The Long-term Stability of SiNx Thin Film Encapsulation Layers and Their Effects on the Optical Property of Organic Light Emitting Diode

Abstract

The most widely used barrier for thin film encapsulation for OLEDs is the silicon nitride (SiNx) deposited at low temperature of 85℃ by plasma enhanced chemical vapor deposition (PECVD). In first study, we investigated the oxidation of SiNx barrier films in the 85℃/85% RH condition and the effect of the optical property of barrier on the reflectance, absorption, and transmittance of multilayer. The SiNx films for the oxidation test were prepared with changing the NH3 and the H2 gas ratio during deposition process. For the as-deposited thin films, the refractive index with increasing NH3/total gas flow rate decreased from 1.93 to 1.81 and the film density reduced from 1.996 to 1.895. On the other hand, with increasing H2 gas ratio, the refractive index increased from 1.80 to 1.90 and the film density increased from 1.852 to 1.922.We estimated the changes in physical properties and chemical bonding structures of the SiNx films with the storing time in the condition of 85℃ and 85% RH. Both the stress and the refractive index of all samples decreased with a duration time in the 85℃/85%RH chamber. Those values changed largely with a larger NH3 or a less H2 gas flow rate during the deposition process. The initial stress with increasing NH3 gas flow rate increased slightly from -153MPa to -163MPa, while the stress with increasing H2 gas flow rate increased from -102MPa to -213MPa. The stresses with NH3 gas flow rate decreased with oxidation test and the variation increased with a larger NH3 gas. The other hand, the stresses of films with H2 gas flow rate also decreased with oxidation time. But the variation was decreased with a higher H2 gas flow rate. The stresses of RH2=0 and 0.48 were changed from -102MPa and -123MPa to nearly +100MPa, respectively, indicating a change from a compressive stress to a tensile. With RNH3=0.073 and 0.122, the refractive index decreased to 1.44 after 240hr, which is similar to the value of SiOx thin film. The [N-H] peak intensities of as-deposited films were enhanced with increasing NH3 gas flow rate, while the intensities after oxidation were more decreased in the films with a larger [N-H] peak. The [Si-H] peak position of as-deposited films with a larger NH3 gas ratio shifted to a longer wavenumber, indicating N-rich SiNx film. The [Si- H] peak intensities increased with increasing H2 gas ratio, while [N-H] peaks decreased. FTIR spectra indicated that the [Si-N] or [N-H] bonds react with H2O to form a thermodynamically more stable [Si-O] and/or [Si-OH] bond. The higher binding energy component of N1s spectrum (XPS) of as-deposited films with a higher NH3 gas ratio corresponds to more [N-H] bonds. The [Si-H] and [N-H] bondings are crucial to the oxidation process. But the oxidation did not depend on [Si-H] because [Si-H] bonding ratio was similar or slightly decreased with increasing NH3 gas ratio. By comparing [Si-H] bonding (FT-IR) and N1s spectrum (XPS) of the films before and after the oxidation represented that the [N-H] groups were a main reactive sites, not the [Si-H]. The surface roughness by AFM increased from 0.88 to 1.20. The SiNx films with increasing NH3 flow rate were well oxidized. Cross-section transmission electron microscopy showed that the oxidation reaction occurred layer-by layer from the film surface and the defect and/or void was the center of oxidation. The WVTR of 0.5um-thick film with RNH3=0.073 was 5*10-5g/m2-day, resulting in the diffusion coefficient of 9.645*10-15cm2/s. After 240hrs in the 85℃/85%RH chamber, the oxidation thickness measured AES data was about 84.8nm and the one calculated from the Deal-Grove model was 76.2nm, being predicted 110nm after 500hrs. Secondly, the effect of thin film encapsulation structure on the device optical properties was studied. The transmittance of multilayered-thin film is affected by the interface reflectance and absorption. In this study, the effect of reflection and absorption was examined separately. The transmittance according to the refractive index and extinction coefficient of barriers showed that the light intensity was decreased when the extinction coefficient was 10-3 or more. The reflection variation according to the first barrier thickness represented a larger variation and a short period when the thickness was less than 1um. The buffer layer formed between barriers had little effect on the reflectance at a thickness of 2um or more. The effect of thin film encapsulation layer on the optical properties was investigated by inserting the light controlling layer between cathode and the first barrier, consisting of a high-/low- refraction layer. This layer showed the maximum reflectance near the total thickness of about 200nm. The range of high-/lowrefraction layer thickness which the reflectance became maximum was 70~100nm/70~120nm for blue, 70~120nm/70~150nm for green, and 70~150nm /50~150nm for red. The reflectance represented maximum value when the total thickness of light controlling layer was about 190nm and the high-/low- refraction layer thickness was between 70~100nm and 120~90nm, respectively. We demonstrated that the maximum efficiency was obtained when the high and low refraction layer thickness were 70nm and 120nm, respectively. Finally, the optical property of the top barrier layer was investigated when the uppermost barrier was oxidized. If the buffer thickness was greater than 2um, the effect of the top barrier oxidation thickness on the optical properties was very small. However, when the buffer thickness was 1um and the barrier was 100nm, only about 30nm oxidation of the top barrier thickness induced the reflectance. In order to develop a flexible display, the thickness of thin film barrier is becoming thinner. Most research is focused on the barrier performance. However if the barrier is oxidized, the film composition, the bonding structure, and the film stress may be changed so that the optical characteristics and the mechanical stresses of device may be affected. This means that the barrier studies are needed in terms of performance and chemical and mechanical stability.