Microbeam Mass Spectrometry

 

Secondary Ion Mass Spectrometry

Ion beam microanalysis is based on the phenomenon of ion-induced sputtering of a solid sample by an energetic primary ion beam, typically consisting of oxygen or cesium ions at a beam energy of 1 - 20 keV.  The sputtered atoms include ionized species known as secondary ions.  By carrying out mass spectrometry on the secondary ions, qualitative and quantitative analysis of all elements is possible by this technique, which is known as secondary ion mass spectrometry (SIMS).  A type of SIMS instrument available in our division is the secondary ion microscope, which utilizes ion optics to focus the secondary ions into a true ion image with a spatial resolution of the order of 0.5 micrometers.  This image can be viewed by a high gain television camera and a digital imaging system or the image can be directly digitized by means of a pulse-counting image detector called a resistive anode encoder.  Because secondary ions are generated near the surface of the sample, the sampling depth of analysis can be as small as 1 nm.  The sample is progressively eroded by the sputtering process, and an elemental depth profile can be obtained with a depth resolution of less than 10 nm to shallow sputtering depths (< 200 nm) and a relative depth resolution of approximately 1% to total depths as great as 20 micrometers. 

Time of Flight Secondary Ion Mass Spectrometry

A pulsed ion beam/time-of-flight mass spectrometer (TOF-SIMS) extends our SIMS capabilities to a spatial resolution of approximately 200 nm, and provides simultaneous detection and imaging of the elements spanning the entire periodic table.  This instrument is often used for molecular SIMS analysis of organic materials, because characteristic spectra can often be obtained at very low primary ion doses that minimize beam-induced degradation of the surface.

Laser Microprobe Mass Spectrometry

Laser microprobe mass spectrometry (LMMS) uses a pulsed beam of photons to evaporate the sample in a small region, as small as 0.5 µm diameter.  A fraction of the evaporated atoms is ionized by the laser beam, accelerated to a kinetic energy of 3 keV, and then analyzed by means of a time-of-flight mass spectrometer.  This instrument provides detection of all elements and isotopes, with detection limits in the µg/g range from a total sample consumption of 0.1 pg per spectrum.  Absolute detection limits are thus in the femtogram range.  Quantitative analysis is based on the use of sensitivity factors and the accuracy is limited to approximately 20 % relative.  The sample for the transmission geometry LMMS must be in the form of a particle or a thin film.  Typical applications include the analysis of particles as small as 200 nm in diameter, the analysis of biological thin sections and cells, and trace analysis of materials. 

High Performance SIMS

The practical sensitivity of SIMS analysis is usually in the µg/g range, and in many cases, detection levels in the ng/g range can be obtained.  The sample may be in the form of a polished solid or a particle.  Typical applications include the study of the distribution of dopants in semiconductors, detection of trace contaminants in alloys, analysis of particles, and isotopic ratio measurements. A new high sensitivity, high mass resolution instrument will soon be installed at NIST. This instrument is specially constructed to provide the highest possible sensitivity for isotopic SIMS analysis.