New Materials and New Processes for MEMS Applications
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57 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
To illustrate this movement in non-silicon technology for MEMS, we will here survey a selection of recent results. The examples are taken from a wide variety of materials classes, and from widely different methods for synthesis and microprocessing, aiming at a manifold of applications. FREE-SPACE 3D LASER WRITING BY LCVD Thermal LCVD (laser-assisted chemical vapour deposition) is a technique where a focused laser beam is used to locally deposit a solid from a reactive gas mixture [1]. By moving the substrate and by controlling the reaction parameters it is possible to fabricate high-aspect-ratio microstructures in desired shapes and compositions. By moving the focus out from the substrate free-space 3D-laser writing is possible. We have been able, by using this technique, to fabricate structures such as springs, fibers and micro solenoids [2-3]. The LCVD equipment used for the experiments is shown in Fig 1. It consist of a small pressure chamber with two connections, one for the inlet of the precursor gas and one for the outlet of the residual gas. It is also equipped with two quartz windows, one for the laser beam and one for inspection with a stereo microscope. A gas handling system with massflow meters provides the gas mixture into the chamber. The chamber is situated on a Burleigh X-Y-Z micropositioning system and the accuracy of the system is better than 0.1 pjm. In the chamber the substrate is fixed in a goniometer to enable a wobble-free rotation perpendicular to the laser beam. A single mode CW Ark-laser operated at 488/514 nm is used for the deposition process.
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Ls Laser beam __LCVD
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Fig. 1. Schematics of the
equipment
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We grew freeform carbon springs from ethylene by focusing the laser beam at the end of a fibre which was offset from an axis of rotation by the intended helix radius. Once three-dimensional growth began, rotation about the axis commenced, and the structure was drawn downwards. During growth, we held the focus at the tip of the growing helix to keep the growth rate constant, Fig. 2.
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Fig. 2. Schematic principle of the growth of afreeform helix
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Different sizes and shapes of the springs were grown, two of them are displayed in Fig. 3. The carbon springs are extremely flexible and the double-folded extension spring in Fig. 3 b) have a spring constant less than 0.025 gN/m. Springs with such a low constant may be used in micromechanical systems, such as seismometers, low-frequency response accelerometers and oscillators. a) b)
Fig. 3. Carbon microcoils
a) single spring b) double folded extension spring
We also grew free-standing single crystalline tungsten rods from a WFI-H 2 reaction gas mixture by focusing the laser beam on a boron fiber. The rods are square-shaped and grow in the direction. The tip of the rod has a tapered almost pyramidal shape as can be seen in Fig. 4. This is due to the fact that the slowes
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