Gravenkötter, Guido

Park, Sangjun

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PROCEEDINGS            Faculty of Mechanical Engineering   ..........................................................................................  PROSPECTS IN MECHANICAL ENGINEERING  8 - 12 September 2008            www.tu-ilmenau.de     Home / Index: http://www.db-thueringen.de/servlets/DocumentServlet?id=17534 53. IWKInternationales Wissenschaftliches KolloquiumInternational Scientific ColloquiumPublished by Impressum  Publisher Der Rektor der Technischen Universität Ilmenau Herausgeber Univ.-Prof. Dr. rer. nat. habil. Dr. h. c. Prof. h. c. Peter Scharff  Editor Referat Marketing und Studentische Angelegenheiten Redaktion Andrea Schneider    Fakultät für Maschinenbau Univ.-Prof. Dr.-Ing. habil. Peter Kurz, Univ.-Prof. Dr.-Ing. habil. Rainer Grünwald, Univ.-Prof. Dr.-Ing. habil. Prof. h. c. Dr. h. c. mult. Gerd Jäger, Dr.-Ing Beate Schlütter, Dipl.-Ing. Silke Stauche  Editorial Deadline  17. August 2008 Redaktionsschluss  Publishing House Verlag ISLE, Betriebsstätte des ISLE e.V. Verlag Werner-von-Siemens-Str. 16, 98693 llmenau   CD-ROM-Version:  Implementation  Technische Universität Ilmenau Realisierung Christian Weigel, Helge Drumm  Production CDA Datenträger Albrechts GmbH, 98529 Suhl/Albrechts Herstellung  ISBN: 978-3-938843-40-6 (CD-ROM-Version)   Online-Version:  Implementation Universitätsbibliothek Ilmenau Realisierung  Postfach 10 05 65  98684 Ilmenau       © Technische Universität Ilmenau (Thür.) 2008  The content of the CD-ROM and online-documents are copyright protected by law. Der Inhalt der CD-ROM und die Online-Dokumente sind urheberrechtlich geschützt.   Home / Index: http://www.db-thueringen.de/servlets/DocumentServlet?id=17534   53rd Internationales Wissenschaftliches Kolloquium Technische Universität Ilmenau  08 – 12 September 2008  G. Gravenkötter / S. Park    Fluid and van-der-Waals Forces in MEMS Production      INTRODUCTION    The “NEXUS Market Analysis for MEMS and Microsystems III, 2004-2009” report predicts a CAGR (Compound Annual Growth Rate) of 16% for 1st level packaged MEMS components with a market of $25 billion in 2009. These production volumes demand a highly automated assembly line. At the same time the dimensions of the components have been shrinking over the last years, e.g. in the field of minimally invasive surgery, which increasingly challenges the handling of devices. To ensure a cost-effective production, a highly reliable assembly process is vital for a smooth material flow. In this paper, we analyze the motion of sub millimeter sized  parts in the material flow technology.   EXPERIMENTAL SETUP  Often the downscaling of material flow technology is an appropriate and inexpensive approach to deal with smaller part dimensions. This downscaling method reaches its limit when the surface related forces are dominating, because the mass decreases with the third power in contrast to the surface related forces which only scale with the second power. To examine the downscaling effects of material flow technology, experiments were conducted exemplary with a vibratory conveyor. In 80% of the cases a vibratory feeder is used for conveying and orientating bulk goods in assembly processes. For that reason the experimental setup includes a vibration exciter with an aluminum channel, a high-speed camera for analyzing the motion and a laser scanning vibrometer, which measures the conveyor’s displacement and acceleration.    FLUID FORCES  Previous experiments have shown that small silicon parts with dimensions of 1x1x0.4 mm³ stick to the vibratory channel even though the acceleration is greater than twice the acceleration of gravity (>2g).  The vertical component of the channel’s motion is described by  ˆ( ) sin( )y t y tω= ⋅   (1)  where yˆ is the vibration amplitude and ω  the angular frequency, leading to the vertical channel acceleration:  2maxˆy yω=$$  (2)  The channel was vibrating at a preset frequency of 12 Hz (Figure 1). Classic theory would predict a lift off as soon as the plate overcomes the acceleration of gravity. The forces that are twice the amount of what would have been expected prevent the part from lifting off and are the subject of this research project. Figure 2 illustrates all components of the adhesion forces and the fluid forces.  Experiments have shown that capillary forces can be very strong if the relative humidity is too high (>80 % RH). They can be orders of magnitudes higher than the remaining interactions [1].  But while regulating the relative humidity down, the influence of electrostatic forces is growing rapidly. Therefore, measures for ESD-protection (electrostatic discharge) are essential for the setup. It is difficult to calculate the exact amount of the van-der-Waals forces (vdW force), but it is possible to make assumptions about the order of magnitude. In general, the vdW force decreases with the third power of the distance between two planes [2]. But the rough surface topography of the two surfaces makes it difficult to calculate the accurate distance between the two areas.   Figure 1: Lift-off of the micropart at f = 12 Hz for different materials0510152025Silicon,polishedSi,unpolishedAl, PVDAcceleration in m/s²TiltLift-offg  Figure 2: Components of Interaction Forces   Various experiments have proven that fluid forces are mainly responsible for high interaction forces. They only occur during the motion of the feeder. This is indirectly approved by increasing the frequency from 12 Hz step-by-step to 100 Hz. As expected it is more and more difficult to compensate the low pressure between the vibrating plate and the micropart bottom surface in the available period of time. Finally the lift-off acceleration reaches more than 5g at a frequency of 100 Hz.   EFFECT OF PART SIZE  To investigate the effect of part sizes we used three different microparts with the same thickness.  The dimensions for the parts are 370 x 370 x 200 µm3, 540 x 540 x 200 µm3, and 740 x 740 x 200 µm3, respectively.  Since the parts were extremely difficult to move on a flat surface, the surface of silicon vibratory channel was patterned by photolithography and deep reactive ion etching to reduce the area of contact with microparts.  The resulting surface has 35% contact area compared to the flat surface. Figure 3 shows the experimental results. As the part size decreases the liftoff acceleration also de-creases despite the decreased mass.  This result indicates that the surface area related forces dominate the motion of the parts. The lift-off acceleration increases with frequency, which is consistent with the previous results.  Figure 3: Effect of Part Size0510152050 100 150Frequency (Hz)Lift-off Acceleration (g)370x370x200 um 3^540x540x200 um 3^740x740x200 um 3^  EXPRIMENTAL RESULT AT A DECREASED PRESSURE  For additional proof the experimental setup was arranged inside a vacuum chamber. The experiment was conducted without changing the parameters of the vibration exciter, but this time the air pressure was decreased to 7 mbar. As a result the micropart lifted off the surface at a much lower acceleration of 1.3g instead of the previous acceleration of 2g. The reason is that the fluid force (4) is directly dependent on the dynamic viscosity η  of the surrounding medium [3]:    31 db12Ș dpV hx= −$  (3)  33Lb vF( h )h⇒ = − η  (4) ( dp volume flow, L length of gap, b width of gap, h height of gap,  pressure gradientdxV$ )  In this setup the viscosity begins to decrease at a pressure lower than 100 mbar, because the air changes the friction property from viscous to molecular. Since the fluid force decreases with the third power it is no longer relevant as soon as the distance between the two surfaces is large enough. But this experiment proved the strong influence of fluid forces on microparts that are transported with a vibratory feeder.  The conventional model for macroscopic parts is extended to flow forces (4) and vdW forces and explains the observed effects with microparts. The results improve the understanding of the different forces that are influencing the material transport and how they are determined by frequency, amplitude, humidity, surface roughness etc.  ACKNOWLEGMENTS The authors would like to thank Eric Snowdon (MTL)  and Aziel Epilepsia (UW) for the help in experiments and Rajashree Baskaran (UW) for helpful discussions.  References: [1] Scherge, M.; Gorb, S.: Biological Micro- and Nanotribology, Springer 2001 [2] Parsegian, V.A.: van-der-Waals Forces, Cambridge 2006 [3] Schlichting, H.: Grenzschicht-Theorie, 10. Auflage, Springer 2005   Authors:    Dipl.-Wi.-Ing. Guido Gravenkötter / Prof. Dr.-Ing. Rainer Bruns  Machine Elements and Technical Logistics (MTL) Helmut-Schmidt-Universität Hamburg, Holstenhofweg 85 22043 Hamburg, Germany Phone: 040-6541.2653 Fax: 040-6541.2095 guido.gravenkoetter@hsu-hh.de / rainer.bruns@hsu-hh.de  Sangjun Park, PhD. / Prof. Karl F. Böhringer, PhD.  MEMS Laboratory, Department of Electrical Engineering University of Washington, Box 352500 Seattle, WA 98195, U.S.A. Phone: 1-206-221-5177 Fax: 1-206-543-3842 park@ee.washington.edu / karl@ee.washington.edu     

Fluid and van-der-Waals forces in MEMS production

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Fluid and van-der-Waals forces in MEMS production

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