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This paper reports on the fabrication and characterization of high-quality factor (Q) single crystal silicon (SCS) in-plane capacitive beam resonators with sub-100 nm to submicron transduction gaps using the HARPSS process. The resonating element is made of single crystal silicon while the drive and sense electrodes are made of trench-refilled polysilicon, yielding an all-silicon capacitive microresonator. The fabricated SCS resonators are 20-40 μm thick and have self-aligned capacitive gaps. Vertical gaps as small as 80 nm in between 20 μm thick silicon structures have been demonstrated in this work. A large number of clamped-free and clamped-clamped beam resonators were fabricated. Quality factors as high as 177000 for a 19 kHz clamped-free beam and 74000 for an 80 kHz clamped-clamped beam were measured under 1 mtorr vacuum. Clamped-clamped beam resonators were operated at their higher resonance modes (up to the fifth mode); a resonance frequency of 12 MHz was observed for the fifth mode of a clamped-clamped beam with the fundamental mode frequency of 0.91 MHz. Electrostatic tuning characteristics of the resonators have been measured and compared to the theoretical values. The measured Q values of the clamped-clamped beam resonators are within 20% of the fundamental thermoelastic damping limits (QTED) obtained from finite element analysis.