Self-assembled 3-D silicon microscanners with self-assembled electrostatic drives

Abstract

Abstract-Three-dimensional, electrostatically driven resonant torsion mirror microscanners are constructed by surface tension powered out-of-plane rotation of parts formed in bonded silicon-on-insulator. Simultaneous self-assembly of the fixed electrodes using a below-substrate limiter mechanism allows scanning perpendicular to the assembly axis, and direct drive allows high-Q operation. Similar components have been fabricated by surface tension self-assembly, a method of mass-parallel fabrication of 3-D microstructures [5]. Rotation is powered by melting small pads of material (solder, glass, or photoresist) linking movable parts to the substrate. Recent work has used thick resist, with single crystal parts formed in bonded silicon-on-insulator (BSOI). Assembly has involved 45 rotation, with the final geometry fixed by a limiter [6]. Fig. 1(a) shows the mechanism used, which involves simultaneous rotation of two parts (FRAME-L and FRAME-R) in opposite directions. Catches on the parts prevent further motion when each has rotated through 45 . The assembly accuracy is extremely high, but the mechanism is bulky and, hence, most suitable for major component frames. Index Terms-MEMSThis two-part structure has been used in electrostatically driven torsion mirror scanners [6]. FRAME-R carried a mirror mounted on a torsion bar with its axis parallel to the substrate. The mirror carried one half of a comb electrostatic drive, while the other half was attached to the substrate. After assembly, the mirror could be excited into resonant torsional oscillation by a harmonic electrostatic force between the two halves of the drive. Because of the direct nature of the drive, quality factors as high as 60 were demonstrated.Manuscript received April 25, 2000; revised July 7, 2000. This work was supported by EPSRC.The author is with the Department of Electrical and Electronic Engineering, Imperial College, Exhibition Road, London, SW7 2BT, U.K. (e-mail: r.syms@ic.ac.uk).Publisher Item Identifier S 1041-1135(00)09600-2. The design had several limitations. First, the skewed electrode layout resulted in a high drive voltage, because of the weak electrostatic field. Second, there was a trade off between the voltage and the scan angle, because a reduction in separation between the halves of the comb could only be performed by lengthening the moving electrode fingers, reducing the angle of turn before they struck the substrate. Finally, only mirror axes parallel to the substrate could be used, to avoid the electrodes clashing.In this paper, we show how to overcome these difficulties by further self-assembly operations, and introduce a new mechanism that allows parts as small as the fixed half of the electrode to be reconfigured. Fig. 1(a) and (b) shows the mechanism, which is based on two cranks attached to a third movable part DRIVE near its hinge. Motion is prevented when the cranks reach the substrate. The accuracy of this mechanism is inherently low, as we show below, but it is sufficient for sub-component assembly. Here, we show how...

Surface tension-powered self-assembly of microstructures - The state-of-the-art

“…Fig. 10(b) shows a mechanical limiter that uses the substrate itself as an end-stop [110]. This mechanism is very compact, and thus suitable for assembling small parts, but its small lever arm again makes it inaccurate.…”

“…17(c) has perpendicular torsion and assembly axes [110]. To avoid the electrodes clashing when the mirror turns, the fixed half of the drive is also rotated through 45 as shown in Fig.…”

Dynamic Response Modeling of MEMS Micromirror Corner Cube Reflectors With Angular Vertical Combdrives

“…2(b)]. This nonlinear response has been observed in other devices with angular vertical drives [7], [10], [11], and we present a more detailed theoretical model of this aspect in Section III.…”

Silicon technology for optical MEMS

“…Fully automatic techniques are also being developed for assembly of 3D MOEMS. One process based on surface tension force simply requires the melting of small pads of material to rotate parts outof-plane [7], as shown in Figure 5. This method allows angular positions to be set to an accuracy of minutes of arc.…”