Li2CO3-doped ZnO films prepared by RF magnetron sputtering technique for acoustic device application

The effective acoustic impedance matrix (Z(k)) is proposed, and its applications are discussed. After defining (Z(k)), it is shown that (Z(k)) for any kind of multilayered structure can be determined successively from the bottom substrate to the top layer. The use of (Z(k)) obviates some problems often occurring in the numerical calculation of multilayered structures. Using three typical examples, it is shown how (Z(k)) can be conveniently applied to characterize acoustic wave excitation and propagation in certain complicated layered structures as well as in semi-infinite substrates. >

Analysis of acoustic properties of multi-layered structures by means of effective acoustic impedance matrix

CMOS integration in RF MEMS have become dominant due to the growing demand of mobile and wireless communication system. Usage of off chip resonators is not efficient since it leads to large area and high interfacing loss. Integration of CMOS with SAW resonators is a possible solution to reduce the loss. In this work, a SAW resonator was developed using AZO/Al/Si layers in 0.18 μm CMOS technology. AZO was chosen as the piezoelectric layer to achieve high electromechanical coupling coefficient for good SAW performance. This paper highlights the comparative analysis of two different piezoelectric materials: pure Zinc Oxide (ZnO) and Aluminium doped Zinc Oxide (AZO) for GHz CMOS MEMS SAW resonator. S parameter measurements were performed for the fabricated designs to get the resonance frequencies and the electromechanical coupling coefficients were recalculated. The design and finite element modeling simulation was conducted using COMSOLTM to verify the performance of the resonator using both Al-doped ZnO and pure ZnO via the simulation results.

Comparative analysis of zinc oxide and aluminium doped ZnO for GHz CMOS MEMS surface acoustic wave resonator

Acoustic waves based MEMS devices offer a promising technology platform for a wide range of applications due to their high sensitivity and the capability to operate wirelessly. These devices utilize acoustic waves propagating through or on the surface of a piezoelectric material. An acoustic wave device typically consists of two layers, metal transducers on top of piezoelectric substrate or thin films. The piezoelectric material has inherent capabilities of generating acoustic waves related to the input electrical sinusoidal signals placed on the transducers. Using this characteristic, different transducer designs can be placed on top of the piezoelectric material to create acoustic wave based filters, resonators or sensors. 

Historically, acoustic wave devices have been and are still widely used in telecommunications industry, primarily in mobile cell phones and base stations. Surface Acoustic Wave (SAW) devices are capable of performing powerful signal processing and have been successfully functioning as filters, resonators and duplexers for the past 60 years. Although SAW devices are technological mature and have served the telecommunication industry for several decades, these devices are typically fabricated on piezoelectric substrates and are packaged as discrete components. Considering the wide flexibility and capabilities of the SAW device to form filters, resonators there has been motivation to integrate such devices on silicon substrates as demonstrated in (Nordin et al., 2007; M. J. Vellekoop et al., 1987; Visser et al., 1989). One such example is illustrated in (Nordin et al., 2007) where a CMOS SAW resonator was fabricated using 0.6 m AMIs CMOS technology process with additional MEMS post-processing. The traditional SAW structure of having the piezoelectric at the bottom was inverted. Instead, the IDTs were cleverly manufactured using standard complementary-metal-oxide-semiconductor (CMOS) process and the piezoelectric layer was placed on the top. Active circuitry can be placed adjacent to the CMOS resonator and can be connected using the integrated metal layers. 

A SAW device can also be designed to have a long propagation path between the input and output transducer. The propagating acoustic waves will then be very sensitive to ambient changes, allowing the device to act as a sensor. Any variations to the characteristics of the propagation path affect the velocity or amplitude of the wave. Important application for acoustic wave devices as sensors include torque and tire pressure sensors (Cullen et al., 1980; Cullen et al., 1975; Pohl et al., 1997), gas sensors (Levit et al., 2002; Nakamoto et al., 1996; Staples, 1999; Wohltjen et al., 1979), biosensors for medical applications (Andle et al., 1995; Ballantine et al., 1996; Cavic et al., 1999; Janshoff et al., 2000), and industrial and commercial applications (vapor, humidity, temperature, and mass sensors) (Bowers et al., 1991; Cheeke et al., 1996; Smith, 2001; N. J. Vellekoop et al., 1999; Vetelino et al., 1996; Weld et al., 1999). In recent years, the interest in the development of highly sensitive acoustic wave devices as biosensor platforms has grown. For biological applications the acoustic wave device is integrated in a microfluidic system and the sensing area is coated with a biospecific layer. When a bioanalyte interacts with this sensing layer, physical, chemical, and/or biochemical changes are produced. Typically, mass and viscosity changes of the biospecific layer can be detected by analyzing changes in the acoustic wave properties such as velocity, attenuation and resonant frequency of the sensor. An important advantage of the acoustic wave biosensors is simple electronic readout that characterizes these sensors. The measurement of the resonant frequency or time delay can be performed with high degree of precision using conventional electronics.
	
This chapter is focused on two important applications of the acoustic-wave based MEMS devices; (1) biosensors and (2) telecommunications. For biological applications these devices are integrated in a microfluidic system and the sensing area is coated with a biospecific layer. When a bioanalyte interacts with this sensing layer, physical, chemical, and/or biochemical changes are produced. Typically, mass and viscosity changes of the biospecific layer can be detected by analyzing changes in the acoustic wave properties such as velocity, attenuation and resonant frequency of the sensor. An important advantage of the acoustic wave biosensors is simple electronic readout that characterizes these sensors. The measurement of the resonant frequency and time delay can be performed with high degree of precision using conventional electronics. Only few types of acoustic wave devices could be integrated in microfluidic systems without significant degradation of the quality factor. The acoustic wave based MEMS devices reported in the literature as biosensors are film bulk acoustic wave resonators (FBAR) and surface acoustic waves (SAW) resonators and SAW delay lines. Different approaches to the realization of FBARs and SAW resonators and SAW delay lines used for various biochemical applications are presented. Next, acoustic wave MEMS devices used in telecommunications applications are presented. Telecommunication devices have different requirements compared to sensors, where acoustic wave devices operating as a filter or resonator are expected to operate at high frequencies (GHz), have high quality factors and low insertion losses. Traditionally, SAW devices have been widely used in the telecommunications industry, however with advancement in lithographic techniques, FBARs are rapidly gaining popularity. FBARs have the advantage of meeting the stringent requirement of telecommunication industry of having Qs in the 10,000 range and silicon compatibility.

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Acoustic Wave Based MEMS Devices, Development and Applications

In recent years, miniaturization and high accuracy of mobile communication systems have become essential. Because of good phase noise performance, the SAW clock has become a viable option for data transmission. However, a SAW oscillator is usually made up of a SAW device and a CMOS-IC separately, and this large size limits its use in mobile communication systems. This paper covers the development of the smallest monolithic SAW oscillator using thin film ZnO deposited on a CMOS-IC. The SAW device and the IC have been combined into a single unit, and the total thickness is nearly the same as the IC.

https://www.bioee.ee.columbia.edu/courses/upload/Bibliography/CMOS_SAW_2005.pdf?origin=publication_detail

Development of monolithic CMOS-SAW oscillator

The monolithic integration of SAW (surface acoustic wave) devices in a ZnO-SiO/sub 2/-Si layered structure with high-frequency electronic circuitry has been demonstrated. The high-resolution technology for the fabrication of the aluminum interdigital pattern-electron beam lithography, bilayer masking, and dry etching-is explained and illustrated with SEM (scanning electron microscope) photographs. Experimental results on a SAW delay line, a SAW transversal filter, and a SAW resonator filter show the feasibility of obtaining on-chip selectivity with SAW filters. A passivation of the sputtered, piezoelectric ZnO layer has been realized with a thin silicon nitride layer, resulting in an increased stability of the frequency response of the SAW device. A single-chip FM upconversion radio receiver with an on-chip SAW filter is discussed. >

P.J.M. Rek

Papers

Surface acoustic wave filters in ZnO-SiO/sub 2/-Si layered structures