Electrothermal Microelectromechanical Systems Research Articles

Actively Tunable Spectral Filter for Compact Hyperspectral Camera Using Angle‐Sensitive Plasmonic Structures

AbstractHyperspectral imaging provides enhanced classification and identification of veiled features for diverse biomedical applications such as label‐free cancer detection or non‐invasive vascular disease diagnostics. However, hyperspectral cameras still have technical limitations in miniaturization due to the inherently complex and bulky configurations of conventional tunable filters. Herein, a compact hyperspectral camera using an active plasmonic tunable filter (APTF) with electrothermally driven spectral modulation for feature‐augmented imaging is reported. APTF consists of angle‐sensitive plasmonic structures (APS) over an electrothermal MEMS (Microelectromechanical systems) actuator, fabricated by combining nanoimprint lithography, and MEMS fabrication ona 6‐inch wafer. APS have a complementary configuration of Au nanohole and nanodisk arrays supported on asilicon nitride membrane. APTF shows a large angular motion at operational voltages of 5–9 VDC for continuous spectral modulation between 820 and 1000 nm (45 nm/V). The compact hyperspectral camera was fully packaged with a linear polarizer, APTF and amonochromatic camera, exhibiting asize of 16 mm (ϕ) × 9.5 mm (h). Feature‐augmented images of subcutaneous vein and a fresh fruit have been successfully demonstrated after the hyperspectral reconstruction and spectral feature extraction. This functional camera provides a new compact platform for point‐of‐care or in vivo hyperspectral imaging in biomedical applications.

Endoscopic optical coherence tomography angiography using inverse SNR-amplitude decorrelation features and electrothermal micro-electro-mechanical system raster scan.

Angiogenesis is closely associated with tumor development and progression. Endoscopic optical coherence tomography angiography (OCTA) enables rapid inspection of mucosal 3D vasculature of inner organs in the early-stage tumor diagnosis; however, it is limited by instabilities of the optical signal and beam scanning. In the phase-unstable swept source OCTA (SS-OCTA), amplitude decorrelation was used to compute the motion-induced changes as motion contrast. The influence of the random noise-induced amplitude fluctuations on decorrelation was characterized as a function of inverse signal-to-noise ratio (SNR) with a multi-variate time series (MVTS) model and statistical analysis. Then, the noise-induced decorrelation artifacts in static tissue regions were eliminated by applying a flow mask based on the statistical relation between inverse SNR (iSNR) and amplitude decorrelation (IDa), which was named IDa-OCTA. In addition, a distal stepwise raster scan was realized with a low-voltage electrothermal micro-electro-mechanical system (ET-MEMS)-based catheter for endoscopic imaging, whereby the stable and repeatable B-scans at each step suppressed the decorrelation noise induced by the spatial mismatch between paired scans. The derived IDa relation was validated through numerical simulation and flow phantom experiments. In vivo human buccal mucosa imaging was performed to demonstrate the endoscopic IDa-OCTA imaging. In this, the subsurface structure and vasculature were visualized in a rapid and depth-resolved manner. The rapid 3D vasculature visualization realized by the endoscopic IDa-OCTA improves the diagnosis of early tumors in internal organs.

Design and fabrication of an electrothermal MEMS micro-actuator with 3D printing technology

This study presents the design and fabrication results of an electrothermal micro-electro-mechanical system (MEMS) actuator. Unlike traditional one-directional U-shaped actuators, this bi-directional electrothermal (BET) micro-actuator can produce displacements in two directions as a single device. The BET micro-actuator was fabricated using two-photon polymerization (2PP) and digital light processing (DLP) methods, which are 3D printing techniques. These methods have been compared to see the success of BET micro-actuator fabrication. The compound of these methods and the essential coefficients through the 3D printing operation were applied. Evaluation experiments have demonstrated that in both methods, the 3D printer can print materials smaller than 95.7 μm size features. Though the same design was used for the 2PP and DLP methods, the supporting structures were not produced with the 2PP. The BET micro-actuator was manufactured by removing the supports from the original design in the 2PP. The number of supports, the diameter, and height on the arms of the micro-actuator is 18, 4 μm, and 6 μm, respectively. Although 4 μm diameter supports could be produced with the DLP, it was not possible to produce them with 3D printing device based on 2PP. Besides, the DLP was found to be better than the 2PP for the manufacturing of asymmetrical support structures. The fabrication process has been carried out successfully by two methods. When the fabrication success is compared, the surface quality and fabrication speed of the micro-actuator fabricated with DLP is better than the 2PP. Presented results show the efficiency of the 3D printing technology and the simplicity of fabrication of the micro-actuator via 2PP and DLP. An experimental study was carried out to characterize the relationship between displacement and input voltage for the micro-actuator. Experimental results show that the displacement range of the micro-actuator is 8 μm with DLP, while 6 μm with 2PP.

Electrothermal-MEMS-induced nonlinear distortion correction in photoacoustic laparoscopy.

Micro-electro-mechanical systems (MEMS) scanner has significant advantages of miniature size, fast response and high stability, which is particularly applicable to photoacoustic laparoscopy (PAL). However, tilt angle-voltage curve of electrothermal MEMS shows a nonlinear character, which leads to inevitable nonlinear distortion in photoacoustic imaging. To overcome this problem, a nonlinear distortion correction was developed for the high-resolution forward-scanning electrothermal-MEMS-based PAL. The adaptive resampling method (ARM) was introduced to adaptively calibrate the projection of non-uniform scanning region to match the uniform scanning region. The correction performed low time complexity and high portability owing to the adaptive capacity of distortion decomposition in the reconstruction of physical models. Compared with the sample structure, phantom experiments demonstrated that the distortion was calibrated in all directions and the corrected image provided up to 96.82% high structural similarity in local subset. Furthermore, ARM was applied to imaging the abdominal cavity of rat and the vascular morphology was corrected in real-time display within a delay less than 2 seconds. All these results demonstrated that the nonlinear distortion correction possessed timely and effective correction in PAL, which suggested that it had the potential to employ to any other electrothermal-MEMS-based photoacoustic imaging systems for accurate and quantitative functional imaging.

Design and Fabrication of a Kirigami-Inspired Electrothermal MEMS Scanner with Large Displacement.

Large-displacement microelectromechanical system (MEMS) scanners are in high demand for a wide variety of optical applications. Kirigami, a traditional Japanese art of paper cutting and folding, is a promising engineering method for creating out-of-plane structures. This paper explores the feasibility and potential of a kirigami-inspired electrothermal MEMS scanner, which achieves large vertical displacement by out-of-plane film actuation. The proposed scanner is composed of film materials suitable for electrothermal self-reconfigurable folding and unfolding, and microscale film cuttings are strategically placed to generate large displacement. The freestanding electrothermal kirigami film with a 2 mm diameter and high fill factor is completely fabricated by careful stress control in the MEMS process. A 200 μm vertical displacement with 131 mW and a 20 Hz responsive frequency is experimentally demonstrated as a unique function of electrothermal kirigami film. The proposed design, fabrication process, and experimental test validate the proposed scanner’s feasibility and potential for large-displacement scanning with a high fill factor.

Stability Study of an Electrothermally-Actuated MEMS Mirror with Al/SiO2 Bimorphs

Electrothermal actuation is one of the main actuation mechanisms and has been employed to make scanning microelectromechanical systems (MEMS) mirrors with large scan range, high fill factor, and low driving voltage, but there exist long-term drifting issues in electrothermal bimorph actuators whose causes are not well understood. In this paper, the stability of an bimorph electrothermal MEMS mirror operated in both static and dynamic scan mode has been studied. Particularly, the angular drifts of the MEMS mirror plate were measured over 90 h at different temperatures in the range of – °C. The experiments show that the temporal drift of the mirror plate orientation largely depends on the temperature of the electrothermal bimorph actuators. Interestingly, it is found that the angular drift changes from falling to rising as the temperature increases. An optimal operating temperature between °C to °C for the MEMS mirror is identified. At this temperature, the MEMS mirror exhibited stable scanning with an angular drift of less than °/h.

Nondestructive On-Site Detection of Soybean Contents Based on An Electrothermal MEMS Fourier Transform Spectrometer

A portable spectrophotometer has been developed to provide on-site detection of soybeans’ moisture, protein, and fat contents in a nondestructive manner. The core module of this soybean analyzer is a micro Fourier transform infrared spectrometer (μFTIR) based on an electrothermal microelectromechanical systems (MEMSs) mirror. The electrothermal MEMS mirror is capable of generating a large piston motion of 320 μ m at only 8.5 Vdc. The tilting of the MEMS mirror plate during its piston scanning affects the spectral repeatability of the μFTIR. In this paper, the effect of the MEMS mirror tilting on the spectral repeatability is analyzed and an automatic tilt realignment algorithm is developed to maintain high spectral repeatability, which is better than 0.01 in long term use. The μFTIR module has a footprint of 125 mm × 90 mm with a height of 50 mm. The whole instrument is designed for portability and on-site operation as well as the convenience of loading soybeans. More than 300 samples of soybeans are collected, 80% of which are used to establish a prediction model using chemometrics methods. The standard error is no more than 3% when using the prediction model to test the remaining 20% samples.

Ultralow-voltage electrothermal MEMS based fiber-optic scanning probe for forward-viewing endoscopic OCT.

We report an ultralow-voltage, electrothermal (ET) micro-electro-mechanical system (MEMS) based probe for forward-viewing endoscopic optical coherence tomography (OCT) imaging. The fully assembled probe has a diameter of 5.5mm and a length of 55mm, including the imaging optics and a 40mm long fiber-optic cantilever attached on a micro-platform of the bimorph ET MEMS actuator. The ET MEMS actuator provides a sufficient mechanical actuation force as well as a large vertical displacement, achieving up to a 3mm optical scanning range with only a 3 VACp-p drive voltage with a 1.5 VDC offset. The imaging probe was integrated with a swept-source OCT system of a 100kHz A-scan rate, and its performance was successfully demonstrated with cross-sectional imaging of biological tissues ex vivo and in vivo at a speed up to 200 frames per second.

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.

Essential design and fabrication considerations for the reliable performance of an electrothermal MEMS microgripper

Over the past years, microelectromechanical systems (MEMS) have become a vital component within a wide range of technologies, making the study of their performance and operational reliability a very critical aspect for their correct functionality. Microgrippers are one such type of MEMS devices with one of their key applications being the manipulation of biological tissues and cells. This paper presents a microgripper design based on the ‘hot and cold arm’ electrothermal actuation mechanism that is suitable to study the deformability properties of human red blood cells under healthy and pathological conditions. The main scope of this paper is to highlight a number of failure mechanisms that are typical of surface micromachined MEMS microgrippers. These include out-of-plane buckling of the hot arm, stress concentrations, device stiction, and residual stresses. The studied polysilicon microgripper structures were designed and fabricated in line with the specifications of the commercial fabrication process PolyMUMPsTM. The microgripper design was numerically modelled and electrothermomechanically studied using finite element analysis in CoventorWare $$^{\circledR }$$ . Experimental testing on the fabricated structures was used to demonstrate reliable microgripper performance as well as instances of the considered failure mechanisms. Results for the reliable microgripper performance show that the microgripper arms deflected as expected when actuated, with the obtained simulation and experimental results in good agreement. The investigated failure mechanisms have led to the identification of essential design and fabrication considerations whose thorough investigation, with the aid of appropriate modelling approaches, is essential to define improvement solutions and best practices to mitigate the failure or malfunction of the microgripper during operation.

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.

Fourier transform infrared spectrometer based on an electrothermal MEMS mirror.

This paper presents a micro Fourier transform infrared spectrometer (μFTIR), enabled by an H-shaped electrothermal microelectromechanical systems (MEMS) mirror. A special driving method was developed for obtaining a linear, uniform-speed motion of 186μm, and the tilting angle of the MEMS mirror was as small as 0.06°, so there was no need of complex closed-loop control. A telecentric lens was employed in the interferometer of the μFTIR to reduce the influence of the MEMS mirror tilting effect. Also, a new phase interpolation algorithm, instead of the traditional fringe interval method, was applied in the process of the spectral reconstruction to improve the spectral stability. Finally, the new μFTIR was applied in the composition prediction of soybeans, and the experimental results show that it can accurately measure grain moisture, protein, and fat contents.

Integrated Optoelectronic Position Sensor for Scanning Micromirrors.

Scanning micromirrors have been used in a wide range of areas, but many of them do not have position sensing built in, which significantly limits their application space. This paper reports an integrated optoelectronic position sensor (iOE-PS) that can measure the linear displacement and tilting angle of electrothermal MEMS (Micro-electromechanical Systems) scanning mirrors. The iOE-PS integrates a laser diode and its driving circuits, a quadrant photo-detector (QPD) and its readout circuits, and a band-gap reference all on a single chip, and it has been fabricated in a standard 0.5 μm CMOS (Complementary Metal Oxide Semiconductor) process. The footprint of the iOE-PS chip is 5 mm × 5 mm. Each quadrant of the QPD has a photosensitive area of 500 µm × 500 µm and the spacing between adjacent quadrants is 500 μm. The iOE-PS chip is simply packaged underneath of an electrothermally-actuated MEMS mirror. Experimental results show that the iOE-PS has a linear response when the MEMS mirror plate moves vertically between 2.0 mm and 3.0 mm over the iOE-PS chip or scans from −5 to +5°. Such MEMS scanning mirrors integrated with the iOE-PS can greatly reduce the complexity and cost of the MEMS mirrors-enabled modules and systems.

Design and simulation of electro-thermal compliant MEMS logic gates

PurposeThe purpose of this paper is to design an out-of-plane micro electro-thermal-compliant actuator based logic gates which work analogously to complementary metal oxide semiconductor (CMOS) based logic gates. The proposed logic gates used a single-bit mechanical micro ETC actuator per logic instead of using 6-14 individual transistors as in CMOS.Design/methodology/approachA complete analytical modelling is performed on a single ETC vertical actuator, and a relation between the applied voltage and the out-of-plane deflection is derived. Its coupled electro-thermo-mechanical analysis is carried out using micro electro mechanical system (MEMS) CAD tool CoventorWare to illustrate its performance.FindingsThis paper reports analytical and numerical simulation of basic MEMS ETC actuator-based logic gates. The proposed logic gate operates on 5 V, which suits well with conventional CMOS logic, which in turn reduces the power consumption of the device.Originality/valueThe proposed logic gates uses a single-bit MEMS ETC actuator per logic instead of using more transistors as in CMOS. The unique feature of this proposed logic gates is that the basic mechanical ETC actuator is customized in its structure to function as specific logic gates depending upon the given inputs.

Modelling and Experimental Verification of Step Response Overshoot Removal in Electrothermally-Actuated MEMS Mirrors.

Micro-electro-mechanical system (MEMS) mirrors are widely used for optical modulation, attenuation, steering, switching and tracking. In most cases, MEMS mirrors are packaged in air, resulting in overshoot and ringing upon actuation. In this paper, an electrothermal bimorph MEMS mirror that does not generate overshoot in step response, even operating in air, is reported. This is achieved by properly designing the thermal response time and the mechanical resonance without using any open-loop or closed-loop control. Electrothermal and thermomechanical lumped-element models are established. According to the analysis, when setting the product of the thermal response time and the fundamental resonance frequency to be greater than Q/2π, the mechanical overshoot and oscillation caused by a step signal can be eliminated effectively. This method is verified experimentally with fabricated electrothermal bimorph MEMS mirrors.

Design and Fabrication of a 2-Axis Electrothermal MEMS Micro-Scanner for Optical Coherence Tomography

This paper introduces an optical 2-axis Micro Electro-Mechanical System (MEMS) micromirror actuated by a pair of electrothermal actuators and a set of passive torsion bars. The actuated element is a dual-reflective circular mirror plate of 1 in diameter. This inner mirror plate is connected to a rigid frame via a pair of torsion bars in two diametrically opposite ends located on the rotation axis. A pair of electrothermal bimorphs generates a force onto the perpendicular free ends of the mirror plate in the same angular direction. An array of electrothermal bimorph cantilevers deflects the rigid frame around a working angle of 45 for side-view scan. The performed scans reach large mechanical angles of 32 for the frame and 22 for the in-frame mirror. We denote three resonant main modes, pure flexion of the frame at 205 , a pure torsion of the mirror plate at 1.286 and coupled mode of combined flexion and torsion at 1.588 . The micro device was fabricated through successive stacks of materials onto a silicon-on-insulator wafer and the patterned deposition on the back-side of the dual-reflective mirror is achieved through a dry film photoresist photolithography process.

Fringe-tunable electrothermal Fresnel mirror for use in compact and high-speed diffusion sensor.

This paper reports the development of an electrothermal microelectromechanical systems (MEMS) mirror with serpentine shape actuators. A micro Fresnel mirror with fringe-spacing tunability is required to realize a compact and high-speed diffusion sensor for biological samples whose diffusion coefficient changes significantly because of a conformational change. In this case, the measurement time-constant is dependent on the fringe-spacing and diffusion coefficient of the sample. In this study, a fringe-tunable MEMS mirror with an actuation voltage less than 10 V was developed. The characteristics of the fabricated mirror were investigated experimentally. A high-visibility optical interference fringe was successfully demonstrated using both an ultranarrow-linewidth solid-state laser and a low-cost compact laser diode. The experimental results demonstrated a distinct possibility of developing a measurement device using only simple and low-voltage optical components.

A Fourier Transform Spectrometer Based on an Electrothermal MEMS Mirror with Improved Linear Scan Range.

A Fourier transform spectrometer (FTS) that incorporates a closed-loop controlled, electrothermally actuated microelectromechanical systems (MEMS) micromirror is proposed and experimentally verified. The scan range and the tilting angle of the mirror plate are the two critical parameters for MEMS-based FTS. In this work, the MEMS mirror with a footprint of 4.3 mm × 3.1 mm is based on a modified lateral-shift-free (LSF) bimorph actuator design with large piston and reduced tilting. Combined with a position-sensitive device (PSD) for tilt angle sensing, the feedback controlled MEMS mirror generates a 430 µm stable linear piston scan with the mirror plate tilting angle less than ±0.002°. The usable piston scan range is increased to 78% of the MEMS mirror’s full scan capability, and a spectral resolution of 0.55 nm at 531.9 nm wavelength, has been achieved. It is a significant improvement compared to the prior work.

Common-path optical coherence tomography using a microelectromechanical-system-based endoscopic probe.

This paper presents a common-path (CP) swept-source optical coherence tomography (SSOCT) system based on a special endoscopic probe design with an in-line internal reflection as the reference and a two-axis electrothermal microelectromechanical system mirror for image scanning. The rear surface of a gradient reflective index (GRIN) lens inside the probe is set as the reference reflection plane. The length of the GRIN lens is optimized to eliminate the artifacts in SSOCT images successfully. Doppler OCT is also demonstrated based on the CP endoscopic probe. The diameter of the probe is only 2.5mm, so it can be easily inserted into the biopsy channel of traditional endoscopes to access human internal organs for in vivo diagnoses.

A Compact Fourier Transform Spectrometer on a Silicon Optical Bench With an Electrothermal MEMS Mirror

This paper reports a compact Fourier transform spectrometer system with a large-stroke electrothermal microelectromechanical systems (MEMS) mirror and other optical components all integrated on a micromachined silicon optical bench with the footprint of 2 cm $\times \,\, 2$ cm. The linear optical path difference (OPD) scan range generated by the MEMS mirror reaches up to 450 $\mu \text{m}$ , and the tilting of the mirror plate is reduced down to $^{{\mathbf {-1}}}$ , or 1.1 nm at 532 nm, has been achieved. The overall size of the system is reduced dramatically and the performance is improved significantly compared with the prior work. [2015-0298]