Focused Ion Beam (FIB)

Focused Ion Beam (FIB)

Focused ion beams have been used since the 1960s to investigate the chemical and isotopic composition of minerals . A focused ion beam blasts atoms and molecules free from the surface of a small sample of material; some of these free particles are also ions, and these are guided by electric fields to a mass spectrometer which identifies them with great precision.

An ion is an atom or molecule with a net electric charge. Electric fields subject electric charges to forces; therefore, electric fields can be used to move and steer ions. A continuous stream of ions moving together is termed an ion beam; a focused ion beam (FIB) is produced by using electric fields to guide a beam of ions.

In a typical FIB analysis, a narrow beam of argon, gallium, or oxygen ions traveling about 500,000 mph (800,000 kph) is directed at a polished flake of the material to be analyzed. Some of the atoms and molecules in the sample are kicked loose by the beam, a process termed sputtering. Some of these sputtered particles are themselves ions and so can be collected and focused by electric fields. The sputtered ions are directed to a mass spectrometer, which sorts them by mass. Even the very slight mass differences between isotopes of a single element can be distinguished by mass spectrometry; thus, not only the chemical but the isotopic composition of a sample can be determined with great precision. Very small, even microscopic, samples can be analyzed by FIB techniques.

The abundance of trace elements in a mineral can reveal information about the processes that formed it, helping petrologists and geochemists unravel geological history. Further, the decay of radioactive elements into isotopes of other elements acts as a built-in clock recording when the host mineral was formed. The hands of this clock are the relative isotope abundances in the mineral, and these can be determined by FIB analysis. Carbon isotope ratios also reveal whether a carbon-containing mineral was assembled by a living organism or by a nonliving process. Using FIB analysis, scientists have exploited this property of carbon isotopes to show that life existed on earth at least 3.85 billion years ago and that certain rocks originating on Mars and recovered as meteorites lying on the Antarctic ice probably, despite appearances, do not contain fossils of Martian microbes.

FIB facilities are complex and expensive. Accordingly, only about 15 facilities devoted to Earth sciences exist worldwide. FIBs are also used extensively in the manufacture of electronic microchips.

See also Carbon dating; Dating methods; Radioactivity