Microelectromechanical Systems Fabrication Research Articles

Design and Fabrication of a Novel MEMS Capacitive Transducer With Multiple Moving Membrane, ${\rm M}^{3}$-CMUT

A novel capacitive micromachined ultrasonic transducer is designed and fabricated. This transducer employs a stack of two deflectable membranes suspended over a fixed bottom electrode. In this configuration, the two moving membranes deflect simultaneously in response to a bias voltage, which results in a smaller effective cavity height compared with the conventional capacitive transducers. Electromechanical and acoustic analyses are conducted to investigate the transducer properties. A set of seven transducers with radii ranging from 30 to 55 μm were fabricated utilizing a sacrificial microelectromechanical system fabrication technology. Electrical measurements were performed and were compared with results from physical deflection measurements utilizing an optical vibrometer system. The results have been compared with analytical models as well as characterization of a set of five conventional, single membrane, transducers fabricated with the same technology. These experiments indicate a good agreement between the model and measured data. A larger membrane deflection and smaller cavity height are achieved from the double membrane devices. Therefore, this type of device may enhance the transducer acoustic power generation capability as well as increasing its sensitivity both of which result from the reduction in the transducer effective cavity height.

Laser-heating wire bonding on MEMS packaging

Making connections is critical in fabrication of MEMS (Micro-Electro-Mechanical Systems). It is also complicated, because the temperature during joining affects both the bond produced and the structure and mechanical properties of the moving parts of the device. Specifications for MEMS packaging require that the temperature not exceed 240 °C. However, usually, temperatures can reach up to 300 °C during conventional thermosonic wire bonding. Such a temperature will change the distribution of dopants in CMOS (Complementary Metal Oxide Semiconductor) circuits. In this paper we propose a new heating process. A semiconductor laser (wavelength 808 nm) is suggested as the thermal source for wire bonding. The thermal field of this setup was analyzed, and specific mathematical models of the field were built. Experimental results show that the heating can be focused on the bonding pad, and that much lower heat conduction occurs, compared with that during the normal heating method. The bond strength increases with increasing laser power. The bond strengths obtained with laser heating are slightly lower than those obtained with the normal heating method, but can still meet the strength requirements for MEMS.

Wavelength specific excitation of gold nanoparticle thin-films

Advances in microelectromechanical systems (MEMS) continue to empower researchers with the ability to sense and actuate at the micro scale. Thermally driven MEMS components are often used for their rapid response and ability to apply relatively high forces. However, thermally driven MEMS often have high power consumption and require physical wiring to the device. This work demonstrates a basis for designing light-powered MEMS with a wavelength specific response. This is accomplished by patterning surface regions with a thin film containing gold nanoparticles that are tuned to have an absorption peak at a particular wavelength. The heating behavior of these patterned surfaces is selected by the wavelength of laser directed at the sample. This method also eliminates the need for wires to power a device. The results demonstrate that gold nanoparticle films are effective wavelength-selective absorbers. This “hybrid” of infrared absorbent gold nanoparticles and MEMS fabrication technology has potential applications in light-actuated switches and other mechanical structures that must bend at specific regions. Deposition methods and surface chemistry will be integrated with three-dimensional MEMS structures in the next phase of this work. The long-term goal of this project is a system of light-powered microactuators for exploring cellular responses to mechanical stimuli, increasing our fundamental understanding of tissue response to everyday mechanical stresses at the molecular level.

Surface-micromachined Electromagnets for 100μm-scale Undulators and Focusing Optics

Abstract Leveraging recent advances in microelectromechanical system (MEMS) fabrication technologies, 100 μm gap 400 μm period electromagnetic undulators and 200 μm gap quadrupole optics have been successfully fabricated. Measurement of the electromagnet impedance closely matches simulations, corresponding to 0.135 T peak field on-axis in the undulator and 1400 T/m field gradients in the quadrupole for the initially tested devices. These microfabricated undulators and quadrupoles are envisioned as insertion devices and focusing lattices for future compact light sources.

Fabrication process suitability ranking for micro-electro-mechanical systems using a fuzzy inference system

In this paper we present the development of a system to evaluate alternatives for manufacturing process steps for micro-electro-mechanical systems (MEMS). We explain in detail a formal process flow definition for MEMS fabrication. Then, we develop a fuzzy inference system which allows MEMS developers to capture users’ preferences to rank the alternatives available to complete the process steps required to fabricate a device. In the last part to this work, we present two case studies: alternatives evaluation for impurity doping and for lead zirconate titanate (PZT) patterning. Using an assortment of user preference data to rate a variety of criteria for potential alternatives, our approach produces a clear preference for one specific alternative in each case, which exemplifies the usefulness of the system proposed and illustrates how effective this methodology is towards improving the fabrication process for MEMS.

Analysis and Experiment of MEMS Based Microdroplet Ejector by a Piezoelectric Stack Actuator in Microfluidic Application

Micro Electro Mechanical Systems (MEMS) are uncovered to an assortment of liquid environments in applications such as chemical and biological sensors and micro fluidic devices. Green interactions between liquids and micro scale structures can lead to volatile performance of MEMS in liquid environments. In this study, the design and fabrication of a multi-material high-performance micro pump is presented. The micro pumps are fabricated using MEMS fabrication techniques, comprised of silicon and Pyrex micromachining and bonding. Manufacturing steps such as three small bulk cylindrical piezoelectric material elements that are integrated with micro-fabricated Silicon-on-Insulator (SOI) and glass micro machined substrates using eutectic bonding and anodic bonding processes were successfully realized and provide a robust and scalable production technique for the micro pump. Exceptional flow rates of 0.1 mL/min with 1 W power consumption based on piezoelectric stack actuation achieved by appropriate design optimization.

A protocol for improving fabrication yield of thin SU-8 microcantilevers for use in an aptasensor

SU-8 negative photoresist has been extensively employed in the fabrication of microfluidics and microelectromechanical systems. This is due to its advantages including ease of fabrication using limited equipment, biocompatibility, excellent chemical resistance, and compatibility with silicon processing. In addition, its low Young's modulus compared to silicon has made it an excellent choice for microcantilever structure development especially for sensing applications. This paper presents a fabrication protocol for the development of thin SU-8 microcantilevers. Factors such as baking temperatures and release methods that influence the fabrication yield of SU-8 microcantilevers are considered. The influence of the baking temperature on the deformation of the SU-8 structure is investigated using the finite element method and verified experimentally. Three release methods are studied including a dry release method using fluorocarbon film, and two wet release methods using OmniCoat sacrificial layer and polymethyl methacrylate (PMMA) sacrificial layer. The wet release method using PMMA sacrificial layer produced the highest yield. An aptasensor is formed using the SU-8 microcantilever, and its deflection measured for thrombin detection.

Aluminum oxide from trimethylaluminum and water by atomic layer deposition: The temperature dependence of residual stress, elastic modulus, hardness and adhesion

Use of atomic layer deposition (ALD) in microelectromechanical systems (MEMS) has increased as ALD enables conformal growth on 3-dimensional structures at relatively low temperatures. For MEMS device design and fabrication, the understanding of stress and mechanical properties such as elastic modulus, hardness and adhesion of thin film is crucial. In this work a comprehensive characterization of the stress, elastic modulus, hardness and adhesion of ALD aluminum oxide (Al2O3) films grown at 110–300°C from trimethylaluminum and water is presented. Film stress was analyzed by wafer curvature measurements, elastic modulus by nanoindentation and surface-acoustic wave measurements, hardness by nanoindentation and adhesion by microscratch test and scanning nanowear. The films were also analyzed by ellipsometry, optical reflectometry, X-ray reflectivity and time-of-flight elastic recoil detection for refractive index, thickness, density and impurities. The ALD Al2O3 films were under tensile stress in the scale of hundreds of MPa. The magnitude of the stress decreased strongly with increasing ALD temperature. The stress was stable during storage in air. Elastic modulus and hardness of ALD Al2O3 saturated to a fairly constant value for growth at 150 to 300°C, while ALD at 110°C gave softer films with lower modulus. ALD Al2O3 films adhered strongly on cleaned silicon with SiOx termination.

Theoretical analysis and simulations of micro-dosing locomotive robot with drug-release mechanism

A micro-dosing system model of overall size 5×5×4.2 mm3 for drug delivery is proposed and presented. The drug delivery system mainly consists of a micro-wheel and a micro-drug release mechanism. The motion of the micro-wheel is controlled by changing the gravity center of a running disk, which is placed within the hollow micro-wheel and attracted by the actuated micro-solenoids fabricated on the inner wall of micro-wheel in shift. In addition, the micro-wheel is controlled to roll forwards/backwards to the designated location by two sliding mode control strategies: one for long-distance motion (to transport the drug to the vicinity of the spots under disease) and the other for short-distance motion (to decelerate down and stop at the exact drug-release location). On the other hand, the micro-drug release mechanism is composed by a cantilever beam and a chamber filled up by medicine. The pyramid tip of the cantilever beam deflected by the applied electrostatic force is designed to penetrate the micro-film which seals the chamber so that the medicine can be released at the specified spot. The so called “pyramid tip” is, in fact, to replace the conventional medical needle. Its profile is like a pyramid to comply with the MEMS (Micro Electro Mechanical System) fabrication process. This “pyramid” is constructed at the free end of a cantilever beam and hence named as a “pyramid tip”.

Micro supercapacitors based on a 3D structure with symmetric graphene or activated carbon electrodes

This paper presents three-dimensional (3D) micro supercapacitors with thick interdigital electrodes supported and separated by SU-8. Nanoporous carbon materials including graphene and activated carbon (AC) are used as active materials in self-supporting composites to build the electrodes. The SU-8 separators provide mechanical support for thick electrodes and allow a considerable amount of material to be loaded in a limited footprint area. The prototypes have been accomplished by a simple microelectromechanical systems (MEMS) fabrication process and sealed by polydimethylsiloxane (PDMS) caps with ionic liquid electrolytes injected into the electrode area. Electrochemical tests demonstrate that the graphene-based prototype with 100 µm thick electrodes shows good power performance and provides a considerable specific capacitance of about 60 mF cm−2. Two AC-based prototypes show larger capacitance of 160 mF cm−2 and 311 mF cm−2 with 100 µm and 200 µm thick electrodes respectively, because of higher volume density of the material. The results demonstrate that both thick 3D electrode structure and volume capacitance of the electrode material are key factors for high-performance micro supercapacitors, which can be potentially used in specific applications such as power suppliers and storage components for harvesters.

Wire bonded 3D coils render air core microtransformers competitive

We present a novel wafer-level fabrication method for 3D solenoidal microtransformers using an automatic wire bonder for chip-scale, very high frequency regime applications. Using standard microelectromechanical systems fabrication processes for the manufacturing of supporting structures, together with ultra-fast wire bonding for the fabrication of solenoids, enables the flexible and repeatable fabrication, at high throughput, of high performance air core microtransformers. The primary and secondary solenoids are wound one on top of the other in the lateral direction, using a 25 µm thick insulated wire. Besides commonly available gold wire, we also introduce insulated copper wire to our coil winding process. The influence of copper on the transformer properties is explored and compared to gold. A simulation model based on the solenoids’ wire bonding trajectories has been defined using the FastHenry software to accurately predict and optimize the transformer's inductive properties. The transformer chips are encapsulated in polydimethylsiloxane in order to protect the coils from environmental influences and mechanical damage. Meanwhile, the effect of the increase in the internal capacitance of the chips as a result of the encapsulation is analyzed. A fabricated transformer with 20 windings in both the primary and the secondary coils, and a footprint of 1 mm2, yields an inductance of 490 nH, a maximum efficiency of 68%, and a coupling factor of 94%. The repeatability of the coil winding process was investigated by comparing the data of 25 identically processed devices. Finally, the microtransformers are benchmarked to underline the potential of the technology in rendering air core transformers competitive.

Magnetic performance and corrosion resistance of cobalt based magnetic films

To optimise the composition of magnetic materials applied in the fabrication of ultralarge scale integration (ULSI) devices and microelectromechanical systems (MEMS), the magnetic performance and anticorrosion property of cobalt based magnetic films (CoWP, CoPtP and CoMoP) were investigated using vibration sample magnetometer (VSM) and electrochemical technologies (potentiodynamic polarisation curve and electrochemical impedance spectroscopy). Results indicate that CoWP has a small coercivity (592·73 Oe) and large saturation magnetisation value (144·20 emu.g−1); while CoPtP and CoMoP films are hard magnetic materials with coercivity of 1786·57 and 2745·20 Oe, saturation magnetisation value of 91·72 and 120·74 emu.g−1 respectively. Electrochemical measurements reveal the order of corrosion resistance in 3·5%NaCl solution was CoPtP film>CoWP film>CoMoP film. In the fabrication of ULSI devices and MEMS, CoPtP film was recommend to be one of the most suitable magnetic materials. CoPtP film was thick with finer grains (52–78 nm) and lower roughness (18·212 nm).

High-stroke, high-order MEMS deformable mirrors

Monolithic fabrication of continuous facesheet high-aspect ratio gold microelectromechanical systems (MEMS) deformable mirrors (DMs) onto a thermally matched ceramic-glass substrate (WMS-15) has been performed. The monolithic process allows thick layer deposition (tens of microns) of sacrificial and structural materials thus allowing high-stroke actuation to be achieved. The fabrication process does not require wafer bonding to achieve high aspect ratio three-dimensional structures. A gold continuous facesheet mirror with 3.4 nm surface roughness has been deposited on a 16 × 16 array of X-beam actuators on a 1-mm pitch. A stroke of 6.4 μm was obtained when poking two neighboring actuators. Initial electrostatic actuation displacement results for a high-aspect ratio gold MEMS DM with a continuous facesheet will be discussed. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JMM.12.3.033012)

A Novel Three-State RF MEMS Switch for Ultrabroadband (DC-40 GHz) Applications

Design, fabrication, and measurement results of a lateral dc-contact RF microelectromechanical systems switch for ultrabroadband applications are presented in this letter. The switch is driven by a bidirectional cascaded electrothermal actuator, which can generate larger displacements and contact forces at two directions than traditional electrothermal actuators. Because of this bidirectional actuator, the proposed switch can not only realize the off-state to on-state shifting, but also provide an additional deep off-state. The proposed switch is fabricated by MetalMUMPs process, and measurement results show that the insertion loss is less than -0.5 dB and the initial isolation is better than -22.5 dB at 0-40 GHz range. At the deep off-state, the isolation better than -30 dB can be achieved at the whole frequency range 0-40 GHz. The measurement results agree well with the theory and design.

Generalized Thermal Design Model for Comb-Capacitor MEMS Devices Fabricated by Bonding-DRIE Process: A Preliminary Framework

Bonding-deep reaction ion etching (DRIE) is a standard microelectromechanical system (MEMS) fabrication technique, especially for widely-applied comb-capacitor microdevices. Being the key structure in the comb-capacitor devices, long and narrow suspended beams suffer a serious heat transfer problem during their releasing in the fabrication because of the high heat flux input and the large thermal resistance. Temperature increment of the micro beam in the DRIE releasing, especially in the unavoidable over etching stage, may cause serious problem, even lead to a failed etch. This work introduced a generalized thermal design model to estimate the possible temperature increment of microstructure in DRIE. The preliminary results indicated that this model was able to capture the basic trend of temperature variation with the geometrical parameters in a comb-capacitor MEMS device during its DRIE releasing.

Determination of Parameters With Uncertainties for Quality Control in MEMS Fabrication

A reliable technique for rapid monitoring of critical dimensional and material parameters during the fabrication of microelectromechanical systems (MEMS) is still an open task. Electrical excitation is sometimes simply not possible, or an electrical test does not reveal sufficient information for a complete characterization. In this paper, we present a new approach that employs optical testing of MEMS vibration frequencies. It is not necessary to have electrical connections because we have developed probes with transparent electrodes to excite vibrations electrostatically without contact on the wafer level. Broad-bandwidth excitation is applied to detect all relevant resonances interferometrically through the probes in a single broad-band measurement. Peak detection of the resonances results in a measurement vector containing the resonant frequencies. We demonstrate a new method to estimate system parameters by model identification. The analytical models are derived from a general approach based on finite-element analysis. The parameters with uncertainties are finally determined by guaranteed parameter estimation.

Design, fabrication, and characterization of electroless Ni–P alloy films for micro heating devices

In this work electroless nickel–phosphorous coatings were used as the micro heaters for scanning thermal microscopy. The deposition of Ni–P alloys not only simplified the microelectromechanical system fabrication steps but also provided flexibility in the tuning of the resistance of the heating elements.Ni–P films were plated on patterned silicon substrates and silicon with a silicon nitride film. The pre-deposition reactive ion etch (RIE) treatment caused a change in surface roughness that enhanced the adhesion of Ni–P coatings. Optimization of RIE parameters and pH values could achieve selective deposition of Ni–P, thus helped the lift-off of a serpentine circuit pattern.The chemical composition and microstructure of Ni–P films affect the electrical properties of micro heaters. Energy-dispersive X-ray spectroscopy identified the Ni–P composition and confirmed its insignificant level of oxidation. The high-temperature X-ray diffraction indicated that the as-deposited film was crystalline Ni, which later transformed into Ni3P at higher temperature. The resistivity of Ni–P films was tailored between 10−5 and 10−7Ωm via a post-deposition annealing, which also obtained a stable temperature coefficient of resistance. Consequently, the performance of micro heaters could be designed with a high degree of flexibility.

Performance Analysis of Twin Stack Direct Methanol Fuel Cells with Hydrophilic and Hydrophobic Anode Channels

In this study, the materials of polymethyl methacrylate (PMMA) with hydrophilic properties and polydimethylsiloxane (PDMS) with hydrophobic properties were used to form the anode cannels of direct methanol fuel cells (DMFCs). The channels of the DMFCs were fabricated through a microelectromechanical system (MEMS) fabrication process. Two DMFCs were stacked together with a common cathode channel to reduce the volume of the cell and form a twin stack DMFC. The performance of the twin stack DMFC was investigated under different operating conditions, including operation temperature and flow rate. The twin stack DMFC with an anode channel made of PDMS possessed better performance under any specific operating conditions tested in comparison to the DMFC with PMMA anode channel. However, an increase in operation temperature and fuel flow rate resulted in decreased a hydro-resistance reduction effect of PDMS and performance enhancement of the PDMS DMFC was not obvious in high temperature and high flow rate operating conditions. For the PDMS DMFC, the maximum power densities of the stack can be enhanced 70∼80% in comparison to single cells under tested operating conditions.

Fabrication of Monolithic Integrated Bimaterial Resonant Uncooled IR Sensor

In this work we studied the fabrication of a monolithic bimaterial micro-cantilever resonant IR sensor with on-chip drive circuits. The effects of high temperature process and stress induced performance degradation were investigated. The post-CMOS MEMS (micro electro mechanical system) fabrication process of this IR sensor is the focus of this paper, starting from theoretical analysis and simulation, and then moving to experimental verification. The capacitive cantilever structure was fabricated by surface micromachining method, and drive circuits were prepared by standard CMOS process. While the stress introduced by MEMS films, such as the tensile silicon nitride which works as a contact etch stopper layer for MOSFETs and releasing stop layer for the MEMS structure, increases the electron mobility of NMOS, PMOS hole mobility decreases. Moreover, the NMOS threshold voltage (Vth) shifts, and transconductance (Gm) degrades. An additional step of selective removing silicon nitride capping layer and polysilicon layer upon IC area were inserted into the standard CMOS process to lower the stress in MOSFET channel regions. Selective removing silicon nitride and polysilicon before annealing can void 77% Vth shift and 86% Gm loss.

Design and fabrication of independent-cavity FP tunable filter

This paper presents a novel micro-electromechanical systems (MEMS) FP tunable filter with a FP cavity independent of electro-static actuator, which is fabricated by the bulk micromachining processes. The electro-static actuator based on the parallel-plate capacitive actuation is separated with the FP cavity, and it can be designed without the limits to fabricate the FP cavity. The compatible MEMS fabrication processes with micromirror and FP cavity is proposed, in which the optical micromirror is fabricated with an average surface roughness (Ra) less than 6nm. The experimental results show that the fabricated FP tunable filter can be tuned 40nm with a driving voltage from 0V to 15V, its 3dB bandwidth is 0.4nm and the single mode fiber coupling loss is less than −3.7dB.