Fabrication and evaluation of an "electrodeless" solidly mounted thin film electroacoustic resonator

Abstract

A solidly mounted resonator (SMR) that has a bottom electrode with a thickness of one quarter of the acoustic wavelength at resonance has been fabricated. In this configuration, the bottom electrode is uncoupled from the resonance cavity of the SMR rendering it electrodeless. The uncoupling of the bottom electrode is confirmed both by simulation with the one-dimensional Nowotny-Benes model, and experimentally by comparison with a SMR with a standard thin bottom electrode. The electrodeless SMR exhibits reduced series resistance Rs and improved device Q. A solidly mounted resonator (SMR) consists of a piezoelectric thin film, sandwiched between two thin electrodes and is fabricated directly onto a carrier substrate. The resonance frequency of the resonator is determined by the total acoustic thickness of the resonance cavity defined by the piezo- layer and the electrodes. To acoustically isolate the resonator from the substrate, it is fabricated on top of a classical acoustic mirror, here called Bragg reflector. The Bragg reflector consists of a sequence of a quarter wavelength layers of low and high acoustic impedance, respectively (1). In this work the bottom electrode is eliminated from the acoustic cavity of the SMR by incorporating it into the Bragg reflector instead. Choosing a low impedance conducting material as the topmost layer in the reflector, eliminates completely the need for a bottom electrode and brings, in return, a number of significant benefits. Most notably, the ohmic resistance of the resonator is reduced, since the reflector layer is much thicker than the original electrode. Alternatively, the whole stack could be made of conducting materials to reduce ohmic resistance even further. The excitation efficiency is also increased since the bottom electrode, which is piezoelectrically inactive, is no longer part of the resonance cavity. Furthermore, if the acoustic reflector layers are made up entirely of metals, an efficient heat path through the mirror is established, which reduces thermal resistance and should improve the power handling capabilities of the device. By the same token, the top electrode can also be eliminated from the resonance cavity, achieving total acoustic isolation of the resonator from its surrounding in addition to doubling the gains described above. In the present study, a one-port SMR, based on reactively sputtered, c-axis oriented aluminum nitride (AlN) was fabricated according to the schematic in Fig.1. The SMR was fabricated such that the bottom electrode is incorporated into the Bragg reflector. Aluminum was used for the top electrode. The acoustic mirror, underneath the resonators were made of a quarter wavelength aluminum (Al) and molybdenum (Mo) layers, providing electrical contact underneath the piezo-layer. The devices were electroacoustically characterized and the results compared with a thin electrode SMR.