In recent years, along with the development of organic light emitting diodes (OLEDs) with excellent optical and mechanical properties, displays are gradually evolving from a conventional form to a new form, and the development for thinner, curved and flexible displays is rapidly increasing. These types of display also require a high level of properties of encapsulation layers which protect the OLEDs’ organic light emitting layer from moisture and oxygen. Currently, silicon nitride (Si3N4) thin film encapsulation layers deposited via chemical vapor deposition (CVD) have a disadvantage in that it is too thick to impart enough mechanical properties required for the new displays. Therefore, atomic layer deposition (ALD), which is currently attracting attention as high-quality, ultra-thin, conformal and pinhole-free thin film deposition, is able to be one of solution for replacing CVD method.
In this study, a silicon nitride thin film was deposited by plasma enhanced ALD. By applying high-energy nitrogen plasma in process, ALD window extended to the low temperature proper to OLEDs application. Thin film analyses were performed to investigate the compositions, roughness, thickness and density by Auger electron spectroscopy (AES), spectroscopic ellipsometry (SE), X-ray reflectometry (XRR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS).
Atomic layer deposition of silicon nitride using bis(tertiary-butyl-amino) silane (BTBAS) precursors and N2 plasma was deposited on the top of Si substrates and polyethylene naphthalate (PEN) substrates at various deposition process temperature from 300 to 85°C, which is the maximum process temperature of OLEDs. It was confirmed by measuring a refractive index of the deposited thin films, which was ~ 2.0, that the deposited thin films had chemical bond of the same energy as that of pure Si3N4. As the process temperature was lowered, the refractive index dropped to ~ 1.6 and the carbon content in the film increased to about 25 at%. The by-products formed during the plasma-enhanced atomic layer deposition (PEALD) reaction were redeposited through activating in the plasma and deteriorated the quality of the thin film. The film properties were improved by adjusting the plasma exposure time and the gas flow rate, and it was possible to deposit thin film having a refractive index of ~ 1.8 even at the temperature as low as 85°C. In addition, to improve reliability of nitrides which are easily oxidized in air, plasma post-treatment with N2 and Ar was conducted and enhanced the stability of deposited thin films.
Silicon nitride was deposited on PEN and polyimide (PI) substrates by the PEALD process, based on the optimized deposition conditions on silicon substrates. As a result of XPS and scanning electron microscopy (SEM) analyses, it was confirmed that a thin film chemically identical to the thin film deposited on the silicon substrate was deposited, and that the grown per cycle (GPC) value measured by SEM was also similar, indicating that the silicon nitride ALD reaction also occurred stably on the polymer substrates.
In conclusion, through the control of process parameter including plasma pressure and plasma gas flow rate, it is confirmed that silicon nitride thin films were deposited at low temperature of about 85°C and were also deposited reliably on the polymeric substrates.