Optimal Doping for Enhanced SnO2Sensitivity and Thermal Stability

A. Tricoli, S.E. PratsinisParticle Technology Laboratory

Department of Mechanical and Process EngineeringETH Zürich, Switzerland

www.ptl.ethz.ch

M. GrafSensirion AG

Stäfa Zürich, Switzerland

1

2

Metal-Oxide Gas SensorIndustrial Fumes Analysis Street Safety

Working Place SafetyAir Quality Monitoring Electronic Nose

3

From Macro to MicroCommercial Sensors Micro-machined Sensors

Analogcircuitry

Digital circuitry

SensorsArray

M. Graf et al., Analytical Chemistry 78, 6801-6808 (2006)U. Frey et al., Journal of Solid-State Circuits 42, 441-450 (2007)

• Lower power consumption.• Higher portability.• Higher performance.

4

Direct Deposition on Micro-machined Sensors

L. Mädler, A. Roessler, S.E. Pratsinis, T. Sahm , A. Gurlo, N. Barsan, U. Weimar, Sensors and Actuators B 114, (2006), 238-295

Cooling Water

Liquid Precursor

Spray Flame

Shadow Mask

A. Tricoli, M. Graf, S.Kühne, F. Mayer, A. Hierlemann, and S. E. Pratsinis, Adv. Mater. 2008, In Press

5

Layer Disintegration

Cooling Water

Impinging Flame

Xylene

6

Tmax = 250 °C

Tg = 1000 °C

30 S

As-Deposited

In situ Annealed

In Situ Stabilization by Flame Annealing

7

SnO2Layer

S. Kühne, M. Graf, A. Tricoli, H. Meier, F. Mayer, S. E. Pratsinis, and A. Hierlemann, J Micromech Microeng 18 (2008), 035040

Wafer – Level Direct Deposition of SnO2 Layers

8time, min0 2 4 6 8

Res

ista

nce,

Ohm

x 1

07

1 ppm

5 ppm

20 ppm10 ppm

1

10

100

dXRD = 21 nm0.2 wt% Pt

25% r.h.450 °C

CO

1 ppm CO

Self-recovery

CO off

time, min2 3 4

Res

ista

nce,

Ohm

x 1

07

5 ppm CO

10

100

Rair

RCO

S =Rair/RCO= 3Response time = 15 sRecovery time ≈ 30 s

CO Detection

• The sensitivity increases with decreasing grain size.

9

Performance of SnO2 Sensor

C. Xu, J. Tamaki, N. Miura, N Yamazoe, Sensors and Actuators B, 3 (1991), 147-155

2 5 3 2gas adC H OH O CH CHO H O e− −⎯⎯→+ + +←⎯⎯

H. Ogawa, M. Nishikawa, A. Abe, American Institute of Physics 53 (1982), 4448 - 4455

d

d

1 10 100 1000Grain Size, nm

200

600

1000

Ann

ealin

g Te

mpe

ratu

re, °

C 2

• Decreasing the grain size increases sintering rates.

• Only limited increase in sensitivity are possible by decreasing size.

Goal: • Decrease grain size and increase thermal stability

simultaneously.

Drastic sensitivity increase

Adapted from: G. Korotcenkov, Sensors and Actuators B 107, (2005), 209-232

10

Sensitivity vs Sintering

T < 400 °C

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SiO2 – Doping of SnO2 particles

3 nm

100 nm

ac

100 nm

a

3 nm

c

A. Tricoli, M. Graf, and S. E. Pratsinis, Adv. Funct. Mater. 2008, In Press

12

Crystal Structure

13

Higher Thermal Stability – Crystal Size

4 h at TA

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Sintering Inhibition

Inhibition of sintering

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SiO2 Insulates the SnO2 Crystals

Si

SnSn

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SiO2-Doped SnO2 Particles Sensitivity

1.4wt% SiO2

100 ppb EtOH

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Sensing PerformanceS = (Rair – REtOH)/REtOH

10 ppm EtOH

EtOH Off

SnO2

1.4 wt% SiO2

18

Optimal SiO2 Doping

Gra

in G

row

th

Inhi

bitio

n

Insu

latio

n

19

SiO2-Doping Effects

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Conclusions I

• Rapid parallel micro-patterning of thick metal-oxide layers down to 100 μm in Ø

• In situ mechanical stabilization by flame-annealing which preserves small crystal size i.e. higher sensor performance

• Good compatibility with micro machining process and circuitry

• Detection of 100 ppb EtOH by SiO2-doped SnO2 nanostructured layers.

• Inhibition of SnO2 grain and necks growth by SiO2-doping.

• Optimal SiO2-doping for enhanced sensitivity and thermal stability.

21

Conclusions II

Gra

in G

row

th

Inhi

bitio

n

Insu

latio

n

22

Barsan et al., 2001

23

Gas’Kov et al., 2001

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Alcohol Breath Analyzers

EtOHAir Quality Monitoring

CO, NOx , …

Metal-Oxide Gas Sensor

• Nanoparticles are highly performing material for MOx • Materials (market share):

• SnO2 (35%)• Mixed Oxides (13%)• ZnO (10%)• TiO2 and WO3 (each 7%)• others (35%)

G. Eranna et al., Critical Reviews in Solid State and Materials Sciences 29, 111-188 (2004)

• Metal-oxide (MOx) gas sensor are solid state device with detection down to ppb level

25

L. Mädler, T. Sahm , A. Gurlo, N. Barsan, J.-D. Grunwaldt, U. Weimar, S.E. Pratsinis, Journal of Nanoparticle Reasearch, (2006), DOI: 10.1007/s11051-005-9029-6

26B.E. Russ and J. B. Talbot, J. Adhesion 68, 257-268 (1999)

0

2

0

2

Mechanical Stability Analysis

Si-Wafer

SnO2 Layer

CA

Film

Film

Cleared Layer Area

Jet Impingement Test

27Oven Annealing Temperature, °C0 300 600 900

Cle

ared

Are

a D

iam

eter

(dC

A),

mm

0

2

4

6

8

10

12

As-Deposited

Layer Adhesion by Jet Impingement

sintering time = 4h

too high T for circuitry

In-situ Flame

Annealing

450

28

High Sensitivity by Small Crystal Size

2 Θ, degree20 30 40 50 60

Inte

nsity

(sca

led)

, a.u

.

Substrate

Nanoparticles

As-Deposited Layer

In-situ Annealed 12 nm

12 nm

12 nm

29

Deposition – Annealing Cycles

1 cmLaye

r res

ista

nce,

Ω

In-situ annealing

Deposition

As-deposited dXRD = 12 nm50% r.h.

25 °C

102 reduction

30

Parallel Micro Patterning at Wafer‐Level

Cooling Water

Liquid Precursor

Spray Flame

2 cm

Shadow Mask

Ts = 150 °C

Microhotplate

Tg ≈ 500 °CLace-like

Nanoparticle Layer

Micro-machined Si Wafer

A. Tricoli, M. Graf, S.Kühne, F. Mayer, A. Hierlemann, and S. E. Pratsinis, Adv. Mater. 2008, accepted

31

300μm

S.Kühne, M. Graf, A. Tricoli, H. Meier, F. Mayer, S. E. Pratsinis, and A. Hierlemann, presented at Eurosensors X, Göteborg, Sweden, September 17-20, 2006

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Higher Thermal Stability – Time

33

Mechanisms of Cross‐Sensitivity to Humidity of SnO2

S. Emiroglu, N. Barsan, U Weimar, V. Hoffmann, Thin Solid Films 391 (2001), 176 - 185

Three main mechanisms*:

1) Rooted OH group as free charge donor:

2 ( ) ( )gasSn o Sn oH O Sn O Sn OH OH e+ − + −⎯⎯→+ + − + +←⎯⎯

2

34

Interaction of H2O with SnO2

*N. Barsan, U Weimar, Journal of Electroceramics 7 (2001), 143 - 167

e-

Homolytic dissociation

2) Oxygen vacancy as donor:

2

2 2 2 ( ) 2gasSn o Sn oH O Sn O Sn OH V e+ − ++ −⎯⎯→+ ⋅ + ⋅ − + + ⋅←⎯⎯

35

Interaction of H2O with SnO2

*N. Barsan, U Weimar, Journal of Electroceramics 7 (2001), 143 - 167

2e-

OH

3) Interaction with pre-adsorbed OH, H, and O- groups:

36

Interaction of H2O with SnO2

*N. Barsan, U. Weimar, Journal of Electroceramics 7 (2001), 143 - 167

ad

2

1. State of surface play a major role.2. Release of free charge carrier.3. Changes analyte interaction with adsorbed O-.

Example: SnO2 Sensoro Sensing mechanism: Variation of the electron/hole density close to the surface

(Surface sensor)

37

220 °C

,Rai

r/Rga

s

dry air

50% r.h.

T. Sahm, L. Mädler, A. Gurlo, N. Barsan, S.E. Pratsinis, U. Weimar, Sensors and Actuators B 98 (2003), 148-153

> 90%

38

40% r.h. 60% 20% 5%

TiO2–based Sensors o Sensing mechanism*:

• High Temperature (900 - 1200 °C): Diffusion of bulk defects (bulk sensor)• Low temperature ( 200 – 700 °C): Generally doped, intrinsic and extrinsic

defect reaction at the interface.

*U.Kirner, K.D. Schierbaum, W. Göpel, B. Leibold, N. Nicoloso, W. Weppner, D. Fischer, D.F. Chu, Sensors and Actuators B 1 (1990), 103-107**M.C. Carotta, M. Ferroni, D. Gnani, V. Guidi, M. Merli, G. Martinelli, M.C. Casale, M. Notaro, Sensors and Actuators B 58 (1999), 310-317

**> 10%