This paper reports a 80nm air gap double-ended tuning fork (DETF) resonator partially filled with 20nm of HfO2. The device operates at 10.7 MHz with a quality Q-factor of 7930 and a motional resistance, Rm, x18 better than for the measured counterparts in air-gaps. An enhancement of the S21 transmission signal of 6dB in addition to the reduction of the Rm have been achieved thanks to the improvement of the electromechanical coupling factor by partially filling the electrode-to-resonator-gap with an atomic layer deposition (ALD) of 20nm of HfO2 high-k dielectric (εr∼19). This result proves an outstanding enhancement in the detection signal while keeping all the desired properties of flexural resonators for future mechanical sensing applications.