Multiphoton-Excited Deep-Ultraviolet Photolithography for 3D Nanofabrication

Light–matter interactions in the deep-ultraviolet (DUV) wavelength region exhibit a variety of optical effects such as luminescence, photoisomerization, and polymerization in many materials. Despite the rich photochemistry and high spatial resolution due to the short wavelength, the notorious lack of DUV-compatible optical components and devices precludes the use of DUV light in microscopy and lithography as a routine laboratory tool. Here, we present the use of two-photon excitation with visible laser light to realize photopolymerization of molecules with an excitation energy equivalent to DUV light. Using standard optics for visible light, we polymerized methacrylate oligomers with 400 nm femtosecond pulses without any addition of photoinitiators and sensitizers. By scanning the laser focus in 3D, we created a series of fine 3D structures with the smallest resolved line-space features of 80 nm. We found that photopolymerizations induced by two-photon absorption at DUV is surprisingly efficient and requires laser intensity only on the order of 100 kW/cm2. The use of DUV absorption sites natively existing in monomers simplifies polymerization reactions without sacrificing material purities by adding initiators and facilitates applications to diverse materials. We have applied two-photon DUV polymerizations to metal–oxo clusters and the amino acid cysteine. With the variety of successful demonstrations including organic- and inorganic-material-made-structures presented, multiphoton DUV polymerization offers a distinct tool for fabrications of nanodevices in 3D.