Electrothermal Bimorph Actuators Research Articles

An approach for achieving uniform temperature distribution on the bimorphs of electrothermal micromirrors

Bimorph-based electrothermal micromirrors can generate large lateral-shift-free (LSF) vertical displacement by employing electrothermal bimorph actuators with multiple segments of bimorphs. The vertical displacement of an LSF bimorph actuator is maximized when all the bimorph segments have equal temperature change. However, due to the distributed heat conduction and convection, the temperature changes on different bimorph segments are different. We report a new LSF bimorph actuator design with non-uniform width bimorph segments to achieve a uniform temperature distribution among multiple bimorph segments. A model is developed to optimize the new LSF bimorph design with three biomorph segments with the widths of 25 µm, 110 µm and 75 µm, which reaches the same average temperature of 316 K on the three bimorphs with incentive current of 5 mA and convection coefficient of 500 W/m2K.

A 1×20 MEMS mirror array with large scan angle and low driving voltage for optical wavelength-selective switches

Wavelength-selective switches (WSSs) are the core devices for reconfigurable optical add-drop multiplexers (ROADMs) that are essential in all-optical networks. WSSs based on microelectromechanical (MEMS) mirror arrays exhibit high potential due to their high switching speed and large port counts, but there still exist challenges to achieve large optical scanning angle at low voltage. This paper presents a 1×20 MEMS mirror array (MMA) design that can scan over 10° in both axes at less than 5 V. The MMA is based on an unique electrothermal bimorph actuator design that can perform 2D scanning and its fill factor reaches 96 %. The 1×20 MMA design has been successfully fabricated using a combined bulk- and surface- micromachining process. The size of each mirror plate is 240 μm by 500 μm. The measured maximum optical scan angles in the x-axis and y-axis are 29.3° and 11.8°, respectively, at 3 V. The measured response time is about 3.7 ms. The thermal crosstalks between adjacent electrothermal actuators are about 10 %. Long-term stability tests show that the angular drifts of the individual mirrors are about 0.002°/h.

A MEMS lens scanner based on serpentine electrothermal bimorph actuators for large axial tuning.

Confocal microscopes and two-photon microscopes are powerful tools for early cancer diagnosis because of their high-resolution 3D imaging capability, but applying them for clinical use in internal organs is hindered by the lack of axially tunable lens modules with small size, high image quality and large tuning range. This paper reports a compact MEMS lens scanner that has the potential to overcome this limitation. The MEMS lens scanner consists of a MEMS microstage and a microlens. The MEMS microstage is based on a unique serpentine inverted-series-connected (ISC) electrothermal bimorph actuator design. The microlens is an aspheric glass lens to ensure optical quality. The MEMS microstage has been fabricated and the lens scanner has been successfully assembled. The entire lens scanner is circular with an outer diameter of 4.4 mm and a clear optical aperture of 1.8 mm. Experiments show that the tunable range reaches over 200 µm at only 10.5 V and the stiffness of the microstage is 6.2 N/m. Depth scan imaging by the MEMS lens scanner has also been demonstrated with a 2.2 µm resolution, only limited by the available resolution target.

A Customized Two Photon Fluorescence Imaging Probe Based on 2D scanning MEMS Mirror Including Electrothermal Two-Level-Ladder Dual S-Shaped Actuators.

We report the design and characterization of a two-photon fluorescence imaging miniature probe. This customized two-axis scanning probe is dedicated for intraoperative two-photon fluorescence imaging endomicroscopic use and is based on a micro-electro-mechanical system (MEMS) mirror with a high reflectivity plate and two-level-ladder double S-shaped electrothermal bimorph actuators. The fully assembled probe has a total outer diameter of 4 mm including all elements. With a two-lens configuration and a small aperture MEMS mirror, this probe can generate a large optical scan angle of 24° with 4 V drive voltage and can achieve a 450 µm FOV with a 2-fps frame rate. A uniform Pixel Dwell Time and a stable scanning speed along a raster pattern were demonstrated while a 57-fs pulse duration of the excitation beam was measured at the exit of the probe head. This miniature imaging probe will be coupled to a two-photon fluorescence endomicroscope oriented towards clinical use.

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.

Thermal Reliability Study of an Electrothermal MEMS Mirror

Micro-electrical-mechanical mirrors based on electrothermal bimorph actuators have unparalleled advantages of large scan range, low driving voltage, and high fill factor, but their reliability issues are not well understood. One of the main failure phenomena is the burnout of the refractory metal resistor embedded in electrothermal bimorph actuator structures. In this paper, the burnout process was thoroughly studied via both electrical and optical testing. In particular, the current passing through the embedded resistor was monitored throughout the entire burnout process. It was found that the burnout was caused by multiple steps of anomalies, including electrical breakdown at poor step coverage, short circuit, overheating, and electromigration. Electrical models and lumped-element thermal models were established and electrothermomechanical finite-element method simulations were performed. The modeling and simulation results are all in good agreement with the experimental results. These findings will help greatly improve future design for better reliability.

Programmable and functional electrothermal bimorph actuators based on large-area anisotropic carbon nanotube paper

Electro-active polymer (EAP) actuators, such as electronic, ionic and electrothermal (ET) actuators, have become an important branch of next-generation soft actuators in bionic robotics. However, most reported EAP actuators could realize only simple movements, being restricted by the small area of flexible electrodes and simple designs. We prepared large-area flexible electrodes of high anisotropy, made of oriented carbon nanotube (CNT) paper, and carried out artful graphic designs and processing on the electrodes to make functional ET bimorph actuators which can realize large bending deformations (over 220°, curvature > 1.5 cm−1) and bionic movements driven by electricity. The anisotropy of CNT paper benefits electrode designs and multiform actuations for complex actuators. Based on the large-area CNT paper, more interesting and functional actuators can be designed and prepared which will have practical applications in the fields of artificial muscles, complicated actuations, and soft and bionic robotics.

Development of large-movements and high-force electrothermal bimorph actuators based on aligned carbon nanotube reinforced epoxy composites

By using aligned multi-walled carbon nanotube (MWCNT) reinforced epoxy composites possessing a negative coefficient of thermal expansion (CTE) as well as high Young’s modulus and aluminum foils, novel electrothermal bimorph actuators are fabricated. U-shaped bimorph actuators are formed by cutting out the middle part of the composite/aluminum laminates, where the MWCNT-aligned direction is parallel to the length direction of the U-shaped bimorph actuators. We demonstrate that the bimorph actuators with a free length of 16mm show a large bending displacement and force output, and their values are 7.6–10.0mm and 0.8–7.8mN under a DC voltage of 5.2–6.0V. Based on these results, the bending displacement and force output of the bimorph actuator are modeled by combining strength-of-materials theories and rule of mixtures (Voigt model and Turner’s model). By using these models, we indicate contributions from two sources toward the increased bending displacement and force output in the bimorph actuator: (i) designing the Young’s modulus and negative CTE of the composite layer by controlling the MWCNT volume fraction and the dimensional parameters (especially thickness) of the bimorph actuator; (ii) selecting materials having a high Young’s modulus as well as a large CTE as the second layer.

Investigation of dynamic thermal behaviors of an electrothermal micromirror

It is well known that the heat transfer at microscale is quite different from that at macroscale, but the dynamic thermal behavior of micro-actuators with movable microstructures still lacks comprehensive investigation. This work presents a thorough study on thermal behaviors of an electrothermally actuated scanning micromirror, which has a 1mm×1mm central mirror plate supported on four sides by four pairs of electrothermal bimorph actuators. The heat transfer from the thermal actuator to the ambience, including the substrate and the environmental air, are investigated experimentally with several different driving powers and various frequencies under natural convection, forced convection, and in vacuum. The results indicate that the heat conduction dominates the heat transfer process of electrothermal bimorph actuators, while the convection has little impact on the overall heat transfer even under strong forced convection. Furthermore, an equivalent circuit model of the electrothermal micromirror is developed and experimentally verified, from which the effective heat transfer coefficient of the electrothermal bimorphs is extracted to be in the range from 1900 to 2282W/(m2K), which is one to two orders of magnitude larger than that at macroscale.

Soft electrothermal actuators using silver nanowire heaters.

Low-voltage and extremely flexible electrothermal bimorph actuators were fabricated in a simple, efficient and scalable process. The bimorph actuators were made of flexible silver nanowire (AgNW) based heaters, which exhibited a fast heating rate of 18 °C s-1 and stable heating performance with large bending. The actuators offered the largest bending angle (720°) or curvature (2.6 cm-1) at a very low actuation voltage (0.2 V sq-1 or 4.5 V) among all types of bimorph actuators that have been reported to date. The actuators can be designed and fabricated in different configurations that can achieve complex patterns and shapes upon actuation. Two applications of this type of soft actuators were demonstrated towards biomimetic robotics - a crawling robot that can walk spontaneously on ratchet surfaces and a soft gripper that is capable of manipulating lightweight and delicate objects.

Electrothermal Actuators for SiO₂ Photonic MEMS.

This paper describes the design, fabrication and characterization of electrothermal bimorph actuators consisting of polysilicon on top of thick (>10 ) silicon dioxide beams. This material platform enables the integration of actuators with photonic waveguides, producing mechanically-flexible photonic waveguide structures that are positionable. These structures are explored as part of a novel concept for highly automated, sub-micrometer precision chip-to-chip alignment. In order to prevent residual stress-induced fracturing that is associated with the release of thick oxide structures from a silicon substrate, a special reinforcement method is applied to create suspended silicon dioxide beam structures. The characterization includes measurements of the post-release deformation (i.e., without actuation), as well as the deflection resulting from quasi-static and dynamic actuation. The post-release deformation reveals a curvature, resulting in the free ends of 800 long silicon dioxide beams with 5 -thick polysilicon to be situated approximately 80 above the chip surface. Bimorph actuators that are 800 in length produce an out-of-plane deflection of approximately 11 at 60 dissipated power, corresponding to an estimated 240 actuator temperature. The delivered actuation force of the 800 -long bimorph actuators having 5 -thick polysilicon is calculated to be approximately 750 at 120 .

Optical coherence tomography endoscopic probe based on a tilted MEMS mirror.

This paper reports a compact microendoscopic OCT probe with an outer diameter of only 2.7 mm. The small diameter is enabled by a novel 2-axis scanning MEMS mirror with a preset 45° tilted angle. The tilted MEMS mirror is directly integrated on a silicon optical bench (SiOB). The SiOB provides mechanical support and electrical wiring to the mirror plate via a set of bimorph flexure, enabling a compact probe mount design without the requirement of a 45° slope, which is capable to dramatically reduce the probe size and ease the assembly process. Additionally, the SiOB also provides trenches with properly-designed opening widths for automatic alignment of the MEMS mirror, GRIN lens and optical fiber. The 45°-tilted MEMS mirror plate is actuated by four electrothermal bimorph actuators. The packaged 2.7 mm-diameter probe offers 2-axis side-view optical scanning with a large optical scan range of 40° at a low drive voltage of 5.5 Vdc in both axes, allowing a lateral scan area of 2.2 mm × 2.2 mm at a 3 mm working distance. High-resolution 2D and 3D OCT images of the IR card, ex vivo imaging of meniscus specimens and rat brain slices, in vivo imaging of the human finger and nail have been obtained with a TDOCT system.

Low-voltage and high-performance electrothermal actuator based on multi-walled carbon nanotube/polymer composites

Multi-walled carbon nanotubes are used to fabricate a type of environment-friendly electrothermal bimetallic actuators with the matrix of waterborne polyurethane or silicone rubber. Even under a relatively low DC driven voltage of 7V, the actuator can achieve a bending displacement up to 28mm, or a curvature up to 0.29cm−1, which are greater than most of other electrothermal actuators reported. The actuator has considerable controllability, large mechanical output and long life-time, and the working power can be reduced down to 25mW/mm3 in the atmospheric environment. The actuation mechanisms owe not only to the mismatch in the coefficients of thermal expansion, but also to the unique negative temperature coefficient effect of nanotube composites. Furthermore, the bimetallic actuators based on various polymer matrices are supposed to provide revelations for electrothermal bimorph actuators.

Design and Simulation of High Efficient MEMS Electrothermal Actuator

The paper mainly designs an efficient MEMS electrothermal actuator, with lower voltage input but large displacement output. The calculation method of the actuator considers the convection and radiation besides heat conduction mentioned in most literatures. With the multiphysics simulation by the software Comsol of electrothermal bimorph actuator, the transient effects of thermal convection and thermal radiation on the actuator displacement and the response characteristics of an actuator with a separate heating layer were studied.

MEMS micromirror integrated endoscopic probe for optical coherence tomography bioimaging

A prototype of an optical coherence tomography (OCT) bioimaging endoscopic probe utilizing a MEMS micromirror as the light beam manipulator has been developed. The MEMS micromirror and the silicon optical bench (SiOB) assembly are enclosed within a biocompatible, transparent and waterproof polycarbonate tube equipped with toroidal-lens for in vivo diagnostic applications. The effects of the curvatures of the mirror platform and the housing on the optical behavior of the OCT probe have been discussed. The toroidal-lens embedded catheter design is proposed to reduce the cylindrical lens effect induced by the cylindrical shape tube. The two-axis scanning micromirror is driven by four independent electrothermal bimorph actuators at a driving voltage of less than 2 V to produce a tilt angle of 11°. High resolution OCT images of the skin tissue from the hind leg of a mouse can be obtained by integrating the MEMS micromirror probe with a commercially available OCT system.

An Electrothermomechanical Lumped Element Model of an Electrothermal Bimorph Actuator

This paper reports a simple electrothermomechanical lumped element model (ETM-LEM) that describes the behavior of an electrothermal bimorph actuator. The ETM-LEM is developed by integrating an electrothermal LEM of a heater with a thermomechanical LEM of a bimorph actuator. This new LEM uses only one power source in both the electrical and thermal domains. The LEM provides a simple and accurate way of relating the output mechanical response of a bimorph actuator to the electrical inputs. The model shows that the tip angular rotation of the bimorph actuator is linearly proportional to its average temperature change. The LEM predicts a linear relationship between both the average temperature change and bimorph tip angular rotation versus voltage when operated above a certain voltage. The LEM is used to predict the rotation angle of a fabricated electrothermal bimorph micromirror in response to the electrical inputs and produces results that agree with finite element model simulations and experimental data within 15% for all measured parameters.

Polymer constraint effect for electrothermal bimorph microactuators

The authors report on the analysis of the polymer constraint effect and its use for a micromachined electrothermal bimorph actuator. The actuated displacement is enhanced due to the polymer constraint effect. Both the thermal expansion and apparent Young’s modulus of the constrained polymer blocks are significantly improved, compared with the no constraint case. The calculation that agrees well with experimental results provides the means to optimize the design of the constrained polymer stack electrothermal microactuator.

Multistate Latching MEMS Variable Optical Attenuator

A multistate latching variable optical attenuator (VOA) is demonstrated using microelectromechanical systems (MEMS) technology. The mechanism is used to fix the position of a shutter inserted into the optical path between two single-mode fibers. The mechanism and fiber mounts are fabricated in 85 /spl mu/m thick silicon using bonded silicon-on-insulator material, by deep reactive ion etching. The device can be continuously adjusted or latched into a discrete set of attenuation states using a rack-and-tooth mechanism driven by electrothermal shape bimorph actuators. Electromechanical and optical characterization is performed to demonstrate a latching VOA function, with a maximum attenuation of 30 dB.