A method that mass-analyzes secondary ions ejected while the specimen is sputtered by primary ions. It can measure depth profiles of trace elements such as dopants with ppm to ppb sensitivity.
Japanese name: SIMS(二次イオン質量分析法)
SIMS (secondary ion mass spectrometry) appears in 1 practice question across 1 topic, including Materials characterization (SEM, TEM, XRD, XPS).
In SIMS, primary ions accelerated to energies from a few hundred eV to tens of keV strike the specimen and knock out surface atoms (sputtering). The small fraction of ejected particles that are ionized is counted by a mass spectrometer to distinguish elements and isotopes. For elements that readily form positive ions, such as B, Al and Na, oxygen ions (O₂⁺) are used, and for elements that readily form negative ions, such as P, As, O, C and H, cesium ions (Cs⁺); this raises the ionization fraction and the sensitivity. A major feature is the ability to measure everything from hydrogen to heavy elements, distinguishing isotopes as well.
Depth profiles of dopants in semiconductors are measured by dynamic SIMS, in which a high dose of primary ions keeps eroding the specimen. After the measurement, the crater depth is measured with a stylus profilometer, and time is converted to depth assuming a constant sputter rate. Static SIMS, by contrast, uses such a low dose that primary ions strike no more than about 1% of the surface atoms; combined with a time-of-flight (TOF-SIMS) mass spectrometer, it examines organic contamination and surface treatments from fragments of molecules on the outermost surface. The same principle is used very differently depending on the information sought.
The fraction of atoms that become secondary ions not only differs by orders of magnitude between elements but also changes greatly with the composition of the surrounding matrix (the matrix effect). Signal intensity alone therefore cannot be converted to concentration; a standard made by implanting a known dose of the element into the same matrix is measured to obtain relative sensitivity factors. The atomic density of silicon is about 5×10²² cm⁻³, so 1 ppb corresponds to about 5×10¹³ cm⁻³. Where the matrix changes, as at an SiO₂/Si interface, apparent steps in concentration are likely, so care is needed.
In mass spectrometry, other ions with the same mass number can overlap. Phosphorus in silicon, ³¹P⁻, differs in mass from ³⁰Si¹H⁻ by only about 0.008, and separating them requires a mass resolution M/ΔM of about 4000. Measures include using an instrument with high mass resolution or detecting the element as a different molecular ion. In depth, mixing, in which primary ions push surface atoms deeper, smooths the profile, so for shallow junctions the primary ion energy is lowered to improve depth resolution. Whether a steep profile is real is checked by changing the conditions.
Answer: A. SIMS (secondary ion mass spectrometry)
SIMS sputters the specimen surface with primary ions such as O₂⁺ or Cs⁺ and mass-analyzes the ejected secondary ions. It can measure many elements with ppm to ppb sensitivity and a depth resolution of a few nanometers, and it is the standard method for measuring profiles of boron, phosphorus and arsenic in silicon. It is destructive, since it removes material, and accurate quantification requires reference standards of the same matrix. The detection limit of EDS is roughly 0.1 wt%, not sensitive enough to measure dopant concentrations. XRD examines crystal structure and lattice parameters and cannot measure concentration profiles of trace elements.
It depends on the element and conditions, but for boron in silicon and similar cases it reaches about 10¹³–10¹⁴ cm⁻³, that is, the ppb range. For elements also present inside the instrument, such as hydrogen, carbon and oxygen, residual gas raises the limit.
No. It is destructive, measuring while eroding the specimen, and the analyzed region does not come back. Static SIMS, however, removes only a tiny fraction of the outermost surface, so it can examine surfaces under nearly non-destructive conditions.
SIMS is extremely sensitive, at the ppm to ppb level, and can measure depth profiles and isotopes, but quantification requires standards. XPS has a sensitivity of about 0.1 at%, but it reveals chemical state and allows semi-quantitative analysis without standards.
SIMS sputters the specimen surface with primary ions such as O₂⁺ or Cs⁺ and mass-analyzes the ejected secondary ions.
In semiconductor failure analysis, which technique is best …EDS (energy-dispersive X-ray spectroscopy) XRD (X-ray diffraction) Bragg's law Extinction rules Scherrer equation SEM (scanning electron microscope) XPS (X-ray photoelectron spectroscopy)