Low-voltage high-resolution SEM: A valuable resource for the polymer morphologist

Abstract

The elucidation of polymer morphology has historically been accomplished using optical microscopy (OM), conventional transmission electron microscopy (TEM), X-ray scattering and more recently for crystalline polymers, high resolution TEM (HRTEM). Significant work using scanning electron microscopy (SEM) to study synthetic polymers began in the late 1960's, but for twenty years after these first studies, the smallest features imaged by SEM in polymer samples have typically been 1-10 μm. The evolution of the low voltage high resolution SEM (LVHRSEM) now provides polymer scientists with the opportunity to image structures topographically on the order of 50-100 Å, a level of microstructure important in basic polymer physics research. The potential for improved understanding of polymer morphology is significant, especially when HRSEM data is combined with information from other complementary techniques such as TEM, X-ray scattering, and the scanning probe microscopies (SPM, AFM, STM).The advantages of LVHRSEM over conventional thermionic source SEM as a polymer characterization technique include: i) low keV operation reduces or eliminates charging in uncoated insulating polymers, ii) the high brightness, low energy spread, small spot size field emission gun allows for operation at low keV without significant loss of resolution (practically, resolution on the order of 50 Å at 1.0 keV in a polymer sample is attainable), iii) imaging of surface topography is improved at low keV as the beam/sample interaction volume is smaller and closer to the surface, resulting in more secondary electrons being generated within the secondary electron escape depth, and iv) the immersion lens system improves the signal to noise ratio by decreasing the noise component in the secondary electron signal.