensable for the sample preparation and opens a wide field for holographic

applications, although some additional challenges are to be taken into account [24]. It is necessary to allow a vacuum

beam travel within the field of view, which is restricted to less than one µm to the detecting system. Furthermore the

method is very sensitive to variations in sample thickness, so that in practice no curtaining effects (see below) are

allowed.

The key to avoid the curtaining effect, which is mainly caused by cutting through very different material, is to make

sure that the ion beam meets only homogeneous material. This can be performed by removing all material with different

sputtering behaviour before starting the FIB steps [18] or by changing the geometry in a way that the ion beam comes

not from the top direction [25]. The excellent possibilities for controlling the ion beam in a FIB/SEM-combination

brought the required region in the TEM lamella and achieved the desired samples thickness. The results are specimen

with MOS transistors in a short distance to the edge of the sample (fig. 14), enabling a path of the vacuum beam to

bring it to interference.

Fig. 14 Two examples for FIB-based target preparation of electronic structures for electron holography: At the left hand side the

transistor structure was situated at the edge of a H-bar sample and the vacuum beam could be situated beside. In the centre a lift-out

technique was used and the final polishing was realised by a direction, which was former horizontal. The right picture shows the

visualisation of the doped areas (white arrows).

Fig. 13 Grain boundary in solar silicon, to be recognized by the

etching behaviour: The deposited Pt bar defines the location of the in-

situ lift-out for characterization of the grain boundary. It should be

noted, that the lift-out preparation can be applied without a

mechanical treatment of the sample.



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5.2 Electron tomography

For 3D-investigations using electron tomography high tilting angles are necessary [26]. Whereas the electrochemically

thinned or broad ion milled samples can fulfil these conditions in most cases, FIB processing is restricted in removed

volumes due to the long preparation times for removal of larger amounts of material. Using the lift-out techniques the

material surrounding the target has a smaller volume and a complete dispensation can be done. The positioning of the

extracted slice on top of the tooth and the selection of a parallel direction to the rotation axis allows the full range of tilt

angles of the tomography holder (fig. 15).

As shown recently [27] it is recommended to prepare a specimen with pillar shape for tomography. This leads to a

constant thickness at the whole range of acquisition angles.

Fig. 15 TEM-specimen for electron tomography: The left picture demonstrates the principle of the free standing sample and the

rotation over a wide angular range (+/- 70°). The preparation of 3 identical pillars is to seen in the centre, looking down parallel the

ion beam. TEM-overview of the pillar system is demonstrated at the right.

5.3 TEM in multiple viewing directions

There are issues, in which the exact definition of the location of the target cannot be realized before starting the

preparation, like in complex electronic devices, where several transistors are working together in a manner that doesn’t

allow to specify, which transistor is responsible for a dysfunction. Then a rough TEM observation of the whole system

using larger foil thickness and a low magnification can lead to more detailed assumptions about the origin of the failure.

A complete understanding of the failure mechanism can be achieved by a more detailed investigation from another

point of view. Using a careful Electron Beam Induced Deposition (EBID) for protection of the specified location

enables further preparation steps in order to detach a specimen perpendicular to the first plane by second lift-out [28].

The present example shows a partial plane view of an integrated circuit covering the levels from silicon substrate to the

lowest contacts (fig. 16), identifying the probable location of the fail. A lift out of this region from the plane view

sample allows the detailed description of the origin of the fail.

Fig. 16 TEM preparation for two directions of observation a one location: At the left the preparation for plane view of a larger area

before application of lift-out is shown. In the centre the area of special interest (squares) was found by TEM plane view imaging. The

dashed lines describe the location of the cross section, prepared out of the plane view specimen and shown on the right.



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6. Samples damage

6.1 Amorphisation and Ga-penetration

The bombardment of a specimen with 30kV accelerated Ga ions doesn’t occur without strong influences on the

structure. Ga ions penetrate deep in the hard core and destroy the bindings in the same manner as any sputtering process

like in case of a Secondary Ion Mass Spectrometry (SIMS) measurement [29]. The depth for direct and lateral

penetration can be calculated using Monte Carlo Simulations [30].

For a TEM sample this leads to amorphisation of the upper regions and the side wall of the TEM lamella (fig. 17,

[31] [32]). The range of amorphisation depends on the material and achieves more than 20nm in silicon in lateral

direction. Other materials like tungsten or crystals with a high amount of packing density like TiN are less sensitive.

Whereas the destruction on the top of the sample can be easily avoided by deposition of a protective layer or EBID, the

amorphous layers at the sidewalls reduce the amount of useful information in the specimen and downgrade the signal to

noise ratio. Especially for sensitive investigations like High Resolution Imaging and EELS, where often very thin foils

are necessary, this layers should be removed. Actual tools offer the reduction of acceleration voltages for the ion

column down to 1 or 2 kV, applied to the final polishing steps. The reduction of the acceleration voltage to 5kV and 2

kV leads to noticeable thinner amorphous layers, demonstrated at a Si single crystal (fig. 18 [33]).

Fig. 18 Effect of reduced acceleration voltage on the lateral amorphisation of a Si-specimen during FIB-polishing

Using the Time-of-Flight Secondary Ion Mass Spectroscopy (ToFSIMS [34]) an investigation of the penetration

depth detected Ga deeper than the amorphous layer [35]. The so measured Ga depth profile depends on the acceleration

voltage too (fig. 19 [35]). Using the FIB preparation the appearance of Ga in an elemental analysis (e.g. by EDX)

should be always taken into account as a preparation artefact. For investigations, where effects of band structure

influence the result the presence of impurity traps due to low concentrations of Ga cannot be neglected [36].

Fig. 17 Cross section of a TEM lamella, demonstrating the

amorphous region due to influence of highly accelerated Ga ions:

Amorphisation above the silicon occurred during deposition of the Pt-

bar and at the sidewall during final polish using 30kV and a small

current density.



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6.2 Curtaining

An additional effect in FIB preparation is the so called “curtaining” effect [37]. Materials with different sputtering

behaviour above the region of interest influence the angle at the edge of the FIB cut, resulting in different foil

thicknesses below (fig. 20 [19]). Unfortunately this effect increases with lower acceleration voltages (fig. 21) like used

for high quality samples.

In order to avoid this effect, all different material, especially structured details, above the region of interest [38] has

to be removed. Alternatively the direction of the ion beam with respect to the objects to be prepared may be changed,

which requires some special ideas about the samples geometry and the planned kind of TEM-investigation [25].

Fig. 21 Influence of reduced acceleration voltage on curtaining effect: Whereas the full voltage of 30kV polishes leads to minor

curtaining effect, which would be only visible in the TEM (left), a 2kV-polish leads to strong curtaining still at locations with small

differences in the material above (right, see white arrows).

Fig. 19 Lateral depth of Ga

penetration during final FIB polishing

for different acceleration voltages: It

should be noted that the Ga

penetration exceeds the thickness of

the amorphous region significantly.

Fig. 20 FIB-prepared TEM-sample: The wedge-like

interference patterns (a, marked by the white line) allow

an estimation of the regions of equal foil thickness.

Below the presence of more heavy material like WSi2

(b) they come closer to the upper edge of the substrate

and below cavities (c), offering no resistance to the ion

beam, they appear in a larger distance to the edge of the

substrate.



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7. Summary

The utilization of the FIB preparation, particularly in a combination with SEM, enables to solve a lot of very different

requests for TEM investigations at very restricted locations and at challenging combinations of materials. Standard

techniques like the H-Bar technique were developed for the semiconductors industry, where FIB machines are fast and

wide distributed in many laboratories and manufacturing sites. This technique can be applied to a lot of issues in

material science.

The availability of lift-out equipment, especially of micromanipulators for in-situ lift-out enlarges the variability of

preparation techniques and simplifies the mechanical preparation to that of the usual preparation of a metallographic

section.

Acknowledgements Parts of this work were supported by by the Ministry of Education and Research of the Federal Republic of

Germany (Project No. 13N9432). Furthermore the authors thank the company FEI for the cooperation during common tool

development projects and tool demos. Special thanks to the staff of the TEM laboratories of Qimonda/Infineon Dresden and

Globalfoundries Dresden for providing us with of unpublished results and to the technical staff of the Institute of Materials Science at

the TU Bergakademie Freiberg for support.

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