Chemically Selective Soft X-ray Direct-Write Patterning of Multilayer Polymer Films

Chemically selective soft X-ray direct-write patterning of trilayer polymer films was performed in a scanning transmission X-ray microscope, extending recent pioneering work on bilayer polymer films. Two trilayer polymer systems were examined: PMMA/PPC/PAN and PMMA/PEC/PAN, where PMMA = poly(methyl methacrylate), PPC = poly(propylene carbonate), PAN = polyacrylonitrile, and PEC = poly(ethylene carbonate). Each polymer layer was selectively patterned by exposure at its characteristic absorption energy: 288.45 eV for PMMA, 290.40 eV for PPC (PEC), and 286.80 eV for PAN. The patterns were visualized by imaging at these same energies. For the PMMA/PPC/PAN trilayer, highly selective patterning was achieved for the PAN and PPC layers, while the selectivity for the PMMA layer was poor. This was significantly improved by replacing PPC with PEC. The trilayer patterning process was simulated from the X-ray absorption spectra of the polymers, the layer order and thicknesses, and the critical doses for damage of each polymer. The simulations give semiquantitative predictions of the experimental contrast, and are a useful tool to find exposure times that optimize pattern contrast. Methods to improve patterning selectivity are discussed. Full color pattern reproduction with ∼150 nm spatial resolution is demonstrated with several high-resolution patterns created in the PMMA/PEC/PAN trilayer film.