In order to perform measurements that are not affected by interfering elements, the “mass resolution” of the mass spectrometer must be set to a resolution that allows the target element and interfering elements to be separated. Mass resolution, the half-width of the mass spectrum is defined in “m / △ m” using the difference between two mass of interest. For example, when measuring Fe in Si, using a mass 56 Fe (56Fe: 55.93490) with a large isotope abundance ratio, 28Si2 (28Si + 28Si: 55.95386) with the same mass 56 becomes an interfering element. Configuring mass resolution in order to separate both (m / △ m) is 2950 or more (56 / 0.01896 or higher) is required.
The SIMS mass spectrometer used for depth analysis is a double-focusing mass spectrometer (magnetic SIMS) or a quadrupole mass spectrometer (quadrupole SIMS). Please refer to this link for the device configuration. Mass resolution of the magnetic type SIMS is “300-10000”, and approximately “300-1000” in quadrupole SIMS. We, therefore use magnetic sector SIMS capable of obtaining high mass resolution when analyzing an element which is subject to a mass interference.
In the case that contains the unknown material and the high concentration of impurities, it may be difficult to predict a mass interference in advance. The example, when data are greatly different from expectations, it is always wise to consider the possibility of a mass interference.