Shengping Mao

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.

Analysis, simulation and fabrication of MEMS springs for a micro-tensile system

The support spring of a uniaxial micro-tensile system for testing micro-scale thin films is studied in this paper. Stresses of different shape springs are analyzed with the finite element method (FEM). The simulated results show that the stress of an S-shaped spring is lower compared with a U-shaped spring with the same dimensions (100 µm thick, 100 µm wide, 250 µm inner diameter and 5 turns). The maximum stress of the S-shaped spring is about 133 MPa when the displacement of 100 µm is imposed at one end of the spring along the lateral side. The number of turns has the most important effect on stress and stiffness of the S-shaped spring. Moreover, main stress concentration is symmetric in the spring system and it is located in the arc near to the two fixed ends in all springs. The spindle-shape support spring is fabricated by UV-LIGA technology according to analyzed results and calibrated by the specific device.

A latching bistable microswitch using dual-beam electrothermal actuation

This paper reports on design and mechanical characteristic of a latching bistable microswitch with a dual-beam electrothermal actuator. As designed two novel contact pair, a stable state can be acquired without continuous power. The bistable mechanical characteristic of the switch is measured by a bonding strength tester. Testing results are in good agreement with theoretic analysis. The optical micrographs clearly indicate that the switch can stay latched in its two states. This novel design provides an excellent alternative for power and communication applications.

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.

Study on non-uniformity of Through-Mask electroplated Ni thin-film

Through-mask electroplating has been widely used in the fabrication of chips, BGA substrates and PCBs etc. The uniformity of plating thin-film is the major factor contributed to the reliability of the products. Currently, it is usually by setting optimum plating parameters and adopting electrochemical method to achieve the uniformity of plating. However, the problem of non-uniform distribution of electric field, which is the major cause of the non-uniformity of the plating thin-film, has not been solved. In this paper, finite element method (FEM) was developed to analyze the non-uniform distribution of electric field under different conditions in the process of electroplating. The results show that different thickness of photo-resist and size of electroplating cell are two major factors contribute to the uniformity of plating thin-film. The uniform of electroplating cell can be improved by adding in-chip auxiliary electrode. Also better uniformity of the plating film in radial direction can achieved by setting a shield in the proper position of the plating solution and annular out-chip auxiliary electrode (Cu) around the wafer. The simulation results were consistent with experimental results, which proved that finite element method is an effect way to simulate the electroplating process.

Effect of additives on mechanical properties of electroplating nickel

Since the additives is necessary for eliminating the residual stress of the nickel structure in MEMS devices, the effect of additives on mechanical properties of electroplating nickel was studied in this paper. Two types of nickel specimens was electroplated in electrolytes with both saccharine and butynediol (type A), and only saccharine (type B), respectively. The test chips were then tested in our tensile system. The average ultimate tensile strength and hardness of type A and type B specimens are 1954MPa and 4.47GPa, 923Mpa and 2.87GPa, respectively; the average elongation of type A, and type B are 7.94% and 12.38%, respectively, when the gauge length is 100μm. The mean grain sizes of type A and type B specimens are about 20nm, 160nm. The results indicates that nickel film fabricated in an electrolyte added by both saccharine and butynediol leads to much more smooth surface, finer nano-grain, and higher ultimate tensile strength-1954Mpa, which is two times of that of another type-923Mpa. The SEM images also show differences in fractography of two types. All the results indicate that additives could affect the mechanical properties dramatically, and it is essential for the designer to consider this effect during the MEMS devices designing process.

EFFECT OF SPECIMEN SIZE ON YOUNG'S MODULUS OF ELECTRODEPOSITED Ni

The Youngs moduli of electrodeposited Ni with different dimensions were measured carefully in this paper. The dimensions of tensile specimens were 200, 35, or 5 μm thick and 2400, 200 or 50 μm wide. These specimens were measured with three different approaches. The measured Youngs moduli of Ni decrease from 122.1 ± 4.3 to 92 ± 5.2 GPa when the thickness changes from 200 to 5 μm and width changes from 2400 to 50 μm.

Design of testing chip for measuring mechanical properties of thin films

Uniaxial tensile test is the most reliable way to measure mechanical properties of thin films. The difficulties of uniaxial tensile test are how to fabricate small and stress-free specimen, align and trip the specimen, generate small forces and measure strain. A novel tensile testing chip to measure thin film specimens with large elongation was proposed in this paper, and it was fabricated by UV-LIGA (Ultraviolet Lithographie GalVanoformung Abformung) technology. This novel testing chip has good alignment and can endure large deformation.