secondary ion mass spectrometry(sims)

7/22/2019 Secondary Ion Mass Spectrometry(SIMS)

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Secondary Ion

MassSpectrometry(SIMS)

BY

Sangeet Gupta(11BPE044)

Dishank Patel(11BPE048)Deep Chaudhari(11BPE058)

Shashank Jaujaniya(11BPE100)


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Secondary Ion Mass Spectrometry

(SIMS)-An Introduction

Secondary ion mass spectrometry (SIMS) isbased on the observation that charged

particles (Secondary Ions) are ejectedfrom a sample surface when bombardedby a primary beam of

heavy particles.


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What it consist of?A basic SIMS instrument will, therefore,consist of: A primary beam source to supply thebombarding species.

A target or sample that must be solid andstable in a vacuum. A method of collecting the ejectedsecondary ions.

A mass analyser to isolate the ion ofinterest.

An ion detection system to record themagnitude of the secondary ion signal.


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Visualization


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How Does It Work? The interaction of the primary ion beam withthe sample provides sufficient energy to ionizemany elements.

If the primary beam is composed of positively

charged ions, the resultant ionization favorsproduction of negative ions; primary beams ofnegative ions favor generation of positive ions.Although most atoms and molecules removedfrom the sample by the interaction of theprimary beam and the sample surface areneutral, a percentage of these are ionized.

These ions are then accelerated, focused,and analyzed by a mass spectrometer.


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To be known... NASA first developed SIMS in the 1960sto investigate the composition of Moonrocks. The method proved so

successful, that the apparatus wascommercially produced.

A beam of primary ions or neutral

particles impacts the surface withenergies of 3-20 keV.


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Components Of SIMS


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Various components of modern SIMS instrumentsare:

1. Primary Ion Sources Duoplasmatron

Cs Ion Source

2. The Primary Column3. Secondary Ion Extraction4. Secondary Ion Transfer5. Ion Energy Analyzer6. Mass Analyzer7. Secondary Ion Detectors

Electron Multipliers Faraday Cup

8. Electron Charge Neutralization9. Vacuum10. Magnetic Field Control

Hall Probe Detectors

NMR Detectors

Components Of SIMS


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Primary Ion Sources

Modern SIMS primary ion sources areequipped with

Duoplasmatron

Cs ion sourceSome are equipped with both.


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Duoplasmatron

The duoplasmatroncan operate withalmost any gasincluding air. Oxygen

is commonly used The duoplasmatronmay be used toextract either O- ,O-2or O+2 depending

upon the electricalpolarity selected bythe operator.


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Cs ion beams aregenerated by a surfaceionization process. Cs

vapor is produced by theheating of a solid Cscompound (Cs-chromateor Cs-carbonate).

The Cs vapor travels alongthe drift tube and strikes a

tungsten plate where it isthermally ionized. Any atom or molecule

coming from the reservoir isforced to bouncebetween the tungstenplate and the ionizer tip.

This results in most atomsbeing ionized andescaping through the smallhole in the cap.


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Primary Ion Column

The primary ions generated by theion source are passed to thesample via the primary column.

A typical column consists of a massfilter, apertures, lenses anddeflection plates. Their function is tofilter, focus, shape, position and

raster the primary beam. The primary beam mass filter

eliminates any impurities in the gasor generated in the ion source.

The deflectors either steer theprimary beam through the centerof the lenses, shape the beam,

position the beam, or raster thebeam at a high frequency (thusproducing an even beam densityover a large area).


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Secondary Sources


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Secondary Ion Extraction

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

These secondary ions are immediately removed by an extraction, orimmersion lens.

The polarity of the secondary ions is user selected and is independentof the primary beam polarity.

In order to obtain a constant secondary ion beam current, thepotential difference between the sample and the extraction(immersion) lens, must be kept constant.


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Secondary Ion Transfer

Now, the secondary ions are transferred by a transfer lens into themass spectrometer which forms a real magnified image of thesample surface at the position of the field aperture and focuses

the secondary ion beam onto the entrance slit of thespectrometer.

At the entrance slit, the immersion lens and the transfer lenstogether form the microscope.

In cases where the primary beam raster is large, the secondarybeam goes off-axis, and needs to be corrected using dynamic

transfer plates which deflects all the ions back to the axis.


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ION ENERGY ANALYSER

The generated secondary ions have a wide range of energies

which, on passing through the electrostatic energy analyzer, aredeflected as per their energy.

A movable energy slit allows a small portion of dispersedsecondary ions to pass into the magnetic analyzer.


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MASS ANALYSER


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SECONDARY ION

DETECTORS

2 types of secondary ions detectors are used in the modernSIMS instruments, which are :

1.) Electron Multiplier, and

2.) Faraday Cups.


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An electron multiplier consists of a series of electrodes calleddynodes which are all connected to a resistor chain.

The first dynode is at ground potential, so that both positive ornegative 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 strikes the first dynode it may produce a fewsecondary electrons which are accelerated to the second dynodethat is held at a slightly higher positive potential.

1. Electron Multiplier


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2. Faraday Cup Detector

consists of a hollow conductingelectrode connected to ground via ahigh resistance.

The ions hitting the collector cause aflow of electrons.

The resulting potential drop across theresistor is amplified.

Plate held at -80 V in front of thecollector , preventsany ejectedsecondary electrons from escaping.


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ELECTRON CHARGE NEUTRALISATION

Electron Gun :-

Charge build-up occurs on surface of specimens sosome method of charge neutralizing is essential.

If the number of negative particles extracted is muchgreater than one then sample charges positivelyand also vice-versa.

In these conditions the sample must be bombardedwith additional high- or low-energy electrons.


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Electron Gun


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VACUUM

Instrument must be kept under Ultra HighVacuum (UHV).

Vacuum is achieved by pumps: Rotary,

Turbo molecular, Ti-sublimation and Ionpumps.

Vacuum in the analysis chamber can getdown to 5*10-10 Torr equivalent to appro.

1010molecules/Litre.

Secondary ion hitting with a air moleculewithin the instrument is almost zero.


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MAGNETIC FIELD CONTROL

Critical to control magnetic field at theion detectors over long periods of time.

Controlled byeither electronic circuits

provide constant current orbyincorporating measuring device in themagnetic field that provides feedback

information for regulation. Two feedback of devices : Hall Probeand The NMR.


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1. Hall Probe

If electric current flows in a magnetic field,the magnetic field exerts a force on themoving electrons that tends to push themto one side of the conductor.

This produces voltage between the twosides that can be measured and used tofeedback for control of magnetic field.

Disadvantage :- Very sensitive tochangesin the temperature.


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HALL PROBE

DETECTOR


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2. Nuclear Magnetic Resonance Probe(NMR)

Nucleus under magnetic field orientated in the lower

energy state & then subject to an additional radio

frequency EM signal , each nuclear absorbs energyand

flips to higher energy state.

Low or high energy state of the nuclei is linearly relatedto

the magnetic field strength.

By switching off the radio frequency, the system revertsback to their original orientation emitting an EM radio

frequencythat can bemeasured.


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Output of SIMS


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Depth Profile

Monitoring the secondary ion count rate of selectedelements as a function of time leads to depth profiles.


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Bulk Analysis

Samples with homogeneously dispersedanalytes are analyzed by bulk analysistechnique.

Faster sputtering rates increase thesecondary ion signal.

In a typical heteregenous sample, the

analyte is concentrated in smallinclusions that produce spikes in thedata stream.


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Mass Spectra

Mass spectra sample the secondary ions ina preselected mass range by continuouslymonitoring the ion signal while scanning arange of mass-to-charge (m/z) ratios.

The mass spectrum detects both atomic

and molecular ions.


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Ion Imaging

Ion images show secondary ionintensities as a function of location onsample surfaces.

Ion images can be acquired in twooperating modes,

Ion Microscope

Ion Microbeam Imaging


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Ion Imaging

For ion microbeam imaging, a finely focusedprimary ion beam sweeps the sample in araster pattern and software saves secondaryion intensities as a function of beam position.


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Pros

The elements from H to U may bedetected. Most elements may be detected down toconcentrations of 1ppm or 1ppb.

Isotopic ratios may be measured, normallyto a precision of 0.5 to 0.05%. Two dimensional ion images may beacquired.

The volume of material sputtered is small. Little or no sample preparation may beneeded.


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Cons

The material sputtered from the sample surfaceconsists not only of mono-atomic ions butmolecular species that in places can dominatethe mass spectrum, making analysis of someelements impossible.

The sputtering process is poorly understood. The sensitivity of an element is stronglydependent on the composition of the matrixand the type of primary beam used. Standardsshould, therefore, be close to the compositionof the unknown. This is particularity true forisotopic analysis.

Samples must be compatible with an ultra highvacuum.


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Thank You

secondary ion mass spectrometry(sims)

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