Numerical Analysis of High Frequency Surface Acoustic Wave Chemical and Biological Sensors Based on Multilayered Diamond/Aln/LiNbO3 Substrates
Subramanian K.R.S. Sankaranarayanan1, Stefan Cular2, and Venkat Bhethanabotla2. (1) School of Engineering and Applied Sciences, Harvard University, 9 & 15 Oxford Street, Cambridge, MA 02138, (2) Department of Chemical Engineering, University of South Florida, 4202 East Fowler Ave., ENB 118, Tampa, FL 33620
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 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 therefore focused on advanced SAW filters and sensors based on multilayered structures which could allow for operational frequencies in the GHz frequency range . The highest frequency SAW devices can be expected on diamond substrates with an aluminum nitride piezoelectric layer because diamond presents the highest acoustic wave velocity among all materials and aluminum nitride has a very high acoustic wave velocity and a fairly large piezoelectric coupling coefficient along its c-axis, in comparison to other piezoelectric materials. This makes 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 . In the present work, we use 3-D coupled field structural and 2-D fluid-solid interaction finite element models to study the acoustic wave propagation characteristics in these multi-layered piezoelectric surface acoustic wave devices under the influence of fluid loading for applications in chemical and biological sensing. A coupled field finite element structural model is first utilized to study the acoustic wave propagation in Diamond/AlN/IDT/LiNbO3 layered SAW devices. The 3-D FE model describes two-port structures based on configurations depicted in Fig. 1. The simulated device consists of three finger pairs in each port. The fingers are considered as mass-less electrodes to ignore the second-order effects arising from electrode mass, thereby simplifying computation. The periodicity of the finger pairs i.e. λ is 40 microns and the aperture width is 200 microns. The transmitting and receiving IDT's are spaced 130 microns or 3.25λ apart. The simulated SAW device dimensions are 20 λ in propagation length and 12.5 λ wide. The thickness of the diamond film was kept constant at 1 λ, whereas AlN thickness was taken as λ/8. By varying the thickness of LiNbO3 from 1 λ to 10 λ, the relative ratios of the thickness of the three films were varied and the propagation characteristics of acoustic wave as well as dispersion relationship in the multi-layered substrate configurations (Fig. 1) were studied. The simulated models have a total of approx. 250000 nodes and are solved for four degrees of freedom (three displacements and voltage). The model was created to have the highest densities throughout the surface and middle of the substrate . The simulated voltage and displacement profiles obtained at the output IDT's are used to measure wave attenuation and velocity changes and thereby optimize the thickness of the waveguide, i.e. the diamond and AlN layer. Our simulated displacement profiles along the longitudinal, surface normal and shear horizontal directions indicate that Rayleigh waves propagate in the multilayered configurations. Analysis of the particle displacement profiles along the depth of multilayered Diamond/AlN/IDT/LiNbO3 substrates at different locations along the SAW delay path was also carried out to investigate the acoustic energy confinement in multilayered SAW devices for the simulated device design parameters. For Diamond/AlN/IDT/LiNbO3 thickness ratio of 1:0.125:10; we find that the acoustic energy is primarily confined in the LiNbO3 layer as shown in Fig 2 and hence the acoustic wave propagates along surface of the LiNbO3 substrate (Fig. 3). At these thickness ratios, we find that the acoustic velocity attained is not very high (approx. 4100 m/s) and hence this configuration cannot be utilized to attain the GHz frequencies for the simulated IDT periodicity. On the other hand, when the thickness ratios of Diamond/AlN/IDT/LiNbO3 is changed to 1:0.125:1 by reducing the LiNbO3 thickness, the acoustic energy is confined primarily in the Diamond and AlN layers as shown in Fig. 4. At higher simulation times, the wave is also partly radiated into the LiNbO3 layer in the form of bulk waves, thereby increasing the insertion loss of the device. We find that the acoustic wave modes propagating in the diamond film (Fig. 5) attains very high velocities close to 11500 m/s, thereby indicating the possibility of fabricating GHz frequency devices. Simulations of additional devices with varying LiNbO3 thickness in the range 1 to 10 λ to obtain optimum normalized thicknesses are in progress in order to confirm the high potential of the new Diamond/AlN/IDT/LiNbO3 structures for high-performance and high-frequency SAW devices. Efforts are also underway to utilize the optimized configurations in fluid-solid interaction FE model to investigate the influence of fluid loading on Rayleigh wave propagation in multi-layered SAW devices and evaluate their potential use in chemical and biosensing applications.