Magnetic Sector Secondary Ion Mass Spectrometry

Summary

Magnetic sector secondary ion mass spectrometry (SIMS) generates isotopic and elemental information from solid surfaces with depth profiling capabilities to measure the concentration of isotopes and elements as a function of depth into the film.

Description

Magnetic sector SIMS uses high energy “primary” ions to bombard the surface and liberate “secondary” ions much like in ToF-SIMS, but the ion beam is operated continuously to maximize duty cycle. The high ion dose is preferred for depth profiling, where the concentration of elements and isotopes are determined as a function of depth. This is one of the very few techniques where the depth resolution can approach 1 nm in inorganic films. The separation of “secondary” ions by mass occurs through a double-focusing lens, first through an electrostatic sector to filter the ions by energy, then through a magnetic sector to filter the ions by mass. Quantification is achieved by applying relative sensitivity factors (determined from standard samples) to convert secondary ion intensities to concentrations. Precision and accuracy better than 1% can be achieved with detection limits in the parts-per-million (ppm) to parts-per-billion (ppb) for most elements in the periodic table.

 

Figure A. Schematics of the (a) electrostatic and (b) magnetic sectors that are used for the separation of ions by mass. The radius of curvature inside the electrostatic sector is dependent only on the electric field strength and ion energy (mass independent), while the magnetic sector separates the ions based on energy and mass. The detector needs to be “positioned” depending on the mass of the ion being detected.

Creating a test pattern for an effective automated particle mapping capability

Oftentimes, characterization of the size, morphology, and elemental composition of thousands of particles and aerosols become necessary. Automation allows collection of particle data with certain characteristics, such as size, shape, composition, and/or isotopic signature. The SIMS instrument operating in the ion imaging mode SIMS has the sensitivity and image resolution to precisely characterize these particles. However, this ability to characterize particles depend heavily on user setup of the instrument, such as proper image calibration and optimized instrument setup, failure of which may lead to inaccurate results regarding the particles. A standardized test pattern (Figure B) that contains features with known sizes, shapes, height, and composition is currently being produced so that instruments around the world equipped with the automated particle mapping capability can be calibrated to output the same results.

Figure B. A SIMS instrument operating in ion imaging mode is used to image a fabricated test pattern to evaluate the performance of the instrument. Individual particles identified during the screening process can later be re-located and analyzed with higher precision, and software has been developed to automate this process. The sample consists of a collection of Ni features of various sizes on a Si substrate distributed over 500 µm x 500 µm, repeating in a 20 x 20 array. The test sample can be used to investigate spatial resolution limits for particle detection, evaluate algorithms for identifying individual particles, and examine the limits of isotopic ratio calculations from small particles

Nanoparticle Detection in Organisms

Release of nanoparticles into the environment presents a potential health hazard that is not yet well understood. Sensitive measurement tools are needed for the assessment of risk and regulatory decision making. SIMS image depth profiling was used to monitor and visualize the uptake of Au nanoparticles by Caenorhabditis elegans (C. elegans), a simple multicellular organism useful as a model in environmental toxicology studies. The high spatial resolution and sensitivity of makes SIMS a powerful tool for the in-situ study of nanoparticle uptake at environmentally relevant concentrations.

Figure C. Optical micrograph showing the size of the organism in relation to the size of the analysis area, as seen by the square crater. Corresponding secondary ion images showing the distribution of Au- and CsC4- in 2-dimensional space within the organism.