Introduction to Some Mechanical Aspects of MEMS

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Introduction to Some Mechanical Aspects of MEMS
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SEM Image of Early Northeastern University MEMS
Microswitch
The scale bar corresponds to 10 ?m
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SEM Image of Improved Northeastern University
Microswitch
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SEM Micrograph of the Contact Tip
2 ?m diameter
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Top View of NU Micromechanical Switch (all
dimensions in ?m, direction reversed from SEM)
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Side View of NU Micromechanical Switch

• Residual Stresses (Curvature, Take-Off Angle)
• Electrostatic Actuation
• Dynamics and Vibrations
• Squeeze-Film Damping
• Contact/Impact Mechanics
• Adhesion/Stiction
• Thermal Effects

Why model ?
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Modeling of Switch Dynamics and Bounce
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Comparison of Model and Experiment for Switch
Bounce
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Drain Failure at High Current
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Micromirror-Based Imaging System
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Micromirror With Torsion Bars and Actuators
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• Design Requirements
• Achieve a minimum angular rotation of ? 2o
• Operate at 1 kHz
• Limit the applied voltage to 200V to prevent
  surface breakdown
• Limit shear stress of springs to 400-500 MPa
  during operation
• Ensure shock resistance during handling and use

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Simulation and Measurements at 600 Hz, Unbiased
The measured and simulated angle of rotation vs.
time (simulated curve is smooth) and the measured
drive voltage vs. time.
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Simulation and Measurements at 905 Hz, Biased
The measured and simulated angle of rotation vs.
time (simulated curve is smooth) and the measured
drive voltage vs. time.
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Stress and Strain
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Stress vs. Strain Relationships(Constitutive
Relations)
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Macroscale Testing
Tension Test Specimen
Tension Test
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Macroscale Testing
Different Yield Strengths (Y)
Slope of the linear elastic region is called the
Youngs Modulus (E) or elastic modulus Poissons
ratio where usually 0 ?
? ? ½, but -1 ? ? ? ½
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Microsystem Design, Senturia, p. 196
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Macroscale Testing
Liberty Ships were the first all-welded
pre-fabricated cargo ships and were mass produced
in the United States. 2,751 Liberty Ships were
built between 1941 and 1945. Only two now remain
afloat, many of the remainder were destroyed by
cracking of the type shown. Photograph courtesy
of the Principle and Fellows of Newnham College,
Cambridge.

• Macroscale material behavior is now well
  understood.
• Lets try to avoid mistakes like this at the
  micro/nanoscale!

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Microscale Testing

• Indentation tests can give modulus
• Micro-Tension tests using comb drive actuation
• Modulus varies depending on processing
• Yield strength is scale-dependent
• Processing introduces pre-stress which can be
  important
• WWW.MEMSNET.ORG (Material Database)
• Many papers in the literature
• Data is not at all consistent !

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Polysilicon
W.N. Sharpe, K.M. Jackson, K.J. Hemker, Z. Xie,
JMEMS, Vol. 10, 2001, pp. 317-326.
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Variation of Youngs Modulus - Ni
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MEMS-Based Tension Testing
M.A. Haque and M.T.A. Saif, JMEMS, 2001, Vol. 10,
pp. 146-152.
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Schematic of Tensile Specimen Haque Saif
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Fracture of Silicon
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Fracture of Silicon
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Nanoscale Testing

• Nanoindenters Hysitron, Nanoindenter Co., CSM
• Journal and Conference Papers
• CHN Peter Ryan, Juan Carlos Aceros
• Variations greater than at microscale
• Importance of surfaces and defects
• Testing is more important, but much more
  difficult !!
• Need multiple test methods

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Hysitron Nanoindenters
Hysitron Ubi
Hysitron Triboindenter
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Nanoindentation Test
Nanoindentation
Force vs. displacement

• Hardness is depth-dependent
• H ? 2.8Y

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CHN Peter Ryan
SEM of Suspended Chromium NanoCantilevers
Positive Resist Process
Negative Resist Process
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AFM Based Measurements Peter Ryan

• Uses an AFM to scan along the suspended
  cantilever
• Measure force vs. deflection to back-out modulus
  (E ? 80 GPa)

Young's Modulus measured versus position
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CHN Tension Tester Juan Carlos Aceros

• Performs tensile test of suspended nanowire
• using electrostatic comb drive actuator.
• Use an AFM to measure displacement

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Electrostatic Angular Resonator Juan Carlos
Aceros

• Electrostatic force in a comb drive oscillates
  the device at its resonant frequency.
• Nanowire is stretched and defects are induced on
  it.