Amorphous Layer Formed during EM Sample Preparation using FIB
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One key to the success of dopant analysis in nanometer-scale is the specimen preparation to expose the region of interest, for instance, using focused ion beam (FIB) milling. However, FIB preparation can induce surface damage. Amorphization of a FIB-milled crystalline surface may occur because of sufficient atom displacement within the collision cascade, leading to the loss of long-range order when the density of point defects reaches a critical value. The thickness of the amorphous layer formed in FIB milling is theoretically consistent with the penetration of the ion beam calculated by Monte Carlo simulations [1]. In general, the thickness of the amorphization layer on the top of the specimen is thicker than the amorphization damage on the side-walls. Note that even with the double cross-section TEM method, the measurements of the FIB-amorphized layers may not be very accurate (a ±10% inaccuracy is very common) since the diffraction contrast from the crystalline phase in TEM is normally interfered by the amorphous layers.

As an example, Figure 4493a shows the typical thickness of amorphous layer on each surface of silicon (Si) TEM samples experimentally prepared by FIB technique. Especially note that the thickness of the Si amorphous layer induced by 2 kV ions remains 3 nm even though this beam energy is very low in general. Furthermore, if the two surfaces of the TEM samples are prepared by FIB, the total thickness of the amorphous layers is doubled. It was suggested [5] that the amorphous thickness is independent on the beam current over the range of 100-500 pA. Table 4493 lists comparison of thicknesses of the damaged layers caused by FIB Ga ion milling and conventional argon (Ar) milling. Ar milling can be used to remove the damage caused by FIB.

typical thickness of amorphous layer on each Si surface of TEM sample experimentally prepared by FIB technique

Figure 4493a. Typical thickness of the amorphous layer on each surface
of silicon TEM samples experimentally prepared by FIB technique.

Table 4493. Comparison of thicknesses of the damaged layers caused by FIB Ga ion milling and conventional Ar milling.

  30 kV FIB 5 kV FIB 1 kV Argon
Damage thickness
~23 nm
~6.5 nm
1.6 nm

On the other hand, if a high energy beam (e.g. 30 kV) is used to prepare the specimen, a near-surface layer containing defects extends to more than 100 nm in depth other than surface amorphization. Those extended defects can trap the dopants that are present in the semiconducting region. [2 - 4]

Figure 4493b schematically shows the amorphized layers caused by FIB specimen preparation and the effects on TEM observation. This specimen damage will degrade the TEM contrast that is related to crystalline materials. For instance, if a Ga ion beam at 30 kV is used, the amorphized layer created by FIB beam will be about 23 nm into a Si film on each side of the TEM specimen. Therefore, the TEM specimen becomes entirely amorphous when its thickness is about 46 nm.

damaged layers caused by FIB specimen preparation (a), and the effects on TEM observation (b)

Figure 4493b. Schematic illustrations of the damaged layers caused by FIB specimen preparation (a), and the effects on TEM observation (b). The unaffected layer is in blue, while the amorphized layers on both sides are in red.

Gao et al. [6] had applied wedge FIB milling method in combination with double cross-section technique to study silicon(Si-) crystal-amorphization induced by FIB milling. Figure 4493c shows a fully amorphized Si portion of 355 nm.

TEM image of a wedge-shaped sample prepared by wedge FIB milling method in combination with double cross-section technique

Figure 4493c. TEM image of a wedge-shaped sample prepared by wedge FIB milling method in combination with double cross-section technique. [6]

 

 

 

 

 

 

 

 

[1] Evaluation of Top, Angle, and Side Cleaned FIB Samples for TEM Analysis, Eduardo Montoya, Sara Bals, Marta D. Rossell, Dominique Schryvers, and Gustaaf Van Tendeloo, Microscopy Research and Technique 70:1060–1071 (2007).
[2] A. C. Twitchett, R. E. Dunin-Borkowski, R. J. Hallifax, R. F. Broom, and P. A. Midgley, Phys. Rev. Lett. 88, 238302 (2002).
[3] D. Cooper, A. C. Twitchett, P. K. Somodi, I. Farrer, D. A. Ritchie, P. A. Midgley, and R. E. Dunin-Borkowski, Appl. Phys. Lett. 88, 063510
(2006).
[4] D. Cooper, C. Ailliot, R. Truche, J. Hartmann, J. Barnes, and F. Bertin, J. Appl. Phys. 104, 064513 (2008).
[5] N. I. Kato, Y. Kohno, and H. Saka, Side-wall damage in a transmission electron microscopy specimen of crystalline Si prepared by focused ion beam etching, J. Vac. Sci. Technol. A 17(4), 1201, (1999).
[6] Qiang Gao, Mark Chang, Chong Niao, Ming Li, W. T. Kary Chien, Experiment Study on Crystal/Amorphous Structure of TEM Sample Prepared by FIB Milling, Proceedings of the 32nd International Symposium for Testing and Failure Analysis, November 12-16, 2006, Renaissance Austin Hotel, Austin, Texas, USA.

 

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