Effect of process parameters on sidewall roughness in polymeric optical waveguides S.K. Pani a,b, * , C.C. Wong a , K. Sudharsanam b , S.G. Mhaisalkar a , V. Lim b , S. Mohanraj b , P.V. Ramana b a School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Institute of Microelectronics, Singapore Science Park II, 11 Science Park Road, Singapore 117685, Singapore Available online 20 June 2004 Abstract Most polymeric optical devices are mainly fabricated by photolithography and reactive ion etching (RIE). This paper describes the fabrication of fluorinated polyether (FPE) waveguides by using a metallic hard mask and proximity exposure followed by RIE. In particular, we focused on the effects of various fabrication parameters on the sidewall roughness. According to our study, a high-power, low-pressure oxygen RF plasma environment will provide vertical and smooth sidewalls desired in a cladded waveguide. The sidewall roughness increases with pressure in pure oxygen plasma, while adding nitrogen gas to oxygen during RIE creates a more vertical profile and smoother sidewalls along with low vertical and lateral etch rates. A simple technique is presented to quantify sidewall roughness using an atomic force microscope (AFM). D 2004 Elsevier B.V. All rights reserved. Keywords: Reactive ion etching; Sidewall roughness; Atomic force microscopy; Nitrogen 1. Introduction Polymers are promising candidate materials for cost- sensitive optical communication components because of their versatility—they can be tailored to achieve specific optical, physical and mechanical properties. However, the material’s process technology of polymer optical structures is yet to reach full maturity. For optical structures like waveguides, the most critical properties are the smoothness of the sidewalls and the vertical profile. Roughness at the sidewall leads to scattering loss which occurs due to the interaction of wave- guide modes at the boundaries of the waveguides. The increased scattering loss could severely hamper the scaling down of the waveguides and waveguide-based devices, as the scattering loss is proportional to cube of the refractive index difference between core and cladding [1]. Especially high refractive index contrast system possesses the potential to scale down the device sizes because of their lower bending radii. Applying the ray optic approach [2], the number of reflections at the boundaries is higher for systems of high index contrast. Therefore, any irregularities in the surface such as sidewall roughness will enhance the scattering which leads to the scattering loss. Sidewall roughness is mainly induced during reactive ion etching (RIE) which is common- ly used [3] for patterning polymers in oxygen plasma. Hence, it is important to understand the effects of RIE parameters on profile and sidewall roughness of the waveguides. This paper describes the fabrication of polymeric (fluo- rinated polyether, FPE) optical waveguides by metallic hard mask and proximity exposure followed by RIE process. During RIE, pure oxygen, as well as oxygen/nitrogen mixtures, was used. To our knowledge, this is the first report on the usage of oxygen/nitrogen mixtures for the RIE of FPE and its associated effects on sidewall roughness and feature profile. The sidewall roughness is measured using atomic force microscope (AFM), and a simple technique is presented to prepare samples to characterize sidewall rough- ness of the waveguides. 2. Experimental details Fluorinated polyether having refractive indices 1.4840 and 1.5479 were used as the cladding and core layers, 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.05.065 * Corresponding author. Tel.: +65-6790-6161; fax: +65-6790-9081. E-mail address: pb2563001@ntu.edu.sg (S.K. Pani). www.elsevier.com/locate/tsf Thin Solid Films 462 – 463 (2004) 471 – 476