Ion Beam Induced Structural Change/Damage in at Surfaces
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Energetic ions interacting with a surface can result in many microstructural or chemical modifications [1] such as grain growth, recrystallization, preferred crystallographic texture development, amorphization, and formation of additional stable or metastable phases [2 – 15]. There are many theories on how microstructure changes during FIB milling. One commonly-referenced theory is based on how specific orientations of crystals result in channeling (or minimal channeling) of incident energetic ions.

Furthermore, if a high energy Ga (gallium) beam (e.g. 30 kV) in FIB 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. [16 - 18]

Figure 4508 schematically shows the amorphized layers caused by FIB specimen preparation and the effects on TEM observation. 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 4508. 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.

To obtain high quality thin TEM-specimen, damage can be minimized in two ways:
         i) Uses low energy ion milling. However, the curtaining artifacts can be worse at low kV if it occurs.
         ii) Reduces the incidence angle to the surface.

It is very important to note that FIB-induced damage is well known to be material dependent.







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