Influence of Fluid-Structure Interaction on Microcantilever Vibrations: Applications to Rheological Fluid Measurement and Chemical Detection

At the microscale, cantilever vibrations depend not only on the microstructure’s properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantilever’s vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include inplane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range

Effect of Hydrodynamic Force on Microcantilever Vibrations: Applications to Liquid-Phase Chemical Sensing

At the microscale, cantilever vibrations depend not only on the microstructure’s properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantilever’s vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, and improve sensitivity in sensing applications, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include in-plane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. However, such systems require sampling (no in-situ measurements) and a relatively large sample volume (a few milliliters). Moreover, the frequency range is limited to low frequencies (less than 200Hz). To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range

Poly(Dimethylsiloxane) Magnetically-actuated Variable Optical Attenuator

Projecte realitzat en col.laboració amb l'IFCOThe idea of mixing optical and magnetic properties in an only device has been the seed of this MSc Thesis. As a result of all the work done a Magneticallyactuated Variable Optical Attenuator (M-VOA) has been developed. Ferrofluid and poly(Dimethylsiloxane) (PDMS) are the raw and unique material needed for the manufacturing of the M-VOA. The experimental results of the precalibation of the Hall probe and the output voltage in function of the distance between the magnet and the MVOA’s are shown in this MSc Thesis and they are in agreement with the theory

Active metamaterial devices at terahertz frequencies

Electromagnetic metamaterials have emerged as a powerful tool to tailor the electromagnetic material properties and control wave propagation using artificial sub-wavelength structures. During the past fifteen years, metamaterials have been intensively studied over the electromagnetic spectrum (from microwave to visible), giving rise to extraordinary phenomena including negative refractive index, invisibility cloaking, sub-diffraction-limit focusing, perfect absorption, and numerous novel electromagnetic devices and optical components. The terahertz regime, between 0.3 THz and 10 THz, is of particular interest due to its appealing applications in imaging, chemical and biological sensing and security screening. Metamaterials foster the development of terahertz sources and detectors and expand the potential applications of the terahertz technology through the realization of dynamic and tunable devices. The objective of this thesis is to present different mechanisms to implement active terahertz metamaterial devices by incorporating advanced microelectromechanical system technology. First, an optical mechanism is employed to create tunable metamaterials and perfect absorbers on flexible substrates. A semiconductor transfer technique is developed to transfer split ring resonators on GaAs patches to ultrathin polyimide substrate. Utilizing photo-excited free carriers in the semiconductor patches, a dynamic modulation of the metamaterial is demonstrated. Additionally, this thesis investigates how sufficiently large terahertz electric fields drive free carriers resulting in nonlinear metamaterial perfect absorbers. Second, a mechanically tunable metamaterial based on dual-layer broadside coupled split ring resonators is studied with the help of comb drive actuators. One of the layers is fixed while the other is laterally moved by an electrostatic voltage to control the interlayer coupling factors. As demonstrated, the amplitude and phase of the transmission response can be dynamically modulated. Third, a microcantilever array is used to create a reconfigurable metamaterial, which is fabricated using surface micromachining techniques. The separation distance between suspended beams and underlying capacitive pads can be altered with an electrostatic force, thereby tuning the transmission spectrum. The tuning mechanisms demonstrated in this thesis can be employed to construct devices to facilitate the development and commercialization of new compact and mechanically robust metamaterial-based terahertz technologies.2017-11-05T00:00:00

Active metamaterial devices at terahertz frequencies

Electromagnetic metamaterials have emerged as a powerful tool to tailor the electromagnetic material properties and control wave propagation using artificial sub-wavelength structures. During the past fifteen years, metamaterials have been intensively studied over the electromagnetic spectrum (from microwave to visible), giving rise to extraordinary phenomena including negative refractive index, invisibility cloaking, sub-diffraction-limit focusing, perfect absorption, and numerous novel electromagnetic devices and optical components. The terahertz regime, between 0.3 THz and 10 THz, is of particular interest due to its appealing applications in imaging, chemical and biological sensing and security screening. Metamaterials foster the development of terahertz sources and detectors and expand the potential applications of the terahertz technology through the realization of dynamic and tunable devices. The objective of this thesis is to present different mechanisms to implement active terahertz metamaterial devices by incorporating advanced microelectromechanical system technology. First, an optical mechanism is employed to create tunable metamaterials and perfect absorbers on flexible substrates. A semiconductor transfer technique is developed to transfer split ring resonators on GaAs patches to ultrathin polyimide substrate. Utilizing photo-excited free carriers in the semiconductor patches, a dynamic modulation of the metamaterial is demonstrated. Additionally, this thesis investigates how sufficiently large terahertz electric fields drive free carriers resulting in nonlinear metamaterial perfect absorbers. Second, a mechanically tunable metamaterial based on dual-layer broadside coupled split ring resonators is studied with the help of comb drive actuators. One of the layers is fixed while the other is laterally moved by an electrostatic voltage to control the interlayer coupling factors. As demonstrated, the amplitude and phase of the transmission response can be dynamically modulated. Third, a microcantilever array is used to create a reconfigurable metamaterial, which is fabricated using surface micromachining techniques. The separation distance between suspended beams and underlying capacitive pads can be altered with an electrostatic force, thereby tuning the transmission spectrum. The tuning mechanisms demonstrated in this thesis can be employed to construct devices to facilitate the development and commercialization of new compact and mechanically robust metamaterial-based terahertz technologies.2017-11-05T00:00:00

산소 플라즈마 애싱 공정을 이용한 응력 구배 MEMS 외팔보가 있는 Ka밴드 대역 가변형 메타물질 흡수체

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2022. 8. 김용권.This dissertation proposes and realizes the first Ka-band frequency tunable metamaterial absorber with stress-induced MEMS cantilever with oxygen plasma ashing process. To employ a MEMS-driven actuator for LC resonance frequency tuning method in the GHz regime, the split-ring resonator (SRR) structure of the metamaterial unit cell is designed to have a sub-mm scale cantilever as a capacitor element of the unit cell. To enlarge capacitance change, the MEMS cantilever is released with a large out-of-plane deflection by the plasma ashing process. This MEMS cantilever with stress gradient is arranged at four parts of a symmetrical SRR unit cell, and the two cells compose the absorber sample as an array structure. The overall cantilevers of the absorber actuate from the initial bent upward state to pulled down state when the electrostatic voltage is applied. The decrease of deflection reduces the gap between cantilevers and bottom electrodes to increase capacitance for frequency tuning to lower frequency. To verify and improve the uniformity of the mechanical behavior of the absorber, this research proposes and demonstrates 3 different design types of releasing on stress-induced cantilevers. First, the array design of 12 cantilevers with 400 μm in length and 50 μm widths is modified from a cantilever with 400 μm in length and 800 μm widths. To overcome the limitation on the mechanical behavior of cantilever arrays due to their nonuniformity, further modification on etching hole rearrangement is reflected in the 2nd type of rectangular cantilever. The space length of etch hole varies depending on the position from the open end of cantilevers. This incremental space length between 8 μm etch holes from the open end enables sequential releasing of cantilevers during photoresist oxygen plasma ashing. The cyclic process was performed in the ashing process to lower the distribution of fabrication results. Finally, the last design to have a semicircle shape with incremental space length between etching holes to improve the uniformity of the cantilever to prevent such drawbacks of a wrinkled profile which the previous design shows. Also, our last design is driven by a digital drive creating 5 different reconfiguration states. With full-wave simulations, the performance of the proposed absorber demonstrates experimentally in each of 5 different reconfiguration states. The initially measured deflection of the cantilever beam is 51.8 μm on average. At the initial state, the resonant frequency and the absorptivity are 32.95 GHz and 80.95%. When all the cantilevers are pulled down, the frequency shifts a total of 4.08 GHz from the initial state showing a tuning ratio of 12.29 %. The error between the measured value and the simulation value came within 0.39 GHz in all five states. This dissertation demonstrated the potential of MEMS as a tuning method for Ka-band absorbers.이 논문은 산소 플라즈마 애싱 공정을 사용하여 응력구배 MEMS 외팔보를 사용한 최초의 Ka 대역 주파수 가변 메타물질 흡수체를 제안하고 검증하였다. GHz 영역에서 LC 공진 주파수 가변 방식에 MEMS 액추에이터를 구동하기 위해 메타물질 단위 셀인 분할링공진기 구조는 mm 스케의 외팔보를 정전용량의 요소를 갖도록 설계하였다. 정전용량 변화를 최대화하기 위해 MEMS 외팔보는 플라즈마 애싱 공정에 의해 수직 방향으로 큰 편향차를 갖도록 설계하였다. 응력구배 MEMS 외팔보는 대칭의 분할링 공진기 구조의 단위 셀 4곳에 배열되고 두 셀은 배열구조로 설계되었다. 이 때, 풀인 전압 이상의 높은 전압을 인가 시 외팔보는 바닥전극에 붙게 되어 정전용량을 키우고 LC 공진 주파수를 낮춘다. 흡수체의 기계적 거동에 대한 균일성을 개선하기 위해 총 3가지 다른 모양의 외팔보를 설계하였다. 먼저 길이 400μm, 너비 50μm인 외팔보를 12개의 배열상태로 설계하여 컴퓨터 시뮬레이션과 측정값을 확인하였다. 첫 번째 흡수체의 플라즈마 애싱 공정을 통한 외팔보 구현 공정의 결과, 96개의 외팔보의 평균 값은 41.5 μm이고 표준 편차는 15.4 μm였다. 첫 번째 흡수체의 경우 제작 외팔보의 산포가 상당히 큼에도 불구하고, 15 V까지 아날로그 튜닝을 하여, 초기 상태의 28 GHz의 공진주파수에서 25.5 GHz의 공진주파수 변화하여 총 2.5 GHz의 주파수 가변범위를 도달하였다. 반사계수는 초기 -5.68 dB에서 -33.60 dB까지 변화하였고, 투과 계수의 경우 -40에서 -60 dB를 유지하였다. 흡수율 계산 결과, 각 공진 주파수에서의 흡수율은 0 V일 때 72.9%에서 계속 증가하며 15 V일때 99.9%의 흡수율을 도달하였다. 그럼에도 불구하고, 외팔보 어레이 갖는 넓은 편향값 산포가 컴퓨터 시뮬레이션과의 괴리가 있어 개선된 설계를 다시 시도하였다. 앞선 설계의 단점을 극복하기 위해 점진적으로 증가하는 패턴의 식각 구멍을 외팔보에 적용하였다. 이 두 번째 구조 또한 제작, 컴퓨터 계산 및 실험 검증하였다. 이러한 식각 구멍 패턴은 외팔보를 산소 플라즈마 애싱 공정으로 구현 시, 제작 균일성을 크게 증가시킨다. 나아가 플라즈마 애싱 공정 또한 시간을 분할하여 제작함으로써 균일도를 크게 증가시킨다. 또한 두번째 설계부터는 응력 구배로 인한 큰 편향을 갖는 외팔보가 갖는 비평형 구동 방식의 해석 어려움에 따라 전압을 개별적으로 인가하며 on/off 형태의 디지털 구동방식으로만 구동하게끔 시스템 구동방식을 변경하였다. 2개의 메타물질 단위 구조에 4개의 전극을 분리하여 총 5개의 구조적으로 다른 상태를 구현하였다. 모든 외팔보가 위로 휘어진 상태에서 전극에 전압을 순차적으로 인가하여 2개씩 바닥에 붙게 하여 최종적으로 모든 외팔보가 바닥에 붙게 하였다. 두 번째 흡수체의 경우, 외팔보 구현이 크게 개선됨에도 불구하고 초기 32.24 GHz의 공진 주파수 값에서 2.14 GHz만 변화하여 최종 30.10 GHz의 공진주파수 측정 결과를 보였다. 흡수율의 경우에도 초기 83.59%에서 최종 90.75%의 결과를 보였지만 컴퓨터 계산과 많은 차이를 보였다. 최종적으로 앞선 2개의 설계를 보완한, 최종 진화한 형태인, 반원형 응력 구배 외팔보를 갖는 흡수체를 설계, 제작, 및 실험 검증하였다. 특히, 외팔보가 갖는 불안정한 기계적 거동을 단순화하여 디지털 구동을 하게끔 흡수체를 설계하였다. 식각 패턴의 거리를 2 μm씩 늘리며 반원 형태의 외곽으로부터 설계한 결과 재현성과 균일성이 매우 크게 개선되었다. 특히 반원 형태의 외팔보의 경우 최고점 편향 높이가 항상 반원 중간에서 구현되기 때문에 반원형 외팔보 간의 모양이 균일하게 유지된다. 제작된 18개 흡수체 샘플에서 144개의 외팔보를 측정한 결과 평균 편향 높이의 평균 값이 51.8 μm였으며 표준 편차는 3.1μm였다. 4개의 전극에서 기반한 5개 상태의 서로 다른 구조에 따른 반사 계수와 투과 계수를 도파관 측정으로 실험 값을 얻었다. 상용 유한요소법 컴퓨터 계산과 비교 검증하였다. 초기 상태에서 공진 주파수는 32.95 GHz였고, 모든 외팔보가 풀인 전압 인가로 인해 바닥 전극에 붙으면 하면 주파수 28.87 GHz가 되어 총 4.08GHz 이동하여 12.29%의 주파수 가변율을 갖는다. 측정값과 유한요소 시뮬레이션 값의 오차는 5개 상태 모두에서 0.39GHz 이내였다. 흡수율의 경우 각 상태에서 80.95 %, 88.17 %, 86.29 %, 99.21 %, and 86.51% 값을 보였다. 이 논문은 Ka-대역 흡수체의 튜닝 방법으로서 MEMS의 가능성을 보여주었다.CHAPTER 1. Introduction 1 1.1 Background 1 1.1.1 Advent of metamaterial 1 1.1.2 Application of metamaterial 2 1.1.3 Physics of metamaterial 3 1.1.4 Meta-atom 10 1.1.5 Electromagnetic absorber to metamaterial absorber 14 1.1.6 Reconfigurable metamaterial 17 1.1.7 MEMS reconfigurable metamaterial 21 1.1.8 Tunable metamaterial absorber 24 1.1.9 MEMS reconfigurable metamaterial absorber 27 1.1.10 Tunable metamaterial absorber for Ka-band 29 1.2 Originality and contribution 32 1.3 Document structure 33 CHAPTER 2. Stress-induced sub-mm scale cantilever 34 2.1 Initial design 34 2.2 Cantilever arrays with stress gradient 37 2.2.1 Preliminary experiment 37 2.2.2 Design 38 2.2.2 Fabrication results 39 2.3 Rectangular shape sub-mm scale cantilever with incremental etch hole spacing 40 2.3.1 Preliminary experiment 40 2.3.2 Design 42 2.4 Semicircular sub-mm scale cantilever with incremental etch hole 49 2.4.1 Design of semicircular sub-mm scale cantilever with incremental etch hole 49 2.4.2 Fabrication 52 CHAPTER 3. The 1st design of MEMS tunable metamaterial absorber with cantilever arrays for continuous tuning 57 3.1 General overview 57 3.2 Design 59 3.2.1 Split ring resonator and simulation 61 3.2.2 Capacitance of cantilever with stress gradient 64 3.2.3 Electrostatic driving of cantilever 67 3.2.4 Stress analysis & PR ashing 69 3.3 Fabrication 71 3.3.1 Fabrication process 71 3.3.2 Fabrication results 74 3.4 Simulation 77 3.5 Experiment 81 3.5.1 Experiment setup 81 3.5.2 Experiment results 84 3.6 Summary 86 CHAPTER 4. The 2nd design of MEMS tunable metamaterial absorber with rectangular shape sub-mm scale stress-induced cantilever with an incremental etch hole spacing for digital driving 87 4.1 General overview 87 4.2 Design 90 4.3 Fabrication 93 4.4 Simulation 98 4.5 Experiment 100 4.6 Summary 102 CHAPTER 5. The 3rd design of MEMS tunable metamaterial absorber with semicircular sub-mm scale stress-induced cantilever with an incremental etch hole spacing for digital driving 103 5.1 General overview 103 5.2 Design 107 5.2.1 Electromagnetic properties 107 5.2.2 Design parameter 109 5.3 Fabrication 112 5.4 Simulation 119 5.4.1 Simulation setup 119 5.4.2 Simulation results 122 5.5 Experiment 126 5.5.1 Experiment setup 126 5.5.2 Preliminary experiment 130 5.5.2 Experiment results 133 5.6 Further Analysis 137 5.6.1 The waveguide simulation 137 5.6.2 The periodic metamaterial unit cell simulation 141 5.6.3 Analysis on the surface current 148 5.7 Summary 152 5.7.1 Summary of the 1st, 2nd, and 3rd design 152 5.7.2 Comparison with MEMS tunable metamaterial absorber 155 5.7.3 Comparison with Ka-band tunable metamaterial absorber 157 CHAPTER 6. Conclusion 159 Bibliography 161 초록 (국문) 180박

Conceptual MEMS Devices for a Redeployable Antenna

Micro-Electro-Mechanical Systems (MEMS) are becoming an integral part of our lives through a wide range of applications, including MEMS accelerators for air bag deployment in vehicles, micromirrors in projection devices, and various sensors for chemical/biological applications. MEMS are a key aspect of ever-increasing significance in a myriad of commercial and military applications. Because of this importance, this thesis utilizes MEMS devices that can deploy and retract an antenna suitably sized for placement on an insect or microrobot for communication purposes. A target monopole antenna with a length of 1 mm was used as a test metric. From this requirement, several MEMS designs using scratch drives and thermal actuators as the basis for powering the motor were developed. Some of the fabricated and tested designs included a gear with side flaps that flip up perpendicular to the substrate; gears that push an antenna beam off the edge of the substrate; and an antenna beam that is moved upwards such that it stands perpendicular to the substrate. These designs had the highest likelihood of success. Other designs included an array of micro gears and guiding beams, a large wheel powered by scratch drives, and a gear with the pawl requiring assembly. For these designs to be successful, several basic modifications would be necessary. The antenna beam that moves into a position perpendicular to the substrate was successfully self-assembled

Secondary Resonances of Electrostatically Actuated Mems Cantilevers

In this work the behavior of micro-electromechanical (MEMS) cantilever resonators is investigated. The cantilever resonators are electrostatically actuated with hard AC voltage resulting in nine distinct resonances cases including super and subharmonic resonances. The amplitude frequency and amplitude voltage bifurcation diagrams are obtained for each of the nine resonance cases. Reduced order models (ROMs) are developed to include one and two modes of vibration. Three different methods are used to solve the ROMs namely 1) the method of multiple scales (MMS), which is a perturbation method used for one mode of vibration, 2) the homotopy analysis method (HAM), which is an analytical method rooted in topology used for one mode of vibration, and 3) direct numerical integration using two modes of vibration (2T ROM) resulting in time responses of the tip of the MEMS cantilever resonator. The MMS and HAM solutions directly yield steady state solutions for the bifurcation diagrams while the 2T ROM is used to validate and demonstrate the limitations of these two methods. The effects of damping, fringe, voltage, and detuning frequency are investigated