With a new time-based method to control the light from an ultrafast laser, researchers have developed a nanoscale Additive Manufacturing technique that can fabricate tiny structures a thousand times faster than conventional two-photon lithography (TPL), without damaging resolution.

The new parallelized technique (known as femtosecond projection TPL) produces depth resolution of 175 nanometers, which is better than established methods and can fabricate structures with 90-degree overhangs that can’t currently be made.

The technique could lead to manufacturing-scale production of bioscaffolds, flexible electronics, electrochemical interfaces, micro-optics, mechanical and optical metamaterials, and other functional micro- and nanostructures.
The researchers of Lawrence Livermore National Laboratory (LLNL) and The Chinese University of Hong Kong reported the work in the journal Science (“Scalable submicrometer additive manufacturing”). Sourabh Saha, the paper’s lead and corresponding author, is now an assistant professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology.

 A printed micropillar forest submerged in the photopolymer resist prior to development. The forest contains 900 micropillars over a 7 mm × 7 mm area and was printed in less than 90 minutes. (Image: Vu Nguyen and Sourabh Saha)

Additive manufacturing techniques use a single spot of high-intensity light — typically around 700 to 800 nanometers in diameter — to convert photopolymer materials from liquids to solids. Because the point must scan through the entire structure being fabricated, the existing TPL technique can require many hours to produce complex 3D structures, which limits its ability to be scaled up for practical applications.

“Instead of using a single point of light, we project a million points simultaneously(…) 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.”

-Sourabh Saha-

To create a million points, the researchers use a digital mask similar to those used in projectors to create images and videos. In this case, the mask controls a femtosecond laser to create the desired light pattern in the precursor liquid polymer material. The high-intensity light causes a polymerization reaction that turns the liquid to solid to create 3D structures.

Each layer of the fabricated structure is formed by a 35-femtosecond burst of high-intensity light. The projector and mask are then used to create layer after layer until the entire structure is produced. The liquid polymer is then removed, leaving behind the solid. The FP-TPL technique allows the researchers to produce in eight minutes a structure wherein standard processes would take several hours to produce.

“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, LLNL’s Center for Engineered Materials and Manufacturing Director

Unlike consumer Additive Manufacturing that uses particles sprayed onto a surface, the new technique goes deep into the liquid precursor, allowing the fabrication of structures that could not be produced with surface fabrication alone. The technique produces what Saha calls an “impossible bridge” with 90-degree overhangs and with more than a 1,000:1 aspect ratio of length to feature size.

The researchers have printed suspended structures a millimeter long between bases that are smaller than 100 microns by 100 microns. The structure doesn’t collapse while being fabricated because the liquid and solid are about the same density — and the production happens so quickly that the liquid doesn’t have time to be disturbed.

The researchers used conventional polymer precursors, however, Saha believes the technique also works for metals and ceramics that can be generated from precursor polymers.

 A stacked nanoscale 3D ring structure generated by overlapping several projections in 3D space. Arbitrarily complex 3D structures can be generated through this depth-resolved nanoscale 3D printing technique. (Image: Vu Nguyen and Sourabh Saha)

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.”

-Sourabh Saha- 

Researchers had been working for years to accelerate the two-photon lithography process used to produce nanoscale 3D structures. The success of this group came from adopting a different way of focusing the light, using its time-domain properties, which allowed the production of very thin light sheets capable of high resolution — and tiny features.

The use of the femtosecond laser allowed the research team to maintain enough light intensity to trigger the two-photon process polymerization while keeping the point sizes thin. In the FP-TPL technique, the femtosecond pulses are stretched and compressed as they pass through the optical system to implement temporal focusing.

The process, which can generate 3D features smaller than the diffraction-limited, focused light spot, requires that two photons hit the liquid precursor molecules simultaneously.

Saha intends to continue advancing the work with new materials and further scale-up of the process.

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John Toon, Nano Werks 3D printing technique accelerates nanoscale fabrication a thousandfold www.nanowerk.com/nanotechnology-news2/newsid=53737.php published on Oct3 2019, published and re-edited by João Andrade on Oct8 2019;