Stretching the limits of dynamic range, shielding effectiveness, and multiband frequency response

In this dissertation, an RF MEMS variable capacitor suitable for applications requiring ultrawide capacitive tuning ranges is reported. The device uses an electrostatically tunable liquid dielectric interface to continuously vary the capacitance without the use of any moving parts. As compared to existing MEMS varactors in literature, this device has an extremely simple design that can be implemented using simple fabrication methods that do not necessitate the use of clean room equipment. In addition, this varactor is particularly suited for incorporating a wide range of liquid dielectric materials for specific tuning ratio requirements. Additionally, the shielding effectiveness performance of graphene-doped ABS thin films is investigated. The use of graphene as a replacement for metal fillers in composite EMI shielding materials is quickly becoming a widely-investigated field in the electromagnetic compatibility community. By replacing conventional metal-based shielding methods with graphene-doped polymers, low-weight, field-use temporary shielding enclosures can be implemented that do not suffer from mechanical unreliability and corrosion/oxidation like a traditional metal enclosure. While the performance of composite EMI shielding materials has not yet surpassed metals, the advantages of polymer-based shielding methods could find usage in a variety of applications. Finally, mutliband pre-fractal antennas fabricated via 3D printing are reported. These devices are the first to incorporate the advantages of 3D printing (rapid prototyping, fabrication of complex geometries otherwise unobtainable) with the advantages of self-similar antennas (increased gain and multiband performance) in a single device. The Sierpinski tetrahedron-based antenna design was both computationally modeled and physically realized to illustrate its potential as a solution to enable true multiband communication platforms

Piezo-Electrochemical Transducer Effect (PECT) Intercalated Graphite Micro-Electromechanical Actuators

The purpose of this research is to investigate the Piezo-Electrochemical Transducer (PECT) effect in intercalated graphite as a possible mechanism of actuation for micro-electromechanical systems (MEMS). This dissertation presents the results of research into the PECT effect in H2SO4-intercalated graphitized carbon fibers, including both electrical and mechanical characteristics of this effect. PECT fibers achieve up to 1.7% strain at 1.4 V of applied potential. In contrast, the piezoelectric material polyvinylidene difluoride (PVDF) generates only 0.01% strain and polysilicon thermal expansion between 0.02 and 0.06% strain depending on the thermal conductivity of the particular polysilicon that the actuators are fabricated in. This work concludes that PECT carbon fiber actuators achieve two orders of magnitude better strain than PVDF piezoelectric actuators and polysilicon thermal expansion in the same voltage range of operation. In addition to this highly improved strain, the devices, after an initial peak power consumption of 227 micronW, a PECT device uses only 260 nW to hold actuation. Although slow operation and unpractical intercalants are serious drawbacks to PECT actuators, the characteristics of strain and power consumption presented in this dissertation prove that PECT actuators, given some minor modifications, prove to be a competitive alternative to current MEMS actuators

Versatile High Performance Photomechanical Actuators Based on Two-dimensional Nanomaterials

The ability to convert photons into mechanical motion is of significant importance for many energy conversion and reconfigurable technologies. Establishing an optical-mechanical interface has been attempted since 1881; nevertheless, only few materials exist that can convert photons of different wavelengths into mechanical motion that is large enough for practical import. Recently, various nanomaterials including nanoparticles, nanowires, carbon nanotubes, and graphene have been used as photo-thermal agents in different polymer systems and triggered using near infrared (NIR) light for photo-thermal actuation. In general, most photomechanical actuators based on sp bonded carbon namely nanotube and graphene are triggered mainly using near infra-red light and they do not exhibit wavelength selectivity. Layered transition metal dichalcogenides (TMDs) provide intriguing opportunities to develop low cost, light and wavelength tunable stimuli responsive systems that are not possible with their conventional macroscopic counterparts. Compared to graphene, which is just a layer of carbon atoms and has no bandgap, TMDs are stacks of triple layers with transition metal layer between two chalcogen layers and they also possess an intrinsic bandgap. While the atoms within the layers are chemically bonded using covalent bonds, the triple layers can be mechanically/chemically exfoliated due to weak van der Waals bonding between the layers. Due to the large optical absorption in these materials, they are already being exploited for photocatalytic, photoluminescence, photo-transistors, and solar cell applications. The large breaking strength together with large band gap and strong light- matter interaction in these materials have resulted in plethora of investigation on electronic, optical and magnetic properties of such layered ultra-thin semiconductors. This dissertation will go in depth in the synthesis, characterization, development, and application of two- dimensional (2D) nanomaterials, with an emphasis on TMDs and molybdenum disulfide (MoS2), when used as photo-thermal agents in photoactuation technologies. It will present a new class of photo-thermal actuators based on TMDs and hyperelastic elastomers with large opto-mechanical energy conversion, and investigate the layer-dependent optoelectronics and light-matter interaction in these nanomaterials and nanocomposites. Different attributes of semiconductive nanoparticles will be studied through different applications, and the possibility of globally/locally engineering the bandgap of such nanomaterials, along with its consequent effect on optomechanical properties of photo thermal actuators will be investigated. Using liquid phase exfoliation in deionized water, inks based on 2D- materials will be developed, and inkjet printing of 2D materials will be utilized as an efficient method for fast fabrication of functional devices based on nanomaterials, such as paper-graphene-based photo actuators. The scalability, simplicity, biocompatibility, and fast fabrication characteristics of the inkjet printing of 2D materials along with its applicability to a variety of substrates such as plastics and papers can potentially be implemented to fabricate high-performance devices with countless applications in soft robotics, wearable technologies, flexible electronics and optoelectronics, bio- sensing, photovoltaics, artificial skins/muscles, transparent displays and photo-detectors

Lateral bending liquid crystal elastomer beams for microactuators and microgrippers

With the rapid development of microsystems in the last few decades, there is a requirement for high precision tools for micromanipulation and transportation of micro-objects, such as microgrippers, for applications in microassembly, microrobotics, life sciences and biomedicine. Polymer based microgrippers and microrobots executing various tasks have been of significant interest as an alternative to the traditional silicon and metal based counterparts due to the advantages of low cost fabrication, low actuation temperature, biocompatibility, and sensitivity to various stimuli. The exceptional actuation properties of liquid crystal elastomers (LCE) have made these materials highly attractive for various emerging applications in the last two decades. Large programmable deformations and the benefits offered by the elastic, thermal and optical properties of LCEs are suitable for implementing stimuli-responsive microgrippers as well as various biomimetic motion in soft robots. In this thesis, a method and the associated processes for fabrication and molecular alignment in LCE were developed, which enabled new functionality and improved performance of the LCE based microactuators and microgrippers, providing controlled response by thermal and remote photothermal actuation, and allowing easy integration of the LCE end-effectors into robotic systems for automated operation. Lateral bending actuation has been demonstrated in LCE microbeams of 900 µm of length and 40 µm of thickness, owing to the new monolithic micromolding technique using vertical patterned walls for alignment. The effects of parameters such as the beam width, the size of the microgrooves, and the surface treatment method on the behavior of the microactuators were studied; the internal alignment pattern of liquid crystals in the structure was investigated by different microscopy methods. An efficient method for finite element modeling of the bending LCE actuators was developed and experimentally verified, based on the gradient of equivalent thermal expansion in the multi-layer structure, which was able to predict the bending behavior of the actuators in a large range of thicknesses as well as rolling behavior of the actuators of tapered thickness. The novel LCE microgripper with in-plane operation showed efficient thermal and photothermal actuation, achieving the gripping stroke of 64 µm under the light intensity of 239 mW/cm2 for the gripper length of 900 µm, which is more efficient than the typical SU-8 polymer based microgrippers of the same dimensions. The LCE gripper was successfully demonstrated for the application in manipulation of the objects of tens to hundreds of micrometers in size. Therefore, the novel LCE microgripper bridges the gap in the LCE-based gripper technologies for typical object size in applications for systems microassembly, biological and cell micromanipulation. The lateral bending functionality enabled by the proposed method expands design opportunities for thermal and photothermal LCE microactuators, providing an effective route toward realization of new modes of gripping, locomotion, and cargo transportation in soft microrobotics and micromanipulation

CMOS-MEMS Scanning Microwave Microscopy

This thesis presents the design, fabrication and experimental validation of an integrated dual-mode scanning microwave microscopy (SMM)/Atomic Force Microscopy (AFM) system that does not require the use of a conventional laser-based AFM or external scanners. Microfabricated SMM probes are collocated with strain-based piezoresistive AFM probes in a CMOS-MEMS process, and are actuated by integrated electrothermal scanners. Integration of AFM enables dual-mode imaging (topography and electrical properties); it also enables control over tip-sample distance, which is crucial for accurate SMM imaging. The SMM (also known as Scanning Near-field Microwave Microscope and Scanning Evanescent Microwave Microscope) is the most well-known type of Scanning Probe Microscopes (SPM) that can quantify local dielectric and conductivity of materials. It has emerged as the most promising means for the fast, non-contact, and non-destructive study of materials and semiconductor devices. The CMOS-MEMS SMM devices are fabricated by using a standard foundry CMOS process, followed by an in-house mask-less post-processing technique to release them. Single-chip SMM/AFM devices with integrated 1-D and 3-D actuation are introduced. The CMOS-MEMS fabrication process allows external bulky scanners to be replaced with integrated MEMS actuators that are small and immune to vibration and drift. In this work, electrothermal MEMS actuators are utilized to scan the tip over the sample in 3 degrees of freedom, over a 13 µm x 13 µm x 10 µm scan range in the x, y, and z directions, respectively. Furthermore, the availability of polysilicon layers on the CMOS processes allows for on-chip integrated piezoresistive position sensing that obviates the need for the laser system. Vertical tip-sample distance control of a few nanometers is achieved with the integrated piezoresistive position sensors. These devices are used to modulate the tip-sample separation to underlying samples with a periodic signal, improving immunity to long-term system drifts. To improve the sensitivity of the CMOS-MEMS SMM, different types of matching networks for SMMs are thoroughly analyzed and closed form formulas are presented for each type. Based on the analyses, the stub matching method is selected to match the high tip-to-sample impedance to the 50 ohm characteristic impedance of the system. After that, with the help of lumped models and EM simulations, different sections of the CMOS-MEMS SMM system are analyzed and suggestions for selecting the best micro-transmission line and bonding-pad transmission lines are given. A measurement circuit for SMM is then presented and explained, showing how this measurement system can improve the output-signal-to-noise ratio and hence the sensitivity of microwave imaging. Calculations for the entire SMM system indicate that sub-attofarad tip-sample impedance can be measured. It is noteworthy that most of the analyses and suggestions given in this thesis can be applied to any Scanning Microwave Microscopes or, even more generally, to any microwave system that needs to sense a small signal. Finally, the measurement results for the fabricated CMOS-MEMS SMM are presented to verify the proposed methods. Several samples with sub-micron and nanometer feature sizes are imaged. A special test sample with no topography but with buried dielectric materials in grid and stripes is also designed and measured

DEVELOPMENT OF NANO/MICROELECTROMECHANICAL SYSTEM (N/MEMS) SWITCHES

Ph.DDOCTOR OF PHILOSOPH

MEMS Devices for Circumferential-scanned Optical Coherence Tomography Bio-imaging

Ph.DDOCTOR OF PHILOSOPH

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