Precision Engineering Research Group

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Massachusetts Institute of Technology
Jin Qiu, Joachim Sihler, Jian Li, Prof. Alexander
H. Slocum, Victoria Sturgeon, Micah Smith
An Instrument to Measure the Stiffness of MEMS
Mechanisms
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• Motivation
• Design of the Prototype
• Measurement Results
• Conclusions and Future work

Presentation Outline
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Actuation force causes deflection
Double Beam Initial shape after DRIE with no
internal stress
Through Etch
Initial apex height 60 micron
Snap through
Silicon
Second stable position after snap through
Click for a movie
The Bistable Mechanism
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Designed thickness 10 µm Apex height 60
µm Lengh 3 mm Depth 490 µm
Theoretical Data(assumes perfectly fabricated
cross section)
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Front side
Back side
Fabrication Imperfectness
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• Verify / Adjust the theoretical model
• To our knowledge, there is no existing instrument
  available for in plane measurement
• Generic instrument for MEMS stiffness tests

Motivation Summary
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Fundamental goal Measure the stiffness of a MEMS
flexure
MEMS flexure
D
Fundamental Goal
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• Design requirements
• Measure the double-beam structure
• ? Force resolution 0.1mN
• ? Displacement resolution 0.1 µm
• Working volume 4 wafers
• Generically applicable
• ? Scaleable (force displacement range)
• Perfectly lateral motion of mechanical contact
• Measure negative stiffness
• Economically manufacturable

Design Requirements
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reference flexure
MEMS flexure
0
1
2
3
4
5
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3
2
1
0
D
FMEMSkref d2
?d1-d2
0
1
2
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2
1
0
d
d
2
1
Fundamental Principle
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Design Equations
Amplification lever
(Actual behavior mapped through calibration)
Reference flexure
MEMS device
Probe Assembly with Reference Flexure
and Amplification Lever
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Design Evolution
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OMAX Machining Center
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Instrument Fabrication
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Waterjet cut aluminum flexures
Prototype
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Prototype Details
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Resolution 100 nm 0.1µm ? 0.1 mV (Op amp based
I-V converter)
Colin J.H. Brenan et al. Characterization and
use of a novel optical position sensor for
microposition control of a linear motor , Rev.
Sci. Instrum. 64(2), February 1993.
Barcode Readers used as Displacement Sensors
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Create a first look-up table
Displacement (micrometer reading)
V2
Voltage V1
X X X X . . .
Y Y Y Y . . .
V1
Calibrated flexure
Calibration Step 1
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Create a second look-up table
V2
Displacement (micrometer reading)
Voltage V1
Voltage V2
Z Z Z Z . . .
X X X X . . .
Y Y Y Y . . .
V1
Tip locked
Calibration Step 2
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V2
V1
Precision Force Gauge (represented by a spring)
Calibration Step 3
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VI-Interface (Labview)
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Theoretical Data for 6.7 micron thick beam
Theoretical Data for 6.0 micron thick beam
Experimental Data
Measurement Data
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• Proof of Concept Successfully recorded F-d curve
  in real time
• Resolution high enough to characterize the double
  beam MEMS structure
• Effective way for in-plane stiffness test of MEMS
  devices
• Ability to record negative stiffness (negative
  F-d-slope)

Conclusions
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• Adjustability for different force and
  displacement ranges
• ( current setup 0.1mN.1N, 100nm.1mm )
• Measurement perpendicular to the wafer surface
• Decrease overall size
• Integrate imaging system for easier initial
  positioning
• Automation of data acquisition cycle (stepper
  motor drives)
• Need to find a more accurate way for calibration

Future Work
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