Manufacturing of Board Level Waveguide Bus Using Hard Mold Xiaohui Lin 1 , Xinyuan Dou 1 , Amir Hosseini 2 , Alan X. Wang 3 and Ray T. Chen 1,* , Fellow, IEEE 1 Department of Electrical and Computer Engineering, the University of Texas at Austin, Austin, TX, 78758, USA Email: xiaohui.lin@mail.utexas.edu & chen@ece.utexas.edu * 2 Omega Optics, Inc., 103006 Sausalito Dr, Austin, TX 78759, USA 3 School of Electrical Engineering & Computer Science, Oregon State University, Corvallis, OR 97331, USA Abstract— Optical interconnects of straight waveguides and bi-directional bus architecture have been successfully fabricated on flexible substrate using nickel hard mold. Optical out-of-plane loss test and high speed data transmission at 10Gbps have been demonstrated. INTRODUCTION The demand of high speed mass data transmission is pushing nowadays digital equipments to have the capability to meet such requirement. Traditional copper interconnect, however, is facing serious challenges when dealing with high frequency operation domain. Optical interconnects, as an alternative signal transmission method, are attracting more and more attention in different levels including chip-to-chip, board-to-board or even rack-to-rack interconnects. Chen et al investigated the differences of these two methods from many aspects including delay uncertainty, latency, power, and bandwidth density etc [1]. The main concern for the integrated optical interconnect is its cost and performance [2]. Many optical elements such as waveguides and gratings have been demonstrated by many research groups [3, 4]. In this work, we present a low cost molding method has been applied to fabricate optical waveguides. Furthermore, a novel bi-direction bus architecture is introduced and fabricated by this method. To evaluate the performance, an out-of-plan bending test is performed to the straight waveguides. High speed data transmission up to 10Gbps via straight waveguides and a bus structure is also demonstrated. Another highlight in this work is the 45 o embedded mirror which enables vertical coupling from VCSEL. DESIGN AND FABRICATION For optical interconnects, the main element is the waveguide. For parallel transmission, waveguide arrays can be used. Therefore, we design the array having 12 straight waveguides with 50μm ×50μm cross-section and 250μm in spacing. The waveguide materials are polymers that have low propagation loss in 850nm. The device is built on flexible substrate, which is flexible and can be bent to accommodate different application environments. For the bus structure, a 3-node bi-direction bus architecture is presented. The structure is shown in Fig. 1. It has 3 main elements: (1) nodes formed with a pair of laser diode (LD) and detector (D). Each node is able to send/receive signals to/from all other nodes at the same time, maximizing the efficiency of data transmission; (2) Embedded mirrors with 45 o slope. The mirrors enable light coupling into and out of the bus vertically using VCSELS; (3) variable ratio Y- splitter with engineered width ratio. By adjusting the width ratio of each branch, the light splitting ratio can be tuned. Fig. 1 board level, bi-direction optical bus architecture and vertical light coupling with embedded mirror The molding method is applied to achieve both the waveguide array and bus architecture. The main steps together with SEM pictures after each step are shown in Fig. 2. It includes SU8 pre-mold fabrication, nickel metal mold by electroplating, molding process and final device fabrication. SU8 pre-mold fabrication is the key that ensures the quality of all the following processes. The 45 o degree slope is fabricated by tilted exposing the SU8 in DI water [5]. The hard nickel mold is prepared by electroplating desired thickness into the SU8 pattern, on pre-buried seed layer. The plating time, current density and stability during the plating should be carefully controlled to ensure good profile and smooth surfaces. After SU8 removal, the nickel mold is used to mold the bottom cladding polymer that is spin-coated on a flexible device substrate. After UV curing of the polymer, the mold is detached from the device substrate. Here, a thin layer of resist is pre-applied in between the mold and device, serving as release agent. Following that, gold is deposited on the 45 o slope to enhance the mirror reflectivity. The waveguide is finalized by coating the channels with core layer and top cladding layer, with proper UV curing. 139 WD2 (Contributed Oral 16:30 – 00:00 978-1-4577-1619-5/12/$26.00 ©2012 IEEE