1. 1 ME5000 MEMS Technologies This courseware product contains scholarly and technical information and is protected by copyright laws and international treaties. No part of this publication may be reproduced by any means, be it transmitted, transcribed, photocopied, stored in a retrieval system, or translated into any language in any form, without the prior written permission of Acehub Vista Sdn. Bhd. The use of the courseware product and all other products developed and/or distributed by DreamCatcher are subject to the applicable License Agreement. For further information, see the Courseware Product License Agreement. http://dreamcatcher.asia/cw

2. 2 Chapter 4 Micro Actuators and Actuating Systems

3. 3 Outline Introduction Actuation mechanisms –Electrostatic Parallel plate Comb drive –Thermal Asymmetric Bimorph Chevron –Piezoelectric –Magnetic External field Cantilever –Bi-stable References

4. 4 Sensors and Actuators Sensor Input Output Mechanical Optical Thermal Others Electrical Actuator Input Output Mechanical Optical Thermal Others Electrical

5. 5 Examples of Actuators Micro-Mirror Voltage/CurrentRotation RF-Switch Pumping/Displacement Switching Micro-Pump Voltage/Current

6. 6 Actuation Mechanisms (Electrostatic, Thermal, Piezoelectric, Magnetic)

7. 7 Electrostatic Actuation Coulomb’s Law: –Consider two charges with distance d –The force between them is: q1q1 q2q2 Charge q 2 Charge q 1 d Force Coulomb’s constant Permittivity of free space This force becomes significant as the dimensions of the device shrink because it is inversely proportional to d 2

8. 8 Parallel Plates When a parallel plate structure is connected to a voltage, the energy stored is: A d The force applied to each plate is:

9. 9 Tunable Capacitor Assume a parallel-plate capacitor with a plate connected to a spring: Under voltage V: Displacement of the spring by force F: New capacitance value: k d0d0 d By solving these 2 equations, we can calculate the equilibrium condition.

10. 10 Pull-In Point For small voltages, the spring force and the electrostatic force balance each other, and the movable electrode displacement can be calculated by solving the previous slide’s equations When the applied voltage increases, the distance between the electrodes become smaller. At a certain point, the electrostatic force overcomes the spring force and the top electrode will collapse on the fixed electrode. This point is called Pull-In Point At this point, there is no solution for the equations in the previous slide The pull-in point limits the travel range of the movable electrode k V d d d0d0 Pull-in point dpdp VpVp

11. 11 Pull-In Point (cont.) k V d d d0d0 Pull-in point dpdp VpVp For details on calculating the pull-in voltage and gap, please refer to Practical MEMS by Ville Kaajakari or Microsystem Design by Stephen D. Sentura. Pull-in gap: Pull-in voltage:

12. 12 Lateral Comb Drive Consider a lateral capacitor of height h One finger (orange) is between the two electrodes (blue) by a gap d When a voltage is applied, the orange electrode will be displaced by x and the capacitance between the electrodes is: The electrostatic force under voltage V becomes: d d x l Fringing capacitance The force is independent of displacement, x

13. 13 Lateral Comb Drive (cont.) To increase the force, many fingers are used For N overlapping fingers, the electrostatic force is: Compared to the parallel- plate structure, comb drive can travel a larger range. However, its force per volume is less N = 5

14. 14 Example A comb-drive has 100 overlapping fingers. The height, width, and spacing between the fingers is 5  m. a)If an actuation voltage of 20V is applied to this structure, calculate the electrostatic force. b)If the length of each finger is 10  m, calculate the electrostatic force for a parallel capacitor structure with the same overlapping area. a) b)

15. 15 Example: Capacitive RF MEMS switch A surface-micromachine approach was followed for the fabrication of switches They are in coplanar waveguide (CPW) configuration with a suspended metal bridge connecting the lateral ground planes and a dielectric layer on the central conductor which provides a capacitive contribution when the bridge is in down state “Source: Figure 4 in Anna Persano et al, Ta2O5 Thin Films for Capacitive RF MEMS Switches, Journal of Sensors, Volume 2010 (2010), Article ID 487061” Ground lines Signal line Suspended spring

16. 16 Example of Comb Drive Application A torsional ratcheting actuator by Sandia Lab The comb drives vibrate the inner frame The inner frame ratchets its surrounding ring gear “Courtesy of Sandia National Laboratories, SUMMiT(TM) Technologies, www.mems.sandia.gov” Comb-drive Inner frame Ring gear

17. 17 Electrostatic Actuators Summary Low power operation (no current) Fast switching Simple fabrication process XThe created force is low XThe displacement (travel) range is small XRequire high voltage to operate

18. 18 Asymmetric Thermal Actuator The current passes through the structure The resistivity of the hot arm is high so it gets hot and will expand The resistivity of the cold arm is low, so its temperature doesn’t change much The structure will turn towards the cold arm Cold armHot arm Contact pads

19. 19 Bimorph Thermal Actuator Bimorph is a cantilever with two active layers The thermal expansion coefficients of the layers are different A temperature change will cause the structure to bend because one layer expands more than the other layer Material with higher thermal expansion coefficient

20. 20 Chevron Thermal Actuator y L x Thermal Expansion Coefficient

21. 21 Example of Chevron Actuator in RF The structure is a RF Switch At rest, the RF In and RF Out are disconnected Upon applying the voltage V, the actuator arm will move and connect the RF In to the RF Out Connecting a few fingers in parallel will increase the force, however, the displacement remains the same RF InRF Out V

22. 22 Thermal Actuators Summary Low voltage High force Large travel range Simple fabrication XThey are popular in research but not widely used commercially XLow switching speed XHigh power consumption

23. 23 Piezoelectric Actuation F h F F F L Modulus of elasticity Longitudinal piezoelectric coefficient Voltage Transverse piezoelectric coefficient  h is independent of the size  L is proportional to the L/h ratio

24. 24 Stacking Piezoelectric Actuators To increase the displacement, several piezoelectric actuators can be stacked together: V Note: The direction of displacement depends on the direction of the electric field Note: Since the direction of the electric field in the adjacent cells is opposite, the orientation of the adjacent cells must also be opposite to achieve displacement in the same direction

25. 25 Piezoelectric Actuators Summary Low power High force Fast switching Bidirectional displacement with change of voltage direction XComplicated fabrication process XHigh voltage XSmall travel range

26. 26 Magnetic Actuators Magnetic actuation is very popular in macro-scale However, material limitations and fabrication is a major challenge in micro-scale There are several possible scenarios: –A permanent magnet can be placed in an external magnetic field –A current carrying conductor can be located in an external field –A permanent magnet can be placed near a current carrying coil

27. 27 External Magnetic Field Actuator A membrane is created inside the silicon substrate using bulk micromachining A permanent magnetic material is embedded inside the membrane Once the structure is exposed to an external magnetic field, the membrane will deflect Permanent magnet Membrane Magnetic field Silicon

28. 28 Cantilever Magnetic Actuator Permanent magnet array is electroplated on the silicon cantilever structure The silicon wafer is bonded to a glass substrate The cantilever will deflect under an external or internal magnetic field Cantilever Permanent magnet array External/internal magnetic field Silicon Glass

29. 29 Magnetic Actuators Summary Low voltage High force Large travel range Bidirectional displacement XHigh power XComplicated fabrication process XModerate switching speed

30. 30 Micro-Actuators Summary Table ElectrostaticThermalPiezoelectricMagnetic Low voltage X X Low power X X Fast switching X X Large traveling range X X High force X Simple Fabrication Process XX Bidirectional XX

31. 31 Bi-Stable Actuators Thermal and magnetic actuators are power hungry One way to reduce the power consumption is to design the actuators such that they only consume power when there is a need to change the state Bi-stable structure has two stable states Stable State 1 F Stable State 2

32. 32 Example: Bi-Stable Magnetic Actuated RF-Switch The RF signal is passing through the signal line The permanent magnet creates a magnetic filed in Z direction The direction of current is such that the force pulls the actuators outwards Since the actuators are bi-stable, even in the absence of a current, they remain in stable position Permanent magnet Bi-stable magnetic actuators “Source: www.ece.lsu.edu/dyhah/RF_switch.html”

33. 33 Example: Bi-Stable Magnetic Actuated RF-Switch (cont.) For switching, the current is passed though the actuators according to the directions shown in the figure The force will push the bi-stable actuators towards the ground lines The signal line will be grounded and the RF signal will be reflected back Now, even in the absence of a current, the actuators remain in stable position Permanent magnet Ground lines “Source: www.ece.lsu.edu/dyhah/RF_switch.html”

34. 34 Summary Micro actuators are used in many applications such as: –RF MEMS switches and relays –Micro-mirrors Various actuation methods are available: –Electrostatic –Piezoelectric –Thermal –Magnetic The selection criteria depends on the application: –Voltage –Power –Speed –Traveling range –Ease of fabrication –Force

35. 35 Selected References & Further Reading [1]Ville Kaajakari, “Practical MEMS”, 2009, Small Gear Publishing [2]Marc Madou, “Fundamentals of Microfabrication and Nanotechnology”, 3 rd Edition, 2011, CRC Press [3]Simon M. Sze, “Semiconductor Sensors”, 1994, John Wiley [4]Stephen D. Senturia, “Microsystem Design”, 2000, Kluwer Academic [5]www.ece.lsu.edu/dyhah/RF_switch.htmlwww.ece.lsu.edu/dyhah/RF_switch.html [6]www.mems.sandia.govwww.mems.sandia.gov [7]Anna Persano et al, “Ta2O5 Thin Films for Capacitive RF MEMS Switches”, Journal of Sensors, Volume 2010 (2010), Article ID 487061