Experimental studies on polymer deformation and flow in micro hot embossing

In large area micro hot embossing, the process temperature plays a critical role to both the local fidelity of micro structure formation and global uniformity. The significance of low temperature hot embossing is to improve global flatness of embossed devices. This paper reports on experimental studies of polymer deformation and flow modes in micro embossing when the process temperatures are below or near its glass transition temperature (Tg). In this investigation, PMMA (polymethyl methacrylate) films with a glass transition temperature of 105degC were used as the process material, and the process temperature ranged from Tg-55degC to Tg+20degC. The embossed structures included a single micro cavity and groups of micro cavity arrays. It was found that at temperature of Tg-55degC, elastic deformation dominated the formation of micro structures and significant relaxation happened after embossing. From Tg-20degC to Tg, plastic deformation dominated polymer deformation, and permanent cavities could be formed on PMMA substrates without obvious relaxation. However, the formation of protrusive structures as micro pillars was not complete since there was little polymer flow. With an increase in process temperature, the polymer lost its storage modulus and micro structure could be formed under lower loading pressure. Considering the local fidelity and global flatness of embossed substrate, hot embossing at low process temperature would be preferred.