Table of ContentsIntroduction Differences Between SIMS and SNMSHiden SIMS Workstation FeaturesHiden SIMS WorkStationSIMS ModeSIMS FeaturesSNMS ModeSNMS Features Combining SIMS and SNMS in the Same AnalysisConclusionsAbout Hiden Analytical
Secondary Ion Mass Spectrometry (SIMS) and Sputtered Neutral Mass Spectrometry (SNMS) are both surface analysis methods in which the specimen is bombarded by an ion beam and the sputtered particles are detected by a mass spectrometer. The key difference between these two methods is that the SIMS ions are generated directly by sputtering, whereas those detected in SNMS are produced by ionisation of neutral particles some time after leaving the surface.
Differences Between SIMS and SNMS
The differences between the SIMS and SNMS techniques are listed below:
- Since the ionization and sputtering events are different in SNMS, a near-constant ionization probability is possible for each element in spite of the originating material. This phenomenon is in contrast to the well known matrix effect of SIMS where the ionisation probability during sputtering can differ by orders of magnitude based on the surface chemistry.
- The SIMS quantification requirement for matrix matched reference materials is absent from SNMS. The techniques are quantified in several ways and are complimentary in the concentrations that they monitor, with SNMS covering 100% to 0.1% atomic concentration and SIMS typically providing quantification in the range 3% to below 10-6%.
This article aims to show that that SIMS and SNMS data may be recorded from within the same depth profile in a quasi-simultaneous way, such that the SNMS may be used to quantify the matrix composition at each point which can then be used to provide precise quantification of the impurity, by application of the SIMS RSF relevant to the alloy.
Hiden SIMS Workstation Features
Features of Hiden SIMS Workstation are listed below:
- The Hiden SIMS Workstation is designed around high sensitivity quadrupole SIMS analyser (Hiden MAXIM)
- The workstation is fitted with an electron impact ionization cell
- Efficient rejection of SIMS ions during SNMS analysis by means of a lateral electric field takes place
- Rejection of residual gas ions generated in the cell by ion energy analysis
- Automatic tuning of secondary ion column
- Rapid switching between detection modes
- Positive and negative ion SIMS
- Sputtered neutral mass spectrometry
- Residual gas analysis
Hiden SIMS WorkStation
In order to study secondary ion, for example, positive ions, an extraction potential, typically of the order of -20 to -60V is applied to the internal probe structure including the ionisation cage. This generates an extraction field that extends from the entrance aperture which captures ions from the target. A positive target potential assists in this process. Ions are collected from an off-axis position and the parallel steering plates sited immediately behind the cage act as an energy filter directing ions of 5 -10 eV ejection energy onto the axis of the quadrupole mass spectrometer. The filament is not needed for SIMS.
Certain features of SIMS are listed below:
- SIMS signals vary non-linearly with the matrix composition also called the matrix effect often over several orders of magnitude.
- Accurate quantification is possible when the impurity or the dopant is dilute within a uniform matrix less than 3%.
- For quantification, it is essential that matrix matched reference material to provide a relative sensitivity factor (RSF).
- Using SIMS to precisely determine the matrix (alloy) composition is not possible in the majority of materials.
- Detection limit can be low ppb.
An electron impact ionization cell is located near the front of the MAXIM probe in order to subtend the largest solid angle from the sampling point for the collection of neutral particles. Normally electron energy of 40 eV is used in order to reduce the generation of ESD signals, for some elements 20 eV is required. However, there is still considerable ionization of the residual gas even at UHV pressures. The two signals, sputtered neutrals from the sample and those generated by the residual gas may easily be differentiated by considerable difference in their energy distributions, with major amount of the sputtered neutral having energies in excess of 5 – 10 eV and those of the gas below 1eV. This is achieved by biasing the electron impact cage and using the parallel plate filter. SIMS ions are rejected by biasing the target and applying a lateral field at the entrance to the probe, forcing them away from the spectrometer axis.
The features of SNMS are listed below:
- Ionization is separated from sputtering resulting in minimal matrix effects.
- RSF is constant so specialist reference materials are not needed, easily available alloys will be sufficient.
- SNMS signals change linearly with composition and quantification methods are essentially simple.
- Detection range is typically 100% to 0.1%.
- Interference from molecular species is dramatically lower than with SIMS.
Combining SIMS and SNMS in the Same Analysis
The Hiden MASsoft spectrometer control software offers a highly flexible flowchart style control of all of the relevant voltages permitting SNMS and SIMS conditions to be swapped in approximately 500 ms and making it practical to collect both sets of data within the same depth profile analysis. The sample used in this example has a graded SiGe layer of approximately 20% Ge peak concentration and includes boron doping. Analysis was undertaken using 5 keV O2+ primary ions bombarding at 45° in order to provide a good sensitivity for the boron layer.
The conclusions are listed below:
- Combined SNMS and SIMS detection in the same analysis has been shown.
- SNMS can determine the local matrix composition so that the correct SIMS RSFs can be applied.
- SNMS can provide quantification above the dilute limit where the matrix effect prevents accurate application of SIMS.
- Using a sample with composition across the detection boundary of the two techniques permits the SIMS RSF to be determined quickly and easily without the manufacture of a special reference material.
About Hiden Analytical
Hiden Analytical is a leading manufacturer of quadrupole mass spectrometers for both research and for process engineering. Their products Our products address a diverse range of applications including:
- Precision gas analysis
- Plasma diagnostics by direct measurement of plasma ions and ion energies
- SIMS probes for UHV surface science
- Catalysis performance quantification
- Thermo-gravimetric studies
These analytical instruments are designed to work over a pressure range extending from 30 bar processes down to UHV/XHV.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
For more information on this source, please visit Hiden Analytical.