low energy ion scattering

8/8/2019 Low Energy Ion Scattering

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Low-energy ion scatteringFrom Wikipedia, the free encyclopedia

LEIS redirects here; for the Hawaiian garland see Lei (Hawaii).

Image of a Kratos Axis-165 system equipped with XPS, ISS, and AES, from Alberta Centre for Surface

Engineering and Science (ACSES).

Low-energy ion scattering spectroscopy (LEIS), sometimes referred to simply as

ion scattering spectroscopy (ISS), is a surface-sensitive analytical technique used to

characterize the chemical and structural makeup of materials. LEIS involves directing

a stream of charged particles known as ions at a surface and making observations of

the positions, velocities, and energies of the ions that have interacted with the surface.

Data that is thus collected can be used to deduce information about the material such

as the relative positions ofatoms in a surface lattice and theelemental identity of those

atoms. LEIS is closely related to both medium-energy ion scattering (MEIS) and high-

energy ion scattering (HEIS, known in practice asRutherford

backscattering spectroscopy, or RBS), differing primarily in the energy range of the ion

beam used to probe the surface. While much of the information collected using LEIS

can be obtained using other surface science techniques, LEIS is unique in

its sensitivity to both structure and composition of surfaces. Additionally, LEIS is one of

a very few surface-sensitive techniques capable of directly observing hydrogen atoms,an aspect that may make it an increasingly more important technique as the hydrogen

economy is being explored.

Contents

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1 Experimental setup

2 Physics of ion-surface interactions

o 2.1 Elemental composition and two-body collision model

o 2.2 Getting quantitative

o 2.3 Shadowing and blocking

o 2.4 Diffraction does not play a major role

o 2.5 Variations of technique

3 Comparison to other analytical techniques

4 References

5 External links

6 See also

[edit]Experimental setup

LEIS systems consist of the following:

General experimental setup for LEIS.

1. Ion source, used to direct a beam ofions at a target sample. Electron impact

ionization is typically used to ionizenoble gas atoms such asHe, Ne orAr, while

heating of wafers containingalkali atoms is used to create an alkali ion beam.


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Ions thus created hold a positive charge, typically +1, due to ejection

ofelectrons from atoms. The range of energies used most often in LEIS is

500eV to 20 keV. In order to attain good experimentalresolution it is important

to have a narrow energy spread (E/E < 1%) in the outgoing ion beam.

2. Ion beam manipulator, including electrostatic lenses forfocusing and beam-

chopping. Lensesconsist of a series of either plate orcylindergeometries and

serve to collimate the beam as well as to selectively filter the beam based

on mass andvelocity. Beam chopping is performed using apulsed-

wave generator when time-of-flight (TOF) experiments are performed. Ions only

pass through the chopper when there is no applied voltage.

3. Sample manipulator, allows an operator to change the position

and/orangle of the target in order to perform experiments with

varying geometries. Using directional controls,azimuthal (rotational)and incident angle adjustments may be made.

4. Drift tube/drift region, used in TOF setup. TOF measurements are used when

analysis of particle velocity is required. By pulsing ions towards the sample with

a regularfrequency, and observing the time to travel a certain distance after

surface impact to a detector, it is possible to calculate the velocity of ions and

neutrals coming from the surface. An acceleratormay also be used in this

setup, prior to the drift tube, in order to achieve separation of ions

from neutrals when desired.

5. Detector/electrostatic analyzer, used to detect the velocities and/or energies

of scattered particles including ions and, in some cases, neutral species.


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Diagram of an electrostatic analyzer in the hemispherical geometry. Only ions of a selected energy

pass through to the detector.

As opposed to TOF analyzers, electrostatic analyzers achieve ion energy

resolution using electrostatic deflectors to direct only ions of a particular energyrange into a collector, while all other ions are redirected. This type of analyzer

can give good energy resolution (and thus, selectivity) but typically suffers from

poorsensitivity due to the fact that it only detects ions of a certain energy range

and ignores neutral species altogether. Two types of detectors are

used: channel electron multiplier(CEM) and microchannel plate (MCP)

detectors. CEMs operate in a similar manner to photomultipliers, displaying a

cascade of secondary electron emission processes initiated by ion or fast

neutral (energy > 1 keV) impact to give a gain in signal current. In this way it is

possible to efficiently detect even small ion or neutral particle fluxes. MCP

detectors are essentially 2-dimensional arrays of CEMs, and they allow

additional information about particle position to be obtained at the cost of

sensitivity at any given position.

6. Vacuum pumps; Studies are performed in ultra-high vacuum (UHV) conditions

(< 1010torr) in order to prevent unwanted interference with the ion

beam and/orsample. Common UHV pumps

include turbomolecularand ion pumps, with roughing pumping typically

performed using a rotary vane pump. Due to the extreme surface (i.e. first-

layer) sensitivity of LEIS, samples also need to be rigorously cleaned prior to

analysis. Some common processes used to clean samples

include sputtering and annealing. Appropriate equipment for cleaning must be

contained within the vacuum chamber.

7. Other analysis tools; in many cases it is desirable to perform multiple types of

analysis on a sample within the same UHV system, or even at the same time.

Some additional tools may includeAuger electron spectroscopy (AES), low-

energy electron diffraction(LEED), and x-ray photoelectron spectroscopy (XPS).Use of these tools typically requires the presence of additional detectors as well

as electron and/orx-ray sources where applicable.

[edit]Physics of ion-surface interactions


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Diagram of various ion-surface interactions (non-exhaustive). (1) Incoming ion; (2) Scattering; (3) Neutralization

and scattering; (4) Sputtering or recoiling; (5) Electron emission; (6) Photon emission; (7) Adsorption; (8)

Displacement. LEIS is unique in its high sensitivity to the first surface layer in a sample.

Several different types ofevents may take place as a result of the ion beam impinging

on a target surface. Some of these events include electron or photon emission,

electron transfer (both ion-surface and surface-ion),scattering, adsorption,

and sputtering (i.e. ejection of atoms from the surface). For each system and each

interaction there exists an interaction cross-section, and the study of these cross-

sections is a field in its own right. As the name suggests, LEIS is primarily concerned

with scattering phenomena.

[edit]Elemental composition and two-body collision model

Due to the energy range typically used in ion scattering experiments (> 500 eV), effects

of thermal vibrations,phonon oscillations, and interatomic binding are ignored since

they are far below this range (~a few eV), and the interaction of particle and surface

may be thought of as aclassical two-body elastic collision problem. Measuring the

energy of ions scattered in this type of interaction can be used to determine the

elemental composition of a surface, as is shown in the following:

Two-body elastic collisions are governed by the concepts

ofenergy and momentum conservation. Consider a particle with mass mx, velocity v0,


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and energy given as impacting another particle at rest with mass my.

The energies of the particles after collision are and

where and thus . Additionally, we

know . Using trigonometry we are able todetermine

Similarly, we know

In a well-controlled experiment the energy and mass of the primary ions (E0 and mx,

respectively) and the scattering or recoiling geometries are all known,

so determination of surface elemental composition is given by the correlation

between E1 or E2 and my. Higher energy scattering peaks correspond to heavier atoms

and lower energy peaks correspond to lighter atoms.

[edit]Getting quantitative

While obtaining qualitative information about the elemental composition of a surface is

relatively straightforward, it is necessary to understand the statisticalcross-section ofinteraction between ion and surface atoms in order to obtain quantitative information.

Stated another way, it is easy to find out if a particular species is present, but much

more difficult to determine how much of this species is there.

The two-body collision model fails to give quantitative results as it ignores the

contributions ofcoulomb repulsion as well as the more complicated effects of

charge screening by electrons. This is generally less of a problem in MEIS and RBS

experiments but presents issues in LEIS. Coulomb repulsion occurs between positively

charged primary ions and the nuclei of surface atoms. The interaction potential is givenas:

Where and are the atomic numbers of the primary ion and surface atom,

respectively, is the elementary charge, is the interatomic distance, and is the


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screening function. accounts for the interference of the electrons orbiting each

nucleus. In the case of MEIS and RBS, this potential can be used to calculate the

Rutherford scattering cross section :

Repulsive scattering by a point particle.

As shown at right, represents a finite region for an incoming particle, while

represents the solid scattering angle after the scattering event. However, for

LEIS is typically unknown which prevents such a clean analysis. Additionally,

when using noble gas ion beams there is a high probability of neutralization on

impact (which has strong angular dependence) due to the strong desire of these ions

to be in a neutral, closed shell state. This results in poor secondary ion flux. See AISS

and TOF-SARS below for approaches to avoiding this problem.

[edit]Shadowing and blocking


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Shadowing and blocking effects in two dimensions. No ions will be detected at angles below Primary

ions are approaching from the upper left.

Shadowing and blocking are important concepts in almost all types of ion-surface

interactions and result from the repulsive nature of the ion-nucleus interaction. As

shown at right, when a flux of ions flows in parallel towards a scattering

center(nucleus), they are each scattered according to the force of the Coulomb

repulsion. This effect is known as shadowing. In a simple Coulomb repulsion model,

the resulting region of forbidden space behind the scattering center takes the form of

a paraboloid with radius at a distance L from the scattering center.

The flux density is increased near the edge of the paraboloid.

Blocking is closely related to shadowing, and involves the interaction between

scattered ions and a neighboring scattering center (as such it inherently requires thepresence of at least two scattering centers). As shown, ions scattered from the first

nucleus are now on diverging paths as they undergo interaction with the second

nucleus. This interaction results in another shadowing cone now called a blocking

cone where ions scattered from the first nucleus are blocked from exiting at angles

below . Focusing effects again result in an increased flux density near .

In both shadowing and blocking, the "forbidden" regions are actually accessible to

trajectories when the mass of incoming ions is greater than that of the surface atoms

(e.g.Ar+

impacting Si orAl). In this case the region will have a finite but depleted fluxdensity.

For higher energy ions such as those used in MEIS and RBS the concepts of

shadowing and blocking are relatively straightforward since ion-nucleus interactions

dominate and electron screening effects are insignificant. However, in the case of LEIS

these screening effects do interfere with ion-nucleus interactions and the repulsive

potential becomes more complicated. Also, multiple scattering events are very likely

which complicates analysis. Importantly, due to the lower energy ions used LEIS is

typically characterized by large interaction cross-sections and shadow cone radii.For this reason penetration depth is low and the method has much higherfirst-layer

sensitivity than MEIS or RBS. Overall, these concepts are essential for data analysis

in impact collision LEIS experiments (see below).

[edit]Diffraction does not play a major role


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The de Broglie wavelength of ions used in LEIS experiments is given as .

Using a worst-case value of 500 eV for an 4He+ ion, we see is still only 0.006 , still

well below the typical interatomic spacing of 2-3 . Because of this, the effects

ofdiffraction are not significant in a normal LEIS experiment.

ICISS geometry and its relevance to structural characterization of surfaces. The direction and length of the

surface-subsurface bond may be determined from an intensity vs. plot. Red: determining the shape of the

shadow cone; Green: determining surface-subsurface spacing and direction with a known shadow cone shape.

[edit]Variations of technique

Depending on the particular experimental setup, LEIS may be used to obtain a variety

of information about a sample. The following includes several of these methods.

Alkali ion scattering spectroscopy (AISS) uses alkali ions in place of noble gas

ions to give a distinctly different type of interaction. The primary difference between

AISS and normal ISS is the increase in ion survival probability when using alkali

ions. This is due to the relative stability of alkali (+1) ions as opposed to noble gas

ions which have a much strongerenergetic incentive for abstracting electrons from

the sample. Increasing the ion survival probability results in an increase in

ion flux and an improvement in sensitivity, which in turn allows for a reduction in

primary ion flux to a point where the method is almost non-destructive. A

disadvantage of using alkali ions in place of noble gas ions is the increased

likelihood ofadsorption ordeposition to the sample surface.


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Impact-collision ion scattering spectroscopy (ICISS) takes advantage of

shadowing and blocking in order to make precisedeterminations about interatomic

spacing of the first 1-2 layers in a surface. The specific scattering geometry (180

degrees) ensures detection of only those particles which have undergone head-on

collisions with surface atoms (thereby avoiding the complications of multiplescattering events). Starting sampling at a relatively high angle of incidence and

scanning over varying incidence angles, the intensity of one particular energy peak

is monitored. Scattered ions form shadow cones (see above) behind each atom,

which prevents any backscattering at low incidence angles. A peak in scattering

intensity is observed when the cones line up such that each passes over the

adjacent atom. Performing such an analysis on a sample with known interatomic

spacing enables determination of the shape of the shadow cone, where as shown

at right, and .

A graph of intensity versus angle of incidence for scattering from a subsurface atom in the ICISS geometry.

The directionality of the surface-subsurface bond (see diagram above) may be deduced from . The

length of this bond may be deduced from and when the shape of the shadow cone is known.

If the shape of the shadow cone is known, the interatomic spacing between surface

atoms as well as the spacing and directionality between surface and subsurface

atoms can then be calculated from the resulting peak-and-valley structure in a

graph of intensity versus scattering angle. In the graph at right showing scattering

intensity from a subsurface (second layer) atom, corresponds to the middle of


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the "valley" where the atom is being blocked by a surface atom. and

correspond to the peaks due to intersection of the shadow cone with the

subsurface atom. Interatomic spacing can be directly calculated from these values

if the shape of the shadow cone is known.

Neutral Impact-collision ion scattering spectroscopy (NICISS) uses detection of

backscattered projectiles to determine concentration depth profiles of the elements.

The NICISS technique uses noble gas ions (usually He+) of energy 1-5 keV. When

the projectile ions are within a few angstrom of the surface they are neutralised,

and proceed to penetrate into the surface. The projectiles may be backscattered (at

an angle of up to 180) upon collision with a target atom. This backscattering

causes the projectiles to lose energy proportional to the mass of the target and is of

the order of a few hundred eV. The final energy of the projectiles is determined

via time-of-flight (TOF). Hence by knowing the initial and final energies of the

projectile, it is possible to determine the identity of the target atom. The projectiles

also experience an additional energy loss while penetrating through the bulk, of the

order of a few eV per angrstrom. Hence the depth that each target atom was hit can

also be determined. From the TOF spectrum it is then possible to gain the

concentration depth profiles of the elements present in the sample. NICISS is able

able to probe to a depth of approximately 20 nm with a resolution of only a few

angstrom.

Reactive ion scattering (RIS) utilizes a stream of very low-energy (1-100

eV) Cs+ ions to probe molecules adsorbed at the surface of a sample. Upon impact

the ions may interact with and chemically bind to species present at the surface.

These interactions take place on a rapid (picosecond) timescale and can be used to

analyze for the presence of different molecules or molecular fragments by

observing spectra of Cs-X+ coming from the surface.

Time-of-flight scattering and recoiling spectroscopy (TOF-SARS) uses the TOF

analysis setup. Elemental analysis may be performed via observation of in-planescattering, while structural information may be obtained by following certain spectral

peaks while shifting either sample incident or azimuthal angle.

Scattering and recoiling imaging spectroscopy (SARIS) takes advantage of

blocking cone geometries to focus ions in a manner similar to conventional optics.


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This gives very large magnifications (~109) when projected onto a 2-d detector and

may be used to give element-specific images of the sample surface. The use of a

wide 2-d MCP detector greatly reduces sample analysis time as opposed to the

TOF geometry with an inherently narrow-angle detector (see drift tube above). J.

Wayne Rabalais at the University of Houston is one of the pioneers of this method,and a fine image of the output of a SARIS experiment can be found here.

[edit]Comparison to other analytical techniques

Medium energy ion scattering (MEIS) and Rutherford backscattering (RBS)

spectroscopies involve a similar setup to LEIS but use ions in the energy range of

~100 keV (MEIS) and ~1-2 MeV (RBS) to probe surfaces. Surface sensitivity is lost

as a result of the use of higher energy particles, so while MEIS and RBS can still

provide information about a sample they are incapable of providing true first-layersensitivity.

Secondary ion mass spectrometry (SIMS) involves the detection of ionic species

ejected from a surface as a result of energetic particle impact. While SIMS is

capable of giving depth profiles of the elemental composition of a sample, it is an

inherently destructive method and is generally does not give structural information.

X-ray photoelectron spectroscopy (XPS) is capable of surface elemental analysis,

but samples a much more broad region of a sample than LEIS and so is not able todistinguish the first layer from subsurface layers. Since XPS relies on ejection

ofcore-level electrons from atoms it is unable to detect hydrogen orhelium atoms

in a sample.

Low-energy electron diffraction (LEED) is often used in combination with LEIS in

order to facilitate proper sample alignment. LEED can give detailed structural

information about a sample including surface superstructures and alignment

ofadsorbates. LEED is not element-specific and so cannot be used to determine

surface elemental composition.

Auger electron spectroscopy (AES) involves the detection of electrons emitted as a

result of core hole excitation and relaxation processes. Since the process involves

core levels it is insensitive to hydrogen and helium atoms. AES results may typically


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be used to infer information on the chemical environment of particular atoms in a

surface.[edit]

low energy ion scattering

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