Electron microscopy
 
Strain/Stress Analysis with STEM
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Table 1194a lists the possible contrast mechanisms of bright-band in the Si (silicon) crystal at an a-SiO2/c-Si interface or an a-Si/c-Si interface in an ADF-STEM image.

Table 1194a. Possible contrast mechanisms of bright-band of c-Si at a-Si or a-SiO2/c-Si interfaces.

Possible contrast mechanism of bright-band in c-Si
a-SiO2/c-Si interface
a-Si/c-Si interface
Roughness of the interface Yes [2] No [5]
Associated with oxygen segregation Yes [3] --
Dopant with annealing temperature   Yes [6]
With a strain field Yes [4] Yes [5]

Other factors such as sample thickness effect in zone-axis crystals [7] and detector inner angle, as shown in Table 1194b, may also affect the appearance of strain contrast in ADF images. In the existing of strain fields, the scattering of electrons results in different angular distributions and thus the intensity collected by the ADF detector from the strained region can be different than that from the strain-free region, resulting in strain contrast. The thickness dependence of strain contrast originates from the propagation process of the electron beam with the channeling effect [8-10, 1] inside a zone-axis crystal, while such channeling effect is not significant in very thin (<10 nm) TEM samples.

Table 1194b. Strain fields in crystalline silicon layers at a-Si/c-Si interface. [5] If the ADF intensity from the strained region is higher than that from the strain free region, this contrast is then defined as a positive contrast and negative contrast if the strained region appears darker than the strain-free region.
ADF
TEM sample thickness
Strain contrast
LAADF <10 nm Negative
>15 nm Positive
HAADF <10 nm Negative
>15 nm Negative

As shown in Figure 1194a, when the strain amplitude increases, the channeling effect decreases, which is also called dechanneling. Therefore, the strain fields cause dechanneling.

(a) HAADF image along (110) zone axis of silicon (Si), (b) LAADF images

Figure 1194a. (a) HAADF image along (110) zone axis of silicon (Si), (b) LAADF images. The strained region is marked by the red arrow. Adapted from [5]

Figure 1194b shows the integrated ADF-STEM intensities versus strain amplitude of crystalline silicon (Si) over LAADF and the HAADF detectors. The LAADF signal increases when a strain field is introduced in LAADF image for thick TEM samples, while the HAADF signal stays essentially constant as strain amplitude increases.

integrated ADF-STEM intensities versus strain amplitude over LAADF and the HAADF detectors

Figure 1194b. Integrated ADF-STEM intensities versus strain amplitude over LAADF and the HAADF detectors, respectively. [5]

Note that, as shown above, the dechanneling contrast in LAADF images is induced by strain field in crystals, which originates from impurities, interface effect, mechanical bending, etc.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[1] R. R. Vanfleet, M. Robertson, M. McKay, and J. Silcox, Characterization and Metrology for ULSI Technology (American Institute of Physics, New York, 1998), p. 901.
[2] D. A. Muller, T. Sorsch, S. Moccio, F. H. Baumann, K. Evans-Lutterodt, and G. Timp, Nature, (London) 399, 758 (1999).
[3] P. E. Batson, IBM J. Res. Dev. 44, 477 (2000).
[4] G. Duscher, S. J. Pennycook, N. D. Browning, R. Rupangudi, C. Takoudis, H.-J. Gao, and R. Singh, Characterization and Metrology for ULSI Technology (American Institute of Physics, New York, 1998), p. 191.
[5] Zhiheng Yu, David A. Muller, and John Silcox, Study of strain fields at a-Si/c-Si interface, J. Appl. Phys. 95, 3362 (2004).
[6] S. Hillyard and J. Silcox, Ultramicroscopy 58, 6 (1995).
[7] S. Hillyard and J. Silcox, Ultramicroscopy 52, 325 (1993).
[8] D. Hugo, H. Kohl, and H. Rose, Ultramicroscopy 17, 303 (1985).
[9] R. F. Loane, E. J. Kirkland, and J. Silcox, Acta Crystallogr., Sect. A: Found. Crystallogr. 44, 912 (1988).
[10] S. Hillyard, R. F. Loane, and J. Silcox, Ultramicroscopy 49, 14 (1993).

 

 

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