Optimization studies of thermal bimorph cantilevers, electrostatic torsion actuators and variable capacitors

In this dissertation, theoretical analyses and optimization studies are given for three kinds of MEMS devices: thermal bimorph cantilevers, electrostatic torsion actuators, and variable capacitors. Calculation, simulation, and experimental data are used to confirm the device behavior and demonstrate the application of the design approaches. For thermal bimorph cantilevers, an analytical model is presented which allows theoretical analysis and quantitative optimization of the performance based on material properties and device dimensions. Bimorph cantilevers are divided into two categories for deflection optimization: either the total thickness is constant, or the cantilever has one constant and one variable layer thickness. The optimum equations are then derived for each case and can be used as design rules. The results show that substantial improvements are possible over existing design approaches. Other parameters like static temperature distribution, power consumption, and dynamic behavior are also discussed, as are design tradeoffs such as feature size, application constraints, fabrication feasibility, and cost. The electrostatic torsion actuator studies are conducted for two device types: round and rectangular. The first case describes an analytical study of the pull-in effect in round, double-gimbaled, electrostatic torsion actuators with buried, variable length electrodes, designed for optical cross-connect applications. It is found that the fractional tilt at pull-in for the inner round plate in this system depends only on the ratio of the length of the buried electrode to the radius of the plate. The fractional tilt at pull-in for the outer support ring depends only on the ratio of the length of the buried electrode to the outer radius of the ring and the ratio of the ring\u27s inner and outer radii. Expressions for the pull-in voltage are determined in both cases. General relationships are also derived relating the applied voltage to the resulting tilt angle, both normalized by their pull-in values. Calculated results are verified by comparison with finite element MEMCAD simulations, with fractional difference smaller than 4% for torsion mode dominant systems. For the second case, a fast, angle based design approach for rectangular electrostatic torsion actuators based on several simple equations is developed. This approach is significantly more straightforward than the usual full calculation or simulation methods. The main results of the simplified approach are verified by comparing them with analytical calculations and MEMCAD simulations with fractional difference smaller than 3% for torsion mode dominant actuators. Also, good agreement is found by comparison with the measured behavior of a micro-fabricated full-plate device. In the last topic, ultra-thin silicon wafers, SU-8 bonding and deep reactive ion etching technology have been combined for the fabrication of folded spring, dual electrostatic drive, vertical plate variable capacitor devices with displacement limiting bumpers. Due to the presence of the bumpers, the variable capacitor with parallel plate drive electrodes has two tuning voltage regimes: first a parabolic region that achieves roughly a 290% tuning range, then a linear region that achieves an additional 310%, making the total tuning range about 600%. The variable capacitor with comb drive electrodes has a parabolic region that achieves roughly a 205% tuning range, then a linear region that achieves an additional 37%, making its total tuning range about 242%. The variable capacitors have Q factors around 100 owing to the use of silicon electrodes other than lower resistivity metal

Cantilever beam microactuators with electrothermal and electrostatic drive

Microfabrication provides a powerful tool for batch processing and miniaturization of mechanical systems into dimensional domain not accessible easily by conventional machining. CMOS IC process compatible design is definitely a big plus because of tremendous know-how in IC technologies, commercially available standard IC processes for a reasonable price, and future integration of microma-chined mechanical systems and integrated circuits. Magnetically, electrostatically and thermally driven microactuators have been reported previously. These actuators have applications in many fields from optics to robotics and biomedical engineering. At NJIT cleanroom, mono or multimorph microactuators have been fabricated using CMOS compatible process. In design and fabrication of these microactuators, internal stress due to thermal expansion coefficient mismatch and residual stress have been considered, and the microactuators are driven with electro-thermal power combined with electrostatical excitation. They can provide large force, and in- or out-of-plane actuation. In this work, an analytical model is proposed to describe the thermal actuation of in-plane (inchworm) actuators. Stress gradient throughout the thickness of monomorph layers is modeled as linearly temperature dependent Δσ. The nonlinear behaviour of out-of-plane actuators under electrothermal and electrostatic excitations is investigated. The analytical results are compared with the numerical results based on Finite Element Analysis. ANSYS, a general purpose FEM package, and IntelliCAD, a FEA CAD tool specifically designed for MEMS have been used extensively. The experimental results accompany each analytical and numerical work. Micromechanical world is three dimensional and 2D world of IC processes sets a limit to it. A new micromachining technology, reshaping, has been introduced to realize 3D structures and actuators. This new 3D fabrication technology makes use of the advantages of IC fabrication technologies and combines them with the third dimension of the mechanical world. Polycrystalline silicon microactuators have been reshaped by Joule heating. The first systematic investigation of reshaping has been presented. A micromirror utilizing two reshaped actuators have been designed, fabricated and characterized

Programmable microstrip dipole antenna design

The narrow bandwidth of microstrip dipole antennas is a major limitation for many applications. A method to increase the microstrip dipole antenna bandwidth is illustrated in this thesis. The proposed method utilizes micromechanical actuators to adjust the electrical length of the dipole antenna. The length change is realized by the activities of several microactuators arranged on both arms of the antenna. The radiation pattern and input impedance, as well as the microactuator mechanisms are detailed in this thesis. A programmable microstrip dipole antenna including the microactuators has been designed with the feedline taken into consideration. The fabrication techniques for this family of programmable antennas are also described

MEMS micromirrors for imaging applications

Strathclyde theses - ask staff. Thesis no. : T13478Optical MEMS (microelectromechanical systems) are widely used in various applications. In this thesis, the design, simulation and characterisation of two optical MEMS devices for imaging applications, a varifocal micromirror and a 2D scanning micromirror, are introduced. Both devices have been fabricated using the commercial Silicon-on-Insulator multi-users MEMS processes (SOIMUMPs), in the 10 m thick Silicon-on-Insulator (SOI) wafer. Optical MEMS device with variable focal length is a critical component for imaging system miniaturisation. In this thesis, a thermally-actuated varifocal micromirror (VFM) with 1-mm-diameter aperture is introduced. The electrothermal actuation through Joule heating of the micromirror suspensions and the optothermal actuation using incident laser power absorption have been demonstrated as well as finite element method (FEM) simulation comparisons. Especially, the optical aberrations produced by this VFM have been statistically quantified to be negligible throughout the actuation range. A compact imaging system incorporating this VFM has been demonstrated with high quality imaging results. MEMS 2D scanners, or scanning micromirrors, are another type of optical MEMS which have been widely investigated for applications such as biomedical microscope imaging, projection, retinal display and optical switches for telecommunication network, etc. For large and fast scanning motions, the actuation scheme to scan a micromirror in two axes, the structural connections and arrangement are fundamental. The microscanner introduced utilises two types of actuators, electrothermal actuators and electrostatic comb-drives, to scan a 1.2-mm-diameter gold coated silicon micromirror in two orthogonal axes. With assistance of FEM software, CoventorWare, the structure optimisation of actuators and flexure connections are presented. The maximum optical scan angles in two axes by each type of actuator individually and by actuating the two at the same time have been characterised experimentally. By programming actuation signals, the microscanner has achieved a rectangular scan pattern with 7° 10° angular-scan-field at a line-scan rate of around 1656 Hz.Optical MEMS (microelectromechanical systems) are widely used in various applications. In this thesis, the design, simulation and characterisation of two optical MEMS devices for imaging applications, a varifocal micromirror and a 2D scanning micromirror, are introduced. Both devices have been fabricated using the commercial Silicon-on-Insulator multi-users MEMS processes (SOIMUMPs), in the 10 m thick Silicon-on-Insulator (SOI) wafer. Optical MEMS device with variable focal length is a critical component for imaging system miniaturisation. In this thesis, a thermally-actuated varifocal micromirror (VFM) with 1-mm-diameter aperture is introduced. The electrothermal actuation through Joule heating of the micromirror suspensions and the optothermal actuation using incident laser power absorption have been demonstrated as well as finite element method (FEM) simulation comparisons. Especially, the optical aberrations produced by this VFM have been statistically quantified to be negligible throughout the actuation range. A compact imaging system incorporating this VFM has been demonstrated with high quality imaging results. MEMS 2D scanners, or scanning micromirrors, are another type of optical MEMS which have been widely investigated for applications such as biomedical microscope imaging, projection, retinal display and optical switches for telecommunication network, etc. For large and fast scanning motions, the actuation scheme to scan a micromirror in two axes, the structural connections and arrangement are fundamental. The microscanner introduced utilises two types of actuators, electrothermal actuators and electrostatic comb-drives, to scan a 1.2-mm-diameter gold coated silicon micromirror in two orthogonal axes. With assistance of FEM software, CoventorWare, the structure optimisation of actuators and flexure connections are presented. The maximum optical scan angles in two axes by each type of actuator individually and by actuating the two at the same time have been characterised experimentally. By programming actuation signals, the microscanner has achieved a rectangular scan pattern with 7° 10° angular-scan-field at a line-scan rate of around 1656 Hz

Microgripper design and evaluation for automated µ-wire assembly: a survey

Microgrippers are commonly used for micromanipulation of micro-objects from 1 to 100 µm and attain features of reliable accuracy, low cost, wide jaw aperture and variable applied force. This paper aim is to review the design of different microgrippers which can manipulate and assemble µ-wire to PCB connectors. A review was conducted on microgrippers’ technologies, comparing fundamental components of structure and actuators’ types, which determined the most suitable design for the required micromanipulation task. Various microgrippers’ design was explored to examine the suitability and the execution of requirements needed for successful micromanipulation

Segmented Control of Electrostatically Actuated Bimorph Micromirrors

Electrostatic actuating bimorph beams are a MEMS device that can be used to control arrays of small micromirrors for optical beam scanning. Previous research has demonstrated that creating high-angle deflection using long repeating arms of bimorph beams is possible. The current devices lack precise control and measurement of the mirror deflection. A solution to improve control and measurement is by using segmented bias channels to control separate portions of the actuation arm. The amount of mirror deflection will vary depending on which segments of the arm are actuated. This thesis discusses the results of FEA modeling and testing

Design, fabrication and thermomechanical testing of a vertical bimorph sensor in the wafer plane

A bimetallic recurve device was designed, fabricated and tested as a temperature sensor. The device is to be used for sensing temperatures up to 300 C inside oil wells for downhole condition monitoring. Continuous downhole measurements at high temperatures and pressures are required to monitor conditions downhole instruments are exposed to during use. Currently mercury thermometers and resistive temperature detectors (RTD) are used for downhole temperature measurements. Microsensors have potential application downhole, due to their small size and inherent robustness. The principle of a bimetallic beam was used to measure temperature. A bimetallic beam deflects with changes in temperature due to differential thermal expansion of the two materials in the beam. Analytical and numerical models were developed for parametric analysis of recurve thermomechanical elements. Invar was electrodeposited with the desired ratio through the depth of the structures of Ni to Fe as 36%:64%. Prototype recurve structures were fabricated using the LIGA microfabrication process. X-ray masks were designed and fabricated to improve the alignment process. A substrate containing twelve different devices was fabricated using a two mask process, with eleven of the structures successfully released. Recurve structures 8 mm long with a 500 µm X 100 µm cross-sections were tested using a dynamic mechanical analyzer. The prototypes were loaded at room temperature and a heating rate of 10 C/min was applied in 20 C steps, with a one minute hold at each step for deflection measurement, until the temperature reached 300 C. The deflection was measured using a probe tip resting against the sample with a force of 0.0001N. Deflection increased linearly to a maximum deflection of 65 µm at a temperature of 200 C. With further increase in temperature, the deflection was found to decrease linearly. The Curie effect was hypothesized to influence this behavior of the Invar alloy. The analytical model of the bimetallic recurve beam with the given experimental conditions and nominal dimensions predicted a maximum free deflection of 54.17 µm at 200 C. Differences were attributed to differences between the experimental conditions and model assumptions

Two-dimensional microscanners with t-shaped hinges and piezoelectric actuators

For a wide range of application areas such as medical instruments, defense, communication networks, industrial equipment, and consumer electronics, microscanners have been a vibrant research topic. Among various fabrication methodologies, MEMS (microelectromechanical system) stands out for its small size and fast response characteristics. In this thesis, piezoelectric actuation mechanism is selected because of its low voltage and low current properties compared with other mechanisms, which are especially important for the target application of biomedical imaging. Although 1- and 2-dimensional microscanners with piezoelectric actuators have been studied by several other groups, this thesis introduces innovative improvements in design of the piezoelectric MEMS microscanner. A novel T-shaped hinge geometry is proposed, which is flexible in whole six directions and also free from the crosstalk issue found in the earlier designs by other groups. The piezoelectric actuator of the microscanner is comprised of five layers; a top electrode, a piezoelectric layer (lead zirconate titanate or PZT), a bottom electrode, a dielectric layer, and a mechanical support. The microscanners were analyzed using both analytical formulas and numerical simulations. Based on the analysis, the microscanners were designed and fabricated with four mask levels¯top electrodes, bottom electrodes, bonding pads, and substrate etching windows. During the silicon substrate wet etching process in KOH, ProTEK@ B3 was coated in the front to protect the devices. Polarization-voltage (P-V) measurement of deposited PZT was performed using RT66B. Actuation of the piezoelectric cantilevers were observed under a microscope by applying voltage

MEMS micro-bridge actuator for potential application in optical switching

In this thesis, the development of a novel electro-thermally actuated bi-stable out-of-plane two way actuated buckled micro-bridge for a potential application in optical switching is presented. The actuator consists of a bridge supported by 'legs' and springs at its four corners. The springs and the bridge are made of a tri-layer structure comprising of 2.5µm thick low-stress PECVD oxide, 1µm thick high-stress PECVD oxide and 2µm thick heavily phosphorus doped silicon. The legs, on the other hand, are 2µm thick single layer heavily phosphorus doped silicon. Both legs and springs provide elastically constrained boundary conditions at the supporting ends, without of which important features of the micro-bridge actuator could not have been achieved. This microbridge actuator is designed, simulated using ANSYS, fabricated and tested. The results from the testing have shown a good agreement with analytical prediction and ANSYS simulation. The actuator demonstrated bi-stability, two-way actuation and 31µm out-of-plane movement between the two-states using low voltage drive. Buckled shape model, design method for bi-stability and thermo-mechanical model are developed and employed in the design of the micro-bridge. These models are compared with Finite Element (FE) based ANSYS simulation and measurements from the fabricated micro-bridge and have shown a good agreement. In order to demonstrate the potential application of this actuator to optical switching, ANSYS simulation studies have been performed on a micro-mirror integrated with the micro-bridge actuator. From these studies, the optimum micro-mirror size that is appropriate for the integration has been obtained. This optimal mirror size ensures the important features of the actuator. Mirror fabrication experiments in (110) wafer have been carried out to find out the appropriate compensation mask size for a given etch depth and the suitable wafer thickness that can be used to fabricate the integrated system