Fabrication of micromirrors with self-aligned metallization using silicon back-end-of-the-line processes

Abstract

Optical waveguides are being explored for on-chip purposes to overcome the speed limitations of electrical interconnects. Passive optical components like waveguides and vertical outcouplers are important components in such schemes. In this study, we fabricated planar waveguides with integrated vertical micromirrors using silicon back-end-of-the-line processes. Approximately 1.6 μm of a hybrid alkoxy-siloxane polymer with a refractive index of 1.50 at the intended wavelength of 830 nm was used as the core and plasma-deposited silicon oxide with a refractive index of 1.46 was used as the cladding. The angular face in the polymer waveguide that would function as the mirror surface was fabricated by a templated pattern transfer method, which involved transferring the angle in a template to the waveguide using anisotropic etching. The sidewall angle generated in Shipley® S1813, a positive photoresist, on patterning was used as the angle template. The photoresist sidewall angle was controlled using a two-step exposure method. A CF 4/O 2 plasma chemistry was used for the anisotropic reactive ion etch. A gas composition of 50/50 CF 4/O 2 was chosen to minimize the etch-related roughness of the alkoxy-siloxane polymer and the photoresist. The metallization of the mirror faces was done using two of the three proposed self-aligned techniques. A calibration-based technique was developed to measure the reflection efficiency of the micromirrors. In this technique, scattered light was used to estimate the incident power on the micromirror and a reflection efficiency of 83% was obtained at a wavelength of 650 nm.