Ashok Final Presentation

8/2/2019 Ashok Final Presentation

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Presented By:Ashok Kumar(Student Trainee)

DAVIET , jalandhar

Project Guide:Dr.R.K.Nahar(Emeritus Scientist)

SNG Group, CEERI,PILANI


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Contents:-1.introduction

2. Hafnium oxide thin film gas sensor.

3. Thin Deposition Techniques.4. CO-sensing characteristics of hafnium oxide thin film

5. Hafnium oxide Film Applications.

6. Hafnium Oxide Gas Sensor : Experimental.

7. Experimental Results

8. Conclusion


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Introduction Gas sensor measures the concentration of gas in its

vicinity. Gas sensor interacts with a gas to measure

its concentration. The adsorption of a gas onto the surface of a metal

oxide can produce a large change in its electricalresistivity.

These devices offer low cost and relative simplicity,advantages that should work in their favour as newapplications .


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metal oxide for gas sensing

Metal oxides are suitable for detecting combustible,reducing, or oxidizing gases by conductivemeasurements.

metal oxides generally used in gas sensing applicationare: Cr2O3, Mn2O3, Co3O4, NiO, CuO, SrO, In2O3,

WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3,

Ta2O5, La2O3, CeO2, Nd2O3 , HfO2.


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Application of gas detection instruments: 1. Process control industries 2.Environmental monitoring 3.Fire detection 4.Detection of harmful gases in mines 5.Home safety 6.Grading of agro-products like coffee and spices

Issues Sensitivity Selectivity Stability


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Hafnium oxide thin film gas sensor Good chemical stability

Thermal stability

Mechanical stability Easily changing the metal oxidation states

Experimental results show that for the same insulator

thickness, the HfO2 sensor is more sensitive than othermetal oxide sensors. Most of the work on HfO2 has been focused on amorphous

filmsto replace SiO2.


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General Properties of HfO2

Relative static dielectric constant : 15 , 26 Refractive Index : 2.24, 2.45 Enthalpy of formation (eV/O atom) : -5.77

Energy Band Gap : 5.6, 6.2 Conduction band offset with Si (eV) : 2.0b, 1.3d

Valence band offset : 2.5, 3.4d

Structural Property : Cubic & Tetragonal Cubic

structures Hf atoms are in a FCC structure O atoms are at the tetrahedral interstitial sites

Ref. : M. Houssa, High-k Gate Dielectrics, 2004, Page-331


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Thin Deposition TechniquesHafnium oxide (HfO2) films are deposited by a

variety of techniques :

SputteringAtomic Layer Deposition

Plasma Enhanced Atomic Layer Deposition

Chemical Vapor Deposition

Plasma Enhanced Chemical Vapor Deposition

Sol-gel Deposition Technique


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Sputtering

Takes place under vacuum and is a type of physical vapordeposition process.

Process involves bombarding a target with high energy

ions. In conventional sputtering systems, these ions are formed

from argon gas. The target in the system is a solid disc which serves as the

source of the deposition material. Film characteristics controlled by deposition parameters

such as process pressure, power, and temperature ofsubstrate.


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Target is electrically grounded. Argon gas is ionized to a positive charge.

Positively charged atoms are accelerated towards groundedtarget.

Argon atom hit target and sputter target molecules .


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Sputtering

additional methodsReactive sputtering

RF sputtering

Magnetron sputteringCollimated sputtering

Hot sputtering


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RF sputtering DC sputter deposition is not suitable for insulator

deposition, because the positive charge on thetarget surface rejects the ion f lux and stop the

sputtering process. RF voltages can be coupled capacitively through the

insulating target to the plasma, so conductingelectrodes are not necessary.

The RF frequency is high enough to maintain theplasma discharge.


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During the first few complete cycles more electronsthan ions are collected at each electrode (highmobility), and cause to negative charge to be buildup

on the electrodes. Thus, both electrodes maintain a steady-state DC

potential that is negative with respect to plasmavoltage, Vp.

A positive Vp aids the transport of the slower positiveions and slow down the negative electrodes.


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The wafer will be sputtered at the same rate as thetarget since the voltage drops would be the same atboth electrodes for symmetric system.

It would thus be very difficult to deposit any materialin that way.

Smaller electrode requires a higher RF current density

to maintain the same total current as the largerelectrode.


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By making the area of the target electrode smaller thanthe other electrode, the voltage drop at the targetelectrode will be much greater than at the otherelectrode.

Therefore almost all the sputtering will occur at thetarget electrode.


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CO-sensing characteristics of

hafnium oxide thin film Surface interaction of metal oxide with gases species

includes following three steps : adsorption and desorption

, Reactions , Catalyst and catalysis .

Interaction of carbon monoxide with a thin hafnium oxide

film includes two steps.

First, molecular oxygen in the atmosphere reacts with the

oxygen vacancies in the film. Oxygen is chemisorbed in the form of O2, O, O2

depending on the operating temperature of the sensor .

The following reactions take place


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The following reactions take place :-

O2(gas) O2(ads)

O2(ads) + e O2(ads) (Top< 100 C)

O2(ads) + e 2O (ads) (100 300 C)

The initial temperature of the sensor in air was about 70

C which was also the initial sensing temperature of CO. These reactions create oxygen adsorbates and deplete

electrons from the conduction band of the semiconductingfilm.


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At temperatures higher than 200 C, the effect of these

reactions is more significant than thermal excitation, and

the conductivity decreases.

The second step involves the reaction of the tested gas

(CO in our case) with the adsorbed oxygen species.

The relevant reactions are the following:-

2CO + O2 2CO2 +e (Top< 100 C) CO + O CO2 +e (100 300 C)


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Hafnium oxide Film ApplicationsHigh-k gate dielectricsDRAM capacitors & memory applications

HfO2 system in high-quality Anti ReflectivecoatingsHumidity SensorGas sensing applications An incipient application to gas sensors has been proposed Pollution prevention and Evaluation of air quality Process Control The precise control of the air / fuel ratio


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Hafnium Oxide Gas Sensor

Experimental:Cleaning

Silicon wafers cleaned by Piranha cleaning

Chemicals used : Sulfuric Acid (H2SO4 96%) Hydrogen Peroxide (H2O2 30%)

HF (1:20)

Piranha Cleaning: 10-20 minDried in dry N2 immediately before loading in the vacuum

chamber.


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Hafnium oxide deposition done by sputteringmethod.

Target Materials HfO2Vacuum 10-6 TorrAmbient gas high purity argon gasPre-sputtered time 15 minsThin films of HfO2 sputtering voltage 0.8 kV for 60minsFilm thickness measured ~700 (by Ambios step profiler )

Gas characterization done by Kelvin Probemethod for CO and H2 gases.


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Experimental Results

Fig1 . Change in CPD with time at different concentration of CO at

room temperature.


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Fig2. Change in CPD with time at 200ppm concentration ofhydrogen at temperature.


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Fig3. Change in CPD with time at different concentration ofCO at 150,C temperature.


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Fig 4..Hafnium oxide thin film characteristics with dry airafter experiment.


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Conclusion The CO gas showed a reducing effect in fig 1

leading to a decrease in the Contact PotentialDifference (CPD).

In fig 2 concentration of CO is 2ooppm at roomtemperature showed reducing effect leadingdecrease in CPD.

fig 3 showed CO gas exposed at 150C on hafniumoxide film continuously increase in CPD withincrease in concentration of CO.

Ashok Final Presentation

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