Transcript Slide 1

econdary
on
ass
pectrometry
Viraj Jayaweera
Department of Physics & Astronomy
Georgia state University
Outline
• Theory
• Strength
• Accuracy
• Limitations
• Application using GaN as example
SIMS Theory
A well established analytical technique that was first pioneered in 1949
Primary
ion beam
SIMS
is generally
(O-, O2+, Ar+, Cs+, Ga+ are
for energies
oftenused
used with
between
1 and 30
keV)
surface,
bulk,
microanalysis,
Primary ions are implanted
depth
and mix
withprofiling,
sample atoms
to depths
and of 1 to 10 nm.
impurity analysis.
http://atomika.com/
The
primarybombarding
ion beam produces
monatomic
and polyatomic
particles
The bombarding
technique involves
the surface
of a sample
with a beam
of
ofions,
sample
material
and
resputtered
primary
ions,
along
with
electrons
and
photons.
thus emitting secondary ions. These ions are later measured with a
The
secondary
particles
carry negative,
positive,
andorneutral
and they
mass
spectrometer
to determine
either the
elemental
isotopiccharges
composition
of
have
kinetic
energies
that
range
from
zero
to
several
hundred
eV.
the surface of the sample.
Schematic Diagram of a SIMS instrument
Cameca IMS 6f secondary ion mass spectrometer
Faraday Cup
Florescence
Screen
SIMS Theory
The detected secondary ion current for ith element
I  I p f i Ci Sii

i

i
I 
Ip 

fi 
Ci 
Si 
i 

Secondary ion current (ions/s)
Incident ion current (ions/s)
Fraction of the particles sputtered as ions
Concentration of the ith element in the sputtered volume
Sputtering yield of both ions and neutrals
(particles / incident ion)
Collection efficiency of the SIMS instrument
Mass Spectrometer
For an analogy, think of how a
prism refracts and scatters white
light separating it into a
spectrum of rainbow colors.
In a mass spectrometer, ions
travel different paths through the
magnet to the detector due to
their mass/charge ratios. A mass
analyzer
sorts
the
ions
according to mass/charge ratios
and the detector records the
abundance of each ratio.
Mass Spectrometer
Time-of-Flight Mass Analyzer
m 2Vt
 2
z
l
Typical flight times
10 ns to 800 µs
2
The Quadrupole Mass Analyzer
www.chm.bris.ac.uk/ms/theory/quad-massspec.html
The two opposite rods in the quadrupole have a potential of +(U+Vcos(wt)) and the other two
-(U+Vcos(wt)) where 'U' is the fixed potential and Vcos(wt) is the applied RF of amplitude 'V'
and frequency 'w'. This results in ions being able to traverse the field free region along the
central axis of the rods but with oscillations amongst the poles themselves. These oscillations
result in complex ion trajectories dependent on the m/z of the ions. Specific combinations of
the potentials 'U' and 'V' and frequency 'w' will result in specific ions being in resonance
creating a stable trajectory through the quadrupole to the detector. All other m/z values will be
non-resonant and will hit the quadrupoles and not be detected. The mass range and resolution
of the instrument is determined by the length and diameter of the rods.
SIMS Imaging
Depth Profiling
 The measurement of dopant and impurity concentrations with depth in
compound semiconductor is often accomplished by SIMS.
 Monitoring the secondary ion count rate of selected elements as a function of
time leads to depth profiles.
The raw data for a measurement of
phosphorous in a silicon matrix.
The sample was prepared by ion
implantation of phosphorous into a
silicon wafer. The analysis uses
Cs+ primary ions and negative
secondary ions.
Depth Profiling
To convert the time axis into depth, the SIMS analyst uses a profilometer to measure
the sputter crater depth. Total crater depth divided by total sputter time provides the
average sputter rate.
Relative sensitivity factors convert the vertical axis from ion counts into
concentration.
Previous phosphorous depth
profile plotted
on depth and
concentration axes.
Depth Profiling
Depth Resolution
Depth resolution depends on flat bottom craters.
 Modern instruments provide uniform sputter currents by sweeping a finely
focused primary beam in a raster pattern over a square area.
 In some instruments, apertures select secondary ions from the crater
bottoms, but not the edges.
 Alternatively, the data processing system ignores all secondary ions
produced when the primary sputter beam is at the ends of its raster pattern.
Sensitivity and Detection Limits
The SIMS detection limits for most trace elements are between
1012 and 1016 atoms/cm3.
In addition to ionization efficiencies (RSF's), two other factors can limit sensitivity.
The dark current (or dark counts) arises from
stray ions, electrons in vacuum systems, and from cosmic rays
Count rate limited sensitivity occurs when sputtering produces less secondary ion
signal than the detector dark current.
If the SIMS instrument introduces the analyte element, then the introduced level
constitutes background limited sensitivity. Oxygen, present as residual gas in
vacuum systems, is an example of an element with background limited sensitivity.
Analyte atoms sputtered from mass spectrometer parts back onto the sample by
secondary ions constitute another source of background.
Advantages and Weaknesses
Advantages
Weaknesses
High sensitivity, especially for light elements
Destructive method
High surface sensitivity, important for depth
profiling
High selectivity, depending on the element
Information about the chemical surface
composition due to ion molecules
Secondary ion yield for an element varies
with the surrounding elemental composition
(matrix dependence)
Can detect all elements and isotopes,
including H
Interference of molecules and isotopes in the
mass spectrum
Quantitative analysis quiet complicated
SIMS is best fitted to:
Surface analysis
Micro analysis
Doping profiles (i.e. semiconductors)
Analysis of the isotopes (i.e. meteorite, isotopic labeling)
SIMS Application: using GaN as an Example
Detection Limits of Selected Elements in GaN
Source:
SIMS Application: using GaN as an Example
Detection Limits of Selected Elements in GaN
Chu, Gao, and Erickson: Characterization of III nitride materials
J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998
Al Depth Profiles at the Interface of an AlGaN/GaN
Al depth profiles at the interface of an AlGaN/GaN sample. One profile
was acquired on a sample with a high density of visible surface pits,
whereas the other curve was obtained on a sample with few if any visible
Chu, Gao, and Erickson, J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998
Si and Mg Doping Profile of GaN
SIMS depth profile of a p-n
homojunction in GaN using
Mg and Si as p- and n-type
dopants. Common dopants in
GaN such as O and C and
some transition metals such
as Fe, Mo, Cr, and Ni are
also measured.
Chu, Gao, and Erickson, J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998
GaN/InGaN/GaN LED Device
SIMS analysis of a finished LED chip after
de-encapsulation
Optical micrograph depicting the postSIMS measurement crater on the
lower left-hand side and the remnant
of the setup crater on the upper righthand side of the device.
Chu, Gao, and Erickson: Characterization of III nitride materials
J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998
Depth Profiles of GaN/InGaN/GaN LED Device
SIMS depth profiles of dopants compositional
profile for the GaN/InGaN/GaN LED device.
Chu, Gao, and Erickson, J. Vac. Sci. Technol. B, Vol. 16, No. 1, Jan/Feb 1998