Adaptive Focal Plane Array - A Compact Spectral Imaging …Adaptive Focal Plane Array -A Compact Spectral Imaging Sensor 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER

Adaptive Focal Plane Array -A Compact Spectral Imaging Sensor

William Gunning

March 5 2007

Approved for Public Release, Distribution Unlimited

Report Documentation Page Form ApprovedOMB No. 0704-0188

Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if itdoes not display a currently valid OMB control number.

1. REPORT DATE 05 MAR 2007

2. REPORT TYPE N/A

3. DATES COVERED -

4. TITLE AND SUBTITLE Adaptive Focal Plane Array -A Compact Spectral Imaging Sensor

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S) 5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Teledyne Technologies Co.

8. PERFORMING ORGANIZATIONREPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

11. SPONSOR/MONITOR’S REPORT NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited

13. SUPPLEMENTARY NOTES DARPA Microsystems Technology Symposium held in San Jose, California on March 5-7, 2007.Presentations, The original document contains color images.

14. ABSTRACT

15. SUBJECT TERMS

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT

UU

18. NUMBEROF PAGES

17

19a. NAME OFRESPONSIBLE PERSON

a. REPORT unclassified

b. ABSTRACT unclassified

c. THIS PAGE unclassified

Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18


Motivation for LWIR / MWIR Adaptive FPA

• Conventional hyperspectral imaging systems– Large and Heavy– Generate large volumes of data– Typically scanning systems

• Conventional multispectral imaging systems– Fixed detection wavelengths limit

capabilityAFPA Objective: Develop a compact spectral imaging sensor to enable enhanced target detection / ID in a device that can be deployed on SWAP-constrained platforms and provide real time information• Wavelength tuned LWIR (8 – 11 µm / ∆λ FWHM ~ 100 nm)• Simultaneous pixel-registered broadband MWIR (3 – 5 µm)• Spatially resolved, intelligent spectral analysis


AFPA Parameter Objectives

• MEMS tunable filter array integrated with a dual-band focal plane array• Parameters:

– Tuning range (individual filters or checkerboard): 8.0 µm ⇒ 11.0 µm– Filter bandwidth (FWHM): 100 nm ± 20 nm @ 10.0 µm– MWIR detection band: ~ 3.5 – 5 µm (nominal)– Filter dimension: ~ 400µm center-to-center spacing– Filter optical fill factor: ≥ 50%– FPA/ROIC: 640x480 20µm DB-FPA– Filter format: Spectral fovea (nominally 8 x 24 filters)– Operating temperature: ~ 80K– Filter tuning speed: ~ 1 msec


TS&I MEMS Filter / AFPA Architecture (Notional)

Single Filter Pixel

MEMS FilterArray

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Wavelength (Microns)

Rel

ativ

e R

espo

nse

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Cav

ity T

rans

mis

sion

SiliconSubstrate

SiliconFlexure

FixedMirror

MoveableMirror

Mirror Support(Actuation Interconnect)

ActuationElectronics

Dual-Band DetectorArray (HgCdTe)

Dual-Band ReadoutIntegrated Circuit

(ROIC)(CMOS)

Dual-BandFocal Plane Array

Adaptive Focal Plane Array(AFPA)


AFPA Phase II MEMS Tunable Filter Array

Si MEMS Substrate• Lower half of MEMS filter

structure• Includes stationary mirror,

actuation traces

MEMS filter array• Hybridized moveable mirrors• Spectral Fovea• 16 x 48 filter array• Filter footprint 200 x 200 µm

Direct Drive Interconnect Traces

DBFM Imager Area640 x 480 pixels

MechanicalMountingSurface

DBFM (MW / LW) Broadband Imaging Area• 480 x 480 pixels• “Conventional” DB Imaging

MW / LW AR CoatingMAIC Chip• Hybridized to MEMS

substrate• Direct interconnect to

each filter

Si MEMS Substrate• Lower half of MEMS filter

structure• Includes stationary mirror,

actuation traces

MEMS filter array• Hybridized moveable mirrors• Spectral Fovea• 16 x 48 filter array• Filter footprint 200 x 200 µm

Direct Drive Interconnect Traces

DBFM Imager Area640 x 480 pixels

MechanicalMountingSurface

DBFM (MW / LW) Broadband Imaging Area• 480 x 480 pixels• “Conventional” DB Imaging

MW / LW AR CoatingMAIC Chip• Hybridized to MEMS

substrate• Direct interconnect to

each filter

DB-FPA (MW / LW)

MEMS Actuation IC (MAIC) Chip

DB-FPA Imager Area• 640 x 480 pixels


MEMS Fabry-Perot Filter Design

≈ ≈

SiliconSubstrate

Au-Au Thermocompression

bond

AntireflectionCoating

AntireflectionCoating

Reflector CoatingsMovableMirror

MembraneSpring Flexures

Au- AuMirror(Optical Aperture) ThermocompressionBond Spacers

Mirror(Optical Aperture)

Folded Flexure

Filter characteristics • Fabry-Perot filter design• Tuning band determined by reflection

band of dielectric mirrors

Filter Actuation• Filter actuated by applying potential between

moveable mirror and substrate mirror• Displacement driven by electrostatic attraction• Restoring force provided by Si flexure springs• Prototype devices - direct drive

MEMS structure• Bulk micromachining • Hybrid assembly using Au-Au

thermocompression bond


Modeled MWIR / LWIR Spectral Performance(Transmission Averaged over F/6.5 Incident cone)

Filter air gap varied between 3.1 – 5.6 µm

Wavelength (µm)3 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 12

20

40

60

80

100

Tran

smiss

ion

(%)

0


MEMS Filter SEM Images

Top View

Moveable Mirror w/ Patterned AR Coating

Flexures

Supports

AR Coating

Si MirrorMembrane

Si Device Layer

Thinned Flexure

Au Bonding Pads

Mechanical Support

Bottom View

Patterned AR Coating (recessed)


MEMS Tunable Filter Measured Optical Performance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

8.5 8.7 8.9 9.1 9.3 9.5 9.7 9.9

Wavelength (µm)

Tran

smis

sion

(Nor

mal

ized

)

CO2 Laser Wavelength

(µm)

Filter Bandwidth (nm FWHM)

9.23 1449.28 1389.32 1459.49 1089.52 1129.55 1459.62 909.66 129

9 10 11 12Wavelength (µm)

0

100

200

300

400

500

Sign

al (m

V)

25V

20V

0V0V

20V

25VFilter 1Filter 2

25V

20V

0V0V

20V

25VFilter 1Filter 2

IR Microscope Transmission

Scanned Filter Transmission of Tunable CO2 laser

LWIR Detector Spectral Response with Tunable MEMS Filter


Tunable MEMS Filter Mechanical Response

• Low energy dissipation in Si MEMS structure leads to mechanical “ringing” under vacuum operation– 300µs in air, but may be >10’s (or even 1000’s) msec in vacuum

• Exploit gas damping for increased response speed– Requires sealed, backfilled package– Neon gas provides necessary viscosity for 77K operation

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-0.0001 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 0.001

Time (sec)

Rel

ativ

e Po

sitio

n ~ 300 µs settling time in 1 atmosphere of air

Q ~ 1.0

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-0.001 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

Time (sec)

Rel

ativ

e Po

sitio

n

Vacuum ⇒ Q~35

MEMS Filter Response to Voltage Actuation Step


AFPA Phase II Imaging Device Objectives

• Demonstrate full capability MEMS filter array– Individual, independent filter tunability– Extended tuning range: 8.0 – 11.0 µm– Narrower bandwidth: 100 nm ± 20 nm @ 10.0 µm– Design and implement CMOS MEMS Actuation IC (MAIC) for full array actuation

• Demonstrate prototype AFPA sensor– Imaging structure with tunable MEMS array coupled with dual-band FPA– Demonstrate spectral tunability in an imaging array– Spectral Fovea configuration

• Technical challenges– Overcome tuning limit imposed by MEMS snap-down phenomenon

• Optimized optical filter design• Implement negative capacitance MEMS actuation to overcome parasitic• Provide viscous MEMS damping

– Heterogeneous technology integration in an integrated optimal subsystem

• Tunable MEMS filter array coupled to DB-FPA in a compact, gas-filled, optical, cryo-enclosure


MEMS Actuation and Snap-down

VCp CMEMS

Maximum in Q(x) curve corresponds to charge snap-down limit

Theoretical maximum MEMS deflection before snap-down

using voltage control (33% of unactuated gap)

Cp / CMEMS0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

V (x)Q_norm (x, 0.0)Q_norm (x, 0.55)Q_norm (x, 1.5)Q_norm (x, 5)Q_norm (x, 100)

Displacement “x” (fraction of gap)

Cp / CMEMS0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

V (x)Q_norm (x, 0.0)Q_norm (x, 0.55)Q_norm (x, 1.5)Q_norm (x, 5)Q_norm (x, 100)

Displacement “x” (fraction of gap)

• Charge control enables tuning beyond snap-down

• Limited by parasitic capacitance between driver and MEMS device

• Negative capacitance circuit can overcome Cp

• Requires low MEMS Q to prevent oscillation past stable point

• Optimize optical coatings to maximize tuning slope / minimize demands on –Cp tuning


Primary Sources of Parasitic Capacitance

• Parasitic Capacitance dominated by coupling capacitance– Values depend position inside filter array – Largest parasitic cap determines tuning range for entire array

• MAIC will add similar capacitance• Negative capacitance actuation circuit under development

to overcome Cp limited snap-down

C2cpM

C1gndM

C1padM

Capacitor Specific Capacitance Length or area max. total capacitance min. total capacitance Comment[aF/µm] or [aF/µm2] [µm] of [µm2] [fF] [fF]

C2cpM 40 3200 256 1 µm spacing

C1cpM 40 200 8 1 µm spacing

C1gndM 26 3200 83.2 1 µm SiO2 thickness

C16gndM 26 200 5.2 1 µm SiO2 thickness

CpadM 26 100 2.6 2.6 1 µm line widthTotal to GND 85.8 7.8Total coupling 256 8

Total 341.8 15.8


Integrated AFPA Assembly (Conceptual)

Window

MAIC Connector

Vacuum / gas fill pinch-off tube

Removable cover(Indium crush seal)

MEMS array / MAIC / DB-FPA

MEMS filter array

MAIC

In-bump bond interconnect

Dual-band FPA

LCCMAIC

wirebonds

• Gas filled enclosure enables viscous gas damping of MEMS filters

• Resealable cover enables reuse and testing of MEMS filter array component

• MAIC / MEMS array interface key to achieving tuning beyond snap-down


Planned AFPA Prototype Demonstration

• Demonstration of LWIR spectral response tunability– Independent filter actuation

• Demonstration of spectral analysis capability– Synthetic input spectra (filtered illumination)– Target materials if military interest

• Demonstration of spectral imaging of scene (lab)• Demonstration of simultaneous LWIR tuning / broadband MWIR imaging• Future development of field-testable camera with integrated optimal

spectral interrogation and analysis algorithms

Lab bench level testing planned using prototype AFPA sensor


Summary

• Phase I - LWIR tunable MEMS filter capability demonstrated– Tuning range 8.0 – 10.0 µm– Filter bandwidth 90 – 150 nm– Tuning speed ~ 1 msec– Simultaneous broadband MWIR transmission– Filters as small as 280 x 280 µm

• Phase II - Integrated dual-band AFPA sensor configuration established– Spectral fovea configuration– Wide tuning range (8.0 – 11.0 µm) achievable using novel actuation and

optimized optical design– Independent filter tunability– Sensor package combining MEMS array, CMOS MAIC, Dual-band FPA with

mechanical MEMS damping– Optical configuration requires minimal optical imaging sensor modifications


Documents

Adaptive Focal Plane Array - A Compact Spectral Imaging …Adaptive Focal Plane Array -A Compact Spectral Imaging Sensor 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER