Material and process challenges in embedding polymeric waveguides and detectors in system on package (SOP)

Polymer materials are becoming increasingly important for integrated optics, in part due to the easy and low cost manufacturability. Embedded polymer waveguides and other polymer devices such as gratings, micro lenses, switches, splitters, etc. are increasingly used in packages and printed wiring boards (PWBs) for optical interconnection and are becoming an emerging technology to enable high data rates over centimeter lengths on-board. A nano scale local surface roughness is desirable for the implementation of these devices in order to avoid unnecessary optical losses. Yet it is a great challenge to control surface roughness on PWB surfaces to the nanometer scale over distances of a millimeter. In addition, board flexibility, lack of planarity, and thermal expansion create great challenges for the implementation of integrated digital-optical technology on packages or boards. A buffer layer produced by a low cost process and which results in a nano scale local roughness is a critical solution that enables the integration of low loss waveguides and other optical passives on boards. It is one of the objectives of this article to demonstrate how to reduce the "as received" surface roughness of a HDI board, which may additionally contain metal lines up to 18 μm thick, to a roughness of ±20 nm in general and to ±5 nm or less in special cases over distances of 500 μm. We will also discuss fabrication of low loss, low multimode waveguides with 0.24 dB/cm at 1.32 μm and 0.52 dB/cm at 1.55μm, as well as other optical passives such as micro lenses having 50-250 μm base diameter and 80 to 200 μm focal length, surface relief gratings with 250 lines/mm and a 35-degree blaze angle, micro mirrors, as well embedded photo detectors, including I-MSM thin film detectors and off-the-shelf PiN bare die detectors. Finally, metallization of bonding pads structures on waveguide polymer material will also be discussed.