Yields and images of secondary ions from organic materials by different primary Bi ions in time-of-flight secondary ion mass spectrometry.

Abstract

Bi cluster ions are used as a source of primary ions for time-of-flight secondary ion mass spectrometry (TOF-SIMS), and it has been recognized that secondary ion yields of macromolecules are higher with Bi cluster ions than with monomer ions or other cluster ions such as Cs(+), Ga(+) and Aun (+). However, the analysis conditions of Bi cluster TOF-SIMS are not sufficiently established. This study provides information on the secondary ion yields, damage cross-section and spatial resolution obtained with different primary Bi ions. We investigated the secondary ion yields, damage cross-section and spatial resolution using three different primary Bi ions in TOF-SIMS. The primary ions selected were Bi1(+), Bi3(+) and Bi3(2)(+) that were accelerated with 25 kV and the positively charged secondary ions were analyzed. The samples were 1, 2-distearoyl-sn-glycero-3-phosphocholine (C44H88NO8P, DSPC), which is a typical lipid, and N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (C44H32N2, NPD) and 4,4',4"-tris[2-naphthyl(phenyl)amino]triphenylamine (C66H48N4, 2-TNATA), which are organic functional materials. Although the secondary ion yields of DSPC were highest when measured with Bi3(+), the spatial resolution obtained from all DSPC analyses could not be evaluated because of the low intensity of the secondary molecular ions. On the other hand, for both NPD and 2-TNATA, the secondary ion yields were highest when imaged with Bi3(2)(+). Also, we obtained the highest spatial resolution using Bi3(2)(+). In the analysis of all molecules, the damage cross-section obtained with Bi3(2)(+) was also the highest. When secondary ions were sensitively detected, images of the high spatial resolution were obtained by using Bi3(2)(+). On the other hand, when the secondary ion sensitivity was low, the spatial resolution depended on the yields of secondary ions, implying that the selection of the primary ion species is crucial for SIMS analysis of large molecules.