Investigations on the behaviour of C₆₀ as a resist in X-ray lithography

Abstract

Oxygen-free C₆₀ films on various substrates have been used as negative resist in X-ray lithography, yielding pure-carbon microstructures of good quality via synchrotron irradiation through an X-ray mask and subsequent development. While X-ray s induce the polymerization of C₆₀ into insoluble product, large numbers of secondary electrons backscattered from the substrate inhibit this, which we attribute to the different cross sections of X-ray s and electrons in C₆₀. Both electrons and X-ray s generate neutral electronically excited C₆₀ molecules. At a low density of excited C₆₀, as generated by X-ray s, they react predominantly with neighbouring molecules in their ground state via a 2+2-cycloaddition similar to UV polymerization. At a high density of excited C₆₀, as generated by secondary electrons, the excited molecules are not able to react with each other due to orbital symmetry. Instead, the excited states quench each other, thus inhibiting the polymerization. The reduction of the resolution in the C₆₀ pattern, and the inhibition of the polymerization near the interface through backscattered electrons, can be reduced by using substrate materials from which only few electrons emerge. To maintain the density of excited C₆₀ molecules at the interface below the point where the quenching reaction prevails, low synchrotron-radiation intensities are recommended.