High Frequency Surface Acoustic Wave Devices Based On Multilayered LiNbO3/Diamond/AlN Substrates: A Finite Element Study

We have used 3-D coupled field structural finite element models to study the acoustic wave propagation characteristics of diamond/AlN/LiNbO3 multilayered piezoelectric surface acoustic wave devices for applications in chemical and biological sensing. These devices were studied as a method to increase device frequency and sensitivity, and maintain standard fabrication procedures. Although recent experimental investigations have realized GHz frequency devices based on such multilayered substrates, very little is known about the acoustic wave propagation characteristics in these devices. Identifying the optimum configuration and thickness of the various layers involved still represents a challenge which is addressed in this work. Introduction Surface acoustic wave (SAW) devices that operate at high GHz frequencies, present low insertion loss, and retain superior performance are critical for chemical & biological sensing as well as for communications applications. The operating frequency of SAW devices is directly proportional to the substrate’s acoustic wave velocity and inversely proportional to the spatial periodicity of the interdigital transducers (IDTs). This makes the fabrication of GHz frequency devices based on bare substrates such as LiNbO3 difficult as it requires submicron or nanometric IDTs, which are difficult to achieve using standard lithographic techniques. Most of the recent research has focused on advanced SAW filters and sensors based on multilayered structures which could allow for operational frequencies in the GHz frequency range. The operating frequency of SAW devices is directly proportional * Author for correspondence at email: [email protected] ECS Transactions, 13 (25) 1-12 (2008) 10.1149/1.3007994 © The Electrochemical Society 1 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 54.70.40.11 Downloaded on 2018-01-11 to IP to the substrate’s acoustic wave velocity; hence the highest acoustic wave velocity material (diamond) is needed for fabrication of GHz frequency devices. The aluminum nitride piezoelectric layer also has a very high acoustic wave velocity and a fairly large piezoelectric coupling coefficient along its c-axis, in comparison to other piezoelectric materials. Hence, the highest frequency SAW devices can be expected on diamond substrates with an aluminum nitride piezoelectric layer. This would make it possible to realize GHz frequency devices with standard transducer configurations. Although recent experimental investigations have realized GHz frequency devices based on such multilayered substrates, very little is known about the acoustic wave propagation characteristics in these devices. Identifying the optimum configuration (Fig. 1) and thickness of the various layers involved still represents a challenge. Figure 1. Examples of configurations (a) IDT/LiNbO3/AlN/diamond (b) LiNbO3/IDT/AlN/diamond and (c) LiNbO3/AlN/IDT/diamond. The models commonly used to simulate the mechanical and electrical behavior of piezoelectric transducers, to study acoustic wave propagation, generally introduce simplifying assumptions that are often invalid for actual designs. The geometries of practical transducers are often two (2-D) or three dimensional (3-D) . Simulations of piezoelectric media require the complete set of fundamental equations relating mechanical and electrical quantities to be solved. Finite difference or finite element schemes are sufficient to handle the differential equations. The finite element method has been preferred because it allows handling of complex geometries and multilayered composite structures. Our previous investigations also indicate the feasibility of the finite element models to study acoustic wave propagation in piezoelectric devices. In the present work, we use 3-D coupled field structural finite element models to study the acoustic wave propagation characteristics in these multilayered piezoelectric ECS Transactions, 13 (25) 1-12 (2008) 2 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 54.70.40.11 Downloaded on 2018-01-11 to IP SAW devices based on diamond for potential applications in chemical and biological sensing. The displacement and voltage waveforms obtained for varying thicknesses of diamond, AlN and LiNbO3 substrates are analyzed to understand acoustic wave propagation characteristics in multilayered substrates. The feasibility of achieving GHz frequency devices using SAW devices based on diamond layer is investigated in this work. Theory The propagation of acoustic waves in piezoelectric materials is governed by the mechanical equations of motion and Maxwell’s equations for electrical behavior. The constitutive equations of piezoelectric media in linear range coupling the two are given by: k t kij kl E ijkl ij E e S c T − = [1] k S ik kl ikl i E S e D ε + = [2] In the above equations, Tij represent the components of stress, E ijkl c the elastic constant for constant electric field, Skl the strain, Ek the electric field intensity, Di the electric displacement, t kij e the piezoelectric constant, and S ik ε the permittivity for constant strain. The acoustic wave propagation velocity is five orders of magnitude smaller than that of electromagnetic waves. Therefore, the quasistatic assumptions help reduce Maxwell’s