Transcript Slide 1

e- beam
Intrinsic region is also here after
Li drifting during manufacture
2D
sample
Cross sectional diagrams of silicon [Si(Li)]
x-ray detector used in x-ray eds. These
diagrams show two slightly different views.
The intrinsic region of each detector is Li
doped and is the region that counts x-rays
by converting their energy into electron
current.
Normal P (boron doped) or N (phosphorus
doped) type silicon is not a good x-ray
detector because of excessive e- and hole
current even in the absence of an x-ray
event.
e- beam
3D
X-rays
The next 2 slides address spectral artifacts and how the
EDS detector disperses the xray energies for plotting.
sample
You will not be tested on this page but it may help you with other concepts.
The Si detector and the escape peak:
When an x-ray enters detector, it makes a charge pulse that is monitored. This pulse
has size (total # of e- counts in detector) based on the formula:
# of e- counts = Energyx-ray in eV / ε’
(ε’ = 3.8 eV for Si, this is the energy
needed to create an e-/hole pair in our
detector, notice how low this value is, it
must be a bonding (molecular) e-, not
an inner shell e- that is involved)
Usually, this energy is entirely captured by multiple electronic/kinetic events inside the detector. If
it is not, say an inner shell Si electron is displaced and a Si K x-ray (1.74 keV) is generated and
escapes, now the pulse count does not accurately represent the energy of the incoming x-ray as
some energy has escaped. This event causes us to see the ‘escape peak’ which is seen on our
spectrum at 1.74 keV below the actual incident x-ray energy peak of the sample.
8.04keV (Cu K) – 1.74keV (Si K) = 6.3keV (Cu K escape peak)
Top and bottom
spectra are the
same data, just
different scaling.
The electron
beam was
focused on the
sem specimen
stage which is
made of a brass
alloy; mostly
copper and zinc.
main Cu K peak
(Kα) at 8.04 keV
main Cu K peak
(Kα) at 8.04 keV
?
Cu Kα escape
peak
Cu Kα escape
peak
Cu Kα sum peak
Low beam current (30%
dead time on detector)
High beam current (60%
dead time)
Although these
spectra show
system peaks by
definition (these
are x-rays coming
from part of our
microscope
chamber or the
stage) we do not
call them artifacts
because we were
intentionally
focused and
collecting signal
from our sem
stage. In this
case, our system
is our sample!
Why does too much beam current lead to this copper ‘sum peak’ (far right arrow)? What is the apparent benefit
of the higher beam current setting? Can you find the Cu escape peak? See lower right spectrum and look
down energy from the very large Cu peak.