A review of the thin film techniques potentially applicable to cavities

Rosa A. Lukaszew, B. Burton, M. Beebe, William M. Roach,College of William and Mary, Williamsburg, Virginia, USA

C. Clavero, LBNLGrigory V. Eremeev, Charles E. Reece, Anne-Marie Valente-Feliciano, L. Phillips

Thomas Jefferson National Accelerator Facility [TJNAF], Newport News, Virginia, USA

Why thin films?In the 2012 workshop at Jlab L. Phillips pointed out:

• Cost is the main driver– Thin film niobium cavity substrates are of castable metals, such as copper or

aluminum.– Enables integration of many system functions in a single low-cost structure,

i.e. an aluminum casting.– Integrated functions– Cavity RF surface definition– High thermal conductivity substrate– Helium vessel– Cryogenic manifold with heat exchanger– Cavity stiffening, and many more……….

• Future applications of SRF linacs for which cost is a driver: ILC, or LEP3, ADS, medical, light sources, ….

The Challenge of Nb-coated SRF Cavities:Coatings have the promise of very substantial savings

• bulk Nb cavities are very expensive• the gradient challenge: higher gradient

implies less cavities, significant savings• potentially even greater savings with

cavities with Nb films • so far mixed results for sputtered films

– adhesion issues– low RRR– Q-slope at high field– breakdown even at relatively low field

Fig. 4 of C. Benvenuti et al., Physica C: Superconductivity 351, 421 (2001).

best sputtered Nb films on electropolished Cu cavities (Benvenuti)

typical bulk Nb cavities(Padamsee)

o Nb films on copper are considered a proven technology for up to about 10 MV/m

o LEP (1996): 216 cavities with sputtered Nb, 6 MV/m with Q0 = 3.4 x 109 at 4.5 K

o the quality factor Q0 of magnetron-sputtered cavities slopes down with increasing RF electric field

Other advantages

• The micro-structure and contaminant levels of the RF surface can –in principle- be controlled.

• Stable diffusion barriers can be added in situ impeding the development of oxides or other forms of atmospheric degradation after exposure to air.

• Other materials can be considered.But:• Practical issues that need attention:

– Control of film thickness over complex shapes– Materials of relevance are compounds and techniques for coating the

interior of cavities while maintaining stoichiometry and SRF proper ties within restricted limits of film thickness also requires control.

Some History of thin films for SRF cavities

• Since SRF is a surface phenomenon where only ~10 penetration depths are needed (=40 nm for niobium), it was recognized for some time now that it would be economically convenient to use thin film coated cavities.

• Earlier attempts at CERN applied standard sputter PVD methods, but the grain size for the CERN Nb/Cu films was 100 nm, which is 10,000 times smaller than for conventional SRF cavities with the ensuing problems that appear at grain boundaries.

• Thus, these prior attempts showed higher surface resistance and worst Q-slope than bulk.

Previous reviews• A 2006* review by Sergio Calatroni of CERN discusses some of the

problems:– Defects within 1 or 2 l of the surface or on the surface. Insufficient

attention has been paid to this topic, including trapping of impurities like oxygen in defects.

– The grain size for the CERN Nb/Cu films is 100 nm. This is 10,000 times smaller than for conventional SRF cavities, (for which grain sizes are > 1 mm and are not important).Grain boundaries are themselves one-dimensional defects. Grain boundary diffusion is much faster than diffusion in the bulk Nb.

– Local thermal conductivity of the film itself may be poor compared to bulk Nb.

– Interfacial thermal resistance, also known as thermal boundary resistance, or Kapitza resistance at two interfaces: Nb/Cu and Cu/LHe(II)

* Physica C: Superconductivity, Volume 441, Issues 1–2, 15 July 2006, 95–101

Coating Nb on SRF Cavities is promising but challenging

• Electroplating: not clean enough• atomic layer deposition: promising but slow• sputtering in UHV: low RRR, low Q• filtered cathodic arc in UHV: tricky particle and geometry issues• emerging: sputtering technology with ionization

o Film issues include adhesion purity defects (like substrate defects and

particulates) grain size and texture stress (intrinsic and thermal)

thermal conductivity of base material at cryogenic temperature can be better than bulk niobium (copper!)

affecting SRF performance

Classify thin films

• Crystalline– atoms show short and

long range order

• Polycrystalline• Amorphous

– atoms show short range order only

– Glasses; not stable state for most pure metals; generally less dense than crystalline materials.

• Typical defects: – grain boundaries– Dislocations– Point defects– Surface roughness

Can we understand TF nucleation?

• Nucleation from a liquid phase to a solid depends on:– Liquid phase instability (going through a phase change

from higher to lower T)– Diffusion of atoms into clusters (increases with T)

Film formation

• Competing Processes• adding to film:

– impingement (deposition) on surface

• removing from film:– reflection of impinging

atoms– desorption (evaporation)

from surface

• Steps in film formation:1. thermal

accommodation2. binding3. surface diffusion4. nucleation5. island growth6. coalescence7. continued growth

How do nuclei grow initially?

From kinetic theory of gases

• How many gas molecules collide with a surface each second ?

• How long does it take to form a complete layer of gas on a surface?

pressure tm

1 atm 2 x 10-9 sec 10-6 torr 2 seconds 10-9 torr 31 minutes

Contamination

• PROBLEM: residual gas in chamber gives two "sources" impinging

• evaporant:

• residual gas:

Impurity concentration

SOLUTION:

• better vacuum• higher deposition rate

Sputter deposition

• target atoms and ions impinge• electrons impinge• Ar atoms impinge

– Ar pressure about 0.1 torr

• Ar may be incorporated into film• energetic particles may modify

growth• substrates heat up

Variations• Ion assisted deposition (IBAD)

– with evaporation or sputtering (or chemical vapor deposition)

• bombard surface with ions– not necessarily same type as in film

• ions typically NOT incorporated in film

• relatively low voltages (50 - 300 eV)• leads to

– physical rearrangement– local heating

• can change film properties– for better or worse

• disruption of columnar (fiber) growth requires about 20 eV of added energy per depositing atom

• Reactive Sputter deposition• add reactive gas to chamber during

deposition (evaporation or sputtering)– oxygen, nitrogen

• chemical reaction takes place on substrate and target

• can poison target if chemical reactions are faster than sputter rate

• adjust reactive gas flow to get good stoichiometry without incorporating excess gas into film.

Arc• high current, low voltage

discharge initiate by touching electrode surfaces and then separating trigger arc by high voltage breakdown

• produces large numbers of electrons

• very efficient ionization of film atoms (almost 100 %)

• impinging ions may be high energy– enhanced chemical reactions– film densification

Plasma sources• plate electrodes

– low plasma densities (109 - 1010 charged particles per cm3)

– common in sputter deposition

• Inductively coupled plasma (ICP)– high plasma densities (1011 - 1012

charged particles per cm3)– operates well at lower gas

densities (< 50 mTorr)– can be used up to atmospheric

pressures (and beyond)– couple RF energy inductively into

plasma (lossy electrical conductor)– produces more efficient ionization

• Electron cyclotron resonance (ECR)– high plasma densities (1012 - 1013

charged particles per cm3)– operates well at lower gas

densities (down to 0.1 mTorr)– couples microwave energy to

electrons by matching frequency to electron gyration frequency

– produces more efficient ionization– control the plasma density with

microwave power and gas pressure

– can also control ion species created.

A note on metallic thin films• The properties of thin films

depend on their microstructure.• The stability of thin metal films

depends on being deposited on appropriate substrates.

• Important characteristics are residual stress and strain, which often develop in film-substrate combinations An unfavorable consequence of high stress is crack formation, local plastic deformation, and layer delamination.

• Residual stresses, which are commonly assumed to be biaxial in thin films, result from different thermal expansion coefficients of substrate and film (thermal stress) and/or from stress formation during film deposition (grown-in stress)

• In polycrystalline films a central mechanism that governs stress relaxation by inelastic deformation is thought to be atomic diffusion, predominantly along grain boundaries.

Examples

• Quantitative, quasisimultaneous in situ characterizations of the modification of vacancy concentration and of residual strain in metallic films have been carried out for particular cases (e.g Pt thin films, PRL 107, 265501, 2011).

• This work was based on based on x-ray scattering techniques. This has the advantage that the use of synchrotron radiation becomes possible, which allowed to carry out time-resolved studies to measure fast relaxation processes taking place on a time scale of minutes.

• In order to detect directly the modification of the vacancy concentration, x-ray diffractometry (XRD) was used to determine the of the out-of-plane lattice parameter a and x-ray reflectivity (XRR) was used to detect the film thickness.

SRF Thin film coating approaches

• CVD (L. N. Hand, Cornell, USA) and ALD (T. Proslier, ANL) have been explored.

• A hybrid physical-chemical vapor deposition (HPCVD) technique at Temple University has resulted in optimal MgB2 films.

• Energetic PVD processes such as ECR (A.-M. Valente-Feliciano, Jlab; private companies), HiPIMS (A. Anders, LBNL) vacuum-arc (R. Russo) have also been reported as suitable techniques for this application.

Opportunities for energetic PVD Niobium Films

• Two major limitations of conventional magnetron sputtering are:– Low deposition energy of arriving atoms limiting control of film

structure– Presence of argon gas being incorporated into the growing film in

addition to spatial limitations of the plasma• Both issues are eliminated by using niobium ions in vacuum to

deposit films.– Control of film microstructure through energetic condensation– High adatom surface mobility– Sub-plantation– Grain competition driven by incident ion energy selective sputtering,

channeling, surface energy of crystal face

Energetic Condensation• Energetic condensation is a deposition process where a significant fraction of the condensate

has hyper thermal energies (energies ‐ ³10 eV ). A number of surface and subsurface processes are activated or enabled by the energy of the particles arriving at the surface (e.g. desorption of adsorbed molecules, enhanced mobility of surface atoms, and the stopping of arriving ions under the surface). The purpose of using energetic condensation deposition methods is to improve film structure while keeping the substrates at lower temperature by adding energy to the film during condensation to compensate for lack of thermally induced growth processes.

• For example crystalline defects, grains connectivity and grain size may be improved with a higher substrate temperature that provides higher surface mobility. However the substrate used may not allow substantial heating and in such case the missing energy may be supplied by ion bombardment. In bias sputter deposition a third electron accelerates the sputtering gas ions, removing the most loosely bound atoms from the coating, while providing additional energy for higher surface mobility

• One possible process, ion beam assisted deposition (IBAD), uses a secondary source of ions to co bombard the film from conventional sources during growth.‐

• A second process, direct ion deposition, uses vacuum plasmas formed from the material being deposited to produce a film grown from metal ions.

Approach: Use a plasma-based technology for “Energetic Condensation”

A. Anders, Thin Solid Films 518 (2010) 4087

Generalized Structure zone Diagram (2010), derived from Thornton’s diagram for

sputtering (1974)

High Power Impulse Magnetron Sputtering( a form of IBAD)

Copper target 2” magnetron

A. Anders(LBNL)

target

substrate

atoms to substrateions to substrate

Sustained self-sputtering

1

Probability for ions

to return to the target

Ionizationprobability

yield

Self-sputtering runaway1

adapted from: A. Anders, J. Vac. Sci. Technol. A 28 (2010) 783

Illustration of Self-sputtering

Our work using DC magnetron sputtering and reactive sputtering at W&M

• We have investigated the effect of microstructure and morphology on the superconducting properties of Nb thin films deposited onto different ceramic surfaces and metallic surfaces.

• In particular we studied a-plane sapphire and (001) MgO and Cu (001).

• We monitored the microstructure of the films, the morphology of the surface and the superconducting properties as well as the DC properties.

• We explored several aspects in the thin film deposition parameters-space, such as growth rate, substrate temperature during growth, annealing treatments, etc.

Nb growth on a-plane sapphire• Nb can grow epitaxially on a-plane sapphire, with Nb(110)//Al2O3(11-20)

* RRR values for niobium thin films is highly dependent on thickness[1]. S. A. Wolf et al., J. Vac. Sci. Tecnol. A 4 (3), May/June 1986[2] G. Wu et al., Thin Solid Films, 489 (2005) 56-62

Group Nb film thickness (nm)

RRR

Lukaszew 600 97

S. A. Wolf [1] 600 82

G. Wu [2] 235 50.2*

Comparison of RRR values obtained by different groups:

Early stages of growth

1 10 100

0.29

0.30

0.31

0.32

0.33

0.34

0.35

0.36

0.23 2.3 23

Nb thickness (nm)

Lat

tice

par

ame

ter

(nm

)

Nb atomic layers

bulk Nb bcc

[111]Nb ll[0001]Al O2 3

[1120]Nb ll [0001]Al O2 3

a

hcp Nb

bcc Nbhcp

+b

cc N

b

a

• Using Reflection high energy electron diffraction (RHEED), we observed a hexagonal Nb surface structure for the first 3 atomic layers followed by a strained bcc Nb(110) structure and the lattice parameter relaxes after 3 nm.

• RHEED images for the hexagonal phase at the third atomic layer. Patterns repeat every 60 deg.

0 deg 30 deg 60 deg

Susceptibility AC measurements

• The thinner Nb film exhibits two steps in the χ’ susceptibility transition accompanied by two peaks in the χ’’ susceptibility due to strained Nb layers at the interface.

• Growth on a-plane sapphire initially follows a hexagonal surface structure to relax the strain and to stabilize the subsequent growth of bcc Nb(110) phase.

• Such initial layers affect the superconducting properties of the films and these effects must be taken into account in the design of multilayers.

0.0

0.1

-1

0

0.0

0.1

-1

0

7 8 9 10

0.0

0.2

7 8 9 10

-1

0

''

'

600 nm

100 nm

Temperature (K)

Temperature (K)

30 nm

χ(ω)= χ’(ω)+i χ’’(ω)Strain Effects on the Crystal Growth and Superconducting Properties of Epitaxial Niobium Ultrathin Films, C. Clavero, D. B. Beringer, W. M. Roach, J. R. Skuza, K. C. Wong, A. D. Batchelor, C. E. Reece, and R. A. Lukaszew, Cryst. Growth Des., 12 (5), pp 2588–2593 (2012)

400 nm

( a ) 30 nm Nb

.

200 nm

( c ) 600 nm Nb

200 nm

( b ) 100 nm Nb

0 500 1000

0

10

20

30

30 nm

100 nm

heigth

(nm

)

distance (nm)

600 nm

AlO

[000

1]2

3

Al O [1100]2 3

( d )

Biaxial anisotropy is observed for thicknesses up to 100 nm while uniaxial anisotropy is observed. For thicker films

Nb growth on (001) MgO

• Nb can also be epitaxially grown on (001) MgO surfaces.

• Unexpected findings:We have found that depending on the deposition conditions it is possible to tailor different epitaxial possibilities.

RHEED images for Nb(110) on MgO

Scaling of surface features

RRR = 46.5 RMS = 6.51 nm

50 nm 600 nm

Nb (001) on MgO 14.29 nm

0.00 nm

400nm

RRR = 165 RMS = 4.06 nm>200 RRR values!

30.00 nm

0.00 nm

1.0µm

RMS = 2.90 nm

10.00 nm

0.00 nm

400nm

10.00 nm

0.00 nm

200nm

RMS = 1.21 nm

RMS = 1.08 nmD. B. Beringer, W. M. Roach, C. Clavero, C. E. Reece, and R. A. Lukaszew, "Roughness analysis applied to niobium thin films grown on MgO(001) surfaces for superconducting radio frequency cavity applications," Phys. Rev. ST Accel. Beams 16, 022001 (2013).

4 5 6 7 8 9 10-0.2

0.0

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"

Temperature (K)

-1.0

-0.8

-0.6

-0.4

-0.2

0.04 5 6 7 8 9 10

'

SQUID characterization

Tc = 9.2 K!

Possible lossdue to interfacialstrain

Nb on Cu (111)

• Growth at room temperature and annealing at 350 ºC leads to the crystallization of Nb islands in a hexagonal surface structure, even though Nb is expected to growth tetragonal (110).

3.30 Å

0.00 Å

3.3 Å

0 ÅCesar Clavero, Nathan P. Guisinger, Srivilliputhur G. Srinivasan, and R. A. Lukaszew, “Study of Nb epitaxial growth on Cu(111) at sub-monolayer level”, J. Appl. Phys. 112, 074328 (2012).

Nb films on Cu (001) surfaces

(a) RHEED pattern for Nb(110)/Cu(100)/Si(100) along the Si[100] and Si[110] azimuths. (b) A representative 2 µm x 2 µm AFM scan for Nb films on the Cu template.

Possible Nb/Cu(100) epitaxy:

SC properties for different growth T• The films grown at 150 °C have

a very sharp transition from the superconducting state to the normal state that begins at ~9 K while films grown at RT have a much more gradual transition.

• Our results suggest that an increased deposition temperature of Nb onto Cu leads to films with higher crystalline quality (grain size) and thus improved superconducting properties (HC1).

Niobium thin film deposition studies on copper surfaces for superconducting radio frequency cavity applications, W. M. Roach, D. B. Beringer, J. R. Skuza, W. A. Oliver, C. Clavero, C. E. Reece, and R. A. Lukaszew, Phys. Rev. ST Accel. Beams 15, 062002 (2012).

Characterization

1. Property that matters (e.g. SRF impedance, Q, etc)

2. Correlation with microstructure, surface morphology, DC transport (RRR) and DC

magnetic properties (Hc1)

What do we want to know ? How do we find this out ?What does the sample look like ?•on a macroscopic scale•on a microscopic scale•on an atomic scale

•optical microscopy•scanning electron microscopy (SEM)•transmission electron microscopy (TEM)•scanning probe microscopies (STM, AFM ...)

What is the structure of the sample ?•internal structure•density•microscopic and atomic scales

•X-ray diffraction (XRD)•low energy electron diffraction (LEED)•reflection high energy electron diffraction (RHEED)

What is the sample made of ?•elemental composition•impurities•chemical states

•Auger Electron Spectroscopy (AES)•Energy Dispersive Analysis of X-rays (EDAX)•X-ray Photoelectron Spectroscopy (XPS)•Secondary Ion Mass Spectrometry (SIMS)•Rutherford Backscattering (RBS)

What are the optical properties of the sample ?•refractive index, absorption•as a function of wavelength

•ellipsometry

What are the transport properties of the sample?• resistance • Surface impedance

•resistance - four point probe•SIC

What are the mechanical properties of the sample ?•internal stress in films / substrates•adhesion

•stress curvature measurements•adhesion tests

Important

• What exactly are we probing?– E.g. XRD typically probes

films in the growth direction. It provides average microstructure information.

– E.g. the grain size extracted from the width of peaks is along the z-direction.

– RHEED, LEED, TEM provide local microstructure information.

– E. g. SEM provides coarser information regarding surface morphology than AFM/STM.

– Optical techniques (ellipsometry) can provide information regarding density of the films.

– It is important to correlate more than one technique for complementary characterization and acquire a more complete description of the sample.

Good prospects for next SRF films:Energetic condensation (ECR, HiPIMS)

• As a result of these fundamental changes, energetic condensation allows the possibility of

• controlling the following film properties:

• the density of the film may be modified to produce improved optical and corrosion-resistant coatings

• the film composition can be changed to produce a range of hard coatings and low friction surfaces

• crystal orientation may be controlled to give the possibility of low temperature epitaxy.‐

• The additional energy provided by fast particles arriving at a surface can induce the following changes to the film growth process:– residual gases are desorbed from

the substrate surface– chemical bonds may be broken

and defects created thus affecting nucleation processes and film adhesion

– film morphology changes– microstructure is altered– stress in the film is altered

Summary

• Higher energetic condensation offers the most promise for better performing SRF films.

• There is sufficient evidence of better addition as well as conformal growth.

• Still needs more R&D to achieve real “bulk-like” films!