Scanning Electron Microscopy and X-Ray Microanalysis

IX

A negative consequence of using a small aper-

ture to reduce the convergence/divergence angle

is a reduction in beam current.

Vendor software supports collection, dynamic
processing, and interpretation of SEM images,
including extensive spatial measurements. Open
source software such as ImageJ-Fiji, which is
highlighted in this textbook, further extends these
digital image processing capabilities and provides
the user access to a large microscopy community
effort that supports advanced image processing.
General specimen property information that
can be obtained from SEM images:

1. Compositional microstructure (e.g.,. Fig.  4 ).
   Compositional variations of 1 unit differ-
   ence in average atomic number (Z) can be
   observed generally with BSE detection, with
   even greater sensitivity (ΔZ = 0.1) for low
   (Z = 6) and intermediate (Z = 30) atomic num-
   bers. The lateral spatial resolution is generally

limited to approximately 10–100 nm depend-

ing on the specimen composition and the

beam energy selected.

2. Topography (shape) (e.g.,. Fig.  5 ). Topo-
   graphic structure can be imaged with varia-
   tions in local surface inclination as small as
   a few degrees. The edges of structures can
   be localized with a spatial resolution rang-
   ing from the incident beam diameter (which
   can be 1 nm or less, depending on the elec-
   tron source) up to 10 nm or greater, depend-
   ing on the material and the geometric nature
   of the edge (vertical, rounded, tapered, re-
   entrant, etc.).


3. Visualizing the third dimension (e.g.,. Fig.  6 ).
   Optimizing for a large depth-of-field permits
   visualizing the three-dimensional structure
   of a specimen. However, in conventional X-Y
   image presentation, the resulting image is a
   projection of the three dimensional informa-
   tion onto a two dimensional plane, suffering

0.5 nA 20 nA

BSE MAG: 1000 x HV: 20.0 kV WD: 11.0 mm BSE MAG: 1000 x HV: 20.0 kV WD: 11.0 mm

20 mm^20 mm

. Fig. 2 Effect of increasing beam current (at constant pixel dwell time) to improve visibility of low contrast features.
Al-Si eutectic alloy; E 0 = 20 keV; semiconductor BSE detector (sum mode): (left) 0.5 nA; (right) 20 nA; Bar = 20 μm

BSE MAG: 750 x HV: 20.0 kV WD: 11.0mm

4

3

1

2

10 mm

. Fig. 4 Atomic number contrast with backscattered
electrons. Raney nickel alloy, polished cross section;
E 0 = 20 keV; semiconductor BSE (sum mode) detector. Note
that four different phases corresponding to different com-
positions can be discerned; Bar = 10 μm

HV WD magdetmode HFW
11.7 mm NIST FEG ESEM
4 mm
20.00 kV8.0 mm12 711 xETDCustom

. Fig. 3 Large the depth-of-focus; Sn spheres;
E 0 = 20 keV; Everhart–Thornley(positive bias) detector;
Bar = 4 μm (Scott Wight, NIST)

Scanning Electron Microscopy and Associated Techniques: Overview