Nanoscale strain measurements in TEM for electron devices: dual lens dark field electron
holography, high angle annular dark field scanning transmission electron microscopy and
nano-beam electron diffraction
W. Weng*, Y.Y. Wang*, F.H. Baumann*, M.A. Gribelyuk*, D. Cooper**, A. Pofelski*** and L.
Grenouillet**
* IBM Microelectronics Division, Zip 40E, 2070 Route52, Hopewell Junction, NY 12533, USA
** CEA, LETI, Minatec, F-38054 Grenoble, France
*** STMicroelectronics, 850 Rue Jean Monnet, 38926 Crolles Cedex, France
Email : [email protected] TEL : +1-845-894-5944
The introduction of strain into the device channel is known to be an effective way to enhance the
device performance in modern semiconductor circuits. A compressive strain in PMOS devices can increase
the hole mobility, and a tensile strain in NMOS devices can increase the electron mobility. In the research and
development phase, monitoring the strain in the device channel at the nanoscale is critical to the device
learning and process development.
Strain measurements in the transmission electron microscope (TEM) allow us to obtain strain profiles
or maps at the nanometer scale with a high precision. In IBM/LETI/ST three strain measurement methods are
developed and performed routinely, namely dual lens dark field electron holography (DL-DFEH) [1]
, high
angle annular dark field scanning transmission electron microscopy (HAADF-STEM) [2]
and nano-beam
electron diffraction (NBED) [3]
. They were used to characterize a PMOS Ultra Thin Box and Body Fully
Depleted Silicon-On-Insulator (UTBB FDSOI) device, which has a pure Si channel as thin as several
nanometers and SiGe stressors in source/drain as shown in Figure 1. The unique UTBB structure poses big
challenges for nanoscale strain measurements. Sample tilts and possible miscuts between substrate and body
makes the interpretation of DL-DFEH results more difficult. The miscut between substrate and body can also
make the acquisition of STEM image along <110> zone axis harder. The size of electron probe (~5nm)
utilized in NBED is comparable with the thickness of the device channel, which requires minimal sample
drift. Additionally, electron diffraction from such a thin volume shows shape effect, causing streaks in the
diffraction patterns. A reliable algorithm is necessary to do the data processing of NBED patterns.
In DFEH experiment, a dark field hologram (Fig. 2(b)) is formed by overlapping the electron beam
passing through the strained region (body) with that passing through the unstrained region (substrate). This
hologram is analyzed by a geometrical phase analysis (GPA) algorithm, and strain information can be
extracted as shown in Fig. 2(c, d, e). In HAADF-STEM, the atomic positions of the strained region are
compared with those in the unstrained region in one HAADF-STEM image (Fig. 3(a)). Strain (Fig. 3(b, c, d))
can be obtained by performing GPA on the Fourier Transform of the HAADF-STEM image. Unlike DL-
DFEH and HAADF-STEM, NBED method directly measures the displacements of the diffraction spots of the
strained area with respect to the diffraction spots of the unstrained area. Strain values are retrieved using an
in-house program based on a simple formula:-
where ε is the strain, d is the interplanar spacing of the crystal and g is the corresponding reciprocal lattice
vector. <110> strain values in the UTBB FDSOI device channel obtained by different techniques are in good
agreement, with DL-DFEH measuring 0.6 – 0.7% compressive strain in the channel, STEM measuring 0.9%
(± 0.2%) and NBED measuring roughly 0.6%. This demonstrates that DL-DFEH, HAADF-STEM and NBED
techniques are able to measure the strain of electron devices at the nanoscale with a high precision.
References [1] Y.Y. Wang et al., Ultramicroscopy 124 (2013) 117.
[2] D. Cooper et al., Applied Physics Letters 100 (2012) 23321.
[3] A. Béché et al., Journal of Physics: Conf. Series 209 (2010), 012063.
strained
strainedunstrained
unstrained
unstrainedstrained
g
gg
d
dd −=
−=ε
[4] This work was performed by the Research and Development Alliance Teams at various IBM Research
and Development Facilities.
Figure 2. DL-DFEH: (a)
BF-TEM micrograph, (b)
dark field hologram and
(c) strain map of a UTBB
FDSOI device; (d) and (e)
strain profile extracted
along line 1 and line 2 in
(c), respectively.
Figure 3. HAADF-STEM:
(a) HAADF micrograph
(inset: enlargement of the
body and the substrate
showing atomic positions
of Si) and (b) strain map
of a UTBB FDSOI device;
(c) and (d) strain profile
extracted along line 1 and
line 2 in (b), respectively.
Channel SiGe SiGe
001
110
Line 1
(a)
(b)
2
1
SOI SOI
(a) (b)
(d) (e)
(c)
Channel
Line 1
Line 2
(a) (b) (c)
(d)
(a1)
(a2)
(a2)
Figure 4. NBED: (a) STEM micrograph, (b)
strain profile along line 1 and (c) an example of
electron diffraction patterns taken from the body
showing streaks along <001> direction of a
UTBB FDSOI device.
(c)
Figure 1. (a) Schematics and
(b) BF-TEM micrograph of a
UTBB FDSOI device.