Ion-sliced lithium niobate thin films for active photonic devices Gorazd Poberaj * , Manuel Koechlin, Frederik Sulser, Andrea Guarino 1 , Jaroslav Hajfler, Peter Günter Nonlinear Optics Laboratory, Institute of Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland article info Article history: Available online 19 May 2008 Keywords: Lithium niobate Crystal ion slicing Wafer bonding Thin films Integrated optics Photonic devices abstract We report on sub-micrometer thick LiNbO 3 films produced by an improved crystal ion slicing and bond- ing technique using polymer adhesive benzocyclobutene. The latter enables to reproducibly fabricate defect-free films with an area of several cm 2 . The method supports also integration of tuning electrodes enabling realization of complex electro-optically active photonic devices on a single chip. Furthermore, the structuring techniques to produce high-index-contrast (Dn  0.65) single-mode optical waveguides are described. The big potential of the novel LiNbO 3 thin films for high-density integrated optics applica- tions is shown on an example of electro-optically tunable microring resonator. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction As optical components continue to replace electronics for opti- cal signal processing applications, there is a growing impetus to integrate more photonic devices onto a single chip. Compared to electronics, photonic devices offer several advantages, such as large bandwidth operation, wavelength division multiplexing, and also absence of electro-magnetic interference. In the last few years, strong efforts have been made to develop silicon based photonic chips [1]. This field of research has profited considerably from the availability of large-size silicon-on-insulator (SOI) wafers and the advanced semiconductor technology providing solutions for micro- and nano-structuring of photonic devices. A high refractive index contrast between silicon and silicon-oxide is essential for the realization of highly integrated optics devices based on photonic wires [2,3] and photonic band gap structures [4]. On the other hand, ferroelectric materials, such as lithium nio- bate, could provide additional functionality of photonic chips due to their excellent electro-optic and nonlinear optic properties. Unfortunately, at present, there are no practical nonlinear optical integrated circuits of LiNbO 3 other than simple modulators and second harmonics generation devices. These are based on conven- tional waveguide fabrication techniques, such as metal in-diffu- sion, ion exchange or proton exchange [5]. Although these waveguides show very low propagation losses, their optical con- finement is very weak due to a relatively low refractive index con- trast (Dn < 0.1). Consequently, these waveguides suffer from high bending losses, which prevent their use for high-density integrated optics. A key step towards realization of high-index-contrast ferroelec- tric waveguides is to produce single-crystalline thin films, which could be structured and embedded in low-index dielectric materi- als. In this endeavor, many techniques have been studied in the past, such as chemical vapor deposition [6], RF sputtering [7], molecular beam epitaxy [8], sol-gel [9], and pulsed laser deposition [10]. However, all these techniques have difficulty in producing high crystalline quality materials. In addition, the use of substrates with required properties is limited, in particular for epitaxial growth due to lattice matching constraints. More recently, crystal ion slicing (CIS) in combination with wafer bonding techniques has emerged as a very promising technique for fabrication of sin- gle-crystalline ferroelectric thin films. This method generally known under the name of ‘‘Smart Cut” was originally discovered and applied for the fabrication of silicon-on-insulator (SOI) wafers [11]. It uses high-dose implantations of H + and/or He + ions (D  10 16 –10 17 /cm 2 ) for cleaving films from a bulk material. Since the process was disclosed in 1995 [12], several variants of this method have also been studied for the exfoliation and transfer of different ferroelectric thin films. Basically, thin film exfoliation from an implanted LiNbO 3 crystal can be achieved either by wet- etching [13,14] or thermal treatment [15,16]. The reported meth- ods differ also in bonding techniques used for the transfer of brittle exfoliated thin films onto supporting substrates. Bonding is one of the most critical steps in fabrication of large-area thin films. The wet-etching technique exploits the fact that HF solution etches the damaged layer (defined by the penetration depth of ions) at a much higher rate than the rest of a LiNbO 3 crystal. An etching selectivity as high as 10,000 has been reported for an opti- mized process consisting of a rapid thermal annealing followed by 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.12.019 * Corresponding author. E-mail address: poberaj@phys.ethz.ch (G. Poberaj). 1 Present address: Bookham AG, 8045 Zurich, Switzerland. Optical Materials 31 (2009) 1054–1058 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat