Influence of Substrate Materials on Nucleation and Properties of Iridium Thin Films Grown by ALD

Paul Schmitt,Andreas Tünnermann,Adriana V Szeghalmi,Nadja Felde,Felix Otto,Vivek Beladiya,Pallabi Paul,Torsten Fritz

doi:10.3390/coatings11020173

Abstract

Ultra-thin metallic films are widely applied in optics and microelectronics. However, their properties differ significantly from the bulk material and depend on the substrate material. The nucleation, film growth, and layer properties of atomic layer deposited (ALD) iridium thin films are evaluated on silicon wafers, BK7, fused silica, SiO2, TiO2, Ta2O5, Al2O3, HfO2, Ru, Cr, Mo, and graphite to understand the influence of various substrate materials. This comprehensive study was carried out using scanning electron and atomic force microscopy, X-ray reflectivity and diffraction, four-point probe resistivity and contact angle measurements, tape tests, and Auger electron spectroscopy. Within few ALD cycles, iridium islands occur on all substrates. Nevertheless, their size, shape, and distribution depend on the substrate. Ultra-thin (almost) closed Ir layers grow on a Ta2O5 seed layer after 100 cycles corresponding to about 5 nm film thickness. In contrast, the growth on Al2O3 and HfO2 is strongly inhibited. The iridium growth on silicon wafers is overall linear. On BK7, fused silica, SiO2, TiO2, Ta2O5, Ru, Cr, and graphite, three different growth regimes are distinguishable. The surface free energy of the substrates correlates with their iridium nucleation delay. Our work, therefore, demonstrates that substrates can significantly tailor the properties of ultra-thin films.

Highlights

The noble metal iridium (Ir) is applied in various areas, such as for optics [1], sensors [2,3], and catalysts [4,5], as electrodes [6], protective layers [7,8], or hydrogen separation membranes [9]
In order to investigate the nucleation of iridium layers on different substrate materials, scanning electron microscopy (SEM) images were obtained
atomic layer deposition (ALD) deposited Ir forms an island growth mode, which is typical for noble metals on dielectric surfaces

Summary

Introduction

The noble metal iridium (Ir) is applied in various areas, such as for optics [1], sensors [2,3], and catalysts [4,5], as electrodes [6], protective layers [7,8], or hydrogen separation membranes [9]. Iridium is essential for different high-performance optical elements, such as metal wire grid polarizers [10], Fresnel zone plates [11], and curved grazing incidence mirrors [12]. For the conformal coating of these structurally complex elements, atomic layer deposition (ALD) is the method of choice. With optical elements becoming more and more complex, the importance of conformal ALD coatings increases. Ultra-thin metallic films with a thickness of few nanometers are widely desired for optical and electrical applications but challenging to obtain

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