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| What
is ToF-SIMS? |
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| The Secondary Ion Mass Spectrometry
(SIMS) is the mass spectrometry of ionized particles which
are emitted from the surface when energetic primary particles
bombard the surface. The mechanism of TOF-SIMS is shown
in the following illustration. The pulsed primary ions
with the energy of 1-25keV, typically liquid metal ions
such as Ga+, Cs+ and O-,
are used to bombard the sample surface, causing the secondary
elemental or cluster ions to emit from the surface. The
secondary ions are then electrostatically accelerated
into a field-free drift region with a nominal kinetic
energy of: |
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|
A
schematic illustration of ToF-SIMS |
| Where V0
is the accelerating voltage, m the mass of ion,
v the flight velocity of ion, e its charge, you can see from the above formula that the
ion with lower mass has higher flight velocity than one
with higher mass. Thus they will reach the secondary-ion
detector earlier. As a result, the mass separation is
obtained in the flight time t from the sample to the detector.
The flight time t is expressed by: |
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Where L is the effective
length of the mass spectrometer. A variety of mass ions
are recorded by the detector with the time sequence
to give the SIMS spectrum.
The early-stage spectrometer is operated
in a mode of dynamic SIMS (DSIMS). The DSIMS directs
a high flux of primary ion to the sample surface, eroding
the surface rapidly. Therefore, DSISM is not suitable
for ultra-thin surface analysis. A static SIMS (SSIMS)
was developed in 1970’s. PHI TRIFT III is a cutting-edge
SSIMS, which directs a primary ion beam with a very
low current density to the outmost surface (~5 Angstrom)
so as to main the surface integrity for a long period
in excess of the analysis time. Typically, the primary
ion dose in the SSIMS mode is <=1012 ions/cm2
for mass analysis and <=1013 ions/cm2
for imaging. Under such a condition, the lifetime of
the monolayer can keep over several hours to tens of
hours. SSIMS is the most sensitive technique for elemental
analysis, which allows it to detect the trace impurity.
SSIMS is used to analyze not only the elemental composition
but also the chemical structure of surface.
[Reference: Robert Braun, Evans PHI,
and operator’s guide, Physical Electronics] |
| Related
subtopics: |
What does
ToF-SIMS do?
How to prepare SIMS samples?
How to operate
ToF-SIMS? |
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Northwestern University, 2220 Campus Drive #2036, Evanston, IL 60208-3108
Phone: 847-467-2318, Fax: 847-467-6573
Email questions to: nuance@northwestern.edu. |
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