Published on 14 November 2024
IMT Atlantique has developed expertise in parallel photoplotting over many years. The school has therefore taken a leading role in the European FABulous project that will use this method to design functional 3D micro and nano-structures. These high-performance optical and photonic microstructures have applications ranging from the automotive industry to biological cell culture.
Example of test 3D microstructures produced using parallelization techniques with the FABulous photoplotter prototype developed at IMT Atlantique. There are no other techniques capable of producing this type of high-resolution 3D structure. Credits: FABULOUS Project.
Few people still have their photos developed these days. Yet the use of light to transfer a mask design onto a light-sensitive layer is now at the heart of a very promising field of research… but at the microscale! This is the principle behind photolithography, a fundamental technique used to produce the tiny electronic circuits and components in our connected objects.
Kevin Heggarty has been exploring this technology at IMT Atlantique for nearly twenty years, specifically in the area of parallel photoplotting. The researcher and his team are drawing on their expertise to contribute to the European FABulous project aimed at producing high-resolution 3D microscale optical components with functional surfaces.
Photoplotting or photolithography is the process of using a light beam (laser) or electrons to write patterns onto photosensitive materials, typically photopolymerizable resin. This specific type of resin polymerizes when exposed to light: it goes from a liquid-paste-like state to a solid state. The surface is then rinsed to remove the resin that is still in a liquid state so that only the polymerized resin remains.
This technique makes it possible to manufacture 2D structures on thin resin layers as well as 3D structures on thicker resin layers, “like a 3D printer,” Kevin Heggarty explains, “but with submicron resolution.” It then requires the use of an ultrafast laser called a femtosecond laser. “These lasers instantaneously increase the light intensity, with the effect of ‘confining’ 3D polymerization. This is not attainable when simple light sources, like LED and conventional lasers are used, since they polymerize everything they come in contact with. It then becomes very difficult, if not impossible, to make high-resolution 3D structures,” the researcher adds. This specific photolithography technique is called multiphoton.
Although photoplotting technology has been used to write in 2D for nearly forty years, 3D photoplotting has only been marketed for about ten years. The technique has applications in numerous fields, particularly in optics and photonics to manufacture structures the size of a wavelength. It is used, for example, to manufacture color filters and anti-glare filters for automotive cameras. It is also of interest to biologists for the design of micro-nanoscopic scaffolds to help with cell culture. However, the technology is marketed in its single-beam form, meaning that it uses only one beam that plots point by point. “The writing takes a while,” Kevin Heggarty says.
To overcome this obstacle, the researcher and his team dedicated around twenty years of study to a technique that speeds up the process by writing with several laser beams in parallel: parallel photoplotting. Although the process had already been applied to mono-photon (two-dimensional) photoplotting, it had never been combined with multi-photons, that is until recent progress in the field made this possible. After receiving a request for their expertise, Kevin Heggarty’s team joined the PHENOmenon H2020 project “to try to apply our knowledge and skills in the parallelization of the writing process to this new multi-photon lithography technique,” the researcher explains.
Example of the test production of a 3D grid for biologists for use as “bio-scaffold” or biological “scaffold”. Credits: FABULOUS Project.
The IMT Atlantique scientists then developed a photoplotter capable of designing unique microstructures by splitting a powerful laser into thousands or even tens of thousands of beams. The project was a great success, and the research teams were encouraged to follow up on their work. “We did so by setting up FABulous together with a slightly different consortium to overcome the limitations identified in the context of PHENOmenon,” Kevin Heggarty adds. In particular, he explains that “we can observe deviations between the pattern or structure obtained and the object initially programmed based on the data entered into the photoplotter.” This is due to the proximity effects.
In the parallel photoplotting technique, the beams must be very close to each other: the closer they are, the more beams can be written at the same time and quickly. But if they are too close to each other, the beams interact within the resin, producing an unwanted chemical change in the polymerization reaction. This is referred to as proximity effects. When conventional methods are used, the beams write on resin layers that are too thin to allow for any vertical interaction. When producing 3D structures, however, the thickness of the resin layer results in interactions between the beams, not only on the writing plane, but also in the upper and lower planes. The proximity effects are therefore felt in all three dimensions of writing.
These interactions are unavoidable, yet they can be mitigated. “It is possible to manufacture resins that will minimize proximity effects – which is one of the roles of our Greek partner with FABulous – and to work on the light source, which we are doing,” Kevin Heggarty says. “But the key to correcting them will above all be understanding the interactions between the beams and the resin.”
The scientists from Fraunhoffer IISB institute – one of the latest partners to join the FABulous consortium – specialize in modelling the photochemical light-resin interactions. They are therefore able to simulate and predict interactions with the aim of “pre-compensating” for the writing data. These corrective measures involve directing more light to some places than others to compensate for proximity effects – the interactions between the rays of light – to achieve the expected result.
The team is assembling a photoplotter designed for parallel photoplotting in the clean rooms at IMT Atlantique to meet the needs of the FABulous project. Credits: FABULOUS Project.
“The innovation lies in the fact that these models and techniques exist in 2D, but not in 3D!” Kevin Heggarty says. The IMT Atlantique team is therefore using parallel photoplotting to manufacture and characterize microstructures. The plot data – corresponding to the desired object – and the actual dimensions of the microstructures obtained are used to generate the models created at Fraunhoffer IISB, where they are working to expand their expertise to include 3D.
The models are then calibrated to represent what is actually happening in the resin based on the data entered into the machine and, conversely, to provide the corrections to be made to the data to obtain the desired object with high fidelity. “We work with Fraunhoffer IISB to agree on the type of structure that will provide the most information,” Kevin Heggarty says. “We start with simple structures for plotting and analysis; then as the predictive models improve, we begin to introduce more complex structures.” The ultimate goal is to obtain precise models, as close to reality as possible, and structures with the highest possible resolution. This will allow us to move from a small-scale functional process of a few square millimeters to demonstrators on surfaces covering several square centimeters.
The FABulous project (Fabrication of 3D metasurfaces to enable the next generation of high efficiency optical products) was launched on December 1, 2022, for a four-year period. It began as response to a call for projects from the Horizon Europe program on surface treatments and their functional integration, and has been awarded funding of €5.5 million. This amount is divided between the nine partners of the research consortium, in Belgium, France, Germany, Spain and Greece.
More information on the project website: https://fabulous3d.eu/
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