optimal doping for enhanced sno2 sensitivity and thermal stability · 100 ppb etoh. 17 sensing...
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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
11
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
14
Sintering Inhibition
Inhibition of sintering
15
SiO2 Insulates the SnO2 Crystals
Si
SnSn
16
SiO2-Doped SnO2 Particles Sensitivity
1.4wt% SiO2
100 ppb EtOH
17
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
20
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
24
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
32
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%
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