Secondary Ion Mass Spectroscopy

8/2/2019 Secondary Ion Mass Spectroscopy

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1. INTRODUCTION2. SPUTTERING3. INSTRUMENTATION

a. PRIMARY ION SOURCEi. DUOPLASMATRON

ii. Cs ION SOURCEb. PRIMARY ION COLUMNc. SECONDARY ION EXTRACTIONd. SECONDARY ION TRANSFERe. ION ENERGY ANALYSERf. MASS ANALYSER

i. QUADRUPOLEii. MAGNETIC SECTOR

iii. DOUBLE FOCUSSINGiv. TIME OF FLIGHT

g. DETECTORi. ELECTRON MULTIPLIER

ii. FARADAY CUPiii. IMAGE PLATEiv.

RAE IMAGE DETECTOR

4. MODES OF SIMSa. STATIC SIMSb. DYNAMIC SIMSc. IMAGING SIMS

5. SIMS SPECTRA6. DETECTION LIMIT


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SECONDARY ION MASS SPECTROSCOPY

Spectroscopy is the study of the interaction between matter and radiation. Mass spectrometers

use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from

each other. Mass spectrometry is therefore useful for quantitation of atoms or molecules and also for

determining chemical and structural information about molecules. Secondary ion mass spectrometry

(SIMS) is based on the observation that charged particles (Secondary Ions) are ejected from a sample

surface when bombarded by a primary beam of heavy particles.

This method can be used to acquire a variety of information about the surface, near-surface, or bulk

composition of the sample, depending on the instrumental parameters. If the rate of sputtering is

relatively low, a complete mass spectrum can be recorded to provide a surface analysis of the outermost

5 nm of the sample. This is often termed static SIMS. Although a useful mode of operation, it is not yet

a routine analytical technique. Alternatively, the intensity of one or more of the peaks in the mass

spectrum can be continuously recorded at a higher sputtering rate to provide an in-depth concentration

profile of the near-surface region. At very high sputtering rates, trace element or impurity analysis in the

bulk is possible. Finally, a secondary ion image of the surface can be generated to provide a spatially

resolved analysis of the surface, near surface, or bulk of the solid. Secondary Ion Mass Spectrometry

(SIMS) both use high energy atoms to sputter and ionize the sample in a single step. In these techniques,

ions are focused on the liquid or solid sample. The impact of this high energy beam causes the analyte

molecules to sputter into the gas phase and ionize in a single step .The exact mechanism of this process

is not well understood, but these techniques work well for compounds with molecular weights up to a

few thousand Daltons.

The technique of Secondary Ion Mass Spectrometry (SIMS) is the most sensitive of all the commonly-

employed surface analytical techniques - capable of detecting impurity elements present in a surface

layer at < 1 ppm concentration, and bulk concentrations of impurities of around 1 ppb (part-per-billion)

in favorable cases. This is because of the inherent high sensitivity associated with mass spectrometric-

based techniques.


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SPUTTERING

The bombardment of a solid surface with a flux of energetic particles can cause the ejection of atomic

species. This process is termed sputtering and in a more macroscopic sense, it causes erosion or etching

of the solid. The incident projectiles are often ions, because this facilitates production of an intense flux

of energetic particles that can be focused into a directed beam. However, in principle, sputtering (and

secondary ion emission) will also occur under neutral beam bombardment.

Physical sputtering is driven by momentum exchange between the ions and atoms in the

materials, due to collisions. The incident ions set off collision cascades in the target. When such

cascades recoil and reach the target surface with energy above the surface binding energy, an atom can

be ejected. If the target is thin on an atomic scale the collision cascade can reach the back side of the

target and atoms can escape the surface binding energy `in transmission'. The average number of atoms

ejected from the target per incident ion is called the sputter yield and depends on the ion incident angle,

the energy of the ion, the masses of the ion and target atoms, and the surface binding energy of atoms in

the target. For a crystalline target the orientation of the crystal axes with respect to the target surface is

relevant.

The sputtering, or ejection, of target atoms and molecules occurs because much of the momentum

transfer is redirected toward the surface by the recoil of the target atoms within the collision cascade.

Because the lifetime of the collision cascade produced by a single primary ion is much smaller than the

frequency of primary ion impingements (even at the highest primary ion beam current densities), this

process can be viewed as an isolated, albeit dynamic, event. The ejection of target atoms due to a single

binary collision between the primary ion and a surface atom occurs infrequently.

The primary ion undergoes a continuous energy loss due to momentum transfer, and to the

electronic excitation of target atoms. Thus, the primary ion is eventually implanted tens to hundreds of

angstroms below the surface. In general, then, the ion bombardment of a solid surface leads not only to

sputtering, but also to electronic excitation, ion implantation, and lattice damage.

Selective sputtering, or preferential sputtering, can occur in multicomponent, multiphase, or

polycrystalline materials. Thus, it is possible for the composition of alloy surfaces to become modified

during sputtering where the species with the lowest sputtering yield become enriched in the outermost

monolayer, while the species with the highest yield are depleted. In the case of multiphase materials,

those phases with the higher yield will be preferentially etched; this alters the phase composition at the


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surface, and introduces microtopography and roughness. For polycrystalline materials, the variation in

sputter yield with crystallographic orientation can also lead to the generation of surface roughness

during sputtering. All of these effects can influence the quality and interpretation of a SIMS analysis.

INSTRUMENTATION

A basic SIMS instrument will consist of:

A primary beam source (usually O2+, O-, Cs+, Ar+, Ga+ or neutrals) to supply the bombardingspecies.

A target or sample that must be solid and stable in a vacuum. A method of collecting the ejected secondary ions. A mass analyzer to isolate the ion of interest (quadrupole, magnetic sector, double focusing

magnetic sector or time of flight).

An ion detection system to record the magnitude of the secondary ion signal (photographic plate,Faraday cup, electron multiplier or a CCD camera and image plate).

PRIMARY ION SOURCE

Modern SIMS instruments are equipped with a duoplasmatron, a Cs ion source, or a Ga ion

Source

Duoplasmatron

The Duoplasmatron can operate with almost any gas including air. Oxygen is commonly used because it

enhances the yield of electropositive elements such as Al, Si, etc. The duoplasmatron may be used to

extract either O-O

2-or O

2+depending upon the electrical polarity selected by the operator. In negative

mode O-is the most abundant species, while in positive mode O

2+is most abundant. When insulating

samples are analyzed, O- has the advantage of preventing charge build-up on the sample surface. Many

ion species are generated within the duoplasmatron and it is left to the primary beam mass filter to select

the desired ion beam.

Cs ion source

Cs ion beams are generated by a surface ionization process. Cs vapor is produced by the heating of a

solid Cs compound (Cs-chromate or Cs-carbonate). The Cs vapor travels along the drift tube and strikes


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a tungsten plate where it is thermally ionized. Any atom or molecule coming from the reservoir is forced

to bounce between the tungsten plate and the ionizer tip. This results in most atoms being ionized and

escaping through the small hole in the cap.

Cs ion beams are used to enhance the yield of electronegative elements such as C, O, and S, etc. within

the target. The Cs gun can only operate in positive mode. In general Cs beams are smaller than those

generated by the duoplasmatron, and sputter material more effectively because of their greater mass.

However, the Cs gun is expensive to operate and is only routinely used for oxygen, sulphur or carbon

isotopic analysis. When insulators are analyzed some method of neutralizing the positive charge build-

up, created by the Cs+

beam, is required.

In the case of the primary ion beam, several features are of interest:

1. The ability to generate reactive primary ions, for example, 2 O+, O-, and Cs+ requires aduoplasmatron or liquid metal source; the more conventional electron-impact ion guns typically

used in surface analysis are limited to inert species (Ne+

or Ar+). The negative oxygen species

can be of great benefit for the analysis of insulators.

2. The beam diameter and maximum current density characteristics will vary with the type anddesign of the gun. Simple ion guns usually provide beam diameters (spot sizes) of the order of 25

to 250 m; dedicated ion microprobes and ion microscopes can produce spot sizes as small as 1 to

2 m.

3.

The background levels that can influence detection limits are controlled to some degree by theion source. This refers not only to residual gases, but also to metallic species that can become

incorporated in the beam, implanted in the sample, and then ultimately appears in the analysis of

the sample. The ability to pump the ion source differentially (relative to the sample chamber) can

greatly alleviate the background due to residual gases and other contaminants produced in the ion

source.

However, the metallic and other impurities that become ionized and thus incorporated in the primary

ion beam are most effectively eliminated using a primary beam mass filter, essentially a mass

spectrometer that filters the primary ions before they strike the specimens.

A primary beam mass filter will also reject any neutral species in the beam. The use of a primary beam

mass filter is almost a necessity for trace element and low-level dopant analyses. Finally, rastering of the


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primary ion beam over the specimen surface is a requirement in SIMS depth profiling and secondary ion

imaging.

PRIMARY ION COLUMN

The primary ions generated by the ion source are passed to the sample via the primary column. A typical

column consists of a mass filter, apertures, lenses and deflection plates. Their function is to filter, focus,

shape, position and raster the primary beam. The primary beam mass filter eliminates any impurities in

the gas or generated in the ion source. In the case of the duoplasmatron the filter removes O, N, Fe and

Ni. Without the mass filter these and other components would be implanted into the sample surface,

increasing their detection limit. With the mass filter only oxygen ions are allowed to bombard the

sample.

The quality of the vacuum environment during SIMS analysis is important for two reasons.

Any variations in the vacuum pressure can influence the secondary ion intensity due to theeffects of reactive species on ionization probability and, to a lesser extent, on sputtering rate.

Thus, day-to-day reproducibility and the quality of the depth profile, which must be acquired

under constant vacuum pressure, will be affected.

The pressure and composition of the vacuum environment will influence the background levelsand therefore the detection limits for some species. This is usually due to their adsorption onto

the surface and their subsequent ejection as secondary ions. The background species of most

concern are hydrogen, oxygen, and water, because these are readily adsorbed and quite

prevalent in most vacuum systems. Ideally, pressure of the order of 10-9

to 10-10

torr should be

maintained at the specimen surface, especially for hydrogen, oxygen, or carbon analysis.

SECONDARY ION EXTRACTION

Secondary ions are formed at the sample surface by the bombardment of the primary beam. These

secondary ions are immediately removed by an extraction, or immersion lens. In order to obtain a

constant secondary ion beam current, the potential difference between the sample and the extraction

(immersion) lens, must be kept constant. With an insulating sample this is partially achieved by coating

the surface with a thin layer (


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Ideally, the secondary ions are extracted electrostatically from the center of the crater (over the

maximum solid angle) instead of simply collecting that portion of the ejected flux of secondary ions that

intersects the entrance aperture of the analyzer. Many dedicated SIMS instruments include an extraction

lens over the sample, but this limits the sample geometry that the instrument can tolerate and the

capability for a multi-technique apparatus. The extracted secondary ions are then energy analyzed to

select the optimum portion of the energy distribution for subsequent mass filtering; if this is not done,

mass resolution is severely degraded. This is an absolute necessity for quadrupole spectrometers and

single-focusing magnetic sectors, whereas an electrostatic energy filter is an inherent component of

higher resolution double-focusing magnetic mass spectrometers.

SECONDARY ION TRANSFER

After the secondary ions have been extracted from the sample surface by the immersion lens they are

transferred by a second electrostatic (transfer) lens into the mass spectrometer. The purpose of this

transfer lens is to form a real magnified image of the sample surface at the position of the field aperture

and to focus the secondary ion beam onto the entrance slit of the spectrometer. At the same position as

the entrance slit is the contrast aperture. Smaller contrast apertures intercept ions with off-axis

components, resulting in greater spatial resolution but reduced ion intensities. The immersion lens and

the transfer lens together form the ion microscope, which enables an image to be viewed by an

appropriate detector at the position of the field aperture.When the primary beam raster is large, the secondary ion beam can, at times, be off axis. This

result in aberrations at the entrance slit of the spectrometer and prevents clean separation of different

masses. This is corrected using the dynamic transfer plates. These deflect all the secondary ions formed

during the raster, back on axis so they pass through the centre of the transfer lens. However, the direct

imaging capability in microscope mode is lost, but high mass resolution may be achieved for any raster

size.

ION ENERGY ANALYSER

Secondary ions generated during the sputtering process have a wide range of energies. As these

secondary ion pass into the electrostatic energy analyser, the lower energy ions are more strongly

deflected than the high energy ions. A movable energy slit placed after the energy analyser can select a

small portion of the dispersed secondary ions and allow them to pass into the magnetic analyser.


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The inner and outer spherical electrode surfaces of the energy analyser have voltages of opposite

polarity; which is positive and which negative depends on the polarity of the secondary ion beam. As a

consequence of the sputtering process, molecular species are abundant at low energies, while the mono-

atomic species dominate the higher energy spectrum. By moving the energy window so that only the

higher energy ions are accepted, the molecular and often unwanted species can be suppressed. An

identical result may be achieved by lowering the sample voltage and keeping the energy window

centered.

An electrostatic spectrometer lens is placed between the energy analyzer and the magnetic

analyzer. It is an electrostatic lens that aligns the energy filtered ion beam into the magnetic analyzer.

MASS ANALYSER

After ions are formed in the source region they are accelerated into the mass analyzer by an electric

field. The mass analyzer separates these ions according to their m/z value. The selection of a mass

analyzer depends upon the resolution, mass range, scan rate and detection limits required for an

application. Each analyzer has very different operating characteristics and the selection of an instrument

involves important tradeoffs. Analyzers are typically described as either continuous or pulsed.

Continuous analyzers include quadrupole filters and magnetic sectors. These analyzers are similar to a

filter or monochromator used for optical spectroscopy. They transmit a single selected m/z to the

detector and the mass spectrum is obtained by scanning the analyzer so that different mass to chargeratio ions are detected. While a certain m/z is selected, any ions at other m/z ratios are lost, reducing the

S/N for continuous analyzers.

QUADRUPOLE

The quadrupole mass spectrometer is the most common mass analyzer. Its compact size, fast scan rate,

high transmission efficiency and modest vacuum requirements are ideal for small inexpensive

instruments. Most quadrupole instruments are limited to unit m/z resolution and have a mass range of

m/z 1000. Many bench top instruments have a mass range of m/z 500 but research instruments are

available with mass range up to m/z 4000.

In the mass spectrometer, an electric field accelerates ions out of the source region and into the

quadrupole analyzer. The analyzer consists of four rods or electrodes arranged across from each other.

As the ions travel through the quadrupole they are filtered according 16 to their m/z value so that only a

single m/z value ion can strike the detector. The m/z value transmitted by the quadrupole is determined


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by the Radio Frequency (RF) and Direct Current (DC) voltages applied to the electrodes. These voltages

produce an oscillating electric field that functions as a band pass filter to transmit the selected m/z value.

The RF voltage rejects or transmits ions according to their m/z value by alternately focusing them in

different planes.

MAGNETIC SECTOR

The first mass spectrometer used a magnet to measure the m/z value of an electron. Magnetic

sector instruments have evolved from this same concept. Sector instruments have higher resolution and

greater mass range than quadrupole instruments, but they require larger vacuum pumps and often scan

more slowly. The typical mass range is to m/z 5000, but this may be extended to m/z 30,000.

Magnetic sector instruments separate ions in a magnetic field according to the momentum and

charge of the ion. Ions are accelerated from the source region into the magnetic sector by a 1 to 10 kV

electric field. This acceleration is significantly greater than the 100 V acceleration typical for a

quadrupole instrument. Since the ions are charged, as they move through the magnetic sector, the

magnetic field bends the ion beam in an arc. This is the same principal that causes electric motors to

turn. The radius of this arc (r) depends upon the momentum of the ion (), the charge of the ion (C) and

the magnetic field strength (B).

Ions with greater momentum will follow an arc with a larger radius.

The velocity of an ion is determined by the acceleration voltage in the source region (V) and the mass to

charge ratio (m/z) of the ion. Equation 2 rearranges to give the m/z ion transmitted for a given radius,

magnetic field, and acceleration voltage as:

Only one m/z value will satisfy Equation 3 for a given radius, magnetic field and acceleration voltage.

Other m/z ions will travel a different radius in the magnetic sector.

ELECTRIC SECTOR/DOUBLE FOCUSING

An electric sector consists of two concentric curved plates. A voltage is applied across these

plates to bend the ion beam as it travels through the analyzer. The voltage is set so that the beam follows


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the curve of the analyzer. The radius of the ion trajectory (r) depends upon the kinetic energy of the ion

(V) and the potential field (E) applied across the plates.

This shows that an electric sector will not separate ions accelerated to a uniform kinetic energy. The

radius of the ion beam is independent of the ion's mass to charge ratio so the electric sector is not useful

as a standalone mass analyzer. An electric sector is, however, useful in series with a magnetic sector.

The mass resolution of a magnetic sector is limited by the kinetic energy distribution ( V) of the ion

beam. This kinetic energy distribution results from variations in the acceleration of ions produced at

different locations in the source and from the initial kinetic energy distribution of the molecules. An

electric sector significantly improves the resolution of the magnetic sector by reducing the kinetic

energy distribution of the ions. These high resolutions experiments are discussed in the section on mass

spectral interpretation.

TIME-OF-FLIGHT

The time-of-flight (TOF) mass analyzer separates ions in time as they travel down a flight tube. This is

a very simple mass spectrometer that uses fixed voltages and does not require a magnetic field. The

greatest drawback is that TOF instruments have poor mass resolution, usually less than 500. These

instruments have high transmission efficiency; no upper m/z limit, very low detection limits, and fast

scan rates. For some applications these advantages outweigh the low resolution. Recent developments in

pulsed ionization techniques and new instrument designs with improved resolution have renewed

interest in TOF-MS.

DETECTORS

Detection of ions is based upon their charge or momentum. For large signals a faraday cup is used to

collect ions and measure the current. Older instruments used photographic plates to measure the ion

abundance at each mass to charge ratio. Most detectors currently used amplify the ion signal using a

collector similar to a photomultiplier tube. These amplifying detectors include: electron multipliers and

multichannel plates. The gain is controlled by changing the high voltage applied to the detector. A

detector is selected for its speed, dynamic range, gain, and geometry. Some detectors are sensitive

enough to detect single ions.


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ELECTRON MULTIPLIER

The electron multiplier is the most sensitive detector. If protected from stray ions, neutrals and cosmic

rays, then the background count rate is normally less than 0.01 counts per second (c/s). However, the

multiplier must also be protected from intense ion beams (>5x106 c/s) as these can rapidly lead to its

destruction. An electron multiplier consists of a series of electrodes called dynodes. Each dynode is

connected to a resistor chain. The first dynode is at ground potential, so that both positive and negative

ions may be detected. The last dynode can be between +1500 to +3500 V depending on the age and type

of multiplier. When a particle (electron, neutral, ion etc.) strikes the first dynode it may produce a few

(1, 2 or 3) secondary electrons. These secondary electrons are accelerated to the second dynode that is

held at a slightly higher positive potential. On impact more secondary electrons are generated and a

cascade of secondary electrons ensues.

FARADAY CUP

A Faraday cup detector can detect count rates from 5x104

c/s upwards. Unlike the electron multiplier it

does not discriminate between the type of ion or its energy. It is simple and cheap, but its response time

is slow.

The Faraday cup detector consists of a hollow conducting electrode connected to ground via a

high resistance. The ions hitting the collector cause a flow of electrons from ground through the resistor.

The resulting potential drop across the resistor is amplified. A plate held at about -80 V in front of the

collector, prevents any ejected secondary electrons from escaping and causing an anomalous reading. Asingle charge on a single ion is 1.6x10 -19 C. Therefore a count rate of 1x106 c/s would produce a current

of 1.6x10-13

Amps. With a resistor of 10 MW connected to ground, the amplifier must be able to detect a

potential drop of 1.6x10-6

V (0.0016 mV). The detection limit of the Faraday cup is limited by the

thermal noise in the resistor and the quality of the amplifier. Often these components will be enclosed

within an evacuated, thermally controlled chamber.

IMAGE PLATE

An ion image plate consists of an array of miniature electron multipliers composed of lead glass.

Typically the electron multipliers, or channels, are about 10 m in diameter, 400 m long and about 7O

from the perpendicular to the plate face. They are located about 12 m between centres and number up

to 2000 in a 25 mm array. The front face of the plate is held at ground potential, while the back plate

may be between +1000 to +2000 V.


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An ion passing down a channel hits the inner channel wall and produces secondary electrons. The

channels are designed so that these secondary electrons initiate an electron cascade down the channel.

The pulse of electrons from the back of the detector may either be passed to a second micro channel

plate for further gain, or accelerated towards a phosphor screen, where their impact may be viewed

directly.

RAE IMAGE DETECTOR

The resistive anode encoder is a position-sensitive detector. It is used to digitally record ion images. The

background count rate is high, but constant over a period of time, and the maximum count rates must be

less than 4x104

c/s. Because it uses a micro-channel plate for the ion to electron conversion, the detector

discriminates between species. An ion enters a channel in the first of two micro-channel plates. The ion

to electron conversion results in a pulse of electrons that emerge from the back of the first plate to

initiate a second electron cascade in the channels of a second plate. The resulting electron pulse strikes a

resistive plate comprising a thick resistive film, deposited on a ceramic plate. The geometry of the

detector is designed to avoid image distortion. The charge pulse is partitioned off to four electrodes at

the corners of the plate.

MODE OF SIMS

There are a number of different variants of the technique:

Statics SIMS : used for sub-monolayer elemental analysis Dynamic SIMS : used for obtaining compositional information as a function of depth below the

surface

Imaging SIMS : used for spatially-resolved elemental analysis

STATIC SIMS

In static SIMS (SSIMS) the aim is to obtain sufficient signal to provide a compositional analysis of the

surface, without actually removing a significant fraction of a monolayer. This requires the use of very

low ion fluxes (around 1012 cm-2) to ensure that each ion is statistically-likely to impact upon fresh,

undamaged surface and that the sputtered secondary ions are representative of the original surface,

rather than surface that has already been "damaged" by earlier ion impacts. In this form, the technique is


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capable of providing information about adsorbed molecular layers or the topmost atomic layer of the

solid surface.

Dynamic SIMS or DEPTH PROFILE

Dynamic SIMS is the process involved in bulk analysis, closely related to the sputtering process, using a

DC primary ion beam and a magnetic sector or quadrupole mass spectrometer. The primary ion beamexceeds the "static limit" (~1E12 ions/cm2) producing a high yield of secondary ions.

The aim of depth profiling is to obtain information on the variation of composition with depth

below the initial surface - such information is obviously particularly useful for the analysis of layered

structures such as those produced in the semiconductor industry.

Since the SIMS technique itself relies upon the removal of atoms from the surface, it is by its very

nature a destructive technique, but also, ideally suited for depth profiling applications. Thus a depth

profile of a sample may be obtained simply by recording sequential SIMS spectra as the surface is

gradually eroded away by the incident ion beam probe. A plot of the intensity of a given mass signal as a

function of time is a direct reflection of the variation of its abundance/concentration with depth below

the surface.

The ionization yield of most elements varies by decades, depending on the chemical environment. This

property is used in SIMS instruments to increase the sensitivity of the technique: a dynamic SIMS

instrument 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 tothe use of noble gas ions. When sending ions onto a solid surface, atleast three phenomena occur

simultaneously:

Secondary particles ejected by collision cascades from the top atomic monolayers of the target.

Ionization of a small fraction of the secondary particles,

Primary ion implantation in the solid (and associated change of composition, surface work

function, etc...).

Starting from the surface (or going through an interface), while the primary ion dose implanted in the

target increases; the primary species concentration (oxygen or cesium) will reach an equilibrium

depending on the sputtering conditions and the nature of the target. This equilibrium corresponds to a

sputtering steady state, and as soon as it is achieved, reliable quantification is possible with reference

standard samples, using Relative Sensitivity Factors.


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One of the main advantages that SIMS offers over other depth profiling techniques (e.g. Auger depth

profiling) is its sensitivity to very low (sub-ppm, or ppb) concentrations of elements - again this is

particularly important in the semiconductor industry where dopants are often present at very low

concentrations.

The depth resolution achievable (e.g. the ability to discriminate between atoms in adjacent thin layers) is

dependent upon a number of factors which include:

1. The uniformity of etching by the incident ion beam

2. The absolute depth below the original surface to which etching has already been carried out

3. The nature of the ion beam utilized (i.e. the mass & energy of the ions)

as well as effects related to the physics of the sputtering process itself.

Imaging SIMS or ION IMAGING

The acquisition of secondary ion images can be accomplished in one of two ways, depending on

the instrument design.

In the case of the ion microprobe, the incident primary ion beam is focused to a small spot, then

rastered over the sample surface. The analysis is carried out point by point, and the image is constructed

by synchronizing the cathode ray tube (CRT) and the detector with the primary ion beam. That is, a

particular secondary ion (for example, 27Al+, FeO+, and so on) is selected at the mass spectrometer, and

its intensity variation from point to point is displayed on the CRT. This approach is analogous to thatused for x-ray imaging in the electron microprobe. The resolution is determined by the diameter of the

primary ion beam, typically 2 to 5 m.

In contrast, the ion microscope is a direct-imaging system in which the secondary ions are

simultaneously collected over the entire imaged area. A strong electrostatic field between the specimen

and the immersion lens preserves the spatial distribution of the emitted secondary ions. This secondary

ion beam is then mass analyzed (only a double-focusing magnetic mass spectrometer is applicable for

direct imaging), and in essence, the spatial distribution of the secondary ions is transmitted through the

mass spectrometer. The transmitted secondary ions (for example, 27Al+, FeO

+, and so on) then strike a

micro channel plate, where the image is formed. The direct-imaging ion microscope can be compared to

the transmission electron microscope or emission microscope. The resolution is determined by the optics

of the system, for which the size of the imaged area on the specimen, that is, the field aperture, and the

contrast aperture are major factors; however, the resolution is typically of the order of 0.5 to 1 m.


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SECONDARY ION MASS SPECTRA

A secondary ion mass spectrum will usually be presented as a vertical bar graph, in which each bar

represents an ion having a specific mass-to-charge ratio (m/z) and the length of the bar indicates the

relative abundance of the ion (intensity). The most intense ion is assigned an abundance of 100, and it is

referred to as the base peak. Most of the ions formed in a SIMS have a single charge (use of energy

filter), so the m/z value is equivalent to mass itself. Modern mass spectrometers easily distinguish

(resolve) ions differing by only a single atomic mass unit (amu), and thus provide completely accurate

values for the molecular mass of a compound. The highest-mass ion in a spectrum is normally

considered to be the molecular ion, and lower-mass ions are fragments from the molecular ion, assuming

the sample is a single pure compound.

Relative intensities depend on many factors in which the primary ion used is most important.

Primary ion used are divided into three broad categories-

Ions that improve electropositive yield- like oxygen inertlike argon Ions that improve electronegative yield- like Cs

A SIMS spectrum is very complex because of the multitude of the molecular secondary ion matrix

species. Also the presence of the molecular species causes mass spectra. Over which the use of oxygen

ion as primary ion precludes oxygen analysis in the specimen. This causes the interference which

prevents the identification of the trace elements present in the matrix of the sample under observation.

This background can be eliminated completely by energy filtering of the secondary ions. Basically

energy permits only high energy ion to pass blocking the entire low energy ion (dominated by molecular

ions) which generally forms the background.

The spectra generally represent the bulk composition of these samples. This is due in part to the nature

of the specimens, which were not subjected to any surface treatment, but it is also a result of the analysis

mode. The use of an oxygen (or any reactive) primary ion beam requires the attainment of a steady state

in the surface and the detected signal. During this transient time period, the reactive primary ions are

implanted in the surface up to a steady-state concentration determined by the primary beam energy and

flux, the target material, and the total sputtering rate. The ion yields are changing during this period;

therefore, the SIMS spectra generated during the first few minutes of sputtering (in the outermost 10 to

20 nm) are of little analytical value. Moreover, the ability to detect very low concentration levels


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requires the use of high sputtering rates when data acquisition in the surface would be difficult even in

the absence of the transient effect. Clearly, the data represent trace element analyses in the near surface

or bulk, not true surface analyses.

DETECTION LIMITS

It has been emphasized that one of the unique characteristics of SIMS is its high detection sensitivity

relative to other surface micro analytical techniques. Nonetheless, it is important to recognize that the

detection limit varies with the element, the sample matrix, and the operating procedures of the

instrument (primary ion, sputter rate, instrument transmission, and background). For example, the

detection limit in metallic, semiconducting, and other nonoxide matrices will be maximized only when

reactive oxygen or cesium beams are used. When high spatial resolution, high depth resolution, or high

mass resolution is also required, there will be some loss in detection sensitivity. There are at least two

reasons for this.

One of these concerns the rate of material consumption, because this ultimately determines the total

secondary ion count rate. If high spatial resolution is required in a microanalysis problem, for example, a

reduction in the beam diameter will be necessary. This is often accompanied by a decrease in the beam

current and therefore a decrease in the rate of material consumption, even though the sputtering rate may

increase. Similarly, a loss in sensitivity occurs when is reduced at a constant spot size to enhance the

resolution of in-depth profiles.The other reason for less than optimum detection sensitivity concerns the collection and transmission of

the secondary ions. Any electronic or mechanical gating of the sputtered crater, such as that required for

high-resolution in-depth profiling, will reduce the detected signal. Similarly, the need for high mass

resolution will reduce the ion transmission in the mass spectrometer and again result in a loss in

detection sensitivity.

REFERENCE

ASM Handbook, vol 10, material characterization

Microstructural Characterization of Materials-David Brandon & Wayne D. Kaplan

An introduction to mass spectrometry, Scott E. Van Balmer


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IMAGES

Typical schematic of a dynamical SIMS instrument. High energy ions are supplied by an ion gun (1 or

2) and focused on to the target sample (3), which ionizes and sputters some atoms off the surface. These

secondary ions are then collected by ion lenses (5) and filtered according to atomic mass (6), then

projected onto an electron multiplier (7, top), Faraday cup (7, bottom), or CCD screen (8)

The physical effects of primary ion bombardment: implantation and sputtering


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Schematic diagram of the sputtered species ejected during primary ion bombardment of a compound ixjy.

These sputtered species may be monatomic, molecular, and/or incorporate implanted primary ions

Duoplasmatron

Cs ion source


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Secondary ion extraction

Secondary ion transfer

Ion energy analyser


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Quadrupole

Magnetic sector


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Double Focussing

Time of flight

Electron multiplier

Faraday Cup


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Image plate

RAE image detector

Example: static SIMS spectra from the surface of PTFE (polytetrafluoroethylene)

Note the substantial differences between the positive and negative ion spectra. In each case the most

intense peaks correspond to relatively stable positive and negative ions respectively.


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Positive SIMS spectra (in the form of a bar graph) for high-purity silicon under oxygen bombardment inan ion microscope

Positive SIMS spectra for an organometallic silicate film deposited on a silicon substrate. Obtained

using a scanning ion microprobe under inert argon bombardment

Secondary Ion Mass Spectroscopy

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