| Soft x-ray contact microscopy (SXCM) is a form of x-ray lithography however the biological cell is imaged instead of an electron-cut microcircuit mask. Imaging of the biological sample is by soft x-rays within the "water window", which has an energy range, 277-525 eV, or {dollar}lambda{dollar}, 4.5-2.4 nm. The contrast mechanism is based upon the x-ray absorption lengths of carbon and nitrogen, which are {dollar}sim{dollar}10 times shorter than that of oxygen in the "water window". The soft x-rays were generated by a conventional laboratory source or a laser-induced plasma x-ray source. The conventional laboratory soft x-ray source, a modified Henke tube, was a converted electron evaporator. The target materials were vanadium, which can generate V{dollar}sb{lcub}rm Lsbalpha{rcub}{dollar} (511 eV or 2.4 nm), and carbon, C{dollar}sb{lcub}rm Ksbalpha{rcub}{dollar} (277 eV or 4.5 nm). The laser-induced plasma x-ray source, which generates a nanosecond pulse, was the Lawrence Livermore National Laboratory JANUS laser facility. The target material was tantalum. A radiachromic nylon film was used to calibrate the exposure dose. The imaging detector was an x-ray sensitive resist, such as polymethylmethacrylate (PMMA) or a copolymer (90% PMMA and 10% methacrylic acid). A wet cell chamber (WCC) was designed to keep a biological sample alive at atmospheric pressure yet permit imaging by soft x-rays. Si{dollar}sb3{dollar}N{dollar}sb4{dollar} thin windows, {dollar}sim{dollar}120 nm thick, made the WCC a feasible holder within a high vacuum. The limits of resolution required an understanding of the interaction of soft x-rays with matter (the biological cell and the resist), Fresnel diffraction (a wavelength-dependent phenomenon), penumbral blurring (a geometric effect) and secondary electron travel within a resist. The imaging of the biological samples included sample preparation, resist development and electron microscope readout of the resist. The biological samples were fixed erythrocytes and zymogen granules, and critical point dried 9L glialsarcoma cells. The major result of this work was the first pictures of living mammalian cells, CHO-SC1, by SXCM. The LLNL JANUS laser facility was the x-ray source for the living cell x-ray micrographs, which had a resolution of {dollar}sim{dollar}30 nm. |
