MODELING AND CHARACTERIZATION OF LOW STRESS SILICON ELECTROSTATIC TRANSDUCERS FOR HIGH SENSITIVE NON-DESTRUCTIVE TESTING APPLICATION

MODELING AND CHARACTERIZATION OF LOW STRESS SILICON ELECTROSTATIC TRANSDUCERS FOR HIGH SENSITIVE NON-DESTRUCTIVE TESTING APPLICATION.

Content. Introduction Classifications Working Principle Advantages of Electrostatic Transducer Literature Review Scope of the Study Objectives Methodology Tentative Chapterization References.

Non Destructive Testing. Phased Array Ultrasonic Testing (PAUT).

4. RMS RMS I. Fig 1: NDT of Pressure Vessels. Fig 2: NDT of Ship hulls.

Fig 6: Ultrasonic NDT. Radiography Testing X-ray Tube Gamma Source.

Why Ultrasound for NDT?. Depth of penetration : Inspection of thick section/Internal flaws High sensitivity to defects : Small defects (up to nm range) Accuracy : Defect size, depth, location and orientation. Real-time monitoring : During operation or load conditions Versatility : Wide range of materials including metals, plastics and composites. Safety : UT is safe and environmentally friendly.

Frequency Range Description 0.1 MHz to 2 MHz Detectable defect sizes: Typically larger than 3 mm. Used for thick materials and deep penetration applications 2 MHz to 5 MHz Detectable defect sizes: Typically between 1 mm and 3 mm. Used for thin materials and surface-breaking defects 5 MHz to 10 MHz Detectable defect sizes: Typically between 0.5 mm and 1 mm. Used for thin materials and surface-breaking defects >10 MHz Detectable defect sizes: Typically smaller than 0.5 mm. Used for extremely fine surface and near-surface defect detection.

Crack Detection. Crack depth: 0.3 mm Dimension of the plates: 600 mm × 3 mm × 70 mm Notch dimension : 0.3 mm × 0.4 mm Center Frequency : 1 MHz Stand off distance : (50 – 300) mm Acoustic pressure : 320 Pa.

Types of Ultrasonic Transducer. Capacitive transducers measure changes in capacitance, while piezoelectric transducers generate an electric charge in response to mechanical stress..

Piezoelectric transducers leverage the , where certain materials generate an electric charge under mechanical stress. They are widely used in , , and , showcasing their versatility in modern technology..

Capacitive transducers utilize to create a capacitor. Changes in the distance or area between plates affect the , enabling precise measurements of displacement, pressure, and more in various fields such as and ..

Electrostatic transducers operate on the principle of . When an electric field is applied, it induces a in the transducer's diaphragm. This conversion process is essential for both and applications..

Advantages of Electrostatic Transducers. ,. The advantages of electrostatic transducers include , and.

Challenges and Limitations. Low acoustic power output: The acoustic power output of a CMUT is limited by the small size of the membrane. This can be a problem for applications that require high acoustic power, such as medical imaging and non-destructive testing. Sensitivity to environmental conditions: The performance of a CMUT can be affected by environmental conditions such as temperature and humidity. This can be a problem for applications where the CMUT is exposed to harsh environments. Bandwidth: The bandwidth of a CMUT is limited by the resonance frequency of the membrane. This can be a problem for applications that require a wide bandwidth, such as ultrasound imaging..

Literature Review. Suzuki et al. (1989) pioneered a silicon IC process-based transducer with high sensitivity and electronic sector scanning capabilities. Haller & Khuri-Yakub (1996, 1994) further advanced this technology, creating surface micromachined transducers operating at MHz frequencies with improved efficiency and bandwidth compared to conventional PZT transducers. They developed an electrical equivalent circuit model and demonstrated a transmit-receive system with a 34 dB signal-to-noise ratio. Wright et al. (1994) utilized a micromachined silicon electrostatic transducer to detect laser-generated ultrasound in various materials, observing bulk, Rayleigh, and Lamb waves. This non-contact system showed good agreement with contact capacitance device measurements. These studies collectively highlight the potential of silicon-based electrostatic transducers for diverse applications, including ranging, non-destructive evaluation, and materials characterization, with the added benefit of potential integration with electronic scanning circuits..

The development of flexible phased array transducers has significantly improved ultrasonic non-destructive testing of components with complex geometries. These smart transducers can adapt to irregular surfaces, minimizing coupling issues and beam distortions that often plague conventional contact probes (S. Chatillon et al., 2000; O. Roy & M. Chatillon, 2000). The flexible arrays incorporate embedded profilometers to measure surface distortions and compute optimized delay laws in real-time, preserving beam characteristics despite profile variations (O. Casula et al., 2023). This technology has evolved from 2D to 3D flexible probes, enabling efficient inspection of components like elbows, nozzles, and other 3D geometry parts in nuclear power plant cooling circuits (O. Casula et al., 2004). The flexible phased array approach offers improved sensitivity, accurate defect localization, and better coverage of scanned areas compared to traditional methods, making it a valuable tool for inspecting components with irregular geometries in various industrial applications (S. Chatillon et al., 2000; O. Roy & M. Chatillon, 2000)..

Zhang et al. (2014) developed a surface-micromachining process using dc-sputtered amorphous SiC membranes with a thermal budget of 200°C, resulting in high-strength CMUTs. Li et al. (2015) proposed a wafer bonding method using transition metal layers at temperatures below 350°C, improving membrane uniformity. Tsuji et al. (2010) demonstrated a low-temperature (<400°C) wafer bonding technique for CMUTs with single-crystal silicon plates, achieving high yield and consistent performance. Cicek et al. (2017) introduced a multi-level surface microfabrication technology using SiC structural layers at 200°C, allowing independent control of submicron vertical and lateral gaps without high-resolution lithography. These advancements in low-temperature fabrication processes enable the integration of CMUTs and MEMS devices with ICs, potentially improving overall device performance and reducing parasitic capacitance..

Scope of the Study. We opted for electrostatic transducer over PZT due to their high sensitivity, broader bandwidth, higher resolution imaging and improved detection capabilities. Additionally, CMUTs can be miniaturized, making them suitable for applications requiring arrays of transducers which can be difficult with the conventional piezo based processing techniques . Furthermore, CMUTs exhibit better reliability and mechanical strength compared to PZT transducers, reducing the risk of failure during NDT operations. This research aims to advance NDT capabilities by exploring low stress materials for resilient membrane and high young modulus to achieve higher frequencies with optimal impedance matching. With improved material properties and optimizing the design of transducer devices, this study seeks to push the boundaries of current NDT technologies, enabling more accurate and reliable inspections in various industrial applications..

Objectives. To determine the device capacitance and the electrostatic force of attraction with the inclusion of fringing fields. To analyze the membrane displacement and yield under static bias. To analyze the natural frequency of the electrostatic transducer and impedance profile. To determine the transducer efficiency with coupling coefficient and pull-in voltage. To determine the fracture limit of the structure with stress and strain analysis. To analyse the performance of the electrostatic transducer under package stress. To analyze the frequency response of the structure under applied ac signal. To analyse the acoustic pressure for a single cell and 1D array structure..

Methodology. The analytical model based on the electromechanical model is to be measured in the proposed work. From these models, the natural frequencies of the electrostatics transducer can be measured and verified simultaneously. Measuring the active and passive capacitance of the model. Measuring the capacitive force between the electrodes after applying the biasing on the electrodes. Measuring the membrane displacement under different applied bias. Measuring the natural frequency of the membrane from the model. To further investigate the concept and designs, a detailed analytical model analysis based on the results of the FEM simulation will be conducted. Analytical model and FEM simulation results will be taken for validation, and results will be correlated with published experimental work..

Tentative Chapterization. Chapter 1: Introduction. Chapter 2: Review of Electrostatics Transducer Chapter 3: Capacitance and Electrostatic Force Chapter 4: Membrane Displacement Profile Chapter 5: Eigen Frequency and Mechanical Impedance Chapter 6: Acoustic Pressure Characterization Chapter 7: Conclusion and Future scope of the Study References Bibliography Appendices.

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