Materials Science and Engineering - Electronic and Mechanical Properties of Materials Part 10 pps

Tuller-2001 9Sensor Configuration A single 9 mm2 chip sensor array with: • four sensing elements with interdigitated structure electrodes • heater • temperature sensor Schematic Cros

3.225 © H.L Tuller-2001 7

1E-5

1E-4

1E-3

0,01

0,1

T = 850°C

X=0.03

X=0.03

Sr1-xLaxTiO3 porous ceramic

t / h

Transient Behavior of Porous Sr1-xLaxTiO3for x=0.005 and x=0.03

T = 850 °C

Mechanisms in Semiconducting Gas Sensor

• Interface - Gas adsorption

Induce space charge barrier

1 Surface conduction

2 Grain boundary barrier

Grain boundary barrier

modulate

2

=

3.225 © H.L Tuller-2001 9

Sensor Configuration

A single 9 mm2 chip sensor array with:

• four sensing elements with interdigitated structure electrodes

• heater

• temperature sensor

Schematic Cross Section of Mounted Sensor

3.225 © H.L Tuller-2001 11

• ZnO film (150 nm)

• Electrode: Pt(200 nm)/Ta(25 nm) film

• Insulation layer: SiO2 layer (1 µm)

• Substrate: Si wafer

Si wafer ZnO film

H2

H2 H2 H2

Pt electrode

SiO2 layer

Electrical Measurement

-10 0 10 30 50 70 90 100

-10 0 10 30 50 70 90 100

MFC2 Temp NO2 NH3 Feuchte CO NO2kl H2

time / h

100k

Temp:360C, H 2 , CO, NH 3 (10, 50 and 100 ppm), NO 2 (0.2, 0.4, and 2 ppm)

ZnO(Ar:O2=7:3) 1

[ Pfad: \ alpha missy Messungen messplatz_1 ] M Jägle / 27.02.2001

S1219a S1219c

M 9710746 20V Datum: 23.02.2001 - 27.02.2001 Steuerdatei: allgas_h2.stg Meßprotokoll: 273

Schematic of Gas Sensor Structure

MicroElectroMechanical Systems - MEMS

Micromachining - Application of microfabrication tools, e.g lithography, thin

film deposition, etching (dry, wet), bonding

Bulk Micromachining Surface Micromachining

Resp

3.225 © H.L Tuller-2001 13

Gas Sensors and MEMS

• Miniaturization

• Reduced power consumption

• Improved sensitivity

• Decreased response time

• Reduced cost

• Arrays

• Improved selectivity

• Integration

• Smart sensors

Microhotplate

3.225 © H.L Tuller-2001 15

Microhotplate Sensor Platform

NIST Microhotplate Design

Microhotplate Characteristics

• Milli-second thermal rise and fall times

programmed thermal cycling low duty cycle

• Low thermal mass

low power dissipation

• Arrays

enhanced selectivity

3.225 © H.L Tuller-2001 17

Harsh Environment MEMS

• Oxidation resistant

• Chemically inert

• Abrasion resistant

Wide band gap semiconductor/insulator

Photo Electro-chemical Etching - PEC

• materials versatility e.g Si, SiC, Ge, GaAs, GaN, etc

• precise dimensional control down to 0.1 mm

through the use of highly selective p-n junction

etch-stops

• fabrication of structures with negligible internal stresses

• fabrication of structures not constrained by

specific crystallographic orientations

Features:

3.225 © H.L Tuller-2001 19

+

-+

h +

h +

semiconductor

Photo Electro-chemical Etching - PEC

• Electro-chemical

etching

p-type

+

-Light source

• Photo electro­

chemical etching

+

-h +

semiconductor

electrolyte

Light source

n-type

Examples

• Arrays of stress free

4.2 µm thick cantilever

beams

• Photoelectrochemically

micromachined cantilevers

are not constrained to

specific crystal planes or

directions

• Similar structures

successfully

micromachined from SiC

by Boston MicroSystems

personnel

3.225 © H.L Tuller-2001 21

Smart Gas Sensor

A Self Activated Microcantilever-based Gas Sensor

1 A device capable of sensing a change in environment and

responding without need for a microprocessor

2 A device has both gas sensing and actuating function by

integration of semiconducting oxide and piezoelectric thin films

Micro-Processor

Actuator

Sensor

Chemical

Environment

Microfluidic structure

Smart Gas Sensor

1 Semiconducting oxide thin films for high gas sensitivity

: Microstructure (Nano-Structure) and Composition

2 Piezoelectric thin films for providing actuating function

3 Thin film electroceramic deposition methods for integrating with silicon microcantilever beam

: Compatibility with Si micromachining technology

4 Microcantilever structures for the self activated gas

: High performance in chemical environment

sensor

3.225 © H.L Tuller-2001 23

Resonant Gas Sensor

• Resonant Frequency: f R = 1/2l (µo /ρo ) 1/2

where l = resonator thickness, µo = effective shear modulus and ρo = density

• Mass change causes shift in resonant frequency : (m 0 - ∆ m) / m o

Gas Sensor elements :

- stoichiometry change translates into mass change

(II) Resonator transduces mass change into resonance frequency change

∆f

∆m

Electrode

Electrode

Resonator Active layer

Choice of Piezoelectric Materials

• Temperature limitations of piezoelectric materials

Temperature ( o C)

Limitations

Quartz 450 High loss

LiNbO 3 300 Decomposition

Li 2 B 4 O 7 500 Phase transformation

GaPO 4 933 ? Phase transformation

La 2 Ga 5 SiO 4

(Langasite)

1470 ? Melting point

3.225 © H.L Tuller-2001 25

Design Considerations

→ contributes to resonator electrical losses

Modify bulk conductivity - how?

→ limited oxygen non-stoichiometry

→ slow oxygen diffusion kinetics

Defect chemistry and diffusion kinetics study

• f R (T): Temperature dependence of resonant frequency

→ need to differentiate from mass dependence

Minimize @ intrinsic and device-levels

Langasite : Bulk Electrical Properties

• Single activation energy in the temperature range 500

-900 °C

8 9 10 11 12 13

10 -7

10 -6

10 -5

10 -4

Y-cut

σ0 = 2.1 S cm -1

EA = 105 kJ mol -1

10 4

/T [1/K]

-1 ]

900 800 700 600 500

T [°C]