Micromachined Hemispheroidal Cavity Resonators

A high quality factor ( factor) -band micromachined hemispheroidal cavity was designed for use in low phase-noise oscillators. The cavity is micromachined using self-limited isotropic etching of a silicon wafer, which is then metallized with gold and soldered to an alumina wafer using a thin layer of indium. The alumina is patterned with a microstrip feed line having an aperture in the ground plane for coupling to the cavity since the ground plane also forms the top wall of the cavity. An analysis based on equivalent energy aperture coupling theory, and perturbation techniques, was used to design the cavities. The resonant frequency, the unloaded factor, and the loaded factor of the fully assembled micromachined cavity can be calculated using simple equations that agree with both finite-element simulations and measurements. The prototype micromachined cavity has a single measured resonance at 76.39 GHz with a measured unloaded factor of 1426 and a measured loaded factor of 909.