IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 17 (2007) 1593–1597 doi:10.1088/0960-1317/17/8/023 Smoothing dry-etched microstructure sidewalls using focused ion beam milling for optical applications In-Hyouk Song, Yves-Alain Peter and Michel Meunier Department of Engineering Physics, ´ Ecole Polytechnique de Montr´ eal, CP 6079, Succ. Centre-ville, Montr´ eal, Qu´ ebec H3C 3A7, Canada Received 1 May 2007 Published 13 July 2007 Online at stacks.iop.org/JMM/17/1593 Abstract This paper presents a new sidewall smoothing process for Si micro-optical components using focused ion beam (FIB) milling. First, the deep reactive ion etching (DRIE) Bosch process is employed to form a microstructure on a Si substrate. However, scalloping which is induced on the sidewall by the repetition of etching and passivation steps is a major source of light scattering, reducing surface reflectivity in optical applications. In this research, FIB is used to smooth the rough sidewall surface and obtain polished mirror surface. After the DRIE Bosch process, the values of sidewall roughness as measured by atomic force microscopy were 153.0 ± 13.5 nm with an rms value of 28.1 ± 6.4 nm. The FIB smoothing process tremendously improved the surface roughness of etched sidewalls resulting in a maximum peak-to-valley roughness and an rms roughness of 5.7 ± 1.8 nm and 1.6 ± 0.7 nm, respectively. In this paper, the DRIE Bosch process and the FIB smoothing process are described in detail and applications using this technique are discussed. (Some figures in this article are in colour only in the electronic version) 1. Introduction Microelectromechanical systems (MEMS) that include optical components are often referred to as micro-opto-electro- mechanical systems (MOEMS). Recently, Si MEMS have been employed for optical components such as optical switches, micro-mirrors, projection displays, adaptive optics and variable optical attenuator [1–8]. Smoothing the sidewall of microstructures is technologically important for producing optical components. Hence, the smoothing process has been extensively investigated and proposed using wet etching or dry etching techniques to fabricate Si microstructures for optical applications. Yun reported a KOH wet etching process after the deep reactive ion etching (DRIE) process to devise a Si micro-mirror with an rms roughness down to 9.6 nm [7]. However, this technique requires a precise alignment to achieve desired angles since the etching angle is dependent on crystal orientation. In contrast, dry etching methods are more flexible for designing and fabricating micro-optical components, but usually lead to rougher surfaces. Deep reactive ion etching (DRIE) is one of the most used techniques to fabricate Si optical components with nano-scaled roughness. DRIE based on electron cyclotron resonance (ECR) source followed by thermal oxidation or boron diffusion has been used to fabricate high aspect ratio vertical Si micro- mirrors with good optical properties [9]. The additional oxidation and diffusion process after dry etching was reported to help sidewall roughness reducing down to 5 nm. However, this process could only be applied on samples that can be exposed to high temperature (over 1100 ◦ C). In this paper, the DRIE Bosch process based on an inductively coupled plasma (ICP) source was used to form microstructures [10]. The DRIE Bosch process is useful in Si micromachining to fabricate high aspect ratio structures [11, 12]. It has been also reported to fabricate vertical mirrors with several tens of nanometers of rms value of surface roughness [2, 6]. However, it is still necessary to reduce roughness in order to increase reflectivity of optical components since roughness of the order of λ/10 results in significant light scattering for optical surfaces. 0960-1317/07/081593+05$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1593