A BRIEF INTRODUCTION TO
SECONDARY ION MASS
What is SIMS?
WHAT IS SIMS?
SIMS = Secondary Ion Mass Spectrometry, also
known as ion microprobe and ion microscope
Used to analyze the composition of solid surfaces
and thin films
Material surface is first sputtered with a focused
primary ion beam and the ejected secondary ions
are then collected and analyzed
WHAT IS SIMS?
HISTORY OF SIMS
1910: British physicist
J.J. Thomson observed
release of positive ions
and neutral atoms from a
solid surface induced by
1940s: Improved vacuum
1949: First prototype
experiments on SIMS by
Herzog and Viehböck at 1949: Herzog and Viehböck
the University of Vienna,
HISTORY OF SIMS
Early 1960s: two SIMS
Castaing and Slodzian at
the University of Paris for
the PhD thesis of Slodzian
Herzog and Liebel at GCA
Corp., funded by NASA
for analyzing moon rocks
1963: Liebel and Herzog
HISTORY OF SIMS
K.Wittmack and C. Magee developed SIMS
instruments with quadruple mass analyzers
A. Benninghoven introduced method of static SIMS
1980s: ‘Time of Flight’ mass spectrometers were
developed by Benninghoven and cooperators.
Recent developments: focus is on novel primary
ion species like C60 or cluster ions of gold and
PRINCIPLE Typically 10-20keV
device consists of:
Primary ion gun
Primary ion column
High vacuum (<10-6
Ion detection unit
The basis of SIMS is the destructive removal of
material from the sample by sputtering and the
analysis of the ejected material by a mass analyzer.
A primary ion beam impinges on the sample and
atoms from the sample are sputtered or ejected
from the sample.
Most of the ejected atoms are neutral and cannot
be detected by conventional SIMS, but some are
positively or negatively charged. This fraction
was estimated as about 1% of the total.
The mass/charge ratio of the ions is analyzed,
detected as a mass spectrum, as a count, or
displayed on a fluorescent screen.
Sputtering is a process in which incident ions lose their energy
mainly by momentum transfer.
In the process they displace atoms within the sample. Sputtering
takes place when atoms near the surface receive sufficient energy
from the incident ion to be ejected form the sample. The escape
depth of the sputtered atoms is generally a few monolayer for the
primary energies of 10 to 20keV.
The primary ion loses its energy in the process and rest tens of nm
below the sample surface.
Sputtering yield is the average number of atoms
sputtered per incident primary ion. It depends on the
sample, its crystallographic orientation, and the
nature, energy and incidence angle of the primary
ions. The yield for SIMS measurements with Cs+, O2+,
O- and Ar+ ions of 1 to 20keV energy ranges from 1 to
What is important is not the total yield, but the yield
of ionized ejected atoms or the secondary ion yield.
The secondary ion yield is significantly lower than
total yield, but can be influenced by primary ion.
For example: Electronegative oxygen O2+ enhances
species for electropositive elements (e.g., B and Al in
Si) which produce predominantly positive secondary
ions. Electropositive ions like cesium (Cs+) enhances
species for electronegative elements )e.g., P, As and
Sb in Si)
STATIC AND DYNAMIC SIMS
SIMS can give three types of results: (1) mass
spectrum (2) depth profile (3) imaging
Static SIMS offers mass spectrum
Dynamic SIMS offers depth profile and 2-D
Main difference between static and dynamic
SIMS is the incident ion beam current or
sputtering rate. Static SIMS has much lower
incident current and much slower sputtering
rate compared to dynamic SIMS. Various SIMS output signal
Static SIMS measure a spectrum of peaks,
while dynamic SIMS focus on one peak.
For low incident ion beam current or low
sputtering rate (~0.1nm per hour), a complete
mass spectrum can be recorded for surface
analysis of the outer 0.5nm or so. This mode of
operation is known as static SIMS.
Example of a SIMS spectrum SIMS spectrum of Mineral Arsenopyrite
In dynamic SIMS, the intensity of one peak for on
particular mass is recorded as a function of time
as the sample is sputtered at a higher sputter
rate (~10µm per hour), yielding a depth profile.
It is also possible to display the intensity of one
peak as a 2-D or 3-D image.
Depth profile example:
Negative Ion SIMS Depth Profile of Ni Layer Imaging example:
on Cu Substrate a Tinplated Steel Sample Used for Food Canning
DEPTH PROFILE APPLICATION
Show impurities present in a particular layer,
their concentrations, and their depth
Measure the thickness of a film.
Monitor a diffusion profile across an interface.
Show whether segregation occurs at interface.
Detect concentration levels <1e17 atoms/cm3
while providing depth resolution of <5 nm.
Depending on the SIMS type, there are three basic
analyzers available: sector, quadrupole, and time-of-
A sector field mass spectrometer uses a combination
of an electrostatic analyzer and a magnetic analyzer
to separate the secondary ions by their mass to charge
A quadrupole mass analyzer separates the masses by
resonant electric fields, which allow only the selected
masses to pass through.
The time of flight mass analyzer separates the ions in
a field-free drift path according to their kinetic
energy. It requires pulsed secondary ion generation
using either a pulsed primary ion gun or a pulsed
secondary ion extraction.
Quadruple SIMS uses a quadruple mass analyzer
instead of electrostatic-magnetic sector analyzers.
A quadruple mass analyzer consists of four parallel
rods with an oscillating electric field through which
the ions pass.
Pros: more robust, less expensive
Cons: lower resolution, cannot distinguish between
ions with close mass/charge ratios
Quadruple SIMS is suitable for analyzing insulating
TIME-OF-FLIGHT SIMS (TOF-SIMS)
Time-of-flight SIMS is a method of using Time-
of-flight mass analyzer.
A major advantage of TOF-SIMS is the absence
of narrow slits in the spectrometer increasing the
ion collection by 10-50%. This reduces the
sputtering rate greatly, allowing characterization
of organic surface layers.
TOF-SIMS is the standard method for static
Pros: highly sensitive
A major source of the limited sensitivity of SIMS
is the fact that most of the sputtered material is
neutral and cannot be detected.
Secondary neutral mass spectrometry (SNMS)
provides significant sensitivity enhancements,
where neutral atoms are ionized by a laser or by
an electron gas and then detected.
Laser microprobe mass spectrometry (LAMMA)
replaces the primary ion beam in SIMS with a
pulsed laser, providing high sensitivity, high
speed of operation, and is applicable to inorganic
as well as organic samples.
Semiconductor (primary) Displays
Compound Semiconductor Photonics
Data Storage Solar Photovoltaic
For semiconductor characterization:
Dopant and impurity depth profiling
Composition and impurity measurements of thin films (metal,
dielectric, SiGe, III-V, and II-V)
Ultra-high depth resolution profiling of shallow implants and ultra
thin films (ULE implants and gate oxides)
Bulk analysis, including B, C, O, and N in Si
High-precision matching of process tools (ion implanters)
SOME NOTES TO CONSIDER
Reference standards are required beforehand
because secondary ion yields are substrate
SIMS is a destructive technique.
A flat surface is required to obtain the best
lateral and depth resolution.
Samples must be <25 mm in diameter and
preferably <5 mm thick.
COMPANIES DOING SIMS
EXAMPLE: ION-IMPLANTATION PROFILE
The primary ion beam was Cs+ with an energy range
between 0.5 and 3keV, depending on the depth, with an
angle of 60o. The secondary ions were P-, As- or Si-. The
base vacuum was less than 10-8 Pa, and the raster scan
area was 120µm×120µm.
OTHER DEPTH PROFILE EXAMPLES
Ultra Shallow Junctions
Ultra thin Oxynitride gate dielectrics
EXAMPLE: TOF-SIMS IMAGING OF VIAS
Ti ion image Si ion image Overlay
EXAMPLE: SIMS MASS SPECTRUM
Residue ion image
The technique of SIMS, its history, basic
principles and its state-of-the art variants are
The various applications of SIMS are introduced.
SIMS as a surface analysis, depth profiling and
imaging technique is inevitable in the
semiconductor industry, especially in the micro
and nano regime.
Semiconductor material and device
characterization – 3rd Edition (K. Schroder)
THANK YOU FOR YOUR ATTENTION
PHI TRIFT III Time of Flight SIMS