paul ahern - time of flight secondary ion mass spectroscopy [tof-sims] theory & practice

Paul Ahern - http://www.linkedin.com/in/paulahern1

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) – Theory & Practice

Abstract—Time of Flight (ToF) Secondary Ion Mass

Spectroscopy (SIMS) is an extremely sensitive surface analysis

technique where the mass to charge ratio of an ion or molecular

fragment is determined by its velocity in the time domain. The

commercially available ToF SIMS instruments available today

have their roots in close to a century’s worth of academic

research, and their ability to gather elemental and molecular

information with excellent depth resolution and high sensitivity is

incomparable.

Keywords — Nanomaterials, thin films, surface analysis, Time

of Flight, Secondary Ion Mass Spectroscopy, SIMS.

I. INTRODUCTION

The ever-shrinking geometries of new materials and novel

devices built at the nanoscale with ever thinner layers has

driven the requirement for ever more sensitive and insightful

analytical techniques capable of viewing and understanding

matter at closer to the atomic level. Surface science

techniques are now more than ever a pre-requisite to

characterize and understand the complex interaction between

chemical composition and surface morphology of materials.

One such surface analysis instrument which has found

increasing application in the mainstream in the past decade is

Secondary Ion Mass Spectroscopy (SIMS), an experimental

technique which allows the analysis of a material in terms of

its molecular, chemical and elemental structure.

Figure 1 – Photograph of the ToF-SIMS instrument located

at the Environmental Molecular Sciences Laboratory

(EMSL) in Washington, USA (Image courtesy of

www.flickr.com/EMSL under Creative Commons license).

A sub-set of this technique is the so-called “Time-of-Flight”

or “ToF” method, which uses a more sophisticated time-

sensitive detection system to separate ions based upon their

mass, and has the advantage of increased sensitivity compared

to the more traditional magnetic sector or quadrupole

detectors. Detection limits of better than 1 part per million

(ppm) are achievable with excellent isotropic sensitivity and

three dimensional imaging1.

II. HISTORY OF TECHNIQUE DEVELOPMENT

The origins of the Time of Flight technique can be found in

the early experiments of J. J. Thomson in 1909. Thomson

observed that the discharge of a positive secondary ion could

be encouraged by bombardment of a metal surface with a

source of primary ions2. Further refinement of his apparatus

allowed Thomson to separate differently charged isotopes of

Neon with divergent atomic masses of 20 and 223.

K. S. Woodcock built upon the foundations laid by

Thomson, and in 1931 published his studies into the creation

of negative ion spectra from surfaces under positive ion

bombardment4. Woodcock carried out his measurements by

reflecting lithium ions off of a metal surface, with the

innovative step of using an applied electric field to slow down

the positive ions so that the negative ion spectra could be

recorded, a principle that is still used to this day to decelerate

secondary ions for SIMS analysis5.

In 1946 William Stephens of the University of Pennsylvania

had proposed the first concept of a linear “Time-of-Flight

Mass Spectrometer”6 at a meeting of the American Physical

Society. Stephens’s mass spectrometer separated ions based

on their relative speed as they travelled in a straight trajectory

towards a detector. Many would regard the work of Herzog

and Viehboeck of the University of Vienna published in 1949

as being similarly ground-breaking, as they were the first to

develop an instrument which used an electron impact ion

source7 to generate their primary beam, a progression that was

directly enabled by advances in vacuum pump technology in

the previous years.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) - Theory and Practice (April 2013)

Paul Ahern, School of Electronic Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland.

[email protected]

Paul Ahern - ie.linkedin.com/in/paulahern1

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Figure 2 – Schematic from William E. Stephens original

1952 patent for a “Time of Flight” spectrometer8.

Meanwhile, Wiley and McLaren9 built upon Stephens’s

study and via their Bendix Corporation released the first Time-

of-Flight mass spectrometer with a similar “EI” or “electron

ionisation” source in 1955, capable of sensitivities of less than

100 amu for the first time. Another notable development was

the work of Richard Honig of RCA Laboratories in New

Jersey, who through the search for new hot cathode materials

for vacuum tubes led him to assemble first a two-stage mass

spectrometer, and then in 1958 a full secondary ion mass

spectrometry instrument10

.

Further advances came in the 1960’s under the stewardship

of NASA who funded Liebel and Herzog of Geophysics

Corporation of America (GCA) Corp to create an analytical

instrument11

to examine moon rock samples brought back to

earth from the Apollo space flights. In 1967 Liebel, now of

Applied Research Laboratories, created an instrument with an

improved design12

which now used a duoplasmatron as the ion

source and mass separation to improve the purity of the

primary ion beam; the mass spectrometer design used a new

arrangement with no entrance slit and an Einzel lens.

In the early 1970’s Wittmaack13

demonstrated that the

secondary ion yield could be increased to a high value using

O2+ ions as the primary ion beam medium, and that there was

a self-induction effect due to the mechanism of recoil

implantation on the sample surface. The subsequent work by

Magee14

working again with Honig at RCA Labs showed

success with the first quadrupole mass analyser complemented

by a high current density 40

Ar+ primary beam, and presented

an instrument that was capable of speedy depth profiling

where an ultra-high vacuum environment helped to maximize

detection sensitivity. This was also the first time that the issue

of the large amount of output data from SIMS as an analytical

technique was addressed, as Magee’s system had computer

automated control and acquisition modules.

In the 1970’s Prof. Benninghoven15

and co-workers at the

University of Münster developed the first “static” SIMS

instrument, which was deemed to be much less harsh on the

surface under analysis than its dynamic SIMS counterpart.

The reason for the reduction in surface damage was the much

lower dose density of the primary ion beam on the sample

surface, with primary ion beam current densities of less than

1nA per cm2 being typical. This allowed slower rates of

material removal but maintained the high sensitivity which

ToF-SIMS would become noted for, as a sufficient amount of

spectral information could be captured quickly before the

sample surface was modified significantly16

, typically at levels

of less than 1%.

In common usage, though sometime the terms “static” and

“Time of Flight” are used interchangeably to describe this

subset of SIMS analysis techniques, they are actually very

different. The main difference being that in Time of Flight

SIMS, the atomic mass peaks are monitored and recorded

more or less simultaneously, whereas in static SIMS only one

peak can be measured at any one time. Benninghoven showed

the static technique was very useful to analyse organic

molecules which had been deposited onto conducting

substrates.

Subsequent improvements and refinements in the technique

have been directly due to the advances that have been made in

constructing high performance Time of Flight detectors, which

has seen it become a key technique in the study of surfaces,

especially those made up of organic materials.

III. OVERVIEW OF TECHNIQUE

To understand the operating principles of the ToF-SIMS

technique it is useful if we summarise the basic steps of the

process first, before we then break down the instrument into

it's constituent parts and describe the phenomenon happening

in each section in more detail.

Figure 3 – Atomic scale representation of the ion sputtering

process which takes place in SIMS when a primary ion

beam impinges up a sample surface17

.

Overview: In basic terms, the SIMS technique is one where a

vacuum is created and within this vacuum region a beam of

primary ions at an energy of hundreds to thousands of electron

volts (eV) is accelerated onto a focussed spot on the surface of

a sample to be analysed. Various elastic and inelastic

processes occur at the surface (and subsurface) atomic layers,


2


giving rise to the liberation of ionised atoms (secondary ions)

as part of a collision cascade, represented in figure 3.

As the cascade emanates outwards from the path of the

primary beam, its energy can affect atoms far away in the

surface layer with sufficient kinetic energy to break their

lattice bonds and be liberated from the surface as neutral ions,

ionised atoms or molecular clusters18

. The secondary ions are

extracted, accelerated, and fashioned into a beam and then

traverse a flight path of a known length which is long enough

that they have the opportunity to become time-focussed;

lighter ions arriving at a mess spectrometer detector and being

counted before their heaver contemporaries.

Figure 4 – Simplified diagram showing the instrument set-

up for secondary ion mass spectroscopy (SIMS)19

.

SIMS has the ability to detect all elements in the periodic

table, including Hydrogen. In SIMS, the analytical data is

contained in the current signal of an ion with a certain mass:

charge ratio, with little or no background signal to be removed

from the spectrum. Therefore, signal to noise is good and the

minimum detection limit in SIMS can be somewhere of the

parts per million (ppm) to parts per billion (ppb) scale.

A. Primary Ion Beam: The composition of the sample under

investigation is of prime importance, as bombardment from

the primary ion beam will give rise to the secondary ions

which must then be resolved in time. There is no set primary

ion beam medium which must be used for ToF-SIMS, and

many instruments employ the use of more than one beam to

give optimum material removal and sensitivity for the given

sample material

In terms of the atomic interaction of the primary ion beam

and the sample surface, the key factor is the difference in

electronegativity; that is, the ratio of the number of protons in

the nucleus and the number and separation distance of valence

electrons orbiting the atomic core. Consider the element

Boron, which has two shells, with 2 electrons in the innermost

{k} orbital and 3 in the outer {l} orbital. Compare Boron to

Aluminium, which is directly below it on the periodic table

and has 2 electrons in the {k} orbital, 8 in the{ l} orbital and 3

in the {m} orbital. The couloumbic attraction between the

nucleus and the electrons is stronger in Boron than in

Aluminium, as the atom is smaller and thus there is a smaller

distance between the electrons and the nucleus, leading to a

higher value of electronegativity (Boron has an

electronegativity value of 2 "Pauling units", while Aluminium

is 1.6).

In reality, it can be seen that electronegativity values have a

trend across the periodic table, increasing as you move to the

top right hand corner, as illustrated in figure 5 below.

Figure 5 – Pictorial representation of how electronegativity

values vary across the periodic table. Electronegativity is

the propensity for an atom to draw electrons from other

atoms, and rises from left to right and from the bottom to

the top of the periodic table. (Image © Wiley & Sons 2000,

all rights reserved).

The two most commonly used ion beam sources in ToF-

SIMS, Oxygen and Caesium, are widespread because they

generate a sufficiently high secondary ion yield for speedy

depth profiling are respectively positively and negatively

charged.; when using an Oxygen ion beam, the bombardment

of ions on the sample surface will increase the number of

positive ion fragments received at the detector, whilst for a

Caesium source you will increase the number of negative ions

collected.

In specialised instruments which are designed specifically

for image fine focussing techniques, a Gallium (Ga+) source

may also be employed as it has a lower melting point and thus

a higher achievable brightness which makes it worthwhile

despite its relative lower sensitivity. Gallium ions are created

in a liquid metal ion source (LMIS), as shown in figure 6 - the

same method that is used to create a Caesium (Cs+) beam.

Oxygen beams can be created by either using an electron

impact source or a duoplasmatron. In the case of a LMIS,

pulsing of the primary beam to achieve good mass resolution

can be achieved by the inclusion of a set of deflection plates to

quickly blank the beam on and off20

.


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Figure 6 – Diagram of the parts of a liquid metal ion source

(LMIS)21

with the parts as labelled (a) emerging metal ions

(b) extractor plate (c) liquid metal film (d) capillary feed

tube (e) liquid metal reservoir (f) crucible needle.

In electron impact sources, oxygen or another noble gas

flows into an ionisation region where a filament resides.

Electrons from this filament are then accelerated by anodic

grid while an extraction cathode (such as a Weinhalt aperture)

accelerates the ions to the lens array where they are focussed

and rastered if in imaging mode. In the case of a

duoplasmatron, which is common in dynamic SIMS

instruments also, plasma is formed in the extraction region and

a pair of off-setted magnetic lenses is used to form the primary

beam and tune it to a usable outline before it is extracted by

the anode.

Figure 7 – Block diagram of the primary ion column in a

typical SIMS instrument22

.

In instruments which have only one ion source, it is

common to use an element in the middle of the

electronegativity table which can successfully liberate both

negative and positive ions at sufficient amounts. Recent

advance have shown that the use of primary cluster ion

sources such as those derived from Bismuth (Bi1+

, Bi3+

) and

polyatomic Carbon (C60+

) have benefits when used to analyse

biological samples, with Bi3+

showing the best surface

sensitivity for lower atomic-mass molecular fragments23

.

Other recent peptide analysis work with Argon (Arn+

) primary

ion beam sources24

has shown its usefulness in terms of

minimising surface damage and allowing the user to maintain

a constant sputter rate with an associated reduction in

secondary ionization mechanisms.

Once the ions have been formed, they must then be

extracted and focussed into the primary beam before they are

accelerated to the required energy25

.

B. Beam / Sample Interaction: Once the primary ion beam

has been formed, it is focussed and pulsed in short (typically

between a 2ns and 20ns interval) onto the sample surface,

giving rise to sputtering. One consequence of this pulsed ion

beam regime is that a sufficient interval must be allowed so

that the heaviest, and thus slowest-moving, secondary ions can

vacate the detection region before the next bundle of

secondary ion data can be accepted.

Figure 8 – Diagram of some of the myriad and complex

phenomena at play during the interaction between the

primary ion beam and the sample in SIMS26

. The

progression of the ion cascade started by a sole primary ion

can be seen.

Ideally the primary ion beam pulse is kept as short as

possible, by using a device known as an electrodynamic pulse

“buncher” in the primary ion column as shown in figure 9, to

time focus the sputter pulse to a Gaussian profile of typically

less than 1 nanosecond (1ns)27

. This primary beam pulse

width value is termed tbeam, and one other consequence of

this arrangement is that the AC current is only a smaller

fraction of the DC current which further preserves the static

conditions on the sample surface allowing for better chemical

mapping.


4


Figure 9 – Simplified schematic diagram of the construction

of a linear buncher for Time of Flight analysis.

Ions with a small first ionisation potential will be readily

ionised by the primary beam, while those with a greater

potential will form positive ions much less readily; ions with

high electron affinity will preferentially give rise to negative

ions more readily. The timespan for the creation of these

secondary ions is typically in the picosecond (ps) regime.

Figure 10 – representation of total ion yield during

sputtering, {S}, and a function of the incident ion energy

{E}28

.

Ion images are produced in a slightly different way,

whereby the primary ion beam is rastered over the area of

interest and the number of ions as a function of the {x,y} co-

ordinates is presented. Secondary electrons generated as a by-

product of his process can also be collected in a resident

Everhardt-Thornley photomultiplier and used to generate a

standard secondary electron image.

The total sputter yield of secondary ions for a particular

element depends on the incident ion energy as shown in figure

10 and also the angle of incidence when the primary beam

strikes the sample surface as shown in figure 12; the vast bulk

of the species evolved from the sputtered surface will actually

be neutrals but it is only the very small positively or

negatively charge portion of the secondary particle flux that is

detected.

A complete understanding of the formation of secondary

ion species in ToF-SIMS has not yet been accepted, although

many competing thesis have been proposed in the literature

and have had limited success in predicting experimental

results for a narrow subset of defined materials.

i. Local Thermal Equilibrium (LTE) model: This (now

defunct) historical model, sometimes also categorised as the

“surface excitation model” proposed that underneath the

bombardment area, surface plasma was created wherein the

sputtered atoms became ionised.

Under equilibrium conditions, the ionisation potential could

be calculated by the use of the Saha-Eggert ionisation

equation29

in the bombardment region. The only important

factor was deemed to be the plasma temperature30

and this

could be estimated by taking into account the amounts of each

element present.

The results from this model are strictly semi-quantitative as

the exponential term in the S-E equation is compatible with

quantum mechanical terms31

. As V. E. Krohn32

of the US

Dept. of Energy laboratory in Argonne, Illinois summarised

succinctly “Unfortunately, surface ionization is an

equilibrium process, whereas secondary-ion emission is not.”

ii. Electron Tunnelling model: This model of secondary ion

formation (sometimes called the Schroeer model33

) is based on

quantum mechanical principles, and describes how the

electron which sits in the conduction band (CB) has the ability

to tunnel into the valence band (VB) of the ejected atom34

.

The ionisation potential of the sputtered element governs

the probability statistics that govern this phenomenon, as well

as the adiabatic surface ionisation function and the velocity of

the sputtered atom.

However, in line with this model, the work function is

autonomous of how the work function change, , is being

induced - as long as the external ions in the primary beam

used to induce does not change the chemical state of the

target atoms35

, as shown in figure 11 below.

Figure 11 – Simplified energy diagram of a charged particle

separating from a metal surface36

. At the distance ZC, the

atomic level intersects the Fermi level of the metal and

charge exchange can occur by tunnelling.


5


iii. Broken Bond model: In this model, proposed initially by

Šroubek37

the electronic temperature, Te, within the region of

the collision cascade was calculated for different materials

from a starting point of electron transport theory, and these

values were compared to calculated values formulated in

conjunction with empirical SIMS data.

The resultant model tackles the process from the standpoint

of the creation of ionic compounds in an idealised electronic

lattice under bombardment from a primary ion beam of

oxygen, and a prerequisite is that there be present an oxide

layer on the surface of the sample to be analysed. In this

model, the binding electrons stay with the oxygen atom and

there is only emission of positively ionised species.

Figure 12 – Secondary ion sputter yield plotted versus the

angle of incidence, , of the primary ion beam (as measured

referenced to the normal plane)38

.

In the theoretical case whereby all the available secondary

ions could be ionised by the incoming beam, and detected by

the spectrometer, then the signal could be related to the

specimen composition by the equation39

below:

iS = iPS

where iS is the sputtered ion current, is the atomic fraction of

the element of interest, iP is the primary ion beam current, S

the total effective sputter yield, and the specific sensitivity

of the detector (which includes the previously explained effect

of the incident angle of detection). Each spectrometer

configuration will also have an inherent angular acceptance

range and a reduction in mass resolution can be observed

when angular divergence becomes too large.

C. Secondary Ion Acceleration: Typically less than 1% of the

sputtered ions from the primary beam are ionised, with the

resultant cloud of ejected atomic and molecular secondary

ions accelerated by a potential into the Time-of-Flight region.

Since the lighter ions travel faster, they thus arrive at the ion

detection module first and can be counted. The Time-of-

Flight relationship can be understood simply as the travel time

of a secondary ion being proportional to the square root of its

mass, and by this mechanism all secondary ions can be

isolated and detected discretely once they impinge upon the

detector.

The drawing out and collimating of the secondary ion signal

emitted from the sample surface is by means of a combination

of transfer and immersion lenses which control the image

amplification and, by way of apertures, serve to limit the

angular acceptance angle of the mass spectrometer detector.

Secondary particles are accelerated by an applied constant

voltage, Vacc such that they now have a fixed kinetic energy

{E} which overrides their initial kinetic energy at source. In

this case their accelerated velocity in the drift region dpends

solely upon their mass, by the equation40

qVacc = E = ½ mv2

The overall secondary ion transmission can thus be greater

than 40%. To further increase the detector effectiveness for

dense ions (m>1,000 Da) a post –acceleration area (10-20

keV) can be sited between the end of the drift region and the

entrance cone of the detector.

D. Charge Neutralisation: Since many of the common

samples analysed by ToF-SIMS are organic molecules or

polymers and are non-conductive, a separate source of

electrons is needed to provide a tuneable charge compensation

function; this is achieved by producing low energy electrons

and introducing them around the analysis area on the sample

surface.

A recurring issue in achieving good spectral quality with

SIMS is adequate monitoring and control of the sample’s

surface potential41

to ensure that as uniform an ion emission

profile as possible is generated, as even a very slight increase

in the electric field on the sample surface can significantly

shift the energy level accepted by the energy filter into an

undesirable region, whereupon the ion yield being generated is

negligible and a impractical spectra with large intensity losses

will be recorded.

Typically a LaB6 or Tungsten filament electron source is

used, and in some cases the removal of surface charge can be

further facilitated by placing an earthed metal grid (such as a

TEM grid) over or close to the area of interest on the sample

surface. This method is useful as the pulsed low energy

electrons automatically steady themselves further over time, as

a positive surface potential serves to minimise the losses of


6


secondary ions which would serve to decrease the overall

potential of the surface42

. Another complication with

instruments that operate in the time domain such as ToF-SIMS

is that charge neutralisation must typically done in a pulsed,

cyclical manner in between the repeated sequences of

secondary ion generation and acceleration.

E. Mass Analysis Detection: Early SIMS instruments relied

upon the quadrupole mass spectrometer for detection,

consisting of four elements at equal distances whereby

alternating pairs of DC and RF voltages were applied to act as

a filter allowing only ions with a specific charge to mass ratio

to traverse into the detector. However, the disadvantages of

poor transmission (typically <1%, which decreases further

with increasing mass number) and a” lossy” data collection

method as it can only operate in scanning mode, meant that

better detection methods were needed for ToF-SIMS to

advance further.

i. Reflectron Analyser

Today the most common in commercial ToF-SIMS

instrumentation, mass-reflectron analysers were first proposed

by Russian physicists Mamyrin & Karataev43

in 1972. A

modern reflectron detector offers a good balance of much

enhanced mass resolution in a smaller equipment footprint44

.

Figure 13 – Schematic representation of the nascent Time of

Flight instrument (complete with reflectron) used by

Benninghoven and co-workers at the University of

Münster45

. The parts are labelled as shown – (a) Electron

ion source (in this case, Ar+). (b) Ga

+ liquid metal ion

source (LMIS). (c) Sample holder (temperature

controllable). (d) Secondary ion acceleration lens. (e)

Reflectron of the gridless type design. (f) Mass

spectrometer detector.

The method of operation is that the secondary ions are

accelerated towards an ion “mirror” which functions as an

electrostatic reflector that then turns the ions and reflects them

back towards the direction of the detector in a “folding”

arrangement, preserving the time-focussed nature of the ions

and enhancing the mass resolution by an order or magnitude.

The travel time is governed by the equation -

√ √

This setup offers mass resolution of the order of 10,000 amu

when a deceleration & re-acceleration grid is used, but by

moving to a gridless design superior mass resolution of up to

50,000 amu is reliably achievable46

. As measuring the usable

secondary ion yield can be problematic, transmission is

instead estimated using a Thomson distribution47

based on the

incoming kinetic energy of the primary beam; however ,this

approximation neglects any influence from detector efficiency.

The detector itself is normally a combination of a Faraday

cup and microchannel plate48

which contains an array of

electron multipliers, similar in design to the historic

channeltron. The signal amplification is by means of an

electron cascade from the inner lining of leaded glass when it

is struck by an incoming secondary ion with secondary

excitation being provided by a phosphor screen.

Figure 14 - Diagram of secondary ion detection in a ToF-

SIMS instrument with a combination of a Faraday Cup and

an Electron Multiplier49

; the bias voltage is equally divided

across the cathodes & anodes.

To boost the detection of slow-moving, heavy ions a pre-

acceleration region is often incorporated immediately before

the detection plate. Between these two complimentary

detectors the full dynamic range of the mass window can be

sufficiently covered – the Faraday cup being useful at high

count rates of >5x104 c/s and the electron multiplier at lower

intensities of <5x106 c/s.


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ii. TRIFT Analyser:

In a nutshell, a TRIFT (or TRIple Focussing Time-of-flight)

analyser uses three separate hemispherical electro static

analysers (ESA’s) to give an effective 270 drift path between

the secondary ion acceleration region and the detector. Each

of these ESA’s continually refocuses the secondary ions so

they can be resolved more accurately in the time domain at the

detector, allowing the maximum angular and energy

acceptance values.

TRIFT detectors excel at gathering high secondary ion

signal from all areas of the sample without appreciable

“shadowing”, and the resultant ion images have higher depth

of focus than reflectron captured images. High mass resolution

can be obtained with a low signal background, especially on

conductive samples with little topography such as the Silicon

wafers used for integrated circuit manufacturing (This mass

resolution can be of the order of ~ {m/m} >104 at 28

Daltons50

).

Figure 15 – Schematic of a typical set-up for static SIMS

with a TRIFT mass spectrometer51

. The 270 defection

path in the drift region can be clearly seen. (Image ©

Physical Electronic, MN, USA. All rights reserved)

F. Data Reduction & Analysis: All commercially available

SIMS instruments possess an in-built library of secondary ion

data for comparison to the sample being analysed. Sometimes

some ambiguity can be encountered when chemical

compounds of a similar mass overlap; one simple way of

sidestepping this limitation in a production environment is to

supply known “good” samples, so that the spectrum of known

material can be subtracted and only the difference signals of

interest remain to be further analysed. If this is not possible,

then another method of disambiguation is through the use of a

complimentary analytical technique such as Auger Electron

Spectroscopy (AES) or X-ray Photoelectron Spectroscopy

(XPS) may be needed.

There are numerous ways that the output data can be

presented from ToF-SIMS analysis. The most basic and easy

to understand is a standard {x-y} plot of positive or negative

ion intensity (counts at the detector) vs. a linear mass scale of

unified atomic mass units, (u), with the mass being recorded

either as atomic or molecular mass: charge ration (m/q) 52

also

called one “Dalton” – defined as “one twelfth of the mass of

an unbound neutral atom of carbon-12 in its nuclear and

electronic ground state” 53

, which is experimentally known to

be 1.660538921 (73 ) × 10−27

kg.

Sometimes it can be difficult to definitively allocate peaks

to one specific material due to the influence of isotopic effects

due to a large number of secondary isotopes for a particular

element; the sensitivity of the ToF-SIMS technique also

means that surface hydrocarbon contamination is readily seen

and the data must be refined in a sensible way before

meaningful analysis can proceed with the use of applicable

standards to help understand the complex spectra.

G. Vacuum System: Although not strictly a part of the

analytical technique, it is worth mentioning the importance of

the ultra high vacuum (UHV) environment and how it is

generated. Many of the early advances in sensitivity

improvement in mass spectroscopy can be related to

improvements in the vacuum environment, he most notable of

which came in the 1960's due to the invention of the turbo-

molecular pump by Becker54

. These pumps allowed the

realisation of an integrated vacuum system which allowed the

sample surface to be maintained free of contamination, and

maximised the inelastic mean free path that the primary and

secondary ion beams could traverse without encountering

losses due to gas phase scattering phenomena55

.

The vacuum in the analysis chamber of a modern ToF-

SIMS instrument can reach as low as 10-10 Torr and this is

achieved through a holistic vacuum system arranged in series

and using roughing pumps, turbo molecular pumps, ion pumps

and sublimation gettering pumps, all monitored by different

vacuum gauges and computer controlled. High temperature

“baking out” is also needed periodically to maintain adequate

system vacuum levels.


8


IV. SELECTED NOTABLE APPLICATIONS

As previously described, the complex nature of Time of Flight

spectral data means that often the technique is used in a

comparative rather than an absolute sense, with samples

analysed to show the differences between them as a method of

side-stepping the arduous task of trying to understand all the

uncertainties in the analytical results.

ToF-SIMS excels rather at being a very sensitive technique

with high surface specificity and resolution and it is in this

niche that it has found use in fields such as nanoelectronics,

composites, catalyst formulation, biomedical, and general

failure analysis. A select few of these applications are detailed

in the following sections.

Semiconductor Analysis: ToF-SIMS has found a natural home

in the labs of many semiconductor manufacturers where they

leverage the tools analytical capability very successfully,

especially in the arena of thin film analysis and defect

metrology. The ability of SIMS to perform analysis using

depth profiling has meant that three dimensional analyses of

defects and structures has become commonplace, if a little

time consuming. Several ToF-SIMS manufacturers provide

instruments with large, fast entry load locks with differential

pumping and five axes motorised stages which can hold and

navigate a 12 inch silicon wafers.

Figure 16 – Parallel ToF imaging used for defect identification

and analysis56

. Ion maps of C-, Si

3-, InO

- and others used to

show the presence and composition of a defect on a thin film

transistor (TFT) array ( Imaged using a 25 keV Ga+ primary

ion source over 128µm2 area).

Indeed ToF-SIMS allows the analyst to understand

delicate properties, for instance how far an implant has

progressed into the silicon lattice or what exact layer of the

fabrication process that a particle has been introduced. When

used in conjunction with defect isolation information from in-

fab metrology tools, ToF-SIMS can be an excellent method to

understand subtle defects which arise in the manufacturing

process and accurately pinpoint where they come from as

depth profiling allows you to etch back to the layer where the

defect was originally detected in-line even if the wafer has

progressed all the way to end of line electrical test & sort.

Figure 17 – A typical high mass resolution positive ion

ToF-SIMS spectra of a contaminated silicon wafer57

. A

plethora of different contaminants are shown, especially

hydrocarbons which ToF-SIMS is especially sensitive to.

More general analysis of molecular contamination of

incoming virgin silicon and of thin films deposited during the

fabrication process can also be done, and increasingly the

results are used as part of routine process monitoring in certain

sensitive parts of the process; for instance, when selective

epitaxial growth is being carried out, to ensure the starting

substrate is free from contamination; or to monitor surface

oxidation levels on electroplated copper backend layers which

have a narrow time window in which they can be allowed to

progress to the next process step.

Molecular Analysis: ToF-SIMS makes it possible to amply

detect ppb levels of low-volatility molecules such as

hydrocarbons, which is one of the reasons why it is so useful

for molecular and polymer analysis at the nano scale.

Everything from the composition of self-assembled

monolayers, plant herbicides, interplanetary meteorites and

photocopier paper have been analysed by ToF-SIMS.

Polymer chemists have relied upon ToF-SIMS techniques

for many years to understand how they can adjust a polymer’s

composition through process alterations and variations in

formulation. ToF-SIMS ion imaging allows them to visualise

the molecular structure of the surface polymer while depth

profiling brings an understanding of chemical distribution

with submicrometer resolution, for instance when blend and

copolymer thin films undergo understated modification to

their molecular structure during annealing processes.


9


Figure 18 – Ion images obtained by ToF-SIMS from the

“Nakhla” Martian meteorite58

.

Pharmacological & Biomedical Analysis: ToF-SIMS is a

common analytical technique in the field of pharmaceuticals

as it is well suited to the parallel analysis of tablets and

formulations which have numerous organic and inorganic

ingredients.

In the biomedical field, advances in charge neutralisation

and data interpretation have seen ToF-SIMS become more

widely utilised, especially in the analysis of biocompatible

surfaces and coatings. Typically cluster ion sources such as

bismuth are used to generate the secondary ions while caesium

or oxygen are employed as a sputtering medium for rapid

depth profiling.

Figure 19 below shows a typical application, where ToF-

SIMS is used to understand the coating homogeneity and drug

loading of a stent with the aid of reducing inflammatory stent

thrombosis. By calibrating the {x} axis from sputter time to

depth, the film thicknesses can be readily measured.

Figure 19 – Negative ion ToF-SIMS depth profile of a stent

coated with Paclitaxel on a Parylene substrate59

.

Biological samples can also be tackled through the use of

appropriate pre-treatments of the sample. The major

roadblock with adoption of ToF-SIMS in this field is the

requirement for an ultra-high vacuum environment, and the

still improving sensitivity for high mass ions which are typical

in bio-molecular analysis specimens. Figure 20 shows high

resolution parallel ion imaging by ToF-SIMS of human cancer

cells.

Figure 20 – Positive ion images by ToF-SIMS60

of human

breast cancer cells pre-treated with 2-amino-1-methyl-6-

phenylimidazo[4,5-b]pyridine and a fluorescent

carbocyanine dye.

Aside from ion imaging for qualitative analysis, depth

profiling has also be used for the study of single or groups of

cells where the high lateral resolution of the technique has

been very beneficial. Breitenstein61

and co-workers used ToF-

SIMS to profile the levels and types of amino acids present in


10


cells from a rat kidney, while Fletcher62

showed in 3D a

similar result but constrained within a single freeze-dried cell

using a primary beam of clustered C60

.

Figure 21 – Fletcher’s 3D ToF-SIMS depth profile using

C60

clusters of a single freeze-dried lipid cell, showing the

amino acid fragments attributable to different proteins.

V. CONCLUSION & FUTURE DIRECTION

ToF-SIMS as an analytical technique can very successfully be

used for the characterisation of nano-materials and films.

Using it’s combination of depth profiling, ion imaging and

static mode analysis it can give huge amounts of information

relating to chemical, molecular and elemental composition of

the sample under investigation.

This has arisen due to decades of development of all the

constituent parts which together make up the instrument; from

the vacuum system to the ion source, an understanding of the

interaction between the primary beam and the generation of

secondary ions, charge neutralisation, signal detection and

data analysis.

The recent adoption of new techniques, such as the use of

exotic cluster sources with highly sensitive mass

fragmentation patterns for negative ion analysis of ultra thin

layers at low impact energies, should pay dividends and

ensure that ToF-SIMS remains a workhorse for nano-

characterisation for the foreseeable future.

In the longer term, the continuing efforts to refine new

techniques related to ToF SIMS, such as the EU FP7

supported 3D NanoChemiscope tool housed in Switzerland

which blends a Time of Flight detector with an AFM tip

scanning approach to manipulate and analyse matter atom by

atom for the creation of in-situ spatial 3D depth analysis,

should see the skillsets of ToF mass spectrometry scientists

remain useful and in demand for some time to come.

In the rapidly advancing biological space, the advent of

tandem mass spectroscopy has been applied to ToF

instruments where ions have multiple disassociative steps

taking place over time in a hybrid instrument with both a

quadrupole and a Time of Flight detector. This approach has

proven to be very useful in analysing molecular peptide chain

arrays rapidly63

, and may one day be able to easily sequence a

strand of DNA to check for genetic diseases. New

instrumental designs are on the horizon that will allow a

purely DC primary ion beam64

that has many advantages for

use with clustered polyatomic sources.

Across all disciplines the challenge remains of forging a

deeper understanding of the shortcomings presented by low

ionisation efficiency, and the data deconvolution processes

necessitated by sputtering matrix effects in the sample.

VI. ACKNOWLEDGMENT

The author would like to thank Dr. Rajani K. Vijayaraghavan

of the School of Electronic Engineering in Dublin City

University for her proposal of this review topic, as well as her

patience in explaining many of the important theoretical

principles and concepts that underlie the ToF-SIMS analytical

technique as a tool for nanomaterial characterisation.


11


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49

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paul ahern - time of flight secondary ion mass spectroscopy [tof-sims] theory & practice

Technology

time of flight technique

positive secondary ion

flight mass spectrometer6

time of flight spectrometer8

secondary ions

tofsims instrument

socalled time

linear time