To realize SAW devices on non-piezoelectric substrates, films of piezoelectric materials like ZnO, AlN, and CdS are employed in conjunction with interdigital transducers. Substrates like silicon, diamond and sapphire are chosen based on the application and requirement. In 1973, Kino et al.
[20], posted a method for the excitation of surface wave on non-piezoelectric substrates by depositing a thin piezoelectric film over the transducer as shown in Fig. 1.7(a). Electrical excitation to IDT causes acoustic stress in the piezoelectric film, which induces the stress in non- piezoelectric substrate resulting in launching of surface wave. An identical transducer is used for acoustic to electric conversion. In 1976, Hickernell [21] published a fabrication methodology Table 1.1 List of selected reports on implementation of thin films over traditional SAW devices.
SI. No Layered structure Type of surface
wave Application Reference
and year 1. Si3N4/LiNbO3 Rayleigh wave Low loss devices with frequency
tuning and passivation Jacqueline et al. [12] 1995 2. SiO2/LiNbO3 Rayleigh wave High frequency and temperature
stable wide band filters Yamanouchi et al. [13] 1995 3. SiO2/LiTaO3 Love wave Low loss wide band filters Yamanouchi et
al. [13] 1995
4. ZnO/LiTaO3 Love wave Liquid sensors Powell et al.
[17] 2002 5. ZnO/Quartz Love wave Low loss IF SAW filters Kadota et al.
[18] 2003 6. ZnO/ 36° YX
LiTaO3
Shear wave NO2 sensor Ippolito et al.
[16] 2003 7. ZnO nano-rod/
64° YX LiNbO3
Shear wave CO sensor Sadek et al.
[14] 2006 8. ZnO/Quartz Love wave Gas sensors with high K2 and
high temperature stability Moreira et al.
[11] 2008 9. SiO2/LiNbO3 Rayleigh wave High K2 and high temperature
stability Nakai et al. [19]
2008
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of ZnO thin film transducers to excite surface waves on non-piezoelectric substrates as shown in Fig. 1.7(b) and studied the effects of ZnO film properties and deposition conditions. The device performance substantially depends on the quality ZnO film as well as its surface roughness. ZnO quality is determined by the percentage of defects which can be reduced by precise control of substrate temperature, working pressure and percentage of oxygen. Because of high acoustic velocity in diamond and sapphire, ZnO or AlN thin film on diamond or sapphire results in SAW devices with high frequency of operation. Uehara et al. demonstrated a SAW filter with 5 GHz band and 0.1% coupling coefficient using AlN film over sapphire substrate [18]. Using diamond thin films, high frequency devices are implement over silicon substrate, where surface wave transduction is carried out with the help of ZnO thin films. The phase velocity, coupling coefficient, and temperature coefficient of frequency (TCF) characteristics of Rayleigh waves and other higher order modes generated in the ZnO/Diamond/Si structure, and also the change in surface wave characteristics with respect to the thickness of diamond layer has been reported by Nakahata et al [22]. Another high frequency SAW device using bi-layered structure of AlN/nano crystalline diamond (NCD) on silicon with operating frequency of 11.3 GHz has been reported by Ali et al. [23].
Integration of SAW filters (ZnO/SiO2/Si structure) with transistor circuits made on the same Si substrate is first reported by Visser et al. [24] in 1989. He reported the monolithic integration of SAW device and its fabrication process flow and also demonstrated the SAW filter applications in wireless communication systems. In 1994 Vellekoop et al. [25], reported a liquid sensors using acoustic plate modes generated in IDT/ZnO/M/SiO2/Si structure interfaced with transistor circuitry on a single silicon substrate and sensing is carried out by realizing an oscillator circuit with SAW device in the feedback path. Subsequent research on the integration of SAW devices with Si technology is reported by the group of Nordin, and Tigili [26-28]. In 2005 Tigli et al., demonstrated the fabrication of delay lines on silicon integrated monolithically
(a) (b)
Si Si
Input IDT Output IDT Input IDT Output IDT
ZnO
ZnO
Fig. 1.7. Methods of generation and detection of SAW over non-piezoelectric substrates reported by (a) thin film SAW device on silicon substrate with ZnO thin film for the generation and propagation of surface waves, Kino et al. [20] and (b) SAW device on silicon substrate using ZnO thin film transducer for the excitation of SAW that propagates in silicon Hickernell et al. [21].
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with CMOS circuit for chemical and bio-sensing applications. In 2007, Nordin et at. Reported the design of fabrication of ZnO/SiO2/Al/Si configured two port SAW resonators with CMOS circuits on silicon [29]. A consolidated list of reports on implementation of SAW devices on silicon substrate using piezoelectric thin films is given in Table 1.2. It presents various configurations of thin film SAW devices on non-piezoelectric substrates reported in last four decades.
Table 1.2 List of selected reports of SAW devices on non-piezoelectric substrates No SI. Layered structure Type of
surface wave Application Reference and year 1. ZnO/Si,
LiNbO3/Al2O3, ZnO/Al2O3, ZnO/SiO2
Rayleigh wave
Demonstration of excitation of surface waves on non- piezoelectric substrates
G. S. Kino et al.[20]
1973
2. ZnO/Si Rayleigh wave Thin film transducers on
non-piezoelectric substrates Hickernell et al.
[21] 1976 3. ZnO/SiO2/Si Rayleigh wave Amplification of Rayleigh
waves J. H. Visser et al.
[24] 1989 4. ZnO/SiO2/Si Rayleigh wave SAW filters on silicon with
electronic circuitry J. H. Visser et al. [31] 1989
5. ZnO/Si Lamb wave SAW monolithic micro
sensors Vellekoop et al.
[25] 1994 6. ZnO/Diamond/Si,
LiNbO3/Diamond/Si,
SiO2/ZnO/Diamond Rayleigh wave High frequency devices Nakahata et al. [22] 1995
7. AlN/ Al2O3 Rayleigh wave 5 GHz band SAW filter Uehara et al. [32]
2004
8. ZnO/Si Rayleigh wave
and love wave SAW-CMOS chemical sensor Tigli et al. [26]
2005
9. ZnO/SiO2/Si Love wave High frequency devices Krishnamurthy et al. [33] 2006 10. ZnO/SiO2/Metal/Si Rayleigh wave CMOS–SAW resonators Nordin et al. [26]
2007 11. LiNbO3/Si Rayleigh wave
and Love wave High frequency devices Pastureaud et al.
[34] 2007 12. AlN/ZnO/Diamond Waveguiding
layer acoustic wave
Package less SAW devices Laurent et al. [35]
2010 13. ZnO/Si,
AlN/Si Plate waves High coupling coefficient
devices Anisimkin et al.
[36] 2011
14. ZnO/Si Plate waves UV sensors Phan et al. [37]
2011
15. AlN/NCD/Si - High frequency SAW devices Ali B et al. [23]
2012
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In 2005 Tigli et al. [26], published the fabrication and characterization of SAW resonators with CMOS circuits for cancer biomarker sensing. They realized the fabrication of SAW device with CMOS process, where initial metal layers and oxide layers are carried out and IDTs are patterned along with the final metal layer of CMOS process and ZnO is deposited all over the substrate, another lithography process is carried out to opening the contact pads for characterization.