Scanning Electron Microscopy and X-Ray Microanalysis

126

8

The picture element dwell time, τ, can be replaced by the time

to scan a full frame, tF, from the relation

τ=

t

N

F

PE

(8.11)

where NPE is the number of pixels in the entire image.

Substituting Eq. (8.11) into Eq. (8.10),

i

N

B CtF

>

()×

()

()=

410 −^18

2

PE

,DQE

coulombsamperes

ηδ

/

(8.12)

For an image with 1024 × 1024 picture elements, Eq. (8.12)

can be stated as

i

A

B CtF

>

()×

()

410 −^12

ηδ,DQE^2

(8.13)

Equation (8.12) is referred to as the “Threshold Equation”

(Oatley et  al. 1965 ; Oatley 1972 ) because it defines the mini-

mum beam current, the “threshold current,” necessary to

observe a specified level of contrast, C, with a signal produc-

tion efficiency specified by η and/or δ and the detector

. Fig. 8.3 Synthesized digital images: a template; b object S = 5 counts above background, ΔS = 5 = N, S/B = 1.2; c object S = 10 counts above
background, ΔS = 10 = 2 N, S/B = 1.4; d object S = 25 counts above background, ΔS = 25 = 5 N, S/B = 2

Chapter 8 · The Visibility of Features in SEM Images