ID-2-P-1518 Imaging mass spectrometry and depth profiling for organic thin films using laser desorption ionization.

Introduction
The electrical devices composed of organic and inorganic materials such as organic light-emitting diodes (OLED) devices have been widely used. There are various techniques for surface analysis, a scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and so on. However, the analytical techniques to obtain the molecular information on organic layers are limited. In this presentation, we will discuss about the availability of the imaging mass spectrometry (IMS) and depth profiling using laser desorption/ionization time-of-flight mass spectrometer (LDI-TOFMS) as for analyzing organic thin films.

Methods
Three types of OLED compounds thin films deposited on Si wafer were prepared,
(i) α-NPD/Si: α-NPD deposited on Si wafer with 600 nm thickness.
(ii) α-NPD/2-TNATA/Si: 2-TNATA was deposited on Si wafer with 700 nm thickness. The α-NPD was deposited above the 2-TNATA layer.
(iii) α-NPD/2-TNATA/Si(mesh): 2-TNATA deposited on Si wafer with 440 nm thickness. The 880 nm α-NPD was deposited making a mesh pattern of 55 lines per inch above the 2-TNATA layer. The IMS and depth profiling were performed with MALDI-TOFMS (JMS-S3000, JEOL). The α-NPD/Si was also measured with SEM/EDS (JSM-7001FTTLLV, JEOL), XPS (JPS-9010, JEOL) and TOF-SIMS (Ar gas cluster ion source developed in Kyoto Univ. was applied to JEOL’s JMS-T100LP) for supplement measurements.

Results & Discussion
The [M]+ ions of α-NPD with negligibly small fragment ions were observed from α-NPD/Si with LDI-TOFMS(Fig.1). In the case of the TOF-SIMS, not only [M]+ ions but also many kinds of fragment ions were observed. The LDI-TOFMS and TOF-SIMS had an advantage for organic compounds analysis compared to SEM/EDS and XPS which could only obtain elemental or chemical state information. The LDI-TOFMS has lower spatial resolution rather than TOF-SIMS, but clear mass spectrum obtained with LDI-TOFMS has the advantage in degradation analysis, which the measurements of minor components will be often important.
Depth profiling was estimated with two layered thin film: α-NPD/2-TNATA/Si. The accession to the boundary of two layers could understand by turnover of the ion intensities of α-NPD and 2-TNATA(Fig.2). The ionization region in depth direction was depended on the laser intensity. The several hundred nanometer layer structure was clearly observed in appropriate laser intensity. Further investigation about the IMS and depth profiling by changing the laser condition using α-NPD/2-TNATA/Si(mesh) will be given in the presentation.