Taiwanese scientists are researching ways to overcome obstacles in photolithography, detailing their findings in the recently published paper, ‘3-Wavelength (UV, Blue, Red) Controlled Photo-Confinement for 3D-Printing: Kinetics and Modeling .’
Historically, photolithography has been limited in prototyping to fabrication of thin layers of material created via single wavelength light. To expand such techniques, the researchers explored two-color (UV and blue light), direct-write photolithography.
Currently, photopolymerization confinement (PC) is a common technique for continuous printing in additive manufacturing, offering a way to fabricate structures at high linear velocities, along with using only a single exposure.
Streamlined 3-Wavelength PC requires the following:
- Photoinhibition light (UV)
- Numerous, compatible monomers and co-initiators
- Photoinhibition of free-radical chain-growth
- Spatially confined, formulated resins
- Elimination of inhibitor species in the dark or after UV exposure
- Large polymerization inhibition depth adjacent to the projection window
- Continuous part production
“In dual-wavelength systems, the IT may be reduced by decreasing irradiation intensity and thus increase the print speeds as polymerization proceeds closer to the projection window,” stated the researchers. “For a given resin composition, the IT depends on the ratio of inhibitor to initiator absorbance, quantum yield of the radicals, reaction rates among the radicals and monomers, initiator, co-initiator and inhibitor, and the light intensity and dose (or exposure time).”
If the light is varied by pixel, the system can also complete topographical patterning on the surface with a single exposure layer. For a perpendicular scheme, photopolymerizations have been noted as confining ‘in depth the regional polymerized resin,’ with blue and UV lights causing respective polymerization initiation and inhibition.
Overall, the researchers realized that for an efficient PC, they required both a strong N-inhibition for uncured regime with a low conversion, and a weak S-inhibition for high conversion under either the blue light, or the blue- and red-light initiation.
A reverse feature was noted when [B] was higher than the transition value—leading to higher conversion as predicted by the research team’s formula:
“This is one of the new findings of this modeling study, which, however, requires further experimental justification,” stated the authors.
“Higher oxygen concentration leads to a lower conversion, which could be enhanced by reducing the S-inhibition via a red or blue-light pre-irradiation,” concluded the researchers. “The strategy for an efficient PC could be achieved by tailoring the two competing factors, N-inhibition and S-inhibition, independently via the steps of: (i) red-light pre-irradiation for minimal S-inhibition (oxygen-inhibition); and (ii) followed by a combined irradiation of UV and blue light, under the criterion for efficient PC to achieve reduced confinement depth, and high print speed.”
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