Infrared and Optical Emission Spectroscopy Study of the Surface Chemistry in Atmospheric-Pressure Plasma-Enhanced Spatial ALD of Al2O3

Abstract

In the past decade, atmospheric-pressure spatial atomic layer deposition (AP-SALD) has gained momentum as a fast deposition technique in thin-film manufacturing [1]. The key benefits of conventional ALD, such as superior control of layer thickness, conformality and homogeneity, in addition to the high throughput enabled by the spatial separation of the half-reactions and the more cost-effective processing at atmospheric pressure, make AP-SALD an interesting option for industrial large-area applications. Finally, the use of atmospheric pressure plasmas as co-reactants enables to rapidly deposit high-quality, dense films at relatively low temperatures [2]. Despite its industrial potential, the further advancement of atmospheric-pressure plasma-enhanced spatial ALD processing still lacks a detailed understanding of the underlying plasma physics and chemistry. Here, effective ALD metrology techniques are of prime importance to obtain more detailed insights into the nature of the process with the aim to improve process performance and thus to open up new industrial application fields. However, due to the very nature of the spatial concept, monitoring AP-SALD processes is rather challenging: especially in the case of close-proximity systems the substrate is typically at ~100 μm distances from the gas injection head. In this work, we employed optical emission spectroscopy (OES) and infrared spectroscopy on effluent plasma gases as diagnostics tools to study the basic chemistry of the AP-SALD process of Al2O3 films grown from Al(CH3)3 and Ar-O2 plasma in a rotary lab-scale reactor (see Fig. 1 and ref. 1). We identified the main reaction products and studied their behavior as a function of the exposure time to the precursor to verify the ALD layer-by-layer growth characteristics. The results show that using spatial separation of the ALD half-reactions and atmospheric pressure plasma as the reactant gives rise to a complex underlying chemistry (see Fig. 2). Infrared absorbance spectra show CO, CO2, H2O and CH4 as the main ALD reaction by-products originating from 1) combustion-like reactions of the methylated surface with oxygen plasma radicals and ozone, and 2) a concurrent latent thermal component related to the substrate. Moreover, CH2O and CH3OH are identified as ALD reaction by-products either formed at the surface or in the plasma by electron-induced dissociation. The investigated trends in CO2, CO and CH4 formed as a function of the exposure time confirm a self-limiting ALD behavior. Finally, the OES results corroborate that, as soon as the plasma-enhanced SALD process takes place, emission from OH and CH arises while excited oxygen species are being consumed. -------------------------------------------------------------------------------------------------------------------------------------------------------------------- References P. Poodt, A. Lankhorst, F. Roozeboom, C. Spee, D. Maas and A. Vermeer, ‘High-speed atomic layer deposition of aluminum oxide layers for solar cell passivation’, Adv. Mater., 22, 3564 - 3567 (2010).M.A. Mione, I. Katsouras, Y. Creyghton, W. van Boekel, J. Maas, G. Gelinck, F. Roozeboom, and A. Illiberi, ‘Atmospheric Pressure Plasma Enhanced Spatial ALD of ZrO2 for Low-Temperature, Large-Area Applications’, ECS Journal of Solid State Science and Technology, 6 (12) N243-N249 (2017), and refererences therein. -------------------------------------------------------------------------------------------------------------------------------------------------------------------- Figure 1