## 2 Fabrication of 2D Photonic Crystal Cavities and Polymer Waveguides on Amorphous Silicon  
Fabrication of the substrate starts with the deposition of a \(2.1\,\mathrm{\SIUnitSymbolMicro m}\) thick layer of SiO\({}_{2}\) on a single side of a \(100\,\mathrm{mm}\) bulk silicon wafer through plasma-enhanced chemical vapour deposition. Due to the relatively low refractive index of SiO\({}_{2}\), \(n\,=\,1.45\) at \(\lambda\,=\,1550\,\mathrm{nm}\), this SiO\({}_{2}\) layer provides good a refractive index contrast with the a-Si layer (whose refractive index is \(n\,=\,3.5\) at \(\lambda\,=\,1550\,\mathrm{nm}\)[13]), necessary for optical mode confinement in the a-Si layer. Amorphous silicon is deposited on the oxide layer through low pressure chemical vapour deposition up to target thickness of \(220\,\mathrm{nm}\); this process is performed at low temperature, \(T\,=\,350\,\mathrm{\SIUnitSymbolCelsius}\), which makes the deposited a-Si compatible with both top-down and bottom-up CMOS processes [7]. The final surface roughness of the depositeda-Si layer is in the range of a nanometre, thus light scattering phenomena, even in the absence of a chemical-mechanical planarization step, are limited.  
After the low temperature deposition the a-Si is coated in photoresist and patterned with dispersion-adapted (DA) PhC cavities [17] through electron-beam lithography. The material is subsequently dry etched with a reactive ion etcher in an isometric SF\({}_{6}\):CHF\({}_{3}\) plasma. The etching recipe is designed to target smooth and vertical sidewalls of the photonic crystal holes, crucial to minimizing scattering losses. A scanning electron microscope (SEM) image of the fabricated PhC cavities can be seen in Figure 1. The length of the PhC cavity is approximately 10 \(\upmu\)m. Measurements of the PhC hole diameters indicate that it is 190 \(\pm\) 6 nm. SEM images show that holes etched in a-Si are uniform in shape and distribution, which is indicative of a high quality fabrication process. The ratio of hole-radius to lattice constant for these structures is \(r/a\ =\ 0.245\), similar to the value of \(r/a\ =\ 0.28\) reported for the PhC structures in [15].  
The DA PhC cavities are accessed by evanescent coupling through the polymer waveguide fabricated on top of them [16]; see Figure 2 for schematic. The highest coupling quality factor is obtained by interposing a thin spin-on-glass (SOG) between the a-Si cavities and the final polymer waveguide layer. This thin SOG layer is created by spin-coating the sample with Accuglass T11, a methyl-siloxane polymer containing 10% CH\({}_{3}\) groups bonded to Si atoms in the Si-O backbone. The SOG layer is cured in an oxidation furnace with N\({}_{2}\) atmosphere at temperature \(T\ =\ 425\,^{\circ}\)C to minimum the thickness of 200 nm for 1 h. To ensure future CMOS process temperature compatibility, SOG processing could be accomplished at \(T\ =\ 200\,^{\circ}\)C [19]. The cured Accuglass is then etched back to the target thickness of 170 nm in O\({}_{2}\) plasma. The refractive index of the SOG layer is \(n\ =\ 1.4\) at \(\lambda\ =\ 1550\,\)nm [20]. The effect of the SOG layer is to enhance the mechanical stability of the PhC cavity and also to improve the symmetry of the PhC cavity mode.  
The polymer waveguide layer is fabricated by spin-coating the samples with a 2.1 \(\upmu\)m thick layer of SU8, an epoxy-based photoresist characterized by a refractive index of \(n\ =\ 1.56\) at \(\lambda\ =\ 1550\,\)nm. The SU8 is soft-baked on a hotplate at 95 \({}^{\circ}\)C and then patterned with 3 \(\upmu\)m wide waveguides on top of the DA PhC cavities. The SU8 waveguide are finally hard-baked at a temperature of 180 \({}^{\circ}\)C. The location of the polymer waveguide in relation to the PhC cavity can be seen in Figure 2.  
Figure 1: (a) Scanning electron microscope (SEM) image of the abricated photonic crystal (PhC) cavity. Length of the PhC cavity is approximately 10 \(\upmu\)m. (b) Close up of the holes in the PhC cavity. Hole diameter is measured to be 190 \(\pm\) 6 nm.