High-Precision and Rapid Direct Laser Writing Using a Liquid Two-Photon Polymerization Initiator

Two-photon polymerization based direct laser writing(DLW) is anemerging micronano 3D fabrication technology wherein two-photon initiators(TPIs) are a key component in photoresists. Upon exposure to a femtosecondlaser, TPIs can trigger the polymerization reaction, leading to thesolidification of photoresists. In other words, TPIs directly determinethe rate of polymerization, physicochemical properties of polymers,and even the photolithography feature size. However, they generallyexhibit extremely poor solubility in photoresist systems, severelyinhibiting their application in DLW. To break through this bottleneck,we propose a strategy to prepare TPIs as liquids via molecular design.The maximum weight fraction of the as-prepared liquid TPI in photoresistsignificantly increases to 2.0 wt %, which is several times higherthan that of commercial 7-diethylamino-3-thenoylcoumarin (DETC). Meanwhile,this liquid TPI also exhibits an excellent absorption cross section(64 GM), allowing it to absorb femtosecond laser efficiently and generateabundant active species to initiate polymerization. Remarkably, therespective minimum feature sizes of line arrays and suspended linesare 47 and 20 nm, which are comparable to that of the-state-of-the-artelectron beam lithography. Besides, the liquid TPI can be utilizedto fabricate various high-quality 3D microstructures and manufacturelarge-area 2D devices at a considerable writing speed (1.045 m s(-1)). Therefore, the liquid TPI would be one of the promisinginitiators for micronano fabrication technology and pave the way forfuture development of DLW.