Improving the Spatial Resolution in Direct Laser Writing Lithography by Using a Reversible Cationic Photoinitiator

Direct laser writing (DLW) lithography has emerged as a competitive additive tool for the fabrication of detailed three-dimensional (3D) structures with a minimum feature size close to the nanometer scale. However, the minimal distance between adjacently written features with no overlapping, that is the writing resolution, is not in the same scale as the feature size. This is a consequence of the so-called "memory effect", namely, the accumulation of radicals between polymerized structures, which prevents the development of DLW for commercial applications. To overcome these limitations, we propose an original approach based on the reversible formation of the active species triggering the polymerization to decrease the impact of the "memory effect" on the writing resolution. We have selected the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecule as the cationic photoinitiator in combination with an oxidizing agent, AgPF6, to trigger the polymerization of the photoresist. The two-photon absorption (2PA) ability of the PCBM material was explored by using the open aperture z-scan technique, obtaining a 2PA cross-section of ∼400 GM. We have also utilized pump-probe spectroscopy to demonstrate the formation of the radical cation of the PCBM via a photoinduced electron transfer reaction with the Ag+cation (ΔG < 0). Moreover, the regeneration of the primary photoinitiating system PCBM/AgPF6was investigated with the flash photolysis technique, proving the absence of excited species in the μs time scale. This is the key point of our approach: the reversible character of the electron transfer process allows the partial regeneration of the primary photoinitiator in the interstice between polymerized structures avoiding the "memory effect". The implementation of this approach with commonly used resists, SU-8 or Araldite, has resulted in a notable improvement of the spatial resolution, from 600 to 400 nm when using a conventional photoinitiator compared to our PCBM/AgPF6system.