Buried electrode electroacoustic technology for the fabrication of thin film based resonant components

Abstract

A fabrication process for thin film electroacoustic devices utilizing buried electrodes is presented. A one-step lithography process has been developed to bury electrodes resulting in a planarized surface. The proposed technology is expected to bring about a number of benefits concerning the performance of a variety of thin film electroacoustic devices. With respect to thin film plate acoustic resonators (FPAR), burying the reflector electrodes results in improved reflectivity and potentially lower susceptibility to acousto-migration effects. It is also shown that employing the proposed technology for the fabrication of both thin film bulk acoustic resonators (FBAR) and thin film solidity mounted bulk acoustic resonators (SBAR) eliminates certain macro-structural defects in the piezoelectric film which is a prerequisite for substantially improved device performance and higher power handling capability. The buried electrode electroacoustic (EA) technology is demonstrated for a thin aluminium nitride (AlN) piezoelectric film with electrodes of both molybdenum (Mo) and tungsten (W). The latter have been primarily chosen because of their high electroacoustic material quality. Thin film resonant structures produced by this technology are characterized and their features are discussed.

Effect of substrate roughness on c-oriented AlN thin films

“…The only way to change the stress on the oxide surface is to modify the surface properties locally in order to reduce the stress there. A similar problem was encountered by Martin et al 8 during the development of buried Mo electrodes between two AlN films. They observed cracking of the top AlN layer at the Mo-AlN step.…”

SAW Characteristics of AlN/SiO2/3C-SiC Layered Structure With Embedded Electrodes

“…Although Refs. [16] and [17] demonstrated fabrication of similar device configuration, variation of the underneath layer for the AlN growth may cause non-uniform crystal growth and/or cracks [4], [9]. To avoid this difficulty, the authors also investigate the case that SiO2 thickness is bigger (Δh=0.5%λ) than the Al electrodes, as shown in Fig.6.…”

“…However, the main drawback of such layered structure is that the patterned IDT electrodes harms AlN crystalline growth and causes cracks due to the edge discontinuities of the electrodes [4], [9]. The defects limit both device performance and reliability.…”

Surface acoustic wave propagation characteristics of ScAlN/diamond structure with buried electrode

“…Therefore, relatively larger K 2 (~10%) and larger V (~7300m/s) can be simultaneously obtainablefor Sezawa mode on the ScAlN/Diamondstructure with buried electrode, which is a promising candidate for applications in wideband SAW devices operating over 3GHz. Moreover,the buried IDT configuration can protect the electrode from oxidation, which could be more suitable for high temperature environment.The difficulties in growing high quality, high oriented film on most substrate and diamond, in particular, improvingthe structural continuity for layered structure with buried electrode need to overcome, although the improved fabrication technology has already been reported in the literature [7].…”