Fabrication of Microstructures for Microphotonic Circuit
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Fabrication of Microstructures for Microphotonic Circuit Subhasish Chakraborty, D G Hasko and R J Mears University of Cambridge, Department of Engineering, Trumpington Street Cambridge CB2 1PZ
ABSTRACT We describe fabrication of sub-micron photonic bandgap structures on Si/SiO2 optical waveguide, which could be used at λ=1.54µm. INTRODUCTION Control of the propagation of light using the photonic band gap (PBG) effect in photonic devices is the subject of intense international effort [1]-[6]. PBG materials are optical analogs of semiconductors. Main drive is towards making structures that modulate free photon dispersion as much as the same way as semiconductor crystal does for electrons. A high dielectric contrast material system is a fundamental requirement for the existence of a PBG. Silicon microphotonics, which uses Si over SiO2 system, provides a large index difference ( ∆n ≈ 2.0 @ 1.54µm ) between the core and the cladding of the guide. Because of the high confinement of the optical wave, the waveguide cross-section has been miniaturised. Strip waveguides with holes of 200nm diameter have been fabricated. The cross-section of the strip waveguide is 0.26×0.5 µm. We have addressed some of the important issues regarding fabrication. On the simulation front, we have used commercially available software Fimmprop3D for 2D simulation of the device. Figure 1(a) and (b) shows the simulation results. Actual structure is shown in the inset of Figure 1(b). High index contrast structures introduce a process problem because performance is limited by scattering loss from surface roughness. One important challenge towards realizing silicon microphotonics lies in making optically smooth structure to keep the scattering loss as low as possible. The dominant source of loss is the sidewall roughness scattering [7]. The increase is attributed to sidewall roughness created during the waveguide patterning process involving lithography and RIE. We have proposed combined chlorine-fluorine based plasma as a reactive ion-etching recipe for silicon microphotonics. Extremely smooth photonic structures of feature size as small as 0.1µm have been made. FABRICATION PBG effect devices have been made using high-resolution lithographic and pattern transfer processes, which include electron beam lithography and reactive-ion-etching. The first step is the realisation of the plasma etch mask. The poor etch resistance of PMMA is serious limiting factor for pattern transfer. The problem has been overcome by using Al as a metal etch mask.
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optical resonance @1.54micron
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Wavelength (micron) Figure 1. Simulation shows transmission for the waveguide microcavity structure designed for a resonance wavelength at 1.54 micron
In this work we have used Unibond silicon-on –insulator wafer (obtained from SOITEC, France), consisted of 0.26µm crystalline silicon layer on 1.0µm SiO2 cladding layer on silicon substrate. . Direct-write electron-beam lithograp
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