Photoresist for Laser Additive Manufacturing:Status,Challenges,and Opportunities(Invited)
Significance Additive manufacturing(AM)has emerged as a key technology for processing and manufacturing objects with arbitrary three-dimensional(3D)structures.Commonly used AM technologies in recent years include fused deposition modeling(FDM),stereolithography(SLA),and digital light processing(DLP).In traditional laser-induced AM technology,the thermal energy at the laser focal point is too high to generate an obvious heat-affected area and seriously affect the processing quality.In addition,it cannot be used for the fabrication of flexible soft materials and transparent materials,and manufacturing micro-nano 3D structures is difficult.To solve these problems,femtosecond(fs)laser-induced two-photon polymerization(TPP)technology has revolutionized AM technology.As early as 1997,researchers discovered the two-photon absorption effect and used fs lasers instead of UV light sources to process 3D micro-nano structures.Owing to the polymerization threshold effect of fs laser focus,two-photon absorption is initiated when the light intensity at the center of the focal point exceeds the photoresist's two-photon ionization threshold.This transition of photoinitiator molecules from the ground state to the excited state initiates monomer polymerization at the focal point.When the focus is scanned point by point,extremely high processing accuracy and arbitrary 3D structure can be achieved.TPP technology utilizes the fs laser's traits of excellent penetration,robust 3D processing capability,and high resolution.It has excellent prospects for future applications in the fields of micro-nano processing,optical storage,and biomedicine.As an important component of TPP technology,photoresist is one of the core components of AM technology.However,achieving a perfect photoresist demands high purity,complex process formulas,and extensive technology research and development cycles.Progress Because of the ultra-narrow pulse and ultra-high peak power,direct laser writing(DLW)technology with fs laser occupies a key position in the field of additive manufacturing.The main components of the photoresist used in this technology include four main parts:monomer,photoinitiator,solvent,and other additives.The monomer forms the main body of the photoresist and the main body of the 3D structure.The selection and optimization of the material composition are directly affected by application requirements.In addition to traditional olefin monomers based on acrylic resin,the recent addition of different functional materials has greatly enriched the composition of photoresist monomers,such as glass and metal.The use of inorganic materials can remove organic molecules and other heterogeneous material monomers.Photoinitiators are igniters that trigger photochemical reactions in photoresists.Recently,in addition to traditional TPO and coumarin-based organic photoinitiators,there has been a growing interest in using inorganic semiconductor materials as photoinitiators to induce additive manufacturing(AM).In addition to high quantum yield photoresists,recent studies have explored the use of photoinitiators based on two-step absorption and triplet-triplet annihilation methods to develop low-cost pulsed laser additive manufacturing(AM)technology.These aim to replace traditional two-photon absorption photoinitiators,enabling new 3D printing technology to be developed without the need for an fs laser.In addition to monomers and photoinitiators,solvents and other additives also impact the physical and chemical properties of photoresists.Based on the optimization of these additives,photoresists can be made more convenient,and the structure obtained by AM can meet processing and application requirements.Conclusions and Prospects Laser AM technology is a promising emerging technology.As the core photoresist of this technology,this article has detailed that its different compositions and formulas have an important impact on the technology and application of laser AM.However,numerous opportunities for photoresists and laser AM remain unexplored.Especially,application scenarios that currently seem to exist only in science fiction may become a reality when we break the barriers of materials and technologies for photoresists.Laser-driven AM technology represented by DLW has upgraded the traditional object design and manufacturing model.The advancement in photoresist technology allows for the creation of complex and highly functional 3D structures through laser interaction.This notable progress instills confidence in our ability to achieve functionalities akin to those found in sci-fi like applications.In the near future,the development of photoresists will overcome technical limitations.Then,photoresists with ultra-resolution,ultra-high speed,ultra-high corrosion resistance,and ultra-wide processing optical wavelength range will be designed and developed to meet the needs of various application scenarios.This will promote the further development of micro-nano manufacturing technology for applications in fields such as semiconductor technology,photonics,optical communications,and solar cells.
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