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We report the development of tunable comb-resonators that use vertically-shaped comb-fingers as electrostatic springs. By restricting our design modifications to the vertical dimension, the tunability is achieved without increasing the device footprint. Three-dimensional finite element analysis was used to evaluate the effects of geometry and design on electrostatic spring strength and linearity. All structural components were fabricated using gray-scale technology, simultaneously defining all vertical levels using a single lithography and dry-etching step. Subsequent testing achieved bidirectional resonant frequency tuning (> 17%) through the creation of electrostatic spring constants as high as 1.06 N/m (at 70 V) and 1.45 N/m (at 120 V). While the current resonant devices show evidence of nonlinear stiffness coefficients at large oscillation amplitudes (> 10 mum), multiple design options are introduced and simulated as potential solutions.