Electron microscopy
  
Thickness of EM Samples
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Thickness uncertainties in TEM/STEM samples are:
         i) Hard to control TEM/STEM sample thickness to a specific value in sample preparations.
         ii) Curtaining effect in TEM sample preparation due to different materials in the structures.
         iii) There has been interest in determining the three-dimensional distribution of elements by using elemental mapping in tomography, in which elemental maps are acquired in a tilt series, typically through ±70° [1-2]. For specimens tilted to 70°, the specimen thickness in the beam direction effectively triples.

Table 1217a. Thickness dependence of electron scattering from a specimen in EM (electron microscope) measurements.
Scattering direction
EM method
Scattering event/angle
Scattering direction
Electrons
Wave (phase) property
Electron property
Contrast interpretation
Thin specimen TEM Less scattering events and then smaller scattering angle
 Forward scattering More electrons Strong coherent signal Strong wave property Easy

Back scattering Fewer electrons Weak incoherent signal Weak particle property  
Thick specimen TEM/SEM   Forward scattering Fewer electrons Weak coherent signal Weak wave property Middle
Electron is scattered more than once (plural scattering), and then scattering angle is greater Back scattering More electrons Strong incoherent signal Strong particle property  
Very thick or bulk specimen SEM More scattering events and then greater scattering angle Forward scattering Fewest electrons Weakest coherent signal Weakest wave property Difficult
Electron is scattered >20 times (multiple scattering), and then scattering angle is greatest Back scattering Most electrons Strongest incoherent signal Strongest particle property  

 

Table 1217b. Optimized sample thickness for various TEM measurements.
Technique
Quality
Thickness range
Resolution
TEM
 Good ≤50 nm Tool resolution
Applicable 50 ~ 70 nm  
HRTEM
 Best 10 ~ 25 nm Tool resolution
Applicable 25 ~ 100 nm  
(ADF-)STEM
 Best 25 ~ 50 nm ≤ 0.3 nm
Applicable 50 ~ 90 nm ≤ 1.2 nm
TEM + DSE*
Applicable 200 ~ 600 nm  
CBED for strain analysis
 Applicable 200 ~ 500 nm  
Holography
 Applicable 150 ~ 300 nm  
EFTEM/EELS
 Best 25 ~ 50 nm (depending on V and Z)** 0.3 ~ 0.5 nm
Ok 50-100 nm  
EDS
 Applicable 50~100 nm 1 ~ 2 nm
* Dopant selective etching.
** V: Beam energy, Z: atomic number.

 

 

 

 

[1] Midgley PA, Weyland M. Ultramicroscopy 2003; 96:413.
[2] Leapman RD, Kocsis E, Zhang G, Talbot TL, Laquerriere P. Ultramicroscopy 2004;100:115.

 

 
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