Polymeric Strip Waveguides and their Connection to Very Thin Ultrafast Metal-Semiconductor-Metal Detectors
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ABSTRACT
We present data on the fabrication process of optical waveguides from four different polymers, which have been patterned either by standard lithographical masking and reactive ion beam etching (RIE) or by direct lithographical exposure of photosensitive material. Three of the resists were directly photosensitive, they could be exposed and developed. Thereafter they can be cured and remain stable. Waveguide losses of 3.5 dB/cm had to be accepted for the photosensitive materials, while the non-sensitive polymers formed very good guides (0.8 dB/cm), but were more difficult to process. We demonstrate the coupling of the strip waveguides to optical fibers on one side and to very thin metal-semiconductor-metal (MSM) photodetectors at the other side. A beam propagation method computer code has been used to evaluate the best coupling efficiencies between the guides and the detectors, which are ultrafast (3.5 ps FWHM) due to their very thin silicon slab design (Si thickness 400 nm, sandwiched between two Schottky contacts).
INTRODUCTION
Optical interconnects on silicon based optoelectronic devices can be fabricated by several techniques, using polycrystalline Si on S102, SiGe epitaxial heterostructures, silicon-oninsulator (SOI) substrates or silicon-on-sapphire (SOS) [1-4]. In this contribution we describe an approach, which uses a photodefinable polymer on top of a SiO 2 layer. It permits the direct lithographical fabrication of waveguiding structures. These waveguides can be coupled to optical fibers with relative ease. The alignment between the glass fiber and the polymer waveguide is realized with the help of a V-groove, which is lithographically defined and etched into the Si wafer. The use of a polymer guide has the advantage of easy processing and high transparency even at visible wavelengths. These waveguides can be coupled very efficiently to Si photodetectors [5,6]. A theoretical analysis is presented in [6]. WAVEGUIDE FABRICATION
We have fabricated optical waveguides from four different polymers and evaluated the waveguiding properties with a prism coupling setup [7]. Table 1 shows the measured optical
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Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Materials Research Society
indices. The top polymers are photodefinable. Cyclotene 3022 is not photosensitive. Strip waveguides made out of this material have to be patterned by depositing an additional layer of photoresist, followed by lithographical processing and a reactive ion etching step. The use of photosensitive polymers facilitates the fabrication of waveguides significantly. These polymers are directly spun onto the Si0 2 cladding layer and dried on a hotplate. For a spin speed of 5000 rpm the resulting layer thickness is about 3 gm for Ultradel 9120 D and Cyclotene 3022 and a thickness of 1 jim is found for a (3:1) mixture of Probimid: NMP (NMethyl-2-pyrrolidone). The strip waveguides are defined by exposure through a lithographical mask. Subsequently the material is developed and finally cured under nitrogen atmosphere. Probimid 7510 is cure
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