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Yang S H, Ding C L, Zhu D Z, et al. High-speed two-photon lithography based on femtosecond laser[J]. Opto-Electron Eng, 2023, 50(3): 220133. doi: 10.12086/oee.2023.220133
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High-speed two-photon lithography based on femtosecond laser
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Abstract
Two-photon polymerization (TPP) based on femtosecond laser has been a research hotspot in 3D micro/nano writing technology. With the increasing demand for processing complex and large-scale miniaturized 3D devices in the fields of life science, material engineering, micro and nano optics, and etc., the issue of low processing efficiency of TPP is becoming increasingly serious. During the long fabrication period, many disturbances can be introduced in the processing, causing the quality deterioration of the structure and seriously hindering the further popularization and application of these crucial 3D devices. This paper respectively compares the four approaches of single-beam writing, parallel multi-beam writing, pattern projection, and 3D projection exposure based on the TPP lithography efficiency. Moreover, the researches on the optical design of system, the writing accuracy, the fabrication throughput, the writing strategy, and etc. of each approach are also described. And the advantages and disadvantages of these four methods are summerized simultaneously. Finally, we also made a brief prospect to the developing trend of TPL efficiency improvement in the future. -
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References
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Overview
Two-photon lithography (TPL) has been a research hotspot in 3D micro/nano writing technology due to its characteristics of high resolution, low thermal influence, a wide range of processed materials, low environmental requirements, and 3D processing capability. It has shown unique advantages in the fields of life science, material engineering, micro/nano optics, microfluidic, micro machinery, and so on. This paper summarizes the research works done by researchers on different writing methods to improve TPL processing efficiency. Single-beam writing is the main method for TPL, which mainly depends on the speed of the scanning device. Single-beam writing has the advantages of simple system and high-quality beam, and it is easy to combine various effects to improve writing results. It mainly includes scanning modes based on the translation stage, galvo, polygon laser scanner, and acousto-optic deflector (AOD) (Fig. 2). All these modes have advantages and disadvantages. As for the scanning speed comparison, polygon laser scanner and AOD have relatively faster writing rates (faster than m/s). Multi-foci parallel lithography can obviously promote efficiency, elevating the speed by dozens or even hundreds of thousands of times, mainly based on spatial light modulator (SLM), digital micromirror device (DMD), microlens array (MLA), diffractive optical elements (DOE), multi-beam interference, and so on (Figs. 3-15). Multi-foci parallel lithography based on SLM is most widely used owing to its high efficiency and ability to flexible and independent control of each single beam, but the refresh rate is still insufficient. DMD has a higher refreshing rate (32 kHz), but the state-of-the-art beam parallelism realized by DMD is severely limited. More parallel beams are further required for improving the processing efficiency. The 2D pattern exposure method based on SLM or DMD can further improve the TPL efficiency with the superiority of generating flexibly designed pattern (Figs. 16-18). However, the 2D projection exposure technology is still difficult to achieve high writing precision, especially the axial resolution. An available method to improve the axial precision is spatially and temporally focusing an ultrafast laser to implement a strong intensity gradient at the spatial focal plane that restricts polymerization within a thin layer. The 3D projection method will be the most efficient writing method in the future, especially in 3D device processing (Figs. 19-20). Researchers used this technique to make hollow tubular and conical helices structures, increasing the processing speed by 600 times. However, the research results show that the current 3D projection can only process simple 3D structures. Further researches on 3D exposure processing of complex structures are expected, which will effectively expand its application in various fields. Authors believe that with the effort of researchers on efficiency improvement gradually, TPL can further highlight its advantages to promote the development of life science, materials engineering, micro-nano optics, and many other fields.
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Yang S H, Ding C L, Zhu D Z, et al. High-speed two-photon lithography based on femtosecond laser[J]. Opto-Electron Eng, 2023, 50(3): 220133. doi: 10.12086/oee.2023.220133
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Figure 1.
Optical methods of improving the TPL throughput. (a) Single-beam writing; (b) Multi-foci parallel lithography; (c) Pattern projection; (d) 3D projection exposure
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Figure 2.
The scanning scheme for single-beam writing based on (a) motorized stage, (b) galvo, (c) polygon laser scanner, and (d) acousto-optic deflector (AOD)
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Figure 3.
(a) Configuration of the parallel writing system based on diffractive beam splitter (DBS) and multi-beam interference; (b)~(d) The periodic structures written by the foci array generated via the interference of four femtosecond laser beams (800 nm/1 kHz). The exposure time is 30 s, 80 s, and 120 s, respectively; (e)~(g) The dot array structures fabricated by the foci lattice generated via four-beam interference with different interference angles of 10.8°, 21.9°, and 33.6°, respectively; (h) The periodic lattice structure written by the foci array generated via three-beam interference[53-55]
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Figure 4.
(a) Configuration of the multi-photon writing system based on four-beam interference; (b) Pillar array fabricated by once laser exposure (1 s), period 7.5 μm, height 20 μm, and diameter 3 μm; (c) Large-scale periodic structure fabricated by continuously shifting the once-exposure position of foci array with diameter of ~80 μm[56]
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Figure 5.
Femtosecond laser parallel direct writing system based on MLA. (a) Direct writing using MLA foci array; (b) Multi-foci writing by imaging the MLA foci array to the focal plane of the objective[63-64]
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Figure 6.
(a) The overall appearance and optical microscopic image of MLA; (b) SEM of the photopolymerized resin voxels on glass substrate; (c) The 2D array structure of 277 ‘N’ microletters fabricated by 28 foci;(d) The fabricated 3D microspring array[65]
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Figure 7.
(a) SEM of 3D periodic structures coated with silver fabricated on a hydrophobic coated glass surface; (b) Enlarged view of a single uncoated polymerized structure composed of a spring on top of a cube; (c) The polymerized structure of (b) with coated silver[66]
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Figure 8.
(a) DBS-based multi-photon processing system with multiple femtosecond laser beams; (b) Multi-foci pattern generated by DBS; (c)~(f) The structures of designed Logo, assembled microgear set (diameter 22 μm), 3D microfans, and assembled microgear set fabricated with the optical setup of (a)[72]
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Figure 9.
DOE-based mluti-beam parallel femtosecond laser writing. (a) SEM of the DOE surface; (b) Enlarged view of (a); (c) Overall appearance of the DOE; (d) Normalized false-color intensity measurement of the DOE using a CMOS-camera; (e) Parallel 3D two-photon fabricating system based on 3×3 foci generated by DOE; (f) Designed model of the chiral 3D metamaterial cubic cell, a=80 μm; (g) 2.4 mm×2.4 mm×9.6 mm chiral metamaterial structure consisting of 30×30×120=108000 3D cubic cells of (f); (h) Oblique side-view SEM of the structure in (g)73]
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Figure 10.
(a) The SLM-based multi-foci femtosecond two-photon lithography system; (b) The Venus sculptures fabricated by 4×4 foci; (c) The scaffold for cell cultivation fabricated by 4 foci; (d) 36 microneedles processed by 4 foci[74]
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Figure 11.
(a) The SLM-based multi-foci femtosecond laser parallel writing system; (b)~(c) The hologram loaded in SLM and the generated 7 foci pattern, respectively; (d)~(g) Microlens arrays fabricated by SLM with 3 foci, 7 foci, and 4×4 foci respectively; (h)~(i) The overall image and enlarged view of the 3D woodpile photonic structure fabricated by 10 foci; (j) Illustration of the multi-foci parallel fabrication process with the spiral scanning strategy; (k)~(m) 3D spiral photonic structures of ‘L’, ‘Z’, and ‘H’ fabricated by the multi-beam scanning way of (j). The foci pitch is 5.6 μm and the diameters of scanning spiral are 10 μm, 6 μm, and 8 μm, respectively[75]
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Figure 12.
(a) Phase modulation scheme based on two synchronized SLMs; (b) Illustration of the 3D PPI array through duplicating the 2×2 parallel PPI foci; (c) The dot array fabricated with femtosecond foci. The blue arrows indicate the scanning route; (d) The dot array fabricated by PPI foci with inhibition power of 210 mW. Scale bar: 500 nm; (e) SEM of the fabricated letters by 2 × 2 PPI foci (left) and the femtosecond multifocal array (right). Each letter is processed respectively by one focus of the 2×2 multifocal array. Scale bars: 1 μm; (f) The scheme for parallelized PPI 3D writing on different layers with interval of 1.5 μm; (g) 3D dots with interval of 200 nm and size of 80 nm on three different layers which are fabricated by the PPI multifocal array [76]
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Figure 13.
(a) Illustration of fabricating integrated miniaturized structure inside a ‘Y’ shape microchannel by parallel femtosecond multiple foci; (b)~(g) the structures with patterns of ‘LOC’ characters, ‘Cross’ and ‘Svastika’ fabricated on the substrate surface (b)~(d) and in the channel (e)~(g) respectively [77]
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Figure 14.
(a) The configuration of SLM-based multi-foci parallel writing system for micropillar fabrication. The inset illustrates the design of CGH; (b) The schematic diagram of self-assembly structure formation by capillary force on micropillars; (c)~(e) Diverse micro-assemblies with 6, 8, and 12 micropillars respectively fabricated by foci array with different beam quantity and distribution.Scale bar: 2 μm; (f)~(g) In-situ trapping of microparticles by the micro-assemblies fabricated with six-foci beams[78]
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Figure 15.
(a) Optical configuration of DMD-based multi-foci femtosecond laser parallel writing system; (b) Designed laser scanning trajectories, laser writing process, and SEMs of the fabricated woodpile structures with single-focus, two-foci, and three-foci writing respectively. Scale bar: 10 μm[79]
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Figure 16.
(a) Schematic diagram of the DMD-based femtosecond laser projection lithography system; (b) Large-scale periodic microstructures of ‘ORC’ letters fabricated by DMD once projection[80]
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Figure 17.
(a) Schematic diagram of DMD-based projection system using the spatial and temporal focusing of femtosecond laser; (b) 2.2 mm×2.2 mm×0.25 mm cube supported on a U.S. penny fabricated in 8 min 20 s; (c) Fabricated 3D micropillar stacked with2D layers; (d) Spiral structures through once projection within several millisecond; (e) Overhanging 3D structures[30]
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Figure 18.
(a) Schematic diagram of femtosecond laser direct writing. The optical intensity distribution at the focal plane is a round point; (b) Schematic diagram of single-exposure holographic femtosecond laser direct patterning. The optical intensity distribution at the defocused plane is a complex pattern ‘H’ designed by computer generated hologram (CGH). However, the distribution shows low signal-to-noise and leads to low surface quality in fabricating micro/nanostructures; (c) Multiexposure holographic femtosecond laser direct patterning. The optical intensity distribution at the defocused plane is a complex pattern ‘H’ designed by multi-CGH with different phases. Due to the improved strategy, the distribution shows high signal-to-noise and thus high quality micro/nanostructures will be realized[81-82]
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Figure 19.
(a) Experimental setup for single exposure fabrication of microtubes; (b) Calculated light intensity distribution at the focal region in the propagation direction; (c) SEM of microtubes[89]
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Figure 20.
(a) Experimental setup for the fabrication of 3D chiral microstructures by single exposure; (b) The spatial intensity distribution of the modulated beam and the electron microscopy of the writing results[90]
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