An intriguing class of alternative lithography techniques utilize localized heating to create nanoscale features.@footnote 1@ The so-called thermolithography is very interesting since heat conduction in highly disordered polymer layers is a relatively slow diffusive phenomenon, which can be exploited to fabricate 3D nanostructures. We present our study of one type of thermolithography techniques that induces thermochemical cross-linking in select areas of a thin polymer layer. The patterned polymer layer can serve as part of a device or as a mask for subsequent processing. We demonstrate fabrication of T-gate structures and negative-slope resist profiles advantageous for the lift-off process. The thermal transport properties of polymer layers and kinetics of cross-linking processes are key parameters that determine the speed and resolution limit of the thermolithography techniques. We develop thermal transport property measurement techniques based on microfabricated ultra-thin membrane structures. The thermal conductivity and heat capacity of commercially available image reversal photoresists are determined before and after UV exposure, before and after postexposure bake, and also as a function of temperature. We also use micro-fabricated heaters coated with a polymer layer to induce precisely controlled heating and study kinetics of cross-linking. The spatial extent of cross-linking is determined by treating the polymer in a developer and examining the profile of the remaining layer using an SEM. The polymer profiles are compared with numerical heat conduction simulation results to extract the threshold cross-linking temperature as a function of heating duration. The present work demonstrates feasibility of a thermolithography technique and provides important data on the thermal transport properties and kinetics of polymerization that help guide further development of micro- and nano-scale thermolithography techniques.
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@footnote 1@ A. S. Basu, S. McNamara, and Y.B. Gianchandani,J.Vacuum Sci. & Tech. B, 2004, 22, pp. 3217-3220. M. Kuwahara , J.H. Kim , J. Tominaga, Microelectronic Engineering, 2003, 67-68, pp. 651-656