The use of the additive manufacturing technique known as two-photon polymerization lithography is expanding across a variety of industries. Two-photon polymerization lithography utilizes liquid precursor's selective curing to create solid structures.
Two-photon polymerization lithography has applications such as submicron optical structures that utilize photoresist materials IP-L and IP-Dip. For instance, the most recent photoresist IP-Q is designed for structural metamaterials, molds, mounts and size applications.
A significant drawback of two-photon polymerization lithography in general and specifically IP-Q is that there is limited information about material properties for users. For example, Young's modulus and degree of conversion (DC) values for IP-Q were unavailable before this study. Similarly, the elastic properties are hatch strategy, process parameters and structure size dependent.
It is often challenging to apply standardized strain rate-dependent testing techniques for elastic properties to the size of structures that can be produced using two-photon polymerization lithography in an acceptable amount of time.
Technique Used in This Study
The micro- and nanoindentation approach was used for this study because, compared to microtensile experiments, it is a simpler method for tiny structures. One reason is the less involved sample preparation process compared to the time- and labor-intensive design and production of micro tensile tests. The indentation approach is simple to automate, enabling characterization to be completed in a more manageable time.
This study utilizes two-photon polymerization lithography via micro- and nanoindentation and Raman spectroscopy to characterize IP-Q and IP-Dip fabricated samples.
Materials and Experimental Procedure
IP-Q and IP-Dip investigated in this study are composed of radical photoinitiators, solvents, and organic monomers. When photoinitiators are exposed to ultraviolet light, free radical species are produced, which come in contact with solvents to initiate polymerization. Cross-linked chains formed by organic monomers form networks. The increased chain lengths and network entanglement cause the liquid to become gel-like and eventually solid.
Radicals' concentration and degree of conversion depend on the exposure dose of ultraviolet radiation. The degree of conversion represents the ratio of monomers remaining in a volume to the polymer's amount. A larger exposure dose causes a higher degree of conversion and strong cross-linking and entanglement that determine polymerized photoresist's mechanical properties.
In contrast to IP-Q, which uses methacrylates as its reactive monomer, IP-Dip uses acrylic-based materials. They were developed for exposure employing two-photon polymerization lithography and are responsive to the aforementioned radical polymerization. IP-Q was designed for meso to macrofabrication with a focus on fabrication speed, while IP-Dip was intended for microfabrication with a focus on high resolution.
Two-Photon Polymerization Lithography
Additive manufacturing based on two-photon polymerization lithography was used to fabricate cuboid parameter sweep samples using the Photonic Professional GT2 system. A focused and pulsed laser beam initiates a multiple photon absorption process within a photoresist's volume containing the necessary photoinitiators in the Photonic Professional GT2 system.
When the first photon is absorbed, it generates a virtual state from which the next photon initiates the excited state for curing reaction within the focal volume. Employed objectives and exposure dose determine the voxel size and shape. Vertical size of 0.3 to15 μm and a radical size of approximately 0.1 to 5 μm can be addressed this way.
The researchers performed Raman spectroscopy at room temperature. Using a Rayleigh filter with a cut-off wavenumber of 100 cm-1, measurements were made in the Stokes regime.
The system's spectral resolution was smaller than 1.0 cm−1. Raman spectra were acquired using an integration of six successive individual observations with an exposure duration of 10 s for each spectrum. A baseline adjustment was carried out after the capture of the raw spectra.
After that, indentation measurements were conducted to characterize the material properties such as hardness and Young's modulus of Two-photon polymerization structures.
In this study, the researcher explored the polymer structures' mechanical properties fabricated via two-photon polymerization for photoresists IP-Q and IP-Dip, by characterizing parameter sweeps of cuboid polymers structures via nanoindentation and Raman spectroscopy.
The researchers showed the similar and different behavior of IP-Q and IP-Dip and compared IP-Dip's properties with literature. These properties of IP-Q manifest as the material's current knowledge state.
The study confirms that these characterization processes are transferable to other photoresists. Users' needs for specific mechanical behavior are linked with measured Young's modulus via a program. This program has posed parameters that impact the exposure dosage, detectable degree of conversion, and Young's modulus. This programmable mechanical behavior is ideal for creating structures for applications requiring specific mechanical properties such as metamaterials and metagratings. Using Raman spectroscopy to validate the required properties non-destructively by comparing with the study's findings.
Severin Schweiger, Tim Schulze, Simon Schlipf, Peter Reinig, Harald Schenk (2022) Characterization of two-photon-polymerization lithography structures via Raman spectroscopy and nanoindentation. Journal of Optical Microsystems. https://www.spiedigitallibrary.org/journals/journal-of-optical-microsystems/volume-2/issue-3/033501/Characterization-of-two-photon-polymerization-lithography-structures-via-Raman-spectroscopy/10.1117/1.JOM.2.3.033501.full