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Secondary Ion Mass Spectrometry
An Introduction to the Technique
Secondary ion mass spectrometry (SIMS) is a
surface analysis technique in which energetic primary ions are used to bombard
a surface. These primary ions upon impacting the surface generate positively
and negatively charged secondary ions, which are subsequently mass-separated
and detected. SIMS has the capability of analyzing the entire periodic table
with isotopic discrimination and typical detection limits on the ppm - ppb
level. SIMS may be performed either in the dynamic mode, where the primary ion
current density is sufficient to sputter erode the surface, constantly exposing
new, previously buried, layers to the analyzing beam or in the static mode,
where far less than one monolayer of the surface is consumed during the
analysis.
Quantification, the conversion of measured secondary ion intensity to
concentration of a particular analyte, is particularly difficult in SIMS. The
fraction of total atoms sputtered off the surface by the primary ion beam which
bear either a positive or negative charge, and are therefore detectable by the
instrument, may vary by several orders of magnitude depending upon the chemical
composition of the surface from which it was sputtered. For this reason SIMS is
usually applied to the analysis of very low concentration analytes (<1
atomic %) where the changing concentration of the analyte itself over depth
will not vary to a high enough degree to influence the ionization efficiency of
the species of interest. Conversion factors for intensity-to-concentration
calculations (the so called Relative Sensitivity Factors) are commonly derived
using ion implanted standards of the species of interest into a specific matrix
(e.g., B in Si).
SIMS may be used in a spectral acquisition mode to simply identify and quantitate a given species of interest on a surface or may be used in imaging or depth profiling modes. In imaging analysis, the areal distribution of a species is determined by creating an image by measuring the intensity of a ion which is specific to the analyte as a function of the position of the primary ion beam (microprobe measurement) or, in certain instruments, by viewing a stigmatic image of the surface through the mass spectrometer (microscope measurement). In depth profiling applications, characteristic ions are monitored as a function of analysis time. The time scale is then converted to depth, usually by post-analysis measurement of the crater depth, and the ion intensity scale is converted to concentration by application of the appropriate relative sensitivity factors.
What is an Atomic Force Microscope?
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The scanning tunneling microscope (STM) and atomic force microscope (AFM) provide pictures of atoms on or in surfaces. A system that uses variations of the principles used by an STM or AFM to image surfaces is often called a scanning probe microscope (SPM). The AFM works by scanning a fine ceramic or semiconductor tip over a surface much the same way as a phonograph needle scans a record (for those of you that remember what a record player was!). The tip is positioned at the end of a cantilever beam shaped much like a diving board. As the tip is repelled by or attracted to the surface, the cantilever beam deflects. The magnitude of the deflection is captured by a laser that reflects at an oblique angle from the very end of the cantilever. A plot of the laser deflection versus tip position on the sample surface provides the resolution of the hills and valleys that constitute the topography of the surface. The AFM can work with the tip touching the sample (contact mode), or the tip can tap across the surface (tapping mode) much like the cane of a blind person |
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