Key Engineering Materials Vols. 297-300

Abstract: A new method to improve the wet etching characteristics is described. The anisotropic wet-etching of (100) Si with megasonic wave has been studied in KOH solution. Etching characteristics of p-type (100) 6inch Si have been explored with and without megasonic irradiation. It has been observed that megasonic irradiation improves the characteristics of wet etching such as the etch rate, etch uniformity, surface roughness. The etching uniformity was less than ±1% on the whole wafer. The initial root-mean-squre roughness(Rrms) of single crystal silicon is 0.23nm [1]. It has been reported that the roughnesses with magnetic stirring and ultrasonic agitation were 566nm and 66nm [3]. But with megasonic irradiation, the Rrms of 1.7nm was achieved for the surface of 37µm depth. Wet etching of silicon with megasonic irradiation can maintain nearly the original surface roughness during etching. The results have verified that the megasonic irradiation is an effective way to improve the etching characteristics - the etch rate, etch uniformity and surface roughness.

Abstract: It is necessary to slot a groove on the outside of the bearing’s raceway in order to embed micro-sensor for smart bearing. Reasonable choice of slotting parameters for different type of bearings is one of key problems to be solved in smart bearing design. In this paper three-dimensional solid model and corresponding mechanics model for a segment of ball bearing’s out ring are built according to some reasonable simplification and supposition of boundary condition and loading status. Further more the influence of slotting parameters on smart bearing’s mechanical characteristics is studied with FEA method. Choosing 220 type of ball bearing as an example, theoretical analysis results of influence of slotting parameters on the maximum stress and deformation of outer ring are shown. The FEA results showed that micro-sensors for smart bearings should be manufactured flatter or the groove depth for micro-sensor embedding should be smaller soon as possible in order to insure the smart bearing’s original mechanical rigidity and strength.

Abstract: This paper focuses on investigating mechanical properties of micron-thick polycrystalline titanium nitride (TiN) films. We propose a new technique that can directly measure lateral strain of microscale crystalline specimen by X-ray diffraction (XRD) during tensile test. The XRD tensile test can provide not only Young’s modulus but also Poisson’s ratio of TiN films. Micron-thick TiN films were deposited onto both surfaces of single crystal silicon (Si) specimen by r.f. reactive magnetron sputtering. Young’s modulus and Poisson’s ratio of Si specimen obtained by XRD tensile tests were in good agreement with analytical values. TiN films deposited at Ar partial pressure of 0.7Pa had the average values of 290GPa and 0.36 for Young’s modulus and Poisson’s ratio. The elastic mechanical properties of TiN films gradually decreased down to 220GPa and 0.29 with increasing Ar partial pressure up to 1.0Pa, regardless of film thickness. The change in the film properties with Ar partial pressure would be attributed to the change in the film density.

Abstract: Effect of thickness on grain growth in Copper electrodeposits by cyclic stressing was studied by metallographic observation. The thickness of the film was 1µm to 20µm and the number of stress cycles was 3 × 106. Recrystallization by annealing in the films was also examined for comparison. The results show that the nucleation of the grains by cyclic stressing decreases significantly with decreasing thickness, while the grain growth rate is almost independent of thickness. The recrystallized grain size by annealing decreases with decreasing film thickness and is always smaller than the film thickness. The difference in the structural change by cyclic stressing and by annealing is discussed. When the crystal growth caused by cyclic stressing is used as a strain analysis method, the optimum thickness of the deposits is about 10µm.

Abstract: We carry out reliablity tests and investigate the failure mechanisms for wafer level vacuum packaged (WLVP) MEMS resonator using an accelerated degradation test (ADT). ADT is also used to estimate the mean life-time (or failure) of WLVP. The main failure mechanism of WLVP is found to be outgassing inside the package. Failure distribution of log-logistic is well fitted with the failure data from ADT, and Arrhenius model is used as a stress-life model because the main stress parameter is temperature. Acceleration factor (AF) for temperature levels is calculated and used to evaluate mean life-time of the WLVP after redesign. Finally the accuracy of ADT model is examined by verification test. The successful WLVP is achieved by reducing outgassing through the deposition of titanium coating inside the package as a getter material.

Abstract: Reliability is very important for the further development, commercialization and miniaturization of microelectromechanical systems (MEMS). In particular, concern arises about time-dependent degradation such as fatigue for MEMS with flexural elements because they are used in cyclic loading. This study investigated the time-dependent degradation of silicon micro-resonating structures. The test structure, designed and fabricated by micromachining, consisted of suspended beams, shuttle, combs and electrodes. It was operated at resonance mode by applying AC voltage with a function generator and the change of resonant frequency was detected. The failure of a notched beam was detected by the saturation of the decrease in resonant frequency. The test structure showed a decrease in resonant frequency with cycles that was attributed to stiffness degradation due to fatigue crack growth at the notch tip. By analyzing the test structure as a spring-mass system, the variation of stiffness of a notched beam with cycles was obtained from the resonant frequency. From this relation and the stiffness-crack relation, crack growth with cycles was calculated. Finally, the lifetime of the test structure was calculated and compared with experimental results.

Abstract: In this work, flapping wings actuated by IPMCs are designed and simulated to mimick birds wing. In order for the wing to generate lift and thrust during flapping motion, the wing must be able to flap and twist at the same time. For design of such wings, shape of the IPMC actuator need to be designed such that the actuator can create bending and twisting motions during wing strokes. To determine the shape of the IPMC actuator, an equivalent bimorph beam model has been proposed based on the measured force-displacement data of an IPMC. The equivalent model and thermal analogy are used for numerical simulation of IPMC actuated wings to determine suitable shape of the IPMC actuator. In this way, we could select a best performing wing that can create the largest twist motion during flapping of the wing.