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

Ph.DDOCTOR OF PHILOSOPH

Contribution au micro-actionnement multi-stable piloté par radiations optiques

In this work, a bistable mechanism based on antagonistic pre-shaped double beams was proposed. Employing the proposed bistable mechanism, a quadristable micro-actuator was designed. ln order to validate the quadristability of the device, a meso-scaled prototype was fabricated from MDF by laser cutting. After the quadristability was experimentally confirmed, a quadristable micro-actuator was realized on SOl wafer using DRIE technique. Strokes for inner row and outer row were reduced to 300 µm and 200 µm respectively. For the actuation of the quadristable micro-actuator,laser heated SMA elements with deposited Si02 layer were used to realize the optical wireless actuation. With the help of a laser beam steering micro-mirror, both inner row and outer row were successfully actuated. ln order to further reduce the stroke, a bistable actuator with stroke reducing structure was designed and a prototype eut from MDF was tested. Bistability was validated and a stroke of 1µm was experimentally achieved. Based on this bistable module, a multistable nano-actuator, which contains four parallel coupled bistable modules,was designed and simulated. The simulated result have indicated that it was capable of outputs 16 discrete stable positions available from 0 nm to 150 nm with a step of 10 nm between two stable positions.Cette thèse traite le sujet du micro-actionnement multistable employant des radiations optiques pour atteindre les différentes positions offertes par le micro-actionneur. Dans le cadre des travaux réalisés, un mécanisme bistable reposant sur un principe de doubles poutres préformées situées en position antagoniste est proposé, et, sur cette brique élémentaire, un micro-actionneur quadristable a été conçu. Afin de valider le principe de fonctionnement de micro-actionneur, des procédés de fabrication Laser (sur le matériau « médium - MDF») puis DRIE (sur un wafer SOI de silicium) ont été utilisés. Sur le prototype en silicium, permettant une réduction des courses du rang interne et du rang externe du micro-actionneur, celles-ci ont été fixées à 300 µm et 200 µm respectivement. L’actionnement à distance de ce micro-actionneur a été prouvé en utilisant le chauffage laser d’un élément actif en Nitinol structuré par un dépôt de SiO2, ceci générant un effet « deux sens » de l’élément actif permettant d’annuler la charge sur les poutres du micro-actionneur une fois celui-ci déclenché puis en position stable. L’utilisation d’un banc expérimental incluant une membrane MEMS de balayage laser a permis de démontrer la quadristabilité du micro-actionneur sur 90 000 cycles. Afin de réduire davantage la course de ce micro-actionneur, des concepts de dispositifs de réduction de course ont été développés pour démontrer, à partir de prototypes fabriqué en MDF par usinage laser, la capacité à atteindre une course de 1 µm. Enfin, à la suite de ces travaux de réduction de course, un concept de nano-actionneur multistable a été proposé. Ce nano-actionneur est composé de quatre modules bistables liés et disposés en parallèle pour offrir 16 positions discrètes sur une course rectiligne. Les simulations de cet actionneur montrent la possibilité d’atteindre les 15 positions espacées de 10 nm sur une course de 150 nm

Dual-Beam Actuation of Piezoelectric AlN RF MEMS Switches Monolithically Integrated with AlN Contour-Mode Resonators

This work reports on piezoelectric Aluminum Nitride (AlN) based dual-beam RF MEMS switches that have been monolithically integrated with AlN contour-mode resonators. The dual-beam switch design presented in this paper intrinsically compensates for the residual stress in the deposited films, requires low actuation voltage (5 to 20 V), facilitates active pull-off to open the switch and exhibits fast switching times (1 to 2 μs). This work also presents the combined response (cascaded S parameters) of a resonator and a switch that were co-fabricated on the same substrate. The response shows that the resonator can be effectively turned on and off by the switch. A post-CMOS compatible process was used for the co-fabrication of both the switches and the resonators. The single-chip RF solution presented herein constitutes an unprecedented step forward towards the realization of compact, low loss and integrated multi-frequency RF front-ends

Electrostatic zipping actuators and their applications to MEMS

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 161-168).Electrostatic actuation is the most common and well-developed method of generating motion on the micro scale. To overcome the challenge of providing both high force and large displacement, electrostatic zipping actuators have been developed and applied to various devices. As device thicknesses increase, however, conventional laterally- moving zipping actuators become less practical due to their high pull-in voltages caused by their minimum achievable electrode gaps. This thesis presents a fundamental improvement of the laterally-moving electrostatic zipping actuator. Its major contributions are: 1) a compliant starting zone is introduced into the fixed electrode to significantly reduce the pull-in voltage of the zipping electrode; 2) numerical and analytical methods are developed to solve general zipping actuator problems; 3) optimization is performed to minimize the effort required to actuate the zipping electrode and its load; and 4) the improved zipping actuators are designed into a relay to illustrate their use and performance. To design a cross-bar micro relay, two zipping actuators are combined with a curved bistable switch beam and two contacts.(cont.) The micro relay is monolithically fabricated in silicon using deep reactive ion etching to move laterally in the wafer plane. Both actuators provided up to 10 mN of actuation force over their 80 [mu]m of stroke at 140 V, and toggle the bistable relay at a maximum rate of 160 Hz. Pullin voltage, actuation voltage and force-displacement measurements of the actuators and switch beam confirm theoretical expectations based on numerical, analytical and finite element analyses, after accounting for fabrication variations. The shortest pulse required to switch the relay is 400 [mu]s, and the time taken for the actuator to close the relay was approximately 3 ms. The relay was operated at 100 Hz for over 120 hours through more than 40 million cycles without any observed stiction or fracture fatigue. To achieve low contact resistance for a laterally-moving micro relay, wet anisotropically etched silicon [111] planes are developed to form relay contact surfaces that offer flat wiping surfaces and ease of thick metalization. Experimental contacts are fabricated and their average contact resistance is measured to be [approx.] 50 m[omega].(cont.) A process plan is also proposed to combine the [111] plane contacts with the prior zipping actuators and the switch beam to build a micro relay with low contact resistance for power protection applications. The compliant starting zone concept can also be applied to vertically-moving MEMS devices. A MEMS valve is also designed using a zipping actuator having com- pliant starting zones. As another application of the zipping mechanism, a nonlinear spring is also presented and analyzed.by Jian Li.Ph.D

Design, simulation, fabrication and testing of microprobes for a new MEMS wafer probe card

A new type of MEMS cantilever wafer probe card consists of an array of microcantilevers individually actuated by bimorph heating to make contact with the test chip was designed and fabricated. This probe card is called the CHIPP (Conformable, High-Pin count, Programmable ) card and can be designed to contact up to 800 I/O pads along the perimeter of a 1 cm2 chip with a microprobe repeat distance of ~50 µm. Each microcantilever had an internal heater and a separate electrode carrying the signal under test and contained four separate layers plus a fifth material for the contact tip region. Different versions of micro-actuators have been designed and made in this Ph.D. research. Ohmic contacts were made with the lowest contact resistance of 250 mΩ. The deflection efficiency varied from 5.23 to 9.6 µm/mW for cantilever length from 300-500 µm. The maximum reversible deflection was in the range of 270 µm. Video recordings made inside the SEM clearly show that ohmic contact was made to a stationery tungsten electrode. A full dynamic deflection (180 µm) for a 50 x 500 µm cantilever occurred in response to input frequency up to nearly 50 Hz. The motion was damped at higher frequencies, with a strong resonance (for a 50 x 500 µm device) at 8160 Hz. Heat loss for devices operating in air was found to be substantially higher than for vacuum operation with a heat loss ratio of about 2/1 for a heater inside the structure; and 4.25/1 for a structure with the heater as an outer layer of the cantilever

DEVELOPMENT OF NEMS RELAYS IN LOGIC COMPUTATION AND RUGGED ELECTRONICS

Ph.DDOCTOR OF PHILOSOPH

A micromachined zipping variable capacitor

Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components. A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows. In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive. Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 μm

Compliant Torsional Micromirrors with Electrostatic Actuation

Due to the existence of fabrication tolerance, property drift and structural stiction in MEMS (Micro Electro Mechanical Systems), characterization of their performances through modeling, simulation and testing is essential in research and development. Due to the microscale dimensions, MEMS are more susceptible and sensitive to even minor external or internal variations. Moreover, due to the current limited capability in micro-assembly, most MEMS devices are fabricated as a single integrated micro-mechanical structure composed of two essential parts, namely, mass and spring, even if it may consist of more than one relatively movable part. And in such a scale of dimensions, low resonant micro-structures or compliant MEMS structures are hard to achieve and difficult to survive. Another problem arises from the limited visibility and accessibility necessary for characterization. Both of these issues are thus attempted in this research work. An investigation on micromirrors with various actuations and suspensions is carried out, with more attention on the micromirrors with compliant suspensions, electrostatic actuation and capable of torsional out-of-plane motion due to their distinct advantages such as the low resonance and the low drive voltage. This investigation presents many feasible modeling methods for prediction and analysis, aiming to avoid the costly microfabrication. Furthermore, both linear and nonlinear methods for structure and electrostatics are all included. Thus, static and dynamic performances of the proposed models are formularized and compared with those from FEA (Finite Element Analysis) simulation. The nonlinear modeling methods included in the thesis are Pseudo Rigid Body Model (PRBM) and hybrid PRBM methods for complex framed microstructures consisting of compliant beam members. The micromachining technologies available for the desired micromirrors are reviewed and an SOI wafer based micromachining process is selected for their fabrication. Though the fabrication was executed outside of the institution at that time, the layout designs of the micro-chips for manufacture have included all related rules or factors, and the results have also demonstrated the successful fabrication. Then investigation on non-contact test methods is presented. Laser Doppler Vibrometer (LDV) is utilized for the measurement of dynamic performances of proposed micromirrors. Two kinds of photo-sensing devices (PSDs), namely, the digitized PSD formed by CCD arrays and the analog PSD composed of a monolithic photosensing cell, are used for static test set-ups. An interferometric method using Mirau objective along with microscope is also employed to perform static tests of the selected micromirrors. Comparison of the tested results and their related theoretical results are presented and discussed, leading to a conclusion that the proposed hybrid PRBM model are appropriate for prediction or analysis of compliantly suspended micromirrors including issues arising from fabrication tolerance, structural or other parametric variations

A micromachined zipping variable capacitor

Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components.A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows.In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive.Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 µm