A robust and reliable stress-induced self-assembly supporting mechanism for optical devices

Abstract

The stress-induced supporting mechanism consisting of stress-induced bending beams and novel locking component used to assemble the micromachined optical devices has been realized. The tip deflection of the stress-induced bending beams has been exploited to lift up the micromachined devices. Based on the MUMPs platform, this study has successfully established an improved surface micromachining process (MUMPs-like process) to enhance the reliability of the stress-induced beams. According to the reliability test, the variation of the tip displacement is <<1% and the variation of curvature is <<<<1% after six months. Several tests relied on the BellCore standard are also achieved, the results reveal the reliability of dielectric film is much better than the metal film. Moreover, a novel locking component was also developed to accurately position the optical component and to prevent it from deviation as well. Experimental results elaborated the feasibility and stability of the novel locking component. With improved stress-induced beam and stronger locking component, the robust and reliable stress-induced self-assembly supporting mechanism for optical devices becomes available. To demonstrate the applications of the proposed mechanism, two optical devices were accomplished in this study as well. IntroductionMUMPs (Multi-User MEMS process) is regarded as one of the most important fabrication platform for micromachined devices. Origins from the planar integrated circuit process, surface micro-machining technology can be more delicate in planar process than the bulk micro-machining or HARM (high-aspect ratio micromachining) technology. However, due to the nature of surface micro-machining process, a fundamental problem is its inability to produce highly 3D structures hence the applications are limited. To accomplish 3D devices after the process, residual stress of thin film, magnetic force [1], ultrasonic wave [2], photoresist [3], or solder reflow are widely adopted to be the selfassembly approaches. Shortcomings of many existing approaches are the requirements of additional fabrication processes and actuators, the complicated process and the consumption of wafer area are the major limitations. To avoid the additional processes or actuators, the residual stress inherent in the thin film materials during fabrication has been exploited in [4] to construct the 3D devices.Moreover, the applications of the MUMPs devices are limited by the small gap between the surface micromachined structure and the substrate which is defined by the sacrificial layer. Presently, various assembly mechanisms have been exploited to extend the actuation range of optical surface micromachined devices. For instance, a cantilever bent by the residual stresses of Metal/Poly-Si films is employed in [5] to lift up the mirror of a 2D switch. The height of the mirror is determined by the length as well as the bending curvature of the cantilever. In addition, the bent cantilever is also acting as a spring during actuation. In [6], Lucent Technology has d...

Fabrication of three dimensional split ring resonators by stress-driven assembly method

“…On the other hand, self assembly method utilizing residual thin film stress force had been demonstrated to form 3D devices, especially for curving structures [26][27][28][29]. The intrinsic stresses in thin films result from the lattice mismatch, grain boundaries, thermal expansion coefficient difference, impurities in the thin film, and deposition methods during film deposition.…”

Methodology for using residual stresses for self-assembly during deposition and etching of microstructures under external load

“…During subsequent operation, heating of a metallic layer bent the beam toward the substrate. Ho et al [2] used a MUMPS-like process with low-stress 'Si x N y ' and polysilicon in which residual stresses created a pop-up mirror. Xie et al [3] used residual stresses to make a rotational electrostatic comb drive.…”

“…The analysis was based on a lamination theory in which the mismatch strain could vary across the thickness of each layer. Thus, Ni et al provided a systematic way of designing the structures made by researchers in [1][2][3][4][5][6][7][8][9][10][11][12]. However, these previous studies did not include the effects of externally applied loads during deposition or etching.…”

Wafer level self-assembly of microstructures using global magnetic lifting and localized induction welding

“…Micro-actuators have been employed to lift up as well as to position micromachined structures to accomplish 3D devices [3]. Moreover, various approaches such as residual stress of thin film [4], magnetic force [5], ultrasonic wave [6], and solder reflow [7] have also been adopted to assemble micromachined structures after fabrication. In this regard, the locking mechanism is often required to place the micromachined structure at the desired position after assembly [8].…”