Scanning Electron Microscopy and X-Ray Microanalysis

360

22

22.1 What Constitutes “Low” Beam Energy

X-Ray Microanalysis?

The incident beam energy, E 0 , is the parameter that deter-

mines which characteristic X-rays can be excited: the beam

energy must exceed the critical excitation energy, Ec, for an

atomic shell to initiate ionization and subsequent emission of

characteristic X-rays. This dependence is parameterized with

the “overvoltage” U0, defined as

U= 00 E/Ec (22.1)

U 0 must exceed unity for X-ray emission. The intensity, Ich, of

characteristic X-ray generation follows an exponential

relation:

I=iaU

n

ch B0()−^1 (22.2)

where iB is the beam current, a and n are constants, with

1.5 ≤ n ≤ 2.

The intensity of the X-ray continuum (bremsstrahlung),

Icm, also depends on the incident beam energy:

I=cm ibB0ZU()− 1

(22.3)

where b is a constant and Z is the mass-concentration-

averaged atomic number of the specimen.

The peak-to-background is then obtained as the ratio of

Eqs. (22.2) and (22.3):

P/BI/I /Z U

n

=−

−

ch cm 0

1

≈() (^11) ()
(22.4)
The value of n −1 in Eq. (22.4) ranges from 0.5 to 1, so that
the P/B rises slowly as U 0 increases above unity.. Figure 22.1
shows experimental measurements of Si K-L 2 + Si K-M 3
(Si Kα,β) characteristic X-ray intensity as a function of over-
voltage. Near the threshold of U 0 = 1, the intensity drops
sharply, and the Si K-L 2 + Si K-M 3 peak becomes progres-
sively lower relative to the X-ray continuum background, as
shown in. Fig. 22.2 for Si measured over a range of beam
energies. The peak-to-background strongly influences the
limit- of- detection. While X-ray measurements can certainly
be made with 1 < U 0 < 1.25 and the limit-of-detection can be
improved by increasing the integrated spectrum intensity by
extending the counting time, the detectability within a prac-
tical measuring time of a constituent excited in this overvolt-
age range diminishes. While major constituents may be
detected, minor and trace constituents are likely to be below
the limit of detection. Thus the situation for 1 < U 0 < 1.25
must generally be considered “marginally detectable” and is
so marked in. Figs. 22.3, 22.4, 22.5, 22.6, 22.7, and 22.8.
Excitation of Si K-shell X-rays
0246
Overvoltage
81012
Si K X-
rays/e

1e-2

1e-3

1e-4

1e-5

1e-6

1e-7

. Fig. 22.1 Production of silicon
K-shell X-rays with overvoltage

Chapter 22 · Low Beam Energy X-Ray Microanalysis