Design of a g-force meter on Si wafer, based on motor driven by photons


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The fundamental limitations of the nano-scale motors driven by flux of photons are well understood. The amount of torque that can be obtained in any nano-scale structure is very low, presenting a real challenge to any design. For example, a simple laser with an intensity of 5×106 [W/m2] can produce a torque of approximately 10-18 [N-m], which doesn’t take into account the frictional forces present that would cause an even further decrease in output. There is nearly zero movement with structures built on silicon wafers [1], making this kind of configuration unsuitable for conventional applications. Our study demonstrates the design of a micro-motor driven by light, capable of indicating altitude or proximity from astronomical objects. This suggests that the device could be used as a guidance system component, useful in unmanned spacecraft operations. The functioning principle is based on the measurement of frictional changes due to variations in the micro-motor’s weight. Friction in rotating systems is directly related to the weight of the structure, which varies with changes in the magnitude of gravitational forces. Therefore, under a constant incident light source, the variation of rotational velocity, at any instant, is determined by the g-force magnitude, which is dependent upon variations in acceleration relative to the gravitational force of a large mass or celestial body. Our novel design’s concept uses radiation pressure as a method of actuation and dynamic friction as an influencing variable. To obtain maximum torque the incident light must hit the rotor as far from center as possible and an eight arm rotor was selected to ensure continuous light incidence on the structure. As a design parameter, it is assumed that the incoming light will produce a 4µm2 spot; Figure 1 shows the restriction of the arm length to be bigger than 6.8μm. On the other hand, the restrictions of micromachining fabrication dictate the arm length minimum to be 9μm. With this value fixed, the rest of the dimensions are determined as showed in Equations 1 and 2. The final design of the structure has the form shown in Figure 2, with the rotational axis in the center. Modelling of the designed structure’s dynamic behavior, including the frictional forces, is also presented. Using this model, the gravitational acceleration variances can be obtained when measuring the angular velocity of the structure, as shown in Figure 3. To obtain an adequate reflective structure, deep reaction ion etching (DRIE) [2] along with laser assisted mounted processing is specified. Hollow silicon rotor plated by aluminium is shown in Figure 4 in three dimensions. To alleviate electrostatic charging and friction of the moving parts, octadecylphosphonic acid (ODPA) [3] and a dihydroxy derivative of perfluoropolyoxyalkane (Z-DOL) [4] coating for the aluminium and silicon surfaces are proposed [5]. References [1] Gauthier, R. C.,”Theoretical model for an improved radiation pressure micromotor”, Applied Physics Letters — September 30, 1996 — Volume 69, Issue 14, pp. 2015-2017 [2] Rahul Agarwal, Scott Samson, Sunny Kedia, and Shekhar Bhansali, “Fabrication of Integrated Vertical Mirror Surfaces and Transparent Window for Packaging MEMS Devices”, Journal of Microelectromechanical Systems, vol. 16, no. 1, february 2007 [3] B. Bhushan et Al, “Nanotribological characterization of perfluoroalkylphosphonate self-assembled monolayers deposited on aluminum-coated silicon substrates”, Microsystem Technologies, 2006, 12, 588-596 [4] Kawaguchi M., Choi J., Kato K., and Tanaka K.,“A Study of Friction Properties of Zdol on Magnetic Disk Surface”, IEEE Transactions on Magnetics, Vol. 39, No. 5, September 2003 [5] Bhushan, Bharat, “Nanotribological Characterization of Molecularly-Thick Boundary Layers of Perfluoropolyether Lubricants for Applications to MEMS/NEMS by Atomic Force Microscopy”, Ultramicroscopy 97 (2003) 321–340

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Journal: TechConnect Briefs
Volume: 2, Nanotechnology 2011: Electronics, Devices, Fabrication, MEMS, Fluidics and Computational
Published: June 13, 2011
Pages: 286 - 289
Industry sectors: Advanced Materials & Manufacturing | Sensors, MEMS, Electronics
Topic: MEMS & NEMS Devices, Modeling & Applications
ISBN: 978-1-4398-7139-3