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We demonstrate folded waveguide ring resonators for biomolecular sensing. We show that extending the ring cavity length increases the resonator quality factor, and thereby enhances the sensor resolution and minimum level of detection, while at the same time relaxing the tolerance on the coupling conditions to provide stable and large resonance contrast. The folded spiral path geometry allows a 1.2 mm long ring waveguide to be enclosed in a 150 ?m diameter sensor area. The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of ~3 pg/mm_2, or a total mass of ~ 5 fg. The real time measurements are used to analyze the kinetics of biotin-streptavidin binding.