We report improvements in fabrication precision obtained in coupled ring-resonator slow-wave structures on an SOI platform. Optical delay lines were formed by using a cascade of coupled resonant cavities  to slow the propagation of light through the combined effects of resonance and increased optical path length. Mismatch between the resonance frequencies of individual rings has the additional effect of broadening the spectrum – and such mismatch can compromise the correct operation of the fabricated device structures; in particular sub-nanometer precision is required in order to obtain the desired working behaviour. An optimized HSQ electron-beam-resist based fabrication technology makes it possible to achieve low propagation losses (~1 dB/cm)  and a precision of better than 0.5 nm for 500×220 nm2 waveguide dimensions. We have shown that so small a variation in the waveguide width can consistently be achieved when the HSQ resist is diluted immediately prior to use, Fig. 1(b), – but that much less reproducible results are obtained if a single dilution is prepared and used for an extended period, Fig. 1(a). Single-ring resonator, bus-coupled ring resonator all-pass filters (up to 64 rings)  and directly coupled ring resonators (up to 20 rings in a CROW configuration) in apodised structures have been fabricated, in order to assess the variability of the process.
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