Additive microfabrication—three-dimensional (3D) printing on the micron and submicron level—is relatively new and is expected to have a broad niche market especially in biomedical and wearable electronics industries. The available microprinting techniques, however, suffer from either low throughput that constrains their scaling up to mass production, or poor resolution on the micron scale. A team of researchers has increased the printing speed by more than 1000-fold without sacrificing resolution of the printed pattern. The research team at Lawrence Livermore National Laboratory (LLNL) and The Chinese University of Hong Kong, led by Sourabh K. Saha and Shih-Chi Chen, succeeded in parallelizing two-photon lithography (TPL), a higher resolution lithography method.
TPL is typically a serial method where submicron patterns are printed in a sequential manner rendering it too slow to be practical. The researchers developed a technique where TPL was used to print large areas simultaneously without sacrificing the resolution of the structures that reached length scales as small as 130 nm. The technique, as introduced in a recent issue of Science (doi:10.1126/science.aax8760), parallelized TPL-based microfabrication by combining technologies from laser physics, digital optics, and 3D printing.
In two-dimensional (2D) lithography, a 2D pattern is printed on a photosensitive polymer (resist) by exposing it to focused light or a laser through a patterned mask; areas exposed to the light are chemically altered (cured/polymerized) while those covered by the mask are not. By changing the solubility of the exposed volume, a 2D pattern emerges after submerging the polymer into a solvent. The idea was to achieve 3D printing by printing 2D layers on top of each other creating 3D structures.
The main challenge that the researchers overcame was patterning a thin sheet of the polymer while leaving the lower and upper layers unaffected. This allowed layer-by-layer printing by moving a new sheet of the uncured polymer into the focal plane of the laser. The new technique accomplished just that by simultaneously focusing a pulsed near-infrared laser in the time and space domains and thus creating a thin, temporally focused light sheet.
“This was implemented by focusing patterned laser pulses in the time domain such that it has the shortest duration and highest intensity at the spatial focal plane. Basically, the laser pulse was stretched and then compressed in the desired plane, a technique used in designing high-power ultrafast lasers,” says Saha, now an assistant professor at the Georgia Institute of Technology.
“By stretching the pulse, its intensity becomes too weak to polymerize the resist, and by designing the optical system such that the pulse is shortened in the focal plane we managed to control the intensity of the laser and consequently the polymerization of resist in the axial (third) dimension,” says Chen, an associate professor at The Chinese University of Hong Kong. The pulse stretching was achieved by exploiting the patterning mask, a collection of digital micromirrors, as a diffraction grating to disperse a broadband femtosecond laser into its constituent wavelengths and accordingly increasing the pulse duration. The setup could be used to print features as small as ∼130 nm.
The researchers could print large areas of 165 µm × 165 µm in a matter of milliseconds. “In conventional TPL, a focused laser spot traces a pattern. So, printing a centimeter-sized object could take a year,” Chen says. The attained speed did not affect the resolution; structures smaller than the optical diffraction limit were achieved by changing the beam power and exposure due to the nonlinear absorption mechanism.
“The parallel two-photon lithography system breaks the coupling between nanoscale feature size and slow build speeds,” says Christopher Spadaccini, director of the Center for Engineered Materials, Manufacturing and Optimization at LLNL, who was not involved in the study. The technique enabled high resolution to be realized in the axial direction, as among the most challenging goals in additive microfabrication. Chen says, “We managed to confine the laser axially to a couple hundred nanometers, an improvement over conventional methods.” Another important accomplishment of the technique is the capacity to print features difficult to achieve with any other method such as curved structures and suspended bridges.
This achievement opens the door to many applications spanning health care to energy and metamaterials. “The demonstrated 3D microprinting with submicron resolution and unprecedented rates opens numerous applications in micro-robotics, drug delivery, tissue engineering, wearable electronics, and sensors industries,” says Andrei Kolmakov, project leader at the National Institute of Standards and Technology (NIST), who conducts research on high-resolution 3D printing with focused electron and x-ray beams. He was not involved in this study.
“Through this new technique, there is now a path to high-throughput nanoscale printing making it practical for real-world applications beyond the study of nanoscience and engineering. We have the setup built by Dr. Saha in the lab and are excited to work with him to apply it in new areas,” Spadaccini says. Chen shares the same enthusiasm for the next step: “We are now looking on how to scale it up to make functional structures and exploring different optical methods to manipulate the temporally focused light sheet.”
(a) Schematic of the microfabrication setup. L1 refers to the collimating lens and L2 refers to the objective lens that focuses the pulse in the time domain. The mirror is a collection of digital micromirrors that are switched on or off to pattern the reflected laser. (b) Zoomed-in illustration of the temporal focusing where the shortest pulse is only achieved at the focal/print plane. (c) Spiral structures printed in single-digit millisecond time scales without any stage motion. (d) Bridge structure printed by multiple 2D projections. The structure is generally difficult to print using other techniques. Credit: Science.
The technique is a breakthrough for resolutions down to ∼100 nm. Achieving resolution in the single-digit nanometer scale in additive manufacturing remains a challenge. However, groups at Oak Ridge National Laboratory and NIST have already managed to reach features as small as a few nanometers using electron beams. Scaling up printing at such high resolutions is still at the forefront of research in this field.
