Secondary ion emission from polymer surfaces under Ar+, Xe+ and SF5+ ion bombardment
Introduction
Sputtering as well as secondary ion formation is strongly influenced by the properties of the bombarding primary ion species. Mass and energy of the bombarding particles are important parameters which determine the size of and the energy distribution in the generated impact cascade which is responsible for secondary ion generation. Both have a strong influence on the resulting particle emission from the surface as well as on the charge state of these particles, that is on the secondary ion generation.
If molecular primary ions are applied, the effective impact energy of their individual atomic components is much lower than the total primary ion energy, which determines important beam characteristics as achievable ion current and spot size. The impact of several atoms at the same time and at the same place on the surface generates overlapping low impact cascades. Strong effects on the sputter as well as the secondary ion yields may be expected. In addition an appropriate selection of the chemical nature of these molecular primary ions allows a flexible matrix modification by primary particle incorporation into the surface near regime of the bombarded sample area.
Appelhans et al. [1]compared the ion formation efficiency for small organic molecules (pharmaceuticals) under Cs+ and SF06 bombardment, respectively. They found a much higher efficiency for SF06 bombardment. Recently we have successfully applied low energy SF+5 sputtering for depth profiling with a dual beam technique in a time-of-flight mass spectrometer [2]. We could remarkably improve the depth resolution, bringing it close to the limits determined by the statistics of the sputtering process. For this kind of depth profiling no mass separation of the sputtering beam is required. The application of SF+5 as a primary ion in TOF-SIMS, however, requires a pulsed and mass separating electron impact ion source [3]as is available in our TOF-instrument.
We have investigated the positive secondary ion emission from polymer surfaces (PET, PP, PTFE, PS, PC, PMMA and PEG) and some molecular overlayers of organic molecules on Ag and Si surfaces under 10 keV r+, e+ and SF+5 primary ion bombardment. In particular for polymer surfaces, changing from Ar+ to Xe+ and SF+5 bombardment, results in a remarkable increase in secondary ion yields and transformation probabilities.
Section snippets
Experimental
All experiments were carried out in a TOF-SIMS III reflectron based time-of-flight mass spectrometer [3]. The instrument was equipped with a mass separating pulsed electron impact ion source and an additional electron source for charge compensation. Secondary ions passed the drift tube of the TOF-instrument with an energy of 3 keV. They were postaccelerated up to 10 keV and detected by a channelplate-scintillator–multiplier combination. The pulsed primary ion source allows the generation of all
Results and discussion
We compared the secondary ion emission of 3 different groups of molecular surfaces under Ar+, Xe+ and SF+5 primary ion bombardment: surfaces of bulk polymers, spin coated thick polymer layers on silicon and some monomolecular overlayers.
Conclusion
By changing from Ar+ to Xe+ and to SF5+ bombardment we found a strong increase in the secondary ion yield Y (up to a factor of 1000) and a much smaller increase in the corresponding damage cross sections σ (up to a factor of 6) for characteristic molecular secondary ions emitted from polymer surfaces. Both effects are more pronounced in the higher mass range. The increases in the corresponding secondary ion formation efficiencies ranges between a factor of 5 and 50, depending on the polymer,
References (4)
- et al.
Anal Chem.
(1989) - et al.
J. Vac. Sci. Technol. A
(1996)