1.thermal sensors 2.thermal actuators 3.example: flow sensorece434/winter2008/434_4.pdf ·...

(C) Andrei Sazonov 2005, 2006 1

Thermal Microsystems

1.Thermal Sensors

2.Thermal Actuators

3.Example: Flow Sensor


conduction

radiationconvection

Thermal microsystems deal with the measurement and regulation of temperature.

Every physical object/system is characterized by the temperature and has limited working range of temperatures. Temperature measurements are necessary for the system control (self-control).

Temperature measurements are based on the heat transfer from the object under test to the sensor.

Heat transfer modes:

1. Conduction.2. Convection.3. Radiation.

Conduction is the heat flow through a solid (the most relevant in the microsystems) similar to electronic transport: - the driving force is ΔT instead of ΔV;- the proportionality constant is thermal conductivity K[J/(msK] instead of electrical conductivity.

Heat flux: [J/(m2s)]

( Fourier’s law).dxdTKJQ −=


Thermal transducers:

Sensors(thermorersistors,

thermocouples, etc.)

Actuators(thermoelectric

coolers, heat pumps,etc. )

Thermomechanical

Thermocouples

Joule-Thompson devices

Peltier devices

Junction based

Thermoresistors


21021

221

))((6)(

ttTTttLR−−

+=

αα

Thermomechanical sensors are based on the linear coefficient of thermal expansion:αT = dεx/dT=dL/(LdT).

If a sandwich of two materials with different αT changes its temperature from T0 to T, then linear strain mismatch occurs, which results in bending. A thermal bimorph switch is based on this effect.

If α2 > α1, then the radius of curvature is:

where L, t1, t2 – film length and thicknesses.

T0 - off T - on

1 2

Example: bimorph switch to indicate temperature deviation (cooling) beyond acceptable range. Normally off, the bimorph bends and shorts the circuit, turning on poly-Si heater.

The fabrication is a combination of bulk and surface micromachining. On top of Si substrate a sandwich of SiO2/poly-Si/SiO2/Au is deposited and patterned, then bottom side Si wet etching forms a cavity.


Thermoresistive sensors (aka thermistors) are obviously based on the thermal resistivity (or resistance) changes:

R = ρL/AIn this case, the increase (or decrease) in resistivity is much more significant than in

dimensions.

LA

For thermoresistor:RT = RT0(1+αR[T-T0]),

αR – temperature coefficient of resistance.

They are metals, metal oxides, selenides, sulfides, etc.

Applications:• Temperature sensing, switching at temperatures ranging from 60°C to 180°C, e.g. protection of windings in electric motors and transformers.• Solid state fuse to protect against excess current levels, ranging from several mA to several A (25°C ambient) and continuous voltages up to 600V and higher, e.g. power supplies for a wide range of electrical equipment. •Liquid level sensor.


Thermocouples.The junctions of two conductors can generate the voltage if one of junctions is heated

(cooled) – Seebeck effect. The connection of two junctions in parallel thermally but in series electrically is called a thermocouple, many junctions - a thermopile.

++

+

++

+

++

+

++

+

++

+

++

+

++

+

++++

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ΔVhot cold

If one side of thermopile is exposed to optical radiation, it heats, generating a voltage.

- conductor 1; - conductor 2

ΔV

hot

cold

Materials with high electrical conductivity but low thermal conductivity are preferable.

The temperature-voltage relationship is linear:

dV = S dT,S is Seebeck coefficient.For a single material:

For a junction:

∫ −=2

1

.)( 21

T

T

dTSSV

∫=Δ2

1

.T

T

SdTV

Thermocouple equation:V = aΔT + b(ΔT)2,

a, b – constants.


Junction-based thermal sensors.a). Diode sensor.Based on temperature dependence of the diode I-V characteristics under the forward

bias:

If we operate at fixed current I, then by measuring the voltage V across the diode we can obtain the temperature:

V = (1/q)nkT ln(I/Is).

nkTqV

snkTqV

s eIeII ≈⎟⎟⎠

⎞⎜⎜⎝

⎛−= 1

b). Transistor sensor.Similar to diode sensor, a BJT can be used to measure the temperature using

relationship between collector current and base-emitter voltage in normal mode:

If we operate at fixed collector current IC, then by measuring the base-emitter junction voltage VBE we can obtain the temperature:

VBE = (1/q)kT ln(IC/Is).

EkT

qV

sC IeIIBE

α=⎟⎟⎠

⎞⎜⎜⎝

⎛=


PTAT Circuit.

The voltage generated is “proportional to absolute temperature”.

The base-emitter voltage of Q2, VBE, mirrors that of diode-like connected Q1. Therefore, the collector currents of Q3, Q4 are equal. Q4 is also diode-like connected and the difference in the VBE of Q3, Q4 drops across R1.

In mV for RT; r is the scaling factor (the ratio of emitter areas between Q3 and Q4)

)ln(26)ln( rre

kTVBE ≈=Δ

Example:

Assume r = 1. In this case, ΔVBE = 0.Assume r = 10. ΔVBE (300K) = 26 * 2.3 = 60mV.

ΔVBE (310K) = 61,9mV.0.19mV/K!

Assume r = 100. ΔVBE (300K) = 26 * 4.6 = 120mV.ΔVBE (300K) = 133.8mV.0.38mV/K.


Thermal Actuators.

Convert electrical signal into heat. Simplest examples are:- Resistive heater;- RF coil heater.

More efficient and sophisticated devices are utilizing Joule-Thompson or Peltiereffects.

Joule-Thompson devices.If a gas expands through a nozzle, the temperature does not change in case of ideal

gas – internal energy transforms into kinetic energy. However, in real gases, this expansion results in the temperature drop.

Refrigerators based on this effect transform the gas pressure change into temperature change. Most air conditioning systems are based on this effect. Using IC technology, microrefrigerators can be fabricated.

inlet

outlet

expansionnozzle

heat exchanger


Peltier devices.

Peltier effect refers to the heat generation or absorption when dc electric current passes through the junction of two materials. It is valid for metals and semiconductors.

Peltier effect is basically release/absorption of electron energy after they get into material with lower/higher electron energy (which is determined by the work function in metals or Fermi energy in semiconductors).

Metal 1 Metal 2

- Heat released

The rate of heat released/absorbed is proportional to electric current:

dQ/dt = ΠI,Π is Peltier coefficient.In semiconductors, to prevent p-n junction formation and

thus make devices reversible, p- and n- laers are connected using metal intermediate layer (tin).

- p-layer; - n-layer

hot

cold

Peltier heat pumps are using an array of p-n junctions connected electrically in series and thermally in parallel.


Application: heat pumps. Establish temperature gradient between cold and hot sides. To achieve actual cooling, heat should be efficiently removed from the hot side. It can be done by using fans or heat sinks attached.

Industrial Peltier module: - 30x30x4 mm; - 15 V;- 30 W; - 3.9 A.− ΔT = 68 ºC;

Temperature gradient establishes between hot and cold sides. To keep it, thermoelectric materials should have very low heat conductivity whereas electrical conductivity should be very high to pump the heat effectively.

These two requirements cannot be fulfilled simultaneously for elementary materials: it violates Wiedemann-Franz law. Alloys, however, can be fabricated with electrical/thermal conductivity ratio of 10.

Most of these devices are based on bismuth telluride ceramic.


Thermal Flow Sensor.Based on the heat exchange between the heater and flowing gas. Heater

temperature is above that of the ambient; gas absorbs the heat and reduces the heater temperature; the reduction is non-uniform: upstream side cools down more than downstream side.

Gas flow

T

x

heater sensors

no flowgas flow

Sensors here can be thermoresistive, junction based, or thermoelectric.

ΔT

Temperature gradient ΔT between two thermopiles is proportional to the flow velocity, v. The voltage between them, respectively, is proportional to the temperature gradient, and thus to the velocity:

V = Psv;P – dissipated heating power;s – sensitivity [VW-1m-1s].

Thermal flow sensors can operate within the range of 1-1000 sccm.In order to get high sensitivity, the heat conduction away from the heater/sensor area should be minimized.


2D–flow sensor based on thermopiles (top etched):

Si substrate SiO2

Poly-Si heater

Poly-Si/Al thermocouple

circuitry

circuitry

First, signal processing CMOS circuitry is fabricated; then the sensor; after that, the cavity is etched by EDP.

ΔTtc


2D–flow sensor based on thermopiles (bottom etched):

Si substrate SiO2

Poly-Si heater

Poly-Si/Al thermocouple

circuitry

circuitry

First, signal processing CMOS circuitry is fabricated; then the sensor; after that, the device is passivated, and cavity is etched by KOH.


Fabrication process: a combination of bulk and surface micromachining.

1.thermal sensors 2.thermal actuators 3.example: flow sensorece434/winter2008/434_4.pdf ·...

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