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 : wweng@us.ibm.com 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.