Oct 4 2019
Researchers have developed a nanoscale 3D printing technique by using a new time-based method for controlling the light from an ultrafast laser. This new technique is capable of fabricating small structures a thousand times faster than standard two-photon lithography (TPL) techniques, without compromising on the resolution.
Image shows a printed micropillar forest submerged in the photopolymer resist prior to development. The forest contains 900 micropillars over a 7 mm x 7 mm area and was printed in less than 90 minutes as compared to more than a day of printing with serial techniques. (Image credit: Vu Nguyen and Sourabh Saha)
In spite of the high throughput, this new parallelized technique called femtosecond projection TPL (FP-TPL) can produce a depth resolution of 175 nm. This resolution is considered to be better than well-known methods and can also be used to fabricate structures with 90° overhangs that cannot be made at present.
The technique might enable manufacturing-scale production of flexible electronics, bioscaffolds, micro-optics, electrochemical interfaces, optical and mechanical metamaterials, and other functional micro- and nanostructures.
Published on October 3rd, 2019, in the journal Science, the study was carried out by researchers from Lawrence Livermore National Laboratory (LLNL) and The Chinese University of Hong Kong. The paper’s lead and corresponding author, Sourabh Saha, is presently working as an assistant professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology.
To convert photopolymer materials from liquids to solids, the current nanoscale additive manufacturing techniques employ a single spot of high-intensity light—normally around 700 to 800 nm in diameter. The point will have to scan through the whole structure being fabricated, and because of this, the existing TPL technique can need several hours to create complex 3D structures, which restricts its potential to be scaled up for practical applications.
“Instead of using a single point of light, we project a million points simultaneously,” said Saha. “This scales up the process dramatically because instead of working with a single point that has to be scanned to create the structure, we can use an entire plane of projected light. Instead of focusing a single point, we have an entire focused plane that can be patterned into arbitrary structures.”
The researchers created a million points by using a digital mask just like those used in projectors to produce videos and images. The mask, in this case, controls a femtosecond laser to produce the preferred light pattern in the precursor liquid polymer material. The high-intensity light creates a polymerization reaction that transforms the liquid to solid, where preferred, to produce 3D structures.
Every single layer of the fabricated structure is developed by a 35-fs burst of high-intensity light. The mask and projector are next used to create layer after layer until the whole structure is developed. This is followed by removing the liquid polymer, and the solid is left behind. The FP-TPL technique enables the researchers to create within 8 minutes a structure that would actually take several hours to create using earlier processes.
The parallel two-photon system that has been developed is a breakthrough in nanoscale printing that will enable the remarkable performance in materials and structures at this size scale to be realized in usable components.
Chris Spadaccini, Director, Center for Engineered Materials and Manufacturing, LLNL
Unlike consumer 3D printing that makes use of particles sprayed onto a surface, this new technique goes deep into the liquid precursor, permitting the fabrication of structures that could not be developed with only surface fabrication. For example, the technique can create what Saha refers to as an “impossible bridge” with 90° overhangs and with a length to feature size aspect ratio of over 1000:1.
We can project the light to any depth that we want in the material, so we can make suspended 3D structures.
Sourabh Saha, Assistant Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology
The researchers succeeded in printing suspended structures a millimeter long between bases that are considered to be smaller than 100 μm x 100 μm. The structure does not collapse while being fabricated because the solid and liquid are about the same density—and the production takes place so fast that the liquid does not have time to be disturbed.
Apart from bridges, the researchers also created a wide range of other structures chosen to demonstrate the technique, including cuboids, micro-pillars, log-piles, spirals, and wires. Although conventional polymer precursors were used by the researchers, Saha is hopeful that the technique will also be able to work for ceramics and metals that can be produced from precursor polymers.
The real application for this would be in industrial-scale production of small devices that may be integrated into larger products, such as components in smartphones. The next step is to demonstrate that we can print with other materials to expand the material palette.
Sourabh Saha, Assistant Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology
For many years, research groups have been working to accelerate the two-photon lithography process used for creating nanoscale 3D structures. This group succeeded by employing a different way of focusing the light by considering its time-domain properties, which enabled the production of extremely thin light sheets capable of high resolution—and small features.
By using the femtosecond laser, the research team was able to maintain sufficient light intensity to activate the two-photon polymerization process while ensuring that the point sizes are thin.
In the FP-TPL technique, the femtosecond pulses are first stretched and then compressed as they pass via the optical system to apply temporal focusing. The process, which can produce 3D features smaller than the diffraction-limited, focused light spot, will need two photons to simultaneously hit the liquid precursor molecules.
“Traditionally, there are tradeoffs between speed and resolution,” Saha said. “If you want a faster process, you would lose resolution. We have broken this engineering tradeoff, allowing us to print a thousand times faster with the smallest of features.”
At Georgia Tech, Saha plans to further improve the study with new materials and even scale-up of the process.
“So far, we have shown that we can do pretty well on speed and resolution,” he said. “The next questions will be how well we can predict the features and how well we can control the quality over large scales. That will require more work to understand the process itself.”