Mechanism Study And Interface Control For Laser Lift-off Of Ultrathin Flexible Electronics

In general,flexible electronic devices are first fabricated independently on the donor wafer using the well-established standard microfabrication process.Subsequently,the devices should be gently separated from the donor wafer to achieve mechanical flexibility.Many approaches have been developed for this separation process,including stress-controlled exfoliation,and etching of the sacrificial layer.However,issues such as low efficiency,sophisticated process,poor controllability,and limited scalability are inevitable barriers for mass production of flexible electronics.The laser lift-off(LLO)technique can provide a reliable way for the fabrication of large-area flexible electronics and address the above-mentioned issues.Polyimide(PI)is a promising substrate for flexible electronics owing to its flexibility,heat resistance,dimensional stability,mechanical strength,low cost,and material compatibility.The purpose of this dissertation is to provide the necessary theoretical support and technical guidance for the LLO process of flexible electronic devices using ultrathin PI films(< 5 ?m)as flexible substrates.Therefore,the LLO process of PI films is studied from the aspects of experimental phenomena,mechanics modeling,mechanism analysis,and process optimization.The main research effort and contributions of the presented dissertation are introduced as follows:Firstly,an experimental platform of LLO is built independently and the influence of process parameters on the separation of an interface is systematically studied.It is found that the process quality depends first and foremost on the employed laser fluence and accumulated pulse number(APN).The threshold fluence that makes the PI film completely release from the glass substrate is found to be strongly correlative to the thickness of PI film and APN.By measuring the interface bond strength of the PI-glass interface after laser irradiations,the decrease of interface bond strength is discovered resulting from the change of interface microstructures.Secondly,the process mechanism and the reason for the weakening of the interface adhesion strength after laser irradiations are fundamentally revealed.The disclosed mechanism showed that the effect of the gas products generated from laser ablation enables the change of interface microstructures through the spallation process of molten interfacial PI.Sufficient gas products are necessary to avoid any micro-connections between the PI-glass interface,and an evaluation index of process quality is established,which is the ratio of total generated gas products to deformation resistance of the PI film.Thirdly,by applying an incomplete LLO process combined with the mechanical peeling process,a fast and low-cost method has been developed for mass production of PI films with nanostructured surfaces.The cicada wing-inspired nanopillar surfaces result from breakages of micro-connections between the PI-glass interface.These nanostructures are proved to be controlled by the interface cavitation degree before the mechanical peeling process,thus realizing the substantial regulation of the surface morphologies becomes possible.The fabricated PI films with biomimetic surfaces have a good performance for unique features in optical trapping(transmission ratio increases ? 6.6%)and wettability(contact angle changes from 65° to 125°)Fourthly,a comprehensive illustration of the mechanism of the occurrence of wrinkles in ultrathin PI films after the LLO process is provided.A mechanical model is established to describe the deformation of the laser-induced blister and validated by experimental results.The results show that wrinkles are caused by the instantaneous pressure of gas products.By measuring the instantaneous pressure through piezoelectric sensors,the maximum allowable laser fluence without inducing plastic deformation has been found,which decreases linearly with the film thickness.The competing relationship between film peeling and damage is discussed to guide the optimization for the nondestructive LLO process.Finally,the nondestructive LLO process for ultrathin flexible electronics is optimized.A method of on-line measurements of the deformation of the PI film during the LLO process was presented based on built-in strain resistance sensors.Two optimization approaches are proposed,which are bonding thermal release tape on the top of the film or using a multi-pass LLO process with low fluence.The effectiveness of these two methods has been well validated by nondestructive LLO of PI film with a thickness of about 1 ?m.