91 5

The corresponding theoretical S/N ratio is estimated from

the number n of BSE produced, which depends on the inci-

dent beam current IB, the BSE coefficient η, and the dwell

time per pixel τ:

nI= 62. (^4) Bητ (5.19)
where the coefficient 6.24 is appropriate for beam current
expressed in pA and the dwell time expressed in μs.
Because the image pixels are independent and uncorre-
lated, if a mean number n of BSE is produced at each pixel the
expected variance is n1/2:
SN//nn//nI.
/
()theory==^1212 =()B
12
624 ητ
(5.20)
For IB = 4000 pA, ηMo = 0.38, and τ = 64  μs
()SN/.theory=()^624 IBητ^12 / =779 1.
(5.21)
The DQE for this particular detector is thus
DQE=()experimental ()theoretictal

SN//SN/

./ ..

22

297 3 779 10^221146

(5.22)

A similar study for an Everhart–Thornley SE-BSE detector

on an electron probe X-ray microanalyzer is shown in

. Fig. 5.30, for which the DQE is calculated as 0.0016.
. Table 5.1 lists values of the DQE for various detectors,
demonstrating that a large range in values is encountered,
even among detectors of a specific class, for example, the E–T
detector.

References

Everhart T, Thornley R (1960) Wide-band detector for micro-microam-

pere low-energy electron currents. J Sci Instrum 37:246

Fiori C, Yakowitz H, Newbury D (1974) Some techniques of signal pro-

cessing in scanning electron microscopy. In: Johari O (ed) SEM/1974.

IIT Research Institute, Chicago, p 167

Jones R (1959) Phenomenological description of the response and

detecting ability of radiation detectors. Adv Electr Electron Opt

11:88

Joy DC, Joy CS, Bunn RD (1996) Measuring the performance of scanning

electron microscope detectors. Scanning 18:533

Newbury DE (1976) “The utility of specimen current imaging in the

scanning electron microscope” SEM/1976/I. IIT Research Inst, Chi-

cago, p 111

Robinson V (1975) “Backscattered electron imaging” SEM/1975, I. IIT

Research Inst, Chicago, p 51

Wells OC (1957) The construction of a scanning electron microscope

and its application to the study of fibres. Ph. D. Diss., Cambridge

University, Cambridge

Everhart-Thornley detector on electron probe microanalyzer

Y-Intercept = 28.6

300

250

200

150

100

50

0

024 6 81012

Beam current (nA)

Gray level

. Fig. 5.30 Average gray level versus beam current for an Everhart–
Thornley detector on an electron probe microanalyzer. Specimen: Si;
E 0 = 10 keV
. Table 5.1 DQE of electron detectors from different
manufacturers (Joy et al. 1996)

SE detector DQE

Everhart–Thornley 0.56

Everhart–Thornley 0.17

Everhart–Thornley 0.12

Everhart–Thornley 0.017

Everhart–Thornley 0.0008

High performance SEM:

Everhart–Thornley (lower) 0.18

Everhart–Thornley (TTL) 0.76

Microchannel plate 0.029

BSE detector

Scintillator BSE 0.043

Scintillator BSE 0.005

E–T BSE mode (negative bias) 0.001

E–T BSE mode (negative bias) 0.004

Microchannel plate BSE 0.058

Microchannel plate BSE 0.026

References