Scanning Electron Microscopy and X-Ray Microanalysis

384

23

23.2 What Degree of Surface Finish Is

Required for Electron-Excited X-ray

Microanalysis To Minimize Geometric

Effects?

Early in the history of microanalysis by electron-excited X-ray

spectrometry, it was recognized that controlling the surface

condition of a specimen was critical to achieving high-accu-

racy results by reducing geometric effects to a negligible level.

Yakowitz and Heinrich ( 1968 ) performed a series of experi-

ments in which the metallographic preparation sequence of

grinding and polishing was interrupted at various stages.

Materials examined included pure elements with two widely

different characteristic peak energies, for example, Au M 5 -

N6,7 at 2.123 keV and Au L 3 -M4,5 at 9.711 keV, and homoge-

neous binary metal alloys with widely differing characteristic

X-ray energies. For each surface condition, the characteristic

X-ray intensity was then measured at random locations and

along line traverses on the specimen surface to examine the

variation in characteristic X-ray intensity that could be

ascribed to surface roughness. Results for selected surface

conditions for a gold target are listed in. Table 23.1. For the

Au L 3 -M4,5 measurements, a final polish of the surface with

0.5-μm alumina was necessary to reduce the coefficient of

variation for 20 random measurements to a level similar to

the expected variation from the random counting statistics,

expressed as 3 n1/2/n. For the lower photon energy Au M 5 -N6,7

which suffers stronger absorption, it was necessary to improve

the surface polish to 0.1 μm alumina to achieve similar results.

For even lower photon energy peaks, such as those asso-

ciated with low atomic number elements with Z ≤ 9 (fluo-

rine) for which E < 1 keV, even better surface finish is required

to control the geometric effects. Newbury and Ritchie (2013a)

simulated X-ray emission from crenelated surfaces with the

Monte Carlo simulation embedded in NIST DTSA-II to

examine the influence of surface topography on low photon

energy peaks. As shown in. Fig. 23.5 for FeO at an incident

beam energy E 0 = 10  keV, the depth of scratches had to be

reduced below 50 nm to reduce the geometric effects on O K,

Fe L 3 -M4,5, and Fe K-L2,3 to a negligible level.

23.2.1 No Chemical Etching

In addition to achieving a high degree of surface finish to

minimize geometric effects, it is also important to avoid

chemical or electrochemical etching of the final surface. For

effective optical microscopy of microstructures, chemical

FeO E 0 = 10 keV

1.0

0.8

0.6

k-ratio

0.4

0.2

0.0

0.01 0.1

Scratch depth and period (micrometers)

1

EDS

10

O K-L2,3 (0.523 keV)

Fe L (0.704 keV)

FeK-L2,3 (6.400 keV)

. Fig. 23.5 Plots of O K, Fe L 3 -
M4,5, and Fe K-L2,3 as a function
of scratch depth for a crenelated
surface as calculated with the
Monte Carlo simulation embed-
ded in NIST DTSA-II (Newbury
and Ritchie 2013a,b)
. Table 23.1 Characteristic X-ray intensity measured on gold after various stages of grinding and polishing (Yakowitz and Heinrich 1968 )

Surface condition AuMα (2.123 keV)

Coeff. variation,%

AuMα (2.123 keV)

3 n1/2/n,%

AuLα (9.711 keV)

Coeff. variation,%

AuLα (9.711 keV)

n1/2/n,%

600 grit SiC 8.6 0.39 1.8 0.93

0.5 μm Al 2 O 3 0.7 0.39 1.1 0.93

0.1 μm Al 2 O 3 0.46 0.39 0.42 0.93

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