Padua University, 16 September 2013

Sputtered thin films for corrosion

protection of targets for

radiopharmaceutical production

Anna Skliarova

Radioisotope=radionuclide is an

atom with an unstable nucleus.

Can undergo radioactive decay, resulting in the

emission of γ-ray(s) and/or α- or β-particles.

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PET- positron

emission tomography

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Radionuclides have wide medical applications:

SPECT- single photon emission computed tomography

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Radionuclides have wide medical applications:

Diagnostic

imaging

Therapeutic

applications

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SPECT/PET-CT

Amount of radionuclides produced by

cyclotrons increases year by year

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Monthly [18F]FDG activities from January 2003 to August 2011;there is a >5 fold increase in produced activities over this period.

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Cyclotron

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The most used radionuclide in PET [18F]F¯

is produced almost exclusively via

proton irradiation of [18O]H2O

18Op

18F

n

g

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Proton irradiation cause water radiolysis

H2O H2, O2, H2O2, ∙OH, H, e-aq,

HO2∙, O2-, HO2

-, OH-, H+, …

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Proton irradiated water

is extremely corrosive!

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Liquid target structure

Havar®

(Co, Cr, Fe, Ni, W, Mo, Mn, C)

Nb bulk

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Need of corrosion resistant top-coating

onto the Havar® beam window

Havar® foil corroded on beam spot

Problem 1:

Proton irradiated Water corrosion

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Problem 2:

Liquid metal embrittlement

in liquid metal cooling systems

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Liquid metal embrittlement

Problem 1:

Proton irradiated Water corrosion

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Problem 2:

Liquid metal embrittlement

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Can be solved by appropriate

protective coatings

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Refractory metals Nb, Ta, Pt, Zr

have extreme chemical resistance

Chemical inertness is mandatory,

but not enough

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Microstructure has a great influence

on corrosion process

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Which film deposition techniques

do you know?

Which one can provide the

microstructure control?

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SPUTTERING

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Experimental

technique

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Sputtering system

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Control panel

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Sputtering chamber

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Distance from target to sample – 6 cm

• grounded

• No temperature control

• No heating

• No bias

• No plasma cleaning

• liquid nitrogen-cooled

• Cooling by liquid N2

• Possible temperature control

• Possible bias

• Possible plasma cleaning

• No heating

• heated

• Heated

• Temperature control

• No bias

• No plasma cleaning

• water-cooled

• Cooling by water

• Stated temperature

• No bias

• No plasma cleaning

• No heating

Substrate holders

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Requirement for corrosion protective coatings:

Uniform thickness

Absence of pin-holes

Low porosity

Low diffusion across grain boundaries

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Analyzing technique

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Acid porosity test:

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Acid porosity test: 10% HCl , 30°C, 10 min

1 2 3 4 5

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Liquid Gallium test: 200 °C, 30 h

a) corrosion b) resistance

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X-ray diffractometry

2dsinθ=nλ

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X-ray diffractometry

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Scanning Electron Microscopie

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Scanning Electron Microscopie

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FIB SEM

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SEM, FIB SEM

SEM SEM

FIB SEM FIB SEM

Approach to corrosion

resistance

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Coating must be dense with minimal distance between grain boundaries

The best possible Diffusion Barriers are Amorphous!

Microstructure requirements:

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Amorphous films have no typical

structural defects of the crystalline state

(dislocations and grain boundaries)

Literature search for corrosion resistance

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Diffusion through the amorphous layers is very

difficult due to the irregularity of the atomic

structure

crystalline amorphous

long-range order structure lack of long-range order characteristic

How to obtain an

amorphous film?

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Approaches to obtain an amorphous metal film

Substrate Cooling

Alloying with other elements

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Experimental results

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Coating systems investigated:

Nb

Nb2O5

Nb/Nb2O5 multilayers

Nb-Ta, Nb-Zr, Ta-Zr

Nb

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Parameters investigated for Nb coatings:

substrate temperature

• -100°C ÷ 500°C

applied bias

• -150 V ÷ +80 V

sputtering gas pressure

• 310-3 mbar ÷ 310-2 mbar

deposition rate

• 0.5 nm/sec ÷ 5 nm/sec

Temperature influence

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Thornton’s Structure Zone Model

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44

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3

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Temperature influence: acid test

Floating (~250°C)

400°C

500°C

0°C

-100°C

-50°C

300°C

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4 0.5 %

-100 °C

4 0.4 %

0 °C

4 1 %

Floating

3 0.04 %

500 °C

- 0 %

800 °C

Temperature influence: SEM

Acid test (1÷5)

Optical profilometry

(%)

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4 0.5 %

-100 °C

4 0.4 %

0 °C

4 1 %

Floating

3 0.04 %

500 °C

- 0 %

800 °C

Temperature influence: FIB SEM

Substrate bias influence

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What happens if substrate is at

negative potential?

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-80 V DC

-400 V DC

grounded

DC bias -80 V -50 V -80 V -80 V -150 V

Arpressure,

mbar310-2 510-3 510-2 310-3 310-3

Porosity acid test 3 2 1 1 2

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DC-biased MS

DC-biased MS of Nb:

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SEM SEM

FIB SEM

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All Nb coatings are columnar!

Nb2O5

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Reactive sputtering of Nb2O5 :

Sputtering gas pressure

• 810-3 mbar ÷ 710-2 mbar

Stoichiometry: Ar/O2

• Ar/O2

Applied bias

• -80 V ÷ 0 V

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Right Stoichiometry:

25 sccm Ar : 19 sccm O2

Nb2O5 stoichiometry

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Liquid Ga test

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1 2

6 7

1

3

2

4

The less porous

6 7

Acid test

▬ stoichiometric

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XRD of amorphous Nb2O5:

Amorphous Nb2O5 deposition recipe:

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Ar flux:

3 sccm

O2 flux:

7 sccm

Sputtering pressure:

110-2

mbar

IDC:

0.5 A

Amorphous Nb2O5:

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SEM FIB SEM

Nb2O5 has superior corrosion protection

but..

oxides are used to be brittle

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Multilayer Nb/Nb2O5 coatings

combine:

high ductility & thermal conductivity of Nb with

excellent barrier properties of Nb2O5

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Nb/Nb2O5 multilayers

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thin thick thermal layers layers oxidation

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M 10

M 10

M 9

M 9

M 8

M 8

M 9M 10M 8

Multilayered Nb/Nb2O5

(60 nm double-layer) coatings

showed high corrosion resistance

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Best Nb/Nb2O5 multilayer recipe:

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Ar flux:

3 sccm

O2 flux:

0/7 sccm

Sputtering pressure:

310-3/ 110-2

mbar

Layer thickness:

40/20 nm

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FIB SEM FIB SEM

SEM

Thin Nb/Nb2O5 multilayer:

Nb-Ta, Nb-Zr, Ta-Zr

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Nb-Ta, Nb-Zr and Ta-Zr

were co-deposited in different ratios in

order to find amorphous metallic coating

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Sample holder

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Sample-holder for co-deposition

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Stress in thin film

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Nb Ta

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Nb Ta

After deposition ~1 μm film

3E-03 mbar

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Nb Ta

Compressive stress High compressive stress

3E-03 mbar

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Nb Ta

7E-03 mbar

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Nb Ta

Tensile stress Compressive stress

Nb Ta

7E-03 mbar

Most suitable sputtering pressure:

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Nb-Ta

• 7E-03 mbar

Nb-Zr

• 5E-03 mbar

Ta-Zr

• 8E-03 mbar

Nb-Ta

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17% Ta

84% Ta

93% Ta

0% Ta

97% Ta

100% TaSingle target sputtering

Single target sputtering

Co-sputtering

9% Ta

11% Ta

26% Ta

44% Ta

64% Ta

91% Ta

Amorphous-like

110 211200

Nb-Ta

10-85 % atomic Ta in Nb-Ta alloy

coating lead to amorphous-like structures

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Acid test

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1 2 3 4 5 6 7 8 9 10Nb↑ Ta↑Nb Ta

1 2 3

4

7

5

8 9

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Nb-Ta

Films with higher Ta content

are less porous

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Nb-Ta

Nb-Zr

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XRD analysis

Acid test

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Sample position 1 2 3 4 5 6 7 8 9 10

Nb content, % 93 89 85 79 65 46 29 - 16 7

Acid test (1÷5) 4 1 4 4 3 2 2 2 2 2

1 2 3 4 5 6 7 8 9 10Nb↑ Zr↑

1 2

6

3

7 8

4 5

9 10

Nb-Zr 5E-03 mbar

Porosity is decreasing with

decrease of Nb content

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Ta-Zr

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XRD Ta-Zr:

Acid test

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Sample position 1 2 3 4 5 6 7 8 9 10

Ta content, weight % 99 98 97 95 90 74 58 32 40 25

Acid test (1 ÷5) 1 2 1 2 2 1 1 2 1 5

1 2

6

3

7

4 5

8 9 10

All samples besides Ta-Zr 10, behave quiet good in acid test!

1 2 3 4 5 6 7 8 9 10Ta↑ Zr↑

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SEM of cross-section amorphous Ta-Zr:

Ta50Zr50

107Not sputtered Havar® substrate Havar® sputtered with Ta-Zr

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The best recipes for corrosion protection:

DC-biased or heated sputtering of Nb

Reactive sputtering of amorphous Nb2O5

Nb/Nb2O5 thin multilayers

Amorphous Ta-Zr

Thank you

for your attention!

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