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A method for extending displacements of comb drive actuators has been demonstrated in this paper. Differing from other methods which modify the shape of suspension to shift the maximum of the lateral spring constant curve, or adjust the lengths of the individual comb fingers to reduce the equivalent electrostatic spring constant, this method changes the configuration of comb electrodes to shift the minimum of the equivalent spring constant curve. In addition to the original set of comb electrodes, a set of secondary comb electrodes was added into a comb drive actuator. By sequentially powering these two sets of electrodes, the comb drive can acquire more displacement than the original. The feasibility of this method was examined based on the existing theories and simulation software. An inequality that predicts the extending displacement after using this method was given. Based on this inequality, additional displacement could be achieved by adding one set of secondary comb electrodes and switching them sequentially. In our experiment, a case with 40.6% additional displacement has been observed. This value is consistent with the result derived by the model presented in this paper.

A new method is proposed to improve thin-film stress measurement using micromachined bilayer cantilevers. Our previous studies demonstrated that the width does affect the bilayer cantilever deflection, and the effect of width disappears as the width exceeds a critical value, which is usually far larger than the bilayer cantilever length. The existing methods, which employ the narrow micromachined bilayer cantilever to measure the residual stresses, do not consider the width effect, and hence may generate some uncertainties in their measurement results. To remove these uncertainties, a cantilever with a very large width is introduced and used in this proposed method. Its corresponding model, based on the plate theory, has been derived and applied to this new measurement procedure. Following the measurement procedure suggested in this paper would provide measurement results more reliable than the traditional ones which use the narrow bilayer cantilever in determining the stresses of thin films.

In this paper, the effect of cantilever width is demonstrated on the stress-induced bending of micromachined bilayer cantilevers. In previous literature, the bilayer cantilever has been modeled as a two-dimensional structure; consequently, the stress-induced bending of micromachined bilayer cantilever was considered to be independent upon the width. In this study, the fabrication and characterization of various widths of micromachined bilayer cantilever have been performed to observe the out-of-plane deformation caused by residual stresses. A finite element model has been established to analyze such a deformation. With the support of experimental and numerical results, width dependence in the stress-induced bending of micromachined bilayer cantilever has been discussed. As a result, the tip deflection of the bilayer cantilever gradually increases as the width increases until the width-to-length ratio reaches a critical value, in which the micromachined bilayer cantilever can be regarded as an infinitely wide cantilever, and any cross section along the length is in plane strain state. Furthermore, width-independent design, which exists among very wide cantilevers with various widths, has also been reported. Thus a more reliable design can be achieved.

Nested folded-beam suspensions with a low longitudinal spring constant and a high lateral spring constant have been used in comb-drive actuators. In the new design, every two flexible beams and two stiff members form a parallelogram flexure, which is considered as an ‘element’ of the nested folded-beam suspension. A set of these flexures of increasing size were placed one outside another to compose a nested structure. In this way, a serial mechanical connection between adjacent parallelogram flexures was formed; thus, a longer output stroke was obtained by combining the stroke displacements of all flexures in an additive fashion. The designed suspensions were theoretically analyzed and numerically simulated. Furthermore, comb-drive actuators with conventional and new suspensions were fabricated and tested to verify the predicted function. In the testing cases, the longitudinal spring constants of suspensions with two (conventional), three and four parallelogram flexures on each side were measured as 2.77, 1.75 and 1.36 N m−1. The ratio among these three values was approximately 6:4:3, which is consistent with the theoretical predictions and simulation results. Microfabricated folded beams in series were achieved.

This study discusses sacrificial bridges that are used to release MEMS devices. Before being released, sacrificial bridges connect all the component structures into an integral structure. Solder bump bonding is used to mount the MEMS chip on another chip or a printed circuit board (PCB) and to maintain the alignment among all component structures after removal of the sacrificial bridges. Two types of sacrificial bridges were designed, analyzed and fabricated. The fabrication process?which used low resistivity single crystal silicon (SCS) wafers as the device material?was developed to implement the sacrificial bridges. Novel SCS through silicon vias (TSVs), which interconnect stacked chips, was made using the same process. An electrostatic comb drive actuator was fabricated and mounted onto a PCB. The fabricated actuator was tested to demonstrate the feasibility of the fabrication process, sacrificial bridges and SCS TSVs. The results show that the actuator worked well. Its maximum displacement and resonant frequency were 69.9 ?m and 406 Hz, respectively. This method is promising for the delivery of a novel 3D system in package for MEMS devices.

The symmetry of the electric field in a dielectric medium sandwiched between comb-fingers dominates the stability of the comb-drive actuator. The stability can be spontaneously broken by the asymmetry appearing on the two sides of the finger array. A quite universal circuit simulator (Qucs) equivalent circuit model was built to illustrate that the symmetry is not preserved in the outer four comb-fingers, especially the first and last fingers, because they are subjected to unbalanced electrostatic forces. Experiments were conducted to verify the proposed Qucs model, which also demonstrated that symmetry exists everywhere in an imaginary comb-finger array with infinite interdigitaed fingers. In this work, the ideal comb-finger array can be nearly realized by making the two outermost fingers five times wider than the others; as a result, the symmetry is preserved and the instability is effectively suppressed in the comb-finger array.

This paper reports magnetic microrobots with rolling capability. A magnetic object subjected to an externally rotating magnetic field would be rotated due to the tendency of alignment between its internal magnetization and the field. Based on this principle, a magnetic microrobot in a spherical body with a diameter of several hundred microns was designed and fabricated. To remotely power and control the microrobot, a rotating magnet was used to generate a rotating magnetic field. Driven by this field, the microrobot can freely roll on three-dimensional surfaces. These surfaces can be in air, water or silicon oil. In a dry environment, a microrobot with a diameter of 440 µm achieved a maximum linear speed of 13.2 mm s−1.