In-process measurement of gradient boundary of resin in evanescent-wave-based nano-stereolithography using reflection interference near critical angle

Abstract

Evanescent-wave-based nano-stereolithography, which uses the ultra-thin field distribution of evanescent wave to solidify photosensitive resin, provides a sub-micrometer vertical resolution of each layer. In fabrication process, cured resin (solid state) is submerged in uncured resin (liquid state). The interfaces between the cured and the uncured resin are made up of half-cured resin in a state of the uncompleted polymerization. Due to the gradient boundary directly determines the thickness of each fabrication layer and greatly influences the quality of products, it is of great significance to study the gradient boundary in the fabrication process. We proposed in-process measurement of gradient boundary using the reflection interference technique to monitor the formation of cured resin and investigate the gradient boundary. In this method, the variation of refractive index of resin in curing process has been utilized in the measurement. Measurement light was deduced near the critical angle to obtain a susceptible total internal reflection condition. In the verification experiment, a compact experiment system including fabrication and measurement sections has been developed. Two beams of light in the different wavelength have been delivered into the system as fabrication and measurement light, respectively. Resin exposed by increasing time has been measured by our proposed method at various incident angles near the critical angle. The refractive index distribution and the depth gradient boundary have been successfully measured. The results prove that the refractive index of cured resin is different in the position; the span of gradient is not constant as well. The maximum span of gradient boundary in center was measured in around 250 nm. This work that helps us clearly understand the curing process and the formation of the cured layer in EWNSL provides a research basis for further and detailed research in the nanoscale stereolithography.