Scanning Electron Microscopy and X-Ray Microanalysis

180

12

current. To capture 0.95 of the total beam current requires

a radial distance to approximately 190 μm, as shown in

. Fig. 12.7b, and the last 0.05 of the beam electrons are
distributed out to 1000 μm (1 mm). The strong effect of the

gas path length on the skirt radius, which follows a 3/2

exponent in the scattering Eq. 12.1, can be seen in

. Fig. 12.7 by comparing the plots for 3-, 5-, and 10-mm
gas path lengths.

1.0

0.9

0.8

0.7

0.6

0.5

0.4

01020

Radial distance from beam center (micrometers)

VPSEM 100 Pa O 2

Cumulative electron intensity

3 mm GPL

5 mm GPL

10 mm GPL

30 40 50

0

0.4

0.5

0.6

Cumulative electron intensity

0.7

0.8

0.9

1.0

3 mm GPL

5 mm GPL

10 mm GPL

200

Radial distance from beam center (micrometers)

VPSEM 100 Pa O 2

400 600 800 1000

a

b

. Fig. 12.7 a Cumulative
electron intensity as a function of
distance (0–50 μm) from the
beam center for 20-keV electrons
passing through 100 Pa of oxygen
as calculated with NIST DTSA-II. b
Cumulative electron intensity as a
function of distance (0–1000 μm)
from the beam center for 20-keV
electrons passing through 100 Pa
of oxygen as calculated with NIST
DTSA-II (GPL = Gas Path Length)

Chapter 12 · Variable Pressure Scanning Electron Microscopy (VPSEM)