The Researchers’ Introduction to Secondary ion mass spectrometry (SIMS)


SIMS or Secondary ion mass spectrometry is an elution mass spectrometry or MS technique. It is widely used across research and industries. It can be incorporated for obtaining high-resolution two-dimensional and three-dimensional mass spectrometric images along with molecular distribution and accurate atomic distributions as one of the functions of depth profiles. SIMS is used for detecting and characterizing trace elements near or at the surface of a thin or solid film which allows researchers to understand the chemical structure of the surface. SIMS is widely known for its surface analysis like studying atomic scale defects occurring in the manufacturing process of semiconductor materials and analyzing surface contamination. SIMS products include tof sims analysis research instruments and the likes of Cross Beam Deflector Ionizer, Axial Energy Analyzers, and Tandem Ionizer Energy Filters.

Fundamentals of SIMS

Interactions between the primary ion beam and the vacuumed sample tend to provide ample energy for ionizing multiple elements. When the primary beam is made up of several positively charged ions, the primary beams made of negative ions tend to generate positive ions. Usually, most molecules and atoms are removed from the sample due to the interactive tendencies between the primary bean and sputtering (sample surface). These are neutral in nature and a certain percentage among these are ionized. These ions are then focused, accelerated, and analyzed using a mass spectrometer. In the case of DSIMS, the primary ion beam overrides the static limit of (~1E12 ions/cm2). This produces a good yield of the secondary ions. This method is usually used in the case of bulk analysis of isotopes and elements and is ideal for tracing elements in minerals or analyzing isotopes. On the other hand, static SIMS uses a comparatively lower energy primary ion beam which is Cs or Ga. This technique is applicable to analyzing atomic monolayers on the surface of materials for obtaining information. This information is based on the molecular species on surface materials such as organic compounds.

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Modes of SIMS

In the surface analysis field, it is quite common for distinguishing static and dynamic SIMS. Static SIMS is the process which is involved in surface atomic monolayer analytics which is also known as surface molecular analysis. This is usually found with a time of flight mass spectrometer and a pulsed ion beam whereas dynamic SIMS is involved in bulk analysis which is associated with the sputtering process. This uses a DC primary ion beam along with a quadrupole mass spectrometer or a magnetic sector.  DSIMS (Dynamic secondary ion mass spectrometry) is a tool which categorizes surfaces inclusive of molecular, elemental and isotopic compositions. This is used in the semiconductor industry and is used for studying the composition of polymers, structuring thin films and also for surface chemistry. And with Intel, the semiconductor giant investing heavily in Europe, the demand for SIMS aided technologies would be on a sharp rise.

Pros and Cons of SIMS

The main advantages of SIMS are:

  • The overall analysis consumes very little nondestructive sample such as consumption of a few cubic micrometers for a classic U-Th-Pb analysis
  • SIMS is ideal for tracing elements with low concentrations such as interplanetary dust, trace elements excess in meteorites.
  • SIMS allows depth profiling of molecular and elemental abundance along with isotope ratios.
  • For situ analysis, complex samples like minerals can be analyzed directly as thin sections or grain mounts.

Certain limitations for SIMS are:

  • Since both molecular and atomic species are produced via sputtering, quantitative analysis of all samples is not an option.
  • Instrumentation of SIMS is a costly affair where typical instruments can reach up to $2-3 million.
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For geochemical analysis, preservation of sample integrity must be done carefully. In SIMS, one must take special care during the physical preparation of the sample before the analysis. The sample surface for SIMS must be spectacularly polished and coated with pure metal, conducting for avoiding charge buildups. Otherwise, the behavior of the ions might lead to erroneous output.

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