Photoassisted atomic layer deposition of oxides employing alkoxides as single-source precursors

Abstract

Photoassisted atomic layer deposition (photo-ALD) is a variant of an ALD process where photons of ultraviolet or visible range are utilized to supply energy to, and to modify, the ALD surface reactions. In this paper, the authors report photo-ALD processes for titanium, zirconium, hafnium, niobium, and tantalum oxides by employing the corresponding liquid, volatile metal alkoxides as precursors in a single-source approach, i.e., without any additional reactant. The ALD reactor was equipped with a light source delivering photons over a continuous spectrum between 190 and 800 nm in wavelength. The deposition sequence consisted of a precursor pulse, a purge, a photon exposure, and another purge. The process characteristics and film properties were explored. Nb2O5 and Ta2O5 films were amorphous, whereas TiO2, ZrO2, and HfO2 showed an amorphous and polycrystalline structure, depending on the deposition conditions. With photo-ALD, area-selective deposition is realized by shadow masking. The character of the growth process, i.e., whether the chemistry is driven by photolytic or photothermal mechanism, is discussed based on deposition experiments with patterned substrates and optical filtering. Electrical characterization of photo-ALD HfO2 shows promising dielectric properties.