Hydrogen Forward Scattering Spectrometry (HFS)

Hydrogen Forward Scattering Spectrometry (HFS)

Hydrogen Forward Scattering Spectrometry (HFS) is a type of Elastic Recoil Detection Analysis (ERDA), that uses forward recoil to detect and quantify Hydrogen atoms in the surface and near surface layers of solid samples. The principle is that a high energy ion beam striking and penetrating a test sample will cause forward recoil of impacted atomic nuclei that are lighter than the incident ion. In the case of HFS, the incident ion beam comprises positively charged Helium ions that are directed at a grazing angle onto the sample surface, enabling shallow penetration of near surface layers. In accordance with ERDA principles, Hydrogen nuclei, being lighter than Helium, will get Forward scattered upon impact. The incident Helium ions will also be scattered at an angle, due to the inclined nature of the incoming beam. The scattered Hydrogen and Helium ions are then separated from each other by a thin stopping foil that allows energetic Hydrogen to pass but not Helium.  The Hydrogen nuclei can be detected and analysed by an energy-sensitive detector system, yielding an energy spectrum. All other atomic species due to their heavier nuclei, will not be forward scattered by Helium ions. The HFS technique therefore is selective for Hydrogen detection and quantification. The angle of incidence is typically in the range of 15⁰ to 35⁰. HFS is also known by different names, such as Elastic Recoil Spectroscopy (ERS), Forward recoil Spectroscopy (FRS) and Hydrogen Recoil Spectrometry (HRS). Since Hydrogen is a common contaminant in thin films, the HFS technique is widely used for quantification and depth profiling of Hydrogen in such applications.

Common Uses of Hydrogen Forward Scattering Spectrometry (HFS)

• Analysis of Hydrogen contamination in thin films: for example, Silicon Nitride film deposited using some vapour deposition methods using ammonia or Silane (SiH4) can contain substantial amounts of Hydrogen compounds. Depth profiling of composition by HFS can characterize the extent of contamination.

Advantages of Hydrogen Forward Scattering Spectrometry (HFS)

•  Non-destructive analysis, except for Polymer films
• Ability to measure atom density of thin films when thickness is known
• Ability to analyse conductors as well as non-conducting organic or inorganic materials

Limitations of Hydrogen Forward Scattering Spectrometry (HFS)

• The depth resolution is poor (~500 A⁰ at 1000 A⁰ depths)
• The sample should be compatible with vacuum

Industrial Applications of EELS

• Semiconductors
• Display technologies
• Polymeric films and coatings