MEMS Thermal Actuators Research Articles

Analysis of thermo-mechanical coupling damage behavior in long-term performance of microelectromechanical systems actuators

Effective numerical analysis is significant for the optimal design and reliability evaluation of MEMS, but the complexity of multi-physical field couplings and irreversible damage accumulation in long-term performance make the analysis difficult. In the present paper, the continuum damage mechanics method is used to develop a creep damage model and conduct long-term performance analysis for MEMS thermal actuators with coupled thermo-mechanical damage behavior. The developed damage model can make a connection between the material deterioration due to microstructure changes and the macroscopic responses (the change of thermo-mechanical performance or structure failure). The numerical simulations of coupled thermo-mechanical behavior in long-term performance are implemented using the finite element method, which is validated through comparison with previous literature. The numerical results demonstrate that the proposed damage model and numerical method can provide effective assessment in the long-term performance of MEMS thermal actuators.

Rapid and low power laser actuation of sputter-deposited NiTi shape memory alloy (SMA) MEMS thermal bimorph actuators

NiTi SMA thermal bimorph actuators have potential as high-force, high-displacement MEMS actuators. Historically, even microscale SMA actuation response has been limited to a maximum of ˜100 Hz. As NiTi film and device dimensions are scaled down into the micro and nano scales, heat transfer, and thus device cycling speeds can be significantly improved upon. We have used an in-situ annealed nickel-titanium (NiTi) shape memory alloy (SMA) sputter deposition process to sputter equiatomic NiTi films at 600 °C. We characterized our thin film (270 nm NiTi – 1.6 μm NiTi) material and verified its reversible shape memory effects (SME) using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and wafer bow versus temperature measurements. Upon release, the NiTi material exhibited a reversible phase change around 60 °C with a hysteresis of ˜3 °C. For the substrate confined case (i.e. NiTi on Si or Pt), hysteresis was much larger (i.e. ˜40 °C) with the phase change completed at ˜80 °C. In addition to SMA material characterization, we fabricated NiTi/Pt bimorph actuators at several (100 nm to 1.2 μm) NiTi and Pt thicknesses. Free-standing bimorph actuators were produced via a dry etch release, and temperature dependent curvature of these cantilevers was investigated. To address the low power, and high response time aspects, we performed a dynamic characterization using a 440 mW, 532 nm “green” laser to irradiate devices from 2 to 24 W/cm2 while measuring actuation response times that varied from 3 ms at the highest irradiation fluxes to 240 ms at the lowest. Our results showed decreased actuation powers and faster heating or actuation times compared to past works with NiTi microactuators. The NiTi films with 600 nm thickness on top of 20 nm Pt films exhibited the greatest change in curvature from 200 μm to flat states, and actuated in under 3 ms due to the very small volume of SMA requiring to be heated. These results suggest that NiTi/Pt bimorphs have potential applications as lower-power, faster-response, laser-activated micro shutters or thermal switches without needing a traditional wired power source.

Numerical Investigation of a MEMS Thermal Actuator Performance by Modifying its Geometric Dimensions

Abstract Mechanical displacement and thermal stress investigation of a two hot arm thermal microactuator (polysilicon type) with different structural and geometrical parameters was investigated in this paper with Comsol Multiphysics software, starting from a validation model based on the real dimensions of a two hot arm thermal microactuator, experimentally tested at different actuation voltages. Variation at different actuation voltages (between 1 and 9 V) for the radiative energy received by the cold arm from the short hot arm and the ratio between the resistive heat energy and radiative energy at different cold arm widths dw with gap between arms g = 2 µm, and respectively at different gap widths g with dw = 15 µm was numerically computed here. Total displacement versus actuation voltages for the actuator models was also evaluated, along with von Mises stress and temperature distributions for identification of maximum bending stress along the inner (short) hot arm of the actuator.

Development of Miniature Camera Module Integrated With Solid Tunable Lens Driven by MEMS-Thermal Actuator

The design, fabrication, assembly, and characterization of a miniature adjustable-focus camera module driven by MEMS-thermal actuators are presented. The camera module consists of one solid tunable lens for optical power variation, two identical MEMS-thermal actuators integrated with displacement amplifiers for lens element driving, a CMOS image sensor for recording, and necessary CNC fabricated components for supporting and housing. Fully 3D-multiphysics simulations with valid material properties are performed to explore the design and, hence, optimize the performance of the system. In this paper, the fabricated MEMS-devices are clearly demonstrated, and followed by the detailed characterization of the camera module. Static characterizations of the system, including the temperature distribution on actuators, electric resistance change with temperature increase, displacement-voltage curve of actuator, and the focal length tuning capability, are tested. Dynamic response speed and imaging performance of the camera module are also covered. Results show that the designed MEMS actuator is able to provide a maximum output displacement of 135 μm when a 10 V driving voltage is applied. Driven by the thermal actuator, the solid tunable lens presents a repeatable focal length tuning from 9.2 to 7.9 mm with precise focus control. The adjustable-focus capability of the miniature camera module is experimentally demonstrated by clearly focusing targets placed at different object distances.

面外熱駆動Auナノグレーティングを用いたマイクロ光学位相子の透過率変調

A reconfigurable transmittance modulation device for Au nanograting retarder that consisted of out-of-plane thermally-driven grating was designed, fabricated, and demonstrated. In this study, an Au nanograting that is known to exhibit giant birefringence was constituted by integrating MEMS thermal actuators. The thermally-driven grating was made of Au/SiO2 bimorph beams that period and slit width are 800 nm and 400 nm, respectively. The device is fabricated on a glass substrate with embedded Si as a sacrificial layer. The gap between the grating and the substrate is changed by applying driving current, and transmittance was changed affected by Fabry-Pérot interference. Transmittance peak wavelength for TE polarization was shifted up to 40 nm keeping the retardation of the Au nanograting. A calculation model of transmittance applying multilayer interference was proposed and demonstrated its effectiveness in TE polarization. By optimizing the design dimensions of the Au nanograting, it is expected to realize a microscale waveplate capable of transmittance modulation.

Measurement of the dynamic response of MEMS thermal actuators based on blurred still images by wavelet transform

A novel method to experimentally determine the dynamic response of MEMS thermal actuators based on blurred still images of motion is presented. The apex displacement of thermal actuators proves to be a first-order linear system, whose dynamic response is depended on the system coefficient. In the frequency domain, the system coefficient of actuators excited by sinusoidal wave can be obtained from the fitted curve of vibration amplitude against frequency. An improved wavelet transform detection algorithm is proposed to acquire the vibration amplitude from blurred images taken by an ordinary camera. Compared with the conventional optical measurement method, the proposed method uses only still images instead of complex laser and optical reflecting systems, which has great potential for wide application. For the actuator in this study, the measured response time by blurred images and high-speed camera method are 14.87 and 15.25 ms with a deviation of 2.5%, which indicates the proposed method can obtain sufficient accurate results without sophisticated equipment and high cost.

Temperature-dependent material properties of Z-shaped MEMS thermal actuators made of single crystalline silicon

MEMS thermal actuators have been employed in a broad range of applications, often operating in different environments (e.g. vacuum, air or liquid). Since the involved heat dissipation mechanisms are different in different operating environments, the device performances are expected to be different. In this paper, we report experimental measurement and multiphysics modeling of device performance metrics of a recently introduced thermal actuator, the Z-shaped thermal actuator, including temperature distribution, electric resistance and displacement in both air and vacuum environments. The temperature measurement was based on Raman scattering in air. Fully 3D multiphysics (coupled thermo-electro-mechanical) simulations were performed to treat both air and vacuum environments. Heat conduction through air to neighboring devices is important, while heat convection to air is negligible. The experimental and modeling results agreed well, which demonstrated the accuracy of the temperature-dependent material properties used in the modeling. Fully 3D multiphysics modeling combined with valid material property parameters will enable the exploration of the design space and the optimization of performances of the MEMS thermal actuators for different operating environments.

Analysis of micropatterned wireless resonant heaters for wireless-control of MEMS thermal actuators

This paper reports a study on wireless radiofrequency (RF) power transfer to and heat generation in micropatterned coils for wireless control of thermally-driven microdevices. Inductor-capacitor circuits with the planar spiral coils act as RF receivers as well as frequency-sensitive wireless heaters that selectively produce heat when resonated with external RF magnetic fields. Electrical and thermal characterizations are performed using two sets of microfabricated circuits with varying numbers of turns and fill ratios defined in the spiral coils. The measurements in electrical parameters of the circuits, coupling coefficient, power transfer efficiency, and heat generation have been evaluated and parameters show good consistency between them. These tests reveal maximum power transfer efficiency and steady-state temperature of ~35 and 99 °C, respectively, for the coil with a fill ratio of 0.55. The reported results will serve as a basis for the design of the wireless heater, which provide a promising path to radio-control of thermoresponsive actuators such as shape-memory alloys and hydrogels.

Development of an SU-8 MEMS process with two metal electrodes using amorphous silicon as a sacrificial material

This work presents an SU-8 surface micromachining process using amorphous silicon as a sacrificial material, which also incorporates two metal layers for electrical excitation. SU-8 is a photo-patternable polymer that is used as a structural layer for MEMS and microfluidic applications due to its mechanical properties, biocompatibility and low cost. Amorphous silicon is used as a sacrificial layer in MEMS applications because it can be deposited in large thicknesses, and can be released in a dry method using XeF2, which alleviates release-based stiction problems related to MEMS applications. In this work, an SU-8 MEMS process was developed using α-Si as a sacrificial layer. Two conductive metal electrodes were integrated in this process to allow out-of-plane electrostatic actuation for applications like MEMS switches and variable capacitors. In order to facilitate more flexibility for MEMS designers, the process can fabricate dimples that can be conductive or nonconductive. Additionally, this SU-8 process can fabricate SU-8 MEMS structures of a single layer of two different thicknesses. Process parameters were optimized for two sets of thicknesses: thin (5–10 µm) and thick (130 µm). The process was tested fabricating MEMS switches, capacitors and thermal actuators.

Z-Shaped MEMS Thermal Actuators: Piezoresistive Self-Sensing and Preliminary Results for Feedback Control

Feedback control of microactuators holds potential to significantly improve their performance and reliability. A critical step to realize the feedback control of microactuators is feedback sensing. In this paper, we report the feasibility of using a Z-shaped thermal actuator (ZTA) as a simultaneous force or displacement sensor. An in situ scanning electron microscope nanomanipulation process is used to characterize the piezoresistive response of ZTAs, which shows that ZTAs can be used as piezoresistive sensors. The experimental results agree very well with multiphysics (electric-thermal-structural-piezoresistive) simulations. A new feedback scheme is further explored, where the ZTA is treated as a two-input (applied current and external force) and two-output (displacement and electric resistance) system. Based on the calibrated relationships between the inputs and the outputs, a feedback system is developed, which can simultaneously sense the external force and generate updated current to actuate the ZTA to the desired position. We demonstrate preliminary results of this feedback control by holding the ZTA at a constant position under various external forces. The device and method presented in this paper are valuable for a range of microelectromechanical systems applications, including on-chip nanoscale mechanical testing and nanopositioning.

Measurement of MEMS thermal actuator time constant using image blur

A method of experimentally determining the time constants of MEMS thermal actuators from an analysis of blurred still camera images of high frequency motion is presented. The response of MEMS thermal actuators to harmonic excitation falls off with increasing frequency. An analytical derivation of the thermal frequency response is performed considering actuator arm conduction and conduction through air to the substrate. For the actuator in this study, the predicted time constant is 120–125 µs. Directly measuring this time constant would require sampling in the kHz range, well above the range of most cameras, which blur the image. The thermal time constant is computed by analyzing the sidewall profiles of a stationary actuator and of the actuator excited over a range of frequencies. The observed slope of the blurred sidewall profile changes with the frequency: the larger the amplitude of motion, the shallower the apparent slope. By numerically blurring the stationary profile, the relationship between the blur amplitude and the blurred slope is found. From this and the measured slopes, the blur amplitude is determined at each frequency and the time constant determined by curve fitting. The resultant measured time constant is 130 ± 12 µs, in good agreement with the predicted value.

Optimization Design of MEMS Thermal Actuator for Food Industry

Micro-actuator is the key device for the MEMS to perform physical functions. According to a kind of bimorph thermal actuator, this article presents an optimum design method to improve the sensitivity of MEMS actuator. Based on thermal analysis of software ANSYS, the thermal displacement and distribution of temperature field can be obtained clearly. Then a series of reasonable parameters are determined by optimum calculation. The simulation comparison analyses including thermal displacement, stress distribution and fatigue life are carried out to demonstrate that the sensitivity of the optimized structure has been improved effectively by the presented optimization method.

Thermal Analysis and Simulation of MEMS Thermal Actuator for Food Industry

Micro-actuator technology is important base of design and fabrication MEMS. According to a kind of bimorph thermal actuator, this paper obtains the thermal deflection and temperature distribution of MEMS thermal actuator by using finite element analysis. The relationship between the thermal deflection and some parameter such as length of flexure are analyzed in detail. Through simulation comparison, the influences of the length of flexure on thermal deflection and fatigue life are verified, which provides a good guide for the design of MEMS thermal actuator further.

Polysilicon thermal microactuators for heat scavenging and power conversion

We describe the design and experimental characterization of two optimized thermal actuators devised to operate by means of scavenging heat from the environment. Different from the traditional MEMS thermal actuator that relies on electric current to generate heat by Joule effect, the devices presented here are optimized to absorb external heat and convert it into mechanical displacement and force. The behavior of vertical and horizontal microactuators fabricated in a standard surface micromachining process (PolyMUMPs, Research Triangle Park, North Carolina) demonstrates the viability of exploiting heat from the surrounding medium to realize batteryless microsystems. Analytical and finite element models are provided in support of the design. Results show that fairly large and useful displacements can be achieved at commonly available operating temperatures.

Creep and Fatigue Failure in Single- and Double Hot Arm MEMS Thermal Actuators

Determination of the failure mechanisms of mechanical devices is the key to the design of reliable products. This paper reports an investigation on creep and fatigue failure of microelectromechanical (MEMS) thermal actuators. Finite element modeling is used to predict thermomechanical behavior of actuators under low to moderate voltage differences. The modeling results are compared with experimental results to evaluate the models. Two probable failure modes associated with thermal actuators, that is, fatigue and creep, are investigated, and it is found that creep is the dominant failure mechanism. The creep behaviors of several U-shape and double hot arm thermal MEMS actuators are examined, and their deformation-time curves are obtained numerically and experimentally. The curves follow a typical three-stage creep curve usually observed in metals. The creep life cycles of the devices are compared on the basis of their stress and temperature distributions. This study shows that actuators with the maximum temperature occurring at the location where the high stress is induced have shorter life spans than those experiencing the high stress away from the maximum temperature location. It is concluded that the double hot arm actuators with equal length have longer creep life than the U-shape (single hot arm) actuators.

Influence of target design on the damage threshold for optically powered MEMS thermal actuators

Bent-beam, polycrystalline silicon MEMS thermal actuators were optically powered using laser illumination. The actuators were designed with a target suspended between angled legs that expand creating displacement and force when the target is heated by a laser. The laser power threshold for immediate damage and extent of the damage exhibited a strong dependence on the target design. The optically powered thermal actuators were fabricated using a five-layer surface micromachining process from combinations of 2.25 μm thick polysilicon layers (poly3 and poly4) and a 2.0 μm thick sacrificial layer. The target compositions listed in decreasing order of laser damage threshold are: the poly3–poly4 laminate targets, the poly3–poly4 targets with an intermediate 2.0 μm thick oxide layer, the poly3–poly4 targets with a 2.0 μm thick air gap, and the poly4 only targets. The laser damage thresholds for the poly3–poly4 targets with connecting posts and an air gap increases with increasing post size, which allows for better thermal transport from the irradiated poly4 layer to the underlying poly3 layer. The laser damage thresholds ranged from a maximum of 475 mW for an oval poly3–poly4 laminate actuator target to a minimum of 190 mW for a circular poly4 only actuator target.

Processing and characterization of silicon carbon-nitride ceramics: application of electrical properties towards MEMS thermal actuators

Abstract This paper reports the development of electrically conductive silicon carbon-nitride (SiCN) for harsh-environment MEMS applications. SiCN is a new class of polymer-derived bulk ceramics for ultra-high temperature and corrosive environment applications; however, as-pyrolyzed SiCN is electrically insulating. Through post-pyrolysis annealing at 1400 °C in a hot isostatic press in a nitrogen atmosphere poly(urea-methyl-vinyl)silazane-derived SiCN MEMS devices were fabricated that exhibit a room temperature electrical resistivity of 2–5.5 Ω cm. The material was examined using linear four-probe, X-ray diffraction, Fourier transform infrared spectroscopy and SEM. This integrated materials/micromachining process enables the realization of a new class of (electro-thermo-mechanical) devices not previously possible in SiCN, and thus takes polymer-derived ceramic (PDC) microsystems technology to the next level of complexity. SiCN lateral thermal actuators and micro-grippers for moving chip-sized objects were fabricated to demonstrate an application of this functionalized material. This work is the first to demonstrate electro-thermo-mechanical transducers fabricated from polymer-derived SiCN.