Biomedical Engineering Reference
In-Depth Information
cused, energetic ion beam that sputters atoms, clusters or large molecules (up
to 10,000 amu) off the surface [51-54]. Most of these originate from the top
monolayer. The ionized sputtered secondary particles can be directly detected
with a time-of-flight mass spectrometer (TOF-MS).
Two types of ion source are particularly suited for TOF-SIMS. The first
one produces positive noble gas ions (usually argon or xenon) either by elec-
tron impact (EI) or in a plasma created by a discharge. The ions are then ex-
tracted from the source region, accelerated to the chosen energy and focused
in an electrostatic ion optical column. More recently it has been shown that
the use of primary polyatomic ions such as SF
5
, created in EI sources, could
enhance the molecular secondary ion yield by several magnitudes [38, 55].
The second type of ion gun produces positive ions from a liquid metal
(gallium, indium or gold) [56]. Because the ion production occurs in a very
small volume, gallium liquid metal ion sources have a very high brightness.
As a result, the ion beam may be focused to a fine spot, resulting in a spot
size of 0.2
m at 8-10 keV or about 20 nm at 30 keV, while being pulsed at
frequencies of up to 50 kHz and rastered at the same time.
All ion gun optical columns provide deflection plates for scanning the ion
beam over areas adjustable from many square millimeters to a few square
micrometers. They have been adapted for pulsing by the introduction of de-
flection plates, which rapidly sweep the beam across an aperture. Applying
an ion beam bunching technique, ion pulses of less than 1 ns width can be
produced.
In a TOF mass analyzer (Fig. 9.14), all sputtered ions are accelerated
with an extraction voltage of
U
0
to a given potential, so that all ions possess
the same kinetic energy. The ions are then allowed to drift through a field-
free drift path of a given length
L
before striking the detector. According to
the equation (
mL
µ
2
)=
qU
0
, light ions travel the fixed distance through
the flight tube more rapidly than identically charged heavy ions. Thus, the
measurement of the time,
t
, of ions with mass-to-charge ratio,
m/q
, provides a
simple means of mass analysis with
t
2
)
/
(2
t
2
=(
mL
2
)
/
(2
qU
0
)
∝ m/q
. Because a very
well defined start time is required for the flight time measurement, the primary
ion gun has to be operated in a pulsed mode in order to be able to deliver
discrete primary-ion packages [57]. Electric fields (e.g., ion mirrors [58, 59] or
electrical sectors [60, 61]) are used in the drift path in order to compensate
for different incident energies and angular distributions of the secondary ions.
For good mass resolution, the flight path must be su
ciently long (1-1.5 m),
and very sophisticated high frequency pulsing and counting systems must
be employed to time the flight of the ion to within a sub-nanosecond. One
great advantage of TOF-MS is its ability to provide simultaneous detection
of all masses of the same polarity. Charge compensation for insulator analysis
is possible using pulsed low-energy electrons, which are introduced during
the time interval between ion pulses. With such a TOF-SIMS instrument,
the useful mass range is extended beyond 10,000 amu; the mass resolution,
