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== Secondary Ion Mass Spectrometry ==
== Secondary Ion Mass Spectrometry ==


[[image:SIMScascade.gif|200x200px|right|thumb|Atoms that used to make up the surface are sputtered off by the high energy ions; some are emitted as secondary ions. ]]
[[image:SIMScascade.gif|200x200px|right|thumb|A beam of high energy ions is rastered on the surface of the sample. Some of the atoms that used to make up the surface are sputtered off and emitted as secondary ions. The mass of the these ions is measured with a mass spectrometer.]]
 


When a solid sample is sputtered by primary ions of few keV energy, a fraction of the particles emitted from the target is ionized. Secondary Ion Mass Spectrometry consists of analyzing these secondary ions with a mass spectrometer. Secondary ion emission by a solid surface under ion bombardment supplies information about the elemental, isotopic and molecular composition of its uppermost atomic layers.
When a solid sample is sputtered by primary ions of few keV energy, a fraction of the particles emitted from the target is ionized. Secondary Ion Mass Spectrometry consists of analyzing these secondary ions with a mass spectrometer. Secondary ion emission by a solid surface under ion bombardment supplies information about the elemental, isotopic and molecular composition of its uppermost atomic layers.


{| border="1" cellspacing="0" cellpadding="4" align="center"
{| border="1" cellspacing="0" cellpadding="4" align="center"
!width="100"|
!width="100"|
!width="200" | SEM with EDX
!width="300" | SEM with EDX
!width="200" | SIMS
!width="300" | SIMS
|-
|-
! Full name  
! Full name  
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|-  
|-  
!  Technique
!  Technique
|| Non destructive excitation of X-rays in the sample. The elemental analysis is possible because the energy of these photons is characteristic of the element they emitted from.
|| Non destructive method: X-rays are generated when the primary beam impinge on the sample. The elemental analysis is possible because the energy of these photons is characteristic of the element they emitted from.
|| Destructive method that sputters off surface atoms with heavy ions.   
|| Destructive method: A beam of high energy heavy ions (caesium or oxygen) sputters off surface atoms that are subsequently measured with a mass spectrometer.   
|-
|-
!  What elements are detected
!  What elements are detected
|| Any element heavier than boron/carbon  
|| Every element heavier than boron/carbon  
|| Any element
|| Every element
|-
|-
!  Chemical information
!  Chemical information
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|-
|-
!  Sample limitations  
!  Sample limitations  
|| Vacuum compatible
|| Vacuum compatible.
|| Vacuum compatible
|| Vacuum compatible. The sample needs to be cut into small pieces.
|-
|-
!  Spatial resolution
!  Spatial resolution
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|-
|-
!  Depth resolution
!  Depth resolution
|| The size interaction volume depends on the SEM high voltage and sample density: The higher the SEM high voltage the bigger and deeper the interaction volume. The more dense the material is the smaller is the interaction volume
|| The size interaction volume depends on the SEM high voltage and sample density: The higher the SEM high voltage the bigger and deeper the interaction volume. The more dense the material is the smaller is the interaction volume.
|| Depth profiling is
|| The sputtering of the surface makes it possible to perform detailed depth profiling with extremely good sensitivity and depth resolution.
|-
|-
!  Detection limit
!  Detection limit
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|| Down to 1 ppb for many elements
|| Down to 1 ppb for many elements
|}
|}
SIMS is the most sensitive elemental and isotopic surface analysis technique.
The secondary ion yields will vary greatly according to the chemical environment and the sputtering conditions (ion, energy, angle). This can add complexity to the quantitative aspect of the technique.
(detection limit down to ppb level for many elements), high lateral resolution imaging (down to 40 nm), and a very low background that allows high dynamic range (more than 5 decades). This technique is "destructive" by its nature (sputtering of material). It can be applied to any type of material (insulators, semiconductors, metals) that can stay under vacuum.
It allows molecular as well as elemental characterization of the first top monolayer in the static SIMS mode. It allows also the investigation of bulk composition or depth distribution of trace elements in the dynamic SIMS mode, with a depth resolution ranging from one to 20-30 nanometers.
This is why SIMS is one of the most widespread surface analysis techniques for advanced material research.
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