Device-level vacuum-packaged infrared sensors on flexible substrates

Device-level vacuum-packaged infrared sensors on flexible

substrates

Aamer Mahmood

Advisor:Prof. Donald P. Butler

Microsensors LaboratoryDepartment of Electrical Engineering

University of Texas at ArlingtonArlington, TX 76019

2

Outline

– Introduction– MEMS– Infrared radiation and detection– Bolometers– Flexible substrates

– Bolometers on flexible substrates

– Device-level vacuum-packaged microbolometers

– Fabry Perot cavity based tunable infrared microspectrometer

3

Microelectromechanical Systems (MEMS)

• Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common substrate through microfabrication technology.

4

Infrared radiationPlanck’s law

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

BlackbodyGraybody

No

rmal

ized

exi

tan

ce

Wavelength(m)

5

Infrared Detectors• Photon Detectors

– Incident radiation generates photo carriers• Photovoltaic detectors• Photoconductive detectors• Photoemissive detectors

• Thermal Detectors– Incident radiation causes change in temperature that

causes a change in a detector property e.g.• Bolometers (change in temperature causes the

detector resistance to change)• Pyroelectric detectors (change in temperature causes

the detector capacitance to change)• Thermocouples (use Seebeck effect)

6

Bolometers and IR detection

eff

thth G

C

2)(1 theff

signalac

G

PT

7

Bolometers and IR detection

• Temperature induces a change in the detector resistance

η = absorptivity, = angular frequency of incident radiation, τ = detector thermal time constant, Psignal = the magnitude of the incident flux fluctuation

signaleff

b PG

IRV

2/122 )1(

signaleffb

b PG

V

RR

RI

2/1222 )1()(

Rb

R amp

V

RbI amp

8

Bolometer Figures of Merit

dT

dR

RTCR

1

2/122 )1(

eff

b

signalV G

IR

P

V

2/1222 )1()(

effb

b

signalI G

V

RR

R

P

I

Temperature coefficient of resistance

Responsivity

Normalized change in resistance w.r.t. temperature

Output/Input

9

Bolometer Figures of Merit

Detectivity

nVV V

AfD

nII I

AfD

Signal-to-noise ratio normalized to the detector area and frequency bandwidth

10

Bolometer materials

Material TCR (300K) Salient features

YBCO -3 to -3.5 %K-1 Room temp sputtering, no heat treatment

VOx -2 %K-1 Low noise

A-Si -2.7 %K-1 High doping with impurities, Crystalization by high temp annealing

P-Si -1 to -2 %K-1 High temperature annealing

P-Si_Ge alloy ~ -2 %K-1 High temperature deposition

11

Flexible substrates

• Polyester and Polyimide used as flexible substrates

• Polyimide is thermally stable

• Polyimide has a Tg of ~400°C

• Polyimide is chemically resistant to most clean room etchants

Property Units Polyimide Polyester

Thickness Range mils 0.3 - 5 0.25 - 14

Dielectric Constant 1 Mhz 3.4 3.2

Volume Resistivity W-cm 10 18 10 18

Tensile Strength (at 25oC) psi 40 000 27 000

Tear Initiation Strength gms 800 1200

Operational Temperature min/max oC .-200 to +300 .-60 to +105

Coefficient of Thermal

Expansion (at 20 oC)1/oC 10x10-6 20x10-6

Change in Linear

Dimension(150 oC, 30min)% >0.15 >1.5

Acid Resistance _ Good Good

Alkali Resistance _ Poor Poor

Grease/Oil Resistance _ Good Good

Organic Solvent Resistance

_ Good Good

Water Absorbtion % (24 hrs.) 3 >0.8

http://www-ee.uta.edu/zbutler/Smart_skin_for_web.ppt

12

Flexible systems

• Advantages of flexible substrate micro sensors– Low cost– Lightweight– Conformable to non planar surfaces– Software based printed IC processes– High degree of redundancy

13

Flexible systems

• Flexible electronics for personal communication (flexible electronic paper)

• Smart clothing (Wireless communications with smart sensors and actuators in the ambient)

• BioMEMS (flexible electrodes for neural prostheses, vision prosthesis)

• Conductive polymers (compound eye, piezoresistive strain sensors)

• Flexible energy sources (photovoltaic cells, organic solar cells)

14

Sensors on flexible substrates (Smart Skin)

• Sensor Arrays on flexible substrates– Infrared sensors– Pressure/Tactile Sensors– Flow sensors– Humidity sensors– Velocity sensors

15

Evolution of “smart skin” in the micro sensors lab

• First generation (2001-2002)– Used solid Kapton sheets pasted on to wafers

• Second generation (2003-2004)– Spin on Kapton used (no micromachining, not

separated from carrier wafer)

• Third generation (2004)– Spin on Kapton used (micromachined devices, peeled

off carrier wafer)

• Fourth generation (2005)– Vacuum packaging at the device level

16

Microbolometers on flexible substrates

17

Microbolometer FabricationTrench Geometry

Si

PI5878

Si3N4SrTiO3

PI2610Si3N4

SrTiO3Ti

AuYBCO

Al

18

Microbolometers on a flexible substrate

19

Temperature Coefficient of Resistance (TCR)

dT

dR

RTCR

1

0

5

10

15

20

25

30

-4.5

-4

-3.5

-3

-2.5

-2

240 250 260 270 280 290 300 310 320

Res

ista

nc

e (M

W)

%T

CR

(K-1)

Temperature (K)

20

Effects of Joule Heating

-3

0

3

-2 0 2

Vo

ltag

e (V

)

Current ()

)(2 TRIVIP bI

Iradtheff PGGG -

)()()( 02 TTTRITPTG bIeff -

)exp()( 0 kTERTR a

21

Responsivity/Detectivity

2/122 )1(

eff

v G

IRVR

nvv V

AfRD

100

1000

104

105

106

100

1000

104

105

106

1 10 100 1000

970 na730 na540 na

390 na214 na136 na

970 na730 na540 na

390 na214 na136 na

Re

spo

nsi

vity

(V

/W)

De

tectivity (c

m H

z1

/2/W)

Frequency (Hz)

22

Effects of substrate heating

23

Area scans of bolometersDevice 1b4 (Trench Geometry)

24

Area scans of bolometersDevice DD15 (Mesa Geometry)

25

Conclusion

• Microbolometers on flexible substrates have been fabricated

• Mean measured thermal conductance = 5.61x10-7 W/K • Max room temperature responsivity RV = 7.4x103 V/W• Max room temperature detectivity D*= 6.6x105 cmHz1/2/W• Measured room temperature TCR = -2.63%/K• Measured room temperature resistance = 3.76MΩ

26

Device-level vacuum-packaging

27

Device-level vacuum packaging

Si

PI5878

Si3N4SrTiO3

PI2610TiAu

Al

Si3N4

OTMSYBCO

28

Device-level vacuum packaging

Optical Window

Detector

Al Mirror

Bond Pad

29

Device-level vacuum packaging

Design Considerations

– Optical window transmission characteristics– Optical window structural analyses– Cavity vacuum– Thermal analyses

30

Optical Transmission Characteristics

Transmission characteristics of thin Aluminum Oxide film M. Aguilar-Frutis, M. Garcia, C. Falcony, G. Plesch and S. Jimenez-Sandoval, “A study of the dielectric characteristics of aluminum oxide thin films deposited by spray pyrolysis from Al(acac)3,” Thin Solid Films, vol 389, Issues 1-2, pp 200-206, 15 June 2001.

31

Complex relative permittivity of Al2O3

0

1

2

3

4

5

6

-1

0

1

2

3

4

5

0 5 10 15 20 25 30 35 40

'"

Wavelength(m)

-1

0

1

2

3

4

0 5 10 15 20 25 30 35 40

tan

Wavelength(m)

32

Optical Transmission Characteristics of polyimide

PI 5878G

0

20

40

60

80

100

120

0.9 1.9 2.9 3.9 4.9 5.9 6.9 7.9 8.9 9.9 10.9 11.9 12.9

Wavelength(um)

Tra

ns

mis

sio

n (

%)

TransmissionVerification

NoiseVerification

5 per. Mov. Avg.(TransmissionVerification)

33

Structural analysis of vacuum element

• Mechanical Strength – Ceramic Al2O3 has a tensile strength of 260

MPa– ZnSe has an apparent elastic limit of 55.1

MPa

34

Structural integrity of vacuum element

35

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)

36

Permeability through Al2O3

• Permeability is the flow rate through a specimen once steady state has been achieved

• He Permeability through Al2O3 at room temperature is

~100-1000 atoms/s/cm/atm•

n=number of moles

R=universal gas constant=8.314J/(mole.K) 10-6

10-5

10-4

10-3

10-2

10-1

1 10 100 1000 10000

Permeability=100atoms/s/cm/atm

Permeability=1000atoms/s/cm/atm

Pre

ssu

re (

mT

)

Time (Days)

V

nRTP

37

Thermal analysis (analytic)

Incomplete micromachining

Heat source ((300+ΔT)K)

Heat sink (300K)

Au

Ti arm

Al2O3

Au

Tipatch

Si3N4

Al2O3

Si3N4

Al2O3

Si3N4

PI2610

Top air

Au

Tipatch

Si3N4

Lower air

Si3N4

Lower air

Complete micromachining

Ruptured cavity

GA GB GC GD GE GF GG

38

Thermal analysis (numeric)

Gth ≈ 5x10-6 W/K (Vacuum)

≈10-4 W/K (air)

39

Microbolometer fabrication

Trench Geometry(Not to scale)

40

Fabrication(Silicon wafer)

41

Fabrication(PI 5878G)

42

Fabrication(Nitride)

43

Fabrication(Al)

44

Fabrication(Sacrificial Polyimide PI 2610)

45

Fabrication(Support Nitride)

46

Fabrication(Ti arms)

47

Fabrication(Au contacts)

48

Fabrication(YBCO detector pixel)

49

Fabrication(Photodefinable PI2737 sacrificial mesa)

50

Fabrication(Al2O3)

51

Section of vacuum cavity before micromachining

Al2O3

Sacrificial PI2737 mesa

Sacrificial PI2610

Al mirrorNitride

Nitride

52

Fabrication(Partially micromachined)

53

Fabrication(Fully micromachined)

54

Fabrication(Sealed vacuum cavity)

55

Fabrication(Superstrate PI 5878G)

56

Single microbolometer

57

Fabrication of encapsulated devices

Partially micromachined

device

Fully micromachined

device

SEM graph of an unsealed

micromachined device

58

Fabrication of encapsulated devices

Sealed device SEM graph of sealed device SEM graph of

cross section of vacuum cavity

Vacuum cavity

59

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)

Measured Gth=3.73x10-6 W/K

60

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

W)

TC

R (%

K-1)

Temperature (K)

R(300K)=53.4 MΩ

TCR(300K)=-3.7%/K

61

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)

RI=61.3 μA/W

@ 5Hz

Current Responsivity (RI)

=Output current/Input power

62

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)

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

Detectivity (D*)

= Area and frequency normalized signal to noise ratio

63

Conclusion

• Device level vacuum encapsulated microbolometers on flexible substrates have been fabricated

• Theoretical thermal conductance in vacuum is 5x10-6 W/K • Measured thermal conductance is 3.73x10-6 W/K (Intact

Vacuum cavity)

• Measured room temperature TCR is -3.7%/K, resistance is 53.4MΩ

• Measured RI is 61.3 μA/W, D*=1.76x105cm-Hz1/2/W

64

• This work is supported by the National Science Foundation

The End