Characterization of two-photon polymerization process using Raman microspectroscopy

Two-photon polymerization (TPP) is a promising micro/nanofabrication technique, which is capable of fabricating 3D micro/nanostructures beyond the diffraction limit of light. However, the study of TPP process with a focus on the dependence of degree of conversion on TPP parameters using a non-destructive and efficient method is still lacking. We studied the quantitative relationships between the TPP parameters and the cross-linking of an acrylic-based IP-L 780 photoresist via systematic Raman characterization. The differences in the Raman spectra between the non-polymerized and the polymerized IP-L 780 photoresists were observed by probing the excitation of carbon-carbon double bond (C=C) vibrations. We obtained the relationship between the degree of conversion in TPP and the Raman spectra of the IP-L 780 resin, in which the intensity of the characteristic Raman peak of IP-L 780 at 1635 cm^-1 decreases with the increase of the TPP laser dose. A mathematic model of the degree of conversion with respective to the TPP parameters, including laser average power and writing speed, has been established. The method provides a simple and effective way to characterize and optimize the TPP micro/nanofabrication processes. The established model for the degree of conversion as the function of TPP parameters will contribute to the advanced 3D TPP micro/nanofabrication by providing a guidance to optimize the laser doses, voxel sizes, and the mechanical strength of the polymers.