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Sensor devicesSensor devicesThermal sensors
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OutlineOutline
• 7 Thermal sensors
•IntroductionIntroduction
•Heat transfer
•Thermal structures
•Thermal-sensing elements
•Thermal and Temperature sensors
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IntroductionIntroduction• Physical quantitaties is converted into heat
Th h t i t d i t l t i l titi• The heat is converted into electrical quantities• The process is done into three steps
– Electromagnetic radiation is transduced into a heatElectromagnetic radiation is transduced into a heat flow
– The heat flow is converted into a temperature differencedifference
– The temperature difference is transduced into a electric signal using a temperature (difference) sensorsensor
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Heat transferHeat transfer• Heat
G d Li id “ l it f th l l ”– Gas and Liquid “average velocity of the molecules”– Solids “Phonons, vibrations of atoms in lattice and
transportations of heat by free electrons”• Specific heat and thermal capacitance
– Heat required to increase the temperature with 1 K at constant pressurep
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Heat transferHeat transfer
•Conduction–Conductivity
•Heat will flow from•Heat will flow from hotter to colder region
–Convection•Heat transfer to flowing fluids (Liquid or gas)•Laminar flow•Turbulent flow
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Heat transferHeat transfer•Radiation•Radiation
–Black-body Radiation•A body which absorb all of the radiation “ absorptivity=1” is called a black body•A black-body with absorptivity=1 y p yalso have an emissivity=1•Stefan-Boltzmann law
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Heat transferHeat transfer
•Heat transfer by infrared radiation–Two parallel plates
G
•Silicon is almost transparent above 1.1 um. At room temperature the radiation have its maximum at 10 um wavelength and themaximum at 10 um wavelength and the absorptivity, emissivity is as low as 0.1-0.3T T+ΔΤ
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Heat transfer SummaryHeat transfer SummaryConductionConductionConvectionRadiation
Incoming radiation
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Thermal structuresThermal structures•Purpose •The membrane can be of type
–Reduce “self heating”–Reduce “heat leakage”
In case of temperature difference like sensorIn case of temperature difference like sensor structure, make Rth largeRth=ΔT/Pth
Often solved by using thin membranes
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Thermal structuresThermal structures
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Thermal structuresThermal structures
O di i l h t flOne dimensional heat flow
Thermal resistance 1 dimensionK= thermal conductivity
Thermal resistance of object inThermal resistance of object in fig 5a
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Thermal structuresThermal structures• Floating Membranes
Th l d l
Rbeam
• Thermal model
ParasiticDepended of frequency of incoming signal
Parasitic conductance by convection, radiation etc
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Thermal structuresThermal structuresCantilever beam and bridges
No convection and infrared radiation
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Thermal structuresThermal structuresCantilever beam and bridgesI f h t ti d i f d di tiIn case of heat convection and infrared radiation
heat convection and infraredand infrared radiation
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Thermal structuresThermal structuresClosed membranes
Uniform heating of membrane with power d i P´´density P´´
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Thermal sensing elementsThermal-sensing elements• Resistors
B id l d l ili i t– Bridge coupled mono or poly silicon resistors– Platinum resistors Pt100, 100ohm at 0ºC,
0.38%/K
• Thermopile
• Acoustic sensing elements
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Thermal sensing elementsThermal-sensing elements
T T+ΔT
v
Metal or semiconductor
v
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Thermal sensing elementsThermal-sensing elementsSeebeck Coefficients
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Thermal sensing elementsThermal-sensing elements
S b kSeebeckCoefficients, dependencies ofdependencies of temperature and doping in p-type silicon
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Thermal sensing elementsThermal-sensing elementsThermopile
ΔV=(Sa-Sb)ΔT( a b)
Design rule number of thermopile
Rst=thermal sheet resistance
Rse= electrical sheet resistance
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Thermal sensing elementsThermal-sensing elements• Acoustic wave sensors
f db k l ill t• feedback loop, oscillators• sensitivity ~2.8kHz/ºC
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Thermal and Temperature sensorsThermal and Temperature sensorsThermopile, SU8 with Ni and Ti as thermopile elementelement
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Thermal and Temperature sensorsThermal and Temperature sensors
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Thermal and Temperature sensorsThermal and Temperature sensors
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Thermal and Temperature sensorsThermal and Temperature sensors
Bolometer
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Thermal and Temperature sensorsThermal and Temperature sensors
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Thermal and Temperature sensorsThermal and Temperature sensorsCarbon-dioxide sensor schematic
Filter, bandpass4.25 μm
Ab bi lAbsorbing volume
IR CO2 absorb aIR sensor CO2 absorb a wavelength of 4.25 μm
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ExercisesExercisesProcessing part (chapter 2)
3.1 9 min, 2.3h 4:1 5.8 um, 5.3 um, 47ohm/Square, 60 ohm/square
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ExerciseExercise