  In large area micro hot embossing, the process temperature plays a critical
role to both the local fidelity of microstructure 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 relaxation in micro embossing when the process
temperatures are below or near its glass transition temperature (Tg). In this
investigation, an indentation system and a micro embosser were used to
investigate the relationship of microstructure formation versus process
temperature and load pressure. The depth of indentation was controlled and the
load force at a certain indentation depth was measured. Experiments were
carried out using 1 mm thick PMMA films with the process temperature ranging
from Tg-55 degrees C to Tg +20 degrees C. The embossed structures included a
single micro cavity and groups of micro cavity arrays. It was found that at
temperature of Tg-55 degrees C, elastic deformation dominated the formation of
microstructures and significant relaxation happened after embossing. From Tg-20
degrees C 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, microstructure could be formed under lower loading
pressure. Considering the fidelity of a single microstructure and global
flatness of embossed substrates, micro hot embossing at a low process
temperature, but with good fidelity, should be preferred.
