Scattering loss measurement of SOI waveguides using 5X17 integrated optical star coupler

Abstract

We describe a novel non-destructive technique to measure the sidewall roughness induced scattering loss of SOI ridge waveguides using an integrated 5x17 star coupler. The accuracy of our technique is independent of the coupling efficiency. In our technique, we capture the near field images of the full output waveguides array with varying width ranging from 0.2 to 2.0 micron and use the intensity maps of these images to produce normalized intensity profiles, from which the relative scattering losses of output waveguides are extracted. Using our technique, we have studied and compared the scattering and polarization dependent losses of three different sets of SOI waveguide samples fabricated by different processes. We have determined the root-mean-square roughness and autocorrelation length of these samples using scanning electron microscopy (SEM). Relating the loss and roughness analysis, we have showed that the process utilizing negative e-beam photoresist and Cr-hardmask with inductively coupled plasma (ICP) etching produced the smoothest waveguide sidewalls and lowest scattering losses. We have also successfully modeled the measured ridge waveguide losses as a function of waveguide width and demonstrated that the theoretical sidewall roughness is in reasonable agreement with the measured roughness from SEM. Our technique is capable of studying roughness induced scattering loss and thus provides an efficient way of optimizing and monitoring process parameters that affect sidewall roughness.