Study Of Highly Reliable Laser Lift-Off Of Flexible Polyimide Thin-film

Polyimide(PI) is an ideal target substrate material for flexible electronic manufacturing due to its high-temperature resistance,small thermal deformation,long service life,and perfect biocompatibility.In order to solve the technical challenge of separating and transferring the ultra-thin functional device based on PI from the original substrate in a large scale,high efficiency,and without damage,the laser lift-off process came into being.At present,research for the laser lift-off process is quite scattered.To promote the understanding of the laser lift-off process and for better guidance for the manufacturer by improving its reliability,in this thesis,we systematically investigate four key aspects of the laser lift-off process,including process threshold,process mechanism,process control,and process monitoring.The major work done in this thesis can be concluded as below:(1)A prototype laser lift-off equipment for experimental use is built and the laser fluence threshold at which the PI film can be just separated from glass substrate is measured through experiments conducted under different PI thickness and accumulated irradiation times(AIT).The result shows that there exists a dynamic threshold fluence for laser lift-off of PI,which increases with the rise of the PI thickness and decreases with the increase of the AIT.(2)The micro-morphology evolution of the PI-glass interface is observed.It is found that a large number of nano-scale pillared micro-structures is formed during the PI is being separated from the glass substrate.The numerical simulation of the interface temperature distribution under laser irradiation is carried out by a theoretical model.It is found that the scale of the micro-structures far exceeds the thermal penetration depth of the laser irradiation.Through the residual adhesion energy measurement and the surface micro-morphology observation of the tested surface of the glass substrate after laser irradiation,it is confirmed that the disappearance of the pillared micro-structures reduces the interfacial adhesion energy and finally detach the PI film.Based on these findings,the mechanism of laser liftoff PI film is proposed,which can be assumed that the extremely high pressure formed by the gas produced in the laser ablation of PI lifts the film up and finally leads to the complete separation of the PI film.This mechanism can well explain the dynamic threshold.(3)The upper limit of the process parameters including the laser fluence and the AIT before the quality degradation of the PI film occurs after laser lift-off process is measured,and it is found that the thicker film has a larger process window than the thinner film.By observing the bubble height and cross-section profile of the PI film after laser irradiation,it is confirmed that the reason for the difference between the two process windows is that the thicker film has greater rigidity and suffers less plastic deformation.Based on these findings,two control strategies including process parameters adjustment and upper layer constraint are proposed,which significantly improved the process quality and efficiency of ultra-thin PI film.(4)Real-time monitoring of the impact by the gas produced in the laser lift-off process based on polyvinylidene fluoride(PVDF) piezoelectric sensor is conducted.A good linear relationship between the impact and the laser fluence is found and the upper layer constraint strategy is proved effective to enhance the ability to endure more impact for the PI film.Real-time monitoring of film deformation during the laser lift-off process is realized based on the resistance strain sensor.It is found that there exists a step signal under the process parameters that the film can be peeled off,which provides a possibility to directly judge whether the film is sufficiently peeled off or not.The signals obtained by the two experiments all show significant characteristics and good consistency,which also agree well with many works done in this thesis.