1 Case Study Cantilever Piezoelectric Accelerometer — ERIC SIA SIEW WEI INTRODUCTION Categorized as quantitative study, this case study was created to explore the design, development, analysis, application and communication challenges of a micromachined cantilever piezoelectric accelerometer (CPA). In this context, this case study could contribute towards optimization of structural dynamics in the realm of professional communication. As thorough analyses are potent to refine the existing theories, particularly in optimizing parameters and assessing material compatibility. Terminologically, piezo in Greek defined as pressure thus the term piezoelectric defined as electricity caused by pressure [2]. The piezoelectric effect applies when vibration energy from mechanical strains being converted to electrical potential energy. Coupled with Micro-Electro-Mechanical Systems (MEMs) technology, mankind uses piezoelectric accelerometer to quantitatively measure accelerations due to applied mechanical forces. This case examines the following research questions: • What are the motivation supporting and bottlenecks hindering the technological evolution of CPA? • How is CPA being incorporated into the existing sophisticated sensing devices? • What are the methods to enhance the performance and efficiency of CPA? The rest of this case study is organized as follows. In section II, the significance of the current case study and its relevancy to current issues, practices, education and studies from similar cases will be presented. Section III explains the data collected to address the case, including the data collection methods, justification on using those methods and data credibility. Section IV discusses about the case in detail. In section V, we conclude the findings with the limitations and recommendations. SITUATING THE CASE This section contextualizes our case study within the existing body of literature. First and foremost, this study will outline the background of the CPA identified in previous literature. Next, this case study will explain the literature selection method and criteria for review. This study also highlights innovation in sensor technology and its importance to various industries. How Literature Was Selected The selection of a cantilever piezoelectric accelerometer for research can be driven by several factors, including its unique characteristics, potential applications, and relevance to the research objectives. In selecting cantilever piezoelectric accelerometers for a research project, the literature review process should focus on gathering relevant studies that demonstrate the sensor's capabilities, applications, performance characteristics, and limitations. In this context, the literature was selected with several criteria, for instance, credibility of the source and publication date, relevancy to the subject matter. Firstly, this study searches the literature from credible databases, namely IEEE Xplore [5], Nature [6], etc. Aforementioned databases could be retrieved from interdisciplinary databases like Google Scholar in various formats, including theses, books, abstracts, journal articles, conference papers, encyclopedia and case study. Next, literature containing keywords such as ‘cantilever’, ‘piezoelectric’, ‘accelerometer’ and phase such as ‘acceleration’, ‘sensor’, ‘vibration measurement’ related to the study will be focused. These terms facilitated targeted searches and were used in various combinations to maximize the retrieval of pertinent literature. As for the country of study, it could vary depending on the institution or research organization conducting the study, the location of the authors, or the geographical focus of the research. 2 Related Works TABLE II [3] has presented a hypothesis that piezoelectric accelerometers has an edge over capacitive accelerometers in terms of high dynamic range and quality factor resonances (Q). [4] has suggested the use of Pt/ZnO/ Si3N4 stack, namely selecting Zinc Oxide (ZnO) as the piezoelectric material, Platinum (Pt) as the top electrode, mixture of Polysilicon with Nitride (Si3N4) as the lower electrode, as well as polysilicon cantilever structure with a length 1000 µm and thickness less than 3 µm. The justifications of selecting ZnO were its relatively easier fabrication and integration into Integrated Circuit (IC). PARAMETERS FOR MAXIMUM EXTRACTABLE POWER Powermax m ω vS YS e31, f ε0 εr Qtot A Maximum extractable electric power for a cantilever structure operating in the 31 mode at resonance frequency Mass of the proof mass Natural frequency Poisson’s ratio Young’s modulus of the passive layer for thin film piezoelectric Piezoelectric coefficient Permittivity of free space Relative permittivity Total quality factor Acceleration Barriers to the Evolution of CPA Charge Sensitivity of CPA Performance , Assuming tZnO << tSi, EZnO = ESi TABLE I PARAMETERS FOR CHARGE SENSITIVITY d31 ρ b L tSi tZnO ESi EZnO Transverse piezoelectric coupling coefficient Beam density Beam Width Beam slow Thickness of the polysilicon cantilever substrate Thickness of piezoelectric layer Elastic moduli of the polysilicon cantilever substrate Elastic moduli of piezoelectric layer From the work presented by [3], in 2013, the CPA fabricated together with a proof mass (bulk silicon tip mass) possesses weakness in achieving high resonance frequency (67.4 Hz), albeit the quality factor, Q could be achieved up to 200 under vacuum condition. [3] presented the dielectric loss contributed that main source of noise in the piezoelectric material. [3] also asserted that the method to improve the CPA is to lower the resonance frequency, at the cost of reducing the bandwidth, with the condition that the materials and overall device architecture were the constant variables. Material The performance of CPAs heavily relies on the properties of piezoelectric materials used in their construction. Material limitations, such as limited sensitivity, stability, or durability under certain conditions, for example, temperature extremes and humidity, can impede the evolution of CPAs. Statement of [7] revealed that MEMS piezoelectric accelerometers material incorporating lead. Considering the adverse effects of lead to mankind and the environment, the lead-free CPA has been introduced. Nevertheless, lead-free CPA has relatively lower piezoelectric coefficient, sensitivity, bandwidth than lead CPA [7]. [8] reported some common method to enhance the piezoelectric characteristics such as doping, when using acceptor dopants such as Manganese (Mn3+) in fabrication, it will establish an internal bias field within piezoelectric material, that will reduce the dielectric constant and cause tangent loss. Figure 1: A Cantilever Piezoelectric Accelerometer Maximum extractable electric power Supporting [8], [9] discovered that power is an indispensable factor in measuring performance. , 3 Performance Measuring Choice of a Research Methodology [9] mentioned that it is difficult to measure the performance of CPA through direct testing and measuring of human motion. Qualitative methods Improvements This method includes conducting literature reviews, case studies, and expert interviews to gather insights into the motivations, challenges, and emerging trends in the field of CPA. Performance Quantitative methods [9] suggested that if cantilever of CPA was orientated at 70o, the measurement of generated peak power will be the most accurate. Quantitative research were employed to gather numerical data on CPA performance metrics, such as sensitivity, power output, and fabrication parameters. This data was essential for assessing the technical capabilities and limitations of CPAs, as well as identifying trends and patterns across different studies and applications. From [4], sensitivity can be improved by increasing the planar beam dimensions b and L. The Beam length L is limited by die size, while beam width b is limited primarily by fabrication constraints during the structural release step. Beam length L is limited by die size, while beam width b is limited primarily by fabrication constraints during the structural release step. Material [8] suggest the method to improve the piezoelectric material, such as proper crystallographic orientation, composition control, stress state control, doping, development of imprint. Specifically, Niobium (Nb5+) is a dopant (donor) that could improve dielectric constant and piezoelectric responsiveness though improved domain wall motion [8]. [1] preferred Zinc Oxide (ZnO) over Lead Zirconate Titanate (PZT) due to the large parasitic capacitance within PZT, as PZT has a greater dielectric constant, albeit a greater piezoelectric coefficient. Selection of the Case The selected case needed to align with the research objectives and scope of the study. Moreover, it extensively addressed the detailed fabrication steps, highlighted some performance metrics such as sensitivity and resonant frequency, as well as elaborated of the working principle of CPA. ABOUT THE CASE Description [1] highlighted the structural design of the CPA. In this context, the cantilever was configured at fixedfree configuration. The functional piezoelectric layer (ZnO), top electrode (Platinium) and bottom electrode (PolySi) were fabricated following the Figure 2. Performance Measuring [9] suggested piezoelectric shaker test as a simple and efficient way to gauge the capability of the sensor. HOW THIS CASE WAS STUDIED To situate this case study in the context of existing literature and methodologies, a comprehensive mixed-methods approach was adopted, combining quantitative and qualitative data analysis. By integrating findings from diverse sources, including industry reports, academic journals, and patent databases, the research sought to provide a thorough examination of CPAs' current state, offering insights into the challenges and opportunities within this field. Figure 2: Schematic Diagram of a CPA 4 Working Principle Figure 5: LPCVD of Si (Covering Bare Silicon Surfaces, PolySi and PSG sacrificial Layer) The working principle of the cantilever piezoelectric accelerometer involves the conversion of mechanical stress into an electrical signal. From [1], a vertical acceleration, above the cantilever, capable of deflecting the cantilever, this further creating a longitudinal stress in axis 1. This stress alters the electric polarization within the piezoelectric ZnO layer, which is polarized perpendicularly to the substrate. As a result, an electrical charge is produced, proportional to the applied stress, due to the piezoelectric effect. The generated charge is then collected by the top and bottom conducting layers, creating a measurable voltage output indicative of the acceleration experienced by the sensor. Figure 6: LPCVD of Si3N4 as Stress-compensation Layer Figure 7: RF-magnetron sputtering of ZnO as Piezoelectric Material Fabrication Process Table III outlines various processes involved in the fabrication of CPA. Figure 8: Sputtering of Pt as Top Electrode TABLE III CPA FABRICATION PROCESS Process Chemical Vapor Deposition (CVD) Reactive Ion Etching (RIE) Low-Pressure Chemical Vapor Deposition (LPCVD) RF-magnetron sputtering Sputtering Description Deposition of SiO2 Deposition of Si3N4 Deposition and patterning of ntype PolySi Deposition and patterning of Phosphosilicate glass (PSG) Deposition and patterning of Silicon (Si) Deposition of Si3N4 Deposition of ZnO Thin film sputtering of Platinium (Pt) Parameters 300-400°C, 100 nm thick 700-800°C, 100 nm thick 620-650°C, 300-500 nm thick 400-450°C, 2 µm thick 650-700°C, 2 µm thick 750-800°C, 300 nm thick 200-300°C, 0.5 µm thick Room temperature to 200°C, 0.2 µm thick Figure 3: CVD of SiO2 (bottom) and Si3N4 (top) as Insulating Layers Figure 4: RIE of Phosphorus-doped PolySi as Electrical Contacts (Bottom Electrode) Application [9] mentioned the CPA could be applied in an energy harvesting device from piezoelectric patches located inside the shoe, with the pressure created by human weight. Nevertheless, this technology could not harvest sufficient amount of energy due to the low frequency property of human walking. NVH Acceleration Sensors for E—mobility Testing From the European Test and Telemetry Conference 2022, [10] highlighted the challenges faced on the quality of measurement of the e-vehicles (EVs) related to piezoelectric vibration and acceleration sensor. Specifically, hybrid and EVs leverage Noise, Vibration, Harshness (NVH) testing. However, NVH possess challenges in addressing complex vehicle’s structure and external signal interference such as stray electrical signals presented during testing. Moreover, the technological advancement in emobility and hydrogen vehicles also drives the sensors to have the ability to generate reproducible and reliable data in various environments. [10] suggested the use of ICP® accelerometers (IEPE), abbreviated for “Integrated Circuit Piezoelectric”, to be used for NVH testing. This sensor integrates built-in microelectronics that are responsible for converting the high-impedance charge signal generated by a piezoelectric sensing element into a low-impedance voltage signal. This voltage signal is more easily transmitted over standard two-wire or coaxial cables to any data acquisition system or readout device. Moreover, this sensor could 5 simplify the process of interfacing with piezoelectric sensing elements by providing integrated electronics that handle signal conditioning. (Transducers ’97), pp. 1205–1208, Jun. 1997. doi:10.1109/sensor.1997.635423 [5] IEEE Xplore, IEEE Xplore, https://ieeexplore.ieee.org/Xplore/home.jsp (accessed Mar. 20, 2024). [6] Nature, Nature, https://www.nature.com/ (accessed Mar. 20, 2024). [7] C.-Y. Li et al., “Design and development of a low-power wireless MEMS lead-free piezoelectric accelerometer system,” IEEE Transactions on Instrumentation and Measurement, vol. 72, pp. 1–11, 2023. doi:10.1109/tim.2023.3242016 Figure 9: ICP® System Schematic [10] CONCLUSIONS, LIMITATIONS, AND SUGGESTIONS FOR FUTURE RESEARCH The case study on micromachined CPA provides valuable insights into their design, fabrication, and applications. Several limitations of this case study include limited scope of study, such as limited to specific aspects of CPA design and application, which may overlook some broader contextual factors. Future works in the field of CPA include continued exploration of alternative materials and fabrication techniques to enhance CPA performance and reliability. [8] H. G. Yeo and S. Trolier-McKinstry, “Effect of piezoelectric layer thickness and poling conditions on the performance of cantilever piezoelectric energy harvesters on Ni foils,” Sensors and Actuators A: Physical, vol. 273, pp. 90–97, Apr. 2018. doi:10.1016/j.sna.2018.02.019 [9] I. Izadgoshasb et al., “Optimizing orientation of piezoelectric cantilever beam for harvesting energy from human walking,” Energy Conversion and Management, vol. 161, pp. 66– 73, Apr. 2018. doi:10.1016/j.enconman.2018.01.076 [10] S. Meyer, “4.1 the challenge of E-mobility and evtols on measurement technology with vibration and acceleration sensors,” Proceedings - ettc2022, 2022. doi:10.5162/ettc2022/4.1 REFERENCES [1] C. Liu, V. B. Mungurwadi, and A. V. Nandi, Foundations of MEMS. Prentice Hall, Upper Saddle River, 2012 [2] Walter P. The history of the accelerometer: 1920s-1996—prologue and epilogue, 2006. Sound and Vibration. 2007; 41(1):84–92. [3] N. N. Hewa-Kasakarage, D. Kim, M. L. Kuntzman, and N. A. Hall, “Micromachined Piezoelectric Accelerometers via epitaxial silicon cantilevers and bulk silicon proof masses,” Journal of Microelectromechanical Systems, vol. 22, no. 6, pp. 1438–1446, Dec. 2013. doi:10.1109/jmems.2013.2262581 [4] D. L. DeVoe and A. P. Pisano, “A fully surfacemicromachined Piezoelectric Accelerometer,” Proceedings of International Solid State Sensors and Actuators Conference Eric Sia Siew Wei is currently pursuing his B.Eng. degree in electrical & electronics engineering (Hons.), in field of Electronics & Devices (E&D), at Institute of Technology PETRONAS (UTP), Malaysia, completing it in 2024. His research interests are in computer applications and the Internet of Things.
