Takuya Miyayama oral presentation (FN1-Mon3-2-4)
Possibility of low energy bismuth cluster ion beams for an intact molecule detection in ToF-SIMS
ULVAC-PHI Inc, Enzo370, 2538522 Chigasaki, Japan
There is no perfect primary ion beam of time-of-flight secondary ion mass spectrometry (ToF-SIMS) for intact molecule detection with sub-micron spatial resolution. In the past two decades, a molecular imaging capability of ToF-SIMS has been improved with cluster ion beam techniques such as Aunq+, Binq+, C60q+, Arnq+, (H2O)nq+, EDI, ionic liquid and so on. [1-3] From the practical point of view, Binq+ is one of the most successful technique because of sub-100 nm beam size with high current density and better ion yield of high mass region from 200 to 1000 u. Even for Binq+, it is difficult to avoid the fragmentation during excitation of the secondary ions. As a result, the secondary ion yield of the intact molecule is thought to be depressed. On the other hand, large gas cluster ion beams such as Arnq+ showed an ability of intact molecule detection for more than 1000 u. In addition, better spatial resolution which is around 2 micron was recently reported. This beam size for Arnq+ seems to be the technical limitation at this moment, because of the broad energy distribution of the ion source.
Here, in order to achieve high secondary ion yield of intact molecule with sub-micron spatial resolution, we propose to use low energy (sub-keV) Binq+ cluster ion beam for ToF-SIMS. Generally, fragmentation probability of molecules depends on the energy of the primary ion beam. Especially, energy per atom is essential. From this point, Arnq+ is most promising technique because it easily achieves less than 10 eV/atom. Likewise, if we could provide lower energy per atom for Binq+ cluster ion beam, defragmentation of molecule would be expected.
Our study with 2 keV Bi3+ (666.7 eV/atom) showed much less fragmentation for some organic materials compared with 30 keV Bi3+ which is commonly used for ToF-SIMS measurements. In the case of Tris (8-hydroxyquinolinato) aluminium (Alq3) which is the most common material for the organic light emitting diode, we found there was almost no de-protonated molecule, (M-H)+, of the intact molecule, M, and also there was almost no proton, H+, in the spectrum. Same trends were also seen in the other materials. Those results said that atomic level fragmentation for the molecule was significantly suppressed by sub-keV bismuth ion beam and it could be one of the candidates for intact molecule detection in ToF-SIMS.
In the presentation, we will discuss the possibility of intact molecule detection using sub-keV bismuth primary ion beam with various organic materials.