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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Sputtered Gold Nanostructures

O. Szabó, S. Flickyngerová, V. Tvarožek, and I. Novotný

Abstract - The forming of Au nanostructures on Corning glass substrates and transparent conductive oxide ZnO:Al thin films by the RF diode sequential sputtering is presented. Morphology of Au structures was analysed by scanning electron microscopy with the free ImageJ software and optical transparency was evaluate by UV-Vis spectrometry. Sputtering power density (deposition rate) and nominal Au thickness caused changes of size and nearest neighbour distance of Au nanostructures as well as it lowered the optical transparency.

I. INTRODUCTION

Among nanostructured noble metals, gold is of special

importance due to its stability and unique electrochemical and optical properties. The morphology of gold surface is the most significant feature that influences its functional properties. Gold nanostructures are often prepared by methods of electrochemical deposition [1] but there are a few reports on sputtered micro-/nano- structures [2]-[4]. ZnO doped by Al (ZnO:Al) belongs to transparent conductive oxide (TCO) thin films with specific electronic/optical properties which are applicable in solar cells, optoelectronics and sensorics.

The purpose of our research was to expand on proof-of-concept experiments by developing the technology of the forming of Au nanostructures on Corning glass substrates / TCO ZnO:Al thin films by the RF diode sputtering and to characterize their morphology by scanning electron microscopy (SEM) and optical spectrometry.

II. EXPERIMENTAL

We used the RF diode sputtering system Perkin/Elmer 2400/8L for thin film deposition. with thickness of 560 nm were deposited using a ceramic target (ZnO+2 wt. % Al2O3) in Ar working gas. Post-deposition annealing of ZnO:Al thin films was done at 500 ºC / 30 min. in the forming gas N2:H2 (90:10). Deposition of Au was realised by the sequential (dynamic) layer-by-layer mode of sputtering [5], Au target of 102.4 mm in diameter, Ar pressure of 1.3 Pa. An influence of RF power on the morphology of Au nanostructures was examine. Corning glass substrates (bare or covered by TCO ZnO:Al thin film)

were used. Deposition (sputtering) rates were determined from thicknesses and time of deposition of the homogeneous thin films, which were evaluated by Dektak profilometer. A quantity of sequentially sputtered material was estimated by nominal thickness of film deposited during one-turn period (≈ 9 s) under the target. Sputtering rates of Au were dependent on the RF power density and they were 1.0 nm/s (72 mW/mm2), 0.5 nm/s (36 mW/mm2), 0.25 nm/s (18 mW/mm2) 0.125 nm/s (9 mW/mm2). The morphology of nanostructures was analyzed by SEM JEOL7500F, their optical transparency was measured by AVA Spec-2048 Fiber Optic Spectrometer.

III. RESULTS AND DISCUSSION

For growth of nanostructures the early stages of thin film growth are important: nuclei formation, growth of clusters and islands and their coalescence. Nanostructures of Au showed the tendency to grow in the Stranski - Krastanov hybrid mode [6], i.e. initially 2 D layer-by-layer, then 3 D island growth mechanism, Figs. 1, 2. The growth of nanostructures, i.e. their morphology also, was influenced by the type of substrate: amorphous Corning glass or polycrystalline textured ZnO:Al thin film on Corning glass.

SEM image processing program ImageJ is a public domain developed at the National Institutes of Health [7]. It can calculate area and nearest neighbour (NN) distance of objects specified in pixels, pixel value statistics of user-defined selections and can create density histograms and line profile plots. It supports standard image processing functions such as contrast manipulation, sharpening, smoothing, edge detection and median filtering. Size (area) of nanostructures (islands) is characterized by an area of selection in square pixels. Original SEM images (size of 1280x1024 pixels) were cut for evaluation to size of 640x512 pixels. An example of SEM image processing of Au nanostructures sputtered on Corning glass substrate at temperature 200 ºC are given on Fig. 1. The decrease of RF power density from 72 mW/mm2 to 18 mW/mm2 caused the lowering of both deposition rates from 1.0 nm/s to 0.25 nm/s and nominal thicknesses from 9.0 nm to 2.2 nm. It resulted in the contraction of median of nanostructure (island) areas from 866 nm2 to 67 nm2. Simultaneously, median of nearest neighbour distances of islands dropped down from 34 nm to 12 nm.

The decrease of RF power density 9 mW/mm2 caused the lowering of both deposition rate to 0.125 nm/s and nominal thickness to 1.1 nm.

O. Szabó, S. Flickyngerová, V. Tvarožek, and I. Novotný are with the Institute of Electronics and Photonics, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia, E-mail: vladimir.tvarozek@stuba.sk

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f) Fig. 1. An example of SEM image processing: a) - b) Au nanostructures sputtered on Corning glass substrate at temperature 200 ºC, c) - d) size distribution and e) - f) NN distance distribution of islands. Power density / deposition rate / nominal thickness: a) - 72 mW/mm2/ 1.0 nms-1 / 9 nm, b) – 18 mW/mm2/ 0.25 nms-1 / 2.2 nm.

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Fig. 2. a) SEM image of Au film deposited onto ZnO:Al thin film (Corning glass substrate) of nominal thickness (sputtering rate) 2.2 nm (0.25 nm/s), b) size distribution and c) NN distance distribution of islands.

Morphology of Au nanostructures formed on ZnO:Al thin film was affected by the power density (deposition rate) and their nominal thicknesses as well as by the polycrystalline texture character of the film surface, Fig. 2.

Medians of nanostructure (island) areas / NN distances were falling down to 18 nm2 / 8.5 nm for power density of 18 mW/mm2, deposition rate 0.25 nm/s and nominal thickness of 2.2 nm, Fig. 2. Statistical analysis of SEM images showed the Poisson (LogNormal) distribution of the size of nanostructures and their NN distances demonstrated Gaussian distribution.

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Fig. 3. Spectral transmittance of Au nanostructures sputtered on ZnO:Al thin film / Corning glass substrate at different power densities and an uncoated ZnO:Al film / Corning glass substrate.

Spectral optical transmittances of an uncoated ZnO:Al / Corning glass substrate and covered by Au at different power densities are shown in Fig. 3. The range of power densities corresponded to the nominal thickness interval 1.1 ≤ dAu ≤ 9 nm.

Transmittance of ZnO:Al film / Corning glass substrate (~ 90 %) decreased down to ~ 50 % with an increase of sputtering power densities (i.e. also with Au nanostructure sizes).

IV. CONCLUSION

Presented results confirmed the ability of the RF diode sputtering to prepare gold nanostructures of variable morphologies by the change of RF power density. Sequential mode of sputtering seems to be well-controllable and precise technology with low deposition rate in the range of 0.1 – 1 nm/s.

Research of Au nanostructures sputtered on textured and columnar TCO ZnO:Al thin films is going on because its outputs are applicable in biochemical sensors.

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ACKNOWLEDGEMENT

The presented work was supported by the SK VEGA Project 1/0459/12 and by the Competence Center for SMART Technologies for Electronics and Informatics Systems and Services, ITMS 26240220072, funded by the R&D OP Programme ERDF SR.

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