Overcoming a major production constraint
Additive manufacturing (AM) using two-photon polymerization lithography (TPP) has come into use in industry and research. Currently, a major limitation of TPP in general and specifically of the material IP-Q (Nanoscribe GmbH, Germany) is the limited access of users to knowledge about material properties. Due to the nature of the process, especially the elastic properties depend not only on the material used, but also on the structure size, process and manufacturing parameters. For example, before research recently published in the Journal of Optical Microsystemsno degree of conversion (DC) and Young’s modulus (E) values for IP-Q were reported.
Due to the nature of the process, especially the elastic properties depend not only on the material used, but also on the structure size, process parameters and hatching strategy. A common approach uses a combination of Raman spectroscopy and nanoindentation to characterize the DC from monomer to polymer, measurable via Raman spectroscopy, which can then be related to the mechanical behavior of the material, measurable via nanoindentation .
Ongoing research on acoustic metagratings and metamaterials fabricated on MEMS would benefit from optimized elastic parameters to provide adaptability of the acoustic behavior as they directly influence the characteristic acoustic impedance. AM includes processes by which objects can be created three-dimensionally from a technical drawing. The data is sent to an AM system, which then performs the manufacturing. AM via TPP is based on selective curing of a liquid precursor to create solid structures in a monomer droplet. Then the remaining liquid is washed away. Known TPP applications are submicron optical structures, where the photoresist IP-Dip (Nanoscribe GmbH, Germany) is commonly used. The more recently developed photoresist IP-Q was designed by the same manufacturer for larger applications such as assemblies, molds and structural metamaterials. Sample structures from each of the two photoresists were produced in parameter sweeps. This allows process parameters to be compared with the resulting characteristics. Raman spectroscopy, a non-contact analysis method for material characterization that scatters monochromatic light from the material, was used.
The reflection includes not only the emitted wavelength but also Raman scattering. The characteristic peaks of the Raman scattering spectrum can be used for chemical identification. In our work, it was used to determine the ratio of monomer to polymer – or DC – in the TPP samples.
Micro and nano indentation were used to test the mechanical properties of the samples. A hard tip whose mechanical properties are known is pressed into the sample whose properties are unknown. From the slope of the load vs. displacement curve, E values were calculated.
Finally, parameter sweeps of cubic sample structures fabricated using TPP were examined across the laser power and scan speed parameters to find dependent properties. The photoresists used were examined by Raman spectroscopy to find the DC from monomer to polymer, and then micro or nano indentation was used to detect E.
For IP-Dip, the DC and E achieved ranged from 20 to 45% and 1 to 2.1 GPa, respectively. The results were compared with reports found in the literature. For IP-Q, the DC and E achieved ranged from 53 to 80% and 0.5 to 1.3 GPa, respectively. The characterized properties of IP-Q manifest themselves as the current state of knowledge of the material.
“In this way, providing an approach to optimize elastic parameters of TPP fabricated structures will be beneficial for several ongoing research topics. A promising application for this method is the characterization of the elastic parameters of acoustic metagratings and metamaterials fabricated on MEMS. devices can then be advantageously implemented in life science, mobility and industrial applications,” said Severin Schweiger of the Fraunhofer Institute of Photonic Microsystems and the Brandenburg University of Technology in Germany.
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Severin Schweiger et al, Characterization of two-photon polymerization lithography structures via Raman spectroscopy and nanoindentation, Journal of Optical Microsystems (2022). DOI: 10.1117/1.YOM.2.3.033501
Provided by SPIE–International Society for Optics and Photonics
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