Optimizing depth resolution in confocal Raman microscopy: a comparison of metallurgical, dry corrected, and oil immersion objectives.

Spherical aberration is probably the most important factor limiting the practical performance of a confocal Raman microscope. This paper suggests some simple samples that can be readily fabricated in any laboratory to test the performance of a confocal Raman microscope under realistic operating conditions (i.e., a deeply buried interface, rather than the often-selected alternative of a bare silicon wafer or a thin film in air). The samples chosen were silicon wafers buried beneath transparent polymeric or glass overlayers, and a polymer laminate buried beneath a cover glass. These samples were used to compare the performance of three types of objectives (metallurgical, oil immersion, and dry corrected) in terms of depth resolution and signal throughput. The oil immersion objective gave the best depth resolution and intensity, followed by a dry corrected (60x, 0.9 numerical aperture) objective. The 100x metallurgical objective was the worst choice, with degradations of approximately 5x and 8x in the depth resolution and signal from a silicon wafer, comparing a bare wafer with one buried under a 150 microm cover glass. In particular, the high signal level obtained makes the immersion objective an attractive choice. Results from the buried laminate were even more impressive; a 30x improvement in spectral contrast was obtained using the oil immersion objective to analyze a thin (19 microm) coating on a PET substrate, buried beneath a 150 microm cover glass, compared with the metallurgical objective.