Nanometer-Thick SiN Films as Gas Barrier Coatings Densified by Vacuum UV Irradiation

Abstract

Gas barrier films are widely used in electronic and packaging applications. They are also critical components of flexible organic light-emitting diodes (FOLEDs) that require high gas barrier performance. Among the various film manufacturing techniques, solution-processed thin-film encapsulation (TFE) represents a low-cost FOLED fabrication method. The nanometerthick SiN films produced following the vacuum ultraviolet (VUV)-induced densification of solution-processed perhydropolysilazane (PHPS) films in a N 2 atmosphere can potentially serve as TFE barrier films. However, the nanometerthick PHPS densification process has not been examined in sufficient detail. We investigated and discussed the effects of the Si−N bond number, PHPS film composition, and free volume (present in the produced Si−N network) on the VUV-induced PHPS densification process. It was found that VUV irradiation caused rapid hydrogen release and film densification through the formation of Si−N bonds. The results obtained using the X-ray photoelectron spectroscopy and dynamic secondary ion mass spectrometry techniques, and the calculated residual hydrogen ratios, revealed that the film composition was strongly related to the number of residual hydrogen atoms and Si−N bonds. Notably, nanometer-thick PHPS film densification was a relatively slow process, in which the free volume in the Si−N network was considerably reduced by the atomic rearrangement induced by the simultaneous cleavage of several Si−N bonds during VUV irradiation. We believe that the results presented herein can potentially serve as a guideline for developing solution-processed nanometer-thick SiN films with relatively high density and excellent gas barrier performance (that is comparable to that exhibited by vacuum-processed barrier films).

Solution‐Processed Gas Barriers with Glass‐Like Ultrahigh Barrier Performance

“…In fact, the thickness of the PHPS layer decreased by ≈10% after VUV irradiation (12 000 mJ cm −2 ) although it depended on the initial thicknesses of the PHPS layers. [ 42 ] In addition to H 2 release, according to past reports [ 51–53 ] and our findings, [ 42 ] it is also suggested that outgases can be released from the layers, such as NH 3 and PHPS oligomers, during the VUV irradiation process. Thermal desorption spectrometry (TDS), which analyzes the gas molecules released from a layer by heating in a vacuum, was conducted to detect the gas molecules released from the PDSN layers (Figure S5, Supporting Information).…”

“…The PHPS and PDSN layers were fitted using a single‐layer model with a Gaussian oscillator and a four‐layer model containing SiO 2 at the surface and three single‐layer models, respectively, referring to previous work. [ 42 ] Although the small penetration depth of VUV light owing to the high absorption coefficient in the VUV region of the PHPS layers causes a continuous refractive index distribution in the direction of the layer thickness, the experimental ellipsometric parameters (Ψ and Δ) of the PDSN layers were fitted well by the four‐layer model, as shown in Figure S3a, Supporting Information. The refractive indices of the Top and Mid layers increased as a result of the VUV irradiation, whereas that of the Bot layers almost did not change.…”

“…In 2021, we analyzed the VUV‐induced densification process to develop guidelines for improving the barrier performance of PDSN layers. [ 42 ] We found that dense PDSN layers with refractive index distribution in the direction of thickness were formed by VUV‐induced SiN network formation and atomic rearrangement that can decrease the free volume in the PDSN layers. Based on these results, we can expect that the thickness and amount of irradiated VUV light (VUV dose) are key parameters in determining the barrier performance.…”

Design of a Flexible Thin-Film Encapsulant with Sandwich Structures of Perhydropolysilazane Layers

“…It was observed that the SiO x N y layer, which gradually thickened according to the VUV irradiation time, makes the overall refractive index larger. 39 The thickness reduction of the PHPS layer saturated around 150 nm, confirming a slowdown of the PHPS-to-silica transformation; accordingly, the optimal PHPS curing time was selected as 40 min. We then fabricated multilayer PHPS/PHPS/PET stacks to improve the barrier properties.…”

“…The overall refractive index of the PHPS film was increased from 1.54 to 1.67 in accordance with the VUV irradiation time because SiO x N y has a high refractive index of 1.45–1.90. It was observed that the SiO x N y layer, which gradually thickened according to the VUV irradiation time, makes the overall refractive index larger . The thickness reduction of the PHPS layer saturated around 150 nm, confirming a slowdown of the PHPS-to-silica transformation; accordingly, the optimal PHPS curing time was selected as 40 min.…”