Resolution characterization and nanofabrication for soft X-ray zone plate microscopy.

摘要

Soft x-ray microcopy is a valuable nano-imaging technique in a wide variety of scientific disciplines. It complements other nano-imaging techniques, such as electron and scanning probe microscopy, by offering a unique set of capabilities including elemental and chemical specificity, magnetization sensitivity, as well as in-situ imaging with applied fields, overcoatings, and wet environments. By combining these advantages with high spatial resolution, the full-field transmission microscope, XM-1, operating at wavelengths in the 1 nm to 3 nm range, has yielded valuable knowledge in many areas of the physical and life sciences. A key to optimizing its performance for nanoscale studies is quantitatively determining and maximizing the spatial resolution. In this dissertation, new methods are demonstrated which permit accurate characterization and significant improvement of the spatial resolution. Based on both theoretical and experimental studies of the existing measurement techniques (using test objects such as knife edge, e-beam fabricated test structures, etc.), a new technique that uses multilayer coatings in cross section has been developed, which was shown to provide a more accurate means in quantifying resolution. By imaging this multilayer test object, systematic measurement of the microscope's modulation response as a function of feature periods is demonstrated. The measurement results show that, for the microscope with an objective micro zone plate fabricated using conventional single exposure electron beam lithography, the resolution is near-diffraction-limited at 20 nm. This resolution is limited by the smallest zone width of the micro zone plate, which was limited by the electron beam lithography used for fabricating the zone plate. To obtain better resolution, a new overlay nanofabrication technique has been developed by the nanofabrication team at Lawrence Berkeley National Laboratory (LBNL). This technique, based on sequential fabrication of alternating zone structures, significantly reduces the smallest feature sizes e-beam lithography is capable of fabricating in dense patterns. Using this technique with the LBNL's Nanowriter electron beam writer, zone plates of 15 nm outermost zone width have been fabricated for the first time, with excellent zone placement accuracy of 1.7 nm. Characterization of the microscope using the multilayer test object indicates that sub-15 nm spatial resolution has been achieved with these zone plates.