Scanning Electron Microscopy and X-Ray Microanalysis

451 25

4. T, the sample chamber temperature (K): The scattering
   skirt is reduced by operating at the lowest possible tem-
   perature.


5. L, the gas path length (m). The shorter the gas path
   length, the smaller the gas scattering skirt, as shown in
   . Fig. 25.4, where the skirt is compared for gas path
   lengths of 3 mm, 5 mm, and 10 mm. Note that the gas
   path length appears in Eq. (25.1) with a 3/2 power, so
   that the skirt radius is more sensitive to this parameter
   than the other parameters in Eq. (25.1). A modification
   to the vacuum system of the VPSEM that minimizes the
   gas path length consists of using a small diameter tube to
   extend the high vacuum of the electron column into the
   sample chamber.

25.2.1 Workarounds To Solve Practical

Problems

Gas scattering effects can be minimized but not avoided,
and for many VPSEM measurements and in situ experi-
ments, the microscopist/microanalyst may be significantly
constrained in the extent to which any of the parameters in
Eq. (25.1) can actually be changed to reduce the gas scatter-
ing skirt without losing the advantages of VPSEM opera-
tion. The measured EDS spectrum is always compromised,
but by carefully choosing the problems to study, successful
X-ray analysis can still be performed. A useful way to con-
sider the impact of gas scattering is the general concentra-
tion level at which the remote scattering corrupts the
measured spectrum:

Major constituent: concentration C > 0.1 mass fraction

Minor constituent: 0.01 ≤ C ≤ 0.1

Trace constituent: C < 0.01

Depending on the exact nature of the specimen, gas scat-

tering will almost always introduce spectral artifacts at the

trace and minor constituent levels, and in severe cases arti-

facts will appear at the level of apparent major constitu-

ents.

25.2.2 Favorable Sample Characteristics

Given that the LVSEM operating conditions have been

selected to minimize the gas scattering skirt, what specimen

types are most likely to yield useful microanalysis results? If

most of the gas scattering skirt falls on background material

that contains an element or elements that are different from

the elements of the target and of no interest, then by follow-

ing a measurement protocol to identify the extraneous ele-

ments, the measured spectrum can still have value for

identifying the elements within the target area, always recog-

nizing that the target is being excited by the focused beam

and the skirt.

Particle Analysis

Particle samples comprise a broad class of problems related to

the environment, technology, forensics, failure analysis, and

other areas. Particles are very often insulating in nature so that

a conductive coating is required for examination in the con-

ventional high vacuum SEM, and the complex morphologies

VPSEM 100 Pa 10 mm GPL

1.0

0.9

0.8 He

O 2

0.7

0.6

Cumulative elec

tron intensit

y

0.5

0.4

01020

Radial distance from beam center (micrometers)

30 40 50

. Fig. 25.12 Comparison of the
scattering skirt for He and O 2

25.2 · What Can Be Done To Minimize gas Scattering in VPSEM?