SIMS - Secondary Ion Mass Spectrometry
Secondary ion mass spectrometry (SIMS) is used to analyze the composition of a solid surface by sputtering it with a focused ion beam, which is called primary ion beam. The flux of sputtered particles consists of both ions and neutral atoms. In SIMS the sputtered secondary ions are collected and analyzed. Because the probability to sputter an ion from the surface is between 10-5 to 10-1, only a small fraction of the particles emitted from the sample is in ionic state. The emission depends strongly on the surface composition, so the matrix effect cannot be neglected. The secondary ions are measured with a mass spectrometer in order to determine the elemental, isotopic, or molecular composition of the surface. One of the most sensitive surface analysis techniques is SIMS. The elemental concentration down to ppb range can be detected by SIMS.
Different SIMS techniques are available:
Static SIMS: it is used for sub-monolayer elemental analysis
Dynamic SIMS: it is very convenient to obtain compositional information as a function of depth
Imaging SIMS: it is used for spatially-resolved elemental analysis
All techniques are based on the same basic physical process: the surface of the sample is subjected to high energy ion bombardment which leads to sputtering neutral and ionic elements from the surface. The ejected species may include atoms, clusters of atoms and molecular fragments.
In a traditional SIMS the positive ions are collected and analysed. This was worked out mainly from practical point of view, but it has a great disadvantage: determination of the correct concentration from the intensity of positive ions is not so simply, because the quantity of positive ions is so small that it cannot represent the fraction of the total sputtered species. In most cases Argon ions (Ar+) are used for bombarding the sample surface, but in some applications alkali metals (e.g. Ga+) are used instead of it.
The mass analyser can be a quadrupole type one with a resolution of unit mass. Nowadays time-of-flight (TOF) analysers are also used to provide substantially higher sensitivity and mass range.
Static SIMS
The aim of developing of the static SIMS was to obtain sufficient high signal to execute a compositional analysis of the surface layer without removing the significant fraction of it, i.e., to be able to analyse a monolayer from 1014atoms (molecules) which is less than 10% of the total number of atoms (molecules) found in 1 cm2 size monolayer. By this technique information about the topmost single atomic layer can be collected.
Surface Imaging using SIMS
SIMS is a very useful method to determine the lateral distribution of elements of a sample surface. Using an ion gun with small ion beam diameter and scanning the surface, the necessary information can be collected. In this method the main problem is to ensure sufficient high intensity of the secondary ion signal under the condition of a high spatial resolution. Therefore in modern SIMS equipments pulsed ion sources and time-of-flight (TOF) mass spectrometers are used instead of the traditional instrumental approach, where continuous-flux ion guns and quadrupole mass spectrometers are used. TOF mass spectrometers are very efficient and provide good resolution and sensitivity up to very high masses. The best lateral resolution which can be performed by a modern SIMS is lower than 50 nm.
Depth Profiling
The aim of depth profiling is to obtain information on the variation of element concentration as a function of depth, below the initial surface. Such information is obviously useful for analysis of layered structures produced in the semiconductor industry.
Since the SIMS technique is based on the detection of the ions removed from the surface - this is a destructive technique -, it is ideally suited to determine the depth profile of elements. A depth profile of a sample may be obtained by recording sequential SIMS spectra as the surface is gradually eroded by the incident ion beam. The plot of the intensity of a given mass signal as a function of time is a direct reflection of the variation of its concentration below the surface.
One of the main advantages of SIMS over other depth profiling techniques (e.g. Auger depth profiling) is its sensitivity to very low concentrations (~ppm) of elements. This is particularly important in the semiconductor industry where dopants are often present in very low concentration forms.
The depth resolution depends upon a number of factors:
- uniformity of etching by the incident ion beam
- depth of the crater at the end of ion etching
- nature of the ion beam (the mass & energy of the incident ions)
- the effects related to the physics of the sputtering process