Optical Microelectromechanical Systems Research Articles

Hygrothermal aging of flip-chip assembled MOEMS

Low cost and miniaturized solutions for assembly of optical microelectromechanical systems (MOEMS) are critical for increased exploitation of optical sensors and devices in consumer applications. Examples of applications based on optical sensors and devices already near or on the market are auto-focus lenses and pico-projectors for mobile phones, physiology measurement units for wearables, and high bandwidth free-space optical communication links for our homes. In addition, extensive use of photonic integrated circuits is predicted to be mandatory in order to answer to the future needs for high data transfer rates in data centers. However, the main limitation for several of these applications is the complexity, the related cost of assembly, and the unknown reliability of several lower cost assembly solutions; polymer-based solutions are e.g. prone to fail particularly fast in humid and warm conditions. In this work, a part of a European project called Lab4MEMS II, we study the feasibility of reducing the footprint, complexity and cost of assembly of silicon micro-machined mirrors directly to organic printed circuit boards, without jeopardizing the reliability of the system. We demonstrate flip-chip assembly using novel isotropic conductive adhesives (ICAs) with a particularly low content of silver that earlier indicated stable behaviour during exposure to hygrothermal aging [1]. The new solution removes the earlier needed space around the component for wire bonds, and the assembly is simplified as electrical and mechanical joints are formed in a single step. Dummy mirrors were designed with two different pad layouts that were either optimized for a large pitch (the pads are distributed evenly around the active area) or for mechanical compliance (the pads are concentrated into three groups that are oriented in a triangle). There are 12 pads in total for each design. The outer dimension of the dies is 7 x 7 mm and the diameter of the region reserved for an active area is 5 mm. The dummy samples represent mirrors that are controllable through the inclusion of a 2 Âμm lead zirconate titanate (PZT) piezo-electrical thin film. The PZT film and the bottom (Ti/Pt) and top (TiW/Au) electrode layers are included in the dummy samples for a good representation of the actual surfaces to be bonded, both their materials and their topography. A 100 nm layer of alumina was deposited for passivation. The passivation layer was opened in the contact holes by wet etching. The layout includes both Kelvin structures and daisy chains. A commercially available and widely used ICA, Epotek H20E, was selected as a reference material for two novel ICAs, Mosaic A and Mosaic B. The Epotek material contains solid Ag particles whereas the Mosaic materials contain metal coated polymer spheres with a diameter of 10 Âμm and a Ag layer coating of 140 nm. The ICAs were stencil printed onto boards having ENIG surface finish. The dummy mirror dies were flipped and aligned, and bonded by applying a tool pressure corresponding to 9 MPa as calculated based on the nominal contact hole areas. Curing was performed for one or two hours at 150 °C, depending on the recommendations from the material providers. A total of 14 boards, with 6 dummy mirrors bonded onto each, were assembled for characterization and reliability testing. The assemblies are presently measured electrically in-situ as they are exposed to hygrothermal aging. The humidity level is 85% RH in all tests, whereas the temperature is either 65, 75 or 85 °C to enable recording of lifetime distribution curves at three different temperature loads. No failures have occurred yet, illustrating already the high potential of this assembly solution, but all samples will be run to failure. Curve fitting will be used to identify distribution functions to enable lifetime predictions, but the failures will be analysed using cross-sectioning, light microscopy and scanning electron microscopy, to identify failure modes and thereby verify the correctness of accelerated testing.

A novel technique for die-level post-processing of released optical MEMS

This work presents a novel die-level post-processing technique for dies including released movable structures. The procedure was applied to microelectromechanical systems (MEMS) chips that were fabricated in a commercial process, SOIMUMPs from MEMSCAP. It allows the performance of a clean DRIE etch of sidewalls on the diced chips enabling the optical testing of the pre-released MEMS mirrors through the chip edges. The etched patterns are defined by photolithography using photoresist spray coating. The photoresist thickness is tuned to create photoresist bridges over the pre-released gaps, protecting the released structures during subsequent wet processing steps. Then, the chips are subject to a sequence of wet and dry etching steps prior to dry photoresist removal in O2 plasma. Processed micromirrors were tested and found to rotate similarly to devices without processing, demonstrating that the post-processing procedure does not affect the mechanical performance of the devices significantly.

Devices Based on Co-Integrated MEMS Actuators and Optical Waveguide: A Review.

The convergence of Micro Electro Mechanical Systems (MEMS) and optics was, at the end of the last century, a fertile ground for a new breed of technological and scientific achievements. The weightlessness of light has been identified very early as a key advantage for micro-actuator application, giving rise to optical free-space MEMS devices. In parallel to these developments, the past 20 years saw the emergence of a less pursued approach relying on guided optical wave, where, pushed by the similarities in fabrication process, researchers explored the possibilities offered by merging integrated optics and MEMS technology. The interest of using guided waves is well known (absence of diffraction, tight light confinement, small size, compatibility with fiber optics) but it was less clear how they could be harnessed with MEMS technology. Actually, it is possible to use MEMS actuators for modifying waveguide properties (length, direction, index of refraction) or for coupling light between waveguide, enabling many new devices for optical telecommunication, astronomy or sensing. With the recent expansion to nanophotonics and optomechanics, it seems that this field still holds a lot of promises.

Highly sensitive lateral deformable optical MEMS displacement sensor: anomalous diffraction studied by rigorous coupled-wave analysis.

This paper discusses the pulse signal of a highly sensitive lateral deformable optical microelectromechanical systems (MEMS) displacement sensor based on Wood's anomalies and its corresponding tolerance. The optical reflection amplitude of the device changes with the displacement of the nanostructured grating elements. Unexpectedly, the device's original sinusoidal signal develops into a new signal form (i.e., a pulse signal), when the air gap between the two layers of gratings decreases. Since the slope of the pulse signal, namely 2.5%/nm (i.e., 0.65dB/nm), is eight times higher than that of the original signal form, namely 0.3%/nm (i.e., 0.03dB/nm), the sensitivity of the structure improves by eight times. However, this device is very sensitive to parameters such as its wavelength, period, duty ratio, and air gap. In this paper we used rigorous coupled wavelength analysis (RCWA) to analyze and optimize the respective influence of each parameter on the device's performance. We have introduced two methods to search for the optimal setting and have demonstrated the optimal settings of different incident lights. The simulation results indicate that it is close to 85% possible to achieve an actual device with the highest slope superior to 0.5%/nm and it is 64% possible that the highest slope of an actual device falls in the interval ranging from 1.0%/nm to 2.0%/nm. All the simulated data helped us better understand the tolerance of the pulse signal and guide us toward the development of an actual device.

Detecting explosives and chemical weapons: a review of recent developments

Purpose – The purpose of this paper is to provide details of recent developments in sensors for detecting explosives and chemical warfare agents. Design/methodology/approach – Following an introduction, this paper first discusses a selection of new sensing techniques aimed at detecting explosives and explosive devices. It then considers new developments in sensors for detecting chemical warfare agents. Brief concluding comments are drawn. Findings – This paper shows that a diversity of sensor technologies is being investigated, including various advanced optical methods, nanomaterials, microelectromechanical system, electronic noses, biosensors and electrochemical techniques, several of which offer levels of sensitivity in the parts-per-trillion region. These not only have the potential to yield improved devices for detecting explosives and chemical weapons but may also play a role in health care, environmental monitoring, drug detection and industrial health and safety. Originality/value – In an era of escalating terrorism and military conflicts, this provides a timely review of new technologies for detecting explosives and chemical warfare agents.

Dry etching of SiC using Ar/F2 plasma and XeF2 plasma

We investigated the SiC dry etching process using Ar/F2 plasma and XeF2 plasma. We carried out optical observation of Ar/F2 plasma and XeF2 plasma. The dominant etching species were different between Ar/F2 plasma and XeF2 plasma. The etching rates of SiC were approximately 100 nm/min at 25 sccm and 200 W for Ar/F2 plasma and 45 nm/min at 2.5 sccm and 100 W for XeF2 plasma. Vertical etching profiles and a smooth etched surface were obtained. The average roughness of the etched bottom surface was 1 nm, which satisfied the requirements for optical device fabrication. We believe that the proposed etching process using F2 of zero-global-warming-potential gases is very simple and useful for fabricating optical devices and micro-electromechanical systems (MEMSs).

Performance and Modeling of a Micro-Grating Accelerometer

We design a packaged micro-optical–electromechanical system (MOEMS) – micro-grating accelerometer – that measures the interference of light resulting from its passage through a diffraction grating and reflection from the proof mass. In the present glass–silicon–glass embodiment, the diffraction optical element made of gold and the sensing proof mass supported by eight aluminum symmetrical cantilevers are fabricated on glass and silicon, respectively. Additional system components include a semiconductor laser, photodiodes, and the necessary electronics. First, we analyze the optical model to achieve high optical sensitivity. Then we optimize the structure parameters using the finite element method. Finally, we successfully fabricated the micro-grating accelerometer and packaged it in a small surface-mount package (12 × 12 × 3.6 mm). Our experiments show that the micro-grating accelerometer has a sensitivity of 5.6 V/G in the linear operating region, a bias stability of 0.94 μG, a resolution of 5.6 μG, and a dynamic range of 82.5 dB. Such a MOEMS accelerometer integrates the micro-grating with the proof mass through an anodic bonding procedure with the advantages of small size and high resolution.

Optical MEMS: From Micromirrors to Complex Systems

Microelectromechanical system (MEMS) technology, and surface micromachining in particular, have led to the development of miniaturized optical devices with a substantial impact in a large number of application areas. The reason is the unique MEMS characteristics that are advantageous in fabrication, systems integration, and operation of micro-optical systems. The precision mechanics of MEMS, microfabrication techniques, and optical functionality all make possible a wide variety of movable and tunable mirrors, lenses, filters, and other optical structures. In these systems, electrostatic, magnetic, thermal, and pneumatic actuators provide mechanical precision and control. The large number of electromagnetic modes that can be accommodated by beam-steering micromirrors and diffractive optical MEMS, combined with the precision of these types of elements, is utilized in fiber-optical switches and filters, including dispersion compensators. The potential to integrate optics with electronics and mechanics is a great advantage in biomedical instrumentation, where the integration of miniaturized optical detection systems with microfluidics enables smaller, faster, more-functional, and cheaper systems. The precise dimensions and alignment of MEMS devices, combined with the mechanical stability that comes with miniaturization, make optical MEMS sensors well suited to a variety of challenging measurements. Micro-optical systems also benefit from the addition of nanostructures to the MEMS toolbox. Photonic crystals and microcavities, which represent the ultimate in miniaturized optical components, enable further scaling of optical MEMS. [2013-0297]

Multistep-shaping control based on the static and dynamic behavior of nonlinear optical torsional micromirror

Electrostatically driven torsional micromirrors are suitable for optical microelectromechanical systems due to their good dynamic response, low adhesion, and simple structure for large-scale-integrated applications. For these devices, how to eliminate the excessive residual vibration in order to achieve more accurate positioning and faster switching is an important research topic. Because of the known nonlinearity issues, traditional shaping techniques based on linear theories are not suitable for nonlinear torsional micromirrors. In addition, due to the difficulties in calculating energy dissipation, the existing nonlinear command shaping techniques using energy method have neglected the effect of damping. We analyze the static and dynamic behavior of the electrostatically actuated torsional micromirrors. Based on the response of these devices, a multistep-shaping control considering the damping effects and the nonlinearity is proposed. Compared to the conventional closed-loop control, the proposed multistep-shaping control is a feedforward approach which can yield a good enough performance without extra sensors and actuators. Simulation results show that, without changing the system structure, the preshaping input reduces the settling time from 4.3 to 0.97 ms, and the overshoot percentage of the mirror response is decreased from 33.2% to 0.2%.

2D MEMS based diagonal switching design for simultaneous optical XOR/XNOR operation

Simultaneous logic and its inverse in a single circuit is an important design because input data of a circuit cannot be lost during operation. In this manuscript an optical microelectromechanical system (MEMS) based diagonal switching architecture is proposed and described by which simultaneous n-variable XOR/XNOR are shown. Also numerical simulations have been done to get the performance of the logic unit.

A MEMS platform for in situ, real-time monitoring of electrochemically induced mechanical changes in lithium-ion battery electrodes

We report the first successful demonstration of an optical microelectromechanical systems (MEMS) sensing platform for the in situ characterization of electrochemically induced reversible mechanical changes in lithium-ion battery (LIB) electrodes. The platform consists of an array of flexible membranes with a reflective surface on one side and a thin-film LIB electrode on the other side. The membranes deflect due to the active battery material volume change caused by lithium intercalation (expansion) and extraction (contraction). This deflection is monitored using the Fabry–Perot optical interferometry principle. The active material volume change causes high internal stresses and mechanical degradation of the electrodes. The stress evolution observed in a silicon thin-film electrode incorporated into this MEMS platform follows a ‘first elastic, then plastic’ deformation scheme. Understanding of the internal stresses in battery electrodes during discharge/charge is important for improving the reliability and cycle lifetime of LIBs. The developed MEMS platform presents a new method for in situ diagnostics of thin-film LIB electrodes to aid the development of new materials, optimization of electrode performance, and prevention of battery failure.

Large negative thermal expansion of a polymer driven by a submolecular conformational change

Mechanoresponsive polymers hold great technological potential in drug delivery, 'smart' optical systems and microelectromechanical systems. However, hysteresis and fatigue (associated with large-scale polymer chain rearrangement) are often problematic. Here, we describe a polyarylamide film that contains s-dibenzocyclooctadiene (DBCOD), which can generate unconventional and completely reversible thermal contraction under low-energy stimulation. The films exhibit a giant negative thermal expansion coefficient of approximately -1,200 ppm K(-1) at ambient or near-ambient temperatures, much higher than any known negative-thermal-expansion materials under similar operating conditions. Mechanical characterization, calorimetry, spectroscopic analysis and density-functional theory calculations all point to the conformational change of the DBCOD moiety, from the thermodynamic global energy minimum (twist-boat) to a local minimum (chair), as the origin of this abnormal thermal shrinkage. This newly identified, low-energy-driven, thermally agile molecular subunit opens a new pathway to creating near-infrared-based macromolecular switches and motors, and for ambient thermal energy storage and conversion.

Simulation of Gold Nanoparticles Aggravating MEMS Cantilever Optical Static Detection Biochip

A kind of gene detection biochip model based on biological micro electro mechanical systems (BioMEMS) technology and micro optical electro mechanical systems (MOEMS) technology is designed and simulated. In order to detect whether there are nucleic acid components in the testing samples, the biochip in this study issues horizontal light by laser, then receives and reads the deformation signals of MEMS cantilever by optical detector. The MEMS optical reflecting system can amplify MEMS cantilever deformation signal 22 times by micro reflectors which are set on the side wall of the cantilever free end. In order to improve optical detection sensitivity, gold nanoparticles (GNPs) which are combined with hybridization information is taken to aggravate MEMS cantilever, and employ Au - S chemical bond of GNPs and dithiol HS(CH2)6SH to combine and fix DNA probe, and then employ target DNA which is marked with biotin to combine GNPs by Biotin - Streptavidin combining. The simulation results show that this biochip can detect biological samples fast, high throughput, low cost, high sensitivity and reliably.

Tension–Torsion Combined Loading Test Equipment for a Minute Beam Specimen

In this article, the development of quasi-static tension–torsion combined loading test equipment for a microscale beam specimen is described. The equipment is composed of a piezoelectric actuator in actuator case for uniaxial tensile loading, a load cell for measuring X-Y-Z-axes forces and θ-axis torque, a stepping motor for rotating sample stage, X-Y-Z-stages for alignment, and a CCD camera for measuring tensile elongation using original image analysis software. The shape and dimension of all the mechanical jigs were designed by means of finite element analysis (FEA). The tension and torsion loading systems are able to be individually operated, so that uniaxial tension, pure torsion, and combined tension–torsion loadings can be realized. The specimen that was designed in consideration of typical optical microelectromechanical systems (MEMS) devices consists of a mirror plate supported by two microbeam structures, four springs, and a frame with chucking holes. Single crystal silicon (SCS) specimens were fabricated by deep reactive ion etching (DRIE). It was confirmed that the above-described three types of loadings were able to be successfully applied to the beam specimens. All the specimens fractured in a brittle manner and showed different-shape fracture surfaces under different deformation modes.

Design and Demonstration of an In-Plane Silicon-on-Insulator Optical MEMS Fabry–Pérot-Based Accelerometer Integrated With Channel Waveguides

In this paper, we present a novel optical microelectromechanical systems (MEMS) accelerometer sensor dedicated to space applications. An in-plane Fabry-Pérot (FP) microcavity (FPM) with two distributed Bragg reflectors (DBRs) is used to detect the acceleration. One of the DBR mirrors is attached to two suspended proof masses, allowing the FP gap to change while proof masses experience acceleration. Acceleration is then detected by measuring the spectral shift of the FPM. The optical accelerometer presented here uses silicon strip waveguides integrated with MEMS on a single silicon-on-insulator wafer, making it compact and robust. All of the device components are fabricated using one single fabrication step. Immunity to electromagnetic interference, high sensitivity and resolution capability, integrability, reliability, low cross-sensitivity, simple fabrication, and possibility of having two- and three-axis sensitivities are numerous advantages of our sensor compared to the conventional ones. The sensor performance demonstrated a 90-nm/g sensitivity and 111-μg resolution and better than 250-mg dynamic range.

Fabrication and characterization of a piezoelectric gallium nitride switch for optical MEMS applications

The utilization of gallium nitride (GaN) in the area of microelectronics continues to increase due to the advantages it offers over other semiconductors. This paper presents GaN as a candidate material well suited for novel microelectromechanical systems (MEMS). The potential of GaN for MEMS is demonstrated via the design, analysis, fabrication, testing and characterization of an optical microswitch device actuated by piezoelectric and electrostrictive means. The piezoelectric and electrostrictive properties of GaN and its differences from common piezoelectrics are briefly discussed before touching upon the device configuration used to implement the microswitch device. Subsequently, the development of two recent fabrication technologies, photoelectrochemical etching and bias-enabled dark electrochemical etching, used to realize the three-dimensional device structure in GaN are described in detail. Lastly, an ultra-low-cost, laser-based, non-contact approach to test and characterize the microswitch device is described, followed by the device testing results.

Multifunctional optical MEMS sensor platform with heterogeneous fiber optic Fabry–Pérot sensors for wireless sensor networks

A multifunctional optical sensor platform based on the microelectromechanical systems (MEMS) technology is developed with the aim of miniaturizing conventional bulky optical measurement systems for application in future optical wireless sensor networks (WSNs). Low-coherence interferometry is used as the sensing principle, which allows for high performance sensing with multiple heterogeneous sensors. The entire platform with the associated electric circuit has a footprint of 2in.×1.8in.×0.75in., which are integrated with a conventional WSN module to form an optical WSN node. The multifunctionality of the platform is successfully demonstrated for simultaneous interrogation of a Fabry–Pérot (FP) pressure sensor and a FP temperature sensor. To prove the optical sensor platform's multifunctionality, chemical sensing is also demonstrated with a FP sensor. The multifunctional optical sensor platform and the optical WSN node will enable optical WSNs, which are expected to impact many fronts including those space-constrained applications and monitoring in harsh environments.

Crank‐shaped and Y‐shaped through‐hole interconnections filled with Au‐Sn solder

AbstractCrank‐shaped and Y‐shaped through‐hole interconnections (THI) filled with Au‐Sn solder are demonstrated in this study. They are expected to realize a higher‐density wafer‐level package (WLP) for electrical devices including the fields of MEMS (Micro Electro‐Mechanical Systems) and MOEMS (Micro Optical Electro‐Mechanical Systems) compared with conventional WLP using THI consisting of straight through‐holes. In this paper, formation techniques for crank‐shaped and Y‐shaped THI are described. In order to make crank‐shaped and Y‐shaped through‐holes in a substrate, both femtosecond laser irradiation and wet chemical etching are used. Crank‐shaped through‐holes with an 80‐μm opening and 400 μm deep have also been achieved. Gold (Au)‐tin (Sn) solder was filled into the holes using the molten metal suction method (MMSM). Airtightness of the THI was examined using the helium leakage test, and the estimated leakage rate was less than 1.0 × 10−9 Pa·m3/s, which is sufficient for use in WLP applications. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(1): 54–62, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21134

An Equivalent Circuit Model for Vertical Comb Drive MEMS Optical Scanner Controlled by Pulse Width Modulation

An equivalent circuit model for the resonant-type vertical comb-driven optical MEMS (micro electro mechanical system) scanner has been developed based on the simple extension of the parallel-plate model. Both the electrostatic torque and the induction charge models are interpreted by using the equation-defined nonlinear dependent current source on the electrical circuit simulator platform. A systematic procedure has been investigated to fit the analytical model with the experimental results by using fitting parameters including the damping coefficient, the suspension width, and the comb gap. The developed simulation model has been used to verify a new control scheme to tune the scanner's oscillation amplitude in resonance by means of the pulse-width modulation of voltage at a given pulse height; the result indicates the effective use of digital electronics for the comb-driven optical scanner.

Design and fabrication of GaN crystal ultra-small lateral comb-drive actuators

Ultra-small electromechanical comb-drive actuators made of GaN crystal were studied in order to apply them to optical micro-electromechanical systems. Using GaN crystals grown on Si substrates by metal-organic chemical vapor deposition, two kinds of electrostatic comb-drive actuators were designed and fabricated. In the fabrication, due to a residual stress of the grown crystal, the movable part of the actuator suffered considerable deformation depending on the growth conditions. The strain-stress issue of the grown crystal layer is discussed on the basis of lattice misfit and thermal expansion. To compensate for a convex deformation, crystallization tension of a thin HfO2 film deposited on a GaN layer was investigated. The displacement of the actuator having dimensions of 52.2 μm in width and 105.4 μm in length was 1.3 μm at 70 V. Several variable systems will be feasible by combining the actuators monolithically with GaN opt-electronic devices. Future applications are also briefly discussed.