Secondary Ion Mass Spectrometry(SIMS)

BYSangeet Gupta(11BPE044)

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

Shashank Jaijaniya(11BPE100)

•Secondary Ion Mass Spectrometry (SIMS)-An Introduction•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.

•What it consist of?A basic SIMS instrument will, therefore, consist of:• A primary beam source to supply the bombarding species.• A target or sample that must be solid and stable in a vacuum.• A method of collecting the ejected secondary ions.• A mass analyser to isolate the ion of interest.• An ion detection system to record the magnitude of the secondary ion signal.

•How Does It Work?

• The interaction of the primary ion beam with the sample provides sufficient energy to ionize many elements.• If the primary beam is composed of positively charged ions, the resultant ionization favors production of negative ions; primary beams of negative ions favor generation of positive ions. Although most atoms and molecules removed from the sample by the interaction of the primary beam and the sample surface are neutral, a percentage of these are ionized.• These ions are then accelerated, focused, and analyzed by a mass spectrometer.

•Visualization

•To be known...•NASA first developed SIMS in the 1960s to investigate the composition of Moon rocks. The method proved so successful, that the apparatus was commercially produced.•A beam of primary ions or neutral particles impacts the surface with energies of 3-20 keV.

Components Of SIMS

Various components of modern SIMS instruments are:

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

•Primary Ion SourcesModern SIMS primary ion sources are equipped with

• Duoplasmatron• Cs ion source

Some are equipped with both.

•Duoplasmatron

• The duoplasmatron can operate with almost any gas including air. Oxygen is commonly used.

• The duoplasmatron may be used to extract either O- ,O-2 or O+2 depending upon the electrical polarity selected by the operator.

Cs ion source•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.

• Cs ion beams are generated by a surface ionization process.

Cs ion source

•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.

Secondary Sources

•Secondary Ion Extraction

• Secondary ions are formed at the sample surface.• Immediately removed by an extraction, or immersion lens.

• The polarity of the secondary ions is user selected.• The potential difference between the sample and the extraction lens must be kept constant.

•Secondary Ion Transfer

• Secondary ions are transferred into the mass spectrometer.

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

• In cases where the primary beam raster is large, the secondary beam goes off-axis.

• Ion Energy Analyser

• The secondary ions pass through the electrostatic energy analyzer and deflect as per their energy.

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

•Mass Analyser

• Here, the ions pass through a magnetic field.• The magnitude of the magnetic field required to deflect the

ion species is given by the equation:

m/q = Mass to charge ratio (Kg and C).B = Strength of the magnetic field (Wb/m2).V = The ion accelerating voltage (V).r = The radius of curvature of the magnetic field (m).

•Secondary Ion Detectors

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

1.) Electron Multiplier, and

2.) Faraday Cups.

• An electron multiplier consists of a series of electrodes called dynodes.

• The first dynode is at ground potential and the last dynode can be between +1500 to +3500 V.

• When a particle strikes the first dynode it may produce a few secondary electrons which are accelerated to the second dynode.

1. Electron Multiplier

• On impact more secondary electrons are generated.• The factors governing the magnitude of the pulse recorded at the end of the dynode chain are :

Type of dynodes composition (Cu/Be, Al2O3, Ni, Ag, etc.) of dynodes number of dynodes the accelerating voltage between dynodes the impact energy and type of the charged particles

• The older the multiplier, lesser is the efficiency due to carbon contamination.

•2. 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.• The resulting potential drop across the resistor is amplified.• Plate held at -80 V in front of the collector , prevents any ejected secondary electrons from escaping.

• Detect count rates from 5x104 c/s upwards. • Unlike the electron multiplier it does not discriminate between the type of ion or its energy.• Faraday cup is limited by the thermal noise in the resistor and the quality of amplifier. • Components will be enclosed within an evacuated, thermally controlled chamber.

• ELECTRON CHARGE NEUTRALISATIONElectron Gun :-

• Charge build-up occurs on surface of specimens so some method of charge neutralizing is essential.

• If the number of negative particles extracted is much greater than one then sample charges positively and also vice-versa.

• In these conditions the sample must be bombarded with additional high- or low-energy electrons.

•Electron Gun

•VACUUM• Instrument must be kept under Ultra High Vacuum (UHV).• Vacuum is achieved by pumps: Rotary, Turbo molecular, Ti-sublimation and Ion pumps.• Vacuum in the analysis chamber can get down to 5*10-10 Torr equivalent to appro. 1010 molecules/Litre.• Secondary ion hitting with a air molecule within the instrument is almost zero.

•MAGNETIC FIELD CONTROL•Critical to control magnetic field at the ion detectors over long periods of time.•Controlled by either electronic circuits provide constant current or by incorporating measuring device in the magnetic field that provides feedback information for regulation.•Two feedback of devices : Hall Probe and The NMR.

•1. Hall Probe• If electric current flows in a magnetic field, the magnetic field exerts a force on the moving electrons that tends to push them to one side of the conductor.

• This produces voltage between the two sides that can be measured and used to feedback for control of magnetic field.

• Disadvantage :- Very sensitive to changes in the temperature.

HALL PROBE DETECTOR

• 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 energy and flips to higher energy state.

• Low or high energy state of the nuclei is linearly related to the magnetic field strength.

• By switching off the radio frequency, the system reverts back to their original orientation emitting an EM radio frequency that can be measured.

Output of SIMS

•Depth ProfileMonitoring the secondary ion count rate of selected elements as a function of time leads to depth profiles.

•Bulk Analysis•Samples with homogeneously dispersed analytes are analyzed by bulk analysis technique.• Faster sputtering rates increase the secondary ion signal.• In a typical heteregenous sample, the analyte is concentrated in small inclusions that produce spikes in the data stream.

•Mass Spectra

• Mass spectra sample the secondary ions in a preselected mass range by continuously monitoring the ion signal while scanning a range of mass-to-charge (m/z) ratios.

• The mass spectrum detects both atomic and molecular ions.

• Ion Imaging• Ion images show secondary ion intensities as a function of location on sample surfaces.• Ion images can be acquired in two operating modes,

Ion MicroscopeIon Microbeam Imaging

• Ion Imaging

• For ion microbeam imaging, a finely focused primary ion beam sweeps the sample in a raster pattern and software saves secondary ion intensities as a function of beam position.

•Pros

• The elements from H to U may be detected.• Most elements may be detected down to concentrations of 1ppm or 1ppb.• Isotopic ratios may be measured, normally to a precision of 0.5 to 0.05%.• 2D ion images may be acquired.• The volume of material sputtered is small.• Little or no sample preparation may be needed.

•Cons• The material sputtered from the sample surface consists not only of mono-atomic ions but molecular species that in places can dominate the mass spectrum, making analysis of some elements impossible.• The sputtering process is poorly understood. • The sensitivity of an element is strongly dependent on the composition of the matrix and the type of primary beam used. Standards should, therefore, be close to the composition of the unknown. This is particularity true for isotopic analysis.• Samples must be compatible with an ultra high vacuum.

• www.geos.ed.ac.uk/facilities/ionprobe/SIMS4.pdf

• Wikipedia(https://en.wikipedia.org/wiki/Secondary_ion_mass_spectrometry)

• http://www.authorstream.com/Presentation/sreeramdileep-584348-mass-spectroscopy/

• www.google.co.in/search?sclient=psy-ab&q=secondary+ion+mass+spectroscopy&btnG=

•References

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