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Dissertation Defense – Ankit Kumar Singh
MSE Grad Presentation
Friday, March 16, 2018 - 4:00pm
Marcus Nanotechnology Building 1116
Committee Members: Prof. Samuel Graham, Advisor, ME Prof. Preet Singh, MSE Prof. Mark Losego, MSE Prof. Elsa Reichmanis, CHBE Prof. Maysam Ghovanloo, ECE
"Study of ALD Films for Improving the Stability and Reliability of Electronic Devices in Harsh Environments"
Abstract: Over the past decade, a wide range of electronic devices have been developed that require barrier film technologies to improve their lifetime. For example, organic electronic devices have the ability to produce light weight, scalable and flexible electronics. Despite having several advantages over their peers, the commercial application of such devices is still challenging as they are prone to rapid degradation on exposure to atmospheric species like oxygen and water vapor. The stability of such devices can be increased by using a barrier layer that prevents the ingress of oxygen and water vapor to the active layers in the devices. More recently, perovskite solar cells (PSCs) have attracted significant attention from the photovoltaics community through their rapid advancement in power conversion efficiency which has gone above 20% in less than a decade. However, their poor environmental stability is a major challenge that needs to be addressed before any practical application. Just like organic electronic devices, PSCs are also prone to degradation by the atmospheric species. Other miniaturized electronic devices, including bioimplants and wearables, have been developed which operate either inside or worn outside the human body. Such devices are exposed to different bodily fluids which are corrosive in nature.
Atomic layer deposition (ALD) has shown the potential for making ultra-thin barrier films for these applications because of their ability to form conformal and pinhole free films. Recent studies have demonstrated water vapor transmission rates of plasma enhanced-ALD films to be on the order of 10-6 g/m2/day which is desirable for commercial applications. However, the existence of defects in the barrier films significantly deteriorate their quality. Various architectures using nanolaminates and hybrid structures have been tested to minimize the impact of defects in the barrier, but none have been able to achieve the desired level of permeation resistance against atmospheric species in the presence of particle contamination. In this dissertation, to address the issue of defect formation due to particle contamination, the impact of particles with different mechanical properties on crack formation in ALD barrier films has been investigated. Subsequently, the healing of cracks created due to particle contamination is studied. It is experimentally shown that the performance of an ALD barrier film consisting of several cracks can be improved significantly by filling the crack openings using PECVD SiNx. In case of indirect encapsulation, despite having high quality barrier films, atmospheric species can permeate to the active regions of the devices through the interface between the barrier and sealant. In this dissertation, the strong dependency of side permeation rates on the contact angles of water with the barrier materials in contact with an edge sealant is demonstrated. It is shown that the rate of side permeation can be controlled by changing the materials at the interface.
In addition to permeation barriers for atmospheric species, ALD films can also have application as protective barriers for electronic devices in harsher environments consisting of various ionic and biological species. To extend the domain of ALD barriers from just gas permeation barriers, the chemical stability of ALD materials was investigated in a variety of solutions including ionic and biological media. The direct deposition of ALD on PSCs also has additional issues beyond defects in the barriers, namely in the chemical compatibility with the deposition process. The adhesive material, used during indirect encapsulation, can interact chemically with the PSCs leading to their degradation. The chemical sensitivity of the PSCs makes it difficult to directly integrate many known encapsulation processes. To overcome this issue, the concept of using PECVD SiNx on top of the PSCs as a chemical buffer layer prior to applying various encapsulation materials is introduced. The results from this study highlight the possibility of using a variety of indirect encapsulation techniques and barrier structures for improving the lifetime of PSCs.