Introduction to acoustic wave devices
The types of acoustic waves generated in the piezoelectric material mainly depend on the properties of the substrate material, the cut of the crystal and the structure of the electrode used in the transducer [11, 12]. SAW is an acoustic wave, which concentrates most of the energy on the surface of the solid body.
Generation of SAW in piezoelectric medium
When the applied potential is sinusoidal with period T, the wave generated by a pair of IDT fingers travels a distance of half wavelength (λ/2) in a time interval of half period (T/2) at the speed of the SAW phase velocity. The stress waves generated by each pair of fingers add constructively to the stress waves generated by other pairs of fingers in subsequent cycles of input excitation, resulting in resonance.
Basic configurations of SAW devices
SAW delay line
Important design parameters of the SAW delay line are the periodicity of the transducers, the number of IDT finger pairs, the distance between the IDT centers, and the acoustic opening. The frequency response of time-delay SAW devices depends on the finger width, the finger spacing, and the acoustic aperture w.
SAW resonators
The absorber is used at the ends of the device so that wave is absorbed and not reflected back to the IDTs. When an electrical excitation is given to the transmitter IDT, SAW is generated and it travels to the receiver IDT where it is converted back to electrical signal.
Operation of SAW devices in sensor applications
The change in delay time is measured and is related to the concentration of the chemical or gas. The interaction of a selective chemical or gas with the sensor layer causes a change in the sensor's resonant frequency and is measured.
Literature based on SAW sensors
The interaction of the sensing film with a selective chemical or gas results in a change in the properties and dimensions of the film and alters the speed, amplitude and phase of the SAW [12]. -Blodgett (LB) poly(vinyltetradecanoal) film. vapor LOD=300ppm Pfeifer et al. coated polypyrrole NH3 Penza et al.
State-of-the-art and motivation
Viespe and Miu [77] developed a SAW resonator sensor for volatile organic compounds using a nanocomposite sensing layer of Fe3O4 nanoparticles embedded in a polyethyleneimine polymer and reported up to five times improvement in LOD after embedding the nanoparticles into the polymer. However, these studies do not involve any change in the basic structure of the SAW device and use SAW devices, either delay line or resonator, having the conventional construction shown in Figure 1.4.
Problem definition
To perform FE simulations to analyze single-port SAW resonator with slots made in the space between IDT fingers, optimize the design parameters and demonstrate the proposed structure for gas sensing application by placing sensing material on the bottom of the slot. To perform FE simulations to analyze single-port SAW resonator with a SiO2 HAR structure created over each IDT finger, optimize the SiO2 height and demonstrate the application of the device in detecting gases through sensing material in the space between IDT fingers.
Scope of the thesis
To analyze, using FE simulations, a single-gate SAW resonator with thick IDT fingers as HAR structures, optimize the IDT thickness, and demonstrate the use of a gas sensing device by depositing sensing material in the space between the thick IDT fingers. To fabricate the proposed single-gate SAW resonators with SiO2 HAR structures with a sensing film placed in the space between the HAR structures, and experimentally validate the application of the proposed gas sensing device.
Organization of thesis
Validation of experimental results with FE simulation of the proposed SAW one-port resonator structure is performed. 𝑇𝑖𝑗= ∑ ∑ 𝑐𝑘 𝑙 𝑖𝑗𝑘𝑙𝐸 𝑆𝑘𝑙− ∑ 𝑒𝑘 𝑘 and permittivity tensor 𝜀𝑖𝑗 of the dielectric medium.
SAW device modelling
- Delta function model
- Impulse response model
- Equivalent circuit model
- Coupling of modes (COM) model
The IDT is represented as an equivalent three-port receiver network and is shown in Figure 2.4. 𝑃12 and 𝑃21 are the transfer coefficients, 𝑃33 represents the IDT admittance, 𝑃13 and 𝑃23 are the currents produced by the waves and is calculated using 𝜌𝑒. 𝑘), which is the Fourier transform of the electrostatic charge density in IDT [84].
Finite element method
Approximations in FEM
The IDT consists of a large number of periodically spaced electrodes, therefore infinite periodic boundary conditions with only one wavelength (λ) of the wave or antiperiodic boundary conditions with half wavelength (𝜆 2⁄ ) of the wave can be used for unit simulation. Since SAW carries energy mostly near the surface, only 5 to 10 wavelengths of the substrate depth can be considered, reducing the number of calculations when solving a model.
SAW device simulation using FEM
FE Simulation of One-port SAW resonator device
Simulation geometry
Aluminum IDT with 𝜆 4⁄ finger width is made on the top surface of the resonator. Stress-free boundary conditions are used at all boundaries except the bottom boundary of the device.
Mesh Refinement
It calculates the homogeneous solutions of the relevant differential equations and is used for the detailed analysis of the device.
Analysis of one-port SAW resonator
The frequency analysis of one-port SAW resonator is performed to confirm the resonant frequency of the device. The total displacement and harmonic admittance versus frequency for the SAW one-port resonator are plotted in Figure 2.10.
Operation of one-port SAW resonator sensor
Exposure to gas will vary the density and thickness of the polymer sensing film, and the variations for different concentrations due to gas absorption are given as [67]. FEM simulation of the proposed SAW devices using COMSOL Multiphysics is described below.
Summary
Structure of one-port SAW resonator with trenches
A single-port SAW resonator with an infinite number of IDT fingers fabricated on a piezoelectric substrate is considered in the simulation. The resonator is treated as an infinitely extended periodic structure of one unit cell, so a half cell with antiperiodic boundary conditions is used to model the proposed single-gate SAW resonator.
Simulation methodology
The 2D geometry of a single-port SAW resonator and the slots included in the simulation is shown in Figure 3.2. The dimensions used in the simulation are as follows: IDT finger width 𝜆 4⁄, electrode spacing 𝜆 2⁄, depth of the piezo substrate 5 λ in –x3 direction, and thickness of IDT fingers 0.05 µm.
Results and discussions
It can be seen from the graph that mode 0 is dominant and the susceptance crosses zero at a frequency of 308.041 MHz, which is considered to be the resonant frequency of the device.
Simulation of one-port SAW resonator devices for sensing DMMP gas
Simulation methodology
The device operates at a frequency of 311.4 MHz, which is close to the resonant frequency of the proposed device. The properties of BSP3 film and DMMP gas are described in [3] and used in our simulation.
Results and Discussions
The resonance frequency for each DMMP gas concentration is recorded and the change in resonance frequency noted as the sensor response is calculated. The main factors affecting the resonance frequency of the devices are changes in the density and thickness of the sensor film due to the absorption of DMMP gas.
Simulation of proposed one-port SAW resonator device for sensing
Simulation methodology
The dimensions used for the simulation are as follows: finger width 4 µm, finger pitch 8 µm, depth of substrate 64 µm (5λ). The values of Young's modulus, density and thickness of Pd for different hydrogen concentrations in Pd used for simulation [91] are given in Table 3.1.
Results and Discussions
Summary
Simulation methodology
The periodic boundary condition is applied to the left and right sides of the device as given in Hamodon et al. The eigenmodes of the device are calculated with zero voltages applied to the IDT electrodes.
Results and discussions
A potential of 1 V is applied to the IDT to obtain the harmonic access of the device.
Analysis of one-port SAW resonator with thick IDT for sensing DMMP
Simulation methodology
The properties of the SXFA film and the DMMP gas are reported in [107] and used in the simulation. The resonance frequency for each DMMP gas concentration is recorded and the shift in the resonance frequency is calculated.
Results and discussions
The eigenmode analysis is performed to obtain the resonance frequency of the proposed and conventional devices. The changes in density and thickness of sensing film due to absorption of DMMP gas are the main factors affecting the resonance frequency of the devices.
Analysis of one-port SAW resonator with SiO 2 HAR structures over IDT
- Simulation methodology
- One-port SAW resonator with SiO 2 HAR structure over IDT for
- One-port SAW resonator with SiO 2 HAR structure over IDT for
- One-port SAW resonators with SiO 2 HAR structure over IDT for
The resonant frequency of the device is noted in the presence and absence of DMMP gas. The resonant frequency of the device is noted in the presence and absence of TCE vapor.
Summary
An increase in the resonance frequency is observed for increasing the concentration of H2 in the proposed device using the Pd sensor film. The swelling of the SXFA film due to the absorption of DMMP gas will decrease the effective height of the SiO2 HAR structure in the proposed sensor and lead to the increase of the resonance frequency.
Process flow
- Design of SAW device
- Preparation of Mask
- Photolithography for IDT
- Metal Deposition and Lift-off
The process steps involved in the fabrication of SAW single-port resonator are the layout design, mask preparation, photolithography and metal deposition on the wafer and lift-off. On the fabricated SAW devices, SiO2 is deposited on the surface using plasma enhanced chemical vapor deposition (PECVD). The second stage photolithography is performed, followed by dry etching of SiO2 to form HAR structures over the IDT fingers.
Fabrication of SiO 2 HAR structures
Deposition of SiO 2
The SiO2 layer is deposited over the wafer containing single port SAW resonator devices using plasma enhanced chemical vapor deposition (PECVD) process using PlasmaLab machine (Oxford Instruments, UK). The wafer is held on the sample holder on a silicon carrier wafer along with some supports so that the wafer does not move during deposition.
Photolithography for SiO 2 HAR structure
The deposition is uniform and a small piece of wafer sample is kept on the side of the main wafer to measure the thickness of the SiO2 layer. The deposition rate was 40 nm/min, and the machine was operated for about 150 minutes to obtain a thickness of about 3.5 µm as verified by the profilometer.
Etching of SiO 2
HAR structure is found to be 3.57 µm uniform across the IDT fingers and at least 3 µm between fingers where profilometer tip did not reach the bottom due to high aspect ratio of the structure.
Testing of fabricated devices for sensing H 2 using Pd
- Coating of Pd film between SiO 2 HAR structures
- Dicing
- Experimental setup
- Device characterization
- Experimental results
- Comparison with simulated results
- Comparison with recently reported SAW H 2 gas sensors
For comparison, the simulation for sensing H2 gas concentration using SAW device is performed from dimensions of the fabricated device. As seen in the optical image of the fabricated device given in figure 5.4 (b), the width of SiO2 HAR structure is about 7 µm instead of 5 µm which reduces space for Pd deposition which gives a width of about 2.4 has.
Testing of fabricated devices for sensing TCE vapor using PIB
Deposition of sensing films and etching
Zipperian, “Acoustic surface gas sensor based on film conductance changes,” Sensors and Actuators , vol. Huang, “Gas-sensing characteristics of surface acoustic wave ammonia sensors,” Sensors and Actuators B: Chemical, vol.
