Chapter 5
Ion methods of surface analysis
5.1 Ion sources
In case of surface and thin ﬁlms analysis can ion sources can be used. Suitable sources provide
proper ion beams for surface cleaning and for surface layer sputtering in order obtain a depth
proﬁle (such as XPS or AES), primary beam for static and dynamic SIMS. Another sources are
represented by the particles accelerators used for energetic ion analysis (such as RBS, PIXE, NRA,
ERDA etc.)
The ionization methods can be classiﬁed according to:
• electron impact
• fast atom bombardment
• electro-spray ionization
• gas discharge ion sources
• solid state surface atom ionisation sources
• high electric ﬁeld desorption
Ion beam focus:
• wide ion beam sources
• narrow focused ion beam sources
5.1.1 Electron impact ionisation sources
The principle of ionization source is similar as a ionisation gauge. The only modiﬁcation include
a geometry of gauge and ion collector must be replaced by the system which is able to extract
the produced ions. Ionisation space is inside the cylinder - screened by anode with the ion trap
at the end. Electrons are accelerated from a thermionic emission cathode by high voltage (100 -
500 V). The electrons travel through the ionisation space and ionize gas molecules around them.
1
2 CHAPTER 5. ION METHODS OF SURFACE ANALYSIS
The resulting ions are collected at a negative trap electrode - thus resulting in an ion beam. The
operating pressure is lower 0.01 Pa and the source provides high focused ion beam with beam
current to 10 µA. Due to its properties it is possible another manipulation in ion optics.
Another source modiﬁcation - double anode source. Two parallel bars - anodes - are placed
along the cylindrical cathode (voltage up to 10 kV). Electrostatic ﬁeld with a saddle point is form
on anode axis. Electrons oscillate around this point and they are eﬀectively ionise the gas. The
produced ions are accelerated along the plane symmetry to cathode with the outlet. Operation
pressure is tenths of Pa, ion current is tens of µA. Electrons are ejected in the form of diverging
beam with wide energy distribution → they are inappropriate to another manipulation in ion
optics. It causes a high chromaticism. In consequence, it is use to surface sputtering.
5.1.2 Plasma sources
Plasma sources generate plasma - excitation of the gas requires ionization of neutral atoms and
molecules. High intensity electric ﬁeld extracts ions from the plasma surface (negative charged
electrode). The dependency of extracts ion current I and electrode surface voltage Uex on the
thickness d is described by Child-Langmuir law
I ≈ U3/2
ex /d2
. (5.1)
Two types of plasma sources are most often use to surface and thin ﬁlms analyses. In van
der Graﬀ accelerators, high frequency discharge are the ion sources generally use to analysis with
ion energy in order MeV. The duoplasmatron is a type of ion source compresses the plasma by
a magnetic ﬁeld in front of the extraction system. The plasma is so dense that matching to the
extraction ﬁeld strength requires an expansion cup to lower the current density. Most often is
used as a source of Ar and O ions.
High frequency ion source
Ions can be created in an inductively or capacitively coupled plasma. The electric ﬁeld has a
frequency in order tenth of MHz, the power is tenth to hundreds of W, the pressure is units of Pa,
current is from tenth of µA to tenth of mA. The ionization eﬃciency can be enhanced by magnetic
ﬁeld.
Two types of inductively coupled RF structures are most common. In one type, the induction coil
is placed outside a glass or quartz cylindrical tube in which the plasma is generated. The other
type has a metallic discharge chamber with an RF antenna inside the discharge plasma.
Although capacitively coupled RF sources are widely used in plasma etching processes, they are
seldom employed for the production of ion beams. Capacitively coupled discharges form a very
high positive plasma potential. As a result, material is sputtered from the chamber wall to form
impurity ions in the extracted beam.
Duoplasmatron
The duoplasmatron operates as follows: a cathode ﬁlament emits electrons into a vacuum chamber,
then the gas is introduced in very small quantities into the chamber, where it becomes charged
5.1. ION SOURCES 3
or ionized through interactions with the free electrons from the cathode, forming a plasma. The
plasma is then accelerated through a series of at least two highly charged grids, and becomes
an ion beam, moving at fairly high speed from the aperture of the device. Duoplasmatrons are
characterized by high eﬃciency - usually above 90 %.
The duoplasmatron components are:
• cathode (K)- generally the heated cathode (thermionic emission), sometimes (for oxygen
ions) is use the hollow cold cathode
• intermediate electrode (IE),
• anode (A).
The intermediate electrode and anodes are ferromagnetic and create the part of magnetic circuit.
Demirkhanov installed a ferromagnetic anticathode electrode to ensure oscillation of electrons in
the anode region of the duoplasmatron. The anticathode was located behind a copper anode and
had a potential close to the cathode potential. Electrons oscillating in the magnetic ﬁeld (1500
G) eﬃciently ionize the gas at a low pressure of ∼5–10˘3
Torr. When the anticathode negative
bias relative to the anode was increased from 0 to 100 V, the ion current increased by a factor of
ﬁve. The source, which had an outlet aperture 6 mm in diameter and an expansion cup 50 mm in
diameter, provided a pulsed hydrogen ion current of up to 1.5 A at a discharge current of 20 A.
Figure 5.1: Duoplasmatron scheme.
Duoplasmotrons can have diﬀerent ion sources:
• Penning-type electrode system (PIG) to maintain a high current discharge by electrons
supplied through a double electric layer from a plasma cathode, has been embodied most
completely in the DuoPIGatron type ion source. The magnetic ﬁeld diverges toward the
ion optics, ensuring oscillation of primary electrons between the intermediate and screen
electrodes and formation of a large, uniform plasma surface. To increase the cathode lifetime,
4 CHAPTER 5. ION METHODS OF SURFACE ANALYSIS
an inert gas (argon) is fed into the cathode stage and oxygen into the anode stage of the
source. An ion optics system with 13 holes of diameter 5 mm provided an ion beam current
up to 170 mA at an accelerating voltage of up to 50 kV. After mass separation of the beam
and acceleration of ions to energy of 200 keV, a beam of O+
ions with current 100 mA was
obtained. The ﬁlament cathode lifetime is over 25 hours.
• periplasmatron-type ion source has a unique electrode system design, which decreases heating
of the screen electrode by thermal radiation of the cathode, reduces the eﬀect of secondary
electron back ﬂow from the accelerating gap, and ensures a high uniformity of plasma ﬂow
to the screen grid. A rectangular variant of this source provided a hydrogen ion current of
96 A from an extractor 40 x 16 cm in size.
• magnetic-ﬁeld-free source for use in neutral beam injectors has been developed in which
the plasma is generated by a diﬀuse low-pressure high-current discharge, with a distributed
thermionically emitting cathode. Twenty hairpin ﬁlaments, 0.5 mm in diameter, are installed
around the periphery of the discharge chamber, 14 cm in diameter, near the cylindrical wall,
which serves as the anode. The peripheral placement of the cathode provides generation of
uniform (± 6 % ), oscillation-free plasma of 12 cm diameter. For a discharge current of 1000
A the deuterium ion beam current density is 0.5 A cm˘2
and the beam current 15 A at an
extraction voltage of 15 kV.
5.1.3 Surface ionization sources
Surface ionization sources are commonly used for producing a Cs+
ion beam. The surface ionization
source emits ions from a hot plate. This conﬁguration is ideal for designing beam optics
with minimum aberration because the emitting surface is a solid boundary. In fact, the surface
ionization source can be designed to produce a beam with very large aperture (and therefore high
current per beam) because of the rigid emitting surface. The ion temperature from a surface
ionization source is typically low, of the order of a fraction of an eV; therefore the beam emittance
is kept low, even when the source diameter is large. The two main types of surface ionization
sources are contact ionizers and aluminosilicate sources.
The ration of density of emitted positive ions ﬂux j+ to density of emitted atoms ﬂux ja is
deﬁned
j+/ja =
g+
ga
e−
Vi−φ
kT , (5.2)
where g+ a ga are statistical weight of ionized and atomic state of adsorbed atom, Vi is ionization
energy of adsorbed atom, φ represents work function of solid, T is the temperature of solid and k
is Boltzmann constant.
5.1.4 Field ionization
Field emission from sharp needle points at which very intense electric ﬁelds are created can be
used to extract either electrons or ions from the solid (or liquid) state:
5.2. ION SCATTERING SPECTROSCOPY - ISS 5
• liquid metal ion sources (LMIS) - is used in liquid metal ion sources to produce beams of
species of low melting point metals such as Ga, In, Bi, Al, Sn, ... with extraordinarily high
brightness
• gas ﬁeld ion source (GFIS)
In both types the electric ﬁeld with intensity 10 V/nm is use.
5.2 Ion scattering spectroscopy - ISS
Sometimes called Low Energy Ion Scattering Spectroscopy (LEIS) is a surface sensitive analytical
technique used to characterize the chemical and structural properties of materials. The range of
ion energies is from hundreds of eV to units keV. When a beam of ions hits a solid surface part
of the projectiles will be scattered back into the vacuum after one or more collisions with target
atoms of the layer. Measurement of the energy of the backscattered particles is use to identify
chemical structure of solid. primary ion energies of 0.5 – 5 keV are used with noble gas ions (He,
Ne, Ar) and also alkali ions (Li, Na, K). With this method, information is obtained from the
topmost atomic layer, under certain circumstances also from the second or third layer.
Figure 5.2: Comparison of RBS and ISS techniques.
6 CHAPTER 5. ION METHODS OF SURFACE ANALYSIS
5.2.1 Qualitative analysis
From the applied conservation and momentum laws in the case of two-body collisions it is possible
to calculate the ion kinetic energy after a collision with a surface atom without knowing the
interaction potential.
If the incident ion has energy E0, the mass M1 and the surface atom M2, we obtain the energy
and mass conservation relation for the backscatter energy E1:
E1
E0
=
cos θ ± [(M2/M1)2
− sin2
θ]1/2
M2/M1 + 1
2
, (5.3)
When this relation is derived, the kinetic energy of atom should be neglected
For 90◦
and 180◦
: backscattering angles the relation is simpliﬁed:
E1
E0
=
M2 − M1
M2 + M1
for θ = 90◦
(5.4)
E1
E0
=
(M2 − M1)2
(M2 + M1)2
for θ = 180◦
(5.5)
The ration K = E1/E0 is named kinematic factor. The above relations show that the energy
of ion after collision is given by the projectile and the target atom weight and the backscattered
angle (for constant E0) =⇒ determination of target atom mass M2. The corresponding expression
for the recoiling target atom is:
E2
E0
=
4A
(1 + A)2
cos2
θ2 (5.6)
Scattered-ion energy spectra are transformed into mass spectra by the backscattered relation for
θ = 180◦
. Consequently, the mass resolution also can be calculated from this equation for the
special case of θ = 90◦
:
M2
∆M2
=
E
∆E
2A
A2 − 1
(5.7)
Assuming a constant relative energy resolution of the detector of E/∆E = 100 the mass resolution
is better for large scattering angles and about equal ion and target atom masses. So the primary
projectile mass has to be selected accordingly if mass resolution is important.
5.3 Rutherford backscattering spectroscopy - RBS
In Rutherford Backscattering Spectroscopy (RBS) the primary ion energy ranges from about 100
keV (for H+
) to several MeV (for He+
and heavier ions). The ion-target atom interaction can
be described using the Coulomb potential from which the Rutherford scattering cross-section is
derived,which allows absolute quantiﬁcation of the results. Information in principle arises from a
thickness of the order of 100nm (10−5
cm), but analysis of surface layers is also possible by using
channelling/blocking techniques. Scattering of H+
with energies around 100 keV is sometimes
referred to as MEIS (Medium Energy Ion Scattering), probably because it only needs a smaller
type of accelerator, but physically it is within the RBS regime.
5.3. RUTHERFORD BACKSCATTERING SPECTROSCOPY - RBS 7
The physical principles are the same for both techniques (ISS and RBS): an ion beam is directed
onto a solid surface, a part of the primary projectiles is backscattered from the sample and
the energy distribution of these ions is measured. Since the ion–target atom interaction can be
described by two-body collisions, the energy spectra can be easily converted intomass spectra. The
diﬀerence between ISS and RBS arises from the diﬀerence in the cross-sections and the inﬂuence of
electronic excitations and charge exchange processes, which result indiﬀerent information depths.
In both cases, structural information is obtained from crystalline samples by varying the angles
between beam and sample. Deduction of structural information from the data is straightforward,
since both techniques are "real space" methods which are based on fairly simple concepts. With
ion scattering only the individual atoms of an element can be detected and no information on
compounds or molecules can be gained.
In terms of analytical characteristics, is important the variable cross section for elastic scattering,
which integrates a number of dispersed particles with the number of atoms in the substance
to be examined. Diﬀerential scattering cross section at the angle θ in the laboratory system is
then:
dσR
dΩ
=
Z1Z2e2
2E0
2
(M2
2 − M2
1 sin2
θ)1/2
+ M2 cos θ
2
M2 sin4
θ(M2
2 − M2
1 sin2
θ)1/2
, (5.8)
where Z1 and Z2 are the atomic numbers of the ion and the target.
Analysis using heavier particles and lower energies can play a signiﬁcant role in electron screening.
In such cases the ﬁrst correct approximation of the cross section is multiply by the correction
factor F in the form:
F =
σ
σR
= 1 −
0, 049Z1Z
4/3
2
ECM
, (5.9)
where ECM is the energy of particles expressed in keV. In a conventional design RBS the correction
is below 5 %. Signiﬁcant deviation of the cross section appear when the energy of the particles is
increased , or particles with lower atomic number are used. The eﬀect of nuclear forces appears
when the scattered are scattered at large angles and energies E0 > Ecritical
Ecritical = 103
Z1Z2M
−1/3
2 . (5.10)
For example, nuclear eﬀects occur when the α particles (Z1 = 2, M1 = 4)on Si atoms (Z2 = 14,
M2 = 28) sat energiesEkrit > 9 MeV are scattered.
5.3.1 Elemental composition determination
The basic relationship is
Y = QN
∆ω
dσ
dΩ
dΩ, (5.11)
where Y is the number of the dispersed particles registered, Q is the number of incident particles,
N is the area density in the layer (atoms/cm2
) and ∆ω is the interval of the solid angle from which
is detected the scattered particle. The equation can be applied in a layer containing multi-elements
or a thin layer on a stronger substrate.
8 CHAPTER 5. ION METHODS OF SURFACE ANALYSIS
5.3.2 Energy loss
Only a small fraction of the primary ions come close enough to a target nucleus (impact parameters
of the order of 10−12
cm) to undergo an elastic nuclear collision which is described by the kinematics
given in the previous section. If such an ion is backscattered, its ﬁnal energy is determined by the
elastic nuclear collision in a certain depth of the sample and the additional inelastic energy loss
to electrons on its way in and out of the target.
The relation for energy loss per unit length, − − dE/dx given in eV/Åand commonly called
stopping power:
S = −
dE
dx
(keV.cm−1
), (5.12)
The stopping cross section (eV/(atoms/cm2
)) relates this quantity to the atomic density N and
is therefore more speciﬁc for a
=
1
N
S (keV.cm2
), (5.13)
The stopping power curve exhibits a broad maximum around 1 MeV and that is the operational
regime of RBS: here, the stopping power does not depend very much on the ion energy and hence
can be assumed to be constant as a suﬃcient approximation in many cases; the stopping power
there has its maximum value and therefore RBS its best depth resolution and in this energy range
the nuclear interaction is exactly given by the Coulomb potential, giving RBS the advantage of
an absolute analytical method. Stopping cross section for a particle with energy E and a mixture
of AmBn elements is
AmBn
= m A
(E) + n B
(E), (5.14)
where A
(E) ans B
(E) are the stopping cross sections for the elements A and B.
5.4 Detection of ejected atoms - ERDA
Detection of light elements by RBS is diﬃcult due to the low eﬀective elastic scattering crosssections
and often because the signal lies on a high background originated by scattering on heavier
elements. Therefore, the method used is ERDA (Elastic recoil detection analysis).
In this method, atoms or ions are detected which are removed from the surface by one single
collision with an incoming projectile. They can therefore be identiﬁed by their kinetic energy.
Directly recoiling particles are therefore diﬀerent from those secondary particles that originate
from a collision cascade in a sputtering process and have a broad energy distribution around one
to two eV. They are used in secondary ion or neutral mass spectroscopy.Recoil cross-sections are
generally of the same order as scattering cross-sections. They are largest towards θ2 = 90◦
, but
then the recoil energy approaches zero. A useful energy range is obtained between 30◦
and 60◦
.
ERDA is also often done using a relatively low energy (2 MeV) He beam speciﬁcally to depth
proﬁle hydrogen. In this technique multiple detectors are used, at backscattering angles to detect
heavier elements by RBS and a forward (recoil) detector to simultaneously detect the recoiled
hydrogen (the cross-section for which reaction is strongly non-Rutherford at these energies). The
recoil detector has to have a range foil: a thin ﬁlm (typically 6 micrometres of PET ﬁlm) to
preferentially stop the incident He beam scattered into the forward direction.
5.5. SECONDARY ION MASS SPECTROMETRY - SIMS 9
5.5 Secondary ion mass spectrometry - SIMS
Secondary Ion Mass Spectrometry (SIMS) is a relatively new technique for surface chemical analysis
compared with Auger enectron spectroscopy (AES) and X - ray photoelectron spectroscopy
(XPS). SIMS examines the mass of ions escaped from a solid surface to obtain information on
surface chemistry.
Figure 5.3: SIMS instumentation.
SIMS uses energized primary particles, usually ions such as Ar+
, Ga+
and Cs+
, to bombard
a solid surface in order to induce sputtering of secondary particles from an area. The interactions
between primary ions and the solid are rather complicated. First, the secondary particles include
electrons, neutral species of atoms or molecules, and ions. The majority of the secondary particles
are neutral and not useful in SIMS. Only the secondary ions generated by the bombarding process
carry chemical information. Second, the interactions are often more than a simply one-to-one
knock-out of a surface ion by a primary ion. Commonly, the primary ions induce a series of
collisions (collision cascade) in a solid because the energy of a primary ion is transferred by
collisions between atoms in a solid before secondary ions on the surface are emitted.
The ionization probability is strongly aﬀected by the electronic properties of the sample matrix.
Ionization directly aﬀects the signal intensity of secondary ions as shown in the basic equation of
secondary ion yield.
J±
s = JpcY β±
f, (5.15)
where Jp is the primary ion ﬂux, Y is the sputter yield, c is concentration of species m in the
surface layer, β±
represents the probability for positive ions and f is the transmission of the
detection system. The transmission is deﬁned as the ratio of the ions detected to ions emitted,
and it varies from 0 to 1 depending on the analyser. The yield of elemental secondary ions can
vary by several orders of magnitude across the periodic table.
10 CHAPTER 5. ION METHODS OF SURFACE ANALYSIS
5.5.1 Ion sputtering
Emission of secondary particles can result from collision sputtering or from other processes such
as thermal sputtering. Collision sputtering includes direct collision sputtering and slow collision
sputtering. The former can be considered as a direct impact between a primary ion and a surface
atom. Direct collision sputtering is extremely fast, and occurs in the range of 10−15
to 10−14
seconds after the primary ion strikes a surface. Slow collision sputtering, represents the case that
a primary ion never has chance to collide with a surface atom; instead, atoms in the solid transfer
the impact energy to surface atoms after a series of collisions. The time scale of slow collision
sputtering is in the range of 10−14
to 10−12
seconds.
Only a small portion of secondary particles (∼ 1% of total secondary particles) are ionized and
become the secondary ions that are analysed in SIMS. A sputtered particle faces competition
between ionization and neutralization processes when it escapes a sample surface. Ionization
probability represents the chance of a sputtered particle being an ion.
5.5.2 Ion sources
The commonly used primary ions include argon (Ar+
), xenon (Xe+
), oxygen (O2+
), gallium (Ga+
)
and caesium (Cs+
) ions. Heavier metal ions such as bismuth (Bi+
) are also available in modern
static SIMS instruments. The ions of elements normally occurring in gaseous phases, such as
oxygen and argon, are produced by electron bombardment sources or plasma ion sources. Electron
bombardment sources use a circular ﬁlament cathode surrounding a cylindrical grid anode. The
gas to be ionized is injected into the open space of the grid, and the gas molecules are bombarded
by the electrons emitted from the cathode. The electrons travel in orbit inside the grid in order to
increase their chances of striking gas molecules. An extraction ﬁeld inducted in the grid will draw
the gas ion beam through an opening in the center of the extractor. The electron bombardment
sources provide moderate brightness of primary ions (∼ 105
Am−2
per solid angle).
Figure 5.4: Secondary particle generation by an energetic primary particle.
5.5. SECONDARY ION MASS SPECTROMETRY - SIMS 11
5.5.3 Mass analysis system
The mass analysis system collects and analyses the ion masses to produce mass spectra with the
assistance of a computer. The extractor ﬁlter extracts secondary ions from the surface, selects a
mass range of ions to analyse, and eliminates scattered primary ions from a mass spectrum. The
mass analyser, the critical component for secondary ion mass analysis, can be one of the following
types:
• magnetic sector analyser,
• quadrupole analyser,
• time-of-ﬂight analyser.
Magnetic sector analyser
This analyser was the oldest type used for mass spectroscopy. Secondary ions are accelerated by an
extraction potential of 4 kV before entering a magnetic ﬁeld. The magnetic ﬁeld in the magnetic
sector will impose a ﬁeld force in a direction orthogonal to the direction in which the ions travel.
Figure 5.5: The sputtering proces: (a) direct collision sputtering;(b) collision cascade; and (c)
thermal sputtering.
12 CHAPTER 5. ION METHODS OF SURFACE ANALYSIS
The ions with a given kinetic energy will change their travel path to a circular trajectory. The
radius of path curvature R has the following relationship with the ion mass.
R =
(2V )1/2
B
m
z
1/2
(5.16)
where the extraction potential (V ) is constant. We can adjust the magnetic ﬁeld strength B, to
select ions with a certain mass-to-charge ratio (m/z) with a ﬁxed radius of magnetic sector R.
Quadrupole mass analyser
The quadrupole mass analyser selects ions with a certain mz−1
by generating unstable oscillation
travels for the non-selected ions. Oscillations of ion trajectories are created by an electric ﬁeld
combining a constant direct current (DC) and alternating current with a radio frequency (RF).
The secondary ions are accelerated by an extraction ﬁeld, and then the ions travel through the
center of four circular rod electrodes. Combined DC and RF voltages are applied to one pair
of rods and equal but opposite combined voltages are applied to another pair of rods. Such
an arrangement of electric ﬁelds generates ion oscillation. The oscillation can be so severe
that ion trajectories become unstable and ions strike the rods. Ions with unstable trajectories
cannot travel through the exit slit of the analyser to reach the mass detector. Only the ions
with a certain mz−1
have stable trajectories and can pass through the exit slit under a given
ratio of DC:AC voltages. The analyser is a sequential type that only allows the ions with
single mz−1
values to reach the detector. Thus, the quadrupole analyser is a device with low
transmission, as less than 1% of ions can reach the detector at any time. To obtain a whole
mz−1
spectrum, the analyser increases the voltages but keeps the ratio of DC:AC voltages constant.
Time-of-Flight analyzer
The time-of-ﬂight (ToF) analyser is the most widely used analyser in static SIMS. As its name
indicates, for this analyser the ﬂight time of an ion is the parameter for measurement. When ions
are obtained with a constant kinetic energy from an acceleration potential (V ) of 3 – 8 kV, the
ﬂight time of ions through a distance (L) of ﬂight tube to reach a detector is calculated.
t = L(2V )−1/2 m
z
1/2
(5.17)
Thus, the mz−1
of ions is analysed by measuring their ﬂight time in the analyser. Heavier ions
will have longer ﬂight times in the tube. To measure the time of ﬂight, precisely pulsed primary
ions should be used. The pulse is controlled by a highly accurate clock. The pulse periods are
typically in the order of 10 ns. The ﬂight time of ions to the detector is electronically measured
and converted to mz−1
. In a pulse period, all the ions can be measured and a whole mass spectrum
can be obtained almost simultaneously. Again, the initial kinetic energy of secondary ions will
aﬀect the resolution of mass analysis, because the velocity could be diﬀerent when ions with the
same mz−1
enter the ﬂight tube.