MEMS Resonator Filters
The use of MEMS resonators for signal processing is relatively new and has the potential to change the topology of newer generation circuits. New materials, design and fabrication processes, and integration with conventional circuitry will need to be considered. This book explores the challenges and opportunities of developing circuits with MEMS resonator filters. The replacement of classical electrical components with electromechanical components is explored in this book, and the specific properties of MEMS resonators required in various frequency ranges are discussed. Materials and their selection, CAD tools for system design and the integration of MEMS with CMOS circuitry, and the design, fabrication, testing and packaging of MEMS filters themselves are addressed in detail. Case studies where resonator MEMS have been used as components have been included to encourage readers to consider the practical applications of this technology. MEMS Resonator Filters is essential reading for the analogue circuit designer community, particularly those who are designing circuits for wireless communications, and CMOS technology researchers and engineers who are involved in the fabrication of circuits. Designers of sensors and interfacing circuits will also be interested since resonators are also being used as sensors.
Inspec keywords: integrated circuit modelling; three-dimensional integrated circuits; integrated circuit design; surface acoustic wave resonators; surface acoustic wave resonator filters; bulk acoustic wave devices; radiofrequency filters; microfabrication; integrated circuit testing; integrated circuit reliability; finite element analysis; micromechanical resonators; integrated circuit packaging; radiofrequency integrated circuits; radiofrequency oscillators; integrated circuit manufacture
Other keywords: low-frequency resonators fabrication; reliability issues; resonator filters verification; oscillator design; heterogeneous systems integration; filter design; MEMS resonator filters testing; RF MEMS resonators; high-frequency resonators fabrication; finite-element modeling; MEMS resonator filters; 3D packaging; microelectromechanical resonators design; SAW-BAW resonators
Subjects: Finite element analysis; Microwave integrated circuits; Semiconductor integrated circuit design, layout, modelling and testing; General fabrication techniques; Passive filters and other passive networks; Reliability; Product packaging; Production facilities and engineering; General electrical engineering topics; MEMS and NEMS device technology; Oscillators; Acoustic wave devices
- Book DOI: 10.1049/PBCS065E
- Chapter DOI: 10.1049/PBCS065E
- ISBN: 9781785618963
- e-ISBN: 9781785618970
- Page count: 432
- Format: PDF
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Front Matter
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1 Introduction
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In the literature, SAW BAW, FBAR, and CMR are treated as the same class of devices and often treated as micromachined electromechanical devices. BAW and FBAR are differentiated by its mounting technologies. In the literature, some other classes of devices, such as capacitive micromachined ultrasonic transducers (CMUTs), are also sometimes called BAW devices. In general, the devices are called MEMS if they are fabricated by using thin and thick film processes used in integrated circuits (IC) fabrication or popularly known as CMOS process. Today, SAW, SMR, FBAR, and CMR devices can be fabricated within standard IC technologies. Additionally, FBAR manufacturing entails micromachining steps, like MEMS resonator processes. On the other hand, FBARs resonate at far-from-fundamental acoustic modes, instead of purely mechanical modes. Both circumstances have thus created certain controversy regarding whether FBARs are considered as MEMS resonators. However, in this book, any microstructure used for filter applications are referred to as MEMS resonators. The distinct feature of all these components is high Q, indicating that less energy dissipated, helps in low power designs.
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2 Filter design
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Filters are essential parts of electrical/electronic systems. It is difficult to find any modern electronic system without filter/s. In some of the systems such as electronic communication systems, filter design becomes very crucial and many times decides the progress of these systems. Mobile communication systems are one of the examples. The filters by and large affecting the quality of systems and therefore it has been always a matter of research and development. As a result, a lot of work has been done in the last 100 years on the topic of filters. The electronic systems have grown in numbers almost exponentially in these years so is the scientific and engineering efforts on each component and subsystem and filters are no exceptions. Today, the number of mobile phones used is in the range of 5 billion and that means at least those many filters are being used. The effectiveness of filters can immediately be seen on the quality of the systems and thus industries also contributed a lot in these efforts. As seen during the evolution of the filters, there were a lot of scientific challenges that were also posed by the filters and thus filters remained an attractive area for researchers. There are so many names associated with the filers since the beginning and some of them will appear in this book also. The topic of filters, in general, is a vast one and has generated so many texts and handbooks apart from the research papers in journals and various articles in magazines. So even a review of all this work may turn out to be another book. This chapter is written to give a quick perspective of the complicity involved in designing and synthesizing the filters along with the brief history.
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3 Microelectromechanical resonators design: low-frequency resonators
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Low-frequency MEMS resonator device design using a mixed mechanical and electrical coupling scheme is illustrated in this chapter. The closed-form expressions are easier to handle and provide automated analysis before fabrication. The electrical coupling scheme is also useful in suppressing spurious modes due to an increase in the operating temperature of the device by no addition to the responses at the output electrode. Mechanical couplers at equal velocity point aids in minimum variations in mode shapes and resonant frequencies. Thus, mixed electrical and mechanical coupling scheme enhances operation in desired modes and suppresses undesired and spurious modes. The device can be designed using Euler Bernoulli's beam theory. Desirable center frequency and bandwidth can be obtained using well-defined equations and physical parameters. Although resultant frequency and bandwidth can be achieved, other filter parameters such as insertion loss, output power, and stopband rejection which is the function of the transduction gap, the width of the CC beam, and the number of CC beams need careful modeling of the device. A comprehensive model is required that would give complete design automation before fabrication of the device under electrostatic actuation. Filter characteristics, suppression of spurious modes, and thermal stability can be obtained using CC beam array that is anchored to the substrate using rigid contact anchors. These anchors are responsible to dissipate large amounts of energy and reduce Q. Although it is known that a free-free beam achieves high Q. But dual frequency characteristics, desired bandwidth, and suppression of unwanted responses may be difficult to achieve using a free -free beam. Hence it is required to adopt techniques that would reduce anchor losses and enhance Q. RF MEMS resonator filters to be deployed in a transceiver for wireless communication systems needs to satisfy all the stringent specifications given to the filter designer. One of the most important among them is thermal stability. Metals MEMS resonator filter has poor thermal stability hence composite structure using silicon dioxide. To improve further the thermal stability of the device, we suggest fabricating the composite structure of the device using metals and silicon dioxide.
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4 Microelectromechanical resonator design for high frequency
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Electronics have become an essential part of human life. Sir Nikola Tesla said it, science proved it, it is a known fact that everything including our own bodies is made up of energy vibrating at different frequencies. The conventional microelectromechanical systems (MEMS) technology converts energy from mechanical to the electrical domain or vice versa -sensors and actuators play an irreplaceable role in our modern life and are offered by many suppliers. In contrast to their unique function, radio frequency microelectromechanical systems (RF MEMS) process electrical signals using mechanically vibrating structure and have replaced on -chip electrical RF devices to provide frequency control functions due to their extraordinary performance compared to on-chip electrical counterparts. Frequency selective elements such as resonators are being increasingly employed in applications related to timing and frequency control, and as building blocks in micro/nanofabricated oscillators and/or fi lters. With small size, high performance, and complementary metal-oxide-semiconductor (CMOS) compatibility, RF MEMS resonator offer promising technology in contemporary RF front-end in wireless communication systems.
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5 Finite-element modeling of RF MEMS resonators
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In this chapter, the quick review of different structures, modes, and excitation mechanisms is given in the beginning. The mechanical model for the RF MEMS resonator is discussed and ways to extract the important device parameters are given along with the electrical equivalent model. The importance of the physical modeling of a MEMS device is established and various governing partial differential equations (PDEs) are discussed. A brief introduction to the FEM for solving PDEs is given to give the reader a glimpse of the FEM machinery running behind the screens of commercial simulation tools. It discusses a finite element assembly for the Poisson's equation and its solution for demonstration purpose followed by the details of a few commercial tools such as CoventorWare, Intellisuite, and COMSOL Multiphysics with specific examples.
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6 Fabrication of low-frequency resonators
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The device fabrication process for low-frequency MEMS resonator is explained in this chapter. Challenges faced during the fabrication process and experiments performed to solve those issues are also discussed. All the processes such as metal deposition, dielectric deposition, electroplating, and wet etching must be studied by performing experiments to determine its deposition and etching rate, respectively. The rate of deposition in electroplating at room temperature is very less; it can be increased by increasing the current density and temperature of the electroplating solution. Observations and measurements of dimensions and thickness need to be regularly performed after every fabrication step. It helps to confirm the correctness of the process. Physical characterization and electrical characterization need to be performed on the fabricated device to validate the proposed concept.
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7 Fabrication of high-frequency resonators
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This chapter focuses on fabrication laterally vibrating contour mode one port ZnO MEMS resonator for global system for mobile communication (GSM) frequency range. The bottom electrode is not used, thus saving the fabrication step. The resonators have been fabricated on Si/SiO2 by using three mask processes. The device area is small (W x L = 57.12 gm x 18.52 gm) compared to the previous works reported. The thickness of ZnO piezoelectric film was 250 nm in this fabrication. The piezoelectric ZnO thin film has been used due to its better coupling coefficient, low deposition temperature, excellent bonding, and unique semiconducting and optical properties. Among all the piezoelectric materials, zinc oxide is considered as a pollution-free green piezoelectric material. The ZnO is highly tensile and may undergo huge mechanical deformation for a long duration unaffected by the temperature variation. The synthesis of ZnO thin films or nano particles has been investigated in the past. The ZnO thin film can be deposited at room temperature and a variety of acidic etchants are also available.
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8 Filter and oscillator design using SAW/ BAW resonators
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The authors have introduced the operation principle and methods for improving the characteristics of SAW and BAW devices in this chapter. Among the various MEMS resonators, SAW and BAW devices have been among the most successfully commercialised fields, particularly for wireless RF applications. The communication architecture was described, which is the common ground in device development. Also, the improvement of the characteristics has been described in terms of the requirements of such applications. With regard to techniques for improving performance, a very wide range of knowledge of engineering from materials to numerical dispersive analysis is required. The authorshave only given an outline of these fields. Details can be found in the books and papers cited in this chapter.
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9 Testing and verification of MEMS resonator filters
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The testing of MEMS resonators helps not only in verifying if the design specifications have been met but also in further optimization of the design, if necessary. In this chapter we have seen various methods to test MEMS resonator filters. As the frequency of operation of the resonators further increases, we may have to come up with specific calibration mechanisms to remove the parasitic effects.
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10 3D packaging for the integration of heterogeneous systems
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This paper discusses innovative processing technologies that would allow 3D packaging by the post-fab vertical stacking technique, suitable for the packaging industry. These novel simple processes may pave way towards 3D-stacked ultra-thin devices.
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11 Reliability issues of MEMS resonators
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This chapter introduces the terms and concepts needed to describe and evaluate MEMS resonator reliability. It is interesting to note that the reliability theory concepts are also used by actuaries in calculating life insurance premiums. Moreover, the life span of human provides a useful analogy for component reliability as well. This text describes some of the main points of various reliability issues of MEMS resonators arising due to its process technology, operation, and transduction method, the failures due to the packaging of these devices, aging, and frequency drift. Lord Kelvin stated, “When you can measure what you are speaking out and can express it in numbers, you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge of the subject is of a meager and unsatisfactory kind”. So, we will first discuss the quantification of reliability for the system.
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Back Matter
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