The frequency characteristics of SAW devices depend on the IDT structure and material properties of the substrate. The simulations of the proposed ER-type SAW devices on silicon are performed using COMSOL and the device parameters for fabrication are designed using equivalent circuit model.
Generation of waves in elastic medium
We have fabricated the proposed edge-reflection type SAW devices on silicon and testing the fabricated devices confirms the feasibility of the proposed devices. The voltages generated in the piezoelectric film are induced in the material, resulting in the propagation of surface waves along the surface of the non-piezoelectric medium.
Surface acoustic wave (SAW) devices
One IDT acts as a transmitter and the other as a receiver IDT, these two gate resonators are often used in the design of highly stable oscillators. The bandwidth of a two-port resonator is proportional to the number of fingers in IDT, Lc and Lg.
Edge reflection (ER) type SAW devices
Thin films over SAW devices
Temperature Coefficient of Frequency (TCF): Change in device operating frequency with respect to change in temperature, and defines the stability of device with respect to temperature. Insertion Loss (IL): Attenuation of a signal due to its conversion and propagation in a device, typically expressed in dB.
SAW devices on non-piezoelectric substrates
SAW generation and detection methods on non-piezoelectric substrates reported by (a) thin film SAW device on silicon substrate with ZnO thin film for generation and propagation of surface waves, Kino et al. 20] and (b) a SAW device on a silicon substrate using a ZnO thin-film transducer to excite SAW propagating in silicon Hickernell et al.
Problem definition
At oxide thickness of 0.8 µm (0.1λ), the second higher-order state VPT2 exhibits an intriguing characteristic of nearly constant phase velocity above. From (5.1) and (5.3), the frequency of transverse BAW is proportional to the width of ZnO, while the frequency.
Scope of thesis
Thesis organization
We performed mesh optimization for the simulation of thin-film SAW devices on a silicon substrate and calculated the phase velocity. In Chapter 6, we present the simulation of SAW devices with edge reflection on a silicon substrate, which is one of the major contributions of the thesis.
Theory of elasticity
- Piezoelectricity and constitutive equations
- Equation of motion
- Solution of surface wave in piezoelectric media
- Acoustic waves in isotropic media
The boundary conditions at the surface must be satisfied to determine the phase, velocity and amplitude of the waves. The appropriate boundary conditions must be applied to determine the phase velocity and amplitude of the wave [38].
Modeling of SAW devices
- Coupling of modes (COM) model
- Discrete source or delta function method
- Equivalent circuit model
- Design parameters
In the above equations, φT = π and φB = π/2 are the phase shift of the grating and the potential, respectively, and ω =2πf is the angular frequency. It does not take into account the energy, capacitance and electromechanical coupling coefficient of the material used in the device.
Finite element method (FEM)
Input and output information in FEM
Using equations (3.5 and 3.6), the number of finger pairs and the length of the IDT opening are chosen so that the impedance of the IDT at the operating frequency matches the source impedance and the load impedance, typically 50 Ω. The phase velocity dispersion and coupling coefficient characteristics of the surface modes created in the structure are shown in Fig.
Finite element simulations of SAW devices
ZnO thin film based SAW devices on silicon
- Mechanical properties of ZnO
- Effect of aperture length on operating frequency and admittance 35
- ZnO/Si layered configurations
- Results of ZnO/Si SAW resonator
- Results of thin film ZnO/IDT/Si SAW delay line
- Observations
The phase velocity (ν), reflection coefficient (k) and electromechanical coupling coefficient (K2) are calculated using the following expressions and [56]. The 2D schematic of the ZnO/IDT/Si SAW delay line with subdomain conditions is shown in Fig.
Summary
This chapter presents the proposed SAW devices on silicon using periodically patterned ZnO films and their simulation to investigate their performance compared to conventional thin film SAW devices simulated in Chapter 2. Design methodology using equivalent circuit model to fabricate the proposed SAW devices with periodically patterned ZnO are discussed.
Patterned-ZnO/Si structure – Theory and operation
- Structure of one port SAW resonator on silicon with periodically
- Simulation methodology
- Results of 3D simulations one port SAW resonator
- Conclusion based on 3D simulations
- Surface modes generated due to transverse bulk waves in ZnO
- Surface modes generated due to longitudinal bulk waves in ZnO
- Observations from 3D and 2D simulation results
The x and y displacement profiles of the first three vertically polarized surface modes generated in the structure are shown in Fig. The surface modes created in the silicon substrate result from the acoustic coupling of transverse and longitudinal common modes created in the ZnO patterned structure.
Addition of a buffer layer in patterned-ZnO/Si structure
- Patterned-ZnO/SiO 2 /Si structure
- ZnO-patterned/AlN/Si structure
- Simulation methodology
- Results of 2D SAW delay line with patterned structure
Phase velocity and coupling coefficient characteristics of patterned ZnO/SiO2/Si structure with SiO2 thicknesses of 0.8 µm, 1.6 µm and 2.4 µm. The phase velocity and K2 dispersion curves with respect to the height of ZnO in the patterned-ZnO/4µm-AlN/Si structure.
Design of proposed patterned-ZnO/Si SAW devices using equivalent
Calculation of IDT parameters for proposed SAW devices on
The penetration depth of the generated surface modes is estimated by calculating the reflection coefficient k, which is the reflection of the surface wave for one electrode finger in the grid structure and is given by [29]. The minimum number of electrodes in the reflector grid (N) required to ensure complete SAW reflection on the grid structure is calculated using (3.13), assuming Γ = 0.99 and the number of electrodes in the grid structure N = 48.
Summary
Design of SAW devices on silicon
Some of the important criteria to be considered in fabricating SAW devices on silicon are as follows. The IDT dimensions for the fabrication of SAW devices are tabulated in Tables 4.1, 4.2 and 4.3, for IDT metallization ratio of 0.5.
Fabrication of SAW devices on silicon
Optical microscopic images of (a) IDT structure on mask sheet 1 and (b) ZnO pattern on mask sheet 2. Concentration of acetic acid and flow rate of the solution are varied to achieve anisotropic etching of ZnO film.
Notable observations from the fabrication process
ZnO pattern obtained on real device using 0.2M acetic acid with convection flow of 1.6 m/s is shown in Fig. ZnO cartridge obtained with 0.2M acetic acid with convection flow of 1.6 m/s is found to be the most appropriate.
Fabrication of SAW devices on Si using e–beam lithography process
The samples are washed with DI water after ensuring that the removal process is complete. Next, the samples are cleaned with acetone and DI water and then dried using nitrogen flushing.
Summary
This chapter of the thesis presents the testing of proposed ZnO/Si SAW devices with periodic pattern using network analyzer (Agilent E8361A) and RF probe station (Cascade Microtech Summit 9000). The finite element simulations of patterned ZnO SAW devices on silicon substrate are again performed with the as-fabricated dimensions and the simulated results are compared with the measured results for validation.
S parameters of fabricated SAW devices on silicon substrate
The distribution of SAW devices fabricated on a full 2” wafer and labels of each individual SAW device are shown in Figure 55P-65L-0.5 defines a two-port resonator with 55 finger pairs with a gap of 65λ and the distance between IDT and reflector grid is 0.5λ.
S parameters of one port SAW resonator
Simulation of one port SAW resonator as fabricated
To validate the test result and identify the type of surface states generated in the structure, FE simulations are performed with dimensions of the device as manufactured. This confirms the presence of VPL0 mode due to misalignment of ZnO blocks with respect to IDT, and the detailed simulation study on the misalignment is carried out in the next section.
S parameters of two port SAW resonator fabricated on silicon
Simulation of one port SAW resonator on silicon substrate as
S parameters of two port SAW resonator fabricated on silicon
The series and parallel resonant frequencies of surface modes generated in the structure can be obtained from the phase plots (Fig. 5.10) of the fabricated two-port Table 5.4 Comparison of measured S11 results and simulated results for 65P-60L-0.5 two-port SAW resonator. The snapshot of the measured S-parameter characteristics of a two-port SAW resonator labeled 45P-50L-0.5 fabricated on a silicon wafer with a 1.73 µm thick oxide film.
Compensation network to match impedance
The table consists of details of resonance frequencies and insertion loss parameters of the surface modes generated in the structure. The calculated Q-factor for various modes generated in the fabricated SAW devices is tabulated in table 5.7.
The study of effect of misalignment of patterned-ZnO structure
5.13, we see a gradual increase in the magnitude of the VPL0 mode as the ZnO pattern begins to overlap the electrode, increasing the in-line electric field along ZnO blocks. This confirms the generation of the VPL0 mode due to misalignment of the ZnO pattern with respect to IDT.
The study of effect of width of patterned-ZnO structure
To validate the above statement, we perform frequency-dependent analysis on the structure by moving the ZnO pattern along x and the admittance characteristics of the device with different misalignment values of ZnO pattern are shown in Fig. Parametric sweep in eigenmode analysis is performed to obtain the frequency distribution characteristics of surface modes generated in the structure with respect to the width (2b) of ZnO block.
Calculation of K 2 using delay line configurations
However, it is less than the simulated K2 values because, as shown by the FESEM analysis, the ZnO structure is not a complete ZnO pattern and the obtained structure has a residual film thickness in the range of 0.5-1 µm with a ZnO structure with pattern and height. in the range of 3.3–3.8 µm. FE simulations are performed on a periodic structure of IDT with dimensions as fabricated and the K2 calculated using free and metallized surface boundary conditions and the K2 value of 3.85% for VPT0, 4% for VPL0 and 5.6% for VPT1 , which are in good condition. agreement with practically obtained value.
S parameters of SAW devices fabricated through e-beam lithography
One port SAW resonators
As the longitudinal BAW modes are significantly affected by the quality of the ZnO pattern than the transverse BAW, whereas transverse BAWs predominantly depend on the dimensions of the structures. In the case of pattern structure, the operating frequency of the device depends on the dimension of the ZnO pattern.
Two port SAW resonators
Summary
Fabrication and testing of the proposed edge-reflection-type SAW devices on silicon validates the device's applicability. 3D schematics of (a) conventional one-port SAW resonator with reflector gratings and (b) edge-reflection-type SAW resonator with edge electrodes.
Love wave based SAW devices on silicon using ZnO films
6.1(b) shows a 3D schematic of an edge-reflected SAW resonator on a piezoelectric substrate, which consists of an IDT placed between a pair of parallel free edges of the piezoelectric substrate. Limitation of surface waves in ER-type SAW devices is achieved by parallel edges rather than reflector gratings with a large number of electrode fingers, thereby significantly reducing the size of the device.
Proposed edge reflection (ER) type SAW devices on silicon using ZnO
11] reported the generation of both Rayleigh and Love modes in ZnO/AT-Quartz structure and identified the type of surface wave using liquid drop test. The device consists of a silicon substrate with piezoelectric film with an IDT on top, placed between two vertical grooves developed in silicon with λ/8 electrodes of the IDT located at the edges formed by the grooves.
Finite element simulations of ER type SAW devices on silicon
Simulation methodology
ZnO oriented (002) with c-axis along the sagittal plane is chosen because of its electromechanical maximum. a) Cross-sectional view of the proposed ER-type SAW device and (b) 3D geometry with periodic boundaries along the aperture used to obtain phase velocity and dispersion coupling coefficient characteristics as well as input characteristics, and (c) ) ER-type SAW resonator with 1λ grating. An input potential of 1V is assumed for the edge electrode and ground boundary conditions are applied to the middle electrode to obtain the input characteristics of the proposed ER-type SAW device.
Results of ER type one port ZnO/Si SAW resonator
Important observations of K2 dispersion characteristics and phase velocity are listed in Table 6.2. Further frequency-dependent analysis is performed on the IDT/ZnO/Si and ZnO/IDT/Si configurations because they have high coupling coefficient and high phase velocity, respectively, to obtain the L0 admittance and displacement characteristics generated in the structure.
Effect of buffer layers in ER type SAW devices on silicon
- Effect of SiO 2 buffer layer on ZnO/Si structure
- Effect of AlN buffer layer on ZnO/Si structure
- Frequency dependent analysis
- Observations
Proposed edge reflection type SAW devices on silicon after deposition of ZnO using RF sputtering. Proposed edge reflection type SAW devices on silicon with patterned ZnO film developed using hard mask.
ER type SAW device on silicon periodically using patterned-ZnO film 139
Results of ER SAW one-port resonator with patterned-ZnO
The displacement profile and the admittance characteristics of generated surface wave are shown in Fig. From the displacement profiles we observed that the excitation is due to Love wave.
Effect of heavy metals on SAW characteristics
The surface wave displacement distribution in silicon with different metal electrodes and the displacement profile of the simulated structure with gold electrodes are shown in Figs. From the characteristics we observed, the bandwidth increased with heavy metals as well as the coupling coefficient.
Effect of buffer layers in ER type patterned-ZnO/Si structure on
From the characteristics of the proposed interlayer structure, we observed an operating frequency of 585.525 MHz with a bandwidth of 1.67 MHz and a maximum offset of 0.45 µm at the edges and center of the device. By including the oxide layer, the bandwidth of the resonator is increased by a factor of 5, and the coupling coefficient is improved to 2%.
Observations
The device has a low coupling coefficient due to the energy distribution generated by the SH-SAW. The FE simulation results of the proposed structure with an oxide interlayer show a significant improvement in the coupling coefficient compared to the -ZnO/Si sample.
Applications of ER type SAW resonator
Longitudinally coupled resonator filter (LCR)
The use of heavy metals such as gold, tungsten and platinum in the IDT design results in the confinement of surface waves within 1λ depth in the substrate and can also significantly improve the coupling coefficient. The phase velocity characteristic depends on the substrate and buffer layer, while in the case of the proposed device (Chapter 3), the phase velocity mainly depends on the ZnO pattern and the type of bulk mode generated in it.
Transversely coupled resonator filter (TCR)
In the LCR filter, two longitudinal modes (symmetric and asymmetric mode) are combined into a passband region. The velocities of the symmetric and asymmetric modes are similar to those of a single-junction resonator, and the higher-order modes are usually absent or weakly coupled.
Ladder filters
The modes in the two devices cross-couple to form symmetrical and asymmetrical modes in the combined device and work as a bandpass filter with exceptionally narrow bandwidth of 0.1%, low insertion loss and excellent stopband rejection. In the passband region, the L-section behaves as a two capacitor network with parallel resistors and the filter selectivity performance can be improved by adding multiple stages of L-section.
Design of ER type SAW devices on silicon using equivalent circuit
6.23(b) and to achieve the bandpass characteristics, the resonators are designed such that fs_r = fp_a r and resonance and anti-resonance frequencies of parallel and series resonators, respectively, from the edges of the passband. The bandwidth and attenuation parameters depend on the static capacitances of the resonators and capacitance ratios of series and parallel resonators.
Summary
The proposed ER-type SAW devices on silicon using ZnO films and periodically patterned films are fabricated using the design parameters obtained from dispersion characteristics and equivalent circuit model. This chapter presents the fabrication process of edge reflection type SAW devices on silicon using ZnO film and patterned ZnO film.
Fabrication of edge reflection type SAW devices on silicon
Design of edge reflection type SAW devices
To validate the feasibility and bulk fabrication of proposed devices, we fabricated one-port resonators, two-port resonators, cross-coupled filters, and ladder filters using UV lithographic process. Devices are acoustically isolated using 250 µm - 400 µm wide grooves in silicon obtained using deep ion reactive etching (DRIE) process.
Fabrication process
Fabrication process flow of ER-type SAW devices in silicon with end-developed grooves. Fabrication process flow of ER-type SAW devices in grooved silicon developed the first IDT.
Summary
Testing of ER type one port SAW resonator using compensation
Using the network analyzer, the impedance value of the device obtained without compensation network is 18+j2.335 Ω. From the characteristics, the resonance peak is observed at 261.727 MHz with RL -37.19 dB, and 3dB bandwidth of 0.525 MHz is recorded using the data obtained from the network analyzer.
FE Simulation of ER type ZnO/Si one port SAW resonator
Accordingly, the values of the components of the compensation network that provide an impedance of 50 Ω are calculated as a series inductor of 11.86 nH and a parallel capacitor of 18.90 pF.
S parameters of ER type ZnO/Si two port SAW devices
S parameters of ER type SAW devices with pattern-ZnO
Summary
