Applied Surface Science 258 (2012) 2275–2282 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Development of rf plasma sputtered Al 2 O 3 –TiO 2 multilayer broad band antireflecting coatings and its correlation with plasma parameters P. Laha a , A.B. Panda a , S.K. Mahapatra a,1 , P.K. Barhai a , A.K. Das b , I. Banerjee a,∗ a Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi 835215, India b Laser & Plasma Technology Division, Bhabha Atomic Research Center, Mumbai 400085, India a r t i c l e i n f o Article history: Received 7 April 2011 Received in revised form 17 August 2011 Accepted 26 September 2011 Available online 2 October 2011 Keywords: Al2O3 TiO2 X-ray photoemission spectroscopy (XPS) PL Optical properties OES a b s t r a c t TiO 2 /Al 2 O 3 /TiO 2 /Al 2 O 3 multilayer structures were obtained at different oxygen:argon gas ratios of 20:80, 30:70, 50:50 and 60:40 sccm and constant rf power of 200 W using reactive magnetron sputtering. Grain size and elemental distribution in the films were studied from AFM image and XPS spectra respectively. The deposited grain size increased with increasing oxygen:argon gas ratio. The optical band gap, refractive index, extinction coefficient were calculated from UV–vis transmittance and reflectance spectra. It was observed that the value of refractive index, extinction coefficient and band gap increased with increasing oxygen. These variations are due to the defects levels generated by the heterostructure and explained by the PL spectrum. The antireflecting (AR) efficiency of the films was estimated from the reflectance spectra of the films. Broad band antireflecting coating for the visible range was achieved by varying oxygen content in the film. The plasma chemistry controlled the antireflecting property by the interface interdiffusion of atoms during layer transition in multilayer deposition. The in situ investigation of the plasma chemistry was performed using optical emission spectroscopy. The plasma parameters were estimated and correlated with the characteristics of the films. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In advanced technology, transparent, nano structured and mul- tilayer coatings popularly known as artificial super lattices are gaining importance due to their superior optical properties com- pared to single layer [1,2]. Proper combination of refractive indices and interface reflectance losses across the interfaces decides the optical response of a multilayer stack. Therefore, material selection and the interface effect play a major role for fabricating multilayer optical windows. Oxide materials like TiO 2 , SiO 2, HfO 2 , ZrO 2 and Al 2 O 3 , etc. are usually used for multilayer optical windows [2–4]. For antireflection (AR), alternating layers of high and low refractive index materials are used which either lowers the overall inten- sity of the light or restrict the wavelength range of the light due to interference effect. There are many reports on the preparation of such optical coatings based on TiO 2 /SiO 2 and Al 2 O 3 /SiO 2 multi- layer dielectric films but less intensive reports are available on the ∗ Corresponding author. Present address: Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, California 90095, USA. Tel.: +91 651 2275444; fax: +91 651 2275401. E-mail addresses: indranibanerjee@bitmesra.ac.in, indrani@seas.ucla.edu (I. Banerjee). 1 Present address: Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA. combination of TiO 2 /Al 2 O 3 deposited by reactive magnetron sput- tering [5–7]. TiO 2 has wide band gap (3.2 eV) [1–7] along with high refractive index (2.4), whereas, Al 2 O 3 also has wide band gap (8.2 eV) but low refractive index (1.65). However, both these materials transmit visible light (600–800 nm) but absorb UV light (200–300 nm) [8,9]. Moreover, bulk TiO 2 and Al 2 O 3 have no common lattice structure at any temperature or pressure [10]. There is a strong affinity for Ti–O–Al bond formation across a TiO 2 –Al 2 O 3 interface. This can avoid alteration of fundamental optical absorption edge (FOAE) [11]. According to Tikhonravov “maximum principle” theory, two stacks of greatest index contrast are sufficient to provide optimal optical performance at normal angles of incidence for any multistack film [11,12]. Therefore, two layers of TiO 2 and two layers of Al 2 O 3 used alternatively to obtain four layer multilayer stacks for this study. Plasma induced low temperature deposition like magnetron sputtering is the most conventional method for such depositions. The plasma chemistry involves large number of reactive species generated in the reaction zone which plays important role for the endpoint detection and process control. However, the major dis- advantage in deposition process arises due to the target poisoning. The radiation from the generated species in the plasma gives a con- crete signature of the plasma chemistry that can be correlated with the deposited materials characteristics. The optical emission spec- troscopy is a powerful non-intrusive method to monitor the in situ deposition mechanism with respect to plasma radiation. This helps 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.118