Fabrication of smooth silicon optical devices using proton beam writing

Abstract

This work gives a brief review of proton beam writing and electrochemical etching process for the fabrication of smooth optical devices in bulk silicon. Various types of structures such as silicon-on-oxidized porous silicon waveguides, waveguide grating and disk resonators have been produced. Optical characterization has been carried out on the waveguides for both TE and TM polarization using free space coupling at 1.55 μm. Various fabrication and processing parameters have been optimized in order to reduce the propagation loss to approximately 1 dB/cm. A surface smoothening technique based on controlled oxidation has also been used to achieve an RMS roughness better than 3 nm.

Introduction

For many years, achieving lasing and optical modulation in silicon has been a challenge for scientists due to its indirect bandgap and weak electro-optical properties. Recently, several breakthroughs have been made in the development of silicon-based device and technology. Intel has demonstrated the first continuous-wave silicon laser using stimulated Raman emission [1]. It is not much later that they demonstrated the first 40 Gbps optical modulator, which uses free carrier injection to modulate light signals [2].

This paper gives a brief review of a newly developed technique that utilizes ion beam irradiation and electrochemical etching for the fabrication of smooth low loss photonic devices in bulk silicon. Previously, three-dimensional microstructures have been successfully demonstrated using this process [3], [4], [5]. In order to extend this technique to waveguide and optical devices fabrication, we have devised two ways of achieving optical isolation from the substrate. In the first approach, the structure is undercut by prolonged etching so that it becomes surrounded by porous silicon cladding [6]. The second approach uses double energy irradiation to create a free-standing waveguide with air cladding [7]. Surface roughness plays an important role in affecting the quality of the optical devices. We demonstrate that performing a post-oxidation smoothening process enables us to achieve smooth surfaces of less than 3 nm. Other factors such as ion fluence and scanning parameters are also shown to affect the surface roughness and propagation loss.