Akhilesh Jain

Roll-to-Roll Nanoimprint Lithography

Roll-to-Roll nanoimprint lithography (R2RNIL) is a process for imprinting nano- and micro- scale features on flexible substrates for fabrication of printed electronic devices. A typical R2RNIL setup includes a roller with patterned template, a flexible or rigid substrate, an imprint dispenser unit, imprint material, and a heating or UV curing module. The pattern transfer takes place by application of pressure from the roller, as the imprint material moves along with the substrate. The imprint material is cured by a thermal or UV-source after it spreads and fills the features on the template, thus replicating the patterns on the template on the roller permanently. The cured resist layer then peels off from the roller as the resist moves with the substrate completing the pattern transfer process.

R2RNIL provides a considerable advantage over batch nanoimprint lithography by offering higher throughput, lower cost and the ability to pattern large area of flexible or rigid substrate. However, there are a number of challenges to overcome before the process can be successfully implemented in the industry. Thermal R2RNIL suffer from long thermal cycles in which the heated resist is embossed by the mold and then cooled before the mold is peeled off making it a very time consuming process. Mold fabrication is an expensive process and heating and cooling cycle of the mold can lead to high thermal stresses and consequently short life of the mold. Slight misalignment of the substrate and mold can also lead to waste of the entire batch.

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Schematic of a roll-to-roll nanoimprint lithography setup showing a roller of radius R with patterned template, a rigid substrate and a UV source. The imprint material is dispensed as droplets by inkjet-type heads on the substrate which form the patterned resist layer after UV-curing.

Guo et al. proposed using UV radiation curing instead of thermal curing to improve the imprinting rate of the substrate. Low viscosity imprint material used in UV curing requires a shorter filling time as compared to thermally cured materials. The low viscosity of UV-curable imprint material also leads to low forces in the process which reduce the defects in the pattern and increase the life of the mold. Since the introduction of the R2RNIL process, many modifications have been suggested to improve the quality of the pattern replication and the over-all processing time. Most of these suggestions include improvement in the setup of the process like introduction of a conveyor mold to improve the speed of thermal R2RNIL, use of a cheaper flexible mold to reduce the cost of the mold fabrication, and automatic stamp release to reduce the pattern distortion. However, there is still a lack of understanding of the quantitative effects of the process parameters and material properties on the dynamics of the imprinting process. My research focuses on understanding of the behavior of the imprint material for optimal design and control of the process by identifying the process parameters of roll-to-roll UV nanoimprint lithography from an analysis of the fluid, curing and peeling dynamics.