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

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

Development and testing of a micromachined probe card.

This thesis is concerned with the design, fabrication and testing of micro scale probes. The probes were designed to act as temporary electrical connections to allow wafer level testing of integrated circuits. The work initially focused on the creation of free standing nickel cantilevers, angled up from the substrate with probe tips at the free end. These were fabricated using a novel method, combining pseudo grey scale lithography and thick photoresist sacrificial layers. Detailed analysis of the fabrication method, in particular the resist processing and lithography was undertaken and the limitations of the method explored.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Electrostatic Radio Frequency (RF) Microelectromechanical Systems (MEMS) Switches With Metal Alloy Electric Contacts

RF MEMS switches are paramount in importance for improving current and enabling future USAF RF systems. Electrostatic micro-switches are ideal for RF applications because of their superior performance and low power consumption. The primary failure mechanisms for micro-switches with gold contacts are becoming stuck closed and increased contact resistance with increasing switch cycles. This dissertation reports on the design, fabrication, and testing of micro-switches with sputtered bi-metallic (i.e., gold (Au)-on-Au-(6.3at%)platinum (Pt)), binary alloy (i.e., Au-(3.7at%)palladium (Pd) and Au-(6.3at%)Pt), and ternary alloy (i.e., Au-(5at%)Pt-(0.5at%)copper (Cu)) contact metals. Performance was evaluated, in-part, using measured contact resistance and lifetime results. The micro-switches with bi-metallic and binary alloy contacts exhibited contact resistance between 1 - 2 ohms and, when compared to micro-switches with sputtered gold contacts, showed an increase in lifetime. The micro-switches with tertiary alloy contacts showed contact resistance between 0.2-1.8 and also showed increased lifetime. Overall, the results presented in this dissertation indicate that micro-switches with gold alloy electric contacts exhibit increased lifetimes in exchange for a small increase in contact resistance

Multifunctional Tool for Expanding AFM-Based Applications

A multifunctional tool which expands the application field of atomic force microscope-based surface modification is presented. The AFM-probe can be used for surface modification and in-situ characterization at the same time, due to a special configuration with two cantilevers. Various applications from different fields are presented, which were carried out with one and the same tool: in-situ characterization of wear generated with and without local lubrication (tribology), fountain-pen lithography in which material is deposited or removed (physical chemistry), and electrochemical metal deposition (electrochemistry)

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

Laterally Movable Gate Field Effect Transistor (LMGFET) for microsensor and microactuator applications

Laterally Movable Gate Field Effect Transistor (LMGFET) invented at LSU as a microactuator is the subject of study in this research. The gate moving in lateral direction in a LMGFET changes channel width but keeps the channel length and the gap between the metal gate and the gate oxide constant. LMGFET offers linear change in drain current with gate motion and a large displacement range. This research is the first demonstration of LMGFET. In this dissertation, a post-IC LIGA-like process for LMGFET microstructure fabrication has been developed that is compatible with monolithic integration with CMOS circuitry. A two-mask post-IC process has been developed in this research for LMGFET fabrication. This novel process utilizes S1813 photoresist as a sacrificial layer in conjunction with a thicker resist like AZ P4620 or SU-8 as an electroplating mold. New curing temperatures for the sacrificial layer photoresist have been determined for this purpose. LMGFET microstructures have been successfully integrated with CMOS circuitry on the same chip to form integrated microsystem. LMGFET microstructure driven by a comb-drive with serpentine retaining spring shows sensitivities Sel of 2 and 1.43 nA/V respectively at 5 and 25 Hz. These numbers reflect that LMGFET is capable of measuring nm range displacement. Electrical characteristics of a depletion type LMGFET structure are measured and show an average sensitivity Sl of - 4 µA/µm at drain to source voltage VDS of 10 V with the gate shorted to source. Several applications of microsystems utilizing LMGFET microstructures as a position sensor or an accelerometer, a spectrum analyzer or an electro-mechanical filter and a mechanical/optical switch are described

Single Substrate Electromagnetic Actuator

A microvalve which utilizes a low temperature ( <300° C.) fabrication process on a single substrate. The valve uses buckling and an electromagnetic actuator to provide a relatively large closing force and lower power consumption. A buckling technique of the membrane is used to provide two stable positions for the membrane, and to reduce the power consumption and the overall size of the microvalve. The use of a permanent magnet is an alternative to the buckled membrane, or it can be used in combination with the buckled membrane, or two sets of micro-coils can be used in order to open and close the valve, providing the capability for the valve to operate under normally opened or normally closed conditions. Magnetic analysis using ANSYS 5.7 shows that the addition of Orthonol between the coils increases the electromagnetic force by more than 1.5 times. At a flow rate of 1 mL/m, the pressure drop is < 100 Pa. The maximum pressure tested was 57 kPa and the time to open or close the valve in air is under 100 ms. This results in an estimated power consumption of 0.1 mW.Georgia Tech Research Corp

Microfabrication of a MEMS piezoresistive flow sensor - materials and processes

Microelectromechanical systems (MEMS) based artificial sensory hairs for flow sensing have been widely explored, but the processes involved in their fabrication are lithography intensive, making the process quite expensive and cumbersome. Most of these devices are also based on silicon MEMS, which makes the fabrication of out-of plane 3D flow sensors very challenging. This thesis aims to develop new fabrication technologies based on Polymer MEMS, with minimum dependence on lithography for the fabrication of piezoresistive 3D out-of-plane artificial sensory hairs for sensing of air flow. Moreover, the fabrication of a flexible sensor array is proposed and new materials are also explored for the sensing application. Soft lithography based approaches are first investigated for the fabrication of an all elastomer device that is tested in a bench top wind tunnel. Micromolding technologies allow for the mass fabrication of microstructures using a single, reusable mold master that is fabricated by SU-8 photolithography, reducing the need for repetitive processing. Polydimethylsiloxane (PDMS) is used as the device material and sputter deposited gold is used as both the piezoresistive as well as the electrode material for collection of device response. The fabrication results of PDMS to PDMS metal transfer micromolding (MTM) are shown and the limitations of the process are also discussed. A dissolving mold metal transfer micromolding process is then proposed and developed, which overcomes the limitations of the conventional MTM process pertinent to the present application. Testing results of devices fabricated using the dissolving mold process are discussed with emphasis on the role of micro-cr  acking as one failure mode in elastomeric devices with thin film metal electrodes. Finally, a laser microfabrication based approach using thin film Kapton as the device material and an electrically conductive carbon-black elastomer composite as the piezoresistor is proposed and demonstrated. Laminated sheets of thick and thin Kapton form the flexible substrate on which the conductive elastomer piezoresistors are stencil printed. Excimer laser ablation is used to make the micro-stencil as well as to release the Kapton cantilevers. The fluid-structure interaction is improved by the deposition of a thin film of silicon dioxide, which produces a stress-gradient induced curvature, strongly enhancing the device sensitivity. This new approach also enables the fabrication of backside interconnects, thereby addressing the commonly observed problem of flow intrusion while using conventional interconnection technologies like wire-bonding. Devices with varying dimensions of the sensing element are fabricated and the results presented, with smallest devices having a width of 400 microns and a length of 1.5 mm with flow sensitivities as high as 60 Ohms/m/s. Recommendations are also proposed for further optimization of the device.M.S.Committee Chair: Allen, Mark; Committee Member: Allen, Sue Ann Bidstrup; Committee Member: Wong, C.P

DESIGN, FABRICATION AND CHARACTERISATION OF FREE-STANDING THICK-FILM PIEZOELECTRIC CANTILEVERS FOR ENERGY HARVESTING

Research into energy harvesting from ambient vibration sources has attracted great interest over the last few years, largely due to the rapid development in the areas of wireless technology and low power electronics. One of the mechanisms for converting mechanical vibration to electrical energy is the use of piezoelectric materials, typically operating as a cantilever in a bending mode, which generate a voltage across the electrodes when they are stressed. Traditionally, the piezoelectric materials are deposited on a non-electro-active substrate and are physically clamped at one end to a rigid base, which serves as a mechanical supporting platform. In this research, a three dimensional thick-film structure in the form of a free-standing cantilever incorporated with piezoelectric materials is proposed. The advantages of this structure include minimising the movement constraints on the piezoelectric, thereby maximising the electrical output and offering the ability for integration with other microelectronic devices. A series of free-standing composite cantilevers in the form of unimorphs were fabricated and characterised for their mechanical and electric properties. The unimorph structure consists of a pair of silver/palladium (Ag/Pd) electrodes sandwiching a laminar layer of lead zirconate titanate (PZT). An extended version of this unimorph, in the form of multimorph was fabricated to improve the electrical output performance, by increasing the distance of the piezoelectric layer from the neutral axis of the structure. This research also discusses the possibility of using an array of free-standing cantilevers in harvesting vibration energy in a broader bandwidth from an unpredictable ambient environment