Comparisons of waveguide core patterns formed by different fabrication methods. (a) Trapezoidal waveguide core formed when etching polymer using a photoresist as a hard mask. (b) Rectangular waveguide core formed when etching polymer using a metal film as a hard mask.|@|~(^,^)~|@|Waveguide core fabricated with O2 plasma etching using a photoresist as a hard mask. (a) Microscope image of the waveguide pattern observed from the top of the substrate. (b) Cross-section of the waveguide observed with a scanning electron microscope. The waveguide core was covered by upper cladding polymer and the cross-section was formed by cleaving the fabricated device.|@|~(^,^)~|@|Cracks observed when using a metal film as a hard mask for etching. (a) Microscope image of the waveguide pattern with cracks. The cracks appeared during the soft bake of the photoresist film which is coated on top of the 20 nm thick Cr film deposited over the polymer film. (b) Yellow dash lines are drawn along the cracks to emphasize them.|@|~(^,^)~|@|Schematic illustration of the chemical reaction during the atomic layer deposition process. Al2O3 atomic layer is formed using Trimethyl aluminum (TMA) and water (H2O) as precursors.|@|~(^,^)~|@|Experimental equipment used for the atomic layer deposition process. (a) Overview of the equipment and (b) a 4-inch wafer placed on the chuck of the vacuum chamber.|@|~(^,^)~|@|Waveguide fabrication using an atomic layer deposition of Al2O3 as a hard mask. (a) Waveguide patterns formed with photoresist on the Al2O3 film. Note that no cracks are observed over a large area. (b) Top view of the image of the Al2O3 hard mask after etching the polymer core observed using scanning electron microscopy (SEM). The enlarged inset shows the sidewall roughness below 30 nm. (c) Cross-section of the polymer waveguide core and the Al2O3 hard mask observed after cleaving the device (the inset shows an enlarged sidewall image). The disconnection of the hard mask near the end facet is due to the elongation of the polymer during cleaving. (d) Polymer waveguide core pattern formed using O2 plasma etching (output waveguides of the multi-mode interference coupler are shown in the top inset). (e) SEM image of the final waveguide cross-section after removal of the Al2O3 hard mask.|@|~(^,^)~|@|Waveguide mode profile and loss measurements. (a) Mode profile of the fundamental mode. (b) Propagation loss measurement through cut-back measurement.|@|~(^,^)~|@|Polymer waveguide 2 × 2 multi-mode interference (MMI) directional coupler. (a) Waveguide structure of the 2 × 2 MMI coupler. (b) Calculated output powers from the two waveguide output ports and splitting ratio of the 2 × 2 MMI coupler using the 3D beam propagation method (BPM). (c) Acquired optical power distribution using BPM simulation when LMMI = 179 µm. (d) Output waveguide mode image of the 2 × 2 MMI captured using a charge-coupled device.
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