Performance Research Of Substrate Pre-bending Low-temperature Deposited Al₂O₃ Films Based On Atomic Layer Deposition Technology And Flexible Encapsulation Applications

Organic semiconductor thin-film devices have many advantages such as flexibility,ultra-thinness,integratability,and low cost,which have great market potential in wearable and foldable electronic devices,and more importantly,large-area solution processing and fabrication techniques have high application value in industry.However,organic materials in organic thin film devices are rapidly degraded by moisture and oxygen molecules.Glass covers have been used to protect rigid electronic devices with glass or silicon wafers as substrates,and this encapsulation method has a Water Vapor Transmission Tate(WVTR)of 10^-6g·m^-2day^-1,which can effectively prevent water vapor,but in Flexible electronic devices are susceptible to stress.Therefore,Thin Film Encapsulation(TFE)method is often applied to solve this problem,both to resist the stress and to block the water vapor,and to protect the device by growing the encapsulation layer before and after the device preparation.Today,encapsulation films are usually prepared by magnetron sputtering,inkjet printing,chemical vapor deposition(CVD)and atomic layer deposition(ALD)processes.However,crystalline and amorphous films prepared by commonly used coating techniques can have residual stresses due to factors such as plasma power,sputtering power,film thickness and temperature during the deposition process.The stress in the film can have a serious impact on the device,such as delamination or bulging when there is a stress mismatch with the underlying film,affecting the package performance.When bending,the film cracks as a result of the combined effect of additional external and internal stresses,and the film is more likely to fracture when the internal stresses are too high.Si Nx films are widely used in industry for their excellent optical properties,high dielectric constant,high barrier properties,and ease of etching.In order to solve the problems caused by the above mentioned stresses,the industry often controls the stresses in the films by regulating the ratio of nitrogen to silicon elements.This method requires constant adjustment of growth parameters and control of elemental ratios during device preparation,which is a complex process and is only applicable to Si Nx films.Another method that can be applied to all films is the organic-inorganic stacked structure,which can release the residual stress of inorganic films through the elastic deformation of organic layers,however,the organic layers will generate a reaction force after the elastic deformation resulting in incomplete stress release,and it will increase the process time and economic cost.Since the ALD process can produce ultra-thin films with high barrier properties,which are promising for wearable electronic devices,there are not many studies on residual stresses in ALD amorphous films,so it is urgent to develop a method that can be applied to all films to release stresses.In this thesis,we propose a simple process using pre-bending and substrate thermal expansion to achieve regulated internal mechanical equilibrium homeostasis of the film to reduce residual stresses,compare and analyze the key properties of ALD-Al₂O₃ films to determine the optimal process conditions,and additionally adjust the surface area difference between the film and substrate to obtain a flat bottom encapsulated substrate.The pre-bending and substrate thermal expansion process is applicable for all films,and the internal mechanical homeostasis is regulated by thermal stress to maximize the release of residual stresses.It also does not affect the barrier properties of the film,does not require adjustment of growth parameters,and is simple to operate without increasing process time or economic costs.Meanwhile,it is still feasible to use substrates subjected to thermal shrinkage for films with tensile stress,such as PVC,PE,PP,and PVDC materials.The results demonstrate that the ALD-Al₂O₃ films deposited at 40°C produced almost no cracks on the surface after multiple bending at 3 mm bending radius with no significant change in WVTR.In addition,the bending radius and crack saturation density of the first crack generation were significantly optimized,showing good mechanical stability.Meanwhile,it was found during the encapsulation performance test that the measurement of high barrier films usually uses the Ca corrosion test method,but the measurement results of this method are easily disturbed by the particles in the environment and the substrate roughness.In particular,flexible substrates have rougher surfaces compared to rigid substrates,and there will be more defective spots on the film surface,which will provide paths for water vapor penetration and accelerate conductivity changes,and these defective spots are not a quality problem of the film itself.Therefore,we introduce a combined optical and electrical Ca corrosion test method,in which the dynamic corrosion process of defective spots is monitored by an optical camera and the corrosion mechanism is analyzed.The accuracy of this method was verified by using ALD-Al₂O₃ films with different defect densities,and the encapsulation properties of ALD-Al₂O₃ films with low residual stresses prepared in this thesis were measured using this method.ALD-Al₂O₃ films prepared in this thesis,the WVTR did not change after the bending test,and the excellent barrier properties were still preserved.Finally,the feasibility of our prepared monolayer ALD-Al₂O₃ was confirmed by its application in OLEDs,where the encapsulated OLEDs maintained 94%of their initial efficiency after bending and storage in harsh environments for 10 days.In this study,we not only prepared low residual stress ALD-Al₂O₃ films,but also provided a process method to release a large amount of residual stress in the films,which can be applied to reduce the residual stress in the films prepared by any coating process.