Microelectromechanical Systems Fabrication Technology Research Articles

The Effects of Cold Arm Width and Metal Deposition on the Performance of a U-Beam Electrothermal MEMS Microgripper for Biomedical Applications.

Microelectromechanical systems (MEMS) have established themselves within various fields dominated by high-precision micromanipulation, with the most distinguished sectors being the microassembly, micromanufacturing and biomedical ones. This paper presents a horizontal electrothermally actuated ‘hot and cold arm’ microgripper design to be used for the deformability study of human red blood cells (RBCs). In this study, the width and layer composition of the cold arm are varied to investigate the effects of dimensional and material variation of the cold arm on the resulting temperature distribution, and ultimately on the achieved lateral displacement at the microgripper arm tips. The cold arm widths investigated are 14 m, 30 m, 55 m, 70 m and 100 m. A gold layer with a thin chromium adhesion promoter layer is deposited on the top surface of each of these cold arms to study its effect on the performance of the microgripper. The resultant ten microgripper design variants are fabricated using a commercially available MEMS fabrication technology known as a silicon-on-insulator multi-user MEMS process (SOIMUMPs)™. This process results in an overhanging 25 m thick single crystal silicon microgripper structure having a low aspect ratio (width:thickness) value compared to surface micromachined structures where structural thicknesses are of the order of 2 m. Finite element analysis was used to numerically model the microgripper structures and coupled electrothermomechanical simulations were implemented in CoventorWare. The numerical simulations took into account the temperature dependency of the coefficient of thermal expansion, the thermal conductivity and the electrical conductivity properties in order to achieve more reliable results. The fabricated microgrippers were actuated under atmospheric pressure and the experimental results achieved through optical microscopy studies conformed with those predicted by the numerical models. The gap opening and the temperature rise at the cell gripping zone were also compared for the different microgripper structures in this work, with the aim of identifying an optimal microgripper design for the deformability characterisation of RBCs.

MEMS Actuators for Optical Microendoscopy.

Growing demands for affordable, portable, and reliable optical microendoscopic imaging devices are attracting research institutes and industries to find new manufacturing methods. However, the integration of microscopic components into these subsystems is one of today’s challenges in manufacturing and packaging. Together with this kind of miniaturization more and more functional parts have to be accommodated in ever smaller spaces. Therefore, solving this challenge with the use of microelectromechanical systems (MEMS) fabrication technology has opened the promising opportunities in enabling a wide variety of novel optical microendoscopy to be miniaturized. MEMS fabrication technology enables abilities to apply batch fabrication methods with high-precision and to include a wide variety of optical functionalities to the optical components. As a result, MEMS technology has enabled greater accessibility to advance optical microendoscopy technology to provide high-resolution and high-performance imaging matching with traditional table-top microscopy. In this review the latest advancements of MEMS actuators for optical microendoscopy will be discussed in detail.

The Effects of Structure Thickness, Air Gap Thickness and Silicon Type on the Performance of a Horizontal Electrothermal MEMS Microgripper

The ongoing development of microelectromechanical systems (MEMS) over the past decades has made possible the achievement of high-precision micromanipulation within the micromanufacturing, microassembly and biomedical fields. This paper presents different design variants of a horizontal electrothermally actuated MEMS microgripper that are developed as microsystems to micromanipulate and study the deformability properties of human red blood cells (RBCs). The presented microgripper design variants are all based on the U-shape ‘hot and cold arm’ actuator configuration, and are fabricated using the commercially available Multi-User MEMS Processes (MUMPs®) that are produced by MEMSCAP, Inc. (Durham, NC, USA) and that include both surface micromachined (PolyMUMPs™) and silicon-on-insulator (SOIMUMPs™) MEMS fabrication technologies. The studied microgripper design variants have the same in-plane geometry, with their main differences arising from the thickness of the fabricated structures, the consequent air gap separation between the structure and the substrate surface, as well as the intrinsic nature of the silicon material used. These factors are all inherent characteristics of the specific fabrication technologies used. PolyMUMPs™ utilises polycrystalline silicon structures that are composed of two free-standing, independently stackable structural layers, enabling the user to achieve structure thicknesses of 1.5 μm, 2 μm and 3.5 μm, respectively, whereas SOIMUMPs™ utilises a 25 μm thick single crystal silicon structure having only one free-standing structural layer. The microgripper design variants are presented and compared in this work to investigate the effect of their differences on the temperature distribution and the achieved end-effector displacement. These design variants were analytically studied, as well as numerically modelled using finite element analysis where coupled electrothermomechanical simulations were carried out in CoventorWare® (Version 10, Coventor, Inc., Cary, NC, USA). Experimental results for the microgrippers’ actuation under atmospheric pressure were obtained via optical microscopy studies for the PolyMUMPs™ structures, and they were found to be conforming with the predictions of the analytical and numerical models. The focus of this work is to identify which one of the studied design variants best optimises the microgripper’s electrothermomechanical performance in terms of a sufficient lateral tip displacement, minimum out-of-plane displacement at the arm tips and good heat transfer to limit the temperature at the cell gripping zone, as required for the deformability study of RBCs.

CMOS MEMS Fabrication Technologies and Devices.

This paper reviews CMOS (complementary metal-oxide-semiconductor) MEMS (micro-electro-mechanical systems) fabrication technologies and enabled micro devices of various sensors and actuators. The technologies are classified based on the sequence of the fabrication of CMOS circuitry and MEMS elements, while SOI (silicon-on-insulator) CMOS MEMS are introduced separately. Introduction of associated devices follows the description of the respective CMOS MEMS technologies. Due to the vast array of CMOS MEMS devices, this review focuses only on the most typical MEMS sensors and actuators including pressure sensors, inertial sensors, frequency reference devices and actuators utilizing different physics effects and the fabrication processes introduced. Moreover, the incorporation of MEMS and CMOS is limited to monolithic integration, meaning wafer-bonding-based stacking and other integration approaches, despite their advantages, are excluded from the discussion. Both competitive industrial products and state-of-the-art research results on CMOS MEMS are covered.

Development and characterization of hollow microprobe array as a potential tool for versatile and massively parallel manipulation of single cells.

Parallel manipulation of single cells is important for reconstructing in vivo cellular microenvironments and studying cell functions. To manipulate single cells and reconstruct their environments, development of a versatile manipulation tool is necessary. In this study, we developed an array of hollow probes using microelectromechanical systems fabrication technology and demonstrated the manipulation of single cells. We conducted a cell aspiration experiment with a glass pipette and modeled a cell using a standard linear solid model, which provided information for designing hollow stepped probes for minimally invasive single-cell manipulation. We etched a silicon wafer on both sides and formed through holes with stepped structures. The inner diameters of the holes were reduced by SiO2 deposition of plasma-enhanced chemical vapor deposition to trap cells on the tips. This fabrication process makes it possible to control the wall thickness, inner diameter, and outer diameter of the probes. With the fabricated probes, single cells were manipulated and placed in microwells at a single-cell level in a parallel manner. We studied the capture, release, and survival rates of cells at different suction and release pressures and found that the cell trapping rate was directly proportional to the suction pressure, whereas the release rate and viability decreased with increasing the suction pressure. The proposed manipulation system makes it possible to place cells in a well array and observe the adherence, spreading, culture, and death of the cells. This system has potential as a tool for massively parallel manipulation and for three-dimensional hetero cellular assays.

Design and fabrication of a mems test socket with an attached tip for a ball-grid-array integrated circuit package

Microelectromechanical system (MEMS) test sockets with attached tips were developed for a ball-grid-array integrated circuit (BGA IC) package, using MEMS fabrication technology in conjunction with Ni-Co electroplating and XeF2 isotropic silicon etching. The electroplating process was used to create metal tips and thick metal cantilevers, which increased the contact reliability and conductivity. XeF2 isotropic silicon etching was used to release the cantilever array, which reduced the process cost and increased the mechanical stability. A finite element analysis (FEA) simulation was conducted to verify the force-deflection relationships of five types of cantilevers with attached tips. With an input deflection of 40 μm, the measured forces ranged from 12.55 to 26.14 mN, and matched the FEA results. The path resistance between a tip and the appropriate electrode pad was less than 3.49 Ω for all cantilevers when the tips were connected to the corresponding positions with more than 5 mN of contact force. The cantilever design with a length of 430 um and a tip height of 60 um (L430D60) showed the best performance in terms of the force-deflection relationship, a well as resistance. Therefore, the fabricated MEMS test sockets with tips are potentially applicable to actual BGA IC package tests.

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.

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.

Analysis and fabrication of a novel MEMS pendulum angular accelerometer with electrostatic actuator feedback

A new type of sensor to directly detect angular acceleration is essential for inertial and control technology. The above interest motivates us to propose a novel micro electromechanical system (MEMS) pendulum angular accelerometer with electrostatic actuator feedback. It adopts a proof pendulum with optimized moment of inertia, suspended to dual anchors by a pair of torsion spring beams, as sensing component. A pair of electrodes are designed as differential capacitors to detect the torsional angular of pendulum, then measure input angular acceleration in sensing axis. Another pair of electrodes are designed as electrostatic actuators for feedback control loop. The structure and operating principle of the MEMS angular accelerometer are introduced. Then, the structure kinetics analysis and signal detecting scheme based on differential capacitors are provided in detail, and the sensitivity and resolution of sensor are derived. Compared with the other MEMS angular accelerometers, the proof pendulum with optimized moment of inertia improves sensitivity and resolution of sensor. The electrostatic actuators feedback loop optimizes the dynamic capability and nonlinearity characteristic. The sensor is fabricated by MEMS fabrication technology. The ANSYS simulation and test results prove the validity of the theoretical analyses. The MEMS angular accelerometer can be used in industrial robots and aircraft by further implementing the signal processing electrocircuit.

MEMS Capacitive Accelerometer-Based Middle Ear Microphone

The design, implementation, and characterization of a microelectromechanical systems (MEMS) capacitive accelerometer-based middle ear microphone are presented in this paper. The microphone is intended for middle ear hearing aids as well as future fully implantable cochlear prosthesis. Human temporal bones acoustic response characterization results are used to derive the accelerometer design requirements. The prototype accelerometer is fabricated in a commercial silicon-on-insulator (SOI) MEMS process. The sensor occupies a sensing area of 1mm × 1mm with a chip area of 2mm × 2.4mm and is interfaced with a custom-designed low-noise electronic IC chip over a flexible substrate. The packaged sensor unit occupies an area of 2.5mm × 6.2mm with a weight of 25mg. The sensor unit attached to umbo can detect a sound pressure level (SPL) of 60dB at 500Hz, 35dB at 2kHz, and 57dB at 8kHz. An improved sound detection limit of 34-dB SPL at 150Hz and 24-dB SPL at 500Hz can be expected by employing start-of-the-art MEMS fabrication technology, which results in an articulation index of approximately 0.76. Further micro/nanofabrication technology advancement is needed to enhance the microphone sensitivity for improved understanding of normal conversational speech.

The fast‐moving world of MEMS technology

PurposeThe purpose of this paper is to provide a technical review of silicon micro‐electromechanical systems (MEMS) technology and its applications.Design/methodology/approachFollowing an introduction, the paper describes silicon MEMS fabrication and assembly techniques, considers a selection of commercially important products and their applications and concludes with a brief review of power MEMS research.FindingsSilicon MEMS fabrication technology is derived from techniques used in semiconductor manufacture and has yielded a diverse and ever‐growing range of sensors, actuators and other miniaturised devices that find applications in a multitude of industries.Originality/valueThis paper provides a detailed technical review of MEMS technology and its applications.

Tunable Pneumatic Microoptics

A new class of tunable and actuated microoptical devices is presented: pneumatic microoptics. Using microelectromechanical system fabrication technology extended by the use of polydimethylsiloxane (PDMS) membranes, tunable microlenses, and lens arrays, actuated micromirrors with large tilt angles and tip-tilt piston mirrors have been designed and fabricated. Actuation is by pressure: Gas- or liquid-filled microfluidic cavities are employed to distend the microfabricated PDMS structures which then act as a lens surface or as an actuator for a micromirror. Thermopneumatic actuation is also employed for completely integrated tunable optical systems in which all actuator and optical components are fabricated on-chip. The technology is particularly promising for microsystem applications in which significant movement is required but high voltages or external fields are impractical. [2007-0301].

MEMS Based Micro-reformer: Performance with Catalyst Packing and Catalyst Coating

MEMS (micro-electro-mechanical system) fabrication technology was used to reduce the size of reformer and steam reforming catalyst was coated on the wall of micro-channels by a novel process. The comparison of performance with the packing the catalyst inside the micro-channels was also carried out. For that purpose, a process for packing catalyst inside micro- channels was developed. Serpentine type of micro-channels was used for both cases. Different issues for packing and coating inside the micro-channels like stability of the coating and durability of the system were examined.

Characteristics of a commercially available silicon-on-insulator MEMS material

Design of microelectromechanical systems (MEMS) requires an understanding of the material characteristics inherent to the specific processing technologies. In this study, several design-critical characteristics of a silicon-on-insulator (SOI) MEMS fabrication technology (SOIMUMPs) have been examined and compared to a surface micromachined MEMS fabrication technology (polyMUMPs), both produced by MEMSCAP, Inc. Diagnostic structures were utilized to evaluate material characteristics including: minimum achievable feature size, deviation of feature size from nominal design dimensions, through-thickness residual strain gradient, in-plane residual strain, resistivity, surface roughness, and fracture strength. In general, most of the measured characteristics were similar for SOIMUMPs and polyMUMPs, in spite of their very different fundamental process constraints. For example, the characteristic fracture strength was 1.97 and 1.43 GPa, with associated Weibull moduli of 8.9 and 14.0, respectively. These values are similar given the very different sources of silicon: mechanically thinned single crystal silicon wafers versus low-pressure chemical vapor deposition for polycrystalline films. Fractography suggests that fracture strength for the both technologies is controlled by surface topography features: individual sidewall etching defects drive SOIMUMPs failures whereas top-surface grain boundary crevices drive polyMUMPs failures. Discussion focuses on the ramifications of the process-controlled material characteristics in SOIMUMPS and polyMUMPs to the design of MEMS components.

MEMS fabrication based on nickel-nanocomposite: film deposition and characterization

Electrosynthesized metal-nanocomposites represent a unique class of microelectromechanical systems (MEMS) structural material due to their compatibility with MEMS and CMOS fabrication technologies via a one-step, selective on-chip deposition process at low temperatures (∼50–90 °C). In this paper, particle-reinforced, electrosynthesized nickel-nanocomposites of various compositions have been successfully demonstrated as potential structural materials for MEMS. We have achieved low-temperature, stress-free, wafer-level fabrication of nano-composite MEMS via both an electroless nickel (EN) and an electrolytic nickel (EL) deposition process, with the addition of uniformly dispersed micro- and nanoparticles of either cordierite (diameter ≈ 100 nm–5 µm) or diamond (diameter ∼ 4 nm). The as-deposited nickel-cordierite films exhibit better thermal compatibility with silicon, compared to nickel. The measured coefficients of thermal expansion (CTE) of EN-cordierite and EN are 17.34 ppm K−1 and 26.69 ppm K−1, respectively. Stress–temperature measurement of EN-cordierite composite also confirms that residual stress decreases with the incorporation of cordierite. Finally, by adding various concentrations of nanodiamond particles into an EL matrix, it is found that higher diamond concentration renders these films more compressively stressed.

An Analytical Model for EHD-Enhanced Microscale Thin-Film Evaporation

An analytical model has been developed to predict the heat transfer and flow characteristics of electrohydrodynamic (EHD)-enhanced microscale ultra thin-film evaporation. The model described in this paper is based on a previously published microcooling device [1] that incorporated an active evaporative cooling surface, an EHD micropump, and temperature sensors into a single chip. The device was fabricated using microelectromechanical systems fabrication technology, allowing the EHD electrodes and temperature sensors to be integrated directly onto the cooling surface. One end of the device was immersed in a pool of liquid. The film originated at the liquid–vapor interface and flowed upward under the influence of the electric field. The model predicts the film thickness, dryout location, local and average heat transfer coefficients, and velocity profile. The agreement between the model and the experimental data is satisfactory. Both the analytical model developed in this study and the experimental results reported previously will facilitate the design of new microcooling devices capable of operating at high power levels.

Design and simulation of an implantable medical drug delivery system using microelectromechanical systems technology

Abstract A unique design of an implantable micropump for medical drug delivery systems was proposed. The peristaltic pumping principle was selected. Three pump chambers are individually actuated by each bulk PZT (lead zirconate titanate) disk in a peristaltic motion. It is this peristaltic motion that propels the fluid. The design of the micropump includes inlet, three pump chambers, three silicon membranes, three normally closed active valves, three bulk PZT actuators, three actuation reservoirs, flow microchannels, and outlet. To prohibit flow when no power is applied, the micropump was designed to be normally closed. The pump features an integral valve/membrane design such that the pump chambers not only pump the liquid, but also function as the inlet and outlet valves. To determine the dimensions of the proposed micropump, analytical modeling of the micropump chamber was conducted. The design tradeoffs between maximizing the pumped volume and reducing the overall size of the proposed micropump were analyzed. An electromechanical coupled field simulation using the FEA method was employed. Based upon the simulation results, 6 and 12 mm diameter silicon membranes with different thickness of 40 and 80 μm were fabricated using microelectromechanical systems (MEMS) technology. The deflection of these silicon membranes was tested. The PZT actuator was manually glued onto the micropump chamber. The testing data agreed well with the FEA simulation of the deflection. The conductive adhesive layer dramatically reduces the deflection. A 12 mm in diameter and 40 μm thick silicon membrane in each pump chamber is needed to meet the micropump design requirements. The fabrication and experiments of these silicon membranes reported in this paper determine the dimensions and fabrication processes for the complete micropump. A 70 mm ×35 mm ×1.0 mm micropump will be fabricated using MEMS fabrication technology. The complete micropump will be characterized to verify our design.

An improved TMAH Si-etching solution without attacking exposed aluminum

In MEMS fabrication technology, it is often desirable to etch silicon substrate or polysilicon sacrificial layer after metallization is completed. Most of the etching solution would attack the aluminum. In this paper, an improved tetramethyl ammonium hydroxide (TMAH) etching method is reported. The advantages of the process are to achieve high silicon etching rate (0.9–1.0 μm/min at 85°C) and smooth silicon surface (less than 0.2 μm root-mean-square). Another important advantage is that no significant aluminum etching is observed during this process. The etchant used in the study consists of 5 wt.% TMAH solution, 1.4 wt.% (or above) dissolved silicon, and 0.4–0.7 wt.% (NH 4) 2S 2O 8 ammonium peroxodisulfate. The aluminum etching rate is significantly reduced by the addition of the ammonium peroxodisulfate, which also significantly improved the silicon surface roughness. When dissolving a suitable amount of silicon in TMAH etchant, along with a suitable amount of ammonium peroxodisulfate, the aluminum etching rate is reduced to zero. After TMAH etching the aluminum surfaces are protected by the coating of by-products, which prevents the underlying aluminum film from being etched by TMAH solution. The etching process demonstrated in this work is readily applicable to MEMS (micro-electromechanical system) device fabrication, such as polysilicon sacrificial layer removal, after the metallization is completed.

Surface-tension-driven microactuation based on continuous electrowetting

This paper describes the first microelectromechanical systems (MEMS) demonstration device that adopts surface tension as the driving force. A liquid-metal droplet can be driven in an electrolyte-filled capillary by locally modifying the surface tension with electric potential. We explore this so-called continuous electrowetting phenomenon for MEMS and present crucial design and fabrication technology that reduce the surface-tension-driving principle, inherently powerful in microscale, into practice. The key issues that are identified and investigated include the problem of material compatibility, electrode polarization, and electrolysis, as well as the micromachining process. Based on the results from the initial test devices and the design concept for a long-range movement of the liquid-metal droplet, we demonstrate a liquid micromotor, an electrolyte and liquid-metal droplets rotating along a microchannel loop. Smooth and wear-free rotation of the liquid system is shown at a speed of /spl sim/40 mm/s (or 420 r/min along a 2-mm loop) with a driving voltage of only 2.8 V and little power consumption (10-100 /spl mu/W).