Device level vacuum packaged micromachined infrared

detectors on flexible substrates

Aamer MahmoodDonald P. Butler

Zeynep Çelik-Butler

Microsensors Laboratory Department of Electrical Engineering

University of Texas at Arlington,Arlington, TX 76019

2

Outline

Microbolometers Flexible substrates

Device level vacuum packaging Design and fabrication

Characterization Future work

3

Bolometers Bolometers are thermal detectors YBCO is used as the detector material Change in temperature induces a change in the detector

resistance

η = absorptivity, β = TCR, = angular frequency of incident radiation, τ = detector thermal time constant, ΔΦ = the magnitude of the incident flux fluctuation, Geff = thermal conductivity

21221 /

eff )τω(G

βIRηΔV

21221 /

eff )τω(RG

βVηΔI

4

Sensors on flexible substrates

PI 5878G (liquid Kapton) is used as the flexible substrate

Sensor Arrays on flexible substrates (Smart skins) Infrared sensors Pressure/Tactile Sensors Flow sensors Humidity sensors Velocity sensors Accelerometers

Advantages of flexible substrate micro sensors Low cost Lightweight Conformable to non planar surfaces High degree of redundancy

Vacuum packaging brings the best out of many MEMS devices

5

Microbolometer fabrication

Trench Geometry(Not to scale)

6

Fabrication(Silicon wafer)

7

Fabrication(PI 5878G)

8

Fabrication(Nitride)

9

Fabrication(Al)

10

Fabrication(Sacrificial Polyimide PI 2610)

11

Fabrication(Support Nitride)

12

Fabrication(Ti arms)

13

Fabrication(Au contacts)

14

Fabrication(YBCO detector pixel)

15

Fabrication(Photodefinable PI2737 sacrificial mesa)

16

Fabrication(Al2O3)

17

Section of vacuum cavity before micromachining

Al2O3

Sacrificial PI2737 mesa

Sacrificial PI2610

Al mirrorNitride

Nitride

18

Fabrication(Partially micromachined)

19

Fabrication(Fully micromachined)

20

Fabrication(Sealed vacuum cavity)

21

Fabrication(Superstrate PI 5878G)

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Single microbolometer

23

Design considerations Transmission through optical window Structural integrity of vacuum element

Lateral dimensions Cavity resonant wavelength

Axial dimensions

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Structural integrity of vacuum element

25

Al2O3 stress analysis

100

101

102

103

104

0.1 1 10 100

Al2O

3 stress vs. radius of curvature

Mises stress (MPa)Tensile strength (MPa)Compressive strength (MPa)

Str

es

s (M

Pa

)

r (cm)

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Thermal analysis

Gth ≈ 5x10-6 W/K (Vacuum)

≈10-4 W/K (air)

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Fabrication of encapsulated devices

Partially micromachined

device

Fully micromachined

device

SEM graph of an unsealed

micromachined device

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Fabrication of encapsulated devices

Sealed device SEM graph of sealed device SEM graph of

cross section of vacuum cavity

Vacuum cavity

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VI curve

-40

-30

-20

-10

0

10

20

30

40

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

Vol

tage

(V

)

Current (A)

)()(1

)( 20 TRI

dT

dR

GRTR b

th

Gth=3.36x10-6 W/K

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Temperature Coefficient of Resistance (TCR)

40

60

80

100

120

140

160

180

-5

-4

-3

-2

-1

0

280 285 290 295 300 305 310 315

Res

ista

nce

(M

)

TC

R (%

K-1)

Temperature (K)

dT

dR

RTCR

1

R(300K)=53.4 MΩ

TCR(300K)=-3.4%/K

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Current Responsivity (RI)

10-1

100

101

102

1 10 100 1000

10.09V7.20V5.48V3.66V

Res

po

ns

ivit

y (A

/W)

Frequency (Hz)

2/122 )1(

eff

I RG

VR

RI=6.13x10-5 A/W

@ 5Hz

Current Responsivity (RI)

=Output current/Input power

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Detectivity (D*)

102

103

104

105

106

1 10 100 1000

10.09V7.20V5.48V3.66V

Det

ec

tiv

ity

(cm

Hz1/

2 /W)

Frequency (Hz)

nV

AfRD

*

D* = 1.76x105 cm-Hz1/2/W

Detectivity = D*

= Area normalized signal to noise ratio

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Conclusion

Device level vacuum encapsulated microbolometers on flexible substrates have been fabricated

Theoretical thermal conductivity in vacuum is 5x10-6 W/K

Measured thermal conductivity is 3.36x10-6 W/K (Intact Vacuum cavity)

Measured room temperature TCR is -3.4%/K, resistance is 53.4MΩ

Measured RI is 6.13x10-5 A/W, D*=1.76x105cm-Hz1/2/W

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Future work

Incorporating more sensors in the smart skins e.g. pressure/tactile sensors, flow sensors, accelerometers

Cavity design to improve/tune optical response

True integrated flexible system

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This work is supported by the National Science Foundation

ECS-025612

The End