Software-based precision and speed optimisation of two-photon polymerisation 3D printing processes

by

Dr Frank Bokeloh, product marketing specialist, Nanoscribe

The trend towards miniaturisation impacts fields such as optics and mechanical systems and has led to the standard technologies used by the majority of us every day in smartphones and other portable devices. New, advanced microfabrication technologies, which allow for the realisation of submicron structures with nanometre precision, play a key role in these far-reaching developments. These technologies reinforce the trend towards miniaturisation in other fields such as life sciences, photonics and material engineering. In this context, two-photon polymerisation (2PP), also referred to as multiphoton polymerisation, multiphoton lithography or direct laser writing, is a key enabling technology for 3D printing objects and structures of nearly any shape with submicron details. 

Of great importance is a straightforward workflow, which includes software solutions that make the technology accessible to experts of different domains without programming knowledge. Wizard-based software tools simplify the complete 3D printing workflow and field-proven parameter presets provide a good starting point for optimising precision and speed. In addition to the inherent design freedom afforded by the 3D printing approach, 2PP-dedicated software solutions allow for a focus on designing sophisticated applications at the nano-, micro- and meso-scale.

High-precision 3D printing technologyTwo-photon polymerisation belongs to a family of 3D printing technologies, in which light is used to cure a liquid photoresin and thus form a digitally defined structure. The basic physical prerequisite of 2PP is two-photon absorption. Two-photon absorption implies that an atom or molecule absorbs two photons simultaneously, exciting it into an energetically higher state. Here, the medium is typically a liquid resin that is photosensitive and can be cured by ultraviolet (UV) light. 2PP uses light with lower energy, for example, in the near-infrared range (NIR), which causes the printing material to solidify only if the photoresin molecules simultaneously absorb the energy of two photons. This mechanism is only likely in the focal volume of pulsed light because it requires a high intensity inside a volume of the photosensitive material1. Figure 1 illustrates the different polymerisation profiles resulting from one-photon absorption and two-photon absorption. Light and a molecular component, the photoinitiator, trigger a chemical reaction in the photoresin. Upon excitation, the monomers in the photoresin convert to a cross-linked solid state, resulting in a solid polymer. A sharp polymerisation threshold separates the polymerised from the unpolymerised areas. Outside of the focal volume, the intensity falls below the polymerisation threshold and thus the photoresin remains liquid.

High-precision 2PP requires a well-tuned 3D printer incorporating a laser, an objective lens and a liquid photosensitive material. By means of an objective lens, a pulsed laser beam is focused into a very tight region inside a volume of the photosensitive material. Even though the average power of the laser might seem quite low, the laser emits ultrashort pulses of light that contain a high photon density. High-end 2PP systems use an optical autofocus (AF) system to precisely locate the interface between the resin and the substrate to exactly position the printed object. For fast and precise printing, galvanometer scanners move the focal point in the focal plane at speeds of hundreds of millimetres per second. In addition, a stage system that has nanopositioning capabilities in all three dimensions moves the substrate carrying the photoresin to build up the 3D microstructures layer by layer.

Software as an essential component in the 3D printing workflow

Software plays a key role in ensuring efficient operation by controlling all printer hardware and parameters throughout the process. Print job development software such as Nanoscribe’s DeScribe ensures a straightforward workflow, from importing the CAD model to printing the object, and is easy to use because no operator programming knowledge is required.

The software has an import wizard for the loading of stereolithography (STL) files as 3D CAD print files. It also has predefined printing modes that serve as well-balanced starting points for preparing specific microfabrication jobs. These printing modes contain a set of carefully tuned print parameters, such as layer and line spacing of the print structure, scan speed and various printing strategies, thus defining an optimal laser focus path through the resin. Customised parameters for very particular structures or for printing using custom materials can be saved and re-used. Furthermore, this software supports a better understanding by offering a visualisation of the complete printing process.

To make 3D microfabrication accessible to not only experts in the field possessing programming knowledge but also scientists and engineers from different domains, the ideal software must have certain features, namely:

  • a 3D CAD model import wizard to guarantee an intuitive workflow for generating print jobs from standard STL files;
  • field-proven print parameter presets to achieve optimal print results;
  • software-based solutions for improving the shape accuracy of a printed object and identifying fitting parameters for new materials and applications (parameter sweep);
  • a 3D preview and printing simulation for insights into a print project’s print time, scan speed and laser power; and
  • an expert option to create and modify print files with coding.

 Optimising the trade-off between precision and speed

In 3D printing, precision and speed are two crucial and fundamentally linked parameters2. Increasing the printing speed results in a loss of quality of the printed object and vice versa. Therefore, it is critical to identify the best-suited requirements for a print. For example, since the surface quality of a larger microfluidic device is less critical than that of a microoptical lens, it is possible to save printing time by requiring less high precision with respect to its surface quality. The reasons for the dependence of precision and speed can be found in the hardware components, their control units and software features.

3D printer hardware changes can push the limits of precision and speed. Figure 2 illustrates the 2PP hardware setup. In the context of 2PP, the introduction of a galvanometer mirror is an excellent improvement that significantly increases the printing speed. Precision and speed remain inevitably linked and highest resolution printing is only possible if the scan speed of the galvanometer is adjusted accordingly. In addition, the introduction of a lower numerical aperture (NA) objective allows for the printing of a mesoscale object in a reasonable timeframe. The lower NA objective results in larger voxel dimensions, which fill more volume in less time but limit resolution of the printed object.

Smart software solutions also play a key role in finding the right balance between precision and speed in 2PP. Control solutions allow for management of system components, e.g., acceleration and deceleration of the galvanometer mirrors. User solutions allow for generation of 3D CAD print files as well as modification of printing parameters such as laser power, layer and line spacing of the print structure, and scan speed. All these settings can help balance precision and printing speed. Figure 3 illustrates how the size of the voxel, controlled by the laser exposure dose, and the distance between the written lines affect the trade-off between precision and speed.

Smart software features, such as an adaptive slicing mode for improving shape accuracy, can further optimise this balance. This mode uses the surface slope of the print object to automatically adjust the distance to the next slice. A steep slope results in a larger slice distance and the given volume of an object can be printed in less time. A shallow slope, on the other hand, results in a smaller slicing distance and therefore more slices to be written to fill the volume of an object. This slows down printing but results in more accurate structures.

Software recipes for dynamic precision printing

DeScribe software has five dynamic precision printing (DPP) modes, essentially fine-tuned print parameter presets that can be further customised to specific design requirements to achieve the optimum balance between precision and speed. Figure 4 illustrates the precision and speed differences of these five printing modes.

Solid mode is the starting point DPP mode, ensuring high precision, contour quality and shape accuracy. This mode fills the entire volume of the print object with adjacent lines and enables prints with Ra surface roughness values of less than 10 nm. It is particularly useful for microoptics and photonics applications.

Shell/scaffold mode incorporates inner support structures to bypass full-volume printing and therefore prints about five times faster than solid mode while maintaining high contour quality and shape accuracy. This mode is especially suitable for higher volume objects and includes a post-processing UV exposure step for curing or polymerising uncured photoresin trapped inside the pockets of the scaffold structure. Nair et al. used this mode to 3D print sockets for precise connection between fibres and optical components3. Printing these sockets in shell/scaffold mode reduced print time by ~80 percent compared with solid mode.

Pure shell mode is based on the shell/scaffold mode and prints about seven times faster than solid mode. This mode is optimal for smaller objects that are mechanically stable without inner support structures. Zvagelsky et al. used this mode to print 3D polymer wire bonds on a chip using a 63x objective4. By printing the outer shells of the wire bonds and then polymerising the photoresin trapped inside them via the post-processing UV exposure step, the fabrication process was significantly shortened.

Standard swift mode is the fastest of all DPP modes and prints about 10 times faster than pure shell mode while still providing remarkable quality that is functionally adequate for many purposes. This mode is an interesting alternative for high-volume printing where the functionality of the printed object can be ensured at lower precision.

Lastly, balanced swift mode is about six times faster than solid mode and affords significantly improved precision compared with standard swift mode.

Figure 5 shows a microfluidic nozzle printed in all five DPP modes.

Conclusion

Two-photon polymerisation has emerged as a solid technology for high-precision 3D printing at the nano-, micro- and meso-scale. It is a direct laser writing process, meaning the costly creation of masks and the use of multiple lithographic steps to create 3D and 2.5D microstructures are not necessary.

A major challenge in high-precision 3D printing is finding the optimal balance between precision and speed, but DPP offers finely tuned print parameter presets for quickly achieving excellent print results. There are modes that have print parameters for achieving exceptional precision, contour quality and shape accuracy but also modes that have parameters for compromising precision in favour of achieving faster print times. The latter are particularly useful for applications that do not require sub-micrometre resolution, such as many microfluidic or meso-scaled applications.

There is a wide range of applications for 2PP in research, prototyping, mastering and small series production, since nano-, micro- and meso-scale structures can be used directly or serve as polymer masters for replication processes in industrial volume production. Polymer masters are used to fabricate moulds in replication processes to upscale the production of high-precision parts, e.g., by injection moulding, hot embossing and nanoimprint lithography. Furthermore, the 2PP-based 3D printing approach allows for the fabrication of master moulds in 2D and 2.5D and miniaturised, complex structures. It can even be used for in-chip printing, a valuable technique that involves fabricating structures inside pre-manufactured objects such as microfluidic channels5. Figure 6 shows a 3D printed intricate spinneret inside a pre-manufactured microfluidic channel system.

Finally, enormous miniaturisation potential is expected of 2PP in integrated photonics projects that focus on increasing data processing and computing capacities for artificial intelligence (AI) applications. AI needs processing power growing at a rate of more than five times higher than given by Moore’s Law, and this is not possible using current electronic approaches. Photonic integrated circuits (PICs) reduce the sizes and costs of compact and functional photonic components. Applications range from biosensing, environmental monitoring and optical communication to imaging and quantum technologies. The design freedom afforded by 2PP allows for the realisation of different optical interconnects, and there are promising research efforts for, for example, high-precision 3D printed couplers6, waveguides7 and aspherical microlenses8, paving the way for future solutions in the aforementioned applications.

Nanoscribe

www.nanoscribe.com

References

¹Nanoscribe (2020). Additive-manufacturing based on two-photon polymerization—a key enabling technology for high-precision 3D printing [whitepaper].

Available at: https://bit.ly/35TPmCY

²Hahn, V., Kiefer, P., Frenzel, T., Qu, J., Blasco, E., Barner-Kowollik, C. and Wegener M. (2020). Rapid assembly of small materials building blocks (voxels) into large functional 3D metamaterials. Advanced Functional Materials, volume 30, issue 26, article number 1907795.

³Nair S, P., Trisno, J., Wang, H. and Yang, J. K. W. (2021). 3D printed fiber sockets for plug and play micro-optics. International Journal of Extreme Manufacturing, volume 3, article number 015301.

⁴Zvagelsky, R. D., Chubich, D. A., Kolymagin, D. A., Korostylev, V. V., Prokhodtsov, A. I., Tarasov, A. V., Goltsman, G. N. and Vitukhnovsky, A. G. (2020). Three-dimensional polymer wire bonds on a chip: morphology and functionality. Journal of Physics D: Applied Physics, volume 53, article number 53 355102.

⁵Lölsberg, J., Linkhorst, J., Cinar, A., Jans, A., Kuehne, A. J. C. and Wessling, M. (2018). 3D nanofabrication inside rapid prototyped microfluidic channels showcased by wet-spinning of single micrometre fibres. Lab on a Chip, volume 18, issue 9, pp.1341–1348.

⁶Hartmann, W., Varytis, P., Gehring, H., Walter, N., Beutel, F., Busch, K. and Pernice, W. (2020). Waveguide-integrated broadband spectrometer based on tailored disorder. Advanced Optical Materials, volume 8, article number 1901602.

⁷Bumjoon, J., Gargiulo, J., Ando, R. F., Lauri, A., Maier, S. A. and Schmidt, M. A. (2019). Light guidance in photonic band gap guiding dual-ring light cages implemented by direct laser writing. Optics Letter, volume 44, issue 16, pp.4016–4019.

⁸Bogucki, A., Zinkiewicz, L., Greszczyk, M., Pacuski, W., Nogajewski, K., Kazimierczuk, T., Rodek, A., Suffczynski, J., Watanabe, K., Taniguchi, T., Wasylczyk, P., Potemski, M and Kossacki, P. (2020). Ultra-long-working-distance spectroscopy of single nanostructures with aspherical solid immersion microlenses. Light: Science & Applications, volume 9, article number 48.