案例分析

Prevent Hydrocarbon Background Interference During SEM Sample Prep

How do I prevent hydrocarbon background interferences when performing secondary ion mass spectrometry during SEM sample preparation?

A recently renovated geophysical science laboratory in a large Midwestern research university took delivery of a new FIB/FESEM and, with it, sought ways to maximize use and results from their new instrument when preparing samples for microanalysis. With many of their primary applications pertaining to earth science study, they routinely coated and polished sections of rocks and meteorites, and mounts of tiny, sometimes submicron, grains. While in preparing samples for microanalysis, these geologists most commonly coated samples with carbon or gold (carbon minimizes interferences in x-ray microanalysis; gold, for secondary ion mass spectrometry, as it is monoisotopic with a low ionization yield), but also prepared other sample metals. However, it remained of particular and critical importance to the laboratory geologists to minimize hydrocarbon background when coating, as hydrocarbon background can cause interferences when secondary ion mass spectrometry is performed on coated specimens.

The geology laboratory had made long-standing use of a 30-year old diffusion-pumped bell-jar type carbon coater and routinely encountered numerous instances of hydrocarbon background-induced interferences. Thus, with the new FIB/FESEM instrument and refurbished laboratory firmly invested, the geologists sought to minimize hydrocarbon background when preparing their SEM samples while performing their microanalysis.

Solution:

To minimize this hydrocarbon background during sample preparation, Denton Vacuum recommended its Desk V TSC with its turbo molecular pumping package, the industry’s standard in sputter coating tools.

Significant hydrocarbon background peaks often occur due to backstreaming of the hydrocarbons from the oils used in both roughing and diffusion pumps. Such hydrocarbon backgrounds can occur due to byproducts in the oil itself or due to samples which are collected and trapped in the pump oils during the microanalysis. Hydrocarbon background contamination, however, is much less common in a rough pump than it is with a diffusion pump.

Turbomolecular pumps, however, are used in a wide variety of high vacuum applications— any that demand clean, truly oil-free vacuum between 104 and 1010 Torr. The single-ended, turbomolecular pump is often used in replacement of a comparably sized diffusion pump in existing systems. In addition to the absence of oil on the backing-vacuum side that may contribute to significant hydrocarbon background in performing SEM sample preparation, a turbomolecular pump provides far more freedom from wear and maintenance.

The Desk V TSC offers for the following standard features:

  • Hydrocarbon-free sample preparation
  • 70 l/s turbo molecular pump
  • Full range high vacuum gauge
  • Capable of sputtering all metals
  • Provides finer grained films
  • Enhanced touchpad with graphics capability
  • Manual or automatic timed sputtering or etching
  • Choice of adjustable current control to allow either high or low rate sputtering to achieve finer grain structure
  • Sample specimen protection from damage via Denton’s patented anode grid which, in conjunction with the magnetron sputter head to collect stray electrons, prevents them from bombarding/heating the sample

Watch this video to see how the Desk V works.