Barrier properties and analysis of defects of plasma polymerized hexamethyldisilazane-based films

Abstract

A great variety of commercially available goods, e. g. food products, require a degree of protection against gases and vapors. Electronic devices whose active layers are based on organic materials in particular demand extremely low oxygen transmission rates in order to attain adequate lifetimes. In order to do so, an encapsulation of the device by means of a barrier becomes necessary. In case of flexible devices, such as organic light emitting devices (OLEDs), conventional encapsulation methods relying on stiff glass lids cannot be employed. Plasma-enhanced chemical vapor deposition (PECVD) methods on the other hand have been proven to be successful in obtaining thin films (in the range of tens or hundreds of nanometers) which combine good barrier performances with flexibility and other favorable mechanical properties. In the current work, thin silica-like (SiOx) films have been deposited on polyethylene terephthalate (PET) through a low-pressure microwave plasma and a gaseous feed consisting of hexa-methyldisilazane (HMDSN) and oxygen, with the aim of providing flexible oxygen barrier layers with additional properties as transparency, colorlessness, good adhesion to the substrate and resilience. Operational parameters such as the gas feed composition, microwave power and deposition time have been investigated and optimized, thus obtaining inorganic barriers with an optimal thickness in the 50 to 100 nm range and with a barrier improvement, when compared to the uncoated substrates, up to a factor of 100. The defects in the barriers have been investigated by means of a concurrently developed non-destructive method for their localization and identification, based on the precipitation of calcium carbonate crystallites on top of them, which allows the defect to be later retrieved and investigated by means of microscopy methods. Further analyses of the transmission rates have been carried out at different temperatures in order to investigate the permeation mechanisms through the bulk and the defects. The films, when compared to barriers deposited via the common precursor hexamethyldisiloxane (HMDSO), obtained in the same experimental setup, showed consistently better properties in a wider range of conditions, proving HMDSN to be a better precursor for thin films with barrier applications. Multilayer systems, based on the combination of SiOx films and an intermediate organic layer optimized in parallel to the barriers, have been developed, tested and used successfully for the encapsulation of flexible Organic Light Emitting Device (OLED) prototypes printed on polymers.