Fabrication of vertical tapers in polymer thin films by oxygen reactive ion etching with a shadow mask for photonic device applications

The taper structure is the key component of an on-chip mode size transformer for efficient fiber coupling. The taper provides a smooth transition between two waveguide sections with different mode sizes with minimum radiation loss. The tapered mode size transformer has become a standard approach for efficient fiber coupling with semiconductor lasers and amplifiers[1]. Various taper fabrication techniques are developed for semiconductor and LiNbO3 devices[2-5], but most of them are not applicable to polymer waveguides. Oxygen reactive ion etching is the primary method to etch a polymer film. For most polymer waveguides, the major part of fiber coupling loss is due to the mode size mismatch between the waveguide and fiber in the vertical direction, therefore a vertical taper is needed to reduce the fiber coupling loss. In this paper, a reactive ion etching (RIE) technique with a shadow mask is demonstrated. A similar technique has been reported for etching submicron tapers in InP with methane plasma[5]. Our approach can effectively create tapers several microns deep and a few millimeters long with oxygen plasma which is required for multilayer polymer waveguide structures.