Silicon CMOS BEOL Compatible Optical Waveguide Micro-mirrors
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Silicon CMOS BEOL Compatible Optical Waveguide Micro-mirrors Shom Ponoth,a) Navnit Agarwal,b) Peter Persans,c) and Joel Plawskya) a) Department of Chemical Engineering, b)Department of Electrical Engineering and Computer Systems, c)Department of Physics, 110 8th St, Rensselaer Polytechnic Institute, Troy, New York 12180. 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 fabricate planar waveguides with integrated vertical micro-mirrors using standard Back End of the Line silicon (BEOL) CMOS based processes. Around 1.6 µm of a hybrid alkoxy siloxane polymer with a refractive index of ~ 1.50 at the intended wavelength of 830 nm is used as the core and plasma deposited silicon oxide with a refractive index of ~ 1.46 is used as the cladding. The angular face in the polymer waveguide that would function as the mirror surface was fabricated by a pattern transfer method which involves transferring the angle in a template to the waveguide using anisotropic reactive ion etching. The sidewall angle realized in a positive resist on patterning was used as the angle template. Exposure and development conditions were adjusted for Shipley® S1813 photoresist to generate a sidewall angle of ~ 65°. The anisotropic Reactive Ion Etching (RIE) was done using a CF4/O2 plasma chemistry. A gas composition of 50/50 CF4/O2 was chosen in order to minimize the etch related roughness of the polymer and the photoresist. The metallization of the mirror faces was done using a self-aligned maskless technique which ensures metal deposition only on the angular face and also eliminates a lithography step. INTRODUCTION Optical interconnects are an essential part of photonics devices relevant to fiber optic communications like splitters, Arrayed Waveguide Gratings (AWGs) and modulators. With the inability of long electrical interconnects to cope up with the processor speed optical interconnects are also projected to be used within computers at different levels.[1] Optical interconnects, as a part of an optical backplane, are expected to be commercialized in another two to five years and are expected to be implemented on board for chip to chip communication within the next ten years. Optical interconnects are also being investigated for on-chip applications. A possible implementation scheme From board is shown in Figure 1. Micro-mirrors for out of the Laser Mirrors plane coupling are important structures in this Cladding Optical Core Waveguides regard. Cladding Micro-mirrors have been fabricated using a Electrical Interconnects variety of techniques. Among them are excimer laser ablation [2], use of V-shaped 90° diamond Silicon Detector blades to create a 45° cut [3,4] and anisotropic etching of (100) silicon in potassium hydroxide Figure 1: Possible on-chip implementation of optical interconnects.
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solution [5]. Concave micr
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