Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 Dean P. Neikirk and Sangwook Han Microelectronics

Microelectronics Research Center, The University of Texas at AustinMicroelectronics Research Center, The University of Texas at AustinMicrotechnologies for the New Millennium 2005Microtechnologies for the New Millennium 2005

Dean P. Neikirk and Sangwook Han Microelectronics Research Center

Department of Electrical and Computer EngineeringThe University of Texas at Austin

Austin, TX 78712USA

SPIE’s Microtechnologies for the New Millennium15-18 May 2003

Hotel Meliá SevillaSevilla, Spain

Proceedings of SPIE Vol. #5836 : Smart Sensors, Actuators, and MEMSSESSION 16, Room: Arenal I

Wed. May 18, 12.00 to 13.00: Infrared Sensors12.20-12.35: Design of infrared wavelength-selective microbolometers using planar multimode detectors, D. P. Neikirk, S. Han, Univ. of Texas/Austin (USA) [5836-60]

link to pdf of proceedings paper .

Design of Infrared Wavelength-Selective Design of Infrared Wavelength-Selective Microbolometers Using Planar Multimode Microbolometers Using Planar Multimode

DetectorsDetectors

Microelectronics Research Center, The University of Texas at AustinMicroelectronics Research Center, The University of Texas at AustinMicrotechnologies for the New Millennium 2005Microtechnologies for the New Millennium 2005 2

Fabry-Perot Microbolometer Array

dmirror

resistive sheet

http://lep694.gsfc.nasa.gov/code693/tdw03/proceedings/docs/session_2/Ngo.pdf

Conventional microbolometer infrared

focal plane detectors• in an ideal device, the absorber should p

rovide total absorption of the incoming radiation and convert the electromagnetic radiation into heat

• to “match” an absorber to free space requires

– absorber: e.g., a thin conductor with sheet resistance 377 ohms

– mirror placed (odd integer)·/4 behind absorbing layer

– essentially a Fabry-Perot cavity– this is sometimes referred to as “space cl

oth”

incident

thin conductor(absorber)

Mirror layer

Gap

4

odd


Spectral response of Fabry-Perot microbolometers

377

mirror

• is it possible to build “multi-color” IR F-P microbolometer focal plane arrays?– the primary “design variable” is the distance to the mirror

3 m gap

2.5 m gap

2 m gap

LWIR

wavelength (microns)

cou

pli

ng

eff

icie

ncy

4 6 8 10 12 14

0.2

0.4

0.6

0.8

0

1

the bandwidth of conventional Fabry-Perot microbolometers is too wide to allow easy “color” discrimination in the LWIR wavelength band


Single Element in Array

g

w

a

d

metal grid

mirror

resistive microbolometer

Planar Multimode Detector Array

metal grid

mirror

Alternative: planar multimode detectors

• replace standard thin film bolometer with a true antenna coupled microbolometer array– essentially a

resistively loaded inductive/capacitive mesh

– planar multimode detectors were extensively studied for infrared and millimeter wave detection by Rutledge and Schwarz in the late 1970’s


Planar Multimode Detectors

support layerresistive microbolometer

gridmirror

single pixelsingle pixel

η0 η0jBcjBl

Gd

Zin d

grid equivalent mirror equivalentarrayarray

• grid period a < the shortest wavelength• analysis can be performed using a

modification of Eisenhart & Kahn’s waveguide post model

single periodsingle period

a g

d

w


Spectral response of planar multimode grids

Blue

a NA

g NA

w NA

d 2.50

Rs 377Ω

all lengths in [micron]

• grid response depends on array period a, gap g, post width w, distance to mirror d, and sheet resistance RS of microbolometer material

space cloth

7 8 9 10 11 12 13 14wavelength [micron]

0

0.5

1

po

we

r ab

sorp

tio

n e

ffic

ien

cy

*

*

377Ω

2.50

4.50

5.00

7.00

Mag

30Ω

3.61

0.96

1.00

5.33

Green

53.5Ω

3.29

3.00

0.20

6.90

Red

• wide range of achievable bandwidths, from broad to narrow

grida g

dw


Methods of to achieve varied wavelength selectivity

mechanical actuation

• for fixed grid dimensions, vary the distance between the array and the mirror

– three “color” array using three different mirror distances

• for fixed grid dimensions, use an actuator to vary the distance between the array and the mirror


d=0μm

d=6μm

d

10 20 30 40 50 60 70 80 90 100

20

40

60

80

100

120

140

160

180

200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 62 41 3 5d in [μm]

7

14

13

12

11

10

9

8

wav

elen

gth

in [

μm]

power absorption efficiency

7 8 9 10 11 12 13 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength in [micron]

po

we

r a

bs

orp

tio

n e

ffic

ien

cy

d=3.29μm

a=6.90μm

g=0.20μm

w=3.00μm

Rs=53.5Ω

Wavelength selectivity varied by changing distance d to the mirror

• narrow spectral response allows greater “color” sensitivity


Selecting spectral response for “color” pixels

7 8 9 10 11 12 13 140

1

0

1

0

1


pow

er a

bsor

ptio

n ef

fici

ency

7 8 9 10 11 12 13 140

1

0

1

0

1


pow

er a

bsor

ptio

n ef

fici

ency

ideal ambiguous

• if a design produces more than one peak in absorption then the “color” becomes ambiguous

pixel 1

pixel 2

pixel 3

“ghost” peak


Achieving wavelength selectivity by varying d

d=3.29 micron

d=5.70 micron

d

10 20 30 40 50 60 70 80 90 100

20

40

60

80

100

120

140

160

180

200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 62 41 3 5d in [micron]

7

14

13

12

11

10

9

8wav

elen

gth

in [

mic

ron]


7 8 9 10 11 12 13 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


po

we

r a

bs

orp

tio

n e

ffic

ien

cy

a=6.90 microng=0.20 micronw=3.00 micronRs=53.5Ω

““ghost” peak: ghost” peak: badbad

7 8 9 10 11 12 13 14wavelength in [micron]

7 8 9 10 11 12 13 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


po

we

r a

bs

orp

tio

n e

ffic

ien

cy

0

1


GoodGood

GoodGood


7 8 9 10 11 12 13 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


po

we

r a

bs

orp

tio

n e

ffic

ien

cy

d=2.0micron

d=2.5micrond=3.0micronRs=377Ω


1

pow

er a

bsor

ptio

n ef

fici

ency

07 8 9 10 11 12 13 14

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


po

we

r a

bs

orp

tio

n e

ffic

ien

cy

d=1.65micrond=3.75micron

d=5.10micron

a=5.07microng=1.43micronw=0.74micron

Rs=21.0Ω


1

pow

er a

bsor

ptio

n ef

fici

ency

0

Space clothSpace cloth

Optimized 3-color wavelength selectivity by varying only d

optimization performed using a genetic algorithm

– design goal: three minimum width spectral peaks centered at 8, 10 and 12 microns

insufficient spectral insufficient spectral selectivityselectivity


7 8 9 10 11 12 13 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


pow

er

abs

orp

tion

effi

cie

ncy

Problem with variable d: complex fabrication

processes

• requires either– fabrication process with three different

sacrificial layer thicknesses

• or– mems actuator

• in both cases the fabrication would be more complicated than current micromachined microbolometer focal plane array processes

– there is still some color ambiguity for the longest wavelength (12 micron) pixel

• due to “ghost” peak at 7 microns


Wavelength selectivity by varying lithographically-drawn parameters

a g

d

w• potentially simpler process if the sacrificial layer is held

fixed for all pixels– sheet resistance of bolometer material also held

constant (same material for all pixels)– vary ONLY the lithographically drawn features of the

grid• array period a, gap width g, and post width w


Optimized designs• genetic algorithm used for optimization

– design goal: three minimum width spectral peaks centered at 8, 10 and 12 microns– constraints:

• d (distance to mirror) is varied, but must be the same under all three pixels• sheet resistance of bolometer material also held constant• vary only grid period a, gap width g, and post width w

w = 4.57m

wavelength in [micron]8 9 10 11 12 13 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

7

pow

er a

bsor

ptio

n e

ffic

ienc

y

w = 2.80m

w = 1.30m

• results from design optimization

– optimum distance to mirror d = 3.14 m

– optimum RS = 56.6 – all three pixels share co

mmon grid period a and gap width g

• a = 6.80 m• g = 0.20 m

– post width w is critical in determining location of absorption peak


abso

rpti

on

Comparison: power absorption efficienciespower absorption efficiencies

7 8 9 10 11 12 13 140

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po

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cy

7 8 9 10 11 12 13 140

0.1

0.2

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po

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tio

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7 8 9 10 11 12 13 140

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po

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7 14wavelength in [micron]

0

1

7 14 7 14

Fabry-Perot microbolometer

variable mirror grid microbolometer

grid dimensions varied


IR wavelength-selective focal plane arrays

• planar multimode detectors exhibit widely tunable spectral response– can tune for much narrower spectral response than conventional

Fabry-Perot microbolometers

• tuning of wavelength response can be achieved using several methods– for fixed grid dimensions distance to tuning mirror can be used

• multiple sacrificial layer thicknesses

• mechanical actuation

• a wavelength-selective three pixel design, each pixel using different lithographically drawn dimensions with constant mirror separation, shows excellent narrow band response

• through the use of planar multimode detectors color vision in the long wavelength band should be achievable

Documents

Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 Dean P. Neikirk and Sangwook Han Microelectronics