Multiphoton lithography

Multiphoton lithography (also known as direct laser lithography or direct laser writing) of polymer templates has been known for years[timeframe?] by the photonic crystal community. Similar to standard photolithography techniques, structuring is accomplished by illuminating negative-tone or positive-tone photoresists via light of a well-defined wavelength. The fundamental difference is, however, the avoidance of reticles. Instead, two-photon absorption is utilized to induce a dramatic change in the solubility of the resist for appropriate developers.

Hence, multiphoton lithography is a technique for creating small features in a photosensitive material, without the use of complex optical systems or photomasks. This method relies on a multi-photon absorption process in a material that is transparent at the wavelength of the laser used for creating the pattern. By scanning and properly modulating the laser, a chemical change (usually polymerization) occurs at the focal spot of the laser and can be controlled to create an arbitrary three-dimensional periodic or non-periodic pattern. This method has been used for rapid prototyping of structures with fine features.

Two-photon absorption is a third-order with respect to the third-order optical susceptibility ${\displaystyle \chi ^{(3)}}$ and a second-order process with respect to light intensity. For this reason it is a non-linear process several orders of magnitude weaker than linear absorption, thus very high light intensities are required to increase the number of such rare events. For example, tightly-focused laser beams provide the needed intensities. Here, pulsed laser sources are preferred as they deliver high-intensity pulses while depositing a relatively low average energy. To enable 3D structuring, the light source must be adequately adapted to the photoresist in that single-photon absorption is highly suppressed while two-photon absorption is favoured. This condition is met if and only if the resist is highly transparent for the laser light's output wavelength λ and, simultaneously, absorbing at λ/2. As a result, a given sample relative to the focused laser beam can be scanned while changing the resist's solubility only in a confined volume. The geometry of the latter mainly depends on the iso-intensity surfaces of the focus. Concretely, those regions of the laser beam which exceed a given exposure threshold of the photosensitive medium define the basic building block, the so-called voxel. Other parameters which influence the actual shape of the voxel are the laser mode and the refractive-index mismatch between the resist and the immersion system leading to spherical aberration.

It was found that polarization effects in laser 3D nanolithography can be employed to fine-tune the feature sizes (and corresponding aspect ratio) in the structuring of photoresists. This proves polarization to be a variable parameter next to laser power (intensity), scanning speed (exposure duration), accumulated dose, etc.

References

• Deubel M, von Freymann G, Wegener M, Pereira S, Busch K, Soukoulis CM (2004). "Direct laser writing of three-dimensional photonic-crystal templates for telecommunications". Nature Materials. 3 (7): 444–7. Bibcode:2004NatMa...3..444D. doi:10.1038/nmat1155. PMID 15195083.
• Haske W, Chen VW, Hales JM, Dong W, Barlow S, Marder SR, Perry JW (2007). "65 nm feature sizes using visible wavelength 3-D multiphoton lithography". Optics Express. 15 (6): 3426–36. Bibcode:2007OExpr..15.3426H. doi:10.1364/OE.15.003426. PMID 19532584.
• Rekstyte S, Jonavicius T, Gailevicius D, Malinauskas M, Mizeikis V, Gamaly E G, Juodkazis S (2016). "Nanoscale Precision of 3D Polymerization via Polarization Control". Advanced Optical Materials. 4 (8): 1209–14. doi:10.1002/adom.201600155.