Scanning Electron Microscopy and X-Ray Microanalysis

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23

Particle Sample Preparation: Minimizing

Substrate Contributions With a Thin Foil

Substrate

As the particle dimensions decrease below 1 μm, electron

penetration through the particle increases dramatically, rais-

ing the substrate contribution to the X-ray spectrum. Even if

the characteristic peak(s) of the substrate material is not of

interest and can be ignored, the deadtime due to the substrate

spectrum will eventually dominate the overall spectrum

measurement, even if the substrate consists of carbon which

is relatively poorly excited. Moreover, the contribution of the

substrate to the composite spectrum occurs not only from

the characteristic peak(s), for example, C K, but also from the

continuum background (bremsstrahlung) which affects all

photon energies. Increased background from the substrate

has the effect of lowering the peak-to-background for all

characteristic peaks from the elements of the particle, which

degrades all aspects of quantitative analysis but especially

impacts the limit of detection, raising the minimum concen-

tration that can be reliably measured.

The substrate contribution to the composite spectrum can

be minimized by reducing the mass of the substrate. Fine par-

ticles, especially those with nanometer dimensions, can be dis-

persed by various methods, including air-jetting or deposition

from fluid drops, onto thin carbon films, typically 20  nm in

thickness, which are supported on a grid (copper, nickel, car-

bon, etc.), as shown in the sequence of images in. Fig. 23.22.

To further stabilize the particle deposit, it is typical practice to

apply a thin (<10 nm) carbon coating to provide conductivity

and also to provide some mechanical constraint. The contri-

bution to the spectrum from the thin carbon support film plus

the final coating is much reduced compared to the situation on

a bulk or thick tape carbon substrate, as shown in the com-

parison of spectra from K411 particles of similar size (~1 μm

in diameter) shown in. Fig. 23.21. Beam electrons that are

scattered laterally from particles can excite the material of the

grid, as shown by the presence of copper in the K411 particle

spectrum of. Fig. 23.21, but if this system radiation is prob-

lematic, this unwanted spectral contribution can be controlled

by choosing alternative grid materials, such as carbon.

0.0 1.0 2.0

K411_sphere-on-tape_20kV10nA

K411_sphere-on-film_20kV10nA

K411_sphere-on-tape_20kV10nA

K411_sphere-on-film_20kV10nA

3.0 4.0

Photon energy (keV)

K411

E 0 = 20 keV

C – tape substrate

C film (20 nm) on Cu grid

Co

unts

Counts

Photon energy (keV)

5.0 6.0 7.0 8.0 9.0 10.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

. Fig. 23.21 Comparison of EDS spectra similar sized K411 spheres mounted on a bulk carbon substrate (red) and on a 20-nm-thick carbon film
on a copper support grid blue). Note artifact Cu peaks arising from lateral electron scattering from particle to excite the support grid

Chapter 23 · Analysis of Specimens with Special Geometry: Irregular Bulk Objects and Particles