Delivered by Ingenta to: Drexel University Libraries IP: 79.110.18.94 On: Sun, 05 Jun 2016 21:38:07 Copyright: American Scientific Publishers RESEARCH ARTICLE Copyright © 2009 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 9, 3763–3770, 2009 CoatingofNanoporousMembranes:AtomicLayer Deposition versus Sputtering K. Grigoras 1 ∗ , V.-M. Airaksinen 2 , and S. Franssila 1 1 Micro and Nanosciences Laboratory, MICRONOVA, Department of Electrical and Communications Engineering, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland 2 Micro and Nanofabrication Centre MINFAB, MICRONOVA, Department of Electrical and Communications Engineering, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland Nanoporous anodic alumina membranes and silicon samples with plasma etched nanopores have been coated with zinc oxide or gold layer using atomic layer deposition (ALD) or sputtering, respec- tively. In the case of ALD process, the precursor pulses were extended, compared with planar substrate coating. Thick (60 m) anodic alumina membranes have been conformally coated with zinc oxide ALD layer. Metal sputtering technique was used just for opposite purpose—to minimize the penetration of gold into the pores during gold-coating of the top and bottom surfaces of the membrane. Scanning electron microscopy (SEM) has been used to investigate the layer thickness, uniformity and conformality inside the nanopores. Keywords: ALD, Anodic Alumina Membrane, Conformal Coating, SEM. 1. INTRODUCTION Application of nanoporous materials is expanding over their traditional area of membranes and filters, and gaining more and more interest in photonics, fuel cells, microflu- idics, proteomics, and drug delivery. 1–4 Anodic etching of aluminum is an excellent method to produce uniform and deep nanopores. 5 6 Depending on the electrochemical etch- ing conditions (electrolyte composition and applied volt- age) the pore diameter can be tailored in the range of 20–400 nm. Electrochemical etching of silicon results in similar, yet not as uniform pores. 7 Deep reactive etch- ing (DRIE) of silicon can produce arrays of uniform and deep nanopores or nanopillars, with designer pore den- sity and positioning on the wafer. 8 The main advantage of silicon as a nanoporous material is the possibility of implementing different active and passive elements on the same silicon chip. Characteristics and performance of nanoporous materials can be modified or function- alized by various surface coating methods. Taking into account that the thickness of the nanoporous membrane or layer can reach hundred of micrometers (aspect ratios reaching 1000:1), the coating of the pores becomes a challenge. Atomic layer deposition (ALD) provides excellent uniformity, conformality and thickness control of the ∗ Author to whom correspondence should be addressed. deposited thin film in the nanometer range. 9–11 This is based on the self-terminating reactions during each cycle of growth. 10 Application areas of ALD are rapidly extending, and main reasons for this can be the dimen- sional downscaling in microelectronic devices, require- ments for lower film growth temperatures (specially in the case of polymer substrates), and requirements for high quality films without post-deposition annealing. ALD is the most suitable technique when deep trenches with ultra- high aspect ratio must be coated. 12–14 Growth rate in ALD is determined as growth-per-cycle, and usually this value is below 1 Å/cycle. When coating planar or almost planar surface, the cycle lasts about a sec- ond, but it can reach even minutes when deep nanotrenches must be coated, resulting in process times of hours for growing tens of nanometers of layer. Long cycle time is necessitated by slow diffusion of precursor gases inside trenches. 15 One solution enabling reduction of precursor pulses is the modification of ALD reactor to force the flow through the substrate. 16 As a drawback this technique requires a special design for each sample size and shape. Additionally, if there are branched and partially closed pores, they will be not coated uniformly. Therefore, opti- mization of gas pulse duration is very important when high aspect ratio structures must be coated. The list of materials that can be deposited by ALD is quite long. They are mostly oxides but nitrides are growing fast. 10 ZnO films are of interest to be used as conductive J. Nanosci. Nanotechnol. 2009, Vol. 9, No. 6 1533-4880/2009/9/3763/008 doi:10.1166/jnn.2009.NS64 3763