In-situ observation on the real-time filling deformation of PMMA into microchannel during hot embossing

•A novel hot embossing apparatus with an in-situ observation system is developed.•A method is proposed to dig up information about real-time filling ratio and shape evolution of PMMA microstructures.•Effects of process parameters on replication accuracy of PMMA microstructures are evaluated.•A general pattern of the central filling ration with time in the embossing stage is revealed.