Photocatalytic Degradation of Organics

Elizabeth Buitrago

University of Arizona

Department of Chemical and Environmental Engineering

Grad Student Mentor: Mike Schmotzer

Faculty Advisor: Dr. Farhang Shadman

UPW Use

• Wet standard operations account 1/3 of total processing steps:– Standard cleans.– Wet etch processes. – Chemical mechanical planarization (CMP)

• Wafer is redundantly cleaned to remove contaminants and prepare the surfaces between processes.

The Concern

• More than 3000 gallons of UPW can be used to process an 8 inch wafer from start to finish.

• Present semiconductor fabrication facilities (FABS) typically use 1-3 million gallons of UPW per day.

• Final UPW quality highest of any industry.

• Contaminants remaining in water end up in wafer surfaces, render a device non-functional.

Water Quality Parameter

Units Typical Municipal Water Supply

Typical Ultrapure Water Product

Resistivity M ohms-cm 0.004 >18

pH Units 8 6

TOC ppb 3500 <10

Ammonium ppb 300 <1

Calcium ppb 22000 <1

Magnesium ppb 4000 <1

Potassium ppb 4500 <10

Silica ppb 4780 <1

Sodium ppb 29000 <1

Chloride ppb 15000 <1

Fluoride ppb 740 <1

Sulfate ppb 42000 <1

Overview

• Goals and objectives

• Introduction/ background– TiO2 as a photocatalyst/photocatalytic process

– Role of promoters in catalytic oxidation (Ag)

– Effects of nitrogen doping in TiO2

• Experimental

• Results/Highlights

• Future goals

Goals and Objectives

• Develop new method for photocatalytic oxidation of organics:– Lower the energy use through catalytic oxidation

(UV 185nm used 2003 UV 254 nm used 2004).– Reduce the use of chemicals.

Introduction: Photocatalytic Process

Photo-generation

electron/hole pairs

Formation of radicals

Radical oxidation of

Organic compound.

TiO2 hv

e- + h+

h+ + H2O OH + H+

e- + O2 O2-

O2- + H+ HO2

TOC + Ox(radical) TOC (partially oxidized species) + CO2 + H2O

Role of Promoters in Photocatalytic Process

)M(eeM

(energy)TiOhe 2

Photo-generation electron/hole

pairs

Formation of radicals (Ox- radical)

Radical oxidation of organic

compound.

Recombination of electron/hole pair

Metal attracts free electron

slows recombination and

promotes radical formation

TiO2 hv

e- + h+

h+ + H2O OH + H+

e- + O2 O2-

O2- + H+ HO2

TOC + Ox TOC(partially oxidized species) + CO2 + H2O

Conduction Bande- e- e- e- e- e- e- e- e- e- e- e-

Role of Promoters in TiO2 Photocatalytic process

Valence Bandh+ h+ h+ h+ h+ h+ h+ h+ h+ h+

Electron/hole pairrecombination

Electron/hole pair generation

Metallic promoter attracts electrons from TiO2 conduction band and slows recombination reaction

e-(M) <-- M+e-

Eg

Effects of Nitrogen Doping in TiO2

TiO2 BondOrbitals

TiO2-xNx BondOrbitals

Conduction Band

Ti d + (O2p) Ti d +O2p +N2p)

Valence Band

N2p + O2p

O2P + (Ti d) + (Ti d)

Energy

Ti d

O2p

Ti dN2pO2p

Addition of nitrogen increases the size of the bondorbitals, decreasing the energy bandgap

Eg = 3.2 eV Eg = 2.5 eV

Experimental

-Sol-gel method #1 3-TiO2 layers3-bakes

-Sol-gel method #23-TiO2 layers2-extra TiO2 coats Ag dopedbefore 3rth bake

-CVD method N2 doped

-Ethylene glycol-urea-Triton X-100 surfactant contaminants

Preparation of Supported Catalyst by ChemicalVapor Deposition Method (CVD)

Experimental Setup

14

Impregnation chamber

23HP

nitrogen cylinder

Stripper

TiCl4 reservoir

Experimental Setup for Batch Reactivity Testing

UV lamp 254 nm

Water bath/shaker/

lamp holder Coated screens

Ethylene glycol/UV 254 nm

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140

time (hrs)

TO

C (ppb)…

TiO2#2

TiO2#1

CVD

Results and Highlights

Ethylene glycol

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120

time(hrs)

TO

C (ppb)... TiO2

TiO2/Ag

Results and Highlights

Sol-gel method #2 used

Triton X-100

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200time (hrs)

TOC

(ppb

)... TiO2

TiO2/Ag

Urea

0

100

200

300

400

500

600

700

800

900

1000

0 20 40 60 80 100 120

time (hrs)TO

C (ppb)...

TiO2TiO2/Ag

Results and Highlights

Sol-gel method#2 used

Ethylene glycol/ 254 nm

0

100

200

300

400

500

600

700

800

0 20 40 60 80 100 120time (hrs)

TO

C (ppb)...

CVD/ N2blank

Results and Highlights

Model for Photocatalytic Reaction

evap

k

2k

22k

k2

2k

k2

r)(TOC)(OHkdt

d(TOC)

)(OHk)(TOC)(OHk)(hkdt

)d(OH

)(ek))(h(ekSkdt

)d(e

)(hk))(h(ekSkdt

)d(h

)M(eeM

OHOH

OHCOTOCOH

HOHOHh

(energy)TiOhe

hehvTiO

3

432

m11

211

m

4

3

2

1

1

1. Electron/hole formation

2. Electron/hole recombination

3. Radical formation

4. Oxidation of organics

5. Radical combining with X (anything other than TOC)

6. Metal attracts electron = 0 not metal present.

Ethylene glycol/UV 254 nm

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140time (hrs)

TO

C (ppb)

TiO2#2

TiO2#1

CVD

model #2

CVD model

model #1

Photocatalytic Model

TiO2#1 S = 3.5 CVD S = 10 TiO2 #2 S = 14 cm2

S = active surface area

Triton X100/ UV 254 nm

0

200

400

600

800

1000

0 50 100 150 200

time (hrs)

TO

C (ppb) model TiO2/Ag

model TiO2

TiO2

TiO2/Ag

Urea/ UV 254 nm

0

200

400

600

800

1000

0 20 40 60 80 100 120

time (hrs)

TO

C (p

pb)

model TiO2/Ag

model TiO2

TiO2

TiO2/Ag

Ethyl glycol/ UV 254 nm

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140

time (hrs)

TO

C (ppb) TiO2/Ag

TiO2

TiO2/Ag model

TiO2 model

Photocatalytic Model

OHCOTOCOH 22k3 +¾ ®¾+·

Triton X100 k3 = 0.6ethylene glycol k3 = 0.4Urea k3 = 0.05

Future Goals

• Find new substrates for better deposition of TiO2.

• Investigate new ways that would improve our TiO2 loading method.

• Improve CVD method.

• Improve nitridation method.