SECONDARY ION MASS SPECTROMETRY

SIMS is a characterization technique commonly employed in the semiconductor device industry for obtaining information regarding either element concentration at the surface or at various depths below the surface of a sample or thin film. There are two primary types of SIMS techniques that provide data of this sort, and they are called static SIMS and dynamic SIMS, respectively.

Dynamic SIMS:
Dynamic SIMS uses a high-energy primary ion beam and corresponding short times to erode away the material surface and obtain a depth profile of the sample.Dynamic SIMS allows for depth profiling of elemental concentration with a sensitivity in the range of parts per billion (ppb), or in the range of 10¹⁴ atoms*cm^-3. This technique is extremely powerful and currently does not have any complimentary techniques that obtain results on the same level of sensitivity. Dynamic SIMS also provides depth resolution on the level of several nanometers, which is something that no other current technique can claim.
Physical Mechanism of SIMS:
The dynamic SIMS characterization technique works by first striking a sample with a primary ion beam. This operation is performed in an ultra high vacuum environment with typical pressures of about 10-9 to 10-10 torr. Ultra high vacuum is required to prevent any interaction of the primary ion beam or emitted secondary ions with gas molecules.The primary ion beam is created by an ion source of which there are four primary types: electron, plasma, radio frequency, and liquid metal ion sources . The primary ion beam typically ranges in energy from 0.5 to 20 keV, and may consist of reactive ions such as O₂⁺, O^-, Cs⁺, or inert ions like Ar⁺, Xe⁺, or Ga⁺ depending upon the type of ion source.
In dynamic SIMS, primary ions that strike a sample may collide with target surface atoms and displace them if the primary ion energy is larger than the displacement energy (15-40 eV) of the target atoms. The primary ions may also become imbedded in the surface of the sample. Target atoms impacted by primary ions may be displaced toward the sample, and collide with more target atoms.
The target atoms that recoil back through to the surface constitute sputtered material, which gives rise to secondary ion emission. Secondary ion emission will only take place if the target atom strikes the surface with energy greater than the surface binding energy. This process of sputtering classifies SIMS as a destructive process since material is physically removed from the sample.
Ion Collection and Analysis:
The secondary ions that are ejected from the sample may be either positive or negative, and are collected in an energy analyzer, which provides enhanced resolution. From there, the ions are transferred to a mass spectrometer that analyzes the mass to charge ratio for purposes of identifying the element. Finally, the ions travel into a detector, which is similar to a Faraday cup, that allows for current to be measured and data interpreted.
Static SIMS would produce a mass spectrum as the primary result, whereas dynamic SIMS allows the user to obtain a depth profile of elements in the sample. The third type of possible output is an image of the sample by a process called imaging SIMS, but is less relevant to the problem that has been posed.