Supplementary Materials
Plasticity Mechanisms in Ultrafine Grained Freestanding Aluminum Thin Films
revealed by in-situ Transmission Electron Microscopy Nanomechanical Testing
S1. Preparation of freestanding Al beams
The Al beams dedicated for in-situ TEM nanomechanical testing were produced using
microfabrication techniques based on MEMS-type procedures, closely resembling the
technique presented in Refs. [1-3]. These structures are schematically shown in Fig. S1.
FIG S1 (color online) Fabrication involves the successive deposition of the sacrificial and the
specimen layers. (a) Wet etching of the sacrificial layer is performed in 73% pure fluoridric
acid. (b) The resulting freestanding Al beams are grouped in sets all over the wafer.
The handling substrate is a 200 µm thick, 3 in. single-crystal silicon wafer. The fabrication
involves two-layer depositions of sacrificial and specimen layers. The sacrificial layer is made
of a SiO2 1 µm thick plasma-enhanced chemical vapour layer deposited at 300°C using SiH4
(diluted in N2O/N2) as precursor (with gas flow respectively equal to 100/700/1000 sccm).
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The specimen layer is made of a 300 nm thick 99.999% pure Al film deposited by evaporation
at room temperature. Photolithography and dry etching with a Cl2/CCl4 plasma are used to
pattern the specimen layer. Wet etching of the sacrificial layer is performed in 73% pure
fluoridric acid (see Fig. S1(a)). The resulting freestanding Al beams are grouped in sets all
over the wafer (see Fig. S1(b)).
S2. Preparation of freestanding submicron Al beams for in-situ TEM nanomechanical
testing.
The experiments were performed in displacement controlled compression mode using a
conductive diamond flat punch indenter in the PI 95 TEM PicoIndenter from Hysitron.Inc. A
special MEMS device called push-to-pull (PTP) was used (see Fig. S2(a)). Owing to four
identical springs distributed symmetrically at the corners of this device, the compression
(push) motion of the flat punch indenter indicated by the white arrow in Fig. S2(a) is
converted into an uniaxial tensile loading (pull) on the middle gap of the device. The springs
are arranged such that the force acting on them is parallel to the force on the tensile specimen.
In the present study, the stiffness of the springs was equal to 450 N/m and the device middle
gap distance is equal to 2.5 µm. Al freestanding beams were cut by FIB and transferred to the
PTP device using an Omniprobe micromanipulator on a FEI dual beam FIB/SEM instrument
(see Fig. 2(b)). The beams were then attached to the PTP device using electron beam
deposited platinum (Pt) as shown in Fig. S2(c). Eventually, the Al beams were cut from both
sides using the gallium ion beam to achieve a dog-bone shaped smaples with a 2.5 µm length
and 550 nm width (see Fig. S2(c)). It is worth noting here that, in the present study, no effect
of gallium ions on GBs is playing a role since FIB has not been used to produce the
freestanding Al thin films. Indeed, FIB has been only used for the fixation of the Al beams on
the PTP devices while scanning electron microscopy (SEM) has been used to visualize this
process in the dual beam FIB/SEM instrument.
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FIG S2 (a) SEM image of the Push-To-Pull (PTP) device. (b) transfer of an Al freestanding
beam in the middle gap of the PTP device. (c) electron beam Pt fixation and final thinning of
the Al beam.
S3. Characterization of native oxide layer
In the present work, STEM-EELS and EFTEM techniques were used to investigate the
presence of a native oxide layer at the surface of Al thin films. In order to minimize the
damage on the surface during TEM foil preparation by FIB, a Pt protective layer was
deposited on the surface of Al thin film with an electron beam (5kV/0.4nA). The STEMEELS spectra were obtained with a spot size of 0.5 nm. Figure S3(a) shows an annular dark
field STEM image on the Al/Pt interface where the EELS spectra were obtained. Figure S3b
shows a series of EELS spectra with Al K edge transition at the Al/Pt interface. In Fig. S3(c)
the oxygen K edge appears at the Al/Pt interface region. Figure S3(d) shows an EELS
spectrum of oxygen K edge which is extracted from the line profile. The spectra obtained on
both sides of the Al/Pt interface do not exhibit any oxygen K edge. This implies that an oxide
layer was formed on the surface of the Al thin films. Figures S3(e) and S3(f) show an oxygen
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EFTEM map and Al EFTEM map obtained, respectively, on the Al/Pt interface regions. A
clear oxide layer with a thickness around 5.5nm was observed on the film surface. The oxide
layer seems to be homogeneous without imperfections.
FIG S3 (color online) STEM-EELS and EFTEM on the Al/Pt interface. (a) ADF-STEM
image on the Al/Pt interface. (b) EELS line profile on Al K edge. (c) EELS line profile on
oxygen K edge.(d) EELS spectrum of oxygen K edge extracted from (c). (e) an oxygen
EFTEM map on the interface. (f) a Al EFTEM map in the same region as (e).
References
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Mater. 60, 1795 (2012).
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H. Idrissi, B. Wang, M. S. Colla, J. P. Raskin, D. Schryvers and T. Pardoen, Adv.
Mater. 23, 2119 (2011).
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