Optimization of sensitivity and quantification in SIMS

Secondary ion mass spectrometry (SIMS) is a well-established and extremely powerful technique for the chemical analysis of surfaces and thin films. Its main advantages are its excellent sensitivity, high dynamic range, good mass resolution and ability to distinguish between isotopes. Due to its excellent sensitivity and thus low detection limits, SIMS can be used to detect both major and trace elements. SIMS was originally mainly used for depth profiling. Since then, the applications have gradually shifted towards 2D and 3D imaging as a result of the dramatically improved spatial resolution thanks to the progress made on the instrumental side. As a consequence, new fields of application for SIMS, e.g. in life sciences and nanotechnologies, are emerging. In addition, the possibility of detecting several isotopes in parallel opens up even more horizons, mainly in life sciences, where isotopic labelling is an important investigation technique.

 

The ionization of the sputtered particles is heavily dependent on the impinging ions. Properly selecting the primary ion species is therefore crucial to optimizing the sensitivity of the analysis.

 

The use of caesium can dramatically increase the ionization probability of the negative secondary ions M-. We have therefore developed a Cs deposition column enabling the sample surface to be coated with Cs prior to or during SIMS analysis in order to optimize analytical performances. This system presents several advantages:

 

• increased sensitivities in the negative secondary ion mode (ionization probabilities of 100% for elements with high electron affinities)
• minimization or elimination of the matrix effect in the negative secondary ion mode
• optimization of the sensitivities in the MCs_x⁺ mode for the efficient elimination of the matrix effect
• optimized sensitivities in the M^- and MCs_x⁺ modes from the topmost layer (as opposed to conventional Cs⁺ bombardment where the topmost layers are sputtered away before a sufficiently high Cs concentration is reached)

 

The Cs deposition column is available in the following different versions:

 

Standalone Cs deposition system

 

• Cs deposition rates of up to 10 Å/s (measured on integrated quartz microbalance)
• Manipulator for 5 sample holders
• Sample transfer to SIMS instruments using a portable UHV suitcase system (10^-10 mbar)

 

 

Add-on Cs deposition column for existing instruments

 

• Cameca SC-Ultra / Wf
• Cameca IMS 4f/5f/6f/7f
• Ion-Tof instruments