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In recent years, the use of cluster primary ion projectiles for organic secondary ion mass spectrometry (SIMS) has generated considerable interest as a method to improve molecular secondary ion yields, facilitate improved sensitivity for large molecule analysis and minimize the accumulation of beam-induced damage in selected organic materials. In this work, we report on our attempts to combine SF5+ primary ion bombardment with secondary ion imaging on an ion microscope SIMS instrument to produce spatially resolved molecular information as a function of depth. Three dimensional (3D) molecular imaging SIMS is achieved by acquiring a series of characteristic molecular secondary ion images as a function of increasing depth during dynamic SIMS sputtering of thin molecular films using cluster primary ion bombardment. Reconstruction of the resulting image stack provides a 3D volumetric image of the molecular composition of the sample. This approach has been used to examine several different types of samples including thin polymer films, multilayer polymer films,polymer films doped with pharmaceuticals and biological thin sections.
3D SIMS images have been obtained on the NIST ion microscope SIMS instrument using an SF5+ primary ion beam. Microscope imaging is particularly suited for use with cluster ion beam sources because the requirement for a highly focused primary ion beam is eliminated. This allows large diameter, higher current and lower impact energy cluster ion beams to be used which in turn allows for higher sputtering rates, faster analysis times and increased depth resolution. Also, image acquisition rates can be further increased since the secondary ion signal from each pixel in the image is acquired and digitized in parallel. An example of microscope-based molecular image depth profiling is shown in the Figure for a five micrometer thick rat brain section on silicon. In this example we show the cluster SIMS mass spectrum for the tissue section which demonstrates a high signal for the m/z 184 phosphatidylcholine molecular ion. Phosphatidylcholine is a phospholipid that is a major constituent of brain tissue. The distribution of this compounds was mapped by acquiring a series of images as a function of increased sputter time into the tissue sample. No degradation in molecular ion signal was observed during the analysis. In this example, the phosphotidylcholine was non-uniformly distributed as a function of depth and was anticorrelated with the cholesterol molecular ion distribution.