Elsevier

Applied Surface Science

Volume 257, Issue 12, 1 April 2011, Pages 5223-5229
Applied Surface Science

Nano-scale and surface precipitation of metallic particles in laser interference patterned noble metal-based thin films

https://doi.org/10.1016/j.apsusc.2010.11.060Get rights and content

Abstract

Laser interference patterning (also known as “laser interference metallurgy”) is used to locally tailor the microstructure of oxide (Pd0.25Pt0.75Ox) and nitride (Cu3N) thin films to induce a chemical decomposition, which is responsible for a decrease of electrical resistivity. This technique allows hereby a laser-induced chemical decomposition of as-deposited oxide and nitride films, resulting locally in a porous microstructure due to the simultaneous emission of gaseous nitrogen and oxygen. The process locally generates at the nanometer scale metal precipitatation of Pt or Cu in the oxide or nitride matrix. Thus, isolated metallic clusters with low resistivity coexist with a high resistivity phase, establishing a preferential electrical conduction path and giving the system a lower effective resistivity. The decomposition process is investigated by four-point probe method, X-ray diffraction, spectrophotometry, white light interference, scanning and transmission electron microscopies.

Research highlights

▶ One step laser interference patterning of Cu3N and Pd0.25Pt0.75Ox. ▶ Thermal decomposition of metal nitride and oxide thin films. ▶ Laser-induced precipitation of metallic nanoparticles. ▶ Significant lowering of electrical resistivity.

Introduction

Microstructural evolutions are directly responsible for effects on the physical, mechanical, electrical and also chemical behaviours of materials. A fortiori in the case of thin films, size effects may also lead to a more complex interpretation of the results and require more specific attention in their advanced microstructural design as well as the detailed understanding of dependencies on the material response. The thermal decomposition of metal oxide and nitride thin films can be exploited to generate original microstructures such as high specific surface area metal islands on an oxide matrix [1] or dispersed particles with different core and surface compositions [2]. Thereby, physico-chemical properties of the as-grown films are strongly impacted by thermal treatments.

Copper nitride (Cu3N) is a metastable compound in normal conditions well known to decompose in the 100–470 °C range [4], [5], [6], [7]. This compound can easily be formed in thin film form by reactive magnetron sputtering with physical properties highly depending on the nitrogen stoichiometry [8]. Palladium oxide (PdO) and platinum oxide (PtO2) are stable in normal conditions but decompose in air around 850 °C [9] and 450 °C [10], respectively. Thus, the thermal decomposition of complex Pd1−yPtyOx thin films produced by physical vapour deposition methods [2], [3] is rather complex but is characterized for y  0.4 by the crystallisation of metallic nanoparticles below 750 °C in a mixture of methane and oxygen [2], [3].

The principle of interference of light through a high-energy nanosecond pulse laser source, so-called laser interference metallurgy [11] leads to a periodic thermal treatment with a local melting of the surface of the material. This one-step process allows a surface tailoring resulting in periodic structures organized in well-defined geometrical patterns and is hence expected to generate decomposition patterns in Cu3N and Pd1−yPtyOx. In the present paper we investigate the influence of laser interference irradiation within nanosecond single pulses on the microstructure and electrical resistivity of Cu3N and Pd0.25Pt0.75Ox thin films deposited by reactive magnetron sputtering.

Section snippets

Film deposition

A 550 nm thick Cu3N film has been deposited on sodalime glass slide by reactive magnetron sputtering of a copper target (50 mm diameter, 3 mm thick and purity over 99.9%). A 350 nm thick Pd0.25Pt0.75Ox film has been deposited on sodalime glass slide by reactive magnetron co-sputtering using palladium and platinum targets (50 mm diameter, 3 mm thick and purities over 99.9%).

The experimental device used is a sputtering chamber (40 L) pumped down via a combination of a back and a turbomolecular pump

Morphological and topographical evolutions

Fig. 1 shows the scanning electron micrographs of the nitride (Cu3N) and oxide (Pd0.25Pt0.75Ox) thin films irradiated by single pulses within the laser interference system described in the experimental part. The 550 nm thick Cu3N has been irradiated with a laser fluence F of 247 mJ/cm2 and the 350 nm thick Pd0.25Pt0.75Ox with F = 132 mJ/cm2.

The line-like interference patterns result from the superposition of two individual laser beams. As shown in Fig. 1a–c, the surface morphology of both films

Conclusions

A local laser-induced chemical decomposition of Cu3N and Pd0.25yPt0.75Ox, leading to a significant lowering of the electrical resistivity has been reported. After structuring those systems by means of laser interference, metallic low resistive Cu and Pt precipitates have been formed from the nitride and oxide films, together with an evaporation of nitrogen and oxygen, respectively. This evaporation generates a porous and foamy microstructure at the surface of the regions of the samples

Acknowledgements

The authors acknowledge the German Research Foundation (DFG) for funding under project MU 959/20-1 and the European Commission for the Erasmus Mundus scholarship of Y.H. in the framework of the AMASE research Master.

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1

Currently at Paul Scherrer Institute (PSI), CH-5232 Villingen, Switzerland.

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