Elsevier

Thin Solid Films

Volume 520, Issue 16, 1 June 2012, Pages 5405-5408
Thin Solid Films

Overview of optical properties of Al2O3 films prepared by various techniques

https://doi.org/10.1016/j.tsf.2012.03.113Get rights and content

Abstract

We study optical properties of Al2O3 films prepared by various techniques using spectroscopic ellipsometry. The film preparation techniques include conventional pulsed magnetron sputtering in various gas mixtures, high power impulse magnetron sputtering, annealing of as-deposited Al2O3 in an inert atmosphere and annealing of as-deposited Al in air. We focus on the effect of the preparation technique, deposition parameters and annealing temperature on the refractive index, n, and extinction coefficient, k, of stoichiometric Al2O3. At a wavelength of 550 nm we find n of 1.50–1.67 for amorphous deposited Al2O3, 1.65–1.67 for amorphous Al2O3 obtained by Al annealing, 1.46–1.69 for γ-Al2O3 and decreasing n for Al2O3 annealing temperature increasing up to 890 °C. The results facilitate correct interpretation of optical characterization of Al2O3, as well as selection of a preparation technique corresponding to a required Al2O3 structure and properties.

Highlights

► Optical properties of Al2O3 films significantly depend on the preparation technique. ► Furthermore, for a given technique they depend on preparation conditions. ► We present optical properties of 13 Al2O3 samples prepared by 5 different techniques. ► We show how to prepare amorphous or γ-Al2O3 with n550 in the range of 1.46–1.69.

Introduction

Aluminum oxide, Al2O3, is a widely used thin film material. It forms many polymorphs [1] including α (thermodynamically stable phase), γ (the most common metastable phase), κ, θ, δ, η and χ, let alone the amorphous phase. Different properties of the aforementioned polymorphs lead to numerous applications as wear-resistant protective coatings (α and κ phases) [2], [3], in catalysis (γ and χ phases) [4], [5], in microelectronics (e.g. waveguides) or as diffusion and thermal barriers. Properties reported for all the polymorphs include high optical transparency, excellent chemical inertness and zero electrical conductivity.

In case of Al2O3 prepared in the form of thin films, a large experimental and theoretical effort is being devoted to investigating the relationships between deposition technique, deposition parameters and the phase formed [6], [7], [8], [9], [10]. This is a complex task due to a high number of Al2O3 polymorphs, uncertainties concerning the exact atomic structures of metastable Al2O3 (particularly the γ-phase [11], [12]) and the fact that a large portion of as-deposited Al2O3 is amorphous. In this context, several works deal with optical properties (particularly refractive index, n) of Al2O3 prepared using various substrates or deposition techniques [13], [14], [15].

However, there is a gap resulting (1) from the fact that many measurements were performed on materials of unknown structure (let alone measurements on very porous materials which cannot yield a bulk n [15]) and (2) from the wide range of properties reported for the metastable/amorphous phases [13], [14], [15]. This is despite the fact that some authors report only a narrow interval or even a single value for “their” materials. Note that some review sources or databases include only the refractive index of the well described thermodynamically stable α-Al2O3.

In this work, we investigate optical properties of amorphous and γ-Al2O3 prepared in the form of thin films by a wide range of deposition and annealing techniques. Our aim is (1) to provide the range of properties (particularly refractive indices) corresponding the aforementioned phases, and (2) to capture the effect of the preparation technique, deposition parameters, annealing temperature and film substrate.

Section snippets

Methodology

The films prepared and analyzed are listed in Table 1. The films were prepared (1) by high power impulse magnetron sputtering (HIPIMS) in Ar + O2, (2) by pulsed sputtering in Ar + O2, (3) by pulsed sputtering in N2 + O2, (4) by annealing the Al2O3 films deposited by pulsed sputtering in He and (5) by annealing Al films in air. All samples were identified as pure aluminum oxides, except traces of magnesium in samples 13–14. While Table 1 shows only the deposition parameters needed for the discussion

Results

The list of 13 samples taken into account and their optical properties at 550 nm is in Table 1. The table includes samples prepared by HIPIMS (no. 2-5), pulsed sputtering in Ar + O2 plasma (no. 6–8) and N2 + O2 plasma (no. 9–10), annealing of Al2O3 in He (no. 11–12) and annealing of Al in air (no. 13–14). Dispersion of the refractive indices of all samples in the visible range of 400–800 nm is provided in Fig. 2. Literature data for α-Al2O3 [17] are provided for comparison.

All variants of Al2O3 films

Conclusions

We studied optical properties of Al2O3 films prepared by various techniques using spectroscopic ellipsometry. We found n550nm of 1.50–1.67 for amorphous deposited Al2O3, 1.65–1.67 for amorphous Al2O3 obtained by Al annealing, 1.46–1.69 for γ-Al2O3 (partially or fully crystallized), decreasing n for Al2O3 annealing temperature increasing up to 890 °C, increasing k for Al2O3 annealing temperature increasing up to 450 °C and slightly decreasing k for Al2O3 annealing temperature increasing from 450 °C

Acknowledgment

This work was supported by the Ministry of Education of the Czech Republic through project no. MSM 4977751302, and by the European Regional Development Fund through project NTIS, CZ.1.05/1.1.00/02.0090.

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