Fabrication of SiO 2 microdisk arrays for optics and light trapping experiments L.V. Saraf * , M.H. Engelhard, A.S. Lea Pacific Northwest National Laboratory, Richland, WA 99352, United States Received 13 September 2006; received in revised form 11 January 2007; accepted 1 February 2007 Available online 8 February 2007 Abstract We present a simple silicon based microfabrication process that produces an array of SiO 2 microdisks using UV lithography. High- resolution SEM images of these structures indicate a smooth outer microdisk cavity surface. Photoemission measurements were per- formed at different spots on the microdisk and compared with measurements inside the cavity. A silicon to oxygen atomic concentration ratio of 1:2 obtained during depth profiling confirms that the entire microdisk is made up of stoichiometric SiO 2 . In contrast, the inner cavity is mostly silicon with native oxide on top. We discuss the usefulness of SiO 2 microdisks in optics for light trapping experiments. Ó 2007 Elsevier B.V. All rights reserved. PACS: 79.60.i; 85.85.+j; 85.40.Hp; 07.10.Cm Keywords: Microdisks; Photoemission 1. Introduction Historically, isotropic etching solutions such as ‘HNA’, which is a combination of hydrofluoric acid (HF), nitric acid (HNO 3 ) and acetic acid (CH 3 COOH), were among the first reported silicon etchants [1–3]. Their use has been primarily for patterning of films, thinning of planar sur- faces from single crystals and creating smooth surfaces in silicon by removal of roughness [4]. HNA chemical reac- tions are typically diffusion limited, meaning that the etch- ing rate can be increased by reducing the diffusion barrier through solution agitation. This is in contrast to aniso- tropic reactions in which temperature has a strong influ- ence on etching rates. Etching of silicon with KOH solutions is an excellent example of anisotropic etching. It is known that silicon/SiO 2 based microstructures pro- duced by isotropic wet chemical etching produce smoother surfaces than dry chemical etching techniques [4]. The mor- phological smoothness helps to guide light in SiO 2 optical transmission devices with little scattering caused from rough surfaces and thus enhancing the efficiency [5]. Silicon based photonic structures such as ring resonators and disk resonators with waveguide coupling are good examples of structures that require low roughness [6,7]. Microstructures made from SiO 2 microdisks can be useful in ultra-high-Q planar microcavities for light trapping experiments [8]. Due to the simplicity and flexibility of basic UV photoli- thography processes and isotropic wet chemical etching, one of the unique advantages of this procedure is the ease of patterning large arrays of such microdisks quickly. In this report, we discuss formation of an array of SiO 2 microdisks on silicon pillars. High-resolution SEM micro- graphs indicate smooth microdisk edges. We have studied high-resolution depth-dependent photoemission properties at various points on the microdisk. It is confirmed that these microdisks are completely stoichiometric SiO 2 with a 1:2 atomic concentration ratio of silicon to oxygen. Finally, we discuss the usefulness of (SiO 2 ) microdisks array in optics. 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.02.001 * Corresponding author. Tel.: +1 509 376 2006; fax: +1 509 376 5106. E-mail address: Lax.Saraf@pnl.gov (L.V. Saraf). www.elsevier.com/locate/mee Available online at www.sciencedirect.com Microelectronic Engineering 84 (2007) 2799–2803