Congchun Zhang

Design and implementation of a bistable microcantilever actuator for magnetostatic latching relay

This paper presents the design, fabrication, and implementation of a microcantilever actuator with magnetostatic latching for performing low power bistable relay applications. This unique bistable feature consists of a low-stiffness torsion/cantilever beam system with circular-shaped support and a permanent magnet for holding the closed state with a permalloy soft magnetic circuit. The special circular support is designed to enhance the stiffness of the overhang beams. First, mechanical modeling of the leveraged torsion/cantilever beams was performed by Castiglianos theorem so as to deduce the spring stiffness of system. Then the device has been prepared by a laminated photoresist sacrificial layer process (LPSLP). Finally, mechanical performance was characterized by atomic force microscopy (AFM), combined with finite element simulation using ANSYS(TM) package and analysis model as well. Switching between two stable states of the microactuator was successfully validated with WYKO NT1100 optical profiling system.

Deep reactive ion etching of commercial PMMA in O2/CHF3, and O2/Ar-based discharges

The reactive ion etching of PMMA in O2, O2/CHF3 and O2/Ar discharges has been examined as a function of bias voltage, flow rate and composition of the gas mixtures. Etching in O2, O2/CHF3 and O2/Ar plasmas (with a flow ratio of Ar/O2 ≤ 50%) shows higher ion-enhanced chemical etching dependence than in O2/Ar (with a flow ratio of Ar/O2 ≥ 50%). In Ar/O2 plasmas, with an increasing proportion of Ar, the dominant process changed from an ion-enhanced chemical process with high values of etch rate to a physical sputtering etch process. PMMA etched in O2 (10 sccm)/Ar (40 sccm) produced a smooth and vertical sidewall. With an increasing amount of CHF3, the etch rates of PMMA show a larger dependence on physical sputtering and a gradual decrease. When the depth of PMMA etched in O2 (25 sccm)/CHF3 (25 sccm) is about 100 µm the profile is vertical, but the sidewall surface is rough.

Vertically bidirectional bistable microrelay with magnetostatic and thermal actuations

The design and characterization of a novel bidirectional bistable MEMS relay with out-of-plane motion are presented in this paper. The microrelay is actuated vertically by combining with magnetostatic and electrothermal microactuators. This bistability features a permanent magnet actuation without electromagnetic coils where lower power consumption is requested. The device was fabricated by laminated sacrificial thick copper layer process in UV-LIGA. The mechanical performance was characterized using atomic force microscope (AFM) as well as nano indentation tester. Switching between two stable states of the fabricated bidirectional bistable microrelay was successfully validated with WYKO NT1100 optical profiling system.

Torsion/cantilever-based MEMS bistable mechanisms with different support configurations: structure design and comparison

Three types of torsion/cantilever-based MEMS bistable mechanisms (BMs) with different support configurations have been constructed, modeled and experimented. For the support configuration, there is a crisscross-shaped, a ring-shaped and a diamond-shaped support beam, respectively. The proposed MEMS BMs consist of a free–free torsion-based cantilever which forms a symmetrical rocker lever. The free–free cantilever is suspended by a support skeleton which in turn is attached to a torsion cantilever. A permanent magnet is attached beside for holding the closed state with a permalloy soft magnetic circuit. The different special support configurations account for a low torsional compliance with the overhanging beams. In order to deduce the equivalent stiffness coefficient of BM systems, mechanical modeling of three types of torsion/cantilever-based MEMS BMs was performed by the classical beam theorem. Meanwhile, the magnetostatic latching force was also deduced by the Maxwell electromagnetism theory. The performances of these MEMS BMs have been compared by the evaluation of static deformation variations, equivalent stiffness coefficients and dynamical switching characterizations. Finally, mechanical performance was characterized by atomic force microscopy, combined with a Nanoindentation Tester. In addition, bistabilities of the MEMS BMs were proved by theoretical analysis as well as experimental results. Among these BMs, the ring-shaped MEMS BM is extremely prone to deflect due to relatively low stiffness compared with other types. The torsion/cantilever-based MEMS BMs have potential application in the field of latching relays with low power consumption.

An electro-thermal SU-8 cantilever micro actuator based on bimorph effect

An electro-thermal SU-8 cantilever micro actuator with large vertical motion at low voltage is presented in this paper. It is lower in voltage due to employing SU-8 as functional material because of its high coefficient of thermal expansion. A finite element simulation was utilized to determine the optimal parameter. Windows in the center of cantilever can reduce internal residual stress. And we also added a nickel matrix on surface of support base to enhance bonding force between supportive base and driving section. Then actuators were obtained through Micro Fabrication Technology. Finally the Scanning-electron microscope and WYKO NT1100 optical profiling system demonstrated that the electro-thermal actuator possesses a high quality cantilever. Its stiffness is relative large that is sufficient for application in Micro device proved by Bonding Tester. With applied voltage 0.5V the electro-thermal actuator can generate a vertical displacement of 42.9μm.

Laminated Photoresist Sacrificial Layer Process for 3-D Movable Suspension Microstructures in LIGA-Based Surface Micromachining

The fabrication process of three-dimensional (3D) high-aspect-ratio MEMS devices entirely made of electroplated metals with suspending multilayered microstructures is reported. The technology used is a LIGA-liked micromachining process, called the laminated positive photoresist sacrificial layer process (LPSLP). The LPSLP allows in UV-lithography not only for thick resist mould for electroplating of cascaded metal structures but also for the sacrificial layer for supporting mechanically the suspensions. So far the LPSLP procedure has incorporated with more than five sacrificial layers, which allows for the creation of overhanging structures and freely moving parts like out-of-plane cantilever stacks. A description of the underlying fabrication principle and processing details is discussed in this paper. Thus the proposed procedures open a low-cost route for fabricating micro-components such as cantilevers, bridges, movable electrodes, and freestanding parts.

Design and fabrication of a micron scale free-standing specimen for uniaxial micro-tensile tests

This paper presents a novel design and fabrication of test chips with a nickel free-standing specimen for the micro uniaxial tensile test. To fabricate test chips on the quartz substrate significantly reduces the fabrication time, minimizes the number of steps and eliminates the effect of the wet anisotropic etching process on mechanical properties. The test chip can be gripped tightly to the test machine and aligned accurately in the pulling direction; furthermore, the approximately straight design of the specimen rather than the traditional dog-bone structure enables the strain be directly measured by a displacement sensor. Both finite-element method (FEM) analysis and experimental results indicate the reliability of the new design. The test chip can also be extended to other materials. The experimental measured Youngs modulus of a thin nickel film and the ultimate tensile strength are approximately 94.5 Gpa and 1.76 Gpa, respectively. The results were substantially supported by the experiment on larger gauge specimens by a commercial dynamic mechanical analysis (DMA) instrument. These specimens were electroplated under the same conditions. The low Youngs modulus and the high ultimate tensile strength might be explained by the fine grain in the electroplated structure.

Modeling of a bistable MEMS mechanism with torsion/cantilever beams

A mechanical modeling of a novel bistable MEMS mechanism with a torsion/cantilever beam system is presented in this paper. The model includes mathematical solution to the system equivalent stiffness, and simulating analysis for the deformation under load. Meanwhile, the geometrical parameters of bistable system were optimized with Matlab software. And the optimization dimensions involves the length, width and thickness where l<inf>1</inf>=1130µm, w<inf>1</inf>=100µm, t<inf>1</inf>=10µm for cantilever, and l<inf>2</inf>=500µm, w<inf>2</inf>=20µm, t<inf>2</inf>=10µm for torsional flexure, respectively. In addition, modal analysis was performed to determine the nature frequency of the torsion/cantilever beam system. The first four fundamental resonant modes consist of torsional, out-of-plane sliding, in-plane sliding and out-of-plane rocking mode. This proposed bistable mechanism can be suitable for bistable switching relay applications where low power consumption is requested.

Novel electro-thermal latching micro-switch based on Ni/electrophoretic polymer micro-cantilevers

A novel electro-thermal and mechanical latching micro-switch is proposed in this study. Theoretical calculations were carried out to determine the geometric parameters of the micro-switch. An electrophoretic polymer film was introduced as the actuation layer to sustain the nickel resistance heater of the electro-thermal actuator. Compared to SU-8, the electrophoretic polymer film had a large thermal expansion coefficient as well as a high adhesive force between the polymer film and Ni. By optimizing the processes, the micro-switch was finally fabricated using a MEMS process. In order to demonstrate the dynamic performance of the micro-switch, a sequential double-pulsed power was used to actuate the fabricated micro-switch, which was connected with an external circuit. The testing results indicated that the electro-thermal latching micro-switch could control the ON/OFF state of the external circuit.This demonstrated the bistable behaviour of the electro-thermal micro-switch.

A study on utilizing a chloride bath to electroform MEMS devices with high aspect ratio structures

Addressing the problems that occurred for electroforming of MEMS devices with high aspect ratio and geometry-complex structures, the electrochemical properties of a chloride bath are studied in this paper. It has been found that the tensile stress of nickel film from the chloride bath with the addition of saccharine is significantly reduced. Our results also suggested that, compared to the commonly used sulfamate bath, the chloride bath has higher throwing power and covering power, possibly due to its higher conductivity and polarizability. In addition, the presence of saccharine provided a finer grain nickel film with a smoother surface. Furthermore, the MEMS-based latching devices with narrow bars of 50 µm width and 500 µm thickness were obtained from the chloride bath, while the same devices with structure deficiency (incomplete structures) were frequently observed from the sulfamate bath. The finite element method simulations using the ANSYS software for the current density distribution on the surface of different aspect ratio latching structures were carried out. After considering the electrochemical perspectives from both baths, the mechanisms for the deficiency formation from the sulfamate bath and the absence of the deficiencies from the chloride bath were proposed and discussed.