Structural Analysis of a Magnetically Actuated Silicon Nitride

Micro-Shutter for Space Applications

James P. Loughlina, Rainer K. Fettigb, S. Harvey Moseleya, Alexander S. Kutyrevb, D. Brent Motta

Simulation: Timothy Carnahana

aNASA/Goddard Space Flight Center, Greenbelt, MD 20771bRaytheon ITSS, Greenbelt, MD 20771

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Next Generation Space Telescope (NGST)

• Next Generation Space Telescope

– Mission: Investigate Galactic Origins

– Key Instrument: NIR MOS• Near Infrared Multi-Object Spectrometer

– MOS Requirement:

Addressable “Field Selector”

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Micro-shutter Attributes and Requirements

• Pixel Size Requirement: 100µm x 100µm• Torsion Strap: 90µm x 3µm x 0.5µm thick• Mirror Material: Silicon Nitride

– silicon nitride is linear until fracture

• Operational Temperature: 30°K• Rotation

– micro-shutter will rotate 90° to the open position

– micro-shutter will be closed in the off position

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Micro-shutter Array

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Electrostatic Analysis

• 2D Structural/Electrostatic FEM Using ANSYS/Multiphysics v5.7

0 Volts 625 Volts

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Electrostatic Analysis– ANSYS Predicts 625+ Volts Required for 90º Actuation

‼ Limit ~ 100 V

– Parametric Study for Determining Max. Rotation with Limiting Actuation Voltage:

Electrostatic Forces

0.0E+00

1.0E-10

2.0E-10

3.0E-10

4.0E-10

5.0E-10

6.0E-10

7.0E-10

8.0E-10

0 20 40 60 80 100Angle

Torq

ue

(N

-m)

Torsion Strap K

20 volts

40 volts

60 volts

80 volts

100 volts

•100 V Limit Allows for 10º rotation, from 80º to 90 º

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Mechanical Operation

• Produced by Timothy Carnahan, GSFC Code 542

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Electromagnetic Analysis

Magnet Width = 0.03m

• Proposed Tripole Electromagnet

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Micro-shutter Size Relative to Magnetic Poles

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Electromagnetic Analysis

d

• d=distance between shutters and magnet; 40µm ≤ d ≤ 120µm• peak magnetic flux density = 0.23 tesla at magnet centerline

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Deflected Micro-shutters

• Magnetic forces are Applied to the Micro-Shutters. • The force deflects and rotates theShutters.

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Electromagnetic Analysis

• Shutter Hinge Reaction Torque Relative to Magnet Centerline

Shutter Hinge Moment vs. Magnet Centerline

-3.00E-09

-2.50E-09

-2.00E-09

-1.50E-09

-1.00E-09

-5.00E-10

0.00E+00

5.00E-10

1.00E-09

1.50E-09

2.00E-09

-800 -600 -400 -200 0 200 400 600 800 1000

Hinge Location Relative to Magnet Centerline (m)

Hin

ge

Mo

me

nt

(N-m

)

0 degree

45 degree

90 degree

60 degree

70 degree

80 degree

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Electromagnetic Analysis• Shutter Hinge Reaction Torque (N-m) vs. Angular

Deflection

Shutter Torsion Strap

-1.0E-09

-5.0E-10

0.0E+00

5.0E-10

1.0E-09

1.5E-09

2.0E-09

0 20 40 60 80 100

Rotation Angle of Shutter Tip

Ro

tatio

na

l Mo

me

nt

Torsion Strap K

100um

50um

35um

15um

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Model Correlation

• Electromagnetic FEA Indicates Maximum Achievable Degree of Rotation = 86º

• Electrostatic Parametric Study Indicates Required Voltage to Capture Shutter (at 86º Position) = 43 V– Lab Tests Show “Capture” Voltage = 50 V

• Electrostatic FEA Indicates 15 V Required to Maintain 90º “Open” Position– Lab Tests Show “Release” Voltage = 23 V

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Stress Analysis• 3D Structural FEM Using ANSYS/Multiphysics v5.7• Assumed Silicon Nitride Fracture Strength = 6.4 GPa[1]

• Peak Resultant Stress = 1.5 GPa

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Weibull Fracture Probability

Weibull Distribution

-2.00E-01

0.00E+00

2.00E-01

4.00E-01

6.00E-01

8.00E-01

1.00E+00

1.20E+00

0 2000 4000 6000 8000 10000

Stress

Pro

bab

ilit

y o

f F

ailu

re

weibull

• Many SiN cantilevered test samples were manufactured and tested.• Probability of failure is dependent on the surface area of the test structure relative to the surface area of the device.• test/torsion = (Surface Area test/Surface Area torsion)1/m

• Stresses in the 3m x 0.5m torsion strap are approximately 1500 MPa when the shutter is open.• The Probability of Failure at 1500 Mpa is 9.48e-5. (99.99% success rate)

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Conclusions

• FEA Predicts:– Electromagnetic Tripole Will Open a

Magnetized Shutter to 90º Position– Shutter Electrostatically Captured When

Opened >80º Using <100V– Shutter Maintained at 90º for 15V

• Test Data Correlates with Predicts

• Coupled-Field FEA May Be Used for Design Optimization

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References

1) G. Coles, R. L. Edwards, W. Sharpe, The Johns Hopkins University. “Mechanical Properties of Silicon Nitride”. SEM Annual Conference. June 2001.

2) R. Fettig, J. Kuhn, S. H. Moseley, A. Kutyrev, J. Orlof, S. Lu. “Some Aspects on the Mechanical Analysis of Microshutters”. Micromachining and Fabrication, Vol. 3875. September 1999