Developing and implementing a Raman NSOM for the characterization of semiconductor materials

We have designed and constructed a novel Raman near-field scanning optical microscope (NSOM) and evaluated its performance characteristics with the goal of characterizing the strain in nanoscopic silicon structures. The Raman NSOM was built around a commercial Raman microscope to which a custom built stage was added to provide precise control over the tip position above the sample (z) using shear-force microscopy feedback as well as sample scanning in the x-y plane. The motion control axes were calibrated to better than 1 nm in z and approximately 20 nm in x and y. The NSOM provides both topographical images and Raman mapping with a lateral spectral resolution of 150-300 nm. The experiments described herein were enabled by gold-coated chemically etched NSOM tips with aperture diameters ranging between 60 and 150 nm. The sensitivity of the instrument was demonstrated by the high signal-to-noise ratios observed for Raman scattering by diamond and silicon in reflection mode. Spatial resolution and spectral sensitivity were demonstrated by obtaining well-resolved tip-sample separation curves that provide an accurate estimate of tip aperture size during an experiment.