AP 5301/8301Instrumental Methods of Analysis

and Laboratory

Lecture 8

Secondary ion mass spectrometry (SIMS)

Prof YU Kin Man

E-mail: kinmanyu@cityu.edu.hk

Tel: 3442-7813

Office: P6422

1

Lecture 9: outline Introduction: general features of secondary ion mass spectrometry

SIMS theory:─ Ion-solid interactions

─ Sputtering process

─ Ion yield

─ Quantification: relative sensitivity factor

Instrumentation:─ Ion sources

─ Mass spectrometer

─ Ion detector

─ Time-of-flight SIMS

Common modes of SIMS:─ Static SIMS

─ Dynamic SIMS

Depth profiling:─ Crater effects

─ Depth resolution

Strengths and weaknesses

2

The technique involves bombarding the surface of a sample with a beam of

ions, thus emitting secondary ions. These ions are later measured with a

mass spectrometer to determine either the elemental or isotopic composition of

the surface of the sample.

Secondary ion mass spectrometry (SIMS)

A well established analytical technique that was first pioneered in 1949

SIMS is generally used for

surface, bulk, microanalysis,

depth profiling, and

impurity analysis.

Primary ion beam

(O-, O2+, Ar+, Cs+, Ga+ are often

used with energies between 1

and 30 keV)

Primary ions are implanted and

mix with sample atoms to depths

of 1 to 10 nm.

The bombarding primary ion beam produces monatomic and polyatomic particles

of sample material and re-sputtered primary ions, along with electrons and

photons. The secondary particles carry negative, positive, and neutral charges

and they have kinetic energies that range from zero to several hundred eV.

http://atomika.com/

3

SIMS analysis4

Secondary ion mass spectrometry (SIMS) is a technique used to analyze the

composition of solid surfaces and thin films by sputtering the surface of the

specimen with a focused primary ion beam and collecting and analyzing

ejected secondary ions with a mass spectrometer to determine the elemental,

isotopic, or molecular composition of the surface to a depth of 1 to 2 nm.

SIMS analysis5

Cameca IMS 6f secondary ion mass spectrometer6

7

SIMS: comparison with other techniques

8

Ion-solid interaction

Cs+, O2+, Ar+ and Ga+

at energies ~ 1-30 keV

negative, positive, and neutral

charges with kinetic energies ranging

from zero to a few hundred eV.

Energy is transferred from the energetic primary ions to atoms in the sample.

Some of these atoms receive enough energy to escape the sample

Sputtered species:

Monatomic and

polyatomic particles of

sample material (+ve,

-ve or neutral)

Re-sputtered primary

species (+ve, -ve or

neutral)

Electrons

photons

Sputtering9

Sputtering is a process whereby particles are

ejected from a solid target material due to

bombardment of the target by energetic particles.

The kinetic energy of the incoming particles is

typically hundreds eV to keV, leading erosion of the

target materials.

Sputtering is commonly used as a

tool for thin film deposition

Eroding material from a target

source onto a substrate using a

gaseous plasma (Ar)

targets

substrate

For thin film analysis:

Mass analyze the sputtered ejected

ions─SIMS

To expose atoms underneath the

surface for analysis─depth profiling

The Sputtering Process10

Sputter rates in typical SIMS experiments vary between 0.5 and 5 nm/s.

Sputter rates depend on sputter yield, which in turn depends on the primary

beam species, energy, intensity, sample material, and crystal orientation.

Sputter yield: ratio of number of atoms sputtered

to number of impinging ions, typically 5-15

─ Commonly in SIMS, oxygen or cesium is used as a

primary ion source, which chemically changes the

surface and the sputter rate.

Sputter yields of silicon as a function of ion energy for

noble gas ions at normal incidence.

The variation of the sputter yield with angle

for the three metals. Below approximately 60

degrees, the sputter rate increases with

angle before passing through a maximum

Secondary ion yield11

Secondary ion current of species 𝑚

𝐼𝑠𝑚 = 𝐼𝑝𝑦𝑚𝛼+𝜃𝑚𝜂

𝐼𝑝 = primary particle flux

𝑦𝑚 = sputter yield

𝛼+ = ionization probability to positive ions

𝜃𝑚 = fractional concentration of m in the layer

𝜂 = transmission of the analysis systemIon yield is influenced by

─ Matrix effects

─ Surface coverage of reactive elements

─ Background pressure

─ Orientation of crystallographic axes with respect to the sample surface

─ Angle of emission of detected secondary ions

The number of secondary particles (atoms/ions) emitted by the surface for each

impinging primary ion is defined as sputtering yield and can range between 5

and 15. The fraction of ionized emitted particles is called secondary ion yield

and ranges typically between 10-4 to 10-6.

In SIMS, it is the secondary ions that are eventually detected by the mass

spectrometer

Secondary ion yields: primary ion beams

Oxygen works as a medium which strips off electrons from the speeding

sputtered atoms when they leave surface, while Cesium prefers to load an

electron on the sputtered atoms.

𝑶𝟐+ ions beam:

─ Oxygen tends to bind with metal (Me)

atoms, if present in the sample.

─ During secondary emission the Me-O

bonds break thus generating 𝑀𝑒𝑛+

𝑪𝒔+ ions beam:

─ The implanted Cs ions lower the

sample work function

─ More secondary electrons are excited

over the surface potential barrier

─ Increased availability of electrons

leads to increased negative ion

formation especially for elements with

high electron affinity.

12

Secondary ion yield depends critically on the primary ion beam species. Typically

𝐶𝑠+and 𝑂2+ ion beams are used in SIMS measurements.

Selection of primary ions:

Inert gas (Ar, Xe, etc.)

─ Minimize chemical modification

Oxygen

─ Enhance positive ions

Cesium

─ Enhance negative ions

Liquid metal (Ga)

─ Small spot for enhanced

lateral resolution

Secondary ion yields: primary ion beam Oxygen bombardment increases the yield of positive ions

Cesium bombardment increases the yield of negative ions.

The increases can range up to four orders of magnitude.

13

Relative secondary ion yield14

10 20 30 40 50 60 70 80 90 100

Atomic number

106

105

104

103

102

108

107

106

105

104

103

102

Rela

tive s

econdary

positiv

e ion y

ield

Rela

tive s

econdary

negative ion y

ield

0 10 20 30 40 50 60 70 80 90

Atomic number

16.5 keV Cs+ 13.5 keV O-

One of the main obstacles preventing the derivation of a universal theory of the

secondary ion emission is a fact that the secondary ion yield of any chemical

element strongly depends on its chemical environment─matrix effect.

This may cause variations in the ion yield over several orders of the

magnitudes, from one matrix to another.

For example, yields of Al+ ions from Al2O3 and Al metal differ by a factor of 100;

Si+ ion emission from SiO2 is 2500x higher than that from Si.

Quantification in SIMS

The SIMS signal intensity for a particular element M (𝐼𝑀) is related to its

concentration in the analyzable layer (𝐶𝑀) by several parameters:

𝐼𝑀 = 𝐽𝑝𝐴𝑆𝛽𝑀𝑇𝐶𝑀

𝐽𝑝 = primary ion current

𝐴 = analyzed surface area

𝑆 = sputtering yield

𝛽𝑀 = secondary ion yield for element M

𝑇 = transmission of SIMS spectrometer

Since many of these parameters are not known, an approach based on

relative sensitivity factors is adopted in SIMS to evaluate atomic

concentrations of minor constituents when that of the major constituent is

known.

15

Relative sensitivity factor (RSF)16

For absolute quantification using SIMS, standards as similar as possible to

the real sample are needed. It is typical to use implanted samples as

standards.

For example for the concentration profile of an impurity 𝑖 (𝐶𝑖) in a matrix

(𝑚𝑎𝑡), a standard with a known dose (𝐷 𝑖𝑛 𝑎𝑡𝑜𝑚𝑠/𝑐𝑚2) of 𝑖 in the same

matrix is created. So that the relative sensitivity factor

𝑅𝑆𝐹 = 𝐷𝐶𝐼𝑚𝑎𝑡𝑡

𝑧𝐼𝑖 𝑠𝑡𝑑

,

where 𝐶 is the number of data cycles, 𝐼𝑚 is the matrix element secondary ion

intensity (counts/sec), 𝑡 is the count time/cycle, 𝑧 is the depth of the crater, 𝐼𝑖 is the

summation of secondary ion intensity of 𝑖 in counts.

𝐶𝑖 = 𝐼𝑖

𝐼𝑚𝑎𝑡 𝑠𝑎𝑚𝑝𝑙𝑒

∙ 𝑅𝑆𝐹

Implanted standards have the advantages of:

─ Any element (isotope) can be implanted into any matrix

─ Depth and peak concentration can be tuned by the energy and the dose

─ Multiple element can be implanted

─ A detection limit can be established

SIMS: ion implanted standards17

The procedure is based on the exposure, for a controlled time, of the

matrix to a beam of primary ions of the element of interest.

The primary ion energy usually ranges between 50 and 300 keV, whereas

the dose is about 1013-1016 ions/cm2.

After implantation the sample is analyzed under Dynamic SIMS conditions

and the element signal is monitored as a function of time (e.g. of depth

reached due to erosion).

After the SIMS measurement

the crater is measured to reveal

the real depth

the implantation dose/crater

depth ratio provides an estimate

for the average atomic

concentration (atoms/cm3) of

the element in the matrix

A RSF can be established

𝑅𝑆𝐹 = 𝐷𝐶𝐼𝑚𝑎𝑡𝑡

𝑧𝐼𝑖 𝑠𝑡𝑑

SIMS: instrumentation

SIMS CAMECA 6F

Ion Sources

Ion sources with electron impact

ionization - Duoplasmatron: Ar+,

O2+, O-

Ion sources with surface ionization -

Cs+ ion sources

Ion sources with field emission -

Ga+ liquid metal ion sources

Mass Analyzers

Magnetic sector analyzer

Quadrupole mass analyzer

Time of flight analyzer

Ion Detectors

Faraday cup

Dynode electron multiplier

Vacuum < 10−6 torr

18

Schematic Diagram of a SIMS instrument19

20

Ion source: DuoplasmatronA duoplasmatron is an ion source with electron impact ionization

A cathode filament emits electrons into a vacuum chamber

Small quantity of gas (Ar, O2, Ne, etc.) leaks into the chamber and interacts

with the electrons forming a plasma

The plasma is accelerated through a series of highly charged grids to the

desired energy and extracted through the aperture.

It can operate with almost

any gas

When O2 is used, O-, O2- or

O2+ can be extracted

depending on the electrical

polarity selected

Probe diameter typically

between 5 mm to 1 mm

Ion current densities >10

mA/cm2

Ion source: Cs+ source21

The Cs atoms are ionized

during evaporation because

the work function of W (4.52

eV) is substantially greater

then the ionization potential of

Cs (3.88 eV)

The Cs+ ions are extracted and

accelerated to an energy up to

10 keV.

Depending on the gun design,

fine focus or high current can

be obtained.

Cs gun is typically more

expensive to operate

Cs metal (or compound) is heated in the reservoir (~400oC) forming a vapor

The Cs vapor flows through a feed tube to a porous tungsten plug

The Cs vapor diffuses through the pores in the plug to the front of surface

which is maintained at >1100oC by the ionizer heater

Operates with low melting point metals or metallic alloys, which are liquid at

room temperature or slightly above (Ga, Cs).

The liquid metal covers a W tip and emits ions under influence of an intense

electric field.

Ion current densities > 1A/cm2 with sub mm probe diameter.

Beam can be focused to <50 nm with moderate intensity and rastered to provide

secondary electron image or elemental mapping over the specimen surface.

Ion source: Liquid Metal Ion Source (LMIS)

W

Capillary

500 mm

22

Dual source SIMS23

Many SIMS spectrometers are

equipped with two sources,

usually a Cesium gun and an

Oxygen Duoplasmatron

source.

A mass filter (typically a

quadrupole), enables the

selection of the ion of interest.

Selected ions are then focused

and accelerated towards the

sample by electrostatic

lenses.

In the final stage of the dual

source electrostatic deflectors

drive primary ions towards

specific regions of the sample

surface.

Magnet Sector

Electrostatic Analyzer and Mass Spectrometer24

ESA is to minimize fluctuation

of kinetic energy of ions so as

to reduce the interference of

ions, providing a higher

mass resolution of mass

spectrometers

All paraxial ions of particular

energy will follow the central

lines to be focused in a plane

of the ESA slit

Fluctuation in kinetic energy

of ions is substantially

suppressed

The sputtering process produces ions with a range of ion energies. An energy

slit can be set to intercept the high energy ions. Sweeping the magnetic field in

MA provides the separation of ions according to mass-to-charge ratios in

time sequence.

The mass analyzer select the particular

species according to the mass-to-charge ratio

𝑚𝑞 =

𝐵2

2𝑉𝑟2

where B is the magnetic field, V is the ion

accelerating voltage, r is the radius of curvature

of the ion

For an analogy, think of how a prism

refracts and scatters white light separating it

into a spectrum of rainbow colors.

Mass spectrometer

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.

25

Ion Detectors

Faraday cupSecondary electron multiplier

20 dynodes Current gain 107

A Faraday cup measures the ion current

hitting a metal cup, and is sometimes used

for high current secondary ion signals.

With an electron multiplier an impact of a

single ion starts off an electron cascade,

resulting in a pulse of 108 electrons which

is recorded directly.

Usually it is combined with a fluorescent

screen, and signals are recorded either

with a CCD-camera or with a fluorescence

detector.

26

Mass resolution27

Typically a higher mass resolution will accompany a loss of ion intensity

Mass resolution is usually specified in terms of 𝑚/∆𝑚 where 𝑚 is the mass of

the ion and ∆𝑚 is the FWHM of the detected signal.

─ For example, 56Fe+ and 28Si2+ (𝑚/𝑞=55.9349 and 55.9539) require 𝑚/∆𝑚 of

2,950 for separation while Au and 133Cs32S2 (𝑚/𝑞=196.9666 and 196.8496)

require 𝑚/∆𝑚 of 1700.

𝑚/∆𝑚 for the two species is 21160FWHM

∆𝒎

Time of flight SIMS28

Time-of-Flight SIMS (ToF-SIMS) uses a pulsed ion beam to remove

molecules from the very outermost surface of the sample. These particles are

then accelerated into a "flight tube" and their mass is determined by

measuring the exact time at which they reach the detector (i.e. time-of-flight).

ToF-SIMS is based on the fact that ions with the same energy but different

masses travel with different velocities.

mass resolutions >18,000 can be achieved

It also has extremely high transmission with the parallel detection of all

masses and the unlimited mass range.

Time-of-flight mass analyzer

In order to provide higher resolution the pulse should be as narrow as 1-10 ns.

The pulse repetition frequency is usually in a kHz range. Typical flight times

10 ns to 800 µs

During a short pulse of ion beam,

sputtered ions are accelerated

and acquire a constant kinetic

energy:

𝐾𝐸 = 𝑚𝑣2/2

with different 𝑚/𝑞 and velocity 𝑣.

The ions arrive to the detector in

time sequence (𝑡) after traveling

a distance 𝑙.

𝑡 =𝑙

𝑣=

𝑙

2𝑞𝑉𝑚

2

𝑚

𝑞=

2𝑉𝑡2

𝑙2

𝑙

29

Reflectron ToF spectrometer

The kinetic energy distribution in the direction of ion flight can be corrected by

using a reflectron. The reflectron uses a constant electrostatic field to reflect

the ion beam toward the detector.

The more energetic ions penetrate deeper into the reflectron, and take a

slightly longer path to the detector.

Less energetic ions of the same mass-to-charge ratio penetrate a shorter

distance into the reflectron and, correspondingly, take a shorter path to the

detector.

Twice the flight path is achieved in a given length of instrument.

30

SIMS: modes of operation31

Static SIMS: 0.1-10 keV ions are

employed, with current surface

densities in the nA/cm2 range,

Under these conditions the total

erosion of the sample first

monolayer (1 nm) may take even

an hour.

Dynamic-SIMS: 10-30 keV ions,

with current surface densities in

the mA-mA/cm2 range, are used.

Under these conditions the sample

is eroded continuously and the

acquired mass spectra enable the

monitoring of constituting elements

along the sample depth (depth

profiling).

According to the primary ion energy and current, the SIMS technique can be

divided into two variants:

Profiling

Material removal

Elemental analysis

Ultra surface

analysis

Elemental or

molecular analysis

Analysis completed

before significant

fraction of molecules

destroyed

Static SIMS32

Under the ion bombardment, fragment ions or

even intact molecular ions are emitted from

the top monolayer.

If the primary ion dose is limited to a level at

which every primary ion should (statistically)

always hit a fresh area, the (static) SIMS

spectrum reveals molecular information.

Progressively, as the ion dose increases, the molecular signal decreases then

vanishes when the whole area has been damaged.

To stay in static SIMS mode, the primary ion dose must be < 𝟏𝟎𝟏𝟐 𝒊𝒐𝒏𝒔/𝒄𝒎𝟐

Static SIMS gives rise to a fingerprint mass spectrum that contains "low mass"

(< 500 amu) ion fragments, identifying organic surface composition.

Due to the complexity of the static SIMS mass spectrum, it is mostly used as a

qualitative characterization of the molecular composition of the top surface.

By focusing and scanning the primary ion beam, molecular information can be

obtained with sub-micron lateral resolution, and molecular surface distribution

can be imaged.

Static SIMS33

Positive ion TOF mass spectrum of polydimethylsiloxane

contaminated polyethylene terephthalateSilicon wafer contaminated with copper, iron and chromium

Range of elements H to U: all isotopes

Destructive Yes, if sputtered long enough

Chemical bonding Yes

Depth probed Outer 1 to 2 monolayers

Lateral resolution Down to below 100 nm

Imaging/mapping Yes

Quantification Possible with suitable standard

Mass range Typically up to 1000 amu, 10000 amu (ToF)

Main application Surface chemical analysis, organics, polymers

Dynamic SIMS34

Ion dosage and sputter rates are high resulting more

fragmentation.

Must be equipped with Oxygen and Cesium primary

ion beams in order to enhance, respectively, positive

and negative secondary ion intensity by 2 to 3 orders

of magnitude compared to the use of noble gas ions.

As the primary ion dose implanted in the target increases, the primary

species concentration (oxygen or cesium) will reach an equilibrium and this

corresponds to a sputtering steady state when reliable quantification is

possible with reference standard samples, using RSF.

One of the main application of dynamic SIMS is the in-depth distribution

analysis of trace elements (for example, dopant in semiconductors).

Impact ion energy is adjusted depending on the applications.

─ Low energy (down to 150eV) is used to reduce atomic mixing and

improve depth resolution down to the sub-nanometer level.

─ High energy (up to 20 keV) is chosen to investigate deeper (10-20

microns), faster (sputter rate of µm per min range), and improve

detection limits and image resolution.

Dynamic SIMS35

Range of elements H to U: all isotopes

Destructive Yes, material removed during sputtering

Chemical bonding In rare cases only

Depth probed Depth resolution 2-30 nm, probe into mm below surface

Quantification Standard needed

Accuracy 2%

Detection limits 1012-1016 atoms/cm3 (ppb-ppm)

Imaging/mapping Yes

Sample requirements Solid; vacuum compatible

Near surface B depth profiles

from a 2.2 keV BF implant in

Si using different energies O2+

primary beam

SIMS depth profiling: example36

Sputter time: 700 sec

Depth: 9310 Å

Erosion rate:13.3 Å/sec

Using an ion implanted

sample: P dose 1015 P/cm2

RSF: Relate the intensity to

atomic concentration:

𝑅𝑆𝐹 =𝑑𝑜𝑠𝑒

𝐼 𝑥 𝑑𝑥

RSF: 1counts/s=3.4x1015 P/cm3

Phosphorus doped Silicon

Dynamic SIMS – Depth Profiling

Factors affecting depth resolution:

Crater edge rejection:─ Raster beam for flat bottomed crater

─ Accept ions only from the center of crater

Ion beam mixing─ Primary ion mass

─ Impact energy

─ Impact angle

Surface roughness─ Metal worse than single crystal materials

The depth profile can be affected by:

Redeposition by sputtering from

the crater wall onto the analysis

area

Direct sputtering from the crater

wall

37

Crater Effect

(a)

(b)

The analyzed area is usually

required to be much smaller

than the scanned area.

a. Ions sputtered from a selected central

area (using a physical aperture or

electronic gating) of the crater are

passed into the mass spectrometer.

b. The beam is usually swept over a large

area of the sample and signal detected

from the central portion of the sweep.

This avoids crater edge effects.

38

Depth resolution39

Reducing ion penetration depth can reduce

effects of ion mixing, this can be achieved by

─ Larger angle of incidence from normal

─ Lower bombarding energy

─ Increased mass of primary beam

Simulation of the effect of 1% and 10%

unevenness on crater bottom for a sinusoidal

dopant distribution, according to the uneven

etching model

D. S. McPhail, et al., Scanning Microscopy 2, 639 (1989)

Luftman et al., J. Vac. Sci. Technol. B10, 323 (1992)

1.5 keV O2+ beam incident at 60° from

normal on a delta doped Be in GaAs sample

FWHM depth

resolution <3 nm

Sample Rotation Effect

SEM micrographs of a)

aluminum surface, b) bottom of

crater sputtered through 1 μm

aluminum layer into underlying

silicon without rotation and c)

with rotation

F. A. Stevie and J. L. Moore, Surf. Interf.

Anal. 18, 147 (1992)

B B

SiSi

Al Al

No rotation With rotation

SIMS profiles of 11B ion implantation into 1 μm Al/Si. With sample rotation, B at interface is clearly

defined and silicon from Al-Si-Cu layer shows movement to Al/Si interface

Reduction of preferential sputtering of different grains of polycrystalline materials

40

The SIMS detection limits for most trace elements are between 1012 and

1016 atoms/cm3.

The primary limiting factor is ionization efficiencies.

The dark current (or dark counts) arises from stray ions, electrons in

vacuum systems, and from cosmic rays

Count rate limited sensitivity:

─ When sputtering produces less secondary ion signal than the detector dark current.

─ If the SIMS instrument introduces the sample element, then the introduced level

constitutes background limited sensitivity, e.g. Oxygen, present as residual gas in

vacuum systems

Atoms sputtered from mass spectrometer parts by secondary ions constitute

another source of background.

Typically, sensitivity and depth resolution cannot be optimized simultaneously

Best sensitivity is achieved with high sputtering rate and large detected

area

Best depth resolution is achieved with low impact energy, reduced ion

penetration into sample, low sputtering rate and small detected area

Sensitivity and resolution41

42

Element M+ (O2+) M- (Cs+)

Li 3E13 1E16

Be 3E14 1E20

B 1E15 3E15

Na 3E14 2E17

Mg 1E14 1E20

Al 2E15 1E17

K 2E14 2E18

Ca 3E14 1E20

Ti 2E14 1E18

V 1E14 1E17

Cr 1E15 2E17

Mn 3E14 1E18

Fe 1E15 3E17

Ni 1E16 5E17

Cu 3E16 1E16

Zn 1E16 1E20

Sr 5E15 1E20

Y 1E17 1E20

Zr 1E15 4E17

Nb 1E16 1E18

Mo 1E16 1E18

Cd 5E16 1E21

In 3E15 3E17

Detection Limits: in InP, GaAs, GaN (atoms/cm3)

For electropositive elements:

Element M- (Cs+) M+ (O2+)

H 2E17 2E18

C 1E16 2E18

N 5E15 (NGa-) 5E18

O 1E16 1E20

F 2E14 5E16

P 2E15 1E16

Si 2E15 1E16

S 1E15 1E19

Cl 3E15 2E17

Ge 5E15 2E16

Se 5E14 2E17

Br 5E13 1E17

Te 1E15 2E17

Ag 2E16 2E16

Au 1E15 1E17

For electronegative elements:

Comparison: static and dynamic SIMS43

Imaging SIMS

The mass spectrometer is set to

only detect one mass.

The particle beam traces a raster

pattern over the sample with a

low ion flux beam, much like

Static SIMS.

Typical beam particles consists of

Ga+ or In+ and the beam diameter

is approximately 100 nm.

The analysis takes usually less

than 15 min.

The intensity of the signal detected for the particular mass is plotted

against the location that generated this signal.

Absolute quantity is difficult to measure, but for a relatively

homogeneous sample, the relative concentration differences are

measurable and evident on an image.

Images or maps of both elements and organics can be generated.

44

Imaging SIMS

Scanning ion image of granite from the Isle of Skye.

-University of Arizona SIMS 75 x 100 micrometers.

45

Imaging SIMS46

Detection of micrometric spots due to an organic contaminant (pentaerythritol-

tetraoctanoate, C37 H68 O8 , a lubricant) on an hard disk surface.

Charging of insulating samples in SIMS47

A positive charge is accumulated on the sample surface during a

SIMS analysis, due to ionic bombardment.

In insulating samples this charge cannot be neutralized by electrons

drawn from the ground through the sample stage.

Sample charging diffuses the primary beam and diverts it from the

analytical area, changes the energy distribution and direction of

secondary ions.

Several techniques are available to manage sample charging:

─ Flooding the sample surface with a low energy (a few eV) electron

beam, like in the case of XPS

─ Placing a conducting grids over the sample, similarly samples are

often coated with conducting materials such as gold or carbon.

─ Bombarding the sample with negative ions (for example O-)

─ Applying a continuously variable voltage offset to the accelerating

voltage for samples that are only slightly charging.

Example: Gate oxide breakdown48

Example: GaAs quantum well structure49

Negative secondary ions with 5keV

Cs primary ion bombardment

Example: ion beam mixing in isotope superlattice50

SIMS concentration profiles of the stable isotopes 74Ge (upper

solid line) and 70Ge (lower solid line) in crystalline (natGe/70Ge)10

and amorphous (natGe/73Ge)10 as-grown multilayers

The structures were implanted with 310 keV

Ga ions with a dose of 1.0 × 1015 and 3.3 ×1014/cm2.

Self-atom mixing in crystalline Ge is mainly

controlled by radiation enhanced diffusion

during the early stage of mixing before the

crystalline structure turns amorphousBracht et al., J. Appl. Phys. 110, 093502 (2011).

310 keV Ga+ 1.0 × 1015 /cm2

310 keV Ga+ 3.3 × 1014/cm2

Example: self-diffusion un GaSb51

Ga and Sb profiles of the

as-grown 69Ga121Sb/71Ga123Sb heterostructure

After annealing the

isotope structure under

Sb-rich conditions at

700oC for 105 min

After annealing at 700oC

for 18 days

Near the melting

temperature, Ga diffuses

more rapidly than Sb by

over three orders of

magnitude. This surprisingly

large difference in atomic

mobility is a consequence of

reactions between defects

on the Ga and Sb

sublattices, which suppress

the defects that are required

for Sb diffusion.

Bracht et al., Nature 408, 69 (2000).

Advantages and weaknesses of SIMS

Advantages Weaknesses

Excellent sensitivity, especially for

light elements

Destructive method

High surface sensitivity Element specific selectivity

Depth profiling with excellent depth

resolution (nm) (dynamic)

Standards needed for quantification

Good spatial resolution (<1-25 mm) Sample must be vacuum compatible

Small analyzed volume (down to

0.3mm3) so little sample is needed

Sample mist have a flat surface

Information about the chemical

surface composition due to ion

molecules (static)

High equipment cost (>1M-3M USD)

Elements from H to U can be

detected with excellent mass

resolution

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