Spatial Resolution in Electron Microscopes
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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The spatial resolution in microscopes, in general, is defined as the smallest distance (R) between two volumes from which independent microanalysis can be obtained. In the typical case with a Gaussian-function electron probe, 50% of the total beam current is contained within a disk with a diameter of the FWHM of the Gaussian profile and 90% of the total current is contained within the full width at tenth maximum (FWTM). Both widths are used as definitions of the probe size and spatial resolution; however, the former is normally quoted by EM manufacturers. Note if the spatial resolution is defined by FWTH, the spatial resolution for a TEM probe can be more than 40x the FWHM.

Before creation and applications of aberration corrections, a method to improve the capability of electron microscopes (EMs) is to increase the accelerating voltage of the electron gun to ultrahigh voltages in order to penetrate more deeply into thicker samples as listed in Table 4628.

Table 4628. Ultrahigh voltage TEMs without aberration corrections.

Year
Spatial Resolution (Å)
Spatial Frequency (Å-1)
Model
Voltage
Location
Reference
1976
2.50
0.40
Horiuchi
1 MeV
[1]
1979
2.00
0.50
Cambridge
500 keV
[2]
1984
1.60
0.63
JEOL ARM-
1000
1 MeV
[3]
1985
1.70
0.59
4000EX
400 keV
[2]
1991
1.10
0.91
Hitachi H-
1500
1.3 MeV
[4]
1994
1.08
0.93
Ichinose
1250 keV
[5]
1996
1.05
0.95
Stuttgart
1.25MeV
[6]
1999
0.98
1.02
Ichinose
1.25MeV
[7]

The third- and fifth-order spherical aberration coefficients and defocus are present in a perfect, round electromagnetic lens, while all other higher-order coherent aberration coefficients in Table 3740 are caused by lens imperfections. Those higher-order aberration coefficients can become important at high spatial resolutions because of the increasing order of their spatial frequency dependence.

Given the constant brightness and illumination half-angle, to increase the probe current the beam size has to be increased, indicating the spatial resolution will be worse.

Investigating individual point defects, e.g. monovacancies, using TEM-related techniques was believed to be difficult because this requires both atomic sensitivity and atomic resolution and the specimens need to be very thin such that one can detect the individual point defects from the image contrast.

Astigmatism in condenser lens is important because it reduces the coherence of the electron beam, while astigmatism in objective lens is important because it induces a serious degradation of spatial resolution.

Note that, seriously speaking, we need to define spatial resolution as two different classes: i) Instrument resolution, and ii) Effective resolution for a specific specimen.

In conventional TEM system, the existence of spherical aberration requires the utility of very small apertures to maximize the spatial resolution, while the resolution will also be limited by diffraction (see Airy disc) if the apertures are too small.


 

 

 

[1] R.M. Fisher, T. Imura, Ultramicroscopy 3 (1978) 3.
[2] D.J. Smith, Rep. Prog. Phys. 60 (1997) 1513.
[3] K.H. Downing, Hu. Meisheng, H.-R. Wenk, M.A. O’Keefe, Nature, 348 (1990) 525.
[4] S. Horiuchi, Y. Matsui, Y. Kitami, M. Yokoyama, S. Suehara, X.J. Wu, I. Matsui, T. Katsuta, Ultramicroscopy 39 (1991) 231.
[5] H. Ichinose, International Workshop on HVEM & HREM, Stuttgart, 1994.
[6] F. Phillipp, Adv. Solid State Phys. 35 (1996) 257.
[7] H. Ichinose, H. Sawada, E. Takuma, M. Osaki, J. Electron Microsc. 48 (1999) 887.

 

 

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