3.3 Addition of a buffer layer in patterned-ZnO/Si structure
3.3.2 ZnO-patterned/AlN/Si structure
Chapter 3 Design and Simulation of Patterned-ZnO/Si SAW Devices
The geometry comprises of thin film of AlN deposited over the silicon substrate and IDT structure patterned over it, followed by the deposition and patterning of ZnO film. In these simulations, we assume that the ZnO is located exactly in the spaces of IDT. The phase velocity and K2 dispersion curves are obtained using eigenmode analysis and results are shown in Fig. 3.16. From the characteristics, we observe that the patterned-ZnO/AlN/Si structure exhibits high coupling coefficient values compared to conventional thin film AlN/Si structure and the resultant phase velocity depends on the frequency of bulk modes generated in the ZnO pattern. With increase in height of ZnO, the phase velocity of generated SAW decreases rapidly due to the decrease in frequency of bulk modes excited in ZnO structure. As the height of ZnO increases the VPT0 surface mode tends to reach the frequency of ZnO structure.
The higher order transverse mode in ZnO results in generation of VPT1 in the structure at h/λ = 0.225, with SAW phase velocity of 5223 m/s. The second higher mode in ZnO starts at h/λ = 0.425 resulting in VPT2 with SAW phase velocity of 5336 m/s. As the height of ZnO increases, higher order transverse modes generated in ZnO result in corresponding VP modes. VPT3 and VPT4modes are generated at h/λ = 0.65 and h/λ = 0.875 with SAW phase velocities of 5335 m/s and 5316 m/s respectively. The maximum phase velocity of VP modes generated in the structure is equal to shear bulk velocity in silicon (5653 m/s).
The K2 dispersion curves of the first five vertically polarized modes generated in the structure are shown in the Fig. 3.16. For VPT0, the coupling coefficient K2 starts at 2.5% at h/λ = 0.025 with SAW phase velocity of 5212 m/s and reaches a relative maximum of 10.8%
at h/λ = 0.125 with SAW phase velocity of 3359 m/s and reduces gradually and reaches 4.4% at h/λ = 1. For VPT1, K2 starts from 9.5% at h/λ = 0.225 with phase velocity 5223 m/s Fig. 3.16. The phase velocity and K2 dispersion curves with respect to height of ZnO in patterned- ZnO/ 4µm-AlN/Si structure.
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and reaches a relative maximum value of 9.56% at h/λ = 0.24 with phase velocity 4996 m/s and then decreases gradually and reaches 2.8% at h/λ = 1 with phase velocity of 909 m/s.
For VPT2, K2 starts from 1.63% at h/λ = 0.425 with phase velocity of 5336 m/s and reaches a relative maximum of 3.7% at h/λ = 0.6 with phase velocity of 4016 m/s and reaches a value of 2.94% at h/λ = 1 with phase velocity 2110 m/s. For VPT3, K2 starts from 2.13% at h/λ = 0.65 with phase velocity of 5335 m/s and reaches a value of 2.76% at h/λ = 1 with phase velocity of 3422 m/s. For VPT4 the K2 starts from 1.5% at h/λ = 0.875 with phase velocity 5316 m/s and at h/λ = 1 it reaches a value of 2.1% with phase velocity of 4718 m/s.
In general, the K2 values are slightly higher than pattern-ZnO/Si structure, because the AlN buffer layer is piezoelectric in nature as well as it exhibits higher acoustic velocity than silicon.
Further we carried out FE simulations on proposed structure with fixed ZnO height and varying the AlN thickness to study the phase velocity and coupling coefficient characteristics. We consider IDT wavelength of 8 μm and ZnO height of 0.225λ, where the device exhibits high phase velocity and high coupling coefficient. The obtained characteristics are shown in Fig. 3.17. From the dispersion curves, we observe that two transverse modes (VPT0 and VPT1) are generated in the structure and their phase velocities saturate at 1800 m/s and 5180 m/s respectively as AlN thickness increases. The K2 of VPT0
starts from 5.7% and reaches to 8.7% at h/λ = 1 and VPT1 starts from 8.67% at h/λ = 0.0125 and reaches 9.9% at h/λ = 1. From the above simulations, we can conclude that the phase velocity of the surface modes generated in the proposed structure depends on the modes excited in the ZnO pattern and its resonant frequency. As the patterned structures exhibit high resonant frequencies, buffer layers with high acoustic velocity are recommended in order to obtain high coupling and high phase velocity.
Fig. 3.17. The phase velocity and K2 dispersion curves of vertically polarized modes generated by transverse bulk waves in patterned-1.6μm-ZnO/AlN/Si structure.
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Chapter 3 Design and Simulation of Patterned-ZnO/Si SAW Devices 3.3.3 Observations from the study of inclusion of buffer layer (SiO2 and AlN)
The results and observations from the FE simulations of proposed patterned-ZnO/Si SAW devices with the addition of buffer layer are summarized as follows:
1. The dispersion characteristics of patterned-ZnO/SiO2/Si show a reduction in coupling coefficient values compared with patterned-ZnO/Si, due to the large acoustic impedance difference between ZnO block and SiO2.
2. A wide non-dispersive region is observed in the range of 0.2–0.4 of h/λ for patterned-ZnO/0.8µm-SiO2/Si structure with an acceptable coupling coefficient range of 0.8%–0.86%. The drawback of low coupling coefficient can be overcome by using IDT with large number of finger pairs.
3. The dispersion characteristics of patterned-ZnO/AlN/Si show a substantial increase in coupling coefficient values, viz. VPT0 exhibits a maximum value of 10.8% at h/λ = 0.125 with SAW phase velocity of 3359 m/s and VPT1 exhibits a high K2 value of 9.5%
at h/λ = 0.225 with high phase velocity of 5223 m/s.
4. The results show that the inclusion of AlN buffer layer improves the coupling coefficient significantly compared with SiO2 layer. So, buffer layers with high acoustic velocity are recommended in order to obtain high coupling and high phase velocity.
5. Assuming the maximum thickness of 4 µm achievable in ZnO film deposition using RF sputtering, the SAW device with maximum λ of 16 µm can be fabricated with patterned-4µm-ZnO/0.8µm-AlN/Si structure with unique property of high phase velocity and high coupling coefficient.
6. The phase velocity characteristics are predominantly depend on the dimensions of ZnO pattern rather than the thickness of buffer layer (SiO2 and AlN).
3.4. 2D simulation of SAW delay line on silicon using periodically patterned ZnO SAW resonator configuration with periodically patterned-ZnO on silicon substrate is demonstrated in section 3.2. In this section, we employ the proposed patterned-ZnO structure to realize, another popular SAW device, i.e. delay line device on silicon substrate.
2D finite element simulations are performed on delay line configuration with patterned ZnO structures in the spaces of IDT. From the phase velocity and K2 dispersion characteristics, dimensions of IDT and ZnO pattern are chosen to obtain a surface mode with high phase velocity and high coupling coefficient. The simulation results of SAW delay line with
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patterned-ZnO structure is compared with the conventional ZnO thin film SAW delay line on silicon substrate (Section 2.4.7).