Reactive ion etching of waveguide structures in diamond Mark P. Hiscocks a, ⁎, Christopher J. Kaalund a , François Ladouceur a , Shane T. Huntington b , Brant C. Gibson b , Steven Trpkovski b , David Simpson b , Eric Ampem-Lassen b , Steven Prawer b , James E. Butler c a School of EE&T, University of New South Wales, Sydney, NSW, 2052, Australia b Quantum Communications Victoria, School of Physics, University of Melbourne, Melbourne, Victoria, 3010, Australia c Naval Research Laboratory, Washington, DC 20375, United States ABSTRACT ARTICLE INFO Article history: Received 18 March 2008 Accepted 30 April 2008 Available online 10 May 2008 Keywords: Diamond crystal Diamond film Reactive ion etching (RIE) Optical properties Waveguide structures were fabricated in both nanocrystalline CVD diamond (NCD) and HPHT type 1b single crystal diamond using photolithography and reactive ion etching. The combination of these techniques allows the patterning of many long photonic structures simultaneously, making it easily scalable. Emphasis has been placed on reducing sidewall roughness to prevent loss due to scattering. In single crystal diamond a peak-to-peak roughness of approximately 10 nm (estimated from SEM images) was achieved for the majority of the structure sidewall. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Diamond is well known for its range of extreme properties which make it an attractive material for a variety of applications. Some of these properties which are advantageous for photonic integrated devices include its transparency over a broad spectrum, high thermal con- ductivity, large Young's modulus, high density, and high refractive index [1]. Perhaps the most appealing property of diamond, however, is the ability to act as a room temperature single photon source [2]. The single photons are created by optically pumping impurities in the diamond crystal lattice. Two examples of this are the nitrogen vacancy (N-V) centre and the nickel nitrogen complex (NE8), which emit single photons at 637 and 793 nm respectively [3,4]. As diamond can act as both a source and a waveguiding material it is potentially possible to fabricate monolithic devices where the source and interfacing photonic structures are all diamond. Recent advances in the growth of synthetic diamond have made practical devices attainable. Polycrystalline diamond can be deposited by CVD onto a range of substrates whereas large single crystals of diamond can only be grown on a substrate of diamond. Applications of a single photon source in diamond include quantum key distribution (QKD) [5] and also linear optics quantum computing (LOQC) [6]. As well as these, diamond photonic devices are well suited to applications requiring high power and/or high frequencies [7–9]. Prior work on the RIE of polycrystalline CVD diamond has included the fabrication of resonators [10], microcavities [11], photonic crystals [12], MEMS devices [13–16], and grating structures [17–19]. In single crystal diamond, microcylinders [20] and nanorods [21] have been patterned using RIE. Other etching methods have been used to fabricate similar diamond structures with inductively coupled plasma (ICP) etching, producing some of the smoother diamond structures [22–26]. In single crystal diamond guidance has been demonstrated in a waveguide fabricated by a number of the authors of this paper [27]. The limitation of this waveguide is that it was fabricated using Focused Ion Beam (FIB) milling, which is not a practically scalable solution and also limits the length of waveguide which can be fabricated (of the order of 100 μm). The following work investigates etching structures into two types of diamond: nanocrystalline CVD diamond (NCD) films grown by the Naval Research Laboratory (NRL) Washington D.C. onto a substrate of silica-on-silicon and a Sumitomo type 1b HPHT (High Pressure, High Temperature) single crystal diamond. As mentioned previously polycrystalline diamond can be deposited onto substrates other than diamond (e.g. silica [10]), which simplifies the production of waveguides since the substrate has lower refractive index to provide vertical confinement. A disadvantage though is that light guided through polycrystalline diamond may scatter off crystal domains, although to what extent is still unclear. It is hoped that by using polycrystalline diamond with crystal domains that are much smaller than the wavelength of light this scattering will be acceptable. Alternately, single crystal diamond has no crystal boundaries to scatter the light although implementing vertical confinement is much more difficult. This is because single crystal diamond to date cannot be grown onto a substrate of lower refractive index. One method to overcome this problem is to create an airgap beneath the waveguide through ion implantation as in [27]. A preferable solution is to investigate the change in refractive index of diamond due to either varying the Diamond & Related Materials 17 (2008) 1831–1834 ⁎ Corresponding author. E-mail addresses: m.hiscocks@student.unsw.edu.au, m.hiscocks@hotmail.com (M.P. Hiscocks). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.04.019 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond