Extraordinary low transmission effects for ultra-thin patterned metal films D. Reibold1 , F. Shao2 , A. Erdmann3 , and U. Peschel4 1 Erlangen
Graduate School in Advanced Optical Technologies (SAOT) and Chair of Electron Devices, Friedrich-Alexander University Erlangen-Nuremberg, Cauerstr. 6, 91058 Erlangen, Germany
[email protected] 2 Erlangen Graduate School in Advanced Optical Technologies (SAOT) and Fraunhofer Institute of Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany
[email protected]
3 Fraunhofer
Institute of Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany
[email protected]
4 Institute
of Optics, Information and Photonics, Max Planck Research Group, G¨unther-Scharowsky-Straße 1, 91058 Erlangen, Germany
[email protected]
Abstract: Thin metal films show a residual transmission for light in the visible and UV spectral range. This transmission can be strongly reduced by an appropriate sub-wavelength patterning of the metal film. Our investigation is focused on metal films with a thickness much below 100nm, where the transmission response is dominated by the individual posts acting like antennas and cannot be attributed to the excitation of surface plasmons. The almost complete suppression of transmission for ultra-thin metal films depends mainly on the absorber width, but not on the pitch of the pattern. The effect is robust with respect to imperfections of the geometry or larger features imprinted into the sub-wavelength pattern. © 2008 Optical Society of America OCIS codes: (050.1220) Apertures; (050.2770) Gratings; (240.6680) Surface plasmons.
References and links 1. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature (London) 391, 667–689 (1998). 2. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer Verlag, Berlin, 1988). 3. H. J. Lezec and T. Thio, “Diffracted evanescent wave model for enhanced and suppressed optical transmission through subwavelength hole arrays,” Opt. Express 16, 3629–3651 (2004). 4. Q. Cao and P. Lalanne, “Negative Role of Surface Plasmons in the Transmission of Metallic Gratings with Very Narrow Slits,” Phys. Rev. Lett. 88, 057403 (2002). 5. O. T. A. Janssen, H. P. Urbach, and G. W. ’t Hooft, “On the phase of plasmons excited by slits in a metal film,” Opt. Express 14, 11823–11832 (2006). 6. www.rit.edu/ 635dept5/thinfilms/thinfilms.htm 7. K. D. Lucas, H. Tanabe, and A. Strojwas, “Efficient and rigorous three dimensional model for optical lithography simulation,” J. Opt. Soc. Am. A 13, 2187-2199 (1996).
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(C) 2009 OSA
Received 15 Oct 2008; revised 26 Nov 2008; accepted 27 Nov 2008; published 7 Jan 2009
19 January 2009 / Vol. 17, No. 2 / OPTICS EXPRESS 544
8. L. Li, “Use of Fourier series in the analysis of discontinuous periodic structures,” J. Opt. Soc. Am. A 13, 1870– 1876 (1996). 9. P. Evanschitzky and A. Erdmann, “Three dimensional EUV simulations: A new mask near field and imaging simulationsystem,” Proc. SPIE 5992, 1546 (2005). 10. www.drlitho.com 11. L. Novotny, “Effective wavelength scaling for optical antennas,” Phys. Rev. Lett. 98, 266802 (2007).
1.
Introduction
The observation of enhanced transmission through an array of sub-wavelength holes or slits in a metal film by Ebbesen et al. [1] initiated many publications. The majority of these publications explains the mechanism with the excitation of surface plasmons (SP). The wave vector of surface plasmons in x-direction is given by [2] r εd εm kSP,x = k , (1) εd + εm where εd and εm specify the permittivity of the dielectric material and the metal, respectively. The symbol k = 2π /λ represents the wave vector of the incident light, λ is the wavelength in vacuum. There is still some controversy, whether surface plasmons really contribute to the transmission [3], and in which way [4]. A recent publication by Janssen et al. [5] reports that the transmission depends also on the phase of the surface plasmon. It is shown that an array of slits in a 200 nm thick gold film is nearly opaque, if the periodicity is an integer multiple of the SP wavelength. The goal of this paper is to investigate transmission effects of sub-wavelength structures in extremely thin ( 30nm) has nearly no impact on the observed transmission values. Obviously the effect of reduced transmission vanishes only, if there is a shortcut between individual posts thus preventing the antenna action. 6.
Conclusions and outlook
The transmission of thin metal films with appropriate sub-wavelength patterns can be considerably lower than that one of unpatterned metal films. If the thickness of the metal films is 100nm or larger, the observed effect is determined by the periodicity of the sub-wavelength array and can be attributed to the interference of surface plasmons. For very thin metal layers with a thickness of 40nm and below, the magnitude of the low transmission effect is governed by the size of the metallic features (ridges, mesas etc.). In that case, the observed effects can be qualitatively explained by a simple ”antenna model”. The majority of the simulations for this paper were performed for (patterned) silver films and a wavelength of 633nm. However, additional simulations for other metals with a good conductivity, like gold, copper or aluminum showed similar results. The observed effects occur also for small periodic sub-wavelength arrays in the vicinity of larger features like contact holes with diameters in the order of the wavelength or larger. Potentially this opens new possibilities for the contrast enhancement in contact hole imaging for lithographic applications. The observed effects are rather robust regarding small geometry variations such as corner rounding and oblique sidewalls. This alleviates the fabrication tolerances for an experimental verification of the observed effects. Acknowledgments The authors gratefully acknowledge funding of the Erlangen Graduate School in Advanced Optical Technologies (SAOT) and of the Cluster of Excellence Engineering of Advanced Materials - Hierarchical Structure Formation for Functional Devices by the German Research Foundation (DFG) in the framework of the excellence initiative.
#102791 - $15.00 USD
(C) 2009 OSA
Received 15 Oct 2008; revised 26 Nov 2008; accepted 27 Nov 2008; published 7 Jan 2009
19 January 2009 / Vol. 17, No. 2 / OPTICS EXPRESS 551
