Experimental optimization of electrodes for high Q, high frequency HBAR

Abstract

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 ¿m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to 23,600 ¿m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . 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 ¿m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and had Q = 6501 at 7.520 GHz, yielding fQ = 4.49 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> .