<div> Large thermal gradients can cause very nonintuitive effects in the flowfield, as flow motion and even a force (often referred to as a Knudsen thermal force) can be induced even with a freestream velocity of zero. These flows can be exploited on the microscale, where temperature gradients of 10E6K/m are achievable. These flows have been studied experimentally many times, and it has been shown that Knudsen forces have a bimodal relationship with pressure, where the peak is in the transitional flow regime. It has also been shown that these thermal gradients cause thermal diffusion, or species separation in a mixture.</div><div> </div><div> A MEMS based device called the Microscale In-Plane Knudsen Radiometric Actuator (MIKRA) was developed to use Knudsen forces to calculate pressure and gas composition. The direct simulation Monte Carlo (DSMC) method was used to analyze the device to calculate the device forces and calculate the flowfield. DSMC proved to be a reliable method of simulating these types of flows, as the force results agreed well with experiments, and the DSMC results matched the results of other numerical methods.</div><div> </div><div> N2 and H2O mixtures were also simulated, and it was shown that the force is sensitive to the composition. At the same pressure, the force is larger for mixtures dominated by N2. Heat flux is also larger for N2 dominated flows.</div>

Neste trabalho foi desenvolvido um estudo sistemático das propriedades estruturais, mecânicas e ópticas de filmes de carbeto de silício amorfo hidrogenado (a-Si1-xCx:H) produzidos pela técnica de deposição química a vapor assistida por plasma (PECVD) a baixas temperaturas (320°C), utilizando silana (SiH4) e metano (CH4) como gases precursores do silício e carbono, com a finalidade de avaliar sua aplicabilidade em processos de microfabricação. Foram depositadas duas séries de filmes de a-Si1-xCx:H, com e sem diluição de hidrogênio da mistura gasosa, variando alguns parâmetros, como potência de RF, concentração de metano e fluxo de silana. Os filmes depositados com a mistura gasosa diluída em hidrogênio apresentaram valores maiores de módulo de elasticidade, dureza, gap óptico e índice de refração, comparados com os filmes depositados sem a adição de hidrogênio; no entanto esses filmes apresentaram também valores maiores de stress residual, ocasionando deformações e em alguns casos a quebra das microestruturas fabricadas. No caso das amostras depositados sem a adição de hidrogênio na mistura gasosa, os filmes com conteúdo de carbono maior que 45% depositados com baixa densidade de potência (50 mW.cm-2) apresentaram baixos valores de stress residual compressivo (menores que 60 MPa) e, as microestruturas fabricadas com eles mostraram que o material possui uma superfície livre de defeitos e uma excelente aderência ao substrato, mostrando a viabilidade de poder utilizá-lo como material estrutural e de mascaramento em processos de microfabricação. Além disso, esses filmes são transparentes para comprimentos de onda acima de 600 nm na região visível do espectro eletromagnético, apresentando-o como um material promissório para a fabricação de guias de onda. Finalmente, com o incremento da potência de RF para 100 W na deposição do filme quase estequiométrico com a mistura gasosa não diluída em hidrogênio, conseguiu-se duplicar a taxa de deposição para aproximadamente 12 nm/min, incorporando uma maior quantidade de carbono (~57%), porém também aumento o valor do stress residual compressivo para ~267 MPa. No entanto, as estruturas suspensas fabricadas com este material não apresentaram deformações notáveis. / In this work a study of structural, mechanical and optical properties of hydrogenated amorphous silicon carbide (a-Si1-xCx:H) films, obtained by plasma enhanced chemical vapor deposition (PECVD) at low temperatures (320°C), using silane (SiH4) and methane (CH4) as a gaseous precursors of silicon and carbon, respectively is performed, intending to assess their suitability in microfabrication processes. Two series of a-Si1-xCx:H films were deposited, with and without hydrogen dilution of the gaseous mixture, varying parameters, such as RF power, methane concentration and silane flow. The films deposited with hydrogen diluted gaseous mixture showed higher values of elastic modulus, hardness, optical gap and refractive index, compared with films produced without hydrogen dilution; however these films present higher values of compressive residual stress, causing deformations and in some cases cracks in the microstructures utilizing these films as structural material. In the case of the films deposited without hydrogen dilution of the gaseous mixture, those with carbon contents higher than 45% deposited at low power densities (50 mW.cm-2) presented lower values of compressive residual stress (lower than 60 MPa), and the microstructures fabricated with these materials do not show defects in the surface and have a good adherence to the substrate, demonstrating the feasibility of using these materials as structural and masking materials in microfabrication processes. Moreover, these samples are transparent to wavelengths larger than 600 nm in the visible range of the spectrum, making it suitable to the fabrication of waveguides. Finally, with the increase in the RF power to 100 W in the deposition process without hydrogen dilution a sample close to stoichiometry presented an increase in the deposition rate to approximately 12 nm/min and in the carbon content (~57%); however the compressive residual stress also increased. In spite of that, the microstructures fabricated with this material do not show notable deformations.

O presente trabalho teve como foco o desenvolvimento de um software para a simulação de processos de microfabricação em substrato e de microfabricação em superfície baseado em autômatos celulares, o simMEMS. Além disso, visando a futura incorporação de ferramentas para análise das estruturas geradas pelo programa, um módulo com funcionalidades básicas para a análise mecânica de estruturas também foi desenvolvido. No que tange à microfabricação em superfície, o software desenvolvido permite simular a corrosão anisotrópica úmida do Si em KOH e deep reactive ion etching (DRIE). O simulador de corrosão úmida utiliza um autômato celular conhecido como BCA. O simulador de DRIE usa um autômato próprio. Para a simulação dos processos de microfabricação em superfície o software fornece quatro processos: deposição de filmes, corrosão de filmes, fotolitografia e planarização. Para corrosão e deposição de filmes, diversos autômatos celulares da literatura foram analisados e os resultados dessas análises é aqui apresentado. Todos os simuladores, tanto de microfabricação em superfície como em substrato, podem ser utilizados em conjunto. Isso torna o software bastante útil e capaz de simular a fabricação de um grande número de dispositivos. / The main goal of this project is the development of a software capable of simulating both surface and bulk micromachining based on a cellular automata approach. This software has been called simMEMS. In order to enable future versions of the software to also be able to analyze the structures created by the software, a module capable of running a mechanical analysis through the finite element method is also developed. simMEMS allows the user to simulate two bulk micromachining processes: wet anisotropic KOH etching and deep reactive ion etching DRIE. The wet etching simulator uses a cellular automaton known as BCA. The DRIE simulator uses an automaton developed during this project. The surface micromachining simulator allows the user to simulate four types of processes: photolithography, film deposition, film etching and substrate planarization. Several automata for the deposition and etching of films are studied and the results of this study are presented here. All processes, be they for surface or bulk micromachining, can be used on the same substrate to simulate the entire fabrication process for a large array of devices. This makes simMEMS a very useful software.

The production of magnetic materials is of great interest for use in the micro-fabrication industry. In particular, Permalloy (Ni80Fe20) is used in the production of micro-electromechanical systems (MEMS) due to its favourable magnetic properties (high relative permeability, low coercivity and high magnetic saturation). This leads to applications in devices such as inductors, transformers and micro-actuators. The electrodeposition of NiFe is also of fundamental electrochemical interest, as there is anomalous thermodynamic behaviour, with the less noble (iron) metal depositing preferentially to the more noble (nickel) metal. To enable consistent alloy deposition nickel and nickel-iron baths are currently almost exclusively based on boric acid. Boric acid has an important role in the deposition of NiFe films but its role(s) in the electro-deposition mechanism is (are) not wholly understood. Recently (2011) boric acid has been identified as a “substance of very high concern” based on the criteria established by EU chemical regulation, REACH. In anticipation of increased regulation an alternative was sought to provide a benign alternative to boric acid in the NiFe plating bath suitable for use in micro-fabrication. Initial work was performed to benchmark the performance of existing boric acid based electro-deposition baths. Cyclic voltammetry was performed, which demonstrated the deposition of nickel and nickel-iron from boric acid baths. Coulombic efficiencies up to 93 % were measured for the deposition of nickel using the electrochemical quartz crystal microbalance (EQCM) on platinum electrodes. For nickel-iron deposition control of the film composition was demonstrated on copper electrodes through varying the iron (II) concentration, current density and temperature. A citrate bath for the deposition of nickel-iron was then developed and characterised. Cyclic voltammetry was performed in these citrate baths demonstrating the deposition of nickel and nickel-iron. Optimal conditions for depositing Ni80Fe20 were demonstrated to be an elevated temperature (60 °C) with a current density of 20 mA cm-2 and a pH of 3. Using the EQCM the efficiency for nickel deposition was measured to be > 80 %. The effects of sodium saccharin and sodium dodecyl sulfate as additives were investigated; these were shown to influence morphology but not the coulombic efficiency. Decreasing the pH was shown to lower the efficiency of nickel deposition from the citrate bath. Comparisons of key properties were made between NiFe films deposited from a boric acid bath and the citrate bath developed in this work. Test structures were used to compare the strain in the films; no significant difference was found. For 2.2 μm thick Ni80Fe20 films the sheet resistance was measured using Greek cross structures as 0.078 ± 0.004 Ω/square for films deposited from the boric acid bath and 0.090 ± 0.006 Ω/square from the citrate bath. The magnetic saturation, Ms, was measured as 895 ± 66 emu cm-3 for deposits from the boric acid bath and 923 ± 111 emu cm-3 from the citrate bath. These again show no significant difference in these values within experimental error. Coercivities for these films were measured to be between 20 and 120 A m-1. In combination, this work demonstrates the development and characterisation of a new citrate based electrodeposition bath for nickel and nickel-iron. Similar chemical, electrical, mechanical and magnetic properties were found from films deposited from both baths, thus demonstrating the suitability of the citrate bath for the deposition of nickel-iron films in microfabrication.

This Thesis proposes phase modulating interferometry as an alternative to phase stepping and phase-shifting interferometry for use in the shape and displacement characterization of microelectromechanical systems (MEMS) [Creath, 1988; de Groot, 1995a; Furlong and Pryputniewicz, 2003]. A phase modulating interferometer is developed theoretically with the use of a stroboscopic illumination source and implemented on a Linnik configured interferometer using a software control package developed in the LabVIEWâ„¢ programming environment. Optimization of the amplitude and phase of the sinusoidal modulation source is accomplished through the investigation and minimization of errors created by additive noise effects on the recovered optical phase. A spatial resolution of 2.762 µm over a 2.97x2.37 mm field of view has been demonstrated with 4x magnification objectives within the developed interferometer. The measurement resolution lays within the design tolerance of a 500Ã… ±2.5% thick NIST traceable gold film and within 0.2 nm of data acquired under low modulation frequency phase stepping interferometry on the same physical system. The environmental stability of the phase modulating interferometer is contrasted to the phase stepping interferometer, exhibiting a mean wrapped phase drift of 40.1 mrad versus 91 mrad under similar modulation frequencies. Shape and displacement characterization of failed µHexFlex devices from MIT's Precision Compliant Systems Laboratory is presented under phase modulating and phase stepping interferometry. Shape characterization indicates a central stage displacement of up to 7.6 µm. With a linear displacement rate of 0.75 Ã…/mV under time variant load conditions as compared to a nominal rate of 1.0 Ã…/mV in an undamaged structure [Chen and Culpepper, 2006].

One of the key measurement devices used in characterization of microelectromechanical systems (MEMS) is the interferometric microscope. This device allows remote, noninvasive measurements of the surface shape and deformations of MEMS in full-field-of-view with high spatial resolution and nanometer accuracy in near real-time. As MEMS are becoming more prevalent in the areas of consumer products and national defense, the demand for a versatile and easy to use characterization system is very high. This Thesis describes the design, implementation, and use of an interferometric system that is based on modular components which allow for many loading and measurement capabilities, depending on a specific application. The system has modules for subjecting MEMS to vacuum and dry gas environments, mechanical vibration excitation, thermal loads (both heating and cooling), and electrical loads. Three interferometric measurement modules can be interchanged to spatially measure shape and deformation of micro- and/or meso-scale objects, and temporally measure vibrations of these objects. Representative examples of the measurement and loading capabilities of the system are demonstrated with microcantilevers and a microgyroscope.

"Microelectromechanical systems (MEMS) technology has been evolving for about two decades and, now it is integrated in many designs, including radio frequency (RF) switches characterized by µm dimensions. Today, designers are attempting o develop the ideal RF MEMS switch, yet electro-thermo-mechanical (ETM) effects still limit the design possibilities and adversely affect reliability of these microswitches. The ETM effects are a result of Joule heat generated at the microswitch contact areas. This heat is due to the current passing through the microswitch, characteristics of the contact interfaces, and other parameters characterizing a particular design. It significantly raises temperature of the microswitch, thus affecting the mechanical and electrical properties of the contacts, which may lead to welding, causing a major reliability issue. Advanced research was performed, in this thesis, to minimize the Joule heat effects on the contact areas, thus improving performance of the microswitch. Thermal analyses done computationally on a cantilever-type RF MEMS switch indicate heat-effected zones and the influences that various design parameters have on these zones. Uncertainty analyses were also performed to ensure accuracy of the computational results, which indicate contact temperatures on the order of 700˚C, for the cases considered in this thesis. Although these temperatures are well below the melting temperatures of the materials used, new designs of the microswitches will have to be developed, in order to lower their maximum operating temperatures and reduce temporal effects they cause, to increase reliability of the RF MEMS switches."

"Microelectromechanical systems (MEMS) are integrated mechanical and electrical devices that are fabricated with features micrometers in size. MEMS are used as chemical laboratories on a chip, actuators, sensors, etc. To increase their operational capability, various MEMS sensors are being integrated into sensor systems, whose functionality may not decrease when their size decreases. However, before more advancement can be made in the sensor systems, behavior of individual sensors must be better understood. Without the basic knowledge of how and why MEMS sensors react the way they do, it is impossible to determine how MEMS sensor systems will behave. Out of the many sensors that can be included in the system, a MEMS gyroscope was selected for consideration in this paper. More specifically, the effects that suspension has on the topography of the microgyroscopes were studied. In this thesis, the folded springs that support the MEMS gyroscopes were modeled using analytical and computational methods, whose results were verified using experimentation. The analytical results correlated well with the computational and experimental results. The analytical and computational results for the deformations of the cantilever compared within 0.1%. The differences between the analytical and experimental results were on the order of 10%. Knowledge gained from these studies will help in the development of a through methodology for modeling the microgyroscope. This methodology will facilitate insertion of the microgyroscopes into the sensor systems."

Recent instrument development for aerosol measurement has focussed on small-scale, on-line measurements that can be incorporated into miniaturised sensor nodes as part of ambient or personal air quality monitoring networks. As a result, optical particle counters (OPCs) have risen in popularity given their ability to consistently size and count individual particles. OPCs have limitations, however, in their inability to detect ultrafine particles (considered the most influential to human health) or to measure particle mass directly (the standard metric for air quality). The growing field of microelectromechanical systems (MEMS) offers a potential alternative by implementing microresonators as mass sensors. MEMS resonators have high mass sensitivities and have recently seen implementation as particulate matter (PM) monitors. The field of MEMS PM instruments is still limited with a variety of implemented resonator topologies and sampling mechanisms. In general, however, they offer real-time, high sensitivity measurements at low flow rates. The aim of this thesis was to further examine the viability of implementing MEMS resonators for PM measurement with a focus on practical considerations for real-world applications. To this end, a new microresonator-based impactor was developed - the MEMS Impactor Stage (MIS) - capable of accommodating various nozzle and resonator combinations. Square lateral bulk acoustic resonators were the primary topology, but the results within the thesis are widely applicable. A series of laboratory studies covered the resonator lifetime, reusability, detection limits, and response to environmental changes. The resonator displayed a high sensitivity throughout, capable of detecting ultrafine particles, but is vulnerable to misinterpretation. Beyond mass measurement, studies introduced possible extensions to hygroscopicity and compositional applications. Ambient particle measurements with the MIS, simulating a real-world application to air quality monitoring, showed the capabilities as a PM instrument while highlighting concerns to be addressed for future instrument design. A microresonator-based impactor has potential as an alternative to OPCs, but its cross sensitivity to deposition patterns and environmental effects must be accounted for prior to implementation as PM monitor.

The high cost of experimentation and product development in the field of microelectromechanical systems (MEMS) has led to a greater emphasis on simulation-based design for increasing first-pass design success and reliability. The use of compliant or flexible mechanisms can help eliminate friction, wear, and backlash, but compliant MEMS are sensitive to variations in material properties and geometry. This dissertation proposes approaches for design stage uncertainty analysis, model validation, and robust optimization of nonlinear compliant MEMS to account for critical process uncertainties including residual stress, layer thicknesses, edge bias, and material stiffness. Methods for simulating and mitigating the effects of non-idealities such joint clearances, semi-rigid supports, non-ideal loading, and asymmetry are also presented. Approaches are demonstrated and experimentally validated using bistable micromechanisms and thermal microactuators as examples.