The Role Of Thin Film In

Modern Particle Accelerator

Technology

Reza Valizadeh,

on behalf of ASTeC Vacuum Science

Group

Motivation

Higher Accelerating Gradient to shorten the

length of Accelerator

Low outgassing and ESD and hydrogen

diffusion barrier and providing additional

Pumping

Lowering Secondary Electron emisson to

mitigate e-Cloud and Multipacting.

SC Advantages

Power consumption is much less operating cost savings,

better conversion of ac power to beam power

.

•CW operation at higher gradient possible Less klystron power required

capital cost saving

•Need fewer cavities for CW operation Less beam disruption

Superconducting RF Cavities

Nb has the highest Tc among all pure metals and the highest

lower critical field Hc1≈170 mT (2K) among all

superconductors

Advantages of Niobium Thin Film Cavity

Thermal stability

Bulk niobium has a typical heat conductance of about 75W/mK if the purity is high, while the

copper is as high as 300 – 2000 W/mK. For a typical hot spot inside a SC cavity, say, 50-micron

Low cost

The copper is a lot cheaper than niobium, about 1/10 the cost of the niobium. Since the

dimension of the RF cavity is inversely proportional to its resonant frequency, a typical

500MHz cavity needs more than 9 times the material required for a 1,500MHz cavity with a

same cavity thickness.

Thin film is potentially free of undissolved inclusions mostly seen in bulk material

This is mostly a metallurgical issue. During the pressing, rolling and melting, etc., the

niobium sheet or the copper sheet will have some kind of micro-inclusions.

Insensitivity to earth magnetic field trapping

Residual resistance of the niobium surface is caused by many different sources. For an ideally

cleaned cavity, the residual resistance can be caused by trapped magnetic flux, hydrogen

dissolved in the surface layer, surface oxides and even some surface roughness. The residual

resistance of thin film caused by trapped magnetic field can be up to 10 times less than that of a

bulk niobium cavity.

Deposition Facilities and Parameters

Magnetron sputtering from a single

target of correct stoichiometry

(prepared by powder sintering)

Stoichiometry,

Substrate Temperature, (High Ts/Tm,

Ts is the substrate temperature, Tm is

the melting point of depositing

material.)

Deposition Pressure,

Deposition Rates,

Substrate Bias,

Deposition Power

Concurrent Ion Bombardment can be

varied independently

Niobium Pentachloride (V):

• Chosen to obtain metallic Nb layer • Reacts with plasma of H+ to create thin film • Crystalline solid, vapour pressure at 95 – 100 °C to

perform ALD • Very sensitive to moisture, hydrolyzes in NbOCl3

• Requires high substrate temperature to reduce Cl contamination in the film (at least 500 °C)

Tris(diethylamido)(tert-butylimido)niobium (V) • Chosen to obtain NbN layer • Reacts with N2 plasma to create thin film • Liquid, good vapour pressure at 70 °C to perform

ALD • Sensitive to heating, start decomposing at 130 °C • Doesn’t require a high deposition temperature

(250 °C) Suitable for deposition on copper

Initial results samples deposited without a bias

samples deposited with a 50-V bias

Surface Composition and Chemical State

XPS analysis

C 1s

O 1s

Nb 3d5/2

Nb 3d3/2

Nb 3p3/2

Nb 3p1/2

Nb 3s

As received

Ar+ bombarded

x 10 2

10

20

30

40

50

60

70

80 C

PS

600 500 400 300 200 100 0

Binding Energy (eV)

Superconductivity evaluation: RRR

0

5

10

15

20

100 600

Gro

wth

Ra

te (

nm

/min

)

Power (W)

0 V

2 RRR 22 for 70 samples studied.

Samples with RRR ≥ 10 were deposited

with 300 ≤ P ≤ 600 W.

In all cases

RRR is higher for a biased

substrate than with unbiased

However, increasing the bias

further does not always result

with increasing RRR

Film growth rate increases as a function of

power.

Growth rate decreases with biased substrate.

Decrease in film growth with bias is due to

either re-sputtering or fewer voids in denser

films

Superconductivity evaluation: DC SQUID

A typical DC magnetic susceptibility

measurement with the sample parallel

to the magnetic field.

HC2 increases with RRR at T = 6 K

Hc1

Hc2

Hsmp

Substrate

Layer 1: Nb

Layers 2,4,6,8:

MgO

Layers 3,5,7,9:

NbN

Pt

Effect Of Substrate On Epitaxial Growth

Layer 2:

MgO?

Layer 4:

MgO?

Layer 1:

Nb?

Layer 3:

NbN?

Layer 5:

NbN?

Effect Of Substrate On Epitaxial Growth

PECVD Deposition of Nb

Successfull deposition of Nb metal over copper substrate

Preliminary EDX data show lack of Cl in the film, thanks to high deposition

temperature

Low Outgassing And Additional

Pumping Surfaces

What NEG coating does

Reduces gas desorption:

– A pure metal film ~1-m

thick without contaminants.

– A barrier for molecules from

the bulk of vacuum chamber.

Increases distributed pumping

speed, S:

– A sorbing surface on whole

vacuum chamber surface

S = Av/4;

where – sticking probability,

A – surface area,

v – mean molecular velocity

Vacuum NEG Subsurface Bulk

Coating Layers

Quaternary NEG alloy film deposited on Si test

sample from twisted Ti, V, Zr, and Hf wires.

Cylindrical Magnetron: Power = 60 W, PKr > 10-1 mbar, deposition rate = 0.12

nm/s, T = 120°C.

Quaternary NEG alloy film deposited on Si test

sample from twisted Ti, V, Zr, and Hf wires.

Cylindrical Magnetron: Power = 60 W, PKr = 5x10-2 mbar, deposition rate = 0.12 nm/s,

T = 120°C. Very glassy structure.

17

XRD of Film deposited from Ternary and

Quaternary alloy wire as target.

In Both cases there is only one broad peak near 2 = 36.8°

The film is nearly amorphous.

18

TiZrVHf film deposited on Si by cylindrical magnetron using Alloy wire

Region scan of XPS core levels of Ti, Zr, C,Hf and V of a Ti-Zr-V-Hf film

(surface composition and chemical bounding)

Quaternary alloy pumping properties

Ti-Zr-Hf-V is the best

Hf-Zr-V, Ti-Zr-Hf, Ti-Hf-V and Zr

are comparable

Ti-Zr-V is lower

Zr-V (best binary alloy) has the

lowest activation temperature

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

CO

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti-Zr-Hf-V

Hf-Zr-V

Ti-Zr-Hf

Ti-Hf-V

Ti-Zr-V

Ti-Zr

Zr-V

Zr

Activation temperature [ C]

H2

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

10

CO

pum

ping

cap

acity

Mitigation Of e-Cloud And

Multipacting

Mitigation of the electron cloud built up due to

Photoemission and Secondary electron (beam instability,

beam losses, emittance growth, reduction in beam life time,

or additional heat loads on cryogenic vacuum chamber)

Mitigation of multipacting in RF wave guide and space

related high power RF hardware.

How ?

Reduce The Secondary electron Yield:

By Changing surface Chemistry

(deposition of lower SEY material)

By Engineering the surface

roughness

Mixture of the above

Existing Mitigation method

1. Coating with Low SEY Material

Ca Normal

coating

Ti-Zr-V-Hf Ti-Zr-Hf-V-N a-C at CERN

Existing Mitigation method

• Coating with a low SEY material with

submicron size structure

Ti-Zr-V black Ag plating, ion etched with Mo Mask

I. Montero et.al, Proc. e-Cloud12

Existing Mitigation method

• Modifying the surface geometry (making

mechanical grooves)

By courtesy of Y. Suetsugu

By A. Krasnov and

By L Wang et.al

• By Implementing weak solenoidal fields to trap

the electrons

• Using clearing electrode

• Using combinations of the above

Laser Treated Metal Surface

Aluminium Stainless Steel Copper

Nd:YVO4 Laser

Pulse length =12 ns at Repetition Rate = 30 kHz

For Aluminium

Max Average Power = 20 W at =1064 nm

For Copper

Max Average Power = 10 W at 532 nm

Argon or air Atmosphere

Beam Raster scanned in both horizontal and vertical direction

With an average laser energy fluence of just above the ablation

Threshold of the metal.

SEY Measurments

Is is the secondary electron current including elastic and inelastic

processes,

IP is the primary beam current.

IF and IS are the currents on the Faraday cup and the sample, respectively.

The sum of the sample current IS and the Faraday cup current IF represents

the primary energy current IP.

SEY of Cu as a function of

incident Electron energy

Untreated Laser treated

SEY of Al and SS as a function of

incident Electron energy

δmax

as a function of electron

dose for Al,306L SS and Cu

Sample Initial After conditioning to Qmax

δmax

Emax

(eV)

δmax

(Qmax

)

Emax

(eV) Qmax

(Cmm-2

)

Black Cu 1.12 600 0.78 600 3.510-3

Black SS 1.12 900 0.76 900 1.710-2

Black Al 1.45 900 0.76 600 2.010-2

Cu 1.90 300 1.25 200 1.010-2

SS 2.25 300 1.22 200 1.710-2

Al 2.55 300 1.34 200 1.510-2

900 800 700 600 500 400 300 200 100 0

Binding Energy (eV)

x 10 3

2

4

6

8

10

12

14

16

18

20

CP

S

𝟏. 𝟏 × 𝟏𝟎−𝟐 C·mm-2

electron

conditioned HD

50um Cu

Cu2p3/2

Cu2p1/2

Cu LMM

O1s Cu3s Cu3p

Cu2p3/2

Cu2p1/2

Cu LMM

O1s Cu3s Cu3p

Laser surface treatment

Latest result with laser treated

Copper: 0.58 < < 0.8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 100 200 300 400 500 600 700 800 900 1000

δ

Primary electron energy (eV)

As-received 50um Cu in Air

Heating to 250 ⁰C 50um Cu in Air As-received 60um Cu in Air

Heating to 250 ⁰C 60um Cu in Air

XPS analysis of Black Copper

900 800 700 600 500 400 300 200 100 0

Binding Energy (eV)

x 10 3

5

10

15

20

25

30

35

40

CP

S

Cu2p3/

2

Cu2p1/2

O1

s

Cu3s Cu3

p

Cu LMM

Cu2p3/

2

Cu2p1/2

Cu LMM

O1

s Cu3s Cu3p

As-received HD

50um Cu

Summary

In modern particle accelerator Thin Film has become an integrated part

of the accelerator lattice matrix.

Quality of the film (morphology, RRR, Hc2) depends on

deposition parameters such as

Substrate temperature,

Ion/atom arrival ratio,

Substrate bias,

Plasma generation at the target

pulsed or not

HiPIMS

Substrate crystallography

As the beam line diameter get smaller and smaller to save cost,ALD

and CVD will pick up where PVD reaches its limits.

NEG as thin film has dual benefit: pumping surface and hydeogen

diffusion barrier

However, Thin film may not be the correct solution in all cases,

alternative and relatively cheaper solution should be considered.

laser blackening of the metal surface is a very viable solution for

reducing the SEY < 1.