Experimental optimization of electrodes for high Q, high frequency HBAR

In this paper we present experimental work towards the fabrication of a high-overtone bulk acoustic resonator (HBAR) that demonstrates large values of quality factor (Q) at high frequency (> 7GHz). We have optimized electrode design and fabrication technique with respect to deposition conditions, material, active-area size, and thickness. We fabricated devices with gold, aluminum, and molybdenum electrodes. Once the molybdenum electrodes were determined to be superior, we fabricated devices with 11 different active-area sizes, ranging from 5,300 ¿m² to 23,600 ¿m². Finally, we made HBARs with electrode thickness ranging, in 0.02 ¿m increments, from 0.06 ¿m to 0.14 ¿m. The best device utilized 0.14 ¿m thick molybdenum electrodes, an active-area of 6,500 ¿m², and had Q = 6501 at 7.520 GHz, yielding fQ = 4.49 × 10¹³.