Resistive/Semiconductive Sensors: Basics, Fabrication and Testing
References:
1. S.A. Akbar, P.K. Dutta and C.H. Lee, “High-temperature Ceramic Gas Sensors: a Review,” IJACT, 3[4], 302-311 (2006).
2. C.O. Park and S.A. Akbar, “Ceramics for Chemical Sensing,” J. Matls. Sci, 38, 4611-4637 (2003).
3. O. Figueroa, C. Lee, S.A. Akbar, P.K. Dutta, N. Sawaki and H. Verweij, “Temperature-controlled CO, CO2 and NOx Sensing in a Diesel Engine Exhaust Stream,” Sensors and Actuators B, 107, 839-848 (2005).
Supplemental Reading: Chapter 3 in “Chemical Sensing with Solid State Devices” by M.J. Madou and S.R. Morrison, Academic Press (1989).
Ceramic-based Resistive Sensors
• Bulk conduction based (dense)– TiO2, BaTiO3, CeO2 and Nb2O5
• Metal/Oxide junction based– Schottky diode (Pd/SnO2, Pt/TiO2)
– MOS/MISFET
• Surface/intergranular controlled (porous)– SnO2, TiO2
Bulk-conduction based Sensors
nO
ao P
kT
E 1
2exp
Fig. The change in conductivity of BaTiO3 with oxygen pressure at temperatures from 600 to 1000C (50C intervals)
Materials n Ea (eV) Structure
TiO2 -4, -6 1.5 Rutile
BaFe0.8Ta0.2O3 +5 ≈0 Perovskite
n = 4
n = 6
- T dependence in thep-type region is negligible
- Think about a proper dopantthat can make Ea ~ 0
Metal/Oxide Junction Controlled Sensors: a. Schottky-diode type
The I-V characteristic of a Schottky diode is described by the thermo-electronic emission as
* 2 exp( / )[exp( / ) 1]SI A T V kT eV nkT
A*: Richardson const. V: bias voltage Vs: Schottky barrier (depends on occupation of the surface states)
Schottky: Pd/SnO2, Pt/TiO2 Ohmic: Ag, Au
Schottky Contact
OhmicContact
The threshold gate voltage, upon exposure to the hydrogen-containing atmosphere, shifts by V, compared to gatevoltage without hydrogen
OT TV V V
Fig. (A) Schematic picture of the metal-insulator region upon chemisorption of hydrogen atoms (B) the hydrogen-induced shift of the threshold voltage characteristics of MISFET (C) the hydrogen-induced shift of the capacitance-voltage curve in MOS Capacitor.
b. MOS/MISFET type
- Accumulation of H+ creates a dipole layer loweringV- Can be measured by I-V or C-V- Shifts threshold voltage in FET
Surface/intergranular ControlledSemiconductive/resistive
- Practically no energy is needed for physisorption -As the molecule approaches it polarizes the surface inducing dipole/dipole interaction- Further approach increases the energy of the system, so physisorbed species is stably located at rp
- There is a little energy for desorption Hphy < 0.2 eV (~5 kcal/mol)
Physisorption vs. Chemisorption
Lennard-Jones model for physisorption and chemisorption
- Molecule can dissociate into atoms. As the atom approaches, a strong interaction involving e- transfer occurs to form a chemical bond at a distance rc.
- Direct chemisorption is less likely because of large energy for dissociation (ED) - Chemisorption follows physisorption that requires an activation barrier of EA for adsorption and Hchem for desorptionHchem > 1 eV (~23 kcal/mol)
Fig. Typical adsorption isobar (coverage as a function T at a fixed pressure: (I) physisorption, (II) irreversible transition (equilibrium physisorption is readily achieved, equilibrium chemisorption falls short) and (III) chemisorption
Adsorption Isobar (coverage as a function of T at a fixed P)
- Coverage decreaseswith T because ofincreased desorption
- Region I is dominated by physisorption
- In region II, initially chemisorption proceedsreadily since EA is low
- As coverage increases EA increases, but Thelps overcome this aslong as desorption rate is lower than adsorptionrate up to region III
Fig. Schematic plots of equilibrium isobars for and and the resistance of SnO2 as a function of temperature known as the sigmoidal curve
[ ]O
,
[ ]OH
2[ ]H O
2s s sR O OH H O
[ ]s is the concentration ofadsorbates on the surface
O- and OH- increase R,but H2O+ donates an e-to decrease R
I: adsorbed water moleculesdominate, blocking off O-.II: as T rises water desorbspermitting an equilibriumsurface oxygen, alsodissociative chemisorption of water to form OH-
III. Combination of NTC anddesorption of OH-.
2 2sH O O e OH
Depletive adsorption of X as X- on an n-type semiconductior: (a) flat band (b) after adsorption, and adsorption of X as X+ on a p type semiconductor: (c) flat band (d) after adsorption
- Direction of e- transferdepends on the differenceof energy of surface stateand Fermi energy of theoxide.- If surface energy is lower,e- from CB of n-type (depletive adsorption) and VB for p-type (cumulativeadsorption; abundant e-exit in the VB. This is whyp-type is a better catalyst).
n-type: SnO2, ZnO, TiO2
p-type: NiO
- If surface energy is higher,the reverse occurs.
CO CO2
e-
Sensing mechanism of oxide-based sensors
Conduction band electrons
SemiconductorParticles
EF
Ec
Ev
O-
O-
O-O-O-O-
O-O-
O-
O-
O-
O-O-
O- O-
Depletionregion
Absorbedoxygen
qVsqVs
O-
O-
O-
O- O-
O-
O-
O-
O-
O-
O-O-
O-
O-
O-
O-
kT
qVso exp
Sensor Structures and Materials
Bulk Type:
Bulk type gas sensors with: (a) direct heating and (b) indirect heating
For T: 300 – 400 C needs >500 mW; heated by Ir-Pd or Cr alloy wire
Thick Film Type:
Thick film gas sensors (1x2mm) [taken from CAOS Inc.]
Requires < 200 mW
<100 mW (thin-film)
Concentration profile of gas species across the thickness for a thick porous film.
Electrodes can be placed on top, at the bottom or on the side of the sensing layer.
S: side electrode
B: bottom electrode
U: upper electrode.
For B, path 1 or 2 depends on the gap between the electrodes and film thicknessFor narrow gap, sensitivity comes from reaction of the gas with the material in the gapFor wide gap, reactivity and thickness simultaneously control
- if too reactive, it gets reacted at the outer surface, low sensitivity- if low reactivity, high sensitivity
Selective Design
Various positions of electrodes used in the construction of the sensor (b) Various configurations of electrodes used in the construction of the sensor ([i]: interdigitized, [ii]: 4 point micro-contact and [iii]: planar).
Bottom electrode is good for selectivity
- bottom electrode with small gap to detect poorly reactive gas
- if the gap is large compared to thickness, highly reactive gas can be selectively detected.
Fig 34 Various alcohol breath analyzers using a semiconductor sensor (adopted from: http://www.dui.com, http://www.intox.com, and http://www.lifeloc.com)
CO Sensor: Fabrication and Testing
Thick-film Technology
References: 1. Ceramic Materials for Electronics: Processing, Properties and Applications, Edited by Relva C. Buchanan, Marcel Dekker, Inc. (1991), pp. 435-488
2. Handbook of Thick Film Technology by P.J. Holmes and R.G. Loasby, Electrochemical Publications Ltd., 1976.
3. Interfacial Phenomena in Dispersed Systems by Jean-Louis Salager, Universidad de Los Andes (1994).
4. O. Figueroa et al., “Temperature controlled CO, CO2 and NOx sensing in a diesel engine exhaust stream,” Sensors and Actuators B, 107, 839-848 (2005).
Screen Preparation
- stainless steel, polyester or nylon screen
- stretched to a proper tension and attached to a frame
- UV-sensitive emulsion is applied
- exposed through a positive or negative of a pattern
- developed to remove emulsion in the pattern regions
Substrate Requirements- High Electrical resistivity Mechanical strength Dielectric strength Thermal shock resistance Thermal conductivity and heat dissipation
- Low K (dielectric const.) and tan (dielectric loss) to minimize signal delay
- Good matching CTE
Thick-Film Technology: Screen-Printing
Substrate Properties
Screen-printing Process
Ink/Paste PreparationInorganics (sensor material, glass frit for adhesion, catalyst) mixed with organics (ethyl cellulose - EC & butyl carbitol acetate - BCA)
Ink Requirements
- Non-Newtonian (viscosity [] depends on strain rate []; =/; shear stress vs. shear strain rate)
- maintain organics in suspension without agglomeration or excessive settlement; thixotropic (high at low )
- must not lose solvent quickly by evaporation during use
- Good flow under the influence of squeegee; pseudoplastic (low at high )
- must wet the surface of the substrate and adhere to it (low surface tension)
- Each column of ink must pull through the screen mesh without leaving an excessive amount in the screen fabric (low thickness)
- should level with a minimum of side flow and edge slumping (high surface tension and high at low )
- to avoid flow after printing, the ink must be capable of being dried with a large increase in
Film AdhesionThree Primary Methods - reactive, flux and frit bonding
Reactive Bonding
A small amount (0.1 - 1%) of reactive oxide (CuO or CdO) is added to the ink that forms Cu- or Cd-aluminate during firing
Flux Bonding
1-5% of oxide flux that forms a low-melting eutectic (e.g., Bi2O3-Al2O3 eutectic
at 820 °C)
Frit Bonding (most common)
2-10% glass powder is added that wets the substrate and penetrates the film giving mechanical interlocking
A continuous glass film forms at the interface if fired at too high a temperature or for too long - crack can easily propagate at lower stress
Key Parameters in Screen Printing
Driving force for leveling
To ensure spreading, contact angle must be less than some maximum value
- Reducing surface tension (lv) reduces . So, additives (surfactants
/wetting agents) are used.
- Surfactants may also reduce sl, helping further reduce .
- Too small may cause bleed out
Ink viscosity, = /
- = 10 Pa-sec; = 103 sec-1 when squeegee traverses the screen
- < 103 Pa-sec for < 15 min. so that leveling can occur; = 1 sec-1 due to surface tension
- after leveling > 104 Pa-sec; = 10-2 driven by gravity
Key Parameters in Screen Printing
S is the measure of how completely the ink is removed from the screen mesh and depends on ink properties and machine variables
- tacky (sticky) ink gives larger S and can be decreased by added organic additives
- machine variables: screen tension, snap-off distance, squeegee speed
- increase in these will decrease S and increase wet film thickness
Wet film thickness and fired film thickness can be estimated
tw = (tcA0/100)(1-S)
tc: screen cloth thickness
A0: % open area of the screen
S: screen retention factor
tf = twVs/(1-Pf)
Vs: volume fraction of solids in ink
Pf: fraction of fired film porosity
Wetting – main property is the surface tension of the various components.
Wetting is determined by the contact angle – (1) for angle between 90 and 180, no wetting; (2) For angle between 0 and 90, wetting.
Contact angle is determined by surface tension of sv, lv and interfacial (sl). In liquid-based systems, surface tension of lv is usually affected by additives.
Examples – (1) long chain polysiloxanes (can affect adhesion; short chain polysiloxanes do not affect adhesion); (2) polyacrylates low surface tension; they float to the surface decreasing the surface tension
http://www.cibasc.com/index/ind-index/ind-paints_and_coatings/ind-paints_and_coatings_theory.htm
Dispersant – molecules with only one polar group anchors on the surface of particle and the tail (non-polar end) extends in the medium, keeping the particles separate. The molecular weight determines tail length and hence the separation distance. The bonding can be one of the following.
1. Hydrogen bonding (6 kcal/mol); (2) dipole-dipole interaction (3-4 kcal/mol); (3) London-van der Waals (2 kcal/mol)
Example: Polyurethane polymers (PUR, viscosity reducing agent), polyacrylate
Amount to add – 10% of the oil adsorption value (OAV).
TiO2 – OAV (19) – amount of dispersant 1.9%
Fe2O3 – OAV (35) – amount of dispersant 3.5%
http://www.cibasc.com/index/ind-index/ind-paints_and_coatings/ind-paints_and_coatings_theory.htm
Resistive/semiconductive type thick-film sensors
Screen printed sensor
RESPONSE OF A TiO2-BASED SENSOR TOWARD CO
-2
-1
0
1
2
3
4
log R
(k
)
0 1 2 3 4 5 6
% CO
700 °C - reverse
700 °C - forward
600 °C - reverse
600 °C - forward
A Selective CO Sensor
Gas Concentration (ppm)0 200 400 600 800
R/R
0
0.75
0.80
0.85
0.90
0.95
1.00
1.05
COCH4
Anatase (10% La2O3, 2% CuO)
Gas Concentration (ppm)0 200 400 600 800
R/R
0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
COCH4
Anatase
Measurement at 600 C, 5%O2/N2
- CH4 is catalyzed before TiO2 feels the effect- CO selective, but may depend on O2
Selective Sensor Based on a Composite
Anatase
O-
h* h* h* h* + CO: Resistance O-O- O-
e- e- e- e- + CO: Resistance O- O- O-O-
Rutile
What happens when Anatase and Rutile are combined?
Parallel conduction pathways
n/p Composite as a Selective CO Sensor
• 75% Rutile composite has simultaneous n-n and p-p percolation • different single-phase response for each gas
p-pn-n R
1
R
1
R
1
0
0.5
1
1.5
2
0 250 500 750 1000
Gas Concentration (ppm)
0
0.5
1
1.5
2
0 250 500 750 1000
Gas Concentration (ppm)
R/R0
Rutile / CH4
Rutile / CO
Anatase / CH4
Anatase / CO
COCH4
Single-Phase Sensor Response 75% Rutile Composite
Sensor Probe for Field TestConfiguration of Sensor Tested at
CAR
alumina tube
Closed-end 4-borealumina tube
Sensor film
Thermocouple
Goldelectrodes
High-Temp cement
Slit forgas flow
Temperature should be controlled by a heating strip
Sensor should be shielded from high-velocity gases and particulates
CO Probe for Auto-test
FIELD-TEST CONFIGURATION
112
Students Testing Sensor Probes at CAR
