Radiopaque Resists for Two-Photon Lithography To Enable Submicron 3D Imaging of Polymer Parts via X-ray Computed Tomography

Abstract

Two-photon lithography (TPL) is a high-resolution additive manufacturing (AM) technique capable of producing arbitrarily complex three-dimensional (3D) microstructures with features 2-3 orders of magnitude finer than human hair. This process finds numerous applications as a direct route toward the fabrication of novel optical and mechanical metamaterials, miniaturized optics, microfluidics, biological scaffolds, and various other intricate 3D parts. As TPL matures, metrology and inspection become a crucial step in the manufacturing process to ensure that the geometric form of the end product meets design specifications. X-ray-based computed tomography (CT) is a nondestructive technique that can provide this inspection capability for the evaluation of complex internal 3D structure. However, polymeric photoresists commonly used for TPL, as well as other forms of stereolithography, poorly attenuate X-rays due to the low atomic number (Z) of their constituent elements and therefore appear relatively transparent during imaging. Here, we present the development of optically clear yet radiopaque photoresists for enhanced contrast under X-ray CT. We have synthesized iodinated acrylate monomers to formulate high-Z photoresist materials that are capable of forming 3D microstructures with sub-150 nm features. In addition, we have developed a formulation protocol to match the refractive index of the photoresists to the immersion medium of the objective lens so as to enable dip-in laser lithography, a direct laser writing technique for producing millimeter-tall structures. Our radiopaque photopolymer resists increase X-ray attenuation by a factor of more than 10 times without sacrificing the sub-150 nm feature resolution or the millimeter-scale part height. Thus, our resists can successfully replace existing photopolymers to generate AM parts that are suitable for inspection via X-ray CT. By providing the "feedstock" for radiopaque AM parts, our resist formulation is expected to play a critical role in enabling fabrication of functional polymer parts to tight design tolerances.

Polymer-Based Device Fabrication and Applications Using Direct Laser Writing Technology

“…Surprisingly, with continuous development of new polymer composite materials, DLW technology can not only be applied to polymer materials, but can also be widely used in organic molecules, dielectrics, metals, semiconductors, proteins, hydrogels, and other materials [125,126,127,128,129]. Furthermore, for the preparation of polymer-based microstructures, other techniques, such as photo-lithography with exposing masks, imprinting technique, and ink-jet printing technique, also have their own advantages [130,131,132,133,134]. It can be expected that innovation of the processing techniques and combination of the DLW method with other fabrication techniques can be used to process a wide range of new materials to make devices with high complexity, more functionality, and better performance, which will be applied in even wider fields.…”

A Systems Approach to Estimating the Uncertainty Limits of X-Ray Radiographic Metrology

“…X-ray CT metrology offers great potential for metrology of complex micro-/nano-scale structures, through its combination of 3D imaging, nondestructive evaluation of internal features and material distribution, all potentially at sizes below what is possible with optical techniques [9,10]. High performance systems (e.g., nano-CT [11]) have pushed the limits of X-ray CT systems down to the smallest scales and up against the resolution and variation limits inherent to the machines which can distort or hide fine structures.…”

“…X-ray CT is an optimal technique to provide dimensional metrology of complex small-scale structures, such as those produced by additive manufacturing, due to its capability to nondestructively measure internal features and multimaterial components [9,10,13,14]. X-ray CT is mainly used in the following fields: medical imaging, material analysis, and recently dimensional metrology [9,10,13,[15][16][17][18][19]. However, the use of X-ray CT in manufacturing metrology has been limited due (1) to a lack of international standards for metrological testing and uncertainty assessment [13] and (2) an often significant and poorly understood level of variability in the metrology results at the micro/ nanoscale.…”

Conjugated Carbazole‐Based Schiff Bases as Photoinitiators: From Facile Synthesis to Efficient Two‐Photon Polymerization

“…Great efforts have been devoted to development of both materials and methods for 2PP, Among them, two‐photon initiators (2PIs) play key roles in 2PP systems for generating the reactive species, which initiate the polymerization process of multifunctional monomers and oligomers. An efficient 2PP process requires active 2PIs to ensure high writing speeds, low polymerization thresholds, and high‐quality resulting structures.…”