Scanning Electron Microscopy and X-Ray Microanalysis

166

11

The incident beam energy is one of the most useful parame-

ters over which the microscopist has control because it deter-

mines the lateral and depth sampling of the specimen

properties by the critical imaging signals. The Kanaya–

Okayama electron range varies strongly with the incident

beam energy:

RAKO− ()nm = 27.6()/ZE0.89ρ 0 1.67

(11.1)

where A is the atomic weight (g/mol), Z is the atomic num-

ber, ρ is the density (g/cm^3 ), and E 0 (keV) is the incident

beam energy, which is shown graphically in. Fig. 11.1a–c.

11.1 What Constitutes “Low” Beam Energy

SEM Imaging?

The rapid but continuous decrease of the range with E 0 shown

in. Fig. 11.1a raises the question, Where does “low” beam

energy SEM imaging begin? That is, what value of E 0 consti-

tutes the upper bound of “low” beam energy microscopy? As

will be discussed below, useful SEM imaging can now be

accomplished down to remarkably low arrival energies at the

specimen surface, less than 100 eV. The upper bound for E 0 is

arbitrary, but a reasonable limit is the value discussed in the

“Low Beam Energy X-Ray Microanalysis” module, where it is

found that E 0 = 5 keV is the lowest beam energy for which a

useful characteristic X-ray peak can be excited for all ele-

ments of the periodic table, excepting H and He, which do

not produce characteristic X-rays. Thus, the plot of the range

for E 0 ≤ 5 keV shown in. Fig. 11.1b will be taken to define

the range for low beam energy SEM.

11.2 Secondary Electron and Backscattered

Electron Signal Characteristics

in the Low Beam Energy Range

The characteristics of the secondary electron (SE) and back-

scattered electron (BSE) signals observed in conventional

SEM imaging performed at high beam energy (E 0 ≥ 10 keV)

can be summarized as follows: (1) For most elements, η > δ.

0.04

Kanaya-Okayama range

0.03

0.02

0.01

0.00

0.0 0.2 0.4

Beam energy (keV)

0.6 0.8 1.0

Range (

μm)

Carbon

Aluminum

Titanium

Iron

Silver

Gold

Carbon

Aluminum

Titanium

Iron

Silver

Gold

Kanaya-Okayama range

10

8

6

Range (

μm)

4

2

0

0510 15 20 25 30

Beam energy (keV)

a Kanaya-Okayama range

c

b

Carbon

Aluminum

Titanium

Iron

Silver

Gold

0.5

0.4

0.3

0.2

0.1

0.0

0123

Beam energy (keV)

Range (

mm)

45

. Fig. 11.1 Plot of the Kanaya–Okayama range for various elements: a 0–30 keV; b 0–5 keV; c 0–1 keV

Chapter 11 · Low Beam Energy SEM