Saturday, November 19, 2016

Tuning mechanical properties of ATO nanolaminates by ALD

ALD nanolaminates were first introduced in high volume manufacturing in Finland for TFEL-displays. Today even a see through version display is produced by Beneq (Lumineq). Nanolaminates have also found applications in optics by combining materials with a high difference in refractive index to tune optical properties. Also any ALD manufactured devices in the semiconductor industry can be regarded as an ALD nanolaminates like for instance the MIM stack in the DRAM capacitor cell for 90 nm and below or the HKMG stack used for transistors starting at 45 nm.

Since then, ALD nanolaminate barriers have also become technologically very important for highly flexible OLED (light-emitting diode) displays expected to go in production during 2017 for e.g. smart phone displays. Rumors in the industry say that both Samsung Electronics, LG Display and Chinese manufacturers are adopting ALD barrier production technology and that possible one of the next Iphones will have ALD barrier technology.

One of the mostly reported ALD barrier stacks consists of a repetition of a Al2O3/TiO2 bi-layer stack so called ATO nanolaminates or multilayers. These are employed to encapsulate the OLED from gas and water penetration otherwise killing the device. The key is to reduce the thickness drastically as compared to PVD or CVD barriers and thereby gaining flexibility while keeping the barrier properties for gas and water diffusion and therefore most importantly the barriers must also be pinhole free. Any pinhole will kill the diode and lead to a pixel failure. I´ve been told that a typical acceptance level for pixel failure for tablet sized displays is three (3!).


(Color online) Residual stress of the ATO nanolaminate as a function of the (a) ALD temperature, and (b) bilayer thickness. The error bars present the maximum measurement uncertainty as explained in Sec. II. © 2016 American Vacuum Society. Reproduced from J. Vac. Sci. Technol. A 35, 01B105 (2017); http://dx.doi.org/10.1116/1.4966198, with the permission of AIP Publishing and corresponding Author Oili M. E. Ylivaara
 
Until today there are not too many published reports available on residual stress, adhesion and mechanical properties in ATO nanolaminates published. Please find below the link to the Open Source publication from researchers at VTT, University of Jyväskylä and Aalto University in Finland, systematically investigating those properties as dependence from bi-layer thickness and ALD process temperature as shown above and much more. Even though the focus in this study is not on barrier properties I think there is a lot to learn here for the OLED community, especially since you operate under relatively low process temperature conditions (<100C) you must have a lot of residual stress in those flexible barriers.

Besides this report, you can also find an earlier very nice poster from Riikka Puurunen here that focuses on tuning the electrical properties of ATO nanolaminates. All this work were performed in a Picosun SUNALE(TM) R-150 reactor.

Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties

Oili M. E. Ylivaara, Lauri Kilpi, Xuwen Liu, Sakari Sintonen, Saima Ali, Mikko Laitinen, Jaakko Julin, Eero Haimi, Timo Sajavaara, Harri Lipsanen, Simo-Pekka Hannula, Helena Ronkainen and Riikka L. Puurunen
 
J. Vac. Sci. Technol. A 35, 01B105 (2017); http://dx.doi.org/10.1116/1.4966198